ArticlePDF Available

Inclusive, prompt and non-prompt J/ψ production at midrapidity in p-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV

Authors:

Abstract

A bstract A measurement of inclusive, prompt, and non-prompt J/ ψ production in p-Pb collisions at a nucleon-nucleon centre-of-mass energy $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 5 . 02 TeV is presented. The inclusive J/ ψ mesons are reconstructed in the dielectron decay channel at midrapidity down to a transverse momentum p T = 0. The inclusive J/ ψ nuclear modification factor R pPb is calculated by comparing the new results in p-Pb collisions to a recently measured proton-proton reference at the same centre-of-mass energy. Non-prompt J/ ψ mesons, which originate from the decay of beauty hadrons, are separated from promptly produced J/ ψ on a statistical basis for p T larger than 1.0 GeV/ c . These results are based on the data sample collected by the ALICE detector during the 2016 LHC p-Pb run, corresponding to an integrated luminosity $$ \mathcal{L} $$ L int = 292 ± 11 μ b − 1 , which is six times larger than the previous publications. The total uncertainty on the p T -integrated inclusive J/ ψ and non-prompt J/ ψ cross section are reduced by a factor 1.7 and 2.2, respectively. The measured cross sections and R pPb are compared with theoretical models that include various combinations of cold nuclear matter effects. From the non-prompt J/ ψ production cross section, the $$ \mathrm{b}\overline{\mathrm{b}} $$ b b ¯ production cross section at midrapidity, $$ {\mathrm{d}\sigma}_{\mathrm{b}\overline{\mathrm{b}}} $$ d σ b b ¯ / d y , and the total cross section extrapolated over full phase space, $$ {\sigma}_{\mathrm{b}\overline{\mathrm{b}}} $$ σ b b ¯ , are derived.
JHEP06(2022)011
Published for SISSA by Springer
Received:May 27, 2021
Revised:March 22, 2022
Accepted:April 28, 2022
Published:June 3, 2022
Inclusive, prompt and non-prompt J/ψproduction at
midrapidity in p-Pb collisions at sNN = 5.02 TeV
The ALICE collaboration
E-mail: ALICE-publications@cern.ch
Abstract: A measurement of inclusive, prompt, and non-prompt J/ψproduction in p-Pb
collisions at a nucleon-nucleon centre-of-mass energy sNN = 5.02 TeV is presented. The
inclusive J/ψmesons are reconstructed in the dielectron decay channel at midrapidity
down to a transverse momentum pT= 0. The inclusive J/ψnuclear modification factor
RpPb is calculated by comparing the new results in p-Pb collisions to a recently measured
proton-proton reference at the same centre-of-mass energy. Non-prompt J/ψmesons, which
originate from the decay of beauty hadrons, are separated from promptly produced J/ψ
on a statistical basis for pTlarger than 1.0 GeV/c. These results are based on the data
sample collected by the ALICE detector during the 2016 LHC p-Pb run, corresponding to
an integrated luminosity Lint = 292 ±11 µb1, which is six times larger than the previous
publications. The total uncertainty on the pT-integrated inclusive J/ψand non-prompt
J/ψcross section are reduced by a factor 1.7 and 2.2, respectively. The measured cross
sections and RpPb are compared with theoretical models that include various combinations
of cold nuclear matter effects. From the non-prompt J/ψproduction cross section, the bb
production cross section at midrapidity, dσbb/dy, and the total cross section extrapolated
over full phase space, σbb, are derived.
Keywords: Heavy Ion Experiments
ArXiv ePrint: 2105.04957
Open Access, Copyright CERN,
for the benefit of the ALICE Collaboration.
Article funded by SCOAP3.
https://doi.org/10.1007/JHEP06(2022)011
JHEP06(2022)011
Contents
1 Introduction 1
2 Data analysis 3
2.1 Inclusive J/ψ4
2.2 Determination of the non-prompt J/ψfraction 8
3 Results 12
4 Summary 20
The ALICE collaboration 29
1 Introduction
The production of the J meson in hadronic interactions represents a challenging testing
ground for models based on quantum chromodynamics (QCD). In protonproton (pp) and
protonantiproton (pp) collisions, charmonium production has been intensively studied
experimentally at the Tevatron [14], RHIC [5,6] and the LHC [719], and it can be
described in the framework of the non-relativistic quantum chromodynamics (NRQCD)
effective theory [2028].
In heavy-ion collisions, charmonium production is highly sensitive to the nature of
the hot and dense matter created in these collisions, the quarkgluon plasma (QGP),
see refs. [23,24,29] for recent reviews. For a precise interpretation of the heavy-ion
results, detailed comparisons with both the reference results obtained in elementary pp
collisions and those in protonnucleus (pA) collisions are indispensable. The latter is
used to disentangle effects due to interaction between the charmonium states and the QGP
medium created in heavy-ion collisions from those that can be ascribed to cold nuclear
matter (CNM). In fact, the nuclear environment affects the free nucleon parton distribution
functions (PDFs), inducing modifications that depend on the parton fractional momentum
xB, the four-momentum transfer squared (Q2) and the mass number A, as first discovered
by the European Muon Collaboration [30]. The modified distributions can be described
using nuclear parton distribution functions (nPDFs) [3133]. In the xBand Q2domain of
“shadowing” reached for nuclear collisions at LHC energies in charm and beauty production,
the parton density, and most notably the one of the gluons, is reduced with respect to the
free nucleon [3436]. At very small xBvalues, where the gluon density becomes very
large, the nuclear environment is expected to favour a saturation process, which can be
described using the colour glass condensate (CGC) effective theory [3739]. In addition, in
the nuclear environment partons can lose energy via initial-state radiation, thus reducing
1
JHEP06(2022)011
the centre-of-mass energy of the partonic system [40], experience transverse momentum
broadening due to multiple soft collisions before the production of the pair [4143], or
loose energy through coherent effects [44]. Finally, once produced, the charmonium state
could be dissociated via inelastic interactions with the surrounding nucleons [45]. This
process, which is dominant among the CNM effects at low collision energy [46,47], should
become negligible at the LHC, where the crossing time of the two nuclei is much shorter
than the resonance formation time [4850].
The comparison of the production measured in pA collisions to the one in pp collisions
allows the CNM effects to be constrained. The size of these effects can be quantified by
the nuclear modification factor, which is defined as the production cross section in pA
collisions (σpA) divided by that in pp collisions (σpp )scaled by the mass number A,
RpA(y, pT) = 1
A
d2σpA/dydpT
d2σpp/dydpT
,(1.1)
where yis the rapidity of the observed hadron in the nucleonnucleon centre-of-mass frame,
and pTits transverse momentum. In the absence of nuclear effects, RpA is expected to be
equal to unity.
The inclusive J yield is composed of three contributions: prompt J produced
directly in the primary hadronic collision, prompt J produced indirectly via the decay
of heavier charmonium states such as χcand ψ(2S), and non-prompt J from the decay
of beauty hadrons (hb). By subtracting the non-prompt component from the inclusive J
production, more direct comparisons with models describing the charmonium production
can be considered. However, this contribution is not large, in particular at low pT(10
20% for pT<10 GeV/c[51]), where the bulk of the production is located, and the inclusive
J production represents already a valuable observable.
The measurement of the non-prompt component gives access to study open beauty pro-
duction through the inclusive decay channel hbJ + X. In pp collisions, the production
cross section of beauty hadrons can be computed with factorisation approaches, either in
terms of Q2(collinear factorisation) [52] as a convolution of the PDFs of the incoming
protons, the partonic hard-scattering cross sections, and the fragmentation functions, or
of the partonic transverse momentum kT[53]. The cold-medium processes that affect the
charmonium production in protonnucleus and nucleusnucleus collisions can also affect
the beauty hadron production [3143,54]. Also in this case, the nuclear modification factor
RpA can be useful to study these effects.
Charmonium and open-beauty production cross sections in pPb collisions have been
measured at LHC energies by the ALICE [50,51,5559], ATLAS [14,60], CMS [15,6163]
and LHCb [6467] collaborations over a wide range of rapidity and transverse momentum.
Thanks to its moderate magnetic field, particle identification capability, and low material
budget of the tracking system in the central barrel, the ALICE apparatus has a unique
coverage for J measurements at midrapidity and low transverse momentum. Previous
ALICE measurements were published based on the pPb data sample collected in 2013 [51,
56,57,68]. This paper presents new measurements of the pT-differential cross sections for
the inclusive, prompt, and non-prompt J production in pPb collisions at sNN =
2
JHEP06(2022)011
5.02 TeV, using the data sample collected in 2016, which is six times larger than that of
2013. Moreover, the cross section of inclusive J production measured in pp collisions at
s= 5.02 TeV [7] is used to derive the RpA results instead of the interpolation procedure
adopted in the previous p-Pb publication [57]. Therefore, the new results, which are
significantly more precise and are obtained differentially in pTand in finer pTintervals,
supersede the measurements published in refs. [51,57].
2 Data analysis
A complete description of the ALICE apparatus and its performance is presented in
refs. [69,70]. The central-barrel detectors employed for the analysis presented in this
paper are the Inner Tracking System (ITS) and the Time Projection Chamber (TPC). The
ITS [71] provides tracking and vertex reconstruction close to the interaction point (IP). It
is made up of six concentric cylindrical layers of silicon detectors surrounding the beam
pipe with radial positions between 3.9 cm and 43.0 cm. The two innermost layers consist of
Silicon Pixel Detectors (SPD), the two central layers are made up of Silicon Drift Detectors
(SDD), and the two outermost layers of Silicon Strip Detectors (SSD). The TPC [72] con-
sists of a large cylindrical drift chamber surrounding the ITS and extending from 85 cm to
247 cm along the radial direction and from 250 cm to +250 cm along the beam direction
(z) relative to the IP. It is the main ALICE tracking device and allows also charged par-
ticles to be identified through specific energy loss (dE/dx) measurements in the detector
gas. Both the TPC and ITS are embedded in a solenoidal magnet that generates a 0.5 T
magnetic field along the beam direction. They cover the pseudorapidity interval |η|<0.9
and allow J mesons to be reconstructed through the e+edecay channel in the central
rapidity region down to zero pT.
The measurements presented in this paper are based on the set of minimum bias
(MB) pPb collisions at sNN = 5.02 TeV collected in 2016 during the LHC Run 2 data
taking period, corresponding to an integrated luminosity Lint = 292 ±11 µb1. The
latter is determined from the number of MB events and the MB-trigger cross section,
which was measured via a van der Meer scan, with negligible statistical uncertainty and
a systematic uncertainty of 3.7% [73]. Collisions were realized by delivering proton and
Pb beams with energies of 4 TeV and 1.58 TeV per nucleon, respectively. The proton
and Pb beams circulated in the LHC anticlockwise and clockwise, respectively, during the
period of data taking considered for this analysis. The MB trigger condition is provided
by the V0 detector [74]: a system made up of two arrays of plastic scintillators placed on
either side of the IP and covering the full azimuthal angle and the pseudorapidity intervals
2.8< η < 5.1and 3.7< η < 1.7. The trigger condition required at least one hit in
both the two arrays during the nominal bunch crossing time frame, allowing non-single-
diffractive pPb collisions to be selected with an efficiency higher than 99% [75]. The
timing information from the V0 detectors is also used, in combination with that from the
SPD, to implement an offline rejection of beam-induced background interactions occurring
outside the nominal colliding bunch crossings. Events with more than one interaction per
bunch crossing are reduced down to a negligible amount by means of a dedicated algorithm
3
JHEP06(2022)011
employing reconstructed tracks to detect the presence of multiple collision vertices. Only
collision events with a reconstructed primary vertex lying within ±10 cm from the nominal
IP along the beam direction are considered in order to obtain a uniform coverage for the
central-barrel detectors. An event sample of about 6×108MB events is obtained after the
application of the above described selection criteria.
2.1 Inclusive J/ψ
Electron candidates are selected following similar procedures as those described in ref. [57].
The tracks, reconstructed with the ITS and TPC detectors, are required to have a trans-
verse momentum pe
T>1.0GeV/cand pseudorapidity |ηe|<0.9, as well as at least 70 (out
of a maximum of 159) attached TPC clusters and a track fit χ2/dof <2in order to ensure
a uniform tracking efficiency in the TPC. Electron identification is performed by requiring
the measured dE/dxto be compatible with the expected specific energy loss for electrons
within 3σ, with σdenoting the specific energy-loss resolution of the TPC. Tracks compatible
with the pion and proton energy loss expectations within 3σare rejected. At least one hit in
either of the two SPD layers is required to remove background electrons produced from the
conversion of photons in the detector materials at large radii. Additional suppression of this
background is realized by discarding electron (positron) candidates, which are compatible
with a photon conversion when combined with a positron (electron) candidate of the same
event, through the application of dedicated topological selections. These selections were
verified, employing Monte Carlo (MC) simulations, to have a negligible impact on the J
signal. Finally, in order to reduce the overall background at low transverse momentum, a set
of slightly tighter SPD and particle identification (PID) requirements is applied to electrons
and positrons forming candidate pairs with pT<3GeV/c. A hit in the first SPD layer and a
3.5σpion and proton rejection condition is required instead of 3σfor higher pTvalues. The
sample of J candidates is obtained by combining the selected opposite-sign tracks in the
same event and requiring the J rapidity to be within |ylab |<0.9in the laboratory system.
Due to the energy asymmetry of the proton and lead beams, such a requirement corresponds
to a selection of J candidates within 1.37 < y < 0.43 in the nucleonnucleon centre-of-
mass system. The resulting dielectron invariant mass (me+e
) distributions are shown in
figure 1for eight selected transverse momentum intervals, from 0 to 14 GeV/c. The signal
component is characterised by an asymmetric shape, with a long tail towards low invariant
masses due to the J radiative decay channel (J e+eγ) and the bremsstrahlung-
induced energy loss of daughter electrons in the detector material. The background com-
ponent is composed of both a combinatorial and a correlated part, with the latter mainly
originating from the semileptonic decay of correlated open heavy-flavour hadrons.
The inclusive J yield is determined from the invariant mass distributions using the
same technique as described in ref. [7]. At first, the combinatorial background shape is
modelled by means of a mixed event (ME) technique and then scaled to the invariant mass
distribution of like-sign track pairs. Then, the combinatorial background is subtracted from
the opposite-sign dielectron invariant mass distribution, and the correlated background is
evaluated by fitting the resulting distribution with a two-component function composed of
a MC template for the J signal and of an empirical function for the correlated back-
4
JHEP06(2022)011
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4
2
cCounts per 40 MeV/
100
200
300 Unlike-sign pairs
Signal + background
Correlated background
Combinatorial background
c < 1 GeV/
T
p0 <
31±Signal = 269
/dof = 1.13
2
χ
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4
2
cCounts per 40 MeV/
100
200
300 = 5.02 TeV
NN
sPb, ALICE p
< 0.43y1.37 <
c < 2 GeV/
T
p1 <
33±Signal = 373
/dof = 1.26
2
χ
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4
50
100
150
200
c < 3 GeV/
T
p2 <
27±Signal = 395
/dof = 1.27
2
χ
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4
100
200
300
c < 4 GeV/
T
p3 <
31±Signal = 485
/dof = 1.10
2
χ
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4
50
100
150
200
c < 5 GeV/
T
p4 <
24±Signal = 385
/dof = 1.32
2
χ
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4
50
100
150
c < 7 GeV/
T
p5 <
22±Signal = 355
/dof = 1.32
2
χ
)
2
c (GeV/-
e
+
e
m
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4
20
40
60
c < 10 GeV/
T
p7 <
13±Signal = 141
/dof = 1.11
2
χ
)
2
c (GeV/-
e
+
e
m
2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4
5
10
15
c < 14 GeV/
T
p10 <
7±Signal = 36
/dof = 0.56
2
χ
Figure 1. Opposite-sign dielectron invariant mass distributions for the pT-intervals used for this
analysis. The signal plus total background (blue), the combinatorial background (red), and the
correlated background (green), evaluated as described in the text, are shown separately in each
panel. The χ2/ndf values of the signal template plus the total background function are also reported
along with the raw yields in the range 2.92 < me+e
<3.16 GeV/c2.
ground. The latter is defined to be either an exponential or a combination of an exponential
and a polynomial. The former is obtained by a detailed MC simulation of J decays in
the ALICE detectors, based on GEANT3 [76] and the full reconstruction chain as for real
events, which is then also used to correct the raw yield for the selection procedure and de-
tector inefficiencies and described in details in the next paragraph. After subtracting from
the opposite-sign dielectron invariant mass distribution also the correlated background,
the raw J yield is obtained by counting the number of entries within the invariant mass
interval 2.92 < me+e
<3.16 GeV/c2. In figure 1, the different components used in the
procedure to describe the opposite-sign dielectron invariant mass distributions are shown
superimposed. An alternative method was also considered, where the invariant mass distri-
bution after the subtraction of the correlated background is fitted with a Crystal Ball (CB)
function [77] for the signal plus either an exponential or a combination of an exponential
5
JHEP06(2022)011
and a polynomial for the background, and the raw yield is obtained from the integral of
the best-fit CB function. The alternative method yields results compatible with those from
the standard approach.
In order to correct the raw yield for the chosen selection procedure as well as for de-
tector inefficiencies, a MC simulation was implemented by injecting J signal events into
MB pPb collision events simulated with the EPOS-LHC model [78]. The J compo-
nent was generated starting with pTand ydistributions that match well a next-to-leading
order (NLO) Colour Evaporation Model (CEM) calculations [79,80] with the inclusion of
nuclear effects based on the EPS09 parameterisation [81]. The J decay into dielectrons
was simulated using the EvtGen package [82] in combination with the PHOTOS model [83]
in order to provide a proper description of the radiative decay channel. In the simulation,
GEANT3 [76] was used to reproduce the propagation of particles through the ALICE exper-
imental setup, taking into account the response of the detectors. The same reconstruction
procedure used for data was then applied to the simulated events in order to evaluate the
product of acceptance times efficiency (A×), which accounts for: the detector acceptance,
the track quality requirements, the electron identification criteria, and the fraction of the
signal counted within the invariant mass interval 2.92 < me+e
<3.16 GeV/c2. The A×
retrieved from MC exhibits a smooth and mild variation with the J pT, ranging from
8.5% to 16% in the pTrange from 0 to 14 GeV/c. Consequently, the resulting hA×i
correction factor, which is the average of A×over pTin a finite-size pTinterval, shows a
weak dependence on the pTshape assumed in the simulation for the J component. In the
end, the final correction factors were computed by re-weighting the original MC distribu-
tion to best-fit the inclusive J spectrum already measured in p-Pb collisions at the same
energy [57]. The pT-differential cross section for inclusive J production is calculated as
d2σJ
dydpT
=NJ (∆y, pT)
BR J e+e× hA×i(∆y, pT)×y×pT× Lint
,(2.1)
where y= 1.8corresponds to the width of the analysed rapidity interval, pTis
the width of the considered pTinterval, NJ is the raw J yield in the interval, and
BR J e+e= (5.97 ±0.03)% is the branching ratio for J decaying into dielec-
trons [84].
The inclusive J nuclear modification factor is obtained, according to eq. (1.1), by
dividing the pT-differential cross section by the reference cross section measured up to
pT= 10 GeV/cin pp collisions at s= 5.02 TeV [7]. The rapidity shift of y= 0.465
between the p-Pb and pp samples is expected to introduce a 1% effect on the RpPb, which
is negligible with respect to the other uncertainties. An interpolation procedure, which is
described in ref. [51], is adopted for the computation of the reference cross section in the last
pTinterval, 10 < pT<14 GeV/c, that was not measured in pp collisions at this energy. The
interpolated value of d2σpp/dydpTfor this interval amounts to 10 ±2 nb/(GeV/c), where
the quoted uncertainty refers to the total systematic uncertainty arising from the interpo-
lation procedure and is uncorrelated with the uncertainties of the measured d2σpp/dydpT
for pT<10 GeV/c.
6
JHEP06(2022)011
pT(GeV/c)
Source 0–1 1–2 2–3 3–4 4–5 5–7 7–10 10–14
Tracking 4.4 4.4 2.9 2.6 2.6 2.2 2.2 2.7
PID 1.0 1.0 1.2 1.3 1.1 2.1 4.4 6.1
Signal shape 1.9 1.9 1.9 2.1 2.1 2.4 2.4 2.4
Background subtraction 1.9 1.8 2.0 0.9 0.9 0.9 0.9 0.9
MC input 0.1 0.3 0.2 0.2 0.2 0.2 0.7 1.7
σpp 11.2 9.6 10.5 11.3 12.4 12.8 18.5 22.2
Luminosity 3.7
Branching ratio 0.5
Total (d2σJ/dydpT) 6.4 6.4 5.6 5.3 5.2 5.5 6.7 8.1
Total (RpPb) 12.9 11.5 11.9 12.5 13.4 13.9 19.7 23.7
Table 1. Summary of the systematic uncertainties, in percentage, of the inclusive J cross section
d2σJ/dydpTand nuclear modification factor RpPb in different pTintervals. All contributions to
the d2σJ/dydpTuncertainty are considered to be highly correlated over the pTbins, except that
for the background subtraction which is considered as fully uncorrelated. The reported values for the
measured σpp reference cross section, which was determined up to pT= 10 GeV/c[7], are the total
uncertainties, both of statistical and systematic origins. The uncertainty of σpp for the interval 10 <
pT<14 GeV/cis also the total uncertainty from the interpolation procedure as discussed in the text.
The estimated systematic uncertainties affecting the inclusive J measurements are
listed in table 1. The dominant sources of uncertainty are related to the tracking and
electron identification procedures. The remaining contributions are related to the signal
extraction procedure, the J input kinematic distributions used in the MC simulation,
the dielectron decay channel branching ratio, and the integrated luminosity determination.
The uncertainty of the tracking procedure dominates at low pTvalues and is related to
both the ITS-TPC matching efficiency and to the adopted track quality requirements. The
first component is estimated by evaluating the discrepancy in the matching probability of
TPC tracks to ITS hits between data and Monte Carlo [85]. The observed discrepancy is
used to re-scale the tracking efficiency of electrons in MC simulations in order to evaluate
the difference in the resulting number of reconstructed J candidates. The second com-
ponent is assessed by employing several variations to the adopted track selection criteria
and by computing the RMS of the corrected J yield distribution resulting after these
variations. The sum in quadrature of the uncertainties related to both these components
is taken as systematic uncertainty on the tracking procedure. The uncertainty related to
the electron identification is estimated by evaluating the TPC electron PID response for
a clean sample of topologically identified electrons from conversion processes in data and
computing the difference with the corresponding quantity from MC simulations. This per-
track uncertainty is then propagated to the reconstructed J candidates with the use of
7
JHEP06(2022)011
MC simulations. The resulting uncertainty on the J cross section increases up to 6%
towards high pTvalues, where it is the largest uncertainty contribution. The systematic
uncertainty related to the signal extraction procedure is due to both the background sub-
traction and the assumptions on the signal shape. It is estimated as the RMS of the yield
distributions corresponding to variations of the mass interval used for the signal counting,
the alternative parameterisations employed to fit the correlated background, and the al-
ternative method using the CB function to fit the signal. The uncertainty on the signal
shape ranges between 2% at low pTand 2.5% at high pT, whereas the uncertainty due
to the background subtraction varies between 1% and 2% and is largest for the lowest
pTintervals. The systematic uncertainty on the J pTdistribution used as input for
the computation of the efficiency corrections is determined by randomly varying, within
one standard deviation contour, the parameters of a function fitted to the measured pT
distribution in pPb collisions [57], taking into account their correlations. The functional
form used for this fit is discussed in ref. [86] and very well describes the measured J pT
distribution. The uncertainty of the integrated luminosity amounts to 3.7% and is deter-
mined from the visible pPb cross sections measured in van der Meer scans as detailed in
ref. [73]. Both the uncertainty on the integrated luminosity and that on the branching ratio
BR J e+e= (5.97 ±0.03)% [84] constitute global uncertainties for the inclusive
J cross section, fully correlated between all pTintervals. All the other discussed sources
of uncertainty are considered to be highly correlated1over pT, with the exception of the
background uncertainty, which is considered as uncorrelated.
The total relative uncertainty for the reference pp cross section, σpp , is also reported
in table 1. In this case, the values up to pT= 10 GeV/c, are the total uncertainties,
of both statistical and systematic origin, associated to the measurement performed up to
pT= 10 GeV/c[7], while that for the 10 < pT<14 GeV/cinterval is the relative uncertainty
of the interpolated cross section (10 ±2 nb/(GeV/c)) quoted before. The uncertainty of
σpp propagates only to the RpPb observable and it varies between 10% and 20% and is
largest for the highest pTintervals.
2.2 Determination of the non-prompt J/ψfraction
The fraction fbof the J yield originating from b-hadron decays is measured for pT>
1GeV/cby discriminating, on a statistical basis, the reconstructed J candidates accord-
ing to the displacement between their production vertex and the primary pPb collision
vertex. The discrimination is realized by means of an unbinned two-dimensional likelihood
fit, following the same technique adopted in previous analyses for the pp [13], pPb [51],
and Pb-Pb [87] systems. In particular, it is performed by maximising the following log-
likelihood function
lnL=
N
X
1
lnhfSig ×FSig(x)×MSig(me+e
)+ (1 fSig )×FBkg(x)×MBkg(me+e
)i,(2.2)
in which Nindicates the number of e+epairs within the 2.32 < me+e
<4.00 GeV/c2
invariant mass interval. The pseudoproper decay length xis introduced to separate J
1With high correlation, we mean a Pearson coefficient larger than 0.7.
8
JHEP06(2022)011
originating from the decay of b-hadrons from prompt J. It is defined as
x=c×Lxy ×mJ
pT
,(2.3)
where cis the speed of light, Lxy =~
L·~pT/pTis the signed projection of the pair flight
distance, ~
L, onto its transverse momentum vector, ~pT, and mJ is the J pole mass
value [84]. The terms FSig (FBkg ) and MSig (MBkg) in eq. (2.2) represent the probability
density functions (PrDFs) describing the signal (background) pair distributions as a func-
tion of xand me+e
, respectively, whereas fSig denotes the ratio of signal to all candidates
within the considered mass interval. The signal xPrDF is given by
FSig(x) = f0
b×Fb(x) + (1 f0
b)×Fprompt(x),(2.4)
with Fprompt(x)and Fb(x)indicating the prompt and non-prompt J PrDFs, and f0
bbeing
the uncorrected fraction of J coming from b-hadron decays.
The evaluation of the different PrDFs used in eq. (2.2) is performed relying either
on data or on MC simulations and following the same procedures described in previous
analyses [13,87]. For the MC simulations, the prompt J component was generated with
the pTand ydistributions obtained with a procedure analogue to that previously dis-
cussed for the inclusive J analysis, while the non-prompt J component was obtained
using PYTHIA 6.4 [88] with Perugia-0 tuning [89] to simulate the production of beauty
hadrons. Also in this case, the J decay into dielectrons was simulated using the EvtGen
package [82] in combination with the PHOTOS model [83] in order to provide a proper de-
scription of the radiative decay channel. The background xPrDF, FBkg(x), is determined
in three invariant mass ranges by fitting the xdistributions of dielectron candidates in the
lower (2.32 < me+e
<2.68 GeV/c2) and upper (3.20 < me+e
<4.00 GeV/c2) side bands
of the invariant mass distributions and by interpolating the resulting fit functions to the
region under the invariant mass signal peak (2.68 < me+e
<3.20 GeV/c2). The experi-
mental resolution function, R(x), which is the key ingredient in the Fprompt(x),Fb(x)and
FBkg(x)PrDFs, is evaluated from the xdistributions of prompt J in MC simulations,
reconstructed after applying the same selection criteria as in data. In order to improve the
resolution of the secondary decay vertices, it is required that at least one of the two J
candidate decay tracks has a hit in the innermost SPD layer. A tune-on-data procedure [51]
is applied to the MC sample in order to reproduce the observed single-track impact param-
eter distributions. This minimises the discrepancy between data and simulation, reducing
the systematic uncertainty related to the R(x)determination. The Fb(x)PrDF is obtained
as the convolution of the R(x)function and a template of the xdistribution for the mixture
of b-hadrons decaying into J. The latter is obtained with a MC simulation study of the
kinematics of the b-hadron decays. In this simulation, the pT-distribution of the b-hadrons
is obtained from pQCD calculations at fixed order with next-to leading-log re-summation
(FONLL) [90]. The decay description is based on the EvtGen package [82], and the relative
abundance of b-hadron species as a function of pTis based on the precise measurements
reported by the LHCb collaboration in pp collisions [91], which are consistent with those
9
JHEP06(2022)011
)
2
c (GeV/
-
e
+
e
m
2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4
2
cCounts per 40 MeV/
200
400
600
800
1000
1200
= 37/41dof/
2
χ
= 5.02 TeV
NN
sPb, ALICE p
c > 1 GeV/
T
p
data
fit, all
fit, signal
fit, background
m)µPseudoproper decay length (
2000150010005000 500 1000 1500 2000
mµ
Counts per 40
1
10
2
10
3
10
= 101/91dof/
2
χ
2
c < 3.16 GeV/
-
e
+
e
m2.92 <
= 5.02 TeV
NN
s
Pb, ALICE p
c > 1 GeV/
T
p
data
fit, all
ψfit, prompt J/
fit, background
b
from hψfit, J/
Figure 2. Invariant mass (left panel) and pseudoproper decay length (right panel) distributions for
J candidates with pT>1GeV/c. The latter distribution is limited to the J candidates under
the signal peak region 2.92 < me+e
<3.16 GeV/c2. The projections of the maximum likelihood
fit functions are shown superimposed and the χ2values of these projections are also reported for
both distributions.
measured in p-Pb collisions [67]. The xresolution estimated from the MC simulations is
characterised by a pronounced dependence as a function of the J pT: for events with
both J decay tracks yielding a hit in the first SPD layer, the RMS of the R(x)distribu-
tion ranges from 140 µm at pT= 1.5GeV/cto 50 µm at pT>7GeV/c. This allows the
fraction of non-prompt J to be determined for events with J pTgreater than 1 GeV/c
as well as in five transverse momentum intervals (1–3, 3–5, 5–7, 7–10 and 10–14 GeV/c).
The projections of the maximised likelihood fit function superimposed over the me+e
and
xdistributions of J candidates with pT>1GeV/care shown as an example in figure 2.
The fbfraction is obtained after correcting the f0
bvalues (eq. (2.5) ) to account for
slightly different hA×ifactors of prompt and non-prompt J:
fb= 1 + 1f0
b
f0
b×hA×iB
hA×iprompt !1
.(2.5)
This small correction is computed relying on MC simulations assuming prompt J to
be unpolarised. A small residual polarisation, resulting from the admixture of the different
b-hadron decay channels, is assumed for non-prompt J as predicted by EvtGen [82].
Under these conditions, the correction mainly originates from the difference in the pT
distribution between the two components and is found to be significant only for the pT-
integrated case. Small relative variations of the corrected fbvalues, in the order of 1–4%,
are expected in the case of a null-polarisation assumption for non-prompt J [51]. The
variations estimated in extreme polarisation scenarios for prompt J are discussed in
ref. [13]. Considering the null or very small degree of polarisation measured in pp collisions
at the LHC [17,9294], these variations are not further propagated to the final results,
and only the choice of the pTshapes used as input for the MC simulations is taken into
account for the systematic uncertainty evaluation, as discussed below.
The evaluated systematic uncertainties affecting the measurements of fbin the five
pTintervals as well as in the pT-integrated range (pT>1GeV/c) are listed in table 2.
10
JHEP06(2022)011
Most of the listed contributions are due to incomplete knowledge of the different PrDFs
used as input for the likelihood fits. An additional contribution originates from the as-
sumptions on the pTdistributions employed for the computation of the correction factor of
eq. (2.5). The uncertainties affecting the evaluation of the resolution function and of the
background PrDF constitute the largest contributions to the total systematic uncertainty
of the fbmeasurements. The former is estimated by propagating to the R(x)PrDF the
residual discrepancy of the single-track impact parameter distributions between data and
MC simulations after the application of the previously discussed tuning procedure. The
latter is evaluated by repeating the likelihood fits after varying the procedure used for the
determination of the FBkg(x)PrDFs, following the same approach described in ref. [87].
Both uncertainties increase towards low transverse momenta and are largest for the lowest
pTinterval, where they amount to 10% and 8.5%, respectively. The uncertainty related
to the invariant mass PrDF of the J signal is estimated by changing the width of the
CB function used to parameterise the MSig PrDF so as to vary the fraction of signal en-
closed within the 2.92 < me+e
<3.16 GeV/c2interval by ±2.5%. The likelihood fits are
repeated, and the variation of the resulting fbvalues is taken as systematic uncertainty.
The uncertainty related to the invariant mass background PrDF is estimated as the RMS
of the fbvalue distributions obtained after employing different parameterisations and al-
ternative fitting approaches for the evaluation of the MBkg PrDF. The estimate of the
systematic uncertainty affecting the non-prompt J x PrDF is performed by repeating
the likelihood fits after employing PYTHIA 6.4 for the description of the pT-distribution,
decay kinematics, and relative abundance of the beauty hadrons in the MC simulations
used to model the Fb(x)PrDF. The relative variation of the resulting fbvalues, assumed
as systematic uncertainty, increases up to 3% towards low transverse momenta. This
uncertainty also contemplates any conservative assumption for a rapidity dependence of
the relative abundances of beauty hadrons at the LHC. The uncertainty related to the
acceptance times efficiency correction procedure is assessed by testing different hypothesis
for the kinematic pT-spectra used to compute the hA×ivalues that enter into eq. (2.5).
Among the tested variations, a tune-on-data parameterisation based on the Run 1 mea-
surement [51] for prompt J, a pT-distribution based on FONLL calculations [90] for the
non-prompt component, and the inclusion or exclusion of nuclear shadowing modifications
according to the EPPS16 parameterisation [95] are considered. The resulting variations of
the correction factors are largest for the pT-integrated measurement, where they amount
to 3%, while they are smaller than 1% within the analysed pT-intervals. The overall
systematic uncertainty of the fbmeasurements is found to increase up to 13.7% towards
low transverse momenta, mostly as a consequence of both the increasing combinatorial
background and the worsening of the xresolution.
The prompt and non-prompt J nuclear modification factors are computed by com-
bining the measurements of fbwith the previously discussed nuclear modification factors
Rincl.J
pPb of inclusive J:
Rnon-prompt J
pPb =fpPb
b
fpp
b
Rincl.J
pPb , Rprompt J
pPb =1fpPb
b
1fpp
b
Rincl.J
pPb .(2.6)
11
JHEP06(2022)011
pT(GeV/c)
Source >11–3 3–5 5–7 7–10 10–14
Resolution function 6.3 10.0 5.5 2.4 1.3 1.1
xPrDF of non-prompt J 1.7 3.2 0.9 0.5 0.3 0.3
xPrDF of background 5.0 8.5 3.2 3.2 2.6 1.8
Invariant mass PrDF of signal 1.4 1.1 1.3 1.5 1.8 1.8
Invariant mass PrDF of background 1.5 1.8 1.4 1.2 0.8 1.2
Acceptance ×efficiency 3.0 0.3 0.7 0.2
Total 9.0 13.7 6.7 4.5 3.5 3.0
Table 2. List of the systematic uncertainties (in percent) for the fraction of J from b-hadron
decays in the different analysed pTintervals. The symbol denotes a negligible contribution.
The value of fbin pp collision at s= 5.02 TeV, indicated as fpp
bin eq. (2.6), is
determined by means of the same interpolation procedure adopted in previous analyses
for the pPb [51] and Pb-Pb [87] systems. The procedure consists of fitting to existing
midrapidity fbmeasurements at s= 1.96 TeV (from CDF [3]) and s= 7 TeV (from
ALICE [13], ATLAS [96], and CMS [97]) the semi-phenomenological function discussed in
ref. [87], which includes FONLL predictions [90] for the non-prompt J production cross
section. An energy interpolation is then performed to derive the fpp
b(pT)at s= 5.02 TeV
as a function of pT. The average value of fpp
bin a given pTinterval is obtained by weighting
fpp
b(pT)over the inclusive J spectrum in pp collisions in that pTinterval. Compared
to our previous estimates [51], a tune-on-data spectrum based on the inclusive J yield
measured by ALICE in pp collisions at s= 5.02 TeV [7] is now employed for this purpose.
The values of fpp
bat s= 5.02 TeV, computed in the considered momentum intervals, are
reported in table 3. The quoted uncertainties take into account the uncertainties of both
data and FONLL predictions, as well as an additional systematic uncertainty due to the
choice of the functional form (either a linear, or an exponential, or a power law function)
employed for the energy interpolation procedure.
3 Results
The inclusive J cross section is measured in 1.37 < y < 0.43 both for pT>0and differ-
entially in pTconsidering seven pTintervals, with the first and last bins being [01] GeV/c
and [1014] GeV/c, respectively. The value of the pT-integrated inclusive J cross section
per unit of rapidity is dσ/dy= 999 ±33 (stat.)±56 (syst.)µb. The pT-differential cross
section of inclusive J per unit of rapidity, d2σincl.J/dydpT, is shown in figure 3in com-
parison with the cross section measured in pp collisions at s= 5.02 TeV [7] multiplied by
A = 208. The latter extends up to pT= 10 GeV/c. The highest pTpoint for pp collisions,
which is shown in the figure with the empty symbol, was obtained using the interpolation
procedure as described before.
12
JHEP06(2022)011
pT(GeV/c)fpp
bat s= 5.02 TeV
>0 0.135 ±0.013
1–3 0.117 ±0.013
3–5 0.144 ±0.012
5–7 0.188 ±0.014
7–10 0.246 ±0.019
10–14 0.333 ±0.038
Table 3. Fraction of non-prompt J in pp collisions at s= 5.02 TeV computed in different
transverse momentum intervals. The reported values and uncertainties are derived following the
interpolation procedure detailed in the text and in ref. [87].
)c (GeV/
T
p
0 2 4 6 8 10 12 14
)
-1
)cb(GeV/µ (
T
pdy/d
σ
2
d
1
10
2
10
< 0.43y1.37 < Pb, p
(3.7% norm. unc. not shown)
| < 0.9y A, |×pp
JHEP 10 (2019) 084
| < 0.9y A interpolated, |×pp
ALICE
ψinclusive J/
= 5.02 TeV
NN
s
Figure 3. The pT-differential inclusive J cross section per unit of rapidity, d2σincl.J /dydpT, as
a function of pTin p-Pb collisions (red circles) compared with the analogous cross section measured
in pp collisions at s= 5.02 TeV [7] multiplied by the Pb mass number (A = 208) (black closed
squares, shifted horizontally by 150MeV/cfor better visibility). The vertical error bar and the
box on top of each point represent the statistical and systematic uncertainty, respectively. The
open square symbol (also shifted by 150 MeV/c) shows the value for pp collisions in the pTinterval
10-14 GeV/c, which was obtained with the interpolation procedure. In this case the error bar
corresponds to the total uncertainty.
The fraction of J from b-hadron decays in the kinematic range pT>1GeV/cand
1.37 < y < 0.43, which is referred to as “visible region” in the following, is found to be
fb= 0.125 ±0.017 (stat.)±0.011 (syst.), where the first quoted uncertainty is statistical
and the second one is systematic. The fbmeasurements in the five analysed pTintervals
are shown in figure 4in comparison with our previous results [51] and with the results
from the ATLAS collaboration [60], measured for pT>8GeV/cwithin a similar rapidity
interval (1.94 < y < 0). The measurements from the CDF [3], ATLAS [98], and CMS [97]
experiments in pp and ppcollisions at midrapidity are also shown for comparison. With
respect to our previous results [51], the present measurements are performed over a wider
13
JHEP06(2022)011
)c (GeV/
T
p
1 10 2
10
b
f
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
= 5.02 TeV
NN
s < 0, yPb -1.94 < ATLAS, p
= 7 TeVs| < 0.9, y ALICE, pp |
= 7 TeVs| < 0.75, y ATLAS, pp |
= 7 TeVs| < 0.9, y CMS, pp |
= 8 TeVs| < 0.25, y ATLAS, pp |
= 1.96 TeVs| < 0.6, y |p CDF, p
= 5.02 TeV
NN
s < 0.43, yPb -1.37 < ALICE (Run 1), p
= 5.02 TeV
NN
s < 0.43, yPb -1.37 < ALICE, p
95%CL
Figure 4. Fraction of J from b-hadron decays at midrapidity as a function of the J pTin
p-Pb collisions at sNN = 5.02 TeV (red closed circles) compared with results from the ALICE [51]
and ATLAS [60] collaborations in the same collision system (blue and green closed circles, and the
blue arrow that shows an upper limit at 95% CL in the range 1.3< pT<3GeV/c). The results
from CDF [3] in ppcollisions at s= 1.96 TeV and those of the ALICE [13], ATLAS [96,98],
and CMS [97] collaborations in pp collisions at s= 7 TeV or s= 8 TeV are also shown (black
symbols). For all experiments, vertical error bars represent the quadratic sum of the statistical and
systematic uncertainties. The data points of ALICE are placed horizontally at the mean value of the
pTdistribution within each pT-interval, determined from the MC simulations described in the text.
pTrange, with a more granular binning, and show a significantly improved precision, with
about half of the statistical uncertainty within similar pTintervals.
The prompt and non-prompt J production cross sections are obtained from the com-
bination of the fbfractions with the measurements of the inclusive J cross section σJ:
σJ from hb=fb×σJ, σprompt J = (1 fb)×σJ .(3.1)
The non-prompt J cross section in the visible region, σvis
J from hb= 201±28 (stat.)±
21 (syst.)µb, is computed using the inclusive J cross section for pT>1GeV/c, which
amounts to 1603 ±55 (stat.)±89 (syst.)µb.
In order to derive the pT-integrated values of the prompt and non-prompt J cross
section at midrapidity, σvis
J from hbis extrapolated down to pT= 0 following the approach
described in our previous work [51]. The extrapolation is performed assuming the shape of
the pTdistribution of b-quarks obtained from FONLL [90] with the CTEQ6.6 PDFs [99]
modified according to EPPS16 nPDF parameterisation [95]. The fragmentation of b-quarks
into hadrons is then modelled using PYTHIA 6.4 [88] with the Perugia-0 tune [89]. The
ratio of the extrapolated cross section for pT>0and 1.37 < y < 0.43 to that in the
visible region (pT>1GeV/cand 1.37 < y < 0.43) equals 1.127+0.014
0.025 , where the quoted
uncertainty takes into account the FONLL, CTEQ6.6 and EPPS16 uncertainties, as de-
scribed in ref. [51], as well as an additional uncertainty, which is related to that on the
14
JHEP06(2022)011
y
543210 1 2 3 4
b)µ (y/d
σ
d
500
1000
1500
= 5.02 TeV
NN
sPb, p
ψprompt J/
ALICE
LHCb
EPPS16 (Lansberg et al.)
reweighted EPPS16 (Lansberg et al.)
nCTEQ15 (Lansberg et al.)
reweighted nCTEQ15 (Lansberg et al.)
)c (GeV/
T
p
0 2 4 6 8 10 12 14 16 18 20
)
-1
)cb(GeV/µ (
T
pdy/d
σ
2
d
1
10
1
10
2
10
< 0.43y1.37 < , ψ ALICE prompt J/
< 1.5y2 < , ψ ATLAS prompt J/
EPPS16 (Lansberg et al.)
nCTEQ15 (Lansberg et al.)
ψprompt J/
= 5.02 TeV
NN
s
Pb, p
Figure 5. dσprompt J/dyas a function of the rapidity in the centre-of-mass frame (left panel) as
obtained in this work at midrapidity and by the LHCb collaboration in the forward and backward
rapidity regions [64], and d2σprompt J/dydpTas a function of pT(right panel) compared with AT-
LAS results [14] (reported up to pT= 20 GeV/c). The vertical error bars and the boxes on top of
each point represent the statistical and systematic uncertainties. In the left plot, the systematic un-
certainty of the ALICE data point includes also the contribution from the extrapolation procedure to
go from the visible region (pT>1GeV/c) to pT>0, as described in the text. For the measurements
as a function of pT, the data symbols are placed within each bin at the mean of the pTdistribution
determined from MC simulations. The results of a model [100103] including nuclear shadowing
based on the EPPS16 [95] and nCTEQ15 [104] nPDFs are shown superimposed on both panels (see
text for details). In the right panel, the computations refer only to the ALICE rapidity range.
relative abundance of beauty hadron species in the extrapolated range. The latter is esti-
mated to be about 0.4% after changing the assumed fractions of beauty hadrons according
to the recent LHCb measurements [91]. Also, in this case, the considered variation largely
includes a possible dependence of these fractions on rapidity. Thus, the measured cross
section corresponds to more than 85% of the pT-integrated cross section at midrapidity.
Dividing by the rapidity range y= 1.8, the following value is derived for the non-prompt
J cross section per unit of rapidity (pT>0and 1.37 < y < 0.43):
dσJ from hb
dy= 125.6±17.6 (stat.)±13.3 (syst.)+1.6
2.8(extr.)µb.
The corresponding value for the prompt component is obtained as the difference between
the inclusive J cross section, which is measured for pT>0, and that of J from b-
hadron decays, as determined with the extrapolation procedure described above. It is
(pT>0and 1.37 < y < 0.43)
dσprompt J
dy= 873.1±33.6 (stat.)±50.4 (syst.)+1.6
2.8(extr.)µb.
In figure 5(left panel) this result is shown as a function of rapidity together with
the results from the LHCb experiment at positive (“forward”) and negative (“backward”)
rapidity [64], corresponding respectively to the p-going and Pb-going direction. The pT-
differential cross section of prompt J is shown, in comparison with ATLAS measure-
ments [14] at high pTand for 2< y < 1.5, in the right panel of figure 5. The ALICE
15
JHEP06(2022)011
y
420 2 4
b)µ (y/d
σ
d
0
20
40
60
80
100
120
140
160
180
200
220
= 5.02 TeV
NN
sPb, p
ψnon-prompt J/
ALICE extr. unc.
> 0
T
ALICE p
c < 14 GeV/
T
LHCb 0 < p
FONLL + EPPS16
EPPS16 unc.
)c (GeV/
T
p
0 2 4 6 8 10 12 14 16 18 20
)
-1
)cb(GeV/µ (
T
pdy/d
σ
2
d
1
10
1
10
2
10
ψnon-prompt J/
< 0.43y1.37 < ALICE,
< 1.5y2 < ATLAS,
FONLL + EPPS16
EPPS16 unc.
= 5.02 TeV
NN
sPb, p
Figure 6. dσJ from hb/dyas a function of rapidity (left panel) as obtained at midrapidity in this
work for pT>0and by the LHCb collaboration in the forward and backward rapidity regions [64]
for 0< pT<14 GeV/c, and d2σJ from hb/dydpTas a function of pT(right panel) compared
with ATLAS measurements [14] (shown up to pT= 20 GeV/c). The statistical and systematic
uncertainties are shown as vertical error bars and hollow boxes, respectively. The extrapolation
uncertainty related to the procedure to go from the visible region (pT>1GeV/c) to pT>0for the
ALICE data point in the left panel is indicated with a dashed band. For the measurements as a
function of pT, data symbols are placed at the mean value of the pTdistribution within each bin.
The results are compared to FONLL computations [90] with EPPS16 [95] nPDFs, highlighting the
total theoretical uncertainty (empty band) and the contribution from EPPS16 (coloured band). In
the right panel, model computations are obtained in the same rapidity range of the ALICE results,
namely 1.37 < y < 0.43.
results, covering the low pTregion at midrapidity, are complementary to the measurements
from both the LHCb and ATLAS collaborations. The data are reported in comparison with
model calculations for prompt J (Lansberg et al. [100103]) based on the EPPS16 [95]
and the nCTEQ15 [104] sets of nuclear parton distribution functions (nPDFs). In both
cases, the shaded bands represent the envelope of the computations for different assump-
tions of the values of the pQCD factorisation (µF) and renormalisation (µR) scales (varied
within 0.5< µFR<2) computed at the 90% confidence level. The predictions show
good agreement with data within the large model uncertainties, which are dominated by
those on the pQCD scales. The results of a Bayesian reweighting approach from the same
authors [101], employing LHCb measurements of J [66,105] as a constraint for the com-
putations, are also shown in the left panel of figure 5. Both the size of uncertainties and
the difference between the nPDF sets are largely reduced after the reweighting. In figure 6,
the cross sections of non-prompt J, computed either for pT>0(left panel) or differ-
entially in pT(right panel), are reported together with the corresponding results from the
LHCb [64] and ATLAS [14] collaborations. The results are compared with theoretical pre-
dictions based on FONLL pQCD calculations [90] with the inclusion of nuclear shadowing
effects according to the EPPS16 nPDFs [95]. In each panel, the coloured curves delimit the
total theoretical uncertainty on the production cross section, which is dominated by that
of the b-quark mass and the pQCD scales, while the shaded bands refer to the theoretical
uncertainty of the EPPS16 nPDFs.
16
JHEP06(2022)011
y
43210 1 2 3 4
pPb
R
0
0.2
0.4
0.6
0.8
1
1.2
EPPS16 reweight (Lansberg et al.)
nCTEQ15 reweight (Lansberg et al.)
et al.)eCGC + CEM (Duclou
CGC + NRQCD (Ma et al.)
Energy loss (Arleo et al.)
Energy loss + EPS09 NLO (Arleo et al.)
EPS09 LO central set (Ferreiro et al.)
= 1.5 mb (Ferreiro et al.)
abs
σEPS09 LO central set +
ALICE
LHCb
= 5.02 TeV
NN
sPb, p
ψ prompt J/
)c (GeV/
T
p
0 2 4 6 8 10 12 14 16 18 20
pPb
R
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
:ψPrompt J/
EPPS16 reweight (Lansberg et al.)
nCTEQ15 reweight (Lansberg et al.)
Energy loss (Arleo et al.)
Energy loss + EPS09 NLO (Arleo et al.)
= 5.02 TeV
NN
sPb, p
< 0.43y1.37 < , ψALICE prompt J/
< 0.43y1.37 < , ψALICE inclusive J/
< 1.5y2 < , ψATLAS prompt J/
Figure 7.RpPb of prompt J as a function of rapidity (left panel) and as a function of pT
along with that of inclusive J at midrapidity (right panel). Results are shown in comparison
with LHCb measurements [64] at backward and forward rapidity in the left panel and with ATLAS
results [14] (shown up to pT= 20 GeV/c) in the right-hand panel. Statistical uncertainties are
represented by vertical error bars, while open boxes correspond to systematic uncertainties. In
the left panel, the systematic uncertainty of the ALICE data point includes also the contribution
from the extrapolation procedure to go from the visible region (pT>1GeV/c) to pT>0. The
filled box around RpPb = 1 in the right panel indicates the size of the global relative uncertainty of
the ALICE measurements. The results of various model predictions for prompt J implementing
different CNM effects are also shown [44,101,106108].
The nuclear modification factor of inclusive J, measured for pT>0and 1.37 < y <
0.43, amounts to 0.851 ±0.028 (stat.)±0.079 (syst.). This quantity is obtained using the
measured inclusive J cross section in pp collisions at s= 5.02 TeV [7]. Similarly, the
pT-differential RpPb of inclusive J is obtained on the basis of the measured pp reference,
except for the highest pTinterval (10–14 GeV/c), where the statistical sample is limited
and the interpolation procedure [57] is still used. In figure 7, the RpPb of prompt J
is reported either for pT>0in comparison with LHCb measurements [64] at backward
and forward rapidity (left panel) or as a function of pT, computed according to eq. (2.6),
together with that of inclusive J (right panel) in comparison with ATLAS results [14].
The pT-integrated RpPb of prompt J at midrapidity (pT>0and 1.37 < y < 0.43) is
measured to be smaller than unity and amounts to 0.860 ±0.033 (stat.)±0.081 (syst.).
Given also the relatively small fraction of J from b-hadron decays for pT<14 GeV/c, the
RpPb of inclusive J is comparable with that of the prompt component. As shown in the
right panel of figure 7, both trends indicate that the suppression observed at midrapidity
is a low-pTeffect, concentrated for pT.3GeV/c. The measurements are compared with
results from various model predictions which embed different CNM effects into prompt
J production. In addition to the previously described computations by Lansberg et
al., which include a reweighting of the EPPS16 and nCTE15 nPDFs [101], the central
values of a computation based on EPS09 nPDF with or without interaction with a nuclear
medium (Ferreiro et al. [106]) are shown. A calculation including the effects of coherent
energy loss (Arleo et al. [44]), with or without the introduction of nuclear shadowing effects
according to EPS09 nPDF, provides a fairly good description of the measurements either
17
JHEP06(2022)011
y
43210 1 2 3 4
pPb
R
0
0.2
0.4
0.6
0.8
1
1.2
1.4
FONLL + EPPS16
EPPS16 reweight (Lansberg et al.)
nDSg LO
ALICE
LHCb
ALICE extr. unc.
= 5.02 TeV
NN
sPb, pψnon-prompt J/
(GeV/c)
T
p
0 2 4 6 8 10 12 14 16 18 20
pPb
R
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
< 0.43y ALICE, -1.37 <
< 1.5y ATLAS, -2 <
FONLL + EPPS16
EPPS16 reweight (Lansberg et al.)
ψnon-prompt J/
= 5.02 TeV
NN
s
Pb, p
Figure 8. Nuclear modification factor RpPb of non-prompt J as a function of rapidity (left
panel) and as a function of pTat midrapidity (right panel). The results are compared with similar
measurements from the LHCb [64] and ATLAS [14] experiments. Vertical bars and the open boxes
indicate the statistical and systematic uncertainties, respectively. The uncertainty due to the ex-
trapolation to pT= 0 GeV/cfor the ALICE measurement is reported as a shaded box in left panel,
while the filled box around RpPb = 1 in the right-hand panel denotes the size of the global normali-
sation uncertainty. The predicted nuclear modification factors according to the nDSg [109] (central
value shown in the left panel only) and EPPS16 [95] parameterisations (including a reweighted
computation from [101]) are shown superimposed on both panels.
as a function of pTor as a function of rapidity. Two model calculations based on the CGC
effective theory coupled with different elementary production models (Ducloué et al. [107],
Ma et al. [108]), are also reported within their domain of validity, in the forward-yregion.
The nuclear modification factor for non-prompt J, determined according to eq. (2.6),
is shown in figure 8. The measured value of the pT-integrated RpPb for pT>0and
1.37 < y < 0.43 is found to be 0.79 ±0.11 (stat.)±0.13 (syst.)+0.01
0.02 (extr.), suggesting the
presence of nuclear effects also for the non-prompt J component. Within uncertainties,
the measurements are found to be compatible with those of the LHCb collaboration [64] at
both forward and backward rapidity as well as with those of the ATLAS collaboration [14]
for pT&9GeV/c. All data are in fair agreement with the mild degree of suppression pre-
dicted by FONLL computations employing the EPPS16 nPDFs. The nuclear modification
factor, as predicted from the Bayesian reweighting approach [101] of the EPPS16 nPDFs
previously introduced for the prompt component, also provides a good description of the
measurements. The central value of an alternative parameterisation of the nuclear PDF,
nDSgLO [109], is reported in the left-hand panel. Despite the larger relative uncertainties,
the comparison with the results shown for the prompt component (right panel of figure 7)
suggests that a reduced suppression as well as a less pronounced pT-dependence affect the
component of J from b-hadron decays.
Similarly as for our previous work [51], the low pTcoverage of the measured non-
prompt J cross section at midrapidity, now extending down to pT= 1 GeV/c, allows the
pT-integrated bbcross section per unit of rapidity, dσbb/dy, and the total bbproduction
cross section, σ(pPb bb + X), to be derived with small extrapolation uncertainties. By
18
JHEP06(2022)011
using FONLL with CTEQ6.6 and EPPS16 nPDFs as input model for the computation of
the extrapolation factor, the bbproduction cross section at midrapidity is derived as
dσbb
dy=dσmodel
bb
dy×σvis
J from hb
σvis,model
J from hb
.(3.2)
Assuming the average branching ratio of J from b-hadron decays measured at
LEP [110112], BR(hbJ +X) = (1.16 ±0.10)%, for the computation of σvis,model
J from hb,
the resulting midrapidity cross section per unit of rapidity is
dσbb
dy= 5.52 ±0.77 (stat.)±0.75 (syst.)+0.07
0.12 (extr.) mb.
With a similar approach, the total bbproduction cross section is obtained by extrapolating
the visible cross section to the full phase space:
σ(p + Pb bb + X) = α4π
σvis
J from hb
2×BR(hbJ +X),(3.3)
where the factor α4πis defined as the ratio of the yield of non-prompt J produced in
the full phase space to that in the visible region, and the factor 2 takes into account that
hadrons with b or bvalence quark can decay into J. The value of the α4πfactor obtained
from FONLL pQCD calculations with EPPS16 nPDFs, with the b-quark fragmentation
performed using PYTHIA 6.4 with the Perugia-0 tune, is α4π= 4.10 +0.14
0.12. Using PYTHIA
8 instead of FONLL for the generation of bbquark pairs provides the value 4.02, which
is 2% smaller than the used value for the extrapolation based on FONLL. The total cross
section is2
σ(p + Pb bb + X) = 35.5±5.0 (stat.)±4.8 (syst.)+1.2
1.0(extr.) mb.
The reported results for the extrapolated bb production cross sections are consistent with
our previous derivations [51]. The total uncertainty is reduced by a factor of 2 thanks
to the larger data sample, the smaller systematic uncertainty, and the slightly extended
coverage of the visible region where the non-prompt J cross section is measured.
In p-Pb collisions at sNN = 5.02 TeV the LHCb Collaboration measured the non-
prompt J production cross section at forward and backward rapidities for pT<14 GeV/c,
reporting [64]σJ from hb(1.5< y < 4.0) = 166.0±4.1±8.2µb and σJ from hb(5.0<
y < 2.5) = 118.2±6.8±11.7µb, respectively. A more precise estimate of the total bbcross
section can be obtained by repeating the same procedure with including also these results
from the LHCb collaboration [64], to obtain a wider visible region: (5< y < 2.5, pT<
14 GeV/c)(1.37 < y < 0.43, pT>1.0 GeV/c)(1.5< y < 4, pT<14 GeV/c). The cross
section in this wider visible region is obtained as the sum of the cross sections measured in
2The extrapolation uncertainties for the dσbb/dyand for the total bbcross section include the contribu-
tions related to the FONLL, CTEQ6.6 and EPPS16 uncertainties as discussed in ref. [51], and also a minor
contribution of about 0.4% related to the uncertainty on the pT-dependence of the relative abundance of
b-hadron species.
19
JHEP06(2022)011
this work at central rapidity and those from LHCb. All the uncertainties are uncorrelated
except that of the branching ratio. In this case, the α4πfactor, which is calculated as the
ratio of the yield from the model in full phase space to that in the wider region covered by
the ALICE and LHCb experiments, is reduced to 1.60 ±0.02, and the corresponding total
cross section is
σ(p + Pb bb + X) = 33.8±2.0 (stat.)±3.4 (syst.)+0.4
0.5(extr.) mb (ALICE and LHCb).
4 Summary
The production of J mesons in pPb collisions at sNN = 5.02 TeV is studied based on a
data sample about six times larger than that of previously published results yielding smaller
uncertainties and extending the pTcoverage. The inclusive J production cross section
at midrapidity is measured down to pT= 0 after reconstructing the J mesons in the
dielectron decay channel. The fraction of the inclusive J yield originated from b-hadron
decays is then determined on a statistical basis, allowing the prompt and non-prompt J
production cross sections at midrapidity to be derived for pT>1GeV/cand as a function
of pTin five momentum intervals. The results are scaled to reference measurements from
pp collisions at the same centre-of-mass energy in order to investigate the presence of
nuclear effects on J production. The nuclear modification factor of prompt J shows
a significant suppression for pT.3GeV/c, whereas there is a hint of a less pronounced
suppression of the non-prompt component over the inspected pTrange. The results can be
described by theoretical calculations including various combinations of cold nuclear matter
effects, although a precise discrimination among the different models is impaired by the
uncertainties affecting the currently available predictions. Finally, the measurement of the
non-prompt J production cross section is used to derive the extrapolated midrapidity
dσbb/dyand total cross section, σbb , of beauty quark production.
Acknowledgments
The ALICE Collaboration would like to thank all its engineers and technicians for their
invaluable contributions to the construction of the experiment and the CERN accelerator
teams for the outstanding performance of the LHC complex. The ALICE Collaboration
gratefully acknowledges the resources and support provided by all Grid centres and the
Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration ac-
knowledges the following funding agencies for their support in building and running the
ALICE detector: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Insti-
tute) Foundation (ANSL), State Committee of Science and World Federation of Scientists
(WFS), Armenia; Austrian Academy of Sciences, Austrian Science Fund (FWF): [M 2467-
N36] and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Ministry
of Communications and High Technologies, National Nuclear Research Center, Azerbaijan;
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de
Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo
(FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Ministry of
20
JHEP06(2022)011
Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and
National Natural Science Foundation of China (NSFC), China; Ministry of Science and
Education and Croatian Science Foundation, Croatia; Centro de Aplicaciones Tecnológi-
cas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba; Ministry of Education, Youth
and Sports of the Czech Republic, Czech Republic; The Danish Council for Independent
Research | Natural Sciences, the VILLUM FONDEN and Danish National Research Foun-
dation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à
l’Energie Atomique (CEA) and Institut National de Physique Nucléaire et de Physique
des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France;
Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für
Schwerionenforschung GmbH, Germany; General Secretariat for Research and Technol-
ogy, Ministry of Education, Research and Religions, Greece; National Research, Develop-
ment and Innovation Office, Hungary; Department of Atomic Energy Government of India
(DAE), Department of Science and Technology, Government of India (DST), University
Grants Commission, Government of India (UGC) and Council of Scientific and Industrial
Research (CSIR), India; Indonesian Institute of Science, Indonesia; Istituto Nazionale di
Fisica Nucleare (INFN), Italy; Institute for Innovative Science and Technology, Nagasaki
Institute of Applied Science (IIST), Japanese Ministry of Education, Culture, Sports, Sci-
ence and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS)
KAKENHI, Japan; Consejo Nacional de Ciencia (CONACYT) y Tecnología, through Fondo
de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección Gen-
eral de Asuntos del Personal Academico (DGAPA), Mexico; Nederlandse Organisatie voor
Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway, Nor-
way; Commission on Science and Technology for Sustainable Development in the South
(COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry of Edu-
cation and Science, National Science Centre and WUT ID-UB, Poland; Korea Institute of
Science and Technology Information and National Research Foundation of Korea (NRF),
Republic of Korea; Ministry of Education and Scientific Research, Institute of Atomic
Physics and Ministry of Research and Innovation and Institute of Atomic Physics, Roma-
nia; Joint Institute for Nuclear Research (JINR), Ministry of Education and Science of the
Russian Federation, National Research Centre Kurchatov Institute, Russian Science Foun-
dation and Russian Foundation for Basic Research, Russia; Ministry of Education, Science,
Research and Sport of the Slovak Republic, Slovakia; National Research Foundation of
South Africa, South Africa; Swedish Research Council (VR) and Knut & Alice Wallenberg
Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland;
Suranaree University of Technology (SUT), National Science and Technology Development
Agency (NSDTA) and Office of the Higher Education Commission under NRU project of
Thailand, Thailand; Turkish Energy, Nuclear and Mineral Research Agency (TENMAK),
Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facil-
ities Council (STFC), United Kingdom; National Science Foundation of the United States
of America (NSF) and United States Department of Energy, Office of Nuclear Physics
(DOE NP), United States of America. In addition, individual groups and members have
received support from the Horizon 2020 programme, European Union.
21
JHEP06(2022)011
Open Access. This article is distributed under the terms of the Creative