J/psi production at high pT in p+p and A+A collisions at STAR

Abstract

The preliminary results of J/psi spectra at high transverse momentum (5<p_T<14 GeV/c) in p+p and Cu+Cu collisions at s_NN = 200 GeV are reported. The nuclear modification factor is measured to be 0.9+/-0.2 at p_T>5 GeV/c. The correlations between J/psi and charged hadrons are also studied in p+p collisions to understand the J/psi production mechanism at high p_T.

Full text

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arXiv:0804.4846v1 [nucl-ex] 30 Apr 2008
J/ψ production at high pT in p + p and A + A
collisions at STAR
Zebo Tang‡ (for the STAR collaboration)
Dept. of Modern Physics, University of Science and Technology of China, Hefei,
Anhui, China, 230026; Brookhaven National Laboratory, Upton, NY, 11973, USA
E-mail: zbtang@mail.ustc.edu.cn
Abstract.
(5 < pT< 14 GeV/c) in p+p and Cu+Cu collisions at√sNN= 200 GeV are reported.
The nuclear modification factor is measured to be 0.9 ± 0.2 at pT > 5 GeV/c. The
correlations between J/ψ and charged hadrons are also studied in p + p collisions to
understand the J/ψ production mechanism at high pT.
The preliminary results of J/ψ spectra at high transverse momentum
1. Introduction
J/ψ dissociation from color-screening of Quantum Chromodynamics (QCD) in a Quark-
Gluon Plasma (QGP) is one of the major signatures of QCD de-confinement in
relativistic heavy-ion collisions. Recent calculations which assuming the AdS/CFT
duality is valid for QCD expect that a heavy fermion pair bound state (an analog of
quarkonium in QCD) will have an effective dissociation temperature decreasing with pT
[1]. This requires a measurement of J/ψ extending to pT> 5 GeV/c where the effective
J/ψ dissociation temperature is expected to decrease to the temperature reached at
RHIC (∼ 1.5 Tc) . In this paper, we report the J/ψ spectra at high transverse momentum
(5 < pT< 14 GeV/c) in p + p and Cu+Cu collisions at√sNN= 200 GeV. In addition,
we performed an analysis of J/ψ-hadron correlations in p + p collisions to understand
the J/ψ production mechanism at high pT. The technique is similar to that used by
UA1 [2] and dihadron correlations analyzed by STAR [3].
UA1 simulated J/ψ-hadron correlation and found two cases: When a J/ψ originated
from χcthere as no visible near-side correlation, whereas J/ψ’s originating from B meson
decays showed a strong near-side correlation. The large acceptance of STAR Time
Projection Chamber (TPC) [4] and the Barrel Electromagnetic Calorimeter (BEMC)
[4] covering |η| < 1 are very well suited for such analyzes.
‡ The author was supported in part by the National Natural Science Foundation of China under Grant
No. 10610285, 10610286, 10575101 and the Knowledge Innovation Project of Chinese Academy of
Sciences under Grant No. KJCX2-YW-A14.

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J/ψ production at high pT in p + p and A + A collisions at STAR2
2. Data analysis and Results
At STAR, both the TPC and BEMC can provide electron identification [4]. At high
pT, the BEMC is very powerful for electron identification and can also be used to set
up a fast trigger to enrich the electron sample. At moderate pT, the TPC can identify
electrons efficiently. In this analysis, the high pT J/ψ was reconstructed through the
dielectron channel, one electron at high pT identified by combing the BEMC and TPC
and the other electron at lower pT identified by the TPC only. We used the BEMC
triggered data in p + p and Cu+Cu collisions at√sNN = 200 GeV. The integrated
luminosity is ∼ 2.8 (11.3) pb−1for p + p collisions collected in year 2005 (2006) with
transverse energy threshold ET> 3.5 (5.4) GeV, and ∼ 860 µb−1for Cu+Cu collisions
collected in year 2005 with ET> 3.75 GeV.
]
2
(ee) [GeV/c
Inv
M
2.6 2.7 2.8 2.933.1 3.2 3.3 3.4 3.5 3.6
)
2
Counts/(40MeV/c
0
2
4
6
8
10
unlike-sign
like-sign
-e
+
e
→ψ
J/
p+p 2005
STAR Preliminary
]
2
(ee) [GeV/c
Inv
M
2.6 2.7 2.8 2.933.1 3.2 3.3 3.4 3.5 3.6
)
2
Counts/(50MeV/c
0
2
4
6
8
10
12
14
unlike-sign
like-sign
-e
+
e
→ψ
J/
Cu+Cu 2005
STAR Preliminary
Figure 1. The dielectron invariant mass distributions in p + p (left) and Cu+Cu
(right) collisions at√sNN= 200 GeV.
Figure 1 shows the high pT J/ψ signal in p + p (left) and Cu+Cu (right) collisions
at√sNN= 200 GeV. The background is represented by the dashed lines from like-sign
technique. We applied a cut of pT > 2.5 − 4 GeV/c to the EMC triggered electrons
and the cut of pT > 1.2 − 1.5 GeV/c for lower pT electrons. This ensured clean J/ψ
identification. The signal/background (S/B) ratio in the analysis is 22/2 (40/14) in p+p
collisions using year 2005 (2006) data and 17/23 in Cu+Cu collisions. The pTcoverage
in p + p and Cu+Cu collisions taken in year 2005 is 5 < pT< 8 GeV/c, while in p + p
collisions taken in year 2006, the J/ψ pT can reach 14 GeV/c due to higher recorded
luminosity and full BEMC coverage. The J/ψ invariant cross section Bee× Ed3σ/dp3,
after efficiency correction, are shown as symbols in Figure 2 (left).
The invariant cross section of inclusive pion and proton production in high energy
p + p collisions have been found to follow the xT scaling law: Ed3σ
xT = 2pT/√s. The value of the power n depends on the quantum exchanged in the
hard scattering and is related to the number of point-like constituents taking an active
role in parton model. It reaches 8 in the case of a diquark scattering and reaches 4 in
more basic scattering processes (as in QED). Figure 2 (middle) shows the xT scaling of
J/ψ, pion and proton. The power n was found to be 6.5 ± 0.8 for pion and proton [5]
and 5.6 ± 0.2 for J/ψ, which indicates that the high pT J/ψ production mechanism is
closer to parton-parton scattering.
dp3 =
g(xT)
√sn , where

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J/ψ production at high pT in p + p and A + A collisions at STAR3
(GeV/c)
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Two Component Approach, Cu+Cu 0-60%
Two Component Approach, Cu+Cu 0-20%
=200GeV
NN
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Figure 2. Left: J/ψ invariant cross section as a function of pT in p + p and Cu+Cu
collisions at√sNN= 200 GeV. Errors shown are statistical only. Middle: xT scaling
of pions, protons and J/ψs.The data from other measurements can be found in
references [8, 9, 10, 12, 5, 2, 11]. Right: J/ψ RAAas a function of pT. The dot-dashed
line represents the fit by constant to all the data points at 5 < pT < 10 GeV/c. The
boxes on the right show the normalization uncertainty.
Figure 2 (right) shows the J/ψ nuclear modification factor RAAas a function of
pT in 0-20% and 0-60% Cu+Cu from PHENIX [6] and STAR measurements. RAA
tends to increase from low to high pT, although the error bars currently do not allow
to draw strong conclusions. One can nevertheless do a combined fit to all the high-pt
data and find that RAA= 0.9 ± 0.2. This result is in contrast to the expectation from
AdS/CFT-based models [1] and from the Two-Component model [7] which predict a
decreasing RAAwith increasing pT. This result could indicate that other J/ψ production
mechanisms such as virtual photons or formation time [13] play a role at high pT.
With large S/B ratios, the J/ψ-hadron correlations were also measured in p + p
collisions. Figure 3 (left) shows the azimuthal angle correlations between high pT J/ψ
(pT > 5 GeV/c) and charged hadrons.
observed, which is in contrast to the dihadron correlation measurements [3]. Since
the Monte Carlo simulation results show a strong near side correlation if the J/ψ is
produced from B-meson decay, these results can be used to constrain the B-meson
contribution to J/ψ production. Figure 3 (right) shows the associated charged hadron
pTdistribution on the near side and away side with respect to J/ψ triggers and charged
hadron triggers. On the away side, the yields of the associated charged hadrons with
respect to both kinds of triggers are consistent with each other, which indicates that the
hadrons on the away side of J/ψ triggers are from light quark or gluon fragmentation.
On the near side, the associated charged hadron yields with respect to J/ψ triggers are
significantly lower than those with respect to charged hadron triggers. This indicates
that the B-meson is not a dominant contributor to the inclusive high pT J/ψ.
No significant near side correlations were

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J/ψ production at high pT in p + p and A + A collisions at STAR4
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b) Near Side
Figure 3.
collisions at√sNN = 200 GeV. Right: Associated charged hadron pT distribution
on the near and away side with respect to J/ψ triggers and charged hadron triggers.
Left: J/ψ-hadron correlations after background subtraction in p + p
3. Summary
We reported the STAR preliminary results of J/ψ spectra from 200 GeV p + p and
Cu+Cu collisions at high pT(5 < pT< 14 GeV/c) at mid-rapidity through the dielectron
channel. The high pT J/ψ production was found to follow the xT scaling with a beam
energy dependent factor ∼√sNN
Cu+Cu increases from low to high pTwhich challenges some models. The average of RAA
at pT > 5 GeV/c is 0.9 ± 0.2, consistent with no J/ψ suppression. It implies that high
pT J/ψ may be produced from virtual photon or formed outside of the hot interaction
region [13]. The J/ψ-hadron correlations were also discussed. We observed an absence
of charged hadrons accompanying high pT J/ψ on the near side which indicates that
the B-meson is not a dominant contributor to the inclusive high pT J/ψ.
5.6±0.2. The J/ψ nuclear modification factor RAAin
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