Measurement of B(J/psi->eta_c gamma) at KEDR
ABSTRACT We present a study of the inclusive photon spectrum from 6.3 million J/psi decays collected with the KEDR detector at the VEPP-4M e+e- collider. We measure the branching fraction of the radiative decay J/psi -> eta_c gamma, eta_c width and mass. Taking into account an asymmetric photon line shape we obtain: M(eta_c) = (2978.1 +- 1.4 +- 2.0) MeV/c^2, Gamma(eta_c) = (43.5 +- 5.4 +- 15.8) MeV, B(J/psi->eta_c gamma) = (2.59 +- 0.16 +- 0.31)%$. Comment: 6 pages, 1 figure. To be published in the proceedings of the 4th International Workshop on Charm Physics (Charm2010), October 21-24, 2010, IHEP, Beijing
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arXiv:1012.1694v1 [hep-ex] 8 Dec 2010
International Journal of Modern Physics: Conference Series
c ? World Scientific Publishing Company
Measurement of B(J/ψ → ηcγ) at KEDR∗†
V.V. Anashin1V.M. Aulchenko1,2E.M. Baldin1,2A.K. Barladyan1A.Yu. Barnyakov1
M.Yu. Barnyakov1S.E. Baru1,2I.Yu. Basok1I.V. Bedny1O.L. Beloborodova1,2A.E. Blinov1
V.E. Blinov1,3A.V. Bobrov1V.S. Bobrovnikov1A.V. Bogomyagkov1,2A.E. Bondar1,2
A.R. Buzykaev1S.I. Eidelman1,2Yu.M. Glukhovchenko1V.V. Gulevich1D.V. Gusev1
S.E. Karnaev1G.V. Karpov1S.V. Karpov1T.A. Kharlamova1,2V.A. Kiselev1
S.A. Kononov1,2K.Yu. Kotov1E.A. Kravchenko1,2V.F. Kulikov1,2G.Ya. Kurkin1,3
E.A. Kuper1,2E.B. Levichev1,3D.A. Maksimov1V.M. Malyshev1;1)A.L. Maslennikov1
A.S. Medvedko1,2O.I. Meshkov1,2A.I. Milstein1S.I. Mishnev1I.I. Morozov1,2
N.Yu. Muchnoi1,2V.V. Neufeld1S.A. Nikitin1I.B. Nikolaev1,2I.N. Okunev1
A.P. Onuchin1,3S.B. Oreshkin1I.O. Orlov1,2A.A. Osipov1S.V. Peleganchuk1
S.G. Pivovarov1,3P.A. Piminov1V.V. Petrov1A.O. Poluektov1D.N. Shatilov1
G.E. Pospelov1V.G. Prisekin1A.A. Ruban1V.K. Sandyrev1G.A. Savinov1A.G. Shamov1
B.A. Shwartz1,2E.A. Simonov1S.V. Sinyatkin1Yu.I. Skovpen1,2A.N. Skrinsky1
V.V. Smaluk1,2A.V. Sokolov1A.M. Sukharev1E.V. Starostina1,2A.A. Talyshev1,2
V.A. Tayursky1V.I. Telnov1,2Yu.A. Tikhonov1,2K.Yu. Todyshev1,2G.M. Tumaikin1
Yu.V. Usov1A.I. Vorobiov1A.N. Yushkov1V.N. Zhilich1V.V.Zhulanov1,2A.N. Zhuravlev1,2
1 Budker Institute of Nuclear Physics, 11, Lavrentiev prospect, Novosibirsk, 630090, Russia
2 Novosibirsk State University, 2, Pirogova street, Novosibirsk, 630090, Russia
3 Novosibirsk State Technical University, 20, Karl Marx prospect, Novosibirsk, 630092, Russia
1) E-mail: V.M.Malyshev@inp.nsk.su
We present a study of the inclusive photon spectrum from 6.3 million J/ψ decays collected
with the KEDR detector at the VEPP-4M e+e−collider. We measure the branching
fraction of the radiative decay J/ψ → ηcγ, ηc width and mass. Taking into account an
asymmetric photon line shape we obtain: Mηc= (2978.1 ± 1.4 ± 2.0) MeV/c2, Γηc=
(43.5 ± 5.4 ± 15.8) MeV, B(J/ψ → ηcγ) = (2.59 ± 0.16 ± 0.31)%.
Keywords: charmonium; radiative decays
PACS numbers: 13.20.Gd, 13.40.Hq, 14.40.Pq
1. Introduction
J/ψ → ηcγ decay is a magnetic dipole radiative transition with photon energy ω
about 114 MeV, and a relatively large branching fraction of about 2%. This is a
transition between 1S states of charmonium and its rate can be easily calculated
in potential models. The transition does not change a spatial part of the wave
function and its matrix element in the leading approximation equals one. A simple
∗A talk presented at the CHARM2010 conference in Beijing, October 2010
†Partially supported by the Russian Foundation for Basic Research, grants 08-02-00258, 08-02-
00258, and RF Presidential Grant for Sc. Sch. NSh-5655.2008.2.
1
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2 V.V. Anashin et al.
calculation without relativistic corrections gives the result1B(J/ψ → ηcγ) = 3.05%.
It was expected that relativistic corrections are of order 20-30%, similarly to the
case of the electric dipole transitions in charmonium. Therefore, it was surprising
when in 1986 the Crystal Ball Collaboration measured this branching fraction in
the inclusive photon spectrum and obtained (1.27 ± 0.36)%2. There are a lot of
theoretical predictions for this decay rate3−8, based on QCD sum rules, lattice
QCD calculations and so on, but as a rule they give values approximately twice as
large as the Crystal Ball result.
This puzzle remained for more than twenty years. Only in 2009 the CLEO
Collaboration published9the result of a new measurement of this branching fraction
using analysis of 12 exclusive decay modes of ηc. The obtained value B(J/ψ →
ηcγ) = (1.98 ± 0.09 ± 0.30)% is closer to theory. Combining the Crystal Ball and
CLEO results, PDG10got B(J/ψ → ηcγ) = (1.7±0.4)% with a scale factor of 1.6.
In this work we report a result of the new independent measurement.
2. Photon spectrum
The photon spectrum in J/ψ → ηcγ decay can be written as1
dΓ(ω)
dω
=4
3αe2
c
m2
c
ω3|M|2BW(ω). (1)
Here M =< ηc|j0(ωr/2)|J/ψ > is the matrix element of the transition, j0(x) =
sin(x)/x, ecand mcare c-quark charge (in electron charge units) and mass while
BW(ω) is a Breit-Wigner function taking into account a nonzero ηc width. The
matrix element tends to unity when ω tends to zero and decreases slowly with the
photon energy increase.
CLEO found that the photon line shape of this transition is asymmetric, and a
Breit-Wigner function alone provides a poor fit to data. Its modification with ω3
improves the fit around the peak, but gives a diverging tail at higher photon energies.
To suppress this tail, CLEO used in their fit |M|2= exp(−ω2
explaining it by the overlap of the ground state wave functions. However, such a
form of the matrix element is valid for the wave functions of the harmonic oscillator
only. In all other potentials, |M|2dependence will be proportional to some negative
degree of ω when ω tends to infinity.
We tried to fit the CLEO data using another line shape: at photon energy ω
near the resonance the decay probability dΓ/dω is proportional to ω3BW(ω), but
at higher energies the factor ω3is replaced with ω. We found that the function
ω3ω2
0
ωω0+(ω−ω0)2BW(ω), where ω0 =
ω2
0
ωω0+(ω−ω0)2 is a smooth function near the resonance. Results of
fits with the CLEO function and our function are shown in Table 1. One can see
that results on the ηc mass, width, and decay rate are close, and the confidence
level of the fit with our function is also good. Therefore, we use the latter function
in the analysis of our data.
8β2) with β = 65 MeV,
dΓ/dω ∼
M2
J/ψ−M2
2M2
J/ψ
ηc
, is also suitable. Here the
correction factor
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Measurement of B(J/ψ → ηcγ) at KEDR3
Table 1.
NEXC
1S
Results of the fits to CLEO data using various decay probability functions.
is the number of signal photons in the fit.
dΓ/dωMηc, MeV/c2
Γηc, MeVNEXC
1S
χ2/ndf (C.L.)
∼ ω3exp(−ω2
ω3ω2
ωω0+(ω−ω0)2BW(ω)
8β2)BW(ω) 2982.4 ± 0.7 32.5 ± 1.8 6142 ± 430 38.0/38 (0.47)
∼
0
2981.8 ± 0.5 33.6 ± 1.9 6494 ± 36239.1/39 (0.47)
3. KEDR data
The experiment was performed at the KEDR detector11of the VEPP-4M
collider12.
The collider operates with a peak luminosity of about 1.5·1030cm−2s−1near the
J/ψ peak energy. Luminosity is measured with single Bremsstrahlung online and
with small-angle Bhabha scattering offline. Two methods of beam energy determi-
nation are used: resonant depolarization with accuracy of 8 ÷ 30 keV and IR-light
Compton backscattering with accuracy ∼ 100 keV13.
This analysis is based on a data sample of 1.52±0.08 pb−1collected at the J/ψ
peak. Three J/ψ scans were performed. Using a measured beam energy spread as
well as known10Γ(J/ψ → e+e−) and Γ(J/ψ → hadrons), we calculate the J/ψ
production cross section at the peak and get NJ/ψ= (6.3 ± 0.3) · 106.
Event selection was performed in two steps. At the first step, multihadron decays
of J/ψ were selected with the following cuts: total energy in the calorimeters is
greater than 0.8 GeV; at least four clusters with the energy greater than 30 MeV
in the calorimeters are detected; at least one central track in the drift chamber
is reconstructed; there are no muon tubes activated in the third layer of the muon
system. These cuts suppress background from the cosmic rays, beam-gas interactions
and Bhabha events. At the second step, photons in these events were identified.
A photon is a cluster in the liquid krypton calorimeter which is not associated
with the reconstructed tracks in the drift chamber and has no time-of-flight (ToF)
scintillation counters activated in front of it. According to the simulation, the photon
detection efficiency with the above mentioned cuts is about 28% and is almost
constant in the investigated range of the photon energies.
4. Data analysis
The inclusive photon spectrum and a fit to our data is shown in Fig. 1 . The spectrum
was fit with a sum of the signal having a shape dΓ/dω ∼
convolved with the calorimeter response function (logarithmic normal distribution14
with σE = 7.4 MeV at 110 MeV and a=-0.33), and background. The background
shape was taken in the form ln(dN/dω) = a · exp(−ω/b) + p2(ω) + c · MIP(ω),
where the first term describes ”fake” photons appearing due to nuclear interactions
of hadrons in the calorimeter and usually having energies less than 60 MeV, the
second term is the second-order polynomial describing photons arising mainly from
ω3ω2
0
ωω0+(ω−ω0)2BW(ω),
Page 4
4 V.V. Anashin et al.
, MeV
ω
50 100 150 200 250 300 350 400
Events/4 MeV
20
40
60
80
100
120
140
160
180
200
220
3
10
×
Photon spectrum
, MeV
ω
50100 150 200 250300350
Events/4 MeV
-1000
0
1000
2000
3000
Photon spectrum, background subtracted
Fig. 1.Fit of the inclusive photon spectrum.
π0decays, and MIP(ω) is the spectrum of charged particles (a charged particle can
be also misidentified as a photon). The Breit-Wigner function of the form BW(ω) ∼
s/((s − M2
J/ψ− 2ωMJ/ψ, was used in the fit.
We also tried to fit our data using other line shapes. We used BW(ω) alone,
ω3BW(ω), the CLEO function, and our function. Again, as first noted by CLEO,
we found that the BW(ω) function alone gives a shifted value of ηcmass compared
to other functions: M(ηc) = 2974.3±1.4 MeV/c2. The function ω3BW(ω) leads to
a large tail at higher photon energies, giving for the branching fraction B(J/ψ →
ηcγ) = (7.3 ± 0.5)% (here the decay probability function was integrated up to
MJ/ψ/2). The last two functions give close fit results. Since the CLEO function has
an exponential factor, the result for the branching fraction with this function can
be considered as a lower limit. The difference between the results obtained with
the two last functions is used to estimate a systematic error appearing due to the
unknown line shape.
ηc)2+ sΓ2
ηc), where s = M2
5. Systematic errors
Systematic errors of our measurements are listed in Table 2.
The uncertainty of the ηcwidth leads to an error, which we evaluated varying the
ηcwidth in the fit by 2.2 MeV (the current PDG error). A systematic error related
to the background subtraction was estimated by using in the fit a polynomial of the
third order instead of the second-order one, varying ranges of the fit, and applying or
not the ToF veto in photon selection. The error in the number of J/ψ produced was
evaluated using the known uncertainty of the luminosity measurement. Since the
cluster multiplicity is different in the simulation and experimental photon spectrum
in J/ψ decays, the error due to the photon detection efficiency was estimated by
changing by 25% weights of events with small (n < 4) and large (n > 3) track
multiplicities, and taking different MC generators for the ηcdecays. The calibration
Page 5
Measurement of B(J/ψ → ηcγ) at KEDR5
Table 2.Systematic errors.
Systematic errorMηc, MeV/c2
Γηc, MeV
B(J/ψ) → γηc),%
Line shape
ηc width
Background subtraction
Number of J/ψ produced
Photon efficiency
Photon energy scale
0.7
0.4
0.8
2.30.15
0.15
0.17
0.13
0.08
15.6
1.7
Total2.015.8 0.31
of the photon energy scale was performed using π0→ 2γ decays and ψ′→ γχcJ→
γJ/ψ transitions.
6. Results and conclusions
A new direct measurement of J/ψ → ηcγ decay was performed. We measured
the ηc mass, width, and branching fraction of J/ψ → ηcγ decay. The values of
the branching fraction and ηc mass are sensitive to the line shape of the photon
spectrum and it should be taken into account during analysis. Our results on the
ηcmass and width are:
Mηc= 2978.1± 1.4 ± 2.0 MeV/c2,
Γηc= 43.5 ± 5.4 ± 15.8 MeV.
Before our experiment these parameters were measured in J/ψ and B meson
decays, as well as in γγ and p¯ p collisions. Measurements of Crystal Ball, MARK3,
BES, and KEDR were performed using the radiative J/ψ decays, therefore, a mass
shift should be taken into account. Crystal Ball and KEDR made such a correction,
but MARK3 and BES did not. Therefore we believe that MARK3 and BES results
on the ηcmass should be corrected by approximately 4 MeV towards higher values.
Our result on the branching fraction of J/ψ → ηcγ decay is
B(J/ψ → ηcγ) = (2.59 ± 0.16 ± 0.31)%.
It is consistent with that of CLEO, is higher than the old Crystal Ball value and
close to theoretical predictions.
The authors are grateful to N. Brambilla and A. Vairo for useful discussions.
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