√
J/ψ Production in s = 200 GeV p+p Collisions
with the PHENIX Detector at RHIC
H.D. Sato1
for the PHENIX Collaboration
University of Kyoto, Sakyo-ku, Kyoto 606-8502, Japan
Abstract. The PHENIX experiment at the Relativistic Heavy Ion Collider (RHIC) detects J/ψ
particles in the rapidity range |y| < 0.35 using the electron-pair decay channel and in 1.2 < y < 2.2
using the muon-pair decay channel. Cross sections
for the J/ψ production in those rapidity ranges
√
have been measured in p+p collisions at s = 200 GeV in the RHIC Run 2001-2002 period
(Run-2). As a result, Br(J/ψ → µ + µ − ) d σJ/ψ /dy|y=1.7 = 37 ± 7 (stat.) ± 11 (syst.) nb and
Br(J/ψ → e+ e− ) d σJ/ψ /dy|y=0 = 52 ± 13 (stat.) ± 18 (syst.) nb are obtained. Total cross section
has been extracted by fitting these results to be σ J/ψ = 3.8 ± 0.6 (stat.) ± 1.3 (syst.) µ b which is
consistent with the perturbative QCD prediction.
INTRODUCTION
Heavy quarkonium production has been playing a very important role in high energy
hadron physics. Its production in high-energy heavy-ion collisions is considered to be
one of the best probes for the earliest stages of the new state of matter, called “QuarkGluon Plasma” [1, 2]. Also its production asymmetries in longitudinally-polarized p+p
collisions are expected to contain information on the polarized gluon density. The
elucidation of the production mechanism of the heavy quarkonium is a key to this
measurement.
The importance of the measurement of the production cross-section for J/ψ in p+p
collisions is twofold: (1) testing theoretical models for the production mechanism and
(2) providing the reference point for measurements in heavy ion collisions. In this
√ paper,
the first results of the production cross section for J/ψ in p+p collisions at s = 200
GeV are presented.
THE PHENIX EXPERIMENT
The PHENIX experiment consists of two independent spectrometers which cover different pseudo-rapidity (η ) regions. Two Central Arms, West and East Arms, cover
|η | < 0.35, ∆φ (azimuthal coverage) = π and measure hadrons, electrons and photons.
1
a JSPS Research Fellow
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
328
Two Muon Arms, North and South Arms, cover 1.2 < η < 2.4 and −2.2 < η < −1.2
respectively with a full azimuth and measure muons. J/ψ particles are identified with
the invariant mass of e+ e− pairs measured in the Central Arms and µ + µ − pairs measured in the Muon Arms. In Run-2, both Central Arms and the South Muon Arm were
operational. In addition to these Arms, three kinds of interaction trigger counters have
been used during the run. They are Beam-Beam Counters (BBC), Normalization Trigger
Counters (NTC) and Zero-Degree Calorimeters (ZDC), which cover different pseudorapidity ranges. For the J/ψ analysis, only BBC-trigger events are used. The coverage
of the BBC is 3.0 < |η | < 3.9.
An approximately 100 nb−1 integrated-luminosity with a good vertex cut is useful
for physics analyses out of 150 nb −1 recorded during the Run-2. For the J/ψ analysis
described in this paper, 81 nb−1 is used for the muon channel and 48 nb −1 for the
electron channel with appropriate run selections.
MUON CHANNEL MEASUREMENT
The South Muon Arm in PHENIX measures muons in the pseudo-rapidity range −2.2 <
η < −1.2 and with a momentum p > 2 GeV/c. It consists of the Muon Tracking
(MuTr) chambers inside the conical Muon Magnet to measure muon momenta and the
Muon Identifier (MuID) chambers interleaved between the steel absorber to discriminate
muons from charged hadrons and provide triggers.
A simple NIM-logic with a memory look-up is used to trigger events with muons
in p+p collisions. Each MuID plane is divided into four quadrants by both vertical and
horizontal lines at its center. If a muon passes through a certain quadrant, a coincidence
of fired planes gives a “quadrant trigger”. The number of the fired quadrants is counted
and if it is one, a single-muon trigger is issued while a dimuon trigger is issued if it is
more than one. Dimuon-trigger events are used for the J/ψ analysis. Inefficiency due to
hardware dead time is measured to be small (1-2%).
Figure 1 shows invariant mass spectra for both opposite-sign muon pairs and samesign pairs. There is a significant enhancement for the opposite-sign pairs in the J/ψ
mass region. Assuming the same spectra for opposite-sign and same-sign muon pairs
from background, which is confirmed with simulation and real data, the number of J/ψ
is obtained to be 36 ± 7 (stat.) ± 4 (syst.).
Acceptance times reconstruction efficiency for J/ψ → µ + µ − is obtained using a
GEANT [3] simulation with the same reconstruction software as for the real data to be
(1.63 ± 0.31)%. Dominant systematic uncertainties come from (1) MuID chamber efficiencies (11%), (2) MuTr chamber efficiencies (10%) and (3) Unknown spin-alignment
(λ ) of J/ψ (10%). For (3), |λ | < 0.3 is assumed which is consistent with both the lower
energy and higher energy experiments [4, 5].
The integrated luminosity L used in this analysis is obtained as L =
inela σ
9
NBBC /εBBC
inela where NBBC (1.73 × 10 ) is the number of BBC triggers with a
inela
small error, εBBC (0.51) is BBC efficiency for p+p inelastic events obtained using the
PYTHIA [6] event generator and GEANT simulation and σinela is the p+p inelastic
inela σ
cross section which is obtained using the fit in [7] (42 mb). The value ε BBC
inela
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20
Number of Counts
Number of Counts
30
PHENIX Preliminary
p+p, s = 200 GeV
1.2 < y < 2.2
µ +µ µ +µ +, µ - µ -
10
0
1
PHENIX Preliminary
p+p, s = 200 GeV
2x2 Tile Trigger
15
10
5
2
3
4
0
1
5
2
µ µ Invariant Mass (GeV/c )
FIGURE 1. Invariant mass spectra for unlikesign and like-sign muon pairs. The error bars include statistical errors only.
2
3
+ -
4
5
2
e e Invariant Mass (GeV/c )
FIGURE 2. Invariant mass spectrum for e + e−
pairs with a Gaussian fit to the J/ψ peak. The error
bars include statistical errors only.
is compared with the one obtained with the van der Meer scan results and they are
consistent within 20%. BBC efficiency for p+p → J/ψ → µ + µ − events is also needed
for the normalization and obtained to be 0.74 with simulation.
Consequently, the branching fraction for the decay J/ψ → µ + µ − times the
cross section for the J/ψ production in the Muon Arm acceptance Br(J/ψ →
µ + µ − )d σJ/ψ /dy|y=1.7 is obtained to be 37 ± 7 (stat.) ± 11 (syst.) nb.
The average transverse momentum (pT ) of J/ψ is also obtained to be 1.66 ± 0.18
(stat.) ± 0.09 (syst.) GeV/c in the limit of our measurement (0 < p T < 5 GeV/c). A
correction on pT due to missing high p T events is expected to be small (3%) assuming
the function form which describes the pT spectra of the lower energy measurements well
[8].
ELECTRON CHANNEL MEASUREMENT
Electrons are tracked by the Central Tracking Chambers (Drift Chambers and Pad Chambers) and identified by the Ring-Imaging Čerenkov detectors and Electro-Magnetic
Calorimeters (EMCal) in the Central Arms [9].
The EMCal was used also for triggering electrons. A trigger is fired when at least
one energy sum of EMCal 2×2 towers exceeds the threshold, which is 0.8 GeV/c. The
efficiency of this trigger for J/ψ → e+ e− is obtained to be 0.90+0.06
−0.07 with a Monte-Carlo
simulation tuned to reproduce single-photon efficiencies in the real data well.
Figure 2 shows an invariant mass spectrum for e+ e− pairs. The peak of J/ψ is clearly
seen with a small background. The number of J/ψ is obtained to be 24 ± 6 (stat.) ± 4
(syst.).
330
σ J/φ( µb)
Bll dσ J/ψ/dy (nb)
p+p, s =200 GeV
e +e -
60
10
1
40
µ µ
+ -
10
10
20
PHENIX Preliminary
10
PHENIX Preliminary
-1
-2
Color evapor. model + GRV94HO
Color evapor. model + MRS-A
Experimental data
-3
GRV94LO
0
-4
-3
-2
-1
0
1
10
2
3
4
J/ψ Rapidity
-4
10
FIGURE 3. J/ψ rapidity differential cross section including both the e + e− and µ + µ − measurements. The error bars show statistical errors
while systematic errors are shown with the brackets. The curve is for the PYTHIA prediction with
the GRV94-LO PDFs.
10
2
s (GeV)
FIGURE 4. Center-of-mass energy dependence
of the total production cross section for J/ψ
in nucleon-nucleon collisions. The error bars include both statistical and systematic errors added
in quadrature. The two curves show the colorevaporation model predictions described in [10].
Acceptance times reconstruction efficiency for J/ψ → e+ e− is obtained to be (1.63
± 0.20)%. The azimuthal coverage of the Central Arm detectors gives pT dependence
of the acceptance. An systematic error on the acceptance due to the uncertainty on the
pT distribution is estimated to be 7%.
The integrated luminosity used for the electron channel analysis and BBC efficiency
for p+p → J/ψ → e+ e− events are obtained in the same way as the muon analysis.
As a result, the branching fraction times the cross section for the J/ψ production at
mid rapidity Br(J/ψ → e+ e− )d σJ/ψ /dy|y=0 = 52 ± 13(stat.) ± 18(syst.) nb is obtained.
RAPIDITY DISTRIBUTION AND TOTAL CROSS SECTION
Figure 3 shows the rapidity differential cross section for the J/ψ production including both the electron and muon channel measurements, which is consistent with the
PYTHIA distribution with the GRV94-LO parton distribution functions (PDFs). The
choice of PDF affects the rapidity distribution slightly, thus changes the acceptance estimation. This effect, however, is found to be small (3%). Total cross section is extracted
to be σ (p + p → J/ψ X ) = 3.8 ± 0.6(stat.) ± 1.3(syst.)µ b using Br(J/ψ → l + l − ) 2 =
(5.9 ± 0.1)% [7].
Figure 4 shows the center-of-mass energy dependence of the total cross section for the
J/ψ production in nucleon-nucleon collisions including the results of the lower energy
experiments. The solid and dotted lines show the color-evaporation model predictions
with two different sets of PDFs, QCD scales and charm quark masses described in [10].
The color-octet model can also reproduce these experimental results including the color2
The average value of Br(J/ψ → e + e− ) and Br(J/ψ → µ + µ − )
331
octet matrix elements described in [11]. Our result is consistent with the perturbative
QCD prediction with appropriate parameters and gluon density.
CONCLUSION
√
We have successfully identified J/ψ particles in s = 200 GeV p+p collisions
both at forward rapidity (1.2 < η < 2.2) using the muon decay channel and at
central rapidity (|η | < 0.35) using the electron decay channel with the PHENIX
detector at RHIC. Cross sections for the J/ψ production in those rapidity ranges
are obtained to be Br(J/ψ → µ + µ − )d σJ/ψ /dy|y=1.7 = 37 ± 7(stat.) ± 11(syst.)
nb and Br(J/ψ → e+ e− )d σJ/ψ /dy|y=0 = 52 ± 13(stat.) ± 18(syst.) nb respectively. Total production cross section is extracted from these measurements to be
σJ/ψ = 3.8 ± 0.6(stat.) ± 1.3(syst.)µ b which is consistent with the perturbative QCD
calculation.
REFERENCES
1.
2.
3.
4.
T. Matsui and H. Satz, Phys. Lett. B178, 416 (1986).
R.L. Thews et al., Phys. Rev. C63, 054905 (2001).
GEANT User’s Guide, 3.15, CERN Program Library.
C. Akerlof et al., Phys. Rev. D48, 5067 (1993), A. Gribushin et al., Phys. Rev. D53, 4723 (1996),
T. Alexopoulos et al., Phys. Rev. D55, 3927 (1997), C. Biino et al., Phys. Rev. Lett. 58, 2523 (1987).
5. T. Affolder et al., Phys. Rev. Lett. 85, 2886 (2000).
6. T. Sjöstrand, Comp. Phys. Comm. 82, 74 (1994).
7. K. Hagiwara et al., Phys. Rev. D66, 010001 (2002).
8. M. H. Schub et al., Phys. Rev. D52, 1307 (1995), D53, 570 (1996), A. Gribushin et al., Phys. Rev.
D62, 012001 (2000).
9. J. T. Mitchell et al., Nucl. Instr. Meth. A482,491 (2002).
10. J. F. Amundson et al., Phys. Lett. B390, 323 (1997).
11. M. Beneke and I. Z. Rothstein, Phys. Rev. D54 2005 (1996).
332