Nuclear Physics B Proceedings Supplement 00 (2014) 1–3
Nuclear Physics B Proceedings Supplement
J/ψ photoproduction in ultra-peripheral Pb-Pb and p-Pb collisions with the ALICE detector
Jaroslav Adamfor the ALICE collaboration
Faculty of Nuclear Sciences and Physical Engineering Czech Technical University in Prague
Abstract
Ultra-relativistic heavy ions generate strong electromagnetic fields which offer the possibility to study gamma- gamma, gamma-nucleus and gamma-proton processes at the LHC in ultra-peripheral Pb-Pb and p-Pb collisions (UPC).
Exclusive photoproduction ofJ/ψvector mesons is sensitive to the gluon distribution of the interacting target (proton or nucleus). This process is expected to be sensitive to saturation phenomena and nuclear gluon shadowing. Here we report on the ALICE measurement ofJ/ψcoherent photoproduction in Pb-Pb UPC at √
sNN =2.76 TeV at forward and central rapidities. Furthermore, we also present results on theJ/ψphotoproduction in p-Pb UPC at √
sNN=5.02 TeV in the forward and backward rapidities, where the rapidity is measured in the laboratory frame with respect to the proton beam direction. TheJ/ψmesons have been identified through their leptonic decays.
Keywords:
photoproduction, ultra-peripheral collision, gluon saturation, nuclear gluon shadowing
1. Introduction
1
Photoproduction reactions of photons on protons or
2
lead nuclei, γp → J/ψp and γPb → J/ψPb, which
3
leave only a J/ψmeson as a new particle in the final
4
state, allow the study of saturation phenomena and nu-
5
clear gluon shadowing at low Bjorken-x. The cross sec-
6
tion of exclusive photoproduction ofJ/ψin LO pQCD
7
is sensitive to the square of the gluon distribution in the
8
target [1]. The ALICE experiment measured this pro-
9
cess inγp andγPb with ultra-peripheral p-Pb and Pb-Pb
10
collisions (UPC).
11
The ultra-peripheral collision is a collision of Pb-ions
12
at a distance larger than the sum of nuclear radii, or
13
a collision of a proton and a Pb-ion at an impact pa-
14
rameter large enough to suppress hadronic interactions.
15
Therefore the only interaction mechanism in both cases
16
Email address:[email protected](Jaroslav Adamfor the ALICE collaboration)
is through the electromagnetic field, which behaves as a
17
flux of virtual photons. The intensity of such a flux is
18
proportional to the square of the electric charge.
19
Pb-Pb UPC allow us to study γ-Pb reactions, since
20
both of the nuclei can be the source of virtual photons.
21
In p-Pb, on the other hand, we measure γp reactions
22
because the Pb nucleus is most likely the photon source.
23
In p-Pb UPC, the centre-of-mass energy of theγp sys-
24
tem isWγp2 =2EpMJ/ψe−y, whereEpis the proton beam
25
energy, MJ/ψis the mass of theJ/ψandyis the rapid-
26
ity of the J/ψalong the direction of the proton beam.
27
Higher values ofWγpare reached at negative (backward)
28
rapidity corresponding to aJ/ψproduced in the direc-
29
tion opposite to that of the proton beam.
30
J/ψproduction in Au-Au UPC has been measured at
31
RHIC [2]. ExclusiveJ/ψproduction has been measured
32
at HERA [3] [4] using ep collisions, at the Tevatron [5]
33
in p¯p collisions at mid-rapidity and recently by LHCb
34
in pp collisions at forward rapidity [6].
35
The ALICE collaboration has measured coherentJ/ψ
36
J. Adam for the ALICE collaboration/Nuclear Physics B Proceedings Supplement 00 (2014) 1–3 2
production at forward rapidities [7] and coherent and in-
37
coherent J/ψproduction at mid-rapidities [8] in Pb-Pb
38
UPC, and exclusiveJ/ψphotoproduction in p-Pb colli-
39
sions at √
sNN=5.02 TeV [9].
40
2. The ALICE experiment at the LHC
41
The measurements reported here were performed
42
with the ALICE central detectors, which cover the pseu-
43
dorapidity range|η|<0.9, and the forward muon spec-
44
trometer, covering the pseudorapidity interval -4.0 <
45
η <-2.51. Also important for UPC analyses are the
46
forward scintillators VZERO-A and VZERO-C at pseu-
47
dorapidity 2.8< η <5.1 and -3.7< η <-1.7 respectively
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and zero degree neutron and proton calorimeters (ZDC)
49
with acceptance for neutrons of|η| < 8.8 and for pro-
50
tons of 6.5<|η|<7.5 and−9.7◦< φ <9.7◦, whereφis
51
azimuthal angle of the particle.
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The central tracking system is divided into an In-
53
ner Tracking System (ITS), Time Projection Chamber
54
(TPC) and Time of Flight detector (TOF). The ITS is
55
placed closest to the beam pipe, and is subdivided into 6
56
layers of tracking silicon detectors. The two innermost
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layers of the ITS, the Silicon Pixel Detectors (SPD),
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have acceptance extended up to |η| < 1.4. The TPC
59
provides tracking and particle identification according
60
to specific energy loss. The central detectors are sur-
61
rounded by a large solenoid magnet of magnetic fieldB
62
=0.5 T.
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The muon spectrometer is composed of a composite
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absorber, 5 MWPC tracking stations, a trigger system
65
and a dipole magnet creating an integrated field of 3
66
T·m.
67
The trigger inputs relevant for UPC analyses are pro-
68
vided by the SPD, TOF, VZERO detectors and the muon
69
spectrometer.
70
3. Signal extraction for exclusiveJ/ψin p-Pb
71
The requirements on the data selection include no
72
activity in SPD, VZERO-A and ZDCs, activity in
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VZERO-C compatible with the expected muons from
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beam-beam interaction and two unlike-sign tracks in the
75
muon spectrometer passing the tracks quality selection.
76
Candidates in a range in J/ψrapidity produce a mea-
77
surement at a given<Wγp>.
78
The direction of the beams in the LHC was inverted
79
in order to measure dimuons in the forward 2.5<y <
80
1Pseudorapidity intervals given in the laboratory frame
4.0 (p-Pb) and backward -3.6<y<-2.6 (Pb-p) rapid-
81
ity, thus providing the following energy ranges: 21 <
82
Wγp<45 and 577<Wγp<952 GeV.
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Figure 1 (left) shows the invariant mass of the se-
84
lected dimuons for both p-Pb and Pb-p configurations.
85
The fit is performed by a Crystal Ball function [10]
86
for the signal and an exponential function for the back-
87
ground. The parameters of the fit are compatible with
88
the expectations of detailed simulations of the response
89
of the detector to this type of events.
90
In order to extract the number of exclusive J/ψcan-
91
didates, the pT distribution of dimuons within the J/ψ
92
mass peak (inv. mass 2.8 < Mµ+µ− < 3.3 GeV/c2) is
93
described by 3 contributing processes, exclusiveJ/ψin
94
γp,γγ→µ+µ−and non-exclusiveJ/ψ.
95
The fits to the pT distributions are shown in the
96
right part of Figure 1, the shapes for exclusiveJ/ψand
97
γγ → µ+µ− were generated using STARLIGHT [11]
98
and folded with a GEANT3 detector simulation and the
99
contribution of non-exclusiveJ/ψandγγ→µ+µ−was
100
estimated from data.
1.5 2 2.5 3 3.5 4 4.5 5
) 2Dimuon candidates / (50 MeV/c
20 40 60 80 100 120 140 160 180 200
ALICE 2.5<y<4.0
= 5.02 TeV sNN p-Pb
2 ) ( GeV/c - µ + Mµ
1.5 2 2.5 3 3.5 4 4.5 5
0 5 10 15 20 25 30
ALICE -3.6<y<-2.6
= 5.02 TeV sNN Pb-p
ALI−PUB−89255
0 0.5 1 1.5 2 2.5 3 3.5 4
Dimuon candidates / (100 MeV/c) 20 40 60 80 100
= 5.02 TeV sNN ALICE p-Pb 2.5<y<4.0
<3.3 GeV/c2 - µ + 2.8<Mµ
(GeV/c) T Dimuon p
0 0.5 1 1.5 2 2.5 3 3.5 4
0 5 10 15 20 25
Sum Exclusive J/ψ Non-exclusive background
µ- µ+
→ γ γ
+Pb γ
= 5.02 TeV sNN ALICE Pb-p -3.6<y<-2.6
<3.3 GeV/c2 - µ + µ 2.8<M
ALI−PUB−89259
Figure 1: Invariant mass and transverse momentum distributions for forward (up) and backward (down) dimuon samples [9].
101
4. Exclusive J/ψ cross section in p-Pb at √ sNN =
102
5.02 TeV
103
The differential cross section of exclusive J/ψpro-
104
duction fromγp interactions can be calculated with for-
105
mula 3 in [7] using the measured number of exclusive
106
J/ψcandidates, the luminosity and the detection effi-
107
ciency.
108
The cross section of γ+p → J/ψ+p is proportional
109
to the measured differential cross section dσ/dyvia the
110
the photon spectrumdNγ/dk, which is the distribution
111
of photons carrying a momentumk:
112
dσ
dy(p+Pb)=kdNγ
dk σ(γ+p). (1)
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J. Adam for the ALICE collaboration/Nuclear Physics B Proceedings Supplement 00 (2014) 1–3 3
The average photon flux has been calculated from
114
STARLIGHT.
115
The σ(γ+p) cross section measured by ALICE is
116
shown in Figure 2 [9] as a function ofWγp. The upper
117
part of the figure compares the measurement to HERA
118
data [3] [4] and theoretical models, in the lower panel
119
there is a comparison of ALICE results and LHCb solu-
120
tions extracted from pp data.
(GeV)
γp
W
102 103
+p) (nb)ψ J/→+p γ(σ
10 102
103
ALICE (p-Pb) ALICE (Pb-p) Power law fit to ALICE data H1
ZEUS
JMRT LO JMRT NLO b-Sat (eikonalized) b-Sat (1-Pomeron) STARLIGHT parameterization
ALI−PUB−89263
(GeV)
p
Wγ
10 102 103
+p) (nb)ψ J/→+p γ(σ
10 102
103
Power law fit to ALICE data LHCb pp data (W+ solutions) LHCb pp data (W- solutions)
ALI−PUB−89267
Figure 2: ExclusiveJ/ψphotoproduction cross section offprotons [9].
121
A power law fit to ALICE data is in agreement with
122
a similar fit to HERA data. The LHCb solutions are
123
consistent with the power law fit obtained from ALICE
124
results.
125
5. Coherent J/ψcross section in Pb-Pb at √ sNN=
126
2.76 TeV
127
Photoproduction ofJ/ψin Pb-Pb UPC is considered
128
coherentin the case of coherent photon coupling to the
129
nucleus, while coupling to a single nucleon leads to the
130
incoherentphotoproduction.
131
The measurement using the muon spectrometer [7]
132
followed a similar analysis procedure as already de-
133
scribed for the p-Pb case. In addition there was a mea-
134
surement at mid-rapidity [8] based on events withµ+µ−
135
ore+e−pairs reconstructed in the central tracking sys-
136
tem and no other activity registered in ALICE. Muons
137
and electrons were separated by their specific energy
138
loss in the TPC.
139
ALICE results on the coherentJ/ψcross section are
140
shown in Figure 3 along with different theoretical pre-
141
dictions based on pQCD (AB, RSZ), color dipole model
142
(CSS, GM, LM) or a Glauber approach (STARLIGHT).
143
Calculations by Adelui and Bertulani [12] using the
144
EPS09 nuclear prediction is in the best agreement with
145
the measured coherent cross section.
y
-4 -2 0 2 4
/dy (mb)σd
0 1 2 3 4 5 6 7 8
RSZ-LTA STARLIGHT GM AB-EPS09 AB-MSTW08
AB-EPS08 AB-HKN07 CSS
LM-fIPSat
ALICE Coherent J/ψ Reflected
a) = 2.76 TeV sNN
Pb+Pb+J/ψ Pb+Pb →
ALI−PUB−66209
Figure 3: Cross section of coherentJ/ψphotoproduction in Pb-Pb UPC [8].
146
6. Conclusions
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ALICE measurements of J/ψ photoproduction in
148
ultra-peripheral p-Pb and Pb-Pb collisions (UPC) have
149
been described. Changes in gluon density from HERA
150
to LHC do not affect the J/ψ photoproduction. The
151
cross section of coherent J/ψ in Pb-Pb favors models
152
which include nuclear gluon shadowing consistent with
153
the EPS09 parametrization.
154
Acknowledgements
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This work was supported by grant LK11209 of
156
M ˇSMT ˇCR.
157
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