Quarkonia production in ultra-peripheral PbPb collisions
at LHCb∗
arXiv:2301.00222v1 [hep-ex] 31 Dec 2022
Xiaolin Wang on behalf of the LHCb collaboration
Guangdong Provincial Key Laboratory of Nuclear Science, Guangdong-Hong Kong
Joint Laboratory of Quantum Matter, Institute of Quantum Matter, South China
Normal University, Guangzhou, China
Received January 3, 2023
Measurements of coherent charmonium production cross sections together with their ratio in ultra-peripheral PbPb collisions are studied at
a nucleon-nucleon centre-of-mass energy of 5.02 TeV, the differential crosssections are measured as a function of rapidity and transverse momentum,
separately. The photo-production of J/ψ mesons at low transverse momentum is studied in peripheral PbPb collisions, which confirms coherent J/ψ
production in hadronic collisions. These latest results significantly improve
previous measurements and are compared with some theoretical predictions.
1. Introduction
The LHCb detector is a single-arm forward spectrometer covering the
pseudorapidity range 2 < η < 5 [1, 2], which has great performance with precise vertex reconstruction, high particle momentum resolution, and excellent
particle identification capability. Measurements of quarkonia production in
(ultra-)peripheral heavy-ion collisions play an important role in studying
the photon-nucleus interaction, the mechanism of vector meson production,
and the partonic structure of nuclei. Meanwhile, coherent photo-production
would provide an excellent laboratory to study the nuclear shadowing effects
and the initial state of heavy ion collisions at small-x at the LHC [3].
2. Charmonia production in ultra-peripheral PbPb collisions at
LHCb [4]
Ultra-peripheral collisions(UPCs) occur when the impact parameter is
larger than the sum of the radii of two incoming nuclei [5]. The two ions
∗
Presented at ”Diffraction and Low-x 2022”, Corigliano Calabro (Italy), September
24-30, 2022
(1)
2
template
printed on January 3, 2023
interact via their cloud of semi-real photons, and photon-nuclear interactions
dominate. In UPCs, J/ψ and ψ(2S) mesons are produced from the colorless
reaction of a photon from one nucleus and a Pomeron from the other. If the
photon interacts with a Pomeron emitted by the whole nucleus, there would
be no nucleus break-up, and this is called coherent production, which is the
process we are going to study.
The charmonia candidates are reconstructed through the J/ψ → µ+ µ−
and ψ(2S) → µ+ µ− decay channels using a PbPb data sample corresponding
to an integrated luminosity of 228 ± 10 µb−1 , which was collected by the
LHCb experiment in 2018. According to the characteristics of UPCs, only
events with low activity could be kept in the selection. After that, the signal
extraction is done through two steps. A fit on the dimuon invariant mass
spectrum is needed to estimate the non-resonant background yields within
the J/ψ and ψ(2S) mass windows, then the fits to J/ψ and ψ(2S) transverse
momentum distributions of selected candidates are performed to determine
the coherent production events from the inclusive charmonia yields, as shown
in Fig. 1.
The measured differential cross sections as a function of y ∗ and p∗T of
coherent J/ψ and ψ(2S) are shown in Fig. 2, where the starred notation
indicates that the observable is defined in the nucleus-nucleus centre-ofmass frame. The differential cross-section ratio of coherent ψ(2S) to J/ψ
production is calculated as a function of rapidity for the first time and
shown in Fig. 3. Compared with theoretical predictions, which are grouped
as perturbative-QCD calculations [6, 7] and colour-glass-condensate (CGC)
models [8–15], it could be found that the measurements are in agreement
with most of the prediction curves in general.
3. Study of J/ψ photo-production in PbPb peripheral collisions
√
at sN N = 5 TeV [16]
In peripheral collisions, the impact parameter is less than the sum of
radii of the two colliding nuclei, so there is not only photo-nuclear interaction
but also hadronic interaction. For hadronic productions, the gluon-gluon
interaction produces a pair of charm quarks and lastly goes into the J/ψ
meson. In this case, J/ψ meson will have transverse momentum typically
between 1 and 2 GeV/c, and for coherent photo-production, J/ψ would have
a very small transverse momentum, less than 300 MeV/c. Thanks to the
high precision of the LHCb experiment, there are two visible clear peaks
in the distribution of dimuon ln(p∗2
T ), which could be used to discriminate
photo-produced and hadronically produced J/ψ , as shown in Fig. 4.
The differential photo-production yields of J/ψ versus the rapidity, the
number of participants in the collision, and the double-differential J/ψ photo-
Candidates / (5 MeV/c2)
template
105
printed on January 3, 2023
3
LHCb
PbPb sNN = 5.02 TeV
2.0 < y* < 4.5
4
10
103
Data
Fit
J/ ψ
ψ (2S)
Background
102
10
3000
3500
4000
1000
800
600
Events / ( 0.3 [ln(GeV2/c2)] )
Events / ( 0.125 [ln(GeV2/c2)] )
mµ +µ - [MeV/c2]
LHCb
J/ψ
PbPb sNN = 5.02 TeV
2.0 < y* < 4.5
Data
Fit
Coherent J/ ψ
400
Incoherent J/ ψ
ψ (2S) → J/ ψ + X
200
0
−15
Non-reson.
−10
−5
0
ln(p *2
) [ln(GeV2/c2)]
T
200 LHCb
180 PbPb sNN = 5.02 TeV
160 2.0 < y* < 4.5
140
Data
120
Fit
100
Coherent ψ (2S)
80
60
Incoherent ψ (2S)
40
Non-reson.
20
0
−15
−10
ψ(2S)
−5
0
ln(p *2
) [ln(GeV2/c2)]
T
Fig. 1: Fit to the invariant mass distribution of dimuon candidates (up) and the
∗
ln(p∗2
T ) distribution fit of dimuon candidates within the 2.0 < y < 4.5 range for
J/ψ candidates (left) and ψ(2S) candidates (right), where the starred notation
indicates that the observable is defined in the nucleus-nucleus centre-of-mass frame.
production yields versus transverse momentum as shown in Fig. 5. For the
phenomenological models, one scenario does not consider the destructive
effect due to the overlap between the two nuclei, whereas the other takes it
into account. In general, the trends of J/ψ photo-production measurements
and theoretical predictions are consistent. Meanwhile, the two theoretical
curves do not show a significant difference because the collisions are peripheral
and the nuclear overlapping effect is expected to increase in more central
collisions. Currently, the measurement is limited to low values of Npart due
to detector limitation, and the measurement of pT is consistent with the
coherent J/ψ photo-production; the peak value of transverse momentum is
around 60 MeV/c.
4
printed on January 3, 2023
LHCb√
PbPb sNN = 5.02 TeV
dσψ(2S)/dy ∗ [mb]
Coherent J/ψ production
Luminosity unc. : 4.4%
0
1
2
3
4
5
Luminosity unc. : 4.4%
20
10
0.05
0.10
0.15
Luminosity unc. : 4.4%
Krelina et al.
GBW+BT
GBW+POW
KST+BT
GG-hs+BG
0.5
Mäntysaari et al.
No fluct. +BG
0
1
2
3
4
y
30
0
0.00
Guzey et al.
LTA W
LTA S
EPS09
Coherent ψ(2S) production
1.0
y∗
LHCb
√
PbPb sNN = 5.02 TeV
Coherent J/ψ production
40
LHCb
√
PbPb sNN = 5.02 TeV
0.0
60
50
data
stat. unc.
syst. unc.
1.5
d2σψ(2S)/dp∗Tdy ∗ [mb/(GeV/c)]
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
d2σJ/ψ /dp∗Tdy ∗ [mb/(GeV/c)]
dσJ/ψ /dy ∗ [mb]
template
0.20
p∗T [GeV/c]
LHCb
√
PbPb sNN = 5.02 TeV
Coherent ψ(2S) production
2 < y ∗ < 4.5
6.0
4.5
Luminosity unc. : 4.4%
Gonçalves et al.
bCGC+BG
bCGC+GLC
5
IP-SAT+BG
∗
IP-SAT+GLC
data
stat. unc.
syst. unc.
Guzey et al.
LTA W
LTA S
EPPS16
Mäntysaari et al.
Is fluct. +BG
No fluct. +BG
Is fluct. +GLC
No fluct. +GLC
3.0
1.5
0.0
0.00
0.05
0.10
0.15
0.20
p∗T [GeV/c]
Fig. 2: Differential cross-section as a function y ∗ (up) and p∗T (down) for coherent J/ψ
(left) and ψ(2S) (right) production. Compared to theoretical predictions, which are
grouped as perturbative-QCD calculations (red lines) and colour-glass-condensate
models (blue lines).
data
stat. unc.
syst. unc.
0.40
0.35
LHCb√
PbPb sNN = 5.02 TeV
σψ(2S)/σJ/ψ
0.30
Coherent ψ(2S)/J/ψ production
0.25
Krelina et al.
GBW+BT
GBW+POW
KST+BT
GG-hs+BG
0.20
0.15
Mäntysaari et al.
No fluct. +BG
0.10
0.05
0.00
Guzey et al.
LTA W
LTA S
EPS09
0
1
2
3
4
y
Gonçalves et al.
bCGC+BG
bCGC+GLC
5
IP-SAT+BG
∗
IP-SAT+GLC
Fig. 3: Differential cross-section ratio of ψ(2S) to J/ψ as a function of y ∗ , compared
to theoretical predictions, which are separated into perturbative QCD calculations
(red lines) and colour-glass-condensate models (blue lines).
220
printed on January
3, 2023
200 LHCb
Data
PbPb sNN = 5 TeV
250
Signal
<N part> = 10.6 ± 2.9
200
Background
Events/(0.40)
Candidates /(5 MeV/c2)
template
LHCb
300
2.0 < y < 4.5
N Total
J/ ψ = 1764 ± 52
150
100
50
0
3000
3050
3100
3150
3200
m(µ +µ −) [MeV/ c2]
180
160
140
120
100
80
60
40
20
0
5
PbPb sNN = 5 TeV
<N part> = 10.6 ± 2.9
2.0 < y < 4.5
N J/ ψ = 277 ± 19
Data
J/ψ hadro-produced
J/ψ photo-produced
5
10
15
20
ln( p 2T / [MeV2/ c2])
−3
LHCb
PbPb sNN = 5 TeV
Photo-produced J/ ψ → µµ
< N part> = 19.7 ± 9.2
0.5
0.4
0.2
0.2
−3
LHCb
PbPb sNN = 5 TeV
Photo-produced J/ ψ → µµ
2.0 < y < 4.5
Data
No overlap effects
Overlap effects
0.1
0.1
0
0.4
0.3
Data
No overlap effects
Overlap effects
0.3
0.5 ×10
d2YJ/ψ/dp Tdy [(GeV/c)-1]
0.6 ×10
dYJ/ψ/dy
dYJ/ψ/dy
Fig. 4: Fit to the invariant mass spectrum of J/ψ candidates (left) and the ln(p2T )
distribution fit for J/ψ candidates (right).
2
3
4
10
y
J/ψ
20
3 ×10
< N part>
LHCb
PbPb sNN = 5 TeV
Photo-produced J/ψ → µµ
2
2.0 < y < 4.5
1.5
Data
< N part> = 19.7 ± 9.2
No overlap effects
Overlap effects
1
0.5
0
0
30
−3
2.5
0.05
0.1
0.15
0.2
p T [GeV/c]
Fig. 5: The results of the differential photo-production yields of J/ψ versus the
rapidity and the number of participants in collisions. Double-differential yields of
the coherent J/ψ produced in peripheral PbPb collisions as a function of pT (right).
4. Conclusion
We report the results of coherent J/ψ , ψ(2S) production cross sections
in UPCs, as well as their ratio. It is the first coherent ψ(2S) and ψ(2S) to
J/ψ ratio measurement in forward rapidity region for UPC at LHC, and
the differential cross section of coherent J/ψ and ψ(2S) production in PbPb
UPC is also measured as a function of p∗T for the first time. Also, it is the
most precise measurement of J/ψ production in UPCs. The production of
photo-produced J/ψ mesons in peripheral PbPb collisions is measured and
by far it is the most precise coherent photo-produced J/ψ by LHCb.
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