CMS-PAS-HIN-22-002

CMS-PAS-HIN-22-002
Photon-nucleus energy dependence of coherent J/$ \psi $ cross session in ultraperipheral PbPb collisions at 5.02 TeV with CMS
CMS Collaboration
6 December 2022

Abstract: To understand the structure and dynamics inside an atomic nucleus at its most fundamental level is a subject at the heart of research on the strong interaction and theory of quantum chromodynamics (QCD). Gluons, the strong interaction carrier, are found to be the dominant constituents inside a nucleus when it is probed at ultra-high energies. Quasi-real photons exchanged in the interactions of highly relativistic heavy ions are powerful and clean probes of the gluonic structures of nuclei and/or nucleons. We report the first measurement of the nuclear gluonic structure probed by high-energy photons via coherent J/$ \psi $ photoproduction as a function of photon-nucleon center-of-mass energy ($ W^{Pb}_{\gamma N} $) up to $ W^{Pb}_{\gamma N} \approx $ 400 GeV. Results are obtained with ultraperipheral PbPb collision data collected by the CMS experiment at the LHC during the 2018 PbPb run, corresponding to an integrated luminosity of 1.52 $ \mathrm{nb}^{-1} $. At high $ W^{Pb}_{\gamma N} $ the coherent J/$ \psi $ cross section is observed to rapidly approach an approximate plateau over 40 $ \leq W^{Pb}_{\gamma N}\leq $ 400 GeV. This observation may provide the first experimental evidence of the black disk limit reached in high energy photons scattering off heavy nuclei, or indicate a gluon density saturation inside the nucleus that is greater than predictions from state-of-the-art QCD theoretical models. The findings open a new domain of QCD studies in the high gluonic density limit and motivate the development of new theoretical models.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The differential coherent $ \mathrm{J}/\psi $ production cross section as a function of rapidity in different neutron multiplicity classes: 0n0n, 0n$ X $n, $ X $n$ X $n, and $ A $n$ A $n. Vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Theoretical predictions from LTA weak/strong shadowing \citep{Guzey:2016piu} and CD_BGK and CD_BGW and CD_IIM [49] are shown in curves. Data for the $ A $n$ A $n case are compared to ALICE [16,20] and LHCb \citep{LHCb:2022ahs} results.
png pdf	Figure 2: The coherent $ \mathrm{J}/\psi $ photo production cross section as a function of the centre-of-mass energy in the $ \gamma $-Pb system, $ W^{\rm{Pb}}_{\gamma\rm{N}} $, measured in 5.02 TeV PbPb UPCs, and compared with ALICE [16,20] and LHCb [23] data at specific rapidity regions. The $ W^{\rm{Pb}}_{\gamma\rm{N}} $ values are evaluated from the center of their corresponding rapidy ranges. Vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Theoretical model calculations (Impulse Approx. [30], CGC IPsat [50], GG-hs [51], LTA-SS and LTA-WS [31], bBK_GG and bBK_A [52], CD_BGK and CD_BGW and CD_IIM [49]) are shown in curves.
png pdf	Figure 3: The nuclear gluon suppression factor as a function of Bjorken $ x $, calculated from the ratio of the coherent $ \mathrm{J}/\psi $ photoproduction cross section in photon-nucleus interactions to that from the Impulse Approx. \citep{Guzey:2013xba}. The $ x $ values are evaluated from the center of their corresponding rapidy ranges. Vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. Theoretical model calculations (GG-hs [51], LTA-SS and LTA-WS [31], bBK_GG and bBK_A [52], CD_BGK and CD_BGW and CD_IIM [49]) are shown as curves.
png pdf	Figure A1: Left: invariant mass distribution of $ \mu^+\mu^- $ pairs within pair $ p_{\mathrm{T}} < $ 0.2 GeV. Right: transverse momentum distribution of $ \mu^+\mu^- $ pairs falling within the $ \mathrm{J}/\psi $ mass window (2.95 $ < m_{\mu^+\mu^-} < $ 3.25 GeV). (Values shown in the figure are with statistical uncertainties only).
png pdf	Figure A1-a: Left: invariant mass distribution of $ \mu^+\mu^- $ pairs within pair $ p_{\mathrm{T}} < $ 0.2 GeV. Right: transverse momentum distribution of $ \mu^+\mu^- $ pairs falling within the $ \mathrm{J}/\psi $ mass window (2.95 $ < m_{\mu^+\mu^-} < $ 3.25 GeV). (Values shown in the figure are with statistical uncertainties only).
png pdf	Figure A1-b: Left: invariant mass distribution of $ \mu^+\mu^- $ pairs within pair $ p_{\mathrm{T}} < $ 0.2 GeV. Right: transverse momentum distribution of $ \mu^+\mu^- $ pairs falling within the $ \mathrm{J}/\psi $ mass window (2.95 $ < m_{\mu^+\mu^-} < $ 3.25 GeV). (Values shown in the figure are with statistical uncertainties only).

Tables
png pdf	Table A1: The average values of incoherent and feed-down fractions for each neutron multiplicity class are shown. The uncertainties are calculated from their respective fits.

Summary

In summary, the first measurement of the coherent $ \mathrm{J}/\psi $ photoproduction cross section off lead nuclei as a function of photon-nucleon center-of-mass energy ($ W^{\rm{Pb}}_{\gamma\rm{N}} $) is presented over a wide energy range. Results are obtained with ultraperipheral PbPb collision data by applying the forward neutron tagging technique. At low $ W^{\rm{Pb}}_{\gamma\rm{N}} $ ($ < $ 40 GeV), the measured cross section rapidity increases with $ W^{\rm{Pb}}_{\gamma\rm{N}} $ as expected from theoretical models. However, it is observed at high $ W^{\rm{Pb}}_{\gamma\rm{N}} $ that the coherent $ \mathrm{J}/\psi $ cross section approaches nearly a plateau up to $ W^{\rm{Pb}}_{\gamma\rm{N}} \approx $ 400 GeV with little energy dependence. This observation may provide the first experimental evidence of black disk limit reached in high energy photons scattering off heavy nuclei, or indicate a gluon density suppression inside a nucleus significantly more than the expectation based on state-of-the-art QCD approaches such as the nuclear shadowing and gluon saturation models. The findings reported in this work suggest that novel methods are needed in this new extreme density region of QCD studies to investigate the complex, multi-gluon system.

References
1	V. N. Gribov	Interaction of gamma quanta and electrons with nuclei at high-energies	Zh. Eksp. Teor. Fiz. 57 (1969) 1306
2	L. Frankfurt, V. Guzey, M. McDermott, and M. Strikman	Revealing the black body regime of small x DIS through final state signals	PRL 87 (2001) 192301	hep-ph/0104154
3	L. Frankfurt, M. Strikman, and M. Zhalov	Signals for black body limit in coherent ultraperipheral heavy ion collisions	PLB 537 (2002) 51	hep-ph/0204175
4	L. Frankfurt, M. Strikman, and C. Weiss	Small-$ x $ physics: From HERA to LHC and beyond	Ann. Rev. Nucl. Part. Sci. 55 (2005) 403	hep-ph/0507286
5	T. Rogers, V. Guzey, M. Strikman, and X. Zu	Determining the proximity of gamma* N scattering to the black body limit using DIS and J / psi production	PRD 69 (2004) 074011	hep-ph/0309099
6	E. Iancu and R. Venugopalan	The Color glass condensate and high-energy scattering in QCD	link	hep-ph/0303204
7	F. Gelis, E. Iancu, J. Jalilian-Marian, and R. Venugopalan	The Color Glass Condensate	Ann. Rev. Nucl. Part. Sci. 60 (2010) 463	1002.0333
8	E. J. Williams	Nature of the high energy particles of penetrating radiation and status of ionization and radiation formulae	PR 45 (1934) 729
9	C. F. von Weizsacker	Radiation emitted in collisions of very fast electrons	Z. Phys. 88 (1934) 612
10	STAR Collaboration	Coherent $ \rho^0 $ production in ultraperipheral heavy ion collisions	PRL 89 (2002) 272302	nucl-ex/0206004
11	STAR Collaboration	Coherent diffractive photoproduction of $ \rho^0 $ mesons on gold nuclei at 200 GeV/nucleon-pair at the Relativistic Heavy Ion Collider	Phys. Rev. C 96 (2017) 054904	1702.07705
12	PHENIX Collaboration	Photoproduction of J/$ \psi $ and of high mass $ e^+e^- $ in ultra-peripheral Au+Au collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 200 GeV	PLB 679 (2009) 321	0903.2041
13	ALICE Collaboration	Coherent $ J/\psi $ photoproduction in ultra-peripheral PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV	PLB 718 (2013) 1273	1209.3715
14	ALICE Collaboration	Charmonium and $ e^+e^- $ pair photoproduction at mid-rapidity in ultra-peripheral PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV	EPJC 73 (2013) 2617	1305.1467
15	ALICE Collaboration	Exclusive $ \mathrm{J/}\psi $ photoproduction off protons in ultra-peripheral p-Pb collisions at $ \sqrt{s_{\rm NN}}= $ 5.02 TeV	PRL 113 (2014) 232504	1406.7819
16	ALICE Collaboration	Coherent J/$ \psi $ photoproduction at forward rapidity in ultra-peripheral Pb-Pb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV	PLB 798 (2019) 134926	1904.06272
17	CMS Collaboration	Coherent J/$ \psi $ photoproduction in ultra-peripheral PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV with the CMS experiment	PLB 772 (2017) 489	CMS-HIN-12-009 1605.06966
18	CMS Collaboration	Measurement of exclusive $ \Upsilon $ photoproduction from protons in pPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV	EPJC 79 (2019) 277	CMS-FSQ-13-009 1809.11080
19	CMS Collaboration	Measurement of exclusive $ \rho(770)^0 $ photoproduction in ultraperipheral pPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV	EPJC 79 (2019) 702	CMS-FSQ-16-007 1902.01339
20	ALICE Collaboration	Coherent $ J/\psi $ and $ \psi^{'} $ photoproduction at midrapidity in ultra-peripheral Pb-Pb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV	EPJC 81 (2021) 712	2101.04577
21	STAR Collaboration	Tomography of Ultra-relativistic Nuclei with Polarized Photon-gluon Collisions		2204.01625
22	LHCb Collaboration	Study of coherent $ J/\psi $ production in lead-lead collisions at $ \sqrt{{\mathrm{s}}_{\mathrm{NN}}} $ = 5 TeV	JHEP 07 (2022) 117	2107.03223
23	LHCb Collaboration	Study of coherent charmonium production in ultra-peripheral lead-lead collisions		2206.08221
24	ZEUS Collaboration	Exclusive photoproduction of J / psi mesons at HERA	EPJC 24 (2002) 345	hep-ex/0201043
25	H1 Collaboration	Elastic J/psi production at HERA	EPJC 46 (2006) 585	hep-ex/0510016
26	H1 Collaboration	Elastic and Proton-Dissociative Photoproduction of J/psi Mesons at HERA	EPJC 73 (2013) 2466	1304.5162
27	LHCb Collaboration	Exclusive $ J/\psi $ and $ \psi $(2S) production in pp collisions at $ \sqrt{s} = $ 7 TeV	JPG 40 (2013) 045001	1301.7084
28	ALICE Collaboration	Energy dependence of exclusive $ \mathrm {J}/\psi $ photoproduction off protons in ultra-peripheral p-Pb collisions at $ \sqrt{s_{\mathrm {\scriptscriptstyle NN}}} = $ 5.02 TeV	EPJC 79 (2019) 402	1809.03235
29	L. Favart, M. Guidal, T. Horn, and P. Kroll	Deeply Virtual Meson Production on the nucleon	no.~6, 158, 2016 Eur. Phys. J. A 52 (2016)	1511.04535
30	V. Guzey, M. Strikman, and M. Zhalov	Disentangling coherent and incoherent quasielastic $ J/\psi $ photoproduction on nuclei by neutron tagging in ultraperipheral ion collisions at the LHC	no.~7, 2014 EPJC 74 (2014)	1312.6486
31	V. Guzey, E. Kryshen, and M. Zhalov	Coherent photoproduction of vector mesons in ultraperipheral heavy ion collisions: Update for run 2 at the CERN Large Hadron Collider	Phys. Rev. C 93 (2016) 055206	1602.01456
32	G. Bayatian et al.	Design, performance and calibration of the CMS forward calorimeter wedges	EPJC 53 (2008) 139
33	O. Sur \'a nyi et al.	Performance of the CMS Zero Degree Calorimeters in pPb collisions at the LHC	JINST 16 (2021) P05008	2102.06640
34	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004
35	CMS Collaboration	The CMS trigger system	JINST 12 (2017) P01020	CMS-TRG-12-001 1609.02366
36	CMS Collaboration	Observation of forward neutron multiplicity dependence of dimuon acoplanarity in ultraperipheral PbPb Collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} $=5.02\,\,TeV	PRL 127 (2021) 122001	CMS-HIN-19-014 2011.05239
37	CMS Collaboration	Performance of CMS muon reconstruction in pp collision events at $ \sqrt{s}= $ 7 TeV	JINST 7 (2012) P10002	CMS-MUO-10-004 1206.4071
38	S. R. Klein et al.	STARlight: A Monte Carlo simulation program for ultra-peripheral collisions of relativistic ions	Comput. Phys. Commun. 212 (2017) 258	1607.03838
39	GEANT4 Collaboration	GEANT 4---a simulation toolkit	NIM A 506 (2003) 250
40	M. Oreglia	A Study of the Reactions $ \psi^\prime \to \gamma \gamma \psi $	other thesis, 12, 1980
41	CMSnoop	A study of the radiative CASCADE transitions between the Upsilon-Prime and Upsilon resonances	\hrefT.~Skwarnicki, . PhD thesis, Cracow, INP, 1986
42	CMS Collaboration	Supplemental Material for details of the coherent J/$\psi$ photoproduction signal extraction procedure
43	Particle Data Group Collaboration	Review of Particle Physics	PTEP 2022 (2022) 083C01
44	CMS Collaboration	Measurement of the inclusive W and Z production cross sections in pp collisions at $ \sqrt{s}= $ 7 TeV	JHEP 10 (2011) 132	CMS-EWK-10-005 1107.4789
45	T. C. collaboration	Performance of CMS muon reconstruction in pp collision events at $ \sqrt{s}= $ 7 TeV	JINST 7 (2012) P10002	1206.4071
46	CMS Collaboration	CMS luminosity measurement using nucleus-nucleus collisions at $ \sqrt{{s_{_{{NN}}}}}= $ 5.02 TeV in 2018	Technical Report, CMS Physics Analysis Summary, 2022 CMS-PAS-LUM-18-001	CMS-PAS-LUM-18-001
47	D. d'Enterria and C. Loizides	Progress in the Glauber Model at Collider Energies	Ann. Rev. Nucl. Part. Sci. 71 (2021) 315	2011.14909
48	ATLAS Collaboration	Exclusive dimuon production in ultraperipheral Pb+Pb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV with ATLAS	Phys. Rev. C 104 (2021) 024906	2011.12211
49	A. \L uszczak and W. Sch ä fer	Coherent photoproduction of $ J/\psi $ in nucleus-nucleus collisions in the color dipole approach	PRC 99 (2019) 044905	1901.07989
50	H. M ä ntysaari and B. Schenke	Probing subnucleon scale fluctuations in ultraperipheral heavy ion collisions	PLB 772 (2017) 832	1703.09256
51	J. Cepila, J. G. Contreras, and M. Krelina	Coherent and incoherent $ \mathrm{J/}\psi $ photonuclear production in an energy-dependent hot-spot model	Phys. Rev. C 97 (2018) 024901	1711.01855
52	D. Bendova, J. Cepila, J. G. Contreras, and M. Matas	Photonuclear $ J/\psi $ production at the LHC: Proton-based versus nuclear dipole scattering amplitudes	PLB 817 (2021) 136306	2006.12980
53	V. Guzey, E. Kryshen, M. Strikman, and M. Zhalov	Evidence for nuclear gluon shadowing from the ALICE measurements of PbPb ultraperipheral exclusive $ J/{\psi} $ production	PLB 726 (2013) 290	1305.1724