CMS-HIN-18-011

CMS-HIN-18-011 ; CERN-EP-2021-071
Azimuthal Correlations within Exclusive Dijets with Large Momentum Transfer in Photon-Lead Collisions
CMS Collaboration
29 April 2022
Phys. Rev. Lett. 131 (2023) 051901
Abstract: The structure of nucleons is multidimensional and depends on the transverse momenta, spatial geometry, and polarization of the constituent partons. Such a structure can be studied using high-energy photons produced in ultraperipheral heavy-ion collisions. The first measurement of the azimuthal angular correlations of exclusively produced events with two jets in photon-lead interactions at large momentum transfer is presented, a process that is considered to be sensitive to the underlying nuclear gluon polarization. This study uses a data sample of ultraperipheral lead-lead collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV, corresponding to an integrated luminosity of 0.38 nb$^{-1}$, collected with the CMS experiment at the LHC. The measured second harmonic of the correlation between the sum and difference of the two jet transverse momentum vectors is found to be positive, and rising, as the dijet transverse momentum increases. A well-tuned model that has been successful at describing a wide range of proton scattering data from the HERA experiments fails to describe the observed correlations, suggesting the presence of gluon polarization effects.
Links: e-print arXiv:2205.00045 [nucl-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Magnitudes of the vector sum ($ Q_{\mathrm{T}} $) and vector difference ($ P_{\mathrm{T}} $) of the two jets (left). The dashed blue line illustrates the $ Q_{\mathrm{T}} < $ 25 GeV requirement. Invariant mass (center) and rapidity (right) of the dijet candidates after all selection requirements. The lines show the RAPGAP MC generated events including detector resolution effects. The statistical uncertainties are covered by the symbol size.
png pdf	Figure 2: The unfolded 1 $ /{N_{\text{events}}} \,\mathrm{d} N/\mathrm{d}\Phi $ distribution (left) and the unfolded $ \langle \cos(2\Phi) \rangle $ values as a function of $ Q_{\mathrm{T}} $ (right). The corresponding distributions from the RAPGAP simulation at the generator level (blue lines) and theoretical calculation by Hatta \textitet al. $ \mbox{et al.} $ [20] for $ \langle \cos(2\Phi) \rangle $ (red dashed lines) are also shown for $ Q_{\mathrm{T}} < $ 15 GeV, consistent with the back-to-back limit in their calculation. The dijet events are found predominantly in the forward direction, with $ {0 < \eta_\text{jet} < 2.4} $. Both the statistical (error bars) and systematic (green boxes) uncertainties are shown.
png pdf	Figure 2-a: The unfolded 1 $ /{N_{\text{events}}} \,\mathrm{d} N/\mathrm{d}\Phi $ distribution. The corresponding distributions from the RAPGAP simulation at the generator level (blue lines) and theoretical calculation by Hatta et al. [20] for $ \langle \cos(2\Phi) \rangle $ (red dashed lines) are also shown for $ Q_{\mathrm{T}} < $ 15 GeV, consistent with the back-to-back limit in their calculation. The dijet events are found predominantly in the forward direction, with $ {0 < \eta_\text{jet} < 2.4} $. Both the statistical (error bars) and systematic (green boxes) uncertainties are shown.
png pdf	Figure 2-b: The unfolded $ \langle \cos(2\Phi) \rangle $ values as a function of $ Q_{\mathrm{T}} $. The corresponding distributions from the RAPGAP simulation at the generator level (blue lines) and theoretical calculation by Hatta et al. [20] for $ \langle \cos(2\Phi) \rangle $ (red dashed lines) are also shown for $ Q_{\mathrm{T}} < $ 15 GeV, consistent with the back-to-back limit in their calculation. The dijet events are found predominantly in the forward direction, with $ {0 < \eta_\text{jet} < 2.4} $. Both the statistical (error bars) and systematic (green boxes) uncertainties are shown.

Tables
png pdf	Table of $ \langle \cos(2\Phi) \rangle $ systematic uncertainties (absolute values). The individual components are discussed in the text. :

Summary

In summary, the exclusive production of two jets in photon-lead interactions with a large rapidity gap has been studied for the first time. The dijet events are characterized by a large momentum transfer. The second harmonic of the angular correlation between the sum and difference of the two jet transverse momenta is found to be positive and rising, with dijet transverse momentum in the measured range 0–25 GeV. An a posteriori calculation that includes the effect of soft gluons from final-state radiation describes the average magnitude of the correlations, but does not rise with the magnitude of the dijet total momentum $ Q_\text{T} $ to the extent found with the CMS data. A model [41] that has been successful at describing a wide range of proton scattering data from the HERA experiments overestimates the strength of the correlations, suggesting the presence of gluon polarization effects. This experiment calls for new theoretical calculations to quantify the theoretical implications and opens a new direction for probing the high-energy limit of strong interactions.

Additional Figures
png pdf	Additional Figure 1: Difference in pseudorapidity between the most backward jet in an event and the most backward track defining the backward rapidity gap, $\eta _{\text {jet}}^{B} - \eta _{\text {track}}^{B}$, shown for data and reconstructed rapgap MC events. The dashed line with the arrow corresponds to the selection applied in the analysis that this difference should be less than 1.
png pdf	Additional Figure 2: Backward rapidity gap, $\Delta \eta ^{B}$, defined as the difference between the minimum pseudorapidity of any high-purity charged track having $ {p_{\mathrm {T}}} > $ 0.2 GeV and the lower limit of the tracker shown for data and reconstructed rapgap MC events. The dashed line with the arrow corresponds to the selection $\Delta \eta ^{B} > $ 1.2 applied in the analysis.
png pdf	Additional Figure 3: Correlation of the magnitudes of the vector sum ($Q_{\mathrm {T}}$) and vector difference ($P_{\mathrm {T}}$) of the two jets for the events that pass the selection criteria.
png pdf	Additional Figure 4: The $\mathrm{d}N/\mathrm{d}\Phi $ distribution shown for the mixed events obtained from data and compared to the rapgap MC prediction after full simulation and reconstruction (blue line). Both the statistical (error bars) and systematic (magenta boxes) uncertainties are shown. Dashed histograms show the toy-MC distribution including the effect of detector smearing.
png pdf	Additional Figure 5: The second harmonic, $\langle \cos(2\Phi) \rangle $, as a function of $Q_{\mathrm {T}}$ shown for the mixed events obtained from data and compared to the rapgap MC prediction after full simulation and reconstruction (blue line). Both the statistical (error bars) and systematic (magenta boxes) uncertainties are shown. Dashed histograms show the toy-MC distribution including the effect of detector smearing.

References
1	J. L. Albacete and C. Marquet	Gluon saturation and initial conditions for relativistic heavy ion collisions	Prog. Part. NP 76 (2014) 1	1401.4866
2	R. Abdul Khalek et al.	Science requirements and detector concepts for the Electron-Ion Collider: EIC yellow report		2103.05419
3	BRAHMS Collaboration	Evolution of the nuclear modification factors with rapidity and centrality in dAu collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 200 GeV	PRL 93 (2004) 242303	nucl-ex/0403005
4	STAR Collaboration	Forward neutral pion production in pp and dAu collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 200 GeV	PRL 97 (2006) 152302	nucl-ex/0602011
5	PHENIX Collaboration	Suppression of back-to-back hadron pairs at forward rapidity in dAu collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 200 GeV	PRL 107 (2011) 172301	1105.5112
6	PHOBOS Collaboration	High transverse momentum triggered correlations over a large pseudorapidity acceptance in AuAu collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 200 GeV	PRL 104 (2010) 062301	0903.2811
7	M. Diehl	Introduction to GPDs and TMDs	EPJA 52 (2016) 149	1512.01328
8	T. Altinoluk, N. Armesto, G. Beuf, and A. H. Rezaeian	Diffractive dijet production in deep inelastic scattering and photon-hadron collisions in the color glass condensate	PLB 758 (2016) 373	1511.07452
9	Y. Hatta, B.-W. Xiao, and F. Yuan	Probing the small-x gluon tomography in correlated hard diffractive dijet production in deep inelastic scattering	PRL 116 (2016) 202301	1601.01585
10	Y. Hagiwara et al.	Accessing the gluon Wigner distribution in ultraperipheral pA collisions	PRD 96 (2017) 034009	1706.01765
11	Y. Hatta, B.-W. Xiao, and F. Yuan	Gluon tomography from deeply virtual Compton scattering at small-x	PRD 95 (2017) 114026	1703.02085
12	H. Mantysaari, N. Mueller, and B. Schenke	Diffractive dijet production and Wigner distributions from the color glass condensate	PRD 99 (2019) 074004	1902.05087
13	A. Dumitru, V. Skokov, and T. Ullrich	Measuring the Weizsacker-Williams distribution of linearly polarized gluons at an electron-ion collider through dijet azimuthal asymmetries	PRC 99 (2019) 015204	1809.02615
14	Y. Hagiwara, Y. Hatta, and T. Ueda	Wigner, Husimi, and generalized transverse momentum dependent distributions in the color glass condensate	PRD 94 (2016) 094036	1609.05773
15	Y. Hagiwara, Y. Hatta, B.-W. Xiao, and F. Yuan	Elliptic flow in small systems due to elliptic gluon distributions?	PLB 771 (2017) 374	1701.04254
16	J. Zhou	Elliptic gluon generalized transverse-momentum-dependent distribution inside a large nucleus	PRD 94 (2016) 114017	1611.02397
17	M. Strikman, R. Vogt, and S. N. White	Probing small-x parton densities in ultraperipheral AA and pA collisions at the CERN Large Hadron Collider	PRL 96 (2006) 082001	hep-ph/0508296
18	V. Guzey and M. Klasen	Diffractive dijet photoproduction in ultraperipheral collisions at the LHC in next-to-leading order QCD	JHEP 04 (2016) 158	1603.06055
19	P. Kotko et al.	Estimating nonlinear effects in forward dijet production in ultra-peripheral heavy ion collisions at the LHC	EPJC 77 (2017) 353	1702.03063
20	Y. Hatta, B.-W. Xiao, F. Yuan, and J. Zhou	Anisotropy in dijet production in exclusive and inclusive processes	PRL 126 (2021) 142001	2010.10774
21	A. J. Baltz et al.	The physics of ultraperipheral collisions at the LHC	PR 458 (2008) 1	0706.3356
22	J. G. Contreras and J. D. Tapia Takaki	Ultra-peripheral heavy-ion collisions at the LHC	Int. J. Mod. Phys. A 30 (2015) 1542012
23	S. Klein and P. Steinberg	Photonuclear and two-photon interactions at high-energy nuclear colliders	Ann. Rev. Nucl. Part. Sci. 70 (2020) 323	2005.01872
24	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
25	CMS Collaboration	Measurement of exclusive $ \rho(770) $ 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
26	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
27	CMS Collaboration	Evidence for light-by-light scattering and searches for axion-like particles in ultraperipheral PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV	PLB 797 (2019) 134826	CMS-FSQ-16-012 1810.04602
28	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
29	ATLAS Collaboration	Observation of light-by-light scattering in ultraperipheral PbPb collisions with the ATLAS detector	PRL 123 (2019) 052001	1904.03536
30	LHCb Collaboration	Central exclusive production of J/$\psi$ and $\psi$(2S) mesons in pp collisions at $ \sqrt{s}= $ 13 TeV	JHEP 10 (2018) 167	1806.04079
31	CDF Collaboration	Observation of exclusive dijet production at the fermilab tevatron $ \mathrm{p\bar{p}} $ collider	PRD 77 (2008) 052004	0712.0604
32	ZEUS Collaboration	Production of exclusive dijets in diffractive deep inelastic scattering at HERA	EPJC 76 (2016) 16	1505.05783
33	CMS Collaboration	HEPData record for this analysis	link
34	CMS Collaboration	The CMS trigger system	JINST 12 (2017) P01020	CMS-TRG-12-001 1609.02366
35	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004	CMS-00-001
36	CMS Collaboration	Constraining gluon distributions in nuclei using dijets in proton-proton and proton-lead collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV	PRL 121 (2018) 062002	CMS-HIN-16-003 1805.04736
37	CMS Collaboration	Particle-flow reconstruction and global event description with the CMS detector	JINST 12 (2017) P10003	CMS-PRF-14-001 1706.04965
38	M. Cacciari, G. P. Salam, and G. Soyez	The anti-$ {k_{\mathrm{T}}} $ jet clustering algorithm	JHEP 04 (2008) 063	0802.1189
39	M. Cacciari, G. P. Salam, and G. Soyez	FastJet user manual	EPJC 72 (2012) 1896	1111.6097
40	CMS Collaboration	CMS tracking performance results from early LHC operation	EPJC 70 (2010) 1165	CMS-TRK-10-001 1007.1988
41	H. Jung	Hard diffractive scattering in high-energy ep collisions and the Monte Carlo generator RAPGAP	CPC 86 (1995) 147
42	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
43	GEANT4 Collaboration	GEANT4: A simulation toolkit	NIMA 506 (2003) 250
44	S. R. Klein and H. Mantysaari	Imaging the nucleus with high-energy photons	Nature Rev. Phys. 1 (2019) 662	1910.10858
45	S. Schmitt	TUnfold: an algorithm for correcting migration effects in high energy physics	JINST 7 (2012) T10003	1205.6201
46	CMS Collaboration	Jet energy scale and resolution in the CMS experiment in pp collisions at 8 TeV	JINST 12 (2017), no. 02, P02014	CMS-JME-13-004 1607.03663
47	A. van Hameren, P. Kotko, K. Kutak, and S. Sapeta	Broadening and saturation effects in dijet azimuthal correlations in pp and pPb collisions at $ \sqrt{s} = $ 5.02 TeV	PLB 795 (2019) 511	1903.01361