CMS-HIN-16-002

CMS-HIN-16-002 ; CERN-EP-2017-285
Study of jet quenching with isolated-photon+jet correlations in PbPb and pp collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV
CMS Collaboration
27 November 2017
Phys. Lett. B 785 (2018) 14
Abstract: Measurements of azimuthal angle and transverse momentum (${p_{\mathrm{T}}}$) correlations of isolated photons and associated jets are reported for pp and PbPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV. The data were recorded with the CMS detector at the CERN LHC. For events containing a leading isolated photon with ${p_{\mathrm{T}}^{\gamma}} > $ 40 GeV/$c$ and an associated jet with ${p_{\mathrm{T}}^{\text{jet}}} > $ 30 GeV/$c$ , the photon+jet azimuthal correlation and ${p_{\mathrm{T}}}$ imbalance in PbPb collisions are studied as functions of collision centrality and ${p_{\mathrm{T}}^{\gamma}}$. The results are compared to pp reference data collected at the same collision energy and to predictions from several theoretical models for parton energy loss. No evidence of broadening of the photon+jet azimuthal correlations is observed, while the ratio ${p_{\mathrm{T}}^{\text{jet}}}/{p_{\mathrm{T}}^{\gamma}}$ decreases significantly for PbPb data relative to the pp reference. All models considered agree within uncertainties with the data. The number of associated jets per photon with ${p_{\mathrm{T}}^{\gamma}} > $ 80 GeV/$c$ is observed to be shifted towards lower ${p_{\mathrm{T}}^{\text{jet}}}$ values in central PbPb collisions compared to pp collisions.
Links: e-print arXiv:1711.09738 [nucl-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The centrality dependence of the shower shape variable $\sigma _{\eta \eta}$ for photons with $ {{p_{\mathrm {T}}} ^\gamma} \in $ (40,50) GeV/$c$. The black points show the PbPb experimental results, the red histograms are the signal templates from PYTHIA+HYDJET simulations, and the green histograms are the background templates obtained from a nonisolated sideband region in data.
png pdf	Figure 2: The azimuthal correlation of photons and jets in five ${{p_{\mathrm {T}}} ^\gamma}$ intervals for 0-30% centrality (top, full circles) and 30-100% centrality (bottom, full squares) PbPb collisions. The smeared pp data (open symbols) are included for comparison. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 3: Distribution of $ {x_{\mathrm {j}\gamma}} = {{p_{\mathrm {T}}} ^\text {jet}} / {{p_{\mathrm {T}}} ^\gamma} $ in five ${{p_{\mathrm {T}}} ^\gamma}$ intervals for 0-30% centrality (top, full circles) and 30-100% centrality (bottom, full squares) PbPb collisions. The smeared pp data (open symbols) are included for comparison. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 4: The $ < x_{\mathrm {j}\gamma} > $ values (top) and ${R_{\mathrm {j}\gamma}}$, the number of associated jets per photon (bottom), in 0-30% centrality (left, full circles) and 30-100% centrality (right, full squares) PbPb collisions. The smeared pp data (open symbols) are added for comparison. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 5: The ${\mathrm {I_\mathrm {AA}^\text {jet}}} $ vs. $ {{p_{\mathrm {T}}} ^\text {jet}}$ for 0-30% centrality (top) and 30-100% centrality (bottom) PbPb collisions. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 6: The centrality dependence of ${x_{\mathrm {j}\gamma}}$ of photon+jet pairs normalized by the number of photons for PbPb (full markers) and smeared pp (open markers) data. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 7: The $ < x_{\mathrm {j}\gamma} > $ (top) and ${R_{\mathrm {j}\gamma}}$ (bottom) as a function of $ < N_{\text {part}} > $ for $ {{p_{\mathrm {T}}} ^\gamma} > $ 60 GeV/$c$ (left) and $ {{p_{\mathrm {T}}} ^\gamma} > $ 80 GeV/$c$ (right). The PbPb results (full markers) are compared to pp results (open markers) smeared by the relative jet energy resolution corresponding to each centrality interval. The vertical lines (bands) through the points represent statistical (systematic) uncertainties.
png pdf	Figure 8: The azimuthal correlation of photons and jets in five ${{p_{\mathrm {T}}} ^\gamma}$ intervals for 0-30% centrality (top, full circles) and 30-100% centrality (bottom, full squares) PbPb collisions. The data points shown are identical to those in Fig. 2. Theoretical calculations from jewel [57,33], LBT [30], and hybrid model [31,32] are included for comparison.
png pdf	Figure 9: The $ {x_{\mathrm {j}\gamma}}$ distributions in five ${{p_{\mathrm {T}}} ^\gamma}$ intervals for 0-30% centrality (top, full circles) and 30-100% centrality (bottom, full squares) PbPb collisions. The data points shown are identical to those in Fig. 3. Theoretical calculations from jewel [57,33], LBT [30], and hybrid model [31,32] are included for comparison.

Tables
png pdf	Table 1: Jet resolution parameters for pp and PbPb collisions. A global uncertainty of 7% is assigned to the smearing parameters, evaluated as described in text.
png pdf	Table 2: Summary of the relative systematic uncertainties (in%) for $ {{p_{\mathrm {T}}} ^\gamma} > $ 40 GeV/$c$.
png pdf	Table 3: Summary of the absolute systematic uncertainties on $({1}/{N_{{\mathrm {j}}\gamma}}) ({{\mathrm {d}}{}N}/{{\mathrm {d}}{}\phi _{\mathrm {j}\gamma}})$ for $ {{p_{\mathrm {T}}} ^\gamma} > $ 40 GeV/$c$, averaged over the ${\Delta \phi _{\mathrm {j}\gamma}}$ distributions.

Summary

Correlations of isolated photons with transverse momentum ${p_{\mathrm{T}}^{\gamma}} > $ 40 GeV/$c$ and pseudorapidity $ | {{\eta^{\gamma}}} | < $ 1.44 and associated jets with ${p_{\mathrm{T}}^{\text{jet}}} > $ 30 GeV/$c$ and ${\eta^{\text{jet}}} < $ 1.6, have been studied for the first time in pp and PbPb collisions at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV, using a large data sample collected by the CMS experiment. No significant azimuthal angular broadening between photons and the associated jets is observed in PbPb data as compared to pp data, for all event centralities and multiple photon ${p_{\mathrm{T}}}$ intervals. The ${x_{\mathrm{j}\gamma}} = {p_{\mathrm{T}}^{\text{jet}}}/{p_{\mathrm{T}}^{\gamma}}$ and the average number of associated jets per photon, ${R_{\mathrm{j}\gamma}} $, are studied in different leading photon ${p_{\mathrm{T}}}$ and PbPb collision centrality intervals. For all ${p_{\mathrm{T}}^{\gamma}} > $ 60 GeV/$c$ intervals, the ${< x_{\mathrm{j}\gamma} >}$ and ${R_{\mathrm{j}\gamma}} $ values in the 0-30% most central PbPb collisions are found to be lower than those in the corresponding pp reference data, indicating that a larger fraction of jets lose energy and thus fall below 30 GeV/$c$ in PbPb collisions. The differences between the pp and PbPb results increase as collisions become more central. By comparing the yields of associated jets in PbPb and pp events that include photons with ${p_{\mathrm{T}}^{\gamma}} > $ 80 GeV/$c$, a shift of the jet spectra towards lower ${p_{\mathrm{T}}^{\text{jet}}}$ is observed. These new results are qualitatively similar to those reported at $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV and to calculations from various theoretical models. The better statistical precision of the new higher energy data provides an opportunity to test theoretical models against data over a wide kinematic range in ${p_{\mathrm{T}}^{\gamma}}$ and ${x_{\mathrm{j}\gamma}} $, and for different event centralities, using a selection of partons with well-defined flavor and initial kinematics.

References
1	J. C. Collins and M. J. Perry	Superdense matter: Neutrons or asymptotically free quarks?	PRL 34 (1975) 1353
2	F. Karsch	The phase transition to the quark gluon plasma: Recent results from lattice calculations	NP A 590 (1995) 367C	hep-lat/9503010
3	J. D. Bjorken	Highly relativistic nucleus-nucleus collisions: The central rapidity region	PRD 27 (1983) 140
4	D. A. Appel	Jets as a probe of quark-gluon plasmas	PRD 33 (1986) 717
5	J. P. Blaizot and L. D. McLerran	Jets in expanding quark-gluon plasmas	PRD 34 (1986) 2739
6	M. Gyulassy and M. Plumer	Jet quenching in dense matter	PLB 243 (1990) 432
7	X.-N. Wang and M. Gyulassy	Gluon shadowing and jet quenching in A+A collisions at $ \sqrt{s} = 200A $ GeV	PRL 68 (1992) 1480
8	R. Baier et al.	Radiative energy loss and $ p_{\perp} $-broadening of high energy partons in nuclei	NPB 484 (1997) 265	hep-ph/9608322
9	B. G. Zakharov	Radiative energy loss of high-energy quarks in finite-size nuclear matter and quark-gluon plasma	JEPTL 65 (1997) 615	hep-ph/9704255
10	STAR Collaboration	Transverse-momentum and collision-energy dependence of high-$ p_{\mathrm{t}} $ hadron suppression in Au+Au collisions at ultrarelativistic energies	PRL 91 (2003) 172302	nucl-ex/0305015
11	PHENIX Collaboration	Suppression pattern of neutral pions at high transverse momentum in Au+Au collisions at $ \sqrt{s_{\rm{NN}}} = $ 200 GeV and constraints on medium transport coefficients	PRL 101 (2008) 232301	0801.4020
12	ALICE Collaboration	Centrality dependence of charged particle production at large transverse momentum in Pb--Pb collisions at $ \sqrt{s_{\rm{NN}}} = $ 2.76 TeV	PLB 720 (2013) 52	1208.2711
13	ATLAS Collaboration	Measurement of charged-particle spectra in Pb+Pb collisions at $ \sqrt{s_{_{\mathrm{NN}}}} = $ 2.76 TeV with the ATLAS detector at the LHC	JHEP 09 (2015) 050	1504.04337
14	CMS Collaboration	Study of high-$ p_{\rm{T}} $ charged particle suppression in PbPb compared to pp collisions at $ \sqrt{s_\mathrm{NN}}= $ 2.76 TeV	EPJC 72 (2012) 1945	CMS-HIN-10-005 1202.2554
15	CMS Collaboration	Charged-particle nuclear modification factors in PbPb and pPb collisions at $ \sqrt{s_{\mathrm{NN}}}= $ 5.02 TeV	JHEP 04 (2017) 039	CMS-HIN-15-015 1611.01664
16	CMS Collaboration	Observation and studies of jet quenching in PbPb collisions at $ \sqrt{s_\mathrm{NN}} = $ 2.76 TeV	PRC 84 (2011) 024906	CMS-HIN-10-004 1102.1957
17	ATLAS Collaboration	Observation of a centrality-dependent dijet asymmetry in lead-lead collisions at $ \sqrt{s_\mathrm{NN}} = $ 2.76 TeV with the ATLAS detector at the LHC	PRL 105 (2010) 252303	1011.6182
18	ATLAS Collaboration	Centrality and rapidity dependence of inclusive jet production in $ \sqrt{s_\mathrm{NN}} = $ 5.02 TeV proton--lead collisions with the ATLAS detector	PLB 748 (2015) 392	1412.4092
19	ALICE Collaboration	Measurement of jet quenching with semi-inclusive hadron-jet distributions in central Pb-Pb collisions at $ \sqrt{s_{\mathrm{NN}}}= $ 2.76 TeV	JHEP 09 (2015) 170	1506.03984
20	CMS Collaboration	Measurement of inclusive jet cross sections in pp and PbPb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 2.76 TeV	PRC 96 (2017) 015202	CMS-HIN-13-005 1609.05383
21	ATLAS Collaboration	Centrality, rapidity and transverse momentum dependence of isolated prompt photon production in lead-lead collisions at $ \sqrt{s_{_{\mathrm{NN}}}} = $ 2.76 TeV measured with the ATLAS detector	PRC 93 (2016) 034914	1506.08552
22	CMS Collaboration	Measurement of isolated photon production in pp and PbPb collisions at $ \sqrt{s_\mathrm{NN}}= $ 2.76 TeV	PLB 710 (2012) 256	CMS-HIN-11-002 1201.3093
23	CMS Collaboration	Study of W boson production in PbPb and pp collisions at $ \sqrt{s_\mathrm{NN}}= $ 2.76 TeV	PLB 715 (2012) 66	CMS-HIN-11-008 1205.6334
24	CMS Collaboration	Study of Z production in PbPb and pp collisions at $ \sqrt{s_{\mathrm{NN}}}= $ 2.76 TeV in the dimuon and dielectron decay channels	JHEP 03 (2015) 022	CMS-HIN-13-004 1410.4825
25	V. Kartvelishvili, R. Kvatadze, and R. Shanidze	On Z and Z+jet production in heavy ion collisions	PLB 356 (1995) 589	hep-ph/9505418
26	X.-N. Wang, Z. Huang, and I. Sarcevic	Jet quenching in the direction opposite to a tagged photon in high-energy heavy-ion collisions	PRL 77 (1996) 231	hep-ph/9605213
27	X.-N. Wang and Z. Huang	Medium-induced parton energy loss in $ \gamma $+jet events of high-energy heavy-ion collisions	PRC 55 (1997) 3047	hep-ph/9701227
28	R. B. Neufeld, I. Vitev, and B. W. Zhang	Physics of $ Z^0/\gamma^* $-tagged jets at energies available at the CERN Large Hadron Collider	PRC 83 (2011) 034902	1006.2389
29	R. B. Neufeld and I. Vitev	$ Z^0 $-tagged jet event asymmetry in heavy-ion collisions at the CERN Large Hadron Collider	PRL 108 (2012) 242001	1202.5556
30	X.-N. Wang and Y. Zhu	Medium Modification of $ \gamma $ Jets in High-Energy Heavy-Ion Collisions	PRL 111 (2013) 062301	1302.5874
31	J. Casalderrey-Solana et al.	A hybrid strong/weak coupling approach to jet quenching	JHEP 10 (2014) 019	1405.3864
32	J. Casalderrey-Solana et al.	Predictions for boson-jet observables and fragmentation function ratios from a hybrid strong/weak coupling model for jet quenching	JHEP 03 (2016) 053	1508.00815
33	R. K. Elayavalli and K. C. Zapp	Simulating V+jet processes in heavy ion collisions with JEWEL	EPJC 76 (2016) 695	1608.03099
34	Z.-B. Kang, I. Vitev, and H. Xing	Vector-boson-tagged jet production in heavy ion collisions at energies available at the CERN Large Hadron Collider	PRC 96 (2017) 014912	1702.07276
35	CMS Collaboration	Studies of jet quenching using isolated-photon+jet correlations in PbPb and pp collisions at $ \sqrt{s_\mathrm{NN}}= $ 2.76 TeV	PLB 718 (2013) 773	CMS-HIN-11-010 1205.0206
36	CMS Collaboration	Study of jet quenching with $ Z $+jet correlations in Pb-Pb and $ pp $ collisions at $ \sqrt{s_\mathrm{NN}} = $ 5.02 TeV	PRL 119 (2017) 082301	CMS-HIN-15-013 1702.01060
37	CMS Collaboration	Azimuthal Anisotropy of Charged Particles at High Transverse Momenta in Pb-Pb Collisions at $ \sqrt{s_\mathrm{NN}}= $ 2.76 TeV	PRL 109 (2012) 022301	CMS-HIN-11-012 1204.1850
38	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004	CMS-00-001
39	CMS Collaboration	The CMS trigger system	JINST 12 (2017), no. 01, P01020	CMS-TRG-12-001 1609.02366
40	CMS Collaboration	Performance of photon reconstruction and identification with the CMS detector in proton-proton collisions at $ \sqrt{s} = $ 8 TeV	JINST 10 (2015) P08010	CMS-EGM-14-001 1502.02702
41	CMS Collaboration	Transverse-momentum and pseudorapidity distributions of charged hadrons in pp collisions at $ \sqrt{s} = $ 7 TeV	PRL 105 (2010) 022002	CMS-QCD-10-006 1005.3299
42	CMS Collaboration	Identification and filtering of uncharacteristic noise in the CMS hadron calorimeter	JINST 5 (2010) T03014	CMS-CFT-09-019 0911.4881
43	CMS Collaboration	Particle-flow reconstruction and global event description with the CMS detector	Submitted to JINST	CMS-PRF-14-001 1706.04965
44	M. Cacciari, G. P. Salam, and G. Soyez	The anti-$ k_t $ jet clustering algorithm	JHEP 04 (2008) 063	0802.1189
45	M. Cacciari, G. P. Salam, and G. Soyez	FastJet user manual	EPJC 72 (2012) 1896	1111.6097
46	O. Kodolova, I. Vardanian, A. Nikitenko, and A. Oulianov	The performance of the jet identification and reconstruction in heavy ions collisions with CMS detector	EPJC 50 (2007) 117
47	CMS Collaboration	Determination of jet energy calibration and transverse momentum resolution in CMS	JINST 6 (2011) P11002	CMS-JME-10-011 1107.4277
48	T. Sjostrand, S. Mrenna, and P. Z. Skands	A brief introduction to PYTHIA 8.1	CPC 178 (2008) 852	0710.3820
49	CMS Collaboration	Event generator tunes obtained from underlying event and multiparton scattering measurements	EPJC 76 (2016) 155	CMS-GEN-14-001 1512.00815
50	I. P. Lokhtin and A. M. Snigirev	A model of jet quenching in ultrarelativistic heavy ion collisions and high-$ p_{\mathrm{T}} $ hadron spectra at RHIC	EPJC 45 (2006) 211	hep-ph/0506189
51	CMS Collaboration	Measurement of the isolated prompt photon production cross section in $ pp $ collisions at $ \sqrt{s} = $ 7 TeV	PRL 106 (2011) 082001	CMS-QCD-10-019 1012.0799
52	T. C. Awes et al.	A simple method of shower localization and identification in laterally segmented calorimeters	NIMA 311 (1992) 130
53	GEANT4 Collaboration	GEANT4---a simulation toolkit	NIMA 506 (2003) 250
54	ATLAS Collaboration	Measurement of the jet radius and transverse momentum dependence of inclusive jet suppression in lead--lead collisions at $ \sqrt{s_\mathrm{NN}} = $ 2.76 TeV with the ATLAS detector	PLB 719 (2013) 220	1208.1967
55	CMS Collaboration	Measurement of transverse momentum relative to dijet systems in PbPb and pp collisions at $ \sqrt{s_{\mathrm{NN}}}= $ 2.76 TeV	JHEP 01 (2016) 006	CMS-HIN-14-010 1509.09029
56	M. L. Miller, K. Reygers, S. J. Sanders, and P. Steinberg	Glauber modeling in high-energy nuclear collisions	Ann. Rev. Nucl. Part. Sci. 57 (2007) 205	nucl-ex/0701025
57	K. C. Zapp	JEWEL 2.0.0: directions for use	EPJC 74 (2014) 2762	1311.0048