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| CMS-HIN-16-006 ; CERN-EP-2017-205 | ||
| Measurement of the splitting function in pp and PbPb collisions at ${\sqrt{\smash [b]{s_{_{\mathrm{NN}}}}}} = $ 5.02 TeV | ||
| CMS Collaboration | ||
| 30 August 2017 | ||
| Phys. Rev. Lett. 120 (2018) 142302 | ||
| Abstract: Data from heavy ion collisions suggest that the evolution of a parton shower is modified by interactions with the color charges in the dense partonic medium created in these collisions. However, the stage in the shower evolution at which the modifications occur is not known. By removing the soft wide-angle radiation inside a jet, the two leading partons can be resolved as subjets. The momentum ratio of these subjets provides information about the corresponding partons at an early stage of the shower. This substructure observable, known as the splitting function, relates to the process of a parton splitting into two other partons. The measurement is performed for jets with transverse momentum larger than 140 GeV and smaller than 500 GeV, using pp and PbPb collision samples collected by the CMS detector at a center-of-mass energy of 5.02 GeV per nucleon pair. In central PbPb collisions, the splitting function is observed to be modified toward a more unbalanced momentum ratio compared to peripheral PbPb and pp collisions. The measurements are compared to the predictions from event generators and analytical calculations using various models for the medium modification of the jet evolution. | ||
| Links: e-print arXiv:1708.09429 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Groomed jet energy fraction in pp and the 10% most central PbPb collisions for jets with 160 $ < {p_{\mathrm {T,jet}}} < $ 180 GeV and $ | \eta _{\text {jet}} | < $ 1.3. The pp data is compared to the PYTHIA 6 event generator and the PbPb data to PYTHIA 6 embedded into the HYDJET event generator. |
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Figure 2:
Splitting function distribution in pp collisions for 160 $ < p_{\mathrm {T,jet}} < $ 180 GeV compared to predictions from event generators. The shaded area indicates the systematic uncertainty while the vertical lines represent the statistical uncertainty. |
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Figure 3:
Splitting function distribution in PbPb collisions for 160 $ < p_{\mathrm {T,jet}} < $ 180 GeV in several centrality ranges compared to pp data smeared to account for the difference in resolution between pp and PbPb collisions for each centrality selection. The shaded area indicates the systematic uncertainty while the vertical lines represent the statistical uncertainty. |
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Figure 4:
Ratio of splitting function distributions in PbPb and smeared pp collisions in the 10% most central events for several ${p_{\mathrm {T,jet}}} $ ranges compared to various jet quenching theoretical calculations. The shaded area indicates the systematic uncertainty while the vertical lines represent the statistical uncertainty. The diagonally hatched band denotes the uncertainty from the treatment of the medium response utilizing the JEWEL event generator. |
| Summary |
| In summary, the first measurement of the splitting function in pp and PbPb collisions at a center-of-mass energy of 5.02 TeV per nucleon pair has been presented. This represents the first application of a grooming technique to PbPb data, allowing for the removal of soft wide-angle radiation from the jet and thereby isolating the two leading subjets. The momentum sharing between these subjets is used to obtain information about hard parton splitting in the early stages of the shower in pp and PbPb collisions. Both the PYTHIA and HERWIG++ event generators agree with the measured splitting function in pp collisions at the level of 15%. This agreement is also observed in peripheral PbPb collisions. In central PbPb collisions, a steeper distribution of $ z_{\mathrm{g}} $ is observed, indicating that the splitting process of a parton is modified when traversing the hot medium created in heavy ion collisions. These results provide new insight into the role of color coherence and other attributes of the early interactions of partons in the quark-gluon plasma. |
| References | ||||
| 1 | M. Gyulassy and M. Plumer | Jet quenching in dense matter | PLB 243 (1990) 432 | |
| 2 | R. Baier, Y. L. Dokshitzer, S. Peigne, and D. Schiff | Induced gluon radiation in a QCD medium | PLB 345 (1995) 277 | hep-ph/9411409 |
| 3 | PHENIX Collaboration | Suppression of hadrons with large transverse momentum in central Au+Au collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 130 GeV | PRL 88 (2002) 022301 | nucl-ex/0109003 |
| 4 | PHENIX Collaboration | Suppressed $ \pi^{0} $ production at large transverse momentum in central Au+Au collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 200 GeV | PRL 91 (2003) 072301 | nucl-ex/0304022 |
| 5 | 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 |
| 6 | BRAHMS Collaboration | Transverse momentum spectra in Au+Au and d+Au collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 200 GeV and the pseudorapidity dependence of high $ p_{\mathrm{T}} $ suppression | PRL 91 (2003) 072305 | nucl-ex/0307003 |
| 7 | PHOBOS Collaboration | Charged hadron transverse momentum distributions in Au+Au collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 200 GeV | PLB 578 (2004) 297 | nucl-ex/0302015 |
| 8 | STAR Collaboration | Dijet imbalance measurements in Au+Au and pp collisions at $ \sqrt{s_{\mathrm{NN}}} = 200 \;$ GeV at STAR | PRL 119 (2017) 062301 | 1609.03878 |
| 9 | STAR Collaboration | Measurements of jet quenching with semi-inclusive hadron+jet distributions in Au+Au collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 200 GeV | Submitted to PRC | 1702.01108 |
| 10 | ALICE Collaboration | Suppression of charged particle production at large transverse momentum in central Pb--Pb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 2.76 TeV | PLB 696 (2011) 30 | 1012.1004 |
| 11 | ALICE Collaboration | Particle-yield modification in jetlike azimuthal dihadron correlations in Pb-Pb collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 2.76 TeV | PRL 108 (2012) 092301 | 1110.0121 |
| 12 | ATLAS Collaboration | Measurement of charged-particle spectra in Pb+Pb collisions at $ \sqrt{{s}_\mathsf{{NN}}} = $ 2.76 TeV with the ATLAS detector at the LHC | JHEP 09 (2015) 050 | 1504.04337 |
| 13 | CMS Collaboration | Study of high-$ p_{\mathrm{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 |
| 14 | ATLAS Collaboration | Observation of a centrality-dependent dijet asymmetry in Lead-Lead collisions at $ \sqrt{\mathrm{s}_{\mathrm{NN}}} = $ 2.76 TeV with the ATLAS detector at the LHC | PRL 105 (2010) 252303 | 1011.6182 |
| 15 | CMS Collaboration | Jet momentum dependence of jet quenching in PbPb collisions at $ \sqrt{s_{\mathrm{NN}}}= $ 2.76 TeV | PLB 712 (2012) 176 | CMS-HIN-11-013 1202.5022 |
| 16 | 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 |
| 17 | ALICE Collaboration | Measurement of charged jet suppression in Pb-Pb collisions at $ \sqrt{\mathrm{s}_{\mathrm{NN}}} = $ 2.76 TeV | JHEP 03 (2014) 013 | 1311.0633 |
| 18 | ALICE Collaboration | Measurement of jet suppression in central Pb-Pb collisions at $ \sqrt{s_{\rm NN}} = $ 2.76 TeV | PLB 746 (2015) 1 | 1502.01689 |
| 19 | CMS Collaboration | Charged-particle nuclear modification factors in PbPb and pPb collisions at $ \sqrt{s_{\mathrm{N}\mathrm{N}}}= $ 5.02 TeV | JHEP 04 (2017) 039 | CMS-HIN-15-015 1611.01664 |
| 20 | 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 |
| 21 | CMS Collaboration | Modification of jet shapes in PbPb collisions at $ \sqrt {s_{\mathrm{NN}}} = $ 2.76 TeV | PLB 730 (2014) 243 | CMS-HIN-12-002 1310.0878 |
| 22 | CMS Collaboration | Measurement of jet fragmentation in PbPb and pp collisions at $ \sqrt{s_{\mathrm{NN}}}= $ 2.76 TeV | PRC 90 (2014) 024908 | CMS-HIN-12-013 1406.0932 |
| 23 | ATLAS Collaboration | Measurement of jet fragmentation in Pb+Pb and pp collisions at $ \sqrt{{s_\mathrm{NN}}} = $ 2.76 TeV with the ATLAS detector at the LHC | EPJC 77 (2017) 379 | 1702.00674 |
| 24 | ALICE Collaboration | First measurement of jet mass in Pb-Pb and p-Pb collisions at the LHC | Submitted to PLB | 1702.00804 |
| 25 | 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 |
| 26 | S. D. Ellis, C. K. Vermilion, and J. R. Walsh | Recombination algorithms and jet substructure: Pruning as a tool for heavy particle searches | PRD 81 (2010) 094023 | 0912.0033 |
| 27 | J. M. Butterworth, A. R. Davison, M. Rubin, and G. P. Salam | Jet substructure as a new Higgs search channel at the LHC | PRL 100 (2008) 242001 | 0802.2470 |
| 28 | D. Krohn, J. Thaler, and L.-T. Wang | Jet trimming | JHEP 02 (2010) 084 | 0912.1342 |
| 29 | M. Dasgupta, A. Fregoso, S. Marzani, and G. P. Salam | Towards an understanding of jet substructure | JHEP 09 (2013) 029 | 1307.0007 |
| 30 | A. J. Larkoski, S. Marzani, G. Soyez, and J. Thaler | Soft drop | JHEP 05 (2014) 146 | 1402.2657 |
| 31 | G. Altarelli and G. Parisi | Asymptotic freedom in parton language | NPB 126 (1977) 298 | |
| 32 | A. J. Larkoski, S. Marzani, and J. Thaler | Sudakov Safety in Perturbative QCD | PRD 91 (2015) 111501 | 1502.01719 |
| 33 | A. Larkoski et al. | Exposing the QCD splitting function with CMS Open Data | 1704.05066 | |
| 34 | A. Tripathee et al. | Jet substructure studies with CMS Open Data | 1704.05842 | |
| 35 | Y. Mehtar-Tani and K. Tywoniuk | Jet (de)coherence in Pb--Pb collisions at the LHC | PLB 744 (2015) 284 | 1401.8293 |
| 36 | J. Casalderrey-Solana, Y. Mehtar-Tani, C. A. Salgado, and K. Tywoniuk | New picture of jet quenching dictated by color coherence | PLB 725 (2013) 357 | 1210.7765 |
| 37 | Y. Mehtar-Tani and K. Tywoniuk | Groomed jets in heavy-ion collisions: sensitivity to medium-induced bremsstrahlung | JHEP 04 (2017) 125 | 1610.08930 |
| 38 | Y.-T. Chien and I. Vitev | Probing the hardest branching of jets in heavy ion collisions | 1608.07283 | |
| 39 | J. G. Milhano, U. A. Wiedemann, and K. C. Zapp | Sensitivity of jet substructure to jet-induced medium response | 1707.04142 | |
| 40 | CMS Collaboration | The CMS experiment at the CERN LHC | JINST 3 (2008) S08004 | CMS-00-001 |
| 41 | CMS Collaboration | Particle-flow reconstruction and global event description with the CMS detector | Submitted to JINST | CMS-PRF-14-001 1706.04965 |
| 42 | M. Cacciari, G. P. Salam, and G. Soyez | Fastjet user manual | EPJC 72 (2012) 1896 | 1111.6097 |
| 43 | M. Cacciari and G. P. Salam | Dispelling the $ N^{3} $ myth for the $ k_t $ jet-finder | PLB 641 (2006) 57 | hep-ph/0512210 |
| 44 | CMS Collaboration | Observation and studies of jet quenching in PbPb collisions at nucleon-nucleon center-of-mass energy = 2.76 TeV | PRC 84 (2011) 024906 | CMS-HIN-10-004 1102.1957 |
| 45 | T. Sjostrand, S. Mrenna, and P. Skands | PYTHIA 6.4 physics and manual | JHEP 05 (2006) 026 | hep-ph/0603175 |
| 46 | CMS Collaboration | Study of the underlying event at forward rapidity in pp collisions at $ \sqrt{s} = $ 0.9, 2.76, and 7 TeV | JHEP 04 (2013) 072 | CMS-FWD-11-003 1302.2394 |
| 47 | CMS Collaboration | Event generator tunes obtained from underlying event and multiparton scattering measurements | EPJC 76 (2016) 155 | CMS-GEN-14-001 1512.00815 |
| 48 | 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 |
| 49 | GEANT4 Collaboration | GEANT4---a simulation toolkit | NIMA 506 (2003) 250 | |
| 50 | T. Sjostrand, S. Mrenna, and P. Skands | A brief introduction to PYTHIA 8.1 | Comp. Phys. Comm. 178 (2008) 852 | 0710.3820 |
| 51 | M. Bahr et al. | Herwig++ physics and manual | EPJC 58 (2008) 639 | 0803.0883 |
| 52 | M. H. Seymour and A. Siodmok | Constraining MPI models using $ \sigma_{\mathrm{eff}} $ and recent Tevatron and LHC underlying event data | JHEP 10 (2013) 113 | 1307.5015 |
| 53 | P. Berta, M. Spousta, D. W. Miller, and R. Leitner | Particle-level pileup subtraction for jets and jet shapes | JHEP 06 (2014) 092 | 1403.3108 |
| 54 | G. Soyez et al. | Pileup subtraction for jet shapes | PRL 110 (2013) 162001 | 1211.2811 |
| 55 | CMS Collaboration | Determination of jet energy calibration and transverse momentum resolution in CMS | JINST 6 (2011) P11002 | CMS-JME-10-011 1107.4277 |
| 56 | Y. L. Dokshitzer, G. D. Leder, S. Moretti, and B. R. Webber | Better jet clustering algorithms | JHEP 08 (1997) 001 | hep-ph/9707323 |
| 57 | CMS Collaboration | Jet energy scale and resolution in the CMS experiment in pp collisions at 8 TeV | JINST 12 (2017) P02014 | CMS-JME-13-004 1607.03663 |
| 58 | K. C. Zapp, F. Krauss, and U. A. Wiedemann | A perturbative framework for jet quenching | JHEP 03 (2013) 080 | 1212.1599 |
| 59 | R. Kunnawalkam Elayavalli and K. C. Zapp | Medium response in JEWEL and its impact on jet shape observables in heavy ion collisions | 1707.01539 | |
| 60 | R. Baier et al. | Radiative energy loss of high-energy quarks and gluons in a finite volume quark-gluon plasma | NPB 483 (1997) 291 | hep-ph/9607355 |
| 61 | R. Baier et al. | Radiative energy loss and $ p_{\mathrm{T}} $ broadening of high-energy partons in nuclei | NPB 484 (1997) 265 | hep-ph/9608322 |
| 62 | N.-B. Chang, S. Cao, and G.-Y. Qin | Probing medium-induced jet splitting and energy loss in heavy-ion collisions | 1707.03767 | |
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