Skip to main content

First measurements of N-subjettiness in central Pb-Pb collisions at \( \sqrt{s_{\mathrm{NN}}} \) = 2.76 TeV

A preprint version of the article is available at arXiv.


The ALICE Collaboration reports the first fully-corrected measurements of the N-subjettiness observable for track-based jets in heavy-ion collisions. This study is performed using data recorded in pp and Pb-Pb collisions at centre-of-mass energies of \( \sqrt{s} \) = 7 TeV and \( \sqrt{s_{\mathrm{NN}}} \) = 2.76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, τ21, which is sensitive to the rate of two-pronged jet substructure, is presented. Energy loss of jets traversing the strongly interacting medium in heavy-ion collisions is expected to change the rate of two-pronged substructure relative to vacuum. The results are presented for jets with a resolution parameter of R = 0.4 and charged jet transverse momentum of 40 ≤ pT,jet ≤ 60 GeV/c, which constitute a larger jet resolution and lower jet transverse momentum interval than previous measurements in heavy-ion collisions. This has been achieved by utilising a semi-inclusive hadron-jet coincidence technique to suppress the larger jet combinatorial background in this kinematic region. No significant modification of the τ21 observable for track-based jets in Pb-Pb collisions is observed relative to vacuum PYTHIA6 and PYTHIA8 references at the same collision energy. The measurements of τ21, together with the splitting aperture angle ∆R, are also performed in pp collisions at \( \sqrt{s} \) = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at \( \sqrt{s} \) = 7 TeV, in order to validate the model as a vacuum reference for the Pb-Pb centre-of-mass energy. The PYTHIA references for τ21 are shifted to larger values compared to the measurement in pp collisions. This hints at a reduction in the rate of two-pronged jets in Pb-Pb collisions compared to pp collisions.


  1. B. Müller and J.L. Nagle, Results from the relativistic heavy ion collider, Ann. Rev. Nucl. Part. Sci. 56 (2006) 93 [nucl-th/0602029] [INSPIRE].

  2. G. Roland, K. Safarik and P. Steinberg, Heavy-ion collisions at the LHC, Prog. Part. Nucl. Phys. 77 (2014) 70 [INSPIRE].

    ADS  Article  Google Scholar 

  3. M. Dasgupta, F.A. Dreyer, G.P. Salam and G. Soyez, Inclusive jet spectrum for small-radius jets, JHEP 06 (2016) 057 [arXiv:1602.01110] [INSPIRE].

    ADS  Article  Google Scholar 

  4. S. Marzani, G. Soyez and M. Spannowsky, Looking inside jets: an introduction to jet substructure and boosted-object phenomenology, Springer, Germany (2019) [arXiv:1901.10342] [INSPIRE].

    Book  Google Scholar 

  5. CMS collaboration, Measurement of jet substructure observables in t\( \overline{t} \) events from proton-proton collisions at \( \sqrt{s} \) = 13TeV, Phys. Rev. D 98 (2018) 092014 [arXiv:1808.07340] [INSPIRE].

  6. A.J. Larkoski, I. Moult and B. Nachman, Jet substructure at the Large Hadron Collider: a review of recent advances in theory and machine learning, Phys. Rept. 841 (2020) 1 [arXiv:1709.04464] [INSPIRE].

    ADS  Article  Google Scholar 

  7. A. Majumder and M. Van Leeuwen, The theory and phenomenology of perturbative QCD based jet quenching, Prog. Part. Nucl. Phys. 66 (2011) 41 [arXiv:1002.2206] [INSPIRE].

    ADS  Article  Google Scholar 

  8. ALICE collaboration, Measurement of jet quenching with semi-inclusive hadron-jet distributions in central Pb-Pb collisions at \( \sqrt{s_{NN}} \) = 2.76 TeV, JHEP 09 (2015) 170 [arXiv:1506.03984] [INSPIRE].

  9. STAR collaboration, Measurements of jet quenching with semi-inclusive hadron+jet distributions in Au+Au collisions at \( \sqrt{s_{NN}} \) = 200 GeV, Phys. Rev. C 96 (2017) 024905 [arXiv:1702.01108] [INSPIRE].

  10. CMS collaboration, Study of Jet Quenching with Z + jet Correlations in Pb-Pb and pp Collisions at \( \sqrt{s_{NN}} \) = 5.02 TeV, Phys. Rev. Lett. 119 (2017) 082301 [arXiv:1702.01060] [INSPIRE].

  11. ATLAS collaboration, Measurement of photon-jet transverse momentum correlations in 5.02 TeV Pb + Pb and pp collisions with ATLAS, Phys. Lett. B 789 (2019) 167 [arXiv:1809.07280] [INSPIRE].

  12. JET collaboration, Extracting the jet transport coefficient from jet quenching in high-energy heavy-ion collisions, Phys. Rev. C 90 (2014) 014909 [arXiv:1312.5003] [INSPIRE].

  13. ALICE collaboration, Medium modification of the shape of small-radius jets in central Pb-Pb collisions at \( \sqrt{s_{NN}} \) = 2.76 TeV, JHEP 10 (2018) 139 [arXiv:1807.06854] [INSPIRE].

  14. CMS collaboration, Modification of Jet Shapes in PbPb Collisions at \( \sqrt{s_{NN}} \) = 2.76 TeV, Phys. Lett. B 730 (2014) 243 [arXiv:1310.0878] [INSPIRE].

  15. CMS collaboration, Observation of medium-induced modifications of jet fragmentation in Pb-Pb collisions at \( \sqrt{s_{NN}} \) = 5.02 TeV using isolated photon-tagged jets, Phys. Rev. Lett. 121 (2018) 242301 [arXiv:1801.04895] [INSPIRE].

  16. ATLAS collaboration, Measurement of jet fragmentation in Pb+Pb and pp collisions at \( \sqrt{s_{NN}} \) = 2.76 TeV with the ATLAS detector at the LHC, Eur. Phys. J. C 77 (2017) 379 [arXiv:1702.00674] [INSPIRE].

  17. ALICE collaboration, First measurement of jet mass in Pb-Pb and p-Pb collisions at the LHC, Phys. Lett. B 776 (2018) 249 [arXiv:1702.00804] [INSPIRE].

  18. CMS collaboration, Measurement of the groomed jet mass in PbPb and pp collisions at \( \sqrt{s_{NN}} \) = 5.02 TeV, JHEP 10 (2018) 161 [arXiv:1805.05145] [INSPIRE].

  19. CMS collaboration, Measurement of the splitting function in pp and Pb-Pb collisions at \( \sqrt{s_{NN}} \) = 5.02 TeV, Phys. Rev. Lett. 120 (2018) 142302 [arXiv:1708.09429] [INSPIRE].

  20. STAR collaboration, Measurement of the shared momentum fraction Zg using jet reconstruction in p+p and Au+Au collisions with STAR, Nucl. Phys. A 967 (2017) 516 [arXiv:1704.03046] [INSPIRE].

  21. ALICE collaboration, Exploration of jet substructure using iterative declustering in pp and Pb-Pb collisions at LHC energies, Phys. Lett. B 802 (2020) 135227 [arXiv:1905.02512] [INSPIRE].

  22. J. Casalderrey-Solana, Y. Mehtar-Tani, C.A. Salgado and K. Tywoniuk, New picture of jet quenching dictated by color coherence, Phys. Lett. B 725 (2013) 357 [arXiv:1210.7765] [INSPIRE].

    ADS  Article  Google Scholar 

  23. F. D’Eramo, M. Lekaveckas, H. Liu and K. Rajagopal, Momentum broadening in weakly coupled quark-gluon plasma (with a view to finding the quasiparticles within liquid quark-gluon plasma), JHEP 05 (2013) 031 [arXiv:1211.1922] [INSPIRE].

    ADS  Article  Google Scholar 

  24. J.M. Butterworth, A.R. Davison, M. Rubin and G.P. Salam, Jet substructure as a new Higgs search channel at the LHC, Phys. Rev. Lett. 100 (2008) 242001 [arXiv:0802.2470] [INSPIRE].

    ADS  Article  Google Scholar 

  25. A.J. Larkoski, S. Marzani, G. Soyez and J. Thaler, Soft drop, JHEP 05 (2014) 146 [arXiv:1402.2657] [INSPIRE].

    ADS  Article  Google Scholar 

  26. H.A. Andrews et al., Novel tools and observables for jet physics in heavy-ion collisions, J. Phys. G 47 (2020) 065102 [arXiv:1808.03689] [INSPIRE].

    ADS  Article  Google Scholar 

  27. T. Sjöstrand, S. Mrenna and P.Z. Skands, PYTHIA 6.4 physics and manual, JHEP 05 (2006) 026 [hep-ph/0603175] [INSPIRE].

  28. M. Cacciari, G.P. Salam and G. Soyez, The anti-kt jet clustering algorithm, JHEP 04 (2008) 063 [arXiv:0802.1189] [INSPIRE].

    ADS  Article  Google Scholar 

  29. J. Thaler and K. Van Tilburg, Identifying boosted objects with N-subjettiness, JHEP 03 (2011) 015 [arXiv:1011.2268] [INSPIRE].

    ADS  Article  Google Scholar 

  30. CMS collaboration, Boosted top jet tagging at CMS, CMS-PAS-JME-13-007 (2014).

  31. Y. Mehtar-Tani and K. Tywoniuk, Groomed jets in heavy-ion collisions: sensitivity to medium-induced bremsstrahlung, JHEP 04 (2017) 125 [arXiv:1610.08930] [INSPIRE].

    ADS  Article  Google Scholar 

  32. S.D. Ellis and D.E. Soper, Successive combination jet algorithm for hadron collisions, Phys. Rev. D 48 (1993) 3160 [hep-ph/9305266] [INSPIRE].

  33. Y.L. Dokshitzer, G.D. Leder, S. Moretti and B.R. Webber, Better jet clustering algorithms, JHEP 08 (1997) 001 [hep-ph/9707323] [INSPIRE].

  34. M. Dasgupta, A. Fregoso, S. Marzani and G.P. Salam, Towards an understanding of jet substructure, JHEP 09 (2013) 029 [arXiv:1307.0007] [INSPIRE].

    ADS  Article  Google Scholar 

  35. J. Thaler and K. Van Tilburg, Maximizing boosted top identification by minimizing N-subjettiness, JHEP 02 (2012) 093 [arXiv:1108.2701] [INSPIRE].

    ADS  Article  Google Scholar 

  36. ALICE collaboration, The ALICE experiment at the CERN LHC, 2008 JINST 3 S08002 [INSPIRE].

  37. ALICE collaboration, Performance of the ALICE experiment at the CERN LHC, Int. J. Mod. Phys. A 29 (2014) 1430044 [arXiv:1402.4476] [INSPIRE].

  38. ALICE collaboration, Charged jet cross sections and properties in proton-proton collisions at \( \sqrt{s} \) = 7 TeV, Phys. Rev. D 91 (2015) 112012 [arXiv:1411.4969] [INSPIRE].

  39. ALICE collaboration, Performance of the ALICE VZERO system, 2013 JINST 8 P10016 [arXiv:1306.3130] [INSPIRE].

  40. ALICE collaboration, Alignment of the ALICE Inner Tracking System with cosmic-ray tracks, 2010 JINST 5 P03003 [arXiv:1001.0502] [INSPIRE].

  41. J. Alme et al., The ALICE TPC, a large 3-dimensional tracking device with fast readout for ultra-high multiplicity events, Nucl. Instrum. Meth. A 622 (2010) 316 [arXiv:1001.1950] [INSPIRE].

    ADS  Article  Google Scholar 

  42. ALICE collaboration, Measurement of jet suppression in central Pb-Pb collisions at \( \sqrt{s_{NN}} \) = 2.76 TeV, Phys. Lett. B 746 (2015) 1 [arXiv:1502.01689] [INSPIRE].

  43. ALICE collaboration, Centrality dependence of charged particle production at large transverse momentum in Pb-Pb collisions at \( \sqrt{s_{NN}} \) = 2.76 TeV, Phys. Lett. B 720 (2013) 52 [arXiv:1208.2711] [INSPIRE].

  44. P.Z. Skands, Tuning Monte Carlo generators: the Perugia tunes, Phys. Rev. D 82 (2010) 074018 [arXiv:1005.3457] [INSPIRE].

    ADS  Article  Google Scholar 

  45. R. Brun, F. Bruyant, M. Maire, A.C. McPherson and P. Zanarini, GEANT3 user’s guide, CERN Data Handling Division DD/EE/84-1 (1985).

  46. M. Cacciari, G.P. Salam and G. Soyez, FastJet user manual, Eur. Phys. J. C 72 (2012) 1896 [arXiv:1111.6097] [INSPIRE].

    ADS  Article  Google Scholar 

  47. M. Cacciari, G.P. Salam and G. Soyez, The catchment area of jets, JHEP 04 (2008) 005 [arXiv:0802.1188] [INSPIRE].

    ADS  Article  Google Scholar 

  48. STAR collaboration, Background fluctuations in heavy ion jet reconstruction, Nucl. Phys. A 855 (2011) 299 [arXiv:1012.2406] [INSPIRE].

  49. P. Berta, M. Spousta, D.W. Miller and R. Leitner, Particle-level pileup subtraction for jets and jet shapes, JHEP 06 (2014) 092 [arXiv:1403.3108] [INSPIRE].

    ADS  Article  Google Scholar 

  50. G. Soyez, G.P. Salam, J. Kim, S. Dutta and M. Cacciari, Pileup subtraction for jet shapes, Phys. Rev. Lett. 110 (2013) 162001 [arXiv:1211.2811] [INSPIRE].

    ADS  Article  Google Scholar 

  51. Roounfold webpage,

Download references

Author information

Authors and Affiliations