A look into the “hedgehog” events in pp collisions


  • Antonio Ortiz ICN-UNAM
  • Guy Paic ICN-UNAM






The UA1 and CDF collaborations have reported the presence of events with a very extended structure of low momentum tracks filling in a uniform way the η − ϕ phase space. However, these events have not caused interest with respect to the details of the interaction that provoke them. We have undertaken to study whether such events are predicted by present event generators like PYTHIA 8. We report the existence of events without any discernible jetty structure in high-multiplicity pp collisions. In the simulations those events originate from several parton-parton scatterings within the same pp collision. We introduce the event shape ρ (f latenicity) as a tool to tag those events in experiments.


W. Busza, K. Rajagopal, and W. van der Schee, Heavy Ion Collisions: The Big Picture, and the Big Questions, Ann. Rev. Nucl. Part. Sci. 68 (2018) 339, 10.1146/annurev-nucl-101917-020852

V. Khachatryan et al., Evidence for collectivity in pp collisions at the LHC, Phys. Lett. B 765 (2017) 193, 10.1016/j.physletb.2016.12.009

J. Adam et al., Enhanced production of multi-strange hadrons in high-multiplicity proton-proton collisions, Nature Phys. 13 (2017) 535, 10.1038/nphys4111

J. L. Nagle and W. A. Zajc, Small System Collectivity in Relativistic Hadronic and Nuclear Collisions, Ann. Rev. Nucl. Part. Sci. 68 (2018) 211, 10.1146/annurev-nucl-101916-123209

S. Acharya et al., Multiplicity dependence of light-flavor hadron production in pp collisions at √ s = 7 TeV, Phys. Rev. C 99 (2019) 024906, 10.1103/PhysRevC.99.024906

C. Quigg, Looking into Particle Production at the Large Hadron Collider, Nuovo Cim. C 033N5 (2010) 327, 10.1393/ncc/i2011-10734-0

A. Ortiz Velasquez and G. Paic, Event Shape Analysis in ALICE (2009)

A. Ortiz, Transverse sphericity of charged particles in minimum bias p-p collision in ALICE at the LHC, In PHYSICS AT LHC 2011 (2011) .

B. Abelev et al., Transverse sphericity of primary charged particles in minimum bias proton-proton collisions at √ s = 0.9, 2.76 and 7 TeV, Eur. Phys. J. C 72 (2012) 2124, 10.1140/epjc/s10052-012-2124-9

A. Ortiz, Experimental results on event shapes at hadron colliders, Adv. Ser. Direct. High Energy Phys. 29 (2018) 343, 10.1142/9789813227767_0016

S. Acharya et al., Charged-particle production as a function of multiplicity and transverse spherocity in pp collisions at √ s = 5.02 and 13 TeV, Eur. Phys. J. C 79 (2019) 857, 10.1140/epjc/s10052-019-7350-y

A. Nassirpour, Probing strangeness production in small systems through new multi-differential measurements with ALICE at the LHC, J. Phys. Conf. Ser. 1602 (2020) 012007, 10.1088/1742-6596/1602/1/012007

T. Martin, P. Skands, and S. Farrington, Probing Collective Effects in Hadronisation with the Extremes of the Underlying Event, Eur. Phys. J. C 76 (2016) 299, 10.1140/epjc/s10052-016-4135-4

A. Ortiz and L. Valencia Palomo, Universality of the underlying event in pp collisions, Phys. Rev. D 96 (2017) 114019, 10.1103/PhysRevD.96.114019

G. Bencedi, A. Ortiz, and A. Paz, Disentangling the hard gluon bremsstrahlung effects from the relative transverse activity classifier in pp collisions, Phys. Rev. D 104 (2021) 016017, 10.1103/PhysRevD.104.016017

C. ALICE, Letter of intent for ALICE 3: A next generation heavy-ion experiment at the LHC, Tech. rep., CERN, Geneva (2022), URL http://cds.cern.ch/record/2803563.

T. Sjöstrand, et al., An introduction to PYTHIA 8.2, Comput. Phys. Commun. 191 (2015) 159, 10.1016/j.cpc.2015.01.024

T. Sjöstrand and M. van Zijl, A multiple-interaction model for the event structure in hadron collisions, Phys. Rev. D 36 (1987) 2019, 10.1103/PhysRevD.36.2019

S. Acharya et al., Underlying Event properties in pp collisions at √ s = 13 TeV, JHEP 04 (2020) 192, 10.1007/JHEP04(2020)192

B. B. Abelev et al., Performance of the ALICE Experiment at the CERN LHC, Int. J. Mod. Phys. A 29 (2014) 1430044, 10.1142/S0217751X14300440 S




How to Cite

Ortiz A, Paic G. A look into the “hedgehog” events in pp collisions. Supl. Rev. Mex. Fis. [Internet]. 2022 Dec. 10 [cited 2024 Jul. 17];3(4):040911 1-4. Available from: https://rmf.smf.mx/ojs/index.php/rmf-s/article/view/6832