Event-shape and multiplicity dependence of freeze-out radii in pp collisions at $$ \sqrt{s} $$ = 7 TeV

Abstract

Two-particle correlations in high-energy collision experiments enable the extraction of particle source radii by using the Bose-Einstein enhancement of pion production at low relative momentum q ∝ 1/R. It was previously observed that in pp collisions at √ s = 7 TeV the average pair transverse momentum k T range of such analyses is limited due to large background correlations which were attributed to mini-jet phenomena. To investigate this further, an event-shape dependent analysis of Bose-Einstein correlations for pion pairs is performed in this work. By categorizing the events by their transverse sphericity S T into spherical (S T > 0.7) and jet-like (S T < 0.3) events a method was developed that allows for the determination of source radii for much larger values of k T for the first time. Spherical events demonstrate little or no background correlations while jet-like events are dominated by them. This observation agrees with the hypothesis of a mini-jet origin of the non-femtoscopic background correlations and gives new insight into the physics interpretation of the k T dependence of the radii. The emission source size in spherical events shows a substantially diminished k T dependence, while jet-like events show indications of a negative trend with respect to k T in the highest multiplicity events. Regarding the emission source shape, the correlation functions for both event sphericity classes show good agreement with an exponential shape, rather than a Gaussian one.

Scattering Studies with Low-Energy Kaon-Proton Femtoscopy in Proton-Proton Collisions at the LHC

“…After the application of the event selection criteria, about 874 million, 374 million, and 1 billion minimum bias pp events were analyzed at ffiffi ffi s p ¼ 5,7,and 13 TeV,respectively. As recently proposed in Ref. [42], in order to reduce the contribution from the minijet background in pp collisions, the events were classified according to their transverse sphericity (S T ), an observable which is known to be correlated with the number of hard parton-parton interactions in each event [43]. An event with only one hard parton-parton interaction will generally produce a jetlike distribution that yields low sphericity, while an event with several independent hard parton-parton interactions can yield higher sphericity.…”

“…To reduce the strong minijet background at low momenta, only events with S T , defined as in Ref. [42], larger than 0.7 were considered in this analysis.…”

RAA em colisões pp de altíssimas energias

“…Algumas análises usam engenharia de formato de evento, como por exemplo artigo do ALICE que diferencia eventos em dois tipos de formato: chamados do tipo duro e do tipo mole [18]. Em mesmo intervalo de multiplicidade separam os eventos que atingiram aquela multiplicidade por presença de jato de partículas (tipo duro) e de grande quantidade de partículas formadas (tipo mole).…”

“…Além da engenharia de formato de evento, o próprio resultado daquele artigo do ALICE poderia ser usado para reunir mais informação sobre a física destes eventos analisados [18]. O artigo media raios de congelamento cinético, ou seja, medidas de PQG, relacionadas ao tamanho dos eventos nos primeiros estágios da colisão.…”

Illuminating Light

“…Transverse sphericity is an event shape observable that has been used throughout the particle physics community as a way to characterize the configuration of the momentum vectors of the final state particles as either pencil-, sphere-like, or anything in between. Pencil-like events are typically associated with events that have a di-jet structure, and sphere-like events with those containing mostly soft processes and an absence of jets [170,171].…”