Quarkonia production as a function of charged-particle multiplicity in pp collisions at s √ =13 TeV with ALICE
QQuarkonia production as a function of charged-particlemultiplicity in pp collisions at √ s = TeV with ALICE
Yanchun Ding a , b , ∗ on behalf of the ALICE Collaboration a Key Laboratory of Quark and Lepton Physics (MOE) and Institute of Particle Physics, Central ChinaNormal University, Wuhan 430079 China b Institut de Physique des 2 Infinis de Lyon, Université Claude Bernard Lyon 1,4 rue Enrico Fermi, 69622 Villeurbanne Cedex, France
E-mail: [email protected]
In pp collisions at LHC energies, heavy quarks are produced in initial hard scatterings and thenthese quarks hadronize in either open heavy-flavor hadrons or quarkonia (e.g. J/ ψ , ψ ( S ) , Υ ).The study of quarkonium production as a function of charged-particle multiplicity links soft andhard processes and allows one to study their interplay. While a linear increase of quarkoniumproduction as a function of charged-particle multiplicity can be reasonably well understood in thecontext of multi-parton interactions, the observation of deviations with respect to a linear increaserequires a more detailed description of the collision.In this contribution, we will present the latest ALICE measurements at forward rapidity for J/ ψ and Υ production as a function of charged-particle multiplicity in pp collisions at √ s =
13 TeV.The first measurement of the double ratios of relative yield of Υ ( S ) over Υ ( S ) and J/ ψ over Υ ( S ) as a function of charged-particle multiplicity will also be shown. The Eighth Annual Conference on Large Hadron Collider Physics-LHCP202025-30 May, 2020online ∗ Speaker © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] S e p uarkonia production as a function of multiplicity Yanchun Ding
1. Introduction
The event-multiplicity dependent production of quarkonium and open heavy-flavor hadrons insmall collision systems such as pp and p–Pb is widely studied at the LHC, because it has the potentialto give new insights on processes at the parton level and on the interplay between the hard and softmechanisms in particle production. ALICE has studied the multiplicity dependence in pp collisionsat √ s =
13 TeV of inclusive J/ ψ production at mid-rapidity [2], which shows a stronger than linearincreasing trend. It is compared with various theoretical models, such as the coherent productionmodel [3], the CGC model [4], the 3-Pomeron CGC model [5], and PYTHIA 8.2 predictions [6, 7].With similar motivations, the recent multiplicity dependence of Υ production at forward rapidityhas been studied, aimed to improve the understanding of the underlying production mechanisms.
2. Analysis strategy
In this analysis, tracklets, i.e. track segments reconstructed in the ALICE Silicon Pixel detector(SPD) [1] with pseudorapidity | η | <
1, are used for the charged-particle multiplicity estimation.The first step of the multiplicity calibration is to correct for the detector inefficiency along theinteraction vertex ( z vtx ), by equalizing the number of tracklets variation as a function of z vtx onan event by event basis. Then, using a Monte Carlo simulation based on the PYTHIA 8.2 [6]and EPOS-LHC [8] event generators, the correlation between the tracklet multiplicity (after the z vtx -correction), N corrtrk , and the generated primary charged particles N ch is determined, as shown inFig. 1. Finally, the self-normalized multiplicity is defined as the ratio between the charged-particledensity (d N ch /d η ) in a given multiplicity interval to the integrated one. Figure 1:
Number of charged particles N ch as a function of trakclets N corrtrk as determined by a Monte Carlosimulation using PYTHIA 8 simulation with superimposed the best fit with a polynomial function. The Υ is reconstructed in the rapidity range − . < y < − . Υ measurement is shown in Fig. 2 for the analyzed data sample. A log-likelihood fit isapplied to extract the signal. The signal shape is described by a Double Crystal Ball (DCB) functionand the background shape is described by a Variable Width Gaussian (VWG) function [9].2 uarkonia production as a function of multiplicity Yanchun Ding ) c (GeV/ µ + µ m c C oun t s pe r M e V / = 13 TeV s pp < 4 y = 36.1 B + S / S (1s): ϒ ALI−PERF−147011 ) c (GeV/ µ + µ m c C oun t s pe r M e V / = 13 TeV s pp < 4 y = 11.8 B + S / S (1s): ϒ High multiplicity
ALI−PERF−147026
Figure 2:
Di-muon invariant mass distribution for integrated over the multiplicity (left) and for highmultiplicity pp collisions, corresponding to the N corrtrk interval bin [ , ] (right).
3. Results and conclusions
The self-normalized yield of Υ is defined as the Υ yield in a given multiplicity interval to themultiplicity-integrated yield. As shown in Fig. 3, an approximately linear increasing behavior isobserved for Υ ( S ) (blue points), Υ ( S ) (green points) and J/ ψ (red points) at forward rapidity.However, a faster than linear increase is presented for J/ ψ (purple points) at mid-rapidity, whenthere might be a correlation between the signal ( | y | < .
9) and the multiplicity estimator ( | η | < Υ ( S ) over Υ ( S ) and Υ ( S ) over J/ ψ , as shown in Fig. 4, are independent on multiplicity and compatiblewith unity within uncertainties. It reveals that there is no dependence on resonance mass and quarkcomponent within uncertainties. Figure 3:
Self-normalized yield of Υ and J/ ψ as a function of normalized charged particle multiplicity. In this contribution, the first results of the Υ ( S ) and Υ ( S ) production as a function of charged-particle multiplicity have been presented. A different behavior is observed compared with the J/ ψ at3 uarkonia production as a function of multiplicity Yanchun Ding
Figure 4: