Abstract
Measurements of the inclusive J/ψ yield as a function of charged-particle pseudorapidity density dNch/dη in pp collisions at s=13 TeV with ALICE at the LHC are reported. The J/ψ meson yield is measured at midrapidity (|y|<0.9) in the dielectron channel, for events selected based on the charged-particle multiplicity at midrapidity (|η|<1) and at forward rapidity (−3.7<η<−1.7 and 2.8<η<5.1); both observables are normalized to their corresponding averages in minimum bias events. The increase of the normalized J/ψ yield with normalized dNch/dη is significantly stronger than linear and dependent on the transverse momentum. The data are compared to theoretical predictions, which describe the observed trends well, albeit not always quantitatively.
Highlights
Hadronic charmonium production at collider energies is a complex and not yet fully understood process, involving hard-scale processes, i.e. the initial heavy-quark pair production, which can be described by means of perturbative quantum chromodynamics, as well as soft-scale processes, i.e. the subsequent binding into a color-neutral charmonium state
The non-relativistic QCD (NRQCD) formalism combined with a Color Glass Condensate (CGC) description of the incoming protons [4] is a recent example of a comprehensive treatment of the transverse momentum pT and rapidity dependent production, in particular extended down to zero transverse momentum
The dashed line shown in the figure is a linear function with the slope of unity. These results include both the minimum bias (MB) and high multiplicity (HM) triggered events, which allow for a coverage of up to 7 times the average chargedparticle multiplicity, when events are selected based on the measured midrapidity multiplicity
Summary
Hadronic charmonium production at collider energies is a complex and not yet fully understood process, involving hard-scale processes, i.e. the initial heavy-quark pair production, which can be described by means of perturbative quantum chromodynamics (pQCD), as well as soft-scale processes, i.e. the subsequent binding into a color-neutral charmonium state The latter stage is addressed via models which assume that it factorizes with respect to the perturbative early stage. The event-multiplicity dependent production of charmonium and open charm hadrons in pp and p–Pb collisions are observables having the potential to give new insights on processes at the parton level and on the interplay between the hard and soft mechanisms in particle production and is widely studied at the LHC.
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