2023-07-21 11:00  Online

Heavy quarkonium production in high multiplicity events

Dr. Kazuhiro Watanabe

The elementary production mechanism of heavy quarkonium, the bound state of a heavy quark and anti-quark, has been an exciting challenge to QCD for almost half a century. Much theoretical effort performed over the last three decades allows us to compare theory predictions within Non-relativistic QCD (NRQCD) factorization with experimental data on the quarkonium production, including its cross-section differential in transverse momentum (pT) at NLO accuracy. However, the simultaneous description of the quarkonium pT spectrum and polarization, which is longitudinal or transverse for a massive quarkonium, still confronts QCD calculations. 

 

Nonetheless, heavy quarkonium is a gripping observable that probes rich QCD phenomena addressed with colliders; in particular, quarkonium production in hadron-hadron and hadron-ion collisions is a valuable tool to look into multi-dimensional parton distributions of hadron and nuclei in high-energy limit, where many gluons can occupy them, which is called Color-Glass-Condensate (CGC), a universal form of the matter. The problem is that it has yet to be discovered in collider experiments thus far. One attractive observable sensitive to CGC is high-multiplicity events in small collision systems. However, note that there are active debates on which CGC effect or hydrodynamical flow effect is more significant to describe data on particle production, including heavy quarkonium. 

 

In this talk, we will discuss how CGC + NRQCD approach describes the pT spectrum and polarization of quarkonium production in minimum bias proton-proton and proton-ion collisions. Then, we will show our model based on CGC to describe quarkonium production in high-multiplicity proton-proton and proton-ion collisions. In the last part of this talk, some prospects for high-multiplicity events will be discussed; we will argue that high-multiplicity events provide an essential playground to deepen our insight into the elementary quarkonium production mechanism as well as hadronization of light particles.