Speaker: Prof. Yen-Jie Lee (MIT)
Talk title: Searches for collective flow in Small Collision Systems
Time: 2024/08/12 (Mon.) 12:30 
Place: R517, New Physics Building 

Abstract:
Collective flow has been considered a key signature of Quark-Gluon Plasma (QGP) formation in relativistic heavy ion collisions. This talk summarizes the lessons learned from the search for collective flow in various collision systems, including e+e-, ep, minimum-bias pp, and ultraperipheral collisions, with a special focus on the following two analyses. The first measurement of two-particle angular correlations for charged particles using LEP data, collected by ALEPH up to 30 years ago at energies from 91 to 209 GeV, revealed a long-range near-side excess in the highest multiplicity interval N_trk > 50 not predicted by simulations. The harmonic anisotropy coefficients v_n were measured for the first time in e+e- data, offering new experimental constraints on collective phenomena. Additionally, a CMS detector analysis at the CERN LHC studied partonic collective effects inside jets in proton-proton collisions at 13 TeV. It found that the long-range elliptic anisotropic harmonic v_2^* decreases with N_{ch}^j for low-multiplicity jets, while a rising trend for v_2^* at N_{ch}^j  >~ 80 hints at the onset of collective behavior, enhancing our understanding of parton fragmentation dynamics. Together with other measurements in small collision systems, these analyses provide new perspectives on the minimum requirements for collective flow and the formation of QGP.