Experimental overview on small collision systems at the LHC

These conferences proceedings summarize the experimental findings obtained in small collision systems at the LHC, as presented in the special session on "QGP in small systems?" at the Quark Matter 2015 conference. (The arXiv version is significantly longer than the printed proceedings, with more details and a short discussion.)Comment: 17 pages, 3 captioned figures, 1 table, extended version of Quark Matter 2015 proceeding

Absence of jet quenching in peripheral nucleus-nucleus collisions

Medium effects on the production of high-$p_{\rm T}$ particles in nucleus-nucleus (AA) collisions are generally quantified by the nuclear modification factor ($R_{\rm AA}$), defined to be unity in absence of nuclear effects. Modeling particle production including a nucleon-nucleon impact parameter dependence, we demonstrate that $R_{\rm AA}$ at midrapidity in peripheral AA collisions can be significantly affected by event selection and geometry biases. Even without jet quenching and shadowing, these biases cause an apparent suppression for $R_{\rm AA}$ in peripheral collisions, and are relevant for all types of hard probes and all collision energies. Our studies indicate that calculations of jet quenching in peripheral AA collisions should account for the biases, or else they will overestimate the relevance of parton energy loss. Similarly, expectations of parton energy loss in light-heavy collision systems based on comparison with apparent suppression seen in peripheral $R_{\rm AA}$ should be revised. Our interpretation of the peripheral $R_{\rm AA}$ data would unify observations for lighter collision systems or lower energies where significant values of elliptic flow are observed despite the absence of strong jet quenching.Comment: 13 pages, 4 captioned figures (the 4th is not in the published version); our code for HG-PYTHIA was ported in 2019 by Austin Baty to run on top of a default PYTHIA installation and can be found at https://github.com/abaty/HGPythi

Charged-particle multiplicity and transverse energy in Pb-Pb collisions at sqrt(snn) = 2.76 TeV with ALICE

The measurements of charged-particle multiplicity and transverse energy at mid-rapidity in Pb-Pb collisions at sqrt(sNN) = 2.76 TeV are reported as a function of centrality. The fraction of the inelastic cross section recorded by the ALICE detector is estimated using a Glauber model. The results scaled by the number of participating nucleons are compared with pp collisions at the same collision energy, to similar results obtained at significantly lower energies, and with models based on different mechanisms for particle production in nuclear collisions.Comment: Contribution to QM 201