Physics Analysis

Jet is a collimated shower of particles produced from the fragmentation and hadronization of hard scattered parton (parton produced in interactions with large momentum transfer Q2) in high energy hadronic and nuclear collisions. Jet is one of the most important probes to the QGP. Experimentally jet can be measured using jet reconstruction algorithms and provides proxy for the parton from which it is produced. In proton-proton (pp) collisions jet provide a testing ground for the pQCD and non-pQCD aspects of jet production and fragmentation as implemented in the theoretical models and forms a baseline for similar measurements in proton-nucleus (p-A) and nucleus-nucleus (A-A) collisions. In the presence of QGP medium, an energetic parton loses its energy via induced gluon radiations and elastic scatterings with the medium partons. Jet production cross section and its properties are therefore modified in presence of medium compared to number of binary collision scaled pp baseline. Jet studies in A–A collisions in comparison to pp allow a better understanding of the medium induced modifications in the fragmentation of hard scattered partons and energy loss mechanisms, whereas similar studies in p–A collisions potentially reveal the effects of (cold) nuclear matter.

Heavy-ion collisions in the RHIC and LHC generate temperatures (T) of the order of 10¹² - 10¹⁵ degrees kelvin. Under these extreme conditions, protons and neutrons melt, freeing the quarks from their bonds with the gluons leading to QGP formation. Thus, recreating in the laboratory conditions similar to those during a few microseconds old Universe. Studying such a phase and its properties tests the theory of quantum chromodynamics (QCD), provides understanding of the phenomenon of confinement, and the physics of chiral-symmetry. While the confinement tells why we observe only colourless states in the particle spectrum in nature, the chiral symmetry on the other hand describes the presence of particles like pions and mass generation of nucleons, which then accounts for most of the mass of all the visible matter. The experiment in conjunction with the theory has measured several properties of the QGP medium like the shear viscosity to entropy density \((\eta/s)\sim (1-2)1/4\pi\), stopping power also called as opacity of the medium \(\sim2-10\) GeV²/fm and diffusion co-efficient times \(2\pi T \sim 1-10\).

Out of the several properties of QGP listed above, the value of \(\eta/s\sim 1/4\pi\) led to the discovery of the emergent property of QCD matter, namely the perfect fluidity. Maxwell had realized that for a dilute gas, the shear viscosity is related to momentum transport by individual molecules and is given as \(\eta = (1/3)npl\). Here \(n\) is the density, \(p\) is the average momentum of the molecules, and \(l\) is the mean free path. As mean free path varies inversely with density, \(\eta\) to a good approximation is independent of density (one interesting consequence is that the damping of a pendulum caused by the surrounding air is independent of atmospheric pressure!). For a fixed density and temperature, as \(\eta\) is proportional to mean free path \((l)\), it will become smaller with stronger interactions among the constituents of the fluids. So, is there a limit to how small a value \(\eta\) can take up? The shear viscosity is a measure of the ability of a fluid to transport momentum from one point to another, and Heisenberg’s uncertainty in quantum mechanics puts limits the accuracy with which momentum and position can be simultaneously determined. This leads to \(pl\geq\hbar\) . Considering entropy density \(s\sim k_Bn\), the lower bound to \(\eta/s\geq \frac{\hbar}{k_B}\) . However, this simple estimate is not the complete physics story, it turns out that a precise value on the bound on the viscosity can come from string theoretical calculations using the anti-de Sitter/Conformal Field Theory conjecture. It was shown by Kovtun, Son, and Starinets (also called KSS bound) \(\eta/s\geq \frac{\hbar}{4\pi k_B}\) (modulo come corrections). The QGP fluid created at RHIC and LHC is found to have a \(\eta/s\) value close to \(1/4\pi\) (KSS bound in natural units, \(\hbar=k_B=1\) ), hence it is termed as a perfect fluid.