CERN Accelerating science

The study of the quark-gluon plasma (QGP), a state of matter which is supposed to have existed after big-bang, is the objective of ultra-relativistic nuclear collisions. At the Large Hadron Collider (LHC), ALICE (A Large Ion Collider Experiment) is designed to produce and investigate the characteristics of QGP, whose presence at high temperature is predicted by Quantum Chromodynamics (QCD). The QCD has also predicted about the local parity (P) violation effects in strong interactions. These parity violation effects when coupled with the strong magnetic field, created by spectator protons in non-central heavy-ion collisions, result into the back-to-back charge separation along the direction of magnetic field. This phenomenon is known as the Chiral Magnetic Effect (CME). The expected charge separation due to CME is very small and can be diluted when searching in a whole sample. So, it is essential to search CME signal in each and every event to isolate potential events exhibiting the back-to-back charge separation with significantly enhanced signal in these events rather than looking the signal taking all events together in a given collision centrality. The noval method, Sliding Dumbbell Method (SDM), is developed to search for the back-to-back charge separation on an event-by-event basis. Events in a given collision centrality are characterised depending on the fractional charge separation across the 90$^{\circ}$ dumbbel using the SDM. Here, azimuthal plane of an event is scanned by sliding the 90$^{\circ}$ dumbbell by 1$^{\circ}$ to get the maximum value of charge separation (fDbCS) across the dumbbell. The obtained (fDbCS) distribution is sliced into ten percentile bins; top 0-10% being highest charge separation and 90-100% being lowest charge separation. Therefore, events are categorised according to their charge separation to get CME-like enrich sample. The charged- particle angular correlations which are the most favourable method to study the charge separation due to CME, are studied for the different fDbCS bins. The SDM is tested on the AMPT generated events samples. The default AMPT with no CME like signal does not differentiate between γ correlator for data and backgrounds. Here, the background contribution is determined by shuffling the charges of the particles in an event and shuffled events are analyzed in similar way as of given data. The varying percentages, i.e., from ∼0.5% to ∼8%, of the CME like signals are externally injected in the events by flipping the charges of pairs of the particles perpendicular to the reaction plane. As the externally injected CME like signal in AMPT is increased, the values of $\gamma$ correlators found to increase. The CME fraction calculated through AMPT samples increases with increase in externally injected signal. The SDM is able to detect the signal as small as ∼0.5%. After testing its credebility, the SDM is then applied on ALICE’s data of Pb-Pb collisions of $\sqrt{SNN}$ = 2.76 eV and $\sqrt{SNN}$ = 5.02TeV. The charge separation binning effect is seen in 3-particle correlator based on 2nd order event plane (γ112). The γ112 increases significantly for the top 10% fDbCS events, while 3-particle correlator based on 3rd order event plane i.e, γ123, remains independent of charge separation binning as expected. Moreover, the magnitudes of γ for same sign charge pair and $\gamma$ 􏰂for 112 112 opposite sign charge pair for the top 10% fDbCS percentile bin are equal. The |$\gamma$| for same sign charge pair is seen to vary linearly with positive charge asymmetry across the dumbbell (term proportional to the parity violating parameter) for the top 10% fDbCS events. Also, it is observed that elliptic flow increases by 5% in the top 10% of fDbCS events, whereas mean multiplicity decreases. Furthermore, out- of-plane correlations are seen to be more prominent in same-sign charge pairs than those of in-plane correlations for the top fDbCS bins. The H-correlator representing the CME signal also enhances many times for the top 10% fDbCS events. The fCME is around 18±8% for 0-50% collision centrality for the top 10% f DbCS events in a given collision centrality which corresponds to 1.8±0.8% for a given collision centrality and seems to be independent of collision energy.