Forward-Backward Net Charge Correlations in pp Collisions at 7 and 13 TeV Using PYTHIA-8

Correlation between the various observables, like, multiplicity, sum of event transverse momenta, or the net charge of particles produced in pp collisions at LHC energies within intervals separated in pseudorapidity and azimuth angle is regarded as a sensitive tool to investigate the collision dynamics and test the models of hadron production. In the present work forward-backward (FB) correlation method is used to measure the net charge angular correlations. Two small windows of width \(\delta \eta \) in pseudorapidity and \(\delta \phi \) in azimuth angle are considered and placed in such a way that their values lie at ( \(\eta _1,\phi _1\) ) in backward and at ( \(\eta _2,\phi _2\) ) in forward region. The value of net charge \(Q = n_+ - n_-\) in these windows on event-by-event basis are estimated and the FB net charge correlation coefficient is calculated using the relation \(b_{QQ} = (\langle Q_FQ_B\rangle - \langle Q_F\rangle \langle Q_B\rangle )/(\langle Q_F^2\rangle - \langle Q_F\rangle ^2)\) . Events corresponding to pp collisions at \(\sqrt{s}\) = 7 and 13 TeV are simulated using the Monte Carlo event generator PYTHIA-8. The events are simulated by setting the color re-connection on/off and Bose-Einstein (B.E) effects on/off. Shown in the figure below are the variations of \(b_{QQ}\) with \(\eta _{sep}\) in different \(\phi \) regions for 7 TeV pp data. It is observed that the value of \(b_{QQ}\) are negative except for \(\eta _{sep}\) =0 and \(\delta \phi = 0\) , when B.E effects are switched as ‘on’. The absolute values of \(b_{QQ}\) are observed to decrease with increasing \(\eta _{sep}\) and approach to zero for \(\eta _{sep} \sim \) 1. This indicates that there is a transition between negative and positive correlation coefficients at higher rapidity gaps. Some structured variations in the values of \(b_{QQ}\) are seen in the region of smaller \(\eta _{sep}\) , which seems to be of interest. Similar structured behavior have been observed in balance function studies too. The effects of Bose-Einstein correlation and the color re-connection on the correlation coefficient may also be noticed in the figure, particularly, in the region of small \(\eta _{sep}\) .