Abstract
First results on the longitudinal asymmetry and its effect on the pseudorapidity distributions in Pb–Pb collisions at sNN = 2.76 TeV at the Large Hadron Collider are obtained with the ALICE detector. The longitudinal asymmetry arises because of an unequal number of participating nucleons from the two colliding nuclei, and is estimated for each event by measuring the energy in the forward neutron-Zero-Degree-Calorimeters (ZNs). The effect of the longitudinal asymmetry is measured on the pseudorapidity distributions of charged particles in the regions |η|<0.9, 2.8<η<5.1 and −3.7<η<−1.7 by taking the ratio of the pseudorapidity distributions from events corresponding to different regions of asymmetry. The coefficients of a polynomial fit to the ratio characterise the effect of the asymmetry. A Monte Carlo simulation using a Glauber model for the colliding nuclei is tuned to reproduce the spectrum in the ZNs and provides a relation between the measurable longitudinal asymmetry and the shift in the rapidity (y0) of the participant zone formed by the unequal number of participating nucleons. The dependence of the coefficient of the linear term in the polynomial expansion, c1, on the mean value of y0 is investigated.
Highlights
In a heavy-ion collision, the number of nucleons participating from each of the two colliding nuclei is finite, and will fluctuate event-by-event
One estimate was based on the charged particle multiplicity reconstructed in the Time Projection Chamber (TPC) and the other was based on the amplitudes in the V0 detectors [20]
The longitudinal asymmetry in a heavy-ion collision has been estimated from the difference in the energy of the spectator neutrons on both sides of the collision vertex
Summary
In a heavy-ion collision, the number of nucleons participating from each of the two colliding nuclei is finite, and will fluctuate event-by-event. The pseudorapidity distribution (dN/dη) of soft particles, averaged over a large number of events, is symmetric in collisions of identical nuclei These distributions were observed to be asymmetric in collisions of unequal nuclei such as d–Au [7] and p–Pb [8,9,10] and have been explained in terms of the rapidity-shift of the participant zone [11]. The effect of the rapidity-shift of the participant zone should be discernible in the distribution of produced particles This small effect can be estimated by taking the ratio of pseudorapidity distributions in events corresponding to different longitudinal asymmetries [2]. The relation between the parameter characterising the change in raw dN/dη distributions is shown for different centralities in Sect. 5, along with its relation to the estimated values of y0
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