First results on the longitudinal asymmetry and its effect on the

pseudorapidity distributions in \PbPb 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 $|\eta| < 0.9$, $2.8 < \eta < 5.1$ and

$-3.7 < \eta < -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 ($y_{\mathrm{0}}$) 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, $c_{\rm 1}$, on the mean value of $y_{\mathrm{0}}$ is investigated.