Forward Energy Flow and Central Track Multiplicities in W and Z boson Events at 7 TeV pp Collisions

In this paper we study the energy flow in the forward detectors, and the correlations of the forward energy flow with the track multiplicity in the central detector using events with centrally produced $W$ and $Z$ bosons, identified with their leptonic decays. The analysis uses the entire 7 TeV pp collision data sample of about 36 pb$^{-1}$, recorded during the 2010 LHC operation and with strict conditions applied for single $pp$ vertex events. The observed forward energy deposits, their correlations and the track multiplicities in the central part of the CMS experiment are compared with different Monte Carlo models. These models describe the $pp \rightarrow X$ interactions at high energies by a perturbative hard interaction of two partons within the protons combined with a non-perturbative multi-parton interaction component, existing in many variations, between the proton remnants. Our analysis of $W$ and $Z$ events shows that the underlying event structure and the observed correlations are not described by the available non-diffractive soft hadron production models. A detailed study of about 300 events featuring a large rapidity gap, with no significant energy deposited in one of the forward calorimeters, is also presented. An indication for a diffractive component in these events comes from the observation that the majority of the charged leptons from the $W(Z)$ decays are found in the hemisphere opposite to the gap. Such an asymmetry is expected from the diffractive PDF component which on average has a lower x than the quark Pdf's. The fraction of events coming from standard vector boson Monte Carlo production models and the one coming from diffractive component using the POMPYT Monte Carlo is determined from a fit with fixed shapes to the signed lepton pseudo rapidity distribution. The diffractive $W$ production component in the LRG events is found to be $50 \pm 9.3(stat.) \pm 5.2(syst.) \%$.