Forward Energy Flow, Central Charged-Particle Multiplicities, and Pseudorapidity Gaps in W and Z Boson Events from pp Collisions at 7 TeV

TL;DR

This paper investigates forward energy flow, central charged-particle multiplicities, and pseudorapidity gaps in W and Z boson events at 7 TeV proton-proton collisions, revealing discrepancies with existing models and evidence for a significant diffractive component.

Contribution

It provides the first detailed analysis of forward energy flow and pseudorapidity gaps in W/Z events, identifying a substantial diffractive component not accounted for by current models.

Findings

Forward energy depositions are not well modeled by existing soft-hadron production models.

Approximately 50% of W/Z events with pseudorapidity gaps have a diffractive component.

Evidence suggests diffractive processes contribute significantly to the observed event topology.

Abstract

A study of forward energy flow and central charged-particle multiplicity in events with W and Z bosons decaying into leptons is presented. The analysis uses a sample of 7 TeV pp collisions, corresponding to an integrated luminosity of 36 inverse picobarns, recorded by the CMS experiment at the LHC. The observed forward energy depositions, their correlations, and the central charged-particle multiplicities are not well described by the available non-diffractive soft-hadron production models. A study of about 300 events with no significant energy deposited in one of the forward calorimeters, corresponding to a pseudorapidity gap of at least 1.9 units, 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. When fitting the signed lepton pseudorapidity distribution of these events with predicted distributions from an admixture of diffractive (POMPYT) and non-diffractive (PYTHIA) Monte Carlo simulations, the diffractive component is determined to be (50.0 +/- 9.3 (stat.) +/- 5.2 (syst.))%.