Soft QCD from ATLAS and CMS

Romain Rougny

doi:10.1051/epjconf/20122802005

Abstract

Measurements of hadron production in pp collisions by the ATLAS and CMS experiments are presented, including charged particle transverse momentum, pseudorapidity and event-by-event multiplicity distributions at sqrt(s) = 0.9, 2.36 and 7 TeV, for NSD and inelastic events. Diffraction is studied with either diffraction enriched or suppressed data samples. Total inelastic cross-section as well as gap cross-section measurements are shown. Measured spectra of identified strange hadrons, reconstructed based on their decay topology, are also discussed. Comparisons to several QCD Monte Carlo models and tunes are exhibited. Results on two-particle angular correlations over a broad range of pseudorapidity and azimuthal angle in pp collisions are presented. Underlying event activity are studied with different hard probes: tracks, trackjets, calorimeter clusters, or in Drell-Yan events.

Highlights

Since the beginning of the collisions at the LHC, the ATLAS and CMS experiments have measured many observables at 0.9, 2.36 and 7 TeV, through a vast physics program
Multiplicity distributions, as well as the pT evolution with the multiplicity were published[4,8], each e√xperiment with their own event selection, and again for s = 0.9, 2.36, and 7 TeV. It was decided by the Minimum Bias Underlying Event Working Group[9] (MBUEWG) to have a common event selection to compare between experiments, ALICE included: events with at least one track in |η| < 0.8 with pT > 0.5 GeV, which led to CMS publishing new results[10]
ATLAS and CMS have proven their general purposefulness with many publications of soft QCD physic results

Summary

Introduction

Since the beginning of the collisions at the LHC, the ATLAS and CMS experiments have measured many observables at 0.9, 2.36 and 7 TeV, through a vast physics program. The results presented here are mainly for soft pp collisions, it is a mandatory step to better understand the physics involved, as QCD still needs to be modeled phenomenologically. As soft QCD is a background to many rare particle searches, improving our knowledge in this low-pT region for 7 TeV will be crucial to many analyses. It is the first time data at such high energy has been acquired, allowing good tuning of the Monte Carlo generators. It allows to discriminate between the handful of theoretical models that were built over the years, by allowing them to be confronted to real measurements

The ATLAS and CMS detectors

Minimum Bias results

Underlying Event results

Conclusion