Diffractive processes in pp collisions at 7 TeV measured with the CMS experiment

Grzegorz Brona

doi:10.22323/1.247.0067

Abstract

The differential diffractive cross section is measured as a function of ξ = M X/s in the region dominated by single dissociation (SD) and double dissociation (DD), where MX is the mass of one of the two final-state hadronic systems separated by the largest rapidity gap in the event. The cross section is also measured as a function of the width of the central rapidity gap in the region dominated by DD, as well as for events with a forward gap over 8.4 units of pseudorapidity. The total SD and DD cross sections are extracted. Single diffraction is one of the main uncertainties both, experimentally and in theoretical calculations, of the particle-production cross section in proton-lead collisions as measured at the centre-of-mass energy per nucleon pair of 5.02 TeV. Furthermore, the observation of a hard color-singlet exchange process in events with a large rapidity gap between two leading jets (jet-gapjet) is reported. The fraction of jet-gap-jet to all dijet events is measured as a function of the second leading jet transverse momentum and the size of the pseudorapidity gap. Presented at DIS2015 XXIII International Workshop on Deep-inelastic scattering and related subjects Diffractive processes in pp collisions at 7 TeV measured with the CMS experiment. Grzegorz Brona∗† Author University of Warsaw E-mail: grzegorz.brona@cern.ch CMS results on the single and double diffractive dissociation in proton-proton collisions at √ s= 7 TeV are presented. The cross sections for both processes are being calculated and compared with previouse LHC and non-LHC measurements. The comparison with phenomenological models is also presented with a focus on the PYTHIA8-MBR which describes the data best. Both cross sections tend to raise weakly with the energy. The results give an insight into mechanism of the Pomeron exchange and will improve modeling of diffraction in event generators. The XXIII International Workshop on Deep Inelastic Scattering and Related Subjects April 27 May 1, 2015 Southern Methodist University Dallas, Texas 75275 ∗Speaker. †For the CMS Collaboration. c © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/ Diffractive processes measured by CMS Grzegorz Brona

Highlights

Diffractive interactions account for about 25% of the total inelastic proton-proton cross section
Double dissociation (DD) when both protons dissociate and the large rapidity gap (LRG) is located between the two areas popoluted by the particles
Central diffraction (CD) which technically is a Pomeron-Pomeron interaction and is leading to the creation of the central system separated from the two intact protons by two LRGs

Summary

Introduction

Diffractive interactions account for about 25% of the total inelastic proton-proton cross section. The barrel and endcaps coverage of the CMS detector spans to |η| < 3, while at higher rapidities Hadronic Forward (HF) calorimeter provides acceptance in 3.0 < |η| < 5.2 range. It consists of steel absorbers with embedded radiation hard quartz fibers, providing fast collection of Cherenkov light. The BSC detectors are located at both sides of the interaction point and are sensitive for charged particles in the 3.23 < η < 4.65 range. The wide geometrical coverage of the experiment and the minimum bias trigger makes the CMS detector a very good tool for the studies of the soft diffraction processes, in which areas devoided of any particles are to be precisely measu√red. For the comparison PYTHIA6-Z2*, and Monte Carlo models based on the Regge-Gribov phenomenology: PHOJET, QGSJET-II and EPOS LHC are used

Event topologies

Forward and central pseudorapidity gaps cross sections

SD and DD cross sections

Findings

Conclusions