Forward energy density per pseudorapdidty and limiting fragmentation studied with CMS at 13TeV

Abstract

The measurement of the energy flow in the forward angular range performed by CMS at LHC is a means of studying soft particle production and testing hadronic interaction models used in collider as well as cosmic ray physics. The two important ingredients necessary for the measurement, forward instrumentation of CMS and the accelerator providing collisions with low pile-up, performed very well in 201 at the start up of the operation at 13 TeV. Presented at DIS 2016 XXIV International Workshop on Deep-Inelastic Scattering and Related Subjects Forward energy flow per pseudorapidity and limiting fragmentation with CMS at 13 TeV Igor Katkov∗† for the CMS Collaboration Institut fur Experimentelle Kernphysik, Karlsruhe Institut fur Technologie (KIT), Karlsruhe, Germany E-mail: katkov@kit.edu The measurement of the energy flow in the forward angular range performed by CMS at LHC is a means of studying soft particle production and testing hadronic interaction models used in collider as well as cosmic ray physics. The two important ingredients necessary for the measurement, forward instrumentation of CMS and the accelerator providing collisions with low pile-up, performed very well in 2015 at the start up of the operation at 13 TeV. XXIV International Workshop on Deep-Inelastic Scattering and Related Subjects 11-15 April, 2016 DESY Hamburg, Germany ∗Speaker. †On leave from Skobeltsyn Institute of Nuclear Physics, M.V. Lomonosov Moscow State University c © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ Energy flow at 13 TeV Igor Katkov 1. Motivation Soft particle production at 13 TeV awaits thorough studies. Any sings of new physics at the LHC are to be uncovered on top of a a large pile of several tens of overlapping soft interactions. Interpretation of high-pT precision measurements of fundamental quantities, for example top quark mass, turns out to require good understanding of soft-inclusive particle production modelling. Basic energy flow measurements provide useful input for tuning of existing hadronic interaction models. They serve also as an important reference for the extrapolation to the highest energies observed in the cosmic rays. So far at LHC the energy flow has been measured by CMS [1] in pp collisions at 900 GeV and 7 TeV [2], in pPb collisions at 5.02 TeV [3] and PbPb collisions at 2.76 TeV [4] per nucleon. The results presented here [5] continue naturally along the same lines focusing on the forward psedorapidity range 3.15 < |η | < 6.6 where most of the energy released in the collision can be observed. 2. Hadronic interaction models Our results are compared to a set of PYTHIA8 [6] tunes and two models, EPOS-LHC [7] and QGSJET II.04 [8], extensively used in the cosmic ray physics. All Monte Carlo models considered are tuned to LHC Run1 data. In the general purpose PYTHIA8 generator hard scattering matrix elements are supplemented by parton showering and string fragmentation in order to provide full collision event record. All aspects of the modelling process, in particular fragmentation, underlying event, diffractive processes, are highly tunable in PYTHIA8. The recent MONASH 2013 tune [9] explicitly aims at improved description of activity the forward angular range. The CMS tunes, CUETP8M1 and CUETP8S1, have been developed based on measurements of the underlying event [10]. The models commonly used in cosmic ray physics, EPOS-LHC and QGSJET II.04, implement the Gribov-Regge multiple scattering approach and use string fragmentation for hadronization. Collective effects are implemented in EPOS-LHC as a hydrodynamic component in a parametrized form. A set of tunable parameters available in EPOS-LHC makes it phenomenologically more flexible compared to theoretically rigour QGSJET II.04. 3. Event selection and measurement procedure The low luminosity data taken by CMS at the start up of the LHC operation at 13 TeV are used in our analysis. We repeat the analysis for two distinct classes of collision events, the softinclusive-inelastic and non-single-diffractive-enhanced events, in several pseudorapidity bins. For soft-inclusive-inelastic events activity above a threshold is required in forward calorimeters on at least one side with respect to the nominal interaction point of CMS. In non-single-diffractiveenhanced events an activity above noise is present in forward calorimeters on both sides. As a measure of the energy flow for every pseudorapidity bin the average of total sum of energies of all reconstructed calorimeter objects is taken. Only energies above a noise threshold 1 Energy flow at 13 TeV Igor Katkov | η | 3.5 4 4.5 5 5.5 6 6.5 | ( G eV )