Study of Very Forward Neutrons with the CMS Zero Degree Calorimeter

Olivér Surányi

doi:10.3390/universe5100210

Abstract

Forward neutrons are studied in proton-lead collisions at the CMS experiment at the CERN LHC. They provide information on the centrality and event plane of collisions and provide an opportunity to study nuclear breakup. At the CMS experiment they are detected by the Zero Degree Calorimeters (ZDCs) in the | η | > 8.5 pseudorapidity range. The ZDCs are quartz fiber Cherenkov calorimeters using tungsten as absorber. Test beam data and events with a single spectator neutron are used for the calibration of these detectors. A Fourier-based method is used correct for the effect of multiple pPb collisions. The corrected ZDC energy distribution is used to calculate centrality percentiles and unfold the neutron multiplicity distribution.

Highlights

Very forward (|η | > 8.5) neutrons are produced in hadron-nucleus and heavy ion collisions
The information gathered from the observation of these neutrons can be used to tag ultraperipheral collisions, calculate the event plane and estimate centrality in heavy ion and hadron-nucleus collisions
Very forward neutrons are produced in hadron-nucleus and heavy ion collisions via intranuclear cascades, evaporation and nuclear resonances

Summary

Introduction

Very forward (|η | > 8.5) neutrons are produced in hadron-nucleus and heavy ion collisions. The information gathered from the observation of these neutrons can be used to tag ultraperipheral collisions, calculate the event plane and estimate centrality in heavy ion and hadron-nucleus collisions. Cascade and evaporation nucleons were observed and studied by a wide range of fix target experiments [4]. In the ALICE experiments at LHC cascade and evaporation neutrons are used for the estimation of centrality [7,8]. The energy spectrum of very forward neutrons produced in proton-proton collisions was measured by the LHCf experiment [9,10]. In the CMS experiment these neutrons can be observed by the Zero Degree. The most important models, which include forward neutron production are DPMJet [2], URQMD [11], Geant4 [12,13], SMASH [14], JAM [15], and PHSD [16]

The Zero Degree Calorimeter of CMS Experiment

Calibration

Pileup Correction

Using ZDC as a Centrality Estimator in pPb Collisions

Unfolding Neutron Multiplicity Distribution

Conclusions