New Fitting Concept in ATLAS muon tracking for the LHC Run-2

William Leight,Edward Moyse,Peter Kluit,Pierre-Francois Giraud,Alan Poppleton,Jochen Meyer

doi:10.1051/epjconf/201921406006

Abstract

Muons with high momentum - above 500 GeV - are an important constituent of new physics signatures in many models. Run-2 of the LHC is greatly increasing ATLAS’s sensitivity to such signatures thanks to an ever-larger dataset of such particles, enhanced further by an increase in the center-of-mass energy. The ATLAS Muon Spectrometer chamber alignment contributes significantly to the uncertainty of the reconstruction of these high-momentum objects. The proper treatment of measurements during tracking and the correct propagation of the alignment effects is extremely challenging. Recently, an innovative approach that imposes Gaussian constraints on ensembles of detector hits was implemented. It provides a significant improvement to high-momentum tracking without increasing the CPU budget. Furthermore, it allows for the verification of the expected alignment quality using high-statistics collision data. A detailed discussion of the algorithmic realization is given, the expected performance gains are presented and prospects for further applications of the approach are outlined.

Highlights

Between the LHC Run-1 and Run-2, the accelerator’s center-of-mass energy was increased from 8 to 13 TeV, allowing it to probe for new particles at ever higher masses, as suggested by a number of proposed extensions to the Standard Model
Final states in which such particles decay to muons are favorable for searches, as they are relatively clean with well-understood backgrounds
The precision with which the muon’s momentum can be measured will be strongly affected by the alignment of the chambers in the Muon Spectrometer (MS): that is, by how much the real position of the chambers differs from the "perfect" detector assumed by the reconstruction software

Summary

Introduction

Between the LHC Run-1 and Run-2, the accelerator’s center-of-mass energy was increased from 8 to 13 TeV, allowing it to probe for new particles at ever higher masses, as suggested by a number of proposed extensions to the Standard Model. The decay of particles with masses of multiple TeV will produce muons with very high momenta, making the accurate reconstruction of muons with momenta. At such momenta, the precision with which the muon’s momentum can be measured will be strongly affected by the alignment of the chambers in the Muon Spectrometer (MS): that is, by how much the real position of the chambers differs from the "perfect" detector assumed by the reconstruction software. For Run-2, a new method of handling misalignments, using Gaussian constraints on ensembles of detector hits, was implemented, resulting in improved reconstruction of muons overall and high-momentum muons in particular. We describe briefly the reconstruction of muons in ATLAS and the measurement of the MS alignment parameters, and how this new method uses the latter to improve the former

Muon reconstruction in the ATLAS detector

Incorporating Alignment Effects in the Track-fitting Procedure

ATLAS Simulation Preliminary

Results and Future