Measurement of the charge ratio of atmospheric muons with the CMS detector

TL;DR

This study delivers a high-precision measurement of the atmospheric muon charge ratio using CMS data from surface and underground cosmic-ray runs. By employing three complementary reconstruction analyses and meticulous corrections for energy loss and detector resolution, the authors extract R across a broad momentum range, confirming a constant value below ~100 GeV/c and a rising trend at higher momenta due to kaon decays. The work also constrains pion/kaon decay fractions via fπ and fK fits, providing valuable input for hadronic-interaction models and atmospheric neutrino flux predictions. The combination of surface and underground CMS data represents a key cross-check and benchmark for cosmic-ray muon studies with a large, modern detector.

Abstract

We present a measurement of the ratio of positive to negative muon fluxes from cosmic ray interactions in the atmosphere, using data collected by the CMS detector both at ground level and in the underground experimental cavern at the CERN LHC. Muons were detected in the momentum range from 5 GeV/c to 1 TeV/c. The surface flux ratio is measured to be 1.2766 \pm 0.0032(stat.) \pm 0.0032 (syst.), independent of the muon momentum, below 100 GeV/c. This is the most precise measurement to date. At higher momenta the data are consistent with an increase of the charge ratio, in agreement with cosmic ray shower models and compatible with previous measurements by deep-underground experiments.

Figures (6)

• Figure 1: Cosmic-ray muons crossing the CMS detector. The upper two pictures display muons from 2008 underground data, leaving signals in the muon system, tracking detectors and calorimeters. A standalone track (top left) and a pair of global half-tracks (top right) are shown. The bottom plot depicts a muon from 2006 surface data crossing the muon chambers at the bottom of CMS.
• Figure 2: (a) Normalized muon $p_{\mathrm{T}}\xspace$ distributions, for the global (closed circles) and standalone-muon analyses (open circles), at the PCA. Differences in the distributions are expected, as the global and standalone-track fits have different momentum resolutions and acceptances. (b) Comparison of the ($q/p_{\mathrm{T}}\xspace$) resolution estimate $d_{C_\mathrm{T}}$ (closed circles) with the true $C_\mathrm{T}$ resolution (hatched histogram), obtained from simulated global muons.
• Figure 3: Uncorrected charge ratio, together with the statistical uncertainty. (a) From 2006 MTCC data, as a function of the measured muon momentum. (b) For the global (closed circles) and standalone-muon analyses (open circles), as a function of the measured $p_{\mathrm{T}}\xspace$ at the PCA.
• Figure 4: Charge ratio for the surface analysis, as a function of the muon momentum, corrected for energy loss in the detector and for charge misassignment, after propagating the muon track to the entry point in CMS. The thick error bars denote the statistical uncertainty and the thin error bars statistical and systematic uncertainties added in quadrature.
• Figure 5: Muon charge ratio as a function of the muon momentum at the Earth's surface for (a) global and (b) standalone-muon analyses. Open squares indicate the uncorrected ratio, including full alignment. Closed circles show the unfolded charge ratio with statistical errors only. The lines denote the statistical and systematic uncertainties added in quadrature.