[Paper Review] Measurement of the atmospheric muon charge ratio with the OPERA detector

OPERA Collaboration, N. Yu. Agafonova+183 more|arXiv (Cornell University)|Mar 9, 2010

Astrophysics and Cosmic Phenomena2 references5 citations

TL;DR

This paper presents a high-precision measurement of the atmospheric muon charge ratio $ R_{\mu} = N_{\mu^+}/N_{\mu^-} $ using the OPERA detector at the Gran Sasso underground laboratory. Analyzing 403,069 atmospheric muons over 113.4 days of livetime, the study corrects for charge-misidentification using Monte Carlo simulations and determines the pion and kaon charge ratios as $ R_\pi = 1.229 \pm 0.001 $ and $ R_K = 2.12 \pm 0.03 $, providing strong constraints on atmospheric muon production models at TeV energies.

ABSTRACT

The OPERA detector at the Gran Sasso underground laboratory (LNGS) was used to measure the atmospheric muon charge ratio in the TeV energy region. We analyzed 403069 atmospheric muons corresponding to 113.4 days of livetime during the 2008 CNGS run. We computed separately the muon charge ratio for single and for multiple muon events in order to select different energy regions of the primary cosmic ray spectrum and to test the charge ratio dependence on the primary composition. The measured charge ratio values were corrected taking into account the charge-misidentification errors. Data have also been grouped in five bins of the "vertical surface energy". A fit to a simplified model of muon production in the atmosphere allowed the determination of the pion and kaon charge ratios weighted by the cosmic ray energy spectrum.

Motivation & Objective

• To measure the atmospheric muon charge ratio $ R_{\mu} $ in the TeV energy range using the OPERA detector at the Gran Sasso underground laboratory.
• To correct for detector-induced charge-misidentification errors in muon charge reconstruction using Monte Carlo simulations.
• To determine the pion and kaon charge ratios $ R_\pi $ and $ R_K $, weighted by the cosmic ray energy spectrum, from the measured $ R_{\mu} $.
• To investigate the potential contribution of prompt muons from charmed particle decays in the high-energy region.
• To assess the sensitivity of the measurement to the primary cosmic ray composition through separate analysis of single- and multiple-muon events.

Proposed method

• The OPERA detector collected 403,069 atmospheric muons over 113.4 days during the 2008 CNGS run, with data grouped by 'vertical surface energy' $ \mathcal{E}_{\mu}\cos\theta $.
• Muon charge ratio $ R_{\mu} $ was computed separately for single- and multiple-muon events to probe different regions of the primary cosmic ray spectrum.
• Charge-misidentification errors were corrected using a matrix unfolding method based on Monte Carlo simulations, assuming symmetric misidentification rates $ \eta^{+-} = \eta^{-+} = \eta $.
• The unfolding formula $ R = \frac{(1-\eta)\hat{R} - \eta}{-\eta\hat{R} + (1-\eta)} $ was applied to obtain the true $ R_{\mu} $ from reconstructed $ \hat{R} $.
• Errors were propagated using the formula $ \delta R = \frac{\sqrt{(1-2\eta)^2(\delta\hat{R})^2 + (\hat{R}^2 - 1)^2(\delta\eta)^2}}{[\eta\hat{R} - (1-\eta)]^2} $, with no regularization to preserve statistical fluctuations.
• A simplified model of atmospheric muon production was fitted to the data to extract $ R_\pi $ and $ R_K $, with no significant change upon inclusion of prompt muon contributions.

Experimental results

Research questions

• RQ1 What is the value of the atmospheric muon charge ratio $ R_{\mu} $ in the TeV energy range, as measured by the OPERA detector?
• RQ2 How do charge-misidentification effects in the detector influence the measured muon charge ratio, and how can they be corrected?
• RQ3 What are the pion and kaon charge ratios $ R_\pi $ and $ R_K $, weighted by the cosmic ray energy spectrum, as inferred from the muon data?
• RQ4 Is there evidence for a non-negligible contribution from prompt muons (e.g., from charmed meson decays) in the high-energy region of the muon spectrum?
• RQ5 Does the measured $ R_{\mu} $ depend on the primary cosmic ray composition, as probed through single- versus multiple-muon event selection?

Key findings

• The measured muon charge ratio $ R_{\mu} $ was corrected for charge-misidentification using Monte Carlo simulations, with $ \eta^{+-} = \eta^{-+} = \eta $, ensuring symmetric detector response.
• The pion charge ratio was determined as $ R_\pi = Z_{N\pi^+}/Z_{N\pi^-} = 1.229 \pm 0.001 $, consistent with expectations from pion decay kinematics.
• The kaon charge ratio was measured as $ R_K = Z_{NK^+}/Z_{NK^-} = 2.12 \pm 0.03 $, indicating a significant asymmetry in kaon production.
• The inclusion of a prompt muon component in the fit did not alter the values of $ R_\pi $ and $ R_K $, suggesting no significant contribution in the measured energy range.
• The measurement lies in a region where charmed particle decays may begin to contribute, highlighting the need for larger statistics or deeper detectors to probe $ \mathcal{E}_{\mu}\cos\theta^* \gtrsim 10 $ TeV.
• The data collected at the end of OPERA's scientific program will enable improved measurements in the high-energy region, particularly for testing prompt muon contributions.