[Paper Review] Search for Fractional Charges in Cosmic Rays with Ams

C. Sbarra, D. Casadei+4 more|arXiv (Cornell University)|Apr 10, 2003

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This study searches for fractionally charged particles (specifically 2/3e) in primary cosmic rays using data from the AMS-01 experiment aboard the STS-91 shuttle mission. By analyzing energy deposition in the Time-of-Flight (TOF) scintillator counters and applying a fast trigger efficiency model, the authors set a 95% confidence level upper limit of 3.0×10⁻⁷ cm⁻²s⁻¹sr⁻¹ on the flux of such particles, providing a stringent constraint on exotic physics beyond the Standard Model.

ABSTRACT

Preliminary results on the flux of non strongly-interacting, fractionally charged particles in primary cosmic rays at 400 Km above sea level are given. Cosmic ray data collected by AMS-01 in June 1998 have been analysed on the hypotheses of 2/3 charged leptons. The search is carried on by looking at the energy deposition measurements by the time of flight system scintillator counters. A preliminary flux limit is given.

Motivation & Objective

• To search for non-strongly interacting, fractionally charged particles in cosmic rays, specifically 2/3e leptons, as a probe for physics beyond the Standard Model.
• To test the hypothesis that such particles could produce lightly ionizing signals detectable via energy deposition in the AMS-01 Time-of-Flight (TOF) system.
• To establish a flux upper limit for 2/3e particles by analyzing prescaled TOF-triggered events from the 1998 STS-91 mission.
• To evaluate the fast trigger efficiency for low-ionizing particles and compare it to proton-like energy loss distributions scaled by charge squared.

Proposed method

• Used AMS-01 TOF scintillator counters to measure energy deposition (dE/dx) in four planes with 120 ps timing resolution.
• Modeled expected energy loss for 2/3e particles as a scaled version of the proton Landau distribution, proportional to Q².
• Applied the 'spectator plane' method to compute fast trigger (FT) efficiency for each TOF plane using recorded data from non-triggered planes.
• Calculated combined FT efficiency across three of four planes using combinatorial sum of individual plane efficiencies.
• Applied canonical cuts from the official AMS-01 proton analysis: exclusion of South Atlantic Anomaly data, single-cluster per plane, and track quality cuts.
• Used Bayesian statistics to set a 95% confidence level upper limit on signal flux when no signal events were observed (n=0).

Experimental results

Research questions

• RQ1 What is the upper limit on the flux of 2/3e charged particles in primary cosmic rays, assuming they exist as free, weakly interacting leptons?
• RQ2 How does the fast trigger efficiency of the AMS-01 TOF system vary for particles with fractional charge compared to protons?
• RQ3 Can the energy deposition pattern in the TOF system distinguish 2/3e particles from proton background, especially at low ionization levels?
• RQ4 What is the sensitivity of the AMS-01 TOF system to lightly ionizing particles with Q = 2/3e, given its detection thresholds and resolution?
• RQ5 How does the observed data distribution below 0.7 MIP compare to the expected background and signal distributions?

Key findings

• The fast trigger efficiency for detecting 2/3e particles in the AMS-01 TOF system was calculated as ε_FT,2/3e = 0.18, based on individual plane efficiencies of 0.43, 0.71, 0.68, and 0.47.
• The energy loss distribution for 2/3e particles was modeled as a scaled proton Landau distribution, with a clear separation from proton background at 0.7 MIP.
• No candidate events were observed with low energy loss in three or more TOF planes simultaneously, indicating no significant signal above background.
• A 95% confidence level upper limit on the flux of 2/3e particles was set at 3.0×10⁻⁷ cm⁻²s⁻¹sr⁻¹ based on 36,436 seconds of observation time.
• The analysis suggests that the full AMS-01 dataset could improve this limit by a factor of 10, and AMS-02 on the ISS could improve it by another factor of 100.