[Paper Review] Physics Results From Alpha Magnetic Spectrometer 1998 Shuttle Flight

M. Huang|arXiv (Cornell University)|Apr 13, 2001

Astrophysics and Cosmic Phenomena3 citations

TL;DR

This paper presents physics results from the Alpha Magnetic Spectrometer (AMS) during its 1998 space shuttle mission, detecting approximately 10⁸ events. It reports an antimatter limit of 1.1×10⁻⁶ for rigidity 1 to 140 GV/c, measures primary cosmic ray spectra with a power-law index γ = 2.78 ± 0.021, and identifies atmospheric albedo particles below the geomagnetic rigidity cutoff, including two distinct populations of short- and long-flight-time particles linked to bounce and drift motions, respectively.

ABSTRACT

The Alpha Magnetic Spectrometer (AMS) is a particle detector designed to detect antimatter. During the 10-day test flight on the space shuttle in June 1998, AMS detected $10^8$ events. Upon analysis, no antimatter was found and the antimatter limit was reduced to $1.1 imes10^{-6}$. The proton spectrum shows some differences with the cosmic ray flux used in atmospheric neutrino simulation. A large amount of protons, positrons, and electrons were found below the geomagnetic rigidity cutoff. The energy of these particles are as high as several GeV, one order of magnitude higher than any previously measured energy in radiation belts. These particles also exhibit many interesting features. This paper reviews the results in the four published papers of the AMS collaboration and provides explanation for some features of the albedo particles.

Motivation & Objective

• To measure cosmic ray fluxes and search for antimatter and dark matter signatures using a space-based magnetic spectrometer.
• To study the behavior of charged particles in the Earth's geomagnetic field, particularly those below the rigidity cutoff.
• To identify and characterize atmospheric albedo particles—secondary particles produced in atmospheric interactions and reflected into space.
• To investigate the origin and distribution of positrons and electrons in the radiation belts, especially the high e⁺/e⁻ ratio observed near the magnetic equator.
• To validate models of particle drift and bounce motions in the magnetosphere using flight-time and rigidity distributions.

Proposed method

• Utilized the AMS-01 prototype, a large acceptance magnetic spectrometer with a 1.9-ton Nd-Fe-B permanent magnet, to measure particle rigidity, charge, and trajectory.
• Employed a multi-layered detector system including double-sided silicon trackers (σy = 20 μm, σx = 33 μm), time-of-flight scintillators (120 ps resolution), aerogel Cherenkov counters, and anticoincidence counters.
• Applied a coordinate system with x (non-bending), y (bending), and z (cylindrical axis) to analyze particle trajectories and rigidity.
• Used flight-time distributions (τd/τb) to classify particles into short-flight-time (SFT) and long-flight-time (LFT) groups based on drift and bounce dynamics.
• Modelled the geomagnetic field using spherical harmonics to account for non-dipolar components and their effects on particle trapping and asymmetries.
• Analyzed particle source distributions and flux ratios (e.g., e⁺/e⁻) as functions of magnetic latitude and longitude to probe the east-west effect and field asymmetries.

Experimental results

Research questions

• RQ1 What is the upper limit on cosmic-ray antimatter flux, particularly for anti-nuclei, in the rigidity range 1–140 GV/c?
• RQ2 How do the spectra of primary cosmic rays (protons, helium, electrons, positrons) measured by AMS-01 compare with existing atmospheric neutrino and cosmic ray models?
• RQ3 Why are high-energy protons, positrons, and electrons detected below the geomagnetic rigidity cutoff, and what mechanisms produce them?
• RQ4 What causes the observed bimodal distribution of source positions for long-flight-time albedo particles, and how do they relate to longitudinal drift motions?
• RQ5 What explains the large e⁺/e⁻ flux ratio (up to 4) near the magnetic equator, and is the east-west effect a sufficient explanation?

Key findings

• The antimatter limit for rigidity 1 to 140 GV/c was measured at 1.1×10⁻⁶, setting a stringent constraint on the existence of primordial antimatter.
• The primary cosmic ray spectrum in the 10–200 GeV rigidity range follows a power law with index γ = 2.78 ± 0.021 and normalization Φ₀ = 17.1 ± 2.0 GeV⁻²·⁷⁸/(m² s sr MeV).
• Particles with energies up to several GeV were detected below the geomagnetic rigidity cutoff, indicating the presence of atmospheric albedo particles from atmospheric interactions.
• Two distinct populations of albedo particles were identified: short-flight-time (SFT) particles linked to bounce motion with uniform source distribution across longitudes, and long-flight-time (LFT) particles associated with longitudinal drift, with sources concentrated at two distinct longitudes (east and west of the magnetic prime meridian).
• The e⁺/e⁻ flux ratio reaches up to 4 near the magnetic equator, and this asymmetry is explained by the east-west effect, where higher fluxes from the west enhance positron yield due to geomagnetic field geometry.
• Particles near the rigidity cutoff, especially in high-latitude regions, exhibit irregular trajectories and can drift over multiple drift periods, with source distributions differing from those of particles well below the cutoff.