Cosmic-ray muon charge ratio
The MINOS collaboration found that the muon charge ratio varies as a function of the muon energy. The black squares represent the MINOS measurements and the triangles represent results by CERN's L3+C experiment.
Physicists typically study pions and kaons by using particle accelerators. Deep underground in a mine in Minnesota, an analysis team using the MINOS detector has discovered how pions and kaons from cosmic rays behave at TeV energies at the surface above the mine.
The MINOS far detector was built in a mine one half-mile underground in northern Minnesota to study neutrinos generated at the Fermilab Main Injector. As is true for other experiments, the MINOS physics reach extends beyond this primary mission. As the first large underground detector with a magnet, a team of physicists exploited the MINOS magnetic field to investigate the electric charge of the cosmic ray muons energetic enough to penetrate to the detector.
The MINOS collaboration measured the muon charge ratio, or the ratio of the number of detected positive to negative muons. The main results of this study, which were reported at the recent International Cosmic Ray Conference in Mexico, are shown in the figure.
Cosmic ray muons, which come mostly from the decay of pions and kaons generated in interactions near the top of the atmosphere, are the only particles - other than neutrinos -- that can reach the detector underground. MINOS measured the charge ratio underground using muons carefully selected to minimize systematic errors and found the value of 1.374 with an error of about 1 percent.
Since muons continuously lose energy at a well-known rate as they pass through rock, the distance that they travel provides a good estimate of their surface energy. The figure shows the charge ratio as a function of the muon energy at the surface. The MINOS measurements are the first high statistics determination of the muon charge ratio above 1 TeV.
In the energy range of 100 MeV to 300 GeV, many experiments find a charge near 1.25, significantly lower than the MINOS value (see figure). Building on previous work, Philip Schreiner, Benedictine University, constructed a model for the energy dependence of the charge ratio that describes this rise as a consequence of the kinematics of pion and kaon decay. The "πK" model provides an excellent fit to the data, as well as a way of estimating the K+/K- and π+/ π- ratios from cosmic rays for the first time. This result was featured prominently as one of the highlights of the ICRC conference. A paper describing these results can be found here.
The analysis team included Maury Goodman (Argonne), Stuart Mufson (Indiana University, pictured), Brian Rebel (Fermilab), Juergen Reichenbacher (Argonne), and Philip Schreiner (Benedictine University).