OPERA measured the cosmic muon charge ratio
Muons from cosmic rays rain down from the atmosphere all the time. If you were an empiricist, what would you do? You might compare the number of positive to the number of negative muons, and find that the ratio (pos/neg) is not one – it is about 1.3.
Why is that interesting now, many decades after the first studies of cosmic rays were conducted? Answer: this ratio is different from one due to identity of the cosmic particles that impinge upon the atmosphere, and due to the particles physics of the cascades they cause. Specifically, the production of pions, kaons and even charmed hadrons play a role that varies with the energy of the cascade. A high charge ratio may be linked to kaons, since there is an asymmetry in the production of K+ and K-, or to a pure proton flux; a ratio close to one hints at heavy nuclei with a nearly equal number of protons and neutrons, and a predominant production of pions in the cascade for which there is no charge asymmetry.
Let me point out that the “chemical composition” of cosmic rays is an interesting topic – see an earlier post about extra-galactic iron nuclei detected by the Pierre Auger Collaboration.
The OPERA experiment was designed and built to study neutrino oscillations. It was installed deep underground in the Gran Sasso Laboratory (Italy), so the cosmic rays that pass through the apparatus have a high energy, on the order of a TeV. During the course of four months, they recorded a few thousand good events with a trajectory that allowed the authors to measure the momentum and charge. They posted their paper on the archives yesterday (arXiv:1003.1907).
For the purposes of this measurement, the OPERA detector is essentially a double-arm spectrometer which can give two measurements of the deflection of a muon in a 1.53 T magnetic field. The size of the deflection tells you the momentum, while the direction of the deflection tells you the charge of the muon. The trajectory is constructed from hits recorded in resistive plate chambers. The problem of false charge measurements due to bad tracking or large multiple scattering can be controlled adequately by comparing the two measurements. The simulation underestimates these effects somewhat, but not enough to spoil the measurement. Special attention is paid to the alignment of the chambers.
The vertical axis is the ratio Rμ of positive to negative muons at the surface of the earth – the authors corrected their observations made in the underground cavern in Gran Sasso for the propagation of muons through the rock above. The horizontal axis is not just the muon energy Eμ, it includes a factor cos(θ*) which takes into account the inclination of the cascade, the radius of the earth and the height of the cascade.
In the figure, the OPERA points are five filled circles above 103 GeV. The curves represent the predictions of four models of cascades in the atmosphere produced by cosmic rays. The OPERA measurements are not able to distinguish those models, but they do agree with and extend prior measurements by MINOS (another underground neutrino experiment) and an experiment in Utah (1975).
This is a nice measurement which adds incrementally to our empirical knowledge of cosmic rays.
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