Precision Measurements in Intermediate Energy Physics

Trustees Of Boston University, Boston

Investigators

Abstract

Muons are very much like the familiar electron, with many of the same characteristics (e.g. intrinsic spin and electric charge) but approximately 200 times more mass. Those similarities and that difference make muons a sensitive tool for exploring new physics and will be exploited in two experiments supported by this award: the MuSun experiment at the Paul Scherrer Institut (Villigen, Switzerland) and on the muon g-2 experiment at the Fermi National Accelerator Laboratory (FNAL,, near Chicago). Proton-proton fusion is the initial nuclear reaction in a chain of reactions which are the source of the energy produced by our sun. The rate of this fusion reaction cannot be measured in the laboratory nor has it been calculated from first principles. In the MuSun experiment, muons will be used to study something like proton-proton fusion in reverse. When a muon is captured by a deuterium nucleus, consisting of a neutron and a proton, the deuterium breaks apart into a pair of neutrons and a muon neutrino. The measurement of muon capture by deuterium will provide the necessary information to make possible a first principles calculation. That calculation will also provide a more solid theoretical foundation for the results concerning neutrino characteristics. The new muon g-2 experiment at FNAL aims to measure the anomalous magnetic moment of the muon with unprecedented precision. Because the muon has intrinsic angular momentum (spin) and an electric charge, it also possesses a magnetic moment, that is, it behaves like a tiny magnet. The relationship between the angular momentum and magnetic moment is described by the gyromagnetic ratio or g factor. For the electron and the muon, g is very slightly greater than 2. This difference from 2 is the so-called anomaly. The anomaly, for both electrons and muons, can be measured and calculated with great precision. Any significant disagreement between the two is a hint of new physics. The technique for measuring the capture rate is simple. Negative muons are stopped in a detector filled with ultra-pure deuterium gas. The experiment will measure the disappearance rate of the muons, similar to any radioactive decay experiment. Because of the capture process, the disappearance rate will be slightly larger than that of free muon decay to electrons plus neutrinos (of both electron and muon types) - the difference is the capture rate. The sensitivity goal of 1.5 percent on the capture rate will require the collection of approximately 20 billion muon disappearance events. To measure the anomaly, muons are injected into a storage ring, a doughnut shaped device, roughly 44 m in circumference, which guides them in roughly circular orbits. As the muons circle the storage ring, their spin vectors, which act like gyroscopes, turn faster than their momentum vectors. The rate of the precession, which is extracted from the time distribution of muon decay electrons, is directly proportional to the anomaly. The experimental goal, a fractional error on the anomaly of approximately one part in ten million, should provide a stringent test of possible new physics.