MeasurementIntro

Introduction to the Measurement

The Mu2e Experiment will measure the rate of muons that convert directly into electrons in the field of a nucleus. This is a process that changes the lepton's "flavor", where the flavor signifies the type as electron, muon or tau. While we have seen flavor oscillation in neutrinos---the neutral leptons that also come in three flavors: electron neutrinos, muon neutrinos and tau neutrinos---this is all but forbidden in the standard model for the charged leptons (electrons, muons and taus.) It is suppressed due to the large masses of the charged leptons. The standard model prediction for the conversion rate of a muon directly into an electron is one in approximately every 10^54. Mu2e will be sensitive to measuring a rate of approximately one in every 10^17 muon decays, so if we see a signal it will be a clear sign of physics beyond the standard model.

The Technique

The typical standard model decay of a muon results in three final state particles: a muon neutrino carries "muon" flavor making this a flavor-conserving process, an electron carries the same electric charge making this a charge-conserving process, and an anti-electron neutrino cancels out the electron flavor that was introduced keeping the decay flavor-conserving. The difference between this standard decay and the decay that we are searching for with Mu2e is that our signal process does not result in the production of those two neutrinos. Neutrinos are notoriously difficult particles to detect so we do not have the luxury of being able to tag them when they are present and notice that they are missing when the signal conversion occurred. Instead, we will use the ideas of conservation of energy and momentum to determine if the muon converted to an electron, or if it proceeded through the standard decay channel and also produced the two neutrinos.

Muons are about 200 times more massive than electrons. In order to account for this mass difference, when a muon decays to an electron and two neutrinos, these three decay products share kinetic energy to account for the missing mass. When, however, the muon converts to an electron, the entire difference in mass between the muon and electron must be converted into kinetic energy of the resulting electron. This means that there is a characteristic energy at which these conversion electrons are produced, at around 104.9 MeV. An electron that was part of a typical three-body decay of a muon would have a smaller amount of kinetic energy to pick up in the decay, since some energy would go to the neutrinos. An electron will exactly this predicted energy is a characteristic of the conversion signal.

One important caveat is that this 104.9 MeV signal will be washed-out if the muon itself carries any kinetic energy when it decays. We therefore have a "stopping target" that will stop the muons before the decay takes place. If the muon is stopped, it only contributes the difference between it's rest mass and the electron's rest mass to the electron energy, and we expect a clear signal at 104.9 MeV.

Why do we refer to this conversion as happening in the field of the nucleus? There is another important conservation principle at work: conservation of momentum. If you stop a muon and want it to convert to an electron, you cannot conserve momentum while also expecting the produced electron to head off in some direction, carrying kinetic energy. In the frame of the stopped muon, you begin with zero momentum, and if the electron moves in some direction---as it has to, to account for its mass difference with the muon---it will have a non-zero momentum. The way we enable this conversion to occur is to stop the muon in the field of a nucleus. Momentum is conserved in the conversion when the nucleus experiences a recoil against the outgoing electron.

Mu2e uses gradient magnetic fields to steer the produced electrons such that they interact with our detector, which has excellent momentum resolution and can efficiently tag electrons that pass through it with the characteristic 104.9 MeV energy predicted in the muon to electron conversion.

Overview of Theories that Mu2e Will Probe

MSSM with right handed neutrinos

SUSY with R-parity Violations

Leptoquarks

New Gauge Bosons

Large Extra Dimensions

Non-Minimal Higgs Structure

A Brief History of the Measurement

The first search for muon to electron conversion was by Lagarrigue and Peyrou in 1952 [1], with many other searches carried out since then [2-9]. In this section we will more briefly describe a few of the more recent searches.

[1] A. Lagarrigue and C. Peyrou, Comptes Rendus Acad. Sci. Paris, 234, 1873(1952). See also J. Steinberger and H. Wolfe, Phys. Rev. 100, 1490 (1955).
[2] M. Conversi et al., Phys. Rev. D122, 687 (1961).
[3] R. Sard et al., Phys. Rev. 121, 619 (1961).
[4] G. Conforto et al., Nuovo Cimento 26, 261 (1962).
[5] J. Bartley et al., Phys. Lett. 13, 258 (1964).
[6] D. Bryman et al., Phys. Rev. Lett. 28, 1469 (1972).
[7] A. Badertscher et al., Phys. Rev. Lett. 39, 1385 (1977).
[8] S. Ahmad et al., Phys Rev. D38, 2102 (1988).
[9] W. Bertl et al., Eur. Phys. J. C47, 337 (2006).