BackgroundsPhysIntro

Introduction

The Mu2e Experiment is hunting for an extremely rare process, even if we are fortunate to have a "strong" signal. It is therefore critical for us to keep our backgrounds low and for these backgrounds to be well-understood. Our backgrounds include anything that can be mistakenly selected as an electron from a muon conversion. This includes electrons from other sources that have either had their momenta mis-measured, or that happen to have the same momentum as our signal electrons. But the backgrounds can also come from particles that are mis-identified as electrons. We have sources of backgrounds both from the running of the experiment (beam-related backgrounds, cascades of particles in the detector due to other interactions) and also from nature (in the form of cosmic rays, or other natural sources of radiation.) Our ability to control and understand these backgrounds is directly tied to the sensitivity of our measurement. The primary sources of background are described below.

Decay In Orbit

When a "free" muon---one that is not in the field of a nucleus---decays, there is a limit to the energy that the outgoing electron can achieve. If we consider a muon at rest, the maximum energy is achieved when the two neutrinos from the decay are produced in the opposite direction of the electron. (The total kinetic energy of the outgoing particles is determined by the difference between the muon rest mass and the masses of the outgoing particles, in order to conserve energy. To maximize the kinetic energy of the outgoing electron and satisfy conservation of momentum, we need to have the electron produced back-to-back with the two neutrinos. Any other arrangement would reduce the kinetic energy of the outgoing electron.) If we consider that we must conserve energy and momentum, this upper limit can be found by subtracting the rest mass of the electron from the rest mass of the muon, and dividing the result by two, giving an upper limit of approximately 53 MeV, or half the mass of the muon. This is safely below our signal value of 105 MeV

If the muon decays while it is in the orbit of a nucleus, however, it can exchange momentum with the nucleus. In the extreme (and rare) case where the outgoing neutrinos carry very little momentum, this can look like the two-body decay that we expect from the muon conversion signal, where the outgoing electron recoils off the nucleus. Precisely measuring the spectrum of these decays, which approach the signal energy, is an important part of the Mu2e analysis plan, and will allow us to predict this small, but challenging, background.

Radiative Muon Capture

We can consider muon capture in two categories: ordinary and radiative. In ordinary muon capture in Al, a neutrino and Magnesium nucleus are the products, with the proton converted to a neutron through the exchange of a W boson. In radiative muon capture, there is also a photon produced. The photon can pair-produce, giving an electron and positron, which can then go on to interact with the Mu2e detector.