Experimental searches for muon decays beyond the Standard Model

Abstract

The study of muon properties and decays played a crucial role in the early years of particle physics and contributed over decades to build and consolidate the Standard Model. At present, searches for muon decays beyond the Standard Model are performed by exploiting intense beams of muons, and plans exist to upgrade the present facilities or build new ones, which would open new prospects for the quest of new physics in this sector. In this paper I review the present status of the search for muon decays beyond the Standard Model, with a special attention to the most conventional muon lepton flavor violation experiments, but also considering more exotic scenarios and future outlooks. Experimental techniques for muon decay studiesAs already mentioned, the muon was discovered and firstly studied in the cosmic ray radiation. A significative step forward in the study of its properties and decay modes was performed when pion beam became available in the years 1950s. Pions can be stopped in a target and the muons produced in their decays can be studied. Finally, the first muon beams were delivered in the years 1970s.Muon beams are usually produced starting from a proton beam impinging on a target and producing pions. Pions decaying in the target itself produce muons that can be collected and transported by a beam line toward the experimental areas where the detectors are installed. The most intense continuous muon beams in the world [10,11] are currently delivered at the Paul Scherrer Institut (PSI), in Villigen (CH). A 2 mA current of protons of 590 MeV kinetic energy from the PSI Ring Cyclotron hits two different graphite targets where, in total, ∼ 18% of the protons are stopped, the rest being preserved to serve a downstream neutron spallation source. The two targets serve different beam lines. In order to get an intense, pure and monoenergetic muon beam, particles with momentum p ∼ 28 MeV/c are selected, corresponding to muons emitted by pions decaying at rest. It happens when the pion decays right on the surface of the production target (surface muons). The beam can be further purified by selecting particles with a given velocity (and hence a given mass) by means of a Wien filter, a superposition of orthogonal electric (E) and magnetic (B) fields through which only particles with the desired velocity v = E/B go straight, while others are removed by collimators. Beams with up to a few 10 8 surface muons per second can be currently delivered at PSI.In the Ring Cyclotron at PSI protons travel in bunches with a repetition rate of about 50 MHz. The pion lifetime (∼ 26 ns) is large enough to spoil the time structure of the beam and make a practically continuous muon beam. For some applications, pulsed beams are instead necessary. For instance, the Delivery Ring at Fermilab (Batavia, USA) used for the Mu2e experiment [12] provides protons in bunches separated in time by 1695 ns. They hit a target and produce muons that mostly arrive to the experimental apparatus within a few hundred ns. After about 700 ns and before the nex...

Testing Lorentz-symmetry violation via electroweak decays

“…eqs. (22), (28), (32) and (44)). It is important to remember that the bounds quoted above were obtained in the rest frame of the decaying particles, but he LSV parameters are not static as seen from the particle's own rest frame, not to mention from Earth's rotating reference frame.…”

“…Now we can translate the bounds in eqs. (22), (28), (32) and (44) -obtained in the rest frame of the decaying particles -into the SCF. In this frame the bounds for time-like components read…”

“…ref. [32] and references therein). By using the data presented in the latest edition of the Particle Data Group [33], we are able to constrain combinations of the LSV coefficients (cf.…”

Modeling that Predicts Elementary Particles and Explains Data about Dark Matter, Early Galaxies, and the Cosmos

“…The four K _ terms correlate with the other term to the right of the equals sign in equation (4). The sum of the two K 0_ terms correlates with the factor D + 2ν SA in equation (7). Equations (7) and (8) characterize solutions.…”

“…Our work does not necessarily automatically embrace assumptions that ongoing modeling tends to embrace. Table 8 provides details that lead to solutions that equations (7) and (8) characterize. We consider equations (3), (4), and (6).…”