The MEG experiment at PSI reaches an unprecedented level of precision in the search for the μ→eγ decay

The international collaboration forming the MEG-experiment, one of PSI’s ¹ “flagship” particle physics experiments at the proton accelerator facility in Switzerland, is publishing its latest results in the search for “New Physics” beyond the Standard Model (SM), in the journal Physical Review Letters. The goal of the experiment is the search for the existence of the decay μ→eγ, a quest that started more than sixty years ago using cosmic-ray muons and now uses one of the world’s most intense surface muon beams. Data collected by MEG in 2009-2010 shows no evidence, as yet, of an excess of events above the expected background. However, even while collecting thirty-million decays a second in the detector, the experiment will require about two more years of data-taking to reach the expected sensitivity. This new MEG result does however exclude, with 90% confidence, that more than one muon in approximately five-hundred billion, does decay into a positron (positive electron) and photon. This therefore translates into the most stringent constraint on the existence of the μ→eγ decay to date (Branching Ratio B ≤ 2.4·10^-12, 90% C.L.) and improves the previous best limit by a factor of five.

Figure 1. Shows a reconstructed candidate event (run77431-event1715) with a high relative signal-like likelihood. The 900 litre C-shaped LXe detector with its 846 photo-multipliers (PMTs), used to detect the light from the interacting photon, is shown on the right. The colour of each PMT reflects the amount of light it has detected (blue=low, orange=high), while the total amount of summed light reflects the energy of the incident photon. The curved track on the left comes from a positron curling-up in the magnetic spectrometer which measures its energy and time using a set of central tracking chambers (yellow) and scintillation counter bars (green) respectively.

The search for this process in a low-energy, high intensity and high precision type of experiment is a powerful means of investigating promising “New Physics” models such as Supersymmetric Grand Unified theories (SUSY-GUT) or theories with extra dimensions and is thus complementary to such searches at high-energy TeV-scale accelerator facilities such as that of LHC at CERN. Contrary to the SM, which is thought to be a low-energy approximation of a more complete theory at high energies and which to date, provides the best overall description of the behaviour of elementary particles but also effectively forbids the μ→eγ decay; there are many candidate models of theories beyond the SM, that predict sizeable μ→eγ decay rates and that are within the reach of the MEG-experiment. Hence, the observation of this particular decay would be an unambiguous signature of “New Physics”, while the lack of observation, at a given sensitivity, would mean excluding these models. To date, the set MEG limit already excludes some large regions of parameter space of certain of the more interesting new physics models.

Owing to the rarity of the simple 2-body muon decay and the high beam intensity required, elaborate, ”fore-front” technology is necessary in order to detect the coincident and back-to-back decay products emerging from the muon stopping target, in particular, the use of the world’s largest liquid xenon detector, containing some three tons of the cryo-fluid to detect the photons from the decay, by means of the ultraviolet light emitted at their interaction in the liquid.

The MEG-experiment is currently data-taking and will continue to do so, in its present form, in order to achieve the increased sensitivity, or as aimed, detect the long sought-after μ→eγ decay; without doubt the MEG-experiment will contribute significantly to a better understanding of the fundamental aspects of the building-blocks of nature.

The MEG collaboration consists of some 60 scientists from Japan (ICEPP Tokyo, Waseda Tokyo, KEK), Italy (University & INFN: Genova, Lecce, Pavia, Pisa & Roma1), Switzerland (PSI), Russia (JINR Dubna and BINP Novosibirsk) and USA (UCI).

1. The Paul Scherrer Institute (PSI) is the largest research centre for natural and engineering sciences within Switzerland, its research activities are concentrated on three main subject areas: Structure of Matter, Energy and the Environment and Health

Contacts