User:Benkrikler/comet

= Introduction = The COMET experiment is searching for the process of COherent Muon to Electron Transitions, whereby a muon converts to an electron in the coulomb field of an atomic nucleus without emitting any neutrinos. Such a process is forbidden in the Standard Model of particle physics and would therefore be a clear sign of New Physics Beyond-the-Standard-Model.

Based at J-PARC in Tokai, Japan the COMET experiment will be built in a staged approach with the final stage, Phase-II scheduled for the early 2020s. The present limit on the conversion rate (see below) has been set by the Sindrum-II collaboration in 2006 at 5x10^-13 \cite(SINDRUM_II) using a gold target nucleus but COMET hopes to extend this by four orders of magnitude to around 10^-17 by Phase-II \cite(TDR2014) with a target made of aluminium.

= Signal and Backgrounds = The process  of muon-to-electron  conversion  begins  with  the stopping  of  a negative muon around the  nucleus of an atom. Once the muon becomes trapped in one of the atomic  orbitals (which are analogous to the  hydrogen atom, but with the  mass of  the electron  replaced  by that  of the  muon) an  electromagnetic cascade producing characteristic X-rays and Auger electrons brings the muon down to the ground  state. From there, the  muon will  either:


 * undergo bound decay (emitting two neutrinos and an electron but  with a spectrum that differs to the normal Michel decay due to the presence of the nucleus),
 * capture against the nucleus in a process similar to inverse beta decay
 * convert (neutrinoless-ly) to an electron

In general, all of these processes have the potential to alter the configuration of the nucleus  itself. However, in the  case of  COMET looking  for coherent muon-to-electon conversion, where the nucleus is treated as  a single particle, the nucleus is  left unchanged. As a result all  of the  free energy  of the initial muon (which is the muon mass, $M_\mu$, minus the atomic binding energy, $E_b$) goes to the out-going particles and since there are only two (the nucleus and the electron) the electron has a mono-energetic energy, $E_e$, of: $$ E_e = M_\mu - E_b - E_{Recoil} $$ where $E_{Recoil}$ is the kinetic energy of the recoiling nucleus. In COMET, which uses a target made of Aluminium, the signal electron's energy is about 104.97 MeV \cite{TDR2014, Czarnecki 2011}

The lifetime of the muon once its in the bound state is an important parameter for COMET. In aluminium the lifetime of the muon is around 847 ns \cite{measday}.

Backgrounds
Since the COMET signal is a single 104.97 MeV electron, any process that could also produce such an event will create a background. As the limits for muon-to-electron conversion are already very tight, it is important that any background processes are well controlled.

One such process is the Standard Model decay of a muon around the nucleus. The decay at rest of a free muon can only produce electrons with energies below half the muon mass (so around 53 MeV). Although this is  considerably below  the muon-to-electron conversion signal, when  the same  process  occurs  in  the  presence of  the  nucleus (bound muon-decay) the spectrum of  electron energies is  considerably modified \cite{Czarnecki 2011}. This results in a long tail extending all the way up to the signal energy. If the detector's resolution is poor the spectra of this steeply falling tail and the mono-energetic conversion signal will be smeared on top of one another.

Another process that could give rise to backgrounds events comes from any pions that remain in the muon beam. The capture of the pions against the nucleus results in gamma and x-ray emission, which could intern produce a signal-like electron by subsequent interaction with material in the target or detector. Since this process is very fast compared to the signal, using a pulsed beam -- where nearly all muons / pions arrive within 150 ns -- allows these prompt processes to be ignored.

Additional backgrounds: - Radiative muon capture: Muons can also produce photons when the decay or capture. These occur in time with the signal process, however since these are generally lower in energy than needed to produce a Signal-like electron, they are generally not a concern for COMET. - Electrons in the beam: Both and pions and muons in the incoming beam can decay in flight to electrons. When this happens, if the parent pion or muon is sufficiently energetic, the outgoing electron can be boosted to the signal region. Delayed timing cuts and tuning of beam acceptance to remove high momentum muons and pions help reduce this background. - Anti-protons in the beam: Anti-protons when captured in the target or other material can produce pions which can then produce signal electrons via a photon. Since the anti-protons are much heavier than the other particles they tend travel much more slowly through the beam line and therefore can arrive much after the rest of the main pulse.

Solutions: - Pulse muon beam, use timing to remove RPC backgrounds - Proton beam below the Anti-proton threshold - Low acceptance beamline for particles with momenta away from optimum muon momentum ~40 MeV

Conversion Rate and Signal Sensitivity
Typically, the rate of conversion is defined with respect to the rate of muon capture: $$ \frac{\Gamma(\mu\textrm{-}e~\textrm{conversion})}{\Gamma(\mu~\textrm{capture})} $$ where $\Gamma(X)$ is the width of the process $X$. This ratio is preferable to the more typical Branching Ratio used in particle physics  since it reduces the impact of uncertainties on the muon wave-function on theoretical calculations.

To quantify the effectiveness of the experiment's ability to observe the signal, typically the value known as the Single-Signal-Event Sensitivity is given which is defined by the minimum conversion rate where a single  signal event would be seen during the lifetime of the experiment. A conversion rate of less than this value and no signal events could be expected. In COMET Phase-I, a Single-Signal-Event Sensitivity of $3\times10^{15}$ is expected but by Phase-II this is expected to reach around $3\times 10^-{17}$.

= Experiment Overview =

COMET (Phase-II)
- C-shaped muon beam transport with a tuneable dipole field - Tungsten production target - Aluminium stopping target - Bent solenoid after target with tuneable dipole field - Straw tracker and ECAL

Phase-I
- Motivation: To understand pion capture and bent solenoid system, proto-type the final detector and to make a physics measurement to 10^-15 - Production target (Graphite, no cooling), bent muon transport solenoid (90 degree) and two different detector systems: - Detectors: * Physics and background measurement: CyDet for low occupancy and reduce impact of beam flash on radiation hardness requirements and detector dead-time * Phase-II prototype, background and beam characterisation: StrECAL, on axis and same detector technology

Schedule
- Construction images

= Theoretical Motivations = Since the discovery of neutrino oscillations, the concept of Lepton Flavour Conservation is known to be invalid. This immediately allows for Flavour violation amongst the Charged Leptons through Feynman Diagrams involving an oscillating neutrino. However, since each of these diagrams will also involve a W or Z boson, they are suppressed to orders of: $$ \frac{\Delta M_\nu^2}{M_W^2} \sim 10^-54 $$

This being well beyond the present experimental capability,  any observation of the  process of  mu-e conversion  would  be a  sign of  physics beyond  neutrino oscillations as well as beyond the Standard Model.

There are  many  models  that   predict  observable  rates  however,  including Supersymmetry, Little Higgs,  Extra Dimension  theories, Lepto-quark  models or Z-prime models. The fact that so many models produce this  process is a double edged sword: whilst  it gives  exposure to  many different  theories should  an observation at COMET  take place it would  be difficult to know  which model was responsible. At that point, the effect of varying the target  material in COMET could be  studied. This consideration  also  high-lights the  complimentarity between COMET and other related  searches such as  for mu->e gamma  and mu->3e, both of which are being studied with dedicated experimental programs.

= The collaboration = - As of September 2015 the collaboration consists of about 170 members, < Collaboration list > - Full collaboration list at:

= See Also = Muon decay Michel parameters Mu2e MEG Mu3e ?

= Links =
 * 1) [1](2014 Phase-I TDR)
 * 2) [2](Yoshi Kuno's muon physics review)
 * 3) [3](Measday et al. muon capture / decay rates)
 * 4) [4](Experimenter's guide to LFV processes)
 * 5) [5](COMET web-site)
 * 6) [6](Czarnecki 2011)