Exploring New Physics with Mu2e: The Search for Leptoquarks

[Naeem Anwar]

Just read a report on Muon to Electron conversion experiment ground breaking at Fermilab today. What kind of new physics one can probe from this flavor changing among leptons?

http://www.symmetrymagazine.org/article/april-2015/mu2e-breaks-ground-on-experiment

 

[Envelope]

Models of new physics that introduce new particle species or new couplings (types of interactions beyond the familiar electromagnetic, weak, and strong) can lead to an observable rate of muon-to-electron conversion, and there are loads of such models in the wild. New particles could come about due to supersymmetry, the presence of a heavy neutrino, or an extended Higgs sector (i.e., more Higgs particles than are absolutely necessary for electroweak symmetry breaking). New heavy bosons could lead to new interactions that we haven't yet probed. (This is analogous to how the weak force is negligible at low energies: it is mediated by heavy gauge bosons. If there are even heavier ones out there, perhaps they lead to additional forces that we have to work hard to detect.) "Compositeness" is the idea that elementary particles aren't actually elementary, and the heretofore unnoticed substructure could lead to anomalous interactions across lepton families. "Leptoquarks" is a generic term for anything that allows the conversion of leptons to quarks, and vice versa, so leptoquarks could evade the approximate lepton flavor conservation in the Standard Model.

All these and more could lead to muon-to-electron conversion. If a signal is observed, it will be Earth shattering, but it will also be difficulty to pick apart what sort of new physics is the cause using just the Mu2e experiment.

 

[ChrisVer]

I think the heavier gauge bosons can't give you a direct muon to electron transition (since they would probably have to couple with the neutrinoes as well). Instead the process they are talking about is (at least what I got from the article) $\mu \rightarrow e \gamma$.
This interaction would be detectable either by looking at the electron's energy (in this case it can be discrete-in contrast to the weak-interaction muon decay) and the photons (for energy conservation) can be detected quite easily. So such a process would give a pretty-clear signal.
This process is obviously violating the leptons' generation number and up to now it's the main reason it's unobserved. Theories that allow for lepton number violation can explain such a process.

 

[mfb]

There are three related muon processes physicists are looking for, experiment names in brackets:

μ -> eee ("Mu3e")
μ -> eγ ("MEG")
μ -> e ("Mu2e" - where 2 means "to", not "two")

Mu2e is looking for a direction conversion inside a nucleus, the signature would be an electron with a kinetic energy of (nearly) the muon energy. The other two experiments look for normal decays and reconstruct the invariant mass of the products.

All three experiments look for peaks at the muon mass, with a continuous background up to this same energy. They all need a good energy resolution to have as few background events as possible in their signal region.

Everything that violates lepton flavor can lead to those processes.
Expected sensitivities for branching fractions I found: about 10^-16 for Mu2e and Mu3e, about 10^-13 for MEG. That allows to test energy scales way beyond the reach of the LHC (1000+ TeV) in some models.

 

[ChrisVer]

Mu2e is looking for a direction conversion inside a nucleus, the signature would be an electron with a kinetic energy of (nearly) the muon energy. The other two experiments look for normal decays and reconstruct the invariant mass of the products.

Didn't we say that this would violate the energy somewhere else? Or is a nucleus recoil going on?

 

[mfb]

The nucleus takes the recoil, right.

 

[ChrisVer]

Ok ;)

 

[Naeem Anwar]

Dear Envelope,

Thanks! its interesting, indeed. Hope so if they are enough lucky they will probe new physics signal by using this single channel.

"Compositeness" is really attractive idea, could you guys explain lepto-quarks a little more? Its expected nature? Quantum numbers etc.