Hunt for a supersymmetric neutrino

The above image shows the expected and observed 95 percent confidence level upper limits on cross section multiplied by a branching ratio of a supersymmetric neutrino produced in quark-antiquark annihilation and decaying via lepton-flavor-violating interactions into (electron and muon) or (muon and tau) or (electron and tau) final states.

Supersymmetry is a theoretical idea that states that for every fermion observed, there is another, yet-undiscovered, boson (and vice versa.) This theory effectively doubles the number of types of particles that physicists expect exist in our universe. It is one of the most popular theories for physical phenomena beyond what we currently know because this extension to our Standard Model allows for the unification of all the forces – something that is currently missing.

Particle physicists have been hunting for the existence of supersymmetry for decades. Furthermore, cosmologists are excited since it predicts a stable particle, which is a leading candidate for dark matter in the cosmos.

If supersymmetry is the correct theory, then a fundamental principle of particle physics, the conservation of quantum properties known as quantum numbers (this analysis focuses particularly on baryon and lepton numbers), is no longer conserved. This principle ensures that, for example, for every lepton created in a collision, an antilepton has to be created as well. Since baryon number and lepton number conservation have been tested very precisely, these couplings need to be very small in order to not conflict with experimental data.

R-parity is a symmetry that particle physicists can use to classify particles — Standard Model particles have an R-parity of one while supersymmetric particles have an R-parity of negative one.

CDF has performed a first search for R-parity violating decays of heavy supersymmetric neutrinos into final states involving a third-generation lepton, the tau, at the Tevatron. This work is based on the Ph.D. thesis of Yanjun Tu and has been recently published in Physical Review Letters. Specifically, CDF scientists searched for a for supersymetric neutrino decaying into (electron and muon) or (muon and tau) or (electron and tau) final states. These processes are not allowed in the Standard Model, because they violate both lepton-number, and R-parity conservation. However, SUSY allows such a process. Any observation therefore will be a proof of new physics and long awaited confirmation that supersymmetry exists.

Technically, the search is challenging; the hadronic decaying modes of tau leptons are necessary to differentiate the taus from the electron and muon directly created from the collision in the final states. The observations are consistent with the Standard Model expectations within the uncertainty of the measurement. The first upper limits on the relevant physics have been set. Confirmation of supersymmetry will have to wait for another day.

— edited by Andy Beretvas

From left: Anadi Canepa, Pennsylvania; Nigel Lockyer, Pennsylvania and director of TRIUMF; Yanjun Tu, Pennsylvania; Pasha Murat, Fermilab.