Supersymmetry with Trilinear R-Parity Violation – Implications for Cosmology and the LHC

The current Standard Model of particle physics is sometimes referred to as one of the
most successful theories ever constructed. When looking at the precision with which
its predictions are being confirmed, it is easy to agree.
Yet no theoretical particle physicist is content with the state of affairs. Some of the
reasons for the dissatisfaction is the non-observation of the Higgs boson, a vital part
of the Standard Model, as well as the theoretical problems associated with it. Most
obvious, though, is the fact that most of the matter density of the universe comes in a
form that cannot be associated with any part of the Standard Model. The nature of this
dark matter is one of the most discussed questions in the field.
Many of the speculations beyond the Standard Model are going in the direction of
supersymmetry, a symmetry that mixes internal symmetries with spacetime symmetries
in a non-trivial way and in the process introduces a symmetry between fermions
and bosons. Due to problems with too fast proton decay and possible presence of other
exotic processes, one usually assumes that R-parity is conserved when studying supersymmetric
models.
This, however, is not necessary and the inclusion of R-parity violating couplings
have interesting implications for the phenomenology both within cosmology and LHC
physics. If R-parity is violated, one has to abandon the neutralino as the dark matter
candidate, but there is a perfectly valid alternative in the gravitino.
Gravitino dark matter that (very slowly) decays, might be a source of measurable
cosmic rays. In fact, the recent anomalies in cosmic ray positrons and electrons, seen
by PAMELA and Fermi LAT, can be well described by decay products from gravitino
dark matter. Unfortunately, the corresponding gamma ray signal was not seen by the
Fermi LAT telescope and although this does not exclude gravitinos as the source of the
cosmic ray anomalies, it makes it less likely to be the correct answer.
It is also worth noticing that the gravitational nature of the gravitino interactions
suppresses the decay rate enough to, in some cases, allow R-parity violating couplings
in the neighborhood of the experimental limits, without coming in conflict with cosmic
ray measurements. This, however, requires a light (GeV scale) gravitino and a coupling
that does not allow a loop decay.
For LHC physics, the implications from violating R-parity can be paramount. However,
the most likely scenario might still be pair production of squarks and gluinos, followed
by a cascade decay down to the neutralino. If the R-parity violating couplings
are large enough, the neutralino will then decay to three Standard Model particles.
This gives rise to multi-jet and/or multi-lepton finals states. This is in general
promising for detection since the detection of especially leptons is much easier than
measuring missing transverse energy as is required in R-parity conserving supersymmetric
models. The large number of invariant mass distributions are also very useful in determining the couplings and measure the neutralino mass. All relevant invariant
mass distributions can be calculated theoretically, at least to a good approximation, and
then compared to data.
For trilinear R-parity violating couplings, it is shown that, due to our good knowledge
of the expected invariant mass distributions, it is clearly possible to identify most
couplings responsible for the decay of the neutralinos. Some problematic cases remain
due to combinatorial jet background, the presence (and associated information-loss to
neutrinos) of taus in the final state as well as phase-space suppression of channels with top quarks