Supersymmetry - A Search For R-Parity Violating SUSY at HERA
By John Loizides
Supervisor Professor Tegid Wyn Jones
What is SUSY?
SUSY is a hypothetical symmetry that describes nature, and says that even though fermions and bosons seem to be very different in their properties and roles in theory they appear in a symmetric way. This relates the masses and couplings of particles of different spins together. So for every fermion and boson in the SM there will be an Sparticle relating to each. If SUSY exist then the standard model will be at least doubled.
Why Haven't They Been Seen Yet?
Well for SUSY to exist it must be a badly broken symmetry. These particles must be much heavier then all known particles to explain why they have not been seen yet. With HERA and the ZEUS detector these particles hope to be discovered at very high Q2 (the four momentum transfer of the exchange boson).
What is R-parity?
It is a quantum number that is put in by hand. For standard model particles it is 1 are for supersymmetric particles it is -1. Therefore if this is a consevered quantity sparticles can only be produced in pairs with the LSP(lightest supersymmetric particle) being stable. At HERA if R-parity is violated this gives rise to an excellent oportinity for discovery, due to the squark being able to decay back to standard model particles which we are able to detect.
Work Carried out.
A search has been made for Supersymmetric particles produced by R-parity violating interactions in ep collisions at a center of mass of 300 Gev, in the ZEUS detector. The ZEUS detector is one of the major experiments at the HERA collider in Germany. Supersymmetric quarks are produced in a eq fusion, which can result in a final state with a lepton that could be a positron or neutrino with multiple jets. The 1997 positron data of 27.8717 pb-1 was used. No evidence for Supersymmetric quark production was found.
The Project comprised of three main areas:-
SUSY is a very attractive extension to the Standard Model (SM) it solves such problems as the hierarchy problem, unification of the coupling constants who's interactions are al dependent upon energy, gravity can also be included within the MSSM (Minimal Supersymmetric Model) as well as solving the fine tunning problem which is linked to the hierarchy problem which is to do eith the smallness in scale of the SM and GUT (Grand Unified Theory).
SUSYGEN is a monte carlo generator extented from e+e- collisions at LEP to ep collisions at HERA. This had to interfaced with Amadeus which is a generic interface for all monte carlo generators at ZEUS. This was needed to be done so the kinematics given out are in a form of which can be passed into the funnelling system at ZEUS. The funnelling system simulates the detector effects.
A generic framework was written and the data and monte carlo were feed through this. The Orange (Overlying Routine for Analysis Ntuple Generation) code at the ZEUS experiment was used. Many subroutines were changed and new variables incorporated into this code. This was then given out in a standard way which was then passed through the analysis code written in C++ in ROOT . Comparisons were made between kinematci variables of the 1997 data, neutral current monte carlo, and SUSYGEN monte carlo data see figures below.
Figure1:
The red line is the signal of the 1997 data and the black is the Nuetral Current Background.
Figure 2:
This is the signal generated by the SUSYGEN monte carlo for a Squark mass of 200GeV
A short movie that I created can be found at www.hep.ucl.ac.uk/~jhl/squark.mov or www.hep.ucl.ac.uk/~jhl/squark.avi. which shows a cartoony representation of what the creation of squark would look like.