Signatures Beyond the Standard Model

In 2007, the Large Hadron Collider (LHC) will be switched on at CERN in Geneva. In 2008, it will collide beams of protons at much higher energies than have been reached before. Our current understanding of known particles is based on the 'Standard Model' (SM) of particle physics, which has been experimentally tested and verified to an extraordinary level of precision. However at the high energies which will be reached at the LHC, the SM requires convenient coincidences unless there is something new at these scales. The exact form that this new physics might take is unknown and deducing a unique underlying theory from the experimental data is the biggest challenge facing particle physicists at the moment, and my area of research to date. There are a lot of possibilities to extend the SM that physicists have been working on for many years but until now we have not been able to produce energies high enough to probe them. One of the most well-motivated extensions to the SM is Supersymmetry (SUSY). This introduces a new symmetry into the SM in such a way that every particle has a new corresponding particle. Another possible model has more than 3 spatial dimensions - the 'extra' dimensions are very small so that we cannot directly detect them. A Universal Extra Dimensions (UED) model gives every SM particle many new partners; if the energy is such that only the lightest of these are produced, it could mimic SUSY. A key difference in the models is in the spin of the partners. Unlike familiar everyday objects, an elementary particle (e.g. electron, quark, photon) can spin only at a certain specific rate. This rotation rate is an intrinsic property of the particle, and we term this its spin. The new partners in SUSY have different spins to the originals, while in UED it is the same. I have worked on determining the spin of particles which have been produced using a method which exploits the fact that the spin of a particle affects the direction in which it is likely to travel. Measurements of spin have usually been done at colliders where the precise energy of impact is controlled, but this cannot be done at a proton-proton collider like the LHC as each proton is made up of smaller particles, called quarks and gluons. Although the energy of the proton is known, this is just the sum of the energy of these other particles, making it impossible to know the exact energy of each collision. The value measured for an angle depends on whether it is measured from the collider perspective, or the perspective of one of the particles involved. This makes them difficult quantities with which to work without knowledge of the collision energy. However, it is possible to construct 'invariants' which are the same from every perspective and can be used instead. When a new particle is produced in a collision and decays, the values of these invariants can be measured. These can be compared to the theoretical curves I have calculated to see which theory best fits the data. I have compared SUSY and UED in this way, and then extended this to include all possibilities for the spins of the new particles without relying on an underlying model. This makes the technique much more versatile. I have completed a similar study for a different decay of a quark partner. I intend to expand this work to consider two or more of the independent invariants together to make maximum use of all the information available. So far, studies of UED models with more than one extra dimension have always assumed that the extra dimensions are flat. If they are curved like a sphere for example, we expect that the possible experimental signatures will be different. I intend to perform a detailed study of this spherical case and its experimental signatures. I then plan to expand this to the case where we have more than two extra dimensions. The methods above may then be applied to these new signatures to assess if we could distinguish these new cases.