Signatures Beyond the Standard Model

Lead Research Organisation: University College London
Department Name: Physics and Astronomy

Abstract

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.

Publications

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Andersen J (2010) Constructing all-order corrections to multi-jet rates in Journal of High Energy Physics

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Ellis J (2009) Measurement of CP violation in stop cascade decays at the LHC in The European Physical Journal C

Related Projects

Project Reference Relationship Related To Start End Award Value
PP/E00427X/1 01/10/2007 30/09/2009 £192,905
PP/E00427X/2 Transfer PP/E00427X/1 01/10/2009 30/09/2010 £65,792
 
Description This was the final year of a grant; the first two years of the grant were held at a different institution and have been reported on elsewhere.

This year was one of great development for the High Energy Jets (HEJ) which I have co-developed. This is a completely fresh approach to making precise predictions for the Large Hadron Collider (LHC). In important areas of phase space, notably including those which are important for Higgs boson studies, standard calculational approaches struggle to describe data where our new approach succeeds. Now, four years later, the framework has been included in many important analyses at both the LHC and TeVatron colliders.

During this time, I also began work on resummation of transverse energy in Higgs boson production. This was the most accurate calculation at the time and has since led to recent work where it was updated.
Exploitation Route My grant was before the time of an official Pathway to Impact statement. The work from this grant is being used in both the theoretical and experimental sections of the particle physics community. It is providing fresh insight into the behaviour of QCD at high energies, and is being used by a number of experimental groups in their LHC jet analyses.
Sectors Other

URL http://cern.ch/hej
 
Description This work has been within fundamental science to further our knowledge of the world around us. It has contributed to the strong position of the UK within the LHC community and therefore to the esteem of the country. It is also notable that this project has really caught the interest and attention of the public, and risen interest in physics among undergraduates for example.
 
Description High Energy Jets 
Organisation European Organization for Nuclear Research (CERN)
Department Theoretical Physics Unit
Country Switzerland 
Sector Academic/University 
PI Contribution I am one half of the High Energy Jets collaboration which resulted in two papers published during the fellowship.
Collaborator Contribution Worked jointly on High Energy Jets project which resulted in two papers published during the fellowship.
Impact Papers published during fellowship: Constructing All-Order Corrections to Multi-Jet Rates, J. R. Andersen and J. M. Smillie, JHEP 1001 (2010) 039 The Factorisation of the t-channel Pole in Quark-Gluon Scattering, J. R. Andersen and J. M. Smillie, Phys. Rev. D81 (2010) 114021
Start Year 2009
 
Description Resummation of Transverse Energy 
Organisation University of Birmingham
Department Nuclear and High Energy Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution I was one third of the team on this calculation which led to a published paper.
Collaborator Contribution Two thirds of the team on this calculation were based in Cambridge. The end result was a published paper.
Impact One published paper: Resummation of Transverse Energy in Vector Boson and Higgs Boson Production at Hadron Colliders, A. Papaefstathio, J. M. Smillie and B. R. Webber, JHEP 1004 (2010) 084 .
Start Year 2009