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New Physics at the LHC

by Yannis Karyotakis - 21 March 2012

Higgs and Electroweak Symmetry Breaking

The main raison d’être of the LHC is to track the Higgs particle and through its discovery as a fundamental object to unveil the nature of the vacuum. In the Standard Model all elementary particle masses derive from one mass, the vacuum expectation value. Yet symmetry breaking might just be a parameterisation of an underlying dynamics like what one is accustomed to in condensed matter. So beyond tracking the Higgs down one is probing a key concept and might discover New Physics.

Is the vacuum expectation value a parameterisation of a condensate?
In that case does the weak interaction get stronger (Higgsless models, technicolour like,..) ?
If there is a Higgs, is it the Standard Model one?
Can one reconstruct the Higgs potential?
In the immediate future the priority is given to the difficult low mass region (115-140GeV) which must be explored before the end of 2012. Members of the Labex are taking active part in this urgent task especially with their expertise in photon physics covering all facets – theoretical, experimental, signal and backgrounds. Beyond 2015, when the LHC runs at nominal conditions, one will need to either study the decay and production of the Higgs separately in as many channels as possible in order to determine its nature and draw a coherent picture with other possible signals of New Physics, or to shift our attention towards the very high mass window if a light Higgs had been excluded by then. In particular we have developed know-how and initiated simulations in technicolour theories and their extended versions. When the full energy and highest luminosity become available, our task is to exploit vector boson scattering.

We are also readying ourselves to the possibility that New Physics will show up in an unexpected way within a framework that one has not thought of. ATLAS, LAPP and LPSC teams will pursue a collaboration around model independent searches and limits based on topologies. Theorists of the team will join in as they have proposed novel ideas concerning the reconstruction of spin and mass measurements (transverse mass techniques). In collaboration with the CKMfitter team (see below) is also foreseen taking advantage of the expertise they have had in combining measurements to constrain parameters.

Physics of Heavy Flavour

From a different angle, it is necessary to pursue the issue of the pattern of mass in the quark sector and how the different quarks share mass and mix among themselves, adding now the top into the picture. It is indeed fascinating that the existence of three families, so far unexplained, permits a rather simple explanation of CP violation, an ingredient that might explain why matter is much more abundant than anti-matter.

New Physics can also be probed in heavy flavour, indirectly through virtual effects notably at LHCb or directly at ATLAS especially as concerns top physics. The LAPP LHCb team building up on a reputation and tradition from analyses at B factories and being a major actor in CKMfitter will spearhead such an activity. A concerted action with LPSC ATLAS top working group will add more insight. One is already in the situation where new Physics indirect effects (eg rare Bd/s→  decay mode) give strong constraints on supersymmetry. The high statistics at LHCb push these tests and constraints for a host of models at an unprecedented level. The precision on the mass patterns and mixing embodied in the CKM matrix will reach a new level of precision (the angle , the Bs CPV mixing angle s, radiative decays such as Bs → K*(, hadronic charmless decays as Bs→ KK/K,,..).