BEYONDSMFROMDATA · Physics beyond the Standard Model: establishing footprints in the light of data

Physics beyond the Standard Model: establishing footprints in the light of data

Project context The project activity was centred around the theoretical interpretation of the results from the Large Hadron Collider (LHC) experiments, currently running at CERN, Switzerland. The LHC is an underground circular accelerator, with a 27 km circumference, colliding two proton beams against each other, at the highest centre-of-mass energy and frequency of collisions ever achieved. At the level of 'numbers', the LHC is just astounding, even for an expert in the field. To give an idea of this, it will suffice to say that a single bunch of protons traveling at full speed has the same kinetic energy (concentrated in a diameter of a few mm) as a one-tonne elephant running at 50 km/h, that one of the detectors (ATLAS) weighs the same as the Eiffel Tower, and that the amount of raw data produced would correspond to 200 000 DVDs per second. Clearly, at the scientific (understanding of the dynamics at ever shorter distances), technological (experiment design and data processing), and human levels (coordination of some 4000 physicists from 35 countries), the LHC concretises one of the most daring global challenges ever achieved by mankind. Project objectives This Marie Curie project focused on the question of 'where to search?' in the above-mentioned amount of data, in particular on the possibility of exploiting the complementary role between existing data, from other collider experiments, and the LHC ones in the search for new physics effects. Specifically, the former data may be used to literally 'guide' the definition of the most promising LHC search channels. Within this strategy, the definition of the theory/calculational framework considered is very important, and in this respect two parallel routes were followed: the model-independent one, and one that is more model-specific and predictive. Project results As far as the first route is concerned, the researcher focused on searching for interesting signatures at the LHC that would hold up independently of the model of new physics being considered. Examples are signatures based on decay topologies, such as the number of charged leptons or the amount of energy imbalance, or the number of clusters of particles (called 'jets') in the event. The researcher has pursued that line of research since end of 2009, in regular collaboration with Prof. Kiwoon Choi (KAIST, Korea) and young scientists from his school. The duration of the fellowship permitted further consolidation of this collaboration which in turn led to the identification of a very general problem connected with the above search strategy - the problem of combinatorial uncertainties. This problem arises every time a topology includes two or more 'indistinguishable' particles, e.g. two (or more) electrons with a large enough transverse momentum. In this case, there are two (or more) equivalent ways of associating these electrons with an assumed topology, only one of which is correct. This problem has been addressed in the literature by proposing the use of special kinematical variables, specifically designed for hadronic colliders, like the LHC. Prof. Choi and his school have made important contributions to the understanding of these variables, in particular the so called M(T2). Still referring to this model-independent line of research, together with the scientist in charge of the present project we have started a project centred on, among other things, the signature represented by two muons, which can come from the decay of a Bs meson, with a very rare rate. The rarity of this decay makes it one of the most formidable search channels for new effects beyond the standard model. Our discussions led to results made public after the interruption of this Marie Curie fellowship. The second line of investigation was instead centred on specific, well-motivated models, among them the so-called 'left-right symmetric' models, that are plausibly a high-energy completion of the standard model. On this topic, the MC fellow has collaborated since 2009 with Prof. Rabindra N. Mohapatra (Maryland University, USA). In the course of this fellowship, the discussions led to the idea of embedding this symmetry within another symmetry group, called 'flavor', namely that which formally organises the existence of 'generations', that is the replication of all matter particles by three, differing only in the mass. In the literature it was shown that this idea, even after confrontation with the existing data, leads to a number of signatures at the LHC that are yet to be explored experimentally.

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