Fresh Summer Harvest of ATLAS Results: Top quarks, Sleptons and Gauginos

Since monday the last of a string of summer High Energy Physics (HEP) conferences is unwinding in Beijing, SUSY-12 and before that in Melbourne ICHEP-2012. Some results with leading contributions from the Stockholm HEP group figure high in the topics of discussion. Finally ATLAS physicists can breathe a  little bit, with most results out for now. The Higgs boson discovery got a lot of attention, and this is great because it is not every day that there is a discovery of that magnitude, but there is an other important reason: simply said, the existence of the Higgs boson is at the basis of much of the research being carried out at Stockholm University and in ATLAS.

One of the mysteries of the Standard Model is the top quark which has a mass (172 GeV) much much higher than the other quarks, this is also much heavier than expected initially and still remains unexplained. But thanks to the Higgs mechanism we know that the mass of a particle is set by how strongly it interacts with the Higgs field, so for some unknown reason the top quark very strongly couples with the Higgs. This is why we decided to study the top quark properties in details. It could be that some of the top quark properties deviate slightly from the standard model of particle physics, or that some of the top quarks are produced via new mechanisms inside the proton-proton collisions at LHC.

Normalised measured top differential cross section as function of the top quark pair invariant mass and theoretical predictions.

New heavy particles could decay into pairs of top quarks. The HEP group in Stockholm has lead the effort of a measurement of the so-called top quark pair differential cross section (see ), which would for instance be able to detect the presence of new particles decaying into top quarks pairs in the invariant mass distribution of the two top quarks. Similar measurements were performed at Tevatron, but thanks to the very high center of mass of the LHC, it is possible to probe new particles to much higher masses. No new particle turned up, and this measurement will have to be redone in 2015 when higher, 14TeV of energy LHC data will be available (and allow to probe even higher particle masses). For now this completely new measurement will allow theorists to tune and improve Monte Carlo simulation programs so that they agree with this new data in a previously unexplored region of phase space.

One of our favourite topics at OKC is dark matter and the ATLAS experiment has a lot to say about this question. This summer saw more than 20 new ATLAS search results in the domain of supersymmetry (SUSY), one of the leading candidate theories of physics beyond the standard model. A full summary of all these results can be found here. Since the LHC is a proton-proton collider, production of colored SUSY particles such as the squarks and gluinos has always been considered as the golden discovery mode for SUSY. Nevertheless the search for these particles has turned nothing so far and since we have a duty to do the best we can with this fantatistic machine, we are also turning to other SUSY production modes.

Slepton exclusion region in the plane (slepton mass,neutralino mass)
Slepton exclusion region in the plane (slepton mass,neutralino mass)

ATLAS could reveal SUSY for instance if the SUSY partners of the leptons, the “sleptons” or the SUSY partners of the gauge bosons the “gauginos” were produced directly in the proton-proton collisions. The Stockholm HEP group has initiated a search for direct slepton production and lead the effort of a multichannel search for direct sleptons and direct gauginos production in two lepton final states ( ). The gaugino modes searched for can be combined with tri-lepton final states and provide very significantly extended limits in the gaugino parameter space M1, M2 and mu over previous results. These parameters are central to constrain the properties of the dark matter particle if it is of SUSY nature. Light sleptons could also play a role in helping SUSY to provide a relic dark matter density consistent with observations. The new ATLAS direct slepton limits are the first limits in the slepton sector since the LEP era and exclude left-handed sleptons up to a mass of 195 GeV for a 20 GeV neutralino. The search for sleptons at LHC is in some sense a benchmark since this is the SUSY production process with the smallest cross section.

The two papers above were prepared with data from 2011 and the analysis of 2012 data is already ongoing. In the case of the top pair differential cross section the precision of the results is limited by systematics and will probably wait for 2015 for a competitive update. The Stockholm team is for this reason now turning to other interesting channels of probing the top sector with the 2012 data. In the case of the slepton and gaugino search, the 2012 data is very valuable (10fb-1 already recorded and another 10fb-1 expcted by the end of the year) so our group is working on the analysis of this new data, which will allow to significantly improve the new limits and perhaps even find something new. Stay tuned!

Christophe Clement & Jörgen Sjölin


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