Search for physics beyond the Standard Model at Uppsalla university Sweden

The Large Hadron Collider (LHC) will start operating at CERN at the end of 2009. During its first year of running, the LHC will produce proton-proton collisions at 10 TeV, a center-of-mass energy never reached before in the laboratory. Installed around one of the LHC collision points, the ATLAS experiment aims at understanding the fundamental particles and forces that have determined the evolution of our universe since the Big Bang.

The latest discovered elementary particle, the top quark (with a mass of 172.6 GeV/c²) will be copiously produced at the LHC. In the Standard Model, the top quark has a lifetime shorter than 10^-24 s. Hence, it does not have time to hadronize and immediately decays into a b quark and a W boson, which can in turn produce either a quark-antiquark pair or a charged lepton and a neutrino. A detailed study of the top quark decays at the LHC will allow precise tests of the Standard Model, which is of crucial interest as a significant deviation from the theoretical predictions will provide evidence for new physics.

We propose to study leptonic decays of the top quarks, i.e. events where a tt pair is produced in the pp collision, leading to final states with two b-quarks, directly observable charged leptons (i.e. electrons and muons, since τ-leptons must be reconstructed from their decay products), as well as neutrinos which escape detection. In particular, we propose to measure the angular distribution of the charged leptons, in order to compare it with that derived from the polarization of the W boson in the Standard Model. New physics may indeed be found if this angular distribution differs from the Standard Model prediction. Such a deviation may be due to the presence of charged Higgs bosons in the top quark decays or to a coupling of the W boson which is not purely left-handed.

Although τ-leptons can not be fully reconstructed due to the presence of undetectable neutrinos in their decay, relevant information can still be extracted from the study of the (hadronic) τ decay products, for instance their polarization. The τ-leptons appear in several physics processes beyond the Standard Model and they provide interesting signatures for such processes. For this reason, we propose to study their identification, reconstruction and characterization.

The successful candidate will be under supervision of Uppsala University (Sweden) and LPSC Grenoble (France). He/she will spend equal amounts of time at both locations. Extended stays at CERN in Geneva will also occur during the thesis, e.g. for data-taking shifts and service work on ATLAS. Experience with the ATLAS analysis software and ROOT is considered as an asset. The successful candidate is expected to attend courses at the post-graduate level. The PhD defense will take place in Uppsala.

Employment as a PhD student is for 4 years and the successful candidate is expected to start in October 2009. Further information regarding the positions can be obtained from Arnaud Ferrari (Arnaud.Ferrari@fysast.uu.se, +46 18-471 5827). The final appointment will be made after interviews with a few top candidates, in Uppsala and Grenoble.

Written applications should be received no later than May 15, 2009. Address: Registrator, UFV-PA 2009/832, Uppsala University, Box 256, S-751 05 Uppsala, Sweden, or by fax +46 18 471 2000 or email registrator@uu.se. Applications should contain a CV, supporting documentation, notarized copies of diplomas/degrees, at least one recommendation letter, and, if not publicly available, copies of any previous research related work. If application is made by fax or email, original documents must be sent by mail within one week.