The interactions of the known particles are described within the Standard Model (SM), where all particles and forces are accounted for. However the SM does not explain the number of particle generations observed or give any indication of the nature of dark matter e.g. Several models beyond the SM try to provide solutions to these and other questions, while predicting new physics signals that could be observed at the Large Hadron Collider (LHC).
Our group is currently involved in searches for scalar leptoquarks (LQ), and is starting to look for Z' and W' bosons. These new particles have similar signatures at the LHC: well isolated, high momentum final state particles and in the case of the W' decays large missing transverse energy, ET, which make these searches ideal analyses for the start of the LHC.
The two-body mass peak of a di-lepton, di-jet or lepton-jet final state in the region of a few hundred GeV up to the TeV scale is the best signature of new physics one can look for. The SM backgrounds to these new signals, mainly Z/W+jets, ttbar+jets and QCD, are expected in general to be small in the TeV range, making it easier to observe a bump in the invariant mass spectrum above the flat background.
These are hypothetical particles predicted by e.g. Grand Unified Theories (GUT), composite models, R-parity violating SUSY. Leptoquarks may have been present in the first few fractions of a second after the Big Bang. Their existence may thus lead to an explanation into the origins of the Universe. Leptoquarks carry both lepton and baryon numbers and couple to both leptons and quarks. They are fractionally charged and can be either scalar (spin = 0) or vector particles (spin = 1). In order to satisfy constraints from flavour-changing neutral currents and rare pion and kaon decays, leptoquarks are restricted to couple to a single lepton-quark generation at a time.
The figure on the right shows the latest LQ results from ATLAS at 7 TeV where LQ with mass < 534 Gev are excluded at 95% confidence level. Click on the plot for a more detailed description.
Quarks and leptons, the two fundamental particles, have different properties, and this is depicted on the diagram on the right as two different-looking halves of the face. It is postulated that the lepton and the quark may be decay products of an exotic particle - the leptoquark. This leptoquark would have properties of both the lepton and the quark.
At the LHC, the two dominant production modes for scalar leptoquarks are through gluongluon fusion and qqbar annihilation, producing a pair of leptoquarks. These processes are shown by the Feynman diagrams on the right.
Several SM extensions like the GUT, Little Higgs Models and the Left-Right Symmetric Models, postulate the existence of a variety of heavy neutral gauge bosons Z' and charged spin-1 bosons W'. The decays of a heavy Z' may be a useful production mechanism for other exotic particles and SUSY (supersymmetric) particles. The size of extra dimensions could depend on the mass of these particles if observed. These heavy bosons could be candidates in the searches for dark matter as well.
The two figures below are event displays showing two high transverse momentum (pT) leptons that were chosen as candidates for the Z' analysis. The diagram on the left shows events containing two high pT electrons, and the diagram on the right shows events containing two high pT muons. These events were used to reconstruct the invariant mass. Click on the diagrams for a more detailed description.
The decay of a Z’ boson into leptons is shown in the diagrams below. The left figure shows the decay of the Z' boson into an electron (e- ) and a positron (e+ ), and the right figure shows the decay of a Z' boson into a muon (μ- ) and an anti-muon (μ+ ).
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