Physics analysis

Our expertise:

• Measurement of the Higgs boson properties in the 4-lepton, tau-tau and Zgamma channels
• Searches for partners of the top and bottom quarks predicted by supersymmetry
• Measurement of the ttZ production cross-section and properties
• Measurement of the double inelastic cross-section in pp collisions

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Background

Members of the Sheffield ATLAS Group participate to a wide range of physics analysis. Furthermore, we have or have had several positions of responsibility within ATLAS.

Dan Tovey was until recently ATLAS physics coordinator and we had conveners of the SUSY (Costanzo, Tovey), e-gamma (Anastopoulos) and validation (Costanzo, Hodgkinson) working groups. Davide Costanzo was a member of the publication committee and the chairperson of the authorship committee.

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Higgs physics

The group played a leading role in direct discovery of the Higgs boson announced in July 2012 by the ATLAS and CMS collaborations. This discovery led to the award of the 2013 Nobel Prize to Peter Higgs and Francois Englert “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles”.

The main interest of the group is the study of the properties of the Higgs boson via its decays into two Z bosons and decays into two tau-leptons.

Additionally, the Sheffield group is heavily involved in the search of SM Higgs boson or a new resonances with mass larger than 250 GeV (BSM), decaying to a Z boson and a photon. The Z bosons are identified through their decays either to charged, light lepton pairs.

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Supersymmetry

Supersymmetry (SUSY) provides a natural explanation to the dark matter puzzle by predicting that each Standard Model particle has a supersymmetric partner, with the lightest supersymmetric particle (LSP) identified as the dark matter constituent.

Sheffield is one of the main groups involved in SUSY searches with ATLAS and we have an extensive involvement in a number of SUSY analyses, including searches for light squarks and gluinos (inclusive jets + MET + 0-lepton analysis), sbottom squarks (2 b-jet + MET+ 0-lepton analysis) and stop squarks (2 b-jets + 6-jets + MET + 0-leptons analysis) . We developed the jet smearing technique which to date has been used for QCD multijet background estimation in many ATLAS SUSY analyses.

Our current activity is mostly in the search for supersymmetric partners of the third generation quarks (aka the s-bottom and the s-top). The LHC Run-2 is a crucial moment to search for these particles.  This work is partly supported by an ERC Advanced Grant (PI Dan Tovey).

Stop searches summary

This plot shows the summary for the search of the s-top quark, a supersymmetric partner of the top quark. 

The phenomenology of the decays depend on the mass of the s-top and of the lightest supersymmetric particle (the neutralino) to which the stop decays to.

Different areas of the plane are covered by different analyses. In Sheffield we worked on the summary of the Run-1 analyses are currently involved in the searched in the 0-lepton channel

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Standard Model and top quark physics

We have a new group funded by an ERC Starting Grant to Kristin Lohwasser working on Standard model diboson measurements. We are also active in fiducial and differential cross-section measurements on associated production of top quark pairs with Z bosons (ttZ), funded by the ERC Advanced Grant of Dan Tovey.