ATLAS Physics

At RAL, the ATLAS group is heavily involved in the silicon tracker, the first-level calorimeter trigger and the software-based higher-level trigger. Besides these activities, we are also participating in the ATLAS physics analysis in the areas of Standard Model, B-physics and Higgs searches. People from our group hold or held significant coordinating roles in the ATLAS and ATLAS-UK physics and performance groups. Bill Murray just finished his coordinating role as ATLAS Higgs coordinator and will become the deputy physics coordinator in October 2012. John Baines is one of the former B-physics group coordinator. Alastair Dewhurst is the current ATLAS UK B-physics coordinator and Julie Kirk held this position in the past. Both Stephen Haywood and Monika Wielers were convenors for the electron and photon combined performance group.

B-physics

One of the main topics of the ATLAS B-physics programme is the precise measurement of weak B-baryon decays. In the Standard Model, all flavour phenomena of weak hadronic decays are described in terms of quark masses and the four independent parameters in the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Enormous quantities of data collected in the past decade by the experiments BaBar, Belle, CDF, and D0 allowed very precise measurements of flavour and CP-violating phenomena, and up to now, no evidence of physics beyond the Standard Model, nor any evidence for CP violation other than that originating from the CKM mechanism, has yet been found. Due to the large beauty production cross-section and the high luminosity of the LHC, the sensitivity of B decay measurements will substantially improve.

At RAL we are focusing on measuring CP violation in the B_s → J/ψ φ channel. The first measurements of this channel focused on the mass and the lifetime of the neutral B_s-meson which is documented in an ATLAS conference note (link opens in a new window). This channel is reconstructed via the decay of the J/ψ in two muons and the decay of the φ into two charged kaons. The muons from the J/ψ decays are used to select such events online. Now the focus of the work has shifted to measure CP violation in this channel. RAL is contributing significantly to the flavour tagging of this channel and plays a crucial role in the ATLAS B-physics trigger aspects.

People involved: John Baines, Will Dearnaley (Ph.D. student), Alastair Dewhurst, Julie Kirk

 

Standard Model

One of the activities within the Standard Model group is the set of measurements involving W and Z-bosons. For example, the study of the W and Z-Boson production in association with jets provides a set of critically important measurements for the first phase of LHC as well as for more mature stages of the experiment. By measuring the inclusive production cross-section, we are able to test theoretical NNLO QCD predictions in a new energy regime. The measurement of the differential production cross-sections, e.g. vs. rapidity, transverse momentum and charge, will give new constraints on the structure functions of the proton as well as on NLO parton emission calculation or resummation models. Knowing these to a high precision is absolutely essential to conduct electroweak precision measurements, such as the future W-mass measurement, which will be valuable in providing improved indirect constraints on e.g. the Higgs boson mass. Moreover, precise understanding of the W- and Z-boson production, especially when associated with jets, is crucial for searches for physics beyond the Standard Model.

At RAL we are involved in the measurement of the W/Z ratio in association with jets. The results based on the 2010 data are summarised in detail in this paper (link opens in a new window). This measurement is done by reconstructing Z-bosons decaying into two electrons or muons. The W's are reconstructed via their semi-leptonic decay into an electron or muon and a neutrino, which is reconstructed indirectly via missing transverse energy. The main background in the electron channel arise from jets misidentified as an electron. Though this mis-identification rate is low, this constitutes a significant background due to the large jet production rate at the LHC. At RAL, our focus is on the data-driven background estimate from QCD jets. Several methods have been developed to measure this background, including the associated systematics. Our group is also involved in the preparations for the precise W-mass measurement. Ultimately we expect to be able to measure the mass with a precision of 15 MeV. Our main contribution to this channel is the development of some special triggers, the detailed understanding of the transverse missing energy arising from the neutrino produced and the effects of the hadronic recoil.

People involved: Rudi Apolle (Ph.D. student), Ellie Davies (Ph.D. student), Stephen Haywood, Monika Wielers

 

Higgs Searches

One of the major motivations of the LHC is the search for the last missing piece of the Standard Model (SM), which is the Higgs boson. Within the SM this particle is responsible for electroweak symmetry breaking and is the particle which will 'give' mass to particles. The Higgs boson has been excluded by LEP up to 114.4 GeV. At the LHC, the search for the Higgs has been focussing so far on the mass range between 110 and 600 GeV. Using the 2011 data amounting to 5 fb^-1 of data, we have observed an excess of events for a Higgs with a mass around 126 GeV. The three most sensitive channels in this mass range are the search for the Higgs in the di-photon decay channel, a Higgs decaying via two Z-bosons into four leptons and a Higgs decaying to two W's which subsequently decay semi-leptonically. In these channels, the local significance at this mass amounts to 2.8, 2.1, and 1.4 standard deviations respectively. This sums up to a local significance of 3.5 standard deviations, or 2.2 standard deviations if the look-elsewhere effect is taken into account. For discovery, we still need more statistics so that we are able to rule out a fluctuation and claim discovery if the significance is at least 5 standard deviations. The latest combined ATLAS Higgs results are summarised here (link opens in a new window). For Higgs searches 2012 will be a crucial year.

At RAL, Tim is involved in the combination of the results from the individual channels measured in ATLAS and thus heavily involved in the statistical methods used. Bill, who has been the coordinator of the Higgs group up to the end of 2011, is active in many areas and recently started to study Z+photon production with the Z decaying into two b-jets, as a calibration for the search for the Higgs decaying to two b-jets. Monika just started to look for a Higgs boson in the decay mode into four leptons.

People involved: Tim Adye, Bill Murray, Monika Wielers