ATLAS at Lancaster

A candidate for the decay of a Higgs boson into four charged leptons in the ATLAS Experiment.
A candidate for the decay of a Higgs boson into four charged leptons in the ATLAS Experiment.

The Lancaster ATLAS group is heavily involved with understanding the early data from the ATLAS experiment at the CERN Large Hadron Collider.

We had a major role in the development and construction of the tracking detectors, and in the development of tracking software. We also lead the ATLAS UK Computing Project, developing distributed computing solutions to handle the approximately 20 PetaBytes of data produced each year, and have held various leadership roles in the experiment itself. More information on this experiment is available here.

The Lancaster Group has a major role in studies using particles called the J/ψ. These particles have been known for a long time, and their properties are very well understood. This means they are ideal for understanding the detector, in particular the tracking and the muon detection. It also allows us to understand some of the important triggers, which are the means by which the huge rate of collisions are reduced to a much lower rate that can be stored, reconstructed and analysed. The importance of these studies was underlined when they became the vital tool for understanding the precise determination of the mass of the new boson, the χ(126), which is thought to be a Higgs boson.

Understanding how these particles are produced in these new very high energy proton-proton collisions is important to the understanding of any new physics processes, and is interesting in itself.

These particles are key to some of the more important B-physics decays that we are using to look for new physics and to study the question of the matter/antimatter asymmetry of the Universe and CP-violation. Our studies of the decay of Bs mesons into a J/ψ and a φ have allowed for much improved determination of the CP violating phase in the decay and world leading measurements concerning the lifetime of the two Bs meson states.

A Monte Carlo simulation of Higgs to tau tau decays
A Monte Carlo simulation of Higgs to tau tau decays.

Lancaster is also participating in the hunt of the Higgs boson. The Standard Model incorporates a spontaneous symmetry breaking mechanism that gives mass to all other particles. This model gives rise to a spin-less particle, the Higgs boson. A candidate for this boson has been found in July 2012. Since then evidence that this candidate is the long sought Higgs boson has been mounting.

The Lancaster ATLAS group has been contributing to this success by investigating Higgs boson decays into two tau-leptons. These are the heaviest leptons and therefore are the leptons with the largest couplings to the Higgs boson. This makes this decay channel a very important one to identify all the properties of the newly found boson. In addition tau leptons have a relatively long lifetime and can decay into hadrons. The Lancaster expertise in tracking and reconstruction of displaced vertexes comes into its own and helps analysing this channel.

Event display of a top anti-top candidate.
Event display of a top anti-top candidate.

The top quark is one the most interesting quarks; it decays before it can hadronise, and it is the most massive of all the fundamental particles. With the discovery of a Higgs-like particle and having measuring its mass we now know that the top quark can decay into a Higgs boson and a lighter quark. The probability of such decay is suppressed in the Standard Model, but may be enhanced in many of its extensions. The Lancaster group is searching for these decays when the decaying Higgs boson produces one or more leptons in the final state. Finding these decays would be evidence for beyond the standard model physics.

The Lancaster group is also heavily involved in searches for new particles that decay to top – anti-top pairs. We are searching for decays where both top quarks decay to a b-quark, a lepton, and a neutrino. This provides a much cleaner signal, at the cost of being a rarer decay.

Latest Progress from ATLAS

You can find more general information aimed at the press and public at the ATLAS public pages.

A particular area of study for the Lancaster involves the J/ψ particles produced and their decays to muons. The following picture shows en event with one such decay.

An event display of a J/ψ → μ<sup>+</sup>μ<sup>-</sup>
An event display of a J/ψ → μ+μ.

We have already shown our understanding of the detector and the data by making very precise measurements of the Bs lifetime.

An event display of a J/ψ → μ<sup>+</sup>μ<sup>-</sup>
Measurement of the lifetime of the Bs-meson.

The Lancaster group are studying the decay of the Bs to J/ψs, in particular the decay Bs to a J/ψ and a φ. We now have an unprecedented number of such decays, which have opened the door to precise measurements of so-called CP violation. Understanding this effect is essential to understand how the universe we live in, in which antimatter is rare and matter abundant, came about. By studying the angular structure of the decay in three angles, along with the mass and lifetime of each decay, the difference between the lifetime of the ‘heavy’ and ‘light’ Bs states, the average lifetime and the CP violating weak phase can be measured. Lancaster work on using various signatures to decide if the initial collision produced a Bs or an anti- Bs allowed us to dramatically reduce the uncertainties on the measurements using the same sample of data. Deviations from the expected lifetime difference (ΔΓ) and weak phase (φs) would be a signature of new physics.

As well as studying the J/ψ, the Lancaster group also study other ‘onia’, which are particles where a quark is coupled with its own antiparticle. This lead to the discovery of a new state, the χb(3P) through its decay to an Υ(nS) state. This had been predicted for many years, but not previously observed, and was the first new particle discovery published from the LHC. It helps us understand the strong interaction that binds particles and nuclei together, and also contributes most of the mass of everyday objects.

The mass distributions of χb→Υ(kS)γ (k = 1, 2) candidates formed using photons which have converted and been reconstructed. The third peak is the χb(3P)