University of Birmingham main campus at Edgbaston

The ATLAS experiment

Birmingham group
Mr BM Allbrooke, Mr HS Bansil, Dr J Bracinik, Prof DG Charlton, Mr AS Chisholm, Mr A Daniels, Mr PJW Faulkner, Dr CM Hawkes, Dr SJ Head, Dr SJ Hillier, Mr T McLaughlan, Mr TA Martin, Mr RD Mudd, Prof PR Newman, Mr JD Palmer, Mr S Pyatt, Mr JA Murillo Quijada, Mr X Serghi, Dr MW Slater, Mr RJ Staley, Dr JP Thomas, Dr PD Thompson, Prof PM Watkins, Dr AT Watson, Dr MF Watson, Dr JA Wilson

ATLAS is one of the two general-purpose experiments currently collecting and analysing data at the large hadron collider (LHC) at CERN.

The Birmingham ATLAS group currently consists of 5 academics members of staff, 11 postdoctoral researchers and engineers and 9 Ph.D. students. Prof Dave Charlton is the deputy spokesperson for the experiment. We have been heavily involved in the construction and operation of the first level calorimeter trigger (L1Calo) and are working towards its upgrade for the next phase of LHC running. We also contribute to the Semiconductor Tracker (SCT), which we partially constructed, and are involved in developing the Atlantis event display program. Our physics analyses are currently focused on Higgs searches, top and heavy flavour physics and diffractive processes.

Detector Development for the ATLAS Experiment

The First-Level Calorimeter Trigger (L1Calo)

The L1Calo system is a vital part of online event selection at ATLAS. It provides the first level trigger decision for all calorimeter based decisions: electrons, taus, jets and missing energy. The system consists of several crates of custom designed electronics which are located in the underground electronics hall next to the experimental cavern. It is required to perform event selection at a rate of 40 MHz within a maximum time of 1 micro-second.
Birmingham was particularly involved with the design and testing of the Cluster Processor Module, which identifies electron and tau candidates, sending their number and location to make the final Level-1 trigger decision and guide the next level of trigger processing. At Birmingham, we continue to maintain and improve the working system by refining calibration and monitoring techniques, as well as developing new algorithms for the future challenges of higher luminosity at LHC.

The Semiconductor Tracker (SCT)

The ATLAS Inner detector (ID) measures with precision and high efficiency the large number of tracks produced at the interaction point. The tracking involves three types of tracker, located in a 2T solenoidal magnetic field: pixel detectors within 15cm of the beam pipe; the SemiConductor Tracker (SCT) which comprises silicon strip detectors, which are planes of strips at radii between 25cm and 60cm. The outermost tracker is the Transition Radiation Tracker which extends out to 1m radius.
The Birmingham group’s involvement is with the SCT whose efficient performance is crucial to ATLAS; it provides four precise space points which are essential in the determination of track momentum. The group built and tested much of the hybrid readout electronics for the Barrel SCT. Currently, our SCT team assists in operating the detector during the LHC runs, in monitoring the data quality and in understanding and solving interesting anomalies.
Beyond the current running at CERN, we are participating in preparations to build a new Silicon Tracker for the upgraded LHC. This involvement is on two fronts: (1) production and testing of hybrid readout (preparations are underway) and (2) use of our Medical Physics cyclotron to irradiate material and devices to fluences equivalent to those expected after running throughout the period of the upgraded LHC.

Current Analysis of ATLAS Data

Hunt for the Higgs-Boson

The Higgs boson is a hypothetical massive elementary particle that is predicted to exist by the Standard Model of particle physics. The Higgs boson is an integral part of the theoretical Higgs mechanism. If shown to exist, this mechanism would help explain why other particles can have mass. It is the only elementary particle predicted by the Standard Model that has not yet been observed in particle physics experiments. The discovery of the Higgs boson and measurement of its properties, notably its couplings to other particles, is the central scientific aim of the LHC experiments.
The Birmingham group participates in analyses of ATLAS data to search for the Higgs boson using its decay into a pair of bottom flavoured quarks in association with either a Z or W boson. These channels are important in the region of low Higgs Boson mass that has yet to be excluded by the LHC data accumulated so far.

Studies of Properties of the top quark

The top quark is the last quark flavour to have been discovered. It has an extraordinarily high mass, nearly 175 times as heavy as a proton, making it the heaviest fundamental particle discovered to date. The Large Hadron Collider, for the first time in particle physics, is providing a very large number of top quarks, so it can also be called a 'Top-Factory'. In the 2011 run period, about 100000 top-quark-pairs have been recorded. This opens up new opportunities to study the properties of the top quark. Its decay modes result in complicated final states to analyse, which require all detector components to be very well-understood and reconstruction techniques to be optimised. The Birmingham group is actively involved in studying those properties, namely the spin correlation of top-quark pairs, lepton identification in top-quark production, and traces of new physics indicated by enhancements in missing momentum in top-quark pair events.

Diffractive Processes at the LHC

Diffractive interactions between protons are a commonplace occurrence in the LHC, making up around 25% of all collisions. In most diffractive interactions, gluons of the strong nuclear force are exchanged between the protons but there is no net flow of colour-charge. This lack of colour-flow results in large regions of space which contain no particles from the collision. By hunting for these large, empty regions in the ATLAS detector we can then investigate in detail the properties of this class of interaction. In every crossing of the two proton beams at the LHC there are now over 15 individual proton-proton interactions. Every so often a rare interaction will occur, such as the creation of top quarks. A quarter of the other interactions happening at the same time will be diffractive in nature so a good understanding of the properties of such events allows us to constrain the underlying models of diffraction and better control the influence of these diffractive `pile up' events on other physics.

Studies of Heavy Quarkonia

Of all the hadrons predicted by the quark model, the heavy quarkonia provide one of the most useful laboratories for investigating the predictions of the Quantum Chromodynamics (QCD) model, the fundamental theory of the strong interaction. Unlike the light quark (u, d, s) mesons, the flavourless bound states of c-cbar (charmonium) and b-bbar (bottomonium) are intrinsically non-relativistic systems. This property of the system permits the use of non-relativistic quantum mechanics to model the bound state through an inter-quark potential. The large quark masses allow a largely perturbative investigation of the system. The Birmingham group is working on the analysis of ATLAS data for decays of the charmonium state chi_c into a J/psi particle, which in turn decays into two muons, plus a low energy photon. Our group has also participated in studying decays of the bottomonium state Chi_b into an upsilon particle and a low energy photon. This study resulted in the first clear observation of a new particle at the LHC, the Chi_b 3P. See: Publication in Physical Review Letters 108, 152001 (2012), also University press release, Dec. 2011 and BBC science news article, Dec. 2011.

Software and Computing Infrastructure

Atlantis Event Display

The Atlantis event display is a software tool to visualise the decay products of the collisions inside the ATLAS detector. It is widely used in the ATLAS collaboration for presentations, posters and visual displays, and also provides a real-time display of ATLAS events. Atlantis consists of interface packages to the standard ATLAS reconstruction software, implemented in C++, and the stand-alone event display itself, which is implemented in Java and therefore runs on all major operating systems. The Birmingham group is working on development, support and maintenance of the Atlantis event display. A spin-off from Atlantis is the Minerva package which has become a popular item in Particle Physics Masterclasses, teacher training and other outreach events.

Distributed Computing: The Grid

Even before being built, it was known that ATLAS (and the other LHC experiments) would produce more data than any other experiment in history and so a new computer system was required to allow the physicists to analyse it all. This new system is called the LHC Computing Grid and consists of a network of computer clusters supplied by participating institutes all around the world. All the data is distributed across these computing 'sites' and can be analysed at the sites using the available computing power. In order to hide the complexity of the underlying system, a large amount of software has been developed so now a physicist just needs to specify what data they want to analyse and how they want to analyse it and the system takes care of everything else. Using the networking and technology that is already in place for the internet, this analysis task gets sent to whichever computing site is deemed the best at that time where it is run and the results sent back. It is analogous to the electrical grid - as a user you don't care where the electricity is generated, you just want it to flick a switch and be able to use it! With thousands of computers available, a physicist can now analyse far more data in a far shorter time than has been possible before. Here in Birmingham we provide nearly 400 'slots' for analysis tasks to be run as well as 200 Terabytes of storage. Over the whole grid, several hundred thousand jobs are run everyday and over 15 Petabytes (15,000,000,000 Megabytes!) of data is stored every year.