The ATLAS experiment
Prof PP Allport, Dr N Andari, Mr MJ Baca, Dr J Bracinik, Mr D Briglin, Mr J Broughton, Prof DG Charlton, Mr A Foster, Dr L Gonella, Dr F Gonnella, Dr CM Hawkes, Dr SJ Hillier, Mr I Iqbal, Dr J Kempster, Mr J Kendrick, Mr J Lindon, Dr A Mazurov, Prof PR Newman, Dr K Nikolopoulos, Mr R Owen, Mr S Pyatt, Mr E Reynolds, Dr MW Slater, Mr RJ Staley, Dr JP Thomas, Dr PD Thompson, Mr R Turner, Mr R Vallance, Prof PM Watkins, Dr AT Watson, Dr MF Watson, Dr JA Wilson, Dr SD Worm
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 8 academics members of staff, 11 postdoctoral researchers and engineers and 9 Ph.D. students. Prof Dave Charlton is the 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, the Tracker Upgrade (ITK) and are involved in developing the Atlantis event display program. Our physics analyses are currently focused on Higgs boson properties, 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) and the Tracker Upgrade (ITk)
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 (ITk) for the upgraded LHC (High Luminosity: 'HL-LHC'). The new tracker is planned to be installed in ATLAS to be ready in 2024. The new system will replace the existing SCT as well as the TRT. Hybrids equipped with prototype front-end chips are being produced and tested in our electronics cleanroom, with a special focus on quality assurance. In the coming years, a large-scale production and test effort is planned for full ITK barrel modules equipped with silicon sensors. This effort is part of the new Birmingham Instrumentation Laboratory for Particle physics and Applications (BILPA) clean-room facility, which opened in 2016.
A further group effort is the use of the Medical Physics MC40 cyclotron at our university 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
Exploring the structure of the Higgs sector
On 4th July 2012, with strong involvement and leadership of the Birmingham group, the ATLAS (arXiv: 1207.7214) and CMS (arXiv: 1207.7235) Collaborations announced the observation of a particle consistent with the Standard Model Higgs boson in the mass region around 125–126 GeV. Further details can be found in the official official CERN Press Release, University press release and also at BBC News, July 2012. Following the discovery, our focus shifted in elucidating the structure of the Higgs sector through measurement of the properties of the observed boson, as well as searches for physics beyond the Standard Model through an extended Higgs sector.
More details can be found in the dedicated page.
Studies of Properties of the top quarkThe 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 the width of the top-quark mass resonance. The group also contributes to the development and maintenance of the analysis software packages.
Diffractive Processes at the LHCDiffractive 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 QuarkoniaMeasurements of the properties and production of heavy quarkonia provide a unique window in the strong interaction. While these heavy quark anti-quark bound states have been known to exist for over forty years, many puzzles still surround our understanding the quarkonium production mechanism in hadron collisions. Members of the Birmingham group have led the ATLAS subgroup concerned with studies of quarkonia and have played an important role in the analysis of the data collected during the first LHC run. The studies of the group have focused on measurements of the chi_c (arXiv:1404.7035) and chi_b states, including the discovery of the chi_b(3P) state (arXiv:1112.5154). Members of the group have also pioneered unique and novel studies of rare processes involving quarkonia such as the associated production of charmonium states with Z bosons (arXiv:1412.6428) and searches for rare Higgs and Z boson decays to a quarkonium state and a photon (arXiv:1501.03276).
The group will continue to study quarkonia with the upgraded ATLAS detector in LHC run 2 starting in 2015, and hope to exploit improved tracking capabilities to perform detailed quarkonium production measurements with the first data collected at 13 TeV to further elucidate the nature of quarkonium production at the LHC.
Software and Computing Infrastructure
Atlantis Event DisplayThe 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. 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 GridEven 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.
The ATLAS detector - schematic view
ATLAS detector - during construction - Nov. 2005
Part of the L1Calo system at the ATLAS electronics hall
L1Calo: Full-crate CP system tests at Birmingham lab in 2006
ATLAS ITK Hybrid with glass chips and wirebonds, 2014
ATLAS Silicon Tracker arriving at CERN in 2005
Top-quark decay into one electron and one muon in ATLAS from 2010 data, shown in Atlantis event display
Atlas-Live website with live events, image from 2011 LHC run period. Click on image for latest event
Illustration of the Worldwide Computing Grid: European sites