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 12 academics members
of staff, 14 postdoctoral researchers and engineers and 12 Ph.D. students.
Prof Dave Charlton is a former 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 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 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 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
Measurements 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 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.
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.
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The ATLAS detector - schematic view
ATLAS detector - during construction - Nov. 2005
Part of the L1Calo system at the ATLAS electronics hall
ATLAS First-Level Calorimeter Trigger Upgrade: FEX Test Module designed and tested in Birmingham (2016)
Full electrical module with two hybrids and 20 frontend chips for ATLAS Tracker Upgrade (ITk), assembled at
BILPA in 2018
Current ATLAS Silicon Tracker (SCT) 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
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