22 Feb 2011



ATLAS is a large general-purpose particle physics experiment. It is designed to study the fundamental constituents of matter, searching for new processes and particles that could change our understanding of energy and mass



                                                ​     ATLAS experiment, with person standing in front of it to show vast scale

ATLAS​ is helping us learn about the fundamental forces that have shaped our universe since the beginning of time and will determine its fate. We have already made a big step forward and discovered an essential missing piece of our so-called Standard Model, which is the Higgs boson. However, we also know that the Standard Model cannot be the ultimate theory. Now, we are searching for physics beyond this model by making precise measurements of our known physics processes. Among the possible unknowns are the origin of mass, extra dimensions of space, microscopic black holes, and evidence for dark matter candidates in the universe.

The RAL PPD group, including PhD students, contains approximately 20 people, some of whom are residents at CERN. ​We have multiple physics interests, including B physics, Higgs measurements, and searches for heavy bosons. We are also heavily involved in the upgrades of the ATLAS detector components, needed for the HL-LHC luminosity upgrade.

To know more about the projects and studentships in which past postgraduate students took part, check the ​​​​ATLAS PhD opportunities page. 



ATLAS SCT under constructionThe current ATLAS tracker system consists of three concentric layers: the pixel detectors, the Semi-Conductor Tracker made of silicon strips and the TRT straw-tube tracker. The construction of the STC was a collaborative effort in which we, PPD, and the Technology Department (TD) worked very closely ​​with UK University groups and colleagues from around the world. ​The group assembled o​ver 700 silicon modules, oversaw the installation of the services on the carbon-fibre support structures, played a substantial part in the creation of the DAQ system and DCS (detector control system), and oversaw the subsequent installation and commissioning of the detector. The tracker has been enormously successful and has performed beyond expectations, at instantaneous luminosities up to twice the design limit.​ 

Now the PPD ATLAS group is actively participating in the operation of this detector and has ongoing responsibility for the readout and detector controls, ensuring the detector works as expected during data-taking.​​


The High-Level Trigger (HLT) Group is led by a PPD staff member and plays a major part in the ATLAS trigger software, as in implementing tracking and selection software for the current trigger and is developing fast-tracking code for Phase-2. Within the UK, PPD provided past and present leadership of the UK-HLT group.

The PPD group made a significant contribution to the Phase I trigger software upgrade, has major roles in Maintenance and Operation and is developing software for Phase II. The trigger software consists of a framework which configures and controls the trigger operation and selection software which reconstructs features of the event data to form trigger decisions, being a major component of the selection software. In particular, PPD delivered a major speedup of the Trigger tracking software for Run 3, added functionality to increase the sensitivity of the trigger to the decays of possible exotic long-lived particles and upgraded the trigger validation infrastructure. For Phase II, there will be a factor of 10 increase in the input event rate and a factor of 4 increase in pile-up requiring significant upgrades of the framework and tracking software. High-level triggering at Phase II will be provided by the Event Filter. PPD staff are coordinating Trigger Core Software development for Phase II. They are also providing staff as UK-HLT co-coordinator for the Event Filter. 

Now, the group is developing fast-tracking software for Phase II for implementation both on CPU and exploiting GPU acceleration.

GROUP LEADER: ​​John Baines​



The ATLAS Level-1 calorimeter trigger selects events based on fast hardware processing of signals in the electromagnetic and hadronic calorimeters.​ 

With the project being a collaboration between UK groups from Birmingham, Cambridge and QMUL, and non-UK groups from ​Germany, Poland, Sweden and USA, the RAL group has played and continues to play a pivotal role in the L1Calo project, from the original Run 1 system through to the proposed upgrades for Run 4. Having built a significant part of the original L1Calo system, the UK has the responsibility to support operations at CERN, being responsible for many operational activities (e.g., software updates, calibration and test runs with calorimeters) and day-to-day run coordination. WiLeft: eFEX modules manufactured and tested at RAL. Right: ATLAS Global Trigger systemth also a leading role in the Global Trigger Project for Run 4, the system will build on the information from the calorimeter and muon trigger systems, along with full granularity information from the calorimeters, to provide refined event selection in the face of intense HL-LHC backgrounds.​

In the LHC Run 3, we have been responsible, with Birmingham, for the Phase-1 Upgrade of the electron feature extractor system (eFEX), which modules identify electron, photon and tau candidates using the high granularity 'supercell' information from LAr. In addition, we have defined the L1Calo standard ATCA backplane usage and communication protocols, along with a custom board shape to better address the demanding data I/O requirements of the trigger. Hosting Ral the principal UK L1Calo integration and testing centre, the ATLAS PPD group has taken a leading role in the operation of the Surface Test Facility (STF) at CERN, which is needed by the L1Calo project and associated detector groups to integrate and test new hardware in a controlled environment prior to installation in the experiment. ​

GROUP LEADER: Robin Middleton     ​​                                                                                            


After over ten years of operation at CERN, the ATLAS inner detector will be degraded by intense radiation. This is why a new upgraded inner detector is required to allow continued detection and data acquisition at a higher rate and to withstand higher radiation environments. 

Prototypes of the Pixel Ring (left) and the Silicon Strip Module (right) for the ITk upgrade

The PPD group has been engaged in the development and preparation for the construction of the ITk, as part of a ​consortium of eleven UK universities and laboratories which will deliver the UK component of the tracker upgrade. It had responsibilities in the construction of both stave and pixel modules for the ITk, as well as simulations of the performance of the detector in the future. In particular, the local support structure of the prototype is based on carbon fibre rings. At the same time, the electrically functional detector modules (11 on each side) were mounted onto these rings using a robotic gantry system. All the assembly and testing of the modules has been undertaken at RAL in 2022, while the prototype will be tested with serial powering at Liverpool University and the cooling system will be tested at Manchester University. 

Working together with the RAL TD colleagues, UK Universities and groups overseas, studies are ongoing to investigate the performance of different layouts in simulation, as investigating the efficiency of reconstructing a hard scatter vertex in high-density events such as those expected at the HL-LHC.

GROUP LEADER: ​Steve McMahon​​