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L1 CALORIMETER TRIGGER UPDATE and RUN-3 PHYSICS ANALYSIS (2019)​​

Co-supervisors:  PPD: D. Sankey, Birmingham: A. Watson

The project will be in the L1 Calorimeter trigger upgrade on ATLAS. It will commence in October 2019 and run for 3.5 years (as is currently the norm for STFC quota places). The studentship would include experimental/operations, calibration/performance and physics analysis components, each potentially providing material for thesis write-up.

The L1Calo groups at Birmingham and RAL are heavily engaged in preparing upgrades for the ATLAS calorimeter trigger which are scheduled to be installed and commissioned during the LHC shutdown in 2019/20. The electronics (the eFex processors) have been successfully prototyped and are about to go into production. Custom test modules have been produced and firmware is in an advanced stage of development by project engineers. The proposed studentship would provide an excellent overlap with the next phase of this activity and afford a number of interesting opportunities (see below).

After an initial period at Birmingham where the student would complete academic and computing courses, he/she would move to CERN to participate in the debugging and commissioning of the eFex processors in readiness for Run-3. This work would sit alongside general operational tasks, giving hands-on experimental experience, including the opportunity to participate in control room shifts and provide first-line expert support. Whilst at CERN there would also be an opportunity to become involved in an analysis group, which would provide a foundation for the physics analysis component of the thesis. After a year to 18 months at CERN, the student would return to RAL where work on eFex calibration and performance would be carried out, alongside ongoing physics analysis. In the final phase, the student would return to Birmingham to complete the study and write ​the thesis.

Currently, the physics analysis topic is still to be selected and of course will depend on the student's interests, the availability of supervisory staff with relevant expertise and the research priorities of the ATLAS groups in PPD and Birmingham.

This project is with the PPD ATLAS High-Level Trigger group. The group is coordinating the ATLAS trigger software upgrade and has strong expertise in trigger tracking.

The project is to optimise, validate and commission the new ATLAS High-Level Trigger software that is being rewritten utilizing modern software techniques to fully exploit new computing architectures. The ATLAS High-Level Trigger decides which collision events are to be recorded for analysis offline and which are to be discarded. Only 1 in 100 events can be kept. The High-Level Trigger does this by reconstructing features characteristic of, for example, electrons, muons, tau-leptons or groups of the nearby particles (jets) using software algorithms that run on a farm of computers. The software has to be fast so that the farm can process 100,000 events per second, so each event must take less than one-fifth of a second. At the moment, each computer processor core works on its own set of events running algorithms sequentially. The software is being upgraded to be fully multi-threaded so that algorithms can run in parallel. You will optimise and validate the software and help to commission the trigger to ensure it is ready for use online in 2021 when the LHC restarts after the current shutdown.

The successful applicant will also participate in physics analysis using ATLAS data, collaborating with other members of the groups at RAL and Glasgow. 

The scholarship covers students'​ stipend and fees at a level required from 'home' students. International students will need to have additional funding to cover full fees at the international level.

Low-mass dimuon or dielectron resonance searches are challenging due to large backgrounds below the Z. Presently such searches are limited by the capability to store all the collisions containing low momentum leptons on disk for future physics analysis. 

In Run 3 we plan to carry out such an analysis in the online selection of the collision events in a so-called Trigger Level Analysis (TLA) for muons (and electrons).  ATLAS successfully implemented a TLA only for dijets in Run 2 and we intend to build on this success. TL analyses enable data to be st​ored with smaller event sizes as only selected data is written out and therefore we can afford to record these collisions which cannot be achieved with our traditional triggers. This is an exciting prospect and opens up the possibility of probing for new physics signals in new regions. In our case, we are looking for deviations from the Standard Model at low invariant mass due to exotic physics, such as dark photons.

After Run 3, the LHC will be upgraded to the High-Luminosity LHC. This will require a major upgrade of the ATLAS detector, including a new all-silicon inner tracking detector (ITk). As part of the ACTS project, the ATLAS track reconstruction software is being rewritten with improved algorithms and modern software engineering practices to take full advantage of the new detector and emerging computing architectures.

On the RHUL side, this project will be supervised by Dr Tracey Berry, an expert (20 years experience) in dilepton searches, supported by RHUL trigger software experts. At RAL, the group has a wealth of expertise in the ATLAS tracking software in the High-Level Trigger and offline systems. The studentship includes developing new track reconstruction software for the ITk, as well as performing an analysis for a low mass resonance search using the new Run 3 data. This is the ideal time and place to perform this research. The Large Hadron Collider will restart in Spring 2022 enabling the student to exploit the new higher energy dataset (@13.6 TeV). At the same time, construction and software development for the Phase II upgrade is ramping up. The research would be published and presented internationally at workshops and conferences.

The Large Hadron Collider (LHC) at CERN will be upgraded to become the High-Luminosity LHC (HL-LHC), providing more proton-proton collisions per second than the current LHC. This will allow for greater precision in physics measurements and further reach in searches for new physics. For this upgrade, the tracking detector which forms the innermost section of the ATLAS Detector closest to the collisions will be replaced with a new tracking detector, the ITk. The ITk will use Silicon sensors to detect and measure the paths and momenta of high-energy particles. It will comprise an inner section of Silicon pixel sensors and an outer section of Silicon strip sensors. Custom high-speed radiation-hard electronics are being developed to read out these sensors. At the Rutherford Appleton Laboratory (RAL), 1500 pixel sensor modules, comprising 6000 front-end chips, must be pre-tested and qualified, mounted to carbon fibre support structures with high precision, and wired into data readout, control, and powering circuitry, before the completed structures are tested for combined operation. A full ITk data readout chain will be used for these tests. Due to a large number of chips and modules to be tested, considerable automation will be required.

As part of the ATLAS collaboration, a student will join RAL and University College London (UCL), to jointly develop ITk pixel data-readout and test infrastructure, building on the considerable ITk and data readout expertise and leadership at these institutes. The student will help to build and develop full pixel readout chains and will identify and develop methods for quantifying and comparing pixel module performance with that rapid automated sensor qualification. The student will also develop and perform the first operation, testing, and qualification of combined pixel structures at RAL. The student will gain and use skills in computer programming, electronics, FPGA firmware development, high-speed digital communication, and international collaboration on a large-scale scientific project.

There will also be an opportunity to pursue physics analysis with the ATLAS Detector utilising the existing ATLAS tracking detector: Unconventional signatures of current great interest are long-lived particles (LLPs), which decay in the detector itself. These LLPs have been predicted by a number of New Physics models, including dark matter candidates. For this task a dedicated track reconstruction algorithm was developed to find tracks starting in the tracking volume. A current analysis using this is the search for exotic decays of the Higgs boson into long-lived particles decaying into b-quarks and tau leptons. An ultimate goal will be placing more model-independent limits on the cross-sections of highly displaced vertices and hence LLP production. The student will join an existing team already involved in this analysis, allowing a greater focus of person-power on this topic, increased progress, and better support. The applicant will have the option to be based at CERN for a period of time and will present their work at international conferences.
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Supervision:
The student will be supervised by Dr Ben Smart, Dr Monika Wielers, and Dr Andreas Korn. Daily local supervision will be supplemented by regular video and in-person team meetings. Existing students and postdocs will lend further support.