The ATLAS experiment at the LHC

UVic scientists have contributed to ATLAS data analyses since the first LHC collisions in 2010, and are currently involved in a variaty of data analyses.

Recent published papers with important contributions from UVic scientists:

• Search for Higgs bosons decaying to new spin-0 or spin-1 particles in four-lepton final states at the ATLAS detector with 139 fb−1 of pp collision data at √s = 13 TeV. JHEP 03 (2022) 041. Submitted 2021/10/26.
  https://arxiv.org/abs/2110.13673
  
  • previous ATLAS result: 
    https://doi.org/10.1007/JHEP06(2018)166 
    arXiv:1802.03388
• Search for associated production of a Z boson with an invisibly decaying Higgs boson or dark matter candidates at √s = 13 TeV with the ATLAS detector.  Phys. Lett. B 829 (2022) 137066.  Submitted 2021/11/16.
  https://arxiv.org/abs/2111.08372
• L. Polson, L.Kurchaninov, M. Lefebvre; Energy reconstruction in a liquid argon calorimeter cell using convolutional neural networks.  2022 JINST 17 P01002.  Submitted 2021/09/10.
  https://arxiv.org/abs/2109.05124
• Search for heavy resonances decaying into a pair of Z bosons in the ℓ+ℓ−ℓ′+ℓ′− and ℓ+ℓ−νν¯ final states using 139 fb−1 of proton-proton collisions at √s = 13 TeV with the ATLAS detector.  Eur. Phys. J. C (2021) 81: 332.  Submitted 2020/09/30.
  arXiv:2009.14791
• Constraints on mediator-based dark matter models using √s = 13 TeV pp collisions at the LHC with the ATLAS detector. JHEP 05 (2019) 142.  Submitted 2019/03/03.
  arXiv:1903.01400
• Constraints on off-shell Higgs boson production and the Higgs boson total width in ZZ→4ℓ and ZZ→2ℓ2ν final states with the ATLAS detector, Phys. Lett. B. 786 (2018) 223.  Submitted 2018/08/03.
  arXiv:1808.01191
• Search for new phenomena using the invariant mass distribution of same-flavour opposite-sign dilepton pairs in events with missing transverse momentum in √s = 13 TeV pp collisions with the ATLAS detector, Eur. Phys. J. C (2018) 78: 625.  Submitted 2018/05/29.
  arXiv:1805.11381
• Search for heavy ZZ resonances in the ℓ+ℓ−ℓ+ℓ− and ℓ+ℓ−νν¯ final states using proton–proton collisions at √s = 13 TeV with the ATLAS detector. Eur. Phys. J. C (2018) 78: 293.  Submitted 2017/12/18.
  arXiv:1712.06386
• Measurement of the Higgs boson coupling properties in the H → ZZ* → 4ℓ decay channel at √s = 13 TeV with the ATLAS detector, J. High Energ. Phys. (2018) 2018: 95.  Submitted 2017/12/06.
  arXiv:1712.02304
• Measurement of the Drell-Yan triple-differential cross section in pp collisions at √s = 8 TeV, the ATLAS collaboration, J. High Energ. Phys. (2017) 2017: 59.  Submitted 2017/10/14.
  arXiv:1710.05167
• Search for an invisibly decaying Higgs boson or dark matter candidates produced in association with a Z boson in pp collisions at √s = 13 TeV with the ATLAS detector, Phys. Lett. B 776 (2017) 318.
  Submitted 2017/08/31.
  arXiv:1708.09624
• Search for new phenomena in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in √s = 13 TeV pp collisions with the ATLAS detector, Eur. Phys. J. C (2017) 77: 144. Submitted 2016/11/17.
  arXiv:1611.05791

Public notes

• Dark matter summary plots for s-channel and 2HDM+a models, ATL-PHYS-PUB-2021-045.
  https://cds.cern.ch/record/2798737
• Combination and summary of ATLAS dark matter searches using 139 fb-1 of √s = 13 TeV pp collision data and interpreted in a two-Higgs-doublet model with a pseudoscalar mediator.
  ATLAS-CONF-2021-036.
  https://cds.cern.ch/record/2777863
• Formulae for estimating significances, ATL-PHYS-PUB-2020-025.
  https://cds.cern.ch/record/2736148.
• LHC Dark Matter Working Group: Next-generation spin-0 dark matter models, CERN-LPCC-2018-02.
  arXiv:1810.09420

Work in progress:

• Measurement of triple gauge boson couplings.
  • related previous ATLAS result:
    • https://doi.org/10.1140/epjc/s10052-019-7027-6
      arXiv:1902.05759 
• Search for pair production of gluinos decaying via third generation squarks resulting in a final state with multiple b-jets and two lightest neutralinos
  • conference note
• Dark matter summary plots for s-channel and 2HDM+a models, public note in preparation.
• Search for dark matter produced in association with a hypothetical scalar boson decaying to W+W−, in the fully hadronic and semi-leptonic final states

Past topics with UVic contributions include:

• Measurement of the properties of the discovered Higgs boson.
• Mesaurement of the WZ diboson production cross section, and measurement of the W and Z polarizations in WZ events.
• Precise measurement of the e⁺e^- and μ⁺μ^- Drell-Yan pairs invariant mass distribution.
• Precise measurement of the transverse momentum of the Z boson.
• Search for supersymmetry with third generation squarks.
• Search for supersymmetry using a Higgs boson in the decay cascade.
• Search for new massive particles decaying to a pair of top quarks.
• Search of quark compositeness through the study of the angular distribution of dijet events.
• Search for long-lived exotic particles.
• Search for contact interactions in dilepton events.

The operation of the ATLAS detector and the assessement and improvement of its performance are critical aspects of the ATLAS program.  Members of the UVic ATLAS group contributions include the following activities:

• Liquid argon calorimeter
  • Run coordination
  • Data quality 
  • Signal pulse shape studies using pp collision data
• Missing transverse momentum high level trigger
  • Monitoring and Operation activities 
  • Algorithm development and pileup mitigation
  • Performance evaluation
    
    • "Performance of the missing transverse momentum triggers for the ATLAS detector during Run-2 data taking",
      JHEP 08 (2020) 80 
      arXiv:2005.09554
    • "Performance of the ATLAS global transverse-momentum triggers at √s = 8 TeV", 
      ATL-DAQ-PUB-2018-001
    • "Analytical description of missing transverse-momentum trigger rates in ATLAS with 7 and 8 TeV data", 
      ATL-DAQ-PUB-2017-002
  • Performance optimization
    • MET algorithm developent for high pileup running

The ATLAS detector was built in the 1990's, and some of its components need to be upgraded, or replaced, to operate in the higher luminosity environment expected at the LHC for Phase-I Run 3 (2021-2023) and for Phase-II running (2026-2037 and beyond).  UVic is contributing to the following ATLAS upgrade projects:

• Phase I and II liquid argon calorimeter readout electronics upgrade
• Phase I new muon small wheel detector

The High Energy Physics Research Computing Group at the University of Victoria is actively engaged in a variety of projects for the analysis of data from particle physics experiments, including ATLAS.  Areas of expertise include:

• High-speed networks
• Management of large scale data storage
• Virtualization
• Grid computing
• Cloud computing

The ATLAS Collaboration was founded in 1992, and includes the University of Victoria as one of its founding institutes. The UVic-ATLAS group was led by Prof. Michel Lefebvre, and included Prof. Alan Astbury and Prof. Richard Keeler. Prof. Lefebvre was the founding Spokesperson of the ATLAS-Canada Collaboration, founded in 1992 with colleagues from the Université de Montréal.  The UVic-ATLAS group has been growing, and now also includes Dr. Justin Albert, Prof. Robert Kowalewski, Prof. Robert McPherson, Prof. Randy Sobie, and Dr Isabel Trigger (TRIUMF).  Both McPherson and Sobie are Institute of Particle Physics Scientists. Prof. McPherson has been the Principal Investigator of the ATLAS-UVic group from 2003 to 2015, and was Spokesperson of the ATLAS-Canada Collaboration from 2007 to 2015; he was appointed Deputy Spokesperson of the ATLAS Collaboration for two years starting February 2015.  Prof. Lefebvre acted as the Principal Investigator of the ATLAS-UVic group from 2015 to 2018; Prof. McPherson is Principal Inverstigator of the ATLAS-UVic group since April 2018.  The particle physics expertise at UVic goes hand in hand with its close relation with TRIUMF, Canada's National Laboratory for Particle and Nuclear Physics. TRIUMF staff located at UVic played crucial roles in the construction of the ATLAS experiment.

Canadian involvement in ATLAS and the LHC has placed us in a prominent position in the forefront international science project of the decade. In total Canada has invested $70 million of the $8 billion total in equipment that is now a crucial part of the CERN LHC accelerator complex and the ATLAS particle physics experiment. Canadian researchers have received an additional $30 million to fund graduate students, postdoctoral researchers and their research on ATLAS. TRIUMF has provided staff and technical support to make these contributions a reality. As a result of these investments and the resulting scientific and technical expertise Canada is a respected partner at CERN and in the international science community.

No single country could afford to build the $8 billion LHC project on its own. ATLAS has been built by researchers from more than 150 universities and laboratories in 35 countries. 150 Canadian scientists (faculty, lab staff, postdoctoral researchers and graduate students) from eleven institutions across the country work at CERN, alongside 2000 other scientists from every corner of the globe, on the ATLAS experiment. Canada has made important contributions to the LHC, ATLAS and the world-wide computing grid now digesting the ATLAS data.

In 1995 TRIUMF was given the mandate to act as Canada's main connection with CERN. It was provided with $42 million of federal funding over ten years to develop and construct components for the LHC. These projects were completed on time and in budget in close collaboration with Canadian industry. Over 90% of our LHC funding has been spent in Canada. There have been a number of spin-offs from this activity. I.E. Power, Inverpower and Digital Predictive Systems in Ontario gained expertise in high current power supply design and fabrication and have competed successfully for an additional $10M in contracts from major international labs. ALSTOM-Canada, in Tracy, Quebec improved assembly tolerances for LHC magnets benefiting their main business, the fabrication of hydro generators. Canadians were instrumental in the construction of the ATLAS detector. ATLAS construction was supported by a $12 million grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). Canadian contributions to the ATLAS detector were completed on time and on budget, are now installed in the ATLAS experiment where they are currently successfully used to record the LHC collisions. 

When taking data, ATLAS produces several Peta-bytes (millions of Giga-bytes) of data per year. Canada has constructed a Tier1 computing centre at TRIUMF funded by the Canadian Foundation for Innovation (CFI) and the BC Knowledge Development Fund (BCKDF) at the levels of $12 million and $4 million, respectively. The primary role of the Tier1 centre is the processing of raw ATLAS data which will be used by physicists to understand what is going on in the high energy proton collisions. The final analyses will be performed largely on the Tier 2 computing centres located at university sites, funded by the CFI National Platforms Fund. The combined Canadian Tier1 and Tier2 centres give us "made in Canada" physics analysis ability, positioning ourselves to be leaders in extracting ATLAS physics over the coming years.

The UVic-ATLAS group is currently composed of over 25 scientists, including students, research associates, technicians, computer experts, engineers and physics professors. Since 1992, the ATLAS project at UVic provides unique opportunities for the training of highly qualified personnel. Many former UVic-ATLAS members now hold permanent positions in top institutions in Canada and abroad.

The UVic-ATLAS group has made crucial contributions to the design, development and construction of the ATLAS detector since 1992. UVic's contributions focused on detector components, called calorimeters, specialized in the measurement of the energy of particles. The chosen technology features liquid argon as active medium, and makes use of a novel geometrical design optimized in part at UVic. From 1992 to 2004 UVic scientists participated in the prototyping of calorimeter detectors, in the testing of these detectors with particle beams, and in the construction of the final full size ATLAS components. Since liquid argon is very cold, about -188 degrees Celsius, the calorimeters must be enclosed in purpose-built cryostats. A critical component of such cryostats are its cryogenic feedthroughs, which allow nearly 200,000 electrical signals from the calorimeters to reach the outside world. Most of these feedthroughs were constructed in UVic between 1997 and 2002, with the support of an $4.0M NSERC Major Installation Grant. Members of the UVic team spent considerable time at CERN integrating these components on the final ATLAS detector.

From 2005 to 2009, the UVic-ATLAS group was heavily involved in the final preparation of the detector leading to first collisions. This commissioning work involved the testing of aspects of the ATLAS detector.  UVic scientists developped and maintained computer software and methods that are used to monitor the detector operations, and to ensure that the recorded data is of the highest quality. UVic scientists also developed strategies and software for the analysis of the collisions.

Since 2010, UVic scientists are actively analyzing the data collected at the LHC, and particlpated in the discovery of a Higgs boson.  Analyses involve Standard Model and Higgs boson measurements, and searches for new phenomena such as supersymmetric particles, or evidence for Dark Matter.  UVic scientists also contribute to the operation of the ATLAS detector, in particlular the operation of the liquid argon calorimetry and the development and operation of the ATLAS high level trigger, and to the upgrade of the ATLAS liquid argn calorimeter electronics and the new muon small wheel.

A Memorial Symposium in honour of our colleague Prof. Alan Astbury (1934-2014) was held at UVic the 27-28 of April 2015 and featured a public talk by the Director General of CERN, Dr Rolf Heuer; this talk was the first of the Astbury Public Lecture series.