High Energy Frontier

Constraints on new phenomena via Higgs boson couplings and invisible decays: A crucial question in particle physics is whether the Higgs boson discovered in 2012 is truly the fundamental scalar predicted by the Standard Model (SM). Strong theoretical arguments suggest that the SM is only an approximation to a more fundamental theory such as supersymmetry or composite Higgs models, which predict modified properties of the Higgs with respect to SM expectations. As published in the Journal of High Energy Physics (2015), the results of several analyses of production and decay rates of the Higgs boson in different channels were combined to determine how the couplings scale with mass and hence put constraints on various extensions of the SM. Vector boson processes and associated WH/ZH production set an upper limit on the Higgs boson decay branching ratio to invisible particles, such as dark matter, of 25%.    

Observation of Higgs boson production in association with a top quark pair: A probe of fundamental interest to further explore the nature of the Higgs boson is to measure its interaction with the top-quark, the most massive particle in the Standard Model. Indirect measurements of this interaction were previously made assuming no contribution from unknown particles. A more direct test of this coupling can be performed through the direct production of the Higgs boson in association with a top-quark pair, ttH. Measuring this process is challenging, because it is extremely rare: only one percent of Higgs bosons are expected to be produced this way. As submit to Physics Letters B (2018), using advanced analysis techniques, several independent searches for ttH production have been performed and combined, yielding the first observation of ttH production with a significance of 6.3 standard deviations relative to the background-only hypothesis.

Exclusion of Obvious and Accessible Supersymmetry: The key feature of a proton collider is copious pair production of strongly interacting particles. The ATLAS collaboration scoured the entire dataset collected in the 8 TeV Large Hadron Collider run for an excess of events containing only particle jets, with an imbalance of transverse momentum. This would be the signature of strongly-produced supersymmetric particles decaying to Standard Model particles and the stable lightest supersymmetric particle, a weakly interacting particle detectable only by the hole it would leave, and thus an excellent candidate for dark matter. As published in the Journal of High Energy Physics (2015), no excess was found, and lower limits on masses of a large number of supersymmetric particles were obtained in a wide variety of benchmark and simplified models. These were in excess of one TeV for most strongly produced particles.    

Elusive dark matter and other exotic phenomena: A number of astrophysical measurements point to the existence of a new form of matter. For instance, the rotational speed of stars and observation of gravitational lensing effects strongly indicate the presence of so-called dark matter, in addition to our ordinary matter, that would compose a large fraction of our universe. Dark matter particles can be directly produced at the Large Hadron Collider, and one striking event signature would be the presence of an energetic jet of ordinary particles (called a monojet) and large missing energy due to dark matter particles escaping the ATLAS detector. As reported in the Journal of High Energy Physics (2018), a monojet final state constitutes a distinctive signature of beyond-Standard Model physics, and is also used to search for extra spatial dimensions and supersymmetry. Constraints have been set on various models.  

Combination of searches for heavy resonances decaying into bosonic and leptonic final states: A generic prediction of many extensions of the Standard Model (SM) is the existence of heavy bosons decaying into pairs SM gauge bosons, as well as WH, ZH, or a pair of fermions. Specific searches for diboson resonances in several decay channels were combined to set constraints, using simple benchmark models, on the existence of a heavy hypothetical scalar, vector, or tensor particle. Analyses of leptonic final states  and were further combined with the diboson searches. Limit contours were obtained on the couplings of a heavy vector triplet (HVT) to quarks, leptons and the Higgs boson. The data exclude an HVT boson with mass below 5.5 (4.5) TeV in a weakly-coupled (strongly-coupled) scenario. Limits are also set on a Kaluza-Klein graviton.

Vacuum stability and the MSSM Higgs mass: Supersymmetry is a leading candidate for beyond-Standard Model physics. A crucial requirement for supersymmetry to be realized is that it does not lead to the catastrophic destruction of the universe, which can occur if the theory contains new lowest-energy vacuum states that are deeper than the one that we live in. As reported in the Journal of High Energy Physics (2014), TRIUMF theorists investigated the implications of this vacuum stability condition on the minimal supersymmetric extension of the Standard Model (MSSM) by studying the vacuum structure of the theory as well as the quantum transition rates between vacuum states. A close connection was found between the stability of the Universe, the observed mass of the Higgs boson, and the properties of the supersymmetric partner particles of the top quarks which are currently being searched for with CERN's Large Hadron Collider.

Pions, muons and positrons: The M11 beam channel provides low intensity beams of pions, muons and positrons for testing and calibrating detectors for particle physics experiments world-wide.  Three notable examples from the period 2013-2018 are described here. TREK The TREK experiment is searching for new physics beyond the standard model in the rare decay modes of kaons. In November 2013 and June 2014, a group led by Mike Hasinoff (UBC) tested the scintillating fibre target to establish the bias voltage offsets for each individual MPPC detector/scintillating fibre combination. Muon and positron tracks through the detector were measured to compute individual fibre efficiencies. This target was subsequently used for data taking in Experiment 36 at J-PARC in Tokai, Japan. Previous to this, in November 2012, the TREK group tested TOF counter time resolution and e-mu discrimination using a polyethylene block placed in front of a Lead-Glass-Counter to change the shower development. ATLAS Polycrystalline chemical vapor deposition (pCVD) diamond detectors are a candidate for forward calorimetry in the high luminosity environment of the CERN LHC.  One such detector was exposed to particles delivered by M11 to quantify the variation in signal response across the surface of such a detector. A discrepancy was observed in the diamond detector’s response to beam particles at different bias polarities.  This study was documented in a Carleton MSc thesis, "Characterization of diamond sensors for use in ATLAS calorimetry upgrades", by Joshua Turner, 2012, Super-B and BELLE The Super-B drift chamber group undertook two tests of drift chamber prototypes in the M11 beam line in 2013. These tests were the first to demonstrate that the particle identification capabilities of drift chambers could be significantly improved by counting individual ionization events in the gas (“cluster counting”), rather than just measuring the total energy deposited in each cell. The results were published as NIM A735, 169-183 (2014), and formed a key component of the PhD thesis of Jean-Francois Caron (UBC, 2015). After the merging of the Super-B and BELLE-II collaborations, the Belle II Canada group used the M11 beamline to test pure and thallium CsI crystals with various readout options in the summer of 2015. The group subsequently decided not to pursue pure CsI. The thallium-doped CsI data have been used, along with data collected at the proton irradiation facility at TRIUMF, to develop a new method of distinguishing hadronic from electromagnetic energy deposits using pulse shape discrimination. This technique is being implemented for the Belle II calorimeter, and will form an important component of the PhD thesis of S. Longo (Victoria).  

2013 Nobel Prize in Physics and the characterization of the Higgs boson: The 2013 Nobel Prize in Physics was awarded jointly to François Englert and Peter Higgs "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider”. The 2012 discovery paper constituted the “Observation of a new particle in the search of the Standard Model Higgs boson”. The scientific justification from the Nobel Prize Committee cited in addition two ATLAS papers in Physics Letters B  (2013). The first and second papers showed the consistency with the spin-0 and even parity characteristics of the discovered particle as well as couplings to bosons that were as expected for a Higgs boson.