Canada

Since the discovery of the Higgs boson at the LHC in 2012, no sign of new physics beyond the Standard Model has been found. The SUSY and exotic particles searches have not uncovered signs of new physics, as the model-dependent searches. In recent years, multiple unsupervised machine learning methods have been proposed to search for new physics at the LHC. This poster will explore the use of a variational auto-encoder (VAE) to perform a general search in proton-proton collisions at the LHC using large radius jets in ATLAS simulation data. To test our workflow, we trained the algorithm on low-level jet information to differentiate between the dominant QCD background and a chosen test signal corresponding to top quark jets. The most anomalous jets predicted by the VAE were selected to plot the invariant mass spectrum and to find the top quark mass peak. Our study found an important correlation between the jet invariant mass and the loss function of the VAE, resulting in a sculpting of our background and preventing the apparition of the top peak. We successfully used a mass-decorrelation method based on Outlier Exposure to prevent this sculpting.

CAP Virtual Congress 2021

Search for New Physics Inside Boosted Jets at the ATLAS Detector Using Anomaly Detection

boosted jets

new physics

atlas

particle physics

anomaly detection

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Telescope Array experiment has recorded several short time bursts of air shower like events. These bursts are very distinct from conventional single showers, and are found to be strongly correlated with lightnings. We proposed these bursts represent the direct manifestation of the dark matter annihilation events within the so-called axion quark nugget model. We discuss how to test this proposal by searching for the radio signals in frequency band (0.5−200) MHz which must be synchronized with the Telescope Array bursts.

Dark matter, axion quark nugget, and mysterious bursts observed by telescope array

Longstanding evidence from observational astronomy indicates that non-luminous "dark matter" constitutes the majority of all matter in the universe, yet this mysterious form of matter continues to elude experimental detection. The study presented in this talk is part of an ongoing programme to search for dark matter production in high-energy proton-proton collisions at the Large Hadron Collider (LHC) at CERN. This search targets a model in which dark matter is produced in association with the emission of a hypothesized heavy Higgs boson in the dark sector, which then decays to a pair of W bosons. The final-state signature of this model would be an excess of missing transverse energy in the detector due to undetected dark matter production, along with two reconstructed W bosons. A search was recently performed targeting this final state in the 'hadronic' decay channel, wherein both W bosons decay to a pair of quarks. The semi-leptonic WW decay channel, in which one of the bosons instead decays to a lepton and neutrino, will complement and extend the reach of the existing search in the hadronic channel. A dark matter search in this semi-leptonic WW decay channel is presented.

Search for Dark Matter Produced in Association with a Dark Sector Higgs Boson in Proton-Proton Collisions with the ATLAS Detector

DEAP-3600 is a liquid argon detector designed to directly detect dark matter by searching for nuclear recoil (NR) events caused by elastically scattered weakly interacting massive particles, a prime candidate for dark matter. Pulse-shape discrimination properties of the argon allow for significant separation between electromagnetic recoil (ER) events and NR events. The majority of the events in the ER spectrum are beta decays of Ar-39, which is an isotope naturally present in argon. Several efforts are underway to study the Ar-39 spectrum in DEAP-3600. In order to perform these studies, we must have an extensive understanding of the possible background in the ER spectrum. A possible background contribution to this spectrum are events from Kr-85, a radioactive isotope that can potentially bypass the purification methods used to fill the detector with argon. Trace amounts of Kr-85 are produced in the atmosphere via interactions with cosmic rays and Kr-84, which could be present in atmospheric argon. Given that the beta spectrum endpoint (Q-value) of Kr-85 is slightly higher than Ar-39, excess events in the region just above the Q-value of Ar-39 are expected if there is any contamination by Kr-85. A full spectrum fit was done to search for potential contributions from this isotope.

Constraining contributions from Kr-85 in DEAP-3600

DEAP-3600, hosted at SNOLAB, has been designed for the search of WIMPs, Weakly Interacting Massive Particles; its target of 3.3 t of liquid argon is the largest direct detection experiment. In addition to its sensitivity to WIMPs, DEAP-3600 is sensitive to super-massive dark matter candidates with masses up to the Planck scale. At dark matter-argon cross-sections above 10^{-24} cm^2 and dark matter masses above 10^{16} GeV, dark matter particles are expected to reach an underground experiment and give a detectable signal in liquid argon; indeed particles at lower masses and such high cross-sections are eventually stopped by the overburden. Moreover, due to the large cross-section, all MIMPs that enter the detector are expected to produce signals; as a consequence, the sensitivity to high masses depends on the cross-sectional area of the detector. The expected signal in DEAP-3600 is a collinear sequence of nuclear recoils in the same acquisition window, giving a very clear and unique signature;hence, we refer to them as MIMPs, Multi-interacting Massive Particles. In the present talk, the search for MIMPs in three years of data-taking is presented, showing the first search for multi-scattering dark matter in noble liquids ever performed.

Multi-Interacting Massive Particles in DEAP-3600

The highest energy range of the solar neutrino spectrum is dominated by $^8$B neutrinos produced in the pp-chain in the Sun and by hep neutrinos. R.S. Raghavan, K. Bhattacharya, and others have predicted that neutrino absorption with $^{40}$Ar is a possible interaction for neutrinos with energies above 3.9 MeV. In this case, neutrino induced nuclear transitions from $^{40}$Ar to $^{40}$K are feasible. One possible transition produces a delayed coincidence signature with a mean lifetime of 480 ns. A search for this process relies on understanding the backgrounds for this search, specifically neutron capture gammas. These neutrons include both radiogenic neutrons from PMTs and detector materials and cosmogenic neutrons from muon interactions with the surrounding rock. We propose to search for this process using 3 years of data from the DEAP-3600 dark matter experiment and present the latest efforts in this on-going study. DEAP-3600 is a liquid argon (LAr) direct dark matter experiment based at SNOLAB that is designed to detect WIMP-nucleon scattering in argon. The experiment's ultra-low background, high sensitivity and its large target mass could make it possible to observe this process for the first time.

A search for neutrino absorption with 40Ar using the DEAP-3600 detector

EMPHATIC (Experiment to Measure the Production of Hadrons At a Testbeam In Chicagoland) is a low-cost, table-top-sized, hadron-production experiment located at the Fermilab Test Beam Facility (FTBF) that will measure hadron scattering and production cross sections that are relevant for neutrino flux predictions. High statistics data will be collected using a minimum bias trigger, enabling measurements of both interacting and non-interacting cross sections. Particle identification will be done using a compact aerogel+heavy gas hybrid ring imaging Cherenkov (RICH) detector, a time-of-flight (ToF) wall, and a lead glass calorimeter array. The ARICH focuses on the kaons, pions and protons separation with multi-track capability up to 8 GeV/c. This presentation is about the implementation of optical reflectors in the ARICH system to reflect Cherenkov light outside of the PMT array acceptance onto the PMT array increasing the angular acceptance of the experiment with a low cost improvement.

Optical reflectors in an ARICH detector for a hadron production experiment

From 2015 to 2018, the Large Hadron Collider (LHC) collided protons at an unprecedented center of mass energy of s=√13 TeV. The ATLAS detector recorded an integrated luminosity of 139fb$^{-1}$ of these collisions. This offers an unprecedented opportunity for physicists to test the Standard Model by measuring predicted but yet unobserved rare processes. The triboson W$\gamma\gamma$ production is one of these unobserved processes. Its sensitivity to the electroweak trilinear and quartic gauge couplings make it a great probe of new physics phenomena as Beyond Standard Model processes could affect the effective strength of these couplings. The dominant source of background to the search for W$\gamma\gamma$ production are jets being misidentified as photons. The advanced data-driven technique used to estimate this background will be presented. Furthermore, preliminary studies of the dominant systematic uncertainties impacting the expected significance of the measurement will be presented.

Study of Wyy tri-boson production in proton-proton collisions with the ATLAS detector

The major ongoing upgrade of the ATLAS detector at the Large Hadron Collider at CERN consists in the replacement of parts of its muon spectrometer. The so-called New Small Wheels (NSWs) will be covered with two detector types that must trigger on and track outgoing particles - one type is small-strip thin gap chambers (sTGCs) assembled into modules of four layers. Canadian-built sTGC modules are characterized at McGill University using cosmic rays before being shipped to CERN for integration into the wheels. To achieve the design tracking performance, misalignments between sTGC layers must be corrected for. The charge profile left by an x-ray gun and coordinate measuring machine (CMM) measurements of quadruplet layers are being used to define these parameters. Work on using cosmic ray data to validate misalignment parameters derived using the above-mentioned methods will be presented.

Validating misalignment measurements between particle detectors for the ATLAS New Small Wheels

MATHUSLA (MAssive Timing Hodoscope for Ultra-Stable neutraL pArticles) is a long-lived particle (LLP) detector which would be constructed on the surface above CMS and is currently in its planning stages. This large-area detector would be composed of several layers of solid plastic scintillator, with wavelength-shifting fibers connected to silicon photomultipliers (SiPMs), allowing us to monitor an empty air-filled decay volume. The purpose of this research is to examine the behavior of a candidate SiPM and signal amplifier for this detection array. The SiPM in question is the SensL MicroFB-30035 and the amplifier is the Broadcom AFBR-S4E001. This goal was accomplished by simulating these components in Ngspice, an electronic circuit simulator. Using this simulation method, we were able to measure the amplified signal output and produce voltage-time plots for it. We will next build this setup in the lab and compare our results to other candidate SiPMs. This research will be useful in developing other aspects of the detection array, the triggering system, and the readout system.

Analyzing the behavior of a Candidate SiPM and Signal Amplifier for MATHUSLA

The NEWS-G experiment uses a spherical proportional counter filled with gas in order to detect potential dark matter particles that can ionize the gas after a nuclear recoil. The detector works by attracting the free electrons towards the center of the sphere where there is a high voltage anode inducing a radial electric field. Near the anode, the accelerated electrons then cause a Townsend avalanche that produces many drifting ions, which creates an identifiable electrical signal. Since this method has been most efficient at energies below 1 GeV, that is where keeping the background contamination at minimum is of upmost importance. However, there is approximately 20 mBq of alpha particles coming from impurities in the copper surface of the sphere that create a sudden influx of events close to the region of interest for up to five seconds. These alphas also lead to fluctuations in the electric field, which in turn alters the time taken for electrons to reach the anode at the center. This presentation aims to show how the consequences of alpha particles in the detector can be characterized, as well as how those consequences can be used to better identify alpha events in order to remove the correlated influx of low energy events.
*On behalf of the NEWS-G collaboration

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Search for New Physics Inside Boosted Jets at the ATLAS Detector Using Anomaly Detection

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