Loukas Gouskos
· Christopher M. Barter Assistant Professor of PhysicsBrown University · Physics
Active 2008–2024
About
Professor Loukas Gouskos joined the Physics Department at Brown University in 2023, after serving as an Experimental Physics Research Staff at the European Organization for Nuclear Research (CERN). He is an experimental particle physicist specializing in the study of the Higgs boson and the search for physics beyond the Standard Model. His work at the Large Hadron Collider (LHC) involves handling enormous volumes of data, where he has been at the forefront of developing and applying cutting-edge Artificial Intelligence (AI) and Deep Learning (DL) techniques since the early days of LHC operations. His contributions span multiple areas within high-energy physics, leveraging advanced algorithms to address the extreme complexity of data produced by particle collisions. In addition to his physics analysis and AI/DL work, Gouskos is heavily involved in the research and development of detector upgrades and the design of future experiments, including the Future Circular Collider (FCC).
Research topics
- Physics
- Nuclear physics
- Particle physics
- Computer Science
- Optics
- Artificial Intelligence
- Astrophysics
- Mechanics
- Algorithm
- Operating system
- Programming language
- Theoretical computer science
- Quantum mechanics
- Geography
Selected publications
Review of top quark mass measurements in CMS
OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information) · 2025 · 8 citations
- Particle physics
- Physics
- Nuclear physics
The top quark mass is one of the most intriguing parameters of the standard model (SM). Its value indicates a Yukawa coupling close to unity, and the resulting strong ties to Higgs physics make the top quark mass a crucial ingredient for understanding essential aspects of the electroweak sector of the SM. This review offers the first comprehensive overview of the top quark mass measurements performed by the CMS Collaboration using the data collected at centre-of-mass energies of 7, 8, and 13 TeV. Moreover, a detailed description of the top quark event reconstruction is provided and dedicated studies of the dominant uncertainties in the modelling of the signal processes are discussed. The interpretation of the experimental results on the top quark mass in terms of the SM Lagrangian parameter is challenging and is a focus of an ongoing discussion in the theory community. The CMS Collaboration has performed two main types of top quark mass measurements, addressing this challenge from different perspectives: highly precise ‘direct’ measurements, based on reconstructed top quark decay products and relying exclusively on Monte-Carlo simulations, as well as ‘indirect’ measurements, where the simulations are employed to determine parton-level cross sections that are compared to fixed-order perturbative calculations. Recent mass extractions using Lorentz-boosted top quarks open a new avenue of measurements based on top quark decay products contained in a single particle jet, with promising prospects for accurate theoretical interpretations.
Performance of CMS muon reconstruction from proton-proton to heavy ion collisions
OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information) · 2024 · 3 citations
- Physics
- Nuclear physics
- Particle physics
Abstract The performance of muon tracking, identification, triggering, momentum resolution, and momentum scale has been studied with the CMS detector at the LHC using data collected at √( s NN ) = 5.02 TeV in proton-proton (pp) and lead-lead (PbPb) collisions in 2017 and 2018, respectively, and at √( s NN ) = 8.16 TeV in proton-lead (pPb) collisions in 2016. Muon efficiencies, momentum resolutions, and momentum scales are compared by focusing on how the muon reconstruction performance varies from relatively small occupancy pp collisions to the larger occupancies of pPb collisions and, finally, to the highest track multiplicity PbPb collisions. We find the efficiencies of muon tracking, identification, and triggering to be above 90% throughout most of the track multiplicity range. The momentum resolution and scale are unaffected by the detector occupancy. The excellent muon reconstruction of the CMS detector enables precision studies across all available collision systems.
Physical Review Letters · 2024 · 96 citations
- Physics
A search is reported for near-threshold structures in the J/ψJ/ψ invariant mass spectrum produced in proton-proton collisions at sqrt[s]=13 TeV from data collected by the CMS experiment, corresponding to an integrated luminosity of 135 fb^{-1}. Three structures are found, and a model with quantum interference among these structures provides a good description of the data. A new structure is observed with a local significance above 5 standard deviations at a mass of 6638_{-38}^{+43}(stat)_{-31}^{+16}(syst) MeV. Another structure with even higher significance is found at a mass of 6847_{-28}^{+44}(stat)_{-20}^{+48}(syst) MeV, which is consistent with the X(6900) resonance reported by the LHCb experiment and confirmed by the ATLAS experiment. Evidence for another new structure, with a local significance of 4.7 standard deviations, is found at a mass of 7134_{-25}^{+48}(stat)_{-15}^{+41}(syst) MeV. Results are also reported for a model without interference, which does not fit the data as well and shows mass shifts up to 150 MeV relative to the model with interference.
Search for heavy neutral leptons in final states with electrons, muons, and hadronically decaying tau leptons in proton-proton collisions at $ \sqrt{s} $ = 13 TeV
2024
- Physics
- Particle physics
- Nuclear physics
A search for heavy neutral leptons (HNLs) of Majorana or Dirac type using proton-proton collision data at $\sqrt{s}$ =13 TeV is presented. The data were collected by the CMS experiment at the CERN LHC and correspond to an integrated luminosity of 138 fb$^{-1}$. Events with three charged leptons (electrons, muons, and hadronically decaying tau leptons) are selected, corresponding to HNL production in association with a charged lepton and decay of the HNL to two charged leptons and a standard model (SM) neutrino. The search is performed for HNL masses between 10 GeV and 1.5 TeV. No evidence for an HNL signal is observed in data. Upper limits at 95% confidence level are found for the squared coupling strength of the HNL to SM neutrinos, considering exclusive coupling of the HNL to a single SM neutrino generation, for both Majorana and Dirac HNLs. The limits exceed previously achieved experimental constraints for a wide range of HNL masses, and the limits on tau neutrino coupling scenarios with HNL masses above the W boson mass are presented for the first time.
Reports on Progress in Physics · 2024 · 16 citations
- Physics
- Nuclear physics
- Particle physics
. The measurement is performed by studying the multiplicity dependence of the average transverse momentum of charged particles emitted in head-on PbPb collisions. Our findings reveal that the speed of sound in this matter is nearly half the speed of light, with a squared value of0.241±0.002(stat)±0.016(syst)in natural units. The effective medium temperature, estimated using the mean transverse momentum, is219±8(syst)MeV. The measured squared speed of sound at this temperature aligns precisely with predictions from lattice quantum chromodynamic (QCD) calculations. This result provides a stringent constraint on the equation of state of the created medium and direct evidence for a deconfined QCD phase being attained in relativistic nuclear collisions.
Journal of High Energy Physics · 2023 · 69 citations
- Physics
- Particle physics
- Nuclear physics
A bstract Three searches are presented for signatures of physics beyond the standard model (SM) in ττ final states in proton-proton collisions at the LHC, using a data sample collected with the CMS detector at $$ \sqrt{s} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msqrt> <mml:mi>s</mml:mi> </mml:msqrt> </mml:math> = 13 TeV, corresponding to an integrated luminosity of 138 fb − 1 . Upper limits at 95% confidence level (CL) are set on the products of the branching fraction for the decay into τ leptons and the cross sections for the production of a new boson ϕ , in addition to the H(125) boson, via gluon fusion (gg ϕ ) or in association with b quarks, ranging from $$ \mathcal{O} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>O</mml:mi> </mml:math> (10 pb) for a mass of 60 GeV to 0.3 fb for a mass of 3.5 TeV each. The data reveal two excesses for gg ϕ production with local p -values equivalent to about three standard deviations at m ϕ = 0 . 1 and 1.2 TeV. In a search for t -channel exchange of a vector leptoquark U 1 , 95% CL upper limits are set on the dimensionless U 1 leptoquark coupling to quarks and τ leptons ranging from 1 for a mass of 1 TeV to 6 for a mass of 5 TeV, depending on the scenario. In the interpretations of the $$ {M}_{\textrm{h}}^{125} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>M</mml:mi> <mml:mi>h</mml:mi> <mml:mn>125</mml:mn> </mml:msubsup> </mml:math> and $$ {M}_{\textrm{h},\textrm{EFT}}^{125} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>M</mml:mi> <mml:mrow> <mml:mi>h</mml:mi> <mml:mo>,</mml:mo> <mml:mi>EFT</mml:mi> </mml:mrow> <mml:mn>125</mml:mn> </mml:msubsup> </mml:math> minimal supersymmetric SM benchmark scenarios, additional Higgs bosons with masses below 350 GeV are excluded at 95% CL.
Physics Letters B · 2023 · 75 citations
- Computer Science
- Physics
- Algorithm
Measurements are presented of the Bs0→μ+μ− branching fraction and effective lifetime, as well as results of a search for the B0→μ+μ− decay in proton-proton collisions at s=13TeV at the LHC. The analysis is based on data collected with the CMS detector in 2016–2018 corresponding to an integrated luminosity of 140fb−1. The branching fraction of the Bs0→μ+μ− decay and the effective Bs0 meson lifetime are the most precise single measurements to date. No evidence for the B0→μ+μ− decay has been found. All results are found to be consistent with the standard model predictions and previous measurements.
Performance of the CMS High Granularity Calorimeter prototype to charged pion beams of 20–300 GeV/c
Journal of Instrumentation · 2023 · 13 citations
- Computer Science
- Physics
- Nuclear physics
Abstract The upgrade of the CMS experiment for the high luminosity operation of the LHC comprises the replacement of the current endcap calorimeter by a high granularity sampling calorimeter (HGCAL). The electromagnetic section of the HGCAL is based on silicon sensors interspersed between lead and copper (or copper tungsten) absorbers. The hadronic section uses layers of stainless steel as an absorbing medium and silicon sensors as an active medium in the regions of high radiation exposure, and scintillator tiles directly read out by silicon photomultipliers in the remaining regions. As part of the development of the detector and its readout electronic components, a section of a silicon-based HGCAL prototype detector along with a section of the CALICE AHCAL prototype was exposed to muons, electrons and charged pions in beam test experiments at the H2 beamline at the CERN SPS in October 2018. The AHCAL uses the same technology as foreseen for the HGCAL but with much finer longitudinal segmentation. The performance of the calorimeters in terms of energy response and resolution, longitudinal and transverse shower profiles is studied using negatively charged pions, and is compared to GEANT4 predictions. This is the first report summarizing results of hadronic showers measured by the HGCAL prototype using beam test data.
HAL (Le Centre pour la Communication Scientifique Directe) · 2023 · 7 citations
- Physics
- Particle physics
- Nuclear physics
A data sample containing top quark pairs ($t\overline{t}$) produced in association with a Lorentz-boosted $Z$ or Higgs boson is used to search for signs of new physics using effective field theory. The data correspond to an integrated luminosity of $138\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of proton-proton collisions produced at a center-of-mass energy of 13 TeV at the LHC and collected by the CMS experiment. Selected events contain a single lepton and hadronic jets, including two identified with the decay of bottom quarks, plus an additional large-radius jet with high transverse momentum identified as a $Z$ or Higgs boson decaying to a bottom quark pair. Machine learning techniques are employed to discriminate between $t\overline{t}Z$ or $t\overline{t}H$ events and events from background processes, which are dominated by $t\overline{t}+\text{jets}$ production. No indications of new physics are observed. The signal strengths of boosted $t\overline{t}Z$ and $t\overline{t}H$ production are measured, and upper limits are placed on the $t\overline{t}Z$ and $t\overline{t}H$ differential cross sections as functions of the $Z$ or Higgs boson transverse momentum. The effects of new physics are probed using a framework in which the standard model is considered to be the low-energy effective field theory of a higher energy scale theory. Eight possible dimension-six operators are added to the standard model Lagrangian, and their corresponding coefficients are constrained via fits to the data.
arXiv (Cornell University) · 2023 · 1 citations
- Physics
- Particle physics
- Nuclear physics
A search for long-lived heavy neutral leptons (HNLs) is presented, which considers the hadronic final state and coupling scenarios involving all three lepton generations in the 2-20 GeV HNL mass range for the first time. Events comprising two leptons (electrons or muons) and jets are analyzed in a data sample of proton-proton collisions, recorded with the CMS experiment at the CERN LHC at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 138 fb$^{-1}$. A novel jet tagger, based on a deep neural network, has been developed to identify jets from an HNL decay using various features of the jet and its constituent particles. The network output can be used as a powerful discriminating tool to probe a broad range of HNL lifetimes and masses. Contributions from background processes are determined from data. No excess of events in data over the expected background is observed. Upper limits on the HNL production cross section are derived as functions of the HNL mass and the three coupling strengths $V_{\ell\mathrm{N}}$ to each lepton generation $\ell$ and presented as exclusion limits in the coupling-mass plane, as lower limits on the HNL lifetime, and on the HNL mass. In this search, the most stringent limit on the coupling strength is obtained for pure muon coupling scenarios; values of $\lvert V_{\mu\mathrm{N}}\rvert^{2}\gt $ 5 (4)$\times$10$^{-7}$ are excluded for Dirac (Majorana) HNLs with a mass of 10 GeV at a confidence level of 95% that correspond to proper decay lengths of 17 (10) mm.
Frequent coauthors
- 2108 shared
M. Titov
Institut de Recherche sur les Lois Fondamentales de l'Univers
- 2088 shared
G. Hamel de Monchenault
Université Paris-Saclay
- 2040 shared
A. Rosowsky
Institut de Recherche sur les Lois Fondamentales de l'Univers
- 1974 shared
M. Besançon
CEA Paris-Saclay
- 1898 shared
M. Lethuillier
Institute of Nuclear Physics of Lyon
- 1761 shared
A. Meyer
Deutsches Elektronen-Synchrotron DESY
- 1756 shared
S. Perriès
Institute of Nuclear Physics of Lyon
- 1749 shared
J. Andreä
Institut Pluridisciplinaire Hubert Curien
Labs
Education
- 2015
Ph.D.
National and Kapodistrian University of Athens
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