Selected Publications by D. Stuart

• Search for long-lived particles decaying in the CMS muon detectors in proton-proton collisions at sqrt(s)=13 TeV PhysRevD.110.032007
  

  A search for long-lived particles (LLPs) decaying in the CMS muon detectors is presented. A data sample of proton-proton collisions at sqrt(s)=13 TeV corresponding to an integrated luminosity of 138 fb−1, recorded at the LHC in 2016–2018, is used. The decays of LLPs are reconstructed as high multiplicity clusters of hits in the muon detectors. In the context of twin Higgs models, the search is sensitive to LLP masses from 0.4 to 55 GeV and a broad range of LLP decay modes, including decays to hadrons, τ leptons, electrons, or photons. No excess of events above the standard model background is observed. The most stringent limits to date from LHC data are set on the branching fraction of the Higgs boson decay to a pair of LLPs with masses below 10 GeV. This search also provides the best limits for various intervals of LLP proper decay length and mass. Finally, this search sets the first limits at the LHC on a dark quantum chromodynamic sector whose particles couple to the Higgs boson through gluon, Higgs boson, photon, vector, and dark-photon portals, and is sensitive to branching fractions of the Higgs boson to dark quarks as low as 2×10⁻³.

• Technical Proposal for the milliQan sub-detector CERN-LHCC-2021-022
  

  A dedicated detector, milliQan, is proposed for detecting new particles with small electric charge (down to 10^-3 of the charge of the electron) produced by proton collisions at CERN's LHC Point 5. Such particles could be related to the dark matter in the Universe. The apparatus consists of two sub-detectors, each with four segmented layers of scintillating plastic optically coupled to high-gain photomultipliers, that will be installed before the end of LS2 in the existing PX56 drainage gallery above UXC. With the dataset that LHC Run3 will provide, milliQan will significantly extend the parameter space explored for new particles with small charges, and masses above 100 MeV.

• Sensitivity to millicharged particles in future proton-proton collisions at the LHC arXiV:2104.07151
  

  We report on the expected sensitivity of dedicated scintillator-based detectors at the LHC for elementary particles with charges much smaller than the electron charge. The dataset provided by a prototype scintillator-based detector is used to characterise the performance of the detector and provide an accurate background projection. Detector designs, including a novel slab detector configuration, are considered for the LHC Run 3 and for the HL-LHC. With the Run 3 dataset, the existence of new particles with masses between 10 MeV and 45 GeV could be excluded at 95% confidence level for charges between 0.003e and 0.3e, depending on their mass. With the HL-LHC dataset, the expected limits would reach between 10 MeV and 80 GeV for charges between 0.0018e and 0.3e, depending on their mass.

• Search for millicharged particles in proton-proton collisions at sqrt(s)=13 TeV Phys.Rev.D 102 (2020) 3, 032002
  

  We report on a search for elementary particles with charges much smaller than the electron charge using a data sample of proton-proton collisions provided by the CERN Large Hadron Collider in 2018, corresponding to an integrated luminosity of 37.5 fb^-1 at a center-of-mass energy of 13 TeV. A prototype scintillator-based detector is deployed to conduct the first search at a hadron collider sensitive to particles with charges ≤0.1e. The existence of new particles with masses between 20 and 4700 MeV is excluded at 95% confidence level for charges between 0.006e and 0.3e, depending on their mass. New sensitivity is achieved for masses larger than 700 MeV.

• Search for long-lived particles using nonprompt jets and missing transverse momentum with proton-proton collisions at √s= 13 TeV, CMS Collaboration. Phys.Lett. B797 (2019) 134876
  

  A search for long-lived particles decaying to displaced, nonprompt jets and missing transverse momentum is presented. The data sample corresponds to an integrated luminosity of 137 fb^-1 of proton-proton collisions at a center-of-mass energy of 13 TeV collected by the CMS experiment at the CERN LHC in 2016-2018. Candidate signal events containing nonprompt jets are identified using the timing capabilities of the CMS electromagnetic calorimeter. The results of the search are consistent with the background prediction and are interpreted using a gauge-mediated supersymmetry breaking reference model with a gluino next-to-lightest supersymmetric particle. In this model, gluino masses up to 2100, 2500, and 1900 GeV are excluded at 95% confidence level for proper decay lengths of 0.3, 1, and 100 m, respectively. These are the best limits to date for such massive gluinos with proper decay lengths greater than ~0.5 m.

• Search for R-parity violating supersymmetry in pp collisions at √s = 13 TeV using b jets in a final state with a single lepton, many jets, and high sum of large-radius jet masses Phys. Lett. B 783 (2018) 114
  

  Results are reported from a search for physics beyond the standard model in proton-proton collisions at a center-of-mass energy of √s= 13 TeV. The search uses a signature of a single lepton, large jet and bottom quark jet multiplicities, and high sum of large-radius jet masses, without any requirement on the missing transverse momentum in an event. The data sample corresponds to an integrated luminosity of 35.9 fb^-1 recorded by the CMS experiment at the LHC. No significant excess beyond the prediction from standard model processes is observed. The results are interpreted in terms of upper limits on the production cross section for R-parity violating supersymmetric extensions of the standard model using a benchmark model of gluino pair production, in which each gluino decays promptly via gluino → tbs. Gluinos with a mass below 1610 GeV are excluded at 95% confidence level.

• Searches for R-parity-violating supersymmetry in pp collisions at sqrt(s)=8 TeV in final states with 0-4 leptons, CMS Collaboration. Phys. Rev. D 94, 112009 (2016)
  

  Results are presented from searches for R-parity-violating supersymmetry in events produced in pp collisions at sqrt(s)=8 TeV at the LHC. Final states with 0, 1, 2, or multiple leptons are considered independently. The analysis is performed on data collected by the CMS experiment corresponding to an integrated luminosity of 19.5 fb-1. No excesses of events above the standard model expectations are observed, and 95% confidence level limits are set on supersymmetric particle masses and production cross sections. The results are interpreted in models featuring R-parity-violating decays of the lightest supersymmetric particle, which in the studied scenarios can be either the gluino, a bottom squark, or a neutralino. In a gluino pair production model with baryon number violation, gluinos with a mass less than 0.98 and 1.03 TeV are excluded, by analyses in a fully hadronic and one-lepton final state, respectively. An analysis in a dilepton final state is used to exclude bottom squarks with masses less than 307 GeV in a model considering bottom squark pair production. Multilepton final states are considered in the context of either strong or electroweak production of superpartners and are used to set limits on the masses of the lightest supersymmetric particles. These limits range from 300 to 900 GeV in models with leptonic and up to approximately 700 GeV in models with semileptonic R-parity-violating couplings.

• Search for supersymmetry in pp collisions at sqrt s = 13 TeV in the single-lepton final state using the sum of masses of large-radius jets, CMS Collaboration, PhysRevLett.119.151802(2017)
  

  Results are reported from a search for supersymmetric particles in proton-proton collisions in the final state with a single lepton; multiple jets, including at least one b-tagged jet; and large missing transverse momentum. The search uses a sample of proton-proton collision data at sqrt(s) = 13 TeV recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 35.9 inverse femtobarns. The observed event yields in the signal regions are consistent with those expected from standard model backgrounds. The results are interpreted in the context of simplified models of supersymmetry involving gluino pair production, with gluino decay into either on- or off-mass-shell top squarks. Assuming that the top squarks decay into a top quark plus a stable, weakly interacting neutralino, scenarios with gluino masses up to about 1.9 TeV are excluded at 95% confidence level for neutralino masses up to about 1 TeV.

• A Letter of Intent to Install a milli-charged Particle Detector at LHC P5, A. Ball, et al., arxiv:1607.04669
  

  In this LOI we propose a dedicated experiment that would detect "milli-charged" particles produced by pp collisions at LHC Point 5. The experiment would be installed during LS2 in the vestigial drainage gallery above UXC and would not interfere with CMS operations. With 300 fb^-1 of integrated luminosity, sensitivity to a particle with charge O(10^-3) e can be achieved for masses of O(1) GeV, and charge O(10^-2) e for masses of O(10) GeV, greatly extending the parameter space explored for particles with small charge and masses above 100 MeV.

• Search for supersymmetry in pp collisions at sqrt(s)=13 TeV in the single-lepton final state using the sum of masses of large-radius jets, CMS Collaboration, J. High Energ. Phys. 1608 (2016) 122
  

  Results are reported from a search for supersymmetric particles in proton-proton collisions in the final state with a single, high transverse momentum lepton; multiple jets, including at least one b-tagged jet; and large missing transverse momentum. The data sample corresponds to an integrated luminosity of 2.3 inverse femtobarns at sqrt(s) = 13 TeV, recorded by the CMS experiment at the LHC. The search focuses on processes leading to high jet multiplicities, such as gluino pair production with gluinos to t t-bar neutralino₁. The quantity M_J, defined as the sum of the masses of the large-radius jets in the event, is used in conjunction with other kinematic variables to provide discrimination between signal and background and as a key part of the background estimation method. The observed event yields in the signal regions in data are consistent with those expected for standard model backgrounds, estimated from control regions in data. Exclusion limits are obtained for a simplified model corresponding to gluino pair production with three-body decays into top quarks and neutralinos. Gluinos with a mass below 1600 GeV are excluded at a 95% confidence level for scenarios with low neutralino₁ mass, and neutralinos with a mass below 800 GeV are excluded for a gluino mass of about 1300 GeV. For models with two-body gluino decays producing on-shell top squarks, the excluded region is only weakly sensitive to the top squark mass.

• Search for supersymmetry in pp collisions at sqrt(s)=8 TeV in events with a single lepton, large jet multiplicity, and multiple b jets, CMS Collaboration, Phys.Lett. B733 (2014) 328-353
  

  Results are reported from a search for new physics processes in events containing a single isolated high-transverse-momentum lepton (electron or muon), energetic jets, and large missing transverse momentum. The analysis is based on a 4.98 inverse femtobarn sample of proton-proton collisions at a center-of-mass energy of 7 TeV, obtained with the CMS detector at the LHC. Three separate background estimation methods, each relying primarily on control samples in the data, are applied to a range of signal regions, providing complementary approaches for estimating the background yields. The observed yields are consistent with the predicted standard model backgrounds. The results are interpreted in terms of limits on the parameter space for the constrained minimal supersymmetric extension of the standard model, as well as on cross sections for simplified models, which provide a generic description of the production and decay of new particles in specific, topology based final states.

• Search for supersymmetry in pp collisions at sqrt(s) = 7 TeV in events with a single lepton, jets and missing transverse momentum, CMS Collaboration, JHEP 8, 156 (2011)
  

  Results are reported from a search for physics beyond the standard model in proton-proton collisions at a center-of-mass energy of 7 TeV, focusing on the signature with a single, isolated, high-transverse-momentum lepton (electron or muon), energetic jets, and large missing transverse momentum. The data sample comprises an integrated luminosity of 36 inverse picobarns, recorded by the CMS experiment at the LHC. The search is motivated by models of new physics, including supersymmetry. The observed event yields are consistent with standard model backgrounds predicted using control samples obtained from the data. The characteristics of the event sample are consistent with those expected for the production of t t-bar and W +jets events. The results are interpreted in terms of limits on the parameter space for the constrained minimal supersymmetric extension of the standard model.

• Search for supersymmetry in events with b jets and missing transverse meomentum at the LHC, CMS Collaboration, JHEP 7, 113 (2011)
  

  A search for supersymmetry is presented using a sample of events with b jets and missing transverse momentum. The search uses a data sample of proton-proton collisions at a centre-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 35 inverse picobarns, collected with the CMS detector. A total of 0.33^+0.43_-0.33 (stat.) ± 0.13 (syst.) events is predicted, using control samples in the data, to arise from standard model processes, and one event is observed in the data. Upper limits are set at the 95% confidence level on the cross sections of benchmark supersymmetric models.

• Fast Track Finding using Radially Pointing Scintillating Fibers
  David Stuart JINST 5, C07006 (2010).
  

  A detector layout using O(cm) long, radially pointing, scintillating fibers at large radius is investigated. Such a geometry allows discrimination between high and low pT particles based on their angle of incidence and because high pT particles deposit large ionization in one fiber, while low pT particles deposit small ionization in many fibers. A pixelated array of these fibers provides a phi-z projection of particle trajectories, from which the track parameters can be calculated. The simulated track parameter resolutions are presented, and some of the unique detector and electronics issues associated with such a design are discussed.

• Background Modeling in New Physics Searches Using Forward Events at LHC.
  Victor Pavlunin and David Stuart. PhysRevD78:035012 (hep-ex:0806.2338).
  

  We present a method to measure dominant Standard Model backgrounds using data containing high rapidity objects in pp collisions at the Large Hadron Collider (LHC). The method is developed for analyses of early LHC data when robustness against imperfections of background modeling and detector simulation can be a key to the discovery of new physics at LHC.

• Search for New Heavy Particles Decaying to Z0 Z0 ---> eeee in p - anti-p Collisions at s**(1/2) = 1.96-TeV.
  T. Aaltonen et al. PhysRevD78:012008 (hep-ex:0801.1129)
  

  We report the results of a search for the anomalous production of a massive particle decaying to four electrons via two Z^0 bosons in 1.1 fb^{-1} of p-pbar collisions at sqrt(s)=1.96 TeV collected by the CDF II detector at Fermilab. We employ optimized electron identification criteria to maximize acceptance and efficiency. We estimate the backgrounds in the invariant mass range 500 - 1000 GeV/c^2 to be 0.028 +/- 0.009 (stat) +/- 0.011 (syst) events. We observe zero events in this search region. Assuming a Randall-Sundrum graviton production model, we set 95% CL limits on \sigma times BF(G -> Z^0 Z^0) < 4 - 6 pb, depending on the graviton mass.

• Search for New Particles Leading to Z+jets Final States in p anti-p Collisions at s**(1/2) = 1.96-TeV.
  T. Aaltonen et al. Phys.Rev.D76:072006,2007
  

  We present the results of a search for new particles that lead to a \Z boson plus jets in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV using the Collider Detector at Fermilab (CDF II). A data sample with a luminosity of 1.06 \ifb\ collected using \Z boson decays to $ee$ and $\mu\mu$ is used. We describe a completely data-based method to predict the dominant background from standard-model \Z+jet events. This method can be similarly applied to other analyses requiring background predictions in multi-jet environments, as shown when validating the method by predicting the background from $W$+jets in \ttbar production. No significant excess above the background prediction is observed, and a limit is set using a fourth generation quark model to quantify the acceptance. Assuming $BR(b' \to b\Z) = 100%$ and using a leading-order calculation of the $b'$ cross section, $b'$ quark masses below 268 $\gev/c^2$ are excluded at 95% confidence level.

• Search for new high mass particles decaying to lepton pairs in p anti-p collisions at s**(1/2) = 1.96-TeV.
  A. Abulencia et al. Phys.Rev.Lett.95:252001,2005
  

  A search for new particles (X) that decay to electron or muon pairs has been performed using approximately 200 pb**(-1) of pbarp collision data at sqrt(s)=1.96 TeV collected by the CDF II experiment at the Fermilab Tevatron. Limits on sigma(ppbar -> X)*BR(X -> ll) are presented as a function of dilepton invariant mass m(ll) > 150 GeV/c**2, for different spin hypotheses (0, 1, or 2). The limits are approximately 25 fb for m(ll) > 600 GeV/c**2. Lower mass bounds for X from representative models beyond the Standard Model including heavy neutral gauge bosons are presented.

• Measurement of the forward-backward charge asymmetry from W ---> e nu production in p anti-p collisions at s**(1/2) = 1.96 TeV.
  D. Acosta et al., Phys.Rev.D71:051104,2005
  

  We report a measurement of the forward-backward charge asymmetry of electrons from W boson decays in p anti-p collisions at s**(1/2) = 1.96 TeV using a data sample of 170 pb-1 collected by the Collider Detector at Fermilab. The asymmetry is measured as a function of electron rapidity and transverse energy and provides new input on the momentum fraction dependence of the u and d quark parton distribution within the proton.

• Search for long lived charged massive particles in anti-p p collisions at s**1/2 = 1.8-TeV.
  D. Acosta et al., Phys.Rev.Lett.90:131801,2003
  

  We report a search for production of long-lived charged massive particles in a data sample of 90 pb^{-1} of \sqrt{s} = 1.8 TeV p anti-p collisions recorded by the Collider Detector at Fermilab (CDF). The search uses the muon-like penetration and anomalously high ionization energy loss signature expected for such a particle to discriminate it from backgrounds. The data is found to agree with background expectations, and cross section limits of \cal{O} (1) pb are derived using two reference models, a stable quark and a stable scalar lepton.

• Charged jet evolution and the underlying event in proton - anti-proton collisions at 1.8-TeV.
  T. Affolder et al., Phys.Rev.D65:092002,2002
  

  The growth and development of "charged particle jets" produced in proton-antiproton collisions at 1.8 TeV are studied over a transverse momentum range from 0.5 GeV/c to 50 GeV/c. A variety of leading (highest transverse momentum) charged jet observables are compared with the QCD Monte Carlo models herwig, isajet, and pythia. The models describe fairly well the multiplicity distribution of charged particles within the leading charged jet, the size of the leading charged jet, the radial distribution of charged particles and transverse momentum around the leading charged jet direction, and the momentum distribution of charged particles within the leading charged jet. The direction of the leading "charged particle jet" in each event is used to define three regions of eta-phi space. The "toward" region contains the leading "charged particle jet," while the "away" region, on the average, contains the away-side jet. The "transverse" region is perpendicular to the plane of the hard 2-to-2 scattering and is very sensitive to the "underlying event" component of the QCD Monte Carlo models. Herwig, isajet, and pythia with their default parameters do not describe correctly all the properties of the "transverse" region.

• Forward - backward charge asymmetry of quark pairs produced at the KEK TRISTAN e+ e- collider.
  D. Stuart et al. Phys.Rev.D49:3098-3105,1994
  

  We report on a measurement of the forward-backward charge asymmetry in e⁺e^---->q qbar at KEK TRISTAN, where the asymmetry is near maximum. We sum over all flavors and measure the asymmetry by determining the charge of the quark jets. In addition we exploit flavor dependencies in the jet charge determination to enhance the contributions of certain flavors. This provides a check on the asymmetries of individual flavors. The measurement agrees with the standard model expectations.

• Some studies of the applications of CsI photocathodes in gaseous detectors.
  G. Charpak, I. Gaudean, Y. Giomataris, V. Peskov, D. Scigocki, F. Sauli, D. Stuart. Nucl.Instrum.Meth.A333:391-398,1993
  

  Cesium-iodide photocathodes have been investigated in combination with gaseous detectors. The quantum efficiency of a semi-transparent CsI photocathode has been measured in the UV range and found to be 10% at 140 nm. We report experimental results obtained with such photocathodes coupled to BaF2 and KMgF3 scintillators and read out by parallel-plate gaseous detectors.

• Measurement of the forward-backward charge asymmetry in electron-positron annihilation into a quark-antiquark pair: A test of electroweak interference.
  D. Stuart. UMI-93-13526, Dec 1992. PhD Thesis
  

  The forward-backward asymmetry of quarks produced in e⁺e^- annihilations, summed over all flavors, is measured at sqrt(s) between 50 and 64 GeV. The asymmetry is found to agree with the electroweak standard model.

• Investigation of operation of a parallel plate avalanche chamber with a CsI photocathode under high gain conditions.
  G. Charpak, P. Fonte, V. Peskov, F. Sauli, D. Seigocki, D. Stuart. Nucl.Instrum.Meth.A307:63-68,1991
  

  We report results of a systematic study of the operational characteristics of a single-step parallel-plate avalanche chamber with CsI photocathode under high-gain conditions at room temperature and 1 atm pressure. Different mixtures of He and Ar with hydrocarbons were tested, as well as with ethylferrocene vapor which are known to form an adsorbed photosensitive layer on the CsI photocathode. The chamber can reach high gains, up to 106, has a very good time resolution (500 ps FWHM), and an energy resolution of 8.2% FWHM for 3 × 103 primary photoelectrons with a quantum efficiency of the CsI photocathode of about 20% at 193 nm. Photon feedback, caused by avalanche emission with wavelength longer than 200 nm, was observed for large total charge and found to be nearly independent of the concentration of quencher in the range 7 to 70 Torr. Breakdown appears at a total charge of 1010 electrons and is always of the slow type. There is good proportionality up to the breakdown limit.

• Forward - backward charge asymmetry in e+ e- ---> hadron jets.
  D. Stuart et al. Phys.Rev.Lett.64:983,1990
  

  The forward-backward asymmetry of quarks produced in e⁺e^- annihilations, summed over all flavors, is measured at sqrt(s) between 50 and 60.8 GeV. Methods of determining the charge direction of jet pairs are discussed. The asymmetry is found to agree with the five-flavor standard model.