Papers by Sebastian White
Journal of Instrumentation, 2022
Many measurements at the LHC require efficient identification of heavy-flavour jets, i.e. jets or... more Many measurements at the LHC require efficient identification of heavy-flavour jets, i.e. jets originating from bottom (b) or charm (c) quarks. An overview of the algorithms used to identify c jets is described and a novel method to calibrate them is presented. This new method adjusts the entire distributions of the outputs obtained when the algorithms are applied to jets of different flavours. It is based on an iterative approach exploiting three distinct control regions that are enriched with either b jets, c jets, or light-flavour and gluon jets. Results are presented in the form of correction factors evaluated using proton-proton collision data with an integrated luminosity of 41.5 fb-1 at √s = 13 TeV, collected by the CMS experiment in 2017. The closure of the method is tested by applying the measured correction factors on simulated data sets and checking the agreement between the adjusted simulation and collision data. Furthermore, a validation is performed by testing the met...
In this thesis, detailed HBT measurements obtained using the PHENIX detector at the Relativistic ... more In this thesis, detailed HBT measurements obtained using the PHENIX detector at the Relativistic Heavy Ion Collider (RHIC) are presented for three beam collision energies (√sNN= 39, 62, and 200 GeV) and three collision species (d+Au,Cu+Cu, and Au+Au). The measurements are studied for their dependence on collision geometry and transverse mass (mT), and observations are made on how the small asymmetric system, d+Au, compares to the A+A systems for these dependencies.
Journal of High Energy Physics, 2021
A measurement of the top quark mass is performed using a data sample enriched with single top qua... more A measurement of the top quark mass is performed using a data sample enriched with single top quark events produced in the t channel. The study is based on proton- proton collision data, corresponding to an integrated luminosity of 35.9 fb−1, recorded at $$ \sqrt{s} $$ s = 13 TeV by the CMS experiment at the LHC in 2016. Candidate events are selected by requiring an isolated high-momentum lepton (muon or electron) and exactly two jets, of which one is identified as originating from a bottom quark. Multivariate discriminants are designed to separate the signal from the background. Optimized thresholds are placed on the discriminant outputs to obtain an event sample with high signal purity. The top quark mass is found to be $$ {172.13}_{-0.77}^{+0.76} $$ 172.13 − 0.77 + 0.76 GeV, where the uncertainty includes both the statistical and systematic components, reaching sub-GeV precision for the first time in this event topology. The masses of the top quark and antiquark are also determin...
We calculate photoproduction rates for several hard processes in ultraperipheral proton-lead and ... more We calculate photoproduction rates for several hard processes in ultraperipheral proton-lead and lead-lead collisions at the LHC with √ sNN = 8.8 and 5.5 TeV respectively which could be triggered in the large LHC detectors. We use ATLAS as an example. The lead ion is treated as a source of (coherently produced) photons with energies and intensities greater than those of equivalent ep collisions at HERA. We find very large rates for both inclusive and diffractive production which will extend the HERA x range by nearly an order of magnitude for similar virtualities. We demonstrate that it is possible to reach the kinematic regime where nonlinear effects are larger than at HERA.
Nuclear Physics B - Proceedings Supplements, 2005
The heavy ion physics approach to global event characterization has led us to instrument the forw... more The heavy ion physics approach to global event characterization has led us to instrument the forward region in the PHENIX experiment at RHIC. In heavy ion collisions this coverage yields a measurement of the "spectator" energy and its distribution about the beam direction. This energy flow is the basis of event-by-event determination of the centrality and reaction plane which are key to analyzing particle production in heavy ion collisions. These same tools have also enabled a unique set of measurements on inelastic diffraction with proton, deuteron and gold ion beams in the PHENIX experiment. We present first new results on this topic and discuss briefly the opportunity for diffractive physics with Heavy Ion beams at the LHC.
Physical Review C, 2015
Measurements of bottomonium production in heavy ion and p+p collisions at the Relativistic Heavy ... more Measurements of bottomonium production in heavy ion and p+p collisions at the Relativistic Heavy Ion Collider (RHIC) are presented. The inclusive yield of the three Υ states, Υ(1S + 2S + 3S), was measured in the PHENIX experiment via electron-positron decay pairs at midrapidity for Au+Au and p+p collisions at √ s N N = 200 GeV. The Υ(1S + 2S + 3S) → e + e − differential cross section at midrapidity was found to be Beedσ/dy = 108 ± 38 (stat) ± 15 (syst) ± 11 (luminosity) pb in p+p collisions. The nuclear modification factor in the 30% most central Au+Au collisions indicates a suppression of the total Υ state yield relative to the extrapolation from p+p collision data. The suppression is consistent with measurements made by STAR at RHIC and at higher energies by the CMS experiment at the Large Hadron Collider.
Physics Letters B, 2012
A measurement of angular correlations in Drell-Yan lepton pairs via the φ * η observable is prese... more A measurement of angular correlations in Drell-Yan lepton pairs via the φ * η observable is presented. This variable probes the same physics as the Z /γ * boson transverse momentum with a better experimental resolution. The Z /γ * → e + e − and Z /γ * → μ + μ − decays produced in proton-proton collisions at a centre-of-mass energy of √ s = 7 TeV are used. The data were collected with the ATLAS detector at the LHC and correspond to an integrated luminosity of 4.6 fb −1. Normalised differential cross sections as a function of φ * η are measured separately for electron and muon decay channels. These channels are then combined for improved accuracy. The cross section is also measured double differentially as a function of φ * η for three independent bins of the Z boson rapidity. The results are compared to QCD calculations and to predictions from different Monte Carlo event generators. The data are reasonably well described, in all measured Z boson rapidity regions, by resummed QCD predictions combined with fixed-order perturbative QCD calculations or by some Monte Carlo event generators. The measurement precision is typically better by one order of magnitude than present theoretical uncertainties.
AIP Conference Proceedings, 2012
The ATLAS detector is capable of resolving the highest energy pp collisions at luminosities suffi... more The ATLAS detector is capable of resolving the highest energy pp collisions at luminosities sufficient to yield 10's of simultaneous interactions within a bunch collision lasting <0.5 nsec. Already in 2011 a mean occupancy of 20 is often found in pp running. In 2004 studies by ATLAS showed that the detector would have excellent performance also for the foreseeable particle multiplicities in the highest energy p-Pb and Pb-Pb collisions that the LHC will produce. These studies resulted in a letter of intent to the LHC committee by ATLAS to do physics with these beams also. In the past 2 years of data taking, ATLAS detector performance studies have confirmed these expectations at the actual multiplicities presented below. The ATLAS program removes an artificial specialization that arose about 30 years ago in high energy physics when the energy and intensity frontier moved to colliders. Before that time, for example, the same experiment that discovered the ϒ (CFS and E605 at Fermilab) also measured the nuclear modification factor in the production of high p T identified charged hadrons using nuclear targets from Beryllium through Tungsten.
The European Physical Journal C, 2014
The integrated elliptic flow of charged particles produced in Pb+Pb collisions at √ s NN = 2.76 T... more The integrated elliptic flow of charged particles produced in Pb+Pb collisions at √ s NN = 2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v 2 , was measured in the pseudorapidity range |η| ≤ 2.5 with the event-plane method. In order to include tracks with very low transverse momentum p T , thus reducing the uncertainty in v 2 integrated over p T , a 1 μb −1 data sample recorded without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v 2 is compared to other measurements obtained with higher p T thresholds. The integrated elliptic flow is weakly decreasing with |η|. The integrated v 2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data. ation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the
Journal of High Energy Physics, 2014
The production of a W boson in association with a single charm quark is studied using 4.6 fb −1 o... more The production of a W boson in association with a single charm quark is studied using 4.6 fb −1 of pp collision data at √ s = 7 TeV collected with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea-quark distributions is determined to be 0.96 +0.26 −0.30 at Q 2 = 1.9 GeV 2 , which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally, the cross-section ratio σ(W + +c)/σ(W − +c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the s-s quark asymmetry.
Journal of High Energy Physics, 2014
Double-differential dijet cross-sections measured in pp collisions at the LHC with a 7 TeV centre... more Double-differential dijet cross-sections measured in pp collisions at the LHC with a 7 TeV centre-of-mass energy are presented as functions of dijet mass and half the rapidity separation of the two highest-p T jets. These measurements are obtained using data corresponding to an integrated luminosity of 4.5 fb −1 , recorded by the ATLAS detector in 2011. The data are corrected for detector effects so that cross-sections are presented at the particle level. Cross-sections are measured up to 5 TeV dijet mass using jets reconstructed with the anti-k t algorithm for values of the jet radius parameter of 0.4 and 0.6. The cross-sections are compared with next-to-leading-order perturbative QCD calculations by NLOJet++ corrected to account for non-perturbative effects. Comparisons with POWHEG predictions, using a next-to-leading-order matrix element calculation interfaced to a partonshower Monte Carlo simulation, are also shown. Electroweak effects are accounted for in both cases. The quantitative comparison of data and theoretical predictions obtained using various parameterizations of the parton distribution functions is performed using a frequentist method. In general, good agreement with data is observed for the NLOJet++ theoretical predictions when using the CT10, NNPDF2.1 and MSTW 2008 PDF sets. Disagreement is observed when using the ABM11 and HERAPDF1.5 PDF sets for some ranges of dijet mass and half the rapidity separation. An example setting a lower limit on the compositeness scale for a model of contact interactions is presented, showing that the unfolded results can be used to constrain contributions to dijet production beyond that predicted by the Standard Model.
Physical Review Letters, 2014
Journal of High Energy Physics, 2014
The results of a search for direct pair production of the scalar partner to the top quark using a... more The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of 20.1 fb −1 of proton-proton collision data at √ s = 8 TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay viat → tχ
Journal of High Energy Physics, 2012
The results of a search for charged Higgs bosons are presented. The analysis is based on 4.6fb−1 ... more The results of a search for charged Higgs bosons are presented. The analysis is based on 4.6fb−1 of proton-proton collision data at $ \sqrt {s} = 7\;TeV $ collected by the ATLAS experiment at the Large Hadron Collider, using top quark pair events with a τ lepton in the final state. The data are consistent with the expected background from Standard Model processes. Assuming that the branching ratio of the charged Higgs boson to a τ lepton and a neutrino is 100 %, this leads to upper limits on the branching ratio of top quark decays to abquarkandachargedHiggsbosonbetween5%and1%forchargedHiggsbosonmasses ranging from 90 GeV to 160 GeV, respectively. In the context of the $ m_h^{\max } $ scenario of the MSSM, tan β above 12-26, as well as between 1 and 2-6, can be excluded for charged Higgs boson masses between 90 GeV and 150 GeV.
Journal of High Energy Physics, 2013
Mass and angular distributions of dijets produced in LHC proton-proton collisions at a centre-of-... more Mass and angular distributions of dijets produced in LHC proton-proton collisions at a centre-of-mass energy $ \sqrt{s}=7 $ TeV have been studied with the ATLAS detector using the full 2011 data set with an integrated luminosity of 4.8 fb−1. Dijet masses up to ~ 4.0 TeV have been probed. No resonance-like features have been observed in the dijet mass spectrum, and all angular distributions are consistent with the predictions of QCD. Exclusion limits on six hypotheses of new phenomena have been set at 95% CL in terms of mass or energy scale, as appropriate. These hypotheses include excited quarks below 2.83 TeV, colour octet scalars below 1.86 TeV, heavy W bosons below 1.68 TeV, string resonances below 3.61 TeV, quantum black holes with six extra space-time dimensions for quantum gravity scales below 4.11 TeV, and quark contact interactions below a compositeness scale of 7.6 TeV in a destructive interference scenario.
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Papers by Sebastian White