1,622 research outputs found

    Jetit CMS-kokeessa

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    Jets are collimated showers of particles originating from quarks and gluons, which are the constituents of protons and neutrons that make up the atomic nuclei. In the CERN's Large Hadron Collider (LHC) protons are collided with the highest energies ever achieved and production of jets is ubiquitous in these collision events. Measuring energies of jets is a complex process and requires sophisticated jet energy calibration methods. In this Master's Thesis a new jet composition driven method for enhancing jet calibration in the Compact Muon Solenoid experiment (CMS) is studied. We study the effects of sensitivity of different detector elements to the jet energy composition and try to find sources of observed discrepancies between composition of Monte Carlo simulated and measured jets. We test three different mis-calibration scenarios in the lightweight FastSim simulation environment and observe encouraging results that are in agreement with the hypothesized mis-calibrations. The FastSim approach proves to be a useful tool for investigating the role of detector calibration on jet composition, at least in the case of hadronic and electromagnetic calorimeters. In the near future the developed method will be applied also to other parts of CMS.Jetit ovat kvarkeista ja gluoneista alkunsa saavia kartiomaisia hiukkasryöppyjä. Protonit ja neutronit, siis atomiytimien rakenneosaset, koostuvat kvarkeista ja gluoneista. Euroopan hiukkastutkimuskeskus CERN:n Suuressa Hadronit törmäyttimessä LHC:ssa törmäytetään protoneja toisiinsa suurilla energioilla ja törmäyksissä syntyy runsaasti kyseisiä hiukkasryöppyjä. Näiden jettien energian mittaaminen on monimutkainen prosessi, joka vaatii pitkälle kehitettyjä kalibrointitekniikoita. Tässä diplomityössä tutkitaan uutta jettien energioiden tarkempaan mittaukseen tähtäävää tekniikkaa, joka perustuu ryöppyjen koostumuksen tutkimiseen. LHC:n CMS-kokeen puitteissa suoritettavassa tutkimuksessa mittaamme CMS:n eri havaintojärjestelmien herkkyystasojen vaikutusta jettien koostumukseen ja etsimme syitä simuloitujen ja mitattujen hiukkasryöppyjen koostumuksien eroihin. Tutkimme kolmen eri herkkyysskenaarion vaikutusta jetteihin käyttäen laskennallisesti kevyttä FastSim-simulaatioympäristöä. Saamamme tulokset ovat yhteensopivia oletettujen virhelähteiden kanssa ja rohkaisevat jatkotutkimuksiin. Käyttämämme menetelmä osoittautuu hyödylliseksi työkaluksi havaintojärjestelmien herkkyyden ja hiukkasryöppyjen koostumuksen välisen yhteyden tutkimiseen. Kehitettyä menetelmää aiotaan tulevaisuudessa soveltaa laajemmin CMS-kokeen eri mittausjärjestelmiin

    Beam test performance of a prototype module with Short Strip ASICs for the CMS HL-LHC tracker upgrade

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    The Short Strip ASIC (SSA) is one of the four front-end chips designed for the upgrade of the CMS Outer Tracker for the High Luminosity LHC. Together with the Macro-Pixel ASIC (MPA) it will instrument modules containing a strip and a macro-pixel sensor stacked on top of each other. The SSA provides both full readout of the strip hit information when triggered, and, together with the MPA, correlated clusters called stubs from the two sensors for use by the CMS Level-1 (L1) trigger system. Results from the first prototype module consisting of a sensor and two SSA chips are presented. The prototype module has been characterized at the Fermilab Test Beam Facility using a 120 GeV proton beam.Peer reviewe

    Measurements of production cross sections of polarized same-sign W boson pairs in association with two jets in proton-proton collisions at root s=13 TeV

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    The first measurements of production cross sections of polarized same-sign (WW +/-)-W-+/- boson pairs in proton-proton collisions are reported. The measurements are based on a data sample collected with the CMS detector at the LHC at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 137 fb(-1). Events are selected by requiring exactly two same-sign leptons, electrons or muons, moderate missing transverse momentum, and two jets with a large rapidity separation and a large dijet mass to enhance the contribution of same-sign (WW +/-)-W-+/- scattering events. An observed (expected) 95% confidence level upper limit of 1.17 (0.88) fbis set on the production cross section for longitudinally polarized same-sign W-+/- W-+/- boson pairs. The electroweak production of same-sign W-+/- W-+/- boson pairs with at least one of the W bosons longitudinally polarized is measured with an observed (expected) significance of 2.3 (3.1) standard deviations. (C) 2020 The Author(s). Published by Elsevier B.V.Peer reviewe

    Measurement of the CP-violating phase phi(s) in the B-s(0) -> J/psi phi(1020) -> mu(+)mu-K+K- channel in proton-proton collisions at root s=13 TeV

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    The CP-violating weak phase ?s and the decay width difference ??s between the light and heavy B0s mass eigenstates are measured with the CMS detector at the LHC in a sample of 48 500 reconstructed B0s? J/I) d (1020) ?11+11? K+K? events. The measurement is based on a data sample corresponding to an integrated luminosity of 96.4 fb?1, collected in proton-proton collisions at ?s = 13 TeV in 2017?2018. To extract the values of ?s and ??s, a time-dependent and flavor-tagged angular analysis of the 11+11?K+K? final state is performed. The analysis employs a dedicated tagging trigger and a novel opposite-side muon flavor tagger based on machine learning techniques. The measurement yields ?s = ?11 ?50 (stat) ? 10 (syst) mrad and ??s = 0.114 ? 0.014 (stat)? 0.007 (syst) ps?1, in agreement with the standard model predictions. When combined with the previous CMS measurement at ?s = 8 TeV, the following values are obtained: ?s = ?21 ? 44 (stat) ? 10 (syst) mrad, ??s = 0.1032 ? 0.0095 (stat) ? 0.0048 (syst) ps?1, a significant improvement over the 8 TeV result. ? 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY licensePeer reviewe

    Development and validation of HERWIG 7 tunes from CMS underlying-event measurements

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    This paper presents new sets of parameters ("tunes") for the underlying-event model of the herwig 7 event generator. These parameters control the description of multiple-parton interactions (MPI) and colour reconnection in herwig 7, and are obtained from a fit to minimum-bias data collected by the CMS experiment at v s = 0.9, 7, and 13 TeV. The tunes are based on the NNPDF3.1 next-to-nextto-leading-order parton distribution function (PDF) set for the parton shower, and either a leading-order or next-to-nextto-leading-order PDF set for the simulation of MPI and the beam remnants. Predictions utilizing the tunes are produced for event shape observables in electron-positron collisions, and forminimum-bias, inclusive jet, top quark pair, and Zand Wboson events in proton-proton collisions, and are compared with data. Each of the new tunes describes the data at a reasonable level, and the tunes using a leading-order PDF for the simulation of MPI provide the best description of the data.Peer reviewe

    Measurement of prompt D-0 and D-0 meson azimuthal anisotropy and search for strong electric fields in PbPb collisions at root S-NN=5.02 TeV

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    The strong Coulomb field created in ultrarelativistic heavy ion collisions is expected to produce a rapiditydependent difference (Av2) in the second Fourier coefficient of the azimuthal distribution (elliptic flow, v2) between D0 (uc) and D0 (uc) mesons. Motivated by the search for evidence of this field, the CMS detector at the LHC is used to perform the first measurement of Av2. The rapidity-averaged value is found to be (Av2) = 0.001 ? 0.001 (stat)? 0.003 (syst) in PbPb collisions at ?sNN = 5.02 TeV. In addition, the influence of the collision geometry is explored by measuring the D0 and D0mesons v2 and triangular flow coefficient (v3) as functions of rapidity, transverse momentum (pT), and event centrality (a measure of the overlap of the two Pb nuclei). A clear centrality dependence of prompt D0 meson v2 values is observed, while the v3 is largely independent of centrality. These trends are consistent with expectations of flow driven by the initial-state geometry. ? 2021 The Author. Published by Elsevier B.V. This is an open access article under the CC BY licensePeer reviewe

    Measurements of production cross sections of the Higgs boson in the four-lepton final state in proton-proton collisions at root s=13 TeV

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    Production cross sections of the Higgs boson are measured in the H -> ZZ -> 4l (l = e, mu) decay channel. A data sample of proton-proton collisions at a center-of-mass energy of 13 TeV, collected by the CMS detector at the LHC and corresponding to an integrated luminosity of 137 fb(-1) is used. The signal strength modifier mu, defined as the ratio of the Higgs boson production rate in the 4l channel to the standard model (SM) expectation, is measured to be mu = 0.94 +/- 0.07 (stat)(-0.08)(+0.09) (syst) at a fixed value of m(H) = 125.38 GeV. The signal strength modifiers for the individual Higgs boson production modes are also reported. The inclusive fiducial cross section for the H -> 4l process is measured to be 2.84(-0.22)(+0.23) (stat)(-0.21)(+0.26) (syst) fb, which is compatible with the SM prediction of 2.84 +/- 0.15 fb for the same fiducial region. Differential cross sections as a function of the transverse momentum and rapidity of the Higgs boson, the number of associated jets, and the transverse momentum of the leading associated jet are measured. A new set of cross section measurements in mutually exclusive categories targeted to identify production mechanisms and kinematical features of the events is presented. The results are in agreement with the SM predictions.Peer reviewe

    Search for dark matter produced in association with a leptonically decaying Z boson in proton-proton collisions at root s=13TeV

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    A Publisher's Erratum to this article was published on 19 April 2021, https://link.springer.com/article/10.1140%2Fepjc%2Fs10052-020-08739-5A search for dark matter particles is performed using events with a Z boson candidate and large missing transverse momentum. The analysis is based on proton-proton collision data at a center-of-mass energy of 13 TeV, collected by the CMS experiment at the LHC in 2016-2018, corresponding to an integrated luminosity of 137 fb(-1). The search uses the decay channels Z -> ee and Z -> mu mu. No significant excess of events is observed over the background expected from the standard model. Limits are set on dark matter particle production in the context of simplified models with vector, axial-vector, scalar, and pseudoscalar mediators, as well as on a two-Higgs-doublet model with an additional pseudoscalar mediator. In addition, limits are provided for spin-dependent and spin-independent scattering cross sections and are compared to those from direct-detection experiments. The results are also interpreted in the context ofmodels of invisible Higgs boson decays, unparticles, and large extra dimensions.Peer reviewe
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