The measurement of the luminosity recorded by the CMS detector installed at LHC interaction point 5, using proton-proton collisions at
in 2015 and 2016, is reported. The absolute luminosity scale is ...measured for individual bunch crossings using beam-separation scans (the van der Meer method), with a relative precision of 1.3 and 1.0% in 2015 and 2016, respectively. The dominant sources of uncertainty are related to residual differences between the measured beam positions and the ones provided by the operational settings of the LHC magnets, the factorizability of the proton bunch spatial density functions in the coordinates transverse to the beam direction, and the modeling of the effect of electromagnetic interactions among protons in the colliding bunches. When applying the van der Meer calibration to the entire run periods, the integrated luminosities when CMS was fully operational are 2.27 and 36.3
in 2015 and 2016, with a relative precision of 1.6 and 1.2%, respectively. These are among the most precise luminosity measurements at bunched-beam hadron colliders.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
A direct search for electroweak production of charginos and neutralinos is presented. Events with three or four leptons, with up to two hadronically decaying
τ
leptons, or two same-sign ...light leptons are analyzed. The data sample consists of 137 fb
−
1
of proton-proton collisions with a center of mass energy of 13 TeV, recorded with the CMS detector at the LHC. The results are interpreted in terms of several simplified models. These represent a broad range of production and decay scenarios for charginos and neutralinos. A parametric neural network is used to target several of the models with large backgrounds. In addition, results using orthogonal search regions are provided for all the models, simplifying alternative theoretical interpretations of the results. Depending on the model hypotheses, charginos and neutralinos with masses up to values between 300 and 1450 GeV are excluded at 95% confidence level.
A
bstract
A search for physics beyond the standard model in events with at least three charged leptons (electrons or muons) is presented. The data sample corresponds to an integrated luminosity of ...137 fb
−
1
of proton-proton collisions at
s
= 13 TeV, collected with the CMS detector at the LHC in 2016–2018. The two targeted signal processes are pair production of type-III seesaw heavy fermions and production of a light scalar or pseudoscalar boson in association with a pair of top quarks. The heavy fermions may be manifested as an excess of events with large values of leptonic transverse momenta or missing transverse momentum. The light scalars or pseudoscalars may create a localized excess in the dilepton mass spectra. The results exclude heavy fermions of the type-III seesaw model for masses below 880 GeV at 95% confidence level in the scenario of equal branching fractions to each lepton flavor. This is the most restrictive limit on the flavor-democratic scenario of the type-III seesaw model to date. Assuming a Yukawa coupling of unit strength to top quarks, branching fractions of new scalar (pseudoscalar) bosons to dielectrons or dimuons above 0.004 (0.03) and 0.04 (0.03) are excluded at 95% confidence level for masses in the range 15–75 and 108–340 GeV, respectively. These are the first limits in these channels on an extension of the standard model with scalar or pseudoscalar particles.
Radiation hard avalanche photodiodes for the CMS detector Antunovic, Z.; Britvitch, I.; Deiters, K. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
01/2005, Letnik:
537, Številka:
1-2
Journal Article
Recenzirano
Odprti dostop
The avalanche photodiodes, developed by Hamamatsu Photonics in collaboration with CMS, which are to be used to read out the lead tungstate crystals in the barrel part of the CMS electromagnetic ...calorimeter, are described. The procedures taken to ensure their long-term reliability in the radiation environment expected in CMS are outlined, as well as the studies made to verify the very high reliability required.
A measurement is presented of the effective leptonic weak mixing angle (
) using the forward-backward asymmetry of Drell-Yan lepton pairs (
and
) produced in proton-proton collisions at
at the CMS ...experiment of the LHC. The data correspond to integrated luminosities of 18.8 and
in the dimuon and dielectron channels, respectively, containing 8.2 million dimuon and 4.9 million dielectron events. With more events and new analysis techniques, including constraints obtained on the parton distribution functions from the measured forward-backward asymmetry, the statistical and systematic uncertainties are significantly reduced relative to previous CMS measurements. The extracted value of
from the combined dilepton data is
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A search is presented for the associated production of a standard model Higgs boson with a top quark-antiquark pair (tt¯H\ \mathrm{t}\overline{\mathrm{t}}\mathrm{H} \), in which the Higgs boson ...decays into a b quark-antiquark pair, in proton-proton collisions at a centre-of-mass energy s=13\ \sqrt{s}=13 \ TeV. The data correspond to an integrated luminosity of 35.9 fb−1 recorded with the CMS detector at the CERN LHC. Candidate tt¯H\ \mathrm{t}\overline{\mathrm{t}}\mathrm{H} \ events are selected that contain either one or two electrons or muons from the tt¯\ \mathrm{t}\overline{\mathrm{t}} \ decays and are categorised according to the number of jets. Multivariate techniques are employed to further classify the events and eventually discriminate between signal and background. The results are characterised by an observed tt¯H\ \mathrm{t}\overline{\mathrm{t}}\mathrm{H} \ signal strength relative to the standard model cross section, μ = σ/σSM, under the assumption of a Higgs boson mass of 125 GeV. A combined fit of multivariate discriminant distributions in all categories results in an observed (expected) upper limit on μ of 1.5 (0.9) at 95% confidence level, and a best fit value of 0.72 ± 0.24(stat) ± 0.38(syst), corresponding to an observed (expected) signal significance of 1.6 (2.2) standard deviations above the background-only hypothesis.
A
bstract
A search for exotic Higgs boson decays to light pseudoscalars in the final state of two muons and two
τ
leptons is performed using proton-proton collision data recorded by the CMS ...experiment at the LHC at a center-of-mass energy of 13 TeV in 2016, corresponding to an integrated luminosity of 35.9 fb
−1
. Masses of the pseudoscalar boson between 15.0 and 62.5 GeV are probed, and no significant excess of data is observed above the prediction of the standard model. Upper limits are set on the branching fraction of the Higgs boson to two light pseudoscalar bosons in different types of two-Higgs-doublet models extended with a complex scalar singlet.
A
bstract
A measurement of the double-differential inclusive jet cross section as a function of the jet transverse momentum
p
T
and the absolute jet rapidity |
y
| is presented. Data from LHC ...proton-proton collisions at
s
=
8
TeV, corresponding to an integrated luminosity of 19.7 fb
−1
, have been collected with the CMS detector. Jets are reconstructed using the anti-
k
T
clustering algorithm with a size parameter of 0.7 in a phase space region covering jet
p
T
from 74 GeV up to 2.5 TeV and jet absolute rapidity up to |
y
| = 3.0. The low-
p
T
jet range between 21 and 74 GeV is also studied up to |
y
| = 4.7, using a dedicated data sample corresponding to an integrated luminosity of 5.6 pb
−1
. The measured jet cross section is corrected for detector effects and compared with the predictions from perturbative QCD at next-to-leading order (NLO) using various sets of parton distribution functions (PDF). Cross section ratios to the corresponding measurements performed at 2.76 and 7 TeV are presented. From the measured double-differential jet cross section, the value of the strong coupling constant evaluated at the Z mass is
α
S
(
M
Z
) = 0.1164
− 0.0043
+ 0.0060
, where the errors include the PDF, scale, nonperturbative effects and experimental uncertainties, using the CT10 NLO PDFs. Improved constraints on PDFs based on the inclusive jet cross section measurement are presented.
A
bstract
A search for direct production of top squark pairs in events with jets and large transverse momentum imbalance is presented. The data are based on proton-proton collisions at a ...center-of-mass energy of 13 TeV, collected with the CMS detector in 2016 at the CERN LHC, and correspond to an integrated luminosity of 35.9 fb
−1
. The search considers a variety of
R
-parity conserving supersymmetric models, including ones for which the top squark and neutralino masses are nearly degenerate. Specialized jet reconstruction tools are developed to exploit the unique characteristics of the signal topologies. With no significant excess of events observed above the standard model expectations, upper limits are set on the direct top squark pair production cross section in the context of simplified supersymmetric models for various decay hypotheses. Models with larger differences in mass between the top squark and neutralino are probed for masses up to 1040 and 500 GeV, respectively, whereas models with a more compressed mass hierarchy are probed up to 660 and 610 GeV, respectively. The smallest mass difference probed is for masses near to 550 and 540 GeV, respectively.
The algorithm developed by the CMS Collaboration to reconstruct and identify τ leptons produced in proton-proton collisions at s=7 and 8 TeV, via their decays to hadrons and a neutrino, has been ...significantly improved. The changes include a revised reconstruction of π0 candidates, and improvements in multivariate discriminants to separate τ leptons from jets and electrons. The algorithm is extended to reconstruct τ leptons in highly Lorentz-boosted pair production, and in the high-level trigger. The performance of the algorithm is studied using proton-proton collisions recorded during 2016 at s=13 TeV, corresponding to an integrated luminosity of 35.9 fb-1. The performance is evaluated in terms of the efficiency for a genuine τ lepton to pass the identification criteria and of the probabilities for jets, electrons, and muons to be misidentified as τ leptons. The results are found to be very close to those expected from Monte Carlo simulation.
PDF
