A
bstract
A measurement is performed of the cross section of top quark pair production in association with a W or Z boson using proton-proton collisions at a center-of-mass energy of 13 TeV at the ...LHC. The data sample corresponds to an integrated luminosity of 35.9 fb
−1
, collected by the CMS experiment in 2016. The measurement is performed in the same-sign dilepton, three- and four-lepton final states. The production cross sections are measured to be
σ
t
t
¯
W
=
0.77
−
0.11
+
0.12
stat
−
0.12
+
0.13
syst
p
b
and
σ
t
t
¯
Z
=
0.99
−
0.08
+
0.09
stat
−
0.10
+
0.12
syst
p
b
. The expected (observed) signal significance for the
t
t
¯
W
production in same-sign dilepton channel is found to be 4.5 (5.3) standard deviations, while for the
t
t
¯
Z
production in three- and four-lepton channels both the expected and the observed significances are found to be in excess of 5 standard deviations. The results are in agreement with the standard model predictions and are used to constrain the Wilson coefficients for eight dimension-six operators describing new interactions that would modify
t
t
¯
W
and
t
t
¯
Z
production.
The nuclear modification factors of
and
mesons are measured in
collisions at a centre-of-mass energy per nucleon pair of
. The analysis is based on
and
data samples collected by CMS at the LHC in ...2015, corresponding to integrated luminosities of 464
and 28
, respectively. The measurements are performed in the dimuon rapidity range of
as a function of centrality, rapidity, and transverse momentum (
) from
in the most forward region and up to 50
. Both prompt and nonprompt (coming from b hadron decays)
mesons are observed to be increasingly suppressed with centrality, with a magnitude similar to the one observed at
for the two
meson components. No dependence on rapidity is observed for either prompt or nonprompt
mesons. An indication of a lower prompt
meson suppression at
is seen with respect to that observed at intermediate
. The prompt
meson yield is found to be more suppressed than that of the prompt
mesons in the entire
range.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A
bstract
A search for a new scalar resonance decaying to a pair of Z bosons is performed in the mass range from 130 GeV to 3 TeV, and for various width scenarios. The analysis is based on ...proton-proton collisions recorded by the CMS experiment at the LHC in 2016, corresponding to an integrated luminosity of 35.9 fb
−1
at a center-of-mass energy of 13 TeV. The Z boson pair decays are reconstructed using the 4
ℓ
, 2
ℓ
2q, and 2
ℓ
2
ν
final states, where
ℓ
= e or
μ
. Both gluon fusion and electroweak production of the scalar resonance are considered, with a free parameter describing their relative cross sections. A dedicated categorization of events, based on the kinematic properties of associated jets, and matrix element techniques are employed for an optimal signal and background separation. A description of the interference between signal and background amplitudes for a resonance of an arbitrary width is included. No significant excess of events with respect to the standard model expectation is observed and limits are set on the product of the cross section for a new scalar boson and the branching fraction for its decay to ZZ for a large range of masses and widths.
A
bstract
A search is performed for a heavy Majorana neutrino (N), produced in leptonic decay of a W boson propagator and decaying into a W boson and a lepton, with the CMS detector at the LHC. The ...signature used in this search consists of two same-sign leptons, in any flavor combination of electrons and muons, and at least one jet. The data were collected during 2016 in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb
−1
. The results are found to be consistent with the expected standard model background. Upper limits are set in the mass range between 20 and 1600 GeV in the context of a Type-I seesaw mechanism, on |
V
eN
|
2
, |
V
μ
N
|
2
, and |
V
eN
V
μN
*
|
2
/(|
V
eN
|
2
+ |
V
μ
N
|
2
), where
V
ℓN
is the matrix element describing the mixing of N with the standard model neutrino of flavor
ℓ
= e
, μ
. For N masses between 20 and 1600 GeV, the upper limits on |
V
ℓN
|
2
range between 2
.
3 × 10
−5
and unity. These are the most restrictive direct limits for heavy Majorana neutrino masses above 430 GeV.
A
bstract
A search for heavy neutral leptons (HNLs), the right-handed Dirac or Majorana neutrinos, is performed in final states with three charged leptons (electrons or muons) using proton-proton ...collision data collected by the CMS experiment at
s
= 13 TeV at the CERN LHC. The data correspond to an integrated luminosity of 138 fb
−
1
. The HNLs could be produced through mixing with standard model neutrinos
ν
. For small values of the HNL mass (
<
20 GeV) and the square of the HNL-
ν
mixing parameter (10
−
7
–10
−
2
), the decay length of these particles can be large enough so that the secondary vertex of the HNL decay can be resolved with the CMS silicon tracker. The selected final state consists of one lepton emerging from the primary proton-proton collision vertex, and two leptons forming a displaced, secondary vertex. No significant deviations from the standard model expectations are observed, and constraints are obtained on the HNL mass and coupling strength parameters, excluding previously unexplored regions of parameter space in the mass range 1–20 GeV and squared mixing parameter values as low as 10
−
7
.
A
bstract
A search for narrow vector resonances decaying into quark-antiquark pairs is presented. The analysis is based on data collected in proton-proton collisions at
s
=
13
TeV with the CMS ...detector at the LHC, corresponding to an integrated luminosity of 35.9 fb
−1
. The hypothetical resonance is produced with sufficiently high transverse momentum that its decay products are merged into a single jet with two-prong substructure. A signal would be identified as a peak over a smoothly falling background in the distribution of the invariant mass of the jet, using novel jet substructure techniques. No evidence for such a resonance is observed within the mass range of 50-300 GeV. Upper limits at 95% confidence level are set on the production cross section, and presented in a mass-coupling parameter space. The limits further constrain simplified models of dark matter production involving a mediator interacting between quarks and dark matter particles through a vector or axial-vector current. In the framework of these models, the results are the most sensitive to date, extending for the first time the search region to masses below 100 GeV.
A
bstract
A search for charged Higgs bosons (H
±
) decaying into a top and a bottom quark in the all-jet final state is presented. The analysis uses LHC proton-proton collision data recorded with the ...CMS detector in 2016 at
s
= 13 TeV, corresponding to an integrated luminosity of 35.9 fb
−
1
. No significant excess is observed above the expected background. Model-independent upper limits at 95% confidence level are set on the product of the H
±
production cross section and branching fraction in two scenarios. For production in association with a top quark, limits of 21.3 to 0.007 pb are obtained for H
±
masses in the range of 0.2 to 3 TeV. Combining this with a search in leptonic final states results in improved limits of 9.25 to 0.005 pb. The complementary
s
-channel production of an H
±
is investigated in the mass range of 0.8 to 3 TeV and the corresponding upper limits are 4.5 to 0.023 pb. These results are interpreted using different minimal supersymmetric extensions of the standard model.
A
bstract
A search for new particles has been conducted using events with two high transverse momentum
τ
leptons that decay hadronically and at least two energetic jets. The analysis is performed ...using data from proton-proton collisions at
s
=
13
TeV, collected by the CMS experiment at the LHC in 2016 and corresponding to an integrated luminosity of 35.9 fb
−1
. The observed data are consistent with standard model expectations. The results are interpreted in the context of two physics models. The first model involves right-handed charged bosons, W
R
, that decay to heavy right-handed Majorana neutrinos, N
ℓ
(
ℓ
= e,
μ
,
τ
), arising in a left-right symmetric extension of the standard model. The model considers that N
e
and N
μ
are too heavy to be detected at the LHC. Assuming that the N
τ
mass is half of the W
R
mass, masses of the W
R
boson below 3.50 TeV are excluded at 95% confidence level. Exclusion limits are also presented considering different scenarios for the mass ratio between N
τ
and W
R
, as a function of W
R
mass. In the second model, pair production of third-generation scalar leptoquarks that decay into
ττ
bb is considered, resulting in an observed exclusion region with leptoquark masses below 1.02 TeV, assuming a 100% branching fraction for the leptoquark decay to a
τ
lepton and a bottom quark. These results represent the most stringent limits to date on these models.
A
bstract
The WZ production cross section is measured in proton-proton collisions at a centre-of-mass energy
s
= 13 TeV using data collected with the CMS detector, corresponding to an integrated ...luminosity of 35.9 fb
−1
. The inclusive cross section is measured to be
σ
tot
(pp → WZ) = 48.09
− 0.96
+ 1.00
(stat)
− 0.37
+ 0.44
(theo)
− 2.17
+ 2.39
(syst) ± 1.39(lum) pb, resulting in a total uncertainty of −2.78/+2.98 pb. Fiducial cross section and ratios of charge-dependent cross section measurements are provided. Differential cross section measurements are also presented with respect to three variables: the Z boson transverse momentum
p
T
, the leading jet
p
T
, and the
M
(WZ) variable, defined as the invariant mass of the system composed of the three leptons and the missing transverse momentum. Differential measurements with respect to the W boson
p
T
, separated by charge, are also shown. Results are consistent with standard model predictions, favouring next-to-next-to-leading-order predictions over those at next-to-leading order. Constraints on anomalous triple gauge couplings are derived via a binned maximum likelihood fit to the
M
(WZ) variable.
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.
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