A search for the pair production of new light bosons, each decaying into a pair of muons, is performed with the CMS experiment at the LHC, using a dataset corresponding to an integrated luminosity of ...20.7 fb−1 collected in proton–proton collisions at center-of-mass energy of s=8 TeV. No excess is observed in the data relative to standard model background expectation and a model independent upper limit on the product of the cross section, branching fraction, and acceptance is derived. The results are compared with two benchmark models, the first one in the context of the next-to-minimal supersymmetric standard model, and the second one in scenarios containing a hidden sector, including those predicting a nonnegligible light boson lifetime.
A
bstract
A search for the standard model Higgs boson produced in association with a top-quark pair
t
t
¯
H
is presented, using data samples corresponding to integrated luminosities of up to 5.1 fb
...−1
and 19.7 fb
−1
collected in pp collisions at center-of-mass energies of 7 TeV and 8 TeV respectively. The search is based on the following signatures of the Higgs boson decay: H → hadrons, H → photons, and H → leptons. The results are characterized by an observed
t
t
¯
H
signal strength relative to the standard model cross section,
μ
=
σ/σ
SM
,under the assumption that the Higgs boson decays as expected in the standard model. The best fit value is
μ
= 2.8 ± 1.0 for a Higgs boson mass of 125.6 GeV.
The first study of W boson production in pPb collisions is presented, for bosons decaying to a muon or electron, and a neutrino. The measurements are based on a data sample corresponding to an ...integrated luminosity of 34.6 nb−1 at a nucleon–nucleon centre-of-mass energy of sNN=5.02 TeV, collected by the CMS experiment. The W boson differential cross sections, lepton charge asymmetry, and forward–backward asymmetries are measured for leptons of transverse momentum exceeding 25 GeV/c, and as a function of the lepton pseudorapidity in the |ηlab|<2.4 range. Deviations from the expectations based on currently available parton distribution functions are observed, showing the need for including W boson data in nuclear parton distribution global fits.
The W boson helicity fractions from top quark decays in tt‾ events are measured using data from proton–proton collisions at a centre-of-mass energy of 8TeV. The data were collected in 2012 with the ...CMS detector at the LHC, corresponding to an integrated luminosity of 19.8fb−1. Events are reconstructed with either one muon or one electron, along with four jets in the final state, with two of the jets being identified as originating from b quarks. The measured helicity fractions from both channels are combined, yielding F0=0.681±0.012(stat)±0.023(syst), FL=0.323±0.008(stat)±0.014(syst), and FR=−0.004±0.005(stat)±0.014(syst) for the longitudinal, left-, and right-handed components of the helicity, respectively. These measurements of the W boson helicity fractions are the most accurate to date and they agree with the predictions from the standard model.
Results are reported of a general search for pair production of heavy resonances decaying to pairs of hadronic jets in events with at least four jets. The study is based on up to 19.4 fb−1 of ...integrated luminosity from proton–proton collisions at a center-of-mass energy of 8 TeV, recorded with the CMS detector at the LHC. Limits are determined on the production of scalar top quarks (top squarks) in the framework of R-parity violating supersymmetry and on the production of color-octet vector bosons (colorons). First limits at the LHC are placed on top squark production for two scenarios. The first assumes decay to a bottom quark and a light-flavor quark and is excluded for masses between 200 and 385 GeV, and the second assumes decay to a pair of light-flavor quarks and is excluded for masses between 200 and 350 GeV at 95% confidence level. Previous limits on colorons decaying to light-flavor quarks are extended to exclude masses from 200 to 835 GeV.
A
bstract
Production of prompt J/
ψ
meson pairs in proton-proton collisions at
s
= 7 TeV is measured with the CMS experiment at the LHC in a data sample corresponding to an integrated luminosity of ...about 4.7 fb
−1
. The two J/
ψ
mesons are fully reconstructed via their decays into
μ
+
μ
−
pairs. This observation provides for the first time access to the high-transverse-momentum region of J/
ψ
pair production where model predictions are not yet established. The total and differential cross sections are measured in a phase space defined by the individual J/
ψ
transverse momentum (
p
T
J/
ψ
) and rapidity (|
y
J/
ψ
|): |
y
J/
ψ
| < 1.2 for
p
T
J/
ψ
> 6.5 GeV/
c
; 1.2 < |
y
J/
ψ
| < 1.43 for a
p
T
threshold that scales linearly with |
y
J/
ψ
| from 6.5 to 4.5 GeV/
c
; and 1.43 < |
y
J/
ψ
| < 2.2 for
p
T
J/
ψ
> 4.5 GeV/
c
. The total cross section, assuming unpolarized prompt J/
ψ
pair production is 1.49 ± 0.07 (stat) ±0.13 (syst) nb. Different assumptions about the J/
ψ
polarization imply modifications to the cross section ranging from −31% to +27%.
The pseudorapidity distribution of charged hadrons in pp collisions at s=13 TeV is measured using a data sample obtained with the CMS detector, operated at zero magnetic field, at the CERN LHC. The ...yield of primary charged long-lived hadrons produced in inelastic pp collisions is determined in the central region of the CMS pixel detector (|η|<2) using both hit pairs and reconstructed tracks. For central pseudorapidities (|η|<0.5), the charged-hadron multiplicity density is dNch/dη||η|<0.5=5.49±0.01(stat)±0.17(syst), a value obtained by combining the two methods. The result is compared to predictions from Monte Carlo event generators and to similar measurements made at lower collision energies.
The ratio of the top-quark branching fractions R=B(t→Wb)/B(t→Wq), where the denominator includes the sum over all down-type quarks (q=b,s,d), is measured in the tt¯ dilepton final state with ...proton–proton collision data at s=8 TeV from an integrated luminosity of 19.7 fb−1, collected with the CMS detector. In order to quantify the purity of the signal sample, the cross section is measured by fitting the observed jet multiplicity, thereby constraining the signal and background contributions. By counting the number of b jets per event, an unconstrained value of R=1.014±0.003(stat.)±0.032(syst.) is measured, in a good agreement with current precision measurements in electroweak and flavour sectors. A lower limit R>0.955 at the 95% confidence level is obtained after requiring R≤1, and a lower limit on the Cabibbo–Kobayashi–Maskawa matrix element |Vtb|>0.975 is set at 95% confidence level. The result is combined with a previous CMS measurement of the t-channel single-top-quark cross section to determine the top-quark total decay width, Γt=1.36±0.02(stat.)−0.11+0.14(syst.) GeV.
A search for supersymmetry with R-parity conservation in proton–proton collisions at a centre-of-mass energy of 7 TeV is presented. The data correspond to an integrated luminosity of 35 pb−1 ...collected by the CMS experiment at the LHC. The search is performed in events with jets and significant missing transverse energy, characteristic of the decays of heavy, pair-produced squarks and gluinos. The primary background, from standard model multijet production, is reduced by several orders of magnitude to a negligible level by the application of a set of robust kinematic requirements. With this selection, the data are consistent with the standard model backgrounds, namely tt¯, W+jet and Z+jet production, which are estimated from data control samples. Limits are set on the parameters of the constrained minimal supersymmetric extension of the standard model. These limits extend those set previously by experiments at the Tevatron and LEP colliders.
We present a measurement of b jet transverse momentum (pT) spectra in proton-lead (pPb) collisions using a dataset corresponding to about 35 nb−1 collected with the CMS detector at the LHC. Jets from ...b quark fragmentation are found by exploiting the long lifetime of hadrons containing a b quark through tagging methods using distributions of the secondary vertex mass and displacement. Extracted cross sections for b jets are scaled by the effective number of nucleon–nucleon collisions and are compared to a reference obtained from pythia simulations of pp collisions. The pythia-based estimate of the nuclear modification factor is found to be 1.22±0.15(stat+syst pPb)±0.27(syst pythia) averaged over all jets with pT between 55 and 400 GeV/c and with |ηlab|<2. We also compare this result to predictions from models using perturbative calculations in quantum chromodynamics.
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