A
bstract
The first observation of the Z boson decaying to four leptons in proton-proton collisions is presented. The analyzed data set corresponds to an integrated luminosity of 5.02 fb
−1
at
TeV ...collected by the CMS detector at the Large Hadron Collider. A pronounced resonance peak, with a statistical significance of 9.7
σ
, is observed in the distribution of the invariant mass of four leptons (electrons and/or muons) with mass and width consistent with expectations for Z boson decays. The branching fraction and cross section reported here are defined by phase space restrictions on the leptons, namely, 80 <
m
4ℓ
< 100 GeV, where
m
4ℓ
is the invariant mass of the four leptons, and
m
ℓℓ
> 4 GeV for all pairs of leptons, where
m
ℓℓ
is the two-lepton invariant mass. The measured branching fraction is
and agrees with the standard model prediction of 4.45 × 10
−6
. The measured cross section times branching fraction is
fb, also consistent with the standard model prediction of 120 fb. The four-lepton mass peak arising from
Z
→ 4
ℓ
decays provides a calibration channel for the Higgs boson search in the
H
→
ZZ
→ 4
ℓ
decay mode.
A
bstract
A measurement of the mass difference between the top and the antitop quark (Δ
m
t
=
m
t
−
) is performed using events with a muon or an electron and at least four jets in the final state. ...The analysis is based on data collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 4
.
96±0
.
11 fb
−1
, and yields the value of Δ
m
t
= −0
.
44±0
.
46 (stat.)±0
.
27 (syst.) GeV. This result is consistent with equality of particle and antiparticle masses required by CPT invariance, and provides a significantly improved precision relative to existing measurements.
A
bstract
A search is performed for long-lived neutral particles decaying into a photon and invisible particles. An example of such a signature is the decay of the lightest neutralino with nonzero ...lifetime into a gravitino and a photon in gauge-mediated supersymmetry, with the neutralino as the next-to-lightest supersymmetric particle and the gravitino as the lightest. The search uses events containing photons, missing transverse energy, and jets. The impact parameter of the photon relative to the beam-beam collision point can be reconstructed using converted photons. The method is sensitive to lifetimes of the order of 0.1 to 1 ns. The data sample corresponds to an integrated luminosity of 2.23 fb
−1
in pp collisions at
TeV, recorded in the first part of 2011 by the CMS experiment at the LHC. Cross-section limits are presented on pair production for such particles, each of which decays into a photon and invisible particles. The observed 95% confidence level limits vary between 0.11 and 0.21 pb, depending on the neutral particle lifetime.
A
bstract
The
production cross section
is measured in proton-proton collisions at
TeV in data collected by the CMS experiment, corresponding to an integrated luminosity of 2.3 fb
−1
. The measurement ...is performed in events with two leptons (electrons or muons) in the final state, at least two jets identified as jets originating from b quarks, and the presence of an imbalance in transverse momentum. The measured value of
for a top-quark mass of 172.5 GeV is
, consistent with the prediction of the standard model.
A
bstract
Jet fragmentation in pp and PbPb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair was studied using data collected with the CMS detector at the LHC. Fragmentation ...functions are constructed using charged-particle tracks with transverse momenta
p
T
> 4 GeV/
c
for dijet events with a leading jet of
p
T
> 100 GeV/
c
. The fragmentation functions in PbPb events are compared to those in pp data as a function of collision centrality, as well as dijet-
p
T
imbalance. Special emphasis is placed on the most central PbPb events including dijets with unbalanced momentum, indicative of energy loss of the hard scattered parent partons. The fragmentation patterns for both the leading and subleading jets in PbPb collisions agree with those seen in pp data at 2.76 TeV. The results provide evidence that, despite the large parton energy loss observed in PbPb collisions, the partition of the remaining momentum within the jet cone into high-
p
T
particles is not strongly modified in comparison to that observed for jets in vacuum.
A
bstract
A search for supersymmetry or other new physics resulting in similar final states is presented using a data sample of 4.73 fb
−1
of pp collisions collected at
TeV with the CMS detector at ...the LHC. Fully hadronic final states are selected based on the variable
M
T2
, an extension of the transverse mass in events with two invisible particles. Two complementary studies are performed. The first targets the region of parameter space with medium to high squark and gluino masses, in which the signal can be separated from the standard model backgrounds by a tight requirement on
M
T2
. The second is optimized to be sensitive to events with a light gluino and heavy squarks. In this case, the
M
T2
requirement is relaxed, but a higher jet multiplicity and at least one b-tagged jet are required. No significant excess of events over the standard model expectations is observed. Exclusion limits are derived for the parameter space of the constrained minimal supersymmetric extension of the standard model, as well as on a variety of simplified model spectra.
A
bstract
Combined results are reported from searches for a fermiophobic Higgs boson in the γγ, WW, and ZZ decay modes in proton-proton collisions at
TeV. The explored Higgs boson mass range is ...110–300 GeV. The data sample corresponds to an integrated luminosity of 4.9–5.1 fb
−1
. A fermiophobic Higgs boson is excluded at 95% confidence level in the mass range 110–194 GeV, and at 99% confidence level in the mass ranges 110–124.5 GeV, 127–147.5 GeV, and 155–180 GeV.
A
bstract
A search is presented for a massive particle, generically referred to as a Z
′
, decaying into a
pair. The search focuses on Z
′
resonances that are sufficiently massive to produce highly ...Lorentz-boosted top quarks, which yield collimated decay products that are partially or fully merged into single jets. The analysis uses new methods to analyze jet substructure, providing suppression of the non-top multijet backgrounds. The analysis is based on a data sample of proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 5 fb
−1
. Upper limits in the range of 1 pb are set on the product of the production cross section and branching fraction for a topcolor Z
′
modeled for several widths, as well as for a Randall-Sundrum Kaluza-Klein gluon. In addition, the result contrain any enhancement in
production beyond expectations of the standard mode for tt invariant mass larger than 1 TeV/
c
2
.
A
bstract
The first measurement of the charged component of the underlying event using the novel “jet-area/median” approach is presented for proton-proton collisions at centre-of-mass energies of 0.9 ...and 7 TeV. The data were recorded in 2010 with the CMS experiment at the LHC. A new observable, sensitive to soft particle production, is introduced and investigated inclusively and as a function of the event scale defined by the transverse momentum of the leading jet. Various phenomenological models are compared to data, with and without corrections for detector effects. None of the examined models describe the data satisfactorily.
A
bstract
A search for new physics is performed using isolated same-sign dileptons with at least two b-quark jets in the final state. Results are based on a 4.98 fb
−1
sample of proton-proton ...collisions at a centre-of-mass energy of 7 TeV collected by the CMS detector. No excess above the standard model background is observed. Upper limits at 95% confidence level are set on the number of events from non-standard-model sources. These limits are used to set constraints on a number of new physics models. Information on acceptance and efficiencies are also provided so that the results can be used to confront additional models in an approximate way.
PDF