83 research outputs found
Neutral-Current Four-Fermion Production in e+e- Interactions at LEP
Neutral-current four-fermion production, e+e- -> ffff is studied in 0.7/fb of
data collected with the L3 detector at LEP at centre-of-mass energies
root(s)=183-209GeV. Four final states are considered: qqvv, qqll, llll and
llvv, where l denotes either an electron or a muon. Their cross sections are
measured and found to agree with the Standard Model predictions. In addition,
the e+e- -> Zgamma* -> ffff process is studied and its total cross section at
the average centre-of-mass energy 196.6GeV is found to be 0.29 +/- 0.05 +/-
0.03 pb, where the first uncertainty is statistical and the second systematic,
in agreement with the Standard Model prediction of 0.22 pb. Finally, the mass
spectra of the qqll final states are analysed to search for the possible
production of a new neutral heavy particle, for which no evidence is found
Measurement of Exclusive rho+rho- Production in Mid-Virtuality Two-Photon Interactions and Study of the gamma gamma* -> rho rho Process at LEP
Exclusive rho+rho- production in two-photon collisions between a quasi-real
photon, gamma, and a mid-virtuality photon, gamma*, is studied with data
collected at LEP at centre-of-mass energies root(s)=183-209GeV with a total
integrated luminosity of 684.8pb^-1. The cross section of the gamma gamma* ->
rho+ rho- process is determined as a function of the photon virtuality, Q^2,
and the two-photon centre-of-mass energy, W_gg, in the kinematic region:
0.2GeV^2 < Q^2 <0.85GeV^2 and 1.1GeV < W_gg < 3GeV. These results, together
with previous L3 measurements of rho0 rho0 and rho+ rho- production, allow a
study of the gamma gamma* -> rho rho process over the Q^2-region 0.2GeV^2 < Q^2
< 30 GeV^2
Measurement of the Running of the Electromagnetic Coupling at Large Momentum-Transfer at LEP
The evolution of the electromagnetic coupling, alpha, in the
momentum-transfer range 1800GeV^2 < -Q^2 < 21600GeV^2 is studied with about
40000 Bhabha-scattering events collected with the L3 detector at LEP at
centre-of-mass energies 189-209GeV. The running of alpha is parametrised as:
alpha(Q^2) = alpha_0/(1-C Delta alpha(Q^2)), where alpha_0=\alpha(Q^2=0) is the
fine-structure constant and C=1 corresponds to the evolution expected in QED. A
fit to the differential cross section of the e+e- ->e+e- process for scattering
angles in the range |cos theta|<0.9 excludes the hypothesis of a constant value
of alpha, C=0, and validates the QED prediction with the result: C = 1.05 +/-
0.07 +/- 0.14, where the first uncertainty is statistical and the second
systematic
Study of Inclusive Strange-Baryon Production and Search for Pentaquarks in Two-Photon Collisions at LEP
Measurements of inclusive production of the Lambda, Xi- and Xi*(1530) baryons
in two-photon collisions with the L3 detector at LEP are presented. The
inclusive differential cross sections for Lambda and Xi- are measured as a
function of the baryon transverse momentum, pt, and pseudo-rapidity, eta. The
mean number of Lambda, Xi- and Xi*(1530) baryons per hadronic two-photon event
is determined in the kinematic range 0.4 GeV < pt< 2.5 GeV, |eta| < 1.2.
Overall agreement with the theoretical models and Monte Carlo predictions is
observed. A search for inclusive production of the pentaquark theta+(1540) in
two-photon collisions through the decay theta+ -> proton K0s is also presented.
No evidence for production of this state is found
Z-boson production with two unobserved, back-to-back, hard photons at LEP
The double-radiative process e+e- -> Z gamma gamma -> q q~ gamma gamma, where
the two hard photons escape detection at low polar angles into opposite
directions, is studied in 0.62/fb of data collected with the L3 detector at LEP
at centre-of-mass energies between 188.6GeV and 209.2GeV. The cross sections
are measured and found to be consistent with the Standard Model expectations
Measurement of Exclusive rho^0 rho^0 Production in Mid-Virtuality Two-Photon Interactions at LEP
Exclusive rho^0 rho^0 production in two-photon collisions between a
quasi-real and a mid-virtuality photon is studied with data collected at LEP at
centre-of-mass energies 183GeV < sqrt{s} < 209GeV with a total integrated
luminosity of 684.8/pb. The cross section of the process gamma gamma* -> rho^0
rho^0 is determined as a function of the photon virtuality, q^2, and the
two-photon centre-of-mass energy, Wgg, in the kinematic region: 0.2GeV^2 < q^2
< 0.85GeV^2 and 1.1GeV < Wgg < 3GeV
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
Measurement of isolated photon production in pp and PbPb collisions at sqrt(sNN) = 2.76 TeV
Isolated photon production is measured in proton-proton and lead-lead
collisions at nucleon-nucleon centre-of-mass energies of 2.76 TeV in the
pseudorapidity range |eta|<1.44 and transverse energies ET between 20 and 80
GeV with the CMS detector at the LHC. The measured ET spectra are found to be
in good agreement with next-to-leading-order perturbative QCD predictions. The
ratio of PbPb to pp isolated photon ET-differential yields, scaled by the
number of incoherent nucleon-nucleon collisions, is consistent with unity for
all PbPb reaction centralities.Comment: Submitted to Physics Letters
Measurement of the Photon Structure Function F2gamma with the L3 Detector at LEP
The e+e- -> e+e- hadrons reaction, where one of the two electrons is detected
in a low polar-angle calorimeter, is analysed in order to measure the hadronic
photon structure function F2gamma . The full high-energy and high-luminosity
data set, collected with the L3 detector at centre-of-mass energies 189-209GeV,
corresponding to an integrated luminosity of 608/pb is used. The Q^2 range
11-34GeV^2 and the x range 0.006-0.556 are considered. The data are compared
with recent parton density functions
Observation of a new Xi(b) baryon
The first observation of a new b baryon via its strong decay into Xi(b)^-
pi^+ (plus charge conjugates) is reported. The measurement uses a data sample
of pp collisions at sqrt(s) = 7 TeV collected by the CMS experiment at the LHC,
corresponding to an integrated luminosity of 5.3 inverse femtobarns. The known
Xi(b)^- baryon is reconstructed via the decay chain Xi(b)^- to J/psi Xi^- to
mu^+ mu^- Lambda^0 pi^-, with Lambda^0 to p pi^-. A peak is observed in the
distribution of the difference between the mass of the Xi(b)^- pi^+ system and
the sum of the masses of the Xi(b)^- and pi^+, with a significance exceeding
five standard deviations. The mass difference of the peak is 14.84 +/- 0.74
(stat.) +/- 0.28 (syst.) MeV. The new state most likely corresponds to the
J^P=3/2^+ companion of the Xi(b).Comment: Submitted to Physical Review Letter
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