131 research outputs found
Dark matter and sub-GeV hidden U(1) in GMSB models
Motivated by the recent PAMELA and ATIC data, one is led to a scenario with
heavy vector-like dark matter in association with a hidden sector
below GeV scale. Realizing this idea in the context of gauge mediated
supersymmetry breaking (GMSB), a heavy scalar component charged under
is found to be a good dark matter candidate which can be searched for direct
scattering mediated by the Higgs boson and/or by the hidden gauge boson. The
latter turns out to put a stringent bound on the kinetic mixing parameter
between and : . For the typical range
of model parameters, we find that the decay rates of the ordinary lightest
neutralino into hidden gauge boson/gaugino and photon/gravitino are comparable,
and the former decay mode leaves displaced vertices of lepton pairs and missing
energy with distinctive length scale larger than 20 cm for invariant lepton
pair mass below 0.5 GeV. An unsatisfactory aspect of our model is that the
Sommerfeld effect cannot raise the galactic dark matter annihilation by more
than 60 times for the dark matter mass below TeV.Comment: 1+15 pages, 4 figures, version published in JCAP, references added,
minor change
Measurement of the W<SUP>+</SUP>W<SUP>-</SUP>γ cross section and direct limits on anomalous quartic Gauge Boson couplings at LEP
The process e<SUP>+</SUP>e<SUP>−</SUP>→W<SUP>+</SUP>W<SUP>-</SUP>γ is analysed using the data collected with the L3 detector at LEP at a centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 176.8 pb<SUP>−1</SUP>. Based on a sample of 42 selected W<SUP>+</SUP>W<SUP>−</SUP> candidates containing an isolated hard photon, the W<SUP>+</SUP>W<SUP>−</SUP>γ cross section, defined within phase-space cuts, is measured to be: σ<SUB>WWγ</SUB>=290±80±16 fb, consistent with the Standard Model expectation. Including the process e<SUP>+</SUP>e<SUP>−</SUP>→νν̅γγ, limits are derived on anomalous contributions to the standard model quartic vertices W<SUP>+</SUP>W<SUP>−</SUP>γ and W<SUP>+</SUP>W<SUP>−</SUP>Zγ at 95% CL: −0.043 GeV<SUP>−2</SUP> < a<SUB>o</SUB> / Λ<SUP>2</SUP> < −0.043 GeV<SUP>−2</SUP>, −0.08 GeV-2 < a<SUB>c</SUB> / Λ<SUP>2</SUP> < 0.13 GeV<SUP>−2</SUP>, -0.41 GeV<SUP>−2</SUP> < a<SUB>n</SUB> / Λ<SUP>2</SUP> < 0.37 GeV<SUP>−2</SUP>
Measurement of the W-pair production cross section and W-decay branching fractions in e<SUP>+</SUP>e<SUP>-</SUP> interactions at √s = 189 GeV
The data collected by the L3 experiment at LEP at a centre-of-mass energy of 188.6 GeV are used to measure the W-pair production cross section and the W-boson decay branching fractions. These data correspond to an integrated luminosity of 176.8 pb<SUP>-1</SUP>. The total cross section for W-pair production, combining all final states, is measured to be σ<SUB>WW</SUB>=16.24±0.37 (stat.)±0.22 (syst.) pb. Including our data collected at lower centre-of-mass energies, the hadronic branching fraction of the W-boson is determined to be B(W→qq)=[68.20±0.68 (stat.)±0.33 (syst.)]%. The results agree with the Standard Model predictions
Search for a Higgs boson decaying into two photons in e<SUP>+</SUP>e<SUP>-</SUP> interactions at √s = 189 GeV
A search is performed for a Higgs boson produced in association with a Z boson and decaying into two photons, using the L3 data collected at LEP at a centre-of-mass energy of 189 GeV. All decay modes of the Z are considered. No signal is observed and limits on the branching fraction of the Higgs boson decay into two photons as a function of the Higgs mass are derived assuming a Standard Model production rate. A lower limit on the mass of a fermiophobic Higgs is set at 94.9 GeV at 95% confidence level
Study of neutral-current four-fermion and ZZ production in e<SUP>+</SUP>e<SUP>−</SUP> collisions at √s= 183 GeV
A study of neutral-current four-fermion processes is performed using a data sample corresponding to 55.3 pb<SUP>−1</SUP> of integrated luminosity collected by the L3 detector at LEP at an average centre-of-mass energy of 183 GeV. The neutral-current four-fermion cross sections for final states with a pair of charged leptons plus jets and with four charged leptons are measured to be consistent with the Standard Model predictions. Events with fermion pair masses close to the Z boson mass are selected in all observable final states and the ZZ production cross section is measured to be σ <SUB>ZZ</SUB> = 0.30<SUP>+0.220+0.07</SUP><SUB>−0.16−0.03</SUB> pb, in agreement with the Standard Model expectation. No evidence for the existence of anomalous triple gauge boson ZZZ and ZZγ couplings is found and limits on these couplings are set
Search for neutral Higgs boson production through the process e<SUP>+</SUP>e<SUP>−</SUP> → Z<SUP>∗</SUP> H<SUP>0</SUP>
The results of the searches for neutral Higgs boson production in the process e+e− → Z∗H0 are reported, focusing on Higgs boson masses below 70 GeV. The data sample consists of three million hadronic Z0 decays collected by the L3 experiment at LEP from 1991 through 1994. No signal is found leading to a lower limit on the mass of the Standard Model Higgs boson of 60.2 GeV at 95% C.L. These results are also interpreted in the framework of the General Two Doublet Model and limits on the nonstandard Higgs boson production through the process e+e− → Z∗H0 are set. A lower limit of 66.7 GeV at 95% C.L. is obtained for the case where the Higgs decays into an invisible final state
Measurement of Z<SUP>0</SUP> → bb̅ decay properties
We have measured the properties of Z0 → bb̅ decays using a sample of 944 inclusive muon events, corresponding to 18 000 hadron events obtained with the L3 detector at LEP. We measured the partial decay width of the Z0 into bb̅, γbb̅=353 ± 48 MeV, and we determined the vector coupling of the Z0 to the b quark; grmv2(b)=0.095±0.047. We measured the forward-backward charge asymmetry in e+e− → bb̅ events at √s≈Mv, and obtained Abb̅=13.3 ± 9.9%
Mass limits for excited electrons and muons from Z<SUP>0</SUP> decay
We searched for excited electrons and muons from Z0 decay in the channels ee→eeγ, ee→eeγγ, ee→(e)eγ, ee→μμγ, ee→μμγγ. using the L3 detector at LEP. The lower mass limit for e∗ is 45.0 GeV and for μ∗ 45.3 GeV. The upper limits of ℓℓ∗Z0 and ℓℓ∗γ couplings at 95% CL have been determined up to m ℓ∗ close to the Z0 mass
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