Forward physics at the LHC (Elba 2010)

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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 $U(1)_X$ sector below GeV scale. Realizing this idea in the context of gauge mediated supersymmetry breaking (GMSB), a heavy scalar component charged under $U(1)_X$ 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 $U(1)_X$ and $U(1)_Y$: $\theta \lesssim 10^{-6}$. 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> &lt; a<SUB>o</SUB> / Λ<SUP>2</SUP> &lt; −0.043 GeV<SUP>−2</SUP>, −0.08 GeV-2 &lt; a<SUB>c</SUB> / Λ<SUP>2</SUP> &lt; 0.13 GeV<SUP>−2</SUP>, -0.41 GeV<SUP>−2</SUP> &lt; a<SUB>n</SUB> / Λ<SUP>2</SUP> &lt; 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

Search for new particles in hadronic events with isolated photons

A search for the neutral Higgs boson in the processes e+e−→ Z &#8594; H0γ → qq̅ and e+e− → Z → Z∗H0 → qq̅γγ has been performed using 2.8 million hadronic Z decays collected with the L3 detector at LEP from 1991 through 1994. No evidence for these processes has been observed. Upper limits at 95% confidence level for the corresponding cross sections have been set and the results have been compared with theoretical predictions beyond the Standard Model

Search for the neutral Higgs bosons of the minimal supersymmetric standard model from Z<SUP>0</SUP> decays

We present the results from a search for the light neutral scalar Higgs boson h° and the pseudoscalar Higgs boson A° of the minimal super-symmetric standard model. The analysis is based on a data sample corresponding to 71 000 hadronic Z° decays recorded with the L3 detector at LEP. No evidence for the existence of the neutral Higgs bosons h° and A° has been found. The region of h° and A° masses up to 41.5 GeV is excluded at 95% confidence level