arXiv subjects
Publications and source records attributed to B. Mele.
The results obtained by the Working Group on Supersymmetry at the 1999 Les Houches Workshop on Collider Physics are summarized. Separate chapters treat "general" supersymmetry, R-parity violation, gauge mediated supersymmetry breaking, and anomaly mediated supersymmetry breaking.
The status of two on-going studies concerning important aspects of the phenomenology of gauge-mediated supersymmetry breaking (GMSB) models at TeV colliders is reported. The first study deals with the characteristics of the light Higgs boson spectrum allowed by the (minimal and non-minimal) GMSB framework. Today's most accurate GMSB model generation and two-loop Feynman-diagrammatic calculation of m_h have been combined. The Higgs masses are shown in dependence of various model parameters at the messenger and electroweak scales. In the minimal model, an upper limit on m_h of about 124 GeV is found for m_t = 175 GeV. The second study is focused on the measurement of the fundamental SUSY breaking scale sqrt(F) at the LHC in the GMSB scenario where a stau is the next-to-lightest SUSY particle (NLSP) and decays into a gravitino with c*tau_NLSP in the range 0.5 m to 1 km. This implies the measurement of mass and lifetime of long lived sleptons. The identification is performed by determining the time of flight in the ATLAS muon chambers. Accessible range and precision on sqrt(F) achievable with a counting method are assessed.
The bounds on the anomalous contributions to the ggh and zgh vertices that can be obtained via the process eg->eh are discussed through a model independent analysis based on SU(2) X U(1) invariant operators of dim=6. A light Higgs boson with m_H ~ 120 GeV and center-of-mass energies of \sqrt{S}=500 GeV and \sqrt{S}=1500 GeV are assumed. The process eg->eh is shown to provide an excellent way to strongly constraint both the CP-even and CP-odd anomalous contributions in the ggH and ZgH couplings.
The t->cH decay width has been computed in the standard model with a light Higgs boson. The corresponding branching fraction has been found to be in the range B(t->cH) ~ 10^{-13} - 10^{-14} for M_Z < m_H < 2 M_W. Our results correct the numerical evaluation usually quoted in the literature.
We consider the possibility of studying anomalous contributions to the gamma-gamma-H and Z-gamma-H vertices through the process e-gamma--> e-H at future e-gamma linear colliders, with Sqrt(S)=500-1500 GeV. We make a model independent analysis based on SU(2)xU(1) invariant effective operators of dim=6 added to the standard model lagrangian. We consider a light Higgs boson (mostly decaying in bar(b)-b pairs), and include all the relevant backgrounds. Initial e-beam polarization effects are also analyzed. We find that the process e-gamma--> e-H provides an excellent opportunity to strongly constrain both the CP-even and the CP-odd anomalous contributions to the gamma-gamma-H and Z-gamma-H vertices.
One-loop production of a Higgs boson in electro-photon collisions at future accelerators is studied via the process (e gamma -> e H), for intermediate Higgs masses. Exact cross sections, including the possibility of longitudinally polarized initial beams, are presented. Confirming previous estimates made in the Weizsäcker-Williams approximation, they are found to be more than two orders of magnitude larger than the cross sections for the crossed process (e e-> gamma H), in the energy range $\sqrt{s}=(0.5-2)$ TeV. We show that, not only (e gamma -> e H) has a similar potential as the (gamma gamma -> H) process for testing the one-loop gamma-gamma-H vertex, but, by requiring a final electron tagged at large angle in (e gamma -> e H), the (H e) production provides an excellent way of testing the Z-gamma-H vertex, too. Kinematical distributions for the (e gamma -> e H -> e bb) process with a tagged final electron are analyzed, and strategies for controlling the main irreducible background are found. Initial-state-radiation effects are checked to be within a few percent.
We study the bounds on the anomalous contributions to the gamma-gamma-H and Z-gamma-H vertices that can be obtained via the process e-gamma-->e H. We consider the representative case of an intermediate Higgs mass production of MH=120 GeV and for a center of mass energy of Sqrt{S}=500 GeV and Sqrt{S}=1500 GeV. We use a model independent analysis based on SU(2)xU(1) invariant operators of dimension 6 added to the Standard Model lagrangian. We find that this process provides an excelent way to put strong constraints both in the sector of CP-even and CP-odd anomalous couplings contribution to the gamma-gamma-H and Z-gamma-H vertices.
The rarest cosmic rays above the GZK cut'off $(E_{CR} \tilde{>} 10^{19} ÷10^{20} eV)$ are probably born at cosmic distances ($\tilde{>}$ tens Mpc) by AGNs (QSRs, BLac, Blazars...). Their puzzling survival over $2.75 K^o$ BBR radio waves opacities (the ``GZK cut off'') might find a natural explanation if the traveling primordial cosmic rays were UHE neutrinos (born by UHE photopion decay) which are transparent to $γ$ or $ν$ BBR. These UHE$% ν$ might scatter onto those (light and cosmological) relic neutrinos clustered around our galactic halo. The branched chain reactions from a primordial nucleon (via photoproduction of pions and decay to UHE neutrinos) toward the consequent beam dump scattering on galactic relic neutrinos is at least three order of magnitude more efficient than any known neutrino interactions with Earth atmosphere or direct nucleon propagation. Therefore the rarest cosmic rays (as the 320 EeV event) might be originated at far $(\tilde{>} 100 Mpc)$ distances (as Seyfert galaxy MCG 8-11-11). The needed UHE radiation power is in rough agreement with the NCG 8-11-11 observed in MeV gamma energy total output power. The final chain products observed on Earth by the Fly's Eye detector might be mainly neutron and antineutrons as well as, at later stages, protons and antiprotons. These hadronic products are most probably secondaries of $W^+ W^-$ or $ZZ$ pair productions and might be consistent with the last AGASA discoveries of 6 doublet and one triplet event.
The diffuse relic neutrinos with light mass are transparent to Ultrahigh energy (UHE) neutrinos at thousands EeV, born by photoproduction of pions by UHE protons on relic 2.73 K BBR radiation and originated in AGNs at cosmic distances. However these UHE $ν$s may interact with those (mainly heaviest $ν_{μ_r}$, $ν_{τ_r}$ and respective antineutrinos) clustered into HDM galactic halos. UHE photons or protons, secondaries of $νν_r$ scattering, might be the final observed signature of such high-energy chain reactions and may be responsible of the highest extragalactic cosmic-ray (CR) events. The chain-reactions conversion efficiency, ramifications and energetics are considered for the October 1991 CR event at 320 EeV observed by the Fly's Eye detector in Utah. These quantities seem compatible with the distance, direction and power (observed at MeV gamma energies) of the Seyfert galaxy MCG 8-11-11. The $νν_r$ interaction probability is favoured by at least three order of magnitude with respect to a direct $ν$ scattering onto the Earth atmosphere. Therefore, it may better explain the extragalactic origin of the puzzling 320 EeV event, while offering indirect evidence of a hot dark galactic halo of light (i.e., $m_ν\sim$ tens eV) neutrinos, probably of tau flavour.
We present a new calculation of the t->cH decay rate in the standard model. We find that the corresponding branching fraction is B(t->cH) \sim 10^{-13} - 10^{-14} for M_Z < m_H < 2 M_W, that is about 6 orders of magnitude less than previously evaluated, and usually quoted in the literature.
Higgs production in $eγ$ collisions, through the one-loop reaction $eγ\to eH$ at large $p_T$, can provide a precise determination of the $ZγH$ vertex.
We show that the Higgs production in $eγ$ collisions, through the one-loop reaction $eγ\to eH$ at large transverse momentum, can provide a precise determination of the $ZγH$ vertex. We concentrate on the analysis of signal rates vs background looking for strategies to improve the ratio $S/B$. Possible bounds on anomalous contributions to the $ZγH$ vertex are derived.
The one-loop process e,gamma->e,h for intermediate Higgs masses is considered. Exact cross sections for unpolarized and longitudinally polarized beams are computed and found to be more than two orders of magnitude larger than the rates for the crossed process e,e->gamma,h, in the energy range \sqrt{s}=(0.5-2) TeV. We show that, apart from being competitive with the gamma,gamma->H process for testing the one-loop gamma-gamma-H vertex, by requiring a final electron tagged at large angle the channel e,gamma->e,h provides an excellent way of testing the Z-gamma-H vertex.
Within the minimal supersymmetric extension of the Standard Model, the best fit to the most recent precision-measurement data requires charginos and neutralinos, with dominant Higgsino components and with masses within the reach of LEP1.5 ($\sqrt{s}=140$ GeV). In this work, we present a detailed analysis of the neutralino and chargino production processes for the favoured region of parameter space, that is low values of $|μ|$ and either low or large values of $\tanβ$. We find that chargino and neutralino searches can cover the Higgsino region in the ($μ,M_2$) plane for values of $M_2 \simlt 1$ TeV, at the next phases of the LEP collider. We also show that, due mainly to phase-space constraints, the lightest neutralinos should be more easily detectable than charginos in most of the parameter space preferred by precision-measurement data.
The production of invisible pairs of lightest neutralinos accompanied by a large-angle hard photon in the reaction $e^+ e^- \to χ^0_1 χ^0_1 γ$ is studied at LEP2 energies. The most general gaugino/higgsino composition of the $χ^0_1$ within the Minimal Supersymmetric Standard Model is assumed. The spectrum of the observed photon is derived within the framework of the $p_t$-dependent structure-function approach, whose accuracy is assessed to be within the foreseen experimental accuracy at LEP2. Higher-order QED corrections due to undetected initial-state radiation are also included. A comparison with the Standard Model main background from $e^+ e^- \to ν\bar νγ$ is performed for optimized photon kinematical cuts. Quantitative conclusions on the signal/background ratio are given for a wide range of values of the SUSY parameters.
Review of prospects for discovery of new physics signals at LEP2. The areas covered include SUSY, exotic fermions, BESS models, leptoquarks, virtual effects and CP violating observables.
A complete phenomenological study of the next-to-lightest neutralino decays is performed in the MSSM. The widths and branching ratios for all the possible decay channels (including the radiative decay X0(2) --> X0(1) gamma and the decay into a light Higgs X0(2) --> X0(1) h0) are studied in detail as functions of all the SuSy parameters of the model. Particular attention is paid to situations that are interesting for LEP2 searches. Non-trivial decay patterns are found that critically depend on the region of the parameter space considered.