17,367 research outputs found

    Exclusive electroproduction revisited: treating kinematical effects

    Full text link
    Generalized parton distributions of the nucleon are accessed via exclusive leptoproduction of the real photon. While earlier analytical considerations of phenomenological observables were restricted to twist-three accuracy, i.e., taking into account only terms suppressed by a single power of the hard scale, in the present study we revisit this differential cross section within the helicity formalism and restore power-suppressed effects stemming from the process kinematics exactly. We restrict ourselves to the phenomenologically important case of lepton scattering off a longitudinally polarized nucleon, where the photon flips its helicity at most by one unit.Comment: 22 pages, 1 figur

    An Updated Historical Profile of the Higgs Boson

    Full text link
    The Higgs boson was postulated in 1964, and phenomenological studies of its possible production and decays started in the early 1970s, followed by studies of its possible production in electron-positron, antiproton-proton and proton-proton collisions, in particular. Until recently, the most sensitive searches for the Higgs boson were at LEP between 1989 and 2000, which were complemented by searches at the Fermilab Tevatron. Then the LHC experiments ATLAS and CMS entered the hunt, announcing on July 4, 2012 the discovery of a "Higgs-like" particle with a mass of about 125~GeV. This identification has been supported by subsequent measurements of its spin, parity and coupling properties. It was widely anticipated that the Higgs boson would be accompanied by supersymmetry, although other options, like compositeness, were not completely excluded. So far there are no signs any new physics, and the measured properties of the Higgs boson are consistent with the predictions of the minimal Standard Model. This article reviews some of the key historical developments in Higgs physics over the past half-century.Comment: 22 pages, 5 figures, update of arXiv:1201.6045, to be published in the volume "The Standard Theory of Particle Physics", edited by Luciano Maiani and Gigi Roland

    Roles and regulation of membrane-associated serine proteases

    Get PDF
    Pericellular proteolytic activity affects many aspects of cellular behaviour, via mechanisms involving processing of the extracellular matrix, growth factors and receptors. The serine proteases have exquisitely sensitive regulatory mechanisms in this setting, involving both receptor-bound and transmembrane proteases. Receptor-bound proteases are exemplified by the uPA (urokinase plasminogen activator)/uPAR (uPAR receptor) plasminogen activation system. The mechanisms initiating the activity of this proteolytic system on the cell surface, a critical regulatory point, are poorly understood. We have found that the expression of the TTSP (type II transmembrane serine protease) matriptase is highly regulated in leucocytes, and correlates with the presence of active uPA on their surface. Using siRNA (small interfering RNA), we have demonstrated that matriptase specifically activates uPAR-associated pro-uPA. The uPA/uPAR system has been implicated in the activation of the plasminogen-related growth factor HGF (hepatocyte growth factor). However, we find no evidence for this, but instead that HGF can be activated by both matriptase and the related TTSP hepsin in purified systems. Hepsin is of particular interest, as the proteolytic cleavage sequence of HGF is an ‘ideal substrate’ for hepsin and membrane-associated hepsin activates HGF with high efficiency. Both of these TTSPs can be activated autocatalytically at the cell surface, an unusual mechanism among the serine proteases. Therefore these TTSPs have the capacity to be true upstream initiators of proteolytic activity with subsequent downstream effects on cell behaviour

    What if the Higgs Boson Weighs 115 GeV?

    Get PDF
    If the Higgs boson indeed weighs about 114 to 115 GeV, there must be new physics beyond the Standard Model at some scale \la 10^6 GeV. The most plausible new physics is supersymmetry, which predicts a Higgs boson weighing \la 130 GeV. In the CMSSM with R and CP conservation, the existence, production and detection of a 114 or 115 GeV Higgs boson is possible if \tan\beta \ga 3. However, for the radiatively-corrected Higgs mass to be this large, sparticles should be relatively heavy: m_{1/2} \ga 250 GeV, probably not detectable at the Tevatron collider and perhaps not at a low-energy e^+ e^- linear collider. In much of the remaining CMSSM parameter space, neutralino-stau coannihilation is important for calculating the relic neutralino density, and we explore implications for the elastic neutralino-nucleon scattering cross section.Comment: 17 pages, 5 eps figure

    Differential Cross Sections for Higgs Boson Production at Tevatron Collider Energies

    Full text link
    The transverse momentum QTQ_T distribution is computed for inclusive Higgs boson production at S=1.96\sqrt{S} = 1.96 TeV. We include all-orders resummation of large logarithms associated with emission of soft gluons at small QTQ_T. We provide results for Higgs boson and ZZ^* masses from MZM_Z to 200 GeV. The relatively hard transverse momentum distribution for Higgs boson production suggests possibilities for improvement of the signal to background ratio.Comment: 12 pages, latex, 7 figure

    Charged-Lepton-Flavour Violation in the Light of the Super-Kamiokande Data

    Get PDF
    Motivated by the data from Super-Kamiokande and elsewhere indicating oscillations of atmospheric and solar neutrinos, we study charged-lepton-flavour violation, in particular the radiative decays mu -> e gamma and tau -> mu gamma, but also commenting on mu -> 3e and tau -> 3 mu/e decays, as well as mu - e conversion on nuclei. We first show how the renormalization group may be used to calculate flavour-violating soft supersymmetry-breaking masses for charged sleptons and sneutrinos in models with universal input parameters. Subsequently, we classify possible patterns of lepton-flavour violation in the context of phenomenological neutrino mass textures that accommodate the Super-Kamiokande data, giving examples based on Abelian flavour symmetries. Then we calculate in these examples rates for mu -> e gamma and tau ->mu gamma, which may be close to the present experimental upper limits, and show how they may distinguish between the different generic mixing patterns. The rates are promisingly large when the soft supersymmetry-breaking mass parameters are chosen to be consistent with the cosmological relic-density constraints. In addition, we discuss mu -> e conversion on Titanium, which may also be accessible to future experiments.Comment: 29 pages, 12 figures. References added, typos correcte

    Astrophysical Probes of the Constancy of the Velocity of Light

    Get PDF
    We discuss possible tests of the constancy of the velocity of light using distant astrophysical sources such as gamma-ray bursters (GRBs), Active Galactic Nuclei (AGNs) and pulsars. This speculative quest may be motivated by some models of quantum fluctuations in the space-time background, and we discuss explicitly how an energy-dependent variation in photon velocity \delta c/ c \sim - E / M arises in one particular quantum-gravitational model. We then discuss how data on GRBs may be used to set limits on variations in the velocity of light, which we illustrate using BATSE and OSSE observations of the GRBs that have recently been identified optically and for which precise redshifts are available. We show how a regression analysis can be performed to look for an energy-dependent effect that should correlate with redshift. The present data yield a limit M \gsim 10^{15} GeV for the quantum gravity scale. We discuss the prospects for improving this analysis using future data, and how one might hope to distinguish any positive signal from astrophysical effects associated with the sources.Comment: 37 pages LaTeX, 9 eps figures included, uses aasms4.st

    Flash of photons from the early stage of heavy-ion collisions

    Get PDF
    The dynamics of partonic cascades may be an important aspect for particle production in relativistic collisions of nuclei at CERN SPS and BNL RHIC energies. Within the Parton-Cascade Model, we estimate the production of single photons from such cascades due to scattering of quarks and gluons q g -> q gamma, quark-antiquark annihilation q qbar -> g gamma, or gamma gamma, and from electromagnetic brems-strahlung of quarks q -> q gamma. We find that the latter QED branching process plays the dominant role for photon production, similarly as the QCD branchings q -> q g and g -> g g play a crucial role for parton multiplication. We conclude therefore that photons accompanying the parton cascade evolution during the early stage of heavy-ion collisions shed light on the formation of a partonic plasma.Comment: 4 pages including 3 postscript figure

    New Constraints on Neutralino Dark Matter in the Supersymmetric Standard Model

    Full text link
    We investigate the prospects for neutralino dark matter within the Supersymmetric Standard Model (SSM) including the constraints from universal soft supersymmetry breaking and radiative breaking of the electroweak symmetry. The latter is enforced by using the one-loop Higgs effective potential which automatically gives the one-loop corrected Higgs boson masses. We perform an exhaustive search of the allowed five-dimensional parameter space and find that the neutralino relic abundance Ωχh02\Omega_\chi h^2_0 depends most strongly on the ratio ξ0m0/m1/2\xi_0\equiv m_0/m_{1/2}. For ξ01\xi_0\gg1 the relic abundance is almost always much too large, whereas for ξ01\xi_0\ll1 the opposite occurs. For ξ01\xi_0\sim1 there are wide ranges of the remaining parameters for which Ωχ1\Omega_\chi\sim1. We also determine that m_{\tilde q}\gsim250\GeV and m_{\tilde l}\gsim100\GeV are necessary in order to possibly achieve Ωχ1\Omega_\chi\sim1. These lower bounds are much weaker than the corresponding ones derived previously when radiative breaking was {\it not} enforced.Comment: 12 pages plus 6 figures (not included), CERN-TH.6584/92, CTP-TAMU-56/92, UAHEP921

    Production of two ccˉc \bar c pairs in gluon-gluon scattering in high energy proton-proton collisions

    Full text link
    We calculate cross sections for ggQQˉQQˉg g \to Q \bar Q Q \bar Q in the high-energy approximation in the mixed (longitudinal momentum fraction, impact parameter) and momentum space representations. Besides the total cross section as a function of subsystem energy also differential distributions (in quark rapidity, transverse momentum, QQQ Q, QQˉQ \bar Q invariant mass) are presented. The elementary cross section is used to calculate production of (ccˉ)(ccˉ)(c \bar c) (c \bar c) in single-parton scattering (SPS) in proton-proton collisions. We present integrated cross section as a function of proton-proton center of mass energy as well as differential distribution in M(ccˉ)(ccˉ)M_{(c \bar c)(c \bar c)}. The results are compared with corresponding results for double-parton scattering (DPS) discussed recently in the literature. We find that the considered SPS contribution to (ccˉ)(ccˉ)(c \bar c)(c \bar c) production is at high energy (s>\sqrt{s} > 5 TeV) much smaller than that for DPS contribution.Comment: 17 pages, 11 figure
    corecore