17,749 research outputs found
Exclusive electroproduction revisited: treating kinematical effects
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
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
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?
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
The transverse momentum distribution is computed for inclusive Higgs
boson production at TeV. We include all-orders resummation of
large logarithms associated with emission of soft gluons at small . We
provide results for Higgs boson and masses from 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
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
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
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
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 depends most strongly on the
ratio . For the relic abundance is almost
always much too large, whereas for the opposite occurs. For
there are wide ranges of the remaining parameters for which
. We also determine that m_{\tilde q}\gsim250\GeV and
m_{\tilde l}\gsim100\GeV are necessary in order to possibly achieve
. 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 pairs in gluon-gluon scattering in high energy proton-proton collisions
We calculate cross sections for 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, , invariant mass) are presented.
The elementary cross section is used to calculate production of 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 . 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 production is at high energy ( 5 TeV) much smaller than that for DPS contribution.Comment: 17 pages, 11 figure
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