284 research outputs found
Fit to Moments of Inclusive B->Xc lv and B-> Xs gamma Decay Distributions using Heavy Quark Expansions in the Kinetic Scheme
We present a fit to measured moments of inclusive distributions in B->Xc lv
and B-> Xs gamma decays to extract values for the CKM matrix element |Vcb|, the
b- and c-quark masses, and higher order parameters that appear in the Heavy
Quark Expansion. The fit is carried out using theoretical calculations in the
kinetic scheme and includes moment measurements of the Babar, Belle, CDF, CLEO
and DELPHI collaborations for which correlation matrices have been published.
We find |Vcb| = (41.96 +- 0.23(exp) +- 0.35(HQE) +- 0.59(Gamma_SL)) 10^-3 and
m_b = 4.590 +- 0.025(exp) +- 0.30(HQE) GeV where errors are experimental and
theoretical respectively. We also derive values for the heavy quark
distribution function parameters m_b and \mu_\pi^2 in different theoretical
schemes that can be used as input for the determination of |Vub|.Comment: 13 pages, 6 figures. v3: Fig 1+2 show absolute values for moments and
fit rather than the difference. Updated some references. v2: Updated to
include Belle photon moments and PDG 2005 lifetime. Included extrapolation
factors for the BR(B->Xs gamma) to 1.6 Gev based on the HQE parameters from
the fit and added an updated OPE expression for |Vub
Searches for New Physics at CMS
The first searches for New Physics with the CMS detector at the LHC are presented. The discussed analyses are based on the data sample recorded in 2010 at a centre-of-mass energy of 7TeV, which corresponds to an integrated luminosity of about 35 pbâ1. Searches for excited vector bosons, leptoquarks, extra dimensions as well as for supersymmetry in different final states are presented. No significant deviations from Standard Model expectations have been observed and thus limits on the parameter space of different New Physics scenarios are derived
Frequentist Analysis of the Parameter Space of Minimal Supergravity
We make a frequentist analysis of the parameter space of minimal supergravity
(mSUGRA), in which, as well as the gaugino and scalar soft
supersymmetry-breaking parameters being universal, there is a specific relation
between the trilinear, bilinear and scalar supersymmetry-breaking parameters,
A_0 = B_0 + m_0, and the gravitino mass is fixed by m_{3/2} = m_0. We also
consider a more general model, in which the gravitino mass constraint is
relaxed (the VCMSSM). We combine in the global likelihood function the
experimental constraints from low-energy electroweak precision data, the
anomalous magnetic moment of the muon, the lightest Higgs boson mass M_h, B
physics and the astrophysical cold dark matter density, assuming that the
lightest supersymmetric particle (LSP) is a neutralino. In the VCMSSM, we find
a preference for values of m_{1/2} and m_0 similar to those found previously in
frequentist analyses of the constrained MSSM (CMSSM) and a model with common
non-universal Higgs masses (NUHM1). On the other hand, in mSUGRA we find two
preferred regions: one with larger values of both m_{1/2} and m_0 than in the
VCMSSM, and one with large m_0 but small m_{1/2}. We compare the probabilities
of the frequentist fits in mSUGRA, the VCMSSM, the CMSSM and the NUHM1: the
probability that mSUGRA is consistent with the present data is significantly
less than in the other models. We also discuss the mSUGRA and VCMSSM
predictions for sparticle masses and other observables, identifying potential
signatures at the LHC and elsewhere.Comment: 18 pages 27 figure
Two-loop electroweak fermionic corrections to sin^2 theta_{eff}^{b anti-b}
We present the first calculation of the two-loop electroweak fermionic
correction to the flavour-dependent effective weak-mixing angle for bottom
quarks, sin^2 theta_{eff}^{b anti-b}. For the evaluation of the missing
two-loop vertex diagrams, two methods are employed, one based on a
semi-numerical Bernstein-Tkachov algorithm and the second on asymptotic
expansions in the large top-quark mass. A third method based on dispersion
relations is used for checking the basic loop integrals. We find that for small
Higgs-boson mass values, M_H ~ 100 GeV, the correction is sizable, of order
O(10^{-4}).Comment: 17 pages, 2 figures, references added, accepted for publication in
Nucl. Phys.
The Probable Fate of the Standard Model
Extrapolating the Standard Model to high scales using the renormalisation
group, three possibilities arise, depending on the mass of the Higgs boson: if
the Higgs mass is large enough the Higgs self-coupling may blow up, entailing
some new non-perturbative dynamics; if the Higgs mass is small the effective
potential of the Standard Model may reveal an instability; or the Standard
Model may survive all the way to the Planck scale for an intermediate range of
Higgs masses. This latter case does not necessarily require stability at all
times, but includes the possibility of a metastable vacuum which has not yet
decayed. We evaluate the relative likelihoods of these possibilities, on the
basis of a global fit to the Standard Model made using the Gfitter package.
This uses the information about the Higgs mass available directly from Higgs
searches at LEP and now the Tevatron, and indirectly from precision electroweak
data. We find that the `blow-up' scenario is disfavoured at the 99% confidence
level (96% without the Tevatron exclusion), whereas the `survival' and possible
`metastable' scenarios remain plausible. A future measurement of the mass of
the Higgs boson could reveal the fate of the Standard Model.Comment: 16 pp, 7 fig
RG-improved single-particle inclusive cross sections and forward-backward asymmetry in production at hadron colliders
We use techniques from soft-collinear effective theory (SCET) to derive
renormalization-group improved predictions for single-particle inclusive (1PI)
observables in top-quark pair production at hadron colliders. In particular, we
study the top-quark transverse-momentum and rapidity distributions, the
forward-backward asymmetry at the Tevatron, and the total cross section at
NLO+NNLL order in resummed perturbation theory and at approximate NNLO in fixed
order. We also perform a detailed analysis of power corrections to the leading
terms in the threshold expansion of the partonic hard-scattering kernels. We
conclude that, although the threshold expansion in 1PI kinematics is
susceptible to numerically significant power corrections, its predictions for
the total cross section are in good agreement with those obtained by
integrating the top-pair invariant-mass distribution in pair invariant-mass
kinematics, as long as a certain set of subleading terms appearing naturally
within the SCET formalism is included.Comment: 55 pages, 14 figures, 6 table
Quantum gauge models without classical Higgs mechanism
We examine the status of massive gauge theories, such as those usually
obtained by spontaneous symmetry breakdown, from the viewpoint of causal
(Epstein-Glaser) renormalization. The BRS formulation of gauge invariance in
this framework, starting from canonical quantization of massive (as well as
massless) vector bosons as fundamental entities, and proceeding perturbatively,
allows one to rederive the reductive group symmetry of interactions, the need
for scalar fields in gauge theory, and the covariant derivative. Thus the
presence of higgs particles is explained without recourse to a
Higgs(-Englert-Brout-Guralnik-Hagen-Kibble) mechanism. Along the way, we dispel
doubts about the compatibility of causal gauge invariance with grand unified
theories.Comment: 20 pages in two-column EPJC format, shortened version accepted for
publication. For more details, consult version
Measurement of the quasi-elastic axial vector mass in neutrino-oxygen interactions
The weak nucleon axial-vector form factor for quasi-elastic interactions is
determined using neutrino interaction data from the K2K Scintillating Fiber
detector in the neutrino beam at KEK. More than 12,000 events are analyzed, of
which half are charged-current quasi-elastic interactions nu-mu n to mu- p
occurring primarily in oxygen nuclei. We use a relativistic Fermi gas model for
oxygen and assume the form factor is approximately a dipole with one parameter,
the axial vector mass M_A, and fit to the shape of the distribution of the
square of the momentum transfer from the nucleon to the nucleus. Our best fit
result for M_A = 1.20 \pm 0.12 GeV. Furthermore, this analysis includes updated
vector form factors from recent electron scattering experiments and a
discussion of the effects of the nucleon momentum on the shape of the fitted
distributions.Comment: 14 pages, 10 figures, 6 table
Single Higgs-boson production at a photon-photon collider: general 2HDM versus MSSM
We revisit the production of a single Higgs boson from direct \gamma \gamma
-scattering at a photon collider. We compute the total cross section
\sigma(\gamma \gamma \to h) (for h=h0, H0, A0), and the strength of the
effective g_{h \gamma \gamma} coupling normalized to the Standard Model (SM),
for both the general Two-Higgs-Doublet Model (2HDM) and the Minimal
Supersymmetric Standard Model (MSSM). In both cases the predicted production
rates for the CP-even (odd) states render up to 10^4 (10^3) events per 500
\invfb of integrated luminosity, in full consistency with all the theoretical
and phenomenological constraints. Depending on the channel the maximum rates
can be larger or smaller than the SM expectations, but in most of the parameter
space they should be well measurable. We analyze how these departures depend on
the dynamics underlying each of the models, supersymmetric and
non-supersymmetric, and highlight the possible distinctive phenomenological
signatures. We demonstrate that this process could be extremely helpful to
discern non-supersymmetric Higgs bosons from supersymmetric ones. Furthermore,
in the MSSM case, we show that \gamma\gamma-physics could decisively help to
overcome the serious impasse afflicting Higgs boson physics at the infamous
"LHC wedge".Comment: LaTeX, 16 pages, 6 Figures, 2 Tables. Minor typos fixed. Matches
published version in Phys. Lett.
The 2009 World Average of
Measurements of , the coupling strength of the Strong Interaction
between quarks and gluons, are summarised and an updated value of the world
average of is derived. Building up on previous reviews,
special emphasis is laid on the most recent determinations of . These
are obtained from -decays, from global fits of electroweak precision data
and from measurements of the proton structure function \F_2, which are based
on perturbative QCD calculations up to ; from hadronic event
shapes and jet production in \epem annihilation, based on
QCD; from jet production in deep inelastic scattering and from
decays, based on QCD; and from heavy quarkonia based on
unquenched QCD lattice calculations. Applying pragmatic methods to deal with
possibly underestimated errors and/or unknown correlations, the world average
value of results in . The
measured values of , covering energy scales from Q \equiv \mtau
= 1.78 GeV to 209 GeV, exactly follow the energy dependence predicted by QCD
and therefore significantly test the concept af Asymptotic Freedom.Comment: 14 pages, 7 figure
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