190 research outputs found
Inadequacy of zero-width approximation for a light Higgs boson signal
In the Higgs search at the LHC, a light Higgs boson (115 GeV <~ M_H <~ 130
GeV) is not excluded by experimental data. In this mass range, the width of the
Standard Model Higgs boson is more than four orders of magnitude smaller than
its mass. The zero-width approximation is hence expected to be an excellent
approximation. We show that this is not always the case. The inclusion of
off-shell contributions is essential to obtain an accurate Higgs signal
normalisation at the 1% precision level. For gg (-> H) -> VV, V= W,Z, O(10%)
corrections occur due to an enhanced Higgs signal in the region M_VV > 2 M_V,
where also sizable Higgs-continuum interference occurs. We discuss how
experimental selection cuts can be used to exclude this region in search
channels where the Higgs invariant mass cannot be reconstructed. We note that
the H -> VV decay modes in weak boson fusion are similarly affected.Comment: 26 pages, 18 figures, 6 tables; added references, expanded
introduction, version to appear in JHE
On the renormalization of multiparton webs
We consider the recently developed diagrammatic approach to soft-gluon
exponentiation in multiparton scattering amplitudes, where the exponent is
written as a sum of webs - closed sets of diagrams whose colour and kinematic
parts are entangled via mixing matrices. A complementary approach to
exponentiation is based on the multiplicative renormalizability of intersecting
Wilson lines, and their subsequent finite anomalous dimension. Relating this
framework to that of webs, we derive renormalization constraints expressing all
multiple poles of any given web in terms of lower-order webs. We examine these
constraints explicitly up to four loops, and find that they are realised
through the action of the web mixing matrices in conjunction with the fact that
multiple pole terms in each diagram reduce to sums of products of lower-loop
integrals. Relevant singularities of multi-eikonal amplitudes up to three loops
are calculated in dimensional regularization using an exponential infrared
regulator. Finally, we formulate a new conjecture for web mixing matrices,
involving a weighted sum over column entries. Our results form an important
step in understanding non-Abelian exponentiation in multiparton amplitudes, and
pave the way for higher-loop computations of the soft anomalous dimension.Comment: 60 pages, 15 figure
Next-to-eikonal corrections to soft gluon radiation: a diagrammatic approach
We consider the problem of soft gluon resummation for gauge theory amplitudes
and cross sections, at next-to-eikonal order, using a Feynman diagram approach.
At the amplitude level, we prove exponentiation for the set of factorizable
contributions, and construct effective Feynman rules which can be used to
compute next-to-eikonal emissions directly in the logarithm of the amplitude,
finding agreement with earlier results obtained using path-integral methods.
For cross sections, we also consider sub-eikonal corrections to the phase space
for multiple soft-gluon emissions, which contribute to next-to-eikonal
logarithms. To clarify the discussion, we examine a class of log(1 - x) terms
in the Drell-Yan cross-section up to two loops. Our results are the first steps
towards a systematic generalization of threshold resummations to
next-to-leading power in the threshold expansion.Comment: 66 pages, 19 figure
Infrared singularities of QCD scattering amplitudes in the Regge limit to all orders
Scattering amplitudes of partons in QCD contain infrared divergences which
can be resummed to all orders in terms of an anomalous dimension.
Independently, in the limit of high-energy forward scattering, large logarithms
of the energy can be resummed using Balitsky-Fadin-Kuraev-Lipatov theory. We
use the latter to analyze the infrared-singular part of amplitudes to all
orders in perturbation theory and to next-to-leading-logarithm accuracy in the
high-energy limit, resumming the two-Reggeon contribution. Remarkably, we find
a closed form for the infrared-singular part, predicting the Regge limit of the
soft anomalous dimension to any loop order.Comment: 35 pages, 8 figure
The impact of point mutations in the human androgen receptor : classification of mutations on the basis of transcriptional activity
Peer reviewedPublisher PD
Predictions for Higgs production at the Tevatron and the associated uncertainties
We update the theoretical predictions for the production cross sections of
the Standard Model Higgs boson at the Fermilab Tevatron collider, focusing on
the two main search channels, the gluon-gluon fusion mechanism and
the Higgs-strahlung processes with , including all
relevant higher order QCD and electroweak corrections in perturbation theory.
We then estimate the various uncertainties affecting these predictions: the
scale uncertainties which are viewed as a measure of the unknown higher order
effects, the uncertainties from the parton distribution functions and the
related errors on the strong coupling constant, as well as the uncertainties
due to the use of an effective theory approach in the determination of the
radiative corrections in the process at next-to-next-to-leading
order. We find that while the cross sections are well under control in the
Higgs--strahlung processes, the theoretical uncertainties are rather large in
the case of the gluon-gluon fusion channel, possibly shifting the central
values of the next-to-next-to-leading order cross sections by more than
. These uncertainties are thus significantly larger than the
error assumed by the CDF and D0 experiments in their recent
analysis that has excluded the Higgs mass range 162-166 GeV at the 95%
confidence level. These exclusion limits should be, therefore, reconsidered in
the light of these large theoretical uncertainties.Comment: 40 pages, 12 figures. A few typos are corrected and some updated
numbers are provide
Probing natural SUSY from stop pair production at the LHC
We consider the natural supersymmetry scenario in the framework of the
R-parity conserving minimal supersymmetric standard model (called natural MSSM)
and examine the observability of stop pair production at the LHC. We first scan
the parameters of this scenario under various experimental constraints,
including the SM-like Higgs boson mass, the indirect limits from precision
electroweak data and B-decays. Then in the allowed parameter space we study the
stop pair production at the LHC followed by the stop decay into a top quark
plus a lightest neutralino or into a bottom quark plus a chargino. From
detailed Monte Carlo simulations of the signals and backgrounds, we find the
two decay modes are complementary to each other in probing the stop pair
production, and the LHC with TeV and 100 luminosity is
capable of discovering the stop predicted in natural MSSM up to 450 GeV. If no
excess events were observed at the LHC, the 95% C.L. exclusion limits of the
stop masses can reach around 537 GeV.Comment: 19 pages, 10 figures, version accepted by JHE
From Webs to Polylogarithms
We compute a class of diagrams contributing to the multi-leg soft anomalous
dimension through three loops, by renormalizing a product of semi-infinite
non-lightlike Wilson lines in dimensional regularization. Using non-Abelian
exponentiation we directly compute contributions to the exponent in terms of
webs. We develop a general strategy to compute webs with multiple gluon
exchanges between Wilson lines in configuration space, and explore their
analytic structure in terms of , the exponential of the Minkowski
cusp angle formed between the lines and . We show that beyond the
obvious inversion symmetry , at the level of the
symbol the result also admits a crossing symmetry , relating spacelike and timelike kinematics, and hence argue that
in this class of webs the symbol alphabet is restricted to and
. We carry out the calculation up to three gluons connecting
four Wilson lines, finding that the contributions to the soft anomalous
dimension are remarkably simple: they involve pure functions of uniform weight,
which are written as a sum of products of polylogarithms, each depending on a
single cusp angle. We conjecture that this type of factorization extends to all
multiple-gluon-exchange contributions to the anomalous dimension.Comment: 64 pages, 8 figure
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