52,383 research outputs found
Testing perturbation theory on the nf=0 static quark potential
The perturbative expansion of static force and potential is reanalyzed
concerning its practical applicability. A well behaved perturbative prediction
is given by the integration of the renormalization group equation for the
coupling . Since the Lambda-parameter
of the \nf=0 theory is known from previous work, the perturbative prediction
contains no free parameter. It is confronted with recent non-perturbative
results. For where the truncation error of the perturbative
expression is naively estimated to be moderate, it is really quite accurate and
large ``non-perturbative terms'' are excluded.Comment: Added comment on known alpha^4 log(alpha) term (page 8), References
and acknowledgemen
Black Hole Production at the LHC by Standard Model Bulk Fields in the Randall-Sundrum Model
We consider the production of black holes at the LHC in the
Randall-Sundrum(RS) model through the collisions of Standard Model(SM) fields
in the bulk. In comparison to the previously studied case where the SM fields
are all confined to the TeV brane, we find substantial suppressions to the
corresponding collider cross sections for all initial states, i.e., ,
and , where represents a light quark or anti-quark which lie close to
the Planck brane. For quarks, which are closer to the TeV brane, this
suppression effect is somewhat weaker though quark contributions to the
cross section are already quite small due to their relatively small parton
densities. Semi-quantitatively, we find that the overall black hole cross
section is reduced by roughly two orders of magnitude in comparison to the
traditional TeV brane localized RS model with the exact value being sensitive
to the detailed localizations of the light SM fermions in the bulk.Comment: 14 pages, 3 figs; refs and discussion adde
Mapping the secondary star in QQ Vulpeculae
We present high- and medium-resolution phase-resolved far-red spectra of the magnetic cataclysmic variable QQ Vul. The spectra show the Na i doublet absorption features near λ 8190 Å from the cool secondary star, and the lines of He ii, O i, Mg ii, C i, N i, Ca ii and Paschen in emission. Using a Doppler imaging technique, we find that the H i, He ii, C i and O i lines have a narrow component originating near the L1 point and a strong component from the stream, while the Mg ii and Ca ii emission arises solely from the illuminated hemisphere of the red dwarf. We carry out an exhaustive analysis of the emission- and absorption-line velocities and fluxes seen in the QQ Vul spectrum. By simultaneously fitting the radial velocity and flux information we are able to produce surface maps of each line on the secondary star using a technique analogous to the one employed by Davey. The Na i and Mg ii maps show an asymmetric distribution akin to that seen in AM Her. Although the observed velocity semi-amplitudes (K2) of the lines can potentially be corrected for the effects of irradiation, we find that time-dependent changes in the degree of heating on the secondary can lead to large discrepancies in the results, significant enough to give inconsistent values from data taken at different epochs. We discuss the limitations of the surface mapping method as a means of correcting the observed K2. Our results also suggest that the emission features from the red dwarf are likely to be formed at quite high levels of the stellar chromosphere, in some cases probably even beyond the L1 point and inside the Roche lobe of the white dwarf, with the different lines possibly forming at different depths. Using the Na i absorption doublet, we find a velocity semi-amplitude for the secondary star of K2=219±6 km s−1 and a projected rotational velocity of vrot sin i=110±15 km s−1. Thus we estimate the mass ratio to be q=0.54±0.14. Based on the results of the best-fitting surface maps on all the lines, and the nature of the phase-dependent variations of the continuum and lines, we infer a binary inclination of i=65°±7°, and obtain a complete set of binary parameters for QQ Vul. We classify the secondary star as M4V from the TiO band ratios
What is the W b anti-b, Z b anti-b or t anti-t b anti-b irreducible background to the light Higgs boson searches at LHC?
The W b anti-b, Z b anti-b and t anti-t b anti-b production at LHC are
irreducible backgrounds for possible observability of the Standard Model (SM)
and Minimal Supersymmetric Standard Model (MSSM) light Higgs boson respectively
in the associated WH, ZH and t anti-t H production followed by the H -> b
anti-b decay. We present a comparison of the background estimates as obtained
with (a) complete massive matrix element implemented in AcerMC Monte Carlo
generator and (b) PYTHIA implementation of the inclusive W, Z, and t anti-t
production, followed by the parton showering mechanism. Both approaches lead to
a production of the final state of interest, but differ in the approximations
used. We concentrate on the comparison of these approaches for the background
to the light Higgs boson searches at LHC.Comment: EPJC Referee's comments adde
Asking for an extra photon in Higgs production at the LHC and beyond
We study the inclusive production of a Higgs boson in association with a
high- photon at the LHC, detailing the leading-order features of the main
processes contributing to the final state. Requiring an extra hard
photon in Higgs production upsets the cross-section hierarchy for the dominant
channels. The inclusive production comes mainly from photons radiated
in vector-boson fusion (VBF), which accounts for about 2/3 of the total rate,
for GeV, at leading order. On the other hand, radiating a
high- photon in the main top-loop Higgs channel implies an extra parton in
the final state, which suppresses the production rate by a further
power. As a result, the production via top loops at the LHC has rates
comparable with the ones arising from either the production or the
associated production. Then, in order of decreasing cross
section, comes the single-top-plus-Higgs channel, followed in turn by the
heavy-flavor fusion processes and .
The production via electroweak loops has just a minor role. At larger
c.m. energies, the channel surpasses the total contribution
of top-loop processes. In particular, requiring GeV at
TeV, accounts for about of the
inclusive production at leading order, about half of the total being
due to VBF production.Comment: 20 pages, 13 figures, two comments added; one typo corrected; version
published in JHE
On Factorization of Generalized Macdonald Polynomials
A remarkable feature of Schur functions -- the common eigenfunctions of
cut-and-join operators from -- is that they factorize at the
peculiar two-parametric topological locus in the space of time-variables, what
is known as the hook formula for quantum dimensions of representations of
and plays a big role in various applications. This factorization
survives at the level of Macdonald polynomials. We look for its further
generalization to {\it generalized} Macdonald polynomials (GMP), associated in
the same way with the toroidal Ding-Iohara-Miki algebras, which play the
central role in modern studies in Seiberg-Witten-Nekrasov theory. In the
simplest case of the first-coproduct eigenfunctions, where GMP depend on just
two sets of time-variables, we discover a weak factorization -- on a
codimension-one slice of the topological locus, what is already a very
non-trivial property, calling for proof and better understanding.Comment: 8 page
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