409 research outputs found
QCD corrections to massive color-octet vector boson pair production
This paper describes the calculation of the next-to-leading order (NLO) QCD
corrections to massive color-octet vector boson pair production at hadron
colliders. As a concrete framework, a two-site coloron model with an internal
parity is chosen, which can be regarded as an effective low-energy
approximation of Kaluza-Klein gluon physics in universal extra dimensions. The
renormalization procedure involves several subtleties, which are discussed in
detail. The impact of the NLO corrections is relatively modest, amounting to a
reduction of 11-14% in the total cross-section, but they significantly reduce
the scale dependence of the LO result
Anti-B --> X(s) gamma in two universal extra dimensions
We calculate the leading order corrections to the anti-B --> X(s) gamma decay
in the standard model with two large flat universal extra dimensions. We find
that the contributions involving the exchange of Kaluza-Klein modes of the
physical scalar field a^+-_(kl) depend logarithmically on the ultraviolet
cut-off scale Lambda. We emphasize that all flavor-changing neutral current
transitions suffer from this problem. Although the ultraviolet sensitivity
weakens the lower bound on the inverse compactification radius 1/R that follows
from anti-B --> X(s) gamma, the constraint remains stronger than any other
available direct measurement. After performing a careful study of the potential
impact of cut-off and higher-order effects, we find 1/R > 650 GeV at 95%
confidence level if errors are combined in quadrature. Our limit is at variance
with the parameter region 1/R <~ 600 GeV preferred by dark matter constraints.Comment: RevTeX4, 11p
Renormalization and ultraviolet sensitivity of gauge vertices in universal extra dimensions
When computing radiative corrections in models with compactified extra dimen-
sions, one has to sum over the entire tower of Kaluza-Klein excitations inside
the loops. The loop corrections generate a difference between the coupling
strength of a zero-mode gauge boson and the coupling strength of its
Kaluza-Klein excitation, although both originate from the same
higher-dimensional gauge interaction. Furthermore, this dis- crepancy will in
general depend on the cutoff scale and assumptions about the UV completion of
the extra-dimensional theory. In this article, these effects are studied in
detail within the context of the minimal universal extra dimension model
(MUED). The broad features of the cutoff scale dependence can be captured
through the so- lution of the functional flow equation in five-dimensional
space. However, an explicit diagrammatic calculation reveals some modifications
due to the compactification of the extra dimension. Nevertheless, when imposing
a physical renormalization condition, one finds that the UV sensitivity of the
effective Kaluza-Klein gauge-boson vertex is relatively small and not very
important for most phenomenological purposes. Similar conclusions should hold
in a larger class of extra-dimensional models besides MUED.Comment: 20 pages, 5 figure
Radiative corrections to masses and couplings in Universal Extra Dimensions
Models with an orbifolded universal extra dimension receive important
loop-induced corrections to the masses and couplings of Kaluza-Klein (KK)
particles. The dominant contributions stem from so-called boundary terms which
violate KK number. Previously, only the parts of these boundary terms
proportional to have been computed, where is the radius of
the extra dimension and is cut-off scale. However, for typical values
of , the logarithms are not particularly large and
non-logarithmic contributions may be numerically important. In this paper,
these remaining finite terms are computed and their phenomenological impact is
discussed. It is shown that the finite terms have a significant impact on the
KK mass spectrum. Furthermore, one finds new KK-number violating interactions
that do not depend on but nevertheless are non-zero. These
lead to new production and decay channels for level-2 KK particles at
colliders.Comment: V2: KK-top (mass and decays) updated, correcting a mistake in the
literatur
Measuring Sparticles with the Matrix Element
We apply the Matrix Element Method (MEM) to mass determination of squark pair
production with direct decay to quarks and LSP at the LHC, showing that
simultaneous mass determination of squarks and LSP is possible. We furthermore
propose methods for inclusion of QCD radiation effects in the MEM.Comment: 4 pages, 2 figures. To appear in the proceedings of SUSY09, the 17th
International Conference on Supersymmetry and the Unification of Fundamental
Interactions. Figures replaced (corrected y axis labels
Integrating in the Higgs Portal to Fermion Dark Matter
Fermion dark matter (DM) interacting with the standard model through a Higgs
portal requires non-renormalizable operators, signaling the presence of new
mediator states at the electroweak scale. Collider signatures that involve the
mediators are a powerful tool to experimentally probe the Higgs portal
interactions, providing complementary information to strong constraints set by
direct DM detection searches. Indirect detection experiments are less sensitive
to this scenario. We investigate the collider reach for the mediators using
three minimal renormalizable models as examples, and requiring the fermion DM
to be a thermal relic. The Large Hadron Collider in its high-energy,
high-luminosity phase can probe most scenarios if DM is lighter than about 200
GeV. Beyond this scale, future high-energy experiments such as an
electron-positron collider or a 100-TeV proton-proton collider, combined with
future direct detection experiments, are indispensable to conclusively test
these models.Comment: 23 pages; v2: references added and correction of direct detection
limits in section VI.
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