1,882 research outputs found
Completely automated computation of the heavy-fermion corrections to the three-loop matching coefficient of the vector current
We evaluate the corrections to the matching coefficient of the vector current
between Quantum Chromodynamics (QCD) and Non-Relativistic QCD (NRQCD) to
three-loop order containing a closed heavy-fermion loop. The result constitutes
a building block both for the bottom- and top-quark system at threshold. Strong
emphasis is put on our completely automated approach of the calculation
including the generation of the Feynman diagrams, the identification of the
topologies, the reduction to master integrals and the automated numerical
computation of the latter.Comment: 14 page
The process gg -> WW as a background to the Higgs signal at the LHC
The production of W pairs from the one-loop gluon fusion process is studied.
Formulas are presented for the helicity amplitudes keeping the top mass finite,
but all other quark masses zero. The correlations among the leptons coming from
the W bosons are kept. The contribution of this background to the Higgs boson
search in the WW decay mode at the LHC is estimated by applying the cuts
foreseen in experimental searches using the PYTHIA Monte Carlo program.
Kinematic distributions for the final state leptons are compared to those of
the Higgs boson signal and of the q qbar -> WW background. After applying final
cuts, the gg background is found to be large, at the level of 35% of the q qbar
background.The characteristics of the gg background are very similar to those
of the signal. Therefore, an experimental normalization of this background
component appears to be very difficult and the uncertainty must largely be
determined by theory. As a result, the significance of a Higgs signal in the gg
-> H -> WW mode at the LHC is reduced.Comment: 24 pages, 4 figure
Matching QCD and HQET heavy-light currents at three loops
We consider the currents formed by a heavy and a light quark within Quantum
Chromodynamics and compute the matching to Heavy Quark Effective Theory to
three-loop accuracy. As an application we obtain the third-order perturbative
corrections to ratios of B-meson decay constants.Comment: 23 pages, full results are available as Mathematica files at
http://www-ttp.particle.uni-karlsruhe.de/Progdata/ttp09/ttp09-41/ ; v2: an
error in comparison with Ref. [8] fixed ; v3: Journal versio
Multiplicity dependence of the average transverse momentum in pp, p-Pb, and Pb-Pb collisions at the LHC
Multiplicity dependence of the average transverse momentum in pp, p-Pb, and Pb-Pb collisions at the LHC
Cyanobacteria, a group of photosynthetic prokaryotes, oscillate between day and night time metabolisms with concomitant oscillations in gene expression in response to light/dark cycles (LD). The oscillations in gene expression have been shown to sustain in constant light (LL) with a free running period of 24 h in a model cyanobacterium Synechococcus elongatus PCC 7942. However, equivalent oscillations in metabolism are not reported under LL in this non-nitrogen fixing cyanobacterium. Here we focus on Cyanothece sp. ATCC 51142, a unicellular, nitrogen-fixing cyanobacterium known to temporally separate the processes of oxygenic photosynthesis and oxygen-sensitive nitrogen fixation. In a recent report, metabolism of Cyanothece 51142 has been shown to oscillate between photosynthetic and respiratory phases under LL with free running periods that are temperature dependent but significantly shorter than the circadian period. Further, the oscillations shift to circadian pattern at moderate cell densities that are concomitant with slower growth rates. Here we take this understanding forward and demonstrate that the utradian rhythm under LL sustains at much higher cell densities when grown under turbulent regimes that simulate flashing light effect. Our results suggest that the ultradian rhythm in metabolism may be needed to support higher carbon and nitrogen requirements of rapidly growing cells under LL. With a comprehensive Real time PCR based gene expression analysis we account for key regulatory interactions and demonstrate the interplay between clock genes and the genes of key metabolic pathways. Further, we observe that several genes that peak at dusk in Synechococcus peak at dawn in Cyanothece and vice versa. The circadian rhythm of this organism appears to be more robust with peaking of genes in anticipation of the ensuing photosynthetic and respiratory metabolic phases
Light quark mass effects in the on-shell renormalization constants
We compute the three-loop relation between the pole and the minimally
subtracted quark mass allowing for virtual effects from a second massive quark.
We also consider the analogue effects for the on-shell wave function
renormalization constant.Comment: 24 page
Nonperturbative QCD Vacuum Effects in Nonlocal Quark Dynamics
A straightforward calculation reveals the essentially nonlocal character of
the leading heavy interaction arising from nonperturbative gluon
field correlations in the model of a fluctuating QCD vacuum. In light of this
quarkonium spin splitting ratio predictions which have supported the scalar
confinement ansatz are reconsidered as a specific example of possible
consequences for spectroscopy.Comment: Latex, 9 page
Cosmological implications of the Higgs mass measurement
We assume the validity of the Standard Model up to an arbitrary high-energy
scale and discuss what information on the early stages of the Universe can be
extracted from a measurement of the Higgs mass. For Mh < 130 GeV, the Higgs
potential can develop an instability at large field values. From the absence of
excessive thermal Higgs field fluctuations we derive a bound on the reheat
temperature after inflation as a function of the Higgs and top masses. Then we
discuss the interplay between the quantum Higgs fluctuations generated during
the primordial stage of inflation and the cosmological perturbations, in the
context of landscape scenarios in which the inflationary parameters scan. We
show that, within the large-field models of inflation, it is highly improbable
to obtain the observed cosmological perturbations in a Universe with a light
Higgs. Moreover, independently of the inflationary model, the detection of
primordial tensor perturbations through the B-mode of CMB polarization and the
discovery of a light Higgs can simultaneously occur only with exponentially
small probability, unless there is new physics beyond the Standard Model.Comment: 28 LaTeX pages, 6 figure
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