3,125 research outputs found
Information on the structure of the a1 from tau decay
The decay is analysed using different methods to
account for the resonance structure, which is usually ascribed to the a1. One
scenario is based on the recently developed techniques to generate axial-vector
resonances dynamically, whereas in a second calculation the a1 is introduced as
an explicit resonance. We investigate the influence of different assumptions on
the result. In the molecule scenario the spectral function is described
surprisingly well by adjusting only one free parameter. This result can be
systematically improved by adding higher order corrections to the iterated
Weinberg-Tomozawa interaction. Treating the a1 as an explicit resonance on the
other hand leads to peculiar properties
Proton-induced noise in digicons
The Space Telescope, which carries four Digicons, will pass several times per day through a low-altitude portion of the radiation belt called the South Atlantic Anomaly. This is expected to create interference in what is otherwise anticipated to be a noise-free device. Two essential components of the Digicon, the semiconductor diode array and the UV transmitting window, generate noise when subjected to medium-energy proton radiation, a primary component of the belt. These trapped protons, having energies ranging from 2 to 400 Mev and fluences at the Digicon up to 4,000 P+/sec-sq cm, pass through both the window and the diode array, depositing energy in each. In order to evaluate the effect of these protons, engineering test models of Digicon tubes to be flown on the High Resolution Spectrograph were irradiated with low-flux monoenergetic proton beams at the University of Maryland cyclotron. Electron-hole pairs produced by the protons passing through the diodes or the surrounding bulk caused a background count rate. This is the result of holes diffusing over a distance of many diode spacings, causing counts to be triggered simultaneously in the output circuits of several adjacent diodes. Pulse-height spectra of these proton-induced counts indicate that most of the bulk-related counts overlap the single photoelectron peak. A geometrical model will be presented of the charge collection characteristics of the diode array that accounts for most of the observed effects
Hexabromocyclododecane in marine species from the western Scheldt Estuary: diastereoisomer- and enantiomer-specific accumulation
Hexabromocyclododecane (HBCD) is a widely used brominated flame retardant, which is increasingly reported in the environment. Here, we report on the diastereomeric and, for the first time, on the enantiomeric composition of HBCD in muscle and liver of several fish species caught in the Western Scheldt Estuary (The Netherlands). The total HBCD content (sum of alfa-, béta- and gamma-diastereoisomers), as well as the distribution of diastereoisomers and enantiomers, varied between the species. The levels of total HBCD (9-1110 ng/g lipid weight) found in fish tissues were higher than those measured in fish from European rivers with no known point sources of HBCD but lower than in fish samples collected near factories producing or using HBCD. The concentrations of total HBCD expressed on a lipid weight basis were higher in liver than in muscle for bib and whiting, while in sole, HBCD had no preferential distribution between the tissues. A similar pattern for liver and muscle distribution was already observed for polybrominated diphenyl ethers (PBDEs) in these species. The alfa-HBCD diastereoisomer was most abundant in all fish samples with a higher contribution to the total HBCD levels in liver compared to muscle for bib and whiting. The gamma-HBCD diastereoisomer accumulated less in liver than in muscle of sole, bib, and whiting. For the first time, enantiomer fractions were determined for HBCD diastereoisomers in liver of three fish species and in muscle of two fish species. A significant enrichment of the (+) alfa-HBCD enantiomer was found in whiting and bib liver samples. A high enantioselectivity has also been seen for the gamma-HBCD diastereoisomer in whiting liver
Factorization and NNLL Resummation for Higgs Production with a Jet Veto
Using methods of effective field theory, we derive the first all-order
factorization theorem for the Higgs-boson production cross section with a jet
veto, imposed by means of a standard sequential recombination jet algorithm.
Like in the case of small-q_T resummation in Drell-Yan and Higgs production,
the factorization is affected by a collinear anomaly. Our analysis provides the
basis for a systematic resummation of large logarithms log(m_H/p_T^veto) beyond
leading-logarithmic order. Specifically, we present predictions for the
resummed jet-veto cross section and efficiency at next-to-next-to-leading
logarithmic order. Our results have important implications for Higgs-boson
searches at the LHC, where a jet veto is required to suppress background
events.Comment: 28 pages, 5 figures; v2: published version; note added in proo
Consistent Factorization of Jet Observables in Exclusive Multijet Cross-Sections
We demonstrate the consistency at the next-to-leading-logarithmic (NLL) level
of a factorization theorem based on Soft-Collinear Effective Theory (SCET) for
jet shapes in e+e- collisions. We consider measuring jet observables in
exclusive multijet final states defined with cone and k_T-type jet algorithms.
Consistency of the factorization theorem requires that the renormalization
group evolution of hard, jet, and soft functions is such that the physical
cross-section is independent of the factorization scale mu. The anomalous
dimensions of the various factorized pieces, however, depend on the color
representation of jets, choice of jet observable, the number of jets whose
shapes are measured, and the jet algorithm, making it highly nontrivial to
satisfy the consistency condition. We demonstrate the intricate cancellations
between anomalous dimensions that occur at the NLL level, so that, up to power
corrections that we identify, our factorization of the jet shape distributions
is consistent for any number of quark and gluon jets, for any number of jets
whose shapes are measured or unmeasured, for any angular size R of the jets,
and for any of the algorithms we consider. Corrections to these results are
suppressed by the SCET expansion parameter lambda (the ratio of soft to
collinear or collinear to hard scales) and in the jet separation measure 1/t^2
= tan^2(R/2)/tan^2(psi/2), where psi is the angular separation between jets.
Our results can be used to calculate a wide variety of jet observables in
multijet final states to NLL accuracy.Comment: 8 pages, 1 figure, uses elsarticle.cls; v2: minor edits, added
reference
NNLO soft function for electroweak boson production at large transverse momentum
The soft function relevant for the production of an electroweak boson
(photon, W, Z or H) with large transverse momentum at a hadron collider is
computed at next-to-next-to-leading order. This is the first two-loop
computation of a soft function involving three light-cone directions. With the
result, the threshold resummation for these processes can now be performed at
next-to-next-to-next-to-leading logarithmic accuracy.Comment: 16 pages, 3 figure
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