53,565 research outputs found
Resummation and NLO Matching of Event Shapes with Effective Field Theory
The resummed differential thrust rate in e+e- annihilation is calculated
using Soft-Collinear Effective Theory (SCET). The resulting distribution in the
two-jet region T~1 is found to agree with the corresponding expression derived
by the standard approach. A matching procedure to account for finite
corrections at T < 1 is then described. There are two important advantages of
the SCET approach. First, SCET manifests a dynamical seesaw scale q = p^2/Q in
addition to the center-of-mass energy Q and the jet mass scale p ~ Q ~ sqrt(1 -
T). Thus, the resummation of logs of p/q can be cleanly distinguished from the
resummation of logs of Q/p. Second, finite parts of loop amplitudes appear in
specific places in the perturbative distribution: in the matching to the hard
function, at the scale Q, in matching to the jet function, at the scale p, and
in matching to the soft function, at the scale q. This allows for a consistent
merger of fixed order corrections and resummation. In particular, the total NLO
e+e- cross section is reproduced from these finite parts without having to
perform additional infrared regulation.Comment: 18 pages, 1 figure; notation updated and references adde
Low-excitation Herbig-Haro objects and interstellar extinction
Previous work on low-excitation Herbig-Haro (HH) objects has confirmed the existence of a strongly-enhanced hydrogen two-photon continuum which is produced by low-velocity shocks in neutral material. The theoretical two-photon energy distribution was compared with that observed for HH 43. With the addition of optical (3300 lambda 7000A) data to the ultraviolet (1200 lambda 3100A) data obtained with the I.U.E, the wavelength dependence of interstellar extinction toward HH 43 has been determined
Ultraviolet observations of low-excitation Herbig-Haro objects
The Herbig-Haro (HH) objects rank among the faintest objects (v approx. 17) observed with the I.U.E. Expectations that the low-excitation shock-excited HHs should be dominated by H two-photon (2q) UV emission were borne out in the first observation of HH 43 and 47 obtained in 1982. The UV continuous energy distributions in the SWP spectra of these objects were found to peak near 1500 Angstroms as predicted by the 2q hypothesis. In addition, emission lines in the Lyman band of H sub 2, excited by Lyman alpha fluorescence, were detected in HH 43. It is not clear if the molecular gas is coextensive with the atomic gas responsible for the optical emission, or if the molecular gas is immediately adjacent to the region of optical emission. The two-photon energy distribution and H sub 2 fluorescent emission were previously discussed
A-3 scientific results - extragalactic
The results of the HEAO A-3 experiment are summarized. Specific contributions of the experiment to extragalactic astronomy are emphasized. The discovery of relatively condensed X-ray emission in the cores of those clusters of galaxies which are dominated by a giant elliptical or cD galaxy, the discovery of extended X-ray emitting plasma in groups of galaxies, and the demonstration that BL Lac objects are a class of X-ray sources are among the topics discussed
A precise determination of alpha_s from LEP thrust data using effective field theory
Starting from a factorization theorem in Soft-Collinear Effective Theory, the
thrust distribution in e+e- collisions is calculated including resummation of
the next-to-next-to-next-to leading logarithms. This is a significant
improvement over previous calculations which were only valid to next-to-leading
logarithmic order. The fixed-order expansion of the resummed result approaches
the exact fixed-order distribution towards the kinematic endpoint. This close
agreement provides a verification of both the effective field theory expression
and recently completed next-to-next-to-leading fixed order event shapes. The
resummed distribution is then matched to fixed order, resulting in a
distribution valid over a large range of thrust. A fit to ALEPH and OPAL data
from LEP 1 and LEP 2 produces alpha_s(m_Z)= 0.1172 +/- 0.0010 +/- 0.0008
+/-0.0012 +/- 0.0012, where the uncertainties are respectively statistical,
systematic, hadronic, and perturbative. This is one of the world's most precise
extractions of alpha_s to date.Comment: 37 pages, 12 figures; v2: hadronization discussion and appendices
expande
Threshold Hadronic Event Shapes with Effective Field Theory
Hadronic event shapes, that is, event shapes at hadron colliders, could
provide a great way to test both standard and non-standard theoretical models.
However, they are significantly more complicated than event shapes at e+e-
colliders, involving multiple hard directions, multiple channels and multiple
color structures. In this paper, hadronic event shapes are examined with
Soft-Collinear Effective Theory (SCET) by expanding around the dijet limit. A
simple event shape, threshold thrust, is defined. This observable is global and
has no free parameters, making it ideal for clarifying how resummation of
hadronic event shapes can be done in SCET. Threshold thrust is calculated at
next-to-leading fixed order (NLO) in SCET and resummed to
next-to-next-to-leading logarithmic accuracy (NNLL). The scale-dependent parts
of the soft function are shown to agree with what is expected from general
observations, and the factorization formula is explicitly shown to be
renormalization group invariant to 1-loop. Although threshold thrust is not
itself expected to be phenomenologically interesting, it can be modified into a
related observable which allows the jet pT distribution to be calculated and
resummed to NNLL+NLO accuracy. As in other processes, one expects resummation
to be important even for moderate jet momenta due to dynamical threshold
enhancement. A general discussion of threshold enhancement and non-global logs
in hadronic event shapes is also included.Comment: 38 pages, 2 figures; small typos corrected in v
Seeing in Color: Jet Superstructure
A new class of observables is introduced which aims to characterize the
superstructure of an event, that is, features, such as color flow, which are
not determined by the jet four-momenta alone. Traditionally, an event is
described as having jets which are independent objects; each jet has some
energy, size, and possible substructure such as subjets or heavy flavor
content. This description discards information connecting the jets to each
other, which can be used to determine if the jets came from decay of a color
singlet object, or if they were initiated by quarks or gluons. An example
superstructure variable, pull, is presented as a simple handle on color flow.
It can be used on an event-by-event basis as a tool for distinguishing
previously irreducible backgrounds at the Tevatron and the LHC.Comment: 4 pages, 5 figures. Published version. Some clarifications and
references adde
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