2,115 research outputs found
The aT distribution of the Z boson at hadron colliders
We provide the first theoretical study of a novel variable, , proposed
in Ref.[1] as a more accurate probe of the region of low transverse momentum
, for the boson distribution at hadron colliders. The is
the component of transverse to a suitably defined axis. Our study
involves resummation of large logarithms in up to the next-to--leading
logarithmic accuracy and we compare the results to those for the well-known
distribution, identifying important physical differences between the two
cases. We also test our resummed result at the two-loop level by comparing its
expansion to order with the corresponding fixed-order results and
find agreement with our expectations.Comment: 30 pages, 3 figures, JHEP class included. Final version published in
JHE
Non-global logarithms and jet algorithms in high-pT jet shapes
We consider jet-shape observables of the type proposed recently, where the
shapes of one or more high-pT jets, produced in a multi-jet event with definite
jet multiplicity, may be measured leaving other jets in the event unmeasured.
We point out the structure of the full next-to-leading logarithmic resummation
specifically including resummation of non-global logarithms in the leading-Nc
limit and emphasising their properties. We also point out differences between
jet algorithms in the context of soft gluon resummation for such observables.Comment: 22 pages, 4 figures. Title and a few words changed. Several typos
corrected. Version accepted by JHE
Probing the low transverse momentum domain of Z production with novel variables
The measurement of the low transverse momentum region of vector boson
production in Drell-Yan processes has long been invaluable to testing our
knowledge of QCD dynamics both beyond fixed-order in perturbation theory as
well as in the non-perturbative region. Recently the D\O\ collaboration have
introduced novel variables which lead to improved measurements compared to the
case of the standard QT variable. To complement this improvement on the
experimental side, we develop here a complete phenomenological study dedicated
in particular to the new \phi* variable. We compare our study, which contains
the state-of-the-art next-to-next-to-leading resummation of large logarithms
and a smooth matching to the full next-to-leading order result, to the
experimental data and find excellent agreement over essentially the entire
range of \phi*, even without direct inclusion of non-perturbative effects. We
comment on our findings and on the potential for future studies to constrain
non-perturbative behaviour.Comment: 20 pages, 7 figures. Version accepted for publication in JHEP. A
figure with comparison to RESBOS has been adde
Resummed event-shape variables in DIS
We complete our study of resummed event-shape distributions in DIS by
presenting results for the class of observables that includes the current jet
mass, the C-parameter and the thrust with respect to the current-hemisphere
thrust axis. We then compare our results to data for all observables for which
data exist, fitting for alpha_s and testing the universality of
non-perturbative 1/Q effects. A number of technical issues arise, including the
extension of the concept of non-globalness to the case of discontinuous
globalness; singularities and non-convergence of distributions other than in
the Born limit; methods to speed up fixed-order Monte Carlo programs by up to
an order of magnitude, relevant when dealing with many x and Q points; and the
estimation of uncertainties on the predictions.Comment: 41 page
Phenomenology of event shapes at hadron colliders
We present results for matched distributions of a range of dijet event shapes
at hadron colliders, combining next-to-leading logarithmic (NLL) accuracy in
the resummation exponent, next-to-next-to leading logarithmic (NNLL) accuracy
in its expansion and next-to-leading order (NLO) accuracy in a pure alpha_s
expansion. This is the first time that such a matching has been carried out for
hadronic final-state observables at hadron colliders. We compare our results to
Monte Carlo predictions, with and without matching to multi-parton tree-level
fixed-order calculations. These studies suggest that hadron-collider event
shapes have significant scope for constraining both perturbative and
non-perturbative aspects of hadron-collider QCD. The differences between
various calculational methods also highlight the limits of relying on
simultaneous variations of renormalisation and factorisation scale in making
reliable estimates of uncertainties in QCD predictions. We also discuss the
sensitivity of event shapes to the topology of multi-jet events, which are
expected to appear in many New Physics scenarios.Comment: 70 pages, 25 figures, additional material available from
http://www.lpthe.jussieu.fr/~salam/pp-event-shapes
Optimisation of variables for studying dilepton transverse momentum distributions at hadron colliders
In future measurements of the dilepton () transverse momentum,
\Qt, at both the Tevatron and LHC, the achievable bin widths and the ultimate
precision of the measurements will be limited by experimental resolution rather
than by the available event statistics. In a recent paper the variable \at,
which corresponds to the component of \Qt\ that is transverse to the dilepton
thrust axis, has been studied in this regard. In the region, \Qt\ 30 GeV,
\at\ has been shown to be less susceptible to experimental resolution and
efficiency effects than the \Qt. Extending over all \Qt, we now demonstrate
that dividing \at\ (or \Qt) by the measured dilepton invariant mass further
improves the resolution. In addition, we propose a new variable, \phistarEta,
that is determined exclusively from the measured lepton directions; this is
even more precisely determined experimentally than the above variables and is
similarly sensitive to the \Qt. The greater precision achievable using such
variables will enable more stringent tests of QCD and tighter constraints on
Monte Carlo event generator tunes.Comment: 8 pages, 5 figures, 2 table
HIIphot: Automated Photometry of HII Regions Applied to M51
We have developed a robust, automated method, hereafter designated HIIphot,
which enables accurate photometric characterization of HII regions while
permitting genuine adaptivity to irregular source morphology. HIIphot utilizes
object-recognition techniques to make a first guess at the shapes of all
sources then allows for departure from such idealized ``seeds'' through an
iterative growing procedure. Photometric corrections for spatially coincident
diffuse emission are derived from a low-order surface fit to the background
after exclusion of all detected sources. We present results for the
well-studied, nearby spiral M51 in which 1229 HII regions are detected above
the 5-sigma level. A simple, weighted power-law fit to the measured H-alpha
luminosity function (HII LF) above log L_H-alpha = 37.6 gives alpha =
-1.75+/-0.06, despite a conspicuous break in the HII LF observed near L_H-alpha
= 10^38.9. Our best- fit slope is marginally steeper than measured by Rand
(1992), perhaps reflecting our increased sensitivity at low luminosities and to
notably diffuse objects. HII regions located in interarm gaps are
preferentially less luminous than counterparts which constitute M51's
grand-design spiral arms and are best fit with a power-law slope of alpha =
-1.96+/-0.15. We assign arm/interarm status for HII regions based upon the
varying surface brightness of diffuse emission as a function of position
throughout the image. Using our measurement of the integrated flux contributed
by resolved HII regions in M51, we estimate the diffuse fraction to be
approximately 0.45 -- in agreement with the determination of Greenawalt et al.
(1998). Automated processing of degraded datasets is undertaken to gauge
systematic effects associated with limiting spatial resolution and sensitivity.Comment: 41 pages, 14 figures, Postscript version with high-resolution figures
at ftp://ftp.aoc.nrao.edu/staff/dthilker/preprint
Non-Global Logarithms in Filtered Jet Algorithms
We analytically and numerically study the effect of perturbative gluons
emission on the "Filtering analysis", which is part of a subjet analysis
procedure proposed two years ago to possibly identify a low-mass Higgs boson
decaying into b\bar{b} at the LHC. This leads us to examine the non-global
structure of the resulting perturbative series in the leading single-log
large-N_c approximation, including all-orders numerical results, simple
analytical approximations to them and comments on the structure of their series
expansion. We then use these results to semi-analytically optimize the
parameters of the Filtering analysis so as to suppress as much as possible the
effect of underlying event and pile-up on the Higgs mass peak reconstruction
while keeping the major part of the perturbative radiation from the b\bar{b}
dipole.Comment: 47 pages, 25 figures, 1 figure and a few comments added, version
accepted for publication in JHE
The Escape of Ionizing Photons from OB Associations in Disk Galaxies: Radiation Transfer Through Superbubbles
By solving the time-dependent radiation transfer problem of stellar radiation
through evolving superbubbles within a smoothly varying HI distribution, we
estimate the fraction of ionizing photons emitted by OB associations that
escapes the HI disk of our Galaxy into the halo and intergalactic medium (IGM).
We consider both coeval star-formation and a Gaussian star-formation history
with a time spread sigma_t = 2 Myr. We consider both a uniform H I distribution
and a two-phase (cloud/intercloud) model, with a negligible filling factor of
hot gas. We find that the shells of the expanding superbubbles quickly trap or
attenuate the ionizing flux, so that most of the escaping radiation escapes
shortly after the formation of the superbubble. For the coeval star-formation
history, the total fraction of Lyman Continuum photons that escape both sides
of the disk in the solar vicinity is f_esc approx 0.15 +/- 0.05. For the
Gaussian star formation history, f_esc approx 0.06 +/- 0.03, a value roughly a
factor of two lower than the results of Dove & Shull (1994), where superbubbles
were not considered. For a local production rate of ionizing photons Psi_LyC =
4.95 X 10^7 cm^{-2} s^{-1}, the flux escaping the disk is Phi_LyC approx
(1.5-3.0) X 10^6 cm^{-2} s^{-1} for coeval and Gaussian star formation,
comparable to the flux required to sustain the Reynolds layer.Comment: Revised version (expanded), accepted for publication by ApJ, 38
pages, 8 figures, aasms4.sty and aabib.sty files include
Pure Samples of Quark and Gluon Jets at the LHC
Having pure samples of quark and gluon jets would greatly facilitate the
study of jet properties and substructure, with many potential standard model
and new physics applications. To this end, we consider multijet and jets+X
samples, to determine the purity that can be achieved by simple kinematic cuts
leaving reasonable production cross sections. We find, for example, that at the
7 TeV LHC, the pp {\to} {\gamma}+2jets sample can provide 98% pure quark jets
with 200 GeV of transverse momentum and a cross section of 5 pb. To get 10 pb
of 200 GeV jets with 90% gluon purity, the pp {\to} 3jets sample can be used.
b+2jets is also useful for gluons, but only if the b-tagging is very efficient.Comment: 19 pages, 16 figures; v2 section on formally defining quark and gluon
jets has been adde
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