4,181 research outputs found
Finite Temperature Reduction of the SU(2) Higgs-Model with Complete Static Background
Direct evaluation of the 1-loop fluctuation determinant of non-static degrees
of freedom in a complete static background is advocated to be more efficient
for the determination of the effective three-dimensional model of the
electroweak phase transition than the one-by-one evaluation of Feynman
diagrams. The relation of the couplings and fields of the effective model to
those of the four-dimensional finite temperature system is determined in the
general 't Hooft gauge with full implementation of renormalisation effects.
Only field renormalisation constants display dependence on the gauge fixing
parameter. Characteristics of the electroweak transition are computed from the
effective theory in Lorentz-gauge. The dependence of various physical
observables on the three-dimensional gauge fixing parameter is investigated.Comment: 12 pages (LATEX) + 1 table (TEX) appende
"A roller coaster of emotions": a qualitative study of parents very first experiences with their preterm baby
Objectives: To assess parents 19 first experiences of their very preterm babies and the neonatal intensive care unit (NICU).
Design: Qualitative study using semistructured interviews.
Participants: 32 mothers and 7 fathers of very preterm babies (<32 weeks gestation).
Setting: Three neonatal units in tertiary care hospitals in South East England.
Results: Five themes were identified. The first describes parents 19 blurred recall of the birth. The second shows the anticipation of seeing and touching their baby for the first time was characterised by contrasting emotions, with some parents feeling scared and others excited about the event. The third theme describes parents 19 first sight and touch of their babies and their 18rollercoaster 19 of emotions during this time. It also highlights the importance of touch to trigger and strengthen the parent 13baby bond. However, some parents were worried that touching or holding the baby might transmit infection or interfere with care. The fourth theme captures parents 19 impressions of NICU and how overwhelming this was particularly for parents who had not toured NICU beforehand or whose first sight of their baby was on NICU. The final theme captures unique experiences of fathers, in particular that many felt excluded and confused about their role.
Conclusions: This study informs family-centred care by providing insight into the experiences of parents of very preterm infants at a time when they are most in need of support. Clinical implications include the importance of offering parents preparatory tours of the NICU and including fathers
Model independent properties of two-photon exchange in elastic electron proton scattering
We derive from first principles, as the C-invariance of the electromagnetic
interaction and the crossing symmetry, the general properties of two-photon
exchange in electron-proton elastic scattering. We show that the presence of
this mechanism destroys the linearity of the Rosenbluth separation.Comment: 12 pages, no figures- Corrected misprints, changes in P. 7. No
changes in conclusion
Three-loop matching coefficients for hot QCD: Reduction and gauge independence
We perform an integral reduction for the 3-loop effective gauge coupling and
screening mass of QCD at high temperatures, defined as matching coefficients
appearing in the dimensionally reduced effective field theory (EQCD).
Expressing both parameters in terms of a set master (sum-) integrals, we show
explicit gauge parameter independence. The lack of suitable methods for solving
the comparatively large number of master integrals forbids the complete
evaluation at the moment. Taking one generic class of masters as an example, we
highlight the calculational techniques involved. The full result would allow to
improve on one of the classic probes for the convergence of the weak-coupling
expansion at high temperatures, namely the comparison of full and effective
theory determinations of the spatial string tension. Furthermore, the full
result would also allow to determine one new contribution of order O(g**7) to
the pressure of hot QCD.Comment: 19 pages, 2 figures. v2: new Section 6 discussing applications, to
match journal versio
Unintegrated CCFM parton distributions and transverse momentum of gauge bosons
Transverse momentum distribution of gauge bosons and is
calculated using unintegrated parton distributions obtained by solving leading
order CCFM equation(s) in the impact parameter space. For illustration we
compare the results of the fixed-order collinear, soft-gluon resummation and
CCFM approaches. A parameter of a nonperturbative form factor is adjusted to
the D0 data. In contrast to the collinear approach, the leading order
calculation based on the unintegrated CCFM parton distributions generates
transverse momentum distributions of gauge bosons, which are almost consistent
with experimental data provided the parameter of the nonperturbative form
factor is appropriately adjusted. The - asymmetry is calculated as a
function of the boson rapidity and transverse momentum. Predictions are given
for RHIC and LHC.Comment: 19 pages, 8 figure
The Ratio of W + N jets To Z/gamma + N jets As a Precision Test of the Standard Model
We suggest replacing measurements of the individual cross-sections for the
production of W + N jets and Z/gamma + N jets in searches for new high-energy
phenomena at hadron colliders by the precision measurement of the ratios (W+0
jet)/(Z+0 jet), (W+1 jet)/(Z+1 jet), (W+2 jets)/(Z+2 jets),... (W+N jets)/(Z+N
jets), with N as large as 6 (the number of jets in ttbarH). These ratios can
also be formed for the case where one or more of the jets is tagged as a b or c
quark. Existing measurements of the individual cross sections for Wenu + N jets
at the Tevatron have systematic uncertainties that grow rapidly with N, being
dominated by uncertainties in the identification of jets and the jet energy
scale. These systematics, and also those associated with the luminosity, parton
distribution functions (PDF's), detector acceptance and efficiencies, and
systematics of jet finding and b-tagging, are expected to substantially cancel
in calculating the ratio of W to Z production in each N-jet channel, allowing a
greater sensitivity to new contributions in these channels in Run II at the
Tevatron and at the LHC.Comment: 10 pages, 8 figures, added reference
Model independent analysis of polarization effects in elastic electron deuteron scattering in presence of two--photon exchange
The general spin structure of the matrix element, taking into account the
two--photon exchange contribution, for the elastic electron (positron)
--deuteron scattering has been derived using general symmetry properties of the
hadron electromagnetic interaction, such as P--, C-- and T--invariances as well
as lepton helicity conservation in QED at high energy. Taking into account also
crossing symmetry, the amplitudes of scattering can be parametrized
in terms of fifteen real functions. The expressions for the differential cross
section and for all polarization observables are given in terms of these
functions. We consider the case of an arbitrary polarized deuteron target and
polarized electron beam (both longitudinal and transverse). The transverse
polarization of the electron beam induces a single--spin asymmetry which is
non--zero in presence of two--photon exchange. It is shown that elastic
deuteron electromagnetic form factors can still be extracted in presence of two
photon exchange, from the measurements of the differential cross section and of
one polarization observable (for example, the tensor asymmetry) for electron
and positron deuteron elastic scattering, in the same kinematical conditions.Comment: 28 page
Thermalization in Weakly Coupled Nonabelian Plasmas
We investigate how relativistic, nonabelian plasmas approach equilibrium in a
general context. Our treatment is entirely parametric and for small Yang-Mills
coupling . First we study isotropic systems with an initially
nonequilibrium momentum distribution. We consider both the case of initially
very high occupancy and initially very low occupancy. Then we consider systems
which are anisotropic. We consider both weak anisotropy and large anisotropy,
and allow the occupancy to be parametrically large or small. Writing the
typical momentum of an initial excitation as Q and the final temperature as T,
full equilibration occurs in a time t ~ \alpha^{-2}/T for T > Q, and t ~
\alpha^{-2} Q^{1/2} T^{-3/2} for T < Q, unless the initial system is
sufficiently anisotropic and T > \alpha^{2/3} Q, in which case equilibration
occurs somewhat faster, t ~ \alpha^{-13/7} Q^{5/7} T^{-12/7} (or \alpha^{-2}/T
if that is longer).Comment: 55 pages including many figures, but with a comprehensive review of
results in the first 6 pages
Joint resummation in electroweak boson production
We present a phenomenological application of the joint resummation formalism
to electroweak annihilation processes at measured boson momentum Q_T. This
formalism simultaneously resums at next-to-leading logarithmic accuracy large
threshold and recoil corrections to partonic scattering. We invert the impact
parameter transform using a previously described analytic continuation
procedure. This leads to a well-defined, resummed perturbative cross section
for all nonzero Q_T, which can be compared to resummation carried out directly
in Q_T space. From the structure of the resummed expressions, we also determine
the form of nonperturbative corrections to the cross section and implement
these into our analysis. We obtain a good description of the transverse
momentum distribution of Z bosons produced at the Tevatron collider.Comment: 27 pages, LaTeX, 8 figures as eps files. Some additions to earlier
version, this version as published in Phys. Rev. D66 (2002) 01401
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