543 research outputs found
Off-shell Behavior of the Mixing Amplitude
We extend a recent calculation of the momentum dependence of the
mixing amplitude to the pseudoscalar sector. The
mixing amplitude is calculated in a hadronic model where the mixing is driven
by the neutron-proton mass difference. Closed-form analytic expressions are
presented in terms of a few nucleon-meson parameters. The observed momentum
dependence of the mixing amplitude is strong enough as to question earlier
calculations of charge-symmetry-breaking observables based on the on-shell
assumption. The momentum dependence of the amplitude is,
however, practically identical to the one recently predicted for
mixing. Hence, in this model, the ratio of pseudoscalar to vector mixing
amplitudes is, to a good approximation, a constant solely determined from
nucleon-meson coupling constants. Furthermore, by selecting these parameters in
accordance with charge-symmetry-conserving data and SU(3)-flavor symmetry, we
reproduce the momentum dependence of the mixing amplitude
predicted from chiral perturbation theory. Alternatively, one can use
chiral-perturbation-theory results to set stringent limits on the value of the
coupling constant.Comment: 13 pages, Latex with Revtex, 3 postscript figures (not included)
available on request, SCRI-03089
Structure of the Vacuum in Nuclear Matter - A Nonperturbative Approach
We compute the vacuum polarisation correction to the binding energy of
nuclear matter in the Walecka model using a nonperturbative approach. We first
study such a contribution as arising from a ground state structure with
baryon-antibaryon condensates. This yields the same results as obtained through
the relativistic Hartree approximation of summing tadpole diagrams for the
baryon propagator. Such a vacuum is then generalized to include quantum effects
from meson fields through scalar-meson condensates. The method is applied to
study properties of nuclear matter and leads to a softer equation of state
giving a lower value of the incompressibility than would be reached without
quantum effects. The density dependent effective sigma mass is also calculated
including such vacuum polarisation effects.Comment: 26 pages including 5 eps files, uses revtex style; PACS number:
21.65.+f,21.30.+
Two-Loop Calculations with Vertex Corrections in the Walecka Model
Two-loop corrections with scalar and vector form factors are calculated for
nuclear matter in the Walecka model. The on-shell form factors are derived from
vertex corrections within the framework of the model and are highly damped at
large spacelike momenta. The two-loop corrections are evaluated first by using
the one-loop parameters and mean fields and then by refitting the total
energy/baryon to empirical nuclear matter saturation properties. The modified
two-loop corrections are significantly smaller than those computed with bare
vertices. Contributions from the anomalous isoscalar form factor of the nucleon
are included for the first time. The effects of the implicit density dependence
of the form factors, which arise from the shift in the baryon mass, are also
considered. Finally, necessary extensions of these calculations are discussed.Comment: 29 pages in REVTeX, 18 figures, preprint IU/NTC 94-02 //OSU--94-11
Self-consistent solution of the Schwinger-Dyson equations for the nucleon and meson propagators
The Schwinger-Dyson equations for the nucleon and meson propagators are
solved self-consistently in an approximation that goes beyond the Hartree-Fock
approximation. The traditional approach consists in solving the nucleon
Schwinger-Dyson equation with bare meson propagators and bare meson-nucleon
vertices; the corrections to the meson propagators are calculated using the
bare nucleon propagator and bare nucleon-meson vertices. It is known that such
an approximation scheme produces the appearance of ghost poles in the
propagators. In this paper the coupled system of Schwinger-Dyson equations for
the nucleon and the meson propagators are solved self-consistently including
vertex corrections. The interplay of self-consistency and vertex corrections on
the ghosts problem is investigated. It is found that the self-consistency does
not affect significantly the spectral properties of the propagators. In
particular, it does not affect the appearance of the ghost poles in the
propagators.Comment: REVTEX, 7 figures (available upon request), IFT-P.037/93,
DOE/ER/40427-12-N9
The Momentum Dependence of the Mixing Amplitude in a Hadronic Model
We calculate the momentum dependence of the mixing amplitude in
a purely hadronic model. The basic assumption of the model is that the mixing
amplitude is generated by loops and thus driven entirely by the
neutron-proton mass difference. The value of the amplitude at the
-meson point is expressed in terms of only the and the
coupling constants. Using values for these couplings constrained by
empirical two-nucleon data we obtain a value for the mixing amplitude in
agreement with experiment. Extending these results to the spacelike region, we
find a contribution to the NN interaction that is strongly
suppressed and opposite in sign relative to the conventional contribution
obtained from using the constant on-shell value for the mixing amplitude.Comment: 11 pages, SCRI-12219
Belokurov-Usyukina loop reduction in non-integer dimension
Belokurov-Usyukina loop reduction method has been proposed in 1983 to reduce
a number of rungs in triangle ladder-like diagram by one. The disadvantage of
the method is that it works in d=4 dimensions only and it cannot be used for
calculation of amplitudes in field theory in which we are required to put all
the incoming and outgoing momenta on shell. We generalize the
Belokurov-Usyukina loop reduction technique to non-integer d=4-2e dimensions.
In this paper we show how a two-loop triangle diagram with particular values of
indices of scalar propagators in the position space can be reduced to a
combination of three one-loop scalar diagrams. It is known that any one-loop
massless momentum integral can be presented in terms of Appell's function F_4.
This means that particular diagram considered in the present paper can be
represented in terms of Appell's function F_4 too. Such a generalization of
Belokurov-Usyukina loop reduction technique allows us to calculate that diagram
by this method exactly without decomposition in terms of the parameter e.Comment: 6 pages, 3 figure
Pneumoproteins and biomarkers of inflammation and coagulation do not predict rapid lung function decline in people living with HIV
Chronic obstructive pulmonary disease (COPD) is among the leading causes of death worldwide and HIV is an independent risk factor for the development of COPD. However, the etiology of this increased risk and means to identify persons with HIV (PWH) at highest risk for COPD have remained elusive. Biomarkers may reveal etiologic pathways and allow better COPD risk stratification. We performed a matched case:control study of PWH in the Strategic Timing of Antiretoviral Treatment (START) pulmonary substudy. Cases had rapid lung function decline (> 40 mL/year FEV1 decline) and controls had stable lung function (+ 20 to − 20 mL/year). The analysis was performed in two distinct groups: (1) those who were virally suppressed for at least 6 months and (2) those with untreated HIV (from the START deferred treatment arm). We used linear mixed effects models to test the relationship between case:control status and blood concentrations of pneumoproteins (surfactant protein-D and club cell secretory protein), and biomarkers of inflammation (IL-6 and hsCRP) and coagulation (d-dimer and fibrinogen); concentrations were measured within ± 6 months of first included spirometry. We included an interaction with treatment group (untreated HIV vs viral suppression) to test if associations varied by treatment group. This analysis included 77 matched case:control pairs in the virally suppressed batch, and 42 matched case:control pairs in the untreated HIV batch (n = 238 total) who were followed for a median of 3 years. Median (IQR) CD4 + count was lowest in the controls with untreated HIV at 674 (580, 838). We found no significant associations between case:control status and pneumoprotein or biomarker concentrations in either virally suppressed or untreated PWH. In this cohort of relatively young, recently diagnosed PWH, concentrations of pneumoproteins and biomarkers of inflammation and coagulation were not associated with subsequent rapid lung function decline. Trial registration: NCT00867048 and NCT01797367
Combination of searches for invisible decays of the Higgs boson using 139 fb−1 of proton-proton collision data at √s = 13 TeV collected with the ATLAS experiment
Many extensions of the Standard Model predict the production of dark matter particles at the LHC.
Sufficiently light dark matter particles may be produced in decays of the Higgs boson that would
appear invisible to the detector. This Letter presents a statistical combination of searches for H →
invisible decays where multiple production modes of the Standard Model Higgs boson are considered.
These searches are performed with the ATLAS detector using 139 fb−1 of proton–proton collisions at a
centre–of–mass energy of √s = 13 TeV at the LHC. In combination with the results at √s = 7 TeV and
8 TeV, an upper limit on the H → invisible branching ratio of 0.107 (0.077) at the 95% confidence level
is observed (expected). These results are also interpreted in the context of models where the 125 GeV
Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross-section of weakly
interacting massive particles and nucleons
Search for top-philic heavy resonances in pp collisions at s=13 TeV with the ATLAS detector
A search for the associated production of a heavy resonance with a top-quark or a top-antitop-quark pair, and decaying into a tt¯ pair is presented. The search uses the data recorded by the ATLAS detector in pp collisions at s=13 TeV at the Large Hadron Collider during the years 2015–2018, corresponding to an integrated luminosity of 139 fb-1. Events containing exactly one electron or muon are selected. The two hadronically decaying top quarks from the resonance decay are reconstructed using jets clustered with a large radius parameter of R=1. The invariant mass spectrum of the two top quark candidates is used to search for a resonance signal in the range of 1.0 TeV to 3.2 TeV. The presence of a signal is examined using an approach with minimal model dependence followed by a model-dependent interpretation. No significant excess is observed over the background expectation. Upper limits on the production cross section times branching ratio at 95% confidence level are provided for a heavy Z′ boson based on a simplified model, for Z′ mass between 1.0 TeV and 3.0 TeV. The observed (expected) limits range from 21 (14) fb to 119 (86) fb depending on the choice of model parameters
Search for non-resonant Higgs boson pair production in the 2b+2ℓ+ETmiss final state in pp collisions at s = 13 TeV with the ATLAS detector
A search for non-resonant Higgs boson pair (HH) production is presented, in which one of the Higgs bosons decays to a b-quark pair (bb¯) and the other decays to WW*, ZZ*, or τ+τ−, with in each case a final state with ℓ+ℓ−+ neutrinos (ℓ = e, μ). The analysis targets separately the gluon-gluon fusion and vector boson fusion production modes. Data recorded by the ATLAS detector in proton-proton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb−1, are used in this analysis. Events are selected to have exactly two b-tagged jets and two leptons with opposite electric charge and missing transverse momentum in the final state. These events are classified using multivariate analysis algorithms to separate the HH events from other Standard Model processes. No evidence of the signal is found. The observed (expected) upper limit on the cross-section for non-resonant Higgs boson pair production is determined to be 9.7 (16.2) times the Standard Model prediction at 95% confidence level. The Higgs boson self-interaction coupling parameter κλ and the quadrilinear coupling parameter κ2V are each separately constrained by this analysis to be within the ranges [−6.2, 13.3] and [−0.17, 2.4], respectively, at 95% confidence level, when all other parameters are fixed
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