25 research outputs found
Calculation of the two-photon decay width of the f_0(980) scalar meson
The applicability of the quasi-static approximation for calculating the
two-photon annihilation rate of the scalar f_0(980) meson envisaged as a K\bar
K molecule is critically re-examined. It is shown that the validity of this
approximation depends on the detailed interplay between the momentum dependence
of the annihilation amplitude and the momentum transform of the bound state
wave function of the annihilating pair. The approximation becomes invalid when
these two scales of variation are similar. An improved method of calculation
based on the inclusion of electromagnetic corrections to the kernel of the
Bethe-Salpeter equation for the interacting K\bar K pair is outlined to cover
this case and applied to re-evaluate the two-photon decay width for f_0(980) in
a one boson exchange model for the interkaon interaction. The corrections are
significant and result in a much better agreement with experiment.Comment: 14 pages, 3 figures. Fig.3 replaced. Additional remarks with
reference
The Das-Mathur-Okubo sum rule for the charged pion polarizability in a chiral model
The Das-Mathur-Okubo (DMO) sum rule for the polarizability of charged pions
is evaluated for the Nambu-Jona-Lasinio model Lagrangian in both its minimal
and extended forms. A comparison is made with the results obtained using the
same sum rule from chiral perturbation theory (CHPT), approximate QCD sum rule
calculations, explicit calculations on the lattice by Wilcox, and using the
semi-empirical Kapusta-Shuryak spectral densities. The PT results from
Compton scattering are also given. We point to a delicate cancellation between
the intrinsic and recoil contributions to in the DMO sum
rule approach that can lead to calculated polarizabilities of either sign.Comment: 10 LaTeX pages plus one postscript figure, to be published in Physics
Letters
Decay of kaonium in a chiral approach
The decay of the K+K- hadronic atom kaonium is investigated
non-perturbatively using meson-meson interaction amplitudes taken from leading
order chiral perturbation theory in an approach adapted from that proposed by
Oller and Oset [18]. The Kudryavtsev-Popov eigenvalue equation is solved
numerically for the energy shift and decay width due to strong interactions in
the 1s state. These calculations introduce a cutoff ~ 1.4 GeV in O(4) momentum
space that is necessary to regulate divergent loop contributions to the
meson-meson scattering amplitudes in the strong-interaction sector. One finds
lifetimes of 2.2 \pm 0.9 x 10-18s for the ground state of kaonium.Comment: 10 pages, 2 figures. Added new reference to isospin-breaking of
scattering lengt
Extraction of the KK-bar isovector scattering length from pp -> d K^+ K-bar^0 data near threshold
The results of a recent experiment measuring the reaction pp -> dK^+ \bar K^0
near threshold are interpreted in terms of a spectator model that encapsulates
the main features of the observed K^+ \bar K^0 invariant mass distribution. A
chi^2 fit to this data leads to an imaginary part of the isovector scattering
length in the K \bar K channel of Im(a_1) = -(0.63 \pm 0.24) fm. We then use
the Flatte representation of the scattering amplitude to infer a value Re(a_1)
= -(0.02 \pm 0.02) fm for the real part under the assumption that scaling is
approximately satisfied. We show further that it is not possible to exclude the
effects of \pi^+\eta to K^+ \bar K^0 channel coupling within the context of our
model.Comment: 10 pages, 2 figures, 1 tabl
Radiative decays with light scalar mesons and singlet-octet mixing in ChPT
We study different types of radiative decays involving f0(980) and a0(980)
mesons within a unified ChPT-based approach at one-loop level. Light scalar
resonances which are seen in pi pi, pi eta, K K-bar channels of phi(1020)
radiative decays and in J/psi decays are responsible for key questions of
low-energy dynamics in the strong interaction sector, and decays phi(1020) ->
gamma a0(980), phi(1020) -> gamma f0(980), a0(980) -> gamma gamma, f0(980) ->
gamma gamma are of interest for current experimental programs in Juelich,
Frascati and Novosibirsk. From theoretical point of view it is important to
verify whether light scalar mesons are members of some flavor octet or nonet.
We find a value of mixing angle dictated by consistency with experiment and
coupling structures of ChPT Lagrangian. Decay widths f0(980)/a0(980) -> gamma
rho(770)/omega(782), which are not studied experimentally yet, are predicted.
We also obtain several relations between widths, which hold independently of
coupling constants and represent a fingerprint of the model.Comment: 18 pages, 8 figures; misprints in text and tables corrected,
discussion extended, references added; version accepted for publication in
Eur.Phys.J.
f0(980) meson as a K bar K molecule in a phenomenological Lagrangian approach
We discuss a possible interpretation of the f0(980) meson as a hadronic
molecule - a bound state of K and bar K mesons. Using a phenomenological
Lagrangian approach we calculate the strong f0(980) to pi pi and
electromagnetic f0(980) to gamma gamma decays. The compositeness condition
provides a self-consistent method to determine the coupling constant between f0
and its constituents, K and bar K. Form factors governing the decays of the
f0(980) are calculated by evaluating the kaon loop integrals. The predicted
f0(980) to pi pi and f0(980) to gamma gamma decay widths are in good agreement
with available data and results of other theoretical approaches.Comment: 21 pages, 11 figures, revised version accepted for publication in
Eur. Phys. J.
Theory of excited state absorptions in phenylene-based -conjugated polymers
Within a rigid-band correlated electron model for oligomers of
poly-(paraphenylene) (PPP) and poly-(paraphenylenevinylene) (PPV), we show that
there exist two fundamentally different classes of two-photon A states in
these systems to which photoinduced absorption (PA) can occur. At relatively
lower energies there occur A states which are superpositions of one
electron - one hole (1e--1h) and two electron -- two hole (2e--2h) excitations,
that are both comprised of the highest delocalized valence band and the lowest
delocalized conduction band states only. The dominant PA is to one specific
member of this class of states (the mA). In addition to the above class of
A states, PA can also occur to a higher energy kA state whose 2e--2h
component is {\em different} and has significant contributions from excitations
involving both delocalized and localized bands. Our calculated scaled energies
of the mA and the kA agree reasonably well to the experimentally
observed low and high energy PAs in PPV. The calculated relative intensities of
the two PAs are also in qualitative agreement with experiment. In the case of
ladder-type PPP and its oligomers, we predict from our theoretical work a new
intense PA at an energy considerably lower than the region where PA have been
observed currently. Based on earlier work that showed that efficient
charge--carrier generation occurs upon excitation to odd--parity states that
involve both delocalized and localized bands, we speculate that it is the
characteristic electronic nature of the kA that leads to charge generation
subsequent to excitation to this state, as found experimentally.Comment: Revtex4 style, 2 figures inserted in the text, three tables, 10 page
Prompt and non-prompt J/psi elliptic flow in Pb plus Pb collisions at root S-NN=5.02 TeV with the ATLAS detector
The elliptic flow of prompt and non-prompt J/ \u3c8 was measured in the dimuon decay channel in Pb+Pb collisions at sNN=5.02 TeV with an integrated luminosity of 0.42nb-1 with the ATLAS detector at the LHC. The prompt and non-prompt signals are separated using a two-dimensional simultaneous fit of the invariant mass and pseudo-proper decay time of the dimuon system from the J/ \u3c8 decay. The measurement is performed in the kinematic range of dimuon transverse momentum and rapidity 9 < pT< 30 GeV , | y| < 2 , and 0\u201360% collision centrality. The elliptic flow coefficient, v2, is evaluated relative to the event plane and the results are presented as a function of transverse momentum, rapidity and centrality. It is found that prompt and non-prompt J/ \u3c8 mesons have non-zero elliptic flow. Prompt J/ \u3c8v2 decreases as a function of pT, while for non-prompt J/ \u3c8 it is, with limited statistical significance, consistent with a flat behaviour over the studied kinematic region. There is no observed dependence on rapidity or centrality
Search for squarks and gluinos in final states with hadronically decaying tau-leptons, jets, and missing transverse momentum using pp collisions at root s = 13 TeV with the ATLAS detector
A search for supersymmetry in events with large missing transverse momentum, jets, and at least one hadronically decaying τ-lepton is presented. Two exclusive final states with either exactly one or at least two τ-leptons are considered. The analysis is based on proton-proton collisions at √s=13 TeV corresponding to an integrated luminosity of 36.1 fb⁻¹ delivered by the Large Hadron Collider and recorded by the ATLAS detector in 2015 and 2016. No significant excess is observed over the Standard Model expectation. At 95% confidence level, model-independent upper limits on the cross section are set and exclusion limits are provided for two signal scenarios: a simplified model of gluino pair production with τ-rich cascade decays, and a model with gauge-mediated supersymmetry breaking (GMSB). In the simplified model, gluino masses up to 2000 GeV are excluded for low values of the mass of the lightest supersymmetric particle (LSP), while LSP masses up to 1000 GeV are excluded for gluino masses around 1400 GeV. In the GMSB model, values of the supersymmetry-breaking scale are excluded below 110 TeV for all values of tanβ in the range 2 ≤ tanβ ≤ 60, and below 120 TeV for tanβ > 30.M. Aaboud … D. Duvnjak … P. Jackson … J.L. Oliver … A. Petridis … A. Qureshi … A.S. Sharma … M.J. White … et al. [The ATLAS Collaboration
The coupling of phonons to magnetic excitations in a system with two singlet levels
On montre que les excitations magnétiques d'un système ayant deux niveaux singulets donnent naissance à des changements de vitesse des phonons longitudinaux à la fois dans les phases ordonnées et paramagnétiques du système. Dans ce dernier régime, l'effet s'étend sur une gamme de température de l'ordre de la séparation des niveaux. Le changement de vitesse des phonons transverses est limité uniquement à la région ordonnée, où il est strictement proportionnel à m2 quand on l'exprime par un changement de la constante élastique C44 (m est la constante magnétique du réseau ou du sous-réseau).It is shown that the magnetic excitations of a system having two singlet levels give rise to velocity shifts of longitudinal phonons in both the ordered and paramagnetic phase of the system. The effect in the latter regime extends over a temperature range of order of the separation between the levels. The velocity shift of transverse phonons is confined to the ordered region only, where it is strictly proportional to m2 when expressed as a change in the elastic constant C44 (m is the lattice or sublattice magnetization)