arXiv
(422.153 recursos)
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Mostrando recursos 1 - 20 de 65.105
1.
Pseudoscalars Mesons in Hot, Dense Matter - Costa, Pedro; Ruivo, M. C.; Kalinovsky, Yu. L.; de Sousa, C. A.
Phase transitions in hot and dense matter and the in--medium behavior of
pseudoscalar mesons ($\pi^{\pm}, \pi^0, K^{\pm}, K^0 ,\bar K^0,\eta {and} \eta'
$) are investigated, in the framework of the three flavor Nambu--Jona-Lasinio
model, including the 't Hooft interaction, which breaks the $U_A(1)$ symmetry.
Three different scenarios are considered: zero density and finite temperature,
zero temperature and finite density in quark matter with different degrees of
strangeness, and finite temperature and density. At T=0, the role of strange
valence quarks in the medium is discussed, in connection with the phase
transition and the mesonic behavior. It is found that the appearance of strange
quarks, above certain densities, leads to...
2.
Mirror World and its Cosmological Consequences - Berezhiani, Zurab
We briefly review the concept of a parallel `mirror' world which has the same
particle physics as the observable world and couples to the latter by gravity
and perhaps other very weak forces. The nucleosynthesis bounds demand that the
mirror world should have a smaller temperature than the ordinary one. By this
reason its evolution should substantially deviate from the standard cosmology
as far as the crucial epochs like baryogenesis, nucleosynthesis etc. are
concerned. In particular, we show that in the context of certain baryogenesis
scenarios, the baryon asymmetry in the mirror world should be larger than in
the observable one. Moreover, we show that mirror baryons could...
3.
Rho - Omega Splitting and Mixing in Nuclear Matter - Zschocke, S.; Kampfer, B.
We investigate the splitting and mixing of $\rho$ and $\omega$ mesons in
nuclear matter. The calculations were performed on the basis of QCD sum rules
and include all operators up to mass dimension-6 twist-4 and up to first order
in the coupling constants. Special attention is devoted to the impact of the
scalar 4-quark condensates on both effects. In nuclear matter the Landau
damping governs the $\rho - \omega$ mass splitting while the scalar 4-quark
condensates govern the strenght of individual mass shifts. A strong in-medium
mass splitting causes the disappearance of the $\rho - \omega$ mixing.
4.
Little Supersymmetry and the Supersymmetric Little Hierarchy Problem - Birkedal, Andreas; Chacko, Z.; Gaillard, Mary K.
The current experimental lower bound on the Higgs mass significantly
restricts the allowed parameter space in most realistic supersymmetric models,
with the consequence that these models exhibit significant fine-tuning. We
propose a solution to this `supersymmetric little hierarchy problem'. We
consider scenarios where the stop masses are relatively heavy - in the 500 GeV
to a TeV range. Radiative stability of the Higgs soft mass against quantum
corrections from the top quark Yukawa coupling is achieved by imposing a global
SU(3) symmetry on this interaction. This global symmetry is only approximate -
it is not respected by the gauge interactions. A subgroup of the global
symmetry is gauged by...
5.
Perturbative generation of a strange-quark asymmetry in the nucleon - Catani, Stefano; de Florian, Daniel; Rodrigo, German; Vogelsang, Werner
We point out that perturbative evolution in QCD at three loops generates a
strange-antistrange asymmetry s(x)-sbar(x) in the nucleon's sea just from the
fact that the nucleon has non-vanishing up and down quark valence densities.
The recently computed three-loop splitting functions allow for an estimate of
this effect. We find that a fairly sizable asymmetry may be generated. Results
for analogous asymmetries in the heavy-quark sector are also presented.
6.
Electroweak and supersymmetric two-loop corrections to (g-2)_\mu - Heinemeyer, Sven; Stöckinger, Dominik; Weiglein, Georg
We present the up to now most precise evaluation of electroweak and
supersymmetric contributions to the anomalous magnetic moment of the muon \amu,
describing in detail also the calculational techniques. We calculate the
bosonic two-loop contributions in the Standard Model without the approximation
of a heavy Higgs-boson mass, finding corrections up to 0.2*10^{-10} for a light
Higgs boson. In the Minimal Supersymmetric Standard Model the corresponding
two-loop contributions from the two-Higgs-doublet model part differ from the
Standard Model result by up to 0.3*10^{-10}. Finally, we evaluate the diagrams
where a loop of charginos or neutralinos, the superpartners of gauge and Higgs
bosons, is inserted into a two-Higgs-doublet one-loop diagram....
7.
Bounds on neutrino masses from leptogenesis in type-II see-saw models - Sahu, Narendra; Sankar, S. Uma
The presence of the triplet $\Delta_{L}$ in left-right symmetric theories
leads to type-II see-saw mechanism for the neutrino masses. In these models,
assuming a normal mass hierarchy for the heavy Majorana neutrinos, we derive a
lower bound on the mass of the lightest of heavy Majorana neutrino from the
leptogenesis constraint. From this bound we establish a consistent picture for
the hierarchy of heavy Majorana neutrinos in a class of left right symmetric
models in which we identify the neutrino Dirac mass matrix with that of
Fritzsch type charged lepton mass matrix. It is shown that these values are
compatible with the current neutrino oscillation data.
8.
The colour triplet qqqbar cluster and pentaquark models - Hogaasen, H.; Sorba, P.
We study the properties of the colour triplet qqqbar quark cluster when
flavour symmetry is broken. The relevance of such a cluster for some models of
pentaquarks is then examined in the light of recent experimental signals.
9.
Split Supersymmetry - Giudice, G. F.; Romanino, A.
The naturalness criterion applied to the cosmological constant implies a
new-physics threshold at 10^-3 eV. Either the naturalness criterion fails, or
this threshold does not influence particle dynamics at higher energies. It has
been suggested that the Higgs naturalness problem may follow the same fate. We
investigate this possibility and, abandoning the hierarchy problem, we use
unification and dark matter as the only guiding principles. The model recently
proposed by Arkani-Hamed and Dimopoulos emerges as a very interesting option.
We study it in detail, analysing its structure, and the conditions for
obtaining unification and dark matter.
10.
Vacuum Cherenkov radiation - Lehnert, Ralf; Potting, Robertus
Within the classical Maxwell-Chern-Simons limit of the Standard-Model
Extension (SME), the emission of light by uniformly moving charges is studied
confirming the possibility of a Cherenkov-type effect. In this context, the
exact radiation rate for charged magnetic point dipoles is determined and found
in agreement with a phase-space estimate under certain assumptions.
11.
Some comments on B -> pi pi decays - Fayyazuddin
Isospin analysis has been used to constrain the CP-asymmetries in B -> pi pi
decays. In particular correlation between a weak phase \theta and a strong
phase \delta is obtained. Further using the experimental values for the
CP-average branching ratios, the following bounds on direct CP-asymmetries are
obtained: -0.35+/- 0.22=< C_{\pi ^+\pi^-}\leq 0; C_{\pi ^0\pi ^0}=-(2.4\pm 1.0)
C_{\pi ^+\pi ^-}. Constraints on mixing induced CP-asymmetries are also
discussed.
12.
Solution of the Fan Diagram Equation in 2+1 Dimensional QCD - Bartels, J.; Fadin, V. S.; Lipatov, L. N.
We investigate the Balitsky-Kovchegov (BK) equation for D=3 space-time
dimensions, corresponding to one transverse coordinate, and we show that it can
be solved analytically. The explicit solutions are found in the linear
approximation and for the paricular cases when they do not depend on the impact
parameter of the dipole or on its rapidity. It is shown, that in a general case
different solutions are related by an infinite parameter group of
transformations. The key observation is that the equation has the
Kowalewskaya-Painlev\'{e} property, which gives the possibility of reducing the
non-linear problem to the solution of a linear integral equation.
13.
Auxiliary fields and the flux tube model - Buisseret, Fabien; Semay, Claude
It is possible to eliminate exactly all the auxiliary fields (einbein fields)
appearing in the rotating string Hamiltonian to obtain the classical equations
of motion of the relativistic flux tube model. A clear interpretation can then
be done for the characteristic variables of the rotating string model.
14.
A QCD analysis of diffractive deep-inelastic scattering data - Martin, A. D.; Ryskin, M. G.; Watt, G.
We perform a novel type of analysis of diffractive deep-inelastic scattering
data, in which the input parton distributions of the Pomeron are parameterised
using the perturbative QCD expressions. In particular, we treat individually
the components of the Pomeron of different size. We are able to describe
simultaneously both the recent ZEUS and H1 diffractive data. In addition to the
usual two-gluon model for the perturbative Pomeron, we allow for the
possibility that it may be made from two sea quarks.
15.
Monte Carlo Exploration of Warped Higgsless Models - Hewett, J. L.; Lillie, B.; Rizzo, T. G.
We have performed a detailed Monte Carlo exploration of the parameter space
for a warped Higgsless model of electroweak symmetry breaking in 5 dimensions.
This model is based on the $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ gauge group
in an AdS$_5$ bulk with arbitrary gauge kinetic terms on both the Planck and
TeV branes. Constraints arising from precision electroweak measurements and
collider data are found to be relatively easy to satisfy. We show, however,
that the additional requirement of perturbative unitarity up to the cut-off,
$\simeq 10$ TeV, in $W_L^+W_L^-$ elastic scattering in the absence of dangerous
tachyons eliminates all models. If successful models of this class exist, they
must be highly...
16.
Partial wave decomposition of pion and photoproduction amplitudes - Anisovich, A. V.; Klempt, E.; Sarantsev, A.; Thoma, U.
Partial wave amplitudes for production and decay of baryon resonances are
constructed in the framework of the operator expansion method. The approach is
fully relativistically invariant and allow us to perform combined analyses of
different reactions imposing directly analyticity and unitarity constraints.
All formulas are given explicitly in the form used by the Crystal Barrel
collaboration in the (partly forthcoming) analyses of the electro-, photo- and
pion induced meson production data.
17.
New-Physics scenarios in $\Delta S = 2$ decays of the B_c meson - Fajfer, S.; Kamenik, J.; Singer, P.
The $\Delta S = 2$ transition $b \to s s \bar d$ is extremely small in the
standard model, proceeding only via box diagrams. Such decays are thus an
appropriate ground for searching new physics. We present a study of several
two-body and three-body decays of the B_c meson belonging to this class. Our
calculation covers the minimal supersymmetric model with and without R-parity
and a two-Higgs-doublet model. The current limits on the parameters of these
models allow for rather large branching ratios for several two and three-body
hadronic decay modes of B_c, up to the 10^{-4} range in the minimal
supersymmetric model with R-parity violation.
18.
A model for next-to-leading order threshold resummed form factors - Aglietti, Ugo; Ricciardi, Giulia
We present a model for next-to-leading order resummed threshold form factors
based on a time-like coupling recently introduced in the framework of small x
physics. Improved expressions for the form factors in N-space are obtained
which are not plagued by Landau-pole singularities, as the included absorptive
effects -- usually neglected -- act as regulators. The physical reason is that,
because of faster decay of gluon jets, there is not enough resolution time to
observe the Landau pole. Our form factors reduce to the standard ones when the
absorptive parts related to the coupling are neglected. The inverse transform
from N-space to x-space can be done directly without any...
19.
Universality of the rho-meson coupling in effective field theory - Djukanovic, D.; Schindler, M. R.; Gegelia, J.; Japaridze, G.; Scherer, S.
It is shown that both the universal coupling of the rho-meson and the
Kawarabayashi-Suzuki-Riadzuddin-Fayyazuddin expression for the magnitude of its
coupling constant follow from the requirement that chiral perturbation theory
of pions, nucleons, and rho-mesons is a consistent effective field theory. The
prerequisite of the derivation is that all ultraviolet divergences can be
absorbed in the redefinition of fields and the available parameters of the most
general effective Lagrangian.
20.
Leptogenesis in the minimal supersymmetric triplet seesaw model - D'Ambrosio, G.; Hambye, T.; Hektor, A.; Raidal, M.; Rossi, A.
In the supersymmetric triplet (type-II) seesaw model, in which a single
SU(2)_L-triplet couples to leptons, the high-energy neutrino flavour structure
can be directly determined from the low-energy neutrino data. We show that even
with such a minimal triplet content, leptogenesis can be naturally accommodated
thanks to the resonant interference between superpotential and soft
supersymmetry breaking terms.
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