849 research outputs found

### Pion and kaon valence quark distribution functions from Dyson-Schwinger equations

Using realistic quark propagators and meson Bethe-Salpeter amplitudes based
on the Dyson-Schwinger equations, we calculate the pion and kaon's valence
parton distribution functions (PDF) through the modified impulse approximation.
The PDFs we obtained at hadronic scale have the purely valence characteristic
and exhibit both dynamical chiral symmetry breaking and SU(3) flavor symmetry
breaking effects. A new calculation technique is introduced to determine the
valence PDFs with precision. Through NLO DGLAP evolution, our result is
compared with pion and kaon valence PDF data at experimental scale. Good
agreement is found in the case of pion, while deviation emerges for kaon. We
point out this situation can be resolved by incorporating gluon contributions
into the mesons if the pion hosts more gluons than kaon nonperturbatively

### Quantum particle confined to a thin-layer volume: Non-uniform convergence toward the curved surface

We clearly refine the fundamental framework of the thin-layer quantization
procedure, and further develop the procedure by taking the proper terms of
degree one in $q_3$ ($q_3$ denotes the curvilinear coordinate variable
perpendicular to curved surface) back into the surface quantum equation. The
well-known geometric potential and kinetic term are modified by the surface
thickness. Applying the developed formalism to a toroidal system obtains the
modification for the kinetic term and the modified geometric potential
including the influence of the surface thickness.Comment: 9 pages, 3 figure

### Limits on the Dark Matter from AMS-02 antiproton and positron fraction data

We derive limits on the dark matter annihilation cross section and lifetime
using measurements of the AMS-02 antiproton ratio and positron fraction data.
In deriving the limits, we consider the scenario of secondary particles
accelerated in supernova remnants (SNRs) which has been argued to be able to
reasonably account for the AMS-02 high energy positron/antiproton fraction
data. We parameterize the contribution of secondary particles accelerated in
SNRs and then fit the observational data within the conventional cosmic ray
propagation model by adopting the GALPROP code. We use the likelihood ratio
test to determine the 95$\%$ confidence level upper limits of the possible dark
matter (DM) contribution to the antiproton/positron fractions measured by
AMS-02. Our limits are stronger than that set by the Fermi-LAT gamma-ray Pass 8
data of the dwarf spheroidal satellite galaxies. We also show that the solar
modulation (cosmic ray propagation) parameters can play a non-negligible role
in modifying the constraints on the dark matter annihilation cross section and
lifetime for $m_\chi100$ GeV), where $m_\chi$ is the rest
mass of the dark matter particles. Using this results, we also put limits on
the effective field theory of dark matter

### Nature of chiral phase transition in QED$_3$ at zero density

Based on the feature of chiral susceptibility and thermal susceptibility at
finite temperature, the nature of chiral phase transition around the critical
number of fermion flavors ($N_c$) and the critical temperature ($T_c$) at a
fixed fermion flavors number in massless QED$_3$ are investigated. It is showed
that, at finite temperature the system exhibits a second-order phase transition
at $N_c$ or $T_c$ and each of the estimated critical exponents is less than 1,
while it reveals a higher-order continuous phase transition around $N_c$ at
zero temperature.Comment: Accepted by PHYSICAL REVIEW

### Finite volume effects with stationary wave solution from Nambu--Jona-Lasinio model

In this paper, we use the two-flavor Nambu-Jona-Lasinio (NJL) model with the
proper time regularization to study the finite-volume effects of QCD chiral
phase transition.
Within a cubic volume of finite size $L$, we choose the stationary wave
condition (SWC) as the real physical spatial boundary conditions of quark
fields and compare our results with that by means of commonly used
(anti-)period boundary condition (APBC or PBC). It is found that the results by
means of SWC are obviously different to the results from the APBC or PBC.
Although the three boundary conditions give the same chiral crossover
transition curve in the infinite volume limit, the limit size $L_0$ (when
$L\geq L_{0}$, the chiral quark condensate $-\left\langle { \bar \psi \psi}
\right\rangle_L$ is indistinguishable from that at $L=\infty$) using SWC is
$L_0\approx 500$ fm which is much larger than the results obtained using APBC
or PBC. More importantly, $L_0\approx 500$ fm is also much large than the
typical size of the quark-gluon plasma produced by the relativistic heavy ion
collisions. This means that the finite volume effects play a very important
role in Relativistic Heavy Ion Collisions. In addition, we also found that when
$L\leq 2$ fm, even at zero temperature the chiral symmetry is effectively
restored. Furthermore, to quantitatively reflect the finite volume effects on
the QCD chiral phase transition, we introduce a new vacuum susceptibility,
$\chi_{1/L}(T)=-\frac{\partial \left\langle { \bar \psi \psi}
\right\rangle}{\partial (1/L)}$. With this new vacuum susceptibility, it is
very interesting to find $\chi_{1/L}(T=0)=\chi_{1/L}(T=1/L)$ for SWC

### Nonperturbative solutions of Dyson-Schwinger equations in QED$_3$

The studies of Dyson-Schwinger Equations (DSEs) provide us with insights into
nonperturbative phenomenon of quantum field theory. However, DSEs are
essentially an infinite set of coupled Green's functions, it's necessary to
decouple parts of the equations which are thought of major physical importance
to make the solution of these equations possible. Although the results are
model-dependent, no qualitative deviations from exact solutions are expected
with properly chosen truncation scheme. In this article, a globally convergent
numerical method for the solution of the DSEs of QED$_3$ in Euclidean space is
presented. This method can be adapted for more complex problems, however, it
also shows its limitations when adopted in problems such as the searching for
Wigner solutions.Comment: 17 pages, 11 figure

### Modelling the Multi-band Afterglow of GRB 091127: Evidence of a Hard Electron Energy Spectrum with an Injection Break

The afterglow of GRBs is believed to originate from the synchrotron emission
of shock-accelerated electrons produced by the interaction between the outflow
and the external medium. The accelerated electrons are usually assumed to
follow a power law energy distribution with an index of $p$. Observationally,
although most GRB afterglows have a $p$ larger than 2, there are still a few
GRBs suggestive of a hard ($p<2$) electron spectrum. GRB 091127, with
well-sampled broad-band afterglow data, shows evidence of a hard electron
spectrum and strong spectral evolution, with a spectral break moving from high
to lower energies. The spectral break evolves very fast and cannot be explained
by the cooling break in the standard afterglow model, unless evolving
microphysical parameters are assumed. Besides, the multi-band afterglow light
curves show an achromatic break at around 33 ks. Based on the model of a hard
electron spectrum with an injection break, we interpret the observed spectral
break as the synchrotron frequency corresponding to the injection break, and
the achromatic break as a jet break caused by the jet-edge effect. It is shown
that the spectral evolution and the multi-band afterglow light curves of GRB
091127 can be well reproduced by this model.Comment: 25 pages, 2 figure

### On the morphology of $\gamma-$ray emission induced by $e^{\pm}$ from annihilating self-interacting dark matter

With the Fermi-LAT data quite a few research groups have reported a spatially
extended GeV $\gamma$-ray excess surrounding the Galactic Center (GC). The
physical origin of such a GeV excess is still unclear and one interesting
possibility is the inverse Compton scattering of the electrons/positrons from
annihilation of self-interacting dark matter (SIDM) particles with the
interstellar optical photons. In this work we calculate the morphology of such
a kind of $\gamma$-ray emission. For the annihilation channel of
$\bar{\chi}\chi\rightarrow \phi\phi\rightarrow e^{+}e^{-}e^{+}e^{-}$, the
inverse Compton scattering (ICS) dominates over the bremsstrahlung on producing
the GeV $\gamma$-ray emission. For the SIDM particles with a rest mass $m_\chi
\sim$ tens GeV that may be favored by the modeling of the Galactic GeV excess,
the ICS radiation at GeV energies concentrates along the Galactic plane. The
degrees of asymmetry high up to $\geq 0.3$ are found in some regions of
interest, which in turn proposes a plausible test on the SIDM interpretation of
the GeV excess.Comment: 9 pages, 6 figures, Phys. Rev. D in pres

### New algorithm to study the pseudo-Wigner solution of the quark gap equation in the framework of the (2+1)-flavor NJL model

In this paper, we study the pseudo-Wigner solution of the quark gap equation
with a recently proposed algorithm in the framework of the (2+1)-flavor
Nambu-Jona-Lasinio (NJL) model. We find that for the current quark mass $m_{\rm
u,d}=5.5$ MeV and chemical potential $\mu<\mu_{\rm TCP}=272.5$ MeV, the Nambu
solution and the positive pseudo-Wigner solution obtained via this algorithm is
consistent with the physical solution obtained with the iterative method.
Furthermore, the algorithm we used can help to illustrate the evolution of the
solutions of the gap equation from the chiral limit to non-chiral limit and
gives a prediction where the crossover line is located in the phase diagram for
$\mu<272.5$ MeV. In addition, we also study the chiral susceptibilities as well
as the loss of solutions for different chemical potentials.Comment: 9 pages, 12 figure

### The chiral phase transition of QED$_3$ around the critical number of fermion flavors

At zero temperature and density, the nature of the chiral phase transition in
QED$_3$ with $\textit{N}_{f}$ massless fermion flavors is investigated. To this
end, in Landau gauge, we numerically solve the coupled Dyson-Schwinger
equations for the fermion and boson propagator within the bare and simplified
Ball-Chiu vertices separately. It is found that, in the bare vertex
approximation, the system undergoes a high-order continuous phase transition
from the Nambu-Goldstone phase into the Wigner phase when the number of fermion
flavors $\textit{N}_{f}$ reaches the critical number $\textit{N}_{f,c}$, while
the system exhibits a typical characteristic of second-order phase transition
for the simplified Ball-Chiu vertex.Comment: 9 pages, 9 figure

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