153 research outputs found
On the Analytic Structure of the Quark Self-Energy in Nambu-Jona- Lasinio Models
The self-energy of quarks is investigated for various models which are
inspired by the Nambu--Jona-Lasinio (NJL) model. Including, beyond the
Hartree-Fock approximation, terms up to second-order in the quark interaction,
the real and imaginary parts of scalar and vector components of the self-energy
are discussed. The second-order contributions depend on the energy and momentum
of the quark under consideration. This leads to solutions of the Dirac equation
which are significantly different from those of a free quark or a quark with
constant effective mass, as obtained in the Hartree-Fock approximation.Comment: 15 pages LaTeX, 6 figures can be obtained from author
Two tricritical lines from a Ginzburg-Landau expansion: application to the LOFF phase
We study the behavior of the two plane waves configuration in the LOFF phase
close to T=0. The study is performed by using a Landau-Ginzburg expansion up to
the eighth order in the gap. The general study of the corresponding grand
potential shows, under the assumption that the eighth term in the expansion is
strictly positive, the existence of two tricritical lines. This allows to
understand the existence of a second tricritical point for two antipodal plane
waves in the LOFF phase and justifies why the transition becomes second order
at zero temperature. The general analysis done in this paper can be applied to
other cases.Comment: LaTex file, 15 pages, 6 figure
Dependence of interface conductivity on relevant physical parameters in polarized Fermi mixtures
We consider a mass-asymmetric polarized Fermi system in the presence of
Hartree-Fock (HF) potentials. We concentrate on the BCS regime with various
interaction strengths and numerically obtain the allowed values of the chemical
and HF potentials, as well as the mass ratio. The functional dependence of the
heat conductivity of the N-SF interface on relevant physical parameters, namely
the temperature, the mass ratio, and the interaction strength, is obtained. In
particular, we show that the interface conductivity starts to drop with
decreasing temperature at the temperature, , where the mean
kinetic energy of the particles is just sufficient to overcome the SF gap. We
obtain as a function of the mass ratio and the interaction
strength. The variation of the heat conductivity, at fixed temperature, with
the HF potentials and the imbalance chemical potential is also obtained.
Finally, because the range of relevant temperatures increases for larger values
of the mass ratio, we consider the - mixture
separately by taking the temperature dependence of the pair potential into
account.Comment: To appear in Physica C (2012
Coarse graining Nuclear Interactions
We consider a coarse graining of NN interactions in coordinate space very
much in the spirit of the well known Vlowk approach. To this end we sample the
interaction at about the minimal de Broglie wavelength probed by NN scattering
below pion production threshold. This amounts to provide a simple delta-shell
potential plus the standard OPE potential above 2 fm. The possible
simplifications in the Nuclear many body problem are discussed.Comment: Presented by RNP at the Erice School on Nuclear Physics 2011: From
Quarks and Gluons to Hadrons and Nuclei, Erice, Sicily (Italy), 16-24
September 2011, 7 pages, 6 figure
Minimal Cooling of Neutron Stars: A New Paradigm
A new classification of neutron star cooling scenarios, involving either
``minimal'' cooling or ``enhanced'' cooling is proposed. The minimal cooling
scenario replaces and extends the so-called standard cooling scenario to
include neutrino emission from the Cooper pair breaking and formation process.
This emission dominates that due to the modified Urca process for temperatures
close to the critical temperature for superfluid pairing. Minimal cooling is
distinguished from enhanced cooling by the absence of neutrino emission from
any direct Urca process, due either to nucleons or to exotica. Within the
minimal cooling scenario, theoretical cooling models can be considered to be a
four parameter family involving the equation of state of dense matter,
superfluid properties of dense matter, the composition of the neutron star
envelope, and the mass of the neutron star. Consequences of minimal cooling are
explored through extensive variations of these parameters. Results are compared
with the inferred properties of thermally-emitting neutron stars in order to
ascertain if enhanced cooling occurs in any of them. All stars for which
thermal emissions have been clearly detected are at least marginally consistent
with the lack of enhanced cooling. The two pulsars PSR 0833-45 (Vela) and PSR
1706-44 would require enhanced cooling in case their ages and/or temperatures
are on the lower side of their estimated values whereas the four stars PSR
0656+14, PSR 1055-52, Geminga, and RX J0720.4-3125 may require some source of
internal heating in case their age and/or luminosity are on the upper side of
their estimated values. The new upper limits on the thermal luminosity of PSR
J0205+6449 and RX J0007.0+7302 are indicative of the occurrence of some
enhanced neutrino emission beyond the minimal scenario.Comment: Version to appear in ApJ Supplements. Minor modifications in text and
discussion of updated data with new figure
Chiral dynamics and the growth of the nucleon's gluonic transverse size at small x
We study the distribution of gluons in transverse space in the nucleon at
moderately small x (~10^{-2}). At large transverse distances (impact
parameters) the gluon density is generated by the 'pion cloud' of the nucleon,
and can be calculated in terms of the gluon density in the pion. We investigate
the large-distance behavior in two different approaches to chiral dynamics: i)
phenomenological soft-pion exchange, ii) the large-N_c picture of the nucleon
as a classical soliton of the pion field, which corresponds to degenerate N and
Delta states. The large-distance contributions from the 'pion cloud' cause a
\~20% increase in the overall transverse size of the nucleon if x drops
significantly below M_pi/M_N. This is in qualitative agreement with the
observed increase of the slope of the t-dependence of the J/psi photoproduction
cross section at HERA compared to fixed-target energies. We argue that the glue
in the pion cloud could be probed directly in hard electroproduction processes
accompanied by 'pion knockout', gamma^* + N -> gamma (or rho, J/psi) + pi + N',
where the transverse momentum of the emitted pion is large while that of the
outgoing nucleon is restricted to values of order M_pi.Comment: 20 pages, revtex4, 10 eps figure
Inhomogeneous Superconductivity in Condensed Matter and QCD
Inhomogeneous superconductivity arises when the species participating in the
pairing phenomenon have different Fermi surfaces with a large enough
separation. In these conditions it could be more favorable for each of the
pairing fermions to stay close to its Fermi surface and, differently from the
usual BCS state, for the Cooper pair to have a non zero total momentum. For
this reason in this state the gap varies in space, the ground state is
inhomogeneous and a crystalline structure might be formed. This situation was
considered for the first time by Fulde, Ferrell, Larkin and Ovchinnikov, and
the corresponding state is called LOFF. The spontaneous breaking of the space
symmetries in the vacuum state is a characteristic feature of this phase and is
associated to the presence of long wave-length excitations of zero mass. The
situation described here is of interest both in solid state and in elementary
particle physics, in particular in Quantum Chromo-Dynamics at high density and
small temperature. In this review we present the theoretical approach to the
LOFF state and its phenomenological applications using the language of the
effective field theories.Comment: RevTex, 83 pages, 26 figures. Submitted to Review of Modern Physic
Pairing in nuclear systems: from neutron stars to finite nuclei
We discuss several pairing-related phenomena in nuclear systems, ranging from
superfluidity in neutron stars to the gradual breaking of pairs in finite
nuclei. We focus on the links between many-body pairing as it evolves from the
underlying nucleon-nucleon interaction and the eventual experimental and
theoretical manifestations of superfluidity in infinite nuclear matter and of
pairing in finite nuclei. We analyse the nature of pair correlations in nuclei
and their potential impact on nuclear structure experiments. We also describe
recent experimental evidence that points to a relation between pairing and
phase transitions (or transformations) in finite nuclear systems. Finally, we
discuss recent investigations of ground-state properties of random two-body
interactions where pairing plays little role although the interactions yield
interesting nuclear properties such as 0+ ground states in even-even nuclei.Comment: 74 pages, 33 figs, uses revtex4. Submitted to Reviews of Modern
Physic
Global Study of Nuclear Structure Functions
We present the results of a phenomenological study of unpolarized nuclear
structure functions for a wide kinematical region of x and Q^2. As a basis of
our phenomenology we develop a model which takes into account a number of
different nuclear effects including nuclear shadowing, Fermi motion and
binding, nuclear pion excess and off-shell correction to bound nucleon
structure functions. Within this approach we perform a statistical analysis of
available data on the ratio of the nuclear structure functions F_2 for
different nuclei in the range from the deuteron to the lead. We express the
off-shell effect and the effective scattering amplitude describing nuclear
shadowing in terms of few parameters which are common to all nuclei and have a
clear physical interpretation. The parameters are then extracted from
statistical analysis of data. As a result, we obtain an excellent overall
agreement between our calculations and data in the entire kinematical region of
x and Q^2. We discuss a number of applications of our model which include the
calculation of the deuteron structure functions, nuclear valence and sea quark
distributions and nuclear structure functions for neutrino charged-current
scattering.Comment: 67 pages, 18 figures (v3: updated text and references, a new section
with discussion about relation between off-shell effect and modification of
the nucleon size in nuclei, accepted for publication in Nucl. Phys. A
Realistic Model of the Nucleon Spectral Function in Few- and Many- Nucleon Systems
By analysing the high momentum features of the nucleon momentum distribution
in light and complex nuclei, it is argued that the basic two-nucleon
configurations generating the structure of the nucleon Spectral Function at
high values of the nucleon momentum and removal energy, can be properly
described by a factorised ansatz for the nuclear wave function, which leads to
a nucleon Spectral Function in the form of a convolution integral involving the
momentum distributions describing the relative and center-of-mass motion of a
correlated nucleon-nucleon pair embedded in the medium. The Spectral Functions
of and infinite nuclear matter resulting from the convolution formula
and from many-body calculations are compared, and a very good agreement in a
wide range of values of nucleon momentum and removal energy is found.
Applications of the model to the analysis of inclusive and exclusive processes
are presented, illustrating those features of the cross section which are
sensitive to that part of the Spectral Function which is governed by
short-range and tensor nucleon-nucleon correlations.Comment: 40 pages Latex , 16 ps figures available from the above e-mail
address or from [email protected]
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