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1.
We study the chiral phase transition at finite temperature in the linear sigma model by employing a self-consistent Hartree
approximation. This approximation is introduced by imposing self-consistency conditions on the effective meson mass equations
which are derived from the finite temperature one-loop effective potential. It is shown that in the limit of vanishing pion
mass, namely when the chiral symmetry is exact, the phase transition becomes a weak first order accompanying a gap in the
order parameter as a function of temperature. This is caused by the long range fluctuations of meson fields whose effective
masses become small in the transition region. It is shown, however, that with an explicit chiral symmetry breaking term in
the Lagrangian which generates the realistic finite pion mass the transition is smoothed out irrespective of the choice of
coupling strength.
Recieved: 19 September 1997 / Revised version: 30 October 1997 相似文献
2.
A. Jokinen J.C. Wang J. Äystö P. Dendooven S. Nummela J. Huikari V. Kolhinen A. Nieminen K. Peräjärvi S. Rinta-Antila 《The European Physical Journal A - Hadrons and Nuclei》2000,9(1):9-12
Beta decay of a refractory isotope 118Rh produced in symmetric fission and mass separated by the ion guide technique has been applied for the study of low-lying
excited states of 118Pd. The yrast band in 118Pd has been observed up to a 6+ state and the lowest states of the asymmetric γ-band have been identified. The measured half-life of 118Rh is (300±60)ms. The systematics of the excited states in neutron-rich Pd-isotopes implies the saturation towards an O(6) symmetry at N = 70.
Received: 19 June 2000 / Accepted: 1 August 2000 相似文献
3.
Yun Chang Shin Bong Soo Han Myung Ki Cheoun K.S. Kim Il-Tong Cheon 《The European Physical Journal A - Hadrons and Nuclei》2000,9(2):269-276
A SU(2) effective Lagrangian is extended to a SU
L(3) ⊗SU
R(3) by including the vector and axial vector meson. With this effective Lagrangian, electromagnetic form factors of charged
pion and kaon are calculated in both time- and space-like regions. The pseudoscalar meson loops are taken into account. Good
agreement with experimental data is obtained for those form factors and charged pseudoscalar meson radii. Decay widths of
ρ→ππ and φ→K
+
K
- are also calculated and shown to agree with experimental data very well.
Received: 20 December 1999 / Accepted: 12 October 2000 相似文献
4.
E.G. Drukarev M.G. Ryskin V.A. Sadovnikova 《The European Physical Journal A - Hadrons and Nuclei》1999,4(2):171-186
We calculate the contribution of pions to the $\bar qq$-expectation value κ(ρ) =<M|ˉq
q|M> in symmetric nuclear matter. We employ exact pion propagator renormalized by nucleon-hole and isobar-hole excitations. Conventional
straightforward calculation leads to the “pion condensation” at unrealistically small values of densities, causing even earlier
restoration of chiral symmetry. This requires a self-consistent approach, consisting in using the models, which include direct
dependence of in-medium mass values on κ(ρ), e.g. the Nambu–Jona-Lasinio–model. We show, that in the self-consistent approach
the ρ-dependence of the condensate is described by a smooth curve. The “pion condensate” point is removed to much higher values
of density. The chiral restoration does not take place at least while ρ < 2.8ρ0 with ρ0 being the saturation value. Validity of our approach is limited by possible accumulation of heavier baryons (delta isobars)
in the ground state of nuclear matter. For the value of effective nucleon mass at the saturation density we found m
*(ρ0) = 0.6m, consistent with nowadays results of other authors.
Received: 8 October 1998 相似文献
5.
Bing An Li 《The European Physical Journal A - Hadrons and Nuclei》2001,10(3):347-353
Based on a phenomenologically successful effective chiral theory of pseudoscalar, vector, and axial-vector mesons, all the
coefficients of the chiral perturbation theory are predicted. There is no new adjustable parameter in these predictions. Up
to O(m
2
q) the formulas of the masses of the pseudoscalar mesons are the same as the ones obtained by ChPT.
Received: 31 August 2000 / Accepted: 16 March 2001 相似文献
6.
V. Bernard N. Kaiser U.-G. Meißner 《The European Physical Journal A - Hadrons and Nuclei》2001,11(2):209-216
We investigate near-threshold neutral pion photoproduction off protons to fourth order in heavy-baryon chiral perturbation
theory in the light of the new data from MAMI. We show that the unitarity cusp at the secondary π+
n threshold is in agreement with expectations from the final-state theorem. We also analyze the fourth-order corrections to
the P-wave low-energy theorems and show that potentially large Δ isobar contributions are cancelled by sizeable pion loop effects.
This solidifies the parameter-free third-order predictions, which are in good agreement with the data.
Received: 25 May 2001 / Accepted: 21 June 2001 相似文献
7.
A. Montina E. Arimondo 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2001,14(1):105-110
In this article we introduce a differential equation for the first order correlation function G
(1) of a Bose-Einstein condensate at T = 0. The Bogoliubov approximation is used. Our approach points out directly the dependence on the physical parameters. Furthermore
it suggests a numerical method to calculate G
(1) without solving an eigenvector problem. The G
(1) equation is generalized to the case of non zero temperature.
Received 20 September 2000 相似文献
8.
M. Wolf J. Ahrens R. Beck V. Hejny J.D. Kellie M. Kotulla B. Krusche V. Kuhr R. Leukel V. Metag J.C. Nacher R. Novotny V. Olmos de León R.O. Owens F. Rambo A. Schmidt M. Schumacher U. Siodlaczek H. Ströher J. Weiß F. Wissmann 《The European Physical Journal A - Hadrons and Nuclei》2000,9(1):5-8
Double neutral pion photoproduction from the proton has been measured at MAMI for photon energies between threshold and 820
MeV. The reaction was identified by an invariant mass and missing mass analysis. From threshold up to 370 MeV the total cross-section
does not exceed 30 nb. For higher energies it shows a smooth rise until it reaches a maximum of about 10 μb at E
γ = 740 MeV. Dalitz plots of m
2(π0π0) versus
m
2(p,π0) for seven bins of incident photon energy have been analysed. For E
γ > 610 MeV, a strong contribution of a sequential decay is observed with the Δ(1232)-resonance as intermediate state. A comparison
to model calculations shows that these sequential decays presumably originate from the D
13(1520) and also the P
11(1440)-resonance.
Received: 6 June 2000 / Accepted: 18 August 2000 相似文献
9.
V. K. B. Kota 《Pramana》2003,60(1):59-74
It is shown that the proton-neutron interacting boson model (pnIBM) admits new symmetry limits withO(12) algebra which breakF spin but preserves theF
z quantum numberM
F. The generators ofO(12) are derived and the quantum numberU ofO(12) for a given boson numberN is determined by identifying the corresponding quasi-spin algebra. TheO(12) algebra generates two symmetry schemes and for both of them, complete classification of the basis states and typical
spectra are given. With theO(12) algebra identified, complete classification of pnIBM symmetry limits with goodM
F is established. 相似文献
10.
We study the static scalar susceptibility of the nuclear medium, i.e., the change of the quark condensate for a small modification of the quark mass. In the linear sigma model it is linked to
the in-medium sigma propagator and its magnitude increases due to the mixing with the softer modes of the nucleon-hole excitations.
We show that the pseudoscalar susceptibility, which is large in the vacuum, owing to the smallness of the pion mass, follows
the density evolution of the quark condensate and thus decreases. At normal nuclear matter density the two susceptibilities
become much closer, a partial chiral symmetry restoration effect as they become equal when the full restoration is achieved.
Received: 20 July 2002 / Accepted: 14 September 2002 / Published online: 21 January 2003
RID="a"
ID="a"e-mail: chanfray@ipnl.in2p3.fr
Communicated by A. Molinari 相似文献
11.
B. Borasoy E. Epelbaum H. Krebs D. Lee U. -G. Meißner 《The European Physical Journal A - Hadrons and Nuclei》2007,31(1):105-123
We discuss lattice simulations of light nuclei at leading order in the chiral effective field theory. Using lattice pion fields
and auxiliary fields, we include the physics of instantaneous one-pion exchange and the leading-order S-wave contact interactions. We also consider higher-derivative contact interactions which adjust the S-wave scattering amplitude at higher momenta. By construction our lattice path integral is positive definite in the limit
of exact Wigner SU(4) symmetry for any even number of nucleons. This SU(4) positivity and the approximate SU(4) symmetry of the low-energy interactions play an important role in suppressing sign and phase oscillations in Monte Carlo
simulations. We assess the computational scaling of the lattice algorithm for light nuclei with up to eight nucleons and analyze
in detail calculations of the deuteron, triton, and helium-4. 相似文献
12.
E.M. Darwish H. Arenhövel M. Schwamb 《The European Physical Journal A - Hadrons and Nuclei》2003,16(1):111-125
The influence of final-state NN and πN rescattering in incoherent pion photoproduction on the deuteron has been investigated. For the elementary photoproduction
operator an effective Lagrangian model is used which describes well the elementary reaction. The interactions in the final
two-body subsystems are taken in separable form. While NN rescattering shows quite a significant effect, particularly strong for neutral pion production, πN rescattering is almost negligible. Inclusion of such effects leads to an improved and quite satisfactory agreement with experiment.
Received: 12 August 2002 / Accepted: 11 September 2002 / Published online: 17 January 2003
RID="a"
ID="a"Present address: Physics Department, Faculty of Science, South Valley University, Sohag, Egypt.
RID="b"
ID="b"e-mail: arenhoev@kph.uni-mainz.de
Communicated by V. Vento 相似文献
13.
On the basis of the experimental data on diffractive processes in πp, pp and pˉp collisions at intermediate, moderately high and high energies, we restore the scattering amplitude related to the t-channel exchange by vacuum quantum numbers by taking account of the diffractive s-channel rescatterings. At intermediate and moderately high energies, the t-channel exchange amplitude turns, with a good accuracy, into an effective pomeron which renders the results of the additive
quark model. At superhigh energies the scattering amplitude provides a Froissart-type behaviour, with an asymptotic universality
of cross sections such as σtot
πp/σtot
pp→ 1 at s→∞. The quark structure of hadrons being taken into account at the level of constituent quarks, the cross sections of pion
and proton (antiproton) in the impact parameter space of quarks, σπ(r
1⊥, r
2⊥; s) and σp(r
1⊥, r
2⊥, r
3⊥; s), are found as functions of s. These cross sections implicate the phenomenon of colour screening: they tend to zero at |r
i⊥−r
k⊥|→ 0. The effective colour screening radius for pion (proton) is found for different s. The predictions for the diffractive cross sections at superhigh energies are presented.
Received: 15 December 1998 相似文献
14.
Hungchong Kim 《The European Physical Journal A - Hadrons and Nuclei》2000,7(1):121-125
Within the conventional QCD sum rules, we calculate the πNN coupling constant, g
πN, beyond the chiral limit using two-point correlation function with a pion. For this purpose, we consider the Dirac structure,
iγ5, at m
π
2 order in the expansion of the correlator in terms of the pion momentum. For a consistent treatment of the sum rule, we include
the linear terms in quark mass as they constitute the same chiral order as m
π
2. In this sum rule, we obtain g
πN= 13.3 ± 1.2, which is very close to the empirical πNN coupling. This demonstrates that going beyond the chiral limit is crucial in determining the coupling.
Received: 8 July 1999 / Revised version: 20 August 1999 相似文献
15.
Tran Huu Phat Nguyen Van Thu 《The European Physical Journal C - Particles and Fields》2011,71(11):1810
The linear sigma model at finite isospin chemical potential μ and temperature T is systematically studied by means of the Cornwal–Jackiw–Tomboulis (CJT) effective potential calculated in the improved Hartree–Fock
(HF) approximation, where the Goldstone theorem and the thermodynamic consistency are respected. It results that in the chiral
limit, for μ=0 the chiral phase transition is second order as expected from the general universality arguments, and for μ≠0 the phase diagram for the pion condensation in the (T,μ) plane exhibits a tricritical point which is crossover from first-order to second-order phase transitions. In the physical
world, where the chiral symmetry is explicitly broken, the pion condensation occurs at μ=m
π
, the pion mass in vacuum, and its phase diagram is basically in agreement with those found from the chiral perturbation theory.
The chiral symmetry gets restored at high values of T for fixed μ and of μ for fixed T. 相似文献
16.
E. Bogomolny U. Gerland C. Schmit 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(1):121-132
We propose a plasma model for spectral statistics displaying level repulsion without long-range spectral rigidity, i.e. statistics intermediate between random matrix and Poisson statistics similar to the ones found numerically at the critical
point of the Anderson metal-insulator transition in disordered systems and in certain dynamical systems. The model emerges
from Dysons one-dimensional gas corresponding to the eigenvalue distribution of the classical random matrix ensembles by restricting
the logarithmic pair interaction to a finite number k of nearest neighbors. We calculate analytically the spacing distributions and the two-level statistics. In particular we
show that the number variance has the asymptotic form Σ2(L) ∼χL for large L and the nearest-neighbor distribution decreases exponentially when s→∞, P(s) ∼ exp(- Λs) with Λ = 1/χ = kβ + 1, where β is the inverse temperature of the gas (β = 1, 2 and 4 for the orthogonal, unitary and symplectic symmetry class
respectively). In the simplest case of k = β = 1, the model leads to the so-called Semi-Poisson statistics characterized by particular simple correlation functions
e.g.
P(s) = 4s exp(- 2s). Furthermore we investigate the spectral statistics of several pseudointegrable quantum billiards numerically and compare
them to the Semi-Poisson statistics.
Received 13 September 2000 相似文献
17.
R. Metzler 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(2):249-258
A generalised random walk scheme for random walks in an arbitrary external potential field is investigated. From this concept
which accounts for the symmetry breaking of homogeneity through the external field, a generalised master equation is constructed.
For long-tailed transfer distance or waiting time distributions we show that this generalised master equation is the genesis
of apparently different fractional Fokker-Planck equations discussed in literature. On this basis, we introduce a generalisation
of the Kramers-Moyal expansion for broad jump length distributions that combines multiples of both ordinary and fractional
spatial derivatives. However, it is shown that the nature of the drift term is not changed through the existence of anomalous
transport statistics, and thus to first order, an external potential Φ(x) feeds back on the probability density function W through the classical term ∝/
x
(x)W(x, t), i.e., even for Lévy flights, there exists a linear infinitesimal generator that accounts for the response to an external field.
Received 30 June 2000 and Received in final form 12 November 2000 相似文献
18.
T. Matulewicz L. Aphecetche Y. Charbonnier H. Delagrange K.K. Gudima G. Martınez M. Płoszajczak Y. Schutz V.D. Toneev M. Appenheimer R. Averbeck J. Dıaz A. Döppenschmidt M.J. van Goethem S. Hlavač M. Hoefman R. Holzmann F. Lefèvre A. Kugler H. Löhner A. Marın V. Metag R. Novotny R.W. Ostendorf R.H. Siemssen R.S. Simon R. Stratmann H. Ströher P. Tlusty P.H. Vogt V. Wagner J. Weiss H.W. Wilschut F. Wissmann A.R. Wolf M. Wolf 《The European Physical Journal A - Hadrons and Nuclei》2000,9(1):69-72
Proton-π0 coincidences have been measured at the beam energy of 180A MeV in the reaction Ar+Ca studied by TAPS at SIS/GSI. In the proton-π0 invariant mass spectrum we observe a significant excess of strength above the background obtained by event mixing. We attribute
this signal to the strength distribution N
Δ of the Δ baryonic resonance. No correlation is observed in the case of deuteron-π0 coincidences. Assuming isotropic emission of π0 and Δ+ from a midrapidity thermal source and isospin symmetry, we determined the global N
Δ/N
πratio of 0.79 ±0.30(stat) ±0.2(syst). This value indicates that most pions produced at subthreshold energy in heavy-ion reaction
are mediated by the Δ-resonance.
Received: 7 April 2000 / Accepted: 29 August 2000 相似文献
19.
20.
An extensive and systematic study of the recent η photo-production data up to 1.2 GeV is presented within a chiral constituent
quark model. A model embodying all known nucleonic resonances shows clear need for a yet undiscovered third S11 resonance in the second resonance region, for which we determine the mass (1.729 GeV) and the total width (183 MeV). Furthermore,
we extract the configuration mixing angles, an important property of the quark-quark interaction in the quark model, for the
resonances S
11(1535) and S
11(1650), as well as for the resonances D
13(1520) and D
13(1700). Our results agree well with the quark model predictions. In addition, the partial ηN decay widths and/or the photo-excitation helicity amplitudes for the nucleonic resonances S
11(1535), S
11(1650), P
11(1710), P
13(1720), D
13(1520), D
13(1700), D
15(1675), and F
15(1680) are also obtained in this approach.
Received: 14 February 2001 / Accepted: 22 June 2001 相似文献