共查询到20条相似文献,搜索用时 62 毫秒
1.
2.
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
3.
F. Bloch J. Ahrens J. R. M. Annand R. Beck L. S. Fog D. Hornidge S. Janssen M. Kotulla B. Krusche J. C. McGeorge I. J. D. MacGregor J. G. Messchendorp V. Metag R. Novotny R. O. Owens M. Pfeiffer M. Rost R. Sanderson S. Schadmand A. Thomas D. P. Watts 《The European Physical Journal A - Hadrons and Nuclei》2007,32(2):219-228
The photoproduction of π
0
π
0 and π
0
π
± pairs off 40Ca has been investigated with the TAPS detector using the Glasgow photon tagging spectrometer at the Mainz MAMI accelerator.
Data have been taken for incident photon energies in the energy range from 200-820MeV. Total cross-sections have been extracted
from threshold up to the maximum photon energy and invariant-mass distributions of the pion pairs have been obtained for incident
photon energies between 400-500MeV and 500-550MeV. The double π
0 invariant-mass distributions show some relative enhancement with respect to the mixed charge channel at small invariant masses.
The effects are smaller than previously observed for lead nuclei and the distributions do not significantly deviate from carbon
data. The data are in good agreement with the results of recent calculations in the framework of the BUU model, with careful
treatment of final-state interaction effects but without an explicit in-medium modification of scalar, iso-scalar pion pairs.
This means that for Ca most of the experimentally observed effect can be explained by final-state interactions. Only at low
incident photon energies there is still a small low mass enhancement of the double π
0 data over the BUU results. 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.
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
6.
G. Fäldt U. Tengblad C. Wilkin 《The European Physical Journal A - Hadrons and Nuclei》2006,28(2):245-249
The cross-section for the dd →^4Heππ reaction is estimated near threshold in a two-step model where a pion created in a first interaction produces a second
pion in a subsequent interaction. This approach, which describes well the rates of 2π and η production in the pd →^3Heππ and dd →^4Heη reactions, leads to predictions that are much too low compared to experiment. Alternatives to this and the double-Δ
model will have to be sought to explain these data. 相似文献
7.
E Oset D Cabrera H C Chiang C Garcia Recio S Hirenzaki S S Kamalov J Nieves Y Okumura A Ramos H Toki M J Vicente Vacas 《Pramana》2001,57(2-3):417-431
In this talk we briefly describe some basic elements of chiral perturbation theory, xPT, and how the implementation of unitarity and other novel elements lead to a better expansion of the T-matrix for meson-meson and meson-baryon interactions. Applications are then done to the ππ interaction in nuclear matter
in the scalar and vector channels, antikaons in nuclei and K
− atoms, and how the φ meson properties are changed in a nuclear medium. 相似文献
8.
H. -W. Hammer D. R. Phillips L. Platter 《The European Physical Journal A - Hadrons and Nuclei》2007,32(3):335-347
We use an effective field theory (EFT) which contains only short-range interactions to study the dependence of a variety of
three-nucleon observables on the pion mass. The pion-mass dependence of input quantities in our “pionless” EFT is obtained
from a recent chiral EFT calculation. To the order we work at, these quantities are the 1
S
0 scattering length and effective range, the deuteron binding energy, the 3
S
1 effective range, and the binding energy of one three-nucleon bound state. The chiral EFT input we use has the inverse 3
S
1 and 1
S
0 scattering lengths vanishing at m
π
crit = 197.8577 MeV. At this “critical” pion mass, the triton has infinitely many excited states with an accumulation point at
the three-nucleon threshold. We compute the binding energies of these states up to next-to-next-to-leading order in the pionless
EFT and study the convergence pattern of the EFT in the vicinity of the critical pion mass. Furthermore, we use the pionless
EFT to predict how doublet and quartet nd scattering lengths depend on m
π in the region between the physical pion mass and m
π = m
π
crit . 相似文献
9.
D. Cabrera D. Fernández-Fraile A. Gómez Nicola 《The European Physical Journal C - Particles and Fields》2009,61(4):879-892
We present a study of the π
π scattering amplitude in the σ and ρ channels at finite temperature and nuclear density within a chiral unitary framework. Meson resonances are dynamically generated
in our approach, which allows us to analyze the behavior of their associated scattering poles when the system is driven towards
chiral-symmetry restoration. Medium effects are incorporated in three ways: (a) by thermal corrections of the unitarized scattering
amplitudes, (b) by finite nuclear-density effects associated to a renormalization of the pion decay constant, and complementarily
(c) by extending our calculation of the scalar–isoscalar channel to account for finite nuclear-density and temperature effects
in a microscopic many-body implementation of pion dynamics. Our results are discussed in connection with several phenomenological
aspects relevant for nuclear-matter and heavy-ion collision experiments, such as ρ mass scaling versus broadening from dilepton spectra and chiral restoration signals in the σ channel. We also elaborate on the molecular nature of π
π resonances. 相似文献
10.
W. Melnitchouk 《The European Physical Journal A - Hadrons and Nuclei》2003,17(2):223-234
We explore the relationship between exclusive and inclusive electromagnetic scattering from the pion, focusing on the transition
region at intermediate Q2. Combining Drell-Yan data on the leading twist quark distribution in the pion with a model for the resonance region at large
x, we calculate QCD moments of the pion structure function over a range of Q2, and quantify the role of higher twist corrections. Using a parameterization of the pion elastic form factor and phenomenological
models for the π↦ρ transition form factor, we further test the extent to which local duality may be valid for the pion.
Received: 10 February 2003 / Accepted: 12 March 2003 / Published online: 27 May 2003 相似文献
11.
Vladimir G. Neudatchin Igor T. Obukhovsky Ludmila L. Sviridova Nikolai P. Yudin 《Central European Journal of Physics》2004,2(3):511-522
The pion momentum distributions (MDs) in four channels of virtual decay p→B+π, B = N, Δ, N
1/2-(1535), N
1/2+(1440) are calculated in two models, the microscopic model of 3
P
0 scalar q−q fluctuation with the pion as a composite q−q-system and the chiral semi-microscopic model of πq interaction with the pion as a structureless Goldstone boson. The results of the above models are similar for the baryon
states B = N, Δ, N
1/2-(1535) but are rather different for the Roper resonance N
1/2+(1440) which corresponds to excitation of two oscillator quanta in the nucleon. The experimental investigation of pion MDs
by means of the reaction of quasi-elastic knockout of pion by an electron of a few GeV energy p(e, e′ π)B may be very suitable for Jefferson Laboratory, Virginia (JLab). 相似文献
12.
Ye.S. Golubeva L.A. Kondratyuk M. Büscher W. Cassing V. Hejny H. Ströher 《The European Physical Journal A - Hadrons and Nuclei》2001,11(2):237-245
The ω resonance production and its π0γ decay in pA reactions close to threshold is considered within the Intranuclear Cascade (INC) model. The π0γ invariant-mass distribution shows two components which correspond to the ω decay “inside” and “outside” the nucleus, respectively.
The “inside” component is distorted by medium effects, which introduce a mass shift as well as collisional broadening for
the ω-meson and its decaying pion. The relative contribution of the “inside” component is analyzed in detail for different
kinematical conditions and nuclear targets. It is demonstrated that a measurement of the correlation in azimuthal angle between
the π0 and γ momenta allows to separate events related to the “inside”ω decay from different sources of background when uncorrelated
π0's and γ's are produced.
Received: 2 April 2001 / Accepted: 5 June 2001 相似文献
13.
G. Pancheri O. Shekhovtsova G. Venanzoni 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):458-460
Effects due to the non-pointlike behaviour of pions in the process e
+
e
-→π
+
π
-
γ can arise for hard photons in the final state. By means of a Monte Carlo event generator, which also includes the contribution
of the direct decay φ→π
+
π
-
γ, we estimate these effects in the framework of the resonance perturbation theory. We consider angular cuts used in the KLOE
analysis of the pion form factor at threshold. A method to reveal the effects of the non-pointlike behaviour of pions in a
model-independent way is proposed. 相似文献
14.
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
15.
S. Abd El-Samad R. Bilger K. -Th. Brinkmann H. Clement M. Dietrich E. Doroshkevich S. Dshemuchadse A. Erhardt W. Eyrich A. Filippi H. Freiesleben M. Fritsch R. Geyer A. Gillitzer J. Hauffe D. Hesselbarth R. Jaekel B. Jakob L. Karsch K. Kilian H. Koch J. Kress E. Kuhlmann S. Marcello S. Marwinski R. Meier K. Möller H. P. Morsch L. Naumann E. Roderburg P. Schönmeier M. Schulte-Wissermann W. Schroeder M. Steinke F. Stinzing G. Y. Sun J. Wächter G. J. Wagner M. Wagner U. Weidlich A. Wilms S. Wirth G. Zhang P. Zupranski 《The European Physical Journal A - Hadrons and Nuclei》2006,30(2):443-453
The single-pion production reactions pp → dπ+, pp → npπ+ and pp → ppπ0 were measured at a beam momentum of 0.95GeV/c ( T
p ≈ 400MeV) using the short version of the COSY-TOF spectrometer. The implementation of a central calorimeter provided particle
identification, energy determination and neutron detection in addition to time-of-flight and angle measurements. Thus, all
pion production channels were recorded with 1-4 overconstraints. The total and differential cross-sections obtained are compared
to previous data and theoretical calculations. Main emphasis is put on the discussion of the ppπ0 channel, where we obtain angular distributions different from previous experimental results, however, partly in good agreement
with recent phenomenological and theoretical predictions. In particular, we observe very large anisotropies for the π0 angular distributions in the kinematical region of small relative proton momenta revealing there a dominance of proton spinflip
transitions associated with π0
s and d partial waves and emphasizing the important role of π0
d-waves. 相似文献
16.
B. L. Birbrair T. M. Avdeeva Yu. A. Kalashnikov 《Zeitschrift für Physik A Hadrons and Nuclei》1996,355(1):427-432
Starting from the extended tree-level model for theπN interaction [1] (hereafter ETLM) we demonstrated that the observed local pion-nucleus repulsion arises from the reduction
of baryon masses in nucleus due to strong scalar nuclear field [2]. This is rather convincing argument in favour of nuclear
relativism because such repulsion has no explanation within the traditional nonrelativistic approach to nucleus.
Communicated by V.V. Anisovich 相似文献
17.
L.V. Fil'kov V.L. Kashevarov 《The European Physical Journal A - Hadrons and Nuclei》1999,5(3):285-292
The Compton scattering on a charged pion and the process γγ→π0π0 are studied using the dispersion relations. Unknown parameters of the S-wave ππ interaction and a sum and a difference of the π0 meson polarizabilities are found from a fit to the experimental data for the γγ→π0π0 process. The found parameters of the ππ interaction are used for the calculation of the cross section of the elastic γπ± scattering. The analysis of the obtained results shows that the experimental data for the elastic γπ± scattering in the energy region up to 1 GeV together with the data for the γγ→π0π0 process could be used both for a determination of the pion polarizability values and for study of the S-wave ππ interaction.
Received: 20 November 1998 / Revised version: 22 January 1999 相似文献
18.
The reaction γd↦π+nn is calculated up to order χ5/2 in chiral perturbation theory, where χ denotes the ratio of the pion to the nucleon mass. Special emphasis is put on the
role of nucleon recoil corrections that are the source of contributions with fractional power in χ. Using the known near-threshold
production amplitude for γp↦π+n as the only input, the total cross-section for γd↦π+nn is described very well. A conservative estimate suggests that the theoretical uncertainty for the transition operator amounts
to 3% for the computed amplitude near threshold. 相似文献
19.
Scales in nuclear matter: Chiral dynamics with pion nucleon form factors<Superscript>⋆</Superscript>
N. Kaiser M. Mühlbauer W. Weise 《The European Physical Journal A - Hadrons and Nuclei》2007,31(1):53-60
A systematic calculation of nuclear matter is performed which includes the long-range correlations between nucleons arising
from one- and two-pion exchange. Three-body effects from 2π exchange with excitations of virtual Δ(1232)-isobars are also
taken into account in our diagrammatic calculation of the energy per particle ˉ(k
f). In order to eliminate possible high-momentum components from the interactions we introduce at each pion-baryon vertex a
form factor of monopole type. The empirical nuclear matter saturation point, ρ0 ≃ 0.16fm^-3, ˉ0 ≃ - 16MeV, is well reproduced with a monopole mass of Λ ≃ 4πf
π ≃ 1.16GeV. As in the recent approach based on the universal low-momentum NN potential V
low-k, the inclusion of three-body effects is crucial in order to achieve saturation of nuclear matter. We demonstrate that the
dependence of the pion exchange contributions to ˉ(k
f) on the “resolution” scale Λ can be compensated over a wide range of Λ by counterterms with two “running” contact couplings.
As a further application we study the in-medium chiral condensate 〈ˉq〉(ρ) beyond the linear density approximation. For ρ ⩽ 1.5ρ0 we find small corrections from the derivative dˉ(k
f)/dm
π, which are stable against variations of the monopole regulator mass Λ. 相似文献