共查询到20条相似文献,搜索用时 93 毫秒
1.
A calculation of the current-quark mass dependence of hadron masses can help in using observational data to place constraints
on the variation of nature’s fundamental parameters. A hadron’s σ-term is a measure of this dependence. The connection between
a hadron’s σ-term and the Feynman-Hellmann theorem is illustrated with an explicit calculation for the pion using a rainbow-ladder
truncation of the Dyson-Schwinger equations: in the vicinity of the chiral limit σπ = mπ/2. This truncation also provides a decent estimate of σρ because the two dominant self-energy corrections to the ρ-meson’s mass largely cancel in their contribution to σρ. The truncation is less accurate for the ω, however, because there is little to compete with an ω → ρπ self-energy contribution
that magnifies the value of σω by ≲25%. A Poincaré-covariant Faddeev equation, which describes baryons as composites of confined-quarks and -nonpointlike-diquarks,
is solved to obtain the current-quark mass dependence of the masses of the nucleon and Δ, and thereby σN and σΔ. This “quark-core” piece is augmented by the “pion cloud” contribution, which is positive. The analysis yields σN ≃ 60 MeV and σΔ ≃ 50 MeV. 相似文献
2.
A ππ, ˉKK, and ρρ(ωω) fully coupled channel model is used to predict the lowest isospin S, P, D, F-wave phase shifts and inelasticities
for elastic ππ scattering from threshold to 2.0 GeV. As input the S-matrix is required to exhibit poles corresponding to the
meson resonance table of the Particle Data Group. As expected, the ππ inelasticity is very strongly related to the opening
of the ˉK channel near 1 GeV, and the opening of ρρ(4π) and ωω(6π) channels in the 1.5 GeV region. The predictions of this model are
compared to the various elastic ππ→ππ amplitudes, that were obtained from analyses of π− p →π−π+n data. The role of the various resonances, in particular the glueball candidate f
0(1500) and the f
J(1710) is investigated.
Received: 19 November 1997 相似文献
3.
M.V. Polyakov V.V. Vereshagin 《The European Physical Journal A - Hadrons and Nuclei》2001,12(3):349-352
We derive and analyse inequalities relating masses of the lightest ππ resonances (ρ and σ) to coupling constants of the effective
chiral Lagrangian in the limit of a large number of colours.
Received: 15 July 2001 / Accepted: 23 October 2001 相似文献
4.
A. Martínez Torres K. P. Khemchandani E. Oset 《The European Physical Journal A - Hadrons and Nuclei》2008,36(2):211-218
We evaluate the “σ " exchange contribution to the ˉN → ˉN scattering within a chiral unitary approach. We show that the chiral transition potentials for ππ → Kˉ in the t -channel lead to a “σ " contribution that vanishes in the ˉ forward direction and, hence, would produce a null “σ " exchange
contribution to the K- optical potential in nuclear matter in a simple impulse approximation. This is a consequence of the fact that the leading-order
chiral Lagrangian gives an I = 0 ππ → Kˉ amplitude proportional to the squared momentum transfer, q2. This finding poses questions on the meaning or the origin of “σ " exchange potentials used in relativistic mean-field approaches
to the K- nuclear self-energy. This elementary “σ ” exchange potential in ˉN → ˉN is compared to the Weinberg-Tomozawa term and is found to be smaller than the present theoretical uncertainties but will
be relevant in the future when aiming at fitting increasingly more accurate data. 相似文献
5.
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
6.
R. Ya. Kezerashvili V. S. Boyko 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):662-664
We study the meson exchange currents (MEC) mechanism for the pion double-charge exchange (DCX) reaction in a composite-meson
model. The model assumes that the mesons are two-quark systems and can interact with each other only through quark loops.
The contributions of the ρ, σ, and f0 mesons, the four-quark box diagram as well as a contact diagram has been taken into account. It is shown that the contribution
of the ρ, σ, and f0 mesons increases the forward scattering cross-section in an average by 25% and decreases with energy. 相似文献
7.
Martin Hoferichter Daniel R. Phillips Carlos Schat 《The European Physical Journal C - Particles and Fields》2011,71(9):1743
Starting from hyperbolic dispersion relations, we derive a system of Roy–Steiner equations for pion Compton scattering that
respects analyticity, unitarity, gauge invariance, and crossing symmetry. It thus maintains all symmetries of the underlying
quantum field theory. To suppress the dependence of observables on high-energy input, we also consider once- and twice-subtracted
versions of the equations, and identify the subtraction constants with dipole and quadrupole pion polarizabilities. Based
on the assumption of Mandelstam analyticity, we determine the kinematic range in which the equations are valid. As an application,
we consider the resolution of the γγ→ππ partial waves by a Muskhelishvili–Omnès representation with finite matching point. We find a sum rule for the isospin-two
S-wave, which, together with chiral constraints, produces an improved prediction for the charged-pion quadrupole polarizability
(a2-b2)p±=(15.3±3.7)×10-4(\alpha_{2}-\beta_{2})^{\pi^{\pm}}=(15.3\pm3.7)\times 10^{-4} fm5. We investigate the prediction of our dispersion relations for the two-photon coupling of the σ-resonance Γ
σγγ
. The twice-subtracted version predicts a correlation between this width and the isospin-zero pion polarizabilities, which
is largely independent of the high-energy input used in the equations. Using this correlation, the chiral perturbation theory
results for pion polarizabilities, and our new sum rule, we find Γ
σγγ
=(1.7±0.4) keV. 相似文献
8.
G. Y. Wolf 《Pramana》2006,66(4):781-790
One consequence of the chiral restoration is the mixing of parity partners. We look for a possible signature of the mixing
of vector and axial vector mesons in heavyion collisions. We suggest an experimental method for its observation. The dynamical
evolution of the heavy-ion collision is described by a transport equation of QMD-type evolving nucleons,N* and Δ resonances, Λ’s and gS baryons, and furthermore,π’s,η’sρ’sσ’sΩ’s and kaons with their isospin degrees of freedom. The input cross-sections and resonance parameters of the model are fitted
to the available nucleon-nucleon and pion-nucleon cross-sections 相似文献
9.
We calculate the process np → ηd near threshold using a separable potential model of the coupled ηN - πN - ππN subystems, and a relativistic three-body calculation for the ηd scattering amplitude. The ππN channels are represented by an effective σN channel, and we compare the case where the σ and π masses are related by m
σ = 2m
π and no width is considered, to another where the mass and width of the σ -meson are taken from ππ scattering data. The np → ηd cross-section can be well described up to about 60MeV by models where the real part of the ηN scattering length lies between 0.4≤Re(a
ηN)≤0.6 fm which allows us to determine the s -wave ηN scattering amplitude for -60≤E≤60 MeV. 相似文献
10.
M. Tytgat 《The European Physical Journal A - Hadrons and Nuclei》2007,31(4):451-453
Hard exclusive production of mesons in deep-inelastic scattering allows one to probe the so-far unknown Generalized Parton
Distributions (GPDs) of the nucleon. The HERMES experiment has measured several different observables in exclusive meson production
by scattering the 27.6GeV HERA lepton beam off an internal fixed gaseous target. Recent results on exclusive π
+, ρ
0 and pion pair production will be presented. 相似文献
11.
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
13.
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
14.
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
15.
M. V. Polyakov 《JETP Letters》2009,90(4):228-231
We argue that the recent BaBar data on γ → π e.m. transition form factor at large photon virtuality supports the idea that
pion distribution amplitude (DA) is close to unity with ϕπ′(0)/6 ≫ 1 at a normalization point of μ = 0.6–0.8 GeV. Such pion DA can be obtained in the effective chiral quark model. The
possible flat shape of the pion DA implies that the standard expansion of the DA in Gegenbauer polynomials can be divergent.
On basis of chiral models we predict that the two-pion DA should exhibit anomalous endpoint behaviour for pions in the S-wave
and that such feature is absent for higher partial waves. The latter implies that the ρ, f
2, etc. meson DAs have no anomalous endpoint behaviour. Possible implications of such pion DA for other hard exclusive processes
are shortly discussed. 相似文献
16.
G. Chanfray D. Davesne M. Ericson M. Martini 《The European Physical Journal A - Hadrons and Nuclei》2006,27(2):191-198
We study the two-pion propagator in the nuclear medium. This quantity appears in the ππ T-matrix and we show that it also enters the QCD scalar susceptibility. The medium effects on this propagator are due to the
influence of the individual nucleon response to a scalar field through their pion clouds. This response is appreciably increased
by the nuclear environment. It produces an important convergence effect between the scalar and pseudoscalar susceptibilities,
reflecting the reshaping of the scalar strength observed in 2π production experiments. While a large modifications of the
σ propagator follows, due to its coupling to two pion states, we show that the NN potential remains instead unaffected. 相似文献
17.
We review the cross-sections for low-energy π-γ reactions in the framework of chiral perturbation theory. Charged-pion Compton scattering, π-γ → π-γ , is considered up to one-loop order where the pion's internal structure enters through the difference of the electric and
magnetic pion polarizability, απ - βπ . The ongoing COMPASS experiment aims at measuring this important structure constant with high statistics using the Primakoff
effect. In the same way, the reaction π-γ → π-π0 serves as a test of the QCD chiral anomaly (via the γ3π -coupling constant F
γ3π . Furthermore, we calculate the total cross-sections for neutral and charged pion-pair production, π-γ → π-π0π0 and π-γ → π-π+π- , which are governed by the chiral ππ -interaction. Finally, we investigate the radiative (correction) process π-γ → π-γγ and calculate the corresponding two-photon mass spectrum. This information will be useful for analyzing the π- Primakoff scattering events with photons in the final state. 相似文献
18.
D. Blaschke Yu. L. Kalinovsky A. E. Radzhabov M. K. Volkov 《Physics of Particles and Nuclei Letters》2006,3(5):327-330
The properties and temperature behavior of the π and σ bound states are studied in the framework of the nonlocal model with
a separable interaction kernel, based on the quark Dyson-Schwinger and meson Bethe-Salpeter equations. M
π(T), f
π(T), M
σ(T), and Γσ → ππ(T) are considered above and below the deconfinement and chiral restoration transitions.
Talk at the Round-Table Discussion “Searching for a Mixed Phase of Strongly Interacting Matter at the JINR Nuclotron,” Dubna,
July 7–9, 2005.
The text was submitted by the authors in English. 相似文献
19.
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
20.
S. Wolf V. Lisin R. Kondratiev A.M. Massone G. Galler J. Ahrens H.-J. Arends R. Beck M. Camen G.P. Capitani P. Grabmayr F. Härter T. Hehl P. Jennewein K. Kossert A.I. L'vov C. Molinari P. Ottonello R.O. Owens J. Peise I. Preobrajenskij S. Proff A. Robbiano M. Sanzone M. Schumacher M. Schmitz F. Wissmann 《The European Physical Journal A - Hadrons and Nuclei》2001,12(2):231-252