Showing papers on "Pion published in 1998"
TL;DR: In this paper, the pion Bethe-Salpeter amplitude and the quark propagator in the chiral limit were derived and the axial-vector Ward-Takahashi identity was preserved.
393 citations
TL;DR: In this article, single transverse-spin asymmetries for hadronic pion production at large transverse momenta using QCD factorization were analyzed and the leading contributions were naturally produced by twist-3 parton correlation functions that couple quark fields and gluon field strengths.
Abstract: We analyze single transverse-spin asymmetries for hadronic pion production at large transverse momenta using QCD factorization. In the large ${x}_{F}$ region, leading contributions to the asymmetries are naturally produced by twist-3 parton correlation functions that couple quark fields and gluon field strengths. With a simple model for these matrix elements, leading-order asymmetries calculated from QCD are consistent with data on pion production from Fermilab, and can be used to predict single-spin asymmetries at BNL RHIC. We argue that our perturbative calculation for the asymmetries is relevant to pion transverse momenta as low as a few GeV.
283 citations
TL;DR: In this article, the pion-nucleon Lagrangian was constructed to third order in small momenta based on relativistic chiral perturbation theory and all terms up to and including order 1 m 2 with fixed and free coefficients.
265 citations
TL;DR: In this paper, the vertical sea-level muon spectrum at energies above 1 GeV and the muon intensities at depths up to 18 km w.r.t. in different rocks and in water are calculated.
Abstract: The vertical sea-level muon spectrum at energies above 1 GeV and the muon intensities at depths up to 18 km w.e. in different rocks and in water are calculated. The results are particularly collated with a great body of the ground-level, underground, and underwater muon data. In the hadron-cascade calculations, we take into account the logarithmic growth with energy of inelastic cross sections and pion, kaon, and nucleon generation in pion-nucleus collisions. For evaluating the prompt-muon contribution to the muon flux, we apply the two phenomenological approaches to the charm production problem: the recombination quark-parton model and the quark-gluon string model. To solve the muon transport equation at large depths of a homogeneous medium, we used a semianalytical method, which allows the inclusion of an arbitrary ~decreasing! muon spectrum at the medium boundary and real energy dependence of both continuous and discrete muon energy losses. The method is checked for accuracy by direct Monte Carlo calculation. Whenever possible, we give simple fitting formulas describing our numerical results. @S0556-2821~98!00313-0# PACS number~s!: 13.85.Tp, 96.40.Tv
239 citations
TL;DR: Hadron pair production gamma* gamma -> h hbar in the region where the c.m. energy is much smaller than the photon virtuality can be described in a factorized form, as the convolution of a partonic handbag diagram and generalized distribution amplitudes which are new non-perturbative functions describing the exclusive fragmentation of a quark-antiquark pair into two hadrons as discussed by the authors.
Abstract: Hadron pair production gamma* gamma -> h hbar in the region where the c.m. energy is much smaller than the photon virtuality can be described in a factorized form, as the convolution of a partonic handbag diagram and generalized distribution amplitudes which are new non-perturbative functions describing the exclusive fragmentation of a quark-antiquark pair into two hadrons. Scaling behavior and a selection rule on photon helicity are signatures of this mechanism. The case where h is a pion is emphasized.
193 citations
TL;DR: In this article, the authors present results from a Monte Carlo simulation of non-linear shock structure and acceleration coupled with photon emission in shell-like supernova remnants (SNRs) and calculate ion and electron distributions that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron emission.
Abstract: Supernova remnants (SNRs) are widely believed to be the principal source of galactic cosmic rays. Such energetic particles can produce gamma-rays and lower energy photons via interactions with the ambient plasma. In this paper, we present results from a Monte Carlo simulation of non-linear shock structure and acceleration coupled with photon emission in shell-like SNRs. These non-linearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays which deviate from pure power-laws. Such deviations are crucial to acceleration efficiency and spectral considerations, producing GeV/TeV intensity ratios that are quite different from test particle predictions. The Sedov scaling solution for SNR expansions is used to estimate important shock parameters for input into the Monte Carlo simulation. We calculate ion and electron distributions that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron emission, yielding complete photon spectra from radio frequencies to gamma-ray energies. The cessation of acceleration caused by the spatial and temporal limitations of the expanding SNR shell in moderately dense interstellar regions can yield spectral cutoffs in the TeV energy range; these are consistent with Whipple's TeV upper limits on unidentified EGRET sources. Supernova remnants in lower density environments generate higher energy cosmic rays that produce predominantly inverse Compton emission at super-TeV energies; such sources will generally be gamma-ray dim at GeV energies.
177 citations
TL;DR: In this paper, the two-pion exchange diagrams with single and double delta-isobar excitation contributing to elastic NN-scattering were evaluated, within one-loop chiral perturbation theory, and they were shown to produce the correct amount of isoscalar central attraction as needed in the peripheral partial waves with L ⩾ 3.
176 citations
TL;DR: Hadron pair production in the region where the energy is much smaller than the photon virtuality can be described in a factorized form, as the convolution of a partonic handbag diagram and generalized distribution amplitudes which are new nonperturbative functions describing the exclusive fragmentation of a quark-antiquark pair into two hadrons as discussed by the authors.
Abstract: Hadron pair production ${\ensuremath{\gamma}}^{*}\ensuremath{\gamma}\ensuremath{\rightarrow}h\overline{h}$ in the region where the c.m. energy is much smaller than the photon virtuality can be described in a factorized form, as the convolution of a partonic handbag diagram and generalized distribution amplitudes which are new nonperturbative functions describing the exclusive fragmentation of a quark-antiquark pair into two hadrons. Scaling behavior and a selection rule on photon helicity are signatures of this mechanism. The case where $h$ is a pion is emphasized.
175 citations
TL;DR: In this paper, the authors consider the assumption that a tachyonic gluon mass imitates short-distance nonperturbative physics of QCD and propose modifications of the QCD sum rules for correlators of currents with various quantum numbers, which lead to a slight reduction of the values of the running light quark masses from the pseudo-scalar sum rules and of alpha_s(M_\tau) from tau decay data.
Abstract: We consider the assumption that a tachyonic gluon mass imitates short-distance nonperturbative physics of QCD. The phenomenological implications include modifications of the QCD sum rules for correlators of currents with various quantum numbers. The new 1/Q^2 terms allow to resolve in a natural way old puzzles in the pion and scalar-gluonium channels. They lead to a slight reduction of the values of the running light quark masses from the (pseudo)scalar sum rules and of alpha_s(M_\tau) from tau decay data. Further tests can be provided by precision measurements of the correlators on the lattice and by the e^+e^- --> hadrons data.
167 citations
TL;DR: In the absence of mixing with other states, the radiatively generated splitting is in the range 200{endash}350 MeV for m{approx_gt} (1) /(2) thinspm{sub Z} as mentioned in this paper.
Abstract: Massive vectorlike electroweak doublets are generic in many extensions of the standard model. Even though one member of the doublet is necessarily electrically charged these particles are not easily detected in collider experiments. The neutral and charged states within the doublet are split by electroweak symmetry breaking. In the absence of mixing with other states, the radiatively generated splitting is in the range 200{endash}350 MeV for m{approx_gt} (1) /(2) thinspm{sub Z} . The charged state decays to the neutral one with an O()>cm decay length, predominantly by emission of a soft charged pion. The best possibility to detect these massive charged particles is to trigger on hard initial state radiation and search for two associated soft charged pions with displaced vertices. The mass reach for this process at LEPII is limited by luminosity rather than kinematics. {copyright} {ital 1998} {ital The American Physical Society}
153 citations
TL;DR: In this article, the two-pion exchange diagrams with single and double delta-isobar excitation contributing to elastic NN-scattering were evaluated within one-loop chiral perturbation theory.
Abstract: We evaluate, within one-loop chiral perturbation theory, the two-pion exchange diagrams with single and double delta-isobar excitation contributing to elastic NN-scattering. We find that virtual $\Delta$-excitation processes (in the static limit) produce the correct amount of isoscalar central attraction as needed in the peripheral partial waves with $L\geq 3$. Furthermore we compute the two-loop diagrams involving the $\pi\pi$-interaction (so-called correlated $2\pi$-exchange). Contrary to common believe these processes lead to negligibly small and repulsive corrections to the NN-potential. The exchange of vector mesons ($\rho, \omega$) turns out to be important for the F-wave phase shifts above $T_{lab}=150$ MeV. Without adjustable parameters we are able to reproduce the empirical NN phase shifts up to 350 MeV for $L\geq 3$ and up to about (50--80) MeV for the D-waves. This is therefore the characteristic window in which the NN-interaction is basically governed by chiral symmetry. Not surprisingly, the lower partial waves require non-perturbative methods and additional short-distance parametrizations of the NN-dynamics.
TL;DR: In this article, the pion-nucleon scattering with a chiral Lagrangian of pions, nucleons, and $\ensuremath{\Delta}$ isobars was studied.
Abstract: We study pion-nucleon scattering with a chiral Lagrangian of pions, nucleons, and $\ensuremath{\Delta}$ isobars. The scattering amplitude is evaluated to one-loop ${Q}^{3}$ order, where $Q$ is a generic small momentum, using an approach which is equivalent to heavy baryon chiral perturbation theory. We obtain a good fit to the experimental phase shifts for pion center-of-mass kinetic energies up to 100 MeV. A $\ensuremath{\sigma}$ term greater than 45 MeV is favored, but the value is not well determined.
TL;DR: In this article, heavy mass methods, previously applied to chiral perturbation theory calculations involving the interactions of nucleons and pions, can be generalized to include interactions with the (1232) in a systematic formalism which they call the ''small-scale expansion''.
Abstract: We demonstrate how heavy mass methods, previously applied to chiral perturbation theory calculations involving the interactions of nucleons and pions, can be generalized to include interactions with the (1232) in a systematic formalism which we call the `small-scale expansion'.
TL;DR: In this article, the vector and scalar form factors of the pion to two loops in Chiral Perturbation Theory were calculated and the unknown (p6) constants were estimated using resonance exchange.
Abstract: We calculate the vector and scalar form factors of the pion to two loops in Chiral Perturbation Theory. We estimate the unknown (p6) constants using resonance exchange. We make a careful comparison to the available data and determine two (p4) constants rather precisely, and two (p6) constants less precisely. We also use Chiral Perturbation Theory to two loops to extract in a model-independent manner the charge radius of the pion from the available data, and obtain r2πV = 0.437±0.016 fm2.
TL;DR: In this paper, the authors studied the low energy expansion of the nucleon's electroweak form factors in the framework of an effective chiral Lagrangian including pions, nucleons and the Δ(1232).
TL;DR: In this article, the low energy scattering of pions was investigated in the presence of electromagnetic interactions at leading order and at next-to-leading order for the amplitudes involving at most one pair of charged pions.
TL;DR: In this paper, the authors calculate the cross sections for dissociation of pions and φ mesons under the framework of a meson exchange model and find that these cross sections are small at center-of-mass energies less than 1 GeV above threshold, and that dissociation rates are less than $0.01\mathrm{fm}/c$ in a thermal meson gas at temperatures where such a description makes sense.
Abstract: We calculate the cross sections for dissociation of $J/\ensuremath{\psi}$ by pions and \ensuremath{\rho} mesons within the framework of a meson exchange model. We find that these cross sections are small at center-of-mass energies less than 1 GeV above threshold, and that dissociation rates are less than $0.01\mathrm{fm}/c$ in a thermal meson gas at temperatures where such a description makes sense.
TL;DR: In this article, the free parameters were determined by a fit to the most recent data for π + and π 0 production on the proton and the neutron, in the energy range 160 MeV ⊽ E γ ⩽ 420 MeV, and the position of the resonance pole was obtained in exellent agreement with pion-nucleon scattering.
TL;DR: In this paper, the Kogut-Susskind quark action was extended to full QCD simulations, and it was shown that the improvements previously demonstrated in the quenched approximation also apply to dynamical quark simulations.
Abstract: We extend tests of {open_quotes}Naik{close_quotes} and {open_quotes}fat link{close_quotes} improvements of the Kogut-Susskind quark action to full QCD simulations, and verify that the improvements previously demonstrated in the quenched approximation also apply to dynamical quark simulations. We extend the study of flavor symmetry improvement to the complete set of pions and find that the nonlocal pions are significantly heavier than the local non-Goldstone pion. These results can be used to estimate the lattice spacing necessary for realistic simulations with this action. {copyright} {ital 1998} {ital The American Physical Society}
TL;DR: In this paper, a model for the NN → NNππ reaction was developed and cross sections for the different charged channels were evaluated, and a rough agreement with the experimental data was obtained in most channels.
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TL;DR: In this article, a statistical model of the early stage of central nucleus-nucleus (A+A) collisions is developed, which leads to the conclusion that a Quark Gluon Plasma is created in central nucleus nucleus collisions at the SPS.
Abstract: A statistical model of the early stage of central nucleus--nucleus (A+A) collisions is developed. We suggest a description of the confined state with several free parameters fitted to a compilation of A+A data at the AGS. For the deconfined state a simple Bag model equation of state is assumed. The model leads to the conclusion that a Quark Gluon Plasma is created in central nucleus--nucleus collisions at the SPS. This result is in quantitative agreement with existing SPS data on pion and strangeness production and gives a natural explanation for their scaling behaviour. The localization and the properties of the transition region are discussed. It is shown that the deconfinement transition can be detected by observation of the characteristic energy dependence of pion and strangeness multiplicities, and by an increase of the event--by--event fluctuations. An attempt to understand the data on J/psi production in Pb+Pb collisions at the SPS within the same approach is presented.
TL;DR: In this article, an elastic and proton-dissociative reaction was studied for photon-proton center-of-mass energies in the range of 50 < W < 100.
Abstract: Elastic and proton-dissociative
$\rho^0$
photoproduction (
$\gamma p \rightarrow \rho^0 p$
,
$\gamma p \rightarrow \rho^0 N$
, respectively, with
$\rho^0 \rightarrow \pi^+\pi^-$
) has been studied in
$ep$
interactions at HERA for photon-proton centre-of-mass energies in the range
$50 < W < 100$
GeV and for
$|t| < 0.5$
GeV
$^2$
, where
$t$
is the square of the four-momentum transfer at the proton vertex; the results on the proton-dissociative reaction are presented for masses of the dissociated proton system in the range
$M_N^2 < 0.1 W^2$
. For the elastic process, the
$\pi^+\pi^-$
invariant mass spectrum has been investigated as a function of
$t$
. As in fixed target experiments, the
$\rho^0$
resonance shape is asymmetric; this asymmetry decreases with increasing
$|t|$
, as expected in models in which the asymmetry is ascribed to the interference of resonant and non-resonant
$\pi^+\pi^-$
production. The cross section has been studied as a function of
$W$
; a fit to the resonant part with the form
$W^a$
gives
$a=0.16\pm 0.06 \mbox{(stat.)} ^{+0.11}_{-0.15} \mbox{(syst.)}$
. The resonant part of the
$\gamma p \rightarrow \pi^+\pi^- p$
cross section is
$11.2\pm 0.1 \mbox{(stat.)} ^{+1.1}_{-1.2} \mbox{(syst.)}$
$\mu$
b at
$\langle W\rangle=71.7$
GeV. The
$t$
dependence of the cross section can be described by a function of the type
$A_{\rho} \exp{(-b_{\rho} |t| + c_{\rho} t^2)}$
with
$b_{\rho}= 10.9 \pm 0.3$
(stat.)
$^{+1.0}_{-0.5}$
(syst.) GeV
$^{-2}$
and
$c_{\rho}= 2.7 \pm 0.9$
(stat.)
$^{+1.9}_{-1.7}$
(syst.) GeV
$^{-4}$
. The
$t$
dependence has also been studied as a function of
$W$
and a value of the slope of the pomeron trajectory
$\alpha_{\mathbb{P}}^{\prime}= 0.23 \pm 0.15 \mbox{(stat.)} ^{+0.10}_{-0.07} \mbox{(syst.)}$
GeV
$^{-2}$
has been deduced. The
$\rho^0$
spin density matrix elements
$r^{04}_{00}$
,
$r^{04}_{1-1}$
and
${\frak R}{e[r^{04}_{10}]}$
have been measured and found to be consistent with expectations based on
$s$
-channel helicity conservation. For proton-dissociative
$\pi^+\pi^-$
photoproduction in the
$\rho^0$
mass range, the distributions of the two-pion invariant mass,
$W$
and the polar and azimuthal angles of the pions in the helicity frame are the same within errors as those for the elastic process. The
$t$
distribution has been fitted to an exponential function with a slope parameter
$5.8 \pm 0.3 \mbox{(stat.)} \pm 0.5 \mbox{(syst.)}$
GeV
$^{-2}$
. The ratio of the elastic to proton-dissociative
$\rho^0$
photoproduction cross section is
$2.0 \pm 0.2 \mbox{(stat.)} \pm 0.7 \mbox{(syst.)}$
.
01 Jan 1998
TL;DR: In this article, a statistical model of the early stage of central nucleus-nucleus (A+A) collisions is developed and a description of the confined state with several free parameters fitted to a compilation of A+A data at the AGS is presented.
Abstract: A statistical model of the early stage of central nucleus–nucleus (A+A) collisions is developed. We suggest a description of the confined state with several free parameters fitted to a compilation of A+A data at the AGS. For the deconfined state a simple Bag model equation of state is assumed. The model leads to the conclusion that a Quark Gluon Plasma is created in central nucleus–nucleus collisions at the SPS. This result is in quantitative agreement with existing SPS data on pion and strangeness production and gives a natural explanation for their scaling behaviour. The localization and the properties of the transition region are discussed. It is shown that the deconfinement transition can be detected by observation of the characteristic energy dependence of pion and strangeness multiplicities, and by an increase of the event–by–event fluctuations. An attempt to understand the data on J/ production in Pb+Pb collisions at the SPS within the same approach is presented.
TL;DR: In this article, the azimuthal angular distribution of K+ mesons has been measured in Au + Au collisions at 1 AGeV and the strength of the anisotropy of the K+ emission is comparable to the one of pions.
Abstract: The azimuthal angular distribution of K+ mesons has been measured in Au + Au collisions at 1 AGeV. In peripheral and semi-central collisions, K+ mesons preferentially are emitted perpendicular to the reaction plane. The strength of the azimuthal anisotropy of K+ emission is comparable to the one of pions. No in-plane flow was found for K+ mesons near projectile and target rapidity.
TL;DR: In this paper, the authors considered several single spin asymmetries in inclusive, transversely polarized proton(antiproton) -proton processes as higher twist QCD contributions, taking into account spin and intrinsic transverse momentum effects in the quark distribution functions.
Abstract: We consider several single spin asymmetries in inclusive, transversely polarized proton(antiproton) - proton processes as higher twist QCD contributions, taking into account spin and intrinsic transverse momentum effects in the quark distribution functions. This approach has been previously applied to the description of the single spin asymmetries observed in transversely polarized proton - proton -> pion + X reactions and all its parameters fixed: we give here predictions for new processes, which agree with experiments for which data are available, and suggest further possible measurements.
TL;DR: In this article, the authors calculate the nonanalytic one-loop corrections of some physical quantities: the chiral condensate, weak decay constants, Goldstone boson masses, B{sub K, and the K{sup + r_arrow} decay amplitude using partially quenched chiral perturbation theory.
Abstract: Partially quenched theories are theories in which the valence- and sea-quark masses are different. In this paper we calculate the nonanalytic one-loop corrections of some physical quantities: the chiral condensate, weak decay constants, Goldstone boson masses, B{sub K}, and the K{sup +}{r_arrow}{pi}{sup +}{pi}{sup 0} decay amplitude, using partially quenched chiral perturbation theory. Our results for weak decay constants and masses agree with, and generalize, results of previous work by Sharpe. We compare B{sub K} and the K{sup +} decay amplitude with their real-world values in some examples. For the latter quantity, two other systematic effects that plague lattice computations, namely, finite-volume effects and unphysical values of the quark masses and pion external momenta, are also considered. We find that typical one-loop corrections can be substantial. {copyright} {ital 1998} {ital The American Physical Society}
TL;DR: In this article, a non-local version of the NJL model based on a separable quark-quark interaction is considered, and the interaction is extended to include terms that bind vector and axial-vector mesons.
TL;DR: In this article, the complete effective chiral pion-nucleon Lagrangian in the presence of virtual photons to one loop was constructed for scattering of neutral pions off nucleons.
TL;DR: In this article, a gauge invariant model based on the Regge trajectory exchanges was proposed to describe the forward charged pion electroproduction reactions above the resonance region and its sensitivity to meson form factors was investigated in kinematics.
Abstract: A gauge invariant model based on $\ensuremath{\pi}$ and $\ensuremath{\rho}$ Regge trajectory exchanges provides a successful and economical description of the forward charged pion electroproduction reactions above the resonance region. Its sensitivity to the meson form factors is investigated in kinematics where the pion electromagnetic form factor will be extracted at high ${Q}^{2}$ at TJNAF. An extension to ${K}^{+}$ electroproduction is presented.
TL;DR: In this article, two-pion correlations from Pb+Pb collisions at 158 GeV/c per nucleon were measured by the NA44 experiment at CERN, and multidimensional fits characterized the emission volume, which was found to be larger than in S-induced collisions.
Abstract: Two-pion correlations from Pb+Pb collisions at 158 GeV/c per nucleon are measured by the NA44 experiment at CERN. Multidimensional fits characterize the emission volume, which is found to be larger than in {ital S}-induced collisions. Comparison to the RQMD model is used to relate the fit parameters to the actual emission volume. {copyright} {ital 1998} {ital The American Physical Society}