Search

The elastic I = 1 / 2 , s - and p -wave kaon-pion scattering amplitudes are calculated using a single ensemble of anisotropic lattice QCD gauge field configurations with N f = 2 + 1 flavors of dynamical Wilson-clover fermions at m π = 230 MeV . A large spatial extent of L = 3.7 fm enables a good energy resolution while partial wave mixing due to the reduced symmetries of the finite volume is treated explicitly. The p -wave amplitude is well described by a Breit–Wigner shape with parameters m K ⁎ / m π = 3.808 ( 18 ) and g K ⁎ K π BW = 5.33 ( 20 ) which are insensitive to the inclusion of d -wave mixing and variation of the s -wave parametrization. An effective range description of the near-threshold s -wave amplitude yields m π a 0 = − 0.353 ( 25 ) . [ABSTRACT FROM AUTHOR]

We present the first QCD-based calculation of hadronic matrix elements with penguin topology determining direct CP-violating asymmetries in D 0 → π − π + and D 0 → K − K + nonleptonic decays. The method is based on the QCD light-cone sum rules and does not rely on any model-inspired amplitude decomposition, instead leaning heavily on quark–hadron duality. We provide a Standard Model estimate of the direct CP-violating asymmetries in both pion and kaon modes and their difference and comment on further improvements of the presented computation. [ABSTRACT FROM AUTHOR]

We show that the interplay of chiral effective field theory and lattice QCD can be used in the evaluation of so-called disconnected diagrams, which appear in the study of the isoscalar and isovector channels of pion–pion scattering and have long been a major challenge for the lattice community. By means of partially-quenched chiral perturbation theory, we distinguish and analyze the effects from different types of contraction diagrams to the pion–pion scattering amplitude, including its scattering lengths and the energy-dependence of its imaginary part. Our results may be used to test the current degree of accuracy of lattice calculation in the handling of disconnected diagrams, as well as to set criteria for the future improvement of relevant lattice computational techniques that may play a critical role in the study of other interesting QCD matrix elements. [ABSTRACT FROM AUTHOR]

We calculate the width of the delta resonance at leading two-loop order in baryon chiral perturbation theory. This gives a correlation between the leading pion–nucleon–delta and pion–delta couplings, which is relevant for the analysis of pion–nucleon scattering and other processes. [ABSTRACT FROM AUTHOR]

We present a dispersive representation of the D-term form factor for hard exclusive reactions, using unsubtracted t -channel dispersion relations. The t -channel unitarity relation is saturated with the contribution of two-pion intermediate states, using the two-pion distributions amplitude for the γ ⁎ γ → π π subprocess and reconstructing the π π → N N ¯ subprocess from available information on pion-nucleon partial-wave helicity amplitudes. Results for the D-term form factor as function of t as well as at t = 0 are discussed in comparison with available model predictions and phenomenological parametrizations. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the phase structure of strongly interacting matter at non-vanishing isospin before the onset of pion condensation in the framework of the unquenched Polyakov–Quark-Meson model with quark flavors. We show results for the order parameters and all relevant thermodynamic quantities. In particular, we obtain a moderate change of the pressure with isospin at vanishing baryon chemical potential, whereas the chiral condensate decreases more appreciably. We compare the effective model to recent lattice data for the decrease of the pseudo-critical temperature with the isospin chemical potential. We also demonstrate the major role played by the value of the pion mass in the curvature of the transition line, and the need for lattice results with a physical pion mass. Limitations of the model at nonzero chemical potential are also discussed. [Copyright &y& Elsevier]

Abstract: We investigate the feasibility of studying in-medium properties of the ω meson in photoproduction experiments via the decay . We use the GiBUU transport model to compare different methods of obtaining in-medium information, such as the invariant mass spectrum, transparency ratio, excitation function and momentum spectrum. We show that the final-state interaction of the pion poses a major obstacle for the interpretation of the invariant mass spectrum. The other three observables turn out to be fairly independent of final-state interactions and thus, taken together, can give access to the ωʼs in-medium properties. [Copyright &y& Elsevier]

Abstract: We extend the usual chiral perturbation theory framework (χPT) to allow the inclusion of a light dynamical isosinglet scalar. Using lattice QCD results, and a few phenomenological inputs, we explore the parameter space of the effective theory. We discuss the S-wave pion–pion scattering lengths, extract the average value of the two light quark masses and evaluate the impact of the dynamical singlet field in the low-energy constants , and of χPT. We also show how to extract the mass and width of the sigma resonance from chiral extrapolations of lattice QCD data. [Copyright &y& Elsevier]

Abstract: We study the correlation among impact parameter, multiplicity and longitudinal rapidity of neutral and charged pions, protons and neutrons produced in the interactions of charged pions with xenon nuclei at momenta of 2.34, 3.5, 9 and 30GeV/c. The correlation is investigated using a JAM modeling code. [Copyright &y& Elsevier]

Abstract: We calculate in chiral perturbation theory the double-pion photoproduction processes and at low energies. At leading order these reactions are governed by the chiral pion-pion interaction. The next-to-leading order corrections arise from pion-loop diagrams and chiral-invariant counterterms involving the low-energy constants , , and . The pertinent production amplitudes and depending on five kinematical variables are given in closed analytical form. We find that the total cross section for neutral pion-pair production gets enhanced in the region by a factor by the next-to-leading order corrections. In contrast to this behavior the total cross section for charged pion-pair production remains almost unchanged in the region in comparison to its tree-level result. Although the dynamics of the pion-pair production reactions is much richer, this observed pattern can be understood from the different influence of the chiral corrections on the pion-pion final state interaction ( versus ). We present also results for the complete set of two-pion invariant mass spectra. The predictions of chiral perturbation theory for the processes can be tested by the COMPASS experiment which uses Primakoff scattering of high-energy pions in the Coulomb field of a heavy nucleus to extract cross sections for reactions with various final states. [Copyright &y& Elsevier]

Abstract: We evaluate the contribution of pion–pion rescattering to the and reactions. We compare our results with the close-to-threshold experimental data. The pion–pion rescattering contribution is found there to be negligible. The predictions for future experiments with PANDA detector at High Energy Storage Ring (HESR) in GSI Darmstadt are presented. It is discussed how to cut off the dominant Roper resonance and double-Δ excitation mechanisms leading to the channel in final state. Differential distributions in momentum transfers, transverse momentum, effective two-pions mass, relative azimuthal angle between pions, and pion rapidities are presented. [Copyright &y& Elsevier]

Abstract: We re-investigate the impact of isospin violation for extracting the s-wave ππ scattering phase shift difference from decays. Compared to our previous analysis in 2003, more precise experimental data and improved knowledge of low-energy constants are used. In addition, we employ a more robust data-driven method to obtain the phase shift difference . [Copyright &y& Elsevier]

Abstract: We evaluate differential distributions for exclusive scalar meson (glueball candidate) production for (FAIR@GSI) and (J-PARC@Tokai). Both QCD diffractive, pion–pion meson exchange current (MEC) components as well as double-diffractive mechanism with intermediate pionic loop are calculated for the first time in the literature. The pion–pion component, which can be reliably calculated, dominates close to the threshold while the diffractive component may take over only for larger energies. At the moment only upper limit for the QCD-diffractive component can be obtained. The diffractive component is calculated based on two-gluon impact factors as well as in the framework of Khoze–Martin–Ryskin approach proposed for diffractive Higgs boson production. Different unintegrated gluon distribution functions (UGDFs) from the literature are used. Rather large cross sections due to pion–pion fusion are predicted for PANDA energies, where the gluonic mechanism is shown to be negligible. The production of close to threshold could limit the so-called πNN form factor in the region of larger pion virtualities. We discuss in detail the two-pion background to the production of the meson. [Copyright &y& Elsevier]

Abstract: We argue that in the small x processes, in the black disc QCD regime (BDR) a very forward parton propagating through the nuclear matter should loose a significant and increasing with energy and atomic number fraction of its initial energy as a result of dominance of inelastic interactions, causality and energy–momentum conservation. We evaluate these energy losses and find them to lead to the significant suppression of the forward jet production in the central NA collisions at collider energies with a moderate suppression of recoiling jet at central rapidities. We confront our expectations with the recent RHIC data of the STAR Collaboration on the probability, P, for emission of at least one fast hadron at a central rapidity in association with production of a very forward high neutral pion in pp and dAu collisions. We calculate the A-dependence of P, and find that the data imply a strong suppression of leading pion production at central impact parameters. We also conclude that production of recoil jets in the hard subprocess is not suppressed providing further evidence for the dominance of peripheral collisions. Both features of the data are consistent with the onset of BDR. We suggest new phenomena and new observables to investigate BDR at RHIC and LHC. [Copyright &y& Elsevier]

Abstract: Total inclusive cross sections for neutral and charged pion production in proton–nucleus and nucleus–nucleus reactions have been calculated and compared to experiment. Nucleon–nucleon theoretical cross sections have been scaled up to nuclear collisions using a scaling factor similar to (A P A T)2/3, where A P and A T are the nucleon numbers of the projectile and target nuclei. Variations in the power of this scaling factor have been studied and a good fit to experiment is obtained with a small modification of the power. Theoretical cross sections are written in a form that is very suitable for immediate input into transport codes. [Copyright &y& Elsevier]

Abstract: We measured, with the KLOE detector, the spectrum of invariant mass in a sample of events with the photon at large polar angle () at a centre of mass energy around the ϕ mass. We observe in this spectrum a clear contribution from the intermediate process . A sizeable effect is also observed in the distribution of the pion forward–backward asymmetry. We use different theoretical models to fit the spectrum and we determine the f0 mass and coupling constants to the ϕ, to and to . [Copyright &y& Elsevier]

The electromagnetic corrections to the low energy scattering amplitude involving charged pions only are investigated at leading and next-to-leading orders in the two-flavour chiral expansion. As an application, the corresponding variation in the strong 2S−2P level shift is evaluated. The relative variation is of the order of 6%. [Copyright &y& Elsevier]

Due to a pion-pole contribution, the nucleon axial current matrix elements can be visualized in position space as becoming delocalized as the chiral limit is approached, with one-third of the current at distance of order 1/mπ. However, this delocalization effect will not cause calculations of gA in a finite system with standard boundary conditions (periodic for boson fields, antiperiodic for fermions) to have large finite volume effects. This is seen by calculating gA using a nonstandard current with axial quantum numbers. The matrix elements of this new current lacks a pion pole and is not delocalized; however, its zero-momentum matrix element is identical to that of the standard axial current both for infinite and finite volumes. [Copyright &y& Elsevier]

We calculate the electromagnetic corrections needed to obtain isospin invariant hadronic pion–pion s-wave scattering lengths a0, a2 from the elements acc,a0c of the s-wave scattering matrix for the (π+π−,π0π0) system at the π+π− threshold. The latter can be extracted from experiments on the π+π− atom (pionium). Our calculation uses energy-independent hadronic pion–pion potentials V0 and V2 that satisfactorily reproduce the low-energy phase shifts given by two-loop chiral perturbation theory, with the hadronic mass of the pion taken first as the charged-pion mass and then as the neutral-pion mass. We also take into account an important relativistic effect whose inclusion influences the corrections considerably. [Copyright &y& Elsevier]