Showing papers on "Pion published in 2011"

Review of lattice results concerning low-energy particle physics

Abstract: We review lattice results related to pion, kaon, D- and B-meson physics with the aim of making them easily accessible to the particle physics community. More specifically, we report on the determination of the light-quark masses, the form factor f+(0), arising in semileptonic K -> pi transition at zero momentum transfer, as well as the decay constant ratio fK/fpi of decay constants and its consequences for the CKM matrix elements Vus and Vud. Furthermore, we describe the results obtained on the lattice for some of the low-energy constants of SU(2)LxSU(2)R and SU(3)LxSU(3)R Chiral Perturbation Theory and review the determination of the BK parameter of neutral kaon mixing. The inclusion of heavy-quark quantities significantly expands the FLAG scope with respect to the previous review. Therefore, for this review, we focus on D- and B-meson decay constants, form factors, and mixing parameters, since these are most relevant for the determination of CKM matrix elements and the global CKM unitarity-triangle fit. In addition we review the status of lattice determinations of the strong coupling constant alpha_s.

Pion-pion scattering amplitude. IV. Improved analysis with once subtracted Roy-like equations up to 1100 MeV

Abstract: We improve our description of π π scattering data by imposing additional requirements on our previous fits, in the form of once-subtracted Roy-like equations, while extending our analysis up to 1100 MeV. We provide simple and ready to use parametrizations of the amplitude. In addition, we present a detailed description and derivation of these once-subtracted dispersion relations that, in the 450 to 1100 MeV region, provide an additional constraint which is much stronger than our previous requirements of forward dispersion relations and standard Roy equations. The ensuing constrained amplitudes describe the existing data with rather small uncertainties in the whole region from threshold up to 1100 MeV, while satisfying very stringent dispersive constraints. For the S0 wave, this requires an improved matching of the low and high energy parametrizations. Also for this wave we have considered the latest low energy K_(l4) decay results, including their isospin violation correction, and we have removed some controversial data points. These changes on the data translate into better determinations of threshold and subthreshold parameters which remove almost alldisagreement with previous chiral perturbation theory and Roy equation calculations below 800 MeV. Finally, our results favor the dip structure of the S0 inelasticity around the controversial 1000 MeV region.

Inclusive charged-current neutrino-nucleus reactions

Abstract: We present a model for weak charged-current induced nuclear reactions at energies of interest for current and future neutrino oscillation experiments. This model is a natural extension of the work in Refs. [1,2], where the quasielastic contribution to the inclusive electron and neutrino scattering on nuclei was analyzed. The model is based on a systematic many-body expansion of the gauge boson absorption modes that includes one, two, and even three-body mechanisms, as well as the excitation of $\ensuremath{\Delta}$ isobars. The whole scheme has no free parameters, besides those previously adjusted to the weak pion production off the nucleon cross sections in the deuteron, since all nuclear effects were set up in previous studies of photon, electron, and pion interactions with nuclei. We have discussed at length the recent charged-current quasielastic MiniBooNE cross section data, and showed that two-nucleon knockout mechanisms are essential to describing these measurements.

Production of pions, kaons and protons in pp collisions at root s=900 GeV with ALICE at the LHC

Abstract: The production of π+, π−, K+, K−, p, and \(\overline{\mathrm{p}}\) at mid-rapidity has been measured in proton-proton collisions at \(\sqrt{s} = 900~\mathrm{GeV}\) with the ALICE detector. Particle identification is performed using the specific energy loss in the inner tracking silicon detector and the time projection chamber. In addition, time-of-flight information is used to identify hadrons at higher momenta. Finally, the distinctive kink topology of the weak decay of charged kaons is used for an alternative measurement of the kaon transverse momentum (pt) spectra. Since these various particle identification tools give the best separation capabilities over different momentum ranges, the results are combined to extract spectra from pt=100 MeV/c to 2.5 GeV/c. The measured spectra are further compared with QCD-inspired models which yield a poor description. The total yields and the mean pt are compared with previous measurements, and the trends as a function of collision energy are discussed.

Neutral Pion Emission from Accelerated Protons in the Supernova Remnant W44

Abstract: We present the AGILE gamma-ray observations in the energy range 50 MeV-10 GeV of the supernova remnant (SNR) W44, one of the most interesting systems for studying cosmic-ray production. W44 is an intermediate-age SNR ({approx}20, 000 years) and its ejecta expand in a dense medium as shown by a prominent radio shell, nearby molecular clouds, and bright [S II] emitting regions. We extend our gamma-ray analysis to energies substantially lower than previous measurements which could not conclusively establish the nature of the radiation. We find that gamma-ray emission matches remarkably well both the position and shape of the inner SNR shocked plasma. Furthermore, the gamma-ray spectrum shows a prominent peak near 1 GeV with a clear decrement at energies below a few hundreds of MeV as expected from neutral pion decay. Here we demonstrate that (1) hadron-dominated models are consistent with all W44 multiwavelength constraints derived from radio, optical, X-ray, and gamma-ray observations; (2) ad hoc lepton-dominated models fail to explain simultaneously the well-constrained gamma-ray and radio spectra, and require a circumstellar density much larger than the value derived from observations; and (3) the hadron energy spectrum is well described by a power law (with index s = 3.0 {+-}more » 0.1) and a low-energy cut-off at E{sub c} = 6 {+-} 1 GeV. Direct evidence for pion emission is then established in an SNR for the first time.« less

Evidence for a bound H dibaryon from lattice QCD.

Abstract: We present evidence for the existence of a bound H dibaryon, an I=0, J=0, s=-2 state with valence quark structure uuddss, at a pion mass of m(π)∼389 MeV. Using the results of lattice QCD calculations performed on four ensembles of anisotropic clover gauge-field configurations, with spatial extents of L∼2.0, 2.5, 3.0, and 3.9 fm at a spatial lattice spacing of b(s)∼0.123 fm, we find an H dibaryon bound by B(∞)(H)=16.6±2.1±4.6 MeV at a pion mass of m(π)∼389 MeV.

Precise determination of the f(0)(600) and f(0)(980) pole parameters from a dispersive data analysis

Abstract: We use our latest dispersive analysis of π π scattering data and the very recent K_(l4) experimental results to obtain the mass, width, and couplings of the two lightest scalar-isoscalar resonances. These parameters are defined from their associated poles in the complex plane. The analytic continuation to the complex plane is made in a model-independent way by means of once-and twice-subtracted dispersion relations for the partial waves, without any other theoretical assumption. We find the f(0)(600) pole at [457_(-13)^(+14)] – i[279_(-7)^(+11)+ MeV and that of the f(0)(980) at (996 ± 7) – i[25_(-6)^(+10)] MeV, whereas their respective couplings to two pions are 3.59_(-0.13)^(+0.11) and 2.3 ± 0.2 GeV.

Single spin asymmetries in charged kaon production from semi-inclusive deep inelastic scattering on a transversely polarized He 3 target

Abstract: We report the first measurement of target single spin asymmetries in the semi-inclusive ^3He(e,e^′π^±)X reaction on a transversely polarized target The experiment, conducted at Jefferson Lab using a 59 GeV electron beam, covers a range of 016

Resonance parameters of the ρ meson from lattice QCD

Abstract: We perform a non-perturbative lattice calculation of the P-wave pion-pion scattering phase in the rho-meson decay channel using two flavors of maximally twisted mass fermions at pion masses ranging from 480 MeV to 290 MeV. Making use of finite-size methods, we evaluate the pion-pion scattering phase in the center-of-mass frame and two moving frames. Applying an effective range formula, we find a good description of our results for the scattering phase as a function of the energy covering the resonance region. This allows us to extract the rho-meson mass and decay width and to study their quark mass dependence.

Flavor blindness and patterns of flavor symmetry breaking in lattice simulations of up, down, and strange quarks

Abstract: QCD lattice simulations with $2+1$ flavors (when two quark flavors are mass degenerate) typically start at rather large up-down and strange quark masses and extrapolate first the strange quark mass and then the up-down quark mass to its respective physical value. Here we discuss an alternative method of tuning the quark masses, in which the singlet quark mass is kept fixed. Using group theory the possible quark mass polynomials for a Taylor expansion about the flavor symmetric line are found, first for the general $1+1+1$ flavor case and then for the $2+1$ flavor case. This ensures that the kaon always has mass less than the physical kaon mass. This method of tuning quark masses then enables highly constrained polynomial fits to be used in the extrapolation of hadron masses to their physical values. Numerical results for the $2+1$ flavor case confirm the usefulness of this expansion and an extrapolation to the physical pion mass gives hadron mass values to within a few percent of their experimental values. Singlet quantities remain constant which allows the lattice spacing to be determined from hadron masses (without necessarily being at the physical point). Furthermore an extension of this program to include partially quenched results is given.

Improved stochastic estimation of quark propagation with Laplacian Heaviside smearing in lattice QCD

Abstract: A new method of stochastically estimating the low-lying effects of quark propagation is proposed which allows accurate determinations of temporal correlations of single-hadron and multi-hadron operators in lattice QCD. The method is well suited for calculations in large volumes. Contributions involving quark propagation connecting hadron sink operators at the same final time can be handled in a straightforward manner, even for a large number of final time slices. The method exploits Laplacian Heaviside (LapH) smearing. ZN noise is introduced in a novel way, and variance reduction is achieved using judiciously-chosen noise dilution projectors. The method is tested using isoscalar mesons in the scalar, pseudoscalar, and vector channels, and using the two-pion system of total isospin I = 0,1,2 on large anisotropic 24 3 × 128 lattices with spatial spacing as � 0.12 fm and temporal spacing at � 0.034 fm for pion masses m� � 390 and 240 MeV.

Meson Transition Form Factors in Light-Front Holographic QCD

Abstract: We study the photon-to-meson transition form factors (TFFs) F{sub M{gamma}}(Q{sup 2}) for {gamma}{gamma}* {yields} M using light-front holographic methods. The Chern-Simons action, which is a natural form in 5-dimensional anti-de Sitter (AdS) space, leads directly to an expression for the photon-to-pion TFF for a class of confining models. Remarkably, the predicted pion TFF is identical to the leading order QCD result where the distribution amplitude has asymptotic form. The Chern-Simons form is local in AdS space and is thus somewhat limited in its predictability. It only retains the q{bar q} component of the pion wavefunction, and further, it projects out only the asymptotic form of the meson distribution amplitude. It is found that in order to describe simultaneously the decay process {pi}{sup 0} {yields} {gamma}{gamma} and the pion TFF at the asymptotic limit, a probability for the q{bar q} component of the pion wavefunction P{sub q{bar q}} = 0.5 is required; thus giving indication that the contributions from higher Fock states in the pion light-front wavefunction need to be included in the analysis. The probability for the Fock state containing four quarks (anti-quarks) which follows from analyzing the hadron matrix elements, P{sub q{bar q}q{bar q}} {approx} 10%, agrees with the analysis of the pion elastic form factor using light-front holography including higher Fock components in the pion wavefunction. The results for the TFFs for the {eta} and {eta}{prime} mesons are also presented. The rapid growth of the pion TFF exhibited by the BABAR data at high Q{sup 2} is not compatible with the models discussed in this article, whereas the theoretical calculations are in agreement with the experimental data for the {eta} and {eta}{prime} TFFs.

Measurements of Cross Sections and Charged Pion Spectra in Proton-Carbon Interactions at 31 GeV/c

Abstract: Interaction cross sections and charged pion spectra in p+C interactions at 31 GeV/c were measured with the large-acceptance NA61/SHINE spectrometer at the CERN SPS. These data are required to improve predictions of the neutrino flux for the T2K long-baseline neutrino oscillation experiment in Japan. A set of data collected during the first NA61/SHINE run in 2007 with an isotropic graphite target with a thickness of 4% of a nuclear interaction length was used for the analysis. The measured p+C inelastic and production cross sections are 257.2 {+-} 1.9 {+-} 8.9 and 229.3 {+-} 1.9 {+-} 9.0 mb, respectively. Inclusive production cross sections for negatively and positively charged pions are presented as functions of laboratory momentum in ten intervals of the laboratory polar angle covering the range from 0 up to 420 mrad. The spectra are compared with predictions of several hadron production models.

Cross Sections and Charged Pion Spectra in Proton-Carbon Interactions at 31 GeV/c

Abstract: As neutrino long baseline experiments enter a new domain of precision, the careful study of systematic errors due to poor knowledge of production cross sections for pions and kaons require more dedicated measurements for precise neutrino flux predictions. The cosmic ray experiments require dedicated hadron production measurements to tune simulation models used to describe air shower profiles. Among other goals, the NA61/SHINE (SPS Heavy Ion and Neutrino Experiment) experiment at the CERN SPS aims at precision measurements (5% and below) for both neutrino and cosmic ray experiments: those will improve the prediction of the neutrino flux for the T2K experiment at J-PARC and the prediction of muon production in the propagation of air showers for the Auger and KASCADE experiments. NA61/SHINE took data during a pilot run in 2007 and in 2009 and 2010 with different carbon targets. The NA61/SHINE set-up and spectra for positive and negative pions obtained with the 2007 thin (4% interaction length) carbon target data are presented.

Nucleon strange quark content from two-flavor lattice QCD with exact chiral symmetry

Abstract: We calculate the strange quark content of the nucleon $⟨N|\overline{s}s|N⟩$ in $2+1$ -flavor lattice QCD. Chirally symmetric overlap fermion formulation is used to avoid the contamination from up and down quark contents due to an operator mixing between strange and light scalar operators, $\overline{s}s$ and $\overline{u}u+\overline{d}d$. At a lattice spacing $a=0.112(1)\text{ }\text{ }\mathrm{fm}$, we perform calculations at four values of degenerate up and down quark masses ${m}_{ud}$, which cover a range of the pion mass ${M}_{\ensuremath{\pi}}\ensuremath{\simeq}300--540\text{ }\text{ }\mathrm{MeV}$. We employ two different methods to calculate $⟨N|\overline{s}s|N⟩$. One is a direct method where we calculate $⟨N|\overline{s}s|N⟩$ by directly inserting the $\overline{s}s$ operator. The other is an indirect method where $⟨N|\overline{s}s|N⟩$ is extracted from a derivative of the nucleon mass in terms of the strange quark mass. With these two methods we obtain consistent results for $⟨N|\overline{s}s|N⟩$ with each other. Our best estimate ${f}_{{T}_{s}}={m}_{s}⟨N|\overline{s}s|N⟩/{M}_{N}=0.009(15{)}_{\mathrm{stat}}(16{)}_{\mathrm{sys}}$ is in good agreement with our previous studies in two-flavor QCD.

Light Cone Sum Rules for the pi0-gamma*-gamma Form Factor Revisited

Abstract: We provide a theoretical update of the calculations of the {pi}{sup 0}{gamma}{sup *}{gamma} form factor in the light cone sum rules framework, including up to six polynomials in the conformal expansion of the pion distribution amplitude and taking into account twist-six corrections related to the photon emission at large distances. The results are compared with the calculations of the B{yields}{pi}l{nu} decay and pion electromagnetic form factors in the same framework. Our conclusion is that the recent BABAR measurements of the {pi}{sup 0}{gamma}{sup *}{gamma} form factor at large momentum transfers [B. Aubert et al. (The BABAR Collaboration), Phys. Rev. D 80, 052002 (2009)] are consistent with QCD, although they do suggest that the pion distribution amplitude may have more structure than usually assumed.

Roy–Steiner equations for γγ→ππ

Abstract: 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–Omnes 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 $(\alpha_{2}-\beta_{2})^{\pi^{\pm}}=(15.3\pm3.7)\times 10^{-4}$ 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.

Effect of charmed meson loops on charmonium transitions

Abstract: The effects of intermediate charmed mesons on charmonium transitions with the emission of one pion or eta are studied systematically. Based on a nonrelativistic effective field theory we show that charmed meson loops are enhanced compared to the corresponding tree-level contributions for transitions between two S-wave charmonia as well as for transitions between two P-wave charmonia. On the contrary, for the transitions between one S-wave and one P-wave charmonium state, the loops need to be analyzed case by case and often appear to be suppressed. The relation to and possible implications for an effective Lagrangian approach are also discussed. This study at the same time provides a cross-check for the numerical evaluations.

Perturbative Renormalizability of Chiral Two Pion Exchange in Nucleon-Nucleon Scattering

Abstract: We study the perturbative renormalizability of chiral two pion exchange for the singlet and triplet channels within effective field theory, provided that the one pion exchange piece of the interaction has been fully iterated. We determine the number of counterterms/subtractions needed in order to obtain finite results when the cut-off is removed, resulting in three counterterms for the singlet channel and six for the triplet. The results show that perturbative chiral two pion exchange reproduce the data up to a center-of-mass momentum of k � 200 300MeV in the singlet channel and k � 300 400MeV in the triplet.

Measurement of neutrino-induced charged-current charged pion production cross sections on mineral oil at Eν∼1GeV

Abstract: Using a high-statistics, high-purity sample of {nu}{sub {mu}-}induced charged current, charged pion events in mineral oil (CH{sub 2}), MiniBooNE reports a collection of interaction cross sections for this process. This includes measurements of the CC{pi}{sup +} cross section as a function of neutrino energy, as well as flux-averaged single- and double-differential cross sections of the energy and direction of both the final-state muon and pion. In addition, each of the single-differential cross sections are extracted as a function of neutrino energy to decouple the shape of the MiniBooNE energy spectrum from the results. In many cases, these cross sections are the first time such quantities have been measured on a nuclear target and in the 1 GeV energy range.

Two-nucleon electromagnetic current in chiral effective field theory: One-pion exchange and short-range contributions

Abstract: We derive the leading one-loop contribution to the one-pion exchange and short-range two-nucleon electromagnetic current operator in the framework of chiral effective field theory. The derivation is carried out using the method of unitary transformation. Explicit results for the current and charge densities are given in momentum and coordinate space.

Effects of momentum conservation and flow on angular correlations observed in experiments at the BNL Relativistic Heavy Ion Collider

Abstract: Correlations of azimuthal angles observed at the Relativistic Heavy Ion Collider have gained great attention due to the prospect of identifying fluctuations of parity-odd regions in the field sector of QCD. Whereas the observable of interest related to parity fluctuations involves subtracting opposite-sign from same-sign correlations, the STAR collaboration reported the same-sign and opposite-sign correlations separately. It is shown here how momentum conservation combined with collective elliptic flow contributes significantly to this class of correlations, although not to the difference between the opposite- and same-sign observables. The effects are modeled with a crude simulation of a pion gas. Although the simulation reproduces the scale of the correlation, the centrality dependence is found to be sufficiently different in character to suggest additional considerations beyond those present in the pion gas simulation presented here.

Observation of transverse polarization asymmetries of charged pion pairs in e +e - annihilation near √s=10.58GeV

Abstract: The interference fragmentation function translates the fragmentation of a quark with a transverse projection of the spin into an azimuthal asymmetry of two final-state hadrons. In e(+)e(-) annihilation the product of two interference fragmentation functions is measured. We report nonzero asymmetries for pairs of charge-ordered π(+)π(-) pairs, which indicate a significant interference fragmentation function in this channel. The results are obtained from a 672 fb(-1) data sample that contains 711 × 10(6) π(+)π(-) pairs and was collected at and near the Υ(4S) resonance, with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider.

Mixing of Active and Sterile Neutrinos

Abstract: We investigate mixing of neutrinos in the νMSM (neutrino Minimal Standard Model), which is the MSM extended by three right-handed neutrinos. Especially, we study elements of the mixing matrix ΘαI between three left-handed neutrinos ν Lα(α = e, μ, τ) and two sterile neutrinos N I (I =2, 3) which are responsible to the seesaw mechanism generating the suppressed masses of active neutrinos as well as the generation of the baryon asymmetry of the universe. It is shown that Θ eI can be suppressed by many orders of magnitude compared with ΘμI and ΘτI , when the Chooz angle θ13 is large in the normal hierarchy of active neutrino masses. We then discuss the neutrinoless double beta decay in this framework by taking into account the contributions not only from active neutrinos but also from all the three sterile neutrinos. It is shown that N 2 and N 3 give substantial, destructive contributions when their masses are smaller than a few 100 MeV, and as a results Θ eI receive no stringent constraint from the bounds on such decays. Finally, we discuss the impacts of the obtained results on the direct searches of N 2,3 in meson decays for the case when N 2,3 are lighter than pion mass. We show that there exists the allowed region for N 2,3 with such small masses in the normal hierarchy case even if the current bound on the lifetimes of N 2,3 from the big bang nucleosynthesis is imposed. It is also pointed out that the direct search by using π+ → e + + N 2,3 and K + → e + + N 2,3 might miss such N 2,3 since the branching ratios can be extremely small due to the cancellation in Θ eI , but the search by K + → μ+ + N 2,3 can cover the whole allowed region by improving the measurement of the branching ratio by a factor of 5.

Roy-Steiner equations for $\gamma\gamma\to\pi\pi$

Abstract: Starting from hyperbolic dispersion relations, we present a system of Roy--Steiner equations for pion Compton scattering that respects analyticity and unitarity requirements, gauge invariance, as well as crossing symmetry, and thus all symmetries of the underlying quantum field theory. To suppress the dependence on the high-energy region, we also consider once- and twice-subtracted versions of the equations, where the subtraction constants are identified with dipole and quadrupole pion polarizabilities. We consider the resolution of the $\gamma\gamma\to\pi\pi$ partial waves by a Muskhelishvili-Omnes representation with finite matching point, and discuss the consequences for the two-photon coupling of the $\sigma$ resonance as well as its relation to pion polarizabilities.