35,444 research outputs found

    Exclusive two-photon annihilation at large energy or large virtuality

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    I review recent progress in the theory of gamma gamma annihilation into meson or baryon pairs at large energy, and of the process gamma* gamma* -> pi0 at large photon virtuality.Comment: 6 pages, 4 figures. Presented at PHOTON 2003, Frascati, Italy, 7-11 April 200

    News from Cosmic Gamma-ray Line Observations

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    The measurement of gamma rays at MeV energies from cosmic radioactivities is one of the key tools for nuclear astrophysics, in its study of nuclear reactions and how they shape objects such as massive stars and supernova explosions. Additionally, the unique gamma-ray signature from the annihilation of positrons falls into this same astronomical window, and positrons are often produced from radioactive beta decays. Nuclear gamma-ray telescopes face instrumental challenges from penetrating gamma rays and cosmic-ray induced backgrounds. But the astrophysical benefits of such efforts are underlined by the discoveries of nuclear gamma~rays from the brightest of the expected sources. In recent years, both thermonuclear and core-collapse supernova radioactivity gamma~rays have been measured in spectral detail, and complement conventional supernova observations with measurements of origins in deep supernova interiors, from the decay of 56^{56}Ni, 56^{56}Co, and 44^{44}Ti. The diffuse afterglow in gamma rays of radioactivity from massive-star nucleosynthesis is analysed on the large (galactic) scale, with findings important for recycling of matter between successive stellar generations: From 26^{26}Al gamma-ray line spectroscopy, interstellar cavities and superbubbles have been recognised in their importance for ejecta transport and recycling. Diffuse galactic emissions from radioactivity and positron-annihilation γ\gamma~rays should be connected to nucleosynthesis sources: Recently new light has been shed on this connection, among others though different measurements of radioactive 60^{60}Fe, and through spectroscopy of positron annihilation gamma~rays from a flaring microquasar and from different parts of our Galaxy.Comment: Invited talk at the international symposium "Nuclei in the Cosmos XIV", June 2016, at Niigata, Japan. Six pages, two figures. Accepted for publication in JPS (Japan Physical Society) Conference Proceedings (http://jpscp.jps.jp/

    Critical Casimir amplitudes for nn-component ϕ4\phi^4 models with O(n)-symmetry breaking quadratic boundary terms

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    Euclidean nn-component ϕ4\phi^4 theories whose Hamiltonians are O(n) symmetric except for quadratic symmetry breaking boundary terms are studied in films of thickness LL. The boundary terms imply the Robin boundary conditions nϕα=c˚α(j)ϕα\partial_n\phi_\alpha =\mathring{c}^{(j)}_\alpha \phi_\alpha at the boundary planes Bj=1,2\mathfrak{B}_{j=1,2} at z=0z=0 and z=Lz=L. Particular attention is paid to the cases in which mjm_j of the nn variables c˚α(j)\mathring{c}^{(j)}_\alpha take the special value c˚mj-sp\mathring{c}_{m_j\text{-sp}} corresponding to critical enhancement while the remaining ones are subcritically enhanced. Under these conditions, the semi-infinite system bounded by Bj\mathfrak{B}_j has a multicritical point, called mjm_j-special, at which an O(mj)O(m_j) symmetric critical surface phase coexists with the O(n) symmetric bulk phase, provided dd is sufficiently large. The LL-dependent part of the reduced free energy per area behaves as ΔC/Ld1\Delta_C/L^{d-1} as LL\to\infty at the bulk critical point. The Casimir amplitudes ΔC\Delta_C are determined for small ϵ=4d\epsilon=4-d in the general case where mc,cm_{c,c} components ϕα\phi_\alpha are critically enhanced at both boundary planes, mc,D+mD,cm_{c,D} + m_{D,c} components are enhanced at one plane but satisfy asymptotic Dirichlet boundary conditions at the respective other, and the remaining mD,Dm_{D,D} components satisfy asymptotic Dirichlet boundary conditions at both Bj\mathfrak{B}_j. Whenever mc,c>0m_{c,c}>0, these expansions involve integer and fractional powers ϵk/2\epsilon^{k/2} with k3k\ge 3 (mod logarithms). Results to O(ϵ3/2)O(\epsilon^{3/2}) for general values of mc,cm_{c,c}, mc,D+mD,cm_{c,D}+m_{D,c}, and mD,Dm_{D,D} are used to estimate the ΔC\Delta_C of 3D Heisenberg systems with surface spin anisotropies when (mc,c,mc,D+mD,c)=(1,0)(m_{c,c}, m_{c,D}+ m_{D,c}) = (1,0), (0,1)(0,1), and (1,1)(1,1).Comment: Latex source file with 5 eps files; version with minor amendments and corrected typo

    Energy Momentum Tensor in Conformal Field Theories Near a Boundary

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    The requirements of conformal invariance for the two point function of the energy momentum tensor in the neighbourhood of a plane boundary are investigated, restricting the conformal group to those transformations leaving the boundary invariant. It is shown that the general solution may contain an arbitrary function of a single conformally invariant variable vv, except in dimension 2. The functional dependence on vv is determined for free scalar and fermion fields in arbitrary dimension dd and also to leading order in the \vep expansion about d=4d=4 for the non Gaussian fixed point in ϕ4\phi^4 theory. The two point correlation function of the energy momentum tensor and a scalar field is also shown to have a unique expression in terms of vv and the overall coefficient is determined by the operator product expansion. The energy momentum tensor on a general curved manifold is further discussed by considering variations of the metric. In the presence of a boundary this procedure naturally defines extra boundary operators. By considering diffeomorphisms these are related to components of the energy momentum tensor on the boundary. The implications of Weyl invariance in this framework are also derived.Comment: 22 pages, TeX with epsf.tex, DAMTP/93-1. (original uuencoded file was corrupted enroute - resubmitted version has uuencoded figures pasted to the ended of the Plain TeX file

    Dynamic surface scaling behavior of isotropic Heisenberg ferromagnets

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    The effects of free surfaces on the dynamic critical behavior of isotropic Heisenberg ferromagnets are studied via phenomenological scaling theory, field-theoretic renormalization group tools, and high-precision computer simulations. An appropriate semi-infinite extension of the stochastic model J is constructed, the boundary terms of the associated dynamic field theory are identified, its renormalization in d <= 6 dimensions is clarified, and the boundary conditions it satisfies are given. Scaling laws are derived which relate the critical indices of the dynamic and static infrared singularities of surface quantities to familiar static bulk and surface exponents. Accurate computer-simulation data are presented for the dynamic surface structure factor; these are in conformity with the predicted scaling behavior and could be checked by appropriate scattering experiments.Comment: 9 pages, 2 figure

    Hard exclusive reactions and hadron structure

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    The generalized Bjorken regime of exclusive reactions opens new ways to explore the hadron structure. We shortly review the present status of this domain where generalized parton distributions, generalized distribution amplitudes and transition distribution amplitudes describe various aspects of confinement physics.Comment: 7 pages LATEX, 5 figures, invited talk at the Vth Conference on Hadronic Physics held at ICTP, Trieste, Italy, May 200

    Exclusive production of pentaquarks in the scaling regime

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    We investigate two exclusive reactions with a Theta+ pentaquark in the final state: electroproduction of a K meson on the nucleon, and K+ scattering on a neutron target producing a lepton pair. These reactions offer unique opportunities to investigate the structure of pentaquark baryons at parton level. We discuss the generalized parton distributions for the N --> Theta+ transition and give the leading order amplitude for these processes in the Bjorken regime.Comment: 5 pages in LATEX, 1 .eps figure, talk at the Baryons04 Conference, October 25-29, 2004, Palaiseau, Franc

    Boundary critical behavior at m-axial Lifshitz points for a boundary plane parallel to the modulation axes

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    The critical behavior of semi-infinite dd-dimensional systems with nn-component order parameter ϕ\bm{\phi} and short-range interactions is investigated at an mm-axial bulk Lifshitz point whose wave-vector instability is isotropic in an mm-dimensional subspace of Rd\mathbb{R}^d. The associated mm modulation axes are presumed to be parallel to the surface, where 0md10\le m\le d-1. An appropriate semi-infinite ϕ4|\bm{\phi}|^4 model representing the corresponding universality classes of surface critical behavior is introduced. It is shown that the usual O(n) symmetric boundary term ϕ2\propto \bm{\phi}^2 of the Hamiltonian must be supplemented by one of the form λ˚α=1m(ϕ/xα)2\mathring{\lambda} \sum_{\alpha=1}^m(\partial\bm{\phi}/\partial x_\alpha)^2 involving a dimensionless (renormalized) coupling constant λ\lambda. The implied boundary conditions are given, and the general form of the field-theoretic renormalization of the model below the upper critical dimension d(m)=4+m/2d^*(m)=4+{m}/{2} is clarified. Fixed points describing the ordinary, special, and extraordinary transitions are identified and shown to be located at a nontrivial value λ\lambda^* if ϵd(m)d>0\epsilon\equiv d^*(m)-d>0. The surface critical exponents of the ordinary transition are determined to second order in ϵ\epsilon. Extrapolations of these ϵ\epsilon expansions yield values of these exponents for d=3d=3 in good agreement with recent Monte Carlo results for the case of a uniaxial (m=1m=1) Lifshitz point. The scaling dimension of the surface energy density is shown to be given exactly by d+m(θ1)d+m (\theta-1), where θ=νl4/νl2\theta=\nu_{l4}/\nu_{l2} is the anisotropy exponent.Comment: revtex4, 31 pages with eps-files for figures, uses texdraw to generate some graphs; to appear in PRB; v2: some references and additional remarks added, labeling in figure 1 and some typos correcte

    Generalized parton distributions: recent results

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    I review progress on selected issues connected with generalized parton distributions. Topics range from the description of hard exclusive reactions to the spatial distribution of quarks in the nucleon and the contribution of their orbital angular momentum to the nucleon spin.Comment: 9 pages, 5 figures. To appear in the proceedings of the Particles and Nuclei International Conference (PANIC 05), Santa Fe, NM, USA, 24-28 Oct 200

    Gamma-ray line measurements from supernova explosions

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    Gamma ray lines are expected to be emitted as part of the afterglow of supernova explosions, because radioactive decay of freshly synthesised nuclei occurs. Significant radioactive gamma ray line emission is expected from 56Ni and 44Ti decay on time scales of the initial explosion (56Ni, tau~days) and the young supernova remnant (44Ti,tau~90 years). Less specific, and rather informative for the supernova population as a whole, are lessons from longer lived isotopes such as 26Al and 60Fe. From isotopes of elements heavier than iron group elements, any interesting gamma-ray line emission is too faint to be observable. Measurements with space-based gamma-ray telescopes have obtained interesting gamma ray line emissions from two core collapse events, Cas A and SN1987A, and one thermonuclear event, SN2014J. We discuss INTEGRAL data from all above isotopes, including all line and continuum signatures from these two objects, and the surveys for more supernovae, that have been performed by gamma ray spectrometry. Our objective here is to illustrate what can be learned from gamma-ray line emission properties about the explosions and their astrophysics.Comment: 7 pages, 4 figures. IAU Symposium 331 "SN1987A 30 years after", La Reunion, Feb. 2017. Accepted for publication in IAU Conf Pro