18,714 research outputs found

    Yukawa Bound States of a Large Number of Fermions

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    We consider the bound state problem for a field theory that contains a Dirac fermion χ\chi that Yukawa couples to a (light) scalar field ϕ\phi. We are interested in bound states with a large number NN of χ\chi particles. A Fermi gas model is used to numerically determine the dependence of the radius RR of these bound states on NN and also the dependence of the binding energy on NN. Since scalar interactions with relativistic χ\chi's are suppressed two regimes emerge. For modest values of NN the state is composed of non-relativistic χ\chi particles. In this regime as NN increases RR decreases. Eventually the core region becomes relativistic and the size of the state starts to increase as NN increases. As a result, for fixed Yukawa coupling and χ\chi mass, there is a minimum sized state that occurs roughly at the value of NN where the core region first becomes relativistic. We also compute an elastic scattering form factor that can be relevant for direct detection if the dark matter is composed of such χ\chi particles.Comment: 14 pages, 7 figure

    Structure- and laser-gauges for the semiconductor Bloch equations in high-harmonic generation in solids

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    The semiconductor Bloch equations (SBEs) are routinely used for simulations of strong-field laser-matter interactions in condensed matter. In systems without inversion or time-reversal symmetries, the Berry connections and transition dipole phases (TDPs) must be included in the SBEs, which in turn requires the construction of a smooth and periodic structure gauge for the Bloch states. Here, we illustrate a general approach for such a structure-gauge construction for topologically trivial systems. Furthermore, we investigate the SBEs in the length and velocity gauges, and discuss their respective advantages and shortcomings for the high-harmonic generation (HHG) process. We find that in cases where we require dephasing or separation of the currents into interband and intraband contributions, the length gauge SBEs are computationally more efficient. In calculations without dephasing and where only the total current is needed, the velocity gauge SBEs are structure-gauge independent and are computationally more efficient. We employ two systems as numerical examples to highlight our findings: an 1D model of ZnO and the 2D monolayer hexagonal boron nitride (h-BN). The omittance of Berry connections or TDPs in the SBEs for h-BN results in nonphysical HHG spectra. The structure- and laser-gauge considerations in the current work are not restricted to the HHG process, and are applicable to all strong-field matter simulations with SBEs

    Effective Theory and Simple Completions for Neutrino Interactions

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    We consider all the dimension 6 operators as well as some simple extensions of the standard model that give new contributions to neutrino interactions with matter. Such interactions are usually parametrized by ϵαβ\epsilon_{\alpha \beta}, where α\alpha and β\beta are neutrino flavor indices taking the values ee, μ\mu and τ\tau. In the simple models we consider the ϵαβ\epsilon_{\alpha \beta}'s are much more constrained than in the operator-based model-independent approach. Typically the ϵαβ\epsilon_{\alpha \beta}'s are restricted to be smaller in magnitude than around 10−310^{-3}. In some of the leptoquark models, a specific pattern for the leptoquark Yukawa couplings allows the diagonal element ϵττ\epsilon_{\tau\tau} to be as large as ∼0.1\sim0.1, or one of ϵee\epsilon_{ee}, ϵμμ∼0.01\epsilon_{\mu\mu}\sim0.01. We discuss the interplay between neutrino physics and leptoquark searches at the LHC.Comment: 12 pages, 2 figure

    Lepton Flavorful Fifth Force and Depth-dependent Neutrino Matter Interactions

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    We consider a fifth force to be an interaction that couples to matter with a strength that grows with the number of atoms. In addition to competing with the strength of gravity a fifth force can give rise to violations of the equivalence principle. Current long range constraints on the strength and range of fifth forces are very impressive. Amongst possible fifth forces are those that couple to lepton flavorful charges Le−LμL_e-L_{\mu} or Le−LτL_e-L_{\tau}. They have the property that their range and strength are also constrained by neutrino interactions with matter. In this brief note we review the existing constraints on the allowed parameter space in gauged U(1)Le−Lμ,LτU(1)_{L_e-L_{\mu}, L_{\tau}}. We find two regions where neutrino oscillation experiments are at the frontier of probing such a new force. In particular, there is an allowed range of parameter space where neutrino matter interactions relevant for long baseline oscillation experiments depend on the depth of the neutrino beam below the surface of the earth.Comment: 6 pages, 5 figure

    Strong CMB Constraint On P-Wave Annihilating Dark Matter

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    We consider a dark sector consisting of dark matter that is a Dirac fermion and a scalar mediator. This model has been extensively studied in the past. If the scalar couples to the dark matter in a parity conserving manner then dark matter annihilation to two mediators is dominated by the P-wave channel and hence is suppressed at very low momentum. The indirect detection constraint from the anisotropy of the Cosmic Microwave Background is usually thought to be absent in the model because of this suppression. In this letter we show that dark matter annihilation to bound states occurs through the S-wave and hence there is a constraint on the parameter space of the model from the Cosmic Microwave Background.Comment: 5 pages, 3 figure

    Supersymmetry and Goldstino-like Mode in Bose-Fermi Mixtures

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    Supersymmetry is assumed to be a basic symmetry of the world in many high energy theories, but none of the super partners of any known elementary particle has been observed yet. We argue that supersymmetry can also be realized and studied in ultracold atomic systems with a mixture of bosons and fermions, with properly tuned interactions and single particle dispersion. We further show that in such non-releativistic systems supersymmetry is either spontaneously broken, or explicitly broken by a chemical potential difference between the bosons and fermions. In both cases the system supports a sharp fermionic collective mode or the so-called Goldstino, due to supersymmetry. We also discuss possible ways to detect the Goldstino mode experimentally.Comment: 4 pages. V4: published versio

    Nanocrystalline iron at high pressure

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    X-ray diffraction measurements were performed on nanocrystalline iron up to 46 GPa. For nanocrystalline epsilon-Fe, analysis of lattice parameter data provides a bulk modulus, K, of 179±8 GPa and a pressure derivative of the bulk modulus, K[prime], of 3.6±0.7, similar to the large-grained control sample. The extrapolated zero-pressure unit cell volume of nanocrystalline epsilon-Fe is 22.9±0.2 Å^3, compared to 22.3±0.2 Å^3 for large-grained epsilon-Fe. No significant grain growth was observed to occur under pressure
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