A Method to Determine ∣Vcb∣|V_{cb}| at the Weak Scale in Top Decays at the LHC

Until now, the Cabibbo Kobayashi Maskawa matrix element, $|V_{cb}|$, has always been measured in $B$ decays, i.e.~at an energy scale $q_b\sim \frac{m_b}{2}$, far below the weak scale. We consider here the possibility of measuring it close to the weak scale, at $q_W\sim m_W$, in top decays at the Large Hadron Collider (LHC). Our proposed method would use data from the LHC experiments in hadronic top decays $t\rightarrow bW\rightarrow b\overline{b} c$, tagged by the semileptonic decay of the associated top. We estimate the uncertainty of such a measurement, as a function of present and potential future experimental jet flavour-tagging performances, and conclude that first measurements using the data collected during 2016 - 2018 could yield a fractional error on \Vcb\ of order 7\% per experiment. We also give projected performances at higher luminosities, which could yield sensitivity to any Standard Model running of \Vcb\ below the weak scale, if present.Comment: 15 pages, 1 figures. Changes for V3: removed earlier Fig. 1, associated text and citations, added one new citatio

A NuSTAR observation of the fast symbiotic nova V745 Sco in outburst

The fast recurrent nova V745 Sco was observed in the 3-79 keV X-rays band with NuSTAR 10 days after the optical discovery. The measured X-ray emission is consistent with a collisionally ionized optically thin plasma at temperature of about 2.7 keV. A prominent iron line observed at 6.7 keV does not require enhanced iron in the ejecta. We attribute the X-ray flux to shocked circumstellar material. No X-ray emission was observed at energies above 20 keV, and the flux in the 3-20 keV range was about 1.6 $\times$ 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$. The emission measure indicates an average electron density of order of 10$^7$ cm$^{-3}$. The X-ray flux in the 0.3-10 keV band almost simultaneously measured with Swift was about 40 times larger, mainly due to the luminous central supersoft source emitting at energy below 1 keV. The fact that the NuSTAR spectrum cannot be fitted with a power law, and the lack of hard X-ray emission, allow us to rule out Comptonized gamma rays, and to place an upper limit of the order of 10$^{-11}$ erg cm$^{-2}$ s$^{-1}$ on the gamma-ray flux of the nova on the tenth day of the outburst.Comment: in press in Monthly Notices of the Royal Astronomical Society, 201

Generating anisotropic fluids from vacuum Ernst equations

Starting with any stationary axisymmetric vacuum metric, we build anisotropic fluids. With the help of the Ernst method, the basic equations are derived together with the expression for the energy-momentum tensor and with the equation of state compatible with the field equations. The method is presented by using different coordinate systems: the cylindrical coordinates $\rho, z$ and the oblate spheroidal ones. A class of interior solutions matching with stationary axisymmetric asymptotically flat vacuum solutions is found in oblate spheroidal coordinates. The solutions presented satisfy the three energy conditions.Comment: Version published on IJMPD, title changed by the revie

Oscillating Neutrinos from the Galactic Center

It has recently been demonstrated that the $\gamma$-ray emission spectrum of the EGRET-identified, central Galactic source 2EG J1746-2852 can be well fitted by positing that these photons are generated by the decay of $\pi^0$'s produced in p-p scattering at or near an energizing shock. Such scattering also produces charged pions which decay leptonically.The ratio of $\gamma$-rays to neutrinos generated by the central Galactic source may be accurately determined and a well-defined and potentially-measurable high energy neutrino flux at Earth is unavoidable. An opportunity, therefore, to detect neutrino oscillations over an unprecedented scale is offered by this source. In this paper we assess the prospects for such an observation with the generation of neutrino \v{C}erenkov telescopes now in the planning stage. We determine that the next generation of detectors may find an oscillation signature in the Galactic Center (GC) signal.Comment: 45 pages, LaTeX, uses ApJ style, some minor revisions, this final version to be published in ApJ

Magnetic confinement of the superconducting condensate in superconductor/ferromagnet hybrid composites

The influence of an inhomogeneous magnetic field on the magnetoresistance of thin Al films, used in different superconductor/ferromagnet hybrids, has been investigated. Two contrasting magnetic textures with out-of-plane magnetization are explored, namely (i) a plain film in a multidomain state and (ii) an array of micro-sized dots. The stray fields of the ferromagnetic structures confine the superconducting condensate and, accordingly, modify the condition for the nucleation of superconductivity. By switching between different magnetic states of the ferromagnet, this confinement can be tuned at will, hereby reversibly changing the dependence of the critical temperature Tc on an external magnetic field H. In particular, the continuous evolution from a conventional linear Tc(H) dependence with a single maximum to a reentrant superconducting phase boundary with multiple Tc peaks has been demonstrated

Determining parameters of the Neugebauer family of vacuum spacetimes in terms of data specified on the symmetry axis

We express the complex potential E and the metrical fields omega and gamma of all stationary axisymmetric vacuum spacetimes that result from the application of two successive quadruple-Neugebauer (or two double-Harrison) transformations to Minkowski space in terms of data specified on the symmetry axis, which are in turn easily expressed in terms of multipole moments. Moreover, we suggest how, in future papers, we shall apply our approach to do the same thing for those vacuum solutions that arise from the application of more than two successive transformations, and for those electrovac solutions that have axis data similar to that of the vacuum solutions of the Neugebauer family. (References revised following response from referee.)Comment: 18 pages (REVTEX

Spectral Changes in the Hyperluminous Pulsar in NGC 5907 as a Function of Super-Orbital Phase

We present broad-band, multi-epoch X-ray spectroscopy of the pulsating ultra-luminous X-ray source (ULX) in NGC 5907. Simultaneous XMM-Newton and NuSTAR data from 2014 are best described by a multi-color black-body model with a temperature gradient as a function of accretion disk radius significantly flatter than expected for a standard thin accretion disk (T(r) ~ r^{-p}, with p=0.608^{+0.014}_{-0.012}). Additionally, we detect a hard power-law tail at energies above 10 keV, which we interpret as being due to Comptonization. We compare this observation to archival XMM-Newton, Chandra, and NuSTAR data from 2003, 2012, and 2013, and investigate possible spectral changes as a function of phase over the 78d super-orbital period of this source. We find that observations taken around phases 0.3-0.4 show very similar temperature profiles, even though the observed flux varies significantly, while one observation taken around phase 0 has a significantly steeper profile. We discuss these findings in light of the recent discovery that the compact object is a neutron star and show that precession of the accretion disk or the neutron star can self-consistently explain most observed phenomena.Comment: 7 pages, 5 figures, submitted to ApJ; comments welcom

Finite pseudo orbit expansions for spectral quantities of quantum graphs

We investigate spectral quantities of quantum graphs by expanding them as sums over pseudo orbits, sets of periodic orbits. Only a finite collection of pseudo orbits which are irreducible and where the total number of bonds is less than or equal to the number of bonds of the graph appear, analogous to a cut off at half the Heisenberg time. The calculation simplifies previous approaches to pseudo orbit expansions on graphs. We formulate coefficients of the characteristic polynomial and derive a secular equation in terms of the irreducible pseudo orbits. From the secular equation, whose roots provide the graph spectrum, the zeta function is derived using the argument principle. The spectral zeta function enables quantities, such as the spectral determinant and vacuum energy, to be obtained directly as finite expansions over the set of short irreducible pseudo orbits.Comment: 23 pages, 4 figures, typos corrected, references added, vacuum energy calculation expande

Ultra-high energy cosmic rays from Quark Novae

We explore acceleration of ions in the Quark Nova (QN) scenario, where a neutron star experiences an explosive phase transition into a quark star (born in the propeller regime). In this picture, two cosmic ray components are isolated: one related to the randomized pulsar wind and the other to the propelled wind, both boosted by the ultra-relativistic Quark Nova shock. The latter component acquires energies $10^{15} {\rm eV}<E<10^{18} {\rm eV}$ while the former, boosted pulsar wind, achieves ultra-high energies $E> 10^{18.6}$ eV. The composition is dominated by ions present in the pulsar wind in the energy range above $10^{18.6}$ eV, while at energies below $10^{18}$ eV the propelled ejecta, consisting of the fall-back neutron star crust material from the explosion, is the dominant one. Added to these two components, the propeller injects relativistic particles with Lorentz factors $\Gamma_{\rm prop.} \sim 1-1000$, later to be accelerated by galactic supernova shocks. The QN model appears to be able to account for the extragalactic cosmic rays above the ankle and to contribute a few percent of the galactic cosmic rays below the ankle. We predict few hundred ultra-high energy cosmic ray events above $10^{19}$ eV for the Pierre Auger detector per distant QN, while some thousands are predicted for the proposed EUSO and OWL detectors.Comment: 20 pages, 1 figure. Major revisions in the text. Accepted for publication in the Astrophysical Journa

Fully Electrified Neugebauer Spacetimes

Generalizing a method presented in an earlier paper, we express the complex potentials E and Phi of all stationary axisymmetric electrovac spacetimes that correspond to axis data of the form E(z,0) = (U-W)/(U+W) , Phi(z,0) = V/(U+W) , where U = z^{2} + U_{1} z + U_{2} , V = V_{1} z + V_{2} , W = W_{1} z + W_{2} , in terms of the complex parameters U_{1}, V_{1}, W_{1}, U_{2}, V_{2} and W_{2}, that are directly associated with the various multipole moments. (Revised to clarify certain subtle points.)Comment: 25 pages, REVTE