Mon·Sep 14, 2020Monday·September 14, 2020

1. 01*

   Revisiting the top quark chromomagnetic dipole moment in the SM

   J. I. Aranda🇲🇽 · T. Cisneros-Pérez🇲🇽 · J. Montaño🇲🇽 · B. Quezadas-Vivian🇲🇽 · F. Ramírez-Zavaleta🇲🇽 · E. S. Tututi🇲🇽

   We revisit the anomalous chromomagnetic dipole moment in the Standard Model and show that its triple gluon vertex contribution, with the on-shell gluon ($q^2=0$), generates an infrared divergent pole. Consequently, the chromomagnetic dipole should not be perturbatively evaluated at $q^2=0$. Focusing on this top quark anomaly, denoted as ${\hat{\mu }}_t(q^2)$, we compute it with the off-shell gluon with a large momentum transfer, just as the ${\alpha }_s(m_Z^2)$ convention scale, for both spacelike $q^2=-m_Z^2$ and timelike $q^2=m_Z^2$ cases. We found that ${\hat{\mu }}_t(-m_Z^2)$ $=$ $-0.0224$ $-0.000925i$ and ${\hat{\mu }}_t(m_Z^2)$ $=$ $-0.0133$ $-0.0267i$. Our $\mathrm{Re}{\hat{\mu }}_t(-m_Z^2)$ matches well with the current experimental value ${\hat{\mu }}_t^{\mathrm{Exp}}=-0.024_{-0.009}^{+0.013}(\mathrm{stat}{)}_{-0.011}^{+0.016}(\mathrm{syst})$, and the $\mathrm{Im}{\hat{\mu }}_t(-m_Z^2)$ part is induced by flavour changing charged currents.

   hep-phEur.Phys.J.Plus(2021)·17 citations
2. 02*

   Discriminating among interpretations for the $X(2900)$ states

   T. J. Burns🇬🇧 · E. S. Swanson🇺🇸

   We make predictions for the production and decays of $X(2900)$ states, and their possible charged partners, in $B^{+}$ and $B^0$ decays, considering a number of competing models for the states, including triangle diagrams mediated by quark exchange or pion exchange, and resonance scenarios including molecules and tetraquarks. Assuming only isospin symmetry and the dominance of colour-favoured weak decays, we find characteristic differences in the predictions of the different models. Future experimental studies can therefore discriminate among the competing interpretations for the states.

   hep-phPRD(2021)·52 citations
3. 03*

   Manifestly Causal Loop-Tree Duality

   Zeno Capatti🇨🇭 · Valentin Hirschi🇨🇭 · Dario Kermanschah🇨🇭 · Andrea Pelloni🇨🇭 · Ben Ruijl🇨🇭

   Loop-Tree Duality (LTD) is a framework in which the energy components of all loop momenta of a Feynman integral are integrated out using residue theorem, resulting in a sum over tree-like structures. Originally, the LTD expression exhibits cancellations of non-causal thresholds between summands, also known as dual cancellations. As a result, the expression exhibits numerical instabilities in the vicinity of non-causal thresholds and for large loop momenta. In this work we derive a novel, generically applicable, Manifestly Causal LTD (cLTD) representation whose only thresholds are causal thresholds, i.e. it manifestly realizes dual cancellations. Consequently, this result also serves as a general proof for dual cancellations. We show that LTD, cLTD, and the expression stemming from Time Ordered Perturbation Theory (TOPT) are locally equivalent. TOPT and cLTD both feature only causal threshold singularities, however LTD features better scaling with the number of propagators. On top of the new theoretical perspectives offered by our representation, it has the useful property that the ultraviolet (UV) behaviour of the original 4D integrand is maintained for every summand. We show that the resulting LTD integrand expression is completely stable in the UV region which is key for practical applications of LTD to the computation of amplitudes and cross sections. We present explicit examples of the LTD expression for a variety of up to four-loop integrals and show that its increased computational complexity can be efficiently mitigated by optimising its numerical implementation. Finally, we provide computer code that automatically generates the LTD expression for an arbitrary topology.

   hep-phhep-th60 citations
4. 04*

   Signals of the QCD axion with mass of 17 MeV/c^2: nuclear transitions and light meson decays

   Daniele S. M. Alves🇺🇸

   The QCD axion remains experimentally viable in the mass range of O(10 MeV) if (i) it couples predominantly to the first generation of SM fermions; (ii) it decays to $e^{+}{e}^{-}$ with a short lifetime ${\tau }_a\lesssim 10^{-13}$s; and (iii) it has suppressed isovector couplings, i.e., if it is piophobic. Remarkably, these are precisely the properties required to explain recently observed anomalies in nuclear de-excitations, to wit: the $e^{+}{e}^{-}$ emission spectra of isoscalar magnetic transitions of ${}^8$Be and ${}^4$He nuclei showed a "bump-like" feature peaked at $m_{e^{+}{e}^{-}}\sim 17$ MeV. In this article, we argue that on-shell emission of the QCD axion (with the aforementioned properties) provides an extremely well-motivated, compatible explanation for the observed excesses in these nuclear de-excitations. The absence of anomalous features in other measured transitions is also naturally explained: piophobic axion emission is strongly suppressed in isovector magnetic transitions, and forbidden in electric transitions. This QCD axion hypothesis is further corroborated by an independent observation: a $2-3\sigma$ deviation in the measurement of $\Gamma ({\pi }^0\to e^{+}{e}^{-})$ from the Standard Model theoretical expectation. This article also includes detailed estimations of various axionic signatures in rare light meson decays, which take into account contributions from low-lying QCD resonance exchange, and, in the case of rare Kaon decays, the possible effective implementations of $\Delta S=1$ octet enhancement in chiral perturbation theory. These inherent uncertainties of the effective description of the strong interactions at low energies result in large variations in the predictions for hadronic signals of the QCD axion; in spite of this, the estimated ranges for rare meson decay rates obtained here can be probed in the near future in $\eta /{\eta }^{\prime }$ and Kaon factories.

   hep-phhep-exnucl-exnucl-thPRD(2021)·55 citations
5. 05*

   Adaptive Multidimensional Integration: VEGAS Enhanced

   G. Peter Lepage🇺🇸

   We describe a new algorithm, VEGAS+, for adaptive multidimensional Monte Carlo integration. The new algorithm adds a second adaptive strategy, adaptive stratified sampling, to the adaptive importance sampling that is the basis for its widely used predecessor VEGAS. Both VEGAS and VEGAS+ are effective for integrands with large peaks, but VEGAS+ can be much more effective for integrands with multiple peaks or other significant structures aligned with diagonals of the integration volume. We give examples where VEGAS+ is 2-19 times more accurate than VEGAS. We also show how to combine VEGAS+ with other integrators, such as the widely available MISER algorithm, to make new hybrid integrators. For a different kind of hybrid, we show how to use integrand samples, generated using MCMC or other methods, to optimize VEGAS+ before integrating. We give an example where preconditioned VEGAS+ is more than 100 times as efficient as VEGAS+ without preconditio ing. Finally, we give examples where VEGAS+ is more than 10 times as efficient as MCMC for Bayesian integrals with D = 3 and 21 parameters. We explain why VEGAS+ will often outperform MCMC for small and moderate sized problems.

   ↳ physics.comp-phhep-phJ.Comput.Phys.(2021)·199 citations
6. 06*

   Formalism of hydrodynamics with spin degrees of freedom

   Rajeev Singh🇵🇱

   In this article we review the perfect-fluid hydrodynamics with spin framework proposed recently. This framework generalises the standard relativistic hydrodynamics framework to include spin degrees of freedom and provides a natural method to describe the spin polarization evolution of massive spin 1/2 particles. This formalism is based on the GLW (de Groot - van Leeuwen - van Weert) energy-momentum tensor and spin tensor. We show here using Bjorken model that how this spin hydrodynamics framework may be used for the determination of the observables which describes the particle polarization measured in the experiment.

   ↳ nucl-thhep-phPoS(2021)·2 citations
7. 07*

   Strangeness-changing Rates and Hyperonic Bulk Viscosity in Neutron Star Mergers

   Mark G. Alford🇺🇸 · Alexander Haber🇺🇸

   In this paper we present a computation of the rates of strangeness-changing processes and the resultant bulk viscosity in matter at the densities and temperatures typical of neutron star mergers. To deal with the high temperature in this environment we go beyond the Fermi surface approximation in our rate calculations and numerically evaluate the full phase space integral. We include processes where quarks move between baryons via meson exchange: these have generally been omitted in previous analyses but provide the dominant contribution to the rates of strangeness-changing processes and the bulk viscosity. The calculation of these rates is an essential step towards any calculation of dissipation mechanisms in hyperonic matter in mergers. As one application, we calculate the dissipation times for density oscillations at the frequencies seen in merger simulations. We find that hyperon bulk viscosity for temperatures in the MeV regime can probably be neglected in this context, but becomes highly relevant for keV-range temperatures.

   ↳ nucl-thastro-ph.HEhep-phPRC(2021)·61 citations
8. 08*

   Limiting Superluminal Neutrino Velocity and Lorentz Invariance Violation by Neutrino Emission from the Blazar TXS 0506+056

   Kai Wang🇨🇳 · Shao-Qiang Xi🇨🇳 · Lijing Shao🇨🇳 · Ruo-Yu Liu🇨🇳 · Zhuo Li🇨🇳 · Zhong-Kai Zhang🇩🇪

   The detection of high-energy neutrino coincident with the blazar TXS 0506+056 provides a unique opportunity to test Lorentz invariance violation (LIV) in the neutrino sector. Thanks to the precisely measured redshift, i.e., $z=0.3365$, the comoving distance of the neutrino source is determined. In this work, we obtain and discuss the constraints on the superluminal neutrino velocity ${\delta }_{\nu }$ and the LIV by considering the energy loss of superluminal neutrino during propagation. Given superluminal electron velocity (${\delta }_e\ge 0$), a very stringent constraint on superluminal neutrino velocity can be reached, i.e., ${\delta }_{\nu }\lesssim 1.3\times 10^{-18}$, corresponding to the quantum gravity (QG) scale $M_{\mathrm{QG},1}\gtrsim 5.7\times 10^3{M}_{\mathrm{Pl}}$ and $M_{\mathrm{QG},2}\gtrsim 9.3\times 10^{-6}{M}_{\mathrm{Pl}}$ for linear (quadratic) LIV, which are $\sim 12$ orders of magnitude tighter for linear LIV and $\sim 9$ orders tighter for quadratic LIV compared to the time-of-flight constraint from MeV neutrinos of SN 1987A. While given the subluminal electron velocity, a weaker constraint on the superluminal neutrino velocity is obtained, i.e., ${\delta }_{\nu }\lesssim 8\times 10^{-17}$, which is consistent with the conclusions of previous works. We also study the neutrino detection probability due to the distortion of neutrino spectral shape during propagation, which gives slightly weaker constraints than above by a factor of $\sim 2$.

   ↳ astro-ph.HEgr-qchep-phPRD(2020)·16 citations
9. 09*

   GW190521 as a merger of Proca stars: a potential new vector boson of $8.7\times 10^{-13}$ eV

   Juan Calderón Bustillo🇪🇸 · Nicolas Sanchis-Gual🇵🇹 · Alejandro Torres-Forné🇩🇪 · José A. Font🇪🇸 · Avi Vajpeyi🇦🇺 · Rory Smith🇦🇺 · Carlos Herdeiro🇨🇱 · Eugen Radu🇨🇱 · Samson H. W. Leong🇨🇳

   Advanced LIGO-Virgo reported a short gravitational-wave signal (GW190521) interpreted as a quasi-circular merger of black holes, one populating the pair-instability supernova gap, forming a remnant black hole of $M_f\sim 142M_{\odot }$ at a luminosity distance of $d_L\sim 5.3$ Gpc. With barely visible pre-merger emission, however, GW190521 merits further investigation of the pre-merger dynamics and even of the very nature of the colliding objects. We show that GW190521 is consistent with numerically simulated signals from head-on collisions of two (equal mass and spin) horizonless vector boson stars (aka Proca stars), forming a final black hole with $M_f=231_{-17}^{+13}{M}_{\odot }$, located at a distance of $d_L=571_{-181}^{+348}$ Mpc. The favoured mass for the ultra-light vector boson constituent of the Proca stars is ${\mu }_{\mathrm{V}}=8.72_{-0.82}^{+0.73}\times 10^{-13}$ eV. This provides the first demonstration of close degeneracy between these two theoretical models, for a real gravitational-wave event. Confirmation of the Proca star interpretation, which we find statistically slightly preferred, would provide the first evidence for a long sought dark matter particle.

   ↳ gr-qcastro-ph.HEhep-phPRL(2021)·290 citations
10. 10*

    Galaxy imaging surveys as spin-sensitive detector for cosmological colliders

    Kazuhiro Kogai🇯🇵 · Kazuyuki Akitsu🇯🇵 · Fabian Schmidt🇩🇪 · Yuko Urakawa🇯🇵

    Galaxy imaging surveys provide us with information on both the galaxy distribution and their shapes. In this paper, we systematically investigate the sensitivity of galaxy shapes to new physics in the initial conditions. For this purpose, we decompose the galaxy shape function into spin components, and compute the contributions to each spin component from both intrinsic alignment and weak lensing. We then consider the angular-dependent primordial non-Gaussianity, which is generated by a non-zero integer spin particle when active during inflation, and show that a galaxy imaging survey essentially functions as a spin-sensitive detector of such particles in the early universe. We also perform a forecast of the PNG generated from a higher spin particle, considering a Rubin Observatory LSST-like galaxy survey.

    ↳ astro-ph.COastro-ph.HEgr-qchep-ph+1JCAP(2021)·56 citations
11. 11*

    Lattice QCD constraints on the parton distribution functions of ${}^3\text{He}$

    William Detmold🇺🇸 · Marc Illa🇪🇸 · David J. Murphy🇺🇸 · Patrick Oare🇺🇸 · Kostas Orginos🇺🇸 · Phiala E. Shanahan🇺🇸 · Michael L. Wagman🇺🇸 · Frank Winter🇺🇸

    The fraction of the longitudinal momentum of ${}^3\text{He}$ that is carried by the isovector combination of $u$ and $d$ quarks is determined using lattice QCD for the first time. The ratio of this combination to that in the constituent nucleons is found to be consistent with unity at the few-percent level from calculations with quark masses corresponding to $m_{\pi }\sim 800$ MeV, extrapolated to the physical quark masses. This constraint is consistent with, and significantly more precise than, determinations from global nuclear parton distribution function fits. Including the lattice QCD determination of the momentum fraction in the nNNPDF global fitting framework results in the uncertainty on the isovector momentum fraction ratio being reduced by a factor of 2.5, and thereby enables a more precise extraction of the $u$ and $d$ parton distributions in ${}^3\text{He}$.

    ↳ hep-lathep-phnucl-thPRL(2021)·33 citations
12. 12*

    Holographic Anisotropic Model for Light Quarks with Confinement-Deconfinement Phase Transition

    Irina Aref'eva🇷🇺 · Kristina Rannu🇷🇺 · Pavel Slepov🇷🇺

    We present a five-dimensional anisotropic holographic model for light quarks supported by Einstein-dilaton-two-Maxwell action. This model generalizing isotropic holographic model with light quarks is characterized by a Van der Waals-like phase transition between small and large black holes. We compare the location of the phase transition for Wilson loops with the positions of the phase transition related to the background instability and describe the QCD phase diagram in the thermodynamic plane -- temperature $T$ and chemical potential $\mu$. The Cornell potential behavior in this anisotropic model is also studied. The asymptotics of the Cornell potential at large distances strongly depend on the parameter of anisotropy and orientation. There is also a nontrivial dependence of the Cornell potential on the boundary conditions of the dilaton field and parameter of anisotropy. With the help of the boundary conditions for the dilaton field one fits the results of the lattice calculations for the string tension as a function of temperature in isotropic case and then generalize to the anisotropic one.

    ↳ hep-thhep-lathep-phJHEP(2021)·57 citations
13. 13*

    Padé Approximants and the analytic structure of the gluon and ghost propagators

    Alexandre F. Falcão🇵🇹

    In a Quantum Field Theory, the analytic structure of the 2-points correlation functions, ie the propagators, encloses information about the properties of the corresponding quanta, particularly if they are or not confined. However, in Quantum Chromodynamics (QCD), we can only have an analytic solution in a perturbative picture of the theory. For the non-perturbative propagators, one resorts on numerical solutions of QCD that accesses specific regions of the Euclidean momentum space, as, for example, those computed via Monte Carlo simulations on the lattice. In the present work, we rely on Padé Approximants (PA) to approximate the numerical data for the gluon and ghost propagators, and investigate their analytic structures. In a first stage, the advantages of using PAs are explored when reproducing the properties of a function, focusing on its analytic structure. The use of PA sequences is tested for the perturbative solutions of the propagators, and a residue analysis is performed to help in the identification of the analytic structure. A technique used to approximate a PA to a discrete set of points is proposed and tested for some test data sets. Finally, the methodology is applied to the Landau gauge gluon and ghost propagators, obtained via lattice simulations. The results identify a conjugate pair of complex poles for the gluon propagator, that is associated with the infrared structure of the theory. This is in line with the presence of singularities for complex momenta in theories where confinement is observed. Regarding the ghost propagator, a pole at $p^2=0$ is identified. For both propagators, a branch cut is found on the real negative $p^2$-axis, which recovers the perturbative analysis at high momenta.

    ↳ hep-lathep-phhep-th1 citation

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.