Fri·Jan 15, 2021Friday·January 15, 2021

1. 01*

   Magnetars and Axion-like Particles: Probes with the Hard X-ray Spectrum

   Jean-François Fortin🇨🇦 · Huai-Ke Guo🇺🇸 · Steven P. Harris🇺🇸 · Elijah Sheridan🇺🇸 · Kuver Sinha🇺🇸

   Quiescent hard X-ray and soft gamma-ray emission from neutron stars constitute a promising frontier to explore axion-like-particles (ALPs). ALP production in the core peaks at energies of a few keV to a few hundreds of keV; subsequently, the ALPs escape and convert to photons in the magnetosphere. The emissivity goes as $\sim T^6$ while the conversion probability is enhanced for large magnetic fields, making magnetars, with their high core temperatures and strong magnetic fields, ideal targets for probing ALPs. We compute the energy spectrum of photons resulting from conversion of ALPs in the magnetosphere and then compare it against hard X-ray data from NuSTAR, INTEGRAL, and XMM-Newton, for a set of eight magnetars for which such data exists. Upper limits are placed on the product of the ALP-nucleon and ALP-photon couplings. For the production in the core, we perform a calculation of the ALP emissivity in degenerate nuclear matter modeled by a relativistic mean field theory. The reduction of the emissivity due to improvements to the one-pion exchange approximation is incorporated, as is the suppression of the emissivity due to proton superfluidity in the neutron star core. A range of core temperatures is considered, corresponding to different models of the steady heat transfer from the core to the stellar surface. For the subsequent conversion, we solve the coupled differential equations mixing ALPs and photons in the magnetosphere. The conversion occurs due to a competition between the dipolar magnetic field and the photon refractive index induced by the external magnetic field. Semi-analytic expressions are provided alongside the full numerical results. We also present an analysis of the uncertainty on the axion limits we derive due to the uncertainties in the magnetar masses, nuclear matter equation of state, and the proton superfluid critical temperature.

   hep-phastro-ph.HEnucl-thJCAP(2021)·41 citations
2. 02*

   First Order Electroweak Phase Transition from Weakly Coupled sub-GeV Physics and Possible Connection to Fermion Flavor

   Hooman Davoudiasl🇺🇸

   We propose that the dynamics of a scalar $\varphi$ of mass $O(10)$ MeV that is weakly coupled to the Higgs can lead to a first order electroweak phase transition, fulfilling a key requirement for baryogenesis. Stability of the model near the weak scale requires a suppressed - possibly vanishing - top Yukawa coupling to the Higgs before the transition which rises to the Standard Model value afterwards. This can be accomplished through the dynamics of $\varphi$ via a dimension-5 operator. We conjecture that the entire Standard Model flavor structure could turn on, mutatis mutandis, after the electroweak phase transition, via dimension-5 interactions of $\varphi$ suppressed by scales ranging from $O(10^3)$ TeV to near Planck mass. Due to its suppressed couplings, $\varphi$ is long-lived and can lead to missing energy signals in rare kaon decays, which can be probed by the KOTO experiment.

   hep-phhep-exPRD(2021)·3 citations
3. 03*

   Test of the 4-th quark generation from the Cabibbo-Kobayashi-Maskawa matrix

   S.R. Juárez Wysozka🇲🇽 · P. Kielanowski🇲🇽

   The structure of the mixing matrix, in the electroweak quark sector with four generations of quarks is investigated. We conclude that the area of the unitarity quadrangle is not a good choice as a possible measure of the CP~violation. In search of new physics we analyze how the existence of the 4-th quark family may influence on the values of the Cabibbo-Kobayashi-Maskawa matrix and we show that one can test for the existence of the 4-th generation using the Jarlskog invariants of the known quarks only. The analysis based on the measured unitary triangle exhibits some tension with the assumption of three quark generations. The measurement of the unitarity triangle obtained from the scalar product of the second row/column of the CKM matrix by the complex conjugate of third row/column can provide information about the existence of the fourth generation of quarks.

   hep-phInt.J.Mod.Phys.A(2021)·0 citations
4. 04*

   Search for the elusive jet-induced diffusion wake in $Z/\gamma$-jets with 2D jet tomography in high-energy heavy-ion collisions

   Zhong Yang🇨🇳 · Wei Chen🇨🇳 · Yayun He🇨🇳 · Weiyao Ke🇺🇸 · Longgang Pang🇨🇳 · Xin-Nian Wang🇨🇳

   Diffusion wake is an unambiguous part of the jet-induced medium response in high-energy heavy-ion collisions that leads to a depletion of soft hadrons in the opposite direction of the jet propagation. New experimental data on $Z$-hadron correlation in Pb+Pb collisions at the Large Hadron Collider show, however, an enhancement of soft hadrons in the direction of both the $Z$ and the jet. Using a coupled linear Boltzmann transport and hydro model, we demonstrate that medium modification of partons from the initial multiple parton interaction (MPI) gives rise to a soft hadron enhancement that is uniform in azimuthal angle while jet-induced medium response and soft gluon radiation dominate the enhancement in the jet direction. After subtraction of the contributions from MPI with a mixed-event procedure, the diffusion wake becomes visible in the near-side $Z$-hadron correlation. We further employ the longitudinal and transverse gradient jet tomography for the first time to localize the initial jet production positions in $Z/\gamma$-jet events in which the effect of the diffusion wake is apparent in $Z/\gamma$-hadron correlation even without the subtraction of MPI.

   hep-phPRL(2021)·77 citations
5. 05*

   Real scalar phase transitions: a nonperturbative analysis

   Oliver Gould🇫🇮

   We study the thermal phase transitions of a generic real scalar field, without a $Z_2$-symmetry, referred to variously as an inert, sterile or singlet scalar, or ${\varphi }^3+{\varphi }^4$ theory. Such a scalar field arises in a wide range of models, including as the inflaton, or as a portal to the dark sector. At high temperatures, we perform dimensional reduction, matching to an effective theory in three dimensions, which we then study both perturbatively to three-loop order and on the lattice. For strong first-order transitions, with large tree-level cubic couplings, our lattice Monte-Carlo simulations agree with perturbation theory within error. However, as the size of the cubic coupling decreases, relative to the quartic coupling, perturbation theory becomes less and less reliable, breaking down completely in the approach to the $Z_2$-symmetric limit, in which the transition is of second order. Notwithstanding, the renormalisation group is shown to significantly extend the validity of perturbation theory. Throughout, our calculations are made as explicit as possible so that this article may serve as a guide for similar calculations in other theories.

   hep-phastro-ph.COcond-mat.stat-mechhep-lat+1JHEP(2021)·68 citations
6. 06*

   $U(1{)}^{\prime }$ extensions of the $\mu \nu$SSM

   J. A. Aguilar-Saavedra🇪🇸 · I. Lara🇵🇱 · D. E. Lopez-Fogliani🇦🇷 · C. Munoz🇪🇸

   In the $\mu \nu$SSM, the presence of $R$-parity violating couplings involving right-handed (RH) neutrinos solves simultaneously the $\mu$- and $\nu$-problems. We explore extensions of the $\mu \nu$SSM adding a $U(1{)}^{\prime }$ gauge group, which provides the RH neutrinos with a non-vanishing charge. In these models, dubbed U$\mu \nu$SSM, the anomaly cancellation conditions impose the presence of exotic quarks in the spectrum that are vector-like under the standard model (SM) gauge group: either three pairs $SU(2)$ quark singlets, or a pair of quark singlets together with a pair of quark doublets. Several extra singlets under the SM group can also be present, with the $U(1{)}^{\prime }$ charges making distinctions among them, and therefore allowing different types of couplings. Some of these singlets dynamically generate Majorana masses for the RH neutrinos, and others can be candidates for dark matter. The useful characteristics of models with $U(1{)}^{\prime }$s are also present in U$\mu \nu$SSM models: baryon-number-violating operators as well as explicit Majorana masses and $\mu$ terms are forbidden, and the domain wall problem is avoided. The phenomenology of U$\mu \nu$SSM models is very rich. We analyze the experimental constraints on their parameter space, specially on the mass and mixing of the new $Z^{\prime }$ boson. In addition to the exotic quarks, which can hadronize inside the detector or decay producing SM particles, the U$\mu \nu$SSM models can also have new signals such as decays of the $Z^{\prime }$ to sparticle pairs like right sneutrinos, charginos or neutralinos. Besides, $Z^{\prime }$ and Higgs mediated annihilations and interactions with the visible sector of WIMP dark matter particles, can also be present.

   hep-phhep-exhep-thEPJC(2021)·6 citations
7. 07*

   Searching for lepton number violating $\Lambda$ baryon decays mediated by GeV-scale Majorana neutrino with LHCb

   Guangshuai Zhang🇨🇳 · Bo-Qiang Ma🇨🇳

   We consider the lepton-number-violating processes in ${\Lambda }_b$/${\Lambda }_c$ decays mediated by on-shell GeV-scale sterile neutrino. We calculate the branching ratio for the following processes: ${\Lambda }_b^0\to {\mathcal{B}}^{+}{\mu }^{-}{\mu }^{-}{\pi }^{+}$, where ${\mathcal{B}}^{+}$ is ${\Lambda }_c^{+}$ or proton, and ${\Lambda }_c^{+}\to \Lambda {\mu }^{+}{\mu }^{+}{\pi }^{-}$, as function of the mass of the sterile neutrino $m_N$ and the heavy-light mixing coefficient of the extended Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix $|U_{\mu N}{|}^2$. The effect of finite detector size is included in our calculation. After comparing the theoretical effective branching ratio with the expected experimental ability of LHCb, we give the sensitivity upper bounds on $|U_{\mu N}{|}^2$ on the ($|U_{\mu N}{|}^2$-$m_N$) plane. These channels give comparable results with the bounds given by different search strategies (e.g., at Belle, NuTeV, DELPHI and BEBC) in the mass region $m_N\simeq$ 0.25 GeV - 4.5 GeV, and the limits are stronger in the mass range 2 GeV $<m_N<$ 4.5 GeV.

   hep-phPRD(2021)·14 citations
8. 08*

   Natural Understanding of Sterile Neutrino by Relativistic Equation

   Keiichi Kimura🇯🇵 · Akira Takamura🇯🇵

   We derive the neutrino oscillation probabilities including sterile neutrinos by using the Dirac equation. If neutrinos have both the Dirac and the Majorana mass terms, left-handed neutrino ${\nu }_L$ and right-handed anti-neutrino ${\nu }_R^c$ enter the same multiplet and can transfer each other. Sterile neutrinos have been introduced by hand as the fourth generation neutrino in many papers, however, we can understand sterile neutrinos naturally in the framework of three generations. We also point out that the oscillations into sterile neutrinos strongly suggest the existence of both the Dirac and the Majorana mass terms, and for neutrinos to be the Majorana particles.

   hep-ph0 citations
9. 09*

   Dark Matter production from relativistic bubble walls

   Aleksandr Azatov🇮🇹 · Miguel Vanvlasselaer🇮🇹 · Wen Yin🇯🇵

   In this paper we present a novel mechanism for producing the observed Dark Matter(DM) relic abundance during the First Order Phase Transition (FOPT) in the early universe. We show that the bubble expansion with ultra-relativistic velocities can lead to the abundance of DM particles with masses much larger than the scale of the transition. We study this non-thermal production mechanism in the context of a generic phase transition and the electroweak phase transition. The application of the mechanism to the Higgs portal DM as well as the signal in the Stochastic Gravitational Background are discussed.

   hep-phastro-ph.COJHEP(2021)·157 citations
10. 10*

    The Polarized Transition Matrix Element $A_{gq}(N)$ of the Variable Flavor Number Scheme at $O({\alpha }_s^3)$

    A. Behring🇩🇪 · J. Blümlein🇩🇪 · A. De Freitas🇩🇪 · A. von Manteuffel🇺🇸 · K. Schönwald🇩🇪 · C. Schneider🇦🇹

    We calculate the polarized massive operator matrix element $A_{gq}^{(3)}(N)$ to 3-loop order in Quantum Chromodynamics analytically at general values of the Mellin variable $N$ both in the single- and double-mass case in the Larin scheme. It is a transition function required in the variable flavor number scheme at $O({\alpha }_s^3)$. We also present the results in momentum fraction space.

    hep-phhep-thNPB(2021)·38 citations
11. 11*

    Long Range Interactions in Cosmology: Implications for Neutrinos

    Ivan Esteban🇺🇸 · Jordi Salvado🇪🇸

    Cosmology is well suited to study the effects of long range interactions due to the large densities in the early Universe. In this article, we explore how the energy density and equation of state of a fermion system diverge from the commonly assumed ideal gas form under the presence of scalar long range interactions with a range much smaller than cosmological scales. In this scenario, "small"-scale physics can impact our largest-scale observations. As a benchmark, we apply the formalism to self-interacting neutrinos, performing an analysis to present and future cosmological data. Our results show that the current cosmological neutrino mass bound is fully avoided in the presence of a long range interaction, opening the possibility for a laboratory neutrino mass detection in the near future. We also demonstrate an interesting complementarity between neutrino laboratory experiments and the future EUCLID survey.

    hep-phastro-ph.COJCAP(2021)·74 citations
12. 12*

    Fermion mass hierarchies from vector-like families with an extended 2HDM and a possible explanation for the electron and muon anomalous magnetic moments

    A. E. Cárcamo Hernández🇨🇱 · S. F. King🇬🇧 · H. Lee🇬🇧

    We study an extended 2 Higgs doublet model (2HDM) in which the Standard Model (SM) Yukawa interactions are forbidden due to a global $U(1{)}^{\prime }$ symmetry, but may arise via mixing with vector-like families. In this model, the hierarchical structure of Yukawa couplings of quarks and leptons in the SM arises from the heavy masses of the fourth and fifth vector-like families. Within this model, we consider various non-standard contributions to the electron and muon anomalous magnetic moments. We first consider the $W$ exchange at one-loop level, consistent with the $\mu \to e\gamma$ constraint, and show that it yields a negligible contribution to both electron and muon anomalous magnetic moments. We then consider Higgs scalar exchange, together with vector-like leptons, at one-loop level and show that it is possible to have non-standard contributions to the electron and muon anomalous magnetic moments within the $1\sigma$ constraint of certain experiments. We present some benchmark points for both the muon and the electron anomalies, together with some numerical scans around these points, which indicate the mass regions of the Higgs scalars of the 2HDM in this scenario.

    hep-phPRD(2021)·40 citations
13. 13*

    Positron Effects on Polarized Images and Spectra from Jet and Accretion Flow Models of M87* and Sgr A*

    Razieh Emami (Center for Astrophysics | Harvard and Smithsonian)🇺🇸 · Richard Anantua (Center for Computational Astrophysics and Center for Astrophysics | Harvard and Smithsonian)🇺🇸 · Andrew A Chael (Princeton University)🇺🇸 · Abraham Loeb (Center for Astrophysics | Harvard and Smithsonian)🇺🇸

    We study the effects of including a nonzero positron-to-electron fraction in emitting plasma on the polarized SEDs and sub-millimeter images of jet and accretion flow models for near-horizon emission from M87* and Sgr A*. For M87*, we consider a semi-analytic fit to the force-free plasma regions of a general relativistic magnetohydrodynamic jet simulation which we populate with power-law leptons with a constant electron-to-magnetic pressure ratio. For Sgr A*, we consider a standard self-similar radiatively inefficient accretion flow where the emission is predominantly from thermal leptons with a small fraction in a power-law tail. In both models, we fix the positron-to-electron ratio throughout the emission region. We generate polarized images and spectra from our models using the general-relativistic ray tracing and radiative transfer from GRTRANS. We find that a substantial positron fraction reduces the circular polarization fraction at infrared and higher frequencies. However, in sub-millimeter images higher positron fractions increase polarization fractions due to strong effects of Faraday conversion. We find a M87* jet model that best matches the available broadband total intensity and 230 GHz polarization data is a sub-equipartition, with positron fraction of $\simeq$ 10%. We show that jet models with significant positron fractions do not satisfy the polarimetric constraints at 230 GHz from the Event Horizon Telescope (EHT). Sgr A* models show similar trends in their polarization fractions with increasing pair fraction. Both models suggest that resolved, polarized EHT images are useful to constrain the presence of pairs at 230 GHz emitting regions of M87* and Sgr A*.

    ↳ astro-ph.HEastro-ph.COastro-ph.GAgr-qc+1ApJ(2021)·29 citations
14. 14*

    Exact Symmetries and Threshold States in Two-Dimensional Models for QCD

    Ross Dempsey🇺🇸 · Igor R. Klebanov🇺🇸 · Silviu S. Pufu🇺🇸

    Two-dimensional SU$(N)$ gauge theory coupled to a Majorana fermion in the adjoint representation is a nice toy model for higher-dimensional gauge dynamics. It possesses a multitude of "gluinoball" bound states whose spectrum has been studied using numerical diagonalizations of the light-cone Hamiltonian. We extend this model by coupling it to $N_f$ flavors of fundamental Dirac fermions (quarks). The extended model also contains meson-like bound states, both bosonic and fermionic, which in the large-$N$ limit decouple from the gluinoballs. We study the large-$N$ meson spectrum using the Discretized Light-Cone Quantization (DLCQ). When all the fermions are massless, we exhibit an exact $\mathfrak{osp}(1|4)$ symmetry algebra that leads to an infinite number of degeneracies in the DLCQ approach. More generally, we show that many single-trace states in the theory are threshold bound states that are degenerate with multi-trace states. These exact degeneracies can be explained using the Kac-Moody algebra of the SU$(N)$ current. We also present strong numerical evidence that additional threshold states appear in the continuum limit. Finally, we make the quarks massive while keeping the adjoint fermion massless. In this case too, we observe some exact degeneracies that show that the spectrum of mesons becomes continuous above a certain threshold. This demonstrates quantitatively that the fundamental string tension vanishes in the massless adjoint QCD${}_2$.

    ↳ hep-thcond-mat.str-elhep-phJHEP(2021)·47 citations
15. 15*

    Anisotropic separate universe and Weinberg's adiabatic mode

    Takahiro Tanaka🇯🇵 · Yuko Urakawa🇯🇵

    In the separate universe approach, an inhomogeneous universe is rephrased as a set of glued numerous homogeneous local patches. This is the essence of the gradient expansion and the $\delta N$ formalism, which have been widely used in solving a long wavelength evolution of the universe. In this paper, we show that the separate universe approach can be generically used, as long as a theory under consideration is local and preserves the spatial diffeomorphism invariance. Focusing on these two conditions, we also clarify the condition for the existence of the so-called Weinberg's adiabatic mode. Remarkably, the separate universe approach and the $\delta N$ formalism turn out to be applicable also to models with shear on large scales and also to modified theories of gravity, accepting violation of four-dimensional diffeomorphism invariance. The generalized $\delta N$ formalism enables us to calculate all the large scale fluctuations, including gravitational waves. We also argue several implications on anisotropic inflation and ultra slow-roll inflation.

    ↳ astro-ph.COgr-qchep-phhep-thJCAP(2021)·19 citations
16. 16*

    Can the gamma-ray bursts travelling through the interstellar space be explained without invoking the drastic assumption of Lorentz invariance violation?

    Masud Chaichian🇫🇮 · Iver Brevik🇳🇴 · Markku Oksanen🇫🇮

    Experimental observations indicate that gamma-ray bursts (GRB) and high-energy neutrino bursts may travel at different speeds with a typical delay measured at the order of hours or days. We discuss two potential interpretations for the GRB delay: dispersion of light in interstellar medium and violation of Lorentz invariance due to quantum gravitational fluctuations. Among a few other media, we consider dispersion of light in an axion plasma, obtaining the axion plasma frequency and the dispersion relation from quantum field theory for the first time. We find that the density of axions inferred from observations is far too low to produce the observed GRB delay. However, a more precise estimation of the spatial distribution of axions is required for a conclusive result. Other known media are also unable to account for the GRB delay, although there remains uncertainties in the observations of the delays. The interpretation in terms of Lorentz invariance violation and modified dispersion relation suffers from its own problems: since the modification of the dispersion relation should not be dependent on particle type, delays between photons and neutrinos are hard to explain. Thus neither interpretation is sufficient to explain the observations. We conclude that a crucial difference between the two interpretations is the frequency dependence of the propagation speed of radiation: in dispersive plasma the group speed increases with higher frequency, while Lorentz invariance violation implies lower speed at higher frequency. Future experiments shall resolve which one of the two frequency dependencies of GRB is actually the case.

    ↳ astro-ph.HEhep-phhep-thPoS(2021)·6 citations
17. 17*

    Radiation Reaction from Soft Theorems

    Paolo Di Vecchia🇩🇰 · Carlo Heissenberg🇸🇪 · Rodolfo Russo🇬🇧 · Gabriele Veneziano🇨🇭

    Radiation reaction (RR) terms at the third post-Minkowskian (3PM) order have recently been found to be instrumental in restoring smooth continuity between the non-relativistic, relativistic, and ultra-relativistic (including the massless) regimes. Here we propose a new and intriguing connection between RR and soft (bremsstrahlung) theorems which short-circuits the more involved conventional loop computations. Although first noticed in the context of the maximally supersymmetric theory, unitarity and analyticity arguments support the general validity of this 3PM-order connection that we apply, in particular, to Einstein's gravity and to its Jordan-Brans-Dicke extension. In the former case we find full agreement with a recent result by Damour obtained through a very different reasoning.

    ↳ hep-thgr-qchep-phPLB(2021)·186 citations
18. 18*

    Post-Merger Jets from Supermassive Black Hole Coalescences as Electromagnetic Counterparts of Gravitational Wave Emission

    Chengchao Yuan🇺🇸 · Kohta Murase🇺🇸 · B. Theodore Zhang🇺🇸 · Shigeo S. Kimura🇯🇵 · Peter Mészáros🇺🇸

    As a powerful source of gravitational waves (GW), a supermassive black hole (SMBH) merger may be accompanied by a relativistic jet that leads to detectable electromagnetic (EM) emission. We model the propagation of post-merger jets inside a pre-merger circumnuclear environment formed by disk winds, and calculate multi-wavelength EM spectra from the forward shock region. We show that the non-thermal EM signals from SMBH mergers are detectable up to the detection horizon of future GW facilities such as the Laser Interferometer Space Antenna (LISA). Calculations based on our model predict slowly fading transients with time delays from days to months after the coalescence, leading to implications for EM follow-up observations after the GW detection.

    ↳ astro-ph.HEgr-qchep-phhep-thApJL(2021)·37 citations
19. 19*

    Remarks on nuclear matter: how an ${\omega }_0$ condensate can spike the speed of sound, and a model of $Z(3)$ baryons

    Robert D. Pisarski🇺🇸

    I make two comments about nuclear matter. First, I consider the effects of a coupling between the $O(4)$ chiral field, $\vec{\varphi }$, and the ${\omega }_{\mu }$ meson, $\sim +{\omega }_{\mu }^2{\vec{\varphi }}^2$; for any net baryon density, a condensate for ${\omega }_0$ is unavoidably generated. I assume that with increasing density, a decrease of the chiral condensate and the effective ${\omega }_0$ mass gives a stiff equation of state (EoS). In order to match that onto a soft EoS for quarkyonic matter, I consider an $O(N)$ field at large $N$, where at nonzero temperature quantum fluctuations disorder any putative pion "condensate" into a quantum pion liquid (Q$\pi$L) (arXiv:2005.10259). In this paper I show that the Q$\pi$L persists at zero temperature. If valid qualitatively at $N=4$, the ${\omega }_0$ mass goes up sharply and suppresses the ${\omega }_0$ condensate. This could generate a spike in the speed of sound at high density, which is of relevance to neutron stars. Second, I propose a toy model of a $Z(3)$ gauge theory with three flavors of fermions, where $Z(3)$ vortices confine fermions into baryons. In $1+1$ dimensions this model can be studied numerically with present techniques, using either classical or quantum computers.

    ↳ nucl-thhep-lathep-phPRD(2021)·49 citations

* 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.