Tue·Apr 7, 2020Tuesday·April 7, 2020

1. 01*

   Degenerate behavior in nonlinear vacuum electrodynamics

   C. A. Escobar🇲🇽 · R. Potting🇵🇹

   We study nonlinear vacuum electrodynamics in the first-order formulation proposed by Plebański. We analyze in detail the equations of motion, and identify conditions for which a singularity can occur for the time derivative of one of the field components. The resulting degenerate behavior can give rise to a shock wave with a reduction of the local number of degrees of freedom. We use an example model to illustrate the occurrence of superluminal propagation for field values approaching the singularity.

   hep-phhep-thPhys.Scripta(2020)·6 citations
2. 02*

   Charged Lepton Flavor Violating processes in Neutrinophilic Higgs+Seesaw model

   Naoyuki Haba🇯🇵 · Tsuneharu Omija🇯🇵 · Toshifumi Yamada🇯🇵

   We investigate charged lepton flavor violating (CFLV) processes in the `neutrinophilic Higgs+seesaw model', in which right-handed neutrinos couple only with an extra Higgs field which develops a tiny VEV and the right-handed neutrinos also have Majorana mass. The model realizes a seesaw mechanism around TeV scale without extremely small Dirac Yukawa couplings. A phenomenological feature of the model is CFLV processes induced by loop diagrams of the charged scalar particles and heavy neutrinos. Therefore, first we constrain the model's parameter space from the search for $\mu \to e\gamma$. Next, we predict the branching ratios of other CFLV processes including the $\mu \to 3e$, $\mu +\mathrm{Al}\to e+\mathrm{Al}$, $\mu +\mathrm{Ti}\to e+\mathrm{Ti}$, $Z\to e\mu$, $Z\to e\tau$, $Z\to \mu \tau$, $h\to e\tau$, $h\to \mu \tau$ processes, and discuss their detectability in future experiments.

   hep-phPTEP(2020)·2 citations
3. 03*

   Electromagnetic dipole, quadrupole moments and parity and time reversal invariance interactions of ${\Omega }^{\pm }$ baryons in bent and straight crystals

   V.G. Baryshevsky🇧🇾

   Experiments with ${\Omega }^{\pm }$ baryons moving in a crystal at LHC in addition to EDM measurement open possibility to study electric quadrupole moment, to observe birefringence effect (spin oscillations and spin dichroism) for ${\Omega }^{\pm }$ baryons and to get information about constants describing P-odd, T-odd effects at ${\Omega }^{\pm }$ baryon interactions with electrons and nuclei

   hep-ph2 citations
4. 04*

   A new approach to radial spectrum of hadrons in bottom-up holographic QCD

   S. S. Afonin🇷🇺

   Within the AdS/CFT correspondence, for description of $\mathcal{N}=4$ super Yang-Mills theory in four dimensions one needs not only low-energy supergravity on AdS${}_5$ but also the whole infinite tower of massive Kaluza-Klein (KK) states on AdS${}_5\times$S${}_5$ which appear after the KK-compactification on five-dimensional sphere. The latter aspect is usually ignored in phenomenological AdS/QCD models. The emerging massive 5D fields on AdS${}_5$ are dual to higher-dimensional operators in 4D gauge theory, with masses being known polynomial functions of canonical dimensions of these operators. Motivated by this observation, we propose to calculate the spectrum of radially excited hadrons in bottom-up holographic QCD models as spectrum of zero KK modes of massive 5D fields dual to higher dimensional operators in QCD. A relevant physical motivation is suggested. The radial states with growing masses are then enumerated by growing dimensions of interpolating QCD operators. We tested the proposal in the Soft Wall and Hard Wall holographic models in the sector of light mesons. The spectrum of Soft Wall model turns out to be unchanged in the new description. But in the Hard Wall model, our approach is shown to lead to a much better phenomenological spectrum of vector radial excitations than the traditional description.

   hep-phhep-thActa Phys.Polon.Supp.(2021)·1 citation
5. 05*

   Probing $Zt\bar{t}$ couplings using $Z$ boson polarization in $ZZ$ production at hadron colliders

   Qing-Hong Cao🇨🇳 · Bin Yan🇺🇸 · C.-P. Yuan🇺🇸 · Ya Zhang🇨🇳

   We propose to utilize the polarization information of the $Z$ bosons in $ZZ$ production, via the gluon-gluon fusion process $gg\to ZZ$, to probe the $Zt\bar{t}$ gauge coupling. The contribution of longitudinally polarized $Z$ bosons is sensitive to the axial-vector component ($a_t$) of the $Zt\bar{t}$ coupling. We demonstrate that the angular distribution of the charged lepton from $Z$ boson decays serves well for measuring the polarization of $Z$ bosons and the determination of $a_t$. We show that $ZZ$ production via the $gg\to ZZ$ process complement to $Zt\bar{t}$ and $tZj$ productions in measuring the $Zt\bar{t}$ coupling at hadron colliders.

   hep-phhep-exPRD(2020)·34 citations
6. 06*

   $T^{\prime }$ Models with High Quality Flaxions

   Christopher D. Carone🇺🇸 · Marco Merchand🇺🇸

   The "gauged'' Peccei-Quinn (PQ) mechanism of Fukuda, Ibe, Suzuki and Yanagida is implemented in the flavorful axion model of Carone and Merchand. This model of flavor is similar to other successful ones based on the double tetrahedral group, but the flavor symmetry includes a global U(1) factor that leads to the presence of a flavorful axion. Here we gauge that U(1) symmetry and introduce a heavy sector that includes (1) the fermions necessary to cancel anomalies and (2) a second scalar flavon field that spontaneously breaks the U(1) symmetry. The full theory has an accidental U(1)$\times$U(1)${}^{\prime }$ global symmetry, anomalous with respect to QCD; U(1)${}_{\text{PQ}}$ emerges as a linear combination. The gauged flavor symmetry restricts the possible PQ symmetry-breaking higher-dimension operators so that sufficient axion quality is preserved. We provide a model of the quark sector, as a proof of principle, and then a model which incorporates the standard model charged leptons as well. In both cases, the charge assignments that lead to acceptable axion quality also lead to a multiplicity of some of the heavy sector states; we check that the Landau pole for hypercharge remains above the cut off of the effective theory. We consider relevant phenomenological constraints on these models including those on the predicted axion couplings.

   hep-phPRD(2020)·7 citations
7. 07*

   $QQ\bar{q}\bar{q}$ in a chiral constituent quark model

   Yue Tan🇨🇳 · Weichang Lu🇨🇳 · Jialun Ping🇨🇳

   Inspired by ${\Xi }_{cc}$ reported by LHCb Collaboration and $X(5568)$ reported by $D0$ Collaboration, the $QQ\bar{q}\bar{q}$ ($Q=c,b,s$, $q=u,d$) tetraquark states, are studied in the present work. With the help of gaussian expansion method, two structures, diquark-antiquark and meson-meson, with all possible color configurations are investigated systematically in a chiral quark model to search for the possible stable states. The results show that there is no bound state in the isovector $QQ\bar{q}\bar{q}$ system, while there are rather deep bound states in the isoscalar $bb\bar{q}\bar{q}$, $cc\bar{q}\bar{q}$ and $bc\bar{q}\bar{q}$ systems. There are also several shallow bound states in $Q{Q}^{\prime }$ system. Mixing two structures of diquark-antidiquark and meson-meson can introduce more attractions and convert some unbound isoscalar states into shallow bound states. The large mass of the heavy quark is beneficial to the formation of the bound state. The separations between quarks are calculated to unravel the spacial structure of the system.

   hep-phEur.Phys.J.Plus(2020)·71 citations
8. 08*

   New study of the Boer-Mulders function: Implications for the quark and hadron transverse momenta

   E. Christova🇧🇬 · D. Kotlorz🇵🇱 · E. Leader🇬🇧

   In series of papers the Boer-Mulders function for a given quark flavour has been extracted: (i) from data on semi-inclusive deep inelastic scattering, using the simplifying, but theoretically inconsistent, assumption that it is proportional to the Sivers function for each quark flavour and (ii) from data on Drell-Yan reactions. In earlier papers, using the semi-inclusive deep inelastic COMPASS deuteron data on the $⟨\cos {\varphi }_h⟩$ and $⟨\cos 2{\varphi }_h⟩$ asymmetries, we extracted the collinear $x_{{}_B}$ dependence of the Boer-Mulders function for the sum of the valence quarks $Q_V=u_V+d_V$ using a small number of model dependent assumptions and found a significant disagreement with the analysis in (i). In the present paper, we provide a more complete analysis of the semi-inclusive deep inelastic scattering reaction, including a discussion of higher twist and interaction-dependent terms, and also a comparison with the Boer-Mulders function extracted from data on the Drell-Yan reaction. We confirm that the proportionality relation of the BM function to the Sivers function, for each quark flavour, fails badly, but find that it holds rather well if applied to the nonsinglet valence-quark combination, $Q_V$. We also find good agreement with the results of the Drell-Yan analysis. Furthermore, we obtain interesting information on the quark transverse momentum densities in the nucleon and on the hadron transverse momentum dependence in quark fragmentation.

   hep-phPRD(2020)·7 citations
9. 09*

   Kinetic description of vacuum $e^{+}{e}^{-}$ production in strong electric fields of arbitrary polarization

   I. A. Aleksandrov🇷🇺 · V. V. Dmitriev🇷🇺 · D. G. Sevostyanov🇷🇺 · S. A. Smolyansky🇷🇺

   We present a detailed analysis of the self-consistent system of kinetic equations (KEs) describing electron-positron pair production from vacuum under the action of a spatially homogeneous time-dependent electric field of arbitrary polarization. The physical significance of all the basic functions of the kinetic theory is ascertained. It is demonstrated that the total system of the KEs consists of two coupled quasiparticle and spin subsystems with their integrals of motion. A projection method is proposed in order to obtain the KE system in two particular cases: linearly polarized external electric field and (2+1)-dimensional description of quasiparticles in graphene. We also address the energy conservation law taking into account the internal plasma field and describe an alternative rigorous derivation of the KE system.

   hep-phphysics.plasm-phEur.Phys.J.ST(2020)·19 citations
10. 10*

    Brief history of the search for critical structures in heavy-ion collisions

    Marek Gazdzicki🇵🇱 · Mark Gorenstein🇺🇦 · Peter Seyboth🇵🇱

    The paper briefly presents history, status, and plans of the search for the critical structures - the onset of fireball, the onset of deconfinement, and the deconfinement critical point - in high energy nucleus-nucleus collisions. First, the basic ideas are introduced, the history of the observation of strongly interacting matter in heavy ion collisions is reviewed, and the path towards the quark-gluon plasma discovery is sketched. Then the status of the search for critical structures is discussed - the discovery of the onset of deconfinement, indications for the onset of fireball, and still inconclusive results concerning the deconfinement critical point. Finally, an attempt to formulate priorities for future measurements - charm quarks vs the onset of deconfinement and detailed study of the onset of fireball - closes the paper.

    hep-phnucl-exnucl-thActa Phys.Polon.B(2020)·10 citations
11. 11*

    Dirac neutrinos in the seesaw mechanism. Dirac lepton number violation

    Igor T. Dyatlov🇷🇺

    The seesaw mechanism explains the exclusive smallness of neutrino masses by the presence of very heavy Majorana masses and leads to the appearance of Majorana particles and to the direct lepton number violation. The author proposes a seesaw scenario that produces only Dirac neutrinos with the same violation. This scenario appears possible for heavy neutrinos with non-perturbative Higgs boson H couplings. Such a scenario could be accomplished in the model describing the structure of weak mixing matrices for quarks and leptons through the existence of very heavy mirror analogs of Standard Model fermions. The non-perturbativity of the problem hinders the analytical solution, but the conditions derived indicate that the mechanism under consideration preferentially generates only Dirac neutrinos. This phenomenon may have relevance for leptogenesis processes if all existing neutrinos turn out to be of the Dirac type.

    hep-phPhys.Atom.Nucl.(2020)·0 citations
12. 12*

    Improved opacity expansion at NNLO for medium induced gluon radiation

    João Barata🇪🇸 · Yacine Mehtar-Tani🇺🇸

    When an energetic parton propagates in a hot and dense QCD medium it loses energy by elastic scatterings or by medium-induced gluon radiation. The gluon radiation spectrum is suppressed at high frequency due to the LPM effect and encompasses two regimes that are known analytically: at high frequencies $\omega >{\omega }_c=\hat{q}{L}^2$, where $\hat{q}$ is the jet quenching transport coefficient and $L$ the length of the medium, the spectrum is dominated by a single hard scattering, whereas the regime $\omega <{\omega }_c$ is dominated by multiple low momentum transfers. In this paper, we extend a recent approach (dubbed the Improved Opacity Expansion (IOE)), which allows an analytic (and systematic) treatment beyond the multiple soft scattering approximation, matching this result with the single hard emission spectrum. We calculate in particular the NNLO correction analytically and numerically and show that it is strongly suppressed compared to the NLO indicating a fast convergence of the IOE scheme and thus, we conclude that it is sufficient to truncate the series at NLO. We also propose a prescription to compare the GW and the HTL potentials and relate their parameters for future phenomenological works.

    hep-phJHEP(2020)·54 citations
13. 13*

    Gravitational Waves from Pati-Salam Dynamics

    Wei-Chih Huang🇩🇰 · Francesco Sannino🇩🇰 · Zhi-Wei Wang🇩🇰

    We show that it is possible to use gravitational wave detectors to observe the occurrence of a first order phase transition in Pati-Salam extensions of the Standard Model. We show that the peak frequency of the expected gravitational wave signals ranges within $0.1-10$ Hz. We find amusing that the next generation of gravity waves detectors are able to explore time honoured extensions of the Standard Model occurring at energy scales inaccessible by present and future particle physics accelerators.

    hep-phastro-ph.COhep-thPRD(2020)·72 citations
14. 14*

    Canonical interpretations of the newly observed ${\Xi }_c(2923{)}^0$, ${\Xi }_c(2939{)}^0$ and ${\Xi }_c(2965{)}^0$ resonances

    Qi-Fang Lü🇨🇳

    Three neutral resonances ${\Xi }_c(2923{)}^0$, ${\Xi }_c(2939{)}^0$ and ${\Xi }_c(2965{)}^0$ have been observed in the ${\Lambda }_c^{+}{K}^{-}$ mass spectrum by the LHCb Collaboration. Given the ${\Xi }_c$ and ${\Xi }_c^{\prime }$ mass spectra predicted by the constituent quark models, these three resonances are tentatively treated as the $\lambda -$mode ${\Xi }_c(2S)$, ${\Xi }_c^{\prime }(2S)$, ${\Xi }_c^{\prime \ast }(2S)$, and ${\Xi }_c^{\prime }(1P)$ states, and their strong decay behaviors are calculated within the ${}^3{P}_0$ model. Our results indicate that the ${\Xi }_c(2923{)}^0$ and ${\Xi }_c(2939{)}^0$ can be clarified into the $J^P=3/2^{-}$ and $5/2^{-}$ ${\Xi }_c^{\prime }(1P)$ states respectively, and the ${\Xi }_c(2965{)}^0$ can be regarded as the $J^P=1/2^{+}$ ${\Xi }_c^{\prime }(2S)$ state. Also, we suggest that the previous observed ${\Xi }_c(2930)$ may be the overlap of two structures ${\Xi }_c(2923)$ and ${\Xi }_c(2939)$, and the ${\Xi }_c(2970)$ should be the same state as ${\Xi }_c(2965)$. Other theoretical information on the missing ${\Xi }_c(2S)$, ${\Xi }_c^{\prime \ast }(2S)$, and ${\Xi }_c^{\prime }(1P)$ states may be helpful for future experimental searches.

    hep-phhep-exnucl-thEPJC(2020)·32 citations
15. 15*

    Three Dimensional Imaging of Proton in Basis Light-Front Quantization

    Siqi Xu🇨🇳 · Chandan Mondal🇨🇳 · Jiangshan Lan🇨🇳 · Xingbo Zhao🇨🇳 · Yang Li · James P. Vary🇺🇸

    We employ an effective Hamiltonian that includes the transverse and longitudinal confinement and the one-gluon exchange interaction with fixed coupling constant. By solving the eigenvalue equation in basis light-front quantization (BLFQ), we generate the light-front wavefunctions (LFWFs) for the nucleon in the valence quark Fock space. Fitting the model parameters, we obtain high-quality descriptions of electromagnetic form factors and radius for proton while the results deviate somewhat from experimental data for neutron.

    hep-phhep-thHadron 2019, 607-611·2 citations
16. 16*

    Helicity form factors for $D_{(s)}\to A\ell \nu$ process in the light-cone QCD sum rules approach

    S. Momeni🇮🇷

    The helicity form factors of the $D_{(s)}\to A{\ell }^{+}\nu$ with $A=a_1^{-},a_1^0,b_1^{-},b_1^0,K_1(1270)$ and $K_1(1400)$ are calculated in the light-cone sum rules approach, up to twist-3 distribution amplitudes of the axial vector meson $A$. In the helicity form factors parametrization the unitarity constraints are applied to the fitting parameters. In addition, the effects of the low-lying resonances are included in series expansions of aforementioned form factors. The properties of the $D_{(s)}\to A{\ell }^{+}\nu$ semileptonic decays are studied by extending the form factors to the whole physical region of $q^2$. For a better analysis, a comparison is also made between our results and the predictions obtained using transition form factors via LCSR, 3PSR and CLFQM methods.

    hep-phEPJC(2020)·11 citations
17. 17*

    Production of exotic particles in electron-positron collisions

    Werner K. Sauter🇧🇷

    In this work is presented several results for cross sections of exotic particles production in electron-positron collisions in future linear electron-positron colliders. It is shown that the central production of exotic leptons, dilatons, radions, axions and technipions in this type of collision can be measurable for the predicted kinematic parameters for linear colliders.

    hep-phBraz.J.Phys.(2023)·1 citation
18. 18*

    The color dipole picture for prompt photon production in $pp$ and $pPb$ collisions at the CERN-LHC

    G. Sampaio dos Santos🇧🇷 · G. Gil da Silveira🇧🇷 · M. V. T. Machado🇧🇷

    A study on the prompt photon production within the QCD color dipole picture with emphasis in $pp$ and $pA$ collisions at the LHC energy regimes is performed. We present predictions for the differential cross section as a function of photon transverse momentum at different rapidity bins considering updated phenomenological color dipole models, which take into account the QCD gluon saturation physics. The results are directly compared to the recent experimental measurements provided by CMS and ATLAS Collaborations, showing a reasonable agreement in all rapidity bins with no free parameters. Special attention is given to the IPSAT model given its good description of the data in all rapidity bins from low- to high-$p_T$ ranges. As a result, a free-parameter approach has succeeded in describing the LHC data for prompt photon production, while new predictions for the 13-TeV data is presented in view of new data to confirm such prospect.

    hep-phhep-exEPJC(2020)·15 citations
19. 19*

    Searching for solar axions using data from the Sudbury Neutrino Observatory

    Aagaman Bhusal🇨🇳 · Nick Houston🇨🇳 · Tianjun Li🇨🇳

    We explore a novel detection possibility for solar axions, which relies only on their couplings to nucleons, via the axion-induced dissociation of deuterons into their constituent neutrons and protons. An opportune target for this process is the now-concluded Sudbury Neutrino Observatory (SNO) experiment, which relied upon large quantities of heavy water to resolve the solar neutrino problem. From the full SNO dataset we exclude in a model-independent fashion isovector axion-nucleon couplings $|g_{aN}^3|\equiv \frac{1}{2}|g_{an}-g_{ap}|>2\times 10^{-5}$GeV${}^{-1}$ at 95 % C.L. for sub-MeV axion masses, covering previously unexplored regions of the axion parameter space. In the absence of a precise cancellation between $g_{an}$ and $g_{ap}$ this result also exceeds comparable constraints from other laboratory experiments, and excludes regions of the parameter space for which astrophysical constraints from SN1987A and neutron star cooling are inapplicable due to axion trapping.

    hep-phastro-ph.COPRL(2021)·65 citations
20. 20*

    A bottom-up approach within the electroweak effective theory: constraining heavy resonances

    Antonio Pich🇪🇸 · Ignasi Rosell🇪🇸 · Juan José Sanz-Cillero🇪🇸

    The LHC has confirmed the existence of a mass gap between the known particles and possible new states. Effective field theory is then the appropriate tool to search for low-energy signals of physics beyond the Standard Model. We adopt the general formalism of the electroweak effective theory, with a non-linear realization of the electroweak symmetry breaking, where the Higgs is a singlet with independent couplings. At higher energies we consider a generic resonance Lagrangian which follows the above-mentioned non-linear realization and couples the light particles to bosonic heavy resonances with $J^P=0^{\pm }$ and $J^P=1^{\pm }$. Integrating out the resonances and assuming a proper short-distance behavior, it is possible to determine or to constrain most of the bosonic low-energy constants in terms of resonance masses. Therefore, the current experimental bounds on these bosonic low-energy constants allow us to constrain the resonance masses above the TeV scale, by following a typical bottom-up approach, i.e., the fit of the low-energy constants to precise experimental data enables us to learn about the high-energy scales, the underlying theory behind the Standard Model.

    hep-phPRD(2020)·18 citations
21. 21*

    Hunting for Scalar Lepton Partners at Future Electron Colliders

    Sebastian Baum🇺🇸 · Pearl Sandick🇺🇸 · Patrick Stengel🇸🇪

    New physics close to the electroweak scale is well motivated by a number of theoretical arguments. However, colliders, most notably the Large Hadron Collider (LHC), have failed to deliver evidence for physics beyond the Standard Model. One possibility for how new electroweak-scale particles could have evaded detection so far is if they carry only electroweak charge, i.e. are color neutral. Future $e^{+}{e}^{-}$ colliders are prime tools to study such new physics. Here, we investigate the sensitivity of $e^{+}{e}^{-}$ colliders to scalar partners of the charged leptons, known as sleptons in supersymmetric extensions of the Standard Model. In order to allow such scalar lepton partners to decay, we consider models with an additional neutral fermion, which in supersymmetric models corresponds to a neutralino. We demonstrate that future $e^{+}{e}^{-}$ colliders would be able to probe most of the kinematically accessible parameter space, i.e. where the mass of the scalar lepton partner is less than half of the collider's center-of-mass energy, with only a few days of data. Besides constraining more general models, this would allow to probe some well motivated dark matter scenarios in the Minimal Supersymmetric Standard Model, in particular the incredible bulk and stau co-annihilation scenarios.

    hep-phhep-exPRD(2020)·18 citations
22. 22*

    Flavor Constraints from Unitarity and Analyticity

    Grant N. Remmen🇺🇸 · Nicholas L. Rodd🇺🇸

    We use unitarity and analyticity of scattering amplitudes to constrain fermionic operators in the standard model effective field theory. For four-fermion operators at mass dimension 8, we scatter flavor superpositions in fixed standard model representations and find the Wilson coefficients to be constrained so that their contraction with any pair of pure density matrices is positive. These constraints imply that flavor-violating couplings are upper-bounded by their flavor-conserving cousins. For instance, LEP data already appears to preclude certain operators in upcoming $\mu \to 3e$ measurements.

    hep-phhep-thPRL(2020)·90 citations
23. 23*

    Gauge Boson Signals at the Cosmological Collider

    Lian-Tao Wang🇺🇸 · Zhong-Zhi Xianyu🇺🇸

    We study the production of massive gauge bosons during inflation from the axion-type coupling to the inflaton and the corresponding oscillatory features in the primordial non-Gaussianity. In a window in which both the gauge boson mass and the chemical potential are large, the signal is potentially reachable by near-future large scale structure probes. This scenario covers a new region in oscillation frequency which is not populated by previously known cosmological collider models. We also demonstrate how to properly include the exponential factor and discuss the subtleties in obtaining power dependence of the gauge boson mass in the signal estimate.

    hep-phastro-ph.COhep-thJHEP(2020)·90 citations
24. 24*

    Fingerprint of Low-Scale Leptogenesis in the Primordial Gravitational-Wave Spectrum

    Simone Blasi🇩🇪 · Vedran Brdar🇩🇪 · Kai Schmitz🇨🇭

    The dynamical generation of right-handed-neutrino (RHN) masses in the early Universe naturally entails the formation of cosmic strings that give rise to an observable signal in gravitational waves (GWs). Here, we show that a characteristic break in the GW spectrum would provide evidence for a new stage in the cosmological expansion history and a suppression of the RHN mass scale compared to the scale of spontaneous symmetry breaking. The detection of such a spectral feature would thus represent a novel and unique possibility to probe the physics of RHN mass generation in regions of parameter space that allow for low-scale leptogenesis in accord with electroweak naturalness.

    hep-phastro-ph.COgr-qcPRResearch(2020)·126 citations
25. 25*

    Probing the Relaxed Relaxion at the Luminosity and Precision Frontiers

    Abhishek Banerjee🇮🇱 · Hyungjin Kim🇮🇱 · Oleksii Matsedonskyi🇮🇱 · Gilad Perez🇮🇱 · Marianna S. Safronova🇺🇸

    Cosmological relaxation of the electroweak scale is an attractive scenario addressing the gauge hierarchy problem. Its main actor, the relaxion, is a light spin-zero field which dynamically relaxes the Higgs mass with respect to its natural large value. We show that the relaxion is generically stabilized at a special position in the field space, which leads to suppression of its mass and potentially unnatural values for the model's effective low-energy couplings. In particular, we find that the relaxion mixing with the Higgs can be several orders of magnitude above its naive naturalness bound. Low energy observers may thus find the relaxion theory being fine-tuned although the relaxion scenario itself is constructed in a technically natural way. More generally, we identify the lower and upper bounds on the mixing angle. We examine the experimental implications of the above observations at the luminosity and precision frontiers. A particular attention is given to the impressive ability of future nuclear clocks to search for rapidly oscillating scalar ultra-light dark matter, where the future projected sensitivity is presented.

    hep-phphysics.atom-phJHEP(2020)·85 citations
26. 26*

    Relativistic Borromean States

    Ziyue Wang🇨🇳 · Shao-Jian Jiang🇨🇳 · Yin Jiang🇨🇳

    In this work the existence of Borromean states has been discussed for bosonic and fermionic cases in both the relativistic and non-relativistic limits from the 3-momentum shell renormalization. With the linear bosonic model we checked the existence of Efimov-like states in the bosonic system. In both limits a geometric series of singularities are found in the 3-boson interaction vertex, while the energy ratio is reduced by around 70\% in the relativistic limit because of the anti-particle contribution. Motivated by the quark-diquark model in heavy baryon studies, we have carefully examined the p-wave quark-diquark interaction and found an isolated Borromean pole at finite energy scale. This may indicate a special baryonic state of light quarks in high energy quark matters. In other cases trivial results are obtained as expected. In relativistic limit, for both bosonic and fermionic cases, potential Borromean states are independent of the mass, which means the results would be valid even in zero-mass limit as well.

    hep-phcond-mat.quant-gashep-thnucl-thCPC(2021)·0 citations
27. 27*

    Threshold ${\pi }^{-}$ Photoproduction on the Neutron

    W.J. Briscoe (GWU)🇺🇸 · A.E. Kudryavtsev (ITEP)🇺🇸 · I.I. Strakovsky (GWU)🇺🇸 · V.E. Tarasov (ITEP)🇷🇺 · R.L. Workman (GWU)🇺🇸

    Recent data from the PIONS$@$MAX-lab Collaboration, measuring the total cross section of the pion incoherent photoproduction $\gamma d\to {\pi }^{-}pp$ near threshold, have been used to extract the E${}_{0+}$ multipole and total cross section of the reaction $\gamma n\to {\pi }^{-}p$, also near threshold. These are the first measurements of the reaction $\gamma d\to {\pi }^{-}pp$ in the threshold region. The value of E${}_{0+}$ is extracted through a fit to the deuteron data in a photoproduction model accounting for final-state interactions. The model takes an $S$-wave approximation for the elementary reaction $\gamma n\to {\pi }^{-}p$ with E${}_{0+}=$ const in the threshold region. The obtained value E${}_{0+}=-31.86\pm 0.8$ (in $10^{-3}/m_{{\pi }^{+}}$ units) is in agreement with other existing results. Model predictions for the total cross section $\sigma (\gamma n\to {\pi }^{-}p)$ are also given.

    ↳ nucl-thhep-exhep-phnucl-exEPJA(2020)·16 citations
28. 28*

    Correlation between mean transverse momentum and anisotropic flow in heavy-ion collisions

    Giuliano Giacalone🇫🇷 · Fernando G. Gardim🇧🇷 · Jacquelyn Noronha-Hostler🇺🇸 · Jean-Yves Ollitrault🇫🇷

    The correlation between the mean transverse momentum of outgoing particles, $⟨p_t⟩$, and the magnitude of anisotropic flow, $v_n$, has recently been measured in Pb+Pb collisions at the CERN Large Hadron Collider, as a function of the collision centrality. We confirm the previous observation that event-by-event hydrodynamics predicts a correlation between $v_n$ and $⟨p_t⟩$ that is similar to that measured in data. We show that the magnitude of this correlation can be directly predicted from the initial condition of the hydrodynamic calculation, for $n=2,3$, if one replaces $v_n$ by the corresponding initial-state anisotropy, ${\epsilon }_n$, and $⟨p_t⟩$ by the total energy per unit rapidity of the fluid at the beginning of the hydrodynamic expansion.

    ↳ nucl-thhep-phnucl-exPRC(2021)·83 citations
29. 29*

    Necessity and Insufficiency of Scale Invariance for solving Cosmological Constant Problem

    Taichiro Kugo🇯🇵

    A scenario based on the scale invariance for explaining the vanishing cosmological constant (CC) is discussed. I begin with a notice on the miraculous fact of the CC problem that the vacuum energies totally vanish at each step of hierarchical and successive spontaneous symmetry breakings. I then argue that the classical scale invariance is a necessary condition for the calculability of the vacuum energy. Next, I discuss how sufficient the scale invariance is for solving the CC problem. First in the framework of classical field theory, the scale invariance is shown to give a natural mechanism for realizing the miracle of vanishing vacuum energies at every step of spontaneous symmetry breakings. Then adopting Englert-Truffin-Gastmans' prescription to maintain the scale invariance in quantum field theory, I point out that the quantum scale invariance alone is not yet sufficient to avoid the superfine tuning of coupling constants for realizing vanishingly small cosmological constant, whereas the hierarchy problem may be solved. Another symmetry or a mechanism is still necessary which protects the flat direction of the potential against the radiative corrections.

    ↳ hep-thhep-phPoS(2020)·4 citations
30. 30*

    Causality and Stability Conditions of a Conformal Charged Fluid

    Farid Taghinavaz🇮🇷

    In this paper, I study the conditions imposed on a normal charged fluid so that the causality and stability criteria hold for this fluid. I adopt the newly developed General Frame (GF) notion in the relativistic hydrodynamics framework which states that hydrodynamic frames have to be fixed after applying the stability and causality conditions. To my purpose, I take a charged conformal matter in the flat and $3+1$ dimension to analyze better these conditions. The causality condition is applied by looking to the asymptotic velocity of sound hydro modes at the large wave number limit and stability conditions are imposed by looking to the imaginary parts of hydro modes as well as the Routh-Hurwitz criteria. By fixing some of the transports, the suitable spaces for other ones are derived. I have observed that in a dense medium with finite $U(1)$ charged chemical potential ${\mu }_0$, negative values for transports appear and the second law of thermodynamics has not ruled out the existence of such values. Sign of scalar transports are not limited by any constraints and just a combination of vector transports is limited by the second law of thermodynamic. Also numerically it is proved that the most favorable region for transports ${\tilde{\gamma }}_{1,2}$, coefficients of the dissipative terms of the current, is of negative values.

    ↳ hep-thhep-phnucl-thJHEP(2020)·14 citations
31. 31*

    First Measurement of Electron Neutrino Scattering Cross Section on Argon

    ArgoNeuT Collaboration: R. Acciarri🇺🇸 · C. Adams🇺🇸 · J. Asaadi🇺🇸 · B. Baller🇺🇸 · V. Basque🇬🇧 · T. Bolton🇺🇸 · C. Bromberg🇺🇸 · F. Cavanna🇺🇸 · D. Edmunds🇺🇸 · R. S. Fitzpatrick🇺🇸 · B. Fleming🇺🇸 · P. Green🇬🇧 and 11 other authors

    We report the first electron neutrino cross section measurements on argon, based on data collected by the ArgoNeuT experiment running in the GeV-scale NuMI beamline at Fermilab. A flux-averaged ${\nu }_e+{\overline{\nu }}_e$ total and a lepton angle differential cross section are extracted using 13 ${\nu }_e$ and ${\overline{\nu }}_e$ events identified with fully-automated selection and reconstruction. We employ electromagnetic-induced shower characterization and analysis tools developed to identify ${\nu }_e/{\overline{\nu }}_e$-like events among complex interaction topologies present in ArgoNeuT data ($⟨E_{{\bar{\nu }}_e}⟩=4.3$ GeV and $⟨E_{{\nu }_e}⟩=10.5$ GeV). The techniques are widely applicable to searches for electron-flavor appearance at short- and long-baseline using liquid argon time projection chamber technology. Notably, the data-driven studies of GeV-scale ${\nu }_e/{\overline{\nu }}_e$ interactions presented in this Letter probe an energy regime relevant for future DUNE oscillation physics.

    ↳ hep-exhep-phPRD(2020)·35 citations
32. 32*

    Analytic Study of Magnetic Catalysis in Holographic QCD

    Song He🇨🇳 · Yi Yang🇹🇼 · Pei-Hung Yuan🇨🇳

    We explore the effect of the magnetic field on the QCD phase transition through AdS/CFT correspondence. By introducing an anisotropic magnetic field in the Einstein-Maxwell-Scalar system, a family of analytic solutions is obtained by the potential reconstruction method where the contribution of the magnetic field in the blackening background can be analytically derived. After imposing the kinetic gauge function by requesting the linear Regge spectrum of $J/\psi$ mesons, the contribution of the magnetic field phase diagram can be demonstrated. The results show that the transition temperature will be raising as the magnetic field increases, which is the so-call magnetic catalysis effect. However, if the system is in a strong enough magnetic environment, the transition temperature will be cool down and display the inverse catalysis effect.

    ↳ hep-thhep-ph42 citations
33. 33*

    Presupernova neutrinos: directional sensitivity and prospects for progenitor identification

    Mainak Mukhopadhyay🇺🇸 · Cecilia Lunardini🇺🇸 · F. X. Timmes🇺🇸 · Kai Zuber🇩🇪

    We explore the potential of current and future liquid scintillator neutrino detectors of O (10) kt mass to localize a pre-supernova neutrino signal in the sky. In the hours preceding the core collapse of a nearby star (at distance D < 1 kpc), tens to hundreds of inverse beta decay events will be recorded, and their reconstructed topology in the detector can be used to estimate the direction to the star. Although the directionality of inverse beta decay is weak (~8% forward-backward asymmetry for currently available liquid scintillators), we find that for a fiducial signal of 200 events (which is realistic for Betelgeuse), a positional error of ~60 degrees can be achieved, resulting in the possibility to narrow the list of potential stellar candidates to less than ten, typically. For a configuration with improved forward-backward asymmetry (~40%, as expected for a lithium-loaded liquid scintillator), the angular sensitivity improves to ~15 degrees, and - when a distance upper limit is obtained from the overall event rate - it is in principle possible to uniquely identify the progenitor star. Any localization information accompanying an early supernova alert will be useful to multi-messenger observations and to particle physics tests using collapsing stars.

    ↳ astro-ph.HEhep-phnucl-thApJ(2020)·61 citations
34. 34*

    Beauty mesons in $N_f=2+1+1+1$ lattice QCD with exact chiral symmetry

    Ting-Wai Chiu🇹🇼

    We present the first study of $N_f=2+1+1+1$ lattice QCD with domain-wall quarks. The $(b,c,s)$ quarks are physical, while the $(u,d)$ quarks are heavier than their physical masses, with the pion mass $\sim 700$ MeV. The gauge ensemble is generated by hybrid Monte Carlo simulation with the Wilson gauge action for the gluons, and the optimal domain-wall fermion action for the quarks. Using point-to-point quark propagators, we measure the time-correlation functions of quark-antiquark meson interpolators with quark contents $\bar{b}b$, $\bar{b}c$, $\bar{b}s$, and $\bar{c}c$, and obtain the masses of the low-lying mesons. They are in good agreement with the experimental values, plus some predictions which have not been observed in experiments. Moreover, we also determine the masses of $(b,c,s)$ quarks.

    ↳ hep-lathep-phPRD(2020)·7 citations
35. 35*

    Evidence for a vector charmonium-like state in $e^{+}{e}^{-}\to D_s^{+}{D}_{s2}^{\ast }(2573{)}^{-}+c.c.$

    Belle Collaboration: S. Jia🇨🇳 · C. P. Shen🇨🇳 · I. Adachi🇯🇵 · H. Aihara🇯🇵 · S. Al Said🇸🇦 · D. M. Asner🇺🇸 · H. Atmacan🇺🇸 · V. Aulchenko🇷🇺 · R. Ayad🇸🇦 · I. Badhrees🇨🇦 · P. Behera🇮🇳 · K. Belous🇷🇺 and 163 other authors

    We report the measurement of $e^{+}{e}^{-}\to D_s^{+}{D}_{s2}^{\ast }(2573{)}^{-}+c.c.$ via initial-state radiation using a data sample of an integrated luminosity of 921.9 fb${}^{-1}$ collected with the Belle detector at the ${\rm Y}(4S)$ and nearby. We find evidence for an enhancement with a 3.4$\sigma$ significance in the invariant mass of $D_s^{+}{D}_{s2}^{\ast }(2573{)}^{-}+c.c.$ The measured mass and width are $(4619.8_{-8.0}^{+8.9}(stat.)\pm 2.3(syst.))\mathrm{MeV}/c^2$ and $(47.0_{-14.8}^{+31.3}(stat.)\pm 4.6(syst.))\mathrm{MeV}$, respectively. The mass, width, and quantum numbers of this enhancement are consistent with the charmonium-like state at 4626 MeV/$c^2$ recently reported by Belle in $e^{+}{e}^{-}\to D_s^{+}{D}_{s1}(2536{)}^{-}+c.c.$ The product of the $e^{+}{e}^{-}\to D_s^{+}{D}_{s2}^{\ast }(2573{)}^{-}+c.c.$ cross section and the branching fraction of $D_{s2}^{\ast }(2573{)}^{-}\to {\bar{D}}^0{K}^{-}$ is measured from $D_s^{+}{D}_{s2}^{\ast }(2573{)}^{-}$ threshold to 5.6 GeV.

    ↳ hep-exhep-phPRD(2020)·29 citations
36. 36*

    Sparse modeling approach to obtaining the shear viscosity from smeared correlation functions

    Etsuko Itou🇯🇵 · Yuki Nagai🇯🇵

    We propose the sparse modeling method to estimate the spectral function from the smeared correlation functions. We give a description of how to obtain the shear viscosity from the correlation function of the renormalized energy-momentum tensor (EMT) measured by the gradient flow method ($C(t,\tau )$) for the quenched QCD at finite temperature. The measurement of the renormalized EMT in the gradient flow method reduces a statistical uncertainty thanks to its property of the smearing. However, the smearing breaks the sum rule of the spectral function and the over-smeared data in the correlation function may have to be eliminated from the analyzing process of physical observables. In this work, we demonstrate that the sparse modeling analysis in the intermediate-representation basis (IR basis), which connects between the Matsubara frequency data and real frequency data. It works well even using very limited data of $C(t,\tau )$ only in the fiducial window of the gradient flow. We utilize the ADMM algorithm which is useful to solve the LASSO problem under some constraints. We show that the obtained spectral function reproduces the input smeared correlation function at finite flow-time. Several systematic and statistical errors and the flow-time dependence are also discussed.

    ↳ hep-latcond-mat.str-elhep-phnucl-thJHEP(2020)·30 citations
37. 37*

    Recent Progress in Basis Light-front Quantization

    Xingbo Zhao🇨🇳 · Kaiyu Fu🇨🇳 · Hengfei Zhao🇨🇳 · Jiangshan Lan🇨🇳 · Chandan Mondal🇨🇳 · Siqi Xu🇨🇳 · James P. Vary (BLFQ Collaboration)🇺🇸

    Basis Light-front Quantization (BLFQ) is a nonperturbative approach to quantum field theory. In this paper, we report our recent progress in applying BLFQ to the positronium system in QED and to the meson and the baryon system in QCD. We present preliminary results on the mass spectrum, light-front wave functions and other observables of these systems, where one dynamical gauge boson is retained for the positronium and meson systems.

    ↳ nucl-thhep-ph3 citations
38. 38*

    Relativistic hydrodynamics for spin-polarized media

    Radoslaw Ryblewski🇵🇱 · Rajeev Singh🇵🇱

    We summarize the key ingredients of the recently proposed formalism of relativistic perfect-fluid hydrodynamics with spin. Based on the underlying kinetic theory definitions for the equilibrium distribution functions we obtain the evolution equations governing the system's expansion. Employing Bjorken symmetry we study the spin polarization dynamics of the system.

    ↳ nucl-thhep-phActa Phys.Polon.B(2020)·2 citations
39. 39*

    Extreme mass ratio inspirals with spinning secondary: a detailed study of equatorial circular motion

    Gabriel Andres Piovano🇮🇹 · Andrea Maselli🇮🇹 · Paolo Pani🇮🇹

    Extreme mass-ratio inspirals detectable by the future Laser Interferometer Space Antenna provide a unique way to test general relativity and fundamental physics. Motivated by this possibility, here we study in detail the EMRI dynamics in the presence of a spinning secondary, collecting and extending various results that appeared in previous work and also providing useful intermediate steps and new relations for the first time. We present the results of a frequency-domain code that computes gravitational-wave fluxes and the adiabatic orbital evolution for the case of circular, equatorial orbits with (anti)aligned spins. The spin of the secondary starts affecting the gravitational-wave phase at the same post-adiabatic order as the leading-order self-force terms and introduces a detectable dephasing, which can be used to measure it at $5-25\%$ level, depending on individual spins. In a companion paper we discuss the implication of this effect for tests of the Kerr bound.

    ↳ gr-qcastro-ph.HEhep-phPRD(2020)·94 citations
40. 40*

    Efficiency Parameterization with Neural Networks

    C. Badiali🇮🇹 · F.A. Di Bello🇮🇹 · G. Frattari🇮🇹 · E. Gross🇮🇱 · V. Ippolito🇮🇹 · M. Kado🇮🇹 · J. Shlomi🇮🇱

    Multidimensional efficiency maps are commonly used in high energy physics experiments to mitigate the limitations in the generation of large samples of simulated events. Binned multidimensional efficiency maps are however strongly limited by statistics. We propose a neural network approach to learn ratios of local densities to estimate in an optimal fashion efficiencies as a function of a set of parameters. Graph neural network techniques are used to account for the high dimensional correlations between different physics objects in the event. We show in a specific toy model how this method is applicable to produce accurate multidimensional efficiency maps for heavy flavor tagging classifiers in HEP experiments, including for processes on which it was not trained.

    ↳ hep-exhep-phComput.Softw.Big Sci.(2021)·25 citations
41. 41*

    Graviton-photon mixing. Exact solution in a constant magnetic field

    Damian Ejlli🇬🇧

    In this work I study the effect of graviton-photon mixing in a constant external magnetic field and find for the first time in the literature exact solution of the equations of motion. In particular, I apply the effect of graviton-photon mixing to the case of interaction of gravitational waves with an external magnetic field and calculate the intensity Stokes parameter of the produced electromagnetic radiation. The obtained results are new and extend previous results obtained by using approximation methods.

    ↳ gr-qchep-phhep-thJHEP(2020)·18 citations
42. 42*

    Fluctuation dynamics near the QCD critical point

    Lipei Du🇺🇸 · Ulrich Heinz🇺🇸 · Krishna Rajagopal🇺🇸 · Yi Yin🇨🇳

    The evolution of non-hydrodynamic slow processes near the QCD critical point is explored with the novel Hydro+ framework, which extends the conventional hydrodynamic description by coupling it to additional explicitly evolving slow modes describing long wavelength fluctuations. Their slow relaxation is controlled by critical behavior of the correlation length and is independent from gradients of matter density and pressure that control the evolution of the hydrodynamic quantities. In this exploratory study we follow the evolution of the slow modes on top of a simplified QCD matter background, allowing us to clearly distinguish, and study both separately and in combination, the main effects controlling the dynamics of critical slow modes. In particular, we show how the evolution of the slow modes depend on their wave number, the expansion of and advection by the fluid background, and the behavior of the correlation length. Non-equilibrium contributions from the slow modes to bulk matter properties that affect the bulk dynamics (entropy, pressure, temperature and chemical potential) are discussed and found to be small.

    ↳ nucl-thhep-phhep-thnucl-exPRC(2020)·62 citations
43. 43*

    Realization of a high quality factor resonator with hollow dielectric cylinders for axion searches

    D. Alesini🇮🇹 · C. Braggio🇮🇹 · G. Carugno🇮🇹 · N. Crescini🇮🇹 · D. D' Agostino🇮🇹 · D. Di Gioacchino🇮🇹 · R. Di Vora🇮🇹 · P. Falferi🇮🇹 · U. Gambardella🇮🇹 · C. Gatti🇮🇹 · G. Iannone🇮🇹 · C. Ligi🇮🇹 and 9 other authors

    The realization and characterization of a high quality factor resonator composed of two hollow-dielectric cylinders with its pseudo-TM${}_{030}$ mode resonating at 10.9 GHz frequency is discussed. The quality factor was measured at the temperatures 300 K and 4 K obtaining $\text{mbox}{Q}_{300\text{mbox}K}=(150,000\pm 2,000)$ and $\text{mbox}{Q}_{4\text{mbox}K}=(720,000\pm 10,000)$respectively, the latter corresponding to a gain of one order of magnitude with respect to a traditional copper cylindrical-cavity with the corresponding TM${}_{010}$ mode resonating at the same frequency. The implications to dark-matter axion-searches with cavity experiments are discussed showing that the gain in quality factor is not spoiled by a reduced geometrical coupling $C_{030}$ of the cavity mode to the axion field. This reduction effect is estimated to be at most 20%. Numerical simulations show that frequency tuning of several hundreds MHz is feasible.

    ↳ physics.ins-dethep-exhep-phNucl.Instrum.Meth.A(2021)·34 citations
44. 44*

    An Upper Limit on the Initial Temperature of the Radiation-Dominated Universe

    Betty X. Hu🇺🇸 · Abraham Loeb🇺🇸

    Gravitational waves (GWs) are produced by colliding particles through the gravitational analogue of electromagnetic bremsstrahlung. We calculate the contribution of free-free emission in the radiation-dominated Universe to the stochastic GW background. We find that the energy density of the resulting GW radiation is heavily dependent on the number of elementary particles, $N_{\mathrm{tot}}$, and the maximum initial temperature, $T_{\max }$. We rule out $N_{\mathrm{tot}}\gtrsim N_{\mathrm{SM}}$ for $T_{\max }\sim T_{\mathrm{Planck}}\approx 10^{19}$ GeV and $N_{\mathrm{tot}}\gtrsim 10^{13}\times N_{\mathrm{SM}}$ for $T_{\max }\sim 10^{16}$ GeV, where $N_{\mathrm{SM}}$ is the number of particles in the Standard Model. In the case of inflation, existing cosmological data constrain $T_{\max }\lesssim 10^{16}$ GeV. However, alternative models to inflation such as bouncing cosmologies allow for $T_{\max }$ near $T_{\mathrm{Planck}}$. At the energy scales we are considering, the extra number of particles arise naturally in models of extra dimensions.

    ↳ astro-ph.COhep-phhep-thJCAP(2021)·5 citations
45. 45*

    Scattering of charged fermion to two-dimensional wormhole with constant axial magnetic flux

    Kulapant Pimsamarn🇹🇭 · Piyabut Burikham🇹🇭 · Trithos Rojjanason🇹🇭

    Scattering of charged fermion with $(1+2)$-dimensional wormhole in the presence of constant axial magnetic flux is explored. By extending the class of fermionic solutions of the Dirac equation in the curved space of wormhole surface to include normal modes with real energy and momentum, we found a quantum selection rule for the scattering of fermion waves to the wormhole. The newly found {\it momentum-angular momentum relation} implies that only fermion with the quantized momentum $k=m^{\prime }/a\sqrt{q}$ can be transmitted through the hole. The allowed momentum is proportional to an effective angular momentum quantum number $m^{\prime }$ and inversely proportional to the radius of the throat of the wormhole $a$. Flux dependence of the effective angular momentum quantum number permits us to select fermions that can pass through according to their momenta. A conservation law is also naturally enforced in terms of the unitarity condition among the incident, reflected, and transmitted waves. The scattering involving quasinormal modes~(QNMs) of fermionic states in the wormhole is subsequently explored. It is found that the transmitted waves through the wormhole for all scenarios involving QNMs are mostly suppressed and decaying in time. In the case of QNMs scattering, the unitarity condition is violated but a more generic relation of the scattering coefficients is established. When the magnetic flux $\varphi =mhc/e$, i.e., quantized in units of the magnetic flux quantum $hc/e$, the fermion will tunnel through the wormhole with zero reflection.

    ↳ hep-thcond-mat.mes-hallhep-phquant-phEPJC(2020)·6 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.