arXiv subjects

Hitoshi Murayama

Publications and source records attributed to Hitoshi Murayama.

At least 19 recordsLinked to original sources

arXiv2026

Low-Frequency Gravitational Bremsstrahlung and Memory in a Medium

Gravitational radiation from the early Universe offers a powerful window into physics beyond the Standard Model (SM). In this paper, we investigate the low-frequency tail of gravitational radiation produced by the decay of massive particles. The energy spectrum in Minkowski space-time, $dE/d\omega$, exhibits two distinct regimes: a flat bremsstrahlung/memory spectrum above a characteristic cutoff frequency and a suppressed quadratic frequency scaling below it. The typical rate of momentum change of the decay products determines the cutoff frequency. We calculate $dE/d\omega$ in two independent but equivalent ways: using Weinberg's soft graviton theorem and by solving the equations of motion. This further establishes the connection between Weinberg's soft graviton theorem and the gravitational memory formula, which we generalize to incorporate the effects of final-state scattering. We then compute the cosmological stochastic gravitational energy spectrum $h^2\Omega_{\rm h}$ generated by many particle decays in the early Universe. We demonstrate that $h^2\Omega_{\rm h}$ exhibits linear scaling with frequency above the cutoff frequency, transitioning to cubic scaling below it. Our findings reveal a suppression of the low-frequency signal compared to a naive linear extrapolation, with implications for detection prospects. Our results also reveal the connection between bremsstrahlung and memory in the context of cosmological stochastic gravitational energy spectra from the early Universe.

hep-ph↗

arXiv2026

AMSB in Truly Confining Gauge Theories

We study deformation of supersymmetric $t$-confining (truly confininig) gauge theories with small anomaly mediation of supersymmetry breaking. We identify breaking pattern of global symmetries in the theories. These results can be compared in priciple to lattice simulation of non-supersymmetric theories (except for one of the cases where the theory is pseudoreal and chiral).

hep-th↗

arXiv2026

Minimal Proton-Mass Dark Matter

We present a minimal dark matter scenario: a single complex scalar carrying baryon and lepton number, with no new exact stabilizing symmetry. Its leading interaction is a dimension-7 semileptonic portal that, below confinement, generates a low-energy Yukawa coupling with the proton and electron. Requiring absolute stability of both the proton and dark matter forces the dark matter mass into a narrow window around the proton mass, which may be anthropically selected. Despite its minimal field content, the model can be probed by many observables: proton burning in stars, hydrogen decay, brown dwarfs and neutron star heating, and nucleon decay-like signatures in direct detection. UV-dominated freeze-in produces the observed relic abundance. This framework provides a unique testable example of dark matter arising from a minimal extension of the Standard Model.

hep-ph↗

arXiv2026

Searches for GeV-Scale ALPs at RHIC

We point out that ultra-peripheral Au+Au collision data collected at the Relativistic Heavy Ion Collider, operational during 2000-2026, can be used to search for axion-like particles coupled to photons via the resonant process $\gamma\gamma \to a \to \gamma\gamma$. Exploiting the $Z^4$ enhancement of the two-photon luminosity in heavy-ion collisions and the low photon energy thresholds achievable at RHIC, we simulate signal and background processes, the latter dominated by light-by-light scattering, hadronic resonance production, and misidentified $e^+e^-$ pairs, and estimate upper limits on the ALP-photon coupling $g_{a\gamma\gamma}$ assuming $1.9~\text{nb}^{-1}$ of existing data collected by the PHENIX experiment. We find sensitivity to ALP masses in the range $2~\text{GeV} \lesssim m_a \lesssim 5~\text{GeV}$ with couplings $g_{a\gamma\gamma} \gtrsim 4\times 10^{-4}~\text{GeV}^{-1}$, probing previously unexplored regions of parameter space. Access to larger luminosity datasets could substantially extend the sensitivity of this search, motivating a dedicated analysis of ultra-peripheral collision data collected at RHIC by PHENIX as well as other experiments.

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arXiv2026

Update to the U.S. National Input to the European Strategy Update for Particle Physics

In this document we update the status of U.S. community inputs for the European Strategy for Particle Physics Update (ESPPU) since April 1, 2025, and offer responses to the revised questions. Major new inputs include a long-term strategy report from the National Academies of Sciences, Engineering, and Medicine and the formal formation of a U.S. Muon Collider Collaboration.

hep-ex↗

arXiv2026

GeV-scale QCD Axion

In order to solve the strong CP problem, we study the possibility that the Peccei--Quinn symmetry is broken {\it below}\/ the QCD scale. We find that a QCD axion can be above GeV, and may be among the observed $\eta$ resonances. It is immune to quantum gravity corrections. The only fermion that has a $U(1)$ Peccei--Quinn charge is the right-handed up quark. Flavor-changing neutral currents are surprisingly small. All accelerator and astrophysical limits can be evaded. The most significant constraint is the mass splitting between $\pi^\pm$ and $\pi^0$. In a UV completed model, LHC can look for a heavy quark pair ${\cal D} \overline{\cal D}$ followed by the decay ${\cal D} \rightarrow W+u$ or a single production $q u \rightarrow q\, {\cal U}$ followed by ${\cal U} \rightarrow u Z, uh$. There can be an $O(1)$ contribution $h \rightarrow u\bar{u}\varphi$ in the measurement of $h \rightarrow gg$ or permille effects on the hadronic $Z$ width at a Higgs factory.

hep-ph↗

arXiv2025

AMSB in $Sp(N_c)$ Gauge Theories

We present a careful study of the chiral symmetry breaking minima and other potential minima in supersymmetric symplectic QCD ($Sp(N_c)$ with $N_f$ flavors) perturbed by Anomaly Mediated Supersymmetry Breaking (AMSB). Although the case of $N_f = N_c +1$ requires particular care due to the inherently strongly coupled nature of the quantum modified moduli space, we are able to show that all $Sp(N_c)$ theories to which AMSB can be applied ($N_f < 3(N_c + 1)$) possess stable chiral symmetry breaking minima, which are plausibly continuously connected to the vacua of QCD-like $Sp(N_c)$ theories for large SUSY breaking, and are protected from runaways to incalculable minima.

hep-th↗

arXiv2025

Resonant Annihilation of WIMP Dark Matter for Halo Gamma Ray Signal

We propose a resonant annihilation as a way to reconcile the WIMP annihilation cross sections in a recently reported gamma ray signal from the Milky Way halo with that for the freeze-out and the upper limit from dwarf galaxies. We perform a simple model-independent analysis based on this hypothesis and determine the required parameters. We also present a simple particle-physics model that can accommodate them.

hep-ph↗

arXiv2025

Clearing up the Strong $CP$ problem

The absence of a neutron electric dipole moment (EDM) constrains the quantum chromodynamics (QCD) theta angle to be less than one part in ten billion, posing the Strong $CP$ problem. We revisit two classes of proposed solutions. First, we show that when $P$ or $CP$ is realized as a gauged discrete symmetry - as can arise in quantum gravity - the vacuum necessarily preserves $CP$, contrary to recent claims that discrete-symmetry solutions fail. Gauged discrete models face model-building challenges, such as avoiding contributions to the neutron EDM after spontaneous $P$ or $CP$ breaking, but in principle have no fundamental obstructions. Second, we critically examine recent arguments that the Strong $CP$ problem is illusory, demonstrating that a nonzero neutron EDM at finite $\bar\theta$ follows directly from well-understood QCD dynamics. Taken together, our results reinforce the reality of the Strong $CP$ problem and highlight gauged discrete-symmetry realizations of $P$ or $CP$ as plausible solutions.

hep-ph↗

arXiv2025

Multiple Axions Save High-Scale Inflation

Many models of dark matter QCD axion requires inflation at a scale $H_{\text{inf}} \lesssim 10^{6}$~GeV and hence does not allow for a detectable tensor mode fluctuation. This is because the domain wall problem forces the Peccei--Quinn symmetry to be broken during the inflation and the axions to be produced by the misalignment mechanism. We point out that theories with multiple axions can evade this constraint and allow for a high-scale inflation with detectable tensor mode. It only requires a condition on the anomaly coefficients so that there is a unique minimum for the axion potential without a fine-tuning or small parameters.

hep-ph↗

arXiv2025

Observing Leptogenesis in Action with Gravitational Waves

Leptogenesis is arguably the best motivated theory of baryogenesis given the discovery of finite neutrino masses, yet its experimental test is elusive given its high energy scale. We discuss gravitational waves (GWs) produced via graviton bremsstrahlung in right-handed neutrino decays during leptogenesis. The presence of right-handed neutrinos in the early universe can lead to a period of early matter domination. In this context, the resultant GW spectrum scales quadratically with the right-handed neutrino mass, while its peak frequency scales inversely with the Yukawa coupling. Detecting such a spectrum would provide strong evidence for leptogenesis and the existence of heavy right-handed neutrinos. We also discuss how the GW spectrum emitted from the thermal plasma is altered by an era of early matter domination. We show that it can mimic the effects of additional relativistic degrees of freedom and a higher reheating temperature, and that information from the graviton bremsstrahlung GW spectrum can break this degeneracy.

hep-ph↗

arXiv2025

Phase Transitions at Unusual Values of $\theta$

We calculate the $\theta$ dependence in a cousin of QCD, where the vacuum structure can be analyzed exactly. The theory is $\mathcal{N}=2$ $SU(2)$ gauge theory with $N_F=0,1,2,3$ flavors of fundamentals, explicitly broken to $\mathcal{N}=1$ via an adjoint superpotential, and coupled to anomaly mediated supersymmetry breaking (AMSB). The hierarchy $m_{AMSB}\ll \mu_{\mathcal{N}=1}\ll \Lambda$ ensures the validity of our IR analysis. As expected from ordinary QCD, the vacuum energy is a function of $\theta$ which undergoes 1st order phase transitions between different vacua where the various dyons condense. For $N_F=0$ we find the expected phase transition at $\theta=\pi$, while for $N_F=1,2,3$ we find phase transitions at fractional values of $\pi$.

hep-th↗

arXiv2025

Topological Freeze-out by Semi-Annihilation

We point out that a QCD-like dark sector can be coupled to the Standard Model by gauging the topological Skyrme current, which measures the dark baryon number in the infrared, to give a technically natural model for dark matter. This coupling allows for a semi-annihilation process $\chi \chi \rightarrow \chi X_\mu$, where $X_\mu$ is the gauge boson mediator and $\chi$ a dark pion field, which plays the dominant role in setting the dark matter relic abundance. The topological interaction is purely $p$-wave and so free from indirect detection constraints. We show that the dark matter pion mass needs to be in the range $10$ MeV $\lesssim m_\chi \lesssim$ $1$ TeV; towards the lighter end of this range, there can moreover be significant self-interactions. We discuss prospects for probing this scenario at collider experiments, ranging from the LHC to low-energy $e^+ e^-$ colliders, future Higgs factories, and beam-dump experiments.

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arXiv2025

Near-SUSY to Non-SUSY Crossover

Gauge theories can be solved exactly slightly away from the supersymmetric (SUSY) limit softly broken by anomaly mediation when the size of SUSY breaking is much smaller than the dynamical scale ($m \ll \Lambda$). We show empirical evidence that the near-SUSY limit is continuously connected to the non-SUSY limit ($m \gg \Lambda$) in $\mathrm{SU}(N_c)$ gauge theories with $N_f$ quarks in the fundamental representation. The evidence includes the behavior of quark bi-linear condensate and gluon condensates, light hadron spectra, and consistency with the large $N_c$ limit. In addition, we present new predictions when $N_f/N_c \gtrsim O(1)$.

hep-th↗

arXiv2025

Truly Confining Supersymmetric Gauge Theories

We classify ``truly confining'' (t-confining) supersymmetric gauge theories, in which no center charges can be screened, and Wilson loops in the fundamental representation are therefore expected to exhibit an area law. In all cases, we identify the condensation of certain ``magnetic'' operators. Many of them have more than three branches, and one with vanishing superpotential, a phenomenon not previously observed.

hep-th↗

arXiv2025

A High-Quality Composite Pati-Salam Axion

We present a composite QCD axion model where the Peccei--Quinn (PQ) symmetry emerges as a high-quality, accidental symmetry. The axion potential is only modified by eight-fermion, dimension 12 operators, which if present at the Planck scale, allow for axion dark matter from misalignment while solving the strong CP problem. The model is an $\text{SU}(N_c)$ gauge theory with ten flavors where the Pati--Salam unified subgroup $\text{SO}(6)\times \text{SO}(4) \subset \text{SU}(10)_L$ and $\text{Sp}(10)\subset \text{SU}(10)_R$ are weakly gauged. The dynamics breaks $\text{SU}(10)_L\times \text{SU}(10)_R \rightarrow \text{SU}(10)_V$ and the weakly-gauged groups to $\text{U}(3)\times \text{U}(2) \supset \text{SU}(3)_c \times \text{SU}(2)_L \times \text{U}(1)_Y$, with the QCD axion identified as one of the Nambu-Goldstone bosons. This axion has a relatively large coupling to photons while a residual $\bar{\theta}_{\rm eff}$ may be just below the current limit on the neutron electric dipole moment. If the dimension 12 operators are present near the GUT scale, they can cause domain wall networks to decay, allowing for axion dark matter even for the post-inflationary scenario.

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arXiv2025

Dynamics of $E_6$ Chiral Gauge Theories

We present exact non-perturbative solutions to chiral gauge theories based on the $E_6$ gauge group and several matter fermions in the fundamental $\bf{27}$-dimensional representation. They are obtained when supersymmetric versions are perturbed by small supersymmetry breaking by anomaly mediation. The universality classes obtained are very different from what can be conjectured by the tumbling hypothesis. In particular, the case with three $\bf{27}$s may have an unbroken $\text{SU}(3)$ symmetry with massless composite fermions in $\bf{10}$ of $\text{SU}(3)$. For this case, we employed numerical techniques to obtain the exact ground state.

hep-th↗

arXiv2025

Phases of Dark Matter from Inverse Decays

Inverse decays are an interesting avenue for producing dark matter in the early universe. We study in detail various phases of dark matter parameter space where inverse decays control its abundance, expanding on our work of INDY dark matter and going beyond. The role of initial conditions and the impact of departure from kinetic equilibrium are investigated as well. We show how these inverse decay phases can arise in theories of a kinetically mixed dark photon and dark Higgs, with promising prospects for detection at upcoming experiments.

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