arXiv subjects

Ryosuke Sato

Publications and source records attributed to Ryosuke Sato.

At least 73 records · Page 4Linked to original sources

arXiv2018

Unified Origin of Axion and Monopole Dark Matter, and Solution to the Domain-wall Problem

We propose a scenario where the spontaneous breakdown of the Peccei-Quinn symmetry leads to monopole production. Both the axion and the monopole contribute to dark matter, and the Witten effect on the axion mass is a built-in feature. In the KSVZ-type axion model, seemingly different vacua are actually connected by the hidden gauge symmetry, which makes the axionic string unstable and separate into two Alice strings. The Alice string is attached to a single domain wall due to the QCD instanton effect, solving the domain-wall problem. This is in the same spirit of the Lazarides-Shafi mechanism, although the discrete Peccei-Quinn symmetry is not embedded into the center of the original gauge symmetry. In the DFSZ-type axion model, the domain-wall problem is avoided by the Witten effect. If the Peccei-Quinn symmetry is explicitly broken by a small amount, monopoles acquire a tiny electric charge and become mini-charged dark matter. Interestingly, the quality of the Peccei-Quinn symmetry is closely tied to darkness of dark matter.

hep-ph↗

arXiv2018

Absolute Lower Bound on the Bounce Action

The decay rate of a false vacuum is determined by the minimal action solution of the tunnelling field: bounce. In this Letter, we focus on models with scalar fields which have a canonical kinetic term in $N(>2)$ dimensional Euclidean space, and derive an absolute lower bound on the bounce action. In the case of four-dimensional space, we show the bounce action is generically larger than $24/λ_{\rm cr}$, where $λ_{\rm cr} \equiv {\rm max} [ -4V(ϕ) /|ϕ|^4] $ with the false vacuum being at $ϕ=0$ and $V(0)=0$. We derive this bound on the bounce action \textit{without solving the equation of motion explicitly}. Our bound is derived by a quite simple discussion, and it provides useful information even if it is difficult to obtain the explicit form of the bounce solution. Our bound offers a sufficient condition for the stability of a false vacuum, and it is useful as a quick check on the vacuum stability for given models. Our bound can be applied to a broad class of scalar potential with any number of scalar fields. We also discuss a necessary condition for the bounce action taking a value close to this lower bound.

hep-ph↗

arXiv2018

Lepton Electric Dipole Moment and Strong CP Violation

Contribution of the strong CP angle, $\barθ$, to the Wilson Coefficients of electron and muon electric dipole moment (EDM) operators are discussed. Previously, $\barθ$ contribution to the electron EDM operator was calculated by Choi and Hong. However, the effect of CP-violating three meson coupling was missing there. We include this missing contribution for the first time in the literature, and reevaluate the Wilson coefficients of the lepton EDM operator. We obtain $d_e = - (2.2-8.6) \times 10^{-28} \barθ$ e-cm which is 15 - 70 % of the result obtained in Choi and Hong. We also estimated the muon EDM as $d_μ= - (0.5-1.8) \times 10^{-25} \barθ$ e-cm. Using $|\barθ| \lesssim 10^{-10}$ suggested by the neutron EDM measurements, we obtain $|d_e| \lesssim 8.6 \times 10^{-38}$ e-cm and $|d_μ| \lesssim 1.8 \times 10^{-35}$ e-cm. The $\barθ$ contribution to the muon EDM is much below the sensitivities of the current and near future experiments. Our result shows that the $\barθ$ contribution to $d_{e,μ}$ can be larger than the CKM contributions by many orders of magnitude.

hep-ph↗

arXiv2017

Cosmic rays, anti-helium, and an old navy spotlight

Cosmic-ray anti-deuterium and anti-helium have long been suggested as probes of dark matter, as their secondary astrophysical production was thought extremely scarce. But how does one actually predict the secondary flux? Anti-nuclei are dominantly produced in pp collisions, where laboratory cross section data is lacking. We make a new attempt at tackling this problem by appealing to a scaling law of nuclear coalescence with the physical volume of the hadronic emission region. The same volume is probed by Hanbury Brown-Twiss (HBT) two-particle correlations. We demonstrate the consistency of the scaling law with systems ranging from central and off-axis AA collisions to pA collisions, spanning 3 orders of magnitude in coalescence yield. Extending the volume scaling to the pp system, HBT data allows us to make a new estimate of coalescence, that we test against preliminary ALICE pp data. For anti-helium the resulting cross section is 1-2 orders of magnitude higher than earlier estimates. The astrophysical secondary flux of anti-helium could be within reach of a five-year exposure of AMS02.

astro-ph.HE↗

arXiv2017

Cosmic-ray Antimatter

In recent years, space-born experiments have delivered new measurements of high energy cosmic-ray (CR) $\bar p$ and $e^+$. In addition, unprecedented sensitivity to CR composite anti-nuclei anti-d and anti-He is expected to be achieved in the near future. We report on the theoretical interpretation of these measurements. While CR antimatter is a promising discovery tool for new physics or exotic astrophysical phenomena, an irreducible background arises from secondary production by primary CR collisions with interstellar matter. Understanding this irreducible background or constraining it from first principles is an interesting challenge. We review the attempt to obtain such understanding and apply it to CR $\bar p,\, e^+,$ anti-d and anti-He. Based on state of the art Galactic cosmic ray measurements, dominated currently by the AMS-02 experiment, we show that: (i) CR $\bar p$ most likely come from CR-gas collisions; (ii) $e^+$ data is consistent with, and suggestive of the same secondary astrophysical production mechanism responsible for $\bar p$ and dominated by proton-proton collisions. In addition, based on recent accelerator analyses we show that the flux of secondary high energy anti-He may be observable with a few years exposure of AMS-02. We highlight key open questions, as well as the role played by recent and upcoming space and accelerator data in clarifying the origins of CR antimatter.

astro-ph.HE↗

arXiv2017

$e^+$ and $\bar{p}$ production in $pp$ collisions and the cosmic-ray $e^+/\bar{p}$ flux ratio

Secondary astrophysical production of $e^+$ and $\bar{p}$ cosmic rays is considered. Inclusive $π$, $K$, and $\bar{p}$ production cross sections in $pp$ collisions at large $\sqrt{s}$ are parametrised using recent experimental data at LHC energies. The astrophysical production rate ratio $Q_{e^+}/Q_{\bar{p}}$ is calculated for an input cosmic ray proton flux consistent with local measurements. At $10 100$$\sim$GeV the measured flux ratio coincides with the production rate ratio of the secondary source.

astro-ph.HE↗

arXiv2017

O($N$) Invariance of the Multi-Field Bounce

In his 1977 paper on vacuum decay in field theory: The Fate of the False Vacuum, Coleman considered the problem of a single scalar field and assumed that the minimum action tunnelling field configuration, the bounce, is invariant under O(4) rotations in Euclidean space. A proof of the O(4) invariance of the bounce was provided later by Coleman, Glaser, and Martin (CGM), who extended the proof to $N>2$ Euclidean dimensions but, again, restricted non-trivially to a single scalar field. As far as we know a proof of O($N$) invariance of the bounce for the tunnelling problem with multiple scalar fields has not been reported, even though it was assumed in many works since, being of phenomenological interest. We make progress towards closing this gap. Following CGM we define the reduced problem of finding a field configuration minimizing the kinetic energy at fixed potential energy. Given a solution of the reduced problem, the minimum action bounce can always be obtained from it by means of a scale transformation. We show that if a solution of the reduced problem exists, then it and the minimum action bounce derived from it are indeed O($N$) symmetric.

hep-th↗

arXiv2017

Discrimination of dark matter models in future experiments

Phenomenological aspects of simple dark matter models are studied. We discuss ways to discriminate the dark matter models in future experiments. We find that the measurements of the branching fraction of the Higgs boson into two photons and the electric dipole moment of the electron as well as the direct detection experiments are quite useful in discriminating particle models of dark matter. We also discuss the prospects of finding new particles in dark sector at the LHC/ILC.

hep-ph↗

arXiv2017

Muon specific two-Higgs-doublet model

We investigate a new type of a two-Higgs-doublet model as a solution of the muon $g-2$ anomaly. We impose a softly-broken $Z_4$ symmetry to forbid tree level flavor changing neutral currents in a natural way. This $Z_4$ symmetry restricts the structure of Yukawa couplings. As a result, extra Higgs boson couplings to muons are enhanced by a factor of $\tanβ$, while their couplings to all the other standard model fermions are suppressed by $\cotβ$. Thanks to this coupling property, we can avoid the constraint from leptonic $τ$ decays in contrast to the lepton specific two-Higgs-doublet model, which can explain the muon $g-2$ within the 2$σ$ level but cannot within the $1σ$ level due to this constraint. We find that the model can explain the muon $g-2$ within the 1$σ$ level satisfying constraints from perturbative unitarity, vacuum stability, electroweak precision measurements, and current LHC data.

hep-ph↗

arXiv2016

LHC Future Prospects of the 750 GeV Resonance

A quantitative discussion on the future prospects of the 750 GeV resonance at the LHC experiment is given using a simple effective field theory analysis. The relative size of two effective operators relevant to diphoton decays can be probed by ratios of diboson signals in a robust way. We obtain the future sensitivities of $Zγ$, $ZZ$ and $WW$ resonance searches at the high luminosity LHC, rescaling from the current sensitivities at $\sqrt{s}=13$ TeV. Then, we show that a large fraction of parameter space in the effective field theory will be covered with 300 fb$^{-1}$ and almost the whole parameter space will be tested with 3000 fb$^{-1}$. This discussion is independent of production processes, other decay modes and total decay width.

hep-ph↗

arXiv2016

Self-consistent Calculation of the Sommerfeld Enhancement

A calculation of the Sommerfeld enhancement is presented and applied to the problem of s-wave non-relativistic dark matter annihilation. The difference from previous computations in the literature is that the effect of the underlying short-range scattering process is consistently included together with the long-range force in the effective QM Schrödinger problem. Our procedure satisfies partial-wave unitarity where previous calculations fail. We provide analytic results for some potentials of phenomenological relevance.

hep-ph↗

arXiv2016

Footprints of New Strong Dynamics via Anomaly and the 750 GeV Diphoton

The chiral anomaly provides a smoking-gun evidence of a new confining gauge theory. Motivated by a reported event excess in a diphoton invariant mass distribution at the LHC, we discuss a scenario that a pseudo-Nambu-Goldstone (pNG) boson of a new QCD-like theory is produced by gluon fusion and decays into a pair of the standard model gauge bosons. Despite the strong dynamics, the production cross section and the decay widths are determined by anomaly matching condition. The excess can be explained by the pNG boson with mass of around 750 GeV. The model also predicts exotic hadrons such as a color octet scalar and baryons. Some of them are within the reach of the LHC experiment.

hep-ph↗

arXiv2016

Relation between the rate of convergence of strong law of large numbers and the rate of concentration of Bayesian prior in game-theoretic probability

We study the behavior of the capital process of a continuous Bayesian mixture of fixed proportion betting strategies in the one-sided unbounded forecasting game in game-theoretic probability. We establish the relation between the rate of convergence of the strong law of large numbers in the self-normalized form and the rate of divergence to infinity of the prior density around the origin. In particular we present prior densities ensuring the validity of Erdos-Feller-Kolmogorov-Petrowsky law of the iterated logarithm.

math.PR↗

arXiv2016

SUSY Higgs Mass and Collider Signals with a Hidden Valley

We propose a framework of supersymmetric extensions of the Standard Model that can ameliorate both the SUSY Higgs mass problem and the missing superpartner problem. New vectorlike matter fields couple to the Higgs and provide new loop contributions to its mass. New Yukawa couplings are sizable and large supersymmetry breaking is not needed to lift the Higgs mass. To avoid a Landau pole for the new Yukawa couplings, these fields are charged under a new gauge group, which confines and leads to a Hidden Valley-like phenomenology. The Hidden Valley sector is almost supersymmetric and ordinary sparticles decay to exotic new states which decay back to Standard Model particles and gravitinos with reduced missing energy. We construct a simplified model to simulate this scenario and find a viable parameter space of specific benchmark models which ameliorates both of the major phenomenological problems with supersymmetry.

hep-ph↗

arXiv2016

Composite Accidental Axions

We present several models where the QCD axion arises accidentally. Confining gauge theories can generate axion candidates whose properties are uniquely determined by the quantum numbers of the new fermions under the Standard Model. The Peccei-Quinn symmetry can emerge accidentally if the gauge theory is chiral. We generalise previous constructions in a unified framework. In some cases these models can be understood as the deconstruction of 5-dimensional gauge theories where the Peccei-Quinn symmetry is protected by locality but more general constructions are possible.

hep-ph↗

arXiv2015

Lepton-Specific Two Higgs Doublet Model as a Solution of Muon $g-2$ Anomaly

We discuss the Type-X (lepton-specific) two Higgs doublet model as a solution of the anomaly of the muon $g-2$. We consider various experimental constraints on the parameter space such as direct searches for extra Higgs bosons at the LEP II and the LHC Run-I, electroweak precision observables, the decay of $B_s \to μ^+μ^-$, and the leptonic decay of the tau lepton. We find that the measurement of the tau decay provides the most important constraint, which excludes the parameter region that can explain the muon $g-2$ anomaly at the 1$σ$ level. We then discuss the phenomenology of extra Higgs bosons and the standard model-like Higgs boson ($h$) to probe the scenario favored by the $g-2$ data at the collider experiments. We find that the $4τ$, $3τ$ and $4τ+ W/Z$ signatures are expected as the main signal of the extra Higgs bosons at the LHC. In addition, we clarify that the value of the $hττ$ coupling is predicted to be the standard model value times about $-1.6$ to $-1.0$, and the branching fraction of the $h\to γγ$ mode deviates from the standard model prediction by $-30\%$ to $-15\%$. Furthermore, we find that the exotic decay mode, $h$ decaying into the $Z$ boson and a light CP-odd scalar boson, is allowed, and its branching fraction can be a few percent. These deviations in the property of $h$ will be tested by the precision measurements at future collider experiments.

hep-ph↗

arXiv2015

Quantum corrections to the spin-independent cross section in the inert Higgs doublet model

The inert Higgs doublet model is an extension of the standard model with an extra scalar doublet. The extra doublet is $Z_2$ odd while all the other particles are $Z_2$ even, thus the model contains a dark matter candidate. It is known that the spin-independent cross section of the dark matter and the nucleon is highly suppressed for $m_{\text DM} \sim m_h/2$ at the tree level. In this talk, we show the loop corrections give the significant effects in this regime.

hep-ph↗

arXiv2015

Quantum corrections to the spin-independent cross section of the inert doublet dark matter

The inert Higgs doublet model contains a stable neutral boson as a candidate of dark matter. We calculate cross section for spin-independent scattering of the dark matter on nucleon. We take into account electroweak and scalar quartic interactions, and evaluate effects of scattering with quarks at one-loop level and with gluon at two-loop level. These contributions give an important effect for the dark matter mass to be around m_h/2, because a coupling with the standard model Higgs boson which gives the leading order contribution should be suppressed to reproduce the correct amount of the thermal relic abundance in this mass region. In particular, we show that the dark matter self coupling changes the value of the spin-independent cross section significantly.

hep-ph↗