So Chigusa, LBNL & UC Berkeley

Estimating eV-Scale Background Rates for Dark Matter Direct Detection

Hybrid On-site: Kenkyu Honkan Seminar room 321, 322 Online: Zoom
A promising strategy for direct detection of sub-MeV dark matter is to look for phonon excitations in crystals. The crystal targets used in such experiments are typically not completely pure, and have impurities or defects. The point defects of our interest include the Frenkel defects, hydrogen and oxygen impurities, and so on. These defects can diffuse and recombine to emit energy in the form of phonons, and can potentially create a background for direct detection experiments. We estimate the defect densities produced through thermal excitations as well as radiogenic nuclear recoils. For various defect configurations, we quantify the diffusion and recombination rates for both thermal and quantum tunneling mechanisms. We find that the thermally generated Frenkel defects are effectively frozen at cryogenic temperatures and cannot diffuse to recombine with each other. The hydrogen impurity is the unique point defect that has non-negligible diffusion rate at cryogenic temperatures, and possibly contributes to the eV-scale events. The radiogenic defects produced on the surface can be annealed effectively at room temperature for typical defect configurations, but defects produced through radiogenic nuclear recoils in a shielded environment at cryogenic temperatures during the run-time of the experiment can recombine to produce eV-scale events. We provide estimates of these background event rates and give some remarks on subtleties we need to take care of in this kind of experimental setups.

Kunio Kaneta, Osaka University

Gravity at Work: Understanding Particle Production and Distribution

Hybrid On-site: Kenkyu Honkan Seminar room 321, 322 Online: Zoom
Despite its apparent weakness, gravity is a vital force driving particle production in the early Universe. To explore this production process, two different but well-established frameworks are commonly used, which are known as the Boltzmann and Bogoliubov approaches. I will first discuss a crucial aspect of these frameworks—their equivalence in pure gravitational production—to answer the question of whether these approaches yield consistent outcomes. In doing so, we will see the phase space distribution as a key quantity. This distribution will then serve as a useful tool to address yet another important question: Did the Higgs field predominantly exist as condensate or fluctuations in the earliest times? which will be discussed in the second part of the talk.

望月拓郎, 京都大学数理解析研究所

[金茶会] 第3回 代数・幾何・解析の不思議なつながり- 純ツイスター D-加群とその周辺

小林ホール (リモート会場:J-PARC研究棟2階会議室)


Takashi Shimomura, Miyazaki University

Revisiting sterile neutrino dark matter in gauged B-L model

Hybrid On-site: Kenkyu Honkan Seminar room 321, 322 Online: Zoom
Sterile neutrino is a decaying dark matter candidate which can be produced non-thermally in the early Universe. Gauged B-L symmetric extension of the standard model naturally accommodates such a sterile neutrino dark matter, and also provides new mediators, B-L gauge boson and symmetry breaking scalar boson. Viable ranges of the dark matter mass and coupling have been studied in this mode. In this talk, we reexamine the freeze-in production of the sterile neutrino dark matter in gauged B-L model. Longitudinal mode contributions in scattering processes as well as inverse decays of the B-L gauge boson and scalar boson are taken into account in our analyses. We will discuss the contributions from these for different mass spectra of the dark matter, gauge boson and scalar boson. Then it is shown that the contributions from these are large and the allowed parameter space is changed from previous studies.


Low entanglement states, black holes and computational efficiency

Hybrid On-site: Seminar room, Kenkyu honkan 3F Online: Zoom
In modern statistical physics, typicality plays a crucial role since it states that almost all the microstates possessing a certain energy are locally indistinguishable from the canonical ensemble. Such states are called typical states and possess a large amount of entanglement. On the other hand, states whose entanglement are parameterically lower than a state are rare. We will call such states low entanglement states. In this talk, we will show that low entanglement states, while rare, are sufficient to account for the leading order of the thermodynamic entropy. We will also present a concrete way to construct such a basis. At the end, we will discuss its implication to the black holes, and its applications to computational physics.

Avelino Vicente (IFIC, Valencia (Spain))

The Scotogenic mechanism: from model to paradigm

Hybrid On-site: Kenkyu Honkan Seminar room 321, 322 Online: Zoom
Neutrinos are very light fermions, a fact that can be well understood if their masses are induced at loop level. Many radiative neutrino mass models have been proposed along the years. The Scotogenic model is a very economical and popular setup that induces Majorana neutrino masses at the 1-loop level and includes a viable dark matter candidate. Based on the original model, many variants can be constructed.
We discuss specific variations of the Scotogenic model with alternative representations under the Standard Model gauge group and additional Scotogenic states, perhaps coming from ultraviolet extended setups. These variants of the Scotogenic paradigm have novel phenomenological predictions and may explain some long-standing anomalies.

Keisuke Harigaya, University of Chicago

Parity symmetry breaking scale and Standard Model parameters

Hybrid On-site: Seminar room, Kenkyu honkan 3F Online: Zoom
The strong CP problem can be solved by parity symmetry. We first review two classes of models: the ones with the minimal fermion content and the ones with the minimal Higgs content. We then focus on the latter class of models and show that the parity symmetry breaking scale is predicted to be the energy scale at which the standard model Higgs quartic coupling vanishes. Surprisingly, after fixing the parity symmetry breaking scale in this way, the gauge coupling constants unify at a high energy scale. We also discuss a model with a dark matter candidate and show that the dark matter direct detection rate is predicted as a function of the standard model parameters.

Philip Lu, Seoul National University

Sterile Neutrinos and Primordial Black Holes: Dark Matter Candidates

Hybrid On-site: Seminar room, Kenkyu honkan 3F Online: Zoom
In the first part of this talk, I will discuss sterile neutrino production in modified cosmologies. Although the standard assumption is a radiation dominated universe up to the era of reheating, there are motivated models in which the Hubble rate-temperature relation changes. The abundance of sterile neutrinos produced both resonantly and non-resonantly can be drastically altered with interesting implications for experimental searches. In the second part of my talk, I will present a set of bounds on the primordial black hole (PBH) mass density. By considering stable gas clouds in thermal equilibrium, we can calculate the cooling rate and impose constraints on possible heating processes. Intermediate mass black holes, which can form efficient accretion disks, and light black holes, which emit Hawking radiation, would generate significant amounts of thermal energy. We set limits on the dark matter fraction of PBH as a result and briefly explore the possible contributions from jets. In the final section of the talk, I will discuss two PBH formation models from first order phase transitions. During a first order phase transition, compact remnants in the form of thermal balls and Fermi balls can be formed from particles trapped within the false vacuum. Eventually, after significant cooling, these remnants can collapse into primordial black holes. We consider a delayed formation scenario in which PBH formation occurs after the CMB era. This evades a strong constraint on intermediate mass black holes derived from Planck observations, and opens parameter space for two astrophysically significant populations: BBH progenitors and SMBH seeds. Another PBH formation scenario relies on the decreased pressure forces during a high temperature QCD transition. This produces a peak in the PBH mass distribution at sub-solar mass and even asteroid mass scales, within the PBH mass window.

Zoltan Fodor, Pennsylvania State University

Tension for the anomalous magnetic moment of the muon: 4.2 sigma, indeed?

Hybrid On-site: Seminar room, Kenkyu honkan 3F Online: Zoom
Twenty years ago, in an experiment at Brookhaven National Laboratory, physicists detected what seemed to be a discrepancy between measurements of the muon’s magnetic moment and theoretical calculations of what that measurement should be, raising the tantalizing possibility of physical particles or forces as yet undiscovered. The Fermilab team has announced that their precise measurement supports this possibility. The reported significance for new physics is 4.2 sigma just slightly below the discovory level of 5 sigma. However, an extensive new calculation of the muon’s magnetic moment using lattice QCD by the BMW-collaboration reduces the gap between theory and experimental measurements. In this talk both the theoretical and experimental aspects are summarized with two possible narratives:
a) almost discovery or b) Standard Model re-inforced. Some details of the lattice caluculation are also shown.

Teruaki Suyama, Titech

Kramers-Kronig relation in gravitational lensing

Hybrid On-site: Seminar room, Kenkyu honkan 3F Online: Zoom

Gravitational lensing (GL) has been a powerful method to probe matter inhomogeneities in the Universe. GL of gravitational waves, which will be detected in the near future, will enable us to gain more information of the nature of dark matter and further boost the significance of GL. In light of this situation, it is important to have understanding of basic properties of GL. In my talk, I show that gravitational lensing obeys the causality in the sense that (electromagnetic/gravitational) waves emitted from the source arrive at an observer only after the arrival of the signal in geometrical optics. This leads to the Kramers-Kronig relation, a well-known relation in the field of optics, in GL, as the relation between real and imaginary parts of the amplification factor. I will also show some relations which hold as consequence of the Kramers-Kronig relation. Finally, I argue that examining the violation of the Kramers-Kronig relation may be used for correctly extracting the lensing signal in the gravitational wave observations.

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