2017 Fall Seminar List

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Fall 2017 LeCosPA Cosmology and Particle Astrophysics Seminars

Location : Room 812, Astro-Math Building, NTU
Time : 11:00-12:00, Monday
Time for ASIAA/LeCosPA Joint Seminar : 14:20-15:20 on specific Thursdays
Organizers :Dr. Nugier, Fabien & Dr. TC Liu

Date Name Title Affiliation File
Dr. Ken Chen
Physics of the first Stars, Supernovae, and Galaxies
One of the paramount problems in modern cosmology is to elucidate how the first generation of luminous objects, stars, supernovae, accreting black holes, and galaxies, shaped the early universe at the end of the cosmic dark ages. According to the modern theory of cosmological structure formation, the hierarchical assembly of dark matter halos provided the gravitational potential wells that allowed gas to form stars and galaxies inside them. Modern large telescopes have pushed the detection of galaxies up to a redshift of z ~ 10. However, models of the first luminous objects still require considerable effort to reach the level of sophistication necessary for meaningful predictions, Due to the complexity of involved physical phenomena, this physical understanding may only come by the proper use of numerical simulations. Therefore, I have used state-of-the-art simulations on some of largest supercomputers to study these objects. In my talk, I will discuss the possible physics behind the formation of these first luminous objects by presenting the results from our simulations. I will also give possible observational signatures of the cosmic dawn that will be the prime targets for the future telescopes such as the James Webb Space Telescope (JWST).
File:Chen lecospa 2017.pdf
Dr. YoungMin Yook
Search for the Leptonic B Meson Decays at the Belle Experiment
A search for the purely leptonic $B$ meson decays $B^+\to\ell^+\nu_\ell$, where $\ell=e,~\mu$, using the full $\Upsilon(4S)$ data sample of $772\times 10^6~B\bar{B}$ pairs collected with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider and a sensitivity study of the lepton flavor violating leptonic $B$ meson decay $B^0\to\ell^\pm\tau^\mp$ are presented. In the studies, one of the $B$ mesons from $\Upsilon(4S)\to B\bar{B}$ decay is fully reconstructed in a hadronic mode, while the recoiling side is analyzed for the signal decay.

In the search for the $B^+\to\ell^+\nu_\ell$ decays no evidence of the signal in both decay modes is found. Upper limits of the corresponding branching fractions are determined as ${\cal B}(B^+\to e^+\nu_e)<3.5\times 10^{-6}$ and ${\cal B}(B^+\to \mu^+\nu_\nu)<2.7\times 10^{-6}$ at 90\% confidence level.

Useful variables that can be utilized in the $B^0\to\ell^\pm\tau^\mp$ decays as well as the performance of analysis estimated by an MC study is also presented. Compared to the most recent and stringent result, also performed at the $B$-factory experiment using hadronic tagging, approximately 10\%~(50\%) improvement in the signal efficiency is expected in the $B^0\to e^\pm\tau^\mp$~($B^0\to \mu^\pm\tau^\mp$) mode while reducing the background events to half.

Yonsei Univ.
Prof. Yi-Zen Chu
Cosmological Gravitational Waves: Causal Structure And Memories
[ Exceptional change of time: 10.30am - 11.30am ]
Despite being associated with particles of zero rest mass, electromagnetic and gravitational waves do not travel solely on the null cone in generic curved spacetimes. (That is, light does not always propagate on the light cone.) This inside-the-null-cone propagation of waves is known as the tail effect, and finding novel ways of understanding it in the strong field regime near a black hole may find applications for modeling the gravitational signals sought after by next-generation space-based detectors such as LISA.

Motivated by these considerations -- and as a first step -- I have been exploring techniques to understand the causal structure of scalar, electromagnetic and gravitational waves in cosmological spacetimes. I will describe my efforts to date, which include how the gravitational wave memory effects in 4D asymptotically flat spacetime generalize to the cosmological case.

Prof. Stathes Paganis
New Higgs results from LHC with implications to Cosmology.
The ATLAS and CMS experiments are currently releasing new results

obtained with the full ~36 fb^-1 luminosity 2016 dataset. Higgs related searches are relevant to cosmology and particle astrophysics, since they involve searches for Dark Matter, extra dimensions, studies of the Higgs potential and more. The goal of this talk is to summarize these results to a cosmology audience and give some update on the Taiwanese involvement in improving these searches.

Dr. Rio Saitou
Generic features of kinetically driven inflation
We perform a model-independent analysis of kinetically driven inflation (KDI) which (partially) includes Generalized G-inflation and Ghost inflation. We evaluate the background evolution splitting into the inflationary attractor and the perturbation around it. We also consider the quantum fluctuation of scalar mode with a usual scaling and derive the spectral index ignoring the contribution from the second order products of slow-roll parameters. Using these formalisms, we find that within our generic framework, the models of KDI which possess the shift symmetry of scalar field cannot create the observed quantum fluctuation. Breaking the shift symmetry, we obtain a few essential conditions for viable models of KDI associated with the graceful exit.
Dr. Meng-Ru Wu
Supernova neutrinos: what do we know and what may we learn?
Core-collapse supernovae are among the most explosive astrophysical events in the Universe. Due to the supra-nuclear density of the proto-neutron star resulting from the gravitational collapse of the progenitor, neutrinos are the only Standard Model particles that can transfer the energy inside the central engine and, therefore, dominate the dynamics of the system. In this talk, I will discuss our current understanding of the role of neutrinos in supernovae, their flavor conversions, and what we might be able to learn from the detection of the next Galactic supernova.
IoP, SINICA File:171023 LeCosPA MRW.pdf
Dr. Yuichi Higuchi
Testing LCDM model with weak gravitational lensing
While most outcomes from the cosmic microwave background experiments and galaxy surveys agree with LCDM cosmology, the physical origin of accelerating expansion the Universe is still uncertain and some anomalous features exist in the CMB temperature map. In order to explain these features, some theoretical models like f(R) gravity and supervoid have been proposed. Weak gravitational lensing analysis with on-going and future large-scale galaxy surveys such as Subaru/Hyper Suprime-Cam survey could give tight constrains on the theoretical models. To understand the relationships between the models and lensing observables, we performed ray-tracing simulations of gravitational lensing. In this talk, I will present our recent results and show how far we can constrain the models with the survey data.
Dr. KunXian Huang
Measurement of neutrino-oxygen neutral current quasi-elastic cross-section in the T2K experiment.
The T2K (Tokai to Kamioka) experiment is a long baseline neutrino oscillation

experiment. A 30 GeV proton beam is used to produce the intense muon neutrino beam at J-PARC. The Super-Kamiokande detector serves as a far detector of the T2K experiment, and it observe the neutrino events induced by the T2K beam neutrinos at a distance of 295 km from the J-PARC. I will talk about measurement of the neutrino-oxygen neutral-current quasi-elastic (NCQE) cross section by observing nuclear de-excitation gamma rays at Super- Kamiokande with the T2K neutrino beam. The study of the de-excitation $\gamma$-rays from the NCQE reactions has two motivations. First, a disappearance of neutral current (NC) events can be used to provide an evidence for the existence of sterile neutrinos in T2K. Second, for the supernova relic neutrino (SRN) search at Super-Kamiokande, the de-excitation $\gamma$-ray events from atmospheric neutrinos are one of the main backgrounds. The T2K measurement can also be used to estimate the number of background events from atmospheric neutrinos. The analyzed data are taken from Jan 2010 to May, 2013. As a result, the the fluxaveraged NCQE cross-section is updated as $<\sigma^{obs, updated}_{\nu,NCQE}> =1.75\times10^{-38}$cm$^{2}$ with the 68\% confidence interval of (1.33, 2.52)$\times10^{-38}$cm$^{2}$. Some recent results of neutrino oscillation in T2K experiment are also talked.

Dr. Jiro Matsumoto
Unified description of dark energy and dark matter in mimetic matter model
The existence of dark matter and dark energy is implied by various astronomical observations, however, it is still unclear because they have not been directly detected. In this talk, the possibility for constructing an unified model of dark energy and dark matter is discussed. In particular, we consider the evolution history of the Universe later than inflationary era, the time evolution of the growth rate function of the matter density contrast, the flat rotation curves of the spiral galaxies, and the gravitational experiments in the solar system in mimetic matter model.
File:Jiro Matsumoto.pdf
Louis Yang
Postinflationary Higgs Relaxation and Leptogenesis
The discovery of the Higgs boson with a mass of 125 GeV implies a relatively slow rise of the Standard Model Higgs potential at large energy scales. A scalar field with such a shallow potential can develop a large vacuum expectation value (VEV) during inflation through quantum fluctuation. The relaxation of the large VEV of the Higgs field after inflation may play an important role in the early stage of the universe. In this talk, I will talk about the generation of large Higgs VEV and how to do leptogenesis using the postinflationary relaxation of the Higgs field. I will also discuss one observable consequence of this relaxation leptogenesis model, which could explain the excess found in the cosmic infrared background (CIB) radiation anisotropy.
Kavli, IPMU
File:Louis Yang.pdf

LeCosPA 3rd Symposium LeCosPA
Dr. Haret Rosu
Inverse Black Body Radiation Problem and Astrophysical Applications
An introduction to some theoretical issues related to the general topic of Inverse Black Body Radiation Problem with possible applications to `black' holes and other astrophysical sources is provided.
IPICyT, Mexico
File:Haret Rosu.pdf
No Seminar
Prof. Guey-Lin Lin
Testing Lorentz violation with the measurement of astrophysical neutrino flavor composition
We study Lorentz violation effects on flavor transitions of high energy astrophysical neutrinos. It is shown that the appearance of Lorentz violating Hamiltonian can drastically change the flavor transition probabilities of

astrophysical neutrinos. Predictions of Lorentz violation effects on flavor compositions of astrophysical neutrinos arriving on Earth are compared with IceCube flavor composition measurement which analyzes astrophysical neutrino events in the energy range between $25~{\rm TeV}$ and $2.8~{\rm PeV}$. Such a comparison indicates that the future IceCube-Gen2 will be able to place stringent constraints on Lorentz violating Hamiltonian in the neutrino sector.

We work out the expected sensitivities by IceCube-Gen2 on dimension-$3$ CPT-odd and dimension-$4$ CPT-even operators in Lorentz violating Hamiltonian. The expected sensitivities can improve on the current constraints obtained from other types of experiments by more than two orders of magnitudes for certain range of the parameter space.

Dr. Yi-Kuan Chiang
Which Galactic dust map should I use? Insights from extragalactic tomography
Over the past few years, clustering-based redshift estimation has emerged as a new way to estimate redshifts and perform extragalactic tomography of arbitrary datasets. On a similar timescale, observations by Planck, WISE, Pan-STARRS and 21cm radio surveys have been used to create a multitude of SFD-type Galactic dust maps. I will explain how clustering-based redshift estimation can be used to test the quality of the seven different dust maps currently available and I will show that extragalactic signatures can be revealed in many of them. When such maps are used for correcting optical magnitudes, we therefore expect biases which are likely to affect the precision of cosmological experiments using supernovae, BAOs, or the growth of structures. I will present possible solutions to alleviate this issue and discuss which map should be used depending on which measurement one wishes to make.
Johns Hopkins University