Location: UCLA Physics & Astronomy Building (PAB) Room 4-330
Time: Mondays 3-4 PM (unless otherwise noted)
For additional information regarding seminars please contact Volodymyr Takhistov.
Friday, August 24 (11:00 AM), 2018: Adam Falkowski (CNRS, Univ. Paris-Sud)
"Stronger 21cm absorption from charge sequestration"
The unexpectedly strong 21cm absorption signal detected by the EDGES experiment suggests that the baryonic gas at the end of the dark ages was colder than predicted within the standard cosmological scenario. I will introduce one novel mechanism to lower the baryon temperature after recombination. The model includes a stable, negatively-charged particle with a significant cosmological abundance, such that the universe remains charge-neutral but the electron and proton numbers are no longer equal. The deficit of electrons after recombination results in an earlier decoupling of the baryon and CMB temperatures, and thus in a colder gas at the cosmic dawn. I will discuss the phenomenological constraints and possible smoking guns of this scenario.
Monday, April 23 (11:00 AM), 2018: Hiroyuki Sagawa (ICRR, University of Tokyo)
"Recent results of the Telescope Array experiment on ultra-high energy cosmic rays"
The Telescope Array (TA) is the largest ultra-high-energy cosmic ray (UHECR) detector in the Northern Hemisphere, which explores the origin and nature of UHECRs. TA is located in Utah, consisting of an array of scintillator detectors on the ground to sample the footprint of the air shower induced by UHECRs and telescopes to measure the fluorescence and Cerenkov light of the air shower. TA determines the energy spectrum and chemical composition of the primary particles, and also searches for anisotropy of arrival directions of UHECRs and their sources. We have found evidence of anisotropy of UHECRs in the northern sky. Here the recent measurements for spectrum, composition and anisotropy and the status of the TA extension will be presented.
Monday, April 23 (3:00 PM), 2018: Benjamin Grinstein (UCSD)
"Dark Particle Interpretation of the Neutron Decay Anomaly"
There is a long-standing discrepancy between the neutron lifetime measured in beam and bottle experiments. We propose to explain this anomaly by a dark decay channel for the neutron, involving a dark sector particle in the final state. If this particle is stable, it can be the dark matter. Its mass is close to the neutron mass, suggesting a connection between dark and baryonic matter. In the most interesting scenario a monochromatic photon with energy in the range 0.782 MeV – 1.664 MeV and branching fraction 1% is expected in the final state. We construct representative particle physics models consistent with all experimental constraints.
Monday, April 16, 2018: Nathaniel Craig (UCSB)
"New tools for solving hierarchy problems"
Hierarchies among observed parameters have long been a driver of physics beyond the Standard Model, but conventional explanations for these hierarchies are under increasing pressure from null experimental tests. In this talk I’ll survey a variety of new approaches to solving hierarchy problems within and beyond the Standard Model, including protection via accidental continuous symmetries arising from discrete symmetries; the generation of exponential hierarchies by analogy with condensed matter systems; and controlled violations of effective field theory. I’ll discuss possible applications of these ideas to dark matter and the electroweak hierarchy problem.
Monday, April 2, 2018: Josef Pradler (Institute of High Energy Physics, Austrian Academy of Sciences)
"Signatures of dark matter (well) below the GeV-scale"
I will discuss experimental and observational signatures of dark matter with mass at and very well below the GeV scale. The first signature regards scattering on nuclei and electrons in dark matter direct detection experiments. I will show how smaller but irreducible signal components can be tapped to dramatically extend the low-mass reach of existing searches. The second signature regards dark matter decay into dark radiation states. I will comment on the experimental detectability of dark radiation, and further show that 21 cm astronomy becomes a probe of very light dark radiation when its converted into ordinary photons before reionization. An explanation of the EDGES result is offered.
Wednesday, January 10, 2018: Mariana Carrillo Gonzalez (University of Pennsylvania), "Interacting dark sectors and the mildly non-linear regime"
[* special date, time: 4:30 PM]
In this talk, I will present both theoretical and observational results on an interacting dark sector. Firstly, I will introduce the field theoretical models that underlie the fluid description. The construction of these models leads to a consistent field theory for an interacting dark sector that behaves exactly like the coupled fluids at both the level of linear perturbations and the mildly non-linear regime. I will show that the quantum corrections are under control and that caustics do not appear at quasi-linear scales. Assuming the fluid description, observational constraints on these models are obtained. Here, I will focus on robust constraints that can be obtained using only mildly non-linear scales. This is done by combining lensing, clustering of galaxies, and the Cosmic Microwave Background (CMB) data sets.
Wednesday, October 4, 2017: Nirmal Raj (Notre Dame), "Dark Fires in the Sky: Model-Independent Dark Matter Detection via Kinetic Heating of Neutron Stars"
[* special date, time: 4:30 PM]
I present a largely model-independent probe of dark matter-nucleon interactions. Accelerated by gravity to relativistic speeds, local dark matter scattering against old neutron stars deposits kinetic energy at a rate that heats them to infrared blackbody temperatures. The resulting radiation is detectable by next generation telescopes such as James Webb, the Thirty Meter Telescope, and the European Extremely Large Telescope. While underground direct detection searches are not (or poorly) sensitive to dark matter with sub-GeV masses, higher-than-weak-scale masses, scattering with strong cross-sections, scattering below neutrino floors, spin-dependent per-nucleon scattering below per-nuclear cross-sections, velocity-dependent scattering, and inelastic scattering for inter-state transitions exceeding O(100 keV), the (non-)observation of dark kinetic heating of neutron stars should advance these frontiers by orders of magnitude. Popular dark matter candidates previously suspected elusive, such as the thermal Higgsino, may be discovered.
Monday, June 12, 2017: Tim Tait (UC Irvine), "Recent Developments in the Search for Dark Matter at Colliders"
[* special time: 11 AM]
Abstract: I will discuss recent work related to searching for production of dark matter at high energy colliders such as the LHC, including both the theoretical constructs used to frame the results of such searches, the new experimental picture post-Moriond, and some discussion of how these results fit into the broader picture of particle physics searches for dark matter.
Monday, June 5, 2017: Jessica Turner (IPPP, Durham), "Creating
the Baryon Asymmetry from Lepto-Bubbles"
Abstract: The origin of the asymmetry between matter and anti-matter of the Universe remains one of the most important open questions in particle physics and cosmology. We propose a novel mechanism, the lepto-bubbles mechanism, to dynamically generate this observed asymmetry. The particle that undergoes the phase transition is a SM-singlet scalar which couples to the lepton number violating Weinberg dimension-5 operator. The phase transition provides an out of equilibrium condition and the time dependent complex couplings of the scalar to the Weinberg operator are CP-violating. Therefore, the CPPT mechanism satisfies Sakharov's conditions without the introduction of the right-handed neutrinos or the see-saw mechanism. We calculate the lepton asymmetry produced by the phase transition using the closed time path formalism.
Monday, May 22, 2017: Hai-Bo Yu (UC Riverside), "Explaining the Diverse Rotation Curves of Spiral Galaxies"
Abstract: The rotation curves of spiral galaxies exhibit a diversity that has been difficult to understand in the cold dark matter (CDM) paradigm. In this talk, I will show that the self-interacting dark matter (SIDM) model provides excellent fits to the rotation curves of a sample of galaxies with asymptotic velocities in the 25 to 300 km/s range that exemplify the full range of diversity. We only assume the halo concentration-mass relation predicted by the CDM model and a fixed value of the self-interaction cross section. The impact of the baryons on the SIDM halo profile and the scatter from the assembly history of halos as encoded in the concentration-mass relation can explain the diverse rotation curves of spiral galaxies. I will also discuss other smoking-gun signatures of SIDM in astrophysical observations.
Monday, March 13, 2017: Shigehiro Nagataki (RIKEN), "Astrophysical Big Bangs"
Abstract: Our group focuses on unveiling the many mysteries surrounding astrophysical explosive phenomena such as supernovae and gamma ray bursts from a theoretical point of view. Supernovae and gamma ray bursts are believed to be the most powerful explosions in our Universe, and yet very little about their explosion mechanisms is known to mankind. Members of our group use frontier physics in their daily routine to attack these problems in high-energy astrophysics. In many cases, we seek power from the quickly advancing supercomputing resources in Japan to make progress in our investigations. These 'astrophysical big bangs' continue to fascinate us with their unknown physics and puzzling astronomical phenomena (e.g. gravitational waves, r-process nucleosynthesis, particle acceleration, high-energy neutrinos, ultra high-energy cosmic rays, gamma-ray emission, etc), and they are commonly regarded as the most representative manifestations of the extreme Universe. Through our theoretical and computational endeavors, we strive to reveal the complete pictures of these violent explosions and provide state-of-the-art physical interpretations and predictions for future observations by the next-generation astronomical observatories. We are more than passionate to co-operate with fellow RIKEN researchers and all other interested groups in Japan and the globe, and together we will realize the Utopia for researchers in RIKEN. In this talk, I would like to introduce our research activities, together with short introduction of a new program in RIKEN "Interdisciplinary Theoretical and Mathematical Science Program (iTHEMS)"
Monday, February 27, 2017: Mark Wise (Caltech), "Dark Matter Bound States and Indirect Dark Matter Signals"
Abstract: Dark Matter and its antiparticle may form bound states analogous to the positronium states that an electron and positron form. I discuss how taking such bound states into account changes (in some regions of parameter space) our expectations for the dark matter annihilation rate at the time of recombination and today.
Monday, March 6, 2017: Valerie Domcke (APC, Paris),
"Probing the early Universe with gravitational waves"
Abstract: Gravitational waves are unique messengers to explore the very early universe, probing energy ranges far beyond the reach of photon or even neutrino astronomy. The holy grail in this context is the stochastic gravitational wave background of cosmic inflation, which would shed light on the microphysics of inflation as well as on the entire subsequent cosmological history. In the simplest model of inflation this signal is however beyond the reach of current and planned gravitational wave interferometers. After reviewing this standard picture, I will discuss how modifications of this standard scenario can be a real game-changer, boosting the primordial gravitational wave signal into the range accessible by experiments such as eLISA and LIGO/VIRGO.
Monday, February 13, 2017: Mu-Chun Chen (UC Irvine), "Discrete (Family) Symmetries
and Origin of CP Violation"
Abstract: The origins of the flavor mixing and CP violation remain a mystery in particle physics. The discovery of non-zero neutrino masses leads to yet another puzzle: why the neutrino masses are so small when compared to other fermions, and why two of the three neutrino mixing angles are so large when compared with their quark counterpart. Furthermore, CP violation in the Standard Model is insufficient to explain the observed cosmological baryon number asymmetry. On the other hand, the recent observation of a large value for the third neutrino mixing angle implies good future experimental prospects for discovering a new source of CP violation in the neutrino sector. This new CP violation source may be relevant for the dynamical generation of the cosmological matter-antimatter asymmetry.
In this talk, I will discuss how these outstanding questions in particle physics can be addressed by new physics beyond the Standard Model. In particular, I will focus on models based on discrete family symmetries which give rise to realistic masses and mixing angles of all observed fermions, including the neutrinos, with a significantly reduced number of parameters. I will point out a novel interesting possibility that for certain discrete symmetries, CP violation can be entirely group theoretical in origin. Specifically, we show that physical CP transformations always have to be class-inverting automorphisms of G. We also show that certain operations that have been dubbed generalized CP transformations in the recent literature do not lead to physical CP conservation. It is found that discrete groups can be classified into three types, each with different implications for CP violation.
Tuesday, March 1, 2016: Tomohiro Fujita (KIPAC, SLAC) "Large-scale magnetic fields can explain
the baryon asymmetry of the universe"
Abstract: Helical magnetic fields in the primordial Universe can produce the observed amount of baryon asymmetry through the chiral anomaly without any ingredients beyond the Standard Model of particle physics. While they generate no B — L asymmetry, the generated baryon asymmetry survives the spharelon washout effect, because the generating process remains active until the electroweak phase transition. We show that the baryon asymmetry of our Universe can be explained, if the present large-scale magnetic fields indicated by the blazar observations have a negative helicity and existed in the early Universe before the electroweak phase transition.
Tuesday, April 5, 2016: Jiang-Hao Yu (University of Massachusetts Amherst) "Hidden sector
shining the light: gamma-ray excess and diphoton excess"
Abstract: I will talk about how the hidden sector dark matter produces visible photon signatures. In the first half of my talk, I will focus on a CP violating hidden sector and its explanation on the GeV Gamma-ray excess. In the second half, I will present how a hidden sector and its UV completion explain the 750 GeV diphoton resonance.
Monday, April 18, 2016: Flip Tanedo (UC Irvine) "Dark Earthshine"
Abstract: I propose a search for dark matter annihilating into dark photons at the center of the Earth, focusing on the present and future reach of IceCube and space-based telescopes. I highlight the effect of low-velocity enhancements to dark matter capture and kinematic features that offer an opportunity for a smoking-gun signal. I also present prospects for an analogous search for dark photons from the center of the sun which may be detected by AMS-02, extending earlier work by modeling the effect of the solar wind.
Tuesday, May 17, 2016: Keisuke Harigaya (UC Berkeley) "A composite model for the 750 GeV diphoton excess"
Abstract: We study a simple model in which the recently reported 750 GeV diphoton excess arises from a composite pseudo Nambu-Goldstone boson (hidden pion) produced by gluon fusion and decaying into two photons. The model introduces an extra hidden gauge group at the TeV scale with a vectorlike quark in the bifundamental representation of the hidden and standard model gauge groups. We discuss the collider phenomenology and cosmology of hidden pions and other hidden resonances. We discuss possible theoretical structures above the TeV scale, e.g. conformal dynamics and supersymmetry, and their phenomenological implications.
Monday, May 23, 2016: Tim Tait (UC Irvine) "Evidence for a Photophobic 5th force?"
Abstract: Recently a 6.8σ anomaly has been reported in the opening angle and invariant mass distributions of e+e− pairs produced in 8Be nuclear transitions. The data are explained by a 17 MeV vector gauge boson X that is produced in the decay of an excited state to the ground state, 8Be∗ → 8BeX, and then decays through X→e+e−. The X boson mediates a fifth force with a characteristic range of 12 fm and has milli-charged couplings to up and down quarks and electrons, and a proton coupling that is suppressed relative to neutrons. The photophobic X boson may also alleviate the current 3.6σ discrepancy between the predicted and measured values of the muon's anomalous magnetic moment. I will discuss the experimental results concerning the nuclear transition, the details of a particle physics model which can hope to explain it while satisfying other constraints, and future experiments which can help shed light on the situation.