Seminars take place on Wednesdays 12:00-1:00pm in Randall Laboratory #3481
Organizer: Bibhushan Shakya, Uri Kol, John Golden
Ken Van Tilberg (IAS): "A small weak scale from a small cosmological constant" | Jan 18
I will present a framework in which Weinberg's anthropic explanation of the cosmological constant problem also solves the hierarchy problem. The weak scale is selected by chiral dynamics that controls the stabilization of an extra dimension. When the Higgs vacuum expectation value is close to a fermion mass scale, the radius of an extra dimension becomes large, and develops an enhanced number of vacua available to scan the cosmological constant down to its observed value. At low energies, the radion necessarily appears as an unnaturally light scalar, in a range of masses and couplings accessible to fifth-force searches as well as scalar dark matter searches with atomic clocks and gravitational-wave detectors. The fermion sector that controls the size of the extra dimension consists of a pair of electroweak doublets and several singlets. These leptons satisfy approximate mass relations related to the weak scale and can be produced at the LHC and future colliders. If time permits, I will also present purely four-dimensional realizations of the framework.
Based on arXiv:1609.06320 in collaboration with Asimina Arvanitaki, Savas Dimopoulos, Victor Gorbenko, and Junwu Huang
Vladimir Rosenhaus (UCSB): "Aspects of SYK" | Jan 25
The recently introduced SYK model is a 0+1 dimensional theory of N>>1 Majorana fermions with a q-body, Guassian-random, all-to-all interaction. The model has an infrared fixed point, is solvable, and is maximally chaotic. We discuss some properties of the model and the search for its AdS_2 dual theory.
Luca Visinelli (Stockholm University): "Axions CDM in non-standard cosmologies" | Feb 1
The properties of cold dark matter axions strongly depend on the thermal history of the Universe before BBN. I show that axion cold dark matter may be a good probe of the pre-BBN epoch since observational properties like the axion mass, its velocity dispersion, and the size of axion miniclusters can be used to distinguish among different scenarios.
Janina Renk (Stockholm University): "Signatures of Horndeski's Gravity on ultra-large cosmic scales" | Feb 8
Upcoming galaxy surveys are aiming to map the largest scales of our observable universe in the next decades. With these data we can test gravity on scales near the horizon by studying the signatures of modified gravity models on large scale structure observables. I will present the imprints on the galaxy number counts (GNC's) and on the cross-correlation of the GNC's with CMB temperature anisotropies from certain classes of scalar-tensor theories of gravity embedded in the Horndeski Lagrangian. Furthermore fundamental properties of gravity, e.g. the propagation speed of tensor perturbations, can be tested using an effective field theory approach.Due to the remarkable modifications of relativistic effects which contribute to the considered signals, like the integrated Sachs-Wolfe effect, these observables allow us to constrain or even rule out some alternative models.
Masha Baryakhtar (Perimeter): "Searching for Ultralight Particles with Black Holes and Gravitational Waves" | Feb 22
The LIGO detection of gravitational waves has opened a new window on the universe. I will discuss how the process of superradiance, combined with gravitational wave measurements, makes black holes into nature's laboratories to search for new light bosons, from axions to dark photons. When a bosonic particle's Compton wavelength is comparable to the horizon size of a black hole, superradiance of these bosons into bound "Bohr levels" extracts energy and angular momentum from the black hole. The occupation number of the levels grows exponentially and the black hole spins down. One candidate for such an ultralight boson is the QCD axion with decay constant above the GUT scale. Current black hole spin measurements disfavor a factor of 30 (>100) in axion (vector) mass; future measurements can provide evidence of a new boson. Particles transitioning between levels and annihilating to gravitons may produce thousands of monochromatic gravitational wave signals, and turn LIGO into a particle detector.
Filip Kos (Yale): "Bootstrapping Gross-Neveu Models" | Mar 8
We study the conformal bootstrap for 4-point functions of fermions in parity-preserving 3D CFTs, where fermions transforms as a vector under an O(N) global symmetry. We compute bounds on scaling dimensions and central charges, finding features in our bounds that appear to coincide with the O(N) symmetric Gross-Neveu-Yukawa fixed points. Our computations are in perfect agreement with the 1/N expansion at large N and allow us to make nontrivial predictions at small N.
Yue Zhang (Northwestern): "Bound State Effects on Dark Matter Annihilation" | Mar 15
I will discuss the non-perturbative effects from dark matter bound states and their impact on the searches for dark matter. The goal of this talk is to emphasize the importance of dark bound state channels using three examples. I first discuss two dark sector models where the dark matter interacts through a vector or the Higgs portal, and the bound state channels allow indirect detection to explore a much wider range of the parameter space. I also discuss bound state effects on dark matter relic abundance in a co-annihilation model with a top/bottom quark partner.
Benjamin Mosk (Stanford): "Linearized Gravity as a Gauge-Invariant Wave Equation on Kinematic Space" | Mar 22
Kinematic space was originally defined in AdS3 as the space of geodesics. In this talk, I will generalize the concept of kinematic space to higher dimensions. Fields can be defined on this kinematic space, and these fields can be identified with "OPE blocks," contributions to the OPE from a single conformal family. In holographic theories, the OPE blocks are dual at leading order in 1/N to integrals of effective bulk fields along geodesics or homogeneous minimal surfaces in anti-de Sitter space. Thus, these operators pave the way for generalizing the Ryu-Takayanagi relation to other bulk fields. The dynamics of bulk fields are related to the dynamics of kinematic space fields via the intertwining property of integral transformations. In particular, the linearized gravitational equations are shown to be equivalent to a gauge-invariant wave equation on kinematic space.
References:
1) Equivalent Equations of Motion for Gravity and Entropy.
By Bartlomiej Czech, Lampros Lamprou, Samuel McCandlish, Benjamin Mosk, James Sully.
[arXiv:1608.06282 [hep-th]].
10.1007/JHEP02(2017)004.
JHEP 1702 (2017) 004.
2) Holographic equivalence between the first law of entanglement entropy and the linearized gravitational equations.
By Benjamin Mosk.
[arXiv:1608.06292 [hep-th]].
10.1103/PhysRevD.94.126001.
Phys.Rev. D94 (2016) no.12, 126001.
3) A Stereoscopic Look into the Bulk.
By Bartlomiej Czech, Lampros Lamprou, Samuel McCandlish, Benjamin Mosk, James Sully.
[arXiv:1604.03110 [hep-th]].
10.1007/JHEP07(2016)129.
JHEP 1607 (2016) 129.
Francesco D'Eramo (UCSantaCruz): "You can hide but you have to run: new theory tools to unveil the mystery of dark matter" | Mar 29
The origin and composition of 85% of the matter in the universe is completely unknown. Among several viable options, Weakly Interacting Massive Particles (WIMPs) are motivated dark matter candidates that can be tested by different and complementary search strategies. Crucially, different searches probe WIMP couplings at different energy scales, and such a separation of scales has striking consequences in connecting different experimental probes. This motivates the development of theoretical tools to properly connect the different energy scales involved in constraining WIMP models. I will introduce these tools and I will illustrate with several examples how crucial the inclusion of these effects in WIMP searches is.
Ahmed Almheiri (Stanford): "Linearity of Holographic Entanglement Entropy" | Apr 5
We consider the question of whether the leading contribution to the entanglement entropy in holographic CFTs is truly given by the expectation value of a linear operator as is suggested by the Ryu-Takayanagi formula. We investigate this property by computing the entanglement entropy, via the replica trick, in states dual to superpositions of macroscopically distinct geometries and find it consistent with evaluating the expectation value of the area operator within such states. However, we find that this fails once the number of semi-classical states in the superposition grows exponentially in the central charge of the CFT. Moreover, in certain such scenarios we find that the choice of surface on which to evaluate the area operator depends on the density matrix of the entire CFT. This nonlinearity is enforced in the bulk via the homology prescription of Ryu-Takayanagi. We thus conclude that the homology constraint is not a linear property in the CFT.
Raymond Co (UC Berkeley): "Crucial Effects of Saxion Cosmology on Dark Matter" | Apr 19
In supersymmetry, the lightest supersymmetric particles (LSP) are well-motivated candidatesfor dark matter. When the Peccei-Quinn (PQ) symmetry is evoked to solve the strong CP problem, the axion becomes another viable candidate. The axion's scalar superpartner, the saxion, however significantly changes the conventional picture. During inflation, a potential for saxions is induced and displaces the saxion field value away from the vev today. This results in a large saxion condensate that dominates the energy density of the Universe. The saxion subsequently decays to the particles in the thermal bath, generating a large amount of entropy and diluting the dark matter abundance. We focus on the cases where dark matter is the axion from the misalignment mechanism, or the axino/gravitino LSP that arises from the thermal scattering and Freeze-In processes. The former case allows the PQ symmetry and grand unification to be of the same origin, whereas the latter case allows a high reheat temperature after inflation, solving the axino/graviton problems. We will also discuss interesting phenomenology for this class of theories, e.g. dark radiation and displaced vertices at colliders.
