Michal P. Heller, Max Planck Institute and NCBJ
[QCD theory Seminar] New results on convergence of hydrodynamic gradient expansion
Online (Zoom) Relativistic hydrodynamics, viewed as an effective field theory, is formulated in a derivative expansion. Studies of applicability of relativistic hydrodynamics in nuclear collisions at RHIC and LHC prompted 1302.0697 to explore convergence properties of this expansion. This and subsequent works showed that the Bjorken flow gives rise to a gradient expansion with a vanishing radius of convergence in all but a single studied quark-gluon plasma model. In my talk I will overview the current status of this field, as well as outline new developments on this topic that, for the first time, allow to make definite statements about the
Relativistic hydrodynamics, viewed as an effective field theory, is formulated in a derivative expansion. Studies of applicability of relativistic hydrodynamics in nuclear collisions at RHIC and LHC prompted 1302.0697 to explore convergence properties of this expansion. This and subsequent works showed that the Bjorken flow gives rise to a gradient expansion with a vanishing radius of convergence in all but a single studied quark-gluon plasma model. In my talk I will overview the current status of this field, as well as outline new developments on this topic that, for the first time, allow to make definite statements about the
convergence of hydrodynamic gradient expansion. New results follow from 2007.05524 and an ongoing work.
Tokiro Numasawa, University of Tokyo
Four coupled SYK models and Nearly AdS2 gravities: Phase Transitions in Traversable wormholes and in Bra-ket wormholes
Online (Zoom) https://kds.kek.jp/event/38340/ Wormholes are interesting and important objects in gravity.
Wormholes are interesting and important objects in gravity.
In particular, recently controllable examples of traversable wormholes are constructed.
To understand these wormholes better, we study the two traversable wormhole configurations through the study of four coupled SYK models and nearly AdS2 gravities.
In the SYK model side, we construct a model that couples two copies of two coupled SYK models.
In the nearly AdS2 gravity side, we entangle matter fields in two copies of traversable wormholes.
In both cases, the systems show first order phase transitions at zero temperature by changing couplings, which is understood as the exchange of traversable wormhole configurations.
In nearly AdS2 gravity cases, through the double wick rotation, traversable wormholes are interpreted as bra-ket wormholes, which gives a connection between bra and ket vectors.
In Lorentzian signature, these bra-ket wormholes lead to two closed universes that are entangled with each other as well as matter fields in the flat space without dynamical gravity.
We study the effect of projection or entangling operation for matters on flat spaces and they cause phase transitions in bra-ket wormholes, which leads to the pair annihilation of closed universes.
Using these bra-ket wormholes, we discuss the way to embed states in 2d holographic CFTs into Hilbert space of many 2d free fields.
David Tong, University of Cambridge
How to Give Chiral Fermions a Mass
Online (Zoom), slides (kek.jp only)
Chiral fermions have the property that their left-handed and right-handed components transform differently under some symmetry. Folklore suggests that it is impossible to give such fermions a mass without breaking this symmetry. I’ll show, through a number of examples, why this folklore is wrong. In particular, I’ll show how one generation of fermions in the Standard Model can get a mass without the need for a Higgs boson that breaks electroweak symmetry.
Jun'ya Kume, RESCEU / The University of Tokyo
The role of chiral gravitational anomaly in cosmology
Online (Zoom), indico page, slides (kek.jp only)
Quantum anomaly is ubiquitous and investigated in various fields of physics, including cosmology. In this talk, I introduce two interesting phenomena in the early universe caused by the chiral gravitational anomaly, which relates the left-right asymmetry of the fermions and the helicity of the gravitational waves. The first one is so-called “gravitational leptogenesis” where the primordial lepton asymmetry is produced as a consequence of this anomaly in models of inflation generating helical primordial gravitational waves. I show the difficulties in the minimal model of gravitational leptogenesis and briefly discuss the extended models where the gauge fields are involved. The other one is what we call “chiral gravitational effect”(CGE), which can be understood as the gravitational counterpart of the chiral magnetic effect (CME). In these effects, the primordial chiral asymmetry, which can be generated in well-motivated scenarios like GUT baryogenesis, is a key ingredient. I present how CGE affects the primordial gravitational waves propagating in the chiral primordial plasma, while comparing with the CME.
Hiroshi Suzuki, Kyushu University
Gradient flow exact renormalization group
Online (Zoom) https://kds.kek.jp/event/38153/
The so-called gradient flow, a gauge covariant diffusion of the gauge field, bears a close resemblance to the coarse graining involved in the Wilsonian renormalization group (RG). In scalar field theory, a precise connection between the gradient flow and the Wilsonian RG flow can be made. By imitating the structure of this connection, we propose a Wilsonian RG equation that preserves manifest gauge invariance in vector-like gauge theory. I also mention some ongoing studies on the basis of this formulation.
Kei Yamamoto, Hiroshima University
EFT approach in U(2) and U(3) flavor symmetries
Online (Zoom), indico page, slides (kek.jp only) The measurements of quark flavor-violating observables show a success of the Standard Model (SM). If we insist that new physics (NP) has to emerge in the TeV region, we need to conclude that NP possesses a highly non-generic flavor structure (flavor problem). One of the reasonable solutions to the flavor problem is to introduce flavor symmetry hypothesis. U(3) flavor symmetry is the maximal flavor symmetry allowed by the SM gauge group, and it is known as the Minimal Flavor Violation hypothesis. U(2) is the corresponding subgroup acting only on the first two (light) generations, and shows a good agreement with Yukawa and CKM hierarchical structures. Interestingly, it is shown that U(2) has a good compatibility with the recent B-anomalies. On the other hand, LHC results so far show that there is a mass gap between the SM spectrum and NP. It has motivated the systematic Effective Field Theory (EFT) approach. In this talk, after a brief review of U(3) and U(2) flavor symmetry, I will show how these flavor symmetries act on the SMEFT, providing an organizing principle to classify the large number of dimension-six operators involving fermion fields. The phenomenological study focusing on B-anomalies will be also discussed.
The measurements of quark flavor-violating observables show a success of the Standard Model (SM). If we insist that new physics (NP) has to emerge in the TeV region, we need to conclude that NP possesses a highly non-generic flavor structure (flavor problem). One of the reasonable solutions to the flavor problem is to introduce flavor symmetry hypothesis. U(3) flavor symmetry is the maximal flavor symmetry allowed by the SM gauge group, and it is known as the Minimal Flavor Violation hypothesis. U(2) is the corresponding subgroup acting only on the first two (light) generations, and shows a good agreement with Yukawa and CKM hierarchical structures. Interestingly, it is shown that U(2) has a good compatibility with the recent B-anomalies. On the other hand, LHC results so far show that there is a mass gap between the SM spectrum and NP. It has motivated the systematic Effective Field Theory (EFT) approach. In this talk, after a brief review of U(3) and U(2) flavor symmetry, I will show how these flavor symmetries act on the SMEFT, providing an organizing principle to classify the large number of dimension-six operators involving fermion fields. The phenomenological study focusing on B-anomalies will be also discussed.
References
JHEP 08 (2020) 166, e-Print: 2005.05366 [hep-ph]
Phys.Lett.B 800 (2020) 135080, e-Print: 1909.02519 [hep-ph]
Kinya Oda, Tokyo Woman's Christian University
New effect in wave-packet scatterings of quantum fields: Saddle points, Lefschetz thimbles, and Stokes phenomenon
Online (Zoom), indico page, slides (kek.jp only)
We find a new contribution in wave-packet scatterings, which has been overlooked in the standard formulation of S-matrix. As a concrete example, we consider a two-to-two scattering of light scalars ϕ by another intermediate heavy scalar Φ, in the Gaussian wave-packet formalism: ϕϕ→Φ→ϕϕ. This contribution can be interpreted as an “in-time- boundary effect” of Φ for the corresponding Φ→ϕϕ decay, proposed by Ishikawa et al., with a newly found modification that would cure the previously observed ultraviolet divergence. We show that such an effect can be understood as a Stokes phenomenon in an integral over complex energy plane: The number of relevant saddle points and Lefschetz thimbles (steepest descent paths) discretely changes depending on the configurations of initial and final states in the scattering.
Xin-Li Sheng, Central China Normal Univ.
[QCD theory Seminar] From Kadanoff-Baym to Boltzmann equations for massive spin-1/2 fermions
Online (Zoom)
The quark-gluon plasma (QGP) generated in high-energy heavy-ion collisions is the most vortical system human ever made. The orbital angular momentum of the system will be converted into spin polarization of quarks during the evolution of the QGP. In this work, we use a matrix-valued distribution function for massive spin-1/2 fermions. The diagonal part of this distribution is the particle number density and the remaining parts are spin polarization density in the rest frame. From the Kadanoff-Baym equation, we derive Boltzmann equations for the matrix-valued distribution function, where nonlocal collision terms appear at next-to-leading order in space-time gradient. The nonlocal terms contribute as sources for the spin polarization part of the matrix-valued distribution function. The Boltzmann equations we obtained pave the way for numerically simulating spin-transport processes involving spin-vorticity coupling.
Ryohei Kobayashi, ISSP, The University of Tokyo
Interacting fermionic topological phases with time reversal symmetry
Online (Zoom) https://kds.kek.jp/event/37875/
In this talk, we discuss a recipe to produce a lattice construction of fermionic topological phases of matter on unoriented spacetime, which plays a crucial role to study topological phases or anomalies based on the time reversal symmetry. As an application, we construct a gapped boundary for a large class of fermionic SPT phases protected by finite onsite symmetry, based on our path integral description in the presence of boundaries. We will also formulate a local path integral for the (1+1)d topological superconductor in class BDI classified by Z8, and discuss its application to the problem of finding non-local order parameter for the Z8 classification if time permits.
Jan Pawlowski, Heidelberg University
Emergent Hadrons and Diquarks
online (Zoom) indico page, slides (kek.jp only)
