High Energy Theory Seminar
The seminar starts at 10:30 a.m. in room P7F on the 7th floor.
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Coming Up:
April 20
Dr. Swarup Kumar Majee (NTU)
Universal Extra Dimension at LHC.
Besides supersymmetry, the other prime candidate of physics beyond the Standard Model (SM), crying out for verification at the CERN Large Hadron Collider (LHC), is extra-dimension. To hunt for effects of Kaluza-Klein (KK) excitations of known fermions and bosons is very much in the agenda of the LHC. These KK states arise when the SM particles access theextra space-like dimension(s). We consider here a 5-dimensional universal extra-dimension (UED) scenario. The Kaluza-Klein (KK) number is conserved at all tree level vertices. This entails the production of KK states in pairs and renders the lightest KK particle stable, which leaves the detector carrying away missing energy. The splitting between different KK flavors is controlled by the zero mode masses and the bulk-and brane-induced one-loop radiative corrections. We concentrate on theproduction of an $n=1$ KK electroweak gauge boson in association with an $n=1$ KK quark. This leads to a signal consisting of one jet, one or more leptons and missing $p_T$. For definiteness we usually choose the inverse radius of compactification to be $R^{-1} = 500$ GeV, which sets the scale of the lowest lying KK states. We show that with 10 ${\rm fb}^{-1}$ integrated luminosity at the LHC with $\sqrt{s}$ = 14 TeV thissignal can be detected over the SM background by imposing appropriate kinematic cuts. We record some of the expectations for a possible intermediate LHC run at $\sqrt{s}$ = 10 TeV and also exhibit the integrated luminosity required to obtain a 5$\sigma$ signal as a function of $R^{-1}$.
April 13
Dr. Bo Ning (NTNU)
Self-Dual Warped AdS_3 Black Holes.
A new class of solutions of 3d topological massive gravity are introduced. These solutions can be taken as non-extreme black holes, with their extreme counterparts being discrete quotients of spacelike warped AdS_3 along the U(1)_L isometry. For consistent boundary conditions, the asymptotic symmetry generators form one copy of the Virasoro algebra with central charge c_L, with which the Cardy formula reproduces the black hole entropy. This provides a novel example of warped AdS/CFT correspondence: the self-dual warped AdS_3 black hole is dual to a CFT with nonvanishing left central charge.
March 30
Dr. Keisuke Izumi (NTU)
Nonlinear superhorizon perturbations in Horava-Lifshitz gravity.
I will talk about a fully nonlinear analysis of superhorizon perturbation in Horava-Lifshitz gravity, based on the gradient expansion method. I will present a concrete expression for the solution of gravity equations up to the second order in the gradient expansion, and prove that the solution can be extended to any order.
March 23
Prof. James M. Nester (NCU)
The Hamiltonian boundary term.
The Hamiltonian for interacting classical fields with quite general theories of dynamic geometry generates the evolution of a spatial region along a time-like vector field. It includes a boundary term which determines the value of the Hamiltonian. From this value one obtains the quasi-local quantities: energy-momentum, angular-momentum/center-of-mass. The Hamiltonian boundary term also directly controls the boundary term in the variation of the Hamiltonian. From the latter one obtains the associated built in boundary conditions and an expression for energy flux. Here we extend our preferred boundary term choice for Einstein's GR (which we had identified in 2005) to select a unique boundary term expression for any dynamic geometry gravity theory along with interacting classical fields. The Hamiltonian boundary term depends not only on the dynamical variables but also on their respective reference values, the latter determine the ground state (having vanishing quasi-local values). Wherever the natural field vacuum value---the ground state---has a vanishing value this is the appropriate reference choice. But for dynamic geometry the ground state fields are non-vanishing: they represent Minkowski or (Anti)de-Sitter space. For these theories, just as we have for GR, we propose 4D isometric matching and extremizing the energy to determine the necessary reference geometry metric and connection values on the boundary.
March 16
Dr. Je-An Gu (LeCosPA, NTU)
Issues in Dark Energy and Modified Gravity.
March 2
Dr. Chian-Shu Chen (NCTS)
Vacuum stability, neutrinos, and dark matter
Motivated by the possible discovery hint of the Standard Model (SM) Higgs mass
around 125~GeV at the LHC, we study the vacuum stability and perturbativity
bounds on Higgs scalar of the SM extensions including neutrinos and dark matter (DM).
Guided by the SM gauge symmetry and the minimal changes in the SM Higgs potential we
consider two extensions of neutrino sector (Type-I and Type-III seesaw mechanisms) and DM
sector (a real scalar singlet (darkon) and minimal dark matter (MDM)) respectively. Similar to
the top quark in the SM we find the cause of instability is sensitive to the size of new Yukawa
couplings between heavy neutrinos and Higgs, that is, the scale of seesaw mechanism. The
nontrivial $SU(2)_{L}$ gauge representations of MDM and Type-III seesaw fermion triplets
will bring the additional positive contributions to the gauge coupling $g_{2}$ renormalization
group (RG) evolution and stabilize the electroweak vacuum up to high scale.
Feburary 24
Dr. Hiroaki Kohyama (CYCU)
The regularization dependence on the phase diagram in the Nambu Jona Lasinio model
The phase structure of quark matters has attracted a lot
of attention for decades both in theoretical and experimental
particle physics. It is believed that the transition between
confined and deconfined state is intimately related to the
phenomena of the chiral phase transition. Quarks have non-trivial
constituent mass which is considerably larger than the small
current quark mass due to underlying chiral symmetry breaking.
In this talk, I am doing to study the phase structure of chiral
broken and restored phase through using the Nambu Jona-Lasinio
model with dimensional regularization. Then compare the results
with the model with frequently used cutoff regularization.
January 13
Dr. Yukihiro Mimura (NTU)
TeV-Scale Seesaw with Non-negligible Left-right Neutrino Mixings
We study natural TeV-scale seesaw with non-negligible left-right neutrino mixings as preserving tiny neutrino masses. The left-right neutrino mixing is directly related to the light and heavy neutrino masses in 1-flavor case. In the multi-generation case, however, the direct relation is not valid due to new freedom of generation mixings. We propose a basis of the Dirac and Majorana neutrino mass matrices, which can be always taken by rotations of left-
and right-handed neutrino fields without loss of generality. Under the basis we take, one can clearly find the condition to preserve tiny active neutrino masses with an experimentally non-negligible size of left-right neutrino mixing. We suggest a flavor symmetry as an underlying theory, which naturally realizes our setup to preserve tiny neutrino masses. We also investigate experimental constraints and phenomenology in our setup.
January 6
Mr. Yang-Ting Chien (Harvard U)
Jet Physics from Static Charges in AdS
Soft interactions with high-energy jets are explored in radial coordinates which exploit the approximately conformal behavior of perturbative gauge theories. In these coordinates, the jets, approximated by Wilson lines, become static charges in Euclidean AdS. The anomalous dimension of the corresponding Wilson line operator is then determined by the potential energy of the charges. To study these Wilson lines we introduce a "conformal gauge" which does not have kinetic mixing between radial and angular directions, and show that a number of properties of Wilson lines are reproduced through relatively simple calculations. We also discuss a relationship between Wilson line diagrams and Witten diagrams.
December 30
Mr. Yu-Hsin Tsai (Cornell U)
Current Collider Bounds on DM
The LHC is capable of providing very strong model-independent constraints on dark matter properties.
Using the recent LHC mono-jet results, we set limits on the couplings of dark matter to quarks, and
hence on the rate of direct and indirect detection. The same approach can be used to constrain dark
matter annihilation and gives strong constraints on thermal relics. We also discuss the dark matter
bounds from the mono-photon searches at the LHC and LEP and the future bounds from the invisible Higgs search.
December 23
Dr. Hui-Tzu Tu (NTU)
Probing Modified Gravity with Weak Gravitational Lensing
December 16
Dr. Chih-Hung Wang (Tamkang U)
Gravitational effects of domain walls on primordial quantum fluctuations
We study the gravitational effects of a planar domain wall on quantum fluctuations of a massless
scalar field during inflation. By obtaining an exact solution of the scalar field equation
in de Sitter space, we show that the gravitational effects of the domain wall break the
rotational invariance of the primordial power spectrum without affecting the translational invariance.
The strength of rotational violation is determined by one dimensionless parameter $\beta$,
which is a function of two physical parameters, the domain wall surface tension $\sigma$ and
cosmological constant $\Lambda$. In the limit of small $\beta$, the leading effect of rotational
violation of the primordial power spectrum is scale-invariant.
December 9
Prof. Hoi-Lai Yu (AS)
Some thoughts on proper time
December 2
Dr. Takaaki Nomura (NCU)
A Gauge-Higgs Unification model in six-dimensional spacetime with S^2/Z_2 extra space
In this talk, we review a gauge-Higgs unification model based on gauge theories defined on six-dimensional spacetime with S^2/Z_2 topology in the extra spatial dimensions. On the extra S^2/Z_2 space, non-trivial boundary conditions are imposed. Here we consider a model based on E_6 gauge symmetry and derive a four-dimensional theory from the six-dimensional model. We will see the model leads to a Standard Model-like gauge theory with the SM gauge symmetry with two extra U(1)s and SM fermions in four dimensions. The Higgs sector of the model is also analyzed. The electroweak symmetry breaking can be realized, and the weak gauge boson and Higgs boson masses are obtained.
November 25
Dr. Debaprasad Maity (NTU)
Constraining a model of varying alpha with PCP violation
In this talk I will be discussing about our recent model on PCP violating varying fine structure constant theory. After briefly reviewing our work, I will talk about the constrains on our model parameters considering various observations from laboratory as well as cosmological experiments.
November 18
Dr. Shoichi Kawamoto (Tunghai U)
Matrix model approach to Quantum Gravity and large-N RG analysis
In this talk, I will first give a review on large N matrix model approach to quantum gravity, or string theory, and what kind of non-perturbative aspects of it has been revealed. Then the large-N renormalization group approach to this model, and how this tool will be improved, are explained.
November 11
Prof. Jae Sik Lee (NTHU)
New Physics for the Top Forward-backward Asymmetry and the CDF Dijet Resonance
We try to figure out the characteristic features of the possible New Physics responsible for the anomalies observed at the Tevatron.
November 4
Dr. Masafumi Ishihara (NCTU
Holographic Confining Gauge theory and Response to Electric Field
We study the response of confining gauge theory to the external electric field by using holographic Yang-Mills theories in the large $N_c$ limit. Although the theories are in the confinement phase, we find a transition from the insulator to the conductor phase when the electric field exceeds its critical value. Furthermore, from the analysis of the massless quark, chiral symmetry restoration is observed at the insulator-conductor transition point.
October 28
Dr. Ryo Takahashi (NTHU)
Split Seesaw Mechanism and Flavor Symmetry
A seesaw mechanism in an extra-dimension, known as the split seesaw mechanism, provides a natural way to realize a splitting mass spectrum of right-handed neutrinos. It leads to one keV sterile neutrino as a dark matter candidate and two heavy right-handed neutrinos being responsible for leptogenesis to explain the observed baryon asymmetry of the Universe. We study models based on A4 flavor symmetry in the context of the split seesaw mechanism. It is pointed out that most of known A4 flavor models with three right-handed neutrinos being A4 triplet suffer from a degeneracy problem for the bulk mass terms, which disturbs the split mechanism for right-handed neutrino mass spectrum. Then we construct a new A4 flavor model to work in the split seesaw mechanism. In the model, the experimentally observed neutrino masses and mixing angles can be realized from both type I+II seesaw contributions. The model predicts the mu-tau symmetry in the neutrino mass matrix at the leading order, resulting in the vanishing $\theta_{13}$ and maximal $\theta_{23}$. The flavor symmetry A4 is broken via the flavon vacuum alignment which can be obtained from the orbifold compactification. The model can be consistent with all data of neutrino oscillation experiments, cosmological discussions of dark matter abundance, leptogenesis, and recent astrophysical data.
October 21
Dr. Melin Huang (NTU)
Dependence of Neutrino Mixing Angles and CP-violating Phase on Mixing Matrix Parametrizations
For neutrino oscillation, the CP-violating phase would be very difficult to determine if the value of $\theta_{13}$ is very small with only an upper limit in the conventional mixing matrix parameterization. This study demonstrates that the application of an alternative mixing matrix representation on the global neutrino data produces significant non-zero mixing angles with uncertainties comparable to those extracted from the application of the conventional mixing representation as well as the sign of $\theta_{13}$. Our study found that global solar-only data in which the MSW effect plays a significant role supports a positive $\theta_{13}$ value while the other neutrino samples where the MSW effect is much less significant favor a negative $\theta_{13}$ value.
October 14
Dr. Gaber Faisel (NCU)
supersymmetric contributions to B_s -> phi pi and B_s -> phi rho in SCET
Using Soft Collinear Effective Theory we analyze the supersymmetric contributions to the branching ratios of B_s -> phi pi and B_s -> phi rho decays. We discuss the different SUSY contributions mediated by gluino and chargino exchange. Using mass insertion approximation we show that SUSY contributions mediated by chargino and gluino exchange can enhance the branching ratios with respect to the SM prediction.
October 7
Dr. Jubin Park (NTHU)
Introduction to Gague Higgs unification with a graded Lie algebra and its extension
The original gauge Higgs unification(GHU) models have well-known problems such as weak mixing angle, correct Higgs potential, and Yukawa couplings. In this talk we introduce a new model with a Z_{2} graded Lie algebra unlike usual supersymmetry in order to solve these problem. In addition we also consider its extension to build a more realistic model.
September 30
Dr. Dong-Won Jung (NTHU)
Dynamical symmetry breaking with four-superfield interactions
We investigate the dynamical mass generation resulted from interaction terms with four chiral superfields. The kind of interactions maybe considered a supersymmetric generalization of the four-fermion interactions of the classic Nambu--Jona-Lasinio model. A four-superfield interaction that contains a four-fermion interaction as one of its component terms has been the standard supersymmetrization of the NJL model for decades. Recently, we introduced a holomorphic variant with a dimension five interaction term instead. The latter is a main target of the present analysis. With the introduction of a new perspective on the superfield gap equation, we derive it for each one of the four-superfield interactions, using the supergraph technique. Through analyzing solutions to the gap equations, we illustrate the dynamical generation of superfield Dirac mass, including a supersymmetry breaking part. A dynamical symmetry breaking generally goes along with the dynamical mass generation, for which a bi-superfield condensate is responsible. The explicit illustration of dynamical symmetry breaking from the holomorphic dimension five interaction is reported for the first time. It has rich and novel features, which would be easily missed without
the superfield approach developed here. We also discuss the nature of the bi-superfield condensate and its role of the effective Higgs superfield picture for both cases, illustrating their difference. Note that such a holomorphic quark superfield interaction term can successful account for the electroweak symmetry breaking with Higgs superfields as composites.
September 23
Dr. Yong Tang (NCTS)
Gravitational contribution to the running of gauge coupling
Quantum gravitational effect is small at low energy scale and large near Planck energy. Its contributions can change the running behaviors of gauge coupling constants and therefore alter their unification. Recently, many researchers show that there are various issues involved in the calculation of this gravitational effect. In this talk, I will give an introduction to these work and discuss how to deal with the one-loop gravitational contributions to gauge theories.
September 16
Dr. Antonio Enea Romano (NTU)
Corrections to the apparent value of the cosmological constant due to local inhomogeneities
Supernovae observations strongly support the presence of a cosmological constant, but its value, which we will call apparent, is normally determined assuming that the Universe can be accurately described by a homogeneous model. Even in the presence of a cosmological constant we cannot exclude nevertheless the presence of a small local inhomogeneity which could affect the apparent value of the cosmological constant. Neglecting the presence of the inhomogeneity can in fact introduce a systematic misinterpretation of cosmological data, leading to the distinction between an apparent and true value of the cosmological constant. Modeling the local inhomogeneity with a $\Lambda LTB$ solution we compute the relation between the apparent and true value of the cosmological constant. Contrary to previous attempts to fit data using large void models our approach is quite general. The correction to the apparent value of the cosmological constant is in fact present for local inhomogeneity of any size, and should always be taken appropriately into account both theoretically and observationally.