Yeuk-Kwan Edna Cheung

• Ph.D.
• Professor at Nanjing University

Precise determination of the branching ratio provides an advantageous opportunity for understanding the electroweak structure of the Standard Model, measuring the CKM matrix element , and probing new physics models. In this paper, we discuss the potential of measuring the process with decaying leptonically at the proposed Circular Electron Positron...

The precise determination of the $B_c \to \tau\nu_\tau$ branching ratio provides an advantageous opportunity for understanding the electroweak structure of the Standard Model, measuring the CKM matrix element $|V_{cb}|$ and probing new physics models. In this paper, we discuss the potential of measuring the processes of $B_c \to \tau\nu_\tau$ with...

This study extends the investigation of quantum dissipative effects of a cosmological scalar field by taking into account cosmic expansion and contraction. Cheung, Drewes, Kang, and Kim calculated the effective action and quantum dissipative effects of a cosmological scalar field in a recent work, where analytical expressions for the effective pote...

The proposed Circular Electron Positron Collider (CEPC) is expected to produce up to 1 trillion Z bosons in 2 years and has a great potential on the flavor physics. The \(K^0_S\) and \(\varLambda \) are important physics signatures for many flavor physics measurements and are abundantly produced in the CEPC Z pole program. Using a full simulation t...

We extend our study on the Coupled-Scalar-Tachyon Bounce Universe to obtain its gravitational waves spectrum. We derive in detail the equations of motion for the tensorial modes of primordial metric perturbations in the Coupled-Scalar-Tachyon Bounce Universe. We solve for the gravitational wave equations in the pre-bounce contraction and the post-b...

Non-parametric reconstruction of the dark energy equation of state (EoS) aims to determine the EoS as a function of redshift without invoking any particular dark energy model, so that the resulting EoS can be free of model-induced biases or artifacts. Without proper regularization, however, such reconstruction is often overwhelmed by the noise of p...

Non-parametric reconstruction of the dark energy equation of state (EoS) aims to determine the EoS as a function of redshift without invoking any particular dark energy model, so that the resulting EoS can be free of model-induced biases or artifacts. Without proper regularization, however, such reconstruction is often overwhelmed by the noise of p...

This paper extends the study of the quantum dissipative effects of a cosmological scalar field by taking into account the cosmic expansion and contraction. Cheung, Drewes, Kang and Kim calculated the effective action and quantum dissipative effects of a cosmological scalar field. The analytic expressions for the effective potential and damping coef...

We derive in detail the equations of motion for the tensorial modes of primordial metric perturbations in the Coupled-Scalar-Tachyon Bounce Universe. We solve for the gravitational wave equations in the pre-bounce contraction and the post-bounce expansion epochs. To match the solutions of the tensor perturbations, we idealise the bounce process yet...

In the present work we examine the possibility of detecting electrons in light dark matter searches. These detectors are considered to be the most appropriate for detecting dark matter particles with a mass in the MeV region. We analyze theoretically some key issues involved in such detection. More specifically we consider a particle model involvin...

The cosmic expansion history, mapped by the Hubble parameter as a function of redshift, offers the most direct probe of the dark energy equation of state. One way to determine the Hubble parameter at different redshifts is essentially differentiating the cosmic age or distance with respect to redshift, which may incur large numerical errors with ob...

Assuming locality, Lorentz invariance and parity conservation we obtain a set of differential equations governing the 3-point interactions of massless bosons, which in turn determines the polynomial ring of these amplitudes. We derive all possible 3-point interactions for tensor fields with polarisations that have total symmetry and mixed symmetry...

We investigate the gauge invariance of the three-point scattering amplitudes in the higher spin extension to Yang-Mills gauge theory as proposed by G. Savvidy. We find that the interactive theory does not preserve all the gauge symmetries of the free theory. We focus our analysis on the spin1-spin2-spin2 amplitudes and conclude that no gauge invari...

In \cite{Chen:2016fgi} we proposed a differential operator for the evaluation of the multi-dimensional residues on isolated (zero-dimensional) poles.In this paper we discuss some new insight on evaluating the (generalized) Cachazo-He-Yuan (CHY) forms of the scattering amplitudes using this differential operator. We introduce a tableau representatio...

In \cite{Chen:2016fgi} we proposed a differential operator for the evaluation of the multi-dimensional residues on isolated (zero-dimensional) poles.In this paper we discuss some new insight on evaluating the (generalized) Cachazo-He-Yuan (CHY) forms of the scattering amplitudes using this differential operator. We introduce a tableau representatio...

The recently proposed coupled scalar tachyon bounce (CSTB) model is a bounce universe model based on Type IIB string theory. We investigate the dynamics of fluctuations across the bounce point and check whether the scale invariance of the spectrum of the primordial density perturbations generated during the phase of matter-dominated contraction is...

A bounce universe model, known as the coupled-scalar-tachyon bounce (CSTB) universe, has been shown to solve the Horizon, Flatness and Homogeneity problems as well as the Big Bang Singularity problem. Furthermore a scale invariant spectrum of primordial density perturbations generated from the phase of pre-bounce contraction is shown to be stable a...

We review the recent status of big bounce genesis as a new possibility of using dark matter particles’ mass and interaction cross-section to test the existence of a bounce universe at the early stage of evolution in our currently-observed universe. To study the dark matter production and evolution inside the bounce universe, called big bounce genes...

We review the recent status of big bounce genesis as a new possibility of using dark matter particle's mass and interaction cross section to test the existence of a bounce universe at the early stage of evolution in our currently observed universe. To study the dark matter production and evolution inside the bounce universe, called big bounce genes...

We propose a differential operator for computing the residues associated with a class of meromorphic $n$-forms that frequently appear in the Cachazo-He-Yuan form of the scattering amplitudes. This differential operator is conjectured to be uniquely determined by the local duality theorem and the intersection number of the divisors in the $n$-form....

We propose a differential operator for computing the residues associated with a class of meromorphic $n$-forms that frequently appear in the Cachazo-He-Yuan form of the scattering amplitudes. This differential operator is conjectured to be uniquely determined by the local duality theorem and the intersection number of the divisors in the $n$-form....

In the present work we examine the possibility for detecting electrons in dark matter searches. These detectors are considered to be the most appropriate for detecting light dark matter particles in the MeV region. We analyze theoretically some key issues involved in such a detection and we perform calculations for the expected rates employing reas...

A bounce universe with a scale invariant as well as stable spectrum of primordial density perturbations was constructed from a string theoretical elements, using a tachyon effective potential arisen from the D-brane and anti--D-brane system. The tachyon is stabilized at the top of its potential via a coupling to the Higgs field within. This is call...

A bounce universe model with a scale-invariant and stable spectrum of primordial density perturbations was constructed using a consistent truncation of the D-brane dynamics from Type IIB string theory. A coupling was introduced between the tachyon field and the adjoint Higgs field on the D3-branes to lock the tachyon at the top of its potential hil...

Bipartite on-shell diagram is the latest tool in constructing scattering amplitudes. In this letter we prove that an on-shell diagram process a (rational) top-form if and only if the algebraic ideal comprised of the geometrical constraints can be shifted linearly during successive Britto-Cachazo- Feng-Witten (BCFW) integrations. With a proper geome...

Bipartite on-shell diagrams are the latest tool in constructing scattering amplitudes. In this letter we prove that an on-shell diagram process a rational top-form if and only if the algebraic ideal comprised of the geometrical constraints can be shifted linearly during successive Britto-Cachazo-Feng-Witten integrations. With a proper geometric int...

Scalar fields appear in many theories beyond the Standard Model of particle physics. In the early universe, they are exposed to extreme conditions, including high temperature and rapid cosmic expansion. Understanding their behavior in this environment is crucial to understand the implications for cosmology. We calculate the finite temperature effec...

We consider the possibility of using dark matter particle's mass and its interaction cross section as a smoking gun signal of the existence of a Big Bounce at the early stage in the evolution of our currently observed universe. A study of dark matter production in the pre-bounce contraction and the post bounce expansion epochs of this universe reve...

The leading singularities encode important information of the scattering amplitudes and can be used in the construction of loop-level amplitudes. In this paper we study systematically the on-shell diagrams for the leading singularities of nonplanar amplitudes in N=4 super Yang-Mills (SYM) theory. In particular we prove that a nonplanar leading sing...

We investigate a new scenario of dark matter production in a bouncing universe, in which dark matter was produced completely out of equilibrium in the contracting as well as expanding phase. We explore possibilities of using dark matter as a probe of the bouncing universe, focusing on the relationship between a critical temperature of the bouncing...

We investigate the spectrum of cosmological perturbations in a bounce cosmos modeled by a scalar field coupled to the string tachyon field (CSTB cosmos). By explicit computation of its primordial spectral index we show the power spectrum of curvature perturbations, generated during the tachyon matter dominated contraction phase, to be nearly scale...

We report on the possibility to use dark matter mass and its interaction cross section as a smoking gun signal of the existence of a big bounce at the early stage in the evolution of our currently observed universe. A model independent study of dark matter production in the contraction and expansion phases of the bounce universe reveals a new venue...

We find a permutation relation among Yangian Invariants -- two Yangian Invariants with adjacent external lines exchanged are related by a simple kinematic factor. This relation is shown to be equivalent to U(1) decoupling and Bern-Carrasco-Johansson (BCJ) relation at the level of maximal helicity violating (MHV) amplitudes. Together with the newly...

We find a permutation relation among Yangian Invariants -- two Yangian Invariants with adjacent external lines exchanged are related by a simple kinematic factor. This relation is shown to be equivalent to U(1) decoupling and Bern-Carrasco-Johansson (BCJ) relation at the level of maximal helicity violating (MHV) amplitudes. Together with the newly...

We investigate the spectrum of cosmological perturbations in a bounce cosmos modeled by a scalar field coupled to the string tachyon field (CSTB cosmos). By explicit computation of its primordial spectral index we show the power spectrum of curvature perturbations, generated during the tachyon matter dominated contraction phase, to be nearly scale...

We study cosmological perturbation spectra using the dynamical equations of gauge invariant perturbations with a generalized blue/red-shift term. Combined with the power-law index of cosmological background, {\nu}, we construct a parameter space, ({\nu}, m), to classify all possible perturbation spectra. The magnitude-invariant power spectra occupy...

We discuss a cosmological model in which the string gauge field coupled universally to matter gives rise to an extra centripetal force and will have effects on cosmological and astronomical observations. Several tests are performed using data including galaxy rotation curves of twenty-two spiral galaxies of varied luminosities and sizes, and perihe...

We introduce a string-inspired model for a cyclic universe, utilizing the tachyon-scalar coupling as well as contribution from curvature in a closed universe. The universe undergoes a locked inflation stage after each bounce. Cycles of inflation, decelerating expansion, followed by contraction are made possible because of the negative contribution...

We prove a no-ghost theorem for a bosonic string propagating in Nappi-Witten spacetime. This is achieved in two steps. We first demonstrate unitarity for a class of NW/U(1) modules: the norm of any state which is primary with respect to a chosen timelike U(1) is non-negative. We then show that physical states - states satisfying the Virasoro constr...

Wess-Zumino-Witten (WZW) models are abstract conformal field theories with an infinite-dimensional symmetry which accounts for their integrability, and at the same time they have a sigma-model description of closed-string propagation on group manifolds which, in turn, endows the models with an intuitive geometric meaning. We exploit this dual algeb...

WZW models are abstract conformal field theories with an infinite dimensional symmetry which accounts for their integrability, and at the same time they have a sigma model description of closed string propagation on group manifolds which, in turn, endows the models with an intuitive geometric meaning. We exploit this dual algebraic and geometric pr...

Remarkable fit of galaxy rotation curves is achieved using a simple model from string theory. The rotation curves of the same group of galaxies are also fit using dark matter model with the generalized Navarro-Frenk-White profile for comparison. String model utilizes three free parameters vs five in the dark matter model. The average chi-squared of...

This is a speculative attempt to connect string theory with cosmological observation. Inspired by an exactly solvable model in string theory, and based on the assumption that all matter is made of strings, galaxies will couple to a string gauge field and execute Landau orbits, much like electrons in an external magnetic field. This three-parameter...

The question of how to stabilize all of the modulus fields coming from compactifications of string theory on internal manifolds is one of the outstanding challenges for string cosmology. Here, in the simple example of a toroidal compactification, we study the dynamics of the moduli fields corresponding to the size and shape of the torus along with...

Stabilizing all of the modulus fields coming from compactifications of string theory on internal manifolds is one of the outstanding challenges for string cosmology. Here, in a simple example of toroidal compactification, we study the dynamics of the moduli fields corresponding to the size and shape of the torus along with the ambient flux and long...

We present a new application of Boundary String Field Theory: calculating the induced-gravity action on a D-brane. Using a simple quadratic tachyon potential to model a D-brane fluctuating in the flat target space we derive the effective action in terms of the extrinsic curvature to all orders in α'. We identify both the Born-Infeld structure as we...

In a given 4d spacetime bakcground, one can often construct not one but a family of distinct N = 2 string theories. This is due to the multiple ways an N = 2 superconformal algebra can be embedded in a given worldsheet theory. We formulate the principle of obtaining different physical theories by gauging different embeddings of the same symmetry al...

We provide a complete solution of closed strings propagating in Nappi-Witten space. Based on the analysis of geodesics we construct the coherent wavefunctions which approximate as closely as possible the classical trajectories. We then present a new free field realization of the current algebra using the γ,β ghost system. Finally we construct the q...

We study two topics in string theory: consistency of D- branes and their intersections in topologically nontrivial background, and Noncommutative Geometry and ``Little String Theories.'' When a D-brane wraps around a cycle of a curved manifold, the nontrivial topology of its normal bundle can induce chiral asymmetry in its worldvolume theory. We ob...

We show that the moduli space of the (2,0) and little-string theories compactified on T3 with R-symmetry twists is equal to the moduli space of U(1) instantons on a non-commutative T4. The moduli space of U(q) instantons on a non-commutative T4 is obtained from little-string theories of NS5-branes at Aq−1 singularities with twists. A large class of...

We show that the moduli space of the $(2,0)$ and little-string theories compactified on $T^3$ with R-symmetry twists is equal to the moduli space of U(1) instantons on a non-commutative $T^4$. The moduli space of $U(q)$ instantons on a non-commutative $T^4$ is obtained from little-string theories of NS5-branes at $A_{q-1}$ singularities with twists...

We study the compactification of the (2, 0) and type-II little-string theories on S1, T2 and T3 with an R-symmetry twist that preserves half the supersymmetry. We argue that it produces the same moduli spaces of vacua as compactification of the (1, 0) theory with E8 Wilson lines given by a maximal embedding of SU(2). In certain limits, this reprodu...

We study 0-branes in type IIA on T2 with a background B-field turned on. This enables us to find the matrix model for M-theory on T2 with a background 3-form potential along the torus and the lightlike circle. The theory is a non-local (2 + 1)-dimensional SYM theory on a dual T2 exactly as proposed by Connes, Douglas and Schwarz. We calculate the r...

We study the two-point correlators of the currents of the E8 global symmetry in the N = (1, 0) superconformal six-dimensional theory as well as in the 4D superconformal theories upon toroidal compactification. From the high-energy behavior of the correlator we deduce that in 4D 10 copies of the superconformal theory with E8 global symmetry can be c...

We study the compactification of the $(2,0)$ and type-II little-string theories on $S^1$, $T^2$ and $T^3$ with an R-symmetry twist that preserves half the supersymmetry. We argue that it produces the same moduli spaces of vacua as compactification of the $(1,0)$ theory with $E_8$ Wilson lines given by a maximal embedding of SU(2). In certain limits...

When a D-brane wraps around a cycle of a curved manifold, the twisting of its normal bundle can induce chiral asymmetry in its world-volume theory. We obtain the general form of the resulting anomalies for D-branes and their intersections. They are not cancelled among themselves, and the standard inflow mechanism does not apply at first sight becau...

We study the two-point correlators of the currents of the $E_8$ global symmetry in the $N=(1,0)$ superconformal six-dimensional theory as well as in the 4D superconformal theories upon toroidal compactification. From the high-energy behavior of the correlator we deduce that in 4D 10 copies of the superconformal theory with $E_8$ global symmetry can...