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Wai-Yee Keung

Publications and source records attributed to Wai-Yee Keung.

At least 73 records · Page 4Linked to original sources

T-Anomaly Induced LHC Signals

$\cal T$-parity in the Little Higgs model could be violated by anomalies that allow the lightest $\cal T$-odd $A_H$ to decay into $ZZ$ and $W^+W^-$. We analyze these anomaly induced decays and the two-particle and the three-particle decay modes of other heavy quarks and bosons in this model which yield unique Large Hadron Collider (LHC) signals with fully reconstructable events. $\cal T$-odd quarks in the Little Higgs model are nearly degenerate in mass and they decay by almost identical processes; however, members of the heavy Higgs triplet follow distinct decay modes. The branching fractions of three-body decays increase with the global symmetry-breaking energy scale $f$ and are found to be at the level of a few percent in heavy quark decays while they can reach up to 10% for heavy bosons.

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Collider signals in unparticle physics

Phenomenology of the notion of an unparticle U, recently perceived by Georgi, to describe a scale invariant sector with a non-trivial infrared fixed point at a higher energy scale is explored in details. Behaving like a collection of d_U (the scale dimension of the unparticle operator O_U) invisible massless particles, this unparticle can be unveiled by measurements of various energy distributions for the processes Z to fermion pair + U and e-e+ to gamma U at e-e+ colliders, as well as mono-jet production at hadron colliders. We also study the propagator effects of the unparticle through the Drell-Yan tree level process and the one-loop muon anomaly.

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Collider Phenomenology of Unparticle Physics

Low energy phenomenology of the unparticle physics associated with an exact scale invariant sector possessing a non-trivial infrared fixed point at a higher energy scale is explored for both electron-positron and hadronic colliders. Feynman rules for a spin 0, 1 or 2 unparticle coupled to a variety of standard model gauge invariant operators that are relevant to many low energy processes involving either real emissions of unparticles or their virtual propagator effects are presented. Missing energy and/or recoil mass distributions of the unparticle in the associated production of unparticle together with a photon or $Z$ boson at LEP2 and ILC as well as in $Z$ decay into an unparticle plus a fermion-antifermion pair are studied. Monojet production at hadronic collisions is explored. The complex phase in the unparticle propagator that can give rise to interesting interference effects between an unparticle exchange diagram and the standard model amplitudes are studied in details for the Drell-Yan process as well as muon pair and diphoton production in electron-positron annihilation. These energy and/or recoil mass distributions (with the exception in hadron colliders) and interference effects are found sensitively depending not only on the scale dimension but also on the spin of the unparticle. For the spin-2 unparticle, its physical effects is found to resemble that of a tower of Kaluza-Klein gravitons, which strongly indicates that the underlying unparticle physics may have root in a higher dimensional theory. A connection between unparticle physics and theories of large extra dimension is speculated. Experimental constraints on the unparticle scale are deduced from the LEP2 data on mono-photon production and from the 4-fermion contact interactions.

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Neutralino annihilation to q qbar g

We compute the cross section for chi chi -> q qbar g at order alpha_s^2/M_{squark}^6 arising from interference between the tree-level and loop-induced processes. This interference term is the same order in alpha_s as chi chi -> gg; for mass degenerate squarks M_{squark_R} = M_{squark_L} = M_{squark} we find v_{rel} sigma_{int} = [-2 m_{chi}^2 / 3 M_{squark}^2] v_{rel} sigma(chi chi -> gg).

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Neutralino annihilation beyond leading order

High-precision measurements of the relic dark matter density and the calculation of dark matter annihilation branching fractions in the sun or the galactic halo today motivate the computation of the neutralino annihilation cross section beyond leading order. We consider neutralino annihilation via squark exchange and parameterize the effective annihilation vertex as a dimension-six operator suppressed by two powers of the squark mass and related to the divergence of the axial vector current of the final-state quarks. Since the axial vector current is conserved at tree level in the limit of massless quarks, this dimension-six operator contains a suppression by the quark mass. The quark mass suppression can be lifted in two ways: (1) by corrections to the dimension-six operator involving the anomalous triangle diagram, and (2) by going to dimension-eight. We address the first of these possibilities by evaluating the anomalous triangle diagram, which contributes to neutralino annihilation to gluon pairs. We relate the triangle diagram via the anomaly equation to the decay of a pseudoscalar into two gluons and use the Adler-Bardeen theorem to extract the next-to-leading order (NLO) QCD corrections to chi chi --> gg from the known corrections to pseudoscalar decay. The strong dependence of the dominant chi chi --> q qbar cross section on the relative velocity of the neutralinos makes these NLO corrections unimportant at chi decoupling but significant today.

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Electric Dipole Moment in the Split Supersymmetry Models

We study an important contribution to the electric dipole moment (EDM) of the electron (or quarks) at the two-loop level due to the $W$-EDM in the recently proposed scenario of split supersymmetry. This contribution is independent of the Higgs mass, and it can enhance the previous estimation of the electron (neutron) EDM by $20-50%$ ($40-90%$). Our formula is new in its analytical form.

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Split Supersymmetry, stable gluino, and gluinonium

In the scenario recently proposed by Arkani and Dimopoulos, the supersymmetric scalar particles are all very heavy, at least of the order of $10^9$ GeV but the gauginos, higgsinos, and one of the neutral Higgs bosons remain under a TeV. The most distinct signature is the metastable gluino. However, the detection of metastable gluino depends crucially on the spectrum of hadrons that it fragments into. Instead, here we propose another unambiguous signature by forming the gluino-gluino bound state, gluinonium, which will then annihilate into $t\bar t$ and $b\bar b$ pairs. We study the sensitivity of such signatures at the LHC.

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Natural Kobayashi-Maskawa Model of CP Violation and Flavor Physics

We explore the possible tie between the naturalness of having a very small strong CP $θ$ parameter in the Kobayashi-Maskawa Model and the flavor symmetry. We provide many examples in which the flavor symmetry group at high energy can naturally give rise to Kobayashi-Maskawa Model at low energy with a naturally small theta.

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Belle J/psi+eta_c anomaly and a very light scalar boson

The large rate of the $J/ψ+η_c$ production observed by the Belle collaboration has posed a serious challenge to our understanding of the quarkonium. We examine a scenario that there exists a light scalar boson around 5 GeV, with an enhanced coupling solely to the charm quark. Such a scenario would explain the Belle anomaly. It also predicts a significant increase in $J/ψ$ plus open charm pair production. An immediate test for the scalar boson is to look for a peak around 5 GeV in the recoil mass spectrum of $J/ψ$, because the scalar boson can also be produced in association with the $J/ψ$. Finally, we also point out that the process $e^+ e^- \to H γ$ is sizable for observation.

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Light Gluino, Light Bottom Squark Scenario, and LEP Predictions

The scenario of light gluinos and light sbottoms was advocated to explain the discrepancy between the measured and theoretical production of $b$ quarks at the Tevatron. This scenario will have model-independent predictions for $Z\to q\bar q \tilde{g} \tilde{g}$ at the $Z^0$-pole, and $e^+ e^- \to q\bar q \tilde{g} \tilde{g}$ at LEPII. We show that the data for $Z\to q \bar q g^* \to q\bar q b \bar b$ at LEPI cannot constrain the scenario, because the ratio $Γ(Z \to q\bar q \tilde{g} \tilde{g})/ Γ(Z \to q \bar q g^* \to q\bar q b \bar b) = 0.15 - 0.04$ for $m_{\tilde{g}}= 12-16$ GeV is smaller than the uncertainty of the data. However, at LEPII the ratio $σ(e^+ e^- \to q\bar q \tilde{g} \tilde{g})/ σ(e^+ e^- \to q \bar q g^* \to q\bar q b \bar b) \simeq 0.4 - 0.2$ for $m_{\tilde{g}}= 12-16$ GeV, which may give an observable excess in $q\bar q b\bar b$ events; especially, the $4b$ events.

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Nambu-Goldstone Bosons in CP Violating theory with Majorana Masses

We derive some properties of the Nambu-Goldstone boson coupling in theories that have CP violation and Majorana masses. We show explicitly that its diagonal coupling to a Majorana fermion is pseudoscalar not scalar. This clarifies some confusion in the literature. Some potentially useful off-diagonal properties are also derived. We also show, in the process, that the Goldstone theorem often produces interesting and nontrivial identities in matrix theory which may be hard to prove otherwise.

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Z$ decay into a bottom quark, a light sbottom and a light gluino

The discrepancy between the measured and theoretical production cross section of $b$ quarks at the Tevatron can probably be explained by the recently proposed scenario of light gluinos of mass 12-16 GeV and light sbottoms of mass $2-5.5$ GeV. In this scenario, we study a related process at the $Z$ pole, $Z \to b \tilde{b}_1^* \tilde{g} + \bar b \tilde{b}_1 \tilde{g}$ followed by $\tilde{g} \to b \tilde{b}_1^* / \bar b \tilde{b}_1$. The hadronic branching ratio for this channel is $(1-3) \times 10^{-3}$, which is of order of the size of the uncertainty in $R_b$. We find that a typical event consists of an energetic prompt bottom-jet back-to-back with a ``fat'' bottom-jet, which consists of a bottom quark and two bottom squarks. Such events with a $10^{-3}$ branching ratio may affect the measurement of $R_b$; even more interesting if the ``fat'' bottom jet can be identified.

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New constraint from Electric Dipole Moments on chargino baryogenesis in MSSM

A commonly accepted mechanism of generating baryon asymmetry in the minimal supersymmetric standard model (MSSM) depends on the CP violating relative phase between the gaugino mass and the Higgsino mu term. The direct constraint on this phase comes from the limit of electric dipole moments (EDM) of various light fermions. To avoid such a constraint, a scheme which assumes the first two generation sfermions are very heavy is usually evoked to suppress the one-loop EDM contributions. We point out that under such a scheme the most severe constraint may come from a new contribution to the electric dipole moments of the electron, the neutron or atoms via the chargino sector at the two-loop level. As a result, the allowed parameter space for baryogenesis in MSSM is severely constrained, independent of masses of the first two generation sfermions.

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Squark Mixing Contributions to CP violating phase gamma

We investigate the possibility that the CP violation due to the soft supersymmetry breaking terms in squark mixing can give significant contributions to the various $γ$ related parameters in B decays, different from those of the Standard Model. We derive the new limits on $(δ^u_{12})_{LL,LR,RR}$ and on $(δ^d_{23})_{LL,LR,RR}$ from the recent data on $D^0$--$\bar{D}^0$ oscillation as well as those on $B_s^0$--$\bar{B_s}^0$ oscillation. We show that, together with all the other constraints, the currents limits on these parameters still allow large contributions to the CP violating phases in $B_s^0$--$\bar{B_s}^0$ as well as $D^0$--$\bar{D}^0$ oscillations which will modify some of the proposed measurements of $γ$ parameters in CP violating B decays. However, the current constraints already dictate that the one-loop squark mixing contributions to various B decay amplitudes cannot be competitive with that of the Standard Model (SM), at least for those B decay modes which are dominated the tree level amplitudes within the SM, and therefore they are not significant in contributing to CP asymmetries in the corresponding B decays.

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Models for Geometric CP Violation with Extra Dimensions

In a recent paper, two of us (D.C. and R.N.M.) proposed a new way to break CP symmetry geometrically using orbifold projections. The mechanism can be realized in the five dimensional brane bulk picture. In this paper, we elaborate on this proposal and provide additional examples of models of this type. We also note the phenomenological implications of some of these models.

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Large Two-Loop Contributions to g-2 from a Generic Pseudoscalar Boson

We calculate the dominant contributions to the muon $g-2$ at the two-loop level due to a generic pseudoscalar boson that may exist in any exotic Higgs sector in most extensions of the standard model. The leading effect comes from diagrams of the Barr-Zee type. A sufficiently light pseudoscalar Higgs boson can give rise to contribution as large as the electroweak contribution which is measurable in the next round of $g-2$ experiment. Through the contribution we calculated here, the anticipated improved data in the recent future on the muon $g-2$ can put the best limit on the possible existence of a light pseudoscalar boson in physics beyond the standard model.

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CP asymmetry in the Higgs decay into the top pair due to the stop mixing

We investigate a potentially large CP violating asymmetry in the decay of a neutral scalar or pseudoscalar Higgs boson into the top-anti-top pair. The source of the CP nonconservation is the complex mixing in the (left-right) stop sector. One of the interesting consequence is the different rates of the Higgs boson decays into CP conjugate polarized states.

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