arXiv subjects

Wai-Yee Keung

Publications and source records attributed to Wai-Yee Keung.

At least 37 records · Page 2Linked to original sources

arXiv2013

Collider Signatures of Goldstone Bosons

Recently Weinberg suggested that Goldstone bosons arising from the spontaneous breakdown of some global hidden symmetries can interact weakly in the early Universe and account for a fraction of the effective number of neutrino species N_eff, which has been reported persistently 1 σaway from its expected value of three. In this work, we study in some details a number of experimental constraints on this interesting idea based on the simplest possibility of a global U(1), as studied by Weinberg. We work out the decay branching ratios of the associated light scalar field $σ$ and suggest a possible collider signature at the Large Hadron Collider (LHC). In some corners of the parameter space, the scalar field σcan decay into a pair of pions with a branching ratio of order O(1)% while the rest is mostly a pair of Goldstone bosons. The collider signature would be gluon fusion into the standard model Higgs boson gg -> H or associated production with a W gauge boson q q'-bar -> H W, followed by H -> σσ-> (ππ) (αα), where αis the Goldstone boson.

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arXiv2013

Supernova Bounds on Weinberg's Goldstone Bosons

Recently, Weinberg proposed a scenario where Goldstone bosons may be masquerading as fractional cosmic neutrinos. We calculate the energy loss rates through the emission of these Goldstone bosons in a post-collapse supernova core. Invoking the well established emissivity bound from the Supernova 1987A observations and simulations, we find that nuclear bremsstrahlung processes can notably impose a bound on the Goldstone boson coupling to the Standard Model Higgs, $g$, dependent on the mass of the associated radial field, $m_r$. For $m_r$ large enough compared with the temperature in the post-collapse supernova core, our bound is $|g| \lesssim 0.011\, (m_r / 500~{\rm MeV})^2$, very competitive to that derived from collider experiments.

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arXiv2013

Superheavy Particle Origin of IceCube PeV Neutrino Events

We interpret the PeV shower events observed by the IceCube collaboration as an s-channel enhancement of neutrino-quark scattering by a leptoquark that couples to the tau-flavor and light quarks. With a leptoquark mass of ~ 0.6 TeV and a steep 1/E^{2.3} neutrino flux, charged-current scattering gives shower events at ~ 1 PeV and neutral-current scattering gives shower events at ~ 0.5 PeV. This mechanism is also consistent with the paucity of muon-track events above 100 TeV.

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arXiv2012

Scalar-Top Masses from SUSY Loops with 125 GeV mh and precise Mw, mt

We constrain the masses of scalar-tops (stop) by analyzing the new precision Tevatron measurement of the W-boson mass and the LHC/Tevatron indications of a Higgs boson of mass 125.5+-1 GeV. Our study adopts Natural SUSY with low fine-tuning, which has multi-TeV first and second generation squarks and a light Higgsino mixing parameter mu=150 GeV. An effective Lagrangian calculation is made of mh to 3-loops using the H3m program with weak scale SUSY parameters obtained from RGE evolution from the GUT scale in the Natural SUSY scenario. The SUSY radiative corrections to the Higgs mass imply maximal off-diagonal elements of the stop mass-matrix and a mass splitting of the two stops larger than 400 GeV.

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arXiv2012

Flavor-Tuned 125 GeV SUSY Higgs Boson at the LHC: MSSM and NATURAL SUSY TESTS

We show that an enhanced two-photon signal of the Higgs boson, h, observed with 125 GeV mass by the ATLAS and CMS collaborations, can be obtained if it is identified principally with the neutral $H_u^0$ of the two Higgs doublets of minimal Supersymmetry. We focus on sparticles and the pseudoscalar Higgs $A$ at the TeV scale. The off-diagonal element of the (Hu0,Hd0) mass matrix in the flavor basis must be suppressed, and this requires both a large Higgsino mass parameter, mu \sim TeV, and large tan beta. A MSSM sum rule is derived that relates gamma gamma and bb rates, and the gamma gamma enhancement predicts the bb reduction. On the contrary, Natural SUSY requires | mu | < 0.5 TeV, for which gamma gamma is reduced and $b\bar b$ is enhanced. This conclusion is independent of the mA-value and the SUSY quantum correction Deltab. Relative tau tau to bb rates are sensitive to Deltab.

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arXiv2012

Dipole Moment Dark Matter at the LHC

Monojet and monophoton final states with large missing transverse energy (${\not E}_T$) are important for dark matter (DM) searches at colliders. We present analytic expressions for the differential cross sections for the parton-level processes, $q\bar{q}(qg)\to g(q)χ\barχ$ and $q\bar{q}\to γχ\barχ$, for a neutral DM particle with a magnetic dipole moment (MDM) or an electric dipole moment (EDM). We collectively call such DM candidates dipole moment dark matter (DMDM). We also provide monojet cross sections for scalar, vector and axial-vector interactions. We then use ATLAS/CMS monojet${+\not E}_T$ data and CMS monophoton$+{\not E}_T$ data to constrain DMDM. We find that 7 TeV LHC bounds on the MDM DM-proton scattering cross section are about six orders of magnitude weaker than on the conventional spin-independent cross section.

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arXiv2012

Total Width of 125 GeV Higgs Boson

By using the LHC and Tevatron measurements of the cross sections to various decay channels relative to the standard model Higgs boson, the total width of the putative 125 GeV Higgs boson is determined as 6.1 +7.7-2.9 MeV. We describe a way to estimate the branching fraction for Higgs decay to dark matter. We also discuss a No-Go theorem for the gammagamma signal of the Higgs boson at the LHC.

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arXiv2012

Looking for a light Higgs boson in the overlooked channel

The final state obtained when a Higgs boson decays to a photon and a Z boson has been mostly overlooked in current searches for a light Higgs boson. However, when the Z boson decays leptonically, all final state particles in this channel can be measured, allowing for accurate reconstructions of the Higgs mass and angular correlations. We determine the sensitivity of the Large Hadron Collider (LHC) running at center of masses energies of 8 and 14 TeV to Standard Model (SM) Higgs bosons with masses in the 120 - 130 GeV range. For the 8 TeV LHC, sensitivity to several times the the SM cross section times branching ratio may be obtained with 20 inverse femtobarns of integrated luminosity, while for the 14 TeV LHC, the SM rate is probed with about 100 inverse femtobarns of integrated luminosity.

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arXiv2012

Azimuthal Correlations in Top Pair Decays and The Effects of New Heavy Scalars

We present the azimuthal correlations in the decay products of the top quark pair created in the process $gg \to t \tbar \to b \bbar \wplus \wminus$ calculated in leading-order QCD. We then discuss deviations in this distribution that could arise from New Physics contributions, focusing, in particular, on the case of a new heavy scalar coupling to the top quark and produced via the gluon fusion mechanism. Within this scenario we consider two cases: one in which the top quark is the only particle with a significant coupling to the scalar and another in which there are additional heavy fermions that modify the effective coupling of the scalar to gluons. We discuss the effects on the overall cross section and azimuthal correlations of the top quark decay products, with particular emphasis on the effect of interference between the QCD and New Physics contributions and on the existence of CP-nonconserving couplings.

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arXiv2012

Differentiating the Higgs boson from the dilaton and the radion at hadron colliders

A number of candidate theories beyond the standard model (SM) predict new scalar bosons below the TeV region. Among these, the radion, which is predicted in the Randall-Sundrum model, and the dilaton, which is predicted in spontaneous scale symmetry breaking, have very similar couplings to those of the SM Higgs boson, and it is very difficult to differentiate these three spin-0 particles in the expected signals of the Higgs boson at the LHC and Tevatron. We demonstrate that the observation of the ratio sigma(gamma gamma)/sigma(WW) gives a simple and decisive way, independently of the values of model parameters: the VEVs of the radion and dilaton fields.

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arXiv2012

Dilaton at the LHC

The dilaton, a pseudo-Nambu-Goldstone boson appearing in spontaneous scale symmetry breaking at a TeV scale f, may be found in Higgs boson searches. The dilaton couples to standard model fermions and weak bosons with the same structure as the Higgs boson except for the overall strength. Additionally, the dilaton couples to a Higgs boson pair. The couplings of the dilaton to a gluon pair and a photon pair, appearing at loop level, are largely enhanced compared to the corresponding Higgs couplings. We present regions of the mass and VEV of the dilaton allowed by WW, ZZ, and gammagamma limits from the LHC at 7 TeV with 1.0-2.3 fb-1 integrated luminosity. A scale of f less than 1 TeV is nearly excluded. We discuss how the dilaton chi can be distinguished from the Higgs boson h by observation of the decays chi -> gammagamma and chi -> hh -> (WW)(WW).

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arXiv2012

Constraint on parity-violating muonic forces

Using the nonobservance of missing mass events in the leptonic kaon decay $K \to μX$, we place a strong constraint on exotic parity-violating gauge interactions of the right-handed muon. By way of illustration, we apply it to an explanation of the proton size anomaly that invokes such a new force; scenarios in which the gauge boson decays invisibly or is long-lived are constrained.

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arXiv2012

Bremsstrahlung in dark matter annihilation

We show that the energy spectra from dark matter (DM) annihilation into $f^+f^-γ$ (where $f$ is a light fermion) via chirality preserving interactions are identical in 3 scenarios: (a) DM is a Majorana fermion and the particle exchanged is a scalar, (b) DM is a Majorana fermion and the particle exchanged is a vector and (c) DM is a scalar and the particle exchanged is a fermion. For cases (a) and (c), we also calculate the differential cross section to $\ell^+f^-V$, where $V=W,Z,γ$, and $\ell$ is a light fermion that may or not be the same as $f$. The form of the cross section depends on whether the DM is a Majorana fermion or a scalar but in both cases its form is independent of $V$.

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arXiv2011

Parity Nonconservation in Strong Interactions

For top-quarks produced via the subprocess $q\bar q\to t\bar t$, the longitudinal t-quark polarization ($P_{\parallel}$) vanishes in QCD. $P_{\parallel}$ can be measured by the angular distribution of the lepton in $t$-quark semileptonic decay. New physics contributions that are parity nonconserving will be manifest by non-vanishing $P_{\parallel}$, which may be large. We illustrate this with the s-channel exchange of a massive $X$-gluon with chiral quark couplings.

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arXiv2011

Observing CP-violating Higgs Signals with Heavy Majorana Neutrinos

We demonstrate the ability to measure the CP-phase of a Higgs boson state by looking at the decays of heavy, Majorana neutrinos in the process $pp \to H \to N N$ at the LHC. This is shown for signals with both opposite-sign ($l^{\pm} l^{\mp} W^{\pm} W^{\mp}$) and same-sign ($l^{\pm} l^{\pm} W^{\mp} W^{\mp}$) leptonic final states. These signals are investigated in the general framework of a sequential fourth generation of fermions with an additional right-handed neutrino. Such a scenario would naturally give rise to heavy Majorana neutrinos as well as significant enhancements to Higgs production via gluon fusion due to the contributions of the new, heavy quark states running in the fermion loops. Combined with the low background inherent to a same-sign lepton signal, this could be a useful way to investigate the CP-nature of a Higgs boson state at the LHC.

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arXiv2011

Long Lived Fourth Generation and the Higgs

A chiral fourth generation is a simple and well motivated extension of the standard model, and has important consequences for Higgs phenomenology. Here we consider a scenario where the fourth generation neutrinos are long lived and have both a Dirac and Majorana mass term. Such neutrinos can be as light as 40 GeV and can be the dominant decay mode of the Higgs boson for Higgs masses below the W-boson threshold. We study the effect of the Majorana mass term on the Higgs branching fractions and reevaluate the Tevatron constraints on the Higgs mass. We discuss the prospects for the LHC to detect the semi-invisible Higgs decays into fourth generation neutrino pairs. Under the assumption that the lightest fourth generation neutrino is stable, it's thermal relic density can be up to 20% of the observed dark matter density in the universe. This is in agreement with current constraints on the spin dependent neutrino-neutron cross section, but can be probed by the next generation of dark matter direct detection experiments.

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arXiv2011

Proton size anomaly

A measurement of the Lamb shift in muonic hydrogen yields a charge radius of the proton that is smaller than the CODATA value by about 5 standard deviations. We explore the possibility that new scalar, pseudoscalar, vector, and tensor flavor-conserving nonuniversal interactions may be responsible for the discrepancy. We consider exotic particles that among leptons, couple preferentially to muons, and mediate an attractive nucleon-muon interaction. We find that the many constraints from low energy data disfavor new spin-0, spin-1 and spin-2 particles as an explanation.

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arXiv2011

Tevatron Asymmetry of Tops in a W',Z' Model

Recent results from the CDF Collaboration indicate the the top-pair forward-backward asymmetry is largest in regions of high rapidity difference $|Δy|$ and invariant mass $M_{t\bar t}.$ We show that experimental observations can be explained by our previously proposed Asymmetric Left-Right Model (ALRM). The gauge symmetry $U'(1)\times SU'(2)\times SU(2)$ is broken by a triplet Higgs in the primed sector. The $W'$ boson has a $(t,d)$ right-handed coupling and the $Z'$ boson has diagonal fermion couplings. We determine the model parameters to be $M_{W'} = 700$ GeV, $M_{Z'}=1$ TeV, and charged current coupling $g'_2=3$. The $W'$ and $Z'$ total decay widths are of $\mathcal{O}(100$ GeV). The signals from $Z'\to t\bar t$ and $W'\to tb$ at the LHC will test the model.

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