arXiv subjects

Cheng-Wei Chiang

Publications and source records attributed to Cheng-Wei Chiang.

At least 127 records · Page 7Linked to original sources

arXiv2014

Novel Constraint on Parameter Space of the Georgi-Machacek Model by Current LHC Data

The same-sign diboson process $pp\to W^\pm W^\pm jj$ has been measured at the LHC using leptonic decay channels of the $W$ bosons, with production cross sections of two fiducial regions reported to be consistent with the standard model expectations within 1 sigma. These results constrain new physics models with a modified $W^+W^+W^-W^-$ vertex. We consider in particular the Georgi-Machacek model in which the quartic $W$ boson vertex is effectively modified due to mediations of new Higgs bosons in the model. The relevant gauge-gauge-scalar couplings are all proportional to the vacuum expectation value of the isospin triplets, which can be of $\mathcal{O}(10)$ GeV because of custodial vacuum alignment. Using the current 8-TeV data at the LHC, we exclude parameter space on the plane of the triplet vacuum expectation value and the new Higgs boson mass. The expected discovery reach at the 14-TeV LHC is also studied.

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arXiv2014

Updating B --> PP, VP decays in the framework of flavor symmetry

Current data of charmless B meson decays to two pseudoscalar mesons (PP) and one vector and one pseudoscalar mesons (VP) are analyzed within the framework of flavor SU(3) symmetry, a working principle that we have tested by allowing symmetry breaking factors in the decay amplitudes and found to be a good approximate symmetry. In the PP sector, the color-suppressed tree amplitude is found to be larger than previously known and has a strong phase of ~ -70^o relative to the color-favored tree amplitude. We have extracted for the first time the W-exchange and penguin-annihilation amplitudes. The former has a size of about the QCD-penguin amplitude and a phase opposite to that of the color-favored tree amplitude, while the latter is suppressed in magnitude but gives the dominant contribution to the $B_s^0 \to π^+ π^-$ and $π^0 π^0$ decays. In the VP sector, one striking feature is that the color-suppressed tree amplitude with the spectator quark ending up in the vector meson has a large size and a strong phase of ~ -90^o relative to the color-favored tree amplitudes. The associated electroweak penguin amplitude also has a similar strong phase and a magnitude comparable to the corresponding QCD penguin amplitude. This leads to a large branching fraction of order 10^{-6} for $B_s^0 \to ϕπ^0$. In contrast, the color-suppressed tree, QCD penguin, and electroweak penguin amplitudes with the spectator quark ending up in the pseudoscalar meson have magnitudes more consistent with naive expectations. Besides, current data are not sufficiently precise for us to fix the W-exchange amplitudes. For both the PP and VP sectors, predictions of all the decay modes are made based upon our preferred fit results and compared with data and those made by perturbative approaches. We have identified a few observables to be determined experimentally in order to discriminate among theory calculations.

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arXiv2014

3.5-keV X-ray line from nearly-degenerate WIMP dark matter decays

The unidentified emission line at the energy of $\sim$3.5~keV observed in X-rays from galaxy clusters may originate from a process involving a dark matter particle. On the other hand, a weakly interacting massive particle (WIMP) has been an attractive dark matter candidate, due to its well-understood thermal production mechanism and its connection to physics at the TeV scale. In this paper, we pursue the possibility that the 3.5-keV X-ray arises from a late time decay of a WIMP dark matter into another WIMP dark matter, both of which have the mass of $O(100)$~GeV and whose mass splitting is about 3.5~keV. We focus on the simplest case where there are two Majorana dark matter particles and two charged scalars that couple with a standard model matter particle. By assuming a hierarchical structure in the couplings of the two dark matter particles and two charged scalars, it is possible to explain the 3.5-keV line and realize the WIMP dark matter scenario at the same time. Since the effective coupling of the two different Majorana dark matter particles and one photon violates CP symmetry, the model always contains a new source of CP violation, so the model's connection to the physics of electric dipole moments is discussed. The model's peculiar signatures at the LHC are also studied. We show the prospect of detecting the charged scalars through a detailed collider simulation.

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arXiv2014

Phenomenology of $E_6$-Inspired Leptophobic $Z'$ Boson at the LHC

We study collider phenomenology of a leptophobic $Z'$ boson existing in eight scenarios of the $E_6$ grand unified theory, differing in particle embeddings. We first review the current bound on the $Z'$ mass $m_{Z'}$ based upon the LHC data of $pp\to t\bar{t}$ process at 8 TeV collisions with an integrated luminosity of 19.6 fb$^{-1}$. Most scenarios have a lower bound of about 1 TeV. However, this constraint does not apply to the case where $m_{Z'} < 2 m_t$, and other methods need to be employed for this lower mass regime. Using existing UA2 constraints and dijet data at the LHC, we find that only one of the eight scenarios is excluded at 95\% confidence level. No bound can be obtained from $Wjj$ and $Zjj$ measurements. We propose to use the photon associated production of the $Z'$ boson that subsequently decays into a pair of bottom quarks, $pp\to Z'γ\to b\bar{b}γ$, at the LHC to explore the constraints in the lower mass regime. We compute the expected signal significance as a function of $m_{Z'}$ using detailed simulations of signal and irreducible background events. We find constraints for two more scenarios using the 8-TeV data and taking appropriate kinematical cuts. We also show the discovery reach for each scenario at the 14-TeV LHC machine.

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arXiv2014

Electroweak Phase Transition in Georgi-Machacek Model

The Georgi-Machacek model extends the standard model Higgs sector by adding two isospin triplet scalar fields and imposing global SU(2)$_R$ symmetry on them. A feature of the model is that the triplets can acquire a large vacuum expectation value without conflicting with the current experimental bound on the $ρ$ parameter. We investigate the electroweak phase transition in the Georgi-Machacek model by evaluating the finite-temperature effective potential of the Higgs sector. The electroweak phase transition can be sufficiently strong in a large parameter space when the triplets acquire a vacuum expectation value of $O(10)$ GeV, opening a possibility to realize successful electroweak baryogenesis.

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arXiv2014

Nonabelian Dark Matter with Resonant Annihilation

We construct a model based on an extra gauge symmetry, SU(2)_X x U(1)_{B-L}, which can provide gauge bosons to serve as weakly-interacting massive particle dark matter. The stability of the dark matter is naturally guaranteed by a discrete Z_2 symmetry that is a subgroup of SU(2)_X. The dark matter interacts with standard model fermions by exchanging gauge bosons which are linear combinations of SU(2)_X x U(1)_{B-L} gauge bosons. With the appropriate choice of representation for the new scalar multiplet whose vacuum expectation value spontaneously breaks the SU(2)_X symmetry, the relation between the new gauge boson masses can naturally lead to resonant pair annihilation of the dark matter. After exploring the parameter space of the new gauge couplings subject to constraints from collider data and the observed relic density, we use the results to evaluate the cross section of the dark matter scattering off nucleons and compare it with data from the latest direct detection experiments. We find allowed parameter regions that can be probed by future direct searches for dark matter and LHC searches for new particles.

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arXiv2014

Effects of Family Nonuniversal Z' Boson on Leptonic Decays of Higgs and Weak Bosons

Though not completely a surprise according to the standard model and existing indirect constraints, the Higgs-like particle, h, of mass around 125 GeV recently observed at the LHC may offer an additional window to physics beyond the standard model. In particular, its decay pattern can be modified by the existence of new particles. One of the popular scenarios involves a Z' boson associated with an extra Abelian gauge group. In this study, we explore the potential effects of such a boson with family-nonuniversal couplings on the leptonic decays of h, both flavor-conserving and flavor-changing. For current constraints, we take into account leptonic decays at the Z pole, LEP II scattering data, limits on various flavor-changing lepton transitions, and lepton magnetic dipole moments. Adopting a model-independent approach and assuming that the Z' has negligible mixing with the Z boson, we find that present data allow the Z' effects to reach a few percent or higher on h decays into a pair of leptons. Future measurements on h at the LHC or a linear collider can therefore detect the Z' contributions or impose further constraints on its couplings. We also consider Z'-mediated four-lepton decays of the Z and W bosons.

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arXiv2013

Constraining Nonstandard Neutrino-Electron Interactions due to a New Light Spin-1 Boson

We consider nonstandard interactions of neutrinos with electrons arising from a new light spin-1 particle with mass of tens of GeV or lower and couplings to the neutrinos and electron. This boson is not necessarily a gauge boson and is assumed to have no mixing with standard-model gauge bosons. Adopting a model-independent approach, we study constraints on the flavor-conserving and -violating couplings of the boson with the leptons from a number of experimental data. Specifically, we take into account the (anti)neutrino-electron scattering and e^+ e^- -> nu nubar gamma measurements and keep explicitly the dependence on the new particle mass in all calculations. We find that one of the two sets of data can provide the stronger constraints, depending on the mass and width of the boson. Also, we evaluate complementary constraints on its separate flavor-conserving couplings to the electron and neutrinos from other latest experimental results.

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arXiv2013

Probing Leptonic Interactions of a Family-Nonuniversal Z' Boson

We explore a Z' boson with family-nonuniversal couplings to charged leptons. The general effect of Z-Z' mixing, of both kinetic and mass types, is included in the analysis. Adopting a model-independent approach, we perform a comprehensive study of constraints on the leptonic Z' couplings from currently available experimental data on a number of flavor-conserving and flavor-changing transitions. Detailed comparisons are made to extract the most stringent bounds on the leptonic couplings. Such information is fed into predictions of various processes that may be experimentally probed in the near future.

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arXiv2013

Enhancements of weak gauge boson scattering processes at the CERN LHC

Current CERN Large Hadron Collider data on the 126 GeV standard model-like Higgs boson suggest the possibility of larger Higgs boson couplings with the weak gauge bosons, $g_{hVV}$, than those in the standard model. We use the Georgi-Machacek model as an explicit model to realize such a scenario. We find that the $g_{hVV}$ couplings can be larger than the standard model value by a factor of about 1.3 maximally in the parameter region consistent with the current Higgs boson search data and allowed by various other constraints. We then show how the modified $g_{hVV}$ couplings lead to enhancements in various weak boson scattering processes. This can be clearly observed as excesses in the transverse mass distributions at around 126 GeV and also the mass of heavy Higgs bosons.

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arXiv2013

Implications of Higgs boson search data on the two-Higgs doublet models with a softly broken $Z_2$ symmetry

Based on current data of the Higgs boson search at the Large Hadron Collider, we constrain the parameter space of the two-Higgs doublet models where a softly broken $Z_2$ symmetry is employed to avoid flavor-changing neutral currents at tree level. There are four types of Yukawa interactions under the $Z_2$ charge assignments of the standard model fermions. We find that the model with type-II Yukawa interactions can better explain the experimental data among all. In this scenario, the couplings of the light CP-even Higgs boson $h$ with weak gauge bosons are almost standard model-like or only slightly different in a small range of $\tanβ$. In particular, we scrutinize a well-constrained region previously ignored by other analyses and study the phenomenology of the extra Higgs bosons at the Large Hadron Collider.

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arXiv2013

Examining custodial symmetry in the Higgs sector of Georgi-Machacek model

We study consequences of custodial symmetry in the Higgs sector of the Georgi-Machacek (GM) model. We discuss how the 5-plet and 3-plet Higgs bosons classified under the custodial SU(2) symmetry are produced and decay at the CERN Large Hadron Collider. By determining their masses through proposed channels, we can test the mass degeneracy in each Higgs multiplet.

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arXiv2013

Dark Matter and Higgs Boson in a Model with Discrete Gauge Symmetry

In view of ongoing measurements of the Higgs-like boson at the LHC and direct searches for dark matter, we explore the possibility of accommodating the potential results in a simple new-physics model with discrete gauge symmetry as well as light neutrino masses. Specifically, we study collider and relic-density constraints on the new gauge coupling, predict the cross section of the dark matter scattering off nucleons, and compare it with current direct search data. We also discuss some of the implications if the dark matter is light. The new gauge sector of the model allows it to be compatible with the latest LHC information on the Higgs-like particle and simultaneously satisfy the requirements in its dark matter sector.

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arXiv2013

Higgs boson decays to $γγ$ and $Z γ$ in models with Higgs extensions

The decays of a Higgs boson to the $γγ$ and $Zγ$ final states are purely quantum mechanical phenomena that are closely related to each other. We study the effects of an extended Higgs sector on the decay rates of the two modes. We propose that a simultaneous determination of them and the $ZZ$ mode is a useful way to see whether the Higgs boson recently observed by the LHC experiments is of the standard model type or could be a member of a larger Higgs sector.

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arXiv2012

Testing the custodial symmetry in the Higgs sector of the Georgi-Machacek model

We study how the custodial symmetry in the Higgs sector of the Georgi-Machacek (GM) model can be tested at the LHC. As the minimal extension of the Higgs triplet model, in which tiny neutrino masses are generated via the Type-II Seesaw Mechanism, the GM model keeps the electroweak $ρ$ parameter at unity at tree level. In the GM model, there are 5-plet ($H_5$), 3-plet ($H_3$) and singlet ($H_1$) Higgs bosons under the classification of the custodial $SU(2)_V$ symmetry, in addition to the standard model-like Higgs boson ($h$). These new Higgs bosons have the following characteristic features at the tree level: (1) the masses of the Higgs bosons belonging to the same $SU(2)_V$ multiplet are degenerate; and (2) $H_5$ and $H_1$ couple to the electroweak gauge bosons but not SM quarks, whereas $H_3$ couples to the quarks but not the gauge bosons. We find that the $H_5$ production from the weak vector boson fusion process and the Drell-Yan process associated with $H_3$ are useful in testing the custodial symmetry of the Higgs sector at the LHC. In addition, these processes can also be used to discriminate from other models that contain singly-charged Higgs bosons and extra neutral Higgs bosons. We also investigate a possible enhancement in the $h\to γγ$ as well as $h\to Zγ$ decays.

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arXiv2012

SU(3) symmetry breaking and CP violation in D -> PP decays

Evidence of CP violation in the charm sector has been observed recently by the LHCb and CDF Collaborations. Adopting the topological diagram approach, we study flavor SU(3) symmetry breaking effects in the weak decay tree amplitudes of singly Cabibbo-suppressed $D\to PP$ decays. The symmetry breaking in the color-allowed and color-suppressed amplitudes is estimated with the help of the factorization ansatz, while that in the $W$-exchange amplitude is done by fitting to related branching fraction data. We find that the $W$-exchange amplitudes stay in the second quadrant relative to the color-allowed tree amplitude, albeit there are two possibilities for one type of $W$-exchange amplitude. The weak decay penguin amplitudes, on the other hand, are evaluated within the framework of QCD factorization. Using the input of topological tree amplitudes extracted from the Cabibbo-favored decay modes and the perturbative results for QCD penguin amplitudes, we make predictions for the branching fractions and CP asymmetries of singly Cabibbo-suppressed modes. The predictions of branching fractions are generally improved from those in the SU(3) limit. We conclude that the direct CP asymmetry difference between $D^0 \to K^+ K^-$ and $D^0 \to π^+ π^-$ is about $-(0.139\pm 0.004)%$ and $-(0.151\pm 0.004)%$ for the two solutions of $W$-exchange amplitudes, respectively. We also find that the CP asymmetry of $D^0\to K^0\ov K^0$ dominated by the interference between $W$-exchange amplitudes ranges from $-0.62\times 10^{-3}$ to $-1.82\times 10^{-3}$. We study phenomenological implications of two new physics scenarios for explaining the observed CP asymmetry in the charm sector, one with large penguin amplitudes and the other with a large chromomagnetic dipole operator. We find that the two scenarios can be discriminated by the measurements of CP asymmetries of a set of decay modes.

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arXiv2012

Revisiting Scalar and Pseudoscalar Couplings with Nucleons

Certain dark matter interactions with nuclei are mediated possibly by a scalar or pseudoscalar Higgs boson. The estimation of the corresponding cross sections requires a correct evaluation of the couplings between the scalar or pseudoscalar Higgs boson and the nucleons. Progress has been made in two aspects relevant to this study in the past few years. First, recent lattice calculations show that the strange-quark sigma term $σ_s$ and the strange-quark content in the nucleon are much smaller than what are expected previously. Second, lattice and model analyses imply sizable SU(3) breaking effects in the determination on the axial-vector coupling constant $g_A^8$ that in turn affect the extraction of the isosinglet coupling $g_A^0$ and the strange quark spin component $Δs$ from polarized deep inelastic scattering experiments. Based on these new developments, we re-evaluate the relevant nucleon matrix elements and compute the scalar and pseudoscalar couplings of the proton and neutron. We also find that the strange quark contribution in both types of couplings is smaller than previously thought.

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arXiv2012

Search for doubly charged Higgs bosons using the same-sign diboson mode at the LHC

Doubly charged Higgs bosons are predicted in many new physics models with an extended Higgs sector that contains a Higgs triplet field. Current experimental searches have been focusing mainly on the scenario in which the same-sign dilepton decay modes are the dominant ones. We study the scenario where the vacuum expectation value of the triplet field is sufficiently large so that the associated charged Higgs bosons decay dominantly to a pair of weak gauge bosons instead. A detailed simulation of the signal and the backgrounds is performed for the CERN Large Hadron Collider at the collision energy of 8 TeV and 14 TeV. We find that different cuts should be imposed for the events, depending on whether the doubly charged Higgs boson mass is greater than about 200 GeV. In the higher mass region, the forward jet tagging proves to be useful in enhancing the signal significance. We show the discovery reach of the LHC running at 8 and 14 TeV, with two benchmark triplet vacuum expectation values. With an integrated luminosity of 10 fb${}^{-1}$ at 8 TeV, the doubly charged Higgs boson with a mass of $\sim 180$ GeV can be tested at $5σ$ level in such a scenario.

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