arXiv subjects

Cheng-Wei Chiang

Publications and source records attributed to Cheng-Wei Chiang.

At least 163 records · Page 9Linked to original sources

arXiv2010

The Family SU(2)_l x SU(2)_h x U(1) Model

We consider extension of the standard model $SU(2)_l \times SU(2)_h \times U(1)$ where the first two families of quarks and leptons transform according to the $SU(2)_l$ group and the third family according to the $SU(2)_h$ group. In this approach, the largeness of top-quark mass is associated with the large vacuum expectation value of the corresponding Higgs field. The model predicts almost degenerate heavy $W'$ and $Z'$ bosons with non-universal couplings, and extra Higgs bosons. We present in detail the symmetry breaking mechanism, and carry out the subsequent phenomenology of the gauge sector. We compare the model with electroweak precision data, and conclude that the extra gauge bosons and the Higgs bosons whose masses lie in the TeV range, can be discovered at the LHC.

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arXiv2009

Doubly charged Higgs bosons and three-lepton signatures in the Higgs Triplet Model

Doubly charged Higgs bosons, H^{\pm\pm}, are being searched for in the Tevatron experiments. The most recent search by the D0 collaboration seeks three muons (mu^\pmμ^\pmμ^\mp), which are assumed to originate from the pair-production process, q\overline q\to H^{++}H^{--}, followed by the decay H^{\pm\pm} \to μ^\pm μ^\pm. In this three-lepton (3l) channel there are six distinct signatures for l=e or μ. In the context of the Higgs Triplet Model, we quantify the dependence of the event numbers for the 3l channels on the parameters of the neutrino mass matrix. It is also shown that the inclusion of the production mechanism q \overline{q'} \to H^{\pm\pm}H^{\mp}, followed by the decay H^\pm\to l^ \pmν, would significantly increase the discovery potential in these channels. We then provide perspectives on the production of these channels at the Tevatron and LHC.

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arXiv2009

Looking for New Physics in B --> K^* πand B --> ρK Decays

B --> K^* πand B --> ρK decays involve the same quark-level processes as B --> πK. Analyzing the measurements of the former decays might be able to shed additional light on the new-physics hints in the current B --> πK data. We perform fits to B --> K^* πand B --> ρK decays, and find that the data can be accommodated within the standard model. However, this agreement is due principally to the large errors in the data, particularly the CP-violating asymmetries. If the errors on the B --> K^* πand B --> ρK observables can be reduced, one will have a clearer sense of whether new physics is present in these decays.

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arXiv2009

Diagnostic for new physics in $B \to πK$ decays

A recent analysis of $B \to πK$ decays concludes that present data do not clearly indicate whether (i) the standard model (or $ΔI=0$ new physics) is sufficient, or (ii) $ΔI=1$ new physics is needed. We show that these two possibilities can be distinguished by whether a sum rule relating the CP asymmetries of the four $B \to πK$ decays is valid. If case (i) is favored, the sum rule holds, and one predicts $A_{CP}(π^0 K^0) = -0.15$, while in case (ii) fits to new physics involving large values of a color-suppressed tree amplitude entail $A_{CP}(π^0 K^0) = -0.03$. The current experimental average $A_{CP}(π^0 K^0) = -0.01 \pm 0.10$ must be measured a factor of at least three times more precisely in order to distinguish between the two cases.

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arXiv2009

The B -> pi K Puzzle: 2009 Update

The measurements of B -> pi K decays have been in disagreement with the predictions of the Standard Model (SM) for some time. In this paper, we perform an update of this puzzle using the latest (2008) data. We find that the situation has become far less clear. A fit to the B -> pi K data alone suggests the presence of new physics (NP). Indeed, if one adds a constraint on the weak phase gamma coming from independent measurements -- the SM fit -- one finds that the fit is poor. On the other hand, it is not terrible. If one is willing to accept some deficiencies in the fit, it can be argued that the SM can explain the B -> pi K data. If one assumes NP, it is found to be present only in the electroweak penguin amplitude, as before. However, the fit is fair at best, and the improvement over the SM is not particularly strong. All and all, while the B -> pi K puzzle has not disappeared, it has become weaker.

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arXiv2008

Flavor symmetry analysis of charmless B --> VP decays

Based upon flavor SU(3) symmetry, we perform global fits to charmless B decays into one pseudoscalar meson and one vector meson in the final states. We consider different symmetry breaking schemes and find that the one implied by na{ï}ve factorization is slightly favored over the exact symmetry case. The $(\barρ,\barη)$ vertex of the unitarity triangle (UT) constrained by our fits is consistent with other methods within errors. We have found large color-suppressed, electroweak penguin and singlet penguin amplitudes when the spectator quark ends up in the final-state vector meson. Nontrivial relative strong phases are also required to explain the data. The best-fit parameters are used to compute branching ratio and CP asymmetry observables in all of the decay modes, particularly those in the $B_s$ decays to be measured at the Tevatron and LHC experiments.

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arXiv2008

Theoretical Review on CP Violation in Rare B decays

We discuss several issues related to direct CP violation in rare $B$ meson decays. We review the use of CP asymmetries in extracting information of strong and weak phases, how the experimental data fit into the overall picture, and the current status of the $K π$ puzzle. We also examine the flavor symmetry assumption using closely related decay modes and extract the weak phase $γ$ from certain $B \to K^* π$ and $ρK$ decays.

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arXiv2008

Probing triple Higgs couplings of the Two Higgs Doublet Model at Linear Collider

We study double Higgs production at the future Linear Collider in the framework of the Two Higgs Doublet Models through the following channels: $e^+e^- \to Φ_i Φ_j Z$, $Φ_i=h^0, H^0, A^0, H^\pm$. All these processes are sensitive to triple Higgs couplings. Hence observations of them provide information on the triple Higgs couplings that help reconstructing the scalar potential. We also discuss the double Higgs-strahlung $e^+e^- \to h^0 h^0 Z$ in the decoupling limit where $h^0$ mimics the SM Higgs boson. The processes $e^+e^- \to h^0 h^0 Z$ and $e^+e^- \to h^0 H^0 Z$ are also discussed in the fermiophobic limit where distinctive signatures such as $4γ+X$, $2γ+X$ and $6γ+X$ are expected in the Type-I Two Higgs Doublet Model.

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arXiv2008

Examination of Flavor Symmetry in $B, B_s \to K π$ Decays

We study a relation between the weak phase $γ$ and the rates and CP asymmetries of several $K π$ decays of $B^+$, $B^0$, and $B_s$, emphasizing the impact of the latter measurements. Current data indicate large SU(3) breaking in the strong phases or failure of factorization (including its application to penguin amplitudes) in $K π$ modes of $B^0$ and $B_s$. SU(3) and factorization only remain approximately valid if the branching ratio for $B_s \to K^- π^+$ exceeds its current value of $(5.27 \pm 1.17) \times 10^{-6}$ by at least 42%, or if a parameter $ξ$ describing ratios of form factors and decay constants is shifted from its nominal value by more than twice its estimated error.

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arXiv2008

Partially Strong WW Scattering

What if only a light Higgs boson is discovered at the CERN LHC? Conventional wisdom tells us that the scattering of longitudinal weak gauge bosons would not grow strong at high energies. We show that this is not always true. In some composite models, two-Higgs-doublet models, or even supersymmetric models, the presence of a light Higgs boson does not guarantee the complete unitarization of the $WW$ scattering. After the partial unitarization by the light Higgs boson, the $WW$ scattering becomes strongly interacting until it hits one or more heavier Higgs bosons or other strong dynamics. We analyze how the LHC experiments can reveal this interesting possibility of partially strong $WW$ scattering.

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arXiv2008

CP violation in the secluded U(1)'-extended MSSM

We study the Higgs sector of the secluded $U(1)'$-extended MSSM (sMSSM) focusing on CP violation. Using the one-loop effective potential that includes contributions from quarks and squarks in the third generation, we search for the allowed region under theoretical and experimental constraints. It is found that the possible region for the electroweak vacuum to exist is quite limited, depending on the parameters in the model. The masses and couplings of the Higgs bosons are calculated with/without CP violation. Even at the tree level, CP violation is possible by complex soft SUSY breaking masses. Similar to the CPX scenario in the MSSM, the scalar-pseudoscalar mixing enables the lightest Higgs boson mass to become smaller than the $Z$ boson mass while the coupling with the $Z$ boson is sufficiently suppressed to avoid the LEP experimental constraints. However, unlike the CPX scenario, large $μ$ and $A$ are not required for the realization of large CP violation. The typical spectrum of the SUSY particles is thus different. We also investigate the possible upper bound of the lightest Higgs boson in the case of spontaneous CP violation. The maximal value of it can reach above 100 GeV with maximal CP-violating phases.

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arXiv2007

Top Hypercharge

We propose a top hypercharge model with gauge symmetry SU(3)_C x SU(2)_L x U(1)_1 x U(1)_2 where the first two families of the Standard Model (SM) fermions are charged under U(1)_1 while the third family is charged under U(1)_2. The U(1)_1 x U(1)_2 gauge symmetry is broken down to the U(1)_Y gauge symmetry, when a SM singlet Higgs field acquires a vacuum expectation value. We consider the electroweak constraints, and compare the fit to experimental observables to that of the SM. We study the quark CKM mixing between the first two families and the third family, the neutrino masses and mixing, the flavour changing neutral current effects in meson mixing and decays, the Z' discovery potential at the Large Hadron Collider, the dark matter with a gauged Z_2 symmetry, and the Higgs boson masses.

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arXiv2007

Flavor SU(3) analysis of charmless B->PP decays

We perform a global fits to charmless $B \to PP$ decays which independently constrain the $(\barρ,\barη)$ vertex of the unitarity triangle. The fitted amplitudes and phase are used to predict the branching ratios and CP asymmetries of all decay modes, including those of the $B_s$ system. Different schemes of SU(3) breaking in decay amplitude sizes are analyzed. The possibility of having a new physics contribution to $K π$ decays is also discussed.

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arXiv2007

Update analysis of two-body charmed $B$ meson decays

The charmed B decays, $B\to DP, ~D^*P$ and $DV$, are re-analyzed using the latest experimental data, where $P$ and $V$ denote the pseudoscalar meson and vector meson, respectively. We perform global fits under the assumption of flavor SU(3) symmetry. The size of the decay amplitudes and the strong phases between the topologically distinct amplitudes are studied. Predictions of the related $B_s$ decay rates are made based upon the fitted results. We also note a serious SU(3) symmetry breaking or inconsistency in the $DV$ sector.

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arXiv2006

Flavor SU(3) analysis of charmless B meson decays to two pseudoscalar mesons

Global fits to charmless B --> PP decays in the framework of flavor SU(3) symmetry are updated and improved without reference to the \sin2βmeasured from the charmonium decay modes. Fit results directly constrain the (\barρ,\barη) vertex of the unitarity triangle, and are used to predict the branching ratios and CP asymmetries of all decay modes, including those of the B_s system. Different schemes of SU(3) breaking in decay amplitude sizes are analyzed. The major breaking effect between strangeness-conserving and strangeness-changing decays can be accounted for by including a ratio of decay constants in tree and color-suppressed amplitudes. The possibility of having a new physics contribution to K πdecays is also examined from the data fitting point of view.

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arXiv2006

Flavor Changing Effects in Family Nonuniversal Z' Models

Flavor-changing and CP-violating interactions of Z' to fermions are generally present in models with extra U(1) gauge symmetry that are string-inspired or related to broken gauged family symmetry. We study the consequences of such couplings in fermion electric dipole moments, muon g-2, and K and B meson mixings. From experimental limits or measured values, we constrain the off-diagonal Z' couplings to fermions. Some of these constraints are comparable or stronger than the existing constraints obtained from other observables.

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arXiv2006

Constraints on flavor-changing Z' models by B_s mixing, Z' production, and B_s -> μ^+ μ^-

Certain string-inspired Z' models have non-universal interactions to three families of fermions and induced tree-level flavor-changing couplings. We use recent results on B_s-anti-B_s mixing to constrain the size of the flavor-changing couplings in the b-s sector. In some highly predictive Z' models, such a constraint on b-s coupling can be translated into the flavor-diagonal couplings. Based on the Z' production limits at the Tevatron, we obtain the limit on the leptonic couplings of Z' and then make predictions for B_s -> μ^+ μ^- branching ratios. We conclude that with the present constraints from B_s mixing and Z' production, the muonic decay of $B_s$ may not be observed at the Tevatron if the projected integrated luminosity is less than O(5-10) fb.

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arXiv2006

Single Top-Quark Production in Flavor-Changing Z' Models

In some models with an extra U(1) gauge boson Z', the gauge couplings of the Z' to different generations of fermions may not be universal. Flavor mixing in general can be induced at the tree level in the up-type and/or down-type quark sector after diagonalizing their mass matrices. In this work, we concentrate on the flavor mixing in the up-type quark sector. We deduce a constraint from D0-D0bar mixing. We study in detail single top-quark production via flavor-changing Z' exchange at the LHC and the ILC. We found that for a typical value of M_{Z'}=1 TeV, the production cross section at the LHC can be of the order of 1 fb. However, the background from the Standard Model single top-quark production makes it difficult to detect the flavor-changing Z' signal unless with a decent charm tagging method. On the other hand, at the ILC, the production cross section at the resonance energy of \sqrt{s} \approx M_{Z'} can reach a size of more than 100 fb. Even away from the resonance, the cross section at ILC is shown to be larger than the threshold of observability of 0.01 fb.

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