Wei-Shu Hou

Tree level t→ch0 or h0→tc decays

Abstract In a third type of two Higgs model, where neutral scalar bosons possess flavor changing uiuih0 couplings proportional to m i m j , low energy constraints are evaded. With the top as the heaviest fermion, tree level flavor changing t→ch0 or h 0 → t c decays may be competitive with, if not dominant over, the corresponding t→bW∗ or h0→b b decays. The CDF limit of mt>91 GeV may be evaded by the t→ch0 mode if mh0 h 0 → t c mode may be useful for the study of intermediate mass Higgs bosons at hadronic supercolliders. The scenario can be distinguished from the existence of exotic quarks since flavor changing Z couplings are absent.

Solutions to the B meson semileptonic branching ratio puzzle within two-Higgs-doublet models

Abstract Recent experimental values for BR ( B →l ν l X ) seem low compared to spectator model expectations. Conventional explanations have to be stretched considerably to accommodate this, pointing towards the possibility of new physics explanations. In the two-Higgs-doublet models, the b → c τ ν τ mode or the b→sg mode could get enhanced to the (10–20)% level, thereby suppressing BR ( B →l ν l X ) to the experimental value. The experimental consequences of these scenarios are discussed.

Searching for B→Dτvτ at the 10% level

Abstract The present discrepancy between the experimental value and theoretical expectations of BR(B→e v e X) suggests that there could be some heretofore “hidden” mode in B decays (with bad signature and vanishing standard model expectations) that is in fact at the (10–20)% level. In two Higgs doublet models with very large tan β and light H± bosons, the b→cτ v τ transition could get enhanced to this level. Many interesting consequences follow from this scenario. One of the most tangible at present is that the exclusive B→Dτ v τ decay could be at the 10% level, saturating the b→cτ v τ rate. We discuss why this is so, and propose a method for studying this mode in the λ (4S)→B B environment.

On right-handed b decay dominance

Abstract In the Gronau-Wakaizumi scenario of right-handed b decay dominance, VcdR is vanishingly small while large VtdR and VtsR lead to potential problems with the KL-KS mass difference. Allowing the b → u transition to be dominantly left-handed, we arrive at a more symmetric form of quark mixing matrices, where VL, VR are constituted mostly of mixing between first and second, and second and third generations, respectively. A third possibility of having b → c left-handed as usual, but b → u dominantly right-handed, is ruled out by the ϵ parameter and Bd mixing. Experimental consequences such as Cabibbo suppressed decays, penguin rates, and CP violation, are discussed. The best way to rule out right-handed b → c decay dominance is to find B → D (∗) D −(∗) modes at the 0.1% level.

Isospin splittings in heavy meson systems. Understanding mB0−mB−≈0 MeV

Abstract Existing data is shown to imply that isospin splittings in heavy meson masses induced by m d − m u ≠0 are smaller than in K mesons. It also implies a suppression of SU(3) breaking effects in hyperfine splittings to leading order in 1 m Q . Hence, pseudoscalar and vector heavy mesons should have similar isospin splittings, empirically given by M Q d −M Q u ∼(1.3±0.5)+(5.3±1.0) Q MeV ( Q is the heavy quark charge). Using first order perturbation theory in the light quark masses and a generalized version of vector meson dominance for the QED contribution, we arrive at a similar result. Thus, the recent observation that M B 0 − M B − ≈0 MeV can be accounted for.

Semi-leptonic flavour-changing neutral-current decays of the fourth generation b′-quark

We study the decay modes b′ → bl+l−, b′ → bνν for minb′ ⪅MZ including virtual γ, Z0, and box diagram contributions, the latter complicating the computation considerably. Due to the effect of the Z0 propagator and the finite Z0 width, these decay rates are substantial. The box diagram is subleading but also not negligible. As a consequence, the lepton pair mass distribution is rather flat and high mass l+l− or νν pairs are much more abundant than suggested from phase space. It is found that b′ → bZ∗ (loosely denoting the sum of ifb′ → bl+l−, b′ → νν and b′ → bqq) is of similar order as the b′ → bg decay mode, and is likely to dominate in case mt′ is very heavy. With three light neutrinos, the decay b′ → bνν is similar order as b′ → b γ, however, the b′ → bl+l− mode are relatively suppressed. The flavour-changing neutral-current b′ decay scenario is thus shown to be quite rich. A comprehensive, but theoretical, discussion is given regarding b′ discovery potentials at present and future colliders.

Q→qZ decays at Tevatron and SSC energies

Abstract The possible existence of a new heavy quark Q that decays predominantly viathe flavor changing neutral current transition Q→qZ is discussed. Candidates include a fourth generation charge — 1 3 quark, or a more exotic vector-like quark. Such particles are interesting both as extensions of the standard model and due to their unique decay modes. The primary experimental indication of the pair production and subsequent decay of such quarks is ZZ pair production at an essentially strong interaction rate. This mode can then constitute an unexpected background to the searches for other particles. In particular the channel Q Q →ZZ+X can generate a serious background to the search for the Higgs boson via the H0→ZZ mode at the SSC. Thus it is essential to search for such new heavy quarks at the Tevatron. Possible detection signatures for this purpose are discussed.

Searching for at the 10% level

Abstract The present discrepancy between the experimental value and theoretical expectations of BR(B→e v e X) suggests that there could be some heretofore “hidden” mode in B decays (with bad signature and vanishing standard model expectations) that is in fact at the (10–20)% level. In two Higgs doublet models with very large tan β and light H± bosons, the b→cτ v τ transition could get enhanced to this level. Many interesting consequences follow from this scenario. One of the most tangible at present is that the exclusive B→Dτ v τ decay could be at the 10% level, saturating the b→cτ v τ rate. We discuss why this is so, and propose a method for studying this mode in the λ (4S)→B B environment.

Signatures of the b' quark at e+e- colliders.

We investigate a scenario in which a fourth generation quark {ital b}{prime} exists with mass {ital m}{sub {ital b}{prime}}{lt}{ital m}{sub {ital t}} such that flavor changing neutral current decay modes dominate its width. In this scenario the signatures of {ital b}{prime} quark production will be different than what have been examined so far at the Fermilab Tevatron and there are implications for {ital b}{prime} quark searches at {ital e}{sup +}{ital e}{sup {minus}} colliders. We study a flavor-distinguishing signature of the {ital b}{prime} quark production relevant in this scenario. We show that the background to this signature at CERN LEP 1 is not very large. At LEP 2 a suitable choice of cuts can make the background manageable.

Signatures of the b prime quark at e sup + e sup minus colliders

We investigate a scenario in which a fourth generation quark {ital b}{prime} exists with mass {ital m}{sub {ital b}{prime}}{lt}{ital m}{sub {ital t}} such that flavor changing neutral current decay modes dominate its width. In this scenario the signatures of {ital b}{prime} quark production will be different than what have been examined so far at the Fermilab Tevatron and there are implications for {ital b}{prime} quark searches at {ital e}{sup +}{ital e}{sup {minus}} colliders. We study a flavor-distinguishing signature of the {ital b}{prime} quark production relevant in this scenario. We show that the background to this signature at CERN LEP 1 is not very large. At LEP 2 a suitable choice of cuts can make the background manageable.