arXiv subjects

Wei-Shu Hou

Publications and source records attributed to Wei-Shu Hou.

At least 55 records · Page 3Linked to original sources

arXiv2017

Electroweak Baryogenesis driven by Extra Top Yukawa Couplings

We study electroweak baryogenesis driven by the top quark in a general two Higgs doublet model with flavor-changing Yukawa couplings, keeping the Higgs potential $CP$ invariant. With Higgs sector couplings and the additional top Yukawa coupling $ρ_{tt}$ all of $\mathcal{O}$(1), one naturally has sizable $CP$ violation that fuels the cosmic baryon asymmetry. Even if $ρ_{tt}$ vanishes, the favor-changing coupling $ρ_{tc}$ can still lead to successful baryogenesis. Phenomenological consequences such as $t\to ch$, $τ\to μγ$, electron electric dipole moment, $h\toγγ$, and $hhh$ coupling are discussed.

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arXiv2017

Search for $tZ'$ associated production induced by $tcZ'$ couplings at the LHC

The $P'_5$ and $R_K$ anomalies, recently observed by the LHCb collaboration in $B \to K^{(*)}$ transitions, may indicate the existence of a new $Z'$ boson, which may arise from gauged $L_μ- L_τ$ symmetry. Flavor-changing neutral current $Z'$ couplings, such as $tcZ'$, can be induced by the presence of extra vector-like quarks. In this paper we study the LHC signatures of the induced right-handed $tcZ'$ coupling that is inspired by, but not directly linked to, the $B \to K^{(*)}$ anomalies. The specific processes studied are $cg \to tZ'$ and its conjugate process each followed by $Z'\toμ^+μ^-$. By constructing an effective theory for the $tcZ'$ coupling, we first explore model-independently the discovery potential of such a $Z'$ at the 14 TeV LHC with 300 and 3000 fb$^{-1}$ integrated luminosities. We then reinterpret the model-independent results within the gauged $L_μ- L_τ$ model. In connection with $tcZ'$, the model also implies the existence of a flavor-conserving $ccZ'$ coupling, which can drive the $c \bar c \to Z' \to μ^+μ^-$ process. Our study shows that existing LHC results for dimuon resonance searches already constrain the $ccZ'$ coupling, and that the $Z'$ can be discovered in either or both of the $cg \to tZ'$ and $c \bar c \to Z'$ processes. We further discuss the sensitivity to the left-handed $tcZ'$ coupling and find that the coupling values favored by the $B \to K^{(*)}$ anomalies lie slightly below the LHC discovery reach even with 3000 fb$^{-1}$.

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arXiv2017

Flavor Physics and CP Violation

We currently live in the age of the CKM paradigm. The $3\times 3$ matrix that links $(d,\ s,\ b)$ quarks to $(u,\ c,\ t)$ in the charged current weak interaction, being complex and nominally with 18 parameters, can be accounted for by just 3 rotation angles and one $CP$ violating (CPV) phase, with unitarity and the CKM phases triumphantly tested at the B factories. But the CKM picture is unsatisfactory and has too many parameters. The main aim of Flavor Physics and $CP$ violation (FPCP) studies is the pursuit to uncover New Physics beyond the Standard Model (SM). Two highlights of LHC Run~1 period are the CPV phase $ϕ_s$ of $B_s$ mixing and $B_s \toμ^+μ^-$ decay, which were found to be again consistent with SM, though the saga is yet unfinished. We also saw the emergence of the $P_5'$ angular variable anomaly in $B^0 \to K^{*0}μ^+μ^-$ decay and $R_{K^{(*)}}$ anomaly in $B \to K^{(*)}μ^+μ^-$ to $B \to K^{(*)}e^+e^-$ rate ratios, and the BaBar anomaly in $B \to D^{(*)}τν$ decays, which suggest possible New Physics in these flavor processes, pointing to extra $Z'$, charged Higgs, or leptoquarks. Charmless hadronic, semileptonic, purely leptonic and radiative $B$ decays continue to offer various further windows on New Physics. Away from $B$ physics, the rare $K \to πνν$ decays and $\varepsilon'/\varepsilon$ in the kaon sector, $μ\to e$ transitions, muon $g-2$ and electric dipole moments of the neutron and electron, $τ\to μγ,\ μμμ,\ eee$, and a few charm physics probes, offer broadband frontier windows on New Physics. (See the document for the full abstract.)

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arXiv2017

Probing extra Yukawa couplings by precision measurements of Higgs properties

If one removes any \emph{ad hoc} symmetry assumptions, the general two Higgs doublet model should have additional Yukawa interactions independent from fermion mass generation, in general involving flavor changing neutral Higgs couplings. These extra couplings can affect the discovered Higgs boson $h$ through fermion loop contributions. We calculate the renormalized $hZZ$ coupling at one-loop level %by on-shell and minimal subtraction scheme, and evaluate the dependence on heavy Higgs boson mass and extra Yukawa coupling $ρ_{tt}$. Precision measurements at future colliders can explore the parameter space, and can give stronger bound on $ρ_{tt}$ than the current bound from flavor experiments. As a side result, we find that if $ρ_{tt}\cosγ< 0$, where $\cosγ$ is the exotic Higgs component of $h$, the $ρ_{tt}$-induced top loop contribution cancels against bosonic loop contributions, and one may have alignment without decoupling, namely $\sin(-γ) \simeq 1$, but exotic scalar bosons could have masses at several hundred GeV.

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arXiv2016

Z'-induced FCNC decays of top, beauty and strange quarks

Anomalous b --> s transitions from LHCb data may suggest a new massive gauge boson Z' that couples to the left-handed b --> s current, which in turn implies a coupling to the t --> c current. In this paper, we study flavor-changing neutral current (FCNC) decays of the top quark induced by a Z' boson, namely t --> c Z', based on a model of the gauged L_mu - L_tau symmetry (the difference between the muon and tauon numbers) with vector-like quarks, which was introduced to explain the anomalous LHCb data. We illustrate that searching for t --> c Z' via Z' --> mu^+ mu^- with LHC Run 1 data can already probe a parameter region which is unexplored by B physics for the Z' mass around O(10) GeV or more. We further extend the model to very light Z' with mass below 400 MeV, which is motivated by the muon g-2 anomaly. Taking rare B and K meson decay data into account, we give upper limits on the t --> c Z' branching ratio for the light Z' case, and discuss about its observability at the LHC. We also scrutinize the possibility that the decay K_L --> pi^0 Z' with Z' --> nu nubar may lead to apparent violation of the usual Grossman-Nir bound of B(K_L --> pi^0 nu nubar) < 1.4 x 10^-9.

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arXiv2015

Correlating $B_q^0 \to μ^+μ^-$ and $K_L \to π^0ν\barν$ Decays with Four Generations

The long-awaited $B_s\to μ^+μ^-$ mode has finally been observed at rate consistent with Standard Model, albeit lower by 1.2$σ$. There is some hint for New Physics in the rarer $B_d^0 \to μ^+μ^-$ decay, especially if the currently 2.2$σ$-enhanced central value persists with more data. The measurement of $CP$ violating phase $ϕ_s$, via both $B_s\to J/ψK\bar K$ and $J/ψππ$ modes, has reached Standard Model sensitivity. These measurements stand major improvement when LHC enters Run 2. Concurrently, the $K_L\toπ^0ν\barν$ and $K^+\toπ^+ν\barν$ modes are being pursued in a similar time frame. We illustrate the possible correlations between New Physics effects in these four modes, using the fourth generation as example. While correlations may or may not exist in other New Physics models, the four generation model can accommodate enhancements in both $B_d^0 \to μ^+μ^-$ and $K_L\toπ^0ν\barν$.

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arXiv2015

Flavor Changing Heavy Higgs Interactions at the LHC

A general two Higgs doublet model (2HDM) is adopted to study the signature of flavor changing neutral Higgs (FCNH) decay $ϕ^0 \to t\bar{c}+\bar{t}c$, where $ϕ^0$ could be a CP-even scalar ($H^0$) or a CP-odd pseudoscalar ($A^0$). Measurement of the light 125 GeV neutral Higgs boson ($h^0$) couplings at the Large Hadron Collider (LHC) favor the decoupling limit or the alignment limit of a 2HDM, in which gauge boson and diagonal fermion couplings of $h^0$ approach Standard Model values. In such limit, FCNH couplings of $h^0$ are naturally suppressed by a small mixing parameter $\cos(β-α)$, while the off-diagonal couplings of heavier neutral scalars $ϕ^0$ are sustained by $\sin(β-α) \sim 1$. We study physics background from dominant processes with realistic acceptance cuts and tagging efficiencies. Promising results are found for the LHC running at 13 or 14 TeV collision energies.

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arXiv2015

Loophole in K --> pi nu nubar Search and New Weak Leptonic Forces

Weakly interacting K --> pi X^0 emission with m_{X^0} \simeq m_{pi^0} is out of sight of the current K^+ --> pi^+ nu nubar study, but it can be sensed by the K_L --> pi^0 nu nubar search. This evades the usual Grossman-Nir bound of B(K_L --> pi^0 nu nubar) < 1.4 x 10^-9, thus the KOTO experiment is already starting to probe New Physics. An intriguing possibility is the Z' gauge boson of a weak leptonic force that couples to L_mu - L_tau (the difference between the muon and tauon numbers), which may explain the long-standing "muon g-2" anomaly, but is constrained by nu_mu N --> nu_mu N mu^+ mu^- scattering to m_{Z'} \lesssim 400 MeV. An explicit model for K --> pi Z' is given, which illustrates the link between rare kaon and B --> K mu^+ mu^-, K^{(*)} nu nubar decays. Complementary to these searches and future lepton experiments, the LHC might discover the scalar boson phi responsible for light m_{Z'} generation via phi --> Z' Z' --> 2(mu^+ mu^-).

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arXiv2014

Rates and Asymmetries of B --> pi l+ l- Decays

The LHCb experiment observed B+ --> pi+ mu+ mu- decay with 1.0 fb^-1 data, which is the first measurement of a flavor changing neutral current b --> d l+ l- decay (l = e, mu). Based on QCD factorization, we give Standard Model predictions for the branching ratios, direct CP asymmetries, and isospin asymmetry for B --> pi l+ l- decays, in the kinematic region where the dilepton invariant mass is small. We find that the contribution from weak annihilation enhances the direct CP asymmetry for low l+ l- pair mass. Anticipating improved measurements, we assess the utility of B+ --> pi+ l+ l- observables, when combined with B0 --> pi- l+ nu and B+ --> K+ l+ l-, for determining CKM parameters in the future.

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arXiv2013

When the Higgs meets the Top: Search for t --> ch^0 at the LHC

The newly discovered "Higgs" boson h^0, being lighter than the top quark t, opens up new probes for flavor and mass generation. In the general two Higgs doublet model, new ct, cc and tt Yukawa couplings could modify h^0 properties. If t --> ch^0 occurs at the percent level, the observed ZZ^* and γγsignal events may have accompanying cbW activity coming from t\bar{t} feeddown. We suggest that t --> ch^0 can be searched for via h^0 --> ZZ^*, γγ, WW^* and b\bar{b}, perhaps even τ^+τ^- modes in t\bar{t} events. Existing data might be able to reveal some clues for t --> ch^0 signature, or push the branching ratio B(t --> ch^0) down to below the percent level.

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arXiv2013

Implication of possible observation of enhanced Bd0 --> mu+ mu- decay

The very rare Bd0 --> mu+ mu- decay may be the last chance for New Physics in flavor sector at the LHC, before the 13 TeV run in 2015. Partially motivated by the known tension in sin(2beta/phi_1), enhancement beyond (3-4) x 10^-10 would likely imply the effect of a fourth generation of quarks. If observed at this level, the 126 GeV boson may not be the actual Higgs boson, while the b --> d quadrangle (modulo m_t') would jump out. The 2011-2012 data is likely not sensitive to values below 3 x 10^-10, and the mode should continue to be pursued with the 13 TeV run.

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arXiv2013

Bootstrap Dynamical Symmetry Breaking with New Heavy Chiral Quarks

A Higgs-like new boson with mass around 126 GeV is now established, but its true nature probably cannot be settled with 2011--2012 LHC data. We assume it is a dilaton with couplings weaker than the Higgs boson (except to $γγ$ and $gg$), and explore dynamical symmetry breaking (DSB) by strong Yukawa coupling of a yet unseen heavy chiral quark doublet $Q$. Assuming the actual Higgs boson to be heavy, the Goldstone boson $G$ of electroweak symmetry breaking still couples to $Q$ with Yukawa coupling $λ_Q$. A ``bootstrap" gap equation without a Higgs particle is constructed. Electroweak symmetry breaking via strong $λ_Q$ generates both heavy mass for $Q$, while self-consistently justifying $G$ as a massless Goldstone particle in the loop. The spontaneous breaking of scale invariance in principle \emph{allows} for a dilaton. We numerically solve such a gap equation and find the mass of the heavy quark to be a couple of TeV. We offer a short critique on the results of the scale-invariant model of Hung and Xiong, where a similar gap equation is built with a massless scalar doublet. Through this we show that a light SM Higgs at 126 GeV cannot be viable within our approach to DSB, while a dilaton with weaker couplings is consistent with our main result.

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arXiv2012

Top Decays with Flavor Changing Neutral Higgs Interactions at the LHC

We investigate the prospects for discovering a top quark decaying into one light Higgs boson along with a charm quark in top quark pair production at the CERN Large Hadron Collider (LHC). A general two Higgs doublet model is adopted to study the signature of flavor changing neutral Higgs decay $t \to cϕ^0$, %or $\bar{t} \to \bar{c}ϕ^0$ where $ϕ^0$ could be CP-even ($H^0$) or CP-odd ($A^0$). The dominant physics background is evaluated with realistic acceptance cuts as well as tagging and mistagging efficiencies. For a reasonably large top-charm-Higgs coupling ($λ_{tc}/λ_{t} \agt 0.09$), the abundance of signal events and the %that our acceptance cuts reduction in physics background allow us to establish a $5σ$ signal for $M_ϕ\sim 125$ GeV at the LHC with a center of mass energy ($\sqrt{s}$) of 8 TeV and an integrated luminosity of 20 fb$^{-1}$. The discovery potential will be greatly enhanced with the full energy of $\sqrt{s} = 14$ TeV.

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arXiv2012

Doubly resonant WW plus jet signatures at the LHC

We present search prospects and phenomenology of doubly resonant signals that come from the decay of a neutral weak-singlet color-octet vector state ω_8 into a lighter weak-triplet color-octet scalar π_8, which can arise in several theories beyond the Standard Model. Taking m_{ω_8}-m_{π_8}>m_W, we demonstrate an analysis of the signals pp \to ω_8 \to π^\pm_8 W^\mp (π^0_8 Z) \to g W^\pm W^\mp (g Z Z). The present 8 TeV LHC run is found to have the potential to exclude or discover the signal for a range of masses and parameters. The preferred search channel has a boosted W-tagged jet forming a resonance with a second hard jet, in association with a lepton and missing energy.

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arXiv2012

Heavy QQ(bar) "Fireball" Annihilation to Multiple Vector Bosons

Drawing analogy of replacing the nucleon by heavy chiral quark $Q$, the pion by Goldstone boson $G$, and $πNN$ coupling by $GQQ$ coupling, we construct a statistical model for $Q\bar Q \to nG$ annihilation, i.e. into $n$ longitudinal weak bosons. This analogy is becoming prescient since the LHC direct bound $m_Q > 611$ GeV implies strong Yukawa coupling. Taking $m_Q \in (1, 2)$ TeV, the mean number $ $ ranges from 6 to over 10, with negligible two or three boson production. With individual $t'$ or $b'$ decays suppressed either by phase space or quark mixing, and given the strong Yukawa coupling, $Q\bar Q\to nV_L$ is the likely outcome for very heavy $Q\bar Q$ production at the LHC.

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arXiv2012

Hints for a Low Bs --> mu+ mu- Rate and the Fourth Generation

With full 2011 LHC data analyzed, there is no indication for deviation from Standard Model (SM) in CP violating phase for Bs --> J/psi phi, nor in the forward-backward asymmetry for B0 --> K*0 mu+ mu-. SM sensitivity, however, has been reached for Bs --> mu+ mu- rate, and there may be some hint for a suppression. We illustrate that, if a suppressed B(Bs --> mu+ mu-) bears out with 2012 data, it would imply a lower bound on the fourth generation quark mixing product |V_{t's}^*V_{t'b}|.

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arXiv2012

Some Unfinished Thoughts on Strong Yukawa Couplings

Yukawa couplings of electroweak Goldstone bosons can be inferred from experiment, but the existence of an elementary Higgs boson is not yet an established fact. If a sequential chiral quark generation does exist, it would bring us now into the strong Yukawa coupling regime. Guided by a Bethe--Salpeter equation approach, we postulate that the leading collapse state, the (heavy) isotriplet and color-singlet $π_1$ meson, becomes the Goldstone boson $G$ itself. Viewing it as a deeply bound state, a gap equation is constructed. This "`bootstrap" picture for electroweak symmetry breaking relies on strong Yukawa coupling, without providing any theory of the latter.

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arXiv2011

Measuring the Fourth Generation b --> s Quadrangle at the LHC

We show that simultaneous precision measurement of the CP-violating phase in time-dependent Bs --> J/psi phi study and the Bs --> mu+ mu- rate, together with measuring m_t' by direct search at the LHC, would determine V_{t's}^*V_{t'b} and therefore the b --> s quadrangle in the four-generation standard model. The forward-backward asymmetry in B --> K* l+ l- provides further discrimination.

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