arXiv subjects

Wei-Shu Hou

Publications and source records attributed to Wei-Shu Hou.

At least 37 records · Page 2Linked to original sources

arXiv2020

Muon Flavor Violation in Two Higgs Doublet Model with Extra Yukawa Couplings

The new round of experiments, MEG II, COMET/Mu2e, and Mu3e, would soon start to push the $μ\to eγ$, $μN \to eN$ conversion, and $μ\to 3e$ frontier, while Belle II would probe $τ\to μγ$ and $τ\to 3μ$. In the general two Higgs doublet model with extra Yukawa couplings, we show that all these processes probe the lepton flavor violating (LFV) dipole transition that arises from the two loop mechanism, with scalar-induced contact terms subdominant. This is because existing data suggest the extra Yukawa couplings $ρ_{μe},\, ρ_{ee} \lesssim λ_e$, while $ρ_{τμ},\, ρ_{ττ} \lesssim λ_τ$ and $ρ_{tt} \lesssim λ_t$, with $λ_i$ the usual Yukawa coupling of the Standard Model (SM), where $ρ_{μe}ρ_{tt}$ and $ρ_{τμ}ρ_{tt}$ enter the $μeγ$ and $τμγ$ two loop amplitudes, respectively. With the $B_s \to μμ$ decay rate basically consistent with SM expectation, together with the $B_s$ mixing constraint, we show that $B_s \to ττ$ would also be consistent with SM, while $B_s \to τμ$ and $B \to Kτμ$ decays would be out of reach of projected sensitivities, in strong contrast with some models motivated by the B anomalies.

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arXiv2020

Probing Top Changing Neutral Higgs Couplings at Colliders

The $h(125)$ boson, discovered only in 2012, is lower than the top quark in mass, hence $t \to ch$ search commenced immediately thereafter, with current limits at the per mille level and improving. As the $t \to ch$ rate vanishes with the $h$-$H$ mixing angle $\cosγ\to 0$, we briefly review the collider probes of the top changing $tcH/tcA$ coupling $ρ_{tc}$ of the exotic $CP$-even/odd Higgs bosons $H/A$. Together with an extra top conserving $ttH/ttA$ coupling $ρ_{tt}$, one has an enhanced $cbH^+$ coupling alongside the familiar $tbH^+$ coupling, where $H^+$ is the charged Higgs boson. The main processes we advocate are $cg \to tH/A \to tt\bar c,\; tt\bar t$ (same-sign top and triple-top), and $cg \to bH^+ \to bt\bar b$. We also discuss some related processes such as $cg \to thh$, $tZH$ that depend on $\cosγ$ being nonzero, comment briefly on $gg \to H/A \to t\bar t, t\bar c$ resonant production, and touch upon the $ρ_{tu}$ coupling.

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arXiv2020

Sub-TeV $H^+$ Boson Production as Probe of Extra Top Yukawa Couplings

We suggest searching for the charged Higgs boson at the Large Hadron Collider (LHC) via $cg \to b H^+ \to b t \bar b$. In the general two Higgs Doublet Model, extra top Yukawa couplings $ρ_{tc}$ and $ρ_{tt}$ can drive the disappearance of antimatter from the Universe, while $\bar cbH^+$ and $\bar tbH^+$ couple with strength $ρ_{tc}V_{tb}$ and $ρ_{tt}V_{tb}$, respectively. For $ρ_{tc},\, ρ_{tt} \sim 0.5$ and $m_{H^+} \sim 300$--500 GeV, evidence could emerge from LHC Run 2 data at hand, and discovery by adding Run 3 data in the near future.

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arXiv2020

Constraining $t \to u$ flavor changing neutral Higgs coupling at the LHC

We study the constraints on $t\to u$ flavor changing neutral Higgs (FCNH) coupling, and how it may be explored further at the Large Hadron Collider (LHC). In the general two Higgs doublet model, such transitions can be induced by a nonzero $ρ_{tu}$ Yukawa coupling. We show that such couplings can be constrained by existing searches at the LHC for $m_H$, $m_A$ and, $m_{H^+}$ in the sub-TeV range, where $H$, $A$ and $H^+$ are the exotic $CP$-even, $CP$-odd and charged scalars. We find that a dedicated $ug\to t H/tA \to t t \bar u$ search can probe the available parameter space of $ρ_{tu}$ down to a few percent level for $200\,\mbox{GeV} \lesssim m_H,\,m_A \lesssim 600$ GeV, with discovery possible at high luminosity. Effects of how other extra top Yukawa couplings, such as $ρ_{tc}$ and $ρ_{tt}$, dilute the sensitivity of the $ρ_{tu}$ probe are discussed.

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arXiv2020

The Coming Decade of $h \to τμ$ and $τ\to μγ$ Interplay in $τ$ Flavor Violation Search

We explore the interplay between $h(125) \to τμ$ search at the LHC and $τ\to μγ$ at the up and coming Belle~II experiment, in context of the general two Higgs doublet model with extra Yukawa couplings such as $ρ_{τμ}$. The search for $h \to τμ$ constrains $ρ_{τμ} \cosγ$, where $\cosγ$ is the $h$--$H$ mixing angle of $h$ with the exotic $CP$-even scalar $H$. For $τ\to μγ$, we define the "BSM-benchmark" by setting the extra top Yukawa coupling $ρ_{tt} = 1\ (\cong λ_t)$ in two-loop diagrams, and $\cosγ= 0$ to decouple $h$. We show that this leading effect due to $H$ and $CP$-odd scalar $A$ can be readily probed by Belle~II, even for the conservative value of $ρ_{τμ} = 0.7λ_τ$. We define the subleading "$h$-benchmark" by setting $ρ_{tt} = 0$ in two-loop diagrams, and take the conservative maximal value of $\cosγ= 0.2$. We show that it falls beyond Belle~II reach, but can interfere with the BSM-benchmark effect, which in principle probes the phase of $ρ_{tt}$. We further show that the one-loop $H$, $A$ effect, proportional to $ρ_{τμ}ρ_{ττ}$ in amplitude, is beyond the sensitivity of Belle~II to probe, even for $ρ_{ττ}$ as large as $3λ_τ$. With the working assumption that $ρ_{32}^f, ρ_{33}^f = {\cal O}(λ_3^f)$ for all charged fermions $f$, we find good discovery potential for both $τ$ lepton flavor violation searches in the coming decade.

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arXiv2020

The $tZH$ and $tZh$ production in 2HDM: Prospects for discovery at the LHC

We study the discovery potential of the $cg \to tA\to tZH$ process at the LHC, where $A$ and $H$ are CP-odd and even exotic scalars, respectively. The context is the general Two Higgs Doublet Model, where $cg \to tA$ is induced by the flavor changing neutral Higgs coupling $ρ_{tc}$. We find that the process $cg \to tA\to tZH$ can be discovered for $m_A \sim 400$ GeV, but would likely require high luminosity running of the LHC. Such a discovery would shed light on the mechanism behind the observed Baryon Asymmetry of the Universe. We also study $cg \to tA\to tZh$, where $h$ is the observed 125 GeV scalar, but find it out of reach at the LHC.

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arXiv2020

Cancellation mechanism for the electron electric dipole moment connected with the baryon asymmetry of the Universe

We elucidate a cancellation mechanism for the electric dipole moment of the electron in the general two Higgs doublet model. The impressive improvement by the ACME Collaboration in 2018 suggests the presence of a new electron Yukawa coupling that brings in exquisite cancellations among dangerous diagrams, broadening the solution space for electroweak baryogenesis driven by an extra top Yukawa coupling. The cancellation mechanism calls for the new Yukawa couplings to have hierarchical structures that echo the observed pattern of the Standard Model Yukawa couplings.

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arXiv2019

Scalar leptoquark effects on $B \to μ\barν$ decay

Purely leptonic $B$ meson decays provide unique probes for physics Beyond the Standard Model. We study the impact of a scalar leptoquark, $S_1$, on $B \to μ\barν$ decay. We find that, for $m_{S_1}\sim 1$ TeV, the $S_1$ leptoquark can modify the $B \to μ\barν$ rate significantly. Such a leptoquark can in prinicple also alter the $B \to τ\barν$ rate. However, current searches from LHC and low energy physics provide some constraints on the parameter space.

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arXiv2019

Enhanced $B \to μ\barν$ Decay at Tree Level as Probe of Extra Yukawa Couplings

With no New Physics seen at the LHC, a second Higgs doublet remains attractive and plausible. The ratio ${\cal R}_B^{μ/τ} = {\cal B}(B \to μ\barν)/{\cal B}(B \to τ\barν)$ is predicted at 0.0045 in both the Standard Model and the type II two Higgs doublet model, but it can differ if extra Yukawa couplings exist in Nature, which we deem an experimental issue. Considering recent Belle update on $B \to μ\barν$, we show that in the general two Higgs doublet model, the ratio could be up by a factor of two, which can be probed by the Belle~II experiment with just a few ab$^{-1}$.

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arXiv2019

Implications of Four-Top and Top-Pair Studies on Triple-Top Production

Multi-top quark production is a staple program at the LHC. Single-top and $t\bar t$ productions are studied extensively, while current efforts are zooming in on four-top search, where the Standard Model (SM) cross section is at ${\cal O}(10)$ fb. In contrast, only at the fb level in SM, triple-top production has not been targeted for study so far. But such a small cross section makes it a unique probe for New Physics. Without the usual discrete $Z_2$ symmetry, the general two Higgs doublet model (g2HDM) can naturally raise the triple-top production to pb level. We illustrate how certain signal regions of four-top search can be utilized to constrain triple-top production, but urge a dedicated search. As an aside, we note that the CMS study at 13 TeV of scalar $t\bar t$ resonance interfering with QCD production background indicate some activity at 400 GeV. We comment that this could be explained in principle in g2HDM via the extra top Yukawa coupling.

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arXiv2019

Flavor Changing Heavy Higgs Interactions with Leptons at Hadron Colliders

In a general two Higgs doublet model, we study flavor changing neutral Higgs (FCNH) decays into leptons at hadron colliders, $pp \to ϕ^0 \to τ^\mpμ^\pm +X$, where $ϕ^0$ could be a CP-even scalar ($h^0$, $H^0$) or a CP-odd pseudoscalar ($A^0$). The light Higgs boson $h^0$ is found to resemble closely the Standard Model Higgs boson at the Large Hadron Collider. In the alignment limit of $\cos(β-α) \cong 0$ for $h^0$--$H^0$ mixing, FCNH couplings of $h^0$ are naturally suppressed, but such couplings of the heavier $H^0, A^0$ are sustained by $\sin(β-α) \simeq 1$. We evaluate physics backgrounds from dominant processes with realistic acceptance cuts and tagging efficiencies. We find promising results for $\sqrt{s} = 14$ TeV, which we extend further to $\sqrt{s} = 27$ TeV and 100 TeV future pp colliders.

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arXiv2019

Top-Assisted Di-Higgs boson Production Motivated by Baryogenesis

We study top-assisted di-Higgs production via $cg \to tH \to thh$, where $h$ is the 125 GeV scalar boson, and $H$ is the $CP$-even heavy Higgs. The context is the two Higgs doublet model without a $Z_2$ symmetry, where the extra Yukawa coupling $ρ_{tc}$ generates $tH$ production, with the extra top Yukawa $ρ_{tt} \simeq 0$ to avoid $gg \to H$ constraints. We find that discovery is possible for $m_H$ around 300 GeV or so at the LHC, but would need finite $h$-$H$ mixing angle $\cosγ$ to allow for finite $λ_{Hhh}$ coupling, and $ρ_{tc}$ also needs to be not too small. A sizable $ρ_{tc}$ could drive electroweak baryogenesis, which further motivates the search.

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arXiv2018

Probing for Extra Top Yukawa Couplings in Light of $t\bar th(125)$ Observation

The observation of $t\bar th(125)$ production at the Large Hadron Collider (LHC) is the first direct measurement of the top Yukawa coupling. It opens the window on an extra top Yukawa coupling, $ρ_{tt}$, from a second Higgs doublet, without a $Z_2$ symmetry to forbid flavor changing neutral Higgs couplings. We show that $t\bar th$ and Higgs property measurements at the High Luminosity LHC can constrain the ${\rm Re}\,ρ_{tt}$--${\rm Im}\,ρ_{tt}$ parameter space that could drive electroweak baryogenesis, but the $Γ_h$ width measurement must be considerably improved beyond current projections.

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arXiv2018

Constraining a Lighter Exotic Scalar via Same-sign Top

It was shown recently that, in two Higgs doublet models without $Z_2$ symmetry, extra Yukawa couplings such as $ρ_{tc}$, $ρ_{tt}$ can fuel enough $CP$ violation for electroweak baryogenesis (EWBG). We revisit an old proposal where a pseudoscalar $A^0$ has mass between $t\bar c$ and $t\bar t$ thresholds. With $ρ_{tt}$ small, it evades $gg \to A^0 \to h^0(125)Z$ constraints, where approximate alignment also helps. We find this scenario with relatively light $A^0$ is not yet ruled out, and $cg \to tA^0 \to tt\bar c$ can probe sizable $ρ_{tc}$ at the LHC, giving access to the second mechanism of EWBG provided by such models.

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arXiv2018

Connecting Electroweak Symmetry Breaking and Flavor: A Light Dilaton ${\cal D}$ and a Sequential Heavy Quark Doublet $Q$

The 125 GeV boson is quite consistent with the Higgs boson of the Standard Model (SM), but there is a challenge from Anderson whether this particle is in the Lagrangian. As LHC Run 2 takes its final year of running, we ought to reflect and make sure we have gotten everything right. The ATLAS and CMS combined Run 1 analysis claims 5.4$σ$ measurement of vector boson fusion (VBF) production that is consistent with SM, which seemingly refutes Anderson. However, to verify the source of electroweak symmetry breaking (EWSB), we caution that VBF measurement is too important for us to be imprudent in any way, and gluon-gluon fusion (ggF) with similar tag jets must be simultaneously \emph{measured}, which should be achievable at LHC Run 2. The point is to truly test the dilaton possibility, the pseudo-Goldstone boson of scale invariance violation. We illustrate EWSB by dynamical mass generation of a sequential quark doublet $Q$ via its ultrastrong Yukawa coupling, and argue how this might be consistent with a 125 GeV dilaton, ${\cal D}$. The ultraheavy $2m_Q \gtrsim 4$--5 TeV scale explains the absence of New Physics so far, while the mass generation mechanism shields us from the UV theory for the strong Yukawa coupling. Collider and flavor physics implications are briefly touched upon. Current Run 2 analyses show correlations between the ggF and VBF measurements, but the newly observed $t\bar tH$ production at LHC poses a challenge.

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arXiv2018

Approximate Alignment in Two Higgs Doublet Model with Extra Yukawa Couplings

With discovery of the 125 GeV boson $h^0$, the existence of a second doublet is very plausible. We show that the "alignment" phenomenon, that $h^0$ is found to resemble closely the Standard Model Higgs boson, may correspond to Higgs quartic couplings $η_i$ that are ${\cal O}(1)$ in strength. If the exotic bosons of the second doublet possess extra top Yukawa couplings, which are the least constrained by data, such a two Higgs doublet model could drive electroweak baryogenesis, as well as further "protect" the apparent alignment. The exotic Higgs bosons can be sub-TeV in mass while remaining well hidden so far, with broad parameter space for search at the Large Hadron Collider.

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arXiv2018

Unraveling the couplings of a Drell-Yan produced $Z'$ with heavy-flavor tagging

Despite no new physics so far at the LHC, a $Z'$ boson with $m_{Z'} \sim 100$ GeV could still emerge via Drell-Yan (DY) production, $q \bar q \to Z' \to μ^+ μ^-$, in the next few years. To unravel the nature of the $Z'$ coupling, we utilize the $c$- and $b$-tagging algorithms developed by ATLAS and CMS to investigate $cg \to c Z'$ at 14 TeV LHC. While light-jet contamination can be eliminated, mistagged $b$-jets cannot be rejected in any of the tagging schemes we adopt. On the other hand, for nonzero $bbZ'$ coupling, far superior $b$-tagging could discover the $bg \to b Z'$ process, where again light-jet mistag can be ruled out, but mistagged $c$-jets cannot yet be excluded. Provided that DY production is discovered soon enough, we find that a simultaneous search for $c g \to c Z'$ and $b g \to b Z'$ can conclusively discern the nature of $Z'$ couplings involved.

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arXiv2018

Searching for new scalar bosons via triple-top signature in $cg \to tS^0 \to tt\bar t$

The alignment phenomenon, that the 125 GeV $h^0$ boson so resembles the Standard Model Higgs boson, can be understood in a two Higgs doublet model without discrete symmetry. New Yukawa couplings $ρ_{tt}$ and $ρ_{tc}$ offer new avenues to discover the extra scalar $H^0$ and pseudoscalar $A^0$. We propose to search for $cg \to tH^0$, $tA^0$ followed by $H^0$, $A^0 \to t\bar t$, $t\bar c$, where same-sign dileptons could be the harbinger, with triple-top, in the signature of three leptons plus three $b$-jets, as confirmation. Discovery could touch upon the origin of baryon asymmetry of the Universe.

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