Search

We suggest a resonant $c \bar b \to H^+$ production search, followed by bosonic $H^+ \to W^+ H$ weak decay at the Large Hadron Collider (LHC). In the general two Higgs doublet model (G2HDM) that has flavor-changing neutral Higgs couplings, $H^+$ is resonantly produced via the top-charm $\rho_{tc} V_{tb}$ coupling at tree level, while $H^+ \to W^+ H$ weak decay occurs within the exotic second doublet, leading eventually to same sign dilepton signals. We perform a signal-to-background analysis at the 14 TeV LHC and show that discovery seems possible with LHC Run 2 data already at hand., Comment: 5 pages, 3 figures; v2: title, abstract and introduction updated

Gauge theories with matter fields in various representations play an important role in different branches of physics. Recently, it was proposed that several aspects of the interesting pseudogap phase of cuprate superconductors near optimal doping may be explained by an emergent $SU(2)$ gauge symmetry. Around the transition with positive hole-doping, one can construct a $(2+1)-$dimensional $SU(2)$ gauge theory coupled to four adjoint scalar fields which gives rise to a rich phase diagram with a myriad of phases having different broken symmetries. We study the phase diagram of this model on the Euclidean lattice using the Hybrid Monte Carlo algorithm. We find the existence of multiple broken phases as predicted by previous mean field studies. Depending on the quartic couplings, the $SU(2)$ gauge symmetry is broken down either to $U(1)$ or $\mathbb{Z}_2$ in the perturbative description of the model. We further study the confinement-deconfinement transition in this theory, and find that both the broken phases are deconfining in the range of volumes that we studied. However, there exists a marked difference in the behavior of the Polyakov loop between the two phases., Comment: 11 pages, 11 figures, minor changes to results and conclusions

We study the discovery prospects for a charged Higgs boson via the $b g\to c H^- \to c \bar t b$ process at the Large Hadron Collider (LHC). Focusing on the general Two Higgs Doublet Model (G2HDM) that possesses extra Yukawa couplings, the process is controlled by extra top couplings $\rho_{tc}$ and $\rho_{tt}$, which can drive electroweak baryogenesis (EWBG) to account for the baryon asymmetry of the Universe (BAU). We propose benchmark points (BPs) and demonstrate that evidence could emerge at 14 TeV LHC and luminosity of 300 fb$^{-1}$, with discovery potential at 600 fb$^{-1}$., Comment: v2: 5 pages, 5 figures, matches version accepted for publication in PRD

Baryon asymmetry of the Universe offers one of the strongest hints for physics Beyond the Standard Model (BSM). Remarkably, in the general two Higgs Doublet Model (g2HDM) that possesses a second set of Yukawa matrices, one can have electroweak baryogenesis (EWBG) while the electron electric dipole moment (eEDM) is evaded by a natural flavor tuning that echoes SM. We show that eEDM may first emerge around $10^{-30}\,e$ cm or so, followed by neutron EDM (nEDM) down to $10^{-27}\,e$ cm. We illustrate a cancellation mechanism for nEDM itself, which in turn can be probed when a facility capable of pushing down to $10^{-28}\,e$ cm becomes available., Comment: 4 pages, 4 labeled figures

With the Large Hadron Collider (LHC) running, we should probe electroweak baryogenesis (EWBG) while probing $CP$ violation (CPV) with electron electric dipole moment (eEDM). Rooted in the flavor structure of the Standard Model (SM), the general two Higgs doublet model (g2HDM) with a second set of Yukawa couplings can deliver EWBG while surviving eEDM. We point out a chiral-enhanced top-bottom interference effect that makes $b \to s\gamma$ decay an exquisite window on EWBG and eEDM, and illustrate the importance of the $\Delta A_{CP}$ observable at Belle II., Comment: 5 pages, 3 captioned figures, 1 table, v3: added results for inclusive CPV difference $\Delta A_{CP}$

We study lepton flavor violating (LFV) $B$ decays in a general two Higgs doublet model with sub-TeV exotic scalars. Two different parameter spaces are explored: one dominated by extra top Yukawa coupling $\rho_{tt}$, the other by LFV couplings relevant for the muon $g-2$ anomaly. In the first case, flavor constraints such as $\ell \to \ell^\prime\gamma$, $h \to \ell\ell^\prime$ imply LFV $B$ decays are far below experimental sensitivities. The second case needs to be close to the alignment limit, but $B_q \to \tau\mu$ and $B \to (K, \pi) \tau\mu$ rates can lie within the sensitivities of Belle II and LHCb Upgrade II. Neutral $B$ meson mixings and $B,\, K,\, \pi \to \ell \nu$ decays provide important flavor constraints on parameter space. $B$ decays involving $e$-$\tau$ violation are constrained by $\mu\to e$ processes., Comment: 11 pages, 6 captioned figures, added a discussion of collider signatures in section IV and included a new appendix; matches published version

In the general two Higgs doublet model (g2HDM) that has extra Yukawa couplings, we analyze their New Physics (NP) contributions to kaon mixing parameter $\varepsilon_K$, direct CP violation parameter $\varepsilon^\prime/\varepsilon$, and rare $K^+ \to \pi^+\nu\bar\nu$, $K_L \to \pi^0\nu\bar\nu$, and $K_{L, S} \to \mu^+ \mu^-$ decays. We study correlations between these observables, and call special attention to a unique complementarity of kaon mixing and rare $K\to \pi \nu\bar\nu$ decays in probing the exotic Higgs mass spectrum of g2HDM. The importance of kaon physics in probing NP vis-\`a-vis B physics is stressed. One unexpected feature we uncover is the special sensitivity of $K^+ \to \pi^+\nu\bar\nu$ to TeV scale charged Higgs boson., Comment: 26 pages, 10 captioned figures; v2: fixed typos; v3: added a brief discussion in section 5 about NP effects of down-type flavor violation. Accepted for publication in JHEP

We extend heavy Higgs searches at the Large Hadron Collider for $H \to \tau \mu$ by CMS, and $H \to \tau\tau$ by ATLAS and CMS, to study discovery prospects of extra Higgs states in $pp \to H,A \to \tau \mu, \tau\tau$ with $e\mu + E_T$ and $j_{\tau} \mu + E_T$ final states, where $j_{\tau} = \pi \, , \rho\, , a_1$ and $E_T$ is missing transverse energy. In a general two Higgs doublet model without $Z_2$ symmetry, extra Yukawa couplings $\rho_{\tau\tau}$ and $\rho_{\tau\mu}$ can drive $H,A \to \tau\tau$ and $\tau\mu$ channels at hadron colliders, following gluon-gluon fusion production with extra $\rho_{tt}$ couplings. The light Higgs boson $h(125)$ is found to resemble closely the Standard Model Higgs boson; in the alignment limit of $\cos\gamma \to 0$ for $h$--$H$ mixing, flavor changing neutral Higgs couplings such as $h \to \tau\mu$ are naturally suppressed, but the couplings of the heavier $H$ is optimized by $\sin \gamma \to 1$. We define various signal regions for $H,A \to \tau\mu$ and $\tau \tau$ and evaluate physics backgrounds from dominant processes with realistic acceptance cuts and tagging efficiencies. Two different studies are presented. We first perform a parton level study without any hadronization and with minimal detector smearing. We then include hadronization using PYTHIA 8.2 and fast detector simulation using DELPHES to give event level simulation. Results for $\sqrt{s}= 13$ TeV appear promising, which we extend further to $\sqrt{s} = 14$ TeV for the High Luminosity LHC., Comment: 20 pages, 11 figures, Version to appear in the European Physical Journal C

We study a search for the charged Higgs boson via $e^+e^- \to H^+H^- \to c\bar{b}\bar{c}b$ at the 500 GeV ILC. In a general two Higgs doublet model without $Z_2$ symmetry, extra Yukawa couplings $\rho_{tt}$ and $\rho_{tc}$ can drive baryogenesis, but searches at the HL-LHC may still go empty-handed if the couplings are relatively weak. Taking $m_{H^+ } \simeq m_H \simeq m_A \simeq 200$ GeV, with $\rho_{tt}$, $\rho_{tc}\sim 0.1$ and no $h(125)$-$H$ mixing, $H^+ \to c\bar b$ decay is dominant, and the $c\bar{b}\bar{c}b$ final state is likely overwhelmed by QCD background at the LHC. We show that the electroweak production of $H^+ H^-$ at the ILC is discoverable with integrated luminosity of 1 ab$^{-1}$. Furthermore, we show that $m_{H^+}$ can be extracted by requiring the two pairs of $b$ and light jets be roughly equal in mass, without assuming the mass value. Thus, ILC can probe low mass Higgs bosons in multijet final states to complement HL-LHC in the future, Comment: 15 pages, 6 figures

In a two Higgs doublet model with extra Yukawa couplings, we assess the new physics contributions to charged lepton electric dipole moment. We focus especially on muon (and tau) EDM where in the coming decade several experiments $-$ Muon g-2, J-PARC and PSI (and Belle II) $-$ will push sensitivities down by several orders of magnitude. With the working assumption that extra Yukawa couplings are analogous to SM ones in strength and taking exotic scalar masses in sub-TeV range, we find that $\mu$ and $\tau$ EDM can be enhanced to values larger than new physics scenarios that scale with lepton mass. The main effect comes from the flavor-conserving extra top coupling $\rho_{tt}$ via well-known two-loop diagrams. Deviating from our working assumption, if the muon $g-2$ anomaly arises from the one-loop diagram, driven by singly enhanced lepton flavor violating $\rho_{\tau\mu}$ coupling, it can also induce rather large muon EDM, accessible at upcoming experiments., Comment: 9 pages, 5 captioned figures; matches journal version

The LHC has not discovered any New Physics beyond the anticipated $h(125)$ boson, and new ideas abound for out-of-the-box searches, or Effective Field Theories with multi-TeV cutoff. But, have we exhausted dimension-4 operators involving sub-TeV particles that are {\it not} exotic (non-XLP)? We advocate the existence of an extra Higgs doublet that carry extra Weinberg couplings, where {\it emergent} mass-mixing hierarchies {\it and} alignment have hidden their effects very well so far. ${\cal O}(1)$ Higgs quartics can induce first order electroweak phase transition and imply sub-TeV $H, A, H^\pm$ spectrum. The extra Weinberg couplings, led by $\rho_{tt}$ and $\rho_{tc}$ at ${\cal O}(1)$, can drive electroweak baryogenesis, while $|\rho_{ee}/\rho_{tt}| \propto \lambda_e/\lambda_t$, the ratio of usual electron and top Weinberg couplings, can tame electron EDM. Finding these extra Higgs bosons via $cg \to tH/A \to tt\bar c$, $tt\bar t$ and $cg \to bH^+ \to bt\bar b$ processes (and those allowed by Higgs boson splittings) at the LHC, and pushing the flavor frontier to break the flavor code, would usher in a new Higgs/flavor era. A new scale at 10--20 TeV, possibly related to the Landau pole of the scalar sector, may emerge., Comment: 18 pages, 9 figures, invited review to be submitted to Chinese Journal of Physics

The recent confirmation of the muon $g-2$ anomaly by the Fermilab g-2 experiment may harbinger a new era in $\mu$ and $\tau$ physics. In the context of general two Higgs doublet model, the discrepancy can be explained via one-loop exchange of sub-TeV exotic scalar and pseudoscalars, namely $H$ and $A$, that have flavor changing neutral couplings $\rho_{\tau\mu}$ and $\rho_{\mu\tau}$ at $\sim 20$ times the usual tau Yukawa coupling, $\lambda_\tau$. Taking $\rho_{\ell\ell^\prime}\sim \lambda_{ \rm min(\ell, \ell^\prime)}$, we show that the above solution to muon $g-2$ then predicts enhanced rates of various charged lepton flavor violating processes, which should be accessible at upcoming experiments. We cover muon related processes such as $\mu \to e \gamma$, $\mu \to eee$ and $\mu N \to e N$, and $\tau$ decays $\tau \to \mu \gamma$ and $\tau \to \mu\mu\mu$. A similar one-loop diagram with $\rho_{e\tau}= \rho_{\tau e} = {\cal O}(\lambda_e)$ induces $\mu \to e\gamma$, bringing the rate right into the sensitivity of the MEG~II experiment. The $\mu e\gamma$ dipole can be probed further by $\mu \to 3e$ and $\mu N \to e N$. With its promised sensitivity range and ability to use different nuclei, the $\mu N \to eN$ conversion experiments can not only make discovery, but access the extra diagonal quark Yukawa couplings $\rho_{qq}$. For the $\tau$ lepton, we find that $\tau \to \mu\gamma$ would probe $\rho_{\tau\tau}$ down to $\lambda_\tau$ or lower, while $\tau \to 3\mu$ would probe $\rho_{\mu\mu}$ to ${\cal O}(\lambda_{\mu})$., Comment: 8 pages, 3 figures, v2: matches journal version; v3: includes additional references related to muon g-2 SM prediction

Recent progress on muon $g-2$ measurement prompts one to take it even more seriously. In the general two Higgs doublet model that allows extra Yukawa couplings, we take a simplified approach of single enhanced coupling. We fix the charged lepton flavor violating coupling, $\rho_{\tau\mu} = \rho_{\mu\tau}$, via the one-loop mechanism, for illustrative masses of the heavy scalar $H$ and pseudoscalar $A$, where we assume $m_A = m_{H^+}$. Since extra top Yukawa couplings are plausibly the largest, we turn on $\rho_{tt}$ and find that LHC search for $gg \to H,\,A \to \tau\mu$ gives more stringent bound than from $\tau\to \mu\gamma$ with two-loop mechanism. Turning on a second extra top Yukawa coupling, $\rho_{tc}$, can loosen the bound on $\rho_{tt}$, but LHC constraints can again be more stringent than from $B \to D\mu\nu$ vs $De\nu$ universality. This means that evidence for $H,\,A \to \tau\mu$ may yet emerge with full LHC Run 2 data, while direct search for $\tau^\pm\mu^\mp bW^+$ or $t\bar cbW^+$ (plus conjugate) may also bear fruit., Comment: 7 pages, 5 captioned figures; accepted in PRD

We study the prospect for discovering the $cg\to bH^+\to b A W^+$ process at the LHC. Induced by the top-flavor changing neutral Higgs coupling $\rho_{tc}$, the process may emerge if $m_{H^+} > m_A + m_{W^+}$, where $H^+$ and $A$ are charged and $CP$-odd Higgs bosons in the general two Higgs Doublet Model (g2HDM). We show that the $cg\to bH^+\to b A W^+$ process can be discovered at LHC Run 3, while the full Run 2 data at hand can constrain the parameter space significantly by searching for the same-sign dilepton final state. The process has unique implications on the hint of $gg\to A \to t \bar t$ excess at $m_A\approx 400$ GeV reported by CMS. When combined with other existing constraints, the $cg\to bH^+\to b A W^+$ process can essentially rule out the g2HDM explanation of such an excess., Comment: matches the published version

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 $\cos\gamma \to 0$, we briefly review the collider probes of the top changing $tcH/tcA$ coupling $\rho_{tc}$ of the exotic $CP$-even/odd Higgs bosons $H/A$. Together with an extra top conserving $ttH/ttA$ coupling $\rho_{tt}$, 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 $\cos\gamma$ being nonzero, comment briefly on $gg \to H/A \to t\bar t, t\bar c$ resonant production, and touch upon the $\rho_{tu}$ coupling., Comment: 25 pages, 10 figures, brief review to appear in MPLA

Electroweak baryogenesis in a two-Higgs doublet model is a well-motivated and testable scenario for physics beyond the Standard Model. An attractive way of providing $CP$ violation is through flavor-changing Higgs couplings, where the top-charm coupling is hardly constrained. This minimal scenario can be tested by searching for heavy charged and neutral Higgs bosons at the LHC. While the charged Higgs signature requires a dedicated analysis, the neutral Higgs signature will be covered by a general search for same-sign top pairs. Together, they provide a conclusive test of this kind of baryogenesis., Comment: discussions, new references and figure added, mild modifications made to title and abstract

The new round of experiments, MEG II, COMET/Mu2e, and Mu3e, would soon start to push the $\mu \to e\gamma$, $\mu N \to eN$ conversion, and $\mu \to 3e$ frontier, while Belle II would probe $\tau \to \mu\gamma$ and $\tau \to 3\mu$. 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 $\rho_{\mu e},\, \rho_{ee} \lesssim \lambda_e$, while $\rho_{\tau\mu},\, \rho_{\tau\tau} \lesssim \lambda_\tau$ and $\rho_{tt} \lesssim \lambda_t$, with $\lambda_i$ the usual Yukawa coupling of the Standard Model (SM), where $\rho_{\mu e}\rho_{tt}$ and $\rho_{\tau\mu}\rho_{tt}$ enter the $\mu e\gamma$ and $\tau\mu\gamma$ two loop amplitudes, respectively. With the $B_s \to \mu\mu$ decay rate basically consistent with SM expectation, together with the $B_s$ mixing constraint, we show that $B_s \to \tau\tau$ would also be consistent with SM, while $B_s \to \tau\mu$ and $B \to K\tau\mu$ decays would be out of reach of projected sensitivities, in strong contrast with some models motivated by the B anomalies., Comment: 11 pages, 3 figures. Added new references and fixed typos to conform to the published version

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 $\rho_{tu}$ 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 $\rho_{tu}$ 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 $\rho_{tc}$ and $\rho_{tt}$, dilute the sensitivity of the $\rho_{tu}$ probe are discussed., Comment: 6 pages, 6 figures

We explore the interplay between $h(125) \to \tau\mu$ search at the LHC and $\tau \to \mu\gamma$ at the up and coming Belle~II experiment, in context of the general two Higgs doublet model with extra Yukawa couplings such as $\rho_{\tau\mu}$. The search for $h \to \tau\mu$ constrains $\rho_{\tau\mu} \cos\gamma$, where $\cos\gamma$ is the $h$--$H$ mixing angle of $h$ with the exotic $CP$-even scalar $H$. For $\tau \to \mu\gamma$, we define the "BSM-benchmark" by setting the extra top Yukawa coupling $\rho_{tt} = 1\ (\cong \lambda_t)$ in two-loop diagrams, and $\cos\gamma = 0$ 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 $\rho_{\tau\mu} = 0.7\lambda_\tau$. We define the subleading "$h$-benchmark" by setting $\rho_{tt} = 0$ in two-loop diagrams, and take the conservative maximal value of $\cos\gamma = 0.2$. We show that it falls beyond Belle~II reach, but can interfere with the BSM-benchmark effect, which in principle probes the phase of $\rho_{tt}$. We further show that the one-loop $H$, $A$ effect, proportional to $\rho_{\tau\mu}\rho_{\tau\tau}$ in amplitude, is beyond the sensitivity of Belle~II to probe, even for $\rho_{\tau\tau}$ as large as $3\lambda_\tau$. With the working assumption that $\rho_{32}^f, \rho_{33}^f = {\cal O}(\lambda_3^f)$ for all charged fermions $f$, we find good discovery potential for both $\tau$ lepton flavor violation searches in the coming decade., Comment: 7 pages, 3 figures

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 $\rho_{tc}$ and $\rho_{tt}$ can drive the disappearance of antimatter from the Universe, while $\bar cbH^+$ and $\bar tbH^+$ couple with strength $\rho_{tc}V_{tb}$ and $\rho_{tt}V_{tb}$, respectively. For $\rho_{tc},\, \rho_{tt} \sim 0.5$ 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., Comment: matches the published version

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 $\rho_{tc}$. 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., Comment: matches the published version

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., Comment: 5 pages, 3 figures; v2: some clarifications and references added, version accepted for publication as a Rapid Communication in PRD

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 $B \to \mu \bar\nu$ decay. We find that, for $m_{S_1}\sim 1$ TeV, the $S_1$ leptoquark can modify the $B \to \mu \bar\nu$ rate significantly. Such a leptoquark can in prinicple also alter the $B \to \tau \bar\nu$ rate. However, current searches from LHC and low energy physics provide some constraints on the parameter space., Comment: matches the published version

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., Comment: matches the published version

With no New Physics seen at the LHC, a second Higgs doublet remains attractive and plausible. The ratio ${\cal R}_B^{\mu/\tau} = {\cal B}(B \to \mu \bar\nu)/{\cal B}(B \to \tau \bar\nu)$ 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 $B \to \mu\bar\nu$, 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$^{-1}$., Comment: matches the published version

In a general two Higgs doublet model, we study flavor changing neutral Higgs (FCNH) decays into leptons at hadron colliders, $pp \to \phi^0 \to \tau^\mp\mu^\pm +X$, where $\phi^0$ 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 $\cos(\beta-\alpha) \cong 0$ 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 $\sin(\beta-\alpha) \simeq 1$. 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., Comment: Version to appear in Physics Letters B

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 $\rho_{tc}$ generates $tH$ production, with the extra top Yukawa $\rho_{tt} \simeq 0$ 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 $\cos\gamma$ to allow for finite $\lambda_{Hhh}$ coupling, and $\rho_{tc}$ also needs to be not too small. A sizable $\rho_{tc}$ could drive electroweak baryogenesis, which further motivates the search., Comment: matches with published version

It was shown recently that, in two Higgs doublet models without $Z_2$ symmetry, extra Yukawa couplings such as $\rho_{tc}$, $\rho_{tt}$ 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 $\rho_{tt}$ 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 $\rho_{tc}$ at the LHC, giving access to the second mechanism of EWBG provided by such models., Comment: matches with the published version

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, $\rho_{tt}$, 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 ${\rm Re}\,\rho_{tt}$--${\rm Im}\,\rho_{tt}$ parameter space that could drive electroweak baryogenesis, but the $\Gamma_h$ width measurement must be considerably improved beyond current projections., Comment: matches with the published version

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, $q \bar q \to Z' \to \mu^+ \mu^-$, 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., Comment: matches with published version

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 $\rho_{tt}$ and $\rho_{tc}$ 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., Comment: references added, minor modifications, matched with the published version

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 $\phi_s$ of $B_s$ mixing and $B_s \to\mu^+\mu^-$ 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 $B^0 \to K^{*0}\mu^+\mu^-$ decay and $R_{K^{(*)}}$ anomaly in $B \to K^{(*)}\mu^+\mu^-$ to $B \to K^{(*)}e^+e^-$ rate ratios, and the BaBar anomaly in $B \to D^{(*)}\tau\nu$ 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 $K \to \pi\nu\nu$ decays and $\varepsilon'/\varepsilon$ in the kaon sector, $\mu \to e$ transitions, muon $g-2$ and electric dipole moments of the neutron and electron, $\tau \to \mu\gamma,\ \mu\mu\mu,\ eee$, and a few charm physics probes, offer broadband frontier windows on New Physics. (See the document for the full abstract.), Comment: 71 pages, 19 figures, to be published in Progress in Particle and Nuclear Physics

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 $\eta_i$ 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., Comment: Final version corresponding to acceptance by Euro. Phys. Lett

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 $\rho_{tt}$ all of $\mathcal{O}$(1), one naturally has sizable $CP$ violation that fuels the cosmic baryon asymmetry. Even if $\rho_{tt}$ vanishes, the favor-changing coupling $\rho_{tc}$ can still lead to successful baryogenesis. Phenomenological consequences such as $t\to ch$, $\tau \to \mu\gamma$, electron electric dipole moment, $h\to\gamma\gamma$, and $hhh$ coupling are discussed., Comment: 5 pages, 3 figures; v2: updated to match version published in PLB

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 $\rho_{tt}$. Precision measurements at future colliders can explore the parameter space, and can give stronger bound on $\rho_{tt}$ than the current bound from flavor experiments. As a side result, we find that if $\rho_{tt}\cos\gamma < 0$, where $\cos\gamma$ is the exotic Higgs component of $h$, the $\rho_{tt}$-induced top loop contribution cancels against bosonic loop contributions, and one may have alignment without decoupling, namely $\sin(-\gamma) \simeq 1$, but exotic scalar bosons could have masses at several hundred GeV., Comment: 13 pages, 5 figures

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 $L_\mu - L_\tau$ 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 $Z'\to\mu^+\mu^-$. 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 $L_\mu - L_\tau$ model. In connection with $tcZ'$, the model also implies the existence of a flavor-conserving $ccZ'$ coupling, which can drive the $c \bar c \to Z' \to \mu^+\mu^-$ 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$^{-1}$., Comment: 14 pages, 18 figures, explanations added, matched with the published version

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$\sigma$ 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., Comment: 8 pages, 3 figures, extensive update of paper with current Run 2 data, version corresponding to the one published in \emph{Symmetry}

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., Comment: 18 pages, 12 figures; accepted by PRD, references updated

A general two Higgs doublet model (2HDM) is adopted to study the signature of flavor changing neutral Higgs (FCNH) decay $\phi^0 \to t\bar{c}+\bar{t}c$, where $\phi^0$ 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 $\cos(\beta-\alpha)$, while the off-diagonal couplings of heavier neutral scalars $\phi^0$ are sustained by $\sin(\beta-\alpha) \sim 1$. 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., Comment: 14 pages, 5 figures, to be published in Phys. Lett. B

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^-)., Comment: 5 pages, 3 figures, title changed, version to appear in PRL

The long-awaited $B_s\to \mu^+\mu^-$ mode has finally been observed at rate consistent with Standard Model, albeit lower by 1.2$\sigma$. There is some hint for New Physics in the rarer $B_d^0 \to \mu^+\mu^-$ decay, especially if the currently 2.2$\sigma$-enhanced central value persists with more data. The measurement of $CP$ violating phase $\phi_s$, via both $B_s\to J/\psi K\bar K$ and $J/\psi\pi\pi$ modes, has reached Standard Model sensitivity. These measurements stand major improvement when LHC enters Run 2. Concurrently, the $K_L\to\pi^0\nu\bar\nu$ and $K^+\to\pi^+\nu\bar\nu$ 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 $B_d^0 \to \mu^+\mu^-$ and $K_L\to\pi^0\nu\bar\nu$., Comment: 9 pages, 5 figures for V1; for V2 title modified, minor changes performed to figures, part of contents revised to 7 pages, note added, one of the authors' affiliation changed, accepted for the publication of PLB

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., Comment: 13 pages, 9 figures; version accepted for publication in PRD; references added

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 \gamma\gamma signal 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^*, \gamma\gamma, WW^* and b\bar{b}, perhaps even \tau^+\tau^- 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., Comment: 5 pages, 3 figures, version to appear in PLB

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., Comment: 6 pages, 9 figures; title changed, references added and updated, other minor improvements, to appear in PRD

We present search prospects and phenomenology of doubly resonant signals that come from the decay of a neutral weak-singlet color-octet vector state \omega_8 into a lighter weak-triplet color-octet scalar \pi_8, which can arise in several theories beyond the Standard Model. Taking m_{\omega_8}-m_{\pi_8}>m_W, we demonstrate an analysis of the signals pp \to \omega_8 \to \pi^\pm_8 W^\mp (\pi^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., Comment: 15 pages, 8 figures; references updated