Z′models with less-minimal flavour violation
01 Jan 2019-
TL;DR: In this article, the authors study the phenomenology of simplified Z′ models with a global U(2)3 flavour symmetry in the quark sector, broken solely by the Standard Model Yukawa couplings.
Abstract: We study the phenomenology of simplified Z′ models with a global U(2)3 flavour symmetry in the quark sector, broken solely by the Standard Model Yukawa couplings. This flavour symmetry, known as less-minimal flavour violation, protects ΔF=2 processes from dangerously large new physics (NP) effects, and at the same time provides a free complex phase in b→s transitions, allowing for an explanation of the hints for additional direct CP violation in kaon decays (ϵ′/ϵ) and in hadronic B-decays (B→Kπ puzzle). Furthermore, once the couplings of the Z′ boson to the leptons are included, it is possible to address the intriguing hints for NP (above the 5σ level) in b→sl+l− transitions. Taking into account all flavour observables in a global fit, we find that ϵ′/ϵ, the B→Kπ puzzle and b→sl+l− data can be explained simultaneously. Sizeable CP violation in b→sl+l− observables, in particular A8, is predicted, which can be tested in the near future, and an explanation of the B→Kπ and ϵ′/ϵ puzzles leads to effects in di-jet tails at the LHC, that are not far below the current limits. Once b→sl+l− is included, cancellations in di-muon tails, possibly by a second Z′, are required by LHC data.
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TL;DR: In this paper, the SU(2)L singlet and SU( 2)L triplet were combined with lepton flavor universality (LFU) for semi-leptonic B decays.
Abstract: In recent years, experiments revealed intriguing hints for new physics (NP) in semi-leptonic B decays. Both in charged current processes, involving b → cτν transitions, and in the neutral currents b → sl +l −, a preference for NP compared to the standard model (SM) of more that 3σ and 5σ was found, respectively. In addition, there is the long-standing tension between the theory prediction and the measurement of the anomalous magnetic moment (AMM) of the muon (aμ) of more than 3σ. Since all these observables are related to the violation of lepton flavor universality (LFU), a common NP explanation seems not only plausible but is even desirable. In this context, leptoquarks (LQs) are especially promising since they give tree-level effects in semi-leptonic B decays, but only loop-suppressed effects in other flavor observables that agree well with their SM predictions. Furthermore, LQs can lead to a mt/mμ enhanced effect in aμ, allowing for an explanation even with (multi) TeV particles. However, a single scalar LQ representation cannot provide a common solution to all three anomalies. In this article we therefore consider a model in which we combine two scalar LQs: the SU(2)L singlet and the SU(2)L triplet. Within this model we compute all relevant 1-loop effects and perform a comprehensive phenomenological analysis, pointing out various interesting correlations among the observables. Furthermore, we identify benchmark points which are in fact able to explain all three anomalies (b → cτν, b → sl +l − and aμ), without violating bounds from other observables, and study their predictions for future measurements.
122 citations
TL;DR: In this paper, a global fit to leptophilic Z′ models was performed with the main goal of finding the bounds for the Z′ couplings to lepton. But the results showed that correlations are weak, this changes once additional constraints on the couplings are imposed.
Abstract: New neutral heavy gauge bosons (Z′) are predicted within many extensions of the Standard Model. While in case they couple to quarks the LHC bounds are very stringent, leptophilic Z′ bosons (even with sizable couplings) can be much lighter and therefore lead to interesting quantum effects in precision observables (like (g − 2)μ) and generate flavour violating decays of charged leptons. In particular, $$ \mathrm{\ell}\to \mathrm{\ell}^{\prime }v\overline{v} $$
decays, anomalous magnetic moments of charged leptons, l → l′γ and l → 3l′ decays place stringent limits on leptophilic Z′ bosons. Furthermore, in case of mixing Z′ with the SM Z, Z pole observables are affected. In light of these many observables we perform a global fit to leptophilic Z′ models with the main goal of finding the bounds for the Z′ couplings to leptons. To this end we consider a number of scenarios for these couplings. While in generic scenarios correlations are weak, this changes once additional constraints on the couplings are imposed. In particular, if one considers an Lμ − Lτ symmetry broken only by left-handed rotations, or considers the case of τ − μ couplings only. In the latter setup, on can explain the (g − 2)μ anomaly and the hint for lepton flavour universality violation in $$ \tau \to \mu v\overline{v}/\tau \to ev\overline{v} $$
without violating bounds from electroweak precision observables.
53 citations
TL;DR: In this article, a considerably improved analysis of model-independent bounds on new physics effects in non-leptonic tree-level decays of B-mesons is presented, showing that contributions of about ± 0.1 to the Wilson coefficient of the colour-singlet operator Q2 of the effective weak Hamiltonian can currently not be excluded at the 90% C.L.
Abstract: We present a considerably improved analysis of model-independent bounds on new physics effects in non-leptonic tree-level decays of B-mesons. Our main finding is that contributions of about ±0.1 to the Wilson coefficient of the colour-singlet operator Q2 of the effective weak Hamiltonian and contributions in the range of ±0.5 (both for real and imaginary part) to Q1 can currently not be excluded at the 90% C.L. . Effects of such a size can modify the direct experimental extraction of the CKM angle γ by up to 10° and they could lead to an enhancement of the decay rate difference ∆Γd of up to a factor of 5 over its SM value — a size that could explain the D0 dimuon asymmetry. Future more precise measurements of the semi-leptonic asymmetries $$ {a}_{sl}^q $$
and the lifetime ratio τ (Bs)/τ (Bd) will allow to shrink the bounds on tree-level new physics effects considerably. Due to significant improvements in the precision of the non-perturbative input we update all SM predictions for the mixing obervables in the course of this analysis, obtaining: ∆Ms = (18.77 ± 0.86) ps−1, ∆Md = (0.543 ± 0.029) ps−1, ∆Γs = (9.1 ± 1.3) · 10−2 ps−1, ∆Γd = (2.6 ± 0.4) · 10−3 ps−1, $$ {a}_{sl}^q $$
= (2.06 ± 0.18) · 10−5 and $$ {a}_{sl}^d $$
= (−4.73 ± 0.42) · 10−4.
51 citations
TL;DR: In this article, the authors consider two qualitatively different physics cases: axion-like-particles (ALP) which couple to gluons or electroweak gauge bosons, and a model based on an approximate strange flavor symmetry that leads to a strong violation of the Grossman-Nir bound.
Abstract: The two kaon factories, KOTO and NA62, are at the cutting edge of the intensity frontier, with an unprecedented numbers of long lived and charged Kaons, ∼ 1013, being measured and analyzed. These experiments have currently a unique opportunity to search for dark sectors. In this paper, we demonstrate that searches done at KOTO and NA62 are complementary, both probing uncharted territories. We consider two qualitatively different physics cases. In the first, we analyze models of axion-like-particles (ALP) which couple to gluons or electroweak gauge bosons. In the second, we introduce a model based on an approximate strange flavor symmetry that leads to a strong violation of the Grossman-Nir bound. For the first scenario, we design a new search strategy for the KOTO experiment, KL → π0a → 4γ. Its expected sensitivity on the branching ratio is at the level of 10−9. This demonstrates the great potential of KOTO as a discovery machine. In addition, we revisit other bounds on ALPs from Kaon factories, highlighting the main sources of theoretical uncertainty, and collider experiments, and show new projections. For the second scenario, we show that the model may be compatible with the preliminary analysis of the KOTO-data that shows a hint for New Physics.
39 citations
TL;DR: In this paper, an anomalous U(1)X gauge symmetry was proposed, where X = Y3 + a(Lμ − Lτ)/6, and the heavy Z′ boson associated with spontaneously breaking U( 1)X at the TeV scale mediates the b → sll transitions, and a desire to preserve the accidental symmetries of the Standard Model (SM) was expressed.
Abstract: Motivated by the intriguing discrepancies in b → sll transitions, the fermion mass problem, and a desire to preserve the accidental symmetries of the Standard Model (SM), we extend the SM by an anomalous U(1)X gauge symmetry where X = Y3 + a(Lμ − Lτ)/6. The heavy Z′ boson associated with spontaneously breaking U(1)X at the TeV scale mediates the b → sll anomalies via $$ {\mathcal{O}}_9^{\mu}\sim \frac{1}{\Lambda^2}\left(\overline{s}{\gamma}_{\rho }{P}_Lb\right)\left(\overline{\mu}{\gamma}^{\rho}\mu \right) $$
. We show that this model, which features mixed gauge anomalies involving U(1)X and hypercharge, can be made anomaly-free for any a ∈ ℤ by integrating in a pair of charged fermions whose masses naturally reside somewhere between 1 and 30 TeV. The gauge symmetry permits only the third family Yukawas at the renormalisable level, and so the light quark masses and mixings are controlled by accidental U(2)3 flavour symmetries which we assume are minimally broken alongside U(1)X. The lepton sector is not governed by U(2) symmetries, but rather one expects a nearly diagonal charged lepton Yukawa with me,μ « mτ. The model does not explain the hierarchy me « mμ, but it does possess high quality lepton flavour symmetries that are robust to the heavy physics responsible for generating me,μ. We establish the viability of these models by checking agreement with the most important experimental constraints. We comment on how the model could also explain neutrino masses and the muon g − 2.
35 citations