Approximate Flavor Symmetries Research Articles

Leading directions in the SMEFT

Short-distance new physics at (or slightly) above the TeV scale should not excessively violate the approximate flavor symmetries of the SM in order to comply with stringent constraints from flavor-changing neutral currents. In this respect, flavor symmetries provide an effective organizing principle for the vast parameter space of the SMEFT. In this work, we classify all possible irreducible representations under U(3)5 flavor symmetry of new heavy spin-0, 1/2, and 1 fields which integrate out to dimension-6 operators at the tree level. For a general perturbative UV model, the resulting flavor-symmetric interactions are very restrictive and, in most cases, predict a single Hermitian operator with a definite sign. These leading directions in the SMEFT space deserve particular attention. We derive an extensive set of present experimental constraints by utilizing the existing global SMEFT fits, which incorporate data from top quark, Higgs boson, and electroweak measurements, along with constraints on dilepton and 4-lepton contact interactions. The derived set of bounds comprehensively summarises the present knowledge from indirect searches of flavor-blind new physics mediators.

Exothermic dark mesons in light of electron recoil excess at XENON1T

We consider a novel mechanism to realize exothermic dark matter with dark mesons in the limit of approximate flavor symmetry in a dark QCD. We introduce a local dark U(1)′ symmetry to communicate between dark mesons and the Standard Model via Z′ portal by partially gauging the dark flavor symmetry with flavor-dependent charges for cancelling chiral anomalies in the dark sector. After the dark local U(1)′ is broken spontaneously by the VEV of a dark Higgs, there appear small mass splittings between dark quarks, consequently, leading to small split masses for dark mesons, required to explain the electron recoil excess in XENON1T by the inelastic scattering between dark mesons and electron. We propose a concrete benchmark model for split dark mesons based on SU(3)L× SU(3)R/SU(3)V flavor symmetry and SU(Nc) color group and show that there exists a parameter space making a better fit to the XENON1T data with two correlated peaks from exothermic processes and satisfying the correct relic density, current experimental and theoretical constraints.

KOTO vs. NA62 dark scalar searches

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.

Exploring the flavour structure of the high-scale MSSM

We analyse the sensitivity of quark flavour-changing observables to the MSSM, in a regime of heavy superpartners. We analyse four distinct and motivated frameworks characterising the structure of the soft-breaking terms by means of approximate flavour symmetries. We show that a set of six low-energy observables with realistic chances of improvement in the near future, namely Delta M_{s,d}, epsilon _K, epsilon _K'/epsilon _K, mathcal {B} (Krightarrow pi nu {{bar{nu }}}), and the phase of D–{bar{D}} mixing, could play a very important role in characterising these frameworks for superpartner masses up to mathcal {O}(100) TeV. We show that these observables remain very interesting even in a long-term perspective, i.e. even taking into account the direct mass reach of the most ambitious future high-energy colliders.

New physics from high energy tops

Precision measurements of high energy top quarks at the LHC constitute a powerful probe of new physics. We study the effect of four fermion operators involving two tops and two light quarks on the high energy tail of the toverline{t} invariant mass distribution. We use existing measurements at a center of mass energy of 13 TeV, and state of the art calculations of the Standard Model contribution, to derive bounds on the coefficients of these operators. We estimate the projected reach of the LHC at higher luminosities and discuss the validity of these limits within the Effective Field Theory description. We find that current measurements constrain the mass scale of these operators to be larger than about 1–2 TeV, while we project that future LHC data will be sensitive to mass scales of about 3–4 TeV. We apply our bounds to constrain composite Higgs models with partial compositeness and models with approximate flavor symmetries. We find our limits to be most relevant to flavor non-universal models with a moderately large coupling of the heavy new physics states to third generation quarks.

Quark-flavor symmetries and their violation in quantum chromodynamics

In Quantum Chromodynamics, the hadrons consisting of light (u, d, s) and heavy (c, b) quarks are subject to approximate flavor symmetries, providing the basis for powerful effective theories. I will briefly overview the origin of these symmetries and the scale of their violation. The current precision tests of Standard Model in the electroweak decays of hadrons demand an accurate quantitative account of flavor-symmetry violation effects. I will discuss the continuum (non-lattice) QCD calculation of these effects in hadronic matrix elements, taking as an example the decay constants of heavy–light hadrons.

Erratum: sin θ 13 and neutrino mass matrix with an approximate flavor symmetry

For a neutrino mass matrix whose texture has an approximate flavor symmetry and where one has near degenerate neutrino mass, it is shown that the tribimaximal values for atmospheric angle $\sin^{2}{\theta_{23}}=1/2$ and solar angle $\sin^{2}{\theta_{12}}=1/3$ can be maintained even when the reactor angle $\theta_{13}\neq0$. The non zero $\sin\theta_{13}$ implies approximate $\nu_{\mu}\rightarrow-\nu_{\tau}$ symmetry instead of $\nu_{\mu}\rightarrow\nu_{\tau}$ symmetry.

Sin θ 13 and neutrino mass matrix with an approximate flavor symmetry

For a neutrino mass matrix whose texture has an approximate flavor symmetry and where one has near degenerate neutrino mass, it is shown that the tribimaximal values for atmospheric angle sin2 $ {\theta_{{23}}} = \frac{1}{2} $ and solar angle sin2 $ {\theta_{{12}}} = \frac{1}{3} $ can be maintained even when the reactor angle θ 13 ≠ 0. The non zero sin θ 13 implies approximate ν μ → −ν τ symmetry instead of ν μ → ν τ symmetry.

Vanishing minors in the neutrino mass matrix from abelian gauge symmetries

Augmenting the Standard Model by three right-handed neutrinos allows for an anomaly-free gauge group extension G_max = U(1)_(B-L) x U(1)_(L_e-L_mu) x U(1)_(L_mu-L_tau). While simple U(1) subgroups of G_max have already been discussed in the context of approximate flavor symmetries, we show how two-zero textures in the right-handed neutrino Majorana mass matrix can be enforced by the flavor symmetry, which is spontaneously broken very economically by singlet scalars. These zeros lead to two vanishing minors in the low-energy neutrino mass matrix after the seesaw mechanism. This study may provide a new testing ground for a zero-texture approach: the different classes of two-zero textures with almost identical neutrino oscillation phenomenology can in principle be distinguished by their different Z' interactions at colliders.

SupersymmetricΔACP

There is experimental evidence for a direct CP asymmetry in singly Cabibbo suppressed D decays, \Delta A_{CP} \sim 0.006. Naive expectations are that the Standard Model contribution to \Delta A_{CP} is an order of magnitude smaller. We explore the possibility that a major part of the symmetry comes from supersymmetric contributions. The leading candidates are models where the flavor structure of the trilinear scalar couplings is related to the structure of the Yukawa couplings via approximate flavor symmetries, particularly U(1), [U(1)]^2 and U(2). The recent hints for a lightest neutral Higgs boson with mass around 125 GeV support the requisite order one trilinear terms. The typical value of the supersymmetric contribution to the asymmetry is \Delta A_{CP}^{SUSY}\sim 0.001, but it could be accidentally enhanced by order one coefficients.

Lepton flavor violation andθ13in minimal resonant leptogenesis

We study the impact of minimal non-supersymmetric models of resonant leptogenesis on charged lepton flavour violation and the neutrino mixing angle theta(13). Possible low-scale flavour realisations of resonant tau-, mu- and e-leptogenesis provide very distinct and predictive frameworks to explain the observed baryon asymmetry in the Universe by sphaleron conversion of an individual tau-, mu- and e-lepton-number asymmetry which gets resonantly enhanced via out-of-equilibrium decays of nearly degenerate heavy Majorana neutrinos. Based on approximate flavour symmetries, we construct viable scenarios of resonant tau-, mu- and e-leptogenesis compatible with universal right-handed neutrino masses at the GUT scale, where the required heavy-neutrino mass splittings are generated radiatively. The heavy Majorana neutrinos in such scenarios can be as light as 100 GeV and their couplings to two of the charged leptons may be large. In particular, we explicitly demonstrate the compelling role that the three heavy Majorana neutrinos play, in order to obtain successful leptogenesis and experimentally testable rates for lepton flavour violating processes, such as mu --> e gamma and mu --> e conversion in nuclei.

Common origin of soft μ−τ and CP breaking in neutrino seesaw and the origin of matter

Neutrino oscillation data strongly support μ – τ symmetry as a good approximate flavor symmetry of the neutrino sector, which has to appear in any viable theory for neutrino mass-generation. The μ – τ breaking is not only small, but also the source of Dirac CP-violation. We conjecture that both discrete μ – τ and CP symmetries are fundamental symmetries of the seesaw Lagrangian (respected by interaction terms), and they are only softly broken, arising from a common origin via a unique dimension-3 Majorana mass-term of the heavy right-handed neutrinos. From this conceptually attractive and simple construction, we can predict the soft μ – τ breaking at low energies, leading to quantitative correlations between the apparently two small deviations θ23−45° and θ13−0°. This nontrivially connects the on-going measurements of mixing angle θ23 with the upcoming experimental probes of θ13. We find that any deviation of θ23−45° must put a lower limit on θ13. Furthermore, we deduce the low energy Dirac and Majorana CP violations from a common soft-breaking phase associated with μ – τ breaking in the neutrino seesaw. Finally, from the soft CP breaking in neutrino seesaw we derive the cosmological CP violation for the baryon asymmetry via leptogenesis. We fully reconstruct the leptogenesis CP-asymmetry from the low energy Dirac CP phase and establish a direct link between the cosmological CP-violation and the low energy Jarlskog invariant. We predict new lower and upper bounds on the θ13 mixing angle, 1°≲θ13≲6° . In addition, we reveal a new hidden symmetry that dictates the solar mixing angle θ12 by its group-parameter, and includes the conventional tri-bimaximal mixing as a special case, allowing deviations from it.

Minimally allowed neutrinoless double beta decay rates from approximate flavor symmetries

Neutrinoless double beta decay ($\beta\beta0\nu$) is among the only realistic probes of Majorana neutrinos. In the standard scenario, dominated by light neutrino exchange, the process amplitude is proportional to $m_{ee}$, the $e-e$ element of the Majorana mass matrix. Naively, current data allows for vanishing $m_{ee}$, but this should be protected by an appropriate flavor symmetry. All such symmetries lead to mass matrices inconsistent with oscillation phenomenology. I perform a spurion analysis to break all possible Abelian symmetries that guarantee vanishing $\beta\beta0\nu$ rates and search for minimally allowed values. I survey 230 broken structures to yield $m_{ee}$ values and current phenomenological constraints under a variety of scenarios. This analysis also extracts predictions for both neutrino oscillation parameters and kinematic quantities. Assuming reasonable tuning levels, I find that $m_{ee}>4\times 10^{-6}$ eV at 99% confidence. Bounds below this value might indicate the Dirac neutrino nature or the existence of new light (eV-MeV scale) degrees of freedom that can potentially be probed elsewhere.

Phenomenological aspects of the exotic T quark in 331 models

In the context of 331 models we analyze the phenomenology of exotic T quarks with electric charge 2/3. We establish bounds for the corresponding masses and mixing angles and study the decay modes T→bW, tZ and qH. It is found that the decays into scalars are strongly dependent on the model parameters and can be the dominant ones in a scenario with approximate flavor symmetry.

Continuous global symmetries and hyperweak interactions in string compactifications

We revisit general arguments for the absence of exact continuous global symmetries in string compactifications and extend them to D-brane models. We elucidate the various ways approximate continuous global symmetries arise in the 4-dimensional effective action. In addition to two familiar methods - axionic Peccei-Quinn symmetries and remnant global abelian symmetries from Green-Schwarz gauge symmetry breaking - we identify new ways to generate approximate continuous global symmetries. Two methods stand out, both of which occur for local brane constructions within the LARGE volume scenario of moduli stabilisation. The first is the generic existence of continuous non-abelian global symmetries associated with local Calabi-Yau isometries. These symmetries are exact in the non-compact limit and are spontaneously broken by the LARGE volume, with breaking effects having phenomenologically interesting sizes ( ∼ 0.01) for plausible choices for underlying parameters. Such approximate flavour symmetries are phenomenologically attractive and may allow the fermion mass hierarchies to be connected to the electroweak hierarchy via the large volume. The second is the possible existence of new hyper-weak gauge interactions under which Standard Model matter is charged, with αHW ∼ 10−9. Such groups arise from branes wrapping bulk cycles and intersecting the local (resolved) singularity on which the Standard Model is supported. We discuss experimental bounds for these new gauge bosons and their interactions with the Standard Model particles.

Broadening the Higgs boson with right-handed neutrinos and a higher dimension operator at the electroweak scale

The existence of certain TeV suppressed higher-dimension operators may open up new decay channels for the Higgs boson to decay into lighter right-handed neutrinos. These channels may dominate over all other channels if the Higgs boson is light. For a Higgs boson mass larger than 2m{sub W}, the new decays are subdominant yet still of interest. The right-handed neutrinos have macroscopic decay lengths and decay mostly into final states containing leptons and quarks. A distinguishing collider signature of this scenario is a pair of displaced vertices violating lepton number. A general operator analysis is performed using the minimal flavor violation hypothesis to illustrate that these novel decay processes can occur while remaining consistent with experimental constraints on lepton number violating processes. In this context, the question of whether these new decay modes dominate is found to depend crucially on the approximate flavor symmetries of the right-handed neutrinos.

B→η(′)(l−ν¯l,l+l−,K,K*)decays in the quark-flavor mixing scheme

In the quark-flavor mixing scheme, $\ensuremath{\eta}$ and ${\ensuremath{\eta}}^{\ensuremath{'}}$ are linear combinations of flavor states ${\ensuremath{\eta}}_{q}=(u\overline{u}+d\overline{d})/\sqrt{2}$ and ${\ensuremath{\eta}}_{s}=s\overline{s}$ with the masses of ${m}_{qq}$ and ${m}_{ss}$, respectively. Phenomenologically, ${m}_{ss}$ is strictly fixed to be around 0.69, which is close to $\sqrt{2{m}_{K}^{2}\ensuremath{-}{m}_{\ensuremath{\pi}}^{2}}$ by the approximate flavor symmetry, while ${m}_{qq}$ is found to be $0.18\ifmmode\pm\else\textpm\fi{}0.08\text{ }\text{ }\mathrm{GeV}$. For a large allowed value of ${m}_{qq}$, we show that the branching ratios (BRs) for $B\ensuremath{\rightarrow}{\ensuremath{\eta}}^{(\ensuremath{'})}X$ decays with $X=({\ensuremath{\ell}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\ell}},{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}})$ are enhanced. We also illustrate that $\mathrm{BR}(B\ensuremath{\rightarrow}\ensuremath{\eta}X)>\mathrm{BR}(B\ensuremath{\rightarrow}{\ensuremath{\eta}}^{\ensuremath{'}}X)$ in the mechanism without the flavor-singlet contribution. Moreover, we demonstrate that the decay branching ratios for $B\ensuremath{\rightarrow}{\ensuremath{\eta}}^{(\ensuremath{'})}{K}^{[*]}$ are consistent with the data. In particular, the puzzle of the large $\mathrm{BR}(B\ensuremath{\rightarrow}{\ensuremath{\eta}}^{\ensuremath{'}}K)$ can be solved. In addition, we find that the $CP$ asymmetry for ${B}^{\ifmmode\pm\else\textpm\fi{}}\ensuremath{\rightarrow}\ensuremath{\eta}{K}^{\ifmmode\pm\else\textpm\fi{}}$ can be as large as $\ensuremath{-}30%$, which agrees well with the data. However, we cannot accommodate the $CP$ asymmetries of $B\ensuremath{\rightarrow}\ensuremath{\eta}{K}^{*}$ in our analysis, which could indicate the existence of some new $CP$ violating sources.

Heavy-light [formula omitted] mesons in QCD

This talk smarizes the study of the dynamics of the heavy-light Q ¯ q open charm and beauty mesons obtained in [1] using QCD spectral sum rules (QSSR) and motivated by the recent experimental discovery of the D sJ (2317) and D sJ (2457) mesons. The important rôle of the chiral condensate 〈 ψ ¯ ψ 〉 in the mass-splittings between the scalar- pseudoscalar mesons is emphasized. The emerging value of the running charm quark mass for reproducing the well-known D(0 −) and D s (0 −) masses is: m ¯ c ( m c ) = 1.13 − 0.04 + 0.08 GeV , which confirms previous estimates from this channel [2]. Using this value, the sum rules give: M D s ( 0 + ) ≃ ( 2297 ± 113 ) MeV , and a small SU(3) breaking: M D s ( 0 + ) − M D ( 0 + ) ≈ 25 MeV . Extending the analysis to the B-system; one finds: M B ( 0 + ) − M B ( 0 − ) ≃ ( 422 ± 196 ) MeV ≃ M D s ( 0 + ) − M D s ( 0 − ) . Assuming an approximate (heavy and light) flavour and spin symmetries of the mass-splittings as indicated by the previous results, one also deduces: M D s ∗ ( 1 + ) ≃ ( 2440 ± 113 ) MeV . Finally, one also gets: f D ( 0 + ) ≃ ( 217 ± 25 ) MeV much bigger than ƒ π=130.6 MeV, suggesting a large violation of the 1 / M D scaling, while the size of the SU(3) breaking ratio f D s ( 0 + ) / f D ( 0 + ) ≃ 0.93 ± 0.02 is opposite to the one of the 0 − channel of about 1.14.

Open charm and beauty chiral multiplets in QCD

We study the dynamics of the open charm and beauty mesons using QCD spectral sum rules (QSSR), where we observe the important rôle of the chiral condensate 〈ψ¯ψ〉 in the mass-splittings between the scalar–pseudoscalar mesons. Fixing the sum rule parameters for reproducing the well known D(0−) and Ds(0−) masses, we re-obtain the running charm quark mass: m¯c(mc)=1.13−0.04+0.08 GeV, which confirms our recent estimate from this channel [Phys. Lett. B 520 (2001) 115]. Therefore, using sum rules with no-free parameters, we deduce MDs(0+)≃(2297±113) MeV, which is consistent with the observed Ds(2317) meson, while a small SU(3) breaking of about 25 MeV for the Ds(0+)−D(0+) mass-difference has been obtained. We extend our analysis to the B-system and find MB(0+)−MB(0−)≃(422±196) MeV confirming our old result from moment sum rules [Phys. Lett. B 210 (1988) 238]. Assuming an approximate (heavy and light) flavour and spin symmetries of the mass-splittings as indicated by our results, we also deduce MDs*(1+)≃(2440±113) MeV in agreement with the observed DsJ(2457). We also get: fD(0+)≃(217±25) MeV much bigger than fπ=130.6 MeV, while the size of the SU(3) breaking ratio fDs(0+)/fD(0+)≃0.93±0.02 is opposite to the one of the 0− channel of about 1.14.

Neutrino mass matrix with approximate flavor symmetry

Phenomenological implications of neutrino oscillations implied by recent experimental data on pattern of neutrino mass matrix are disscussed. It is shown that it is possible to have a neutrino mass matrix which shows approximate flavor symmetry; the neutrino mass differences arise from flavor violation in off-diagonal Yukawa couplings. Two modest extensions of the standard model, which can embed the resulting neutrino mass matix have also been discussed.