University of Basel

Department
of Physics

Theoretical Particle Physics and Cosmology - Group Greljo

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Overview

The SM flavor puzzle

The Standard Model describes every particle we have measured, yet it does not explain its own structure. Why are there three generations of matter, with masses spanning more than five orders of magnitude and quarks that barely mix? And if new physics lies near the TeV scale, why has it left no trace in precision flavor measurements?

Our group works on these questions at the interface of theory and experiment. We build explicit models beyond the Standard Model, develop effective field theory methods to confront them with data, and design new observables for running and future experiments.

The full list of our papers is on INSPIRE. Three connected research lines organize this work, as discussed below.

Flavor as the organizing principle beyond the Standard Model

We often treat the two flavor puzzles as one question. The Standard Model flavor puzzle asks where the hierarchies in fermion masses and mixings come from. The new-physics flavor puzzle asks how TeV-scale dynamics can exist without violating the tight bounds on flavor-changing neutral currents. A clever flavor symmetry should address both.

Our Minimal Flavor Protection program identifies the smallest approximate symmetry that does this job. A rank-2 subgroup, SU(2)q × U(1)X, already protects generic TeV-scale physics against the most dangerous effects, such as kaon mixing. A few symmetry-breaking spurions of order 10⁻² then reproduce all observed hierarchies. This opens a much wider space of collider-accessible scenarios than Minimal Flavor Violation or U(2)⁵, with richer patterns of flavor violation to look for.

We also build the dynamics behind the hierarchies. Our models include a gauged SU(2) flavor symmetry in which Yukawa ranks rise generation by generation, U(2) acting on left-handed quarks and right-handed leptons, quark–lepton unification, Froggatt–Nielsen models with a light axion-like particle, and hierarchies from higher flavor representations. Each construction leaves its own fingerprints in neutral meson mixing, rare hadronic decays, charged lepton flavor violation, electric dipole moments and, in some cases, at high-pT colliders and gravitational waves.

Selected papers

Precision probes at present and future colliders

Precision lets us look beyond the energy frontier. A Tera-Z factory such as FCC-ee can test new physics indirectly at scales of several tens of TeV, about an order of magnitude beyond LEP. We ask which measurements carry this reach and what theory precision is needed to exploit them.

For example, we showed that FCC-ee can measure the hadronic ratios Rb, Rc, and Rs above the Z pole up to a hundred times more precisely than today. These ratios probe flavor non-universal four-fermion interactions well beyond current LEP-II and LHC bounds. For natural supersymmetry, the Tera-Z run gives consistently stronger indirect sensitivity than the Higgs run, provided Standard Model uncertainties shrink. We also identified a two-loop SMEFT effect that ties the top–Higgs coupling to the W mass, which future Z and W factories must include to read their data correctly.

The group contributes to the FCC Feasibility Study and the ECFA study of Higgs, electroweak and top factories. At lower energies, we chart signatures for high-intensity experiments. Examples are exotic multi-electron muon decays at MEG II and Mu3e, and correlations between semileptonic B decays and high-mass Drell–Yan tails at the LHC.

Selected papers

Particle cosmology as an indirect probe of fundamental interactions

The Universe reaches energies and feeble couplings that no laboratory can. We use cosmological data to test hidden sectors and very high scales, and we build models in which particle physics explains what cosmology observes.

One theme is the cosmic origin of matter. In a model where the proton is absolutely stable, a gauged combination of baryon number and lepton flavors breaks to a residual ℤ₉ symmetry. The same high-scale breaking yields a seesaw for neutrino masses, minimal thermal leptogenesis, and a majoron dark-matter candidate. The model predicts testable neutrino textures and potential signals in X- and γ-ray lines, neutrino telescopes, and the CMB. We map the window in which majoron dark matter and thermal leptogenesis coexist. In a related work, we show how to turn this majoron into a high-quality QCD axion that is consistent with both pre- and post-inflationary cosmologies.

A second theme is dark energy. Motivated by the DESI preference for an evolving equation of state, we proposed a technically natural axion coupled to N copies of dark QCD related by a ℤN symmetry. The symmetry exponentially suppresses the axion potential and so sets the tiny dark-energy scale. Dark pions make up the dark matter; as they dilute, the axion is released and produces an apparent phantom crossing without tuning.

We also study portals to dark sectors and their flavor structure. Examples are a topological portal linking QCD to dark pions, with distinctive signatures at Belle II, the flavor structure allowed for thermal dark matter, and gravitational-wave backgrounds from flavor-symmetry breaking.

Selected papers

Funding and prospects

Besides the core funding from the University of Basel, our research is supported by the SNSF Project Funding and the CHEF project through SERI; see the link.

We welcome applications from prospective PhD students, postdocs and Master's students interested in any of these lines. See our open positions or write to us directly.

Publications

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