Flavor at the Crossroads

02.430 (Mainz Institute for Theoretical Physics, Johannes Gutenberg University)


Mainz Institute for Theoretical Physics, Johannes Gutenberg University

Staudingerweg 9 / 2nd floor, 55128 Mainz

The flavor physics community is eagerly awaiting the upcoming LHCb and Belle II results, which are expected to deepen our understanding of the Standard Model (SM) flavor sector. During the coming years, a vast amount of new experimental analyses will be presented, providing a rich environment for discussion among theorists and experimentalists. The goal of this Scientific Program is to bring these two communities together to discuss recent experimental results and their theoretical interpretation, as well as new directions for future LHCb and Belle II measurements. In each week of the program, the first day will be dedicated to overview talks presenting the current status of the field. This will set the foundations for the rest of the program, which will contain ample time for discussions.

Contact MITP team:
    • 1
      Latest results and prospects from LHCb
      Speaker: Paula Alvarez Cartelle (University of Cambridge)
    • 2
      Latest results and prospects from Belle II
      Speaker: Gagan Mohanty (Tata Institute of Fundamental Research)
    • 11:00 AM
      Coffee break
    • 3
      b->sll fits overview
      Speaker: Marco Fedele (KIT)
    • 1:00 PM
      Lunch break
    • 4
      Looking for a Muonic Force in the Muon Anomalies

      Significant deviations from SM predictions have been observed in $ b \to s \mu^+ \mu^-$ decays and in the muon (g-2). Scalar leptoquark extensions of the SM are known to be able to address these anomalies, but generically give rise to lepton flavor violation or even proton decay. A possible resolution is to introduce a lepton-flavored U(1) gauge symmetry to preserve the accidental symmetries of the SM. I will discuss various approaches to addressing the anomalies in this framework, with a particular focus on constraining the light gauge boson solution to the g-2. I will also touch on a surprisingly strong protection mechanism preventing proton decay in the presence of a lepton-flavored symmetry

      Speaker: Anders Eller Thomsen (University of Bern)
    • 4:00 PM
      Coffee break
    • 5
      High-pT searches for flavor
      Speaker: Ulrich Haisch (Max Planck Institute, Munich)
    • 8
      Novel CP phases in B-physics anomalies

      I will discuss how the novel CP-phases, potentially present in New physics solutions to observed LFU ratios in b-hadron decays, could be revealed. For R_K() I will argue that a novel CP phase in NP amplitudes of b -> s mu mu could be observed either in mixing-induced asymmetries
      or in direct CP-asymmetries which are enhanced by narrow charmonium resonances. The correlation with constraints from meson mixing will be discussed in concrete models. In charged current amplitudes from NP that explain R_D^{
      } I will show that some leptoquark scenarios require large
      novel CP phase. I will present possibilities to constrain such CP phases in direct collider searches.

      Speaker: Nejc Kosnik (Jozef Stefan Institute)
    • 11:00 AM
      Coffee break
    • 1:00 PM
      Lunch break
    • 9
      Electron and Muon g-2 in a LFUV 2HDM

      In the wake of the recent determinations of the electron and muon anomalous magnetic moments, a lot of attention has been drawn to these observables. There are as well other observables that point to the possibility of having lepton flavor universality violation. Here we present a scalar extension of the SM with a second Higgs doublet that containing non-universal Yukawa coupling to leptons, it is capable of explaining the different signed (g-2) anomalies for the electron and muon. It also offers an explanation for the small bump in the ditau channel at 400GeV observed by ATLAS.

      Speaker: Fernando Cornet Gómez (IFIC, CSIC-Universidad de Valencia)
    • 10
      Automatic one-loop matching for flavour phenomenology

      Comparing the low energy phenomenology of flavour models can be a hard task, especially if one-loop running and matching are taken into account. In this talk, I will briefly review the use of Matchmakereft, an automated tool to compute matching and RGEs of general models into general EFTs, and how it can be used for the study of models addressing flavour anomalies.

      In particular, I will show an example on how a three field extension of the SM (for which there are no full one-loop matching examples in the literature) can open up new ways of explaining the anomalous magnetic moment of the muon, while accommodating some other flavour anomalies.

      Speaker: Pablo Olgoso (Universidad de Granada)
    • 5:00 PM
      Coffee break
    • 11
      New strategies for the full reconstruction of rare semi-invisible B decays

      Kinematic variables designed for pairwise decays to partly undetected final states---the mother of which variables is perhaps the Cambridge M_{(T)2}---have been extensively used in high-p_T collider searches, in particular of supersymmetry. A new range of potential applications at flavour facilities was recently pointed out in 2106.16236.

      We discuss two novel constraints that can be imposed on M_2. We apply them to the benchmark search B -> K tau mu at Belle II. We show that the strategy allows to fully close the kinematics in semi-leptonic tag events, making them as efficient as hadronic tag ones, and effectively doubling the statistics.

      Speaker: Diego Guadagnoli (Annecy, LAPTH)
    • 11:00 AM
      Coffee break
    • 1:00 PM
      Lunch break
    • 12
      Semileptonic transitions at high-pT

      High-pT tail observables at the LHC offer a complementary probe to low-energy experiments for studying the flavor structure of the Standard Model and beyond. We discuss the neutral and charged current Drell-Yan processes contributing to these high-pT tails to probe New Physics (NP) effects in semileptonic transitions. For this purpose, we describe the relevant cross-sections in terms of general form-factors, which are matched to the Standard Model Effective Field Theory (SMEFT) or to the coupling constants of tree-level mediators present in concrete NP models. By using the latest run-2 data from LHC on the relevant mono-lepton and di-lepton production channels, we derive constraints on the SMEFT Lagrangian by using a consistent expansion of the observables in the NP scale $\Lambda$ up to order $\mathcal{O}(\Lambda^{-4})$. We also compare these results with the truncation at order $\mathcal{O}(\Lambda^{-2})$ or when considering only dimension six operators. Furthermore, we present the Mathematica package “HighPT”, which provides a simple way to compute the relevant high-pT tail observables and to extract the complete LHC likelihood for semileptonic effective operators or NP coupling constants. To illustrate the relevance of these results, we revisit the leptoquark explanations of the charged-current B-physics anomalies, by exploring the complementarity of our high-pT constraints with the relevant low-energy observables.

      Speaker: Wilsch Felix (University of Zurich)
    • 5:00 PM
      Coffee break
    • 8:00 PM
      Conference dinner
    • 12:00 PM
      Treetop Climbing Forest Neroberg

      It offers 18 high ropes courses with various
      levels of difficulty. For more information, you can take a look at
      We will meet at Mainz main station at 12:00. The price for the climbing
      forest is 23 EUR per person (plus ca. 6 EUR for the transport).

    • 18
      Flavor hierarchies, flavor anomalies, and Higgs mass from a warped extra dimension

      In recent years a series of anomalies hinting at lepton flavor universality violation in B-meson decays have emerged. Interestingly, these anomalies can be coherently explained at the TeV scale by "4321" gauge models with hierarchical couplings reminiscent of the Standard Model (SM) Yukawas. This provides a tantalizing hint of new physics connected to the SM flavor puzzle at the same scale where the electroweak (EW) hierarchy problem is expected to be resolved. We show that 4321 models can arise as the low-energy limit of a complete theory of flavor, based on a warped fifth dimension where each Standard Model family is quasi-localized in a different brane. The SM Higgs is identified as a pseudo-Nambu-Goldstone boson emerging from the same dynamics responsible for breaking 4321 gauge symmetry. This novel construction unifies quarks and leptons in a flavor non-universal manner, provides a natural description of flavor hierarchies, and addresses the EW hierarchy problem a la Randall-Sundrum.

      Speaker: Ben Stefanek (University of Zurich)
    • 11:00 AM
      Coffee break
    • 19
      Semileptonic tau decays beyond the Standard Model

      Hadronic tau decays are studied as probe of new physics. We determine the dependence of several inclusive and exclusive tau observables on the Wilson coefficients of the low-energy effective theory describing charged-current interactions between light quarks and leptons. The analysis includes both strange and non-strange decay channels. The main result is the likelihood function for the Wilson coefficients in the tau sector, based on the up-to-date experimental measurements and state-of-the-art theoretical techniques. The likelihood can be readily combined with inputs from other low-energy precision observables. We discuss a combination with nuclear beta, baryon, pion, and kaon decay data. In particular, we provide a comprehensive and model-independent description of the new physics hints in the combined dataset, which are known under the name of the Cabibbo anomaly.

      Speaker: David Diaz-Calderon (IFIC)
    • 1:00 PM
      Lunch break
    • 20
      Method of moments for Λb → pKℓ+ℓ−
      Speaker: Anja Beck (University of Warwick)
    • 21
      Angular analysis of Λb → Λ(1520)μ+μ− with the LHCb detector

      Rare decays got recently lots of attention due to the observed anomalies. Of a theoretical point of view these electroweak Penguin diagrams are good know in the Standard Model of particle physics. Since new physics diagrams can change the angular distribution of those decays, the mesurement of those are an important test of the Standard Model of particle physics. For confirming or rejecting the observed tensions, it is from uttermost importance to exploit other b->sl+l- decays.
      The angular analysis of one of the baryonic decays, nameliy Λb -> Λ(1520)μ+μ-, is treated in this talk. The analysis is performed within the LHCb experiment.

      Speaker: Felicia Volle (IJCLab, Orsay)
    • 5:00 PM
      Coffee break
    • 22
      Prospects of |Vub| measurement at Belle II

      Belle II is a B-factory experiment and thus offers a perfect environment for measuring the CKM-matrix element |Vub|. This measurement is particularly important for a precise test of the unitarity condition of the CKM-matrix. Two types of measurement exist to extract |Vub|, both relying on semi-leptonic decays of B mesons: the exclusive measurements (where a particular u-hadron is reconstructed) and the inclusive measurements (they effectively sum over all possible hadronic final states containing a u-quark). The two methods show a long-standing discrepancy of around 3sigma. I will present in this talk the first studies of |Vub| performed at Belle II and I will outline what the prospects for this measurement are.

      Speaker: Tommy Martinov (DESY)
    • 23
      Renormalization Group Evolved 4321 and intriguing behaviours in the UV

      The 4321 model is one of the best model providing an explanation for the B anomalies. Although all of the gauge couplings exhibit asymptotic freedom or run very slowly, we found that Landau poles can arise at one-loop at relatively low scale, from the quartic part of the extended scalar sector. We also discuss the possibility of radiative electroweak symmetry breaking and its incapacity in alleviating the little hierarchy problem brought by the presence of new TeV-scale scalar states.Finally, the idea of coupling/mass unification in the UV is explored, motivated by UV completions of the 4321 model such as PS^3.

      Speaker: Julie Pagès (University of Zurich)
    • 11:00 AM
      Coffee break
    • 1:00 PM
      Lunch break
    • 24
      Explaining the hints for lepton flavour universality violation with three S2 leptoquark generations

      Leptoquarks are prime candidates for explaining the intriguing hints for lepton flavour universality violation. In
      particular, the $SU(2)_L$ doublet of scalar leptoquarks $S_2$ is capable of providing an explanation for the tensions between the measurements and the Standard Model predictions in $(g-2)_\mu$, $b\to s\ell^+ \ell^-$ and $b\to c\tau\nu$ processes, as well as in non-resonant di-electron production. However, in the minimal setup with a single leptoquark generation, a common explanation for all these issues is not possible as this would lead to unacceptably large charged lepton flavour violation. We therefore propose a model with three generations of $S_2$, each coupling exclusively to a single lepton flavour, \textit{i.e.} a model extending the Standard Model particle content by an electroquark, a muoquark and a tauquark. We show that after taking into account other constraints, such as those originating from electroweak precision observables and $\Delta F=2$ processes, it is possible to provide a combined explanation for all these hints of lepton flavour universality violation. Moreover, we find that the presence of the tauquark can generate a dimension-six ${\cal O}_9^U$ operator via off-shell photon penguin diagrams, which, together with the muoquark contribution, further improves the global fit to $b \to s \ell^+ \ell^-$ data.

      Speaker: Luc Schnell (Max Planck Institute for Physics)
    • 5:00 PM
      Coffee break
    • 25
      On the ALP solution to the B anomalies

      Tantalizing anomalies have been observed in tests of lepton flavour universality (LFU) that show significant deviations from the SM prediction. While the most straightforward solutions to these anomalies rely on heavy new physics impacting the vector semi-leptonic effective operators, it is also possible to consider the effects of light particles produced on-shell in the B decays. This is motivated, in particular, by the anomaly in the low-energy bin of RK* that is typically difficult to understand with only heavy mediators.

      Among the possible BSM particles, ALPs are one of the most compelling candidates. Due to its shift-symmetry, the ALP contribution to the LFU ratios is suppressed by the fermion masses. Still, interesting solutions have been found in the literature, assuming lepton flavour universal scenarios, that comply with strong bounds from other observables. I will discuss departures from this assumption, as well as the plausibility of flavour assumptions behind the new regions of parameter space that will open.

      Speaker: Maria Ramos (IFT Madrid)
    • 26
      From light to heavy Z′ to explain b → sℓ+ℓ− data

      Many extensions of the SM have been proposed to explain the discrepancies between b→sℓ+ℓ- data and the corresponding Standard Model predictions. In this talk, I will focus on important phenomenological implications of a prime solution to the anomalies: a neutral bosons (Z’). In particular, concerning a heavy neutral gauge boson, I will discuss the effect of the mixing of the Z′ with the SM Z, which cannot be avoided by any symmetry. I will show that that a mixing of the order of (mZ/mZ′)^2 as expected in generic models, leads to lepton flavour universal contributions, providing a good fit to b → sl+l− data and is also welcome by the global electroweak fit where the tension in the W mass prefers a non-zero value of it with intriguing consequences for model building.
      On the other hand, a light Z’ boson is the only viable single particle solution to the anomalies predicting R(K(*))>0 in high q2 bins, providing an essential consistency test of data which will be available in the near future. I will discuss this solution together with the impact on B->K+invisible, Drell-Yan searches for muon pairs at LHC and e+e- -> μ+μ- + invisible. I will point out that the forthcoming improved limits on these processes, including the experimental sensitivities required for a proper treatment of the necessarily sizable Z’ width, can rule out a Z’ explanation of b→sℓ+ℓ- data with a mass below ~4 GeV.

      Speaker: Claudio Andrea Manzari (University of Zurich)
    • 11:00 AM
      Coffee break
    • 1:00 PM
      Lunch break
    • 27
      Connecting the B anomalies with the hierarchy problem

      Intriguing hints for lepton flavor universality violation have emerged from the measurement of B meson decays by LHCb. These B anomalies point to the new physics at the low TeV scale, exactly where we expect a solution to the hierarchy problem! In this talk, I will present an economical model that can address the two problems at once. The model is based on a SU(4)/SP(4) fundamental composite Higgs model. The hierarchy problem can be solved by the compositeness of the pNGB Higgs, and the observed B anomalies can be explained by the additional Z’ boson in the model. The model connects the energy scales of the EWSB and the B anomalies. I will discuss the experimental constraints from Higgs physics, flavor physics, and LHC direct searches.

      Speaker: Yi Chung (Max-Planck-Institut für Kernphysik)
    • 5:00 PM
      Coffee break
    • 8:00 PM
      Conference dinner @ Restaurant Damas at 20:00

      Please choose what you'd like to eat from the menu at https://damas-shop.de

    • 28
      Considerations on b-hadron production and tree-level decays for theory and experiment

      Measurements of b-hadron production and constraints on their tree-level decays are closely related. With the precision measurements of both performed In recent years, experiments have found curious new phenomena: hadronisation turns out to depend on the kinematics of the b-hadron and the multiplicity of the pp collision, and some tree-level branching fractions seem to deviate from the Standard Model predictions.
      In this talk, I plan to treat recent results, point out how they affect current measurements of new physics in flavour,
      such as BF(Bs->mumu), and discuss the prospects for the near future, including ideas for new measurements.

      Speaker: Mick Mulder (University of Groningen)
    • 29
      A bridge to new physics -- explaining g-2 and connecting it to other flavour anomalies

      In this talk I will go over the study of new physics contributions towards the anomalous magnetic moment of the muon through a specific diagram, which we denote by the bridge. We will classify all possible SM extensions which can generate this chirally enhanced contribution, finding new classes of models to explain g-2. We will focus on a particular 3-field extension which uses the one-loop bridge explanation of g-2 as a connection between tree-level solutions of the neutral B-anomalies and the Cabibbo angle anomaly.

      Speaker: Guilherme Guedes (Universidad de Granada and LIP)
    • 11:00 AM
      Coffee break
    • 1:00 PM
      Lunch break
    • 30
      Towards an All-Orders Flavor Formalism in the (geo)SM(EFT) and Beyond

      I will present model- and basis-independent formulae for calculating fermionic mass, mixing and CP-violation parameters given arbitrary 3x3 complex Yukawa couplings of the fundamental Lagrangian. The formalism is applicable to any construction whose Yukawas transform under certain global U(3) flavor symmetries, including the Standard Model (SM), numerous popular SM extensions and, as I will show, the geometric SM Effective Field Theory (geoSMEFT), which is a novel formulation of the SMEFT valid at all orders in the characteristic EFT expansion parameter v/Lambda. This merger of invariant theory and geoSMEFT technologies therefore allows for compact all-orders expressions for quark masses, CKM mixing angles and the Dirac phase. After showing off the applicability of the formulae, including the rapid derivation of renormalization group equations that are themselves valid at all orders in v/Lambda, I will discuss its future phenomenological and theoretical extensions, most notably in the neutrino flavor sector.

      Speaker: Jim Talbert (DAMTP, Cambridge)
    • 5:00 PM
      Coffee break