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Fajfer, S., Solomonidi, E., & Vale Silva, L. (2024). S-wave contribution to rare D0 → π+ π- l+ l- decays in the standard model and sensitivity to new physics. Phys. Rev. D, 109(3), 036027–24pp.
Abstract: Physics of the up-type flavor offers unique possibilities of testing the standard model (SM) compared to the down-type flavor sector. Here, we discuss SM and new physics (NP) contributions to the rare charmmeson decay D0 -> x+x- l+l-. In particular, we discuss the effect of including the lightest scalar isoscalar resonance in the SM picture, namely, the f0(500), which manifests in a big portion of the allowed phase space. Other than showing in the total branching ratio at an observable level of about 20%, the f0(500) resonance manifests as interference terms with the vector resonances, such as at high invariant mass of the leptonic pair in distinct angular observables. Recent data from LHCb optimize the sensitivity to P-wave contributions that we analyze in view of the inclusion of vector resonances. We propose the measurement of alternative observables that are sensitive to the S-wave and are straightforward to implement experimentally. This leads to a new set of null observables that vanish in the SM due to its gauge and flavor structures. Finally, we study observables that depend on the SM interference with generic NP contributions from semileptonic four-fermion operators in the presence of the S-wave.
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Bolton, P. D., Fajfer, S., Kamenik, J. F., & Novoa-Brunet, M. (2024). Signatures of light new particles in B -> K(*) Emiss. Phys. Rev. D, 110(5), 055001–16pp.
Abstract: The recent Belle II observation of B -* KEmiss challenges theoretical interpretations in terms of Standard Model neutrino final states. Instead, we consider new physics scenarios where up to two new light-invisible particles of spin 0 up to 3/2 are present in the final state. We identify viable scenarios by reconstructing the (binned) likelihoods of the relevant B -* K(*) Emiss and also Bs -* Emiss experimental analyses and present preferred regions of couplings and masses. In particular, we find that the current data prefer two-body decay kinematics involving the emission of a single massive scalar or a vector particle or, alternatively, three-body decays involving pairs of massive scalars or spin 1/2 fermions. When applicable, we compare our findings with existing literature and briefly discuss some model-building implications.
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