A tale of invisibility: constraints on new physics in b → sνν

Abstract

The Belle II experiment will measure the rare decays B → Kνν and B → K∗νν with increased sensitivity which can hence be expected to serve as a very efficient probe of new physics. We calculate the relevant branching ratios in low-energy effective field theory (LEFT) including an arbitrary number of massive sterile neutrinos and discuss the expected sensitivity to the different operators. We also take into account the longitudinal polarisation fraction FL and the inclusive decay rate B → Xsνν. In our investigation we consider new physics dominantly contributing to one and two operators both for massless and massive (sterile) neutrinos. Our results show a powerful interplay of the exclusive decay rates B → Kνν and B → K∗νν, and a surprisingly large sensitivity of the inclusive decay mode to vector operators even under conservative assumptions about its uncertainty. Furthermore, the sensitivity of FL is competitive with the branching ratio of B → K∗νν in the search for new physics contributing to scalar operators and thus also complementary to B → Kνν and B → Xsνν.

Dark sector origin of the KOTO and MiniBooNE anomalies

“…The recently updated measured values of R K ( * ) = B(B → K ( * ) µ + µ − )/B(B → K ( * ) e + e − ) are now fully consistent with the standard model (SM) expectation [1], and have dampened interest in neutral current (NC) B anomalies (although the individual branching fractions remain discrepant [2]). However, a first measurement by Belle II of the branching ratio B(B + → K + ν ν) = (2.4 ± 0.7) × 10 −5 [3], is 2.8σ higher than the SM expectation B(B + → K + ν ν) SM = (5.58 ± 0.38) × 10 −6 [4], and has revived interest in NC B decays [5][6][7][8][9][10]; an earlier upper limit by Belle II [11] also led to a flurry of theoretical activity [12][13][14][15][16][17]. Unlike the dilepton modes in R K ( * ) , the contamination from cc states in B → K ( * ) ν ν can be neglected.…”

FCNC B and K meson decays with light bosonic Dark Matter

“…These processes are further divided into 2-body decays with a single new particle and 3-body decays with a pair of new particles. All the 2-body channels plus the 3-body decay with a pair of new fermions have been extensively studied before [8][9][10][11][12][13][14][16][17][18][19][20]. 1 The table indicates with a double-checkmark processes that have not received much attention, namely three body modes with a pair of vector particles.…”

“…The relevant DM EFT operators in this JHEP03(2023)037 picture can be found in [9,[43][44][45][46]. 6 The FCNC B and K meson decays into fermion DM (or similar invisible particles like sterile neutrinos) have been studied in [9,13,14,[18][19][20]. For this reason, in the following we restrict ourselves to the scalar and vector DM cases and investigate the experimental sensitivity to the interactions in eqs.…”

“…Note that the 2-body modes with a single new fermion are only possible when the meson decays into a baryon and χ[8]. For the 3-body decay with a pair of fermions, χ can be either the neutrino or some DM particle, the former is studied in[12,14,[18][19][20], while the latter in[13].…”

The first limit on invisible decays of $$B_s$$ mesons comes from LEP

“…The b → s flavor-changing neutral currents, which are strongly suppressed in the Standard Model (SM), have gathered a lot of attention in the past few years. The transition involving neutrinos, b → s νν, is theoretically well-suited to test the consistency of the Standard Model [1][2][3][4][5] and offers a window to weakly-coupled light new physics [6][7][8][9][10][11][12][13][14][15][16][17]. However, the associated B meson decay modes are challenging to test experimentally due to the presence of missing energy in the final state, and until recently only upper limits on the B → K νν channels existed [18][19][20][21].…”