Invisible $K_L$ decays as a probe of new physics

Abstract

The decay KL → invisible has never been experimentally tested. In the Standard Model (SM) its branching ratio for the decay into two neutrinos is helicity suppressed and predicted to be Br(KL → ν ν) 10 −10 . We consider several natural extensions of the SM, such as two-Higgs-doublet (2HDM), 2HDM and light scalar, and mirror dark matter models, those main feature is that they allow to avoid the helicity suppression factor and lead to an enhanced Br(KL → invisible). For the decay KL → ν ν the smallness of the neutrino mass in the considered 2HDM model is explained by the smallness of the second Higgs doublet vacuum expectation value. The small nonzero value of the second Higgs isodoublet can arise as a consequence of nonzero quark condensate. We show that taking into account the most stringent constraints from the K → π + invisible decay, this process could be in the region of Br(KL → invisible) ≃ 10 −8 − 10 −6 , which is experimentally accessible. In some scenarios the KL → invisible decay could still be allowed while the K → π + invisible decay is forbidden. The results obtained show that the KL → invisible decay is a clean probe of new physics scales well above 100 TeV, that is complementary to rare K → π + invisible decay, and provide a strong motivation for its sensitive search in a near future experiment.

Rare hyperon decays with missing energy

“…Within the SM, this process is predominantly due to s → dν ν arising from Z-penguin and box diagrams with up-type quarks and the W boson in the loops [1], the neutrino pair (ν ν) being undetected. In the presence of NP, this SM contribution could be altered [2][3][4][5][6][7][8][9], and there might be invisible nonstandard states which are light enough and have sd couplings to give rise to new s → d / E channels [10][11][12][13][14][15][16][17][18][19].…”

Searching for heavy sterile neutrinos in kaon decays

“…5 where we see that that a heavy neutrino could increase Br(K L → νν) up to 1.2 × 10 −10 . Unfortunately this falls short of the of NA64 expected sensitivity to this decay 37 where this decay could be searched for by using K L produced from a K + beam hitting an active target. This motivates the study of other set-ups which could be sensitive to this decay or similar invisible decays.…”

Flavor-changing hyperon decays with light invisible bosons

“…( 4) and (5). Specifically, we will make the 4) GeV and M A = 1.25 (15) GeV, which are in line with the commonly used parametrization in experimental studies of hyperon semileptonic decays [20][21][22][23][24]. Moreover, for the Ω − decay we apply the modification…”

“…As a consequence, φ and φ always appear together in their interactions with SM quarks at low energies. Scenarios where such nonstandard bosons have ds couplings that contribute to kaon processes have been addressed before in the literature under specific NP contexts [12][13][14][15]. Here we pursue a model-independent approach, starting from the most general effective Lagrangian that satisfies SM gauge invariance and contains operators describing dsφφ interactions at leading order.…”