Probing novel scalar and tensor interactions from (ultra)cold neutrons to the LHC

Abstract

Scalar and tensor interactions were once competitors to the now well-established V − A structure of the Standard Model weak interactions. We revisit these interactions and survey constraints from low-energy probes (neutron, nuclear, and pion decays) as well as collider searches. Currently, the most stringent limit on scalar and tensor interactions arise from 0 + → 0 + nuclear decays and the radiative pion decay π → eνγ, respectively. For the future, we find that upcoming neutron beta decay and LHC measurements will compete in setting the most stringent bounds. For neutron beta decay, we demonstrate the importance of lattice computations of the neutronto-proton matrix elements to setting limits on these interactions, and provide the first lattice estimate of the scalar charge and a new average of existing results for the tensor charge. Data taken at the LHC is currently probing these interactions at the 10 −2 level (relative to the standard weak interactions), with the potential to reach the < ∼ 10 −3 level. We show that, with some theoretical assumptions, the discovery of a charged spin-0 resonance decaying to an electron and missing energy implies a lower limit on the strength of scalar interactions probed at low energy.

Prospects for precision measurements in nuclear decay in the LHC era

“…[9], the lower bounds of g S,T are actually a better indicator of the final S,T bounds than the errors δg S,T . It is thus technically more accurate to say that a future determination of S,T from a measurement of b n at the level of 10 −3 will be dominated by the experimental error as long as Once again the use of this approximation is justified to study the effect of NP at the current level of precision, whereas sub-leading corrections have to be taken into account in the calculation of the SM contribution.…”

“…The low energy scale O(1 GeV) effective Lagrangian for semi-leptonic transitions is then given by [9,10] 1 . 1 For the sake of simplicity we do not considered operators involving ν μ or ν τ .…”

“…Furthermore, in neutron decay it is usual to extract the axial-vector form factor g A (or the so-called mixing ratio in the case of nuclear decays) from experiments. The extracted quantity contains an unobservable NP contribution [9,15] …”

“…In the rest of this article we will use the recent lattice QCD determinations g S = 0.8 ± 0.4 and g T = 1.05 ± 0.35 in the MS scheme and at the renormalization scale μ = 2 GeV [9]. Notice that, given the smallness of the scalar and tensor couplings S,T , it is not necessary to have such a precise determination of g S,T as for the g V,A couplings.…”

“…We confront next the most precise results obtained in β decay with constraints obtained from other semileptonic processes and also with results from the LHC [9][10][11], paying special attention to those observables that are linear on the exotic couplings. The use of an Effective Field Theory (EFT) framework allows us to bridge through the low energy and the high energy searches and to compare their sensitivities to new physics.…”