At the TeV scale, low-energy precision observations of neutron characteristics provide unique probes of novel physics. Precision studies of neutron beta decay (𝑛 → 𝑝𝑒¯𝜈¯) observables are susceptible to beyond the Standard Model (BSM) tensor and scalar interactions, as can be seen from the effective Lagrangian
where 𝜖𝑆, 𝜖𝑃, 𝜖𝑇 are the new-physics couplings. To gain sensitivity to these effects the majority of previous and proposed neutron beta decay studies aim to determine one or more of the correlation coefficients included in the differential decay rate for a beam of polarised neutrons. These give bounds on the combinations 𝑔𝑆𝜖𝑆, 𝑔𝑇𝜖𝑇, where 𝑔𝑆 and 𝑔𝑇 are the scalar and tensor nucleon isovector charges. Future experiments including at the Large Hadron Collider (LHC) are currently looking at probing scalar and tensor interactions, so increasing precision results are to be expected. These aim to measure the following combinations
at a level of 10−3, so 𝑔𝑆 and 𝑔𝑇 are needed at this level of precision. 𝑔𝑆 and 𝑔𝑇 can be determined using a lattice simulation and hence bounds can be found on possible non-zero 𝜖𝑆, 𝜖𝑇 taking |b| and ∣𝑏𝜈∣ to be ≤ 10−3. In the figure we use our recent results for 𝑔𝑆 and 𝑔𝑇 [1] leading to bands in the 𝜖𝑆-𝜖𝑇 plane. It is apparent (from the axis scales) that while 𝑔𝑇 is approaching the required accuracy it is necessary to constrain 𝑔𝑆 are better.
Allowed regions in the 𝜖𝑆-𝜖𝑇 plane using the computed tensor and scalar charges (blue and red bands) together with the present experimental result on 𝜖𝑆 (green band).
The simulations and analysis were performed on the DiRAC-3 Extreme Scaling service (Edinburgh) and Data Intensive service (Cambridge). as part of the project ‘Hadron structure: the parton model and beyond’ (dp007).
[1] R. E. Smail et al. [QCDSF/UKQCD/CSSM], Phys. Rev. D 108 (2023) 094511
PI: Roger Horsely
Resources used: Data Intensive Cambridge, Data Intensive Leicester, Extreme Scaling Edinburgh