Overview

In the next decade, an impressive experimental program will test the limits of the Standard Model (SM) and address the most pressing open questions in particle physics, from the nature of dark matter to the origin of neutrino masses, from the dominance of matter over anti-matter in the universe to the absence of strong CP violation and the large hierarchy between the electroweak and the Planck scale. Searches for beyond-the-SM (BSM) physics via low-energy precision experiments will play a crucial role in answering these questions, by looking, in particular, for deviations from symmetry patterns of the SM. Prominent examples involving nuclear systems are searches for lepton number violation via neutrinoless double beta decay, probes of CP violation via EDM experiments with nucleons, nuclei, and diamagnetic atoms, and DM direct detection. Other experiments look for proton decay, neutron-antineutron oscillations, non-standard charge-current interactions in neutron and nuclear β decays, CPT violation, neutrino-scattering experiments, and the list goes on.

These experiments are complementary to high-enery experiments such as the LHC or DUNE, and share the common feature of probing high-energy physics by using nuclear targets. Their interpretation involves physics on a large range of scales, from the eV and MeV scales of atomic and nuclear physics, to the GeV scale where hadronization takes place, all the way to the (multi-)TeV scale where BSM physics potentially originates. In the present unfortunate situation of no hints for any BSM signals at the LHC, low-energy precision searches, where significant improvements in sensitivity can be reached in relatively short time with relatively low costs, can take an even more prominent place in the future of particle physics.

Background

A signal in many of the aforementioned experiments would be an unambiguous sign of BSM physics. To interpret such a signal (or limits) and to discriminate between different BSM models, we require solid theoretical control over a large range of energy scales. Ideally one has

• A model-independent link to other searches for BSM physics such as collider and flavor observables.

• A smooth transition between quark-level and hadronic degrees of freedom, using an interplay of lattice QCD and effective field theory.

• A consistent power counting to determine the hierarchy of contributions to nuclear processes such that nuclear wave functions and external currents can be calculated in a systematic expansion.

• Sufficient control over nuclear theory once the power counting has determined the relevant interactions. For very light nuclei the scattering and bound-state equations can be solved essentially exactly, while for heavier (but still relatively light) systems ab initio methods have made great progress. Heavier systems that are often used in experiments, require nuclear many-body methods.

Program format

Overcoming these challenges requires a multi-disciplinary approach. The first is typically associated to the particle physics community, the second and third to lattice QCD (LQCD) and chiral EFT practitioners, while the fourth is the domain of many-body nuclear-structure community. We therefore divide the program into 4 weeks, where each week has a theme where at least two communities overlap:

• Week 1, (13-17 July): From the TeV scale to QCD. Discussion of aims and motivations for nuclear precision experiments. Comparison to direct high-energy searches. Standard Model Effective Field Theory. LQCD calculations of nucleon observables such as nucleon charges and form factors, electric dipole moments, and low-energy constants for neutrinoless double beta decay processes.

• Week 2, (20-24 July): From QCD to nucleons and light nuclei. Continuation of LQCD calculations of BSM observables, including two-nucleon matrix elements, with the goal of identifying a list of key matrix elements for various BSM searches. Chiral EFT methods for BSM observables in two- and few-nucleon systems. We will address the questions: Can we identify a consistent power-counting for BSM nuclear forces and currents? Can BSM matrix elements in very light nuclei, which can be computed without approximations, be used as benchmark points for more advanced methods such as QMC and NCSM?

• Week 3, (27-31 July): Towards ab initio methods. Further discussion of the BSM potentials and currents. Focus on how to implement them into first-principle nuclear calculations. How can we ensure proper renormalization? Do BSM potentials and currents need to be developed at the same order as the strong chiral potentials? Are ab initio methods mature enough to provide medium-mass benchmark calculations for many-body methods?

• Week 4, (3-7 August): Towards heavy nuclei. Many nuclear BSM experiments take advantage of heavy nuclei that can enhance certain observables such as DM detection (via coherent scattering) and EDMs (avoiding of Schiff screening), or where light nuclei are simply of no use (0νββ). Such nuclei pose severe theoretical challenges as the interpretation of data requires knowledge of nuclear matrix elements that cannot be measured. We will encourage exchange of ideas between the few- and many-body communities, and try to identify observables and calculations that can provide robust tests of important nuclear matrix elements.