# Introduction

High-precision measurements of observables in neutron beta decay address a number of open questions, which are at the forefront of particle physics [Abe08]. Main emphasis lies on the search for new physics beyond the Standard Model (SM) of elementary particles and fields, and in particular, on the question of unification of all forces shortly after the Big Bang. This grand unification is not part of the SM, and new symmetry concepts are needed like left-right symmetry, fundamental fermion compositeness, new particles, leptoquarks, supersymmetry (SUSY), supergravity, and many more.

In the search for new symmetries, we see precision measurements in neutron beta decay as the next high-precision frontier in the domain of low-energy studies. These experiments fit in a greater field of precision measurements comprising cold or ultra-cold neutrons, cold or ultra-cold ions or atoms, protons, electrons, and their antiparticles. A second frontier in the domain of high-energy physics is certainly the Large Hadron Collider (LHC). Low-energy experimentation allows probing these questions in a complementary way to LHC-based experiments or even constitutes a unique way.

With the new facility PERC [Dub08] several symmetry tests based on neutron data become competitive [Kon11]. Interesting are symmetry contributions to neutron beta decay parameters, which are forbidden in the SM but show up in SUSY due to the fact that the natural energy scale of SUSY is not far away from the energy scale of W exchange in weak interaction.

Observables in free neutron decay are abundant: besides the neutron lifetime Ï„_{n}, angular correlations involving the neutron spin as well as momenta and spins of the emitted particles are characterized by individual coefficients, which can be related to the underlying coupling strengths of the weak interaction, including yet unobserved ones. Examples are the neutrino electron correlation coefficient a, the beta asymmetry parameter A, the neutrino asymmetry parameter B (reconstructed from proton and electron moÂmenta), the proton asymmetry parameter C, the triple correlation coefficient D, the Fierz interference term b, and various correlation coefficients involving the electron spin. Each coefficient in turn relates to an underlying broken symmetry.

With PERC, we concentrate on the following precision tests of the SM and searches for physics beyond the SM:

- An improved determination of SM parameters. With a new and precise value of the ratio Î» of the axial to the vector coupling constant, we will cover the demand from particle physics, astro-particle physics, where a better value for this quantity is needed for calculations of the nucleosynthesis after the big bang, the energy production in the sun, the formation of neutron stars, and the calibration of neutrino- and LHC-detectors. A new value for the first generation matrix element
*V*_{ud}gives better insight into quark mixing. - A search for right-handed admixtures to the left-handed feature of the SM. As a natural consequence of symmetry breaking in the early universe, they should be found in neutron beta decay. Signatures are a
*W*_{R}mass with mixing angle Î¶. - A search for scalar and tensor admixtures
*g*_{S}and*g*_{T}to the electroweak interaction.*g*_{S}and*g*_{T }are also forbidden in the SM but SUSY contributions to correlation coefficients can approach the 10^{âˆ’3}level, a factor of five away from the current sensitivity limit [Sch07]. - A first search in neutron beta decay for the Fierz interference term
*b,*which is forbidden in the SM but can approach the 10^{âˆ’3}level from SUSY contributions. - A first measurement of the weak-magnetism form factor
*f*prediction of electroweak theory. Such an experiment would be one of the rare occasions, where a strong test of the underlying structure itself of the SM becomes available._{2}

## References

[Abe08] H. Abele, Prog. Part. Nucl. Phys., **60**, 1 (2008).

[Dub08] D. Dubbers at al., Nucl. Instrum. Meth. A **596**, 238 (2008), for an extended version, see arXiv:0709.4440.

[Kon11] G. Konrad et al., in World Scientific ISBN 978-981-4340-85-4, 660 (2011) and arXiv: 1007.3027v2 (2010).

[Sch07] M. Schumann et al., Phys. Rev. Lett., Phys. Rev. Lett. **99**, 191803 (2007).