Charge Symmetry Breaking and Other Isospin Violations

Scientific Proposal

We propose a week-long workshop on isospin violations with special emphasis on charge symmetry breaking. Recent experimental and theoretical progress make June 2005 a propitious time to bring together experimentalists and theorists to generate new ideas for specific new experiments and calculations. Here we define the subject, explain its timeliness, and discuss its applications and implications. QCD is invariant under the transformation that interchanges the up u and down d quarks: charge symmetry, CS. This is a rotation by 180 degrees in isospin space. (Invariance under a general rotation is called isospin invariance.) CS is broken only by the quark mass difference and by electromagnetic interactions. The u and d quark masses are fundamental parameters of the standard model, and learning the phenomenological influence of their difference is an important way to improve understanding of the standard model.
Cooler CSB Experiment

Charge symmetry breaking (CSB)---has been previously observed in hadronic mass differences, in rho-omega and pi-eta mixing, in the mass differences between mirror nuclei, and in measurements of the analyzing power differences between n and p in n+p elastic scattering. In addition, a detailed understanding of CS is needed to understand a variety of other seemingly-unrelated quantities such as controlling the hadronic contribution to the muonic (g-2), and extracting the strangeness content of the nucleon from parity violating observables. The impetus for a workshop in the near future is provided by two very recent experiments involving the production of a neutral pion at energies just above threshold that have very recently shed light on CSB and its relation with chiral symmetry. Collisions of two deuteron nuclei to form an alpha particle (4He nucleus) and a pi0 were studied at IUCF. In the dd to 4He+pi0 process the initial state is even under CS, but the final state is odd, so CS would prevent the reaction. The IUCF experimental team observed a cross section of 12.7+/-2.2 pb at 228.5 MeV, a value that is about (1/300)^2 times the mu b cross section typical of pion production involving light nuclear targets at low energy. Another experiment, performed at TRIUMF, involves producing a deuteron and a pi0 in neutron-proton collisions: n p to d pi0. CS predicts that the observed cross section is invariant under the exchange of the initial neutrons and protons, i.e. symmetric about 90 degrees. The experiment detected a fore-aft asymmetry of 1.7 +/- .9 \times 10^-3.

Two of the main goals of the workshop are to improve and develop theories of the pion production reactions and to explore the experimental possibilities to further improve our understanding of the standard model at low and intermediate energies. Recent advances in chiral effective field theories have placed the necessary tools in the hands of theorists. We now understand that the bulk of the masses of most particles stems from a mechanism called spontaneous symmetry breaking. As a consequence, the scale for hadronic masses is set through a scale related to this mechanism, such as the chiral condensate, not the quark masses of the standard model. Only the masses of the pseudoscalar mesons are controlled by the non--vanishing light quark masses; access to quark mass differences is provided through study of CSB reactions.

Chiral perturbation theory links hadronic masses, isospin violating IV pion-nucleon scattering and CSB and IV nuclear reactions. To study this link is to study the standard model at low energies. Meeting this ambitious goal requires: improving understanding of meson-mixing and pion production, appreciating the finer details of power counting, using realistic four-body wave functions for both bound and continuum states, and relating these wave functions to underlying theory. Therefore the theoretical analysis of reactions like dd to alpha pi0 poses a significant challenge to theorists. So far, the corresponding calculations are in a preliminary state and bringing together experts from around the world would greatly stimulate progress toward more complete, better calculations and interesting new experiments.

The observation of CSB can not only be used to gain information on quark mass effects, but it may also be used as a tool to study very different aspects of strong interaction physics. One possibility is to improve our understanding of the structure of scalar mesons through the observation of the a_0-f_0 mixing amplitude. In addition, studying the decays of eta and eta' mesons give access to quark masses. The corresponding measurements are an essential part of the future physics program at COSY-Juelich and there is also a program to measure eta decays with the Crystal Ball at MAMI (Mainz).

So far we have mainly discussed CSB and IV in hadronic and nuclear physics, but the implications of this subject are very broad. The origin of the quark masses is not fully understood. In the Standard Model, the Higgs mechanism allows for the generation of such masses, but cannot predict the masses themselves. CSB is concerned with understanding the flavor dependence of the quark mass spectrum: why are the masses of the u and d almost the same, and why are the other four (c, s, t, b) masses so very different?  No fundamental understanding exists. The calculations and new experiments required for relating quark mass differences to observed reactions are not easy, and having a workshop would greatly aid progress. Joint participation of theorists and experimentalists of the field will help to identify relevant observables, and to develop ideas for future experiments at various facilities.