Volume 110, Number 3, May-June 2005 Journal of Research of the National Institute of Standards and Technology [J. Res. Natl. Inst. Stand. Technol. 110, 189-194 (2005)] Neutron Measurements and the Weak Nucleon-Nucleon Interaction Volume 110 Number 3 May-June 2005 W. M. Snow The weak interaction between nucleons Key words: anapole moment; effective remains one of the most poorly-understood field theory; few-body systems; neutral sectors of the Standard Model. A quantita- currents; parity violation; QCD; strong Indiana University/Indiana tive description of this interaction is need- interaction; weak interaction. University Cyclotron Facility ed to understand weak interaction phe- Bloomington, IN 47408 nomena in atomic, nuclear, and hadronic systems. This paper summarizes briefly what is known about the weak nucleon- nucleon interaction, tries to place this phe- Accepted: August 11, 2004 nomenon in the context of other studies of the weak and strong interactions, and out- lines a set of measurements involving low energy neutrons which can lead to signifi- cant experimental progress. Available online: http://www.nist.gov/jres 1. Introduction and Discussion study of the weak nucleon-nucleon (NN) interaction has the potential to improve our understanding of the Despite nearly 40 years of study, the details of the strongly interacting limit of quantum chromodynamics weak interaction between nucleons are not understood. (QCD), which is clearly a problem of fundamental This is mainly due to a paucity of experimental results importance. Like the electromagnetic interaction, the that can be robustly compared with theory. The extreme weak interaction between quarks and leptons is under- “weakness” of this interaction implies that it is only stood at the fundamental level and is weak enough to experimentally accessible through the study of the probe strongly interacting systems without affecting the measurement of small parity-odd interference effects strong dynamics. Unlike the electromagnetic interac- amid the much larger effects of the strong interaction, tion, the range of the weak interaction among the described, described by quantum chromodynamics quarks, set by the masses of the W and Z bosons, is (QCD). Since QCD is a purely vector theory it con- much smaller than the size of the nucleon as set by the serves parity, and so any parity-odd effects must come dynamics of the strongly interacting limit of QCD. At from the weak interaction. However the natural scale the same time the strong repulsion of two nucleons at for the size of parity-odd amplitudes, set by the ratio of short distances, understood qualitatively in terms of the the amplitudes for W and Z exchange to those for Fermi statistics of the quarks in the nucleons and the meson exchange between nucleons, is extremely small high energy cost of flipping a quark spin in the nucle- (≈10–7), and therein lies the experimental challenge. on, means that the dynamical mechanism for the weak It is important to understand the character of weak interaction between nucleons must involve meson interactions between nucleons for a number of reasons. exchange and the soft QCD physics that leads to it. We If one assumes the electroweak theory is correct, a therefore expect that the weak interaction has the 189 Volume 110, Number 3, May-June 2005 Journal of Research of the National Institute of Standards and Technology potential to provide qualitatively new information on tions for quark-quark neutral currents must be due to quark-quark correlations in the strongly interacting strong QCD effects. ground state of QCD and on the underlying physics With experimental information on the low energy behind the meson exchange model of the NN interac- parity-violating (PV) partial waves in the NN system, tion. there is a chance to understand quantitatively for the A physical understanding of the ground state of QCD first time the extensive observations performed in many in the strongly interacting limit does not exist. Such an systems of parity-violating phenomena in nuclei and to understanding is one of the goals of the rich field of use this data to deepen our understanding of nuclear hadron physics which has opened up between “tradi- physics. For example, PV can give information on tional” nuclear physics and high energy collider small components of the nuclear wavefunction. In the physics and which tests the perturbative limit of QCD. nuclear shell model, successive levels alternate in pari- As we look to the future beyond the Standard Model ty, and so parity eigenstates are linear combinations of (SM), most theorists anticipate that the theory that the either even (|nhω >, n even) or odd (|nhω >, n odd) SM is embedded in (technicolor, supersymmetry) will shell states. Since parity violation directly connects possess new strongly interacting sectors. From a theo- these two subspaces, PV observables are linearly sensi- retical point of view, strong coupling is a phenomenon tive to small components of the nuclear eigenstates that is now understood to be the generic norm in quan- from higher shells [2]. There are a number of observa- tum field theories rather than the exception. We already tions of PV in parity doublets in nuclei with A≈20 that know that collective, nonlinear effects are present in the are amenable to a shell model treatment which should QCD ground state. The approximate chiral symmetry be calculable when the PV NN interaction is deter- of QCD and the successful interpretation of pions as the mined. In addition, ideas from quantum chaos and low energy excitations of this broken chiral symmetry nuclear statistical spectroscopy have been used to ana- in the ground state was the first example. The qualita- lyze parity violation in neutron reactions in heavy tive understanding of many aspects of the QCD vacu- nuclei in terms of the effective isovector and isoscalar um and lattice simulations in terms of helicity-flipping, weak NN interaction, and knowledge of PV in the NN quark-quark interactions induced by instantons or other system would allow a quantitative test of the predictive nontrivial gluon field configurations is another intrigu- power of these interesting ideas [3,4]. In both cases, the ing possibility. It has also been shown that color super- PV observables open a new window into specific fea- conducting phases caused by BCS-like, quark-quark tures of the nuclear many-body wave function which correlations can exist in the high-density limit of QCD. can only be exploited if the NN weak interaction ampli- The ground state of QCD is best thought of as a many- tudes are known. body system for which knowledge of both the elemen- NN parity violation also may be relevant to properly tary excitations of the system and their correlations in interpret certain recent and planned measurements the ground state are essential components for a physi- involving PV with electrons. In atoms, the effect of NN cal understanding. The weak interactions between parity violation has been seen recently for the first time quarks in nucleons quietly probe the quark-quark corre- in 133Cs [5] through its contribution to the anapole lations and give us a new opportunity to learn about moment of the nucleus, which is an axial vector cou- QCD. pling of the photon to the nucleus induced mainly by The NN weak interaction is also the only practical the PV NN interaction [6,7]. Anapole moment meas- way to study quark-quark neutral currents at low ener- urements in other atoms are possible, and experiments gy. The neutral weak current conserves quark flavor to are under way [8]. In the heavy nuclei for which the high accuracy in the standard electroweak model (due anapole moment is a well-defined observable, the main to the GIM mechanism). Therefore it is not seen at all contribution to the anapole moment comes from PV in the well-studied strangeness-changing nonleptonic admixtures in the nuclear ground state wave function. weak decays. We therefore know nothing experimental- Although the main contribution to the nuclear anapole ly about how QCD modifies weak neutral currents. The comes from the unpaired valence nucleons as in the effects of quark-quark neutral currents have been seen case of nuclear magnetic moments, one also expects the in collider experiments [1] and the SM predictions have effects of NN correlations to modify these single-parti- been verified at the high momentum transfers reached cle estimates [9]. In electron scattering from nucleons, in these experiments. Therefore, we have confidence PV effects are sensitive to both Z exchange between the that any deviations from the perturbative QCD predic- electron and the quarks in the nucleon as well as the 190 Volume 110, Number 3, May-June 2005 Journal of Research of the National Institute of Standards and Technology coupling of the virtual photon to the axial current from an interesting observation has been made with regard to PV interactions among the quarks in the nucleon. A the weak pion coupling: chiral loop effects not includ- number of experiments to measure PV in electron scat- ed in DDH can have a significant effect on the size of tering in different kinematic regions (SAMPLE, this coupling [18]. In particular, the chiral analysis HAPPEX, PVA4, G0) will be able to isolate different implies that there are significant contributions associat- PV mechanisms and separate out the contribution from ed with disconnected sea quark loops. This is interest- q-q weak interactions [10]. Finally, there have been a ing because there are few probes of the u and d sea number of recent calculations of PV effects in Compton quark component of the QCD ground state in the non- scattering [11] and pion photoproduction [12] which perturbative regime. Finally, preparations have been are sensitive to q-q weak interactions in the nucleon.