Wolfenstein’s research is in the area of elementary particle phenomenology, which means theoretical work that is closely related to experiment. His thesis work under Edward Teller and his early work at Carnegie Tech concerned the scattering of polarized protons. (These are protons with their spins lined up.) In particular he anayzed possible “triple scattering experiments” in which the first scattering polarized the protons, the second scattering changed the magnitude or direction of the spin, and the third scattering analyzed the final polarization. Following this suggestion such experiments were carried out at the Berkeley cyclotron as part of their study of proton-proton scattering. As a result Wolfenstein was invited to spend the spring of 1955 as a visiting lecturer at the University of California, Berkeley.

From 1957 on, Wolfenstein’s work was mainly in the area of weak interactions. In 1957 it was discovered that these interactions violated parity P, the symmetry betwen left and right. The theory that then developed had CP invariance, the symmetry between left-handed particles and right-handed antiparticles. In 1964 a small violation of CP symmetry was found in an experiment at Brookhaven involving particles known as K mesons. Wolfenstein pointed out that this could be due to a new interaction weaker than the usual weak interaction by a factor of 10 billion. This became the “superweak theory.” For the next 30 years a major goal of experiments in this area was to disprove this superweak idea. The first evidence against it came from experiments at CERN and Fermilab in the 1990s.

Wolfenstein continues to do much work on CP violation. Within the standard model of weak interactions, CP violation arises from a phase in a 3x3 matrix that connects the three quarks of charge –1/3 to three quarks of charge +2/3 via the weak interaction. Wolfenstein provided in 1983 a parameterization of this matrix that has become the standard form. From this it became clear that the best way to study CP violation involved particles called B mesons. In 1989 North Holland Press published a book of reprints on CP violation edited by Wolfenstein.

Much of his recent work has involved neutrinos. There are three kinds of neutrinos resulting from different weak processes, called n_e, n_m, and n_t. It was pointed out in 1960 that if neutrinos have mass and the neutrino, e.g. n_e, emitted in a process is in a mixture of mass eigenstates, then as the neutrino moves through the vacuum, n_e may transform into n_m or n_t. The quantum mechanics is the same as that of an electron precessing in a magnetic field. In 1978 Wolfenstein made the important observation that this neutrino oscillation can be modified in a material medium due to the neutrino index of refraction. This idea was applied to neutrinos from the sun by Russian physicists Mikhaeyev and Smirnov and is now known as the MSW effect. It is believed to explain the disappearence of about two-thirds of the n_e emitted from the sun. In the special centenary issue of the Reviews of Modern Physics, March 1999, Wolfenstein wrote the article on neutrino physics.