Early research After receiving his doctorate, Steinberger attended the
Institute for Advanced Study in
Princeton for a year. In 1949 he published a calculation of the lifetime of the neutral
pion, which anticipated the study of anomalies in quantum field theory. Following Princeton, in 1949, Steinberger went to the
Radiation Lab at the
University of California at Berkeley, where he performed an experiment which demonstrated the production of neutral pions and their decay to photon pairs. This experiment utilized the 330 MeV synchrotron and the newly invented scintillation counters. Despite this and other achievements, he was asked to leave the Radiation Lab at Berkeley in 1950, due to his refusal to sign the so-called non-Communist Oath. A direct measurement of the production of pions on a
liquid hydrogen target, then not a common tool, provided the data needed to show that the pion has spin zero. The same target was used to observe the relatively rare decay of neutral pions to a photon, an electron, and a
positron. A related experiment measured the mass difference between the charged and neutral pions based on the angular correlation between the neutral pions produced when the negative pion is captured by the proton in the hydrogen nucleus. Other important experiments studied the angular correlation between electron–positron pairs in neutral pion decays, and established the rare decay of a charged pion to an electron and neutrino; the latter required use of a liquid-hydrogen
bubble chamber.
Investigations of strange particles During 1954–1955, Steinberger contributed to the development of the
bubble chamber with the construction of a 15 cm device for use with the
Cosmotron at
Brookhaven National Laboratory. The experiment used a pion beam to produce pairs of
hadrons with
strange quarks to elucidate the puzzling production and decay properties of these particles. In 1956, he used a 30 cm chamber outfitted with three cameras to discover the neutral
Sigma hyperon and measure its mass. This observation was important for confirming the existence of the SU(3) flavor symmetry which hypothesizes the existence of the strange quark. An important characteristic of the
weak interaction is its violation of
parity symmetry. This characteristic was established through the measurement of the spins and parities of many
hyperons. Steinberger and his collaborators contributed several such measurements using large (75 cm) liquid-hydrogen bubble chambers and separated hadron beams at Brookhaven. One example is the measurement of the invariant mass distribution of electron–positron pairs produced in the decay of
Sigma-zero hyperons to
Lambda-zero hyperons.
Neutrinos and the weak neutral current In the 1960s, the emphasis in the study of the weak interaction shifted from strange particles to neutrinos. Leon Lederman, Steinberger and
Schwartz built large spark chambers at
Nevis Labs and exposed them in 1961 to neutrinos produced in association with muons in the decays of charged pions and kaons. They used the
Alternating Gradient Synchrotron (AGS) at Brookhaven, and obtained a number of convincing events in which muons were produced, but no electrons. This result, for which they received the Nobel Prize in 1988, proved the existence of a type of neutrino associated with the muon, distinct from the neutrino produced in beta decay.
Study of CP violation The
CP violation (
charge conjugation and
parity) was established in the neutral kaon system in 1964. Steinberger recognized that the phenomenological parameter epsilon (
ε) which quantifies the degree of CP violation could be measured in interference phenomena (See CP violation). In collaboration with
Carlo Rubbia, he performed an experiment while on sabbatical at
CERN during 1965 which demonstrated robustly the expected interference effect, and also measured precisely the difference in mass of the short-lived and long-lived neutral kaon masses. Back in the United States, Steinberger conducted an experiment at Brookhaven to observe CP violation in the semi-leptonic decays of neutral kaons. The charge asymmetry relates directly to the epsilon parameter, which was thereby measured precisely. This experiment also allowed the deduction of the phase of epsilon, and confirmed that
CPT is a good symmetry of nature.
CERN In 1968, Steinberger left Columbia University and accepted a position as a department director at
CERN. He constructed an experiment there utilizing multi-wire proportional chambers (
MWPC), recently invented by
Georges Charpak. The MWPCs, augmented by micro-electronic amplifiers, allowed much larger samples of events to be recorded. Several results for neutral kaons were obtained and published in the early 1970s, including the observation of the rare decay of the neutral kaon to a muon pair, the time dependence of the asymmetry for semi-leptonic decays, and a more-precise measurement of the neutral kaon mass difference. A new era in experimental technique was opened. These new techniques proved crucial for the first demonstration of direct
CP-violation. The
NA31 experiment at CERN was built in the early 1980s using the CERN
SPS 400 GeV proton synchrotron. As well as banks of MWPCs and a hadron calorimeter, it featured a liquid
argon electromagnetic calorimeter with exceptional spatial and energy resolution. NA31 showed that direct CP violation is real. Steinberger worked on the
ALEPH experiment at the
Large Electron–Positron Collider (LEP), where he served as the experiment's spokesperson. Among the ALEPH experiment's initial accomplishments was the precise measurement of the number of families of
leptons and
quarks in the
Standard Model through the measurement of the decays of the
Z boson. He retired from CERN in 1986, and went on to become a professor at the
Scuola Normale Superiore di Pisa in Italy. He shared the prize with
Leon M. Lederman and
Melvin Schwartz; at the time of the research, all three experimenters were at
Columbia University. The experiment used charged
pion beams generated with the Alternating Gradient Synchrotron at
Brookhaven National Laboratory. The pions decayed to
muons which were detected in front of a steel wall; the
neutrinos were detected in spark chambers installed behind the wall. The coincidence of muons and neutrinos demonstrated that a second kind of neutrino was created in association with muons. Subsequent experiments proved this neutrino to be distinct from the first kind (electron-type). Steinberger, Lederman and Schwartz published their work in
Physical Review Letters in 1962. He was also awarded the
National Medal of Science in 1988, by the then US president,
Ronald Reagan ==Selected publications==