Professor of Physics David Hitlin has been awarded the 2016 W.K.H. Panofsky Prize in Experimental Particle Physics from the American Physical Society. The $10,000 prize, established in 1985, is given annually to recognize achievements in high energy particle physics. The 2016 prize is shared with Jonathan Dorfan, Stephen Olsen, and Fumihiko Takasaki "for leadership in the BABAR and Belle experiments, which established the violation of CP symmetry in B meson decay, and furthered our understanding of quark mixing and quantum chromodynamics," according to the prize citation.

Hitlin was the founding spokesperson of the BABAR collaboration, and Dorfan directed the construction of the PEP-II asymmetric B factory and was also the technical coordinator during construction of BABAR. Dorfan is the director emeritus of the SLAC National Accelerator Laboratory and president of the Okinawa Institute of Science and Technology. BABAR is a particle detector designed to study unstable elementary particles called neutral B mesons, composed of a third-generation bottom quark and a down quark.

BABAR, and a similar experiment called Belle, carried out simultaneously at the High Energy Accelerator Research Organization in Tsukuba, Japan, led by Stephen Olsen and his colleague Fumihiko Takasaki, made the first measurements of the time-dependent Charge-Parity (CP)-violating asymmetry in the rare decay of neutral B mesons to a J/psi meson and a short-lived neutral K meson. These results established the validity of the model of Makoto Kobayashi and Toshihide Maskawa, who showed that if there were three families of quarks, there would be a CP-violating phase. This phase could cause a difference in the decay rate of neutral B mesons and their antiparticles to this final state. This CP-violating phase could help explain why our universe is dominated by matter, while at the time of the Big Bang there were equal amounts of matter and antimatter.

In 2001, BABAR and Belle convincingly measured this CP-violating phase, finding its value to be consistent with the predictions of the model. Kobayashi and Maskawa subsequently shared the Nobel Prize in Physics in 2008 for "the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature."

"This is an award that is long overdue," says Mark Wise, Caltech's John A. McCone Professor of High Energy Physics. "The measurements by BABAR and Belle of CP non-conservation in B decays showed that the CP non-conservation observed in weak decays is intimately connected with the generation of mass for the quarks. It didn't have to be that way, and this discovery was a milestone achievement in particle physics."

While BABAR ended data taking in 2008, the analysis of the data set continues and so far has led to 550 published papers. BABAR has subsequently refined and extended its measurements to many other B meson decays as well as charm meson and tau lepton decays, that have provided compelling evidence for the validity of the Standard Model of elementary particle physics, which unifies into a single theory the smallest building blocks of matter and three of nature's four forces.

"This award recognizes the contributions of more than 600 physicists and engineers from 75 institutions in 13 countries," Hitlin says. "I was fortunate to have the opportunity to lead this extraordinary collaboration."

Hitlin arrived at Caltech in 1979 as an associate professor and became a full professor in 1986. He was the principal investigator of the Caltech High Energy Physics group from 1994 to 2010 and is a Fellow of the American Physical Society.

The name BABAR is not an acronym, as are the names of most high energy physics experiments. Rather, it is derived from the name of the elephant Babar, the main character of a set of children's books, and refers to the B and anti-B (Bbar) mesons, which are at the heart of the experiment.