Maurice Goldhaber, Ukrainian Jewish-American physicist and academic (d. 2011)

Maurice Goldhaber (April 18, 1911 – May 11, 2011) was an extraordinarily influential American physicist whose illustrious career spanned much of the 20th century. Living to the remarkable age of 100, Goldhaber made profound contributions to our understanding of the fundamental nature of matter, most notably through his pivotal work in particle physics. In 1957, in a groundbreaking collaboration with his colleagues Lee Grodzins and Andrew Sunyar, he definitively established a crucial property of neutrinos: that they possess negative helicity. This experimental confirmation was a cornerstone in developing the modern Standard Model of particle physics, forever changing our view of the universe's weak nuclear force.

A Life Dedicated to Science

Born in Czernowitz, Austria-Hungary (now Chernivtsi, Ukraine), Maurice Goldhaber immigrated to the United States and became a naturalized citizen, embarking on a scientific journey that would span over seven decades. His intellectual curiosity and dedication to experimental physics led him to prominent institutions, including the University of Illinois and later, the Brookhaven National Laboratory (BNL). At BNL, he served with distinction, eventually becoming its Director from 1961 to 1973. His leadership at such a critical research facility underscored his scientific acumen and his commitment to fostering groundbreaking discoveries, further cementing his legacy beyond his experimental achievements.

The Groundbreaking Neutrino Helicity Experiment

The year 1957 proved to be a watershed moment for Goldhaber and his team, as they conducted an experiment that elegantly determined the helicity of neutrinos. This was a critical piece of the puzzle following the revolutionary discovery of parity non-conservation in weak interactions earlier that same year, setting the stage for a deeper understanding of fundamental forces.

Understanding Helicity

In the realm of quantum mechanics, helicity describes the orientation of a particle's spin relative to its direction of motion. Imagine a screw: if its rotation (spin) matches the direction it's moving, it's akin to positive helicity (right-handed); if the rotation is opposite to its motion, it's negative helicity (left-handed). For massless or ultra-relativistic particles like neutrinos, helicity is a particularly significant and well-defined property. The prevailing theoretical framework, the V-A (Vector minus Axial-vector) theory of weak interactions, predicted that neutrinos should only exist as left-handed particles.

The Ingenious Experimental Setup

To experimentally verify this, Goldhaber, Grodzins, and Sunyar devised a clever setup involving Europium-152 (¹⁵²Eu). When ¹⁵²Eu undergoes electron capture, it transforms into an excited state of Samarium-152 (¹⁵²Sm*), emitting a neutrino in the process. This excited Samarium nucleus then quickly decays by emitting a gamma ray. The key insight was that if the neutrino has a specific helicity, the gamma ray emitted in the subsequent decay would carry information about that helicity through its circular polarization. By using a sophisticated magnetic field to select gamma rays that scattered forward or backward relative to their initial emission and precisely measuring their circular polarization, the team was able to infer the helicity of the neutrinos with remarkable accuracy.

Impact and Legacy

The results of the Goldhaber-Grodzins-Sunyar experiment unequivocally showed that neutrinos possess negative helicity, meaning they are "left-handed." This profound discovery provided crucial experimental validation for the V-A theory of weak interactions, which posits that only left-handed particles (and right-handed antiparticles) participate in these fundamental forces. It became a cornerstone of the Standard Model of particle physics, helping to shape our understanding of the universe's fundamental particles and their interactions, and solidifying Maurice Goldhaber’s place as a towering figure in 20th-century physics.

Frequently Asked Questions

What is a neutrino?

A neutrino is a fundamental subatomic particle, famous for being extremely light (though not massless as once thought) and interacting very weakly with other matter. Produced in nuclear reactions like those in the Sun or supernovae, billions of neutrinos pass through us every second, largely unnoticed due to their weak interaction.

What does it mean for a neutrino to have "negative helicity"?

Negative helicity means that the neutrino's intrinsic angular momentum (its spin) is oriented in the opposite direction to its linear momentum (its direction of motion). In simpler terms, if you imagine the neutrino moving forward, its spin would be counter-clockwise from its perspective, making it "left-handed."

Why was Goldhaber's 1957 discovery so important?

It provided crucial experimental proof for the V-A theory of weak interactions, which was a fundamental theoretical framework at the time. This confirmed that neutrinos are indeed left-handed, a key aspect that helped build the Standard Model of particle physics and our understanding of how the universe's fundamental forces operate, particularly the weak nuclear force responsible for radioactive decay.

Who were Maurice Goldhaber's collaborators on this experiment?

He collaborated with fellow physicists Lee Grodzins and Andrew Sunyar at the Brookhaven National Laboratory, forming a highly effective team for this intricate experiment.

What is the V-A theory of weak interactions?

The V-A (Vector minus Axial-vector) theory is a quantum field theory that describes the weak nuclear force, one of the four fundamental forces of nature. It famously predicts that the weak interaction only couples to left-handed particles and right-handed antiparticles, a prediction dramatically confirmed by the Goldhaber experiment regarding neutrinos, and later extended to other particles.