Or Hen, MIT assistant professor of physics and a researcher in the Laboratory for Nuclear Science, is the recipient of the 2018 Guido Altarelli Award. The award honors the memory of the late Italian physicist Guido Altarelli, a pioneer of the unraveling of the strong interaction and the structure of hadrons, an outstanding communicator of particle physics, and a mentor and strong supporter of junior scientists.
The citation of the award is “for his role in uncovering a striking relation of the nuclear EMC effect and the strength of nucleon-nucleon correlations, with implications for the constraint of the down and up quark distribution ratio at large x.” Hen received the award at the 26th International Workshop on Deep-Inelastic Scattering, in Kobe, Japan, where he also presented a plenary talk on this work.
The EMC effect is an observation that the proton and neutron’s building blocks, called quarks, are distributed differently in heavy versus light nuclei. Put into simple words, it implies that the internal structure of protons and neutrons, commonly called nucleons, is modified when bound in an atomic nucleus. The first observation of this effect was very surprising and immediately drew vast attention from theorists trying to explain it and experimentalist measuring it in various nuclei.
Thirty-five years later, with over 1,000 papers written in an attempt to explain it, we still lack an accepted explanation for the origin of the EMC effect. “It’s challenging because of the very different energy scales involved in the problem” says Hen. “The average nuclear binding energy is so small compared to the energy due to the interactions between quarks, that it doesn’t make any sense that the quarks will be significantly impacted by the nuclear environment.” Hen explains, “It’s like the fact that it’s a windy day will affect the way we move around the room, when all the windows are closed.”
To come up with a plausible explanation to this effect, Hen and collaborators took a different view on the problem. Instead of considering ‘static’ effects, that would modify all nucleons all the time, they considered temporal fluctuation effects, that significantly modify some of the nucleons for part of the time. These temporal fluctuations are pairs of nucleons that occasionally get close to each other and experience much stronger interactions than the average. They are referred to as “short-range correlations” and are the primary focus of the research program of Hen and his group at MIT.
By analyzing various data sets, Hen and his collaborators were able to show that the average amount by which a nucleon is modified in a nucleus is linearly correlated with the number of such short-range correlated nucleon pairs. “Prof. Hen’s work provides a vital new direction for future work by both theorists and experimentalists.” says Professor Gerald Miller from the University of Washington. “It also allowed gaining new insight to the structure of the free neutron which has vast implications to our understanding of the strong nuclear interaction.”
The Hen group is also leading the next generation of experiments at the Department of Energy’s Thomas Jefferson National Accelerator Facility located in Virginia, where they will test their theory by studying how the internal structure of correlated nucleon pairs is modified as a function of their relative distance. The experiments rely on state-of-the-art fast neutron detectors and laser calibration systems being developed by Hen’s group at MIT’s Laboratory for Nuclear Science and the BATES engineering research center.
Hen received his undergraduate degree in physics and computer engineering from the Hebrew University and earned his PhD in experimental physics at Tel-Aviv University. Prior to joining the MIT physics faculty in 2017, Hen was an MIT Pappalardo Fellow in Physics. Hen has previously received various prizes and fellowships for his work including: the Bose Fellowship, the Fermi Lab Intensity-Frontier fellowship, the Rothschild Fellowship, and the A. Pazi and J. Eisenberg research awards.
This work is supported by the Office of Science at the U.S. Department of Energy.