The more we learn about sub-atomic particles, the closer we are to understanding nuclear forces and the building blocks of the universe. Illustration: Katie Schmitt
By Jean Andrews
Physics & Astronomy
Dr. Daniel Phillips, Professor of Physics at Ohio University, co-authored a ground-breaking paper describing how patterns of protons scattered in lab experiments can be explained by examining interactions between the quarks that make up the protons.
Phillips and physicists Daris Samart and Carlos Schat published a on paper on “Parity-Violating Nucleon-Nucleon Force in the 1/Nc Expansion” Feb. 10 in Physical Review Letters. Samart is from Rajamangala University of Technology Isan in Thailand. Schat is from the Universidad de Buenos Aires.
“Physicists like to bash things together to find out what they’re made of,” said Phillips, Director of Ohio University’s Institute for Nuclear and Particle Physics. “We’re trying to better understand what happens in the sub-atomic level and, in doing so, gain insight about how nuclei are formed. We reviewed experiments where a powerful proton beam is directed at a proton target to see the direction in which two protons bounce apart after they collide.
“In the case of protons,” he continued, “there is a slight asymmetry, about 1 part in 10 million, between the numbers of times they bounce apart to the left, and the number of times they bounce apart to the right. This is because there is a very small component of the force between the quarks that make up the proton that is not left-right symmetric.”
A First-Time Explanation of Parity-Violating Forces
This breaking of the left-right symmetry, or parity violation, is only a small effect in proton-proton scattering, but theoretical physicists have been trying to understand its connection to the underlying inter-quark forces for more than 40 years. More broadly, understanding how proton-proton, and all nuclear forces emerge from the Standard Model of Particle Physics is a central goal of contemporary nuclear physics.
The work of Phillips, Schat, and Samart is ground-breaking because it is the first time theoretical physicists have been able to explain, starting from the Standard Model, why some parts of the parity-violating force between neutrons and protons are big, and why some are small. Of particular interest is that the pattern of sizes among components of the force is quite different than it is for the dominant, parity-conserving, piece of the nuclear force.
The researchers’ collaboration grew out of a visit by Samart to Ohio University in April 2014—a visit for which Samart received funding from the Government of Thailand. The Thai government is investing in research in order to build up scientific infrastructure in Thailand.
“Our work is already receiving significant attention from the experimentalists who search for left-right asymmetries in nuclear processes. It may prove to be a new paradigm for understanding parity violation in nuclear forces,” Phillips said.
Abstract: Several experimental investigations have observed parity violation (PV) in nuclear systems—a consequence of the weak force between quarks. We apply the 1/Nc expansion of QCD to the P-violating T-conserving component of the nucleon-nucleon (NN) potential. We show there are two leading-order operators, both of which affect p⃗ p scattering at order Nc. We find an additional four operators at order N0csin2θW and six at O(1/Nc). Pion exchange in the PV NN force is suppressed by 1/Nc and sin2θW, providing a quantitative explanation for its nonobservation up to this time. The large-Nc hierarchy of other PV NN force mechanisms is consistent with estimates of the couplings in phenomenological models. The PV observed in p⃗ p scattering data is compatible with natural values for the strong and weak coupling constants: there is no evidence of fine-tuning.
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