Rare Events: Exotic processes probe the heart of matter

Physicists have for the first time unambiguously detected and measured the rates of certain rare reactions among protons, neutrons, and simple atomic nuclei, possibly opening a novel window onto the deepest nature of matter.

At Canada’s national TRIUMF cyclotron in Vancouver, British Columbia, researchers have observed unusual particle trajectories following proton-neutron collisions. At the Indiana University Cyclotron Facility in Bloomington, another team has studied never-before-detected fusions of deuterium nuclei, which contain one proton and one neutron. Both teams reported their results April 5 at a meeting of the American Physical Society in Philadelphia.

In both experiments, scientists focused on how the reactions violate so-called charge symmetry, a type of order among subatomic particles, says Edward Stephenson of Indiana University, leader of the experiment conducted there. Charge-symmetry violations have played an important role in shaping the universe.

Because of charge-symmetry violations, protons and neutrons differ in both charge and mass. In the early universe, the mass differences resulted in vast numbers of neutrons, which were not yet stabilized within nuclei, quickly decaying into protons. That process yielded a huge reservoir of protons that contributed to star formation.

The new results promise to yield important information about the up and down quarks that comprise protons and neutrons. Scientists haven’t had enough information to determine the masses of these quarks. The new cyclotron data ought to usher scientists closer to finding those values, comments theorist Bira van Kolck of the University of Arizona in Tucson.

Besides settling basic quantitative facts about matter, gauging those masses may grant physicists a glimpse within the quarks themselves to learn what, if anything, lurks there–perhaps more fundamental entities known as strings (SN: 9/22/01, p. 184: When Branes Collide), van Kolck adds.

In the experiment at the TRIUMF accelerator, a team lead by Allena K. Opper of Ohio University in Athens beamed neutrons against a tuna-can-size vessel containing protons in the form of liquid hydrogen cooled to 20 kelvins.

Each combination of a neutron and a proton produced both a deuterium nucleus, or deuteron, and a neutral pion particle. As this happened, the researchers observed the directions in which those products shot off from the point of impact. In 17 of every 10,000 events, they observed unusual trajectories emblematic of charge-symmetry breaking, Opper says.

These telling trajectories derived from the quarks’ electromagnetic fields and differences in their masses, Opper says. Her team has teased apart the contributions each effect had on the trajectories.

In the Indiana University study, Stephenson’s team collided a beam of deuterons into a target of deuterium gas. For the most part, the collisions shattered both projectile and target nuclei into loose protons and neutrons. However, in one of every 10 billion collisions, the deuterons fused to form a helium nucleus and a neutral pion. This marks the first observation of this reaction, which physicists have been looking for since the late 1950s.

As in the TRIUMF experiment, electromagnetic and mass-difference effects governed the frequency of the exotic fusion reactions. However, the relative strengths with which those factors act are different from those in the charge-symmetry violation spotted by the Opper group. The combined results may thus yield enough information for theorists to pinpoint the up and down quark masses, van Kolck says.

However, the calculations required for these determinations of mass are fabulously difficult. Spurred by the new experimental results, more than a half-dozen theorists including van Kolck have embarked together on an unusual crash program.

If things go well, in the next several years, the up and down quarks may finally weigh in, van Kolck says.


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