&ball; Physics 14, 155
Preliminary results from new experiments at Jefferson Laboratory suggest a solution to the 38-year-old nuclear physics mystery of why atomic nuclei have different scattering sections than expected.
In 1983, scientists in CERN’s European collaboration made an unexpected observation: the internal structures of protons and neutrons are not the same in the audience as they are in isolation. The team saw this effect in experiments in which iron nuclei and deuterium were bombarded with degenerates. In analyzing the scattering cross-sections of the nuclei – their effective collision zones – the scientists found that they were smaller than expected if you summed up the individual cross-sections of all the nuclei (protons and neutrons) that make up those nuclei.
A phenomenon known as the EMC effect after its discoverers, the non-sum-of-its-parts phenomenon has been opposed to explanation for almost 40 years. But researchers now think they were based on the physical basis of the impact. At the fall meeting of the APS Division in Nuclear Physics, Tyler Kotz of the Massachusetts Institute of Technology (MIT) and Tel Aviv University, Israel, presented preliminary results from experiments at the Thomas Jefferson National Acceleration Facility, Virginia, indicating why Is an interaction between pairs of nucleons. He and his colleagues have additional work to do to confirm their initial findings, but they hope to make the data available to the public soon.
Since the initial observation, scientists have published thousands of articles on the EMC effect. Many explanations have been suggested, but scientists think there are only two viable reasons. One is the so-called mean field change, in which the structure of a nucleon changes only by being in an environment containing other nucleons. The second reason is a different interaction between pairs of short-term coordinated nucleons resulting from their spatial overlap. Indirect experimental tests of the effect indicate that the latter is the most likely cause. But, until now, there has been no direct test of the hypothesis, something the new experiments change.
In their experiments, Kotz and his colleagues bombed a Duterium target with high-energy electrons. An electron can collide with a neutron or proton inside a deuterium nucleus. Previous experiments have only measured the scattered electrons, but only with this information, “it is impossible to really know whether it is scattered from a neutron or a proton,” says Kotz. To identify which nucleon was damaged in a particular event, Thorn and colleagues used detectors for both scattered electrons and for deterrent neutrons. According to these measurements, the team was able to reconstruct the distribution of quarks within the nucleus from which the electron was scattered, and more importantly, the interaction between the pair of protons and neutrons.
The results suggest that the internal structure of a proton changes when a spatial overlap with a neutron in correlation causes a strong short-term interaction between the two particles. “For the first time, we are seeing indications of nuclear interactions affecting the distribution of quarks,” says Or Chen, Kotz’s postdoctoral fellow at MIT and a collaborator on the project. “It’s not something we’ve ever had direct observations of in the past.”
As for why the short-term interaction is causing the structure to change, Hen says they still do not know. He suspects this is related to the confinement effects from the overlap of the quarks when the two nucleons are in this strongly related state, but further models and experiments will be required to confirm this idea. Thorn agrees. “It [result] He hinted at a very moving and seductive suggestion that we might be on to something. “
Catherine Wright is the deputy editor of Physics.