Different particles get different treatment i

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NEWPORT NEWS, VA — For nearly four decades, scientists have known that the protons and neutrons comfortably clustered inside an atom’s nucleus are different from those roaming freely in the cold vacuum of space. Now, for the first time, nuclear physicists at the Department of Energy’s Thomas Jefferson National Accelerator Facility have shown that if the two particles are modified by their residence inside a nucleus, they can be affected differently. The results were recently published in Physical examination letters.

It comes from a new theoretical analysis from the Jefferson Lab Angular Momentum (JAM) collaboration of data from the MARATHON experience at the Jefferson Lab Experimental Room a. The experience used a new experimental technique who took advantage the first tritium target deployed for a scattering experiment in decades. the JAM collaboration also included tons of other experimental data from decades of experiments in their new analysis. Many of these experiments aimed to discover the internal structures of protons and neutrons (nucleons) – but not in the critical test range of the MARATHON data, and not with mirror nuclei.

Comparisons of early data from the 1980s on heavy and light nuclei revealed that the building blocks of the proton, called quarks, are distributed differently inside nuclei compared to free nucleons. This difference was first noticed by the European Muon Collaboration at CERN, the European Organization for Nuclear Research. The phenomenon has therefore been dubbed the “EMC effect”.

“The really exciting thing about this analysis is that it gives us the first empirical clue that nuclear effects on protons and neutrons bound in the nuclear EMF effect are different for protons and neutrons,” Wally explained. Melnitchouk, a Jefferson Lab scientist and , as well as Jefferson Lab theorist Nobuo Sato, spokesperson for the JAM collaboration. “If it’s different for protons and neutrons, then it’s probably different for up and down quarks.”

Protons and neutrons are each composed of three quarks bound by the strong force. Protons have two up quarks and one down quark. Neutrons, on the other hand, have two down quarks and one up quark. So while protons and neutrons are mostly composed of the same two quark flavors, they have different ratios for each.

It was partly an interest in what they could learn from these different quark ratios that originally led nuclear physicists to obtain the data set that made the results of this analysis possible. In 2006, the MARATHON collaboration, led by researchers from Kent State University and Jefferson Lab, proposed to measure the ratio of up quarks to down quarks in mirror nuclei. Mirror nuclei are those that contain opposite numbers of protons and neutrons.

The MARATHON collaboration looked at nuclei found in helium-3 and an isotope of hydrogen called tritium. Helium-3 has two protons and one neutron, while tritium has one proton and two neutrons. The MARATHON physicists designed their experiment to provide the best data ever collected to determine the structures of these nuclei and the distribution of up and down quarks within them.

“The beautiful thing about this is that one of the things that drove the MARATHON experiment was to capture the best information ever about the ratios of the down and up quark distributions in these nuclei,” said Melnichuk. “What we didn’t know when the experiment was proposed was that we would have additional data from other experiments somewhat constraining the ratios of down and up quarks and that MARATHON would be much more sensitive to nuclear corrections.”

One such compelling experiment is the Jefferson Lab Hall B BONUS experiment, which deployed a new labeling technique to create an efficient free neutron target.

These nuclear corrections can be used by nuclear physicists to gain information about the degree to which the distribution of quarks up and down in a particular nucleus is altered from those that are free or inside other cores. Some physicists, including Melnitchouk and his colleagues, expected that these nuclear effects, resulting from the EMF effect, would “cancel” among the sum total of the particles.

This would mean that nuclear physicists could treat the bound particles they can access in experiments as if they were free particles in nature. This would allow a freer understanding of how up and down quarks organize inside protons and neutrons, without the confusing effects of being wrapped inside a nucleus. It would also mean that nuclear physicists could treat the distribution of up quarks in a nucleus as if they were down quarks in a mirror nucleus, allowing for easier comparison.

To test this idea, Melnitchouk and his colleagues in the JAM collaboration, including Christopher Cocuzza of Temple University, used the MARATHON data in their complex and computationally demanding analysis of available experimental data sets.

“The idea was to take the MARATHON data and use it in global analysis machines. We could then simultaneously extract information about the up and down distributions and other quarks and possible differences in nuclear effects between protons and neutrons,” he said.

What the JAM collaboration found is that these effects don’t seem to cancel each other out. The EMC effect exerts more influence on the distribution of down quarks, and therefore of the neutrons in which they reside, inside a nucleus compared to up quarks and protons.

“That’s what the MARATHON data set was able to show us. The effects have a different sign for up and down quarks. Not only different in magnitude but also in sign. Until MARATHON, we had no sign or magnitude information,” he said.

Now, he said, it’s clear that experiments that seek to reveal new information about different flavors of quarks may need to consider how different quarks are processed inside different nuclei.

Cynthia Keppel, Jefferson Lab’s associate director for the Experimental Nuclear Physics division, and co-author of both results, noted that “it’s a tantalizing enough observation that it requires additional experiments to verify it.”

If true, the result could impact experiments in a wide range of fields, including neutrino physics, heavy ion physics and astrophysics.

Melnitchouk pointed out that the result was made possible by the excellent dataset recently obtained through the determined efforts of the MARATHON experiment. “The reason we were able to say something is that the new MARATHON data is remarkable. They give us unique insights into the structures of helium-3 and tritium nuclei that were not possible before,” he said.

Further reading:
C. Cocuzza, CE Keppel, H. Liu, W. Melnitchouk, A. Metz, N. Sato, and AW Thomas (Jefferson Lab Angular Momentum (JAM) Collaboration). “Isovector EMC Effect from Global QCD Analysis with MARATHON Data.” Phys. Rev. Lett. 127242001
Technical proposal of the MARATHON experience
The DOE Explains: Nuclei
The DOE Explains: Quarks and Gluons
Proton’s Party Pals can change their internal structure
Pocket detector

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Jefferson Science Associates, LLC, operates the Thomas Jefferson National Accelerator Facility, or Jefferson Lab, for the U.S. Department of Energy’s Office of Science.

DOEs Office of Science is the largest supporter of basic physical science research in the United States and works to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science.

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