25/02/2021 at 4:22 AM #26337Anonymous UserParticipant
Puzzling asymmetry in the proton
Antimatter in the core component: In the proton, the strong nuclear force constantly creates short-lived quark-antiquark pairs – the so-called sea quarks. An experiment has now revealed that there is an asymmetry in the antimatter content of these quarks: more anti-down quarks are formed than anti-up quarks, as reported by researchers in the journal “Nature”. Why this is so remains unclear. The result could help to narrow down the theories.
The proton is one of the basic building blocks of matter, because together with the neutron it forms the atomic nucleus. According to popular doctrine, the proton consists of three quarks, which are held together by the strong nuclear force and its carrier particles, the gluons. The strange thing is that the mass of these two up and one down quarks alone make up only a fraction of the total proton mass.
“Lake” made up of short-lived quark-antiquark pairs
But where does the remaining mass of the proton come from? Physicists have long suspected that the strong nuclear force constantly creates short-lived particles through quantum physical fluctuations: pairs of quarks and their antimatter counterparts, the antiquarks. This “sea” of volatile pairs of particles – the sea quarks – surrounds the three quarks that shape the proton and gives the proton its additional mass.
“So far, however, we have only incomplete knowledge of how the quarks behave in the proton and how they shape the properties of the core building block,” explains co-author Paul Reimer from the Argonne National Laboratory in Illinois. “The volatile nature of the quark-antiquark pairs makes it difficult to research them.” But according to the models, there should be as many anti-up and anti-down quarks among the antimatter quarks.
Muons as messengers from the inside of the protons
Reimer, first author Jason Dove from the University of Illinois at Urbana-Champaign and her team have now checked whether this is the case with a special experiment. Because when protons collide at high speed, annihilation occurs – the particles and their antiparticles cancel one another out. During this extinction reaction, energy is released and further elementary particles, including muons, are created.
These muons show how many antiquarks were present in the protons and what type they were. “With this experiment we were able to take a closer look at the confusing dynamics inside the proton, says Reimer’s colleague Don Geesaman. “And check out some older concepts about the nature of the proton.”
Asymmetry in the Antiquarks
The experiment revealed: There is a clear asymmetry in the antimatter part of the short-lived quark pairs. Apparently, the short-lived quark-antiquark pairs are not created in arbitrary proportions, but in such a way that one antiquark variety predominates. “The production of muon pairs means that more anti-down quarks are created in the core than anti-up quarks,” the researchers report.
This asymmetry was evident over a wide range of proton energies. This contradicts the results of an earlier experiment in which it initially looked as if the proportions of the various antimatter particles in the proton were reversed at a certain energy of the proton. “Instead, we are now showing that this asymmetry remains constant and that there is no reversal of the antiquark ratios,” says Reimer.
However, why this asymmetry exists and how it arises is still completely unclear. There are some theories about the formation of the sea quarks and their characteristics. But so far there is a lack of clear data to determine which of them is correct. “You need experiments to narrow down the theories; nature itself has to give us an insight into the dynamics of the proton,” explains Geesaman. That is why it is only the combination of theory and experiment that leads to results.
The new results can now help to weed out or support theories, at the same time they also provide valuable information on where to continue experimental research. (Nature, 2021; doi: 10.1038 / s41586-021-03282-z)
Source: DOE / Argonne National Laboratory
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