Why do we exist in a world full of matter rather than antimatter is one of the major scientific questions of our time. Wherever we look, matter appears to have the upper hand over antimatter, despite the fact that we think matter and antimatter were produced in equal proportions shortly after the Big Bang. There must be a difference in how matter and antimatter behave to reconcile these two truths.
Experiments with accelerators that produce heavy quark particles reveal some asymmetry with their antiparticle counterparts, a phenomenon known as Charge-Parity Violation, or CPV.
Despite the fact that there is a variation between heavy particles and their antiparticles, the total influence of these variations is too tiny to explain the early universe’s imbalance between matter and antimatter.
Another possibility is that neutrinos, which are very light neutral particles, behave differently from their antimatter cousins, antineutrinos. Neutrinos are charged particles with a very tiny mass that interact with regular matter extremely weakly. Neutrinos are divided into three categories: electron, muon, and tau neutrinos. They have a strange characteristic known as neutrino oscillations, which allows them to shift from one kind to another during in flight, earning them the Nobel Prize in Physics in 2015.
The symmetry between matter and antimatter is violated in neutrino oscillations, according to a new research study published today in Nature and sponsored by academics from the University of Glasgow.
The T2K cooperation used facilities at the Japan Proton Accelerator Research Complex (J-PARC) facility in Tokai on Japan’s east coast to conduct research for the new article. The T2K team’s accelerator produces muon neutrinos and antineutrinos, which travel 295 kilometers to the massive Super-Kamiokande detector, which is housed in a 50,000-ton water tank beneath a mountain near Kamioka, Japan’s west coast.
The muon neutrinos and antineutrinos convert to electron neutrinos and antineutrinos throughout this journey, exhibiting neutrino oscillations. T2K was the first to notice the arrival of electron neutrinos and antineutrinos in 2011. T2K has discovered a substantial difference between neutrino and antineutrino oscillations for the first time.
Neutrino oscillations are caused by the strange realm of quantum mechanics. The relative phase between the neutrino and anti-neutrino states, termed delta CP, is the parameter that determines the difference between matter and antimatter in neutrino oscillations.
This parameter has a range of -180° to +180°. There is no difference between neutrino and antineutrino oscillations when the parameter is 0° or 180°, thus physicists conclude CP is symmetric. Antineutrinos oscillate at a faster rate than neutrinos when the CP phase is larger than zero, and this difference is greatest at +90°. When the CP phase is lower than zero, neutrinos change in flight more frequently than antineutrinos, and scientists talk about CP violation in both situations.
The CP phase of 0° is disfavored by the experiment at more than 3 standard deviations (3), equivalent to a likelihood of less than 0.3 percent that the results are merely an incidental fluctuation, according to the data released today. In fact, at more than 3 standard deviations, approximately half of the parameter space for the CP phase is eliminated (see Figure 1).
At the J-PARC accelerator facility, muon neutrinos and antineutrinos are produced in large beams. The pattern of light in the Super-Kamiokande detector from the production of electrons and muons in their collisions in the water distinguishes muon and electron neutrinos (and antineutrinos). Super-Kamiokande detected 90 electron neutrinos and just 15 electron antineutrinos after 9 years of data collection.
If the CP phase is -90°, Super-Kamiokande should have seen 82 electron neutrinos and 17 electron antineutrinos, but if the CP phase is +90°, Super-Kamiokande should have seen 56 electron neutrinos and 22 electron antineutrinos. The experiment obviously favors a number less than zero, with a hint that the difference might be approaching its maximum value of -90°.
While the experiment clearly favors neutrino oscillation over antineutrino oscillation, it is still unclear if the CP symmetry is broken. To improve these results, the T2K experiment is improving its experimental facilities, and J-beam PARC’s intensity will be increased to improve experimental sensitivity.
Since 2018, the University of Glasgow team has been a part of the partnership and has helped to build a fresh new detector system as part of J-improved PARC’s facilities.
Dr. Phillip Litchfield, the leader of the Glasgow neutrino oscillation project, has been a T2K member since 2007 and was a co-recipient of the 2016 Breakthrough Prize.
“We’ve known for more than 50 years that a violation of CP symmetry might explain the existence of matter in the cosmos, but we haven’t been able to discover a mechanism that works in practice,” he added. This finding suggests that neutrinos may play a crucial role in that mechanism.”
Professor Paul Soler went on to say: “The characteristics of neutrino and antineutrino oscillations are likely to provide the key to comprehending the distinction between matter and antimatter. This provides the first indications that CP violation in neutrino oscillations is imminent.”
“The disparity between the matter-antimatter asymmetry we notice in experiments at the microscopic scale and what we observe in the universe is one of the big open questions in physics,” said Professor Lars Eklund, “and this result suggest that neutrinos could provide a new mechanism that can generate this asymmetry.”
A postdoctoral research assistant, Dr. John Nugent, said: “This fascinating finding hints to the importance of the CP phase in comprehending the distinction between matter and antimatter in our universe.”