Matter And Anti-Matter

Flavour Detectors.

Neutrinos, subatomic remnants of the early universe, are high-energy particles that pass at nearly the speed of light through everything, our planet, our bodies, while rarely interacting with other matter. Most of them were born in the beginning, nearly 14 billion years ago, though more are continually made in the nuclear reactions of stars, in human-built nuclear reactors and in particle accelerators used for experimentation.

These ghostly particles are of intense interest to physicists because they may be a key player in how the universe came to be; how, in the first moments after the Big Bang, matter managed to annihilate antimatter, allowing a universe of particles to coalesce and form atoms, molecules, elements and compounds, the matter that became and becomes galaxies, black holes, planets and on one planet at least, life.

Physicists at the University of Virginia’s High-Energy Physics Lab are deeply engaged in a neutrino experiment, called NOvA, now under way at the U.S. Department of Energy’s Batavia, Illinois-based Fermilab, to help answer how and why matter came about at all.

They have fabricated key components for the experiment, and their research eventually could help solve a multitude of questions about the fabric of the universe, and possibly even about the mysteries of dark energy, which may make up 68 percent or more of the universe.

“We’re trying to answer fundamental and extremely difficult-to-answer questions about the makings of everything and the physical processes that still are going on in the active universe,” said Craig Dukes, a U.Va. physics professor who has led U.Va.’s NOvA efforts since 2004, and whose research group has built components for NOvA since 2008.

Using a $2.5 million Department of Energy grant, Dukes and his team of postdoctoral fellows, graduate and undergraduate students have designed and built a system that powers the electronics on a $280 million, 14,000-ton particle detector used for the project.

The components are part of a newly commissioned neutrino beam and experiment that is composed of two massive particle detectors: one at Fermilab, called the Near Detector, and the other, the Far Detector, 502 miles away near Ash River, Minnesota. The accelerator sends neutrinos through the detectors to allow scientists to glimpse how they change as they travel, and to catch their rare interactions with matter.

“By observing the behaviour of neutrinos and anti-neutrinos, we may be able to shed light on the asymmetry that caused the abundance of matter over anti-matter, the early universe, in effect, that became the universe we have today,” Dukes said.

For the next six years, Fermilab will send tens of thousands of billions of neutrinos every second in a beam aimed at both detectors, and scientists expect to catch only a few interactions per day.

From these data, scientists hope to learn more about how and why neutrinos change between one type and another. The three types, called flavours, are the muon, electron and tau neutrino. Over distance, neutrinos can flip between these flavours. NOvA is specifically designed to study muon neutrinos changing into electron neutrinos.

If it is found that neutrinos and anti-neutrinos change differently from one type to another, it might explain the process that occurred in the early universe to produce a slight abundance of matter over a nearly equal amount of anti-matter.

The experiments will be conducted over a six-year period. Sorting through the data with high-speed computers for interactions and their implications will continue for many additional years.

Slightly Unequal.

Most of the laws of nature treat particles and antiparticles equally, but stars and planets are made of particles, or matter, and not antiparticles, or antimatter. That asymmetry, which favours matter to a very small degree, has puzzled scientists for many years.

New research by UCLA physicists, published in the journal Physical Review Letters, offers a possible solution to the mystery of the origin of matter in the universe.

Alexander Kusenko, a professor of physics and astronomy in the UCLA College, and colleagues propose that the matter-antimatter asymmetry could be related to the Higgs boson particle, which was the subject of prominent news coverage when it was discovered at Switzerland’s Large Hadron Collider in 2012.

Specifically, the UCLA researchers write, the asymmetry may have been produced as a result of the motion of the Higgs field, which is associated with the Higgs boson, and which could have made the masses of particles and antiparticles in the universe temporarily unequal, allowing for a small excess of matter particles over antiparticles.

If a particle and an antiparticle meet, they disappear by emitting two photons or a pair of some other particles. In the “primordial soup” that existed after the Big Bang, there were almost equal amounts of particles of antiparticles, except for a tiny asymmetry: one particle per 10 billion. As the universe cooled, the particles and antiparticles annihilated each other in equal numbers, and only a tiny number of particles remained; this tiny amount is all the stars and planets, and gas in today’s universe, said Kusenko, who is also a senior scientist with the Kavli Institute for the Physics and Mathematics of the Universe.

The research also is highlighted by Physical Review Letters in a commentary in the current issue.

The 2012 discovery of the Higgs boson particle was hailed as one of the great scientific accomplishments of recent decades. The Higgs boson was first postulated some 50 years ago as a crucial element of the modern theory of the forces of nature, and is, physicists say, what gives everything in the universe mass. Physicists at the LHC measured the particle’s mass and found its value to be peculiar; it is consistent with the possibility that the Higgs field in the first moments of the Big Bang was much larger than its “equilibrium value” observed today.

The Higgs field “had to descend to the equilibrium, in a process of ‘Higgs relaxation,'” said Kusenko, the lead author of the UCLA research.

Two of Kusenko’s graduate students, Louis Yang of UCLA and Lauren Pearce of the University of Minnesota, Minneapolis, were co-authors of the study. The research was supported by the U.S. Department of Energy (DE-SC0009937), the World Premier International Research Center Initiative in Japan and the National Science Foundation (PHYS-1066293).


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