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Posts Tagged ‘CP Violation’

A measurement to watch

Monday, October 12th, 2015

This article appeared in symmetry on Oct. 7, 2015.

Finding a small discrepancy in measurements of the properties of neutrinos could show us how they fit into the bigger picture.

Finding a small discrepancy in measurements of the properties of neutrinos could show us how they fit into the bigger picture.

Physics, perhaps more so than any other science, relies on measuring the same thing in multiple ways. Different experiments let scientists narrow in on right answers that satisfy all parties—a scientific system of checks and balances.

That’s why it’s exciting when a difference, even a minute one, appears. It can teach physicists something about their current model – or physics that extends beyond it. It’s possible that just such a discrepancy exists between a certain measurement of neutrinos coming out of accelerator experiments and reactor-based experiments.

Neutrinos are minuscule, neutral particles that don’t interact with much of anything. They can happily pass through a light-year of lead without a peep. Trillions pass through you every second. In fact, they are the most abundant massive particle in the universe—and something scientists are, naturally, quite keen to understand.

The ghostly particles come in three flavors: electron, muon and tau. They transition between these three flavors as they travel. This means that a muon neutrino leaving an accelerator at Fermi National Accelerator Laboratory in Illinois can show up as an electron neutrino in an underground detector in South Dakota.

Not complicated enough for you? These neutrino flavors are made of mixtures of three different “mass states” of neutrinos, masses 1, 2 and 3.

At the end of the day, neutrinos are weird. They hang out in the quantum realm, a land of probabilities and mixing matrices and other shenanigans. But here’s what you should know. There are lots of different things we can measure about neutrinos—and one of them is a parameter called theta13 (pronounced theta one three). Theta13 relates deeply to how neutrinos mix together, and it’s here that scientists have seen the faintest hint of disagreement from different experiments.

Accelerators vs. reactors

There are lots of different ways to learn about neutrinos and things like theta13. Two of the most popular involve particle accelerators and nuclear reactors.

The best measurements of theta13 come from nuclear reactor experiments such as Double Chooz, RENO and Daya Bay Reactor Neutrino Experiment based in China (which released the best measurement to date a few weeks ago).

Detectors located near nuclear reactors provide such wonderful readings of theta13 because reactors produce an extremely pure fountain of electron antineutrinos, and theta13 is closely tied to how electron neutrinos mix. Researchers can calculate theta13 based on the number of electron antineutrinos that disappear as they travel from a near detector to the far detector, transforming into other types.

Accelerators, on the other hand, typically start with a beam of muon neutrinos. And while that beam is fairly pure, it can have a bit of contamination in the form of electron neutrinos. Far detectors can look for both muon neutrinos that have disappeared and electron neutrinos that have appeared, but that variety comes with a price.

“Both the power and the curse of long-baseline neutrino oscillation is that it’s sensitive to all of neutrino oscillation, not just theta13,” says Dan Dwyer, a scientist at Lawrence Berkeley National Laboratory and researcher on Daya Bay.

With that in mind, we come to the source of the disagreement. The results coming out of accelerator-based experiments, such as the United States-based NOvA and Japan-based T2K, see just a few more electron neutrinos than researchers would predict based on what the reactor experiments are saying.

“The theta13 value that fits the beam experiments, that really describes how much electron neutrino you get, is somewhat larger than what Daya Bay, RENO and Double Chooz measure,” says Kate Scholberg, professor of physics at Duke University and researcher on T2K. “So there’s a little bit of tension.”

Many grains of salt

Data coming out of the accelerator experiments is still very young compared to the strong readings from reactor experiments, and it is complicated by the nature of the beam. No one is jumping on the discrepancy yet because it can be explained in different ways. Most importantly, the accelerator experiments just don’t have enough information.

“We have to wait for T2K and NOvA to get sufficient statistics, and that’s going to take a while,” says Stephen Parke, head of the Theoretical Physics Department at Fermilab. Parke, Scholberg and Dwyer all estimated that about five more years of data collection will be required before researchers are able to start saying anything substantial.

“There’s been a lot of pressure on Daya Bay to try to eke out as precise a measurement as we possibly can,” Dwyer says. “Every bit of increased precision we provide further improves the ability of NOvA and T2K and eventually [proposed neutrino experiment] DUNE to measure the other parameters.”

Finding meaning in neutrinos

If the accelerator experiments gather more data and if a clear discrepancy emerges—a big if—what does it mean?

Turns out there are lots of reasons to love theta13. It’s one of the fundamental parameters that can define our universe. From a practical standpoint, it helps design future experiments to better understand neutrinos. And it could help physicists learn something new.

“We don’t expect things not to agree, but we kind of hope that they won’t,” says André de Gouvêa, professor of physics at Northwestern University. “It means that we’re missing something.”

That something could be CP violation, evidence that neutrinos and antineutrinos behave differently. CP violation has never been seen in neutrinos before, but if researchers observed it with accelerator experiments, it could help explain why our universe is made of matter rather than equal parts of matter and antimatter.

Figuring out if CP violation is occurring means nailing down all of the different neutrino mixing parameters, which in turn means building more powerful, next-generation experiments such as Hyper-K in Japan, JUNO in China and the Deep Underground Neutrino Experiment in the United States. DUNE will build on oscillation experiments like NOvA but will be able to better separate background noise from neutrino events, see a broader energy spectrum of neutrinos and find other neutrino characteristics.

DUNE, which will be built in a repurposed gold mine in South Dakota and detect neutrinos passed 800 miles through the Earth from Fermilab in Illinois, will be one of the best ways to see CP violation and rely on expertise gained from smaller neutrino experiments.

“Developing these types of experiments is very complicated,” de Gouvêa says. One of the major challenges of physics experiments is making sure you are measuring what you think you are measuring. “That’s part of the reason why we have a significant number of neutrino oscillation experiments.”

Ultimately, the neutrino puzzle is still missing many pieces. A variety of experiments are ramping up to fill in the gaps, making it an exciting time to be a neutrino physicist.

“We have to untangle the mysteries of the neutrino, and it’s not easy,” Parke says. “The neutrino doesn’t give up her secrets very easily.”

Lauren Biron

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Where Do I Come From?

Wednesday, February 4th, 2015

It’s the oldest question in the world and it occurs to every child, sooner or later: where do I come from? Mum and Dad of course, but where did they come from? Genetics only takes us so far; our line of ancestors actually stretches back beyond our first single celled forebears. Chemistry proceeds biology, and before that the world was made only of protons, neutrons and electrons. Now this takes us pretty far back, to the first second of the universe. In many ways, our fate was decided in this instant. The protons and neutrons we are made of formed a millionth of a second after the proverbial lights went on, condensing out of quarks. But where did the quarks come from?

Photo courtesy of NASA

Photo courtesy of NASA

Baryogenesis as a concept is not too difficult to follow. Every molecule you see around you is a survivor of a vast catastrophe that struck the early universe, when 30,000,000 of every 30,000,001 quarks in the universe were destroyed. The culprit of this disaster is antimatter – the bizarro version of matter. The crux of the matter is that matter and antimatter have a love-hate relationship; they annihilate each other, but also prefer to be created together. In the present day our universe is just too cold to create matter out of thin air (actually, through interactions with particles like photons), but this was not always so. When we go far enough back, at temperatures of about 10^13 degrees Celsius pair creation kicks off and the universe is filled with massive amounts of matter and antimatter. While this is lukewarm for a particle physicist there are more orders of magnitude between this temperature and the sun’s core than the sun’s core and you. From what I have said, the origin of matter doesn’t seem like much of a mystery; pair creation made matter. The problem is that it also made antimatter, and (according to the Standard Model) in equal amounts. When the universe cooled, matter could no longer be created, only destroyed, and so both matter and antimatter dwindled into nothing.

Creation (or destruction) of an electron-positron pair. Canny readers will notice that I have used this little diagram before.

Creation (or destruction) of an electron-positron pair. Canny readers will notice that I have used this little diagram before.

Clearly this is not the case – as any child can see, our universe is a populated and interesting one, filled with stars and planets and puppies. Above all, our universe is made of matter – no antimatter allowed. So there must have been a kind of discrimination against antimatter for some matter to survive this rampant destruction. Either this asymmetry between matter and antimatter existed from the start, as some sort of initial condition, or it somehow has dynamically evolved since then. Inflation dilutes any primordial asymmetry even more than a homeopathic remedy, so there must have been some matter creating process – baryogenesis. In any case, simply citing “initial conditions” is almost like saying “just because”, which never really seems to work with children.

When you need to explain something, it is usually best to start by what broad features your theory must have. For baryogenesis, Sahkarov did this back in 1967. For any theory (that doesn’t violate CPT) to create an asymmetry between matter and antimatter, three conditions must be satisfied:

  1. Baryon number must be violated. If you wish to count the number of protons and neutrons, it turns out that assigning them a quantity called “baryon number” is useful, a proton and neutron each have a baryon number of 1, and a quark has a baryon number of 1/3. Antimatter versions have a negative baryon number. The process that leads to the predominance of matter over antimatter, and so baryons over anti-baryons, is referred to as “baryogenesis”. It turns out that the total baryon number of the universe is conserved under perturbative effects in the Standard Model, what is known as an “accidental symmetry”. If we want more protons than antiprotons this number cannot be inviolate. There is a similar counting of electrons and neutrinos called lepton number, which is also believed to be broken. Unfortunately as neutrinos are extremely difficult to observe there is no direct evidence of a total lepton asymmetry.
  2. Matter and antimatter must be treated differently. This means that charge conjugation (where you swap particles with antiparticles) and charge-parity conjugation (swap particles with antiparticles and also reflect them like a mirror image) cannot leave the physics unchanged. More succinctly, C and CP must be broken. While C violation is trivial (the weak force violates C maximally), CP is almost entirely preserved in the Standard Model. This is both a major failing of the Standard Model and a fantastic prediction – we know that CP violation is absolutely fundamental to our universe, and that there must be more of it than we have seen so far. You have probably seen CP violation mentioned many times, both on this site and through news reports. The necessity for CP violation to explain our own existence is the real reason why CP violation deserves our attention.
  3. The universe must go out of thermal equilibrium. In thermal equilibrium any process creating a baryon asymmetry would be balanced by its reverse. Fortunately for us, the fact that the universe expands creates periods of thermal non-equilibrium, such as phase changes (like when the Higgs mechanism breaks the electroweak symmetry of the Standard Model).

 

While the Standard Model does technically satisfy all three of these, it does so in a trivial way. The amount of CP violation is far too low, and a universe in which the Standard Model is entirely correct never gets far enough out of equilibrium to allow a large difference in matter and antimatter to form even if it did violate CP more. The only really useful element that the Standard Model has is baryon number violation; a non-perturbative process called sphalerons occurs above the electroweak phase transitions which violates baryon and lepton number. More importantly, it preserves a linear combination of the two, so if you manage to make a baron asymmetry or a lepton asymmetry, you automatically get both. Theories like leptogenesis use this to turn a lepton asymmetry into a baryon asymmetry. While there are many possible scenarios that could have lead to the present day world (my own work is in one of these, asymmetric dark matter), the truth is that we simply don’t know which of these, if any, is correct.

Despite this being a question of the most fundamental kind, baryogenesis does not get nearly the same kind of media attention as dark matter or dark energy. Partly this is because we have little chance of experimentally finding an answer – baryogenesis could have occurred at almost any energy scale, which includes a good many far out of the reach of our colliders. But it is still important to push for an answer. Nothing is a better mark of our progress in understanding our origins than seeing how the question we ask about our origin evolves.

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