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Solving Five Problems of Particle Physics and Cosomolgy in One Stroke

12/09/2016 09:32AM

Solving Five Problems of Particle Physics and Cosomolgy in One Stroke
In the search for the mysterious dark matter, physicists have used elaborate computer calculations to come up with an outline of the particles of this unknown form of matter. To do this, the scientists extended the successful Standard Model of particle physics which allowed them, among other things, to predict the mass of so-called axions, promising candidates for dark matter.

The team of researchers led by Professor Zoltan Fodor of the University of Wuppertal in Germany and Forschungszentrum Julich of Eotvos University in Hungary carried out its calculations on Julich's supercomputer JUQUEEN (BlueGene/Q).

"Dark matter is an invisible form of matter which until now has only revealed itself through its gravitational effects. What it consists of remains a complete mystery," explains Dr. Andreas Ringwald, who is based at the Deutsches Elektronen-Synchrotron (DESY) and who proposed the current research.

Evidence for the existence of this form of matter comes, among other things, from the astrophysical observation of galaxies, which rotate far too rapidly to be held together only by the gravitational pull of the visible matter. High-precision measurements using the European satellite Planck show that almost 85 percent of the entire mass of the universe consists of dark matter. All the stars, planets, nebulae, and other objects in space that are made of conventional matter account for no more than 15 percent of the mass of the universe.

"The adjective "dark" does not simply mean that it does not emit visible light," says Ringwald. "It does not appear to give off any other wavelengths either. Its interaction with photons must be very weak indeed." For decades, physicists have been searching for particles of this new type of matter. What is clear is that these particles must lie beyond the Standard Model of particle physics, and while that model is extremely successful, it currently only describes the conventional 15 percent of all matter in the cosmos. From theoretically possible extensions to the Standard Model physicists not only expect a deeper understanding of the universe, but also concrete clues in what energy range it is particularly worthwhile looking for dark matter candidates.

The unknown form of matter can either consist of comparatively few, but very heavy particles, or a large number of light ones. The direct searches for heavy dark matter candidates using large detectors in underground laboratories and the indirect search for them using large particle accelerators are still going on, but have not turned up any dark matter particles so far. A range of physical considerations make extremely light particles, dubbed axions, very promising candidates. Using clever experimental setups, it might even be possible to detect direct evidence of them. "However, to find this kind of evidence it would be extremely helpful to know what kind of mass we are looking for," emphasizes theoretical physicist Ringwald. "Otherwise the search could take decades, because one would have to scan far too large a range."

The existence of axions is predicted by an extension to Quantum ChromoDynamics (QCD), the quantum theory that governs the strong interaction, responsible for the nuclear force. The strong interaction is one of the four fundamental forces of nature alongside gravitation, electromagnetism, and the weak nuclear force, which is responsible for radioactivity. "Theoretical considerations indicate that there are so-called topological quantum fluctuations in Quantum ChromoDynamics, which ought to result in an observable violation of time reversal symmetry," explains Ringwald. This means that certain processes should differ depending on whether they are running forwards or backwards. However, no experiment has so far managed to demonstrate this effect.

The extension to Quantum ChromoDynamics restores the invariance of time reversals, but at the same time it predicts the existence of a very weakly interacting particle, the axion, whose properties, in particular its mass, depend on the strength of the topological quantum fluctuations.

However, it takes modern supercomputers like J├╝lich's JUQUEEN to calculate the latter in the temperature range that is relevant in predicting the relative contribution of axions to the matter making up the universe. "On top of this, we had to develop new methods of analysis in order to achieve the required temperature range," notes Fodor who led the research.

The results show, among other things, that if axions do make up the bulk of dark matter, they should have a mass of 50 to 1500 micro-electronvolts, expressed in the customary units of particle physics, and thus be up to ten billion times lighter than electrons. This would require every cubic centimeter of the universe to contain on average ten million such ultra-lightweight particles. Dark matter is not spread out evenly in the universe, however, but forms clumps and branches of a web-like network. Because of this, our local region of the Milky Way should contain about one trillion axions per cubic centimeter.

Thanks to the Julich supercomputer, the calculations now provide physicists with a concrete range in which their search for axions is likely to be most promising. "The results we are presenting will probably lead to a race to discover these particles," says Fodor. Their discovery would not only solve the problem of dark matter in the universe, but at the same time answer the question why the strong interaction is so surprisingly symmetrical with respect to time reversal. The scientists expect that it will be possible within the next few years to either confirm or rule out the existence of axions experimentally.

Through a simple extension to the Standard Model, physicists have come up with a new twist that they say solves five of the biggest unanswered questions in modern physics: explaining the weirdness of dark matter, neutrino oscillations, baryogenesis, cosmic inflation, and the strong CP (Charge Parity) problem all at once.

The new model, called SMASH (which stands for Standard Model - Axion - Seesaw - Higgs portal inflation) model, proposes that we only need six new particles to reconcile all of these gaps in the standard model of physics, and the team behind it says it won't be that hard to test.

The model has been developed by a team of French and German physicists, and they say it doesn't require any major changes to the standard model -- just a few new additions.

That's a very attractive proposition because other models designed to explain the mysteries of quantum mechanics (such as supersymmetry) require the addition of hundreds of new particles that we've never even seen traces of.

SMASH, on the other hand, requires just six: three neutrinos, a fermion, and a field that includes two particles. (In physics, a field is a physical or mathematical entity that has a value for each point in space and time. A particle is an excited state of a field.)

These five fundamental problems addressed by SMASH are:

- Dark matter

- Neutrino oscillations

- Baryogenesis

- Cosmic Inflation

- The strong CP (Charge Parity) problem

Dark matter

There is now overwhelming evidence that a large percentage of the Universe is made up of an unidentified type of matter. While we can detect its gravitational force, this unknown matter doesn't appear to emit any form of light or radiation that we can observe.

Despite years of searching, we still have no idea what dark matter actually consists of, but we do know that its presence is crucial to the stability of the Universe.

Neutrino oscillations

Last year, the Nobel Prize in Physics was awarded to two physicists who proved that neutrinos could oscillate between "flavors."

Neutrino oscillation is a quantum mechanical phenomenon where a neutrino created with a specific lepton flavor (such as an electron, a muon, or a tau) can have a different flavor later on.

Because only particles with mass can switch flavors (or oscillate), neutrinos must have mass, and this presents a problem for the standard model, because no one knows where neutrino mass actually comes from.

It could come from the Higgs Boson, but it could also come from an entirely new particle we've yet to discover.


This major unsolved problem in physics can be summed up pretty simply -- Why does the observable Universe have more matter than antimatter?

According to the standard model, the Big Bang would have produced equal amounts of matter and antimatter, and since they annihilate each another on contact, this should have led to a Universe with no particles, just radiation.

Obviously the fact that there are a whole lot of particles in the Universe means that there's something wrong with this scenario, because how can there be so much matter in the Universe now, but almost no antimatter?

Cosmic Inflation

It's thought that within a fraction of a second after the Big Bang, the Universe underwent a period of accelerated expansion called inflation.

While most physicists accept the reality of cosmic inflation, no one has been able to figure out the exact mechanism responsible for making the Universe expand faster than the speed of light, going from a subatomic size to a golf ball size almost instantaneously.

A hypothetical field has been proposed as the main cause of inflation, but we're yet to actually detect it.

The strong CP problem

Described as a serious flaw of the standard model, the strong CP problem helps to explain why there is more matter than antimatter in the Universe, but brings its own unsolved mysteries with it.

This one is a particularly long story, but in a nutshell, the strong CP problem describes how CP violation -- a break in the fundamental symmetry of the Universe -- doesn't occur in Quantum ChromoDynamics, which relates to interactions between quarks and gluons. And no one has been able to figure out why.

A possible solution to the five fundamental problems of physics?

The SMASH model builds on one proposed by physicist Mikhail Shaposhnikov from the Swiss Federal Institute of Technology in Lausanne, Switzerland back in 2005, called the Neutrino Minimal Standard Model (or nMSM).

Back then, it was suggested that the extension of the Standard Model by three right-handed neutrinos with certain masses could simultaneously explain the dark matter and baryon asymmetry of the Universe, while also being consistent with the experiments on neutrino oscillations.

Now, the team led by French physicist Guillermo Ballesteros from the University of Paris-Saclay in France says we can add these three right-handed neutrinos to the three existing neutrinos in the standard model, plus a subatomic particle called a color triplet fermion, to solve the first four problems listed above.

The addition of a new, unidentified field appears to take care of the fifth problem. This field includes two particles: the axion (a candidate for dark matter) and the inflaton, the particle behind inflation.

The team says the fact that their hypothesis could be tested using the next generation of particle accelerators means it's not out of the realm of possibility, and that makes it more convincing than other solutions to these problems that have been proposed in the past.

"The best thing about the theory is that it can be tested or checked within the next 10 years or so," according to Andreas Ringwald from DESY.

"You can always invent new theories, but if they can only be tested in 100 years, or never, then this is not real science but meta-science."

It should be noted that the SMASH model has yet to be published in a peer-reviewed journal, so it still needs to undergo the scrutiny of the particle physics world.

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