A Particle Purely Made of Nuclear Force

Scientists at TU Wien (Vienna) have calculated that the meson f0(1710) could be a very special particle – the long-sought-after glueball, a particle composed of pure force.

 

For decades, scientists have been looking for so-called “glueballs”. Now it seems they have been found at last. A glueball is an exotic particle, made up entirely of gluons – the “sticky” particles that keep nuclear particles together. Glueballs are unstable and can only be detected indirectly, by analysing their decay. This decay process, however, is not yet fully understood.

Professor Anton Rebhan and Frederic Brünner from TU Wien (Vienna) have now employed a new theoretical approach to calculate glueball decay. Their results agree extremely well with data from particle accelerator experiments. This is strong evidence that a resonance called “f0(1710)”, which has been found in various experiments, is in fact the long-sought glueball. Further experimental results are to be expected in the next few months.

Forces are Particles too

Protons and neutrons consist of even smaller elementary particles called quarks. These quarks are bound together by strong nuclear force. “In particle physics, every force is mediated by a special kind of force particle, and the force particle of the strong nuclear force is the gluon”, says Anton Rebhan (TU Wien).

Gluons can be seen as more complicated versions of the photon. The massless photons are responsible for the forces of electromagnetism, while eight different kinds of gluons play a similar role for the strong nuclear force. However, there is one important difference: gluons themselves are subject to their own force, photons are not. This is why there are no bound states of photons, but a particle that consists only of bound gluons, of pure nuclear force, is in fact possible.

In 1972, shortly after the theory of quarks and gluons was formulated, the physicists Murray Gell-Mann and Harald Fritsch speculated about possible bound states of pure gluons (originally called “gluonium”, today the term “glueball” is used). Several particles have been found in particle accelerator experiments which are considered to be viable candidates for glueballs, but there has never been a scientific consensus on whether or not one of these signals could in fact be the mysterious particle made of pure force. Instead of a glueball, the signals found in the experiments could also be a combination of quarks and antiquarks. Glueballs are too short-lived to detect them directly. If they exist, they have to be identified by studying their decay.

Candidate f0(1710) decays strangely

“Unfortunately, the decay pattern of glueballs cannot be calculated rigorously”, says Anton Rebhan. Simplified model calculations have shown that there are two realistic candidates for glueballs: the mesons called f0(1500) and f0(1710). For a long time, the former was considered to be the most promising candidate. The latter has a higher mass, which agrees better with computer simulations, but when it decays, it produces many heavy quarks (the so-called “strange quarks”). To many particle scientists, this seemed implausible, because gluon interactions do not usually differentiate between heavier and lighter quarks. 

Anton Rebhan and his PhD-student Frederic Brünner have now made a major step forward in solving this puzzle by trying a different approach. There are fundamental connections between quantum theories describing the behaviour of particles in our three dimensional world and certain kinds of gravitation theories in higher dimensional spaces. This means that certain quantum physical questions can be answered using tools from gravitational physics.

“Our calculations show that it is indeed possible for glueballs to decay predominantly into strange quarks”, says Anton Rebhan. Surprisingly, the calculated decay pattern into two lighter particles agrees extremely well with the decay pattern measured for f0(1710). In addition to that, other decays into more than two particles are possible. Their decay rates have been calculated too.

Further Data is Expected Soon

Up until now, these alternative glueball decays have not been measured, but within the next few months, two experiments at the Large Hadron Collider at CERN (TOTEM and LHCb) and one accelerator experiment in Beijing (BESIII) are expected to yield new data. “These results will be crucial for our theory”, says Anton Rebhan. “For these multi-particle processes, our theory predicts decay rates which are quite different from the predictions of other, simpler models. If the measurements agree with our calculations, this will be a remarkable success for our approach.” It would be overwhelming evidence for f0(1710) being a glueball. And in addition to that, it would once again show that higher dimensional gravity can be used to answer questions from particle physics – in a way it would be one more big success of Einstein’s theory of general relativity, which turns 100 years old next month.

Technische Universität Wien

New model suggests dark matter is made of electrically charged particles

New 'stealth dark matter' theory may explain mystery of the universe's missing mass

Lawrence Livermore National Laboratory (LLNL) scientists have come up with a new theory that may identify why dark matter has evaded direct detection in Earth-based experiments.

A group of national particle physicists known as the Lattice Strong Dynamics Collaboration, led by a Lawrence Livermore National Laboratory team, has combined theoretical and computational physics techniques and used the Laboratory’s massively parallel 2-petaflop Vulcan supercomputer to devise a new model of dark matter. It identifies it as naturally "stealthy" ( like its namesake aircraft, difficult to detect) today, but would have been easy to see via interactions with ordinary matter in the extremely high-temperature plasma conditions that pervaded the early universe.

“These interactions in the early universe are important because ordinary and dark matter abundances today are strikingly similar in size, suggesting this occurred because of a balancing act performed between the two before the universe cooled,” said Pavlos Vranas of LLNL, and one of the authors of the paper, “Direct Detection of Stealth Dark Matter Through Electromagnetic Polarizability(link is external).” The paper appears in an upcoming edition of the journal Physical Review Letters and is an “Editor’s Choice.”

Dark matter makes up 83 percent of all matter in the universe and does not interact directly with electromagnetic or strong and weak nuclear forces. Light does not bounce off of it, and ordinary matter goes through it with only the feeblest of interactions. Essentially invisible, it has been termed dark matter, yet its interactions with gravity produce striking effects on the movement of galaxies and galactic clusters, leaving little doubt of its existence.

Lawrence Livermore scientists have devised a new model of dark matter. It identifies it as naturally "stealthy" today, but would have been easy to see via interactions with ordinary matter in the extremely high-temperature plasma conditions that pervaded the early universe.

The key to stealth dark matter’s split personality is its compositeness and the miracle of confinement. Like quarks in a neutron, at high temperatures these electrically charged constituents interact with nearly everything. But at lower temperatures they bind together to form an electrically neutral composite particle. Unlike a neutron, which is bound by the ordinary strong interaction of quantum chromodynamics (QCD), the stealthy neutron would have to be bound by a new and yet-unobserved strong interaction, a dark form of QCD.

“It is remarkable that a dark matter candidate just several hundred times heavier than the proton could be a composite of electrically charged constituents and yet have evaded direct detection so far,” Vranas said.

Similar to protons, stealth dark matter is stable and does not decay over cosmic times. However, like QCD, it produces a large number of other nuclear particles that decay shortly after their creation. These particles can have net electric charge but would have decayed away a long time ago. In a particle collider with sufficiently high energy (such as the Large Hadron Collider in Switzerland), these particles can be produced again for the first time since the early universe. They could generate unique signatures in the particle detectors because they could be electrically charged.

“Underground direct detection experiments or experiments at the Large Hadron Collider may soon find evidence of (or rule out) this new stealth dark matter theory,” Vranas said.

The LLNL lattice team authors are Evan Berkowitz, Michael Buchoff, Enrico Rinaldi, Christopher Schroeder and Pavlos Vranas, who is the lead of the team. The Laboratory Directed Research and Development, the LLNL Grand Challenge computation programs, the DOE Office of Science High Energy Theory and the High Energy Physics Lattice SciDAC program supported this research

Other collaborators include researchers Thomas Appelquist and George Fleming of Yale University, Richard Brower, Claudio Rebbi and Evan Weinberg of Boston University, Xiao-Yong Jin  and James Osborn of Argonne Leadership Computing Facility, Joe Kiskis of the University of California, Davis, Graham Kribs of the University of Oregon, Ethan Neil  of the University of Colorado and RIKEN-BNL Research Center at Brookhaven National Laboratory, David Schaich of Syracuse University, Sergey Syritsyn of RIKEN-BNL Research Center at Brookhaven National Laboratory), and Oliver Witzel of Boston University and the University of Edinburgh.

Lawrence Livermore National Laboratory

Nanotechnology World Association

What are WIMPs, and what makes them such popular dark matter candidates?

Invisible dark matter accounts for 85 percent of all matter in the universe, affecting the motion of galaxies, bending the path of light and influencing the structure of the entire cosmos. Yet we don’t know much for certain about its nature.

Most dark matter experiments are searching for a type of particles called WIMPs, or weakly interacting massive particles.

“Weakly interacting” means that WIMPs barely ever “talk” to regular matter. They don’t often bump into other matter and also don’t emit light—properties that could explain why researchers haven’t been able to detect them yet.

Created in the early universe, they would be heavy (“massive”) and slow-moving enough to gravitationally clump together and form structures observed in today’s universe.

Scientists predict that dark matter is made of particles. But that assumption is based on what they know about the nature of regular matter, which makes up only about 4 percent of the universe.

WIMPs advanced in popularity in the late 1970s and early 1980s when scientists realized that particles that naturally pop out in models of Supersymmetry could potentially explain the seemingly unrelated cosmic mystery of dark matter.

Supersymmetry, developed to fill gaps in our understanding of known particles and forces, postulates that each fundamental particle has a yet-to-be-discovered superpartner. It turns out that the lightest one of the bunch has properties that make it a top contender for dark matter.

“The lightest supersymmetric WIMP is stable and is not allowed to decay into other particles,” says theoretical physicist Tim Tait of the University of California, Irvine. “Once created in the big bang, many of these WIMPs would therefore still be around today and could have gone unnoticed because they rarely produce a detectable signal.”

When researchers use the properties of the lightest supersymmetric particle to calculate how many of them would still be around today, they end up with a number that matches closely the amount of dark matter experimentally observed—a link referred to as the “WIMP miracle.” Many researchers believe it could be more than coincidence.

“But WIMPs are also popular because we know how to look for them,” says dark matter hunter Thomas Shutt of Stanford University and SLAC National Accelerator Laboratory. “After years of developments, we finally know how to build detectors that have a chance of catching a glimpse of them.”

Shutt is co-founder of the LUX experiment and one of the key figures in the development of the next-generation LUX-ZEPLIN experiment. He is one member of the group of scientists trying to detect WIMPs as they traverse large, underground detectors.

Other scientists hope to create them in powerful particle collisions at CERN’s Large Hadron Collider. “Most supersymmetric theories estimate the mass of the lightest WIMP to be somewhere above 100 gigaelectronvolts, which is well within LHC’s energy regime,” Tait says. “I myself and others are very excited about the recent LHC restart. There is a lot of hope to create dark matter in the lab.”

A third way of searching for WIMPs is to look for revealing signals reaching Earth from space. Although individual WIMPs are stable, they decay into other particles when two of them collide and annihilate each other. This process should leave behind detectable amounts of radiation.

Researchers therefore point their instruments at astronomical objects rich in dark matter such as dwarf satellite galaxies orbiting our Milky Way or the center of the Milky Way itself.

“Dark matter interacts with regular matter through gravitation, impacting structure formation in the universe,” says Risa Wechsler, a researcher at Stanford and SLAC. “If dark matter is made of WIMPs, our predictions of the distribution of dark matter based on this assumption must also match our observations.”

Wechsler and others calculate, for example, how many dwarf galaxies our Milky Way should have and participate in research efforts under way to determine if everything predicted can also be found experimentally.

So how would researchers know for sure that dark matter is made of WIMPs? “We would need to see conclusive evidence for WIMPs in more than one experiment, ideally using all three ways of detection,” Wechsler says.

In the light of today’s mature detection methods, dark matter hunters should be able to find WIMPs in the next five to 10 years, Shutt, Tait and Wechsler say. Time will tell if scientists have the right idea about the nature of dark matter.

http://www.nanotechnologyworld.org/#!What-are-WIMPs-and-what-makes-them-such-popular-dark-matter-candidates/c89r/55b797610cf228fd5ebad145

An end in sight in the long search for gravity waves

Our unfolding understanding of the universe is marked by epic searches and we are now on the brink of discovering something that has escaped detection for many years.

The search for gravity waves has been a century long epic. They are a prediction of Einstein’s General Theory of Relativity but for years physicists argued about their theoretical existence.

By 1957 physicists had proved that they must carry energy and cause vibrations. But it was also apparent that waves carrying a million times more energy than sunlight would make vibrations smaller than an atomic nucleus.

Building detectors seemed a daunting task but in the 1960s a maverick physicist Joseph Weber, at the University of Maryland, began to design the first detectors. By 1969 he claimed success!

There was excitement and consternation. How could such vast amounts of energy be reconciled with our understanding of stars and galaxies? A scientific gold rush began.

Within two years, ten new detectors had been built in major labs across the planet. But nothing was detected.

Going to need a better detector

Some physicists gave up on the field but for the next 40 years a growing group of physicists set about trying to build vastly better detectors.

By the 1980s a worldwide collaboration to build five detectors, called cryogenic resonant bars, was underway, with one detector called NIOBE located at the University of Western Australia.

These were huge metal bars cooled to near absolute zero. They used superconducting sensors that could detect a million times smaller vibration energy than those of Weber.

Gravity waves caused by two rotating black holes.

Click to enlarge

They operated throughout much of the 1990s. If a pair of black holes had collided in our galaxy, or a new black hole had formed, it would have been heard as a gentle ping in the cold bars… but all remained quiet.

What the cryogenic detectors did achieve was an understanding of how quantum physics affects measurement, even of tonne-scale objects. The detectors forced us to come to grips with a new approach to measurement. Today this has grown into a major research field called macroscopic quantum mechanics.

But the null results did not mean the end. It meant that we had to look further into the universe. A black hole collision may be rare in one galaxy but it could be a frequent occurrence if you could listen in to a million galaxies.

Laser beams will help

A new technology was needed to stretch the sensitivity enormously, and by the year 2000 this was available: a method called laser interferometry.

The idea was to use laser beams to measure tiny vibrations in the distance between widely spaced mirrors. The bigger the distance the bigger the vibration! And an L-shape could double the signal and cancel out the noise from the laser.

Several teams of physicists including a team at the Australian National University had spent many years researching the technology. Laser beam measurements allowed very large spacing and so new detectors up to 4km in size were designed and constructed in the US, Europe and Japan.

The gravity wave facility at Gingin. Australian International Gravitational Research Centre.

Click to enlarge

The Australian Consortium for Gravitational Astronomy built a research centre on a huge site at Gingin, just north of Perth, in Western Australia, that was reserved for the future southern hemisphere gravitational wave detector.

The world would need this so that triangulation could be used to locate signals.

Latest detectors

The new detectors were proposed in two stages. Because they involved formidable technological challenges, the first detectors would have the modest aim of proving that the laser technology could be implemented on a 4km scale, but using relatively low intensity laser light that would mean only a few per cent chance of detecting any signals.

The detectors were housed inside the world’s largest vacuum system, the mirrors had to be 100 times more perfect than a telescope mirror, seismic vibrations had to be largely eliminated, and the laser light had to be the purest light ever created.

A second stage would be a complete rebuild with bigger mirrors, much more laser power and even better vibration control. The second stage would have a sensitivity where coalescing pairs of neutron stars merging to form black holes, would be detectable about 20 to 40 times per year.

Australia has been closely involved with both stages of the US project. CSIRO was commissioned to polish the enormously precise mirrors that were the heart of the first stage detectors.

A gathering of minds

The Australian Consortium gathered at Gingin earlier this year to plan a new national project.

Students at work in the labs at Gingin. University of WA

Click to enlarge

Part of that project focusses on an 80 meter scale laser research facility – a sort of mini gravity wave detector – the consortium has developed at the site. Experiments are looking at the physics of the new detectors and especially the forces exerted by laser light.

The team has discovered several new phenomena including one that involves laser photons bouncing off particles of sound called phonons. This phenomenon turns out to be very useful as it allows new diagnostic tools to prevent instabilities in the new detectors.

The light forces can also be used to make “optical rods” – think of a Star Wars light sabre! These devices can capture more gravitational wave energy – opening up a whole range of future possibilities from useful gadgets to new gravitational wave detectors.

Final stages of discovery

The first stage detectors achieved their target sensitivity in 2006 and, as expected, they detected no signals. You would know if they had!

The second stage detectors are expected to begin operating next year. The Australian team is readying itself because the new detectors change the whole game.

For the first time we have firm predictions: both the strength and the number of signals. No longer are we hoping for rare and unknown events.

We will be monitoring a significant volume of the universe and for the first time we can be confident that we will “listen” to the coalescence of binary neutron star systems and the formation of black holes.

Once these detectors reach full sensitivity we should hear signals almost once a week. Exactly when we will reach this point, no one knows. We have to learn how to operate the vast and complex machines.

If you want to place bets on the date of first detection of some gravity wave then some physicists would bet on 2016, probably the majority would bet 2017. A few pessimists would say that we will discover unexpected problems that might take a few years to solve.

Source: http://theconversation.com/an-end-in-sight-in-the-long-search-for-gravity-waves-22336