Discovery of new particle: 'four-flavored' tetraquark

The new particle is the first tetraquark to contain four quarks of different "flavors." | Photo by Fermilab

Research led by Indiana University physicist Daria Zieminska has resulted in the first detection of a new form of elementary particle: the "four-flavored" tetraquark.

Zieminska, a senior scientist in the IU Bloomington College of Arts and Sciences' Department of Physics, is a lead member of the team responsible for the particle's detection by the DZero Collaboration at the U.S. Department of Energy's Fermi National Laboratory, which announced the discovery Feb. 25.

She also delivered the first scientific seminar on the particle and is an author on a paper submitted to Physics Review Letters, the premier journal in physics, describing the tetraquarks' observation.

"For most of the history of quarks, it's seemed that all particles were made of either a quark and an antiquark, or three quarks; this new particle is unique -- a strange, charged beauty," said Zieminska, who has been a member of the DZero experiment since the project's establishment in 1985. "It's the birth of a new paradigm. Particles made of four quarks -- specifically, two quarks and two antiquarks -- is a big change in our view of elementary particles."

The results could also affect scientists' understanding of "quark matter," the hot, dense material that existed moments after the Big Bang, and which may still exist in the super-dense interior of neutron stars.

Quarks are the building blocks that form subatomic particles, the most familiar of which are protons and neutrons, each composed of three quarks. There are six types, or "flavors," of quarks: up, down, strange, charm, bottom and top. Each of these also has an antimatter counterpart.

A tetraquark is a group of four quarks, the first evidence for which was recorded by scientists on the Belle experiment in Japan in 2008. But the new tetraquark is the first quark quartet to contain four different quark flavors: up, down, strange and bottom.

Currently, Zieminska leads the "heavy flavor" group of the DZero experiment, which encompasses the study of all particles containing one or more "heavy quarks," including the new tetraquark, dubbed X(5568) for its mass of 5568 Megaelectronvolts, roughly 5.5 times the mass of a proton. The DZero experiment is led by Dmitri Denisov, a staff scientist at the U.S. Department of Energy's Fermilab.

"Daria was the lead person on the tetraquark observation and performed calculations, cross-checking and other work required to answer the hundreds of questions of the rest of the team," said Denisov, co-spokesman for the DZero experiment. "She was an active participant in the design and construction of the experiment and in the collection of the data."

A chart compares mesons, composed of two quarks; baryons, composed of three quarks; and the lesser understood tetraquark, composed of four quarks. | Photo by Fermilab

The DZero experiment is also responsible for other fundamental physics discoveries, including the first observation, with the Collider-Detector at Fermilab experiment, of the elusive Higgs boson particle decaying into bottom quarks.

Other IU scientists engaged in the DZero project include the late Andrzej Zieminski, former professor of physics at IU Bloomington, who also joined the project in 1985, and Rick Van Kooten, IU vice provost for research, who joined in 2002 during "phase 2" of the project, which involved upgrades to the detector partially constructed at IU. Hal Evans, professor, and Sabine Lammers, associate professor, both at IU, also contributed to the upgraded detector.

DZero is one of two experiments collecting data from Fermilab's Tevatron proton-antiproton collider, once the most powerful particle accelerator in the world, officially retired in 2011. Zieminska and colleagues uncovered the existence of X(5568) based on analysis of billions of previously recorded events from these collisions.

As with other discoveries in physics, Zieminska said the new tetraquark’s discovery was a surprise. Alexey Drutskoy, a colleague at Russia's National Research Nuclear University, spotted indications of the tetraquark signal in summer 2015, after which Zieminska joined him in the hunt. Only after performing multiple cross-checks, in collaboration with Alexey Popov, another Russian colleague, did the team confirm they were observing evidence for a new particle.

Although nothing in nature forbids the formation of a tetraquark, four-quark states are rare and not nearly as well understood as two- and three-quark states. Zieminska and colleagues plan to deepen their understanding of the tetraquark by measuring various properties of the particle, such as the ways it decays or how much it spins on its axis.

The discovery of the tetraquark also comes on the heels of the first observation of a pentaquark -- a five-quark particle -- announced last year by CERN's LHCb experiment at the Large Hadron Collider.

Zieminska is also a member of the ATLAS Experiment at CERN, the European Organization for Nuclear Research.

A total of 75 institutions from 18 countries are members of the DZero Collaboration.

Indiana University

Solving Hard Quantum Problems: Everything is Connected

Quantum objects cannot just be understood as the sum of their parts. This is what makes quantum calculations so complicated. Scientists at TU Wien (Vienna) have now calculated Bose-Einstein-condensates, revealing the secrets of the particles’ collective behaviour.

 

Quantum systems are extremely hard to analyse if they consist of more than just a few parts. It is not difficult to calculate a single hydrogen atom, but in order to describe an atom cloud of several thousand atoms, it is usually necessary to use rough approximations. The reason for this is that quantum particles are connected to each other and cannot be described separately. Kaspar Sakmann (TU Wien, Vienna) and Mark Kasevich (Stanford, USA) have now shown in an article published in “Nature Physics” that this problem can be overcome. They succeeded in calculating effects in ultra-cold atom clouds which can only be explained in terms of the quantum correlations between many atoms. Such atom clouds are known as Bose-Einstein condensates and are an active field of research.

Quantum Correlations

Quantum physics is a game of luck and randomness. Initially, the atoms in a cold atom cloud do not have a predetermined position. Much like a die whirling through the air, where the number is yet to be determined, the atoms are located at all possible positions at the same time. Only when they are measured, their positions are fixed. “We shine light on the atom cloud, which is then absorbed by the atoms”, says Kaspar Sakmann. “The atoms are photographed, and this is what determines their position. The result is completely random.”

There is, however, an important difference between quantum randomness and a game of dice:  if different dice are thrown at the same time, they can be seen as independent from each other. Whether or not we roll a six with die number one does not influence the result of die number seven. The atoms in the atom cloud on the other hand are quantum physically connected. It does not make sense to analyse them individually, they are one big quantum object. Therefore, the result of every position measurement of any atom depends on the positions of all the other atoms in a mathematically complicated way.

“It is not hard to determine the probability that a particle will be found at a specific position”, says Kaspar Sakmann. “The probability is highest in the centre of the cloud and gradually diminishes towards the outer fringes.” In a classically random system, this would be all the information that is needed. If we know that in a dice roll, any number has the probability of one sixth, then we can also determine the probability of rolling three ones with three dice. Even if we roll five ones consecutively, the probability remains the same the next time. With quantum particles, it is more complicated than that.

“We solve this problem step by step”, says Sakmann. “First we calculate the probability of the first particle being measured on a certain position. The probability distribution of the second particle depends on where the first particle has been found. The position of the third particle depends on the first two, and so on.” In order to be able to describe the position of the very last particle, all the other positions have to be known. This kind of quantum entanglement makes the problem mathematically extremely challenging.

Only Correlations Can Explain the Experimental Data 

But these correlations between many particles are extremely important – for example for calculating the behaviour of colliding Bose-Einstein-condensates. “The experiment shows that such collisions can lead to a special kind of quantum waves. On certain positions we find many particles, on an adjacent position we do not find any”, says Kaspar Sakmann. “If we consider the atoms separately, this cannot be explained. Only if we take the full quantum distribution into account, with all its higher correlations, these waves can be reproduced by our calculations.”

Also other phenomena have been calculated with the same method, for instance Bose-Einstein-condensates which are stirred with a laser beam, so that little vortices emerge – another typical quantum many-particle-effect. “Our results show how important theses correlations are and that it is possible to include them in quantum calculations, in spite of all mathematical difficulties”, says Sakmann. With certain modifications, the approach can be expected to be useful for many other quantum systems as well.

Original paper:

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3631.htmlhttp://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3631.html 

Technische Universität Wien

Quantum physics problem proved unsolvable

A mathematical problem underlying fundamental questions in particle and quantum physics is provably unsolvable, according to scientists at UCL, Universidad Complutense de Madrid – ICMAT and Technical University of Munich. It is the first major problem in physics for which such a fundamental limitation could be proven. The findings are important because they show that even a perfect and complete description of the microscopic properties of a material is not enough to predict its macroscopic behavior.

A small spectral gap – the energy needed to transfer an electron from a low-energy state to an excited state – is the central property of semiconductors. In a similar way, the spectral gap plays an important role for many other materials. When this energy becomes very small – i.e. the spectral gap closes – it becomes possible for the material to transition to a completely different state. An example of this is when a material becomes superconducting.

Mathematically extrapolating from a microscopic description of a material to the bulk solid is considered one of the key tools in the search for materials exhibiting superconductivity at ambient temperatures or other desirable properties. A study, published today in Nature, however, shows crucial limits to this approach. Using sophisticated mathematics, the authors proved that, even with a complete microscopic description of a quantum material, determining whether it has a spectral gap is, in fact, an undecidable question.

“Alan Turing is famous for his role in cracking the Enigma code,” says Co-author, Dr. Toby Cubitt from UCL Computer Science. “But amongst mathematicians and computer scientists, he is even more famous for proving that certain mathematical questions are ‘undecidable' – they are neither true nor false, but are beyond the reach of mathematics. What we’ve shown is that the spectral gap is one of these undecidable problems. This means a general method to determine whether matter described by quantum mechanics has a spectral gap, or not, cannot exist. Which limits the extent to which we can predict the behavior of quantum materials, and potentially even fundamental particle physics.”

One million dollars to win!

The most famous problem concerning spectral gaps is whether the theory governing the fundamental particles of matter itself – the standard model of particle physics – has a spectral gap (the `Yang-Mills mass gap' conjecture). Particle physics experiments such as CERN and numerical calculations on supercomputers suggest that there is a spectral gap. Although there is a $1m prize at stake from the Clay Mathematics Institute for whoever can, no one has yet succeeded in proving this mathematically from the equations of the standard model.

Dr. Cubitt added, “It's possible for particular cases of a problem to be solvable even when the general problem is undecidable, so someone may yet win the coveted $1m prize. But our results do raise the prospect that some of these big open problems in theoretical physics could be provably unsolvable.”

"We knew about the possibility of problems that are undecidable in principle since the works of Turing and Gödel in the 1930s,” added Co-author Professor Michael Wolf from Technical University of Munich. “So far, however, this only concerned the very abstract corners of theoretical computer science and mathematical logic. No one had seriously contemplated this as a possibility right in the heart of theoretical physics before. But our results change this picture. From a more philosophical perspective, they also challenge the reductionists’ point of view, as the insurmountable difficulty lies precisely in the derivation of macroscopic properties from a microscopic description."

Not all bad news

Co-author, Professor David Pérez-García from Universidad Complutense de Madrid and ICMAT, said: “It's not all bad news, though. The reason this problem is impossible to solve in general is because models at this level exhibit extremely bizarre behavior that essentially defeats any attempt to analyze them. But this bizarre behavior also predicts some new and very weird physics that hasn't been seen before. For example, our results show that adding even a single particle to a lump of matter, however large, could in principle dramatically change its properties. New physics like this is often later exploited in technology.”

The researchers are now seeing whether their findings extend beyond the artificial mathematical models produced by their calculations to more realistic quantum materials that could be realized in the laboratory.

The research has been funded by the John Templeton Foundation, the Royal Society (UK), the Spanish Ministry of Economics and Competitiveness (MINECO), the Madrid Regional Government and the European Research Council (ERC).

Technical University of Munich

Physicists Measure Force that Makes Antimatter Stick Together

Peering at the debris from particle collisions that recreate the conditions of the very early universe, scientists have for the first time measured the force of interaction between pairs of antiprotons. Like the force that holds ordinary protons together within the nuclei of atoms, the force between antiprotons is attractive and strong. 

The experiments were conducted at the Relativistic Heavy Ion Collider (RHIC), a U.S. Department of Energy Office of Science User Facility for nuclear physics research at DOE's Brookhaven National Laboratory. The findings, published in the journal Nature, could offer insight into larger chunks of antimatter,including antimatter nuclei previously detected at RHIC, and may also help scientists explore one of science's biggest questions: why the universe today consists mainly of ordinary matter with virtually no antimatter to be found.

"The Big Bang—the beginning of the universe—produced matter and antimatter in equal amounts. But that's not the world we see today. Antimatter is extremely rare. It's a huge mystery!" said Aihong Tang, a Brookhaven physicist involved in the analysis, which used data collected by RHIC's STAR detector. "Although this puzzle has been known for decades and little clues have emerged, it remains one of the big challenges of science. Anything we learn about the nature of antimatter can potentially contribute to solving this puzzle."

RHIC is the perfect place to study antimatter because it's one of the few places on Earth that is able to create the elusive stuff in abundant quantities.

RHIC is the perfect place to study antimatter because it's one of the few places on Earth that is able to create the elusive stuff in abundant quantities. It does this by slamming the nuclei of heavy atoms such as gold into one another at nearly the speed of light. These collisions produce conditions very similar to those that filled the universe microseconds after the Big Bang—with temperatures 250,000 times hotter than the center of the sun in a speck the size of a single atomic nucleus. All that energy packed into such a tiny space creates a plasma of matter's fundamental building blocks, quarks and gluons, and thousands of new particles—matter and antimatter in equal amounts.

"We are taking advantage of the ability to produce ample amounts of antimatter so we can conduct this study," said Tang.

The STAR collaboration has previous experience detecting and studying rare forms of antimatter—including anti-alpha particles, the largest antimatter nuclei ever created in a laboratory, each made of two antiprotons and two antineutrons. Those experiments gave them some insight into how the antiprotons interact within these larger composite objects. But in that case, "the force between the antiprotons is a convolution of the interactions with all the other particles," Tang said. "We wanted to study the simple interaction of unbound antiprotons to get a 'cleaner' view of this force."

To do that, they searched the STAR data from gold-gold collisions for pairs of antiprotons that were close enough to interact as they emerged from the fireball of the original collision. 

"We see lots of protons, the basic building blocks of conventional atoms, coming out, and we see almost equal numbers of antiprotons," said Zhengqiao Zhang, a graduate student in Professor Yu-Gang Ma's group from the Shanghai Institute of Applied Physics of the Chinese Academy of Sciences, who works under the guidance of Tang when at Brookhaven. "The antiprotons look just like familiar protons, but because they are antimatter, they have a negative charge instead of positive, so they curve the opposite way in the magnetic field of the detector."

"By looking at those that strike near one another on the detector, we can measure correlations in certain properties that give us insight into the force between pairs of antiprotons, including its strength and the range over which it acts," he added.

The scientists found that the force between antiproton pairs is attractive, just like the strong nuclear force that holds ordinary atoms together. Considering they'd already discovered bound states of antiprotons and antineutrons—those antimatter nuclei—this wasn't all that surprising. When the antiprotons are close together, the strong force interaction overcomes the tendency of the like (negatively) charged particles to repel one another in the same way it allows positively charged protons to bind to one another within the nuclei of ordinary atoms.

In fact, the measurements show no difference between matter and antimatter in the way the strong force behaves. That is, within the accuracy of these measurements, matter and antimatter appear to be perfectly symmetric. That means, at least with the precision the scientists were able to achieve, there doesn't appear to be some asymmetric quirk of the strong force that can account for the continuing existence of matter in the universe and the scarcity of antimatter today.

But the scientists point out that we wouldn't know that if they hadn't done these experiments. 

"There are many ways to test for matter/antimatter asymmetry, and there are more precise tests, but in addition to precision, it's important to test it in qualitatively different ways. This experiment was a qualitatively new test," said Richard Lednický, a STAR scientist from the Joint Institute for Nuclear Research, Dubna, and the Institute of Physics, Czech Academy of Sciences, Prague.

"The successful implementation of the technique used in this analysis opens an exciting possibility for exploring details of the strong interaction between other abundantly produced particle species," he said, noting that RHIC and the Large Hadron Collider (LHC) are ideally suited for these measurements, which are difficult to assess by other means.

Brookhaven National Laboratory

Physicists create antimatter in record density

Positrons are plentiful in ultra-intense laser blasts 

Physicists from Rice University and the University of Texas at Austin have found a new recipe for using intense lasers to create positrons — the antiparticle of electrons — in record numbers and density.

In a series of experiments described recently in the online journal Scientific Reports published by Nature, the researchers used UT’s Texas Petawatt Laser to make large number of positrons by blasting tiny gold and platinum targets.

Although the positrons were annihilated in a fraction of a microsecond, the experiments have implications for new realms of physics and astrophysics research, medical therapy and perhaps even space travel, said Rice physicist Edison Liang, lead author of the study.

“There are many futuristic technologies related to antimatter that people have been dreaming about for the last 50 years,” said Liang, the Andrew Hays Buchanan Professor of Astrophysics. “One is that antimatter is the most efficient form of energy storage. When antimatter annihilates with matter, it becomes pure energy. Nothing is left behind, unlike in fusion or fission or chemical-based reactions.”

With laser pulses as short as 130 femtoseconds (one femtosecond equals one-quadrillionth of a second), and peak intensity reaching almost 2 billion-trillion watts per square centimeter, the Texas Petawatt Laser is one of the world’s most intense short-pulse, high-energy lasers. In experiments that began in 2012, the Rice-UT team led by Liang completed more than 130 successful laser shots using gold and platinum targets.

“The physical conditions created in our experiments are more extreme than in previous experiments,” Liang said. “Antimatter has been created in accelerators for many decades, but the key difference here is that the laser pulses are very short and ultra intense. The density of antimatter created is high because it’s all concentrated in a tiny amount of space.”

In some shots, the team found they produced jets of particles that included about half as many positrons as electrons. Liang said a high ratio of positrons to electrons is important because physicists would like to produce a mixture that approaches a neutral “pair plasma” with equal numbers of positrons and electrons.

“Pair-dominated plasmas of positrons and electrons are fixtures in the universe,” he said. “They are believed to exist in the winds of pulsars, jets of quasars and gamma-ray bursts, and they are also thought to dominate the universe in the milliseconds following the Big Bang. We hope to use this laboratory platform to simulate some of those phenomena.”

Liang said the group would ultimately like to both create a dense pair-dominated plasma and find a way to capture it in a magnetic bottle for further study.

“Once you trap pure antimatter, the antiparticles can live a long time as long as they don’t touch any matter,” he said. “Since the 1950s, the Air Force, NASA and others have been talking about using antimatter for space travel. In space, weight is the key factor in determining how much fuel you can take with you.

Under the right conditions, positrons and electrons can pair up and orbit one another without touching or annihilating themselves. This paired arrangement, which behaves like a hydrogen atom with almost no mass, is known as positronium.

“But my main interest, which I’ve been concerned with for about 25 years, is to create a very high concentration of antimatter in a very small volume. We want to create a new form of quantum matter, a Bose-Einstein condensate (BEC), of antimatter. That is a really new domain of quantum physics that we could explore.”

A BEC is a collection of atoms or particles that are cooled to such low temperatures that their behavior is dictated by the laws of quantum mechanics. In a BEC, thousands of individual particles act collectively, marching in lock step as if they were a single entity.

“If we have a very large concentration of electrons and positrons, we could imagine making a BEC,” he said. “The beauty of positronium is that it is so light, it can actually become a BEC at relatively mild temperatures — about the temperature at which you would store liquid helium (minus 452 degrees Fahrenheit).”

The research is supported by the Department of Energy and the Rice University Faculty Initiative Fund.

Rice University

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