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

Heavy fermions get nuclear boost on way to superconductivity

Study finds surprise link between nuclear spins, unconventional superconductivity

In a surprising find, physicists from the United States, Germany and China have discovered that nuclear effects help bring about superconductivity in ytterbium dirhodium disilicide (YRS), one of the most-studied materials in a class of quantum critical compounds known as “heavy fermions.”

The discovery, which is described in this week’s issue of Science, marks the first time that superconductivity has been observed in YRS, a composite material that physicists have studied for more than a decade in an effort to probe the quantum effects believed to underlie high-temperature superconductivity.

Rice University physicist and study co-author Qimiao Si said the research provides further evidence that unconventional superconductivity arises from “quantum criticality.”

“There is already compelling evidence that unconventional superconductivity is linked in both copper-based and iron-based high-temperature superconductors to quantum fluctuations that alter the magnetic order of the materials at ‘quantum critical points,’ watershed thresholds that mark the transition from one quantum phase to another,” Si said. “This work provides the first evidence that similar processes bring about superconductivity in the canonical heavy-fermion system YRS.”

Electrons fall within a quantum category called fermions. Heavy fermions are composite materials that contain rare earth elements. Their name stems from the fact that at extremely low temperatures, typically less than 1 kelvin, electrons move through the material as if they were 1,000 times more massive than normal. In the latest experiments, Si said, the measured heat capacity was so large that the electrons behaved as if they were heavier still — about 1 million times heavier than normal. This occurred as the YRS was cooled to just above the point of superconductivity, around 2 millikelvins.

Si, Rice’s Harry C. and Olga K. Wiess Professor of Physics and Astronomy. also directs the Rice Center for Quantum Materials (RCQM). He said the research was conducted in collaboration with RCQM partners in Germany and China. Experiments were performed at the Walther Meissner Institute for Low Temperature Research at the Bavarian Academy of Sciences in Garching, Germany, and at the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany.

Theoretical work was performed at Rice and at Renmin University of China in Beijing.

Experiments overseen by the Meissner Institute’s Erwin Schuberth and the Max Planck Institute’s Frank Steglich offered the first glimpse of YRS’ behavior at the quantum critical point. Schuberth, who has appointments at both institutes as well as the Technical University of Munich, said what appeared to be an increase in apparent mass was actually the clue that nuclear forces were at work.

“Nothing else could have accounted for such a large change,” he said.

The bulk of experiments were performed in Garching, where Schuberth’s team used “adiabatic magnetic cooling” and other specialized techniques to make its YRS samples ultracold, about 10 times colder than those in any previous YRS experiment; this is what allowed the team to discover superconductivity.

In analyzing the evidence, Si and fellow theorist Rong Yu of Renmin University found that the arrangement of inertial spins of the ytterbium nuclei in the YRS composite helped bring about superconductivity. He said the nuclear spins became coupled at extremely low temperatures and arranged in an ordered pattern that exposed the quantum criticality of the electrons.

“In YRS, the spins of electrons are locked in a pattern that varies periodically in space and is the hallmark of an electronic order known as anti-ferromagnetism,” Si said. “An ordered arrangement of the nuclear spins acts to suppress the electronic order, and this exposes the electronic quantum criticality, which in turn drives unconventional superconductivity.”

The discovery of superconductivity in YRS followed a search lasting more than a decade. Steglich said the previous experiments demonstrate that quantum criticality in YRS brings electrons to the verge of being both localized and itinerant, a condition that was predicted by Si and collaborators in a landmark 2001 theory.

Steglich said, “In previous experiments, an external magnetic field revealed a quantum critical point with a host of truly remarkable electronic properties that had been predicted by theory. But the magnetic field also created a condition that is inhospitable to superconductivity.”

The current work succeeded in discovering superconductivity by reaching quantum criticality through the ordering of nuclear spins at ultralow temperatures, without applying an external magnetic field.

“It is remarkable that it takes an act of nuclear spins to produce quantum criticality at zero magnetic field and realize superconductivity,” Steglich said.

Si said the new findings are important for the study of both heavy-fermion superconductivity and, more generally, the physics of quantum criticality.

“The work demonstrates that quantum criticality is a robust mechanism for bringing about unconventional superconductivity, not only in high-temperature superconductors, as had previously been shown, but also in heavy-fermion materials that are the canonical example of quantum critical behavior in every other respect,” Si said.

Study co-authors include Marc Tippmann of the Meissner Institute; Lucia Steinke of both the Meissner Institute and the Max Planck Institute for Chemical Physics of Solids; Stefan Lausberg, Alexander Steppke, Manuel Brando and Christoph Geibel, all of the Max Planck Institute for Chemical Physics of Solids; and Cornelius Krellner of both the University of Frankfurt and the Max Planck Institute for Chemical Physics of Solids.

Rice University

New exotic particle state puzzles theorists

Scientists of the COMPASS-experiment at CERN discover new nuclear particle

Scientists of the COMPASS collaboration at CERN have observed a new exotic combination of light quarks. Researcher from the Cluster of Excellence “Origin and Structure of the Universe” of the Technical University of Munich (TUM) had a leading role in the data analysis of the new finding. So far theoretical physicists are not able to correctly describe all characteristics of this exotic new particle.
 

The standard model of particle physics defines quarks as the fundamental components of atomic nuclei. A proton consists of one “up” and two “down” quarks, a neutron of one “down” and two “up” quarks. With this, the quark particle garden is far from being complete: apart from the two lightest quarks, there are four heavier ones:  “strange”, “charm”, “bottom”, and “top” quarks, plus the corresponding antiparticles, the antiquarks.

All these quarks existed shortly after the Big Bang and played an important role in the early universe. Nowadays, heavy quarks cannot be observed in nature anymore and can only be created in particle physics experiments. Quarks are “glued” together via special adhesive particles, gluons, which mediate the “strong nuclear force”, the strongest of the four fundamental forces of nature.

The basis of all matter

The strong force is described by a theory called Quantum Chromodynamics (QCD), which had been developed in the late 1980s. This set of theories explains the basic principles on which formation of all matter is based, and prescribes which particle configurations occur in nature. In that way QCD predicts a whole set of possible quark combinations.

Some of them are well known: combinations of three quarks (baryons), such as protons and neutrons, and combinations of one quark and one antiquark (mesons), such as pions. According to QCD, some truly exotic combinations, for example molecule-like tetra-quarks or even penta-quarks, are considered as possible. Hints for such a penta-quark have been recently found at LHC.

Understanding the combination rules for quarks has always been a big challenge for theoretical and experimental particles physics, because an extraordinary phenomenon is hindering scientific exploration of the processes that combine quarks together: the force between two quarks increases as the quarks move away from each other, contrary to all other fundamental forces of nature, that always fall off with distance.

The QCD equations that describe the strong force mathematically represent one of the biggest challenges in theoretical physics. They can only be solved by sophisticated computer simulations, which require enormous amounts of computing time. Because of that, not all possible particle combinations have been explored so far.

New, exotic member of the “particle zoo”

In its most recent publication, the COMPASS collaboration announces the existence of an extraordinary meson, which is made out of light quarks and has mass of 1,42 GeV/c2. As numerous investigations have explored this mass range in the last 50 years, the discovery of the new particle at the COMPASS spectrometer at the Proton Synchrotron (SPS) at CERN is indeed a big surprise. It was only possible thanks to the largest worldwide dataset currently available for such investigations.

The new particle called a1(1420) was found during data analyses of experiments, in which pions were shot at a liquid hydrogen target with an impulse of 190 GeV/c2. As this new particle is about 1,000 times rarer than the known mesons, a new, much more complex method of analysis had to be developed, led by scientists from the Cluster of Excellence Universe of the Technical University of Munich (TUM).

Various theoretical explanations for the new particle where proposed, some of which interpret a1(1420) as a molecule composed of known mesons (also known as a tetra-quark-state). Other explanations are based on postulating different long range effects of the strong force, but all of them fail to fully explain the experimental findings.

“This new particle a1(1420) is obviously a new member of the club of unexplained states”, says Stephan Paul, Professor at the Institute for Hadronic Structure and Fundamental Symmetries of Technical University of Munich and coordinator at the Universe Cluster. Now the QCD experts have to solve another difficult problem.

The COMPASS-experiment has been running since 2002 at the Super Proton Synchrotron (SPS), the second largest accelerator at CERN. The collaboration includes about 220 physicists from 13 countries. In Germany, the universities in Bochum, Bonn, Erlangen-Nürnberg, Freiburg and Mainz are involved, as well as the Helmholtz Centre Bonn and the Technical University of Munich (TUM). In Germany, the research was supported by the German Ministry for Education and Research (BMBF), the Excellence Cluster Universe and the computing cluster C2PAP of the Excellence Cluster Universe, the TUM Institute for Advanced Study and the Alexander von Humboldt Foundation. 

Technische Universität München

Nanotechnology World Association

New measurement of the mass of a strange atomic nucleus achieves very high degree of precision

Results obtained at the MAMI particle accelerator in Mainz should add to the understanding of the "strong force"

An international team of physicists working at the Institute of Nuclear Physics at Johannes Gutenberg University Mainz (JGU) has measured the mass of a "strange" atomic nucleus with the aid of an innovative technique that is capable of significantly greater precision than that of previous methods. The researchers were able, for the first time worldwide, to observe the radioactive decay of artificially generated nuclei of super-heavy hydrogen at the Mainz MAMI particle accelerator using a combination of several magnetic spectrometers. They could then precisely determine the mass on the basis of accurate measurement of the decay product. The results have been published in the journal Physical Review Letters.

Read More: http://www.nanotechnologyworld.org/#!New-measurement-of-the-mass-of-a-strange-atomic-nucleus-achieves-very-high-degree-of-precision/c89r/5580aa620cf298dc5b9b648c 

The magic of nuclear physics

The island of stabilityThe white crosses indicate isotopes with ‘magic’ proton or neutron numbers, for example lead-208 (²⁰⁸Pb), which consists of two magic numbers—82 protons and 126 neutrons—making it especially stable. While very heavy atoms with proton numbers in the hundreds are highly unstable, nuclear physicists predict that even some of the heaviest atoms have magic numbers and form an ‘island of stability’. The as-yet-undiscovered ‘doubly magic’ nucleus with 114 protons and 184 neutrons is presumed to be located on this island.© Yuri Oganessian

Except for hydrogen, all other chemical elements originated from violent nuclear processes in stars. Powerful machines known as accelerators allow physicists to study how these elements form and also to create new elements and atomic isotopes. As such, employing accelerators to study atoms not only improves our understanding of the Universe, but also produces synthetic isotopes useful in applications that include medical diagnostics.

Shortly after the Big Bang, the Universe consisted only of hydrogen—the smallest of the chemical elements—and its electrically uncharged partner, the neutron. The diversity of chemical elements that have come into existence since then resulted from nuclear processes taking place within stars. The element helium soon followed hydrogen, forming from the fusion of two hydrogen atoms, and then came carbon, which forms upon the fusion of three helium atoms. Heavier elements, including iron—the most energetically stable element—are created at the end of the life of a star. And elements heavier than iron appear only as a product of catastrophic stellar processes such as supernovae.

Nuclear physicists are using powerful accelerators to shed light on the mechanisms underpinning the Universe’s violent beginnings. Such accelerators can create new atoms with exotic, unstable nuclei that had vital roles in the creation of heavier elements during the explosion of stars.

The dawn of synthetic nuclear physics

Nuclear physics came of age as a research field in the 1920s, when nuclear particles, such as the proton, were discovered. Initial interest focused on understanding the properties of these particles and their involvement in the fundamental nuclear processes that were also discovered at the time: fission and fusion.

Yoshio Nishina was a pioneer of nuclear research in Japan. Following his education at the University of Tokyo and research stints in Europe in the 1920s, Nishina became a chief scientist at RIKEN in 1931. His mission was to establish a nuclear physics laboratory.

One of Nishina’s key research areas—and still the subject of ongoing research—was the study of different nuclear isotopes. Atomic cores consist of two types of particles: the proton, the number of which determines the identity of a chemical element; and the neutron, which is electrically neutral and determines an element’s isotopes. Hydrogen, for example, has one proton and three different isotopes; but heavier chemical elements, such as uranium, can have many different isotopes.

The stability of an isotope strongly depends on its mix of neutrons and protons, and this mix also influences how one element transforms into another during a nuclear reaction. A radioactive element, such as uranium, can follow many different reaction pathways to become a more stable isotope. Even the radioactive decay rates of an element can vary greatly between isotopes, ranging from billions of years to a fraction of a second.

An ideal way to study isotope properties is by synthesizing isotopes in the laboratory with the help of accelerators. These machines accelerate and smash atoms together. At sufficiently high energy, the collision of atoms creates new isotopes, or even new elements. Using the first Japanese accelerator—a cyclotron—Nishina discovered a new isotope of uranium, uranium-237, which differed from the form typically found in nature, uranium-238 (containing 92 protons and 146 neutrons).

Nishina built two cyclotrons at RIKEN but both were destroyed after the Second World War. New cyclotrons followed at RIKEN and the latest, the ninth cyclotron, is the world’s most powerful superconducting ring cyclotron (SRC). The SRC can accelerate heavy atomic nuclei to speeds of up to 70 per cent of the velocity of light and has a beam intensity that is 100 times greater than any other accelerator in the world.

The SRC is part of the nuclear research facility at RIKEN that now bears Nishina’s name—the RIKEN Nishina Center for Accelerator-Based Science (RNC). Inaugurated in 2006, the center hosts some 200 full-time scientists and collaborates with many international institutions.

Demystifying ‘magic numbers’

Experiments using the RNC’s latest generation of cyclotrons are pushing frontiers in nuclear research, particularly in the search for new isotopes with so-called ‘magic numbers’. These isotopes have a set number of protons or neutrons in their nuclei and are surprisingly stable in comparison to other isotopes. Calcium-54, for example, consists of 20 protons and 34 neutrons. This isotope is very unusual: typically, 34 neutrons do not constitute a ‘magic’ isotope; and, its magic properties were theoretically proposed shortly before their experimental discovery by researchers at RIKEN¹.

Explaining how elements as heavy as uranium could have resulted from stellar explosions hinges on understanding magic numbers and the stability of their associated elements.

The intense beams of atomic isotopes generated by RIKEN’s cyclotrons are useful in the search for very heavy elements. As heavy isotopes are accelerated and collide with each other, they re-assemble into different atoms, allowing the discovery of new chemical elements. Such research led to the discovery in 2012 of element 113 at the RNC², after more than nine years of thorough searching.

The synthesis of new chemical elements could lead to a new understanding of nuclear physics. Very heavy atoms are highly unstable and have a very short life, making it difficult to prove their existence following high-energy collisions. However, nuclear physicists predict that even some of the heaviest atoms have magic numbers and could survive for longer after a collision. They are predicted to form a so-called ‘island of stability’ within the short-lived heavy elements, which is typically the domain of elements with approximately 120 protons (Fig. 1).

Since existing accelerators lack the power necessary to reveal new elements within this island of stability, RIKEN plans to replace its synchrotrons 5 and 6 with a new superconducting accelerator. This powerful accelerator would produce beam intensities some 100 times greater than presently possible, securing RIKEN’s leadership in the study of heavy isotopes.

Nuclear research for better living

Atomic accelerators also have application beyond fundamental physics. In medicine, for example, certain radioactive isotopes are used as markers in diagnostic experiments because their distribution within the body can be precisely measured. Many of these isotopes are a by-product of uranium fission in nuclear reactors. As research reactors are being decommissioned, alternative methods to produce radioactive isotopes, including accelerators, have become increasingly important.

The use of high-energy ion beams further extends to agriculture. Naturally occurring radioactivity is one of the causes of mutations in plants. Radioactive rays can destroy DNA or cause small changes to the genome. Over many generations, such changes can accumulate and influence an organism’s evolution.

With ion beams, this process can be sped up by introducing mutations at a faster rate. Careful selection of useful mutations in each generation can produce better plants for cultivation. Developing rice that is tolerant to salty water, or even mixtures containing up to 50 per cent sea water, is one example. Controlled mutations could become increasingly important in ensuring a sufficient food supply for an ever-increasing global population.

After more than 80 years of nuclear physics research at RIKEN, much work is still required to better understand the properties of magic isotopes, the formation of heavy elements, the interplay of protons and neutrons in the nucleus and the internal structure of protons and neutrons. Attaining this understanding will require accelerators with even higher energies and intensities than are presently available. In that quest, the RNC will continue to play an important role. The new generation of accelerators being planned will help us to understand how the elementary particles in an atom interact with each other to form the world around us.

Source: http://www.rikenresearch.riken.jp/eng/perspectives/7757.html