Physicists discover family of tetraquarks

Syracuse University Professor Tomasz Skwarnicki and Ph.D. student Thomas Britton confirm existence of rare 'exotic' particle, find evidence of 3 others

Physicists in the Syracuse University College of Arts and Sciences have made science history by confirming the existence of a rare four-quark particle and discovering evidence of three other "exotic" siblings.

Their findings are based on data from the Large Hadron Collider (LHC), the world's biggest, most powerful particle accelerator, located at the CERN science laboratory in Geneva, Switzerland.

Professor Tomasz Skwarnicki and Ph.D. student Thomas Britton G'16, both members of the Experimental High-Energy Physics Group at Syracuse and the Large Hadron Collider beauty (LHCb) collaboration at CERN, have confirmed the existence of a tetraquark candidate known as X(4140). They also have detected three other exotic particles with higher masses, called X(4274), X(4500) and X(4700).

All four particles were the subject of Britton's Ph.D. dissertation, which he defended in May and then submitted, on behalf of the LHCb collaboration, as a journal article to Physical Review Letters (American Physical Society, 2016).

A tetraquark is a particle made of four quarks: two quarks and two antiquarks.

Tetraquarks--and, by extension, pentaquarks, containing five quarks--are considered exotic because they have more than the usual allotment of two or three quarks.

"Even though all four particles contain the same quark composition, each of them has a unique internal structure, mass and set of quantum numbers," says Skwarnicki, who, in April 2014, confirmed the existence of the world's first charged tetraquark candidate, called Z(4430)+. A year earlier, he and Ph.D. student Bin Gui G'14 determined the quantum numbers of the first neutral, heavy tetraquark candidate, X(3872).

Quantum numbers describe each particle's subatomic properties.

Skwarnicki says the measurement of all four particles is the largest single one of its kind to date. Unlike other exotic particle candidates, his and Britton's do not contain ordinary nuclear matter (i.e., quarks found in protons and neutrons).

"We've never seen this kind of thing before. It's helping us distinguish among various theoretical models of particles," Skwarnicki says.

A rendering of the enormous LHCb detector, which registers approximately 10 million proton collisions per second. Scientists study the debris from these collisions to better understand the building blocks of matter and the forces controlling them.

A fellow of the American Physical Society, Skwarnicki is a longtime member of the LHCb collaboration, involving approximately 800 other scientists from 16 countries. Their goal is to discover all forms of matter, in hopes of explaining why the universe is made of it, instead of anti-matter.

Skwarnicki's work focuses on quarks--fundamental constituents of matter that serve as a kind of scaffolding for protons and neutrons. While most particles have two or three quarks, Skwarnicki and others, in the past decade, have observed ones with four or five.

Last summer, he and doctoral student Nathan Jurik G'16 teamed up with Distinguished Professor Sheldon Stone and Liming Zhang, a professor at Tsinghua University in Beijing, to announce their discovery of two rare pentaquark states. The news made headlines, thrusting Syracuse and CERN into the international spotlight.

According to the Standard Model of particle physics, there are six kinds of quarks, whose intrinsic properties cause them to be grouped into pairs with unusual names: up/down, charm/strange and top/bottom.

Tomasz Skwarnicki

The particles that Skwarnicki and Britton study have two charm quarks and two strange quarks. Charm and strange quarks are the third- and fourth-most massive of all quarks.

That all four quarks in the new family are "heavy" is noteworthy.

"The heavier the quark, the smaller the corresponding particle it creates," says Skwarnicki, adding that the names of the particles reflect their masses. "The names are denoted by mega-electron volts [MeV], referring to the amount of energy an electron gains after being accelerated by a volt of electricity. ... This information, along with each particle's quantum numbers, enhances our understanding of the formation of particles and the fundamental structures of matter."

Evidence of X(4140) first appeared in 2009 at the Fermi National Accelerator Laboratory, outside of Chicago, but the observation was not confirmed until three years later at CERN.

A rendering of the enormous LHCb detector, which registers approximately 10 million proton collisions per second. Scientists study the debris from these collisions to better understand the building blocks of matter and the forces controlling them. Extremely rare and four times heavier than a proton, X(4140) has been initially detected only 20 times out of billions of man-made energy collisions. LHCb is uniquely suited to study such particles, and thus, has gone on to detect X(4140) nearly 560 times.

Skwarnicki attributes the discovery of X(4140)'s three siblings, culled from LHCb data from 2011 to 2012, to increased instrumental sensitivity. It is the energy configuration of the quarks, he explains, that gives each particle its unique mass and identity.

Thomas Britton G'16

"Quarks may be tightly bound, like three quarks packed inside a single proton, or loosely bound, like two atoms forming a molecule," Skwarnicki says. "By examining the particles' quantum numbers, we were able to narrow down the possibilities and rule out the molecular hypothesis."

A snapshot of LHCb detector data, singling out the collisions that have resulted in the four tetraquarks. Not that the process has been easy. An "aporetic saga" is how Britton describes studying molecular structures that seem to "jump out of the data."

"We looked at every known particle and process to make sure that these four structures couldn't be explained by any pre-existing physics," he says. "It was like baking a six-dimensional cake with 98 ingredients and no recipe--just a picture of a cake."

Meanwhile, Skwarnicki, Britton and others face the onerous task of combing through data and developing theoretical models, in an attempt to confirm what they have seen.

"It may be a quartet of entirely new particles or the complex interplay of known particles, simply flipping their identities," Skwarnicki concludes. "Either way, the outcome will shape our understanding of the subatomic universe."

Syracuse University

Physicists measured something new in the radioactive decay of neutrons.

The experiment inspired theorists; future ones could reveal new physics.

A physics experiment performed at the National Institute of Standards and Technology (NIST) has enhanced scientists’ understanding of how free neutrons decay into other particles. The work provides the first measurement of the energy spectrum of photons, or particles of light, that are released in the otherwise extensively measured process known as neutron beta decay. The details of this decay process are important because, for example, they help to explain the observed amounts of hydrogen and other light atoms created just after the Big Bang.

Published in Physical Review Letters, the findings confirm physicists’ big-picture understanding of the way particles and forces work together in the universe—an understanding known as the Standard Model. The work has stimulated new theoretical activity in quantum electrodynamics (QED), the modern theory of how matter interacts with light. The team’s approach could also help search for new physics that lies beyond the Standard Model.

Neutrons are well known as one of the three kinds of particles that form atoms. Present in all atoms except the most common form of hydrogen, neutrons together with protons form the atomic nucleus. However, “free” neutrons not bound within a nucleus decay in about 15 minutes on average. Most frequently, a neutron transforms through the beta decay process into a proton, an electron, a photon, and the antimatter version of the neutrino, an abundant but elusive particle that rarely interacts with matter.

The photons from beta decay are what the research team wanted to explore. These photons have a range of possible energies predicted by QED, which has worked very well as a theory for decades. But no one had actually checked this aspect of QED with high precision.

“We weren’t expecting to see anything unusual,” said NIST physicist Jeff Nico, “but we wanted to test QED’s predictions very precisely in a way no one has done before.”

Nico and his colleagues, who represent nine research institutions, performed their measurements at the NIST Center for Neutron Research (NCNR). It produces an intense beam of slow-moving neutrons whose photon emissions can be detected with the same setup used for earlier precision measurements of the neutron’s lifetime.

The team measured two aspects of neutron decay: the energy spectrum of the photons, and also its branching ratio, which can provide information on how frequently the decays were accompanied by photons above a specific energy. The results of this effort gave them a branching ratio measurement more than twice as accurate as the previous value, and the first measurement of the energy spectrum.

“Everything we found was consistent with the predominant QED calculations,” Nico said. “We got quite a good match with theory on the energy spectrum, and we reduced the uncertainty in the branching ratio.”

According to Nico, the results provided specific information that theoretical physicists are already using to further develop QED to provide more detailed descriptions of neutron beta decay.

The results serve as a needed check on the Standard Model, said Nico, and validates the team’s experimental approach as a way to go beyond it. With better detectors, the approach could be used to search for so-called “right-handed” neutrinos, which have not yet been detected in nature, and potential time-reversal symmetry violations, which could explain why there is much more matter than antimatter in the universe.

Paper: M.J. Bales, R. Alarcon, C.D. Bass, E.J. Beise, H. Breuer, J. Byrne, T.E. Chupp, K.J. Coakley, R.L. Cooper, M.S. Dewey, S. Gardner, T.R. Gentile, D. He, H.P. Mumm, J.S. Nico, B. O'Neill, A.K. Thompson and F.E. Wietfeldt (RDK II Collaboration). Precision measurement of the radiative beta decay of the free neutron. Physical Review Letters. June 14, 2016, DOI: 10.1103/PhysRevLett.116.242501

NIST

UChicago physicists first to see behavior of quantum materials in curved space

Harnessing the shared wave nature of light and matter, researchers at the University of Chicago led by Neubauer Family Assistant Professor of Physics Jonathan Simon have used light to explore some of the most intriguing questions in the quantum mechanics of materials. The topic encompasses complex and non-intuitive phenomena that are often difficult to explain in non-technical language, but which carry important implications to specialists in the field.

In work published online June 6, 2016, in the journal Nature, Simon's group presents new experimental observations of a quantum Hall material near a singularity of curvature in space.

Quantum effects give rise to some of the most useful and promising properties of materials: they define standard units of measurement, give rise to superconductivity, and describe quantum computers. The quantum hall materials are one prominent example in which electrons are trapped in non-conducting circular orbits except at the edges of the material. There, electrons exhibit quantized resistance-free electrical conduction that is immune to disorder such as material impurities or surface defects.

Furthermore, electrons in quantum Hall materials do not transmit sound waves but instead have particle-like excitations, some of which are unlike any other particles ever discovered. Some of these materials also exhibit simultaneous quantum entanglement between millions of electrons, meaning that the electrons are so interconnected, the state of one instantly influences the state of all others. This combination of properties makes quantum Hall materials a promising platform for future quantum computation.

Researchers worldwide have spent the past 35 years delving into the mysteries of quantum Hall materials, but always in the same fundamental way. They use superconducting magnets to make very powerful magnetic fields and refrigerators to cool electronic samples to thousandths of a degree above absolute zero.

Trapping light...

In a new approach, Simon and his team demonstrated the creation of a quantum Hall material made up of light. "Using really good mirrors that are pointed at each other, we can trap light for a long time while it bounces back and forth many thousands of times between the mirrors," explained graduate student Nathan Schine.

In the UChicago experiment, photons travel back and forth between mirrors, while their side-to-side motion mimics the behavior of massive particles like electrons. To emulate a strong magnetic field, the researchers created a non-planar arrangement of four mirrors that makes the light twist as it completes a round trip. The twisting motion causes the photons to move like charged particles in a magnetic field, even though there is no actual magnet present.

"We make the photons spin, which leads to a force that has the same effect as a magnetic field," explained Schine. While the light is trapped, it behaves like the electrons in a quantum Hall material.

First, Simon's group demonstrated that they had a quantum Hall material of light. To do so, they shined infrared laser light at the mirrors. By varying the laser's frequency, Simon's team could map out precisely at which frequencies the laser was transmitted through the mirrors. These transmission frequencies, along with camera images of the transmitted light, gave a telltale signature of a quantum Hall state.

Next, the researchers took advantage of the precise control that advanced optical systems provide to place the photons in curved space, which has not been possible so far with electrons. In particular, they made the photons behave as if they resided on the surface of a cone.

...near a singularity

"We created a cone for light much like you might do by cutting a wedge of paper and taping the edges together," said postdoctoral fellow Ariel Sommer, also a co-author of the paper. "In this case, we imposed a three-fold symmetry on our light, which essentially divides the plane into three wedges and forces the light to repeat itself on each wedge."

The tip of a cone has infinite curvature--the singularity--so the researchers were able to study the effect of strong spatial curvature in a quantum Hall material. They observed that photons accumulated at the cone tip, confirming a previously untested theory of the quantum Hall effect in curved space.

Despite 20 years of interest, this is the first time an experiment has observed the behavior of quantum materials in curved space. "We are beginning to make our photons interact with each other," said Schine. "This opens up many possibilities, such as making crystalline or exotic quantum liquid states of light. We can then see how they respond to spatial curvature."

The researchers say this could be useful for characterizing a certain type of quantum computer that is built of quantum Hall materials.

"While quantum Hall materials were discovered in the eighties, they continue to reveal their fascinating secrets to this day," said Simon. "The final frontier is exploring the interplay of these beautiful materials with the curvature of space. That is what we've begun to explore with our photons."

Reference:

Synthetic Landau levels for photons
Nature (2016) doi:10.1038/nature17943

University of Chicago

Doubling down on Schrödinger’s cat

Yale physicists have given Schrödinger’s famous cat a second box to play in, and the result may help further the quest for reliable quantum computing.

Schrödinger’s cat is a well-known paradox that applies the concept of superposition in quantum physics to objects encountered in everyday life. The idea is that a cat is placed in a sealed box with a radioactive source and a poison that will be triggered if an atom of the radioactive substance decays. Quantum physics suggests that the cat is both alive and dead (a superposition of states), until someone opens the box and, in doing so, changes the quantum state.

This hypothetical experiment, envisioned by one of the founding fathers of quantum mechanics in 1935, has found vivid analogies in laboratories in recent years. Scientists can now place a wave-packet of light composed of hundreds of particles simultaneously in two distinctly different states. Each state corresponds to an ordinary (classical) form of light abundant in nature.

A team of Yale scientists created a more exotic type of Schrödinger’s cat-like state that has been proposed for experiments for more than 20 years. This cat lives or dies in two boxes at once, which is a marriage of the idea of Schrödinger’s cat and another central concept of quantum physics: entanglement. Entanglement allows a local observation to change the state of a distant object instantaneously. Einstein once called it “spooky action at a distance,” and in this case it allows a cat state to be distributed in different spatial modes.

The Yale team built a device consisting of two, 3D microwave cavities and an additional monitoring port — all connected by a superconducting, artificial atom. The “cat” is made of confined microwave light in both cavities.

“This cat is big and smart. It doesn’t stay in one box because the quantum state is shared between the two cavities and cannot be described separately,” said Chen Wang, a postdoctoral associate at Yale and first author of a study in the journal Science, describing the research. “One can also take an alternative view, where we have two small and simple Schrodinger’s cats, one in each box, that are entangled.”

The research also has potential applications in quantum computation. A quantum computer would be able to solve certain problems much faster than classical computers by exploiting superposition and entanglement. Yet one of the main problems in developing a reliable quantum computer is how to correct for errors without disturbing the information.

“It turns out ‘cat’ states are a very effective approach to storing quantum information redundantly, for implementation of quantum error correction. Generating a cat in two boxes is the first step towards logical operation between two quantum bits in an error-correctible manner,” said co-author Robert Schoelkopf, Sterling Professor of Applied Physics and Physics, and director of the Yale Quantum Institute.

Schoelkopf and his frequent collaborators, Michel Devoret and Steve Girvin, have pioneered the field of circuit quantum electrodynamics (cQED), providing one of the most widely used frameworks for quantum computation research. Devoret, Yale’s F.W. Beinecke Professor of Physics, and Girvin, Yale’s Eugene Higgins Professor of Physics and Applied Physics, are co-authors of the paper.

The research builds upon more than a decade of development in cQED architecture. The Yale team designed a variety of new features, including cylindrical 3D cavities with record quantum information storage time of more than 1 millisecond in superconducting circuits, and a measurement system that monitors certain aspects of a quantum state in a precise, non-destructive way. “We have combined quite a lot of recent technologies here,” Wang said.

Additional co-authors from the Yale Departments of Applied Physics and Physics include assistant professor Liang Jiang; senior research scientist Luigi Frunzio; postdoctoral associates Reinier Heeres and Nissim Ofek; graduate students Yvonne Gao, Philip Reinhold, Kevin Chou, Christopher Axline, Matthew Reagor, Jacob Blumoff, and Katrina Sliwa; and former Yale researcher Mazyar Mirrahimi.

Yale

The proof is in the pudding

There is an important difference between knowing something, and being able to prove something. Even in science, there are ideas that everyone shares, but no one can prove.

The link between disorder and metastability in a granular material was one of such ideas, until three researchers devised an experiment to measure the relationship between the two phenomena.

Prof Mahesh Bandi, lead researcher of the Collective Interactions Unit at theOkinawa Institute of Science and Technology Graduate University (OIST), is one of these three researchers. Together with two colleagues from Nagoya University, Naoki Iikawa and Prof Hiroaki Katsuragi, Prof Bandi has recently published inPhysical Review Letters, showing for the first time that it is possible to detect and quantify metastability induced by disorder. "Everyone expected this result," said Bandi, "but before us no one was able to find a way to measure the relationship between disorder and metastability in granular materials."

Metastability is a concept linked with the level of energy in a physics system. When a physics system is stable, it has only one state of low energy. Its energy is like a bucket in a well: when it goes down, it will always end up in the same water pool.

But there are physics systems that can be equated to strange wells. Wells that have many water pools at their bottom. When the bucket goes down in one of these wells, it is not possible to predict with certainty in which pool it will end up. Furthermore, if the bucket is lifted to any height in the well and then released, it may end up in a different pool. These physics systems are metastable: the lowest state of energy for these systems is neither unique nor predictable.

The relationship between metastability and disorder was elusive since the measurements typically used in granular physics are not sensitive enough to detect metastability. Bandi and colleagues succeeded because they thought out of the box. They borrowed a parameter, called 'S', from a different branch of physics, liquid crystal physics. They showed that S can be used to detect the orientation of the forces inside a granular material. These forces, called force chains, are the networks of forces that arise from the contacts among grains. Force chains give granular materials their physical proprieties. To study force chains, the scientists used special disks, called photo-elastic disks, which change their appearance when compressed.

After checking that in an ordered configuration there was no change in S, the scientists set up a random configuration of photo-elastic disks. They took disks of two different diameters, and inserted them in a vertical structure. Inside such configuration, the force chains pointed along any direction. "Then, we tapped the structure," Bandi recollects, "hitting it along the vertical direction."

The configuration of the disks stayed the same, but the force chains slowly oriented themselves from the direction of gravity towards all random directions. The scientists captured high-resolution images of this process. The images showed the changing force chains, and the researchers translated the visual information into numbers. The resulting values of S confirmed the change in the force chains, finally proving the link between disorder, induced by tapping, and the metastable energy state of the granular material, which changed after each tapping like a bucket changing pool in one of those strange wells.

While this result was expected, the introduction of S, the parameter from liquid crystal, represents an important change in the study of granular materials. S switches the focus from the structure of granular materials, to the orientation of the force chains inside granular materials. Such switch of focus provides the researchers with a new way to conceptualize disorder and metastability in granular materials, hopefully leading to new advances in the understanding of these materials.

Reference:

Sensitivity of Granular Force Chain Orientation to Disorder-Induced Metastable Relaxation
N. Iikawa, M. M. Bandi, and H. Katsuragi
Phys. Rev. Lett. 116, 128001 – Published 21 March 2016
DOI: http://dx.doi.org/10.1103/PhysRevLett.116.128001

Okinawa Institute of Science and Technology Graduate University OIST

New Physics and Application of Antiferromagnet Uncovered

The research group of Professor Hideo Ohno and Associate Professor Shunsuke Fukami of Tohoku University has studied the control of magnetization using a current applied to heterostructures comprising an antiferromagnet. They found that the current gives rise to a flow of electron spin in the antiferromagnet, which induces magnetization switching in a neighboring ferromagnet.

The obtained results shed light on a new physics of antiferromagnet and also open various pathways toward ultralow-power integrated circuits and other novel applications such as neuromorphic computing.

Spintronics devices that can store information via the magnetization direction under no power supply, are expected to realize ultralow-power integrated circuits. 　A key issue for the application is how to achieve a fast and reliable magnetization switching with low power consumption.

Recently, a switching scheme utilizing the flow of electron spin, the so-called spin current, originating from the spin-orbit interaction, has attracted a great deal of attention as a new method to achieve fast and reliable control of magnetization. This scheme has been observed in heterostructures typically consisting of ferromagnet and nonmagnetic heavy metal layer and is called the spin-orbit torque induced magnetization switching.

fig. 1: Schematics of the antiferromagnet-ferromagnet bilayer system studied in this work. The current applied to the bilayer gives rise to a flow of electron spin in the perpendicular direction to the film plane. The magnetization of ferromagnet in the vicinity of interface is biased in the film plane direction due to an interaction with the antiferromagnet, which allows for the field-free switching.

The research group investigated the spin-orbit torque induced switching in an antiferromagnet-ferromagnet bilayer system. Until now, the motion of electron spin in antiferromagnetic materials has not yet been studied well. They fabricated switching devices from a stack with an antiferromagnetic PtMn and a ferromagnetic Co/Ni multilayer, and electrically evaluated the switching properties at room temperature. They found that the current flowing in the antiferromagnet generates a spin-orbit torque large enough to induce the magnetization switching in the neighboring ferromagnet.

It is notable that whereas the spin-orbit torque switching in nonmagnet-ferromagnet bilayer systems studied previously requires in-plane external field, the present system allows field-free switching owing to a unique property arising at the antiferromagnet-ferromagnet interface.

fig. 2: The Hall resistance versus applied current measured at zero magnetic fields. The Hall resistance represents the perpendicular component of magnetization. The reversed component of magnetization depends on the magnitude of applied current.

Furthermore, they found that in specific stack structures, the reversed portion of magnetization can be controlled in an analogue manner by the magnitude of the applied current, and this feature can also be attributed to the nature of the antiferromagnet.

This work is significant from both the physics and application points of view. In terms of physics, the obtained results allow for deeper understanding of antiferromagnet and spin transport phenomena, such as the topological Hall effect. In terms of application, the external-field-free switching achieved in this work shows promise for the implementation of spin-orbit torque devices for future ultralow-power integrated circuits. In addition, since the analogue-like behavior observed here resembles the operation manner of synapses in the brain, the present antiferromagnet-ferromagnet device could be key to realizing neuromorphic computing, which is known to achieve efficient information processing.

This work is supported by the ImPACT Program of CSTI and R&D Project for ICT Key Technology to Realize Future Society of MEXT.

Publication Details :

Authors: S. Fukami, C. Zhang, S. Dutta Gupta, A. Kurenkov and H. Ohno
Title: Magnetization switching by spin-orbit torque in an antiferromagnet-ferromagnet bilayer system
Journal: Nature Materials
DOI: 10.1038/NMAT4566

Tohoku University