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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.
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.
Hypothetical shortcuts through the universe, wormholes link separate points in space-time.
Quantum entanglement may explain gravity.
Quantum entanglement is one of the more bizarre theories to come out of the study of quantum mechanics—so strange, in fact, that Albert Einstein famously referred to it as “spooky action at a distance.”
Essentially, entanglement involves two particles, each occupying multiple states at once, for example simultaneously spinning clockwise and counterclockwise. But neither has a definite state until one is measured, causing the other particle to instantly assume a corresponding state. The resulting correlations between the particles are preserved even if they reside on opposite ends of the universe.
But what enables particles to communicate instantaneously—seemingly faster than the speed of light—over such vast distances?
Now an MIT physicist looking at entanglement through the lens of string theory has proposed an answer: the creation of two entangled quarks—the building blocks of matter—simultaneously gives rise to a wormhole connecting the pair.
The theoretical results bolster the relatively new and exciting idea that the laws of gravity holding together the universe may not be fundamental but arise instead from quantum entanglement.
Julian Sonner, a senior postdoc in MIT’s Laboratory for Nuclear Science and Center for Theoretical Physics, has published his results in the journal Physical Review Letters.
To see what emerges from two entangled quarks, he first created a theoretical model of quarks based on the Schwinger effect—a concept in quantum theory that makes it possible to create particles out of nothing. Once extracted from a vacuum, these particles are considered entangled.
Sonner mapped the entangled quarks onto a four-dimensional space, considered a representation of space-time. In contrast, gravity is thought to exist in the next dimension, where, according to Einstein’s laws, it acts to “bend” and shape space-time.
To see what geometry may emerge in the fifth dimension from entangled quarks in the fourth, Sonner employed the string theory concept of holographic duality, used to derive a more complex dimension from the next-lowest dimension.
He found that what emerged was a wormhole connecting the two entangled quarks, implying that the creation of quarks simultaneously creates a wormhole. More fundamentally, he says, gravity itself may be a result of entanglement. What’s more, the universe’s geometry as described by classical gravity may be a consequence of entanglement—pairs of particles strung together by tunneling wormholes.
“It’s the most basic representation yet that we have where entanglement gives rise to some sort of geometry,” Sonner says. “What happens if some of this entanglement is lost, and what happens to the geometry? There are many roads that can be pursued.”
Tiny Drops of Hot Quark Soup—How Small Can They Be?
New analyses of deuteron-gold collisions at RHIC reveal that even small particles can create big surprises
Scientists designed and built the Relativistic Heavy Ion Collider (RHIC) at the U.S. Department of Energy’s Brookhaven National Laboratory to create and study a form of matter that last existed a fraction of a second after the Big Bang, some 13.8 billion years ago. The early-universe matter is created when two beams of gold nuclei traveling close to the speed of light slam into one another. The high-speed particle smashups pack so much energy into such a tiny space that the hundreds of protons and neutrons making up the nuclei “melt” and release their constituent particles—quarks and gluons—so scientists can study these building blocks of matter as they existed at the dawn of time.
Collisions between gold nuclei and deuterons—much smaller particles made of just one proton and one neutron—weren’t supposed to create this superhot subatomic soup known as quark-gluon plasma (QGP). They were designed as a control experiment, to generate data to compare against RHIC’s gold-gold smashups. But new analyses indicate that these smaller particle impacts may be serving up miniscule servings of hot QGP—a finding consistent with similar results from Europe’s Large Hadron Collider (LHC), which can also collide heavy nuclei.
“Considering that the quark-gluon plasma we create in gold-gold collisions at RHIC fills a space that is approximately the size of the nucleus of a single gold atom, the possible hot spots we’re talking about in these deuteron-gold collisions are much, much smaller—and an intriguing surprise,” said Dave Morrison, a physicist at Brookhaven and co-spokesperson for RHIC’s PHENIX collaboration. The collaboration describes their results in two papers just published by Physical Review Letters, one of which is highlighted by the journal.
The findings at RHIC and the LHC have triggered active debate about their interpretation. Said PHENIX co-spokesperson Jamie Nagle of the University of Colorado, “There isn't yet universal agreement about what we’re seeing in these small systems, but if indeed nearly perfect fluid droplets of quark-gluon plasma are being formed, this may be a perfect testing ground for understanding the essential conditions for creating this remarkable state of matter.”
New analyses
The PHENIX scientists’ evidence for the possibility of tiny hot spots comes from taking a closer look at data collected by their namesake detector, one of two large particle trackers at RHIC, during deuteron-gold collisions in 2008. The group was inspired to conduct a new analysis after hints of QGP-like behavior emerged from similar control experiments in the heavy-ion physics program at the LHC.
When not searching for particles such as the Higgs boson, the LHC spends a few weeks during each experimental run colliding beams of lead nuclei to create and study QGP at higher energies than RHIC, using protons instead of deuterons for its control experiments. In those control collisions (between lead nuclei with protons and also in some collisions of protons with protons), LHC scientists observed certain “signatures” that were surprisingly reminiscent of those generated by QGP—namely similar patterns of particle production and correlations in how particles flow out of the collisions.
When PHENIX physicists took a closer look at their data from deuteron-gold collisions at RHIC, they saw the same telltale effects.
Correlations in particle “flow”
This animation shows how hot spots (red) created in a deuteron-gold collision—formed from the impact of the deuteron's proton and neutron with the gold nucleus—evolve into cooler matter that flows out to the sides. This small scale flow is similar to the dramatic elliptical particle flow observed in gold-gold collisions at RHIC. Credit: Produced using the viscous hydrodynamic code of Prof. Paul Romatschke (University of Colorado) run by Jamie Nagle and Mike McCumber.
“As you look around the collision and measure the particles detected at different angles, it’s not just a random array of particles coming out. There are strong correlations among some of the particles,” Morrison said. For example, if a particle emerges in one direction, you are more likely to see a “correlated” particle emerging in a particular, predictable direction (for example, back-to-back or alongside one another), rather than in random directions.
These correlations produce an asymmetric distribution of particles as they expand from the collision zone, with more particles flowing out in an elliptically shaped area close to the plane of the colliding beams than perpendicular to it. This elliptical flow pattern is one of the defining characteristics of the quark-gluon plasma created in gold-gold collisions at RHIC.
Evidence to date suggests that gold-gold collisions the Relativistic Heavy Ion Collider at Brookhaven are indeed creating a new state of hot, dense matter, but one quite different and even more remarkable than had been predicted. Instead of behaving like a gas of free quarks and gluons, as was expected, the matter created in RHIC's heavy ion collisions appears to be more like a "perfect" liquid.
“In the relatively big drop of quark-gluon plasma created in gold-gold collisions, which flows like a friction-free ‘perfect’ liquid, there’s all sorts of collective motion that gives you that pattern. But it was not expected to see this in these smaller systems,” he said.
The strength of these particle correlations in deuteron-gold collisions increases the higher the momentum of the particles, said Brookhaven/PHENIX physicist Anne Sickles, who led the group analyzing and writing up the flow results. That pattern fits the models of “hydrodynamic” flow used to describe the behavior of hot QGP. “Even though the system—the hot spot created in these deuteron-gold collisions—is very small, it shows particle correlations similar to those understood to be triggered by flow in much larger collisions of gold on gold,” Sickles said.
If signs of flow persist to higher levels of correlations—for example, producing flow patterns with three or more lobes instead of simple ellipses—it would be a clear sign that the deuteron is somehow altering the state of the tiny portion of the gold nucleus with which it interacts. Data from the LHC have already revealed some evidence for these higher-level correlations. Future runs colliding deuterons, protons, or helium nuclei with gold at RHIC have the potential to provide the definitive test by changing the shape of the initial hot spot and looking for corresponding changes in particle correlation patterns.
Differential dips in particles counted
Other intriguing findings emerged when PHENIX physicists looked at the relative numbers of particles containing “charm” and “anticharm” quarks counted in RHIC’s various collisions. Charm and anticharm quarks don’t exist in ordinary matter, so all the charm and anticharm particles picked up by RHIC’s detectors are created in the collisions. But there are several ways these particles and their antiparticles can combine, with the charm and anticharm particles in one arrangement (J/psi particles) being more tightly bound together than in others (e.g. psi prime). Counting the numbers of the different combinations emerging from different types of collisions could offer clues about what’s happening inside.
For example, PHENIX scientists compared the numbers of J/psi and psi prime particles counted by their detector in glancing deuteron-gold collisions—where the smaller deuteron just nicks the edge of the big gold nucleus—with those emerging from more central collisions—where the deuteron pierces the center of the gold nucleus.
The glancing collisions produced results very similar to those from proton-proton collisions. “It was as if the deuteron was colliding with a single proton at the edge of the gold nucleus,” said Darren McGlinchey of the University of Colorado, who led the group that wrote the paper on these results.
In the central collisions, however, where the deuteron interacts with more protons and neutrons inside the nucleus, the detectors counted fewer of both types of charm-anticharm particles than would be expected from these multiple interactions. Additionally, the number of the less-tightly bound charm-anticharm pairs (the psi primeparticles) was suppressed more than the tightly bound J/psi’s—and this difference got bigger the more central the collisions were.
“This differential particle suppression suggests that something novel is going on in the more central deuteron-gold collisions,” McGlinchey said.
Is it something that happens in the collision—perhaps interactions of the particles with tiny specks of quark-gluon plasma, causing them to lose energy and get “stuck” as happens in the big drops of QGP formed in gold-gold collisions? Or is it due to some characteristic of the bigger nucleus that exists before the collision even takes place?
That question of before or after—which physicists refer to as initial state vs. final state effects—is a big part of the debate going on right now about these findings from both RHIC and the LHC.
Other explanations
Some theorists, for example, suggest that the correlations in particle flow patterns observed in deuteron-gold and proton-lead collisions could be triggered not by tiny specks of hot quark-gluon plasma, but instead by properties of the “cold” nuclei themselves. These nuclei, they say, reveal a different aspect of their identity when moving close to the speed of light.
Their evidence comes from experiments at other colliders revealing that the quarks and gluons making up protons and neutrons are far from static. They constantly move about and even blink in and out of existence, forming new particles and disappearing like fireflies blinking on and off in the evening sky—with the gluons, in particular, proliferating profusely to the point where they dominate the nuclei.
Because light speed motion stretches time, it effectively freezes this action so the fast moving nuclei appear as dense walls of gluons, called color glass condensates, which are thicker in the center of the speeding nuclei than near their edges. These variations in the distribution of gluons within the nuclei might account for some of the unusual signs observed when smaller particles like deuterons or protons collide with the nuclei. The gluon walls may even play a role in the formation of “large” scale quark-gluon plasma in nucleus-nucleus collisions.
Next steps
Separating the initial state effects from the final state effects is important if scientists want to fully understand the subtle details of the big drops of QGP created at RHIC and the LHC.
“In gold-gold collisions, there are initial state effects and final state effects but they are entangled,” said Morrison. “In deuteron-gold, there are still initial and final state effects, but you would expect initial state effects to be more important than final state ones in the smaller system—because even if you are making tiny drops of quark-gluon plasma, the bulk of the nucleus still exists in its initial state. These deuteron-gold collisions are therefore still valid control experiments for comparison with gold-gold.” Comparing proton-lead collisions at the LHC with RHIC’s existing deuteron-gold collisions—as well as possible future deuteron-gold, proton-gold, and helium-gold collisions—may provide an extra handle that highlights RHIC’s unique versatility.
Another way to disentangle the roles played by conditions before and after the collisions would be to probe the light-speed nuclei with extreme precision using a beam of accelerated electrons. Brookhaven Lab has a vision for transforming RHIC into a machine with that capability—known as an electron ion collider—by adding an electron accelerator ring to the existing RHIC infrastructure sometime in the 2020s. The new facility would be called eRHIC.
“Using electrons and small nuclei such as protons, deuterons, and helium to probe the larger, heavy nuclei are complementary ways to get a complete picture of what’s going on inside the nucleus,” said Nagle. That glimpse of the inner workings of atomic nuclei will help scientists better understand the quark gluon plasma—and possibly how that primordial soup ultimately gave rise to the bulk of visible matter in today’s world.
An international team of high-energy physicists says the discovery of an electrically charged subatomic particle called Zc(4020) is a sign that they have begun to unveil a whole new family of four-quark objects.
The Beijing Spectrometer (BESIII) collaboration, which includes scientists from UH Mānoa, previously announced the discovery of a mysterious four-quark particle called Zc(3900) in April 2013.
“While quarks have long been known to bind together in groups of twos or threes, these new results seem to be quickly opening the door to a previously elusive type of four-quark matter,” said Frederick Harris, a professor of physics and astronomy at UH Mānoa, and a spokesman for the BESIII experiment. “The unique data sample collected by the BESIII collaboration has continued to yield a stream of clues about the nature of multi-quark objects.”
The recent breakthroughs by the BESIII collaboration have come about through a dedicated study of the byproducts of the anomalous Y(4260) particle.
Using the Beijing Electron Positron Collider (BEPCII) in China, scientists tuned the energy at which electrons and positrons annihilate matter to 4260 MeV, which corresponds to the mass of the Y(4260) particle. The BESIII Collaboration used this method to directly produce and collect large samples of the particle’s byproducts, or decays.
This experimental method allowed the BESIII collaboration to first observe the Zc(3900) and then the Zc(4020). Also recently spotted in the decays is the electrically neutral X(3872), a particle that has been experimentally established for more than 10 years, and has long been suspected to be a four-quark object.
“The year 2013 has so far been an exciting one for the BESIII experiment,” Harris said. “Using decays of the Y(4260), a family of four-quark objects has begun to appear. While the theoretical picture remains to be finalized, more and more clues are suggesting that we are witnessing new forms of matter. And while a new ‘zoo’ of mysterious particles is emerging, it seems a new classification system may soon be at hand to understand it.”
About the BESIII Experiment:
The Beijing Spectrometer (BESIII) experiment at the Beijing Electron Positron Collider is composed of about 350 physicists from 50 institutions in 11 countries. U.S. groups include Carnegie Mellon University, Indiana University, The University of Minnesota, The University of Rochester, as well as physicists in the High Energy Physics Group, in the Department of Physics and Astronomy at the University of Hawai‘i at Mānoa.
The scientists have reported their findings to the scientific journal Physical Review Letters, including:
Observation of Z_c(4040) in e+e- --> D*D*- pi+ process at 4.26 GeV arXiv:1308.2760
Observation of a charged charmoniumlike structure Z_c(4020) and search for the Z_c(3900) in e+e- to pi+pi-h_c arXiv:1309.1896
Observation of a charged (DD*bar)- mass peak in e+e- --> pi+(DD*bar)-at Ecm=4.26 GeV arXiv:1310.1163
Observation of the X(3872) in e+e- --> gamma pi+pi- J/psi at sqrt(s) around 4.26 GeV arXiv:1310.4101
