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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.
Key sign of quark-gluon plasma (QGP) and evidence for a long-debated quantum phenomenon
Scientists in the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC), a particle accelerator exploring nuclear physics and the building blocks of matter at the U.S. Department of Energy’s Brookhaven National Laboratory, have new evidence for what’s called a “chiral magnetic wave” rippling through the soup of quark-gluon plasma created in RHIC’s energetic particle smashups.
The presence of this wave is one of the consequences scientists were expecting to observe in the quark-gluon plasma—a state of matter that existed in the early universe when quarks and gluons, the building blocks of protons and neutrons, were free before becoming inextricably bound within those larger particles. The tentative discovery, if confirmed, would provide additional evidence that RHIC’s collisions of energetic gold ions recreate nucleus-size blobs of the fiery plasma thousands of times each second. It would also provide circumstantial evidence in support of a separate, long-debated quantum phenomenon required for the wave’s existence. The findings are described in a paper that will be highlighted as an Editors' Suggestion in Physical Review Letters.
To try to understand these results, let’s take a look deep within the plasma to a seemingly surreal world where magnetic fields separate left- and right-“handed” particles, setting up waves that have differing effects on how negatively and positively charged particles flow.
The presence of this wave is one of the consequences scientists were expecting to observe in the quark-gluon plasma. It also provides circumstantial evidence for a separate, long-debated quantum phenomenon.
“What we measure in our detector is the tendency of negatively charged particles to come out of the collisions around the ‘equator’ of the fireball, while positively charged particles are pushed to the poles,” said STAR collaborator Hongwei Ke, a postdoctoral fellow at Brookhaven. But the reasons for this differential flow, he explained, begin when the gold ions collide.
The ions are gold atoms stripped of their electrons, leaving 79 positively charged protons in a naked nucleus. When these ions smash into one another even slightly off center, the whole mix of charged matter starts to swirl. That swirling positive charge sets up a powerful magnetic field perpendicular to the circulating mass of matter, Ke explained. Picture a spinning sphere with north and south poles.
Within that swirling mass, there are huge numbers of subatomic particles, including quarks and gluons at the early stage, and other particles at a later stage, created by the energy deposited in the collision zone. Many of those particles also spin as they move through the magnetic field. The direction of their spin relative to their direction of motion is a property called chirality, or handedness; a particle moving away from you spinning clockwise would be right-handed, while one spinning counterclockwise would be left-handed.
The STAR detector at RHIC tracks particles emerging from thousands of subatomic smashups per second.
According to Gang Wang, a STAR collaborator from the University of California at Los Angeles, if the numbers of particles and antiparticles are different, the magnetic field will affect these left- and right-handed particles differently, causing them to separate along the axis of the magnetic field according to their “chiral charge.”
“This ‘chiral separation’ acts like a seed that, in turn, causes particles with different charges to separate,” Gang said. “That triggers even more chiral separation, and more charge separation, and so on—with the two effects building on one another like a wave, hence the name ‘chiral magnetic wave.’ In the end, what you see is that these two effects together will push more negative particles into the equator and the positive particles to the poles.”
To look for this effect, the STAR scientists measured the collective motion of certain positively and negatively charged particles produced in RHIC collisions. They found that the collective elliptic flow of the negatively charged particles—their tendency to flow out along the equator—was enhanced, while the elliptic flow of the positive particles was suppressed, resulting in a higher abundance of positive particles at the poles. Importantly, the difference in elliptic flow between positive and negative particles increased with the net charge density produced in RHIC collisions.
According to the STAR publication, this is exactly what is expected from calculations using the theory predicting the existence of the chiral magnetic wave. The authors note that the results hold out for all energies at which a quark-gluon plasma is believed to be created at RHIC, and that, so far, no other model can explain them.
The finding, says Aihong Tang, a STAR physicist from Brookhaven Lab, has a few important implications.
“First, seeing evidence for the chiral magnetic wave means the elements required to create the wave must also exist in the quark-gluon plasma. One of these is the chiral magnetic effect—the quantum physics phenomenon that causes the electric charge separation along the axis of the magnetic field—which has been a hotly debated topic in physics. Evidence of the wave is evidence that the chiral magnetic effect also exists.” Tang said.
The chiral magnetic effect is also related to another intriguing observation at RHIC of more-localized charge separation within the quark-gluon plasma. So this new evidence of the wave provides circumstantial support for those earlier findings.
Finally, Tang pointed out that the process resulting in propagation of the chiral magnetic wave requires that “chiral symmetry”—the independent identities of left- and right-handed particles—be “restored.”
“In the ‘ground state’ of quantum chromodynamics (QCD)—the theory that describes the fundamental interactions of quarks and gluons—chiral symmetry is broken, and left- and right-handed particles can transform into one another. So the chiral charge would be eliminated and you wouldn’t see the propagation of the chiral magnetic wave,” said nuclear theorist Dmitri Kharzeev, a physicist at Brookhaven and Stony Brook University. But QCD predicts that when quarks and gluons are deconfined, or set free from protons and neutrons as in a quark-gluon plasma, chiral symmetry is restored. So the observation of the chiral wave provides evidence for chiral symmetry restoration—a key signature that quark-gluon plasma has been created.
“How does deconfinement restore the symmetry? This is one of the main things we want to solve,” Kharzeev said. “We know from the numerical studies of QCD that deconfinement and restoration happen together, which suggests there is some deep relationship. We really want to understand that connection.”
Brookhaven physicist Zhangbu Xu, spokesperson for the STAR collaboration, added, “To improve our ability to search for and understand the chiral effects, we’d like to compare collisions of nuclei that have the same mass number but different numbers of protons—and therefore, different amounts of positive charge (for example, Ruthenium, mass number 96 with 44 protons, and Zirconium, mass number 96 with 40 protons). That would allow us to vary the strength of the initial magnetic field while keeping all other conditions essentially the same.”
Research at RHIC, a DOE Office of Science User Facility, is supported by the Office of Science (NP) and these agencies and organizations.
Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
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.
