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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
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.
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.
Researchers have come one step closer to understanding unstable atomic nuclei. A team of researchers from RIKEN, the University of Tokyo and other institutions in Japan and Italy has provided evidence for a new nuclear magic number in the unstable, radioactive calcium isotope 54Ca. In a study published today in the journal Nature, they show that 54Ca is the first known nucleus with 34 neutrons (N) where N = 34 is a magic number.
The protons and neutrons inside the atomic nucleus exhibit shell structures in a manner similar to electrons in an atom. For naturally stable nuclei, these nuclear shells fill completely when the number of protons or the number of neutrons is equal to the ‘magic’ numbers 2, 8, 20, 28, 50, 82 or 126.
However, it has recently been shown that the traditional magic numbers, which were once thought to be robust and common for all nuclei, can in fact change in unstable, radioactive nuclei that have a large imbalance of protons and neutrons.
In the current study led by David Steppenbeck of the Center for Nuclear Study, the University of Tokyo, the team of researchers focused on 54Ca, which has 20 protons and 34 neutrons in its nucleus. They were able to study this nucleus thanks to the Radioactive Isotope Beam Factory (RIBF) at RIKEN, which produces the highest intensity radioactive beams available in the world.
In their experiment, a radioactive beam composed of scandium-55 and titanium-56 nuclei travelling at around 60% of the speed of light, was selected and purified by the BigRIPS fragment separator, part of the RIBF. The radioactive beam was focused on a reaction target made of beryllium. Inside this target, projectile fragmentation of the 55Sc and56Ti nuclei occurred, creating numerous new radioactive nuclei, some in excited states. The researchers measured the energy of the γ rays emitted from excited states of the radioactive nuclei using an array of 186 detectors surrounding the reaction target.
The results of the experiment indicate that 54Ca’s first excited state lies at a relatively high energy, which is characteristic of a large nuclear shell gap, thus indicating that N = 34 in 54Ca is a new magic number, as predicted theoretically by the University of Tokyo group in 2001. By conducting a more detailed comparison to nuclear theory the researchers were able to show that the N = 34 magic number is equally as significant as some other nuclear shell gaps.
“Our new measurement provides key data for the understanding of neutron-rich nuclei and will help pin down the treatment of nuclear forces in systems far from stability,” explains David Steppenbeck.
“Enriching our knowledge of the structures of highly unstable nuclei and the nucleon-nucleon forces that drive nuclear shell evolution and the appearance or disappearance of the nuclear magic numbers in radioactive nuclei plays an important role in understanding astrophysical processes such as nucleosynthesis in stars,” he adds.
• First evidence that ultra-cold neutrons interact with moving nano-sized particles provides a new tool for chemists, biologists and engineers
• Billiard-ball collisions may also explain inaccuracies in 60-year-old experiments to measure the lifetime of the neutron and explain the origin of matter in the universe
Physicists working on a 60-year-old experiment to understand the origin of matter in the universe have uncovered a new tool for studying the movement of tiny particles along a surface, such as a virus travelling along a cell membrane. The new tool utilises ultra-cold neutrons (UCNs) that move slower than most people can run and will allow scientists to map the movement of tiny objects with previously unattainable precision. This discovery, made at the Institut Laue-Langevin, the world’s flagship centre for neutron science and the home of ultra-cold neutron research for over 25 years, is published today in Crystallography Reports.
Since their discovery in 1969, UCNs have been used by experimental physicists to answer fundamental questions about the universe, such as the origin of matter and how gravity fits into the standard model of particle physics. They do this by collecting them in traps and monitoring properties such as their energies or lifetime at high precision.
However, the average storage time of the UCNs in their traps was always much lower than expected, affecting the quality of observations. In 1999 Dr Valery Nesvizhevsky and colleagues at the ILL discovered a new phenomenon that might explain these losses. They found that occasionally a UCN in the trap was given a small thermal ‘kick’. This occurred in just 1 out of every 10,000,000 collisions, but the origin of this ‘kick’ was unknown. As other hypotheses were ruled out, Dr Nesvizhevsky started to consider the influence of the nanoparticles or nanodroplets which were known to populate a layer immediately above the surface of most materials, including those of the trap’s interior.
To tests this hypothesis Dr Nesvizhevsky and his colleagues returned to the UCN apparatus at the ILL. They placed samples with nanoparticle surface layers of known size distribution into the UCN trap and observed the interactions. The team discovered that the change in UCN energy was induced by ‘billiard-ball-like’ collisions with moving surface nanoparticles, thus providing the first proof that these nanoparticles are not stationary.
The low energy levels of UCNs means they usually bounce off the inner walls and remain in the trap. However these shots of extra energy caused by the interaction with the nanoparticles gives them just enough energy either to overcome gravity and escape out of the top of the chamber, which is left open, or to pass right through the chamber walls.
This phenomenon has two very dramatic consequences:
It could explain discrepancies in the results of 60-year-old experiments measuring the lifetime of the neutron, the results of which differ by about 10 seconds, much more than the reported uncertainties would allow. A precise figure could affect conclusions on the origin of matter in the early universe, as well as on the number of families of elementary particles existing in nature, and could modify models of star formation.
It also gives science a brand new and uniquely accurate tool for studying for the very first time how nanoparticles move around and interact with material surfaces, particular through van der Waals/Casimir interactions, in all kinds of natural and man-made systems. Currently no other techniques exist to make these measurements.
Potential applications of this technique are vast and include the production of chemicals and semiconductors, catalytic converters, integrated circuits used in electronic devices and silver halide salts used in photographic films. The tool could also be used to study for the first time how biological molecules move along a surface such as viruses along a biological membrane. For their next experiments, Valery and his colleagues have secured further beam time at the ILL and will be inviting scientists from various disciplines to provide samples from their own research for analysis with UCNs to prove the validity of this new technique.
Quotes
Dr Valery Nesvizhevsky said: “We found this brand new scientific tool by chance. We never thought UCNs might have such practical uses. The implications of these findings for fundamental physics are sure to be a hot topic and I expect there will be some debate as to how much these thermal kicks contribute to the uncertainties around the lifetime of the neutron measurements. However the potential in this new technique for studying nanoparticle dynamics is a certainty and we look forward to working with researchers from across the scientific disciplines to realise its potential.”
Dr Valentin Gordeliy is heads of the Membrane Transportation Group at the Institut de Biologie Structurale and has met with Dr Nesvizhevsky to discuss the possible application of the new ultra-cold neutron technique in his own line of research: “This is a great piece of science and if its feasibility can be verified, a potentially very exciting discovery for my own work in structural biology. In the human body there are a great many nano-scale structures, the motions of those correspond to time-scale of this technique. These include viruses, and various proteins and protein complexes including histones, the scaffolds of DNA that regulate the expression of genes, helping turning them on and off. One of the most attractive objects for study would be membrane proteins that help maintain the health of the whole body and are therefore targeted by 60% of existing drugs. Whilst other tools such as electron microscopes can be used they are not suitable to study dynamics. Also the possibility of watching the evolution of complex interactions such as that of a virus and a membrane protein would provide new insights for drug discovery as well as a better understanding of how our body functions.”
