As the paper explains, the calculations are based on the hydrodynamic particle flow seen in head-on collisions of various types of ions at both the LHC and the Relativistic Heavy Ion Collider (RHIC), a DOE Office of Science user facility for nuclear physics research at Brookhaven Lab. With only modest changes, these calculations also describe flow patterns seen in near-miss collisions, where photons that form a cloud around the speeding ions collide with the ions in the opposite beam.

“The upshot is that, using the same framework we use to describe lead-lead and proton-lead collisions, we can describe the data of these ultra-peripheral collisions where we have a photon colliding with a lead nucleus,” said Brookhaven Lab theorist Bjoern Schenke, a coauthor of the paper. “That tells you there’s a possibility that, in these photon-ion collisions, we create a small dense strongly interacting medium that is well described by hydrodynamics — just like in the larger systems.”

Fluid signatures

Observations of particles flowing in characteristic ways have been key evidence that the larger collision systems (lead-lead and proton-lead collisions at the LHC; and gold-gold and proton-gold collisions at RHIC) create a nearly perfect fluid. The flow patterns were thought to stem from the enormous pressure gradients created by the large number of strongly interacting particles produced where the colliding ions overlap.

“By smashing these high-energy nuclei together we’re creating such high energy density — compressing the kinetic energy of these guys into such a small space — that this stuff essentially behaves like a fluid,” Schenke said.

Spherical particles (including protons and nuclei) colliding head on are expected to generate a uniform pressure gradient. But partially overlapping collisions generate an oblong, almond-shaped pressure gradient that pushes more high-energy particles out along the short axis than perpendicular to it.

This “elliptic flow” pattern was one of the earliest hints that particle collisions at RHIC could create a quark-gluon plasma, or QGP — a hot soup of the fundamental building blocks that make up the protons and neutrons of nuclei/ions. Scientists were at first surprised by the QGP’s liquid-like behavior. But they later established elliptic flow as a defining feature of QGP, and evidence that the quarks and gluons were still interacting strongly, even when free from confinement within individual protons and neutrons. Later observations of similar flow patterns in collisions of protons with large nuclei, intriguingly suggest that these proton-nucleus collision systems can also create tiny specks of quark-gluon soup.

“Our new paper is about pushing this to even further extremes, looking at collisions between photons and nuclei,” Schenke said.

Changing the projectile

It has long been known that that ultra-peripheral collisions could create photon-nucleus interactions, using the nuclei themselves as the source of the photons. That’s because charged particles accelerated to high energies, like the lead nuclei/ions accelerated at the LHC (and gold ions at RHIC), emit electromagnetic waves — particles of light. So, each accelerated lead ion at the LHC is essentially surrounded by a cloud of photons.

“When two of these ions pass each other very closely without colliding, you can think of one as emitting a photon, which then hits the lead ion going the other way,” Schenke said. “Those events happen a lot; it’s easier for the ions to barely miss than to precisely hit one another!”

ATLAS scientists recently published data on intriguing flow-like signals from these photon-nucleus collisions.

“We had to set up special data collection techniques to pick out these unique collisions,” said Blair Seidlitz, a Columbia University physicist who helped set up the ATLAS trigger system for the analysis when he was a graduate student at the University of Colorado, Boulder. “After collecting enough data, we were surprised to find flow-like signals that were similar to those observed in lead-lead and proton-lead collisions, although they were a little smaller.”

Schenke and his collaborators set out to see whether their theoretical calculations could accurately describe the particle flow patterns.

They used the same hydrodynamic calculations that describe the behavior of particles produced in lead-lead and proton-lead collision systems. But they made a few adjustments to account for the “projectile” striking the lead nucleus changing from a proton to a photon.

According to the laws of physics (specifically, quantum electrodynamics), a photon can undergo quantum fluctuations to become another particle with the same quantum numbers. A rho meson, a particle made of a particular combination of a quark and antiquark held together by gluons, is one of the most likely results of those photon fluctuations.

If you think back to the proton — made of three quarks — this two-quark rho particle is just a step down the complexity ladder.

“Instead of having a gluon distribution around three quarks inside a proton, we have the two quarks (quark-antiquark) with a gluon distribution around those to collide with the nucleus,” Schenke said.

Accounting for energy

The calculations also had to account for the big difference in energy in these photon-nucleus collision systems, compared to proton-lead and especially lead-lead.

“The emitted photon that’s colliding with the lead won’t carry the entire momentum of the lead nucleus it came from, but only a tiny fraction of that. So, the collision energy will be much lower,” Schenke said.

That energy difference turned out to be even more important than the change of projectile.

In the most energetic lead-lead or gold-gold heavy ion collisions, the pattern of particles emerging in the plane transverse to the colliding beams generally persists no matter how far you look from the collision point along the beamline (in the longitudinal direction). But when Schenke and collaborators modeled the patterns of particles expected to emerge from lower-energy photon-lead collisions, it became apparent that including the 3D details of the longitudinal direction made a difference. The model showed that the geometry of the particle distributions changes rapidly with increasing longitudinal distance; the particles become “decorrelated.”

“The particles see different pressure gradients depending on their longitudinal position,” Schenke explained.

“So, for these low energy photon-lead collisions, it is important to run a full 3D hydrodynamic model (which is more computationally demanding) because the particle distribution changes more rapidly as you go out in the longitudinal direction,” he said.

When the theorists compared their predictions using this lower-energy, full 3D, hydrodynamic model with the particle flow patterns observed in photon-lead collisions by the ATLAS detector, the data and theory matched up nicely, at least for the most obvious elliptic flow pattern, Schenke said.

Implications and the future

“From this result, it looks like it’s conceivable that, even in photon-heavy ion collisions, we have a strongly interacting fluid that responds to the initial collision geometry, as described by hydrodynamics,” Schenke said. “If the energies and temperatures are high enough,” he added, “there will be a quark-gluon plasma.”

“It’s conceivable that, in photon-heavy ion collisions, we have a strongly interacting fluid,” said Brookhaven Lab theorist Bjoern Schenke.

Seidlitz, the ATLAS physicist, commented, “It was very interesting to see these results suggesting the formation of a small droplet of quark-gluon plasma, as well as how this theoretical analysis offers concrete explanations as to why the flow signatures are a bit smaller in photon-lead collisions.”

Additional data to be collected by ATLAS and other experiments at RHIC and the LHC over the next several years will enable more detailed analyses of particles flowing from photon-nucleus collisions. These analyses will help distinguish the hydrodynamic calculation from another possible explanation, in which the flow patterns are not a result of the system’s response to the initial geometry.

In the longer-term future, experiments at an Electron-Ion Collider (EIC), a facility planned to replace RHIC sometime in the next decade at Brookhaven Lab, could provide more definitive conclusions.

Lead author Dr Ian Towle, of the Sir John Walsh Research Institute in the Faculty of Dentistry, says the “surprisingly large” monkey lemur, Archaeolemur, had novel anatomical features not seen in living lemurs, such as lacking a ‘tooth comb’ in the front of the mouth for grooming.

“These extinct lemurs are so different to those alive today. They also show fascinating similarities to monkeys and apes, including humans,” he says.

The study, published in the American Journal of Biological Anthropology, aimed to assess the diet of Archaeolemur by analysing chipping in 447 teeth, comparing chipping frequencies to those of other primates.

The results were surprising — with these remarkable extinct lemurs with dentitions resembling baboons in shape; but presenting tooth chipping patterns similar to fossil hominins such as Neanderthals.

“Archaeolemur tooth chipping patterns are unlike any living primate, with their front teeth showing substantial fractures, often with numerous tooth chips on a single tooth, yet very little chipping on their back teeth.

“Similar tooth fracture patterns are observed in fossil hominins, such as Neanderthals. Typically, in Neanderthals these fracture patterns are thought to be related to tool-use behaviours,” Dr Towle says.

The results fit with previous research on Archaeolemur, in particular evidence that their large and robust front teeth may have been used to process a diet containing hard and tough foods.

Dr Towle thinks the study raises the “fascinating possibility” that stone tools do not necessarily explain the high rate of fractures on Neanderthal teeth.

“Archaeolemur shows similar tooth chipping patterns, yet there is no evidence to suggest they were capable of, or used, such tools.

“Studying extinct primates not only provides crucial insight into their diet and behaviour, but also elucidates our own evolutionary history.”

Given the overlap in skull and dental shape, and potential similarities in diet and behaviour, it is perhaps not surprising that Archaeolemur  was thought to be an ape when first discovered in Madagascar over 100 years ago.

“Archaeolemur is a brilliant example of convergent evolution, showing remarkable similarities to monkeys and apes. This species also highlights the extent to which lemurs in Madagascar diversified into a variety of ecological niches.”

Dr Carolina Loch, also from Sir John Walsh Research Institute, and who mentored Dr Towle during his tenure as postdoctoral fellow at the SJWRI, says the research is another great example of the “breadth and depth of multidisciplinary research” at the Faculty of Dentistry.

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Their results were published this week in the Proceedings of Royal Society B.

“We essentially reverse-engineered the inner working of the wing musculoskeletal system — how the muscles and skeleton work in hummingbirds to flap the wings,” said first author and Penn State mechanical engineering graduate student Suyash Agrawal. “The traditional methods have mostly focused on measuring activity of a bird or insect when they are in natural flight or in an artificial environment where flight-like conditions are simulated. But most insects and, among birds specifically, hummingbirds are very small. The data that we can get from those measurements are limited.”

The researchers used muscle anatomy literature, computational fluid dynamics simulation data and wing-skeletal movement information captured using micro-CT and X-ray methods to inform their model. They also used an optimization algorithm based on evolutionary strategies, known as the genetic algorithm, to calibrate the parameters of the model. According to the researchers, their approach is the first to integrate these disparate parts for biological fliers.

“We can simulate the whole reconstructed motion of the hummingbird wing and then simulate all the flows and forces generated by the flapping wing, including all the pressure acting on the wing,” Cheng said. “From that, we are able to back-calculate the required total muscular torque that is needed to flap the wing. And that torque is something we use to calibrate our model.”

With this model, the researchers uncovered previously unknown principles of hummingbird wing actuation.

The first discovery, according to Cheng, was that hummingbirds’ primary muscles, that is, their flight engines, do not simply flap their wings in a simple back and forth motion, but instead pull their wings in three directions: up and down, back and forth, and twisting — or pitching — of the wing. The researchers also found that hummingbirds tighten their shoulder joints in both the up-and-down direction and the pitch direction using multiple smaller muscles.

“It’s like when we do fitness training and a trainer says to tighten your core to be more agile,” Cheng said. “We found that hummingbirds are using similar kind of a mechanism. They tighten their wings in the pitch and up-down directions but keep the wing loose along the back-and-forth direction, so their wings appear to be flapping back and forth only while their power muscles, or their flight engines, are actually pulling the wings in all three directions. In this way, the wings have very good agility in the up and down motion as well as the twist motion.”

While Cheng emphasized that the results from the optimized model are predictions that will need validation, he said that it has implications for technological development of aerial vehicles.

“Even though the technology is not there yet to fully mimic hummingbird flight, our work provides essential principles for informed mimicry of hummingbirds hopefully for the next generation of agile aerial systems,” he said.

The other authors were Zafar Anwar, a doctoral student in the Penn State Department of Mechanical Engineering; Bret W. Tobalske of the Division of Biological Sciences at the University of Montana; Haoxiang Luo of the Department of Mechanical Engineering at Vanderbilt University; and Tyson L. Hedrick of the Department of Biology at the University of North Carolina.

The Office of Naval Research funded this work.

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While the Diprotodon — the extinct megafauna species that is distantly related to wombats but was the size of a small car — is commonly (but incorrectly) thought of as Australia’s ‘giant wombat’, researchers from Griffith University have shed light on a large species that does belong in the modern-day wombat family.

The complete skull of this true fossil giant wombat, found in a Rockhampton cave in Queensland and estimated to be around 80,000 years old, has been described for the first time by a team led by Associate Professor Julien Louys from Griffith’s Australian Research Centre for Human Evolution.

Associate Professor Louys said the discovery provided unprecedented insights into the biology and appearance of these previously little known ‘gentle giants’.

“The extinct megafauna of Australia never ceases to amaze and intrigue not just Australians, but people all over the world,” he said.

“Although one of the most charismatic of the giant mammals to go extinct, Diprotodon is commonly referred to as a ‘giant wombat’. But this is incorrect as Diprotodon belongs to an entirely different family — equivalent to saying a hippo is just a giant pig.

“There were however, true giant wombats. These have traditionally been poorly known, but the discovery of the most complete skull of one of these giants, Ramsayia, has provided us with an opportunity to reconstruct what this creature looked like, where and when it lived, and how the evolution of giant wombats took place in Australia.”

The cranium and mandible of the Ramsayia magna fossil was discovered from the rear of the front chamber of Lower Johansons Cave in Rockhampton in the early 2000s, but it was only through subsequent excavations and analysis by Associate Professor Louys’s team that is was confirmed as belonging to a previously described but very poorly known species.

Extinct giant wombats of the family Vombatidae (broadly defined as twice the size of modern wombats) are rarer than the fossil diprotodontids that are often popularly — and incorrectly — referred to as giant wombats.

Associate Professor Louys said this giant wombat — Ramsayia — had extensive cranial sinuses, which had not been previously reported for a wombat.

“This indicates that the wombat had a large, rounded skull for the attachment of specific and strong chewing muscles,” he said.

“The giant wombat also possessed a ‘premaxillary spine’, an indication that it had a large, fleshy nose.

“In this paper, we show that all true giant wombats evolved large body sizes first, then individually became quite specialised to eat different types of grasses.

“We also dated this species as being about 80,000 years old. This is the first date for this species and is much earlier than human arrival in Australia, although we still don’t know exactly when or why this species became extinct.”

The research ‘Cranial remains of Ramsayia magna from the Late Pleistocene of Australia and the evolution of gigantism in wombats (Marsupialia, Vombatidae)’ has been published in Papers In Palaeontology.

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