The calculation shows why heavy quarks are caught in the stream

The calculation shows why heavy quarks are caught in the stream

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The data points on this graph show that the heavy quark (Q) interactions with the quark-gluon plasma (QGP) are strongest and have a short mean free path (zig zag) right around the transition temperature (T/Tc = 1) . The interaction strength (the diffusion constant of heavy quarks) decreases and the mean free path lengthens at higher temperatures. Credit: Brookhaven National Laboratory

Using some of the world’s most powerful supercomputers, a group of theorists has produced a major advance in nuclear physics, a calculation of the “diffusion coefficient of heavy quarks.” This number describes how quickly a molten soup of quarks and gluons, the building blocks of protons and neutrons, which are set free in collisions of nuclei in powerful particle accelerators, transfers its momentum to the heavy quarks.

The answer, it turns out, is very fast. As described in an article just published in Physical Review Letters, the transfer of momentum from the “liberated” quarks and gluons to the heavier quarks occurs at the limit of what quantum mechanics will allow. These quarks and gluons have so many short-range strong interactions with the heavier quarks that they drag the “boulder”-like particles along with their flow.

The work was led by Peter Petreczky and Swagato Mukherjee of the nuclear theory group at the US Department of Energy’s Brookhaven National Laboratory and included theorists from the universities of Bielefeld, Regensburg and Darmstadt in Germany and the University of Stavanger in Norway .

The calculation will help explain experimental results showing heavy quarks being captured in the flux of matter generated in heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven and at the Large Hadron Collider (LHC) at CERN’s European laboratory . The new analysis also adds corroborating evidence that this matter, known as “quark-gluon plasma” (QGP), is a near-perfect liquid, with a viscosity so low it even approaches the quantum limit.

“To see the heavy quarks flowing with the QGP at the RHIC and LHC was very surprising initially,” Petreczky said. “It would be like seeing a heavy stone dragged with water in a stream. Usually the water flows but the stone remains”.

The new calculation reveals why that startling image makes sense when you think about QGP’s extremely low viscosity.

Frictionless flow

The low viscosity of matter generated in RHIC’s gold ion collisions, first reported in 2005, was a major reason for the new calculation, Petreczky said. When these collisions melt away the boundaries of individual protons and neutrons to free internal quarks and gluons, the fact that the resulting QGP flows with virtually no resistance is evidence that there are many strong interactions between the quarks and gluons in the hot quark soup.

“The low viscosity implies that the ‘mean free path’ between the ‘molten’ quarks and gluons in the hot dense QGP is extremely small,” said Mukherjee, explaining that the mean free path is the distance a particle can travel before interact with another particle

“If you think about trying to walk in a crowd, that’s the typical distance you can walk before you bump into someone or have to change course,” he said.

With a short mean free path, quarks and gluons interact frequently and strongly. The collisions dissipate and distribute the energy of the fast-moving particles, and the strongly interacting QGP exhibits collective behavior including nearly frictionless flow.

“It’s much more difficult to change the momentum of a heavy quark because it’s like a train to stop,” noted Mukherjee. “It would have to go through a lot of collisions to be pulled along with the plasma.”

But if the QGP is indeed a perfect fluid, the mean free path for the heavy quark interactions should be short enough to make this possible. Calculating the diffusion coefficient of heavy quarks, which is proportional to how strongly heavy quarks interact with plasma, was one way to test this understanding.

Crunching the numbers

The calculations needed to solve the equations of quantum chromodynamics (QCD), the theory that describes the interactions between quarks and gluons, are mathematically complex. Several advances in theory and powerful supercomputers helped pave the way for the new calculus.

“In 2010/11 we started using a mathematical shortcut, which assumed that plasma consisted only of gluons, no quarks,” said Olaf Kaczmarek of the University of Bielefeld, who led the German side of this effort. Thinking only of gluons helped the team come up with their own method using lattice QCD. In this method, scientists run simulations of particle interactions on a discretized four-dimensional space-time lattice.

In essence, they “place” particles at discrete locations on an imaginary 3D grid to model their interactions with nearby particles and see how those interactions change over time (the 4th dimension). They use many different starting arrangements and include varying distances between particles.

After working out the method with just gluons, they figured out how to add the complexity of the quarks.

The scientists loaded a large number of sample configurations of quarks and gluons onto the 4D lattice and used Monte Carlo methods, repeated random sampling, to try to find the most likely distribution of quarks and gluons within the lattice.

“By averaging over these configurations, you get a correlation function related to the diffusion coefficient of the heavy quarks,” said Luis Altenkort, a graduate student at the University of Bielefeld who also worked on this research at the Brookhaven Lab.

As an analogy, think of estimating the air pressure in a room by sampling the positions and motion of molecules. “You try to use the most likely distributions of molecules based on another variable, like temperature, and rule out unlikely configurations like all the air molecules clustered in one corner of the room,” Altenkort said.

In the case of QGP, the scientists were trying to simulate a thermalized system in which even on a tiny time scale of split-second collisions of heavy ion particles, the quarks and gluons reach a certain equilibrium temperature.

They simulated the QGP at a range of fixed temperatures and calculated the heavy quark diffusion coefficient for each temperature to map the temperature dependence of the heavy quark interaction strength (and the mean free path of those interactions).

“These challenging calculations were only possible using some of the world’s most powerful supercomputers,” said Kaczmarek.

Computing resources included Perlmutter at the National Energy Research for Scientific Computing Center (NERSC), a DOE Office of Science user facility located at the Lawrence Berkeley National Laboratory; Juwels Booster at the Juelich Research Center in Germany; Marconi at CINECA in Italy; and dedicated lattice QCD GPU clusters at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) and at the University of Bielefeld.

As Mukherjee noted, “These powerful machines don’t just do the work for us as we sit back and relax; it took years of hard work to develop the codes that can squeeze the most efficient performance out of these supercomputers to do our complex calculations.” .”

Fast thermalization, short-range interactions

Calculations show that the heavy quark’s diffusion coefficient is highest at the very temperature at which QGP forms, and therefore decreases with increasing temperature. This result implies that the QGP reaches an equilibrium very quickly.

“You start with two nuclei, essentially having no temperature, then you crash them together and in less than a quadrillionth of a second you have a thermal system,” Petreczky said. Even heavy quarks are thermalized.

For this to happen, the heavy quarks must undergo a lot of scattering with other particles very quickly, implying that the mean free path of these interactions must be very small. Indeed, calculations show that, at the transition to QGP, the mean free path of the heavy quark interactions is very close to the shortest allowed distance. That so-called quantum limit is set by the inherent uncertainty of knowing both the position and momentum of a particle simultaneously.

This independent ‘measurement’ provides corroborating evidence of QGP’s low viscosity, confirming the picture of its perfect fluidity, the scientists say.

“The shorter the mean free path, the lower the viscosity and the faster the thermalization,” Petreczky said.

Real collision simulation

Now that scientists know how heavy quark interactions with the QGP vary with temperature, they can use this information to improve their understanding of how actual heavy ion collision systems evolve.

“My colleagues are trying to develop more accurate simulations of how QGP interactions affect the motion of heavy quarks,” Petreczky said. “To do this, they have to account for the dynamic effects of how the QGP expands and cools throughout the complicated stages of collisions.”

“Now that we know how the diffusion coefficient of heavy quarks changes with temperature, they can take this parameter and plug it into their simulations of this complicated process and see what else needs to change to make those simulations compatible with the experimental data from RHIC and the LHC.”

This effort is the subject of a major collaboration known as Heavy-Flavor Theory (HEFTY) for the QCD Matter Topical Theory Collaboration.

“We will be able to better model the motion of heavy quarks in the QGP, and thus have a better theory for comparing the data,” Petreczky said.

More information:
Luis Altenkort et al, Heavy quark diffusion from 2+1 flavor lattice QCD with 320 MeV pion mass, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.231902

About the magazine:
Physical Review Letters

#calculation #shows #heavy #quarks #caught #stream

NASA’s heavy metal Psyche asteroid journey is set for October

NASA's heavy metal Psyche asteroid journey is set for October

NASA is on track to launch its mission to asteroid Psyche later this year in October, after correcting several project management issues that led to its initial delay.

Psyche was originally scheduled to launch in August 2022 and cost about $1 billion, but was pushed back when engineers delivered flight guidance, navigation and control (GNC) software eight months later than planned. The setback meant that NASA’s Jet Propulsion Laboratory (JPL) did not have enough time to test vital components of the spacecraft and decided to postpone the launch.

An independent review board conducted an investigation into the delay and released a November 2022 report [PDF] which found that the project was understaffed, workers were suffering from burnout, and top management changes disrupted work. Collaboration and communication between employees has also been made more difficult during the COVID-19 pandemic.

The latest report [PDF], dated May 2023, found that JPL has taken steps to roll over the project. The board now believes NASA’s plan to launch the mission in October 2023 is “credible” and that “the overall likelihood of mission success is high.” The officials leading the Psyche mission recruited more personnel especially in critical areas such as the project’s lead engineer; Experienced CNG Engineer and Lead Fault Protection Engineer.

The board had previously criticized the project for having “serious miscommunications”. Psyche team members reportedly raised issues with management, but felt their concerns weren’t taken seriously or acted upon before it was too late. In response, JPL encouraged employees to come to work together onsite in person rather than independently from home, to improve communication.

“Returning the majority of in-person work has made a huge difference in restoring visibility and informal communications throughout the project,” the report said. “Immediate meetings, coffee social hours, offsite crash courses, and people ‘walking the floor’ have improved team interaction, problem solving, efficiency and confidence. The team is also doing judicious use of remote and hybrid access options as appropriate to ensure flexibility while not compromising their collaboration.”

There are, however, some areas that JPL still needs to work on in preparation for launch, including verification and validation of integrated systems and mission operations. Leaders must establish flight rules, procedures, and train personnel to meet flight readiness requirements.

Nicola Fox, associate administrator of NASA’s Science Mission Directorate, said in a statement, “I am pleased with the Independent Review Board’s overwhelmingly positive assessment of JPL’s hard work in correcting the issues outlined in the board’s original report.”

“We know the work is not done. As we move forward, we will work with JPL to ensure these implemented changes continue to be prioritized to position Psyche and the other missions in JPL’s portfolio for success. We convened this board weeks later My intervention as director and addressing the issues raised was a central focus in my first year as director of JPL.

“The results are gratifying,” added JPL director Laurie Leshin.

“Our goals have gone beyond getting Psyche on the launch pad to improving JPL across the board as we work on missions that will help us better understand Earth, explore the solar system and universe, and look for signs of life. Ours Strong response to the board’s findings reinforce the belief that JPL can solve any problem with the right focus and attention.”

The launch delay means Psyche is not expected to reach its target asteroid, 16 Psyche, until August 2029 rather than 2026. 16 Psyche is a unique rock measuring 220 kilometers (140 miles) in diameter and is described as the largest metal planetary body in solar system. Scientists are debating whether the asteroid could be a leftover metallic core from a failed rocky planet and believe it could reveal secrets about how planets like Earth formed.

#NASAs #heavy #metal #Psyche #asteroid #journey #set #October

Secrets of the hypernuclei flux: first observations at the relativistic heavy ion collider

Particle Collision Hypernuclei Flow Concept

Particle Collision Hypernuclei Flow Concept

Physicists at the Relativistic Heavy Ion Collider (RHIC) have made the first observation of the directed flow of hypernuclei, rare nuclei containing at least one hyperon, in particle collisions. Hyperons, which contain a strange quark, are thought to be abundant in neutron stars, one of the densest objects in the universe. By simulating these conditions in the laboratory, researchers aim to understand the interactions between hyperons and nucleons.

Particle collisions offer a new way to study the interactions of hyperon particles with normal nuclear building blocks, potentially providing insight into the properties of neutron stars.

In a groundbreaking study, RHIC scientists observed the directed flow of hypernuclei during particle collisions. This rare matter, abundant in neutron stars, was examined under simulated conditions, providing information on interactions crucial for understanding[{” attribute=””>neutron star structures. The observations, mirroring regular nuclei flow patterns, will help enhance theoretical models of neutron stars.

Physicists studying particle collisions at the Relativistic Heavy Ion Collider (RHIC) have published the first observation of directed flow of hypernuclei. These short-lived, rare nuclei contain at least one hyperon in addition to ordinary protons and neutrons. Hyperons contain at least one strange quark in place of one of the up or down quarks that make up ordinary nucleons (the collective name for protons and neutrons). Such strange matter is thought to be abundant in the hearts of neutron stars, which are among the densest, most exotic objects in the universe. While blasting off to neutron stars to study this exotic matter is still the stuff of science fiction, particle collisions could give scientists insight into these celestial objects from a laboratory right here on Earth.

The conditions in a neutron star may still be far from what we reach at this moment in the laboratory, but at this stage its the closest we can get, said Xin Dong, a physicist from the U.S. Department of Energys Lawrence Berkeley National Laboratory (LBNL) who was involved in the study. By comparing our data from this laboratory environment to our theories, we can try to infer what happens in the neutron star.

Neutron Star Insight From Particle Collisions

Neutron stars are compact objects formed when massive stars collapse at the end of their lives. Tracking how hypernuclei flow collectively in high-energy heavy ion collisions could help scientists learn about hyperon-nucleon interactions in the nuclear medium and understand the inner structure of neutron stars. Credit: STAR Collaboration

The scientists used the STAR detector at RHIC, a DOE Office of Science user facility for nuclear physics research at Brookhaven National Laboratory, to study the flow patterns of the debris emitted from collisions of gold nuclei. Those patterns are triggered by the enormous pressure gradients generated in the collisions. By comparing the flow of hypernuclei with that of similar ordinary nuclei made only of nucleons, they hoped to gain insight into interactions between the hyperons and nucleons.

In our normal world, nucleon-nucleon interactions form normal atomic nuclei. But when we move into a neutron star, hyperon-nucleon interactionswhich we dont know much about yetbecome very relevant to understanding the structure, said Yapeng Zhang, another member of STAR from the Institute of Modern Physics of the Chinese Academy of Sciences, who led the data analysis together with his student Chenlu Hu. Tracking how hypernuclei flow should give the scientists insight into the hyperon-nucleon interactions that form these exotic particles.

The data, just published in the journal Physical Review Letters, will provide quantitative information theorists can use to refine their descriptions of the hyperon-nucleon interactions that drive the formation of hypernucleiand the large-scale structure of neutron stars.

There are no solid calculations to really establish these hyperon-nucleon interactions, said Zhang. This measurement may potentially constrain theories and provide a variable input for the calculations.

Go with the flow

Previous experiments have shown that the flow patterns of regular nuclei generally scale with massmeaning the more protons and neutrons a nucleus has, the more the nuclei exhibit collective flow in a particular direction. This indicates that these nuclei inherit their flow from their constituent protons and neutrons, which coalesce, or come together, because of their interactions, which are governed by the strong nuclear force.

The STAR results reported in this paper show that hypernuclei follow this same mass-scaling pattern. That means hypernuclei most likely form via the same mechanism.

In the coalescence mechanism, the nuclei (and hypernuclei) form this way depending on how strong the interactions are between the individual components, Dong said. This mechanism gives us information about the interaction between the nucleons (in nuclei) and nucleons and hyperons in hypernuclei.

RHIC STAR Detector

A side view of the Solenoidal Tracker at RHIC (STAR) experiment at the Relativistic Heavy Ion Collider (RHIC), a particle collider for nuclear physics research at the U.S. Department of Energys Brookhaven National Laboratory. Credit: Brookhaven National Laboratory

Seeing similar flow patterns and the mass scaling relationship for both normal nuclei and hypernuclei, the scientists say, implies that the nucleon-nucleon and hyperon-nucleon interactions are very similar.

The flow patterns also convey information about the matter generated in the particle smashupsincluding how hot and dense it is and other properties.

The pressure gradient created in the collision will induce some asymmetry in the outgoing particle direction. So, what we observe, the flow, reflects how the pressure gradient is created inside the nuclear matter, Zhang said.

The measured flow of hypernuclei may open a new door to study hyperon-nucleon interactions under finite pressure at high baryon density.

The scientists will use additional measurements of how hypernuclei interact with that medium to learn more about its properties.

The benefits of low energy

This research would not have been possible without the versatility of RHIC to operate over such a wide range of collision energies. The measurements were made during Phase I of the RHIC Beam Energy Scana systematic study of gold-gold collisions ranging from 200 GeV per colliding particle pair down to 3 GeV.

To reach that lowest energy, RHIC operated in fixed-target mode: One beam of gold ions traveling around the 2.4-mile-circumference RHIC collider crashed into a foil made of gold placed inside the STAR detector. That low energy gives scientists access to the highest baryon density, a measure related to the pressure generated in the collisions.

At this lowest collision energy, where the matter created in the collision is very dense, nuclei and hypernuclei are produced more abundantly than at higher collision energies, said Yue-Hang Leung, a postdoctoral fellow from the University of Heidelberg, Germany. The low-energy collisions are the only ones that produce enough of these particles to give us the statistics we need to do the analysis. Nobody else has ever done this before.

How does what the scientists learned at RHIC relate to neutron stars?

The fact that hypernuclei appear to form via coalescence just like ordinary nuclei implies that they, like those ordinary nuclei, are created at a late stage of evolution of the collision system.

At this late stage, the density for the hyperon-nucleon interaction we see is not that high, Dong said. So, these experiments may not be directly simulating the environment of a neutron star.

But, he added, This data is fresh. We need our theory friends to weigh in. And they need to include this new data on hyperon-nucleon interactions when they build a new neutron star model. We need both experimentalists and our theorists efforts to work towards understanding this data and making those connections.

Reference: Observation of Directed Flow of Hypernuclei 3H and 4H in sNN=3GeV Au+Au Collisions at RHIC by B.E. Aboona et al. (STAR Collaboration), 24 May 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.130.212301

This research was funded by the DOE Office of Science (NP), the U.S. National Science Foundation, and a range of international organizations and agencies listed in the scientific paper. The STAR team used computing resources at the Scientific Data and Computing Center at Brookhaven Lab, the National Energy Research Scientific Computing Center (NERSC) at DOEs Lawrence Berkeley National Laboratory, and the Open Science Grid consortium.


#Secrets #hypernuclei #flux #observations #relativistic #heavy #ion #collider