* Astronomy

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RE: Quantum Chromodynamics

Blobrana · L

Title: Spontaneous CP violation in quark scattering from QCD Z(3) interfaces
Authors: Abhishek Atreya, Anjishnu Sarkar, Ajit M. Srivastava

In this paper, we explore the possibility of spontaneous CP violation in the scattering of quarks and anti-quarks from QCD Z(3) domain walls. The CP violation here arises from the nontrivial profile of the background gauge field (A_{0}) between different Z(3) vacua. We calculate the spatial variation of A_{0} across the Z(3) interface from the profile of the Polyakov loop L(\vec{x}) for the Z(3) interface and calculate the reflection of quarks and antiquarks using the Dirac equation. This spontaneous CP violation has interesting consequences for the relativistic heavy-ion collision experiments, such as baryon enhancement at high P_{T}. It also acts as a source of additional J/ \psi suppression. We also discuss its implications for the early universe.

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Title: Collective behaviour of partons could be a source of energetic hadrons
Authors: M. K. Suleymanov

We discuss the idea that collective behaviour of the quarks/partons, which has been intensely discussed for the last 40 years in relativistic hadron-nuclear and nuclear-nuclear interactions and confirmed by new data coming from the ultrarelativistic heavy ion collisions, can lead to energetic particle production. Created from hadronization of the quark/parton (or quarks/partons), energetic particles could get the energy of grouped partons from coherent interactions. Therefore, we think that in the centre of some massive stars, a medium with high density, close to Quantum Chromodynamic one could be a source of the super high-energy cosmic rays.

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Quantum Chromo Dynamics

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Strong coupling constant

Blobrana · L

RE: Quantum Chromodynamics

Blobrana · L

When atomic nuclei are smashed together at great speed, resulting temperatures exceed one trillion degrees, 200 million times hotter than the surface of the sun. Scientists who study nuclear matter under extreme conditions have a particular interest in the properties of particles of light called photons, which reveal valuable information because they don’t interact strongly with other particles following a nuclear collision. Using the Relativistic Heavy Ion Collider (RHIC) at the U.S. Department of Energy’s Brookhaven National Laboratory, physicist Stefan Bathe has measured characteristics of photons to reveal data about the temperature and density of a nuclear collision.

Bathe will discuss his study of direct photon production at the April meeting of the American Physical Society at the Hyatt Regency in Dallas, Texas. Bathe’s talk will be held on Tuesday, April 25 at 2:06 p.m. in Landmark B. RHIC is a world-class scientific research facility used by hundreds of physicists from around the world to study what the universe may have looked like in the first few moments after its creation. RHIC drives two intersecting beams of gold ions head-on, in a subatomic collision. What physicists learn from these collisions may help us understand more about why the physical world works the way it does, from the smallest subatomic particles, to the largest stars.

Working at the PHENIX detector, Bathe has studied gold-gold, deuteron-gold, and proton-proton collisions to test the theory of quantum chromodynamics, the theory of the strong force that holds atomic nuclei together.

"While gold-gold collisions are the most interesting, we also have to understand simpler systems with fewer particles. We want to understand the strong interactions between quarks and gluons, which are the components of protons and neutrons. In a collision of nuclei, they all break apart, resulting in thousands of particles" - Stefan Bathe.

The vast majority of the particles released in a nuclear collision interact strongly with the nuclear medium and lose large amounts of energy. How much energy they lose reveals information about the medium. By studying the energy spectrum of direct photons, Bathe and his colleagues have been able to determine the temperature and density of the matter, which in turn reveals the phase of the collision.

"The detector will tell us the energy and position where a photon hits it. From the energy and position, we get a spectrum of energy distribution, whose slope tells us the temperature. If you know the temperature and density, you know the phase of the matter. By studying photons, particles that you can see, you can learn about the temperature of the nuclear matter you’ve created" - Stefan Bathe.

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Blobrana · L

Normally, we think of building blocks as static objects. For instance, the brick and mortar used to build the local bank remain pretty much the same from the day it's built to the day it's torn down. But the building blocks of ordinary matter are different.
These bricks are particles called quarks, and the mortar is made up of particles called gluons. Quarks and gluons are in constant motion, taking part in a complicated dance to build protons and neutrons (also called nucleons).

Ian Cloet, a theorist at Jefferson Lab and the Special Research Centre for the Subatomic Structure of Matter at the University of Adelaide, and his colleagues wondered how this quark-gluon dance was affected by the environment around the nucleons in which the quarks and gluons reside.
In particular, they wanted to know how the spin of the quarks and gluons may be modified by the environment around the nucleons they're embedded in and how this modification may affect the spin of the nucleons themselves.

To find out, Cloet and his colleagues, Wolfgang Bentz at Tokai University in Japan and JLab's Tony Thomas, calculated the spin-dependent structure functions of nucleons. These probability distributions provide information on how the quark spins are organized inside the nucleon, and from these distributions, it is possible to determine what fraction of the nucleon spin comes from the quarks and what fraction comes from the gluons.

"The spin-dependent structure function tells us about the spin content of the nucleon. So we know the proton has spin one-half. All the constituents have to add to give you spin one-half, and the structure functions give you information on how that happens" - Ian Cloet.

The calculations took into account two different environments where nucleons are commonly found: inside the nucleus and outside. The researchers calculated the spin-dependent structure functions for a nucleon inside the nucleus and for a free nucleon - one outside the nucleus. They then compared these structure functions in ratio form.
This method revealed that the spin content of a nucleon inside the nucleus is different from one outside.

"So what this is telling us is that how the gluons' and quarks' spins add together to give spin one-half is different for a bound nucleon than it is for a free nucleon" - Ian Cloet.

That means that while a nucleon's spin may remain constant, the proportion of that spin contributed by its constituents, the quarks and gluons, may change as the environment around nucleons change.

"The spin of the nucleon has to be spin one-half, but how you can get one-half can vary. It was thought that the skin of the proton would expel this force from the other nucleons. It just really wouldn't get inside and affect the quarks. People expected the small-scale structures of the nucleon to remain mostly the same whether they're inside or outside the nucleus" -Ian Cloet.

The presence of other nucleons inside the nucleus causes this difference.
One model, the Quark-Meson Coupling Model, assumes that the quarks inside nucleons in a nucleus interact through the exchange of mesons. This model regards nucleons inside a nucleus less like billiard balls and more like squirmy bags that may be modified by other nucleons in the nucleus around them.

"The idea is that this meson field generated by all the other nucleons is felt by the quarks inside the original proton. And this is changing their properties, and therefore changes these structure functions. Outside the nucleus, there are no mesons really interacting with these quarks" - Ian Cloet.

While the new calculation shows a clear difference in how quarks and gluons contribute to a nucleon's spin, it doesn't reveal the exact makeup of these contributions. And the finding hasn't yet been backed up by experiment. But that could change. One of the goals of the 12 GeV Upgrade project at Jefferson Lab is to measure the origin of the basic properties of nucleons, including mass, size, electric field, magnetic field, and spin.

This is the first calculation of the spin-dependent structure functions of nucleons inside the nucleus. This discovery is giving theorists new insight into Quantum Chromodynamics (QCD). QCD is a theory that describes the force, the strong force, that binds quarks into nucleons and nucleons into nuclei.
This newest calculation demonstrates that the nucleus isn't a dull, well understood object: it still has many secrets, and investigating its properties is pushing the boundaries of physicists' understanding of QCD. It also indicates that the nucleus is far from a simple collection of protons and neutrons, but is truly a complex system of interacting quarks and gluons.

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