New Muon g-2 Result Improves the Measurement by a Factor of 2

First results from the Muon g-2 experiment at Fermilab have strengthened evidence of new physics. Credit: Reidar Hahn/Fermilab

At the Fermi National Accelerator Laboratory (aka. Fermilab), an international team of scientists is conducting some of the most sensitive tests of the Standard Model of Particle Physics. The experiment, known as Muon g-2, measures the anomalous magnetic dipole moment of muons, a fundamental particle that is negatively charged (like electrons) but over 200 times as massive. In a recent breakthrough, scientists at Fermilab made the world’s most precise measurement of the muon’s anomalous magnetic moment, improving the precision of their previous measurements by a factor of 2.

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Is the “D-star Hexaquark” the Dark Matter Particle?

The early universe. Credit: Tom Abel & Ralf Kaehler (KIPACSLAC)/ AMNH/NASA

Since the 1960s, astronomers have theorized that all the visible matter in the Universe (aka. baryonic or “luminous matter) constitutes just a small fraction of what’s actually there. In order for the predominant and time-tested theory of gravity to work (as defined by General Relativity), scientists have had to postulate that roughly 85% of the mass in the Universe consists of “Dark Matter”.

Despite many decades of study, scientists have yet to find any direct evidence of Dark Matter and the constituent particle and its origins remain a mystery. However, a team of physicists from the University of York in the UK has proposed a new candidate particle that was just recently discovered. Known as the d-star hexaquark, this particle could have formed the “Dark Matter” in the Universe during the Big Bang.

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The Pressure Inside Every Proton is 10x That Inside Neutron Stars

The first measurement of a subatomic particle’s mechanical property reveals the distribution of pressure inside the proton. Credit: DOE's Jefferson Lab

Neutron stars are famous for combining a very high-density with a very small radius. As the remnants of massive stars that have undergone gravitational collapse, the interior of a neutron star is compressed to the point where they have similar pressure conditions to atomic nuclei. Basically, they become so dense that they experience the same amount of internal pressure as the equivalent of 2.6 to 4.1 quadrillion Suns!

In spite of that, neutron stars have nothing on protons, according to a recent study by scientists at the Department of Energy’s Thomas Jefferson National Accelerator Facility. After conducting the first measurement of the mechanical properties of subatomic particles, the scientific team determined that near the center of a proton, the pressure is about 10 times greater than the pressure in the heart of a neutron star.

The study which describes the team’s findings, titled “The pressure distribution inside the proton“, recently appeared in the scientific journal Nature. The study was led by Volker Burkert, a nuclear physicist at the Thomas Jefferson National Accelerator Facility (TJNAF), and co-authored by Latifa Elouadrhiri and Francois-Xavier Girod – also from the TJNAF.

Cross-section of a neutron star. Credit: Wikipedia Commons/Robert Schulze

Basically , they found that the pressure conditions at the center of a proton were 100 decillion pascals – about 10 times the pressure at the heart of a neutron star. However, they also found that pressure inside the particle is not uniform, and drops off as the distance from the center increases. As Volker Burkert, the Jefferson Lab Hall B Leader, explained:

“We found an extremely high outward-directed pressure from the center of the proton, and a much lower and more extended inward-directed pressure near the proton’s periphery… Our results also shed light on the distribution of the strong force inside the proton. We are providing a way of visualizing the magnitude and distribution of the strong force inside the proton. This opens up an entirely new direction in nuclear and particle physics that can be explored in the future.”

Protons are composed of three quarks that are bound together by the strong nuclear force, one of the four fundamental forces that government the Universe – the other being electromagnetism, gravity and weak nuclear forces. Whereas electromagnetism and gravity produce the effects that govern matter on the larger scales, weak and strong nuclear forces govern matter at the subatomic level.

Previously, scientists thought that it was impossible to obtain detailed information about subatomic particles. However, the researchers were able to obtain results by pairing two theoretical frameworks with existing data, which consisted of modelling systems that rely on electromagnetism and gravity. The first model concerns generalized parton distributions (GDP) while the second involve gravitational form factors.

Quarks inside a proton experience a force an order of magnitude greater than matter inside a neutron star. Credit: DOE’s Jefferson Lab

Patron modelling refers to modeling subatomic entities (like quarks) inside protons and neutrons, which allows scientist to create 3D images of a proton’s or neutron’s structure (as probed by the electromagnetic force). The second model describes the scattering of subatomic particles by classical gravitational fields, which describes the mechanical structure of protons when probed via the gravitational force.

As noted, scientists previously thought that this was impossible due to the extreme weakness of the gravitational interaction. However, recent theoretical work has indicated that it could be possible to determine the mechanical structure of a proton using electromagnetic probes as a substitute for gravitational probes. According to Latifa Elouadrhiri – a Jefferson Lab staff scientist and co-author on the paper – that is what their team set out to prove.

“This is the beauty of it. You have this map that you think you will never get,” she said. “But here we are, filling it in with this electromagnetic probe.”

For the sake of their study, the team used the DOE’s Continuous Electron Beam Accelerator Facility at the TJNAF to create a beam of electrons. These were then directed into the nuclei of atoms where they interacted electromagnetically with the quarks inside protons via a process called deeply virtual Compton scattering (DVCS). In this process, an electron exchanges a virtual photon with a quark, transferring energy to the quark and proton.

The bare masses of all 6 flavors of quarks, proton and electron, shown in proportional volume. Credit: Wikipedia/Incnis Mrsi

Shortly thereafter, the proton releases this energy by emitting another photon while remaining intact. Through this process, the team was able to produced detailed information of the mechanics going on in inside the protons they probed. As Francois-Xavier Girod, a Jefferson Lab staff scientist and co-author on the paper, explained the process:

“There’s a photon coming in and a photon coming out. And the pair of photons both are spin-1. That gives us the same information as exchanging one graviton particle with spin-2. So now, one can basically do the same thing that we have done in electromagnetic processes — but relative to the gravitational form factors, which represent the mechanical structure of the proton.”

The next step, according to the research team, will be to apply the technique to even more precise data that will soon be released. This will reduce uncertainties in the current analysis and allow the team to reveal other mechanical properties inside protons – like the internal shear forces and the proton’s mechanical radius. These results, and those the team hope to reveal in the future, are sure to be of interest to other physicists.

“We are providing a way of visualizing the magnitude and distribution of the strong force inside the proton,” said Burkert. “This opens up an entirely new direction in nuclear and particle physics that can be explored in the future.”

Perhaps, just perhaps, it will bring us closer to understanding how the four fundamental forces of the Universe interact. While scientists understand how electromagnetism and weak and strong nuclear forces interact with each other (as described by Quantum Mechanics), they are still unsure how these interact with gravity (as described by General Relativity).

If and when the four forces can be unified in a Theory of Everything (ToE), one of the last and greatest hurdles to a complete understanding of the Universe will finally be removed.

Further Reading: Jefferson Lab, Cosmos Magazine, Nature

What is Binding Energy?

The atomic structure. Credit: Encyclopædia Britannica, Inc.

Have you ever taken a look at a piece of firewood and said to yourself, “gee, I wonder how much energy it would take to split that thing apart”? Chances are, no you haven’t, few people do. But for physicists, asking how much energy is needed to separate something into its component pieces is actually a pretty important question.

In the field of physics, this is what is known as binding energy, or the amount of mechanical energy it would take to disassemble an atom into its separate parts. This concept is used by scientists on many different levels, which includes the atomic level, the nuclear level, and in astrophysics and chemistry.

Nuclear Force:

As anyone who remembers their basic chemistry or physics surely knows, atoms are composed of subatomic particles known as nucleons. These consist of positively-charged particles (protons) and neutral particles (neutrons) that are arranged in the center (in the nucleus). These are surrounded by electrons which orbit the nucleus and are arranged in different energy levels.

Neils Bohr's model a nitrogen atom. Credit:
Neils Bohr’s model a nitrogen atom. Credit:

The reason why subatomic particles that have fundamentally different charges are able to exist so close together is because of the presence of Strong Nuclear Force – a fundamental force of the universe that allows subatomic particles to be attracted at short distances. It is this force that counteracts the repulsive force (known as the Coulomb Force) that causes particles to repel each other.

Therefore, any attempt to divide the nucleus into the same number of free unbound neutrons and protons – so that they are far/distant enough from each other that the strong nuclear force can no longer cause the particles to interact – will require enough energy to break these nuclear bonds.

Thus, binding energy is not only the amount of energy required to break strong nuclear force bonds, it is also a measure of the strength of the bonds holding the nucleons together.

Nuclear Fission and Fusion:

In order to separate nucleons, energy must be supplied to the nucleus, which is usually accomplished by bombarding the nucleus with high energy particles. In the case of bombarding heavy atomic nuclei (like uranium or plutonium atoms) with protons, this is known as nuclear fission.

Nuclear fission, where an atom of Uranium 96 is split by a free neutron to produce barium and krypton. Credit:
Nuclear fission, where an atom of Uranium 96 is split by a free neutron to produce barium and krypton. Credit:

However, binding energy also plays a role in nuclear fusion, where light nuclei together (such as hydrogen atoms), are bound together under high energy states. If the binding energy for the products is higher when light nuclei fuse, or when heavy nuclei split, either of these processes will result in a release of the “extra” binding energy. This energy is referred to as nuclear energy, or loosely as nuclear power.

It is observed that the mass of any nucleus is always less than the sum of the masses of the individual constituent nucleons which make it up. The “loss” of mass which results when nucleons are split to form smaller nucleus, or merge to form a larger nucleus, is also attributed to a binding energy. This missing mass may be lost during the process in the form of heat or light.

Once the system cools to normal temperatures and returns to ground states in terms of energy levels, there is less mass remaining in the system. In that case, the removed heat represents exactly the mass “deficit”, and the heat itself retains the mass which was lost (from the point of view of the initial system). This mass appears in any other system which absorbs the heat and gains thermal energy.

Types of Binding Energy:

Strictly speaking, there are several different types of binding energy, which is based on the particular field of study. When it comes to particle physics, binding energy refers to the energy an atom derives from electromagnetic interaction, and is also the amount of energy required to disassemble an atom into free nucleons.

Nuclear Physics
Diagram showing the process of nuclear fusion. Credit: Lancaster University

In the case of removing electrons from an atom, a molecule, or an ion, the energy required is known as “electron binding energy” (aka. ionization potential). In general, the binding energy of a single proton or neutron in a nucleus is approximately a million times greater than the binding energy of a single electron in an atom.

In astrophysics, scientists employ the term “gravitational binding energy” to refer to the amount of energy it would take to pull apart (to infinity) an object held together by gravity alone – i.e. any stellar object like a star, a planet, or a comet. It also refers to the amount of energy that is liberated (usually in the form of heat) during the accretion of such an object from material falling from infinity.

Finally, there is what is known as “bond” energy, which is a measure of the bond strength in chemical bonds, and is also the amount of energy (heat) it would take to break a chemical compound down into its constituent atoms. Basically, binding energy is the very thing that binds our Universe together. And when various parts of it are broken apart, it is the amount of energy needed to carry it out.

The study of binding energy has numerous applications, not the least of which are nuclear power, electricity, and chemical manufacture. And in the coming years and decades, it will be intrinsic in the development of nuclear fusion!

We have written many articles about binding energy for Universe Today. Here’s What is Bohr’s Atomic Model?, What is John Dalton’s Atomic Model?, What is the Plum Pudding Atomic Model?, What is Atomic Mass?, and Nuclear Fusion in Stars.

If you’d like more info on binding energy, check out Hyperphysics article on Nuclear Binding Energy.

We’ve also recorded an entire episode of Astronomy Cast all about the Important Numbers in the Universe. Listen here, Episode 45: The Important Numbers in the Universe.


What Are The Parts Of An Atom?

A depiction of the atomic structure of the helium atom. Credit: Creative Commons

Since the beginning of time, human beings have sought to understand what the universe and everything within it is made up of. And while ancient magi and philosophers conceived of a world composed of four or five elements – earth, air, water, fire (and metal, or consciousness) – by classical antiquity, philosophers began to theorize that all matter was actually made up of tiny, invisible, and indivisible atoms.

Since that time, scientists have engaged in a process of ongoing discovery with the atom, hoping to discover its true nature and makeup. By the 20th century, our understanding became refined to the point that we were able to construct an accurate model of it. And within the past decade, our understanding has advanced even further, to the point that we have come to confirm the existence of almost all of its theorized parts.

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[/caption]Ever since Einstein unveiled his theory of relativity, the speed of light has been considered to be the physical constant of the universe, interrelating space and time. In short, it was the speed at which light and all other forms of electromagnetic radiation were believed to travel at all times in empty space, regardless of the motion of the source or the inertial frame of reference of the observer. But suppose for a second that there was a particle that defied this law, that could exist within the framework of a relativistic universe, but at the same time defy the foundations on which its built? Sounds impossible, but the existence of such a particle may very well be necessary from a quantum standpoint, resolving key issues that arise in that chaotic theory. It is known as the Tachyon Particle, a hypothetical subatomic particle that can move faster than light and poses a number intriguing problems and possibilities to the field of physics.

In the language of special relativity, a tachyon would be a particle with space-like four-momentum and imaginary proper time. Their existence was first attributed to German physicist Arnold Sommerfeld; even though it was Gerald Feinberg who first coined the term in the 1960s, and several other scientists helped to advance the theoretical framework within which tachyons were believed to exist. They were originally proposed within the framework of quantum field theory as a way of explaining the instability of the system, but have nevertheless posed problems for the theory of special relativity.

For example, if tachyons were conventional, localizable particles that could be used to send signals faster than light, this would lead to violations of causality in special relativity. But in the framework of quantum field theory, tachyons are understood as signifying an instability of the system and treated using a theory known as tachyon condensation, a process that attempts to resolve their existence by explaining them in terms of better understood phenomena, rather than as real faster-than-light particles. Tachyonic fields have appeared theoretically in a variety of contexts, such as the bosonic string theory. In general, string theory states that what we see as “particles” —electrons, photons, gravitons and so forth—are actually different vibrational states of the same underlying string. In this framework, a tachyon would appear as either indication of instability in the D-brane system or within spacetime itself.

Despite the theoretical arguments against the existence of tachyon particles, experimental searches have been conducted to test the assumption against their existence; however, no experimental evidence for the existence of tachyon particles has been found.

We have written many articles about tachyon for Universe Today. Here’s an article about elementary particles, and here’s an article about Einstein’s Theory of Relativity.

If you’d like more info on tachyon, check out these articles from Science World. Also, you may want to browse through a forum discussion about tachyons.

We’ve also recorded an entire episode of Astronomy Cast all about the Theory of Special Relativity. Listen here, Episode 9: Einstein’s Theory of Special Relativity.


Subatomic Particles

Fine Structure Constant


Not long ago, scientists believed that the smallest part of matter was the atom; the indivisible, indestructible, base unit of all things. However, it was not long before scientists began to encounter problems with this model, problems arising out of the study of radiation, the laws of thermodynamics, and electrical charges. All of these problems forced them to reconsider their previous assumptions about the atom being the smallest unit of matter and to postulate that atoms themselves were made up of a variety of particles, each of which had a particular charge, function, or “flavor”. These they began to refer to as Subatomic Particles, which are now believed to be the smallest units of matter, ones that composenucleons and atoms.

Whereas protons, neutrons and electrons have always been considered to be the fundamental particles of an atom, recent discoveries using atomic accelerators have shown that there are actually twelve different kinds of elementary subatomic particles, and that protons and neutrons are actually made up of smaller subatomic particles. These twelve particles are divided into two categories, known as Leptons and Quarks. There are six different kinds, or “flavors”, of quarks (named because of their unusual behavior). These include up, down, charm, strange, top, and bottom quark, each of which possesses a charge that is expressed as a fraction (+2/3 for up, top and charm,-1/3 for down, bottom and strange) and have variable masses. There are also six different types of Leptons, which include Electrons, Muons, Taus, Electron Neutrinos, Muon Neutrinos, and Tau Neutrinos. Whereas electrons and Muons both have a negative charge of -1 (Muons having greater mass), Neutrinos have no charge and are extremely difficult to detect.

In addition to elementary particles, composite particles are another category of subatomic particles. Whereas elementary particles are not made up of other particles, composite particlesare bound states of two or more elementary particles, such as protons or atomic nuclei. For example, a proton is made of two Up quarks and one Down quark, while the atomic nucleus of helium-4 is composed of two protons and two neutrons.In addition, there are also the subatomic particles that fall under the heading of Gauge Bosons, which were identified using the same methods as Leptons and Quarks. These are classified as “force carriers”, i.e. particles that act as carriers for the fundamental forces of nature. These include photons that are associated with electromagnetism, gravitons that are associated with gravity, the three W and Z bosons of weak nuclear forces, and the eight gluons of strong nuclear forces. Scientists also predict the existence of several more, what they refer to as “hypothetical” particles, so the list is expected to grow.

Today, there are literally hundreds of known subatomic particles, most of which were either the result of cosmic rays interacting with matter or particle accelerator experiments.

We have written many articles about the subatomic particles for Universe Today. Here’s an article about the atomic nucleus, and here’s an article about the atomic theory.

If you’d like more info on the Atom, check out the Background on Atoms, and here’s a link to the NASA’s Understanding the Atom Page.

We’ve also recorded an entire episode of Astronomy Cast all about the Composition of the Atom. Listen here, Episode 164: Inside the Atom.