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

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 Are The Parts Of An Atom?

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.

Continue reading “What Are The Parts Of An Atom?”

Astronomy Cast Ep. 395: The Standard Model – Baryons and Beyond

In the last few episodes, we’ve been talking about the standard model of physics, explaining what everything is made up of. But the reality is that we probably don’t know a fraction of how everything is put together. This week we’re going to talk about baryons, the particles made up of quarks. The most famous ones are the proton and the neutron, but that’s just the tip of the baryonic iceberg. And then we’re going to talk about where the standard model ends, and what’s next in particle physics.
Continue reading “Astronomy Cast Ep. 395: The Standard Model – Baryons and Beyond”

Astronomy Cast Ep. 393: The Standard Model – Leptons & Quarks

Physicists are getting a handle on the structure of the Universe, how everything is made of something else. Molecules are made of atoms, atoms are made of protons, neutrons and electrons, etc. Even smaller than that are the quarks and the leptons, which seem to be the basic building blocks of all matter.
Continue reading “Astronomy Cast Ep. 393: The Standard Model – Leptons & Quarks”

Macro View Makes Dark Matter Look Even Stranger

We know dark matter exists. We know this because without it and dark energy, our Universe would be missing 95.4% of its mass. What’s more, scientists would be hard pressed to explain what accounts for the gravitational effects they routinely see at work in the cosmos.

For decades, scientists have sought to prove its existence by smashing protons together in the Large Hadron Collider. Unfortunately, these efforts have not provided any concrete evidence.

Hence, it might be time to rethink dark matter. And physicists David M. Jacobs, Glenn D. Starkman, and Bryan Lynn of Case Western Reserve University have a theory that does just that, even if it does sound a bit strange.

In their new study, they argue that instead of dark matter consisting of elementary particles that are invisible and do not emit or absorb light and electromagnetic radiation, it takes the form of chunks of matter that vary widely in terms of mass and size.

As it stands, there are many leading candidates for what dark matter could be, which range from Weakly-Interacting Massive Particles (aka WIMPs) to axions. These candidates are attractive, particularly WIMPs, because the existence of such particles might help confirm supersymmetry theory – which in turn could help lead to a working Theory of Everything (ToE).

According to supersymmetry, dark-matter particles known as neutralinos (which are often called WIMPs) annihilate each other, creating a cascade of particles and radiation that includes medium-energy gamma rays. If neutralinos exist, the LAT might see the gamma rays associated with their demise. Credit: Sky & Telescope / Gregg Dinderman.
According to supersymmetry, dark-matter particles known as neutralinos (aka WIMPs) annihilate each other, creating a cascade of particles and radiation. Credit: Sky & Telescope / Gregg Dinderman.

But so far, no evidence has been obtained that definitively proves the existence of either. Beyond being necessary in order for General Relativity to work, this invisible mass seems content to remain invisible to detection.

According to Jacobs, Starkman, and Lynn, this could indicate that dark matter exists within the realm of normal matter. In particular, they consider the possibility that dark matter consists of macroscopic objects – which they dub “Macros” – that can be characterized in units of grams and square centimeters respectively.

Macros are not only significantly larger than WIMPS and axions, but could potentially be assembled out of particles in the Standard Model of particle physics – such as quarks and leptons from the early universe – instead of requiring new physics to explain their existence. WIMPS and axions remain possible candidates for dark matter, but Jacobs and Starkman argue that there’s a reason to search elsewhere.

“The possibility that dark matter could be macroscopic and even emerge from the Standard Model is an old but exciting one,” Starkman told Universe Today, via email. “It is the most economical possibility, and in the face of our failure so far to find dark matter candidates in our dark matter detectors, or to make them in our accelerators, it is one that deserves our renewed attention.”

After eliminating most ordinary matter – including failed Jupiters, white dwarfs, neutron stars, stellar black holes, the black holes in centers of galaxies, and neutrinos with a lot of mass – as possible candidates, physicists turned their focus on the exotics.

Particle Collider
Ongoing experiments at the Large Hadron Collider have so far failed to produce evidence of WIMPs. Credit: CERN/LHC/GridPP

Nevertheless, matter that was somewhere in between ordinary and exotic – relatives of neutron stars or large nuclei – was left on the table, Starkman said. “We say relatives because they probably have a considerable admixture of strange quarks, which are made in accelerators and ordinarily have extremely short lives,” he said.

Although strange quarks are highly unstable, Starkman points out that neutrons are also highly unstable. But in helium, bound with stable protons, neutrons remain stable.

“That opens the possibility that stable strange nuclear matter was made in the early Universe and dark matter is nothing more than chunks of strange nuclear matter or other bound states of quarks, or of baryons, which are themselves made of quarks,” said Starkman.

Such dark matter would fit the Standard Model.

This is perhaps the most appealing aspect of the Macros theory: the notion that dark matter, which our cosmological model of the Universe depends upon, can be proven without the need for additional particles.

Still, the idea that the universe is filled with a chunky, invisible mass rather than countless invisible particles does make the universe seem a bit stranger, doesn’t it?

Further Reading: Case Western

How CERN’s Discovery of Exotic Particles May Affect Astrophysics

You may have heard that CERN announced the discovery (confirmation, actually. See addendum below.) of a strange particle known as Z(4430).  A paper summarizing the results has been published on the physics arxiv, which is a repository for preprint (not yet peer reviewed) physics papers.  The new particle is about 4 times more massive than a proton, has a negative charge, and appears to be a theoretical particle known as a tetraquark.  The results are still young, but if this discovery holds up it could have implications for our understanding of neutron stars.

A periodic table of elementary particles. Credit: Wikipedia
A periodic table of elementary particles.
Credit: Wikipedia

The building blocks of matter are made of leptons (such as the electron and neutrinos) and quarks (which make up protons, neutrons, and other particles).  Quarks are very different from other particles in that they have an electric charge that is 1/3 or 2/3 that of the electron and proton.  They also possess a different kind of “charge” known as color.  Just as electric charges interact through an electromagnetic force, color charges interact through the strong nuclear force.  It is the color charge of quarks that works to hold the nuclei of atoms together. Color charge is much more complex than electric charge.  With electric charge there is simply positive (+) and its opposite, negative (-).  With color, there are three types (red, green, and blue) and their opposites (anti-red, anti-green, and anti-blue).

Because of the way the strong force works, we can never observe a free quark.  The strong force requires that quarks always group together to form a particle that is color neutral. For example, a proton consists of three quarks (two up and one down), where each quark is a different color.  With visible light, adding red, green and blue light gives you white light, which is colorless. In the same way, combining a red, green and blue quark gives you a particle which is color neutral.  This similarity to the color properties of light is why quark charge is named after colors.

Combining a quark of each color into groups of three is one way to create a color neutral particle, and these are known as baryons.  Protons and neutrons are the most common baryons.  Another way to combine quarks is to pair a quark of a particular color with a quark of its anti-color.  For example, a green quark and an anti-green quark could combine to form a color neutral particle.  These two-quark particles are known as mesons, and were first discovered in 1947.  For example, the positively charged pion consists of an up quark and an antiparticle down quark.

Under the rules of the strong force, there are other ways quarks could combine to form a neutral particle.  One of these, the tetraquark, combines four quarks, where two particles have a particular color and the other two have the corresponding anti-colors.  Others, such as the pentaquark (3 colors + a color anti-color pair) and the hexaquark (3 colors + 3 anti-colors) have been proposed.  But so far all of these have been hypothetical.  While such particles would be color neutral, it is also possible that they aren’t stable and would simply decay into baryons and mesons.

There has been some experimental hints of tetraquarks, but this latest result is the strongest evidence of 4 quarks forming a color neutral particle.  This means that quarks can combine in much more complex ways than we originally expected, and this has implications for the internal structure of neutron stars.

Very simply, the traditional model of a neutron star is that it is made of neutrons.  Neutrons consist of three quarks (two down and one up), but it is generally thought that particle interactions within a neutron star are interactions between neutrons.  With the existence of tetraquarks, it is possible for neutrons within the core to interact strongly enough to create tetraquarks.  This could even lead to the production of pentaquarks and hexaquarks, or even that quarks could interact individually without being bound into color neutral particles.  This would produce a hypothetical object known as a quark star.

This is all hypothetical at this point, but verified evidence of tetraquarks will force astrophysicists to reexamine some the assumptions we have about the interiors of neutron stars.

Addendum: It has been pointed out that CERN’s results are not an original discovery, but rather a confirmation of earlier results by the Belle Collaboration.  The Belle results can be found in a 2008 paper in Physical Review Letters, as well as a 2013 paper in Physical Review D.  So credit where credit is due.

Proton Parts

The proton has three parts, two up quarks and one down quark … and the gluons which these three quarks exchange, which is how the strong (nuclear) force works to keep them from getting out.

The proton’s world is a totally quantum one, and so it is described entirely by just a handful of numbers, characterizing its spin (a technical term, not to be confused with the everyday English word; the proton’s spin is 1/2), electric charge (+1 e, or 1.602176487(40)×10-19 C), isospin (also 1/2), and parity (+1). These properties are derived directly from those of the proton parts, the three quarks; for example, the up quark has an electric charge of +2/3 e, and the down -1/3 e, which sum to +1 e. Another example, color charge: the proton has a color charge of zero, but each of its constituent three quarks has a non-zero color charge – one is ‘blue’, one ‘red’, and one ‘green’ – which ‘sum’ to zero (of course, color charge has nothing whatsoever to do with the colors you and I see with our eyes!).

Murray Gell-Mann and George Zweig independently came up with the idea that the proton’s parts are quarks, in 1964 (though it wasn’t until several years later that good evidence for the existence of such parts was obtained). Gell-Mann was later awarded the Nobel Prize of Physics for this, and other work on fundamental particles (Zweig has yet to receive a Nobel).

The quantum theory which describes the strong interaction (or strong nuclear force) is quantum chromodynamics, QCD for short (named in part after the ‘colors’ of quarks), and this explains why the proton has the mass it does. You see, the up quark’s mass is about 2.4 MeV (mega-electron volts; particle physicists measure mass in MeV/c2), and the down’s about 4.8 MeV. Gluons, like photons, are massless, so the proton should have a mass of about 9.6 MeV (= 2 x 2.4 + 4.8), right? But it is, in fact, 938 MeV! QCD accounts for this enormous difference by the energy of the QCD vacuum inside the proton; basically, the self-energy of ceaseless interactions of quarks and gluons.

Further reading: The Physics of RHIC (Brookhaven National Lab), How are the protons and neutrons held together in a nucleus?, and Are protons and neutrons fundamental? (the Particle Adventure) are three good places to go!

Some of the Universe Today articles relevant to proton parts are: Final Detector in Place at the Large Hadron Collider, Hidden Stores of Deuterium Discovered in the Milky Way, and New Study Finds Fundamental Force Hasn’t Changed Over Time.

Two Astronomy Cast episodes you won’t want to miss, on proton parts: The Strong and Weak Nuclear Forces, and Inside the Atom.

Sources:
Chem4Kids
Wikipedia