How did the proton get its spin?

March 31, 2017 by Shannon Brescher Shea
How did the Proton Get Its Spin?
In the 1980s, scientists discovered that a proton's three valance quarks (red, green, blue) account for only a fraction of the proton's overall spin. More recent measurements have revealed that gluons (yellow corkscrews) contribute as much as or possibly more than the quarks. Credit: Brookhaven National Laboratory

Calculating a proton's spin used to be an easy college assignment. In fact, Carl Gagliardi remembers answering that question when he was a physics graduate student in the 1970s. But the real answer turned out not to be simple at all. Even Gagliardi's "right" response was disproven by experiments a few years later that turned the field upside-down.

Protons are one of the three particles that make up atoms, the building blocks of the universe. A proton's spin is one of its most basic properties. Because protons are in part made up of quarks, scientists presumed the proton spins were just the sum of the quark spins.

But studies in the 1980s showed that reality is far more complex. Since then, Gagliardi and other researchers have used the unique DOE Office of Science User Facilities at Thomas Jefferson National Accelerator Facility (Jefferson Lab) and Brookhaven National Laboratory to explore this fundamental phenomenon.

Investigating a Force of Nature

Protons always have "spin." The direction and strength of a proton's spin determines its magnetic and electrical properties. Changes to the proton's spin also alter its structure.

"By understanding how [a proton's components] play off of each other to produce spin, we can learn about how Mother Nature builds a proton," said Gagliardi, now a researcher at the Cyclotron Institute at Texas A&M. He collaborates on work at the Relativistic Heavy Ion Collider (RHIC), a DOE Office of Science User Facility at Brookhaven National Laboratory on Long Island, New York.

Having a better grasp of protons' spin and structure could lead to unexpected benefits. Just as James Clerk Maxwell's discoveries about electromagnetism in the 1860s set the foundation for today's electronics, some scientists think understanding proton spin could lead to similar advances.

"[Maxwell's equations] were mankind's mastery over a fundamental force of nature, electromagnetism," said John Lajoie, an Iowa State researcher who works on RHIC. "What we are trying to do is gain a fundamental understanding of the force that binds the quarks to the proton."

Unexpected Findings

"Studying spin in physics has led to a lot of surprises," said Elke-Caroline Aschenauer, who leads Brookhaven's research group focused on proton spin. But nature hasn't given up its secrets easily.

Researchers first thought that each proton consisted entirely of only three quarks, which together determined the spin. Quarks are elementary particles that scientists have not been able to break down into smaller parts.

But the closer they looked, the more complex the picture got. The initial experiment at the European Organization for Nuclear Research (CERN) suggested quark spins contributed almost nothing to the proton spin. Since then, more precise experiments have raised the quark spin contribution to 25 to 30 percent. That leaves a good deal unaccounted for.

Rather than being disappointed, many physicists were thrilled.

"I live for being wrong," said Lajoie. "That's where we learn."

Banging Particles Together

To investigate protons and other subatomic particles, scientists use accelerators to collide them at velocities near the speed of light.

"Particle physicists have not really evolved much further than the days of the cavemen in terms of banging two rocks together," joked Lajoie.

(left) The Relativistic Heavy Ion Collider at Brookhaven National Laboratory. (right) The Continuous Electron Beam Accelerator Facility at Thomas Jefferson National Accelerator Laboratory. Photo courtesy of Jefferson Laboratory. Credit: Brookhaven National Laboratory

The accelerators at Brookhaven and Jefferson Labs have the unique ability to polarize streams of particles. This means that they coordinate the particles' spins so that they are aligned in the same direction.

At the Continuous Electron Beam Accelerator Facility (CEBAF), a DOE Office of Science User Facility at Jefferson Lab in Newport News, Virginia, the machine shoots a polarized beam of electrons into a stationary target. The target is also polarized. Colliding the beam of electrons with the protons or neutrons in the target gives scientists particularly good insight into quarks' contributions to spin. When the beam strikes the target, particles scatter off at different angles. An electron spectrometer then identifies what types and how many particles resulted from the experiment.

RHIC at Brookhaven sends two beams of protons through a four-mile-long tunnel. When they collide, the particles tear each other apart and regroup immediately. They strike two house-sized detectors that collect data on their direction, momentum, and energy.

"It's just an amazing accomplishment of humankind," said Ernst Sichtermann, a researcher at DOE's Lawrence Berkeley National Laboratory and deputy spokesperson for one of RHIC's experiments.

As the only facility that can polarize and collide protons, RHIC is useful for understanding gluons' contribution. Gluons are the particles that hold quarks together to form protons and neutrons.

Comparing and contrasting results is an essential part of proton spin research. Both laboratories conduct experiments that examine what happens when you collide particles that are spinning in the same direction versus those spinning in opposite directions. To determine how a specific particle, such as a gluon or , contributes to spin, researchers compare the number and type of that result from different configurations of the beams and target.

One of the main challenges is collecting and analyzing the incredible amount of data. Much of the work focuses on collecting the correct data and minimizing errors or biases.

"That is where one becomes a real physicist," said Gagliardi. "Ninety-five percent of the scientific analysis time is devoted to identifying, quantifying and limiting those biases."

Understanding the Contributions

Using these tools, physicists realized that the proton's structure isn't simple at all. It's an ocean of shifting quarks and gluons. In addition, gluons rapidly split into short-lived pairs of quarks and anti-quarks (known as sea quarks). Anti-quarks have similar characteristic to quarks, except the opposite charge.

A number of experiments have examined possible sources of spin.

One experiment at RHIC found that the spins of anti-quarks often aren't aligned in the same direction. As a result, it's unlikely they contribute much to the 's spin.

Another study tackled the role of gluons. In 2014, scientists found experimental data that demonstrated gluons contribute significantly to . In fact, they contribute about 20 to 30 percent of it.

A follow-on experiment focused on "wimpy" gluons with low momentum. Previous studies had underestimated the contribution of these gluons. But collisions at much higher energies found that while single "wimpy" gluons contribute almost nothing, the sheer number of them results in quite a bit of influence.

There's one major source that researchers haven't yet explored: . Orbital angular momentum comes from the movement of the quarks and gluons relative to each other. While theorists have developed simulations that model this contribution, scientists haven't had the equipment to test them.

That will change with the opening of a major upgrade to CEBAF. Doubling the accelerator's energy and providing better resolution will allow scientists to study orbital angular momentum. The lab staff members expect to have the upgraded accelerator fully running in the next year.

"There's no other beam like it elsewhere in the world," said Robert McKeown, Jefferson Lab's deputy director of research.

Explore further: Model identifies a high degree of fluctuations in gluons as essential to explaining proton structure

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39 comments

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antialias_physorg
5 / 5 (19) Mar 31, 2017
For all the science deniers and scientist critics on this site:
"I live for being wrong," said Lajoie. "That's where we learn."

Read this. Read it again. Then read it again.

Then realize that while YOU do the first part, you never bother to do the second part.
axemaster
4.7 / 5 (13) Mar 31, 2017
YES! I second this comment, a hundred times over.

Also, people REALLY need to learn to separate their ideas from their personal identity. For example, I would be shocked to learn that climate change isn't real, but I wouldn't be DISAPPOINTED. The *idea* is not part of who *I* am.

This is also why I wish we wouldn't call people climate deniers. By identifying *who* someone is according to their ideas, you obviously are conflating their ideas with their personal identity. And then you call it/them a piece of crap. I wonder why they become so resistant to change???
Dark_Solar
5 / 5 (3) Mar 31, 2017
As above, so below.

This tends to confirm something I have long suspected but will likely never have the means investigate i.e.--that the observable universe is full of mega-systems which exhibit this same particle (planetary masses) behavior at extremely long time-scales. Consider the extreme distances involved in the orbital arrangements of solar systems and weight that against what we've learned about relative sizes as compared to the internal structure of an atom and it's components; it would make sense that the rules governing all bodies with mass, spin and vector magnitude should scale all the way down to the subatomic/quark level. This opens some interesting avenues of research; assuming that we can create precise macro-scale analogs of quarks in normal, magnetic matter, it should then be possible to run experiments in micro-gravity conditions which simulate particle interactions...assuming we can find a way to create true mono-polar material..
antialias_physorg
4.6 / 5 (9) Mar 31, 2017
This is also why I wish we wouldn't call people climate deniers

True. On the other hand if we would call them what they really are (i.e. "willfully obtuse scientifically illiterate morons") doesn't sound any better. At least 'science deniers' gives them the benefit of the doubt about all the other areas of their lives.
Dark_Solar
5 / 5 (1) Mar 31, 2017
RE: my earlier comment

Additionally, macro-scale modelling of subatomic particles and how they interact in micro-gravity might also make it possible to determine the shape of quarks (assuming they even have anything approaching what we could reasonably define as a "shape").

An afterthought: if spin is a fundamental property of the universe/matter, by extension, all matter in the universe could reasonably be considered to have an inherent, base-line torque value (yes, I know measurement of torque requires a point of resistance and for this thought experiment, I'm substituting other atoms/particles or even just space-time as the resistance point).
Captain Stumpy
4.2 / 5 (5) Mar 31, 2017
This is also why I wish we wouldn't call people climate deniers. By identifying *who* someone is according to their ideas, you obviously are conflating their ideas with their personal identity
@axe
more often than not the reason for their denial of climate science is directly due to their personal identity, be it religious, political, peer identity or conspiracist ideation

case in point: on this site alone there are several posters who show intelligence and functional ability in science, so we can consider them scientifically literate, however, when the topic is climate science the predominant argument comes from politics, conspiracy or religion with heavy politically influenced misrepresentation of science

this can't entirely be related to being scientifically illiterate, therefore the *primary cause* must come from other sources, and those must be sources with strong ties to identity to continue to cling in the face of irrefutable evidence
NIPSZX
1.7 / 5 (6) Mar 31, 2017
People are finally starting to realize that 40% of science is based on unproven theories, but those theories spur future knowledge. The key is to separate fact from prophecy in science. The problem is that most theories are not provable in this time era. It is up to the individual to study the research and conclude for themselves and their families what they believe to be true and false. Luckily, this long and overdue skepticism toward science and scientists will strengthen the science community to rely on hard proven facts and work harder to prove theories quicker in the future.
Captain Stumpy
4.3 / 5 (6) Mar 31, 2017
People are finally starting to realize that 40% of science is based on unproven theories
@NIPSZX
you do realise the problem with that, right?

here is an accurate definition of a scientific theory (I've checked the source material so this is accurate)
A scientific theory is an explanation of some aspect of the natural world that can, in accordance with the scientific method, be repeatedly tested, using a predefined protocol of observations and experiments. Established scientific theories have withstood rigorous scrutiny and are a comprehensive form of scientific knowledge.
https://en.wikipe...c_theory

in light of the very definition of scientific theory, the rest of your post makes absolutely no sense at all whatsoever

-just sayin'
Seeker2
3 / 5 (2) Mar 31, 2017
@CS
here is an accurate definition of a scientific theory (I've checked the source material so this is accurate)... A scientific theory is an explanation of some aspect of the natural world that can, in accordance with the scientific method, be repeatedly tested, using a predefined protocol of observations and experiments. Established scientific theories have withstood rigorous scrutiny and are a comprehensive form of scientific knowledge.
So no theories in cosmology like the big bang or whatever are scientific, at least in this life, since I don't think we'll be around to repeat them. :(
Da Schneib
5 / 5 (3) Apr 01, 2017
What's really interesting is that all these complicated quark and gluon spins added together always add up to 1/2 or -1/2 anytime the proton interacts with anything.

Protons are, after all, fermions. This is experimentally confirmed. And the only available spin is +/- 1/2 for a fermion. And it's quantized. Not just according to theory, but according to experimental fact.
Seeker2
2 / 5 (1) Apr 01, 2017
"In 2014, scientists found experimental data that demonstrated gluons contribute significantly to proton spin. In fact, they contribute about 20 to 30 percent of it."

Interesting. Per http://physics.ap...s/v10/23
"the quark contribution to the proton (30%) comes entirely from the valence quarks"
" Gluon spin at this resolution thus contributes about 50% of the proton's spin"

This site discusses the strong force and how virtual pions interact with gluons to bind the quarks. Leading one to wonder if the magnetic force is held together by the interaction of photons with electron/positron pairs to produce the magnetic field.

Note really neat diagram of proton structure at that site.
Da Schneib
5 / 5 (3) Apr 01, 2017
@Seeker, you need to understand quantization.
Dingbone
Apr 01, 2017
This comment has been removed by a moderator.
Seeker2
5 / 5 (1) Apr 01, 2017
@DS
@Seeker, you need to understand quantization.
Especially the relation between angular momentum and spin. Good point.
Da Schneib
5 / 5 (3) Apr 01, 2017
Ummm, spin is actually Spin Angular Momentum, or SAM. Might want to look that up.
Seeker2
5 / 5 (2) Apr 01, 2017
Sounds good to me. Is there a problem?
Da Schneib
5 / 5 (2) Apr 01, 2017
Yes, there's also OAM. And J. And classical angular momentum.
Seeker2
5 / 5 (1) Apr 01, 2017
So not everything is quantized. That is a tricky one.
antialias_physorg
5 / 5 (4) Apr 01, 2017
People are finally starting to realize that 40% of science is based on unproven theories,

...aaaand the rest (i.e. those who aren't dumb) realize that you just made a number up and then went on from there.

Sooo...are you going to prove your number in the near future as you demand of everyone else? Thought not.
Da Schneib
5 / 5 (4) Apr 01, 2017
So not everything is quantized. That is a tricky one.
Hmmm, for an individual proton, actually, everything *is* quantized. A proton has SAM of +/- 1/2. The question here is exactly how this is composed from three SAM 1/2 quarks and a multitude of SAM 1 colored gluons. There are deeper questions when we start talking about the nuclear physics, but that one will do for now.

And the answer to the question hinges upon SAM, OAM, and J- all of which are quantized. There are a lot of people using up a lot of chalk, whiteboard markers, paper, pencils, ink, and burning a lot of electrons in a lot of computers working on that. This information will help them.
Captain Stumpy
4 / 5 (4) Apr 01, 2017
So no theories in cosmology like the big bang or whatever are scientific, at least in this life
@seeker
1- WTF are you going on about there? were you going for sarcasm, satire or some other form of humour or...???

2- if it is a scientific theory it is based upon evidence that is repeatable - and in astrophysics this means a lot of things like plasma physics (pppl.gov), or thermodynamics (mit.edu) or... well, you get the point

so i am hoping that was meant in humour, hence my rating

if not, then i suggest you utilise this link ( https://ocw.mit.e...ophysics ) and show me where there is an astrophysics theory that isn't based on valid repeatable physics experimentation

note: don't confuse "theory" with "hypothesis"
thanks
jross
1 / 5 (2) Apr 01, 2017
Proton spin is described in my 2-24-16 paper "Particle Spins are Real" at my web site "tronnies.com". I also calculate spins of photons and electrons. The positron has the same spin as the electron. These calculations are all based on my model of the Cosmos as described in my book. "Tronnies-The Source of the Coulomb Force", available at "amazon.com". (Just GOOGLE "tronnies". At my web site, click on "News".) Spin cannot be calculated based on the Standard Model. The calculations are easy with my model. (In the electron, three tronnies (two minus and one plus) spin at 160 trillion-trillion cycles per second and in the proton one high-energy electron and two positrons spin at 0.1765 trillion-trillion cycles per second.) Most of the basic concepts of the Standard Model are incorrect. See my recent proposal to the NSF also under "News" at tronnies.com.
John Ross
RealityCheck
2.3 / 5 (3) Apr 01, 2017
Hi jross. :)

Firstly let me sincerely commend you on a valiant effort to formulate an alternative (to GR types) gravitational theory, including the entities and dynamics/forces which you have provided the logics/values/interactions etc of in your 'model'. Kudos.

I skimmed through PDFs @:

http://tronnies.com

I shall of course peruse all of it more closely when I have more time; but I have already identified a source of concern regarding the logics of the comparable gravity 'backreaction' of emitted/passing Neutrino Photons in the cases of similar-massed BH and Star.

QUESTION: Assuming an ISOLATED Black Hole and an ISOLATED Star having similar mass, and assuming your Neutron Photons 'production/re-emission' logics in both cases, THEN how do you reconcile the logics that such a BH, which produces much MORE outgoing NF 'flux', still has a SIMILAR 'gravitational well' profile/strength at similar 'altitude' well away from respective surfaces/centers of mass?

Thanks. :)
ZergSurfer
Apr 01, 2017
This comment has been removed by a moderator.
Seeker2
1 / 5 (1) Apr 01, 2017
So no theories in cosmology like the big bang or whatever are scientific, at least in this life
@seeker
1- WTF are you going on about there?
Hard to tell. Quoted out of context.
Captain Stumpy
3.7 / 5 (3) Apr 01, 2017
Hard to tell. Quoted out of context
@seeker
you could always just look at your original post above... so let me help you

i stated-and you quoted in your post-
here is an accurate definition of a scientific theory (I've checked the source material so this is accurate)... A scientific theory is an explanation of some aspect of the natural world that can, in accordance with the scientific method, be repeatedly tested, using a predefined protocol of observations and experiments. Established scientific theories have withstood rigorous scrutiny and are a comprehensive form of scientific knowledge
to which you replied
So no theories in cosmology like the big bang or whatever are scientific, at least in this life, since I don't think we'll be around to repeat them
so let me ask again: WTF are you going on about there?

were you going for sarcasm, satire or some other form of humour or...???
Seeker2
1 / 5 (1) Apr 02, 2017
@CS
were you going for sarcasm, satire or some other form of humour or...???
Sarcasm yes. Satire yes. Humor? Not really funny as I see it. Serious issue when you're trying to experiment with things like the big bang. People do their best actually trying to simulate conditions at the big bang. Have to give them credit for that. Inflation? Testing the inflaton? Black holes? Let's see how they go about repeating those experiments. Good luck.
Seeker2
1 / 5 (1) Apr 02, 2017
cont
Established scientific theories...were you going for sarcasm, satire or some other form of humour or...???
Speaking of humor, you could probably get so good laughs in the field of cosmology with your established scientific theories. I'm looking for plausibility, and if possible, falsifiability.
Seeker2
1 / 5 (1) Apr 02, 2017
sorry
some good laughs
A typical reaction to your philosophy would probably be where is this guy coming from? Or perhaps lighten up, Larry.
Seeker2
1 / 5 (1) Apr 02, 2017
cont
Don't misunderstand me. Established scientific theories are fine as long as they're relevant to the issue at hand. It's like we're on the cutting edge. But it's true, the knife can cut in both directions.
Seeker2
1 / 5 (1) Apr 02, 2017
And then there's the problem of established scientific theories which shouldn't be. Like curved spacetime. That would be asymmetrical and violate conservation of energy. Spacetime is stretched, compressed, twisted, or maybe warped. That's it. And the gravitational well demonstrations in the children's museums. There has to be a better way of demonstrating stretched spacetime around gravitating objects.
Captain Stumpy
3 / 5 (2) Apr 02, 2017
A typical reaction to your philosophy would probably be where is this guy coming from?
@seeker
not really
Don't misunderstand me. Established scientific theories are fine as long as they're relevant to the issue at hand. It's like we're on the cutting edge. But it's true, the knife can cut in both directions
this, however, is nonsensical to me
i mean - science follows the evidence, and the evidence, to be considered a scientific fact, must be validated, so...

and it's not like scientists don't like to be wrong...
you could probably get so good laughs in the field of cosmology with your established scientific theories
not sure what you mean here: i aint rc, juli penrod, xtian creationist or the eu cult

Seeker2
1 / 5 (2) Apr 02, 2017
@CS
...science follows the evidence, and the evidence, to be considered a scientific fact, must be validated, so...
Just thinking about all the cosmology that's been validated. Surely must be something out there?
Seeker2
Apr 02, 2017
This comment has been removed by a moderator.
Seeker2
1 / 5 (2) Apr 02, 2017
cont
Note you can validate man-made global warming all you want. But if it doesn't fit your agenda the uncertainty principle seems to creep in there somehow.
nikola_milovic_378
1 / 5 (1) Apr 02, 2017
Many theories have been done so that fitted with the data that will satisfy the desires and "evidence" of these researchers.
Science has not yet proven true movement of celestial bodies, convince us that they have solved the enigma of the micro systems. In both of the celestial bodies with carbon, the system of the body mass, rotates around the center of mass of the system, so that the body rotates about its axis (spin), but at the same time rotates about the center of mass with the same size of spin, but the opposite direction. Code subatomic particles, you first need to know how these particles are formed, and it is easy to determine their behavior. You need to know how it came gluon and how it behaves. When the gluon partially transformed into a photon, then the electron and positron rotate like planets. How this is done, it is left for the Nobel Prize.
Captain Stumpy
5 / 5 (1) Apr 02, 2017
@CS
...science follows the evidence, and the evidence, to be considered a scientific fact, must be validated, so...
Just thinking about all the cosmology that's been validated. Surely must be something out there?
@seeker
and again, since you ignored it the first few times: then i suggest you utilise this link ( https://ocw.mit.e...ophysics ) and show me where there is an astrophysics theory that isn't based on valid repeatable physics experimentation

and i better repeat this too - do not confuse hypothesis with theory

i'll await your reply...
Captain Stumpy
5 / 5 (1) Apr 02, 2017
cont
Note you can validate man-made global warming all you want. But if it doesn't fit your agenda the uncertainty principle seems to creep in there somehow.
@seeker
this is completely irrelevant and stupid to boot

for starters, there is no agenda to the science
it simply is science

inferring that there is an agenda intimates your complete lack of understanding of physics or the requisite methodology whence the results come from

it is also considered conspiracist ideation and we all know that is based upon delusional and irrational thinking

feel free to make a concerted scientific assessment of the data and prove there is some agenda or conspiracy, but just make sure it adheres to the scientific method and can be validated - you will surely become the hero of the denier movement if you succeed

i won't go into this further as it's OT and irrelevant, so i will not continue on that topic
feel free to bring it up on a climate thread though
Seeker2
1 / 5 (1) Apr 02, 2017
@CS
cont
Note you can validate man-made global warming all you want. But if it doesn't fit your agenda the uncertainty principle seems to creep in there somehow.
@seeker
this is completely irrelevant and stupid to boot

for starters, there is no agenda to the science
it simply is science
I believe that. It's the people who use it. Or deny it. For starters.

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