How does the proton get its spin?

February 17, 2010 by Anne Trafton, Massachusetts Institute of Technology

Artist's rendering of the inner structure of a proton showing quarks and quark-antiquark pairs, with springs showing the gluons that bind the quarks. Image courtesy of Klaus Rith
( -- At a meeting this week of the American Physical Society in Washington, MIT Associate Professor of Physics Bernd Surrow reported on new results from the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) that provide a better understanding of the internal structure of the proton, the basic building block of all nuclei.

The world’s only polarized collider, at Brookhaven National Laboratory in Upton, N.Y., RHIC is used by MIT physicists to understand how the proton gets its , a fundamental quantum mechanical property (spin manifests itself as an intrinsic , a property that is the basis of magnetic resonance imaging, or MRI). In 2009, spin-polarized protons were collided in RHIC at a record high center-of-mass energy of 500 giga electron volts (GeV). At this high energy — an energy 250 times the mass of the two individual protons making the collision — the protons are moving essentially at the and the inside the proton are able to “see” each other at a resolution that is very small compared to the size of the proton. This allows scientists to study the proton's internal structure.

Nobody has yet succeeded in performing a decomposition for the proton spin in terms of its constituent quarks and . The accompanying cartoon shows a model of how complicated the “simple” proton actually is; its structure arises through the strong force and is described by the of quarks and gluons known as quantum chromodynamics (QCD). This theory has thus far been unable to explain the origin of proton spin, so new insight is obtained from experiments. It has been established that the quarks themselves account for only about 25 percent of the proton’s spin and previous Brookhaven data provided by Surrow’s team indicate that the gluon’s contribution is also small.

Protons have a spin of ½, a number whose simplicity is compelling, considering that the proton is made up of several constituent particles.

The results presented in Washington this week have established a new way to explore the spin structure of the proton by using the weak interaction, which is responsible for radioactive β-decay (a process that converts a neutron into a proton while emitting an electron and an electron antineutrino). The weak interaction is mediated by very massive (about 80 GeV) particles known as W bosons. At RHIC, the longitudinal polarization of the colliding polarized proton beams at high energies allows the experiments to directly observe the weak interactions by detecting the decay electrons of the W bosons produced. This process gives rise to a large parity violating signal. (Parity violation means that physics results are different in processes occurring in left- and right-handed systems.) Such a signal has now been established for the first time by careful measurement of the spin-dependent cross section under the leadership of Surrow's group at the MIT Laboratory for Nuclear Science. The STAR experiment is well suited to carry out such measurements because of its large acceptance for the detection of the decay electron produced by the W boson. In addition, it can discriminate against background processes from the strong interaction.

The production of W bosons provides an ideal tool to study the quark spin structure of the proton. W bosons are produced in quark-antiquark collisions and can be detected through their respective decay electrons. The analysis distinguishes the electric charge sign of the decay product, which provides a direct view of the quark polarization at high energies where fundamental calculations are well under control. The results from Surrow’s group clearly establish the different polarization patterns of up and down quarks. They are consistent with fundamental calculations within the Standard Model (SM) of particle physics. This new technique allows direct sensitivity to the polarization of anti-quarks.

Surrow’s group is also developing new tracking detectors that will greatly increase the ability to detect the parity-violating W events in further data-taking planned for 2011/2012. These measurements will focus on measuring the polarization of the anti-quarks, which only live fleetingly in the proton.

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4 / 5 (4) Feb 17, 2010
The Title? I don't see the answer. So I presume this article is about studies into the question.
3.7 / 5 (6) Feb 17, 2010
Another article that fails to answer its titular question...

If the question isn't going to be answered in the text, why ask the question at all? Especially in the title.

It would be more aptly titled "We still don't know how the proton gets its spin. Despite the latest research..."
4.2 / 5 (5) Feb 17, 2010
There is no definitive rule stating that if you ask a question in a title it must be answered in the following text. The title is valid - they're asking the question and following with how they will attempt to answer it with new insights. I'd have more of a problem with them titling it along the lines of a statement like "How the Proton Gets its Spin".

It will be interesting to see what the final results are in any case.

3 / 5 (2) Feb 17, 2010
The title is valid - they're asking the question
I admit, the "measurement of the spin-dependent cross section during collisions of spin-polarized protons" isn't so catchy name for most of laymans. Such title works like packet of product in cheap supermarket: it's promising more, then it provides, but some people prefer to be cheated.
3.5 / 5 (6) Feb 18, 2010
Why all this critique and nit-picking of how the article is written? The title is in the form of a rhetorical question, which is a way of telling you the aim of the research. Are you just poking it with a stick because you don't understand what it is? And don't bother pointing out that I'm critiquing your critique, it just that I see these type of responses often at this site.
not rated yet Feb 18, 2010
Noumenon, I think the major issue is we get excited by the title, thinking that the problem has been solved, then we are let down when the article is not what we expected.

Not so much the fault of the writing as it is a fault of the way we interpret it.

I appreciate these kinds of articles, I'd just rather not have my hopes go up with the title, and down with the article.
2.3 / 5 (3) Feb 18, 2010
Geez! I can imagine how disappointed you would be with an article titled "Is there a God?" or "Are we alone in the Universe?"...
Feb 18, 2010
This comment has been removed by a moderator.
not rated yet Feb 18, 2010
Well, there you can find something about subject...

5 / 5 (2) Feb 18, 2010
My question is:

How do they get those tiny, very tiny springs into the proton?

And, what are they made of?
not rated yet Feb 18, 2010
Another explanation for the 1/2-spin could be a much simpler model of the proton, like the one described on http://classicala...ons.html

But the convention requires quarks and gluons, of course...
not rated yet Feb 18, 2010
How do they get those tiny, very tiny springs into the proton?
They're supposedly formed by density fluctuations of every dense fluid, the extremely dense nuclear fluid in particular. We can observe them inside of supercritical fluid, for example.


be a much simpler model of the proton
At the case of electron, the Mobius strip model was proposed a long time ago already (Wasserman, Spec.Sc.Tech.(1992)). This model was detailed later:

IMO strip model of hadrons correspond the path of quarks triplets which are revolving each other like system of gravitating bodies. More massive down-quarks (3.5–6.0 MeV/c2) are concentrated bellow up-quark (1.5–3.3 MeV/c2) near the center of neutron, like inside of gravitationaly coupled Eefimov state of three massive bodies of different mass.

not rated yet Feb 18, 2010
Indeed this model doesn't explain 1/2 spin of particles, but the coincidence of charge value of electron with those of calculated indicates, Mobius strip is quite close model of internal structure of particles. At the case of proton particle is composed of three 2/3 parts of Mobius strips, which are representing valence quarks. These parts are joined in characteristic ratios, described by CKM matrix of mixing angles.

not rated yet Feb 18, 2010
There real is no such thing as "spin" in the physical sense for a proton..

Why dont they just call it a magnetic field???
Which is what it is.
not rated yet Feb 19, 2010
But proton has no magnetic field around it, until it doesn't move with respect to observer.
not rated yet Feb 19, 2010
There real is no such thing as "spin" in the physical sense for a proton..

Look at quantum rotation operators - spin means that 'quantum phase' (whatever it means...) makes something like this
on each plane orthogonal to spin axis, what through Aharonov-Bohm effect connects with its internal magnetic structure ... for me it also automatically leads to proton's internal structure without any problems with spin (4th section of ) which also while deep inelastic scattering would look to be made of three parts which carry small part of its mass and fulfilling asymptotic freedom ...
Feb 20, 2010
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not rated yet Feb 20, 2010
What I like is that our poor twin travels to a distant system taking longer then we expect and coming back less aged than we yet the proton travels almost at the speed of light, still takes the expected amount of time, and doesn't drop into an energy level less than a proton. Go figure?

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