Particle physicists measure the spin contribution of the proton's antiquark

August 14, 2014 by Jennifer Chu
The STAR detector, used in the researchers' experiment, measures the energy and angle of the electron from the W boson decay produced in the proton collision. Credit: STAR Collaboration

What causes a proton to spin? This fundamental question has been a longstanding mystery in particle physics, although it was once thought that the answer would be fairly straightforward: The spin of a proton's three subatomic particles, called quarks, would simply add up to produce its total spin.

But a series of experiments in the 1980s threw this theory for a loop, proving that the spins of the quarks are only partially responsible for the 's overall spin. Thus emerged what physicists now refer to as the "proton spin crisis," prompting a decades-long search for the missing pieces, or contributors, to a proton's spin.

Now an international team of more than 300 researchers, including MIT physicists, has placed new constraints on the spin of the proton's antiquarks—the antiparticles of quarks that are thought to arise when the bonds between quarks break. The researchers say these measurements may help to identify the antiquark's role in the proton's spin, as well as the mechanism by which antiquarks are produced.

"We'd like to understand the spin contributions of the inside the proton, to learn something fundamental about their interactions," says Justin Stevens, a postdoc in MIT's Laboratory for Nuclear Science (LNS). "Now we have a new method sensitive to the antiquark spin, which can shed light on where these quarks and antiquarks come from."

Stevens and Jan Balewski, a research scientist in the LNS, led the analysis of more than 1 billion recorded produced by the Relativistic Heavy Ion Collider (RHIC), a particle accelerator at Brookhaven National Laboratory. Using the facility's STAR detector, which tracks the particles produced by each collision, the team identified 3,500 proton collisions that produced a W boson—an elementary particle that, when generated, temporarily inherits the spin of a proton's antiquark.

"We measured the decay product of the W boson, and from this, we could infer the spin of the antiquark, and how it relates to the spin of the mother proton," Balewski explains. "It turns out the antiquark polarization is marginal, and contributes very little to the polarization of the proton."

Stevens, Balewski, and their collaborators publish their experimental results today in the journal Physical Review Letters.

When protons collide

According to the Standard Model of particle physics, the proton is a composite particle composed of three quarks, each of a distinct type, or "flavor": two "up" quarks, and one "down" quark. These quarks are bound together by particles called gluons which, when temporarily broken, are thought to give rise to pairs of short-lived quarks and antiquarks. Since a proton's spin cannot be fully explained by the spin of its quarks, physicists have looked to other possible contributors, such as the spins of gluons and antiquarks, or their orbital motion inside the proton.

Stevens, Balewski, and their colleagues identified the antiquarks by measuring the decay of W bosons following collisions of polarized protons. As Balewski explains it, at any given moment, a polarized or spinning proton contains pairs of quarks and antiquarks that "pop out and disappear, pop out and disappear."

"When they show up, the spin of the proton is passed to those antiquarks to some degree," Balewski says. "Now this antiquark, for a fraction of a second, shows up in the proton exactly at the moment when this proton collides with this other proton, and the W boson is produced."

The researchers observed a significant difference in the number of W bosons produced when the proton's spin was oriented in the same direction as its motion compared to cases where the proton's spin was oriented in the opposite direction. The antiquark spin was then inferred by measuring this difference for various orientations of the electrons produced as the W boson decays.

Exploring antiquarks' origins

The researchers' results provide significant new constraints on the spin of the proton's antiquarks, which contribute a small fraction to the total spin of the proton. In addition, they observed a slightly larger-than-expected spin effect for a subset of up-flavored antiquarks compared to down-flavored antiquarks. Stevens says this asymmetry in antiquark spin may help to identify how a proton's antiquarks arise in the first place.

"The naive picture of how these quark/antiquark pairs pop in and out of existence is that gluons split to form these pairs," Stevens says. "But if that were the case, you'd expect to get equal numbers of up- and down-flavored quarks. Our measurements provide some new information, which could tell us something about how these and antiquarks are produced."

Marco Stratmann, a staff scientist at the Institute for Theoretical Physics at the University of Tubingen, says the group's data "provide us with 'snapshots' of the proton's spin and flavor structure. … In [future] analyses of the proton's structure, this data will provide a novel and particularly clean probe of the up and down antiquark polarizations, largely free of theoretical uncertainties."

"We're tightening up the constraints on what this antiquark polarization looks like," Stevens says. "And with future data, that constraint will get even better."

Explore further: New tool for proton spin

More information: Paper: "Measurement of Longitudinal Spin Asymmetries for Weak Boson Production in Polarized Proton-Proton Collisions at RHIC"

Related Stories

New tool for proton spin

May 6, 2011

How the particles that constitute a proton give rise to is to its rotation, or ‘spin’, is an intriguing open question of contemporary particle physics. A technique that could provide some answers has been developed ...

How does the proton get its spin?

February 17, 2010

( -- 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 ...

Quarks' spins dictate their location in the proton

April 2, 2013

A successful measurement of the distribution of quarks that make up protons conducted at DOE's Jefferson Lab has found that a quark's spin can predict its general location inside the proton. Quarks with spin pointed in the ...

Moving Quarks Help Solve Proton Spin Puzzle

September 11, 2008

( -- New theory work at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility has shown that more than half of the spin of the proton is the result of the movement of its building blocks: ...

Recommended for you

Tiny magnetic tremors unlock exotic superconductivity

June 26, 2017

Deep within solids, individual electrons zip around on a nanoscale highway paved with atoms. For the most part, these electrons avoid one another, kept in separate lanes by their mutual repulsion. But vibrations in the atomic ...


Adjust slider to filter visible comments by rank

Display comments: newest first

Phil DePayne
5 / 5 (9) Aug 14, 2014
Maybe your wrong
Aug 14, 2014
This comment has been removed by a moderator.
5 / 5 (10) Aug 14, 2014
"This my comment probably is not suitable for today's state of the science . . ."

One would be hard pressed to provide a more accurate assessment.
5 / 5 (7) Aug 14, 2014
I just 5'd Arom by accident. I need to go sit in my shame-corner now. :(
Aug 15, 2014
This comment has been removed by a moderator.
5 / 5 (3) Aug 15, 2014
Gluon spin - now there's a neat thought. If we can add up the spin contributions of the quarks, subtract that from the proton spin and find out the gluon spin contribution that may allow a calculation of the gluon exchange rate between quarks - which should be a fixed number (otherwise you would get a non-quantized spin for the proton).
3 / 5 (3) Aug 15, 2014
How about preons, a new idea on the block, another layer of matter's onion. Possible these layers will be found, one after another till we get to Planck scale. And even then !!?? Contention is that every stage smaller in size will have accession energies orders of magnitude higher. Think the difference of energies between nuclear and chemical; and then think of the energies of intranuclear reactions between quarks as being or orders higher than nuclear. Then move on to intraparticular dynamics of preons within quarks. To liberate and expose that energy may take a supercollider in space over 1000 miles in diameter with decillions of watts of power to access, but a generator of power from those reactions would produce new physics beyond the dreams of avarice.
5 / 5 (4) Aug 15, 2014
Antialias: We have attempted to measure the contribution from gluons within the proton too. It seems that to get the *total* spin of the proton you need not just the spins of the particles within, but also their several angular momenta. Somehow this whole bag of particles always has exactly spin 1/2 too. It's a real tough nut to crack because the maths are insane.
5 / 5 (4) Aug 15, 2014
Osiris1: I like this article about why preons, while not impossible, are unlikely.

Namely that the mass of preons can't be small (or we'd have seen them) so it must be big. And then the preons must be in a state that their masses almost, but not quite, exactly cancel out, leaving only the mass of a quark behind. Which... is questionable. Again, not impossible, just...
5 / 5 (1) Aug 15, 2014
Couldn't these same arguments be used against quarks. Ultraviolet confinement provides the answer.
Da Schneib
5 / 5 (1) Aug 16, 2014
Motl is kind of... unusual. He's also an AGW denier.
1 / 5 (2) Aug 16, 2014
You mean sociopath who calls himself a "humble correspondent"? Not quite accidentally he's also a supporter of Crimea annexation and Russian maskirovka.
5 / 5 (1) Aug 17, 2014
Sorry but I just cannot secretly read his blog and publicly defame him. Motl is like fugu sushi; once you resect his poisonous political positions you are treated with perfectly fine physics. This being said Zephir, I would probably be more critical if he was a compatriot.
4.5 / 5 (2) Aug 17, 2014
Shavera: it isn't a "bag of particles". And note http://en.wikiped...finement where you can read this: "as opposed to virtual ones found in ordinary hadrons". There are no real gluons in a proton. And see Matt Strassler's article on virtual particles: . They aren't popping in and out of existence, that's a popscience myth. For the spin ½ electron look to Dirac's belt: http://www.mathpa...h619.htm . For the proton it's similar but more complicated. See http://www.math.i...en/tqft/ and look at those blue trefoil knots at the top. Pick one, start at the bottom left, and trace around it anticlockwise calling out the crossing-over directions: up down up.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.