# Spin Structure of Protons and Neutrons

##### October 10, 2005

Normally, we think of building blocks as static objects. For instance, the brick and mortar used to build the local bank remain pretty much the same from the day it's built to the day it's torn down. But the building blocks of ordinary matter are different. These bricks are particles called quarks, and the mortar is made up of particles called gluons. Quarks and gluons are in constant motion, taking part in a complicated dance to build protons and neutrons (also called nucleons).

Image: An artist's depiction of quarks as spinning tops inside protons, with the protons embedded in a nucleus.

Ian Cloet, a theorist at Jefferson Lab and the Special Research Centre for the Subatomic Structure of Matter at the University of Adelaide, and his colleagues wondered how this quark-gluon dance was affected by the environment around the nucleons in which the quarks and gluons reside. In particular, they wanted to know how the spin of the quarks and gluons may be modified by the environment around the nucleons they're embedded in and how this modification may affect the spin of the nucleons themselves.

To find out, Cloet and his colleagues, Wolfgang Bentz at Tokai University in Japan and JLab's Tony Thomas, calculated the spin-dependent structure functions of nucleons. These probability distributions provide information on how the quark spins are organized inside the nucleon, and from these distributions, it is possible to determine what fraction of the nucleon spin comes from the quarks and what fraction comes from the gluons.

"The spin-dependent structure function tells us about the spin content of the nucleon. So we know the proton has spin one-half. All the constituents have to add to give you spin one-half, and the structure functions give you information on how that happens," Cloet says.

The calculations took into account two different environments where nucleons are commonly found: inside the nucleus and outside. The researchers calculated the spin-dependent structure functions for a nucleon inside the nucleus and for a free nucleon - one outside the nucleus. They then compared these structure functions in ratio form. This method revealed that the spin content of a nucleon inside the nucleus is different from one outside.

"So what this is telling us is that how the gluons' and quarks' spins add together to give spin one-half is different for a bound nucleon than it is for a free nucleon," Cloet says. That means that while a nucleon's spin may remain constant, the proportion of that spin contributed by its constituents, the quarks and gluons, may change as the environment around nucleons change. "The spin of the nucleon has to be spin one-half, but how you can get one-half can vary," Cloet explains.

This result comes as something of a surprise. "It was thought that the skin of the proton would expel this force from the other nucleons. It just really wouldn't get inside and affect the quarks. People expected the small-scale structures of the nucleon to remain mostly the same whether they're inside or outside the nucleus," Cloet remarks.

He says the presence of other nucleons inside the nucleus causes this difference. One model, the Quark-Meson Coupling Model, assumes that the quarks inside nucleons in a nucleus interact through the exchange of mesons. This model regards nucleons inside a nucleus less like billiard balls and more like squirmy bags that may be modified by other nucleons in the nucleus around them.

"The idea is that this meson field generated by all the other nucleons is felt by the quarks inside the original proton. And this is changing their properties, and therefore changes these structure functions," Cloet explains, "Outside the nucleus, there are no mesons really interacting with these quarks."

He says that while the new calculation shows a clear difference in how quarks and gluons contribute to a nucleon's spin, it doesn't reveal the exact makeup of these contributions. And the finding hasn't yet been backed up by experiment. But that could change. One of the goals of the 12 GeV Upgrade project at Jefferson Lab is to measure the origin of the basic properties of nucleons, including mass, size, electric field, magnetic field, and spin.

In the meantime, Cloet says this is the first calculation of the spin-dependent structure functions of nucleons inside the nucleus. He says this discovery is giving theorists new insight into Quantum Chromodynamics (QCD). QCD is a theory that describes the force, the strong force, that binds quarks into nucleons and nucleons into nuclei.

Cloet says this newest calculation demonstrates that the nucleus isn't a dull, well understood object: it still has many secrets, and investigating its properties is pushing the boundaries of physicists' understanding of QCD. It also indicates that the nucleus is far from a simple collection of protons and neutrons, but is truly a complex system of interacting quarks and gluons.

Source: Thomas Jefferson National Accelerator Facility

Explore further: Quarks' spins dictate their location in the proton

## Related Stories

#### 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 ...

#### Quarks take wrong turns

April 13, 2004

Physicists peering inside the neutron are seeing glimmers of what appears to be an impossible situation. The vexing findings pertain to quarks, which are the main components of neutrons and protons. The quarks, in essence, ...

#### The first supercomputer simulations of 'spin–orbit' forces between neutrons and protons in an atomic nucleus

July 25, 2014

Protons and neutrons are held together at the center of an atom by powerful nuclear forces. A theory that can describe the interaction between just two of these subatomic particles could potentially be extended to predict ...

#### New measurement of electron–quark scattering

February 5, 2014

From matching wings on butterflies to the repeating six-point pattern of snowflakes, symmetries echo through nature, even down to the smallest building blocks of matter. Since the discovery of quarks, the building blocks ...

#### G-Zero Finds that Ghostly Strange Quarks Influence Proton Structure

June 20, 2005

In research performed at the Department of Energy's Jefferson Lab, nuclear physicists have found that strange quarks do contribute to the structure of the proton. This result indicates that, just as previous experiments have ...

#### HAPPEx results hint at strangely magnetic proton

April 21, 2005

New results from research performed at the Department of Energy's Jefferson Lab hint that strange quarks may contribute to the proton's magnetic moment. If confirmed by data to be taken later this year, these surprising results ...

## Recommended for you

#### New Horizons returns last bits of 2015 flyby data to Earth

October 28, 2016

NASA's New Horizons mission reached a major milestone this week when the last bits of science data from the Pluto flyby – stored on the spacecraft's digital recorders since July 2015 – arrived safely on Earth.

#### Researchers uncover the origin of atmospheric particles

October 28, 2016

In a study led by the University of Leeds, scientists have solved one of the most challenging and long-standing problems in atmospheric science: to understand how particles are formed in the atmosphere.

#### More than 15,000 near-Earth objects and counting

October 28, 2016

The international effort to find, confirm and catalogue the multitude of asteroids that pose a threat to our planet has reached a milestone: 15 000 discovered – with many more to go.

#### How planets like Jupiter form

October 28, 2016

Young giant planets are born from gas and dust. Researchers of ETH Zürich and the Universities of Zürich and Bern simulated different scenarios relying on the computing power of the Swiss National Supercomputing Centre ...

#### Novel light sources made of 2-D materials

October 28, 2016

Physicists from the University of Würzburg have designed a light source that emits photon pairs, which are particularly well suited for tap-proof data encryption. The experiment's key ingredients: a semiconductor crystal ...

#### Physicists make it possible to 3-D print your own baby universe

October 28, 2016

Researchers have created a 3D printed cosmic microwave background - a map of the oldest light in the universe - and provided the files for download.