A joint Fermilab/SLAC publication
Technicians work inside ALICE
Courtesy of CERN

Scientists search for origin of proton mass


Only 1% of the mass of the proton comes from the Higgs field. ALICE scientists examine a process that could help explain the rest.

When protons and nuclei inside the Large Hadron Collider smash directly into each other, their energy can transform into new types of matter such as the famed Higgs boson, known for its association with a field that gives fundamental particles mass. But when nuclei merely graze each other, a different amazing thing happens: They generate some of the strongest magnetic fields in the universe. 

These ultra-intense magnetic fields are enabling nuclear physicists to peer inside atoms to answer a fundamental question: How do protons get most of their mass?

Protons are made up of fundamental particles called quarks and gluons. The quarks in protons are very light, and, as far as scientists know, gluons have no mass at all. Yet protons are much heavier than the combined masses of the three quarks they each contain.

“There is a lot of publicity about the origin of mass because of the Higgs boson,” says Dmitri Kharzeev, a nuclear theorist with a joint appointment at Stony Brook University and the Department of Energy’s Brookhaven National Laboratory. “But the Higgs is responsible for the mass of the quarks. The rest of it has a different origin.”

The origin of mass

The quarks in protons are very light, accounting for only about 1% of the proton’s overall mass. The plausible—yet still unproven—theoretical explanation for this discrepancy is related to how quarks move through the vacuum.

This vacuum is not empty, says Sergei Voloshin, a professor at Wayne State University and a member of the ALICE experiment at CERN. The vacuum is actually filled with undulating fields that constantly burp particle-antiparticle pairs into and out of existence. 

The three quarks that give protons their identity are forever jostling with these ethereal particle-antiparticle pairs. When one of these quarks gets too close to a vacuum-produced antiquark, it is annihilated and disappears in a burst of energy.

But the proton doesn’t wither and die when its quark is zapped out of existence; rather, the partner quark from the vacuum-produced particle-antiparticle pair steps in and takes the annihilated quark’s place (a plot twist straight out of The Talented Mr. Ripley). 

Scientists think that this incessant interchange of quarks is responsible for making a proton appear more massive than the sum of its quarks.

“Ninety-nine percent of mass might originate from this process of chirality-flipping in the vacuum.”

A matter of handedness

From the outside, not much appears to change in this swap. The annihilated quark is immediately replaced by a seemingly identical twin, making this process difficult to observe. Luckily for LHC scientists, they are not exactly identical: Quarks, like people, can be left- or right-handed, a concept called chirality.

Chirality is related to a quantum mechanical property called spin and roughly translates to whether the quark spins clockwise or counterclockwise as it moves along a particular direction through space. (Visualize beads spinning as they slide along a wire.) 

Because of the properties of the vacuum, the replacement quark will always have the opposite handedness from the original. That constant flipping of quarks from one handedness to the other is how theorists explain the majority of the proton’s mass.

“Ninety-nine percent of mass might originate from this process of chirality flipping in the vacuum,” Kharzeev says. “When we step on a scale, the number we see might be the result of these chirality-flipping transitions.”

Physics inside a magnetic field

In 2004, when Kharzeev was the head of the Nuclear Theory Group at Brookhaven Lab, he had an idea for how they could experimentally search for evidence of quark chirality flipping, which had never been observed. 

Because quarks are charged, they should interact with a magnetic field. “Normally, we never think about this interaction, because the magnetic fields we can create in the laboratory are extremely weak compared to the strength of quarks’ interactions with each other,” Kharzeev says. “However, we realized that when charged ions are colliding, they are accompanied by an electromagnetic field, and this field can be used to probe the chirality of quarks.”

When they did the math, they found that positively charged ions grazing each other inside a particle collider like the LHC will generate a magnetic field two orders of magnitude stronger than the one at the surface of the strongest magnetic field known to exist. This would be enough to override the quarks’ strong attraction to each other.

“Measuring the magnetic field’s strength and its lifetime was the primary goal of a recent ALICE data analysis,” says Voloshin. “The study yielded somewhat unexpected results, but they were still consistent with the existence of the strong magnetic field required for sorting of quarks according to their handedness.”

Within a strong magnetic field, a quark’s motion is no longer random. The magnetic field automatically sorts quarks according to their chirality, with their handedness steering them toward either the field’s north or south pole.

A hearty, hot soup of quarks

It’s nearly impossible to catch a quark flipping its chirality inside a proton, Kharzeev says.

“Inside a proton, left-handed quarks transition into right-handed quarks, and right-handed quarks transition back into left-handed quarks,” he says. “We will always see a mixture of left- and right-handed quarks.”

To study whether quark chirality flipping happens, physicists need to catch several large and unexpected imbalances between the number of right- and left-handed quarks. 

Luckily, heavy nuclei collisions produce the perfect conditions for quarks to change their handedness. When two nuclei hit each other at high speeds, their protons and neutrons melt into a quark-gluon plasma, which is one of the hottest and densest materials known to exist in the universe. The liberated quarks swimming through this plasma can shift their identities with ease.

“It’s like pretzels before they’re baked,” Kharzeev says. “You can easily mold the dough and change the twist.” 

The vacuum of space is not homogeneous—there are knots of gluon field that preferentially twist these doughy quarks one way or the other. If chirality flipping is happening, then scientists should catch an imbalance in the number of left- and right-handed quarks that shoot out from the plasma.

“The average handedness over all the collisions should be the same,” Kharzeev says, “but the fluctuations from collision to collision should be very large; we should see some quark-gluon plasmas that are preferentially righted-handed and others that are preferentially left-handed.” Due to the presence of magnetic field, the handedness of the plasma translates into observable charge asymmetry of produced particles—this is the “chiral magnetic effect” proposed by Kharzeev.

Shortly after Kharzeev proposed the idea of sorting quarks according to their handedness in the strong magnetic field of colliding nuclei, Voloshin designed a way to test this theory using the ALICE experiment, whose US participation is funded by the Department of Energy. The initial results show evidence for quarks sorting themselves according to chirality. But more research needs to be done before scientists can be sure.