New Data Casts Doubt Over Newly-Discovered 'Tetraquark'

Mar 28, 2016

A schematic shows the decay channels of the four-flavored tetraquark. New data analysis has cast doubt on the particle's discovery.
Credit Fermi National Accelerator Laboratory /

Physicists around the world were thrilled last month when news broke about a new type of subatomic particle—the so-called “tetraquark”, an unheard-of arrangement of four different types of quarks.

But last week, scientists at the Large Hadron Collider near Geneva, Switzerland, looked at their own data, and they’ve said they’ve tried and failed to find concurring evidence.

An Indiana University physicist who helped lead the original research is, for the moment, standing by the findings.

The existence of the four-flavored tetraquark, also known as X(5568), was posited by a group of researchers called the DZero Experiment. DZero worked off data from a particle accelerator at Chicago’s Fermilab. The discovery of X(5568) excited physicists because it offered a peek into how subatomic particles are held together, a notoriously difficult and confounding area of science.

That makes the news that scientists working at the LHC, the largest particle accelerator in the world, couldn’t find their own tetraquark evidence, disheartening.

“Yeah, it’s a little disappointing, it would have been nice if we would have seen it,” says Syracuse Professor Sheldon Stone, who led the analysis of the Large Hadron Collider data as part of a group of particle physicists called the Large Hadron Collider Beauty Experiment, or LHCb.

“Everybody would have been happy,” he says. “But we don’t see it.”

Stone says the LHC, which boasts a 17-mile circumference, gives physicists much more information to work with than that collected from the Fermilab’s four-mile Tevatron.

“We have 20 times more data than they do and we do not see any evidence of this state,” he says.

He continues: “Science is a human activity, people sometimes don’t get things quite right…It’s very possible to see something that isn’t really there.”

Stone attributes the DZero discovery to wishful thinking.

“If you’re not careful, you can enhance fluctuations in the data and make them seem more important than they really are.”

IU Physicist Daria Zieminska, a member of the DZero Experiment for three decades and lead of the tetraquark observation, says for now, she’s holding out hope for the tetraquark.

“There are several differences between the LHCb conditions and ours,” she says.

Zieminska says, for example, in order to study the particles, the LHC smashes different types of matter together than Fermilab does to study their decay (Fermilab used a proton and an antiproton. The LHC used two protons). Thus, it’s possible those conditions are more amenable to tetraquark formation than those at the LHC.

She also says the DZero scientists have only been studying a single so-called “decay channel” of the hypothetical tetraquark. More research on different decay channels might lead to more proof of its existence.

(A decay channel refers to smaller bits of energy the larger particle leaves behind when it breaks apart. The types of heavy particles created by large-scale accelerators such as Fermilab’s Tevatron are so short-lived scientists never actually witness them; they infer their existence from the leftover “daughter” and “granddaughter” particles the meson, or two-quark particle, leaves behind.)

“We are only looking at the B_s [meson], which is the heavy meson,” Zieminska says. “It decays in several different ways, we were looking only at one way, which is the cleanest and best choice,” she says.

She says DZero can’t re-do the experiment. “We cannot make a better selection in retrospect, it has to be applied blindly.”  However, “we have done so many tests that we are confident we see it in this particle channel, tracing back from a given set of particles, and reconstructing this state,” she says. Zieminska is confident further research will verify X(5568)’s existence.

Sheldon stone also isn’t ruling out the four-flavored tetraquark.

“I think the community believes it is a yet-to-be-discovered state,” he says. “We’re very secure in our beliefs that we did it right. We will continue on our quest.”