Twice the Charm: A New Creature Joins the Particle Zoo

If you’re in need of some charming news and a break from weather-related disaster coverage, this is your story. Today, in the American Physical Society’s journal Physical Review Letters, a several-hundred-member team of CERN researchers announced the first unambiguous sighting of a baryon with two heavy quarks.

So what exactly does that mean?

Matter, all the visible stuff around us, is made of atoms. Atoms are made of three subatomic particles: protons, neutrons, and electrons. As far as we can tell, electrons are fundamental particles—they can’t be broken down into smaller components. However, protons and neutrons are in a class of particles called baryons that are composed of three fundamental particles of a different kind, called quarks.

Quarks come in six creatively named types, or flavors, including up, down, charm, strange, top (also known as truth), and bottom (also known as beauty). Different groupings of quarks lead to different baryons. For example, a proton has two up quarks and one down quark (uud). A neutron has two down quarks and one up quark (ddu). The up and down quarks are the lightest and most stable of the six. The heavier ones have fleeting lifetimes and we only see them in high energy collisions. The charm, top, and bottom quark are much more massive than the other three, so they are often referred to as the heavy quarks.

According to particle physics theory, baryons can be composed of three quarks of any type, with the exception of top quarks because they don’t live long enough to form baryons. However, despite years of searching scientists have only observed baryons with zero or one heavy quark—they have never definitively* detected a baryon with two or three heavy quarks. Until now.

In the new journal article, the LHCb collaboration at CERN shares their discovery of “a highly significant structure” in their data—one that represents a baryon composed of two charm quarks and one up quark (ccu). In their paper, the researchers refer to the baryon with the greek letter Ξcc++
(pronounced “sai” or “zai”).

An illustration of the newly discovered baryon.   
Image Credit: CERN

LHCb, which stands for the Large Hadron Collider beauty experiment is one of the four large experiments based at the CERN laboratory near Geneva, Switzerland. With this long-running experiment, a team of more than 800 scientists supported by more than 400 technicians and engineers study particles that contain bottom (beauty) or charm quarks as a way of learning about the differences between antimatter and matter. For more on the LHCb, check out this 2015 Buzz post, Surprises from the LHC’s “Beauty factory”.

Although this doubly charmed baryon was predicted years ago, finding it was quite a challenge; the particle’s lifetime is a tiny fraction of a second, and it is only produced in high energy collisions. This means that the discovery requires finding just the right signal amidst all of the chaos that results when beams of protons traveling at nearly the speed of light collide. The LHCb detector registers millions of proton collisions per second, each one producing particles that can decay into other particles.

To comb through this huge amount of data, LHCb relies on a state-of-the-art electronic system. To put it simply, scientists study and simulate the collisions they would like to analyze. Then, they use this information to optimize a sophisticated computer program, telling it what to look for. During data collection, sub-detectors within LHCb measure the properties of particles produced in real collisions. The computer program acts like a gatekeeper, retaining only the events that look like they might match what the scientists are interested in analyzing.

The new baryon was discovered in data taken in 2016, confirmed through cross-checks, and then verified in data taken by LHCb under different conditions. From the experimental results, the scientists determined the mass of the baryon to be about four times heavier than the mass of a proton. Its mass and other measured properties match theoretical predictions for this particle well, another good sign.

Part of the fun in this story is that it’s a departure from news stories that impact daily life as dramatically as hurricanes and earthquakes, at least in some concrete sense. However, this discovery contains important information for scientists trying to put together the story of our universe and the fundamental particles of which we are made. LHCb collaborators are optimistic that studying this new baryon will help us better understand how quarks interact, how matter is held together, and how baryons with two heavy quarks behave. This, in turn, should help us detect even more of them.

[*The term “definitively” is included in this statement because a 2002 experiment at Fermilab saw evidence of a baryon with two charm quarks and one down quark (ccd), but some of its properties were outside of expectations and the finding has yet to be verified by other experiments. Scientists are working to reconcile this finding with the new one from CERN, but so far they seem at odds.]

Kendra Redmond

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