Physicists discover new class of pentaquarks

Large Hadron Collider
Large Hadron Collider dipole magnets. Credit: CERN

Tomasz Skwarnicki, professor of physics in the College of Arts and Sciences at Syracuse University, has uncovered new information about a class of particles called pentaquarks. His findings could lead to a new understanding of the structure of matter in the universe.

Assisted by Liming Zhang, an associate professor at Tsinghua University in Beijing, Skwarnicki has analyzed data from the Large Hadron Collider beauty (LHCb) experiment at CERN's Large Hadron Collider (LHC) in Switzerland. The experimental physicist has uncovered evidence of three never-before-seen pentaquarks, each divided into two parts.

"Until now, we had thought that a pentaquark was made up of five [called quarks], stuck together. Our findings prove otherwise," says Skwarnicki, a Fellow of the American Physical Society.

Skwarnicki is part of a team of researchers, including members of Syracuse's High-Energy Physics (HEP) Group, studying fundamental particles and forces in the Universe. Most of their work takes place at the CERN laboratory, whose LHC is the biggest, most powerful particle detector in the world.

It is within the LHC that protons are flung together at high energies, only to collide with one another. What lies inside the particles, when cracked open, helps scientists probe the mysteries of the fundamental universe.

Studying from 2015-18, Skwarnicki has confirmed the existence of substructure within a pentaquark. The giveaway, he says, was a trio of narrow peaks in the LHC kinematic data.

Each peak refers to a particular pentaquark—specifically, one divided into two parts: a baryon, containing three quarks, and a meson, with two quarks.

A peak also suggests resonance, a short-lived phenomenon during , in which one unstable particle transforms into several others. Resonance happens when protons (a type of baryon) meet—or, more accurately, glide into one another—during an LHC collision.

What is unique about each of these three pentaquarks is that its mass is slightly lower than the sum of its parts—in this case, the masses of the baryon and meson. "The pentaquark didn't decay by its usual easy, fall-apart process," Skwarnicki says. "Instead, it decayed by slowly and laboriously rearranging its quarks, forming a narrow resonance."

Understanding how interact with and bind together is Skwarnicki's specialty. In 2015, he and then Ph.D. student Nathan Jurik G'16, Distinguished Professor Sheldon Stone and Zhang made headlines with their role in LHCb's detection of a pentaquark. Theorized a half century earlier, their discovery drew on LHC data from 2011-12.

LHCb's latest data utilized an energy beam that was nearly twice as strong. This method, combined with more refined data-selection criteria, produced a greater range of proton collisions.

"It also gave us 10 times more data and enabled us to observe pentaquark structures more clearly than before," Skwarnicki says. "What we thought was just one pentaquark turned out to be two narrow ones, with little space between them."

The data also revealed a third "companion" pentaquark. "All three pentaquarks had the same pattern—a baryon with a meson substructure. Their masses were below appropriate the baryon-meson thresholds," he adds.

Skwarnicki's discovery occurred relatively fast, considering that LHCb stopped collecting data less than three months ago.

Eric Sedore, associate CIO for infrastructure services in Information Technology Services (ITS), played a supporting role. His Research Computing Team provided the necessary computer firepower for Skwarnicki to achieve his goals.

In addition to Skwarnicki and Stone, HEP includes Professors Marina Artuso and Steven Blusk and Assistant Professor Matthew Rudolph. The group currently is building an apparatus on campus called the Upstream Tracker (UT), being shipped to and installed at CERN next year as part of a major LHCb upgrade.

"The UT will significantly enhance LHCb, which is composed of about 10 different sub-detectors. I am hopeful that the UT will lead to more discoveries," says Skwarnicki, adding that Artuso and Stone are the UT Project's leader and deputy, respectively.

Skwarnicki is excited about LHCb because it helps explain how the smallest constituents of matter behave. His latest discovery, for instance, proves that pentaquarks are built the same way as protons and neutrons, which are bound together in the nucleus of an atom.

"Pentaquarks may not play a significant role in the matter we are made of," he says, "but their existence may significantly affect our models of the matter found in other parts of the universe, such as neutron stars."


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CERN's LHCb experiment reports observation of exotic pentaquark particles

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Mar 26, 2019
Just thinking about quarks' quirks makes my head hurt...

Like why they have 1/3 or 2/3 the charge of 'indivisible' electrons or positrons...
D'uh...

Mar 27, 2019
Like why they have 1/3 or 2/3 the charge of 'indivisible' electrons or positrons...
D'uh...
It has long been speculated that electrons might be composite items. Here's a paper of 30 years ago by a Nobel Prize winner, Prof. Hans Demehlt pdfs.semanticscholar.org/f325/735eb19efcbada86521981f19332498421e0.pdf
More recently, ensembles of electrons in certain quantum states behave as though having 1/3 'charge' https://www.scien...-nature/

Back in 1982, seeming fractional charge was determined for the electron, under special conditions.

Mar 27, 2019
adendum: This, of course, leads naturally to a more normalized view that electrons and quarks have (integral) charges of 3, 2 and 1, and that there exists a naked, massless, spinless, particle and its antiparticle having only the property of charge (of 1) and the ability to pair/bind with one or two more of its own.

Mar 27, 2019
Better article on the LHCb site: http://lhcb-publi...l#Pentaq

Has a link to the presentation they made at the ongoing quark physics conference; I haven't seen that yet.

Mar 27, 2019
The Moriond presentation is a PowerPoint, and is damn near impenetrable unless you're a particle physicist. I may eventually glean some data from it if this is interesting enough.

But quite frankly, it's much more interesting as an indicator to some new physics than it is in and of itself. And I don't mean Earth-shattering stuff; just more details on how quarks and hadrons can interact, rather than some wild new paradigm or something.

The structure is duuc^c, that is, a down, two ups, a charm, and an anticharm. In other words, among other possible interpretations, a proton and a psi/J meson. There are indications in the data that this may in fact be a charmonium nucleus, that is, a proton and a psi/J particle associated briefly by the strong force, until the psi/J decays. There's probably some data in the Moriond PowerPoint about the decay paths.

Confidence is very high; these are results in excess of 7-sigma, far beyond the confidence level needed to announce a discovery.

Mar 28, 2019
SD, I am way over my head here. Reading this article is humbling realization of my lack of understanding for the Physics & supporting technology.

But I an a wordsmith, to everybody's annoyance.
What has attracted my curiosity is the choice of terminology in the following paragraph,

"... What is unique about each of these three pentaquarks is that its mass is slightly lower than the sum of its parts ..."

Could you please explain what you think they meant? I doubt if my opinion of "lower mass" is the same as the researcher's intended.

Mar 28, 2019
What they're saying is that these particles are less likely to decay than their constituents. Just like nuclei do, when the constituent particles team together, they lose mass, which must be added back for them to decay. Assuming I'm right about the charmonium, this means it's stable until the psi/J particle decays.

Mar 28, 2019
yeek! but DS, I guess a more correct question would be "What do they lose mass to?" ???
Tnless it's one of those weird quantum disappearening & reappearing at inconvenient moments sort of things.

If it is an involved answer?
Which honestly, I probably wouldn't understand anyway!
Please put up a link or perhaps a grade-school textbook you would suggest for my level of perpetual baffle.

Please don't waste any more of your time than you have to spare.

Thank you - R

Mar 28, 2019
Just a photon, I would expect. Same as protons lose when they fuse into a helium nucleus.

Mar 28, 2019
I dunno, I suspect that damn cat is somehow involved. Thank you for a clear & concise explanation.
I'll go wiki themthere proton thingofmanobs & how that fuse changing handyworks.

Mar 28, 2019
"Fuse" is a reference to fusion. It's the verb that describes what happens when hydrogen fuses to helium. Hydrogen nuclei are protons.

Mar 29, 2019
This comment has been removed by a moderator.

Mar 29, 2019
sorry DS, changing the fuse/fusion was another bad joke & a reference to my antiquity with ancient technology.

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