Discovery Of Double Charm Tetraquark ‘The impossible particle’

Colorful illustration in which a sphere divided into four quadrants is surrounded by spheres divided into halves and thirds.]
A simple model can explain groupings of two or three quarks, but it fails to explain tetraquarks.
Cr: Samuel Velasco/Quanta Magazine

Ivan Polyakov revealed this spring at a meeting of Syracuse University’s quark physics group that he had discovered the fingerprints of a semi-mythical particle.

Polyakov left to confirm his data analysis from the Large Hadron Collider beauty (LHCb) experiment, which the Syracuse group is a part of. The proof was sufficient. It demonstrated that, contrary to popular opinion, a specific set of four fundamental particles known as quarks can make a close clique. At a symposium in July, the LHCb consortium announced the finding of the composite particle, named the double-charm tetraquark, as well as two publications published earlier this month that are currently under peer review.

Quarks, the particles that make up the tetraquark, are some of the most fundamental building blocks of matter, and they come in six flavors, each with its unique mass and charge: up, down, top, bottom, strange, and charm. Physicists have recently identified lots of new tetraquarks, but this one which is a mix of two antimatter quarks and two charms is the first “doubly charmed” one. This indicates that it has two charm quarks but no charm antiquarks to even them out.

Impacts of the Discovery

The unanticipated finding of the double-charm tetraquark reveals an unpleasant truth. while physicists realize the exact equation that defines the strong force. This strong force is known as the fundamental force that binds quarks together to shape neutrons and protons in the hearts of atoms, as well as other composite particles like tetraquarks, which hardly ever solve this extraordinary, infinitely repeated equation, making the expectation of strong force effects difficult.

The tetraquark now gives theorists a physical goal to measure their mathematical machinery against in order to approximate the strong force. Physicists’ biggest hope for knowing how quarks act inside and outside atoms — and for separating quark effects from small signals of new fundamental particles that physicists are investigating — is to fine-tune their approximations.

The discovery advances our insight into the underlying fundamental laws that regulate how these unusual particles interact and also lays the possibility for the discovery of even heavier exotic hadrons in the future.

Quark Cartoon

The strange thing about quarks is that they can be approached at two varying degrees of complexity by physicists. Faced with a zoo of recently found composite particles in the 1960s, they devised the cartoonish “quark model,” which states clearly that quarks bunch up together in supplementary groups of three to form the neutron, proton, and other so-called baryons, while pairs of quarks form multiple kinds of “meson” particles.

However, quantum chromodynamics (QCD) arose as a stronger theory throughout time. It showed the proton as a roiling mass of quarks bound together through tangled threads of “gluon” particles, the strong force’s carriers. Many parts of QCD have been validated by experiments, but no known mathematical approaches have been able to thoroughly untangle the theory’s core equation.

It was discovered in the wreckage of about 200 collisions at the LHCb experiment, where protons collide 40 million times per second, providing quarks untold possibilities to play in all the manners that nature allows. There’s enough production of energy by each of the 200 collisions to create two charm-flavored quarks, which are lighter than the massive “beauty” quarks and heavier than the light quarks that make up protons that are the LHCb’s primary target. The middleweight charm quarks also came near enough to entice each other and attract two light antiquarks to join them. According to Polyakov’s calculations, the four quarks bonded for 12 sextillionths of a second until an energy fluctuation produced two more quarks, and the grouping split into three mesons.

That’s an eternity for a tetraquark. Past tetraquarks comprised quarks and their equally opposing heavy antiquarks, and they inclined to escalate thousands of times quicker. The research team was astonished by the new tetraquark’s production and consequent durability, as they expected charm quarks to engage each other even weaker than quark-antiquark pairs that bond more ephemeral tetraquarks. It’s a new piece of information in the puzzle of the strong force.

Rules of Thumb for Quark

Jean-Marc Richard, presently at the Institute of Physics of the Two Infinities in Lyon, France, was one of the few theorists who foresaw why two charm quarks would interact.

He and two colleagues examined a simple quark model in 1982 and discovered that four quarks would form two pairs — mesons. A quark pair can waltz together in the same way as a proton and electron can. When they added two more, the newcomers tend to obstruct the bond, diminishing the attraction and hamstringing the aggregate particle.

However, theorists discovered a flaw: lopsided quartets can cling together if the heavier pair can ignore the weaker pair. How distorted would the masses have to be, was the question? What was humiliating was the inability to understand the pattern [of tetraquarks and pentaquarks].

According to additional research by Richard and a colleague, it is not required to reach all the way to the heaviest massive quarks; a tetraquark might be anchored by a pair of middleweight charm quarks. However, other extensions of the quark model anticipated various critical thresholds, raising doubts about the reality of the double-charm tetraquark.

The LHCb experiment has now confirmed that charm quarks can connect a tetraquark together. (scientists estimate that two mesons would win out if the composite particle had just one-hundredth of a percent more mass.) Theorists now have a new standard to compare their models to.




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