An “impossible” particle adds a piece to the puzzle of strong force

This spring at At a meeting of the quark physics group at Syracuse University, Ivan Polyakov announced that he had discovered the fingerprints of a semi-mythical particle.

“We said, ‘It’s impossible. What mistake are you making? ‘ ”, Recalls Sheldon Stone, the leader of the group.

Polyakov left and rechecked his analysis of the data from the Large Hadron Collider (LHCb) beauty experiment of which the Syracuse group is a part. The proof held. He showed that a particular set of four fundamental particles called quarks can form a tight clique, contrary to the belief of most theorists. The LHCb collaboration reported the discovery of the composite particle, dubbed the double-charm tetraquark, at a conference in July and in two papers published earlier this month that are currently under peer review.

The unexpected discovery of the Double Charm Tetraquark brings to light an uncomfortable truth. While physicists know the exact equation that defines the strong force – the fundamental force that binds quarks together to form protons and neutrons in the hearts of atoms, as well as other composite particles like tetraquarks – they rarely can solve this strange endless iterative equation. , they therefore find it difficult to predict the effects of the strong force.

The tetraquark now presents theorists with a solid target against which to test their mathematical machine to get closer to the strong force. Refining their approximations is the main hope for physicists to understand how quarks behave inside and outside atoms, and to distinguish the effects of quarks from the subtle signs of new fundamental particles that physicists are looking for.

Quark cartoon

The odd thing about quarks is that physicists can approach them at two levels of complexity. In the 1960s, grappling with a zoo of newly discovered composite particles, they developed the cartoonish “quark model,” which simply says that quarks come together in complementary sets of three to form the proton, neutron and other baryons, while pairs of quarks constitute various types of meson particles.

Gradually, a deeper theory known as quantum chromodynamics (QCD) emerged. He painted the proton as a bubbling mass of quarks linked together by entangled chains of particles of “gluons”, the carriers of powerful force. Experiments have confirmed many aspects of CDQ, but no known mathematical technique can systematically unravel the central equation of theory.

Either way, the quark model can replace the much more complicated truth, at least when it comes to the menagerie of baryons and mesons discovered in the 20th century. But the model failed to anticipate the ephemeral tetraquarks and five-quark “pentaquarks” that began to appear in the 2000s. These exotic particles surely come from QCD, but for nearly 20 years theorists have been arguing. ask how.

“We just don’t know the model yet, which is embarrassing,” said Eric Braaten, a particle theorist at Ohio State University.

The newest tetraquark sharpens the mystery.

It appeared in the debris of about 200 collisions from the LHCb experiment, where protons smash 40 million times per second, giving quarks countless opportunities to frolic in any way nature allows them. Quarks come in six mass “flavors”, with heavier quarks appearing more rarely. Each of these 200 or so collisions generated enough energy to produce two charming-flavored quarks, which weigh more than the light quarks which contain protons, but less than the gigantic “beauty” quarks which are the main quarry of LHCb. The middleweight charmed quarks also came close enough to attract each other and chain into two light antiquarks. Polyakov’s analysis suggested that the four quarks clustered for 12 sextillionths of a second before an energy fluctuation evoked two more quarks and the cluster disintegrated into three mesons.

About Roberto Frank

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