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First Open Charm Tetraquark Discovered

Tetraquark
Tetraquark
Source : physicsworld

The LHCb experiment at CERN has developed a penchant for locating exotic combinations of quarks, the elementary particles that close to offer us composite particles like the more familiar proton & neutron. LHCb has observed several tetraquarks, which name suggests, are made from four quarks (or rather 2 quarks and 2 antiquarks). Observing these unusual particles helps scientists advance our knowledge of the strong force interaction , one among the four known fundamental forces within the universe. At a CERN seminar held virtually on 12 Aug, LHCb announced the 1st signs of a completely new quite tetraquark with a mass of 2.9 GeV/c²: the first such particle with just 1 charm quark .

First predicted to exist in 1964, scientists have observed 6 sorts of quarks (and their antiquark counterparts) within the laboratory: up, down, charm, strange, top and bottom. Since quarks cannot exist freely, they group to make composite particles: 3 quarks or 3 antiquarks form “baryons” just like the proton, while a quark and an antiquark form “mesons”.

The LHCb detector at the Large Hadron Collider (LHC) is dedicated to the study of B mesons, which contain either a bottom or an antibottom. Shortly after being produced in proton–proton collisions at the LHC, these heavy mesons transform—or “decay”—into a spread of lighter particles, which can undergo further transformations themselves. LHCb scientists observed signs of the new tetraquark in one such decay, during which the +ve(positively) charged B meson transforms into a positive D meson, a negative D meson and a positive kaon: B+→D+D−K+. In total, they studied around 1300 candidates for this particular transformation altogether the info the LHCb detector has recorded thus far .

The well-established quark model predicts that a number of the D+D− pairs during this transformation might be the results of intermediate particles—such because the ψ(3770) meson—that only manifest momentarily: B+→ψ(3770)K+→D+D−K+. However, theory doesn’t predict meson-like intermediaries leading to a D−K+ pair. LHCb were therefore surprised to ascertain a clear band in their data like an intermediate state transforming into a D−K+ pair at a mass of around 2.9 GeV/c², or around 3 times the mass of a proton.

The data are interpreted because the first sign of a new exotic state of 4 quarks: an anticharm, an up, a down and an antistrange. All previous tetraquark-like states observed by LHCb always had a charm–anticharm pair, leading to net-zero “charm flavor.” The newly observed state is that the first time a tetraquark containing a sole charm has been seen, which has been dubbed an “open-charm” tetraquark.

“When we first saw the surplus in our data, we thought there was an error ,” says Dan Johnson, who led the LHCb analysis. “After years of analyzing the info , we accepted that there really are some things surprising!”

Why is this important? It so happens that the jury remains out on what a tetraquark really is. Some theoretical models favor the notion that tetraquarks are pairs of distinct mesons bound together temporarily as a “molecule,” while other models like better to consider them as one cohesive unit of 4 particles. Identifying new sorts of tetraquarks and measuring their properties—such as their quantum spin (their intrinsic spatial orientation) and their parity (how they seem under a mirror-like transformation) – will help paint a clearer picture of those exotic inhabitants of the subatomic domain. Johnson adds: “This discovery also will allow us to stress-test our theories in a completely new domain.”

While LHCb observation is a crucial initiative , more data are going to be needed to verify the character of the structure observed within the B+ decay. The LHCb collaboration also will anticipate independent verification of their discovery from other dedicated B-physics experiments like Belle II. Meanwhile, the LHC continues to supply new and exciting results for experimentalists and theorists alike to probe.

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