First Open Charm Tetraquark Discovered

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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