Twice the Charm: Long-Lived Exotic Particle Discovered at Large Hadron Collider

 

An artist’s impression of Tcc+, a tetraquark composed of two charm quarks and an up and a down antiquark. Credit: CERN

 

Discovery of a new mysterious hadron having two charm quarks and an up and a down antiquark.

 

The Large Hadron Collider (LHC) experiment at CERN revealed a new discovery at the European Physical Society Conference on High Energy Physics (EPS-HEP). The newly discovered particle by LHC, labeled as Tcc+, is a tetraquark – an exotic hadron comprising of two quarks and two antiquarks. It is the longest-lived exotic matter particle discovered, and the first one to contain two heavy quarks and two light antiquarks.

 

Quarks are the vital building blocks from which matter is built. They combine to form hadrons, i.e. baryons, such as the proton and the neutron, which contain three quarks, and mesons, which are formed as quark-antiquark pairs. In recent times a number of so-called exotic hadrons – particles with four or five quarks, instead of the two or three – have been discovered. Today’s discovery is of a principally unique exotic hadron, an exotic hadron if you like.

 

The new particle comprises of two charm quarks and an up and a down antiquark. Numerous tetraquarks have been discovered in recent times (including one with two charm quarks and two charm antiquarks), but this is the first ever that contains two charm quarks, without antiquarks to stable them. Scientists call this “open charm” (in this case, “double open charm”). Particles having a charm quark and a charm antiquark have “hidden charm” – the charm quantum number for the entire particle adds up to zero, just like a positive and a negative electrical charge do. At this time the charm quantum number adds up to two, so it has twice the charm!

 

The quark content of Tcc+ has other exciting features in addition to being open charm. It is the first particle discovered that fit in a class of tetraquarks with two heavy quarks and two light antiquarks. These particles decay by converting into a pair of mesons, each from one of the heavy quarks and one of the light antiquarks. According to some theoretical hypothesis, the mass of tetraquarks of this particular type should be very close to the total masses of the two mesons. Such proximity in mass causes the decay “difficult”, causing an extended lifetime of the particle, and indeed Tcc+ is the longest-lived exotic hadron ever discovered.

 

The discovery paves the way for a quest for heavier particles of the same particular type, with one or two charm quarks substituted by bottom quarks. The particle with two bottom quarks is particularly exciting: according to estimations, its mass should be smaller than the total of the masses of any pair of B mesons. This would make the decay not only improbable, but in fact forbidden: the particle would not be able to decay through the solid interaction and would have to do so via the feeble interaction instead, which would make its lifetime many orders of magnitude longer than any previously discovered exotic hadron.

 

 

References:

https://scitechdaily.com/twice-the-charm-long-lived-exotic-particle-discovered-at-large-hadron-collider/#:~:text=The%20new%20particle%20discovered%20by,quarks%20and%20two%20light%20antiquarks.

https://www.livescience.com/double-charm-tetraquark-found-lhc.html