A baryon is a composite subatomic particle. It is made up of three quarks. The most stable baryons are protons and neutrons, so most building blocks of matter are baryons.
Baryon comes from the Greek word βαρύς (barys) for heavy. At the time of their naming, most known elementary particles had lower masses than the baryons.
Quarks and Hadrons
Quarks have many properties, including electric charge, mass, color charge, and spin. There are six types of quarks, known as flavors: up, down, strange, charm, top, and bottom quarks. Up and down quarks have the lowest masses of all quarks. Only up and down quarks are stable; all heavier quarks decay very quickly into these.
For every quark flavor, there is a corresponding type of antiparticle, known as an antiquark. So each baryon has a corresponding antiparticle, an antibaryon. Here, quarks are replaced by their corresponding antiquarks.
A composite particle made of quarks is called a hadron (Greek: ἁδρός, hadrós, “stout, thick”). Baryons belong to the hadron family of particles. Baryons (3 quarks) and mesons (2 quarks) as well as exotic conditions such as pentaquarks with 5 quarks, dibaryon with 6 quarks belong to the hydron family. For reasons of symmetry, particles of only 1 quark are not allowed.
Baryon with Two Heavy Quarks
In July 2017, physicists at CERN first demonstrated a baryon with two heavy quarks [1]. It consists of two heavy charm quarks and a light up quark and is double positively charged. It is the first proof of a baryon with two equally heavy quarks. The existence of such doubly charged baryon has been considered possible since the 1970s, but it has never been demonstrated so far.
The researchers discovered the baryon named Ξcc++ using the Large Hadron Collider beauty (LHCb) detector at the Large Hadron Collider (LHC) at CERN. The new particle has a mass of 3,621 megaelectron volt (MeV). This makes it about four times as heavy as a proton. The reason for this is the two heavy charm quarks, each weighing 1.275 MeV.
[1] Observation of the doubly charmed baryon Ξcc++, Phys. Rev. Lett. 2017. submitted (Link)
Also of interest
- What is a Dibaryon?, ChemistryViews.org 2015.
