Capital "H", lower case "b".
Avoid the tautology "Higgs-boson particle". A boson is a particle; there is no need to repeat yourself.
Named after the English physicist Peter Higgs, the Higgs boson is a subatomic particle whose existence is predicted by the theory which unified the weak and electromagnetic interactions.
The particle provides a mechanism called the Brout-Englert-Higgs mechanism to explain how subatomic particles get mass: they interact with the Higgs field. On 4 July 2012, the ATLAS and CMS experiments at CERN announced they had each observed a new particle consistent with the Higgs boson at a mass of around 126 gigaelectronvolts (GeV).