Image courtesy of ATLAS Collaboration
The mass spectrum of the bottomonium particle: the left-most peak is the so-called 1P state, the middle peak is the 2P state, and the right-most peak is the recently discovered 3P state.
Since the discovery of a Higgs-like boson was announced in 2012, it has garnered much of the attention afforded high energy physics. But other particles have been found as well, including a previously unseen excited state of the quark/anti-quark composite particle, bottomonium.
The newly-discovered 3P state of bottomonium joins a class of particles whose masses are well known. Knowledge of these experimentally-derived masses allows physicists to validate key theoretical simulation tools that can be applied to other problems in particle physics. Study of bottomonium and its excited states may lead to a better understanding of the strong force and the role it plays in holding quarks and atoms together.
When two quarks combine to make a particle, physicists call that particle a meson. Pions (symbol π) are the lightest mesons. They’re composed of various combinations of up and down quarks and their anti-quark partners. (Up and down quarks are the lightest quarks.) A special group of mesons are collectively called quarkonia. These are mesons composed of heavier quarks and their anti-quark partners. Bottomonium, for example, is composed of two quarks, a b quark and an anti-b quark. Because bottomonium has integer spin, it is a boson. But unlike the Higgs, it is not an elementary particle, but a composite particle whose total spin is integer. It is the strong force that binds these quarks together. The so-called 1P and the 2P states of bottomonium and their decay modes have been long known. The new particle is in the 3P state and like its sister states ultimately decays to two muons and a photon (although with different energies). Reconstructing the event leads to a mass of approximately 10.5 GeV for this new state. Knowledge of the masses of all these states allow physicists to test and refine their understanding of the strong force, because, after all, the Higgs may give mass to some fundamental particles, but it is the strong force that binds quarks, atoms, and ultimately the universe together.
Basic research: Office of Science High Energy Physics program.
ATLAS Collaboration, “Observation of a new χ b state in radiative transitions to Υ(1S) and Υ(2S) at ATLAS.” Phys. Rev. Lett. 108, 152001 (2012).
D0 Collaboration, ”Observation of a narrow mass state decaying into Υ(1S)+γ in p pbar collisions at √s=1.96TeV” Phys. Rev. D 86, 031103(R) (2012).
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