χb (3P) discovered at the ATLAS

The ATLAS detector at the CERN confirmed the existence of another particle predicted by the Standard Model, this one much less prolific than its apotheosized cousin, the Higgs boson, on December 22. It's the Upsilon meson, which is a coupled state of the bottom quark and the bottom antiquark. Such a coupling makes the meson a quarkonium, and so it also assumes the name of bottomonium.

Such a flavourless meson, also called a χ (chi) particle, exists at various excited states, or excitation modes, just like a hydrogen atom, and it decays to each state by the radiation of a photon (γ). The Upsilon meson was first observed in 1977 in an experiment headed by Leon Lederman at the Fermilab, Chicago, with a mass of 9.46 GeV/c². The ATLAS discovery corresponds to a higher excitation state, χ_b (3P), wherein the particle's mass is 10.539 ± 0.004 GeV/c².

[caption id="attachment_21088" align="aligncenter" width="600" caption="The spike in activity corresponds to a detection. The Upsilon meson was observed in three independent channels."][/caption]

Like the electronic orbitals in an atom, each decay state is designated as belonging to an S, P or D energy-state, and the discovery is that of a χ_b (3P): a flavourless meson at the 3P excitation mode. In the second diagram, the particle shows up as the right-most peak on the purple (lowermost) curve, corresponding to ~10.5 GeV on the x-axis. It hasn't shown up in earlier experiments because only the Large Hadron Collider has been able to supply the high rate of collisions required to record the particle's existence before it decays in 1.21×10⁻²⁰ seconds (at 9.46 GeV/c²).

The corresponding pre-print paper submitted by the ATLAS Collaboration is available here.