25 Days of Particles: Day 16

Bottom Quark

Classification: quark, fermion
Fundamental: yes
Family: third
Mass: 4190 - 4670 MeV
Interactions: Weak, Electromagnetic (with charge -1/3), Strong, Gravity
Spin: 1/2
Lifetime: depends on meson, but often ~1e-12 s

The bottom, or beauty, or just b, quark is the down-type member of the third family. It was hypothesized in the mid-1970s before being discovered, which has become to typical method for particle discovery. By that time, with two families already known, postulating a third wasn't particularly radical. Also, having three generations of quarks allowed for a good mathematical theory describing CP violation, or when processes that we would expect to be symmetric (perhaps between matter and antimatter) turn out not to be symmetric. This can be mathematically described by the mixing between quarks, but only if there are three generations of them.

Of course, then you also have to have the b quark's partner. But the b is lighter and was discovered first, so I'm talking about it first.
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Many of the mesons that contain b quarks display CP violation in their decays, making them excellent ways to study CP violation and its possible effect on the balance of matter and antimatter in the universe. Several experiments such as Babar (from b-b-bar, and yes their slogan involved an elephant) and Belle specialized in producing and studying b mesons for exactly that reason.

The b quark also plays in an important role in many other physics processes. Since the b is fairly heavy, heavier things tend to decay into it. Also, b hadrons have a long lifetime compared to other hadrons, which means the b quarks created by the heavier things tend to travel away from where they were created before decaying. The decay of the b quark produces a jet of hadrons. So physicists studying this can find jets that originate a distance away from the main particle interaction, and be fairly confident that those jets are from b quarks and not from the other, lighter quarks. This is called b-tagging, and it gives physicists another way to either select interesting particle events from the data or reject things that they don't want to study. So even those of us who don't like to study jets salute those brave persons who look at the dozens of particles in a jet that splatter in the detector and try to sort out where they came from. 'Tis a difficult, difficult task.