Bottom quark
Composition:	Elementary particle
Statistical behavior:	Fermion
Group:	Quark
Generation:	Third
Interaction:	Strong, Weak, Electromagnetic force, Gravity
Symbol(s):	b
Antiparticle:	Bottom antiquark (b)
Theorized:	Makoto Kobayashi and Toshihide Maskawa (1973)[1]
Discovered:	Leon M. Lederman et al. (1977)[2]
Mass:	4.20+0.17−0.7 GeV/c2 (MS scheme)[3] 4.68+0.17−0.7 GeV/c2 (1S scheme)[3]
Decays into:	Charm quark, up quark
Electric charge:	−1⁄3 e
Color charge:	Yes
Spin:	1⁄2
Weak isospin:	LH: −1⁄2, RH: 0
Weak hypercharge:	LH: 1⁄3, RH: −2⁄3

The bottom quark is a third-generation quark with a charge of −¹⁄₃ e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large mass (around 4,200 MeV/c²,^[3] a bit more than four times the mass of a proton), combined with low values of the CKM matrix elements V_ub and V_cb, gives it a distinctive signature that makes it relatively easy to identify experimentally (using a technique called B-tagging). Because three generations of quark are required for CP violation (see CKM matrix), mesons containing the bottom quark are the easiest particles to use to investigate the phenomenon; such experiments are being performed at the BaBar and Belle experiments. The bottom quark is also notable because it is a product in almost all top quark decays, and would be a frequent decay product for the hypothetical Higgs boson if it is sufficiently light.

The bottom quark was theorized in 1973 by physicists Makoto Kobayashi and Toshihide Maskawa to explain CP-violation,^[1] discovered in 1977 by the Fermilab E288 experiment team led by Leon M. Lederman, when collisions produced bottomonium.^[2][4][5] Kobayashi and Maskwawa won the 2008 Nobel Prize in Physics for their explanation of CP-violation.^[6][7] On its discovery, there were efforts to name the bottom quark "beauty", but "bottom" became the predominant usage.

The bottom quark can decay into either an up or charm quark via the weak interaction. Both these decays are suppressed by the CKM matrix, making lifetimes of most bottom particles (~10⁻¹² s) somewhat higher than those of charmed particles (~10⁻¹³ s), but lower than those of strange particles (from ~10⁻¹⁰ to ~10⁻⁸ s).

Contents 1 Hadrons containing bottom quarks 2 References 3 Further reading 4 External links

[edit] Hadrons containing bottom quarks

Some of the hadrons containing bottom quarks include:

• B mesons contain a bottom quark (or its antiparticle) and an up or down quark.
• B_c and B_s mesons contain a bottom quark along with a charm quark or strange quark respectively.
• There are many bottomonium states, for example the ϒ meson. These consist of a bottom quark and its antiparticle.
• Bottom baryons have been observed, and are named in analogy with strange baryons (e.g. Λ0b).

[edit] References

1. ^ ^{a b} M. Kobayashi, T. Maskawa (1973). "CP-Violation in the Renormalizable Theory of Weak Interaction". Progress of Theoretical Physics 49 (2): 652–657. doi:10.1143/PTP.49.652. http://ptp.ipap.jp/link?PTP/49/652/pdf. 
2. ^ ^{a b} Fermilab (7 August 1997). "Discoveries at Fermilab - Discovery of the Bottom Quark". Press release. http://www.fnal.gov/pub/inquiring/physics/discoveries/bottom_quark_pr.html. Retrieved 2009-07-24. 
3. ^ ^{a b c} C. Amsler et al. (Particle Data Group) (2008). "Review of Particle Physics: Bottom Quark". Physics Letters B 667 (1–5): 1–1340, and 2009 partial update for the 2010 edition. http://pdg.lbl.gov/2009/listings/rpp2009-list-b-quark.pdf. 
4. ^ L.M. Lederman (2005). "Logbook: Bottom Quark". Symmetry Magazine 2 (8). http://www.symmetrymagazine.org/cms/?pid=1000195. 
5. ^ S.W. Herb et al. (1997). "Observation of a Dimuon Resonance at 9.5 GeV in 400-GeV Proton-Nucleus Collisions". Physical Review Letters 39: 252. doi:10.1103/PhysRevLett.39.252. 
6. ^ 2008 Physics Nobel Prize lecture by Makoto Kobayashi
7. ^ 2008 Physics Nobel Prize lecture by Toshihide Maskawa

[edit] Further reading

• L. Lederman (1978). "The Upsilon Particle". Scientific American 239 (4): 72. 
• J. Yoh (1997). "The Discovery of the b Quark at Fermilab in 1977: The Experiment Coordinator’s Story". Proceedings of Twenty Beautiful Years of Bottom Physics. Fermilab. http://lss.fnal.gov/archive/1997/conf/Conf-97-432-E.pdf. Retrieved 2009-07-24. 

[edit] External links

• History of the discovery of the bottom quark / Upsilon meson

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