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Primordial Black Holes and Cosmological Phase Transitions Report ...

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PBHs <strong>and</strong> <strong>Cosmological</strong> <strong>Phase</strong> <strong>Transitions</strong> 41<br />

A baryon consists of a triplet of quarks. This means that we have 35 different<br />

possible combinations13 . The most common baryons in the present universe are<br />

nucleons, i.e., protons <strong>and</strong> neutrons. The proton consists of two up quarks <strong>and</strong><br />

one down quark (uud) <strong>and</strong> the neutron consists of one up quark <strong>and</strong> two down<br />

quarks (udd).<br />

Hyperons are baryons containing at least a strange quark, but no charm<br />

or bottom quarks (e.g. Σ− ,Σ0 ,Σ + ,Ξ− ,Ξ0 ,Λ0 ,Ω− ). Charmed baryons are<br />

baryons containing at least a charm quark, but no bottom quarks (e.g. Ξ + c ,Ξ0c ,<br />

Ξ + cc ,∆+ c ,Ω0c ). Bottom baryons are baryons containing at least a bottom quark<br />

(e.g. Ξ −<br />

b ,Ξ0b ,∆0b ).<br />

Some of the 35 triplets have never been observed (e.g. ubb, sbb). However,<br />

the number of known baryons is much larger than the number of different quark<br />

triplets. That is due to the existence of many excited states for each configuration.<br />

For example, in the case of the proton (uud) there are at least 25 known<br />

excited states (e.g. N(2190) + with mass ≈ 2190 MeV, N(1710) + with mass<br />

≈ 1710 MeV, ∆ + with mass ≈ 1232 MeV) 14 . On table 8 we show the most<br />

stable known baryons.<br />

Antibaryons are triplets made of antiquarks. For each baryon there is an<br />

antibaryon, which is an antiparticle with the same mass <strong>and</strong> opposite electric<br />

charge, obtained by replacing each quark by the corresponding antiquark15 .<br />

For example, the antiparticle of the proton (uud), is the antiproton (ūū ¯ d), an<br />

antibaryon with m ≈ 938.272 MeV <strong>and</strong> e = −1.<br />

13 The 35 different quark triplets are: uuu, uud, uus, uuc, uub, udd, uds, udc, udb, uss, usc,<br />

usb, ucc, ucb, ubb, ddd, dds, ddc, ddb, dss, dsc, dsb, dcc, dcb, dbb, sss, ssc, ssb, scc, scb, sbb,<br />

ccc, ccb, cbb <strong>and</strong> bbb. The top quark was left outside because the probability of formation of<br />

a top baryon is negligibly small.<br />

14 For a complete list of currently known baryons (including excited states) see<br />

http://pdg.lbl.gov/2007/tables/contents tables.html.<br />

15 Baryons are matter (they are made of quarks) <strong>and</strong> antibaryons are antimatter (they are<br />

made of antiquarks). The same idea does not apply to mesons <strong>and</strong> antimesons. That is<br />

because a meson (or antimeson) consists of a quark–antiquark pair.

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