antibaryon پادباریون pâdbâriyon
*Fr.: antibaryon*
The → *antiparticle* of a → *baryon*. → *anti-*; → *baryon*. |

baryon باریون bâriyon (#)
*Fr.: baryon*
Any of the class of the heaviest → *subatomic particle*s that
includes → *proton*s, → *neutron*s,
as well as a number of short-lived particles
whose decay products include protons. Baryons obey the
→ *Fermi-Dirac statistics*. They form a subclass
of the → *hadron*s and
are further subdivided into → *nucleon*s
and → *hyperon*s. Gk. *barys* "heavy" + → *-on*, from "fermi*on*." |

baryon acoustic oscillation (BAO) نوش ِ صداییک ِ باریونی naveš-e sedâyik-e bâryoni
*Fr.: oscillation acoustique baryonique*
In cosmology, one of a series of peaks and troughs that are present in the power spectrum
of matter fluctuations after the → *recombination era*, and on large
scales. At the time of the Big Bang, and for about 380,000 years
afterwards, Universe was ionized and photons and baryons were tightly
coupled. Acoustic oscillations arose from perturbations in the
primordial plasma due to the competition between gravitational
attraction and gas+photons pressure. After the epoch of recombination, these
oscillations froze and imprinted their signatures in both the
→ *CMB* and matter distribution. In the case of the photons,
the acoustic mode history is manifested as the high-contrast Doppler peaks in the
temperature anisotropies. As for baryons, they were in a similar
state, and when mixed with the non-oscillating → *cold dark matter*
perturbations, they left a small residual imprint in the clustering of
matter on very large scales, ~100 *h*^{-1}Mpc (*h* being the
→ *Hubble constant*
in units of 100 km s^{-1} Mpc^{-1}).
The phenomenon of BAOs, recently discovered
using the Sloan Digital Sky Survey data, is a confirmation of the current model of cosmology.
Like → *Type Ia supernova*e, BAOs
provide a → *standard candle* for determining cosmic distances.
The measurement of BAOs is therefore a powerful new technique for probing how
→ *dark energy* has affected the expansion of the Universe
(see, e.g., Eisenstein 2005, New Astronomy Reviews 49, 360; Percival et al. 2010, MNRAS 401,
2148). → *baryon*; → *acoustic*;
→ *oscillation*. |

baryon asymmetry ناهمامونی ِ باریون nâhamâmuni-ye bariyon
*Fr.: asymmétrie baryonique*
The observation that in the present → *Universe*
there is → *matter* but not much
→ *antimatter*. Observations do not show the presence of
galaxies made of antimatter, nor gamma rays are observed
that would be produced if large entities of
antimatter would undergo → *annihilation*
with matter. However, the → *early Universe*
could have been baryon symmetric, and for some reason the matter excess has
been generated, through some process called → *baryogenesis*.
→ *Sakharov conditions*. → *baryon*; → *asymmetry*. |

baryon number عدد ِ باریونی adad-e bâriyoni (#)
*Fr.: nombre baryonique*
1) The difference between the total number of → *baryon*s and
the total number of → *antibaryon*s in a system of
→ *subatomic particle*s.
It is a measure of → *baryon asymmetry* and is
defined by the quantity
η = (*n*_{b} - *n*_{b-})/*n*_{γ},
called the → *baryon-photon ratio*,
where *n*_{b} is the → *comoving* number
density of baryons, *n*_{b-} is the number of
antibaryons, and *n*_{γ} is that of photons. The value of η for
the → *cosmic microwave background radiation (CMBR)*
has been very well determined by the → *WMAP* satellite to be
η = (6.14 ± 0.25) x 10^{-10}. The baryon number is assumed to be
constant. The photons created in
stars amount to only a small fraction, less than 1%, of those in the CMBR.
2) A property of an → *elementary particle* represented by a
→ *quantum number*. It is
equal to +1 for a baryon and -1 for an antibaryon.
→ *Boson*s, → *lepton*s, and
→ *meson*s have a baryon number B = 0.
→ *Quark*s and → *antiquark*s
have baryon numbers of B = +1/3 and -1/3, respectively.
The baryon number is → *conserve*d in all observed types of
particle-particle interaction. → *baryon*; → *number*. |

baryon-photon ratio وابر ِ باریون-فوتون vâbar-e bâriyon-foton
*Fr.: rapport baryon-photon*
The → *baryon number* compared with the number of photons in the
→ *Universe*. The baryon-photon ratio can be estimated in a
simple way. The
→ *energy density* associated with
→ *blackbody radiation* of → *temperature*
*T* is *aT*^{4}, and the mean energy per photon is
~*kT*. Therefore, the number density of blackbody photons for *T* = 2.7 K is:
*n*_{γ} = *aT*^{4}/*kT* = 3.7 x 10^{2}
photons cm^{-3}, where *a* = 7.6 x 10^{-15}
erg cm^{-3} K^{-4} (→ *radiation density constant*)
and *k* = 1.38 x 10^{-16} erg K^{-1}
(→ *Boltzmann's constant*). The number density of baryons can be
expressed by ρ_{m}/*m*_{p},
where ρ_{m} is the mass density of the Universe and
*m*_{p} is the mass of the → *proton*
(1.66 x 10^{-24} g). → *CMB*
measurements show that the baryonic mean density is ρ_{m} =
4.2 x 10^{-31} g cm^{-3} (roughly 5% of the
→ *critical density*). This leads to the value of ~ 2 x 10^{-7}
for the number density of baryons.
Thus, the baryon/photon ratio is approximately equal to
η = *n*_{b}/*n*_{γ} =
2 x 10^{-7}/3.7 x 10^{2} ~ 5 x 10^{-10}. In other words,
for each baryon in the Universe there is 10^{10} photons. This estimate
is in agreement with the precise value of the baryon-photon ratio
6.14 x 10^{-10} derived with the → *WMAP*.
Since the photon number and the baryon number are conserved,
the baryon-photon ratio stays constant as the Universe expands. → *baryon*; → *photon*;
→ *ratio*. |

baryonic dark matter مادهی ِ سیاه ِ باریونی mâde-ye siyâh-e bâriyoni
*Fr.: matière noire baryonique*
→ *Dark matter* made up of → *baryon*s
that are not luminous enough to produce any detectable radiation. It is
generally believed that most dark matter is → *non-baryonic*.
The baryonic dark matter could reside in a number of forms, including cold gas and
compact objects. → *baryonic*; → *dark*;
→ *matter*. |

baryonic matter مادهی ِ باریونی mâde-ye bâriyoni (#)
*Fr.: matière baryonique*
Ordinary matter composed of → *baryon*s, i.e.
→ *proton*s and → *neutron*s,
as distinct from → *non-baryonic*, exotic forms. Adj. of → *baryon*; → *matter*. |

non-baryonic dark matter مادهی ِ سیاه ِ ناباریونی mâde-ye siyâh-e nâbâriyoni
*Fr.: matière noire non-baryonique*
→ *Dark matter* composed of
→ *non-baryonic* particles. → *non-*; → *baryonic*;
→ *dark*; → *matter*. |

non-baryonic matter مادّهی ِ ناباریونی mâdde-ye nâbâriyoni (#)
*Fr.: matière non-baryonique*
Matter that, unlike the ordinary matter, is not made of baryons
(including the neutrons and protons). It is proposed as a possible
constituent of dark matter. → *non-*; → *baryonic matter*. |

photon-baryon plasma پلاسما فوتون-باریون plâsmâ foton-bâriyon
*Fr.: plasma photon-baryon*
The plasma filling space before the → *recombination epoch*
that mainly consisted of → *cosmic microwave background radiation*
photons, electrons, protons, and → *light element*s. → *photon*; → *baryon*;
→ *plasma*. |