Radiation astronomy/Baryons

From Wikiversity
Jump to navigation Jump to search

A baryon is a composite subatomic particle bound together by the strong interaction, whereas leptons are not. The most familiar baryons are the protons and neutrons that make up most of the mass of the visible matter in the universe. Electrons (the other major component of the atom) are leptons. Each baryon has a corresponding antiparticle (antibaryon).

Baryonic matter is matter composed mostly of baryons (by mass), which includes atoms of any sort (and thus includes nearly all matter that may be encountered or experienced in everyday life).

Omegas[edit]

Ω0
c
has a rest mass of 2695.2 ± 1.7 MeV/c2.[1]

Ω
b
has a rest mass of 6071 ± 40 MeV/c2.[1]

Xis[edit]

Ξ0 has a rest mass of 1314.86 ± 0.20 MeV/c2.[1]

Ξ has a rest mass of 1321.71 ± 0.07 MeV/c2.[1]

Ξ0
b
has a rest mass of 5787.8 ± 5.0 ± 1.3 MeV/c2.[1]

Sigmas[edit]

Σ+ has a rest mass of 1189.37 ± 0.07 MeV/c2.[1]

Σ0 has a rest mass of 1192.642 ± 0.024 MeV/c2.[1]

Σ
b
has a rest mass of 5815.5 +0.6 −0.5 ± 1.7 MeV/c2.[1]

Lambdas[edit]

Λ0 has a rest mass of 1115.683 ± 0.006 MeV/c2.[2]

Λ+
c
has a rest mass of 2286.46 ± 0.146 MeV/c2.[2]

On 13 July 2015, the LHCb collaboration at CERN reported results consistent with pentaquark states in the decay of bottom Lambda baryons (Λ0
b
).[3]

Rest mass = 5619.4 ± 0.6 MeV/c2.

Deltas[edit]

Δ++ has a rest mass of 1,232 ± 2 MeV/c2.[2]

Δ+ has a rest mass of 1,232 ± 2 MeV/c2.[2]

Δ0 has a rest mass of 1,232 ± 2 MeV/c2.[2]

Δ has a rest mass of 1,232 ± 2 MeV/c2.[2]

Neutrons[edit]

The neutron is a subatomic hadron particle which has the symbol n or n0
, no net electric charge and a mass slightly larger than that of a proton.

Outside the nucleus, free neutrons are unstable and have a mean lifetime of 885.7±0.8 s (about 14 minutes, 46 seconds); therefore the half-life for this process (which differs from the mean lifetime by a factor of ln(2) = 0.693) is 613.9±0.8 s (about 10 minutes, 11 seconds).[4] Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay:[5]

n0
=> p+
+ e
+ ν
e

Because free neutrons are unstable, they can be obtained only from nuclear disintegrations, nuclear reactions, and high-energy reactions (such as in cosmic radiation showers or accelerator collisions).

The neutron has a negatively charged exterior, a positively charged middle, and a negative core.[6]

Antiprotons[edit]

The antiproton (p, pronounced p-baer) is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived since any collision with a proton will cause both particles to be annihilated in a burst of energy.

Antiprotons have been detected in cosmic rays for over 25 years, first by balloon-borne experiments and more recently by satellite-based detectors. The standard picture for their presence in cosmic rays is that they are produced in collisions of cosmic ray protons with nuclei in the interstellar medium, via the reaction, where A represents a nucleus:

p +  A → p + p + p + A

The secondary antiprotons (p) then propagate through the galaxy, confined by the galactic magnetic fields. Their energy spectrum is modified by collisions with other atoms in the interstellar medium. The antiproton cosmic ray energy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions.[7]

Canal rays[edit]

In 1886, Eugen Goldstein discovered canal rays (also known as anode rays) and showed that they were positively charged particles (ions) produced from gases. However, since particles from different gases had different values of charge-to-mass ratio (e/m), they could not be identified with a single particle, unlike the negative electrons discovered by J. J. Thomson.

Protons[edit]

The proton is a subatomic particle with the symbol p or p+
and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number.

Nucleon spin structure describes the partonic structure of proton intrinsic angular momentum (spin). The key question is how the nucleon's spin, whose magnitude is 1/2ħ, is carried by its [suggested] constituent partons (quarks and gluons). In the late 1980s, the European Muon Collaboration (EMC) conducted experiments that suggested the spin carried by quarks is not sufficient to account for the total spin of [protons]. This finding astonished particle physicists at that time, and the problem of where the missing spin lies is sometimes referred to as the "proton spin crisis".

Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) and the COMPASS experiment at CERN, experiments E154 and E155 at [SLAC National Accelerator Laboratory] SLAC, HERMES at DESY, experiments at [Thomas Jefferson National Accelerator Facility] JLab and RHIC, and others. Global analysis of data from all major experiments confirmed the original EMC discovery and showed that the quark spin [may] contribute about 30% to the total spin of the nucleon.

New measurements performed by European scientists reveal that the radius of the proton is 4 percent smaller than previously estimated.[8]

References[edit]

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 J. Beringer et al. (2012) and 2013 partial update for the 2014 edition: Particle summary tables – Baryons
  2. 2.0 2.1 2.2 2.3 2.4 2.5 C. Amsler et al. Particle Data Group (2008). "Review of Particle Physics". Physics Letters B 667 (1): 1. doi:10.1016/j.physletb.2008.07.018. 
  3. R. Aaij et al. LHCb collaboration (2015). "Observation of J/ψp resonances consistent with pentaquark states in Λ0
    b
    →J/ψKp decays". Physical Review Letters 115 (7). doi:10.1103/PhysRevLett.115.072001.
     
  4. K. Nakamura et al. (Particle Data Group), JP G 37, 075021 (2010) and 2011 partial update for the 2012 edition
  5. Particle Data Group Summary Data Table on Baryons
  6. G.A. Miller (2007). "Charge Densities of the Neutron and Proton". Physical Review Letters 99 (11): 112001. doi:10.1103/PhysRevLett.99.112001. 
  7. Dallas C. Kennedy (2000). "Cosmic Ray Antiprotons". Proc. SPIE 2806: 113. doi:10.1117/12.253971. https://books.google.com/books?hl=en&lr=&id=8tnDViJoOIYC&oi=fnd&pg=PA438&ots=7WbrnBWJDS&sig=W5AePyLLDvDbnJd43a8wcBRCYe8#v=onepage&f=false. 
  8. Proton's radius revised downward. ScienceNews. 23 February 2013. Retrieved 22 April 2013.