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Web address:
http://www.sciencedaily.com/releases/2012/12/
121213085106.htm
12 Matter Particles Suffice in Nature: Limited Number of Fermions in Standard
Model, Physicists Say
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Based on the data produced by big detectors like CMS at CERN, Geneva, the
structure of matter can be studied in detail. (Credit: KIT/Markus Breig)
Dec. 13, 2012 How many matter particles exist in nature? Particle physicists
have been dealing with this question for a long time. The 12 matter particles
contained in the standard model of particle physics? Or are there further
particles with too high a mass to be produced by the experiments performed so
far? These questions are now answered by researchers of KIT, CERN, and Humboldt
University in the current issue of the Physical Review Letters.
Matter particles, also called fermions, are the elementary components of the
universe. They make up everything we see on earth or through telescopes. "For a
long time, however, it was not clear whether we know all components," explains
Ulrich Nierste, Professor at KIT. The standard model of particle physics knows
12 fermions. Based on their similar properties, they are divided into three
generations of four particles each. Only the first generation of particles
occurs in appreciable amount outside of particle accelerators. Among these
particles are the electron, the electron neutrino, and the up-quark and
down-quark. Up- and down-quarks form heavier particles, such as protons and
neutrons and, hence, all elements of the periodic system.
"But why does nature have second and third generations, if these are hardly
needed? And are there maybe more generations of particles?," ask the main
authors of the article, Martin Wiebusch and Otto Eberhardt. At least, the
latter question is answered: "There are exactly three fermion generations in
the standard model of particle physics!"
For their analysis, the researchers combined latest data collected by the
particle accelerators LHC and Tevatron with many known measurements results
relating to particles, such as the Z-boson or the top-quark. The result of the
statistical analysis is that the existence of further fermions can be excluded
with a probability of 99.99999 percent (5.3 sigma). The most important data
used for this analysis come from the recently discovered Higgs particle.
The Higgs particle gives all other particles their mass. As additional fermions
were not detected directly in accelerator experiments, they have to be heavier
than the fermions known so far. Hence, these fermions would also interact with
the Higgs particle more strongly. This interaction would have modified the
properties of the Higgs particle such that this particle would not have been
detected. With the exclusion of the fourth fermion generation the first open
question of particle physics is now answered by the measurements made at the
new LHC particle accelerator ring of CERN.
"Within the standard model the number of fermions is now firmly established,"
explains Nierste. However, some interesting questions remain. The properties of
the just discovered Higgs particle still have to be determined and it has to be
found out why there is more matter than antimatter in the universe.
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Story Source:
The above story is reprinted from materials provided by Karlsruhe Institute
of Technology.
Note: Materials may be edited for content and length. For further
information, please contact the source cited above.
Journal Reference:
Otto Eberhardt, Geoffrey Herbert, Heiko Lacker, Alexander Lenz, Andreas
Menzel, Ulrich Nierste, Martin Wiebusch. Impact of a Higgs Boson at a Mass of
126 GeV on the Standard Model with Three and Four Fermion Generations. Physical
Review Letters, 2012; 109 (24) DOI: 10.1103/PhysRevLett.109.241802
Need to cite this story in your essay, paper, or report? Use one of the
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Karlsruhe Institute of Technology (2012, December 13). 12 matter particles
suffice in nature: Limited number of fermions in standard model, physicists
say. ScienceDaily. Retrieved December 14, 2012, from
http://www.sciencedaily.com /releases/2012/12/121213085106.htm
Note: If no author is given, the source is cited instead.
Disclaimer: Views expressed in this article do not necessarily reflect those of
ScienceDaily or its staff.
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