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. (DOI: 10.1103/PhysRevLett.109.241802)
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.
Helmholtz Association of German Research Centres: http://www.helmholtz.de/en/index.html
This press release was posted to serve as a topic for discussion. Please comment below. We try our best to only post press releases that are associated with peer reviewed scientific literature. Critical discussions of the research are appreciated. If you need help finding a link to the original article, please contact us on twitter or via e-mail.
A sophisticated prediction software ran tens of thousands of scenarios and picked one team to win 60 percent of the time
The Insurance Institute For Highway Safety tested more than three dozen late-model vehicles to see if new safety features are preventing deaths. Chip Reid reports on the results.
Physicist Charles Townes died Tuesday. He was a key inventor of the laser and won the Nobel Prize for his discovery in 1964. But his career didn't end there.
In order to learn how lava travels before spurting out in an eruption, researchers designed a small rolling robot that can go deep inside a volcano and send live images to the surface.
It may not sound like the most useful of scientific endeavours, but the methods used to turn a hard-boiled egg back into its liquid state could bring major benefits to areas as diverse as cheese-making and cancer research
The science behind how water releases the funk from all the yeasts and bacteria hiding in your dog's fur.
The oldest rocky planets yet are 11.2 billion years old, just a little younger than the universe - meaning the galaxy made an early start on planet building
It's usually only possible to see the spot where a laser lands rather than its path, but now an ultrafast camera has caught those photons mid-flight
A team of Indian physicists has made a mathematical model that purports to explain why ants don't have traffic jams. NPR's Joe Palca explains as part of his series, Joe's Big Idea.
On a fall morning in 2009, a team of three young physicists huddled around a computer screen in a small office overlooking Broadway in New York. They were dressed for success—even the graduate student’s shirt had buttons—and a bottle of champagne was at the ready.