The particle is dedicated to the discipline of physics the study of particles , especially the elementary particles . While this was limited to molecules and atoms until the end of the 19th century , the focus today is on elementary particles and hadrons .
In modern particle physics, physical models are checked primarily through colliding beam experiments at particle accelerators , in which different particles are shot at one another (for example electrons on positrons ). The resulting reaction products, their distribution in the particle and radiation detectors as well as the energy and momentum balance provide information about the properties of known or suspected “new” particle types.
These experiments require accelerated particle beams of very high energy. This is why high-energy physics is often used instead of particle physics; However, this term is also used for heavy ion physics experiments at high energies.
As more and more particles became known, they began to organize their properties according to their properties and also began to make predictions about particles that had not yet been observed. The current state of particle physics - and many of its predictions - are summarized in the so-called standard model .
Standard model of elementary particle physics
Today's knowledge about elementary particles and their interactions is summarized in the standard model of elementary particle physics. The standard model allows a consistent description of the strong , the weak and the electromagnetic interaction in the form of quantum field theories .
In the standard model there are twelve particles and twelve antiparticles , which are divided into leptons and quarks . The forces that act between these particles are mediated by the exchange of calibration bosons . For the electromagnetic interaction this is the massless photon , for the weak interaction these are the massive W bosons and the also massive Z boson , while the strong interaction is mediated by eight massless gluons . There is also the assumption that a graviton could exist which mediates gravitation .
An important difference compared to ideas of the everyday world and classical physics is that the Standard Model is very holistic . If several building blocks combine to form a single new object, one imagines that the building blocks are still present in the new object and continue to exist there; If the new object disintegrates, you get the original building blocks again , just like when you dismantle a Lego model. In the Standard Model, too, two colliding particles (e.g. an electron and a positron ) can combine to form a single one (e.g. a photon ). The new particle, however, is not thought of as being composed of the two original ones, but is again an “indivisible” elementary particle (i.e. without an internal structure). This idea corresponds to the observation that the new particle can decay into particles of different types (e.g. muons ) than those from which it was created.
The Higgs boson is also predicted as part of the standard model . In July 2012, the research institute CERN announced the detection of a particle at the Large Hadron Collider , which could be the Higgs boson. The fact that this is actually the Higgs boson is now so confirmed that Peter Higgs and François Englert received the 2013 Nobel Prize in Physics for predicting the Higgs boson.
If the Higgs boson had not been proven with this particle accelerator, the theory of the existence of the particle would have had to be rejected. The Higgs mechanism (which inevitably requires the existence of the Higgs boson) can theoretically elegantly explain why (almost) all other particles are not massless (such as the photon ), but have mass .
It is clear from theoretical considerations that the Standard Model cannot provide a correct description of the world above certain particle energies. For this reason, extensions of the Standard Model were developed even without empirical data indicating a failure of the Standard Model. Of these, supersymmetry and string theory are mentioned here .
Experimental particle physics
In particle physics, scattering experiments are carried out, mostly in the form of colliding beam experiments.
The largest international laboratories for particle physics are:
- CERN , on the French-Swiss border near Geneva . The main accelerators are the now dismantled LEP ring (Large Electron Positron Collider) and the LHC (Large Hadron Collider).
- DESY in Hamburg (Germany). The main accelerator was HERA (shutdown on June 30, 2007). Here electrons or positrons were made to collide with protons .
- SLAC , near Palo Alto (USA). The main accelerator is PEP-II, where electrons are collided with positrons .
- Fermilab , near Chicago (USA). The main accelerator was the Tevatron (shut down on September 30, 2011), which made protons collide with antiprotons .
- Brookhaven National Laboratory , Long Island (USA). The main accelerator is the RHIC (Relativistic Heavy Ion Collider), which brings heavy ions (e.g. gold ) or protons to collision.
- KEK , Tsukuba ( Japan ). The main accelerator is KEKB , now expanded to the SuperKEKB accelerator , for investigations of B mesons .
In addition, there are many other particle accelerators which work in different energy ranges depending on the physical issue.
Before the development of accelerators for the GeV energy range, the only source of high-energy particles was cosmic radiation , then mostly called cosmic radiation . Many particles, e.g. B. muon, pion, kaon, were first discovered in cosmic rays. For this purpose, measurements were made on mountain peaks or with photosensitive plates carried by free balloons .
- G. Barr et al: Particle Physics in the LHC Era. Oxford University Press, 2016, ISBN 978-0-19-874855-7 .
- Comprehensive website on particle physics
- KworkQuark (DESY)
- Particle physics at student level ( LEIFIphysik )
- Cahn, Goldhaber, The experimental foundations of particle physics
- ↑ CERN experiments observe particle consistent with long-sought Higgs boson. CERN press release, July 4, 2012, accessed October 15, 2012 .