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Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Experimental Particle Physics

4th Generation

Vierte Generation

The Standard Model of Elementary Particle Physics

The known elementary particles are divided into fermions and gauge bosons based on the magnitude of a particle spin, so fermions are spin 1/2 particles and bosons are particles with integer spin.  Fermions can be divided into quarks and leptons, which are arranged into three generations with two leptons and two quarks in each generation.  The two leptons can be classified into one with an electric charge -1 (electron-like) and into one with zero charge (neutrino); the two quarks can be classified into one with charge -1/3 (down-type) and into one with charge +2/3 (up-type).  Overall there are three electron-like leptons, electron, muon and tau, along with three types of three corresponding neutrinos, electron-neutrino, muon-neutrino and tau-neutrino.  And there are six types of quarks: up, down, strange, charm, bottom, and top.  Elementary particles interact with each other by an exchange of intermediate (gauge) boson.  The known bosons are: photon - carrier of the electromagnetic interactions, eight gluons - carriers of strong (or colour) interactions and the massive W+, W- and Z0 bosons - carriers of weak interactions.  The fundamental fermions and their interactions by exchange of intermediate bosons are described by the Standard Model of particle physics.  The Standard Model predicts also the existence of the Higgs boson to be discovered, which is responsible for the generation of particles' masses within the Standard Model.

An extension by a fourth generation

The three above mentioned fermion generations along with the gauge bosons have been experimentally verified.  Theoretical interest in a fourth generation of heavy fermions come from its impact on electroweak symmetry breaking and a possible large CP violation in the 4x4 CKM matrix which might play a crucial role in understanding the baryon asymmetry in the Universe. Thus, there are significant efforts ongoing to search for the fourth generation.
Research interests of the working group Experimental Particle Physics II (Head: Prof. Dr. Heiko Lacker) in this context focus on direct searches for fourth generation quarks with the ATLAS experiment at the LHC as well as on a phenomenological analysis using the CKMfitter package combining direct Higgs searches, electroweak precision data and flavor observables.
The strongest (model dependent) limits on the fourth generation quarks exclude currently the down-type b' quark mass below 611 GeV/c2 and the up-type t' quark mass below 557 GeV/c2.  Direct Higgs searches might exclude the simple standard model with a fourth generation.  In this case, one needs to enlarge the simple Higgs sector.

All the Standard Model particles along with hypothetical fourth generation fermions including their essential properties are shown in the graphics above.

Last update: June 2012


Matter | Dark Matter | Higgs-Particle | 4th Generation | Heavy Quarks