Leptoquark

Leptoquarks ( X and Y bosons ) are hypothetical elementary particles that couple to quarks and leptons at the same time . They are postulated in a number of models beyond the standard model of particle physics , e.g. B. in field theoretical GUT models such as the Georgi-Glashow model , but could not be proven experimentally so far.

Leptoquarks enable the conversion of leptons into quarks and vice versa and explain the equality of the charge of protons and electrons . Their existence could also explain why there are as many quarks as there are leptons, as well as many other similarities between the quark and lepton sectors.

properties

The twelve leptoquarks were first introduced by Jogesh Pati and Abdus Salam in a SU (4) model, in which the lepton number was treated as the fourth color charge . According to this model, they have integer spin (0 or 1) and carry electrical charge and color:

Leptoquarks
(electrical charges Q in units of the elementary charge )
Color charge		red	green	blue
	Q	X bosons
Particle	+4/3	${\ displaystyle X ^ {r} \! \,}$	${\ displaystyle X ^ {g} \! \,}$	${\ displaystyle X ^ {b} \! \,}$
Antiparticle	−4/3	${\ displaystyle {\ bar {X}} ^ {r}}$	${\ displaystyle {\ bar {X}} ^ {g}}$	${\ displaystyle {\ bar {X}} ^ {b}}$
	Q	Y bosons
Particle	+1/3	${\ displaystyle Y ^ {r} \! \,}$	${\ displaystyle Y ^ {g} \! \,}$	${\ displaystyle Y ^ {b} \! \,}$
Antiparticle	−1/3	${\ displaystyle {\ bar {Y}} ^ {r}}$	${\ displaystyle {\ bar {Y}} ^ {g}}$	${\ displaystyle {\ bar {Y}} ^ {b}}$

Strong bounds on their coupling constant products - especially in the case of leptoquarks that couple to left- and right-handed quarks - can be derived from leptonic mesons decay (e.g. pion decay).

The Leptoquark Lagrangian contains, in addition to terms that have the same shape as in the supersymmetric Lagrangian, other, pseudoscalar interactions. By excluding these pseudoscalar interactions, the corresponding bounds on R-parity violating supersymmetric interactions can be obtained from the bounds on leptoquark interactions .

Classification

The classification of Buchmüller , Rückl and Wyler (BRW classification) divides leptoquarks according to the spin (0 or 1), the fermion number (0 or 2), the weak isospin and the coupling to left- or right-handed fermions .

Decay modes

An X boson would have the following decay mode :

X → u + u

X → e ⁺ + d

the two decay products each having opposite chirality .

A Y boson would have the following decay mode:

Y → e ⁺ + u

Y → d + u

Y → d + ν _e

the first decay product being left-handed and the second right-handed.

Here u denotes the up quark , d the down quark , e ⁺ the positron (anti-electron) and ν _e the electron antineutrino . There are similar decay products for the other particle generations .

In these reactions, neither the lepton number are L nor the baryon number B received (which the proton decay allowed), but the difference B - L .

Different decay rates of the X boson and its antiparticle (similar to the K meson ) could explain the baryogenesis at the beginning of our universe . It is believed that leptoquarks only existed for a very short period, at the end of the GUT era shortly after the Big Bang . Then they disintegrated into quarks and leptons and, according to theories, formed the asymmetry between matter and antimatter .

Footnotes

↑ Sometimes the X and Y bosons are identified by their charges, then the letter X can generally be used. One speaks then only of X bosons and means all leptoquarks.
^ ^A ^b Ta-Pei Cheng, Ling-Fong Li: Gauge Theory of Elementary Particle Physics, Oxford University Press 1984 [corrected reprint 1988, 2000], ISBN 0-19-851961-3 .

literature

JC Pati and A. Salam, Unified Lepton - Hadron Symmetry And A Gauge Theory Of The Basic Interactions , Phys. Rev. D 8 (1973) 1240, Phys. Rev. Lett. 31 (1973) 661, Phys. Rev. D 10 (1974) 275.
W. Buchmüller, R. Rückl and D. Wyler, Leptoquarks In Lepton Quark Collisions , Phys. Lett. B 191 (1987) 442 [Erratum-ibid. B 448 (1999) 320].
J. Blümlein and R. Rückl, Production of scalar and vector leptoquarks in e + e- annihilation , Phys. Lett. B 304 (1993) 337.
A. Blumhofer and B. Lampe, A low-energy compatible SU (4) -type model for vector leptoquarks of mass </ = 1TeV , Eur. Phys. J. C 7 (1999) 141.
Dieter B. Herrmann : Antimatter: in search of the opposite world , 2nd updated edition, Munich, Beck, 2004, ISBN 3-406-44504-7
Chris C. King: Dual-Time Supercausality (1989) Physics Essays 2/2 128-151 ( PDF )

[1] Sometimes the X and Y bosons are identified by their charges, then the letter X can generally be used. One speaks then only of X bosons and means all leptoquarks.

[Cheng-Li-2] A ^b Ta-Pei Cheng, Ling-Fong Li: Gauge Theory of Elementary Particle Physics, Oxford University Press 1984 [corrected reprint 1988, 2000], ISBN 0-19-851961-3 .