rpp2010-rev-leptoquark-quantum-numbers

rpp2010-rev-leptoquark-quantum-numbers - 1 LEPTOQUARKS...

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–1– LEPTOQUARKS Written November 2007 by S. Rolli (Tufts U.) and M. Tanabashi (Nagoya U.) Leptoquarks are hypothetical particles carrying both baryon number (B) and lepton number (L). The possible quantum num- bers of leptoquark states can be restricted by assuming that their direct interactions with the ordinary SM fermions are di- mensionless and invariant under the standard model (SM) gauge group. Table 1 shows the list of all possible quantum numbers with this assumption [1]. The columns of SU (3) C , (2) W , and U (1) Y in Table 1 indicate the QCD representation, the weak isospin representation, and the weak hypercharge, respec- tively. The spin of a leptoquark state is taken to be 1 (vector leptoquark)or0(sca larleptoquark) . Table 1: Possible leptoquarks and their quan- tum numbers. Spin 3 B + LS U (3) c (2) W U (1) Y Allowed coupling 0 2 ¯ 31 1 / q c L ± L or ¯ u c R e R 0 2 ¯ 4 / 3 ¯ d c R e R 0 2 ¯ 33 1 / q c L ± L 1 2 ¯ 32 5 / q c L γ μ e R or ¯ d c R γ μ ± L 1 2 ¯ 1 / u c R γ μ ± L 00 7 / q L e R or ¯ u R ± L 1 / 6 ¯ d R ± L 10 2 / q L γ μ ± L or ¯ d R γ μ e R 5 / u R γ μ e R 2 / q L γ μ ± L If we do not require leptoquark states to couple directly with SM fermions, diFerent assignments of quantum numbers become possible [2,3]. Leptoquark states are expected to exist in various extensions of SM. The Pati-Salam model [4] is an example predicting the existence of a leptoquark state. Vector leptoquark states also exist in grand uni±cation theories based on (5) [5], CITATION: K. Nakamura et al. (Particle Data Group), JPG 37 , 075021 (2010) (URL: http://pdg.lbl.gov) July 30, 2010 14:34
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–2– SO (10) [6], which includes Pati-Salam color SU (4), and larger gauge groups. Scalar quarks in supersymmetric models with R-parity violation may also have leptoquark-type Yukawa couplings. The bounds on the leptoquark states can therefore be applied to constraining R-parity-violating supersymmetric models. Scalar leptoquarks are expected to exist at TeV scale in extended technicolor models [7,8] where leptoquark states appear as the bound states of techni-fermions. Compositeness of quarks and leptons also provides examples of models which may have light leptoquark states [9]. Bounds on leptoquark states are obtained both directly and indirectly. Direct limits are from their production cross sections at colliders, while indirect limits are calculated from the bounds on the leptoquark-induced four-fermion interactions, which are obtained from low-energy experiments, or from collider experi- ments below threshold.
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This note was uploaded on 06/07/2011 for the course PHYS 4132 taught by Professor Kutter during the Spring '11 term at University of Florida.

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rpp2010-rev-leptoquark-quantum-numbers - 1 LEPTOQUARKS...

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