Quantitative study on helicity inversion in Majorana neutrino decays at the LHC

Richard Ruiz (Centre for Cosmology, Particle Physics and Phenomenology (CP3), Université Catholique de Louvain, Chemin du Cyclotron, Louvain la Neuve B-1348, Belgium and Institute of Nuclear Physics, Polish Academy of Sciences, ul. Radzikowskiego, Cracow 31-342, Poland)

We report an analytical and numerical investigation into the impact of helicity inversion in LHC processes that do not conserve lepton number (L). As a case study, we focus on the production and decay of Majorana neutrinos (N) through on- and off-shell W bosons in the phenomenological type I seesaw model. Using the Monte Carlo event generator madgraph5_amc@nlo in conjunction with the heavyn model libraries, we perform exact matrix element (ME) computations without the narrow width approximation. Despite helicity inversion appearing explicitly in MEs, we report the absence of helicity suppression of L-violating collider observables for 14 and 24 processes that are dominated by resonant N production. We attribute this incongruity to the different scalings of 4-momenta and squared 4-momenta in MEs and squared MEs, with exact cancelations occurring in the latter when N goes on-shell in the small-width limit. In off-shell regimes, total suppression/enhancement of L violation can emerge. Implications for other neutrino mass models are discussed.

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      "source": "APS", 
      "value": "We report an analytical and numerical investigation into the impact of helicity inversion in LHC processes that do not conserve lepton number (<math><mi>L</mi></math>). As a case study, we focus on the production and decay of Majorana neutrinos (<math><mi>N</mi></math>) through on- and off-shell <math><mi>W</mi></math> bosons in the phenomenological type I seesaw model. Using the Monte Carlo event generator madgraph5_amc@nlo in conjunction with the heavyn model libraries, we perform exact matrix element (ME) computations without the narrow width approximation. Despite helicity inversion appearing explicitly in MEs, we report the absence of helicity suppression of <math><mi>L</mi></math>-violating collider observables for <math><mn>1</mn><mo>\u2192</mo><mn>4</mn></math> and <math><mn>2</mn><mo>\u2192</mo><mn>4</mn></math> processes that are dominated by resonant <math><mi>N</mi></math> production. We attribute this incongruity to the different scalings of 4-momenta and squared 4-momenta in MEs and squared MEs, with exact cancelations occurring in the latter when <math><mi>N</mi></math> goes on-shell in the small-width limit. In off-shell regimes, total suppression/enhancement of <math><mi>L</mi></math> violation can emerge. Implications for other neutrino mass models are discussed."
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Published on:
20 January 2021
Publisher:
APS
Published in:
Physical Review D , Volume 103 (2021)
Issue 1
DOI:
https://doi.org/10.1103/PhysRevD.103.015022
arXiv:
2008.01092
Copyrights:
Published by the American Physical Society
Licence:
CC-BY-4.0

Fulltext files: