Distinguishing seesaw models at LHC with multi-lepton signals (0808.2468v2)

Abstract: We investigate the LHC discovery potential for electroweak scale heavy neutrino singlets (seesaw I), scalar triplets (seesaw II) and fermion triplets (seesaw III). For seesaw I we consider a heavy Majorana neutrino coupling to the electron or muon. For seesaw II we concentrate on the likely scenario where the new scalars decay to two leptons. For seesaw III we restrict ourselves to heavy Majorana fermion triplets decaying to light leptons plus gauge or Higgs bosons, which are dominant except for unnaturally small mixings. The possible signals are classified in terms of the charged lepton multiplicity, studying nine different final states ranging from one to six charged leptons. Using a fast detector simulation of signals and backgrounds, it is found that the trilepton channel l+- l+- l-+ is by far the best one for scalar triplet discovery, and for fermion triplets it is as good as the like-sign dilepton channel l+- l+-. For heavy neutrinos with a mass O(100) GeV, this trilepton channel is also better than the usually studied like-sign dilepton mode. In addition to evaluating the discovery potential, we make special emphasis on the discrimination among seesaw models if a positive signal is observed. This could be accomplished not only by searching for signals in different final states, but also by reconstructing the mass and determining the charge of the new resonances, which is possible in several cases. For high luminosities, further evidence is provided by the analysis of the production angular distributions in the cleanest channels with three or four leptons.

• The paper demonstrates that multi-lepton signals at the LHC can effectively differentiate between Type I, II, and III seesaw mechanisms.
• It details how distinct lepton multiplicities and invariant mass peaks serve as key signatures for each seesaw model.
• The study provides concrete strategies for suppressing standard model backgrounds to enhance the discovery potential in neutrino mass searches.

Distinguishing Seesaw Models at LHC with Multi-Lepton Signals

The paper of neutrino masses and the mechanisms that generate them—known as seesaw models—constitutes an essential aspect of modern particle physics. This paper provides a comprehensive examination of the discovery potential of various seesaw models at the Large Hadron Collider (LHC) by focusing on multi-lepton signals. Specifically, the paper explores the distinguishing features of three types of seesaw models: the Type I seesaw involving heavy neutrino singlets, the Type II seesaw with scalar triplets, and the Type III seesaw comprised of fermion triplets.

The paper sets a context where these new particles can be probed at LHC energies, assuming they inhabit the electroweak scale. Each model predicts different lepton multiplicities in decay channels, which in turn can be used as potential signals at the LHC for model discrimination.

Analysis of Seesaw Models

1. Type I Seesaw: In this mechanism, heavy neutrino singlets are considered. Their production and decays at the LHC involve certain signatures that primarily consist of the production of a charged lepton alongside a heavy neutrino, which subsequently decays into another lepton and a gauge boson. The trilepton final states are identified as better channels for discovery compared to the like-sign dilepton channels due to the larger backgrounds in the latter.
2. Type II Seesaw: This model involves scalar triplets with distinct doubly charged and singly charged components. The paper highlights that the discovery potentials lie strongly in the trilepton signals, as opposed to like-sign dilepton or four-lepton signals. Scalar triplet states can yield signatures of four, three, or two leptons, depending on the hierarchy and mass settings, with pronounced peaks observed in invariant mass distributions which aid in their discovery.
3. Type III Seesaw: In cases of fermion triplets, the signals can extend to five or six lepton final states, providing unique opportunities distinguishable from the Type I and Type II seesaw scenarios. The trilepton and like-sign dilepton channels offer significant discovery potential, accentuated by additional jet activity that accompanies fermion triplet decays.

Implications and Future Prospects

The analyses provided steer the roadmap for future experimental searches for neutrino mass mechanisms at the LHC, focusing on multi-lepton final states. The paper also gives clear strategies on how to segregate different seesaw mechanisms by examining their unique signal characteristics, like distinct invariant mass distributions, missing energy profiles, and lepton multiplicity events. The methodology outlined mandates high precision in multivariate data analyses at the LHC to efficiently suppress standard model backgrounds and thoroughly investigate the physics beyond the standard model.

Notably, there exist indirect measurements and their eventual manifestation as collider signatures that are also touched upon, which might be pivotal for painting a holistic picture of neutrino mass generation.

In conclusion, this paper offers significant contributions by elaborating robust strategies for identifying and distinguishing between the different seesaw models at the LHC. The methods and results laid out provide a strong stepping stone for ongoing and future experiments aimed at unraveling the mystery of neutrino masses, thereby contributing valuable insights to the field of particle physics.