Lepton flavour violating signature in supersymmetric U(1)′ seesaw models at the LHC

Journal of High Energy Physics, May 2015

We consider a U(1)′ supersymmetric seesaw model in which a right-handed sneutrino is a thermal dark matter candidate whose relic density can be in the right range due to its coupling to relatively light \( \tilde{Z}^{\prime } \), the superpartner of the extra gauge boson Z ′. Such light \( \tilde{Z}^{\prime } \) can be produced at the LHC through cascade decays of colored superparticles, in particular, stops and sbottoms, and then decay to a right-handed neutrino and a sneutrino dark matter, which leads to lepton flavor violating signals of same/opposite-sign dileptons (or multileptons) accompanied by large missing energy. Taking some benchmark points, we analyze the opposite- and same-sign dilepton signatures and the corresponding flavour difference i.e., (2e − 2μ). It is shown that 5σ signal significance can be reached for some benchmark points with very early data of ∼ 2 fb−1 integrated luminosity. In addition, 3ℓ and 4ℓ signatures also look promising to check the consistency in the model prediction, and it is possible to reconstruct the \( \tilde{Z}^{\prime } \) mass from jjℓ invariant mass distribution.

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Lepton flavour violating signature in supersymmetric U(1)′ seesaw models at the LHC

Received: January Lepton flavour violating signature in supersymmetric U 0 1 2 seesaw 0 1 2 Priyotosh Bandyopadhyay 0 1 2 4 Eung Jin Chun 0 1 2 3 Open Access 0 1 2 c The Authors. 0 1 2 0 Hoegiro 85 , Dongdaemun-gu, Seoul 130-722 , South Korea 1 Via Arnesano , 73100, Lecce , Italy 2 FIN-00014 , Helsinki , Finland 3 Korea Institute for Advanced Study 4 Department of Physics, University of Helsinki and Helsinki Institute of Physics We consider a U(1)0 supersymmetric seesaw model in which a right-handed sneutrino is a thermal dark matter candidate whose relic density can be in the right range due to its coupling to relatively light Z0, the superpartner of the extra gauge boson Z0. Such light Z0 can be produced at the LHC through cascade decays of colored superparticles, in particular, stops and sbottoms, and then decay to a right-handed neutrino and a sneutrino dark matter, which leads to lepton flavor violating signals of same/opposite-sign dileptons (or multileptons) accompanied by large missing energy. Taking some benchmark points, we analyze the opposite- and same-sign dilepton signatures and the corresponding flavour difference i.e., (2e 2). It is shown that 5 signal significance can be reached for some benchmark points with very early data of 2 fb1 integrated luminosity. In addition, 3` and 4` signatures also look promising to check the consistency in the model prediction, and it is possible to reconstruct the Z0 mass from jj` invariant mass distribution. Supersymmetry Phenomenology 1 Introduction 2 Z0 phenomenology 3 Experimental bounds and benchmark points 4 Production rates and decays 5 LHC phenomenology 5.1 5.2 Reconstruction of RHN 6 Conclusion Introduction Among various reasons for requiring theories beyond the Standard Model (SM), experimental evidences for tiny neutrino masses and dark matter, and a theoretical requirement for naturalness of the electroweak scale would be key elements and related to each other. The first candidate model addressing these features would be Minimal Supersymmetric Standard Model (MSSM) where the neutrino masses and mixing are explained by R-parity and lepton-number violation [1] and the dark matter consists of a slowly decaying gravitino as the lightest supersymmetric particle (LSP). In this paper, we wish to explore another possibility which can provide richer collider signatures. A sumpersymmetric seesaw model [2] associated with an additional gauge symmetry U(1)0 [3] would be one of such examples. In supersymmetric theories with R-parity, the LSP is stable and thus a neutral LSP, typically a linear combination of neutral gauginos and Higgsinos, becomes a good thermal dark matter (DM) candidate if supersymmetry (SUSY) is broken around the TeV scale [4]. The SUSY breaking can radiatively induce the U(1)0 breaking in addition to the usual electroweak symmetry breaking, which determines the seesaw scale also at O(TeV) [5]. A the presence of right-handed neutrinos (RHNs) for the anomaly-free condition, and thus realizes the TeV-scale seesaw mechanism. Let us note that we do not assume any grand unification theory as the origin of our model and the grand unification structure will be used for a convenient guide to a theoretically consistent model gauranteeing the anomaly free condition, and so on. In our framework, a right-handed sneutrino (RHsN) can be the LSP and thus another good dark matter candidate whose thermal relic density is in the right range if the U(1)0 gaugino Z0, the superpartner of the U(1)0 gauge boson Z0, is relatively light [6]. In this paper, we analyze LHC signatures of the extra gaugino Z0 which can be pairly produced mainly through third generation squark cascades and then decay to a RHN and a RHsN DM. Due to the Majorana nature of a RHN, N , it can decay to bothdilepton (SSD) events in addition to the usual opposite-sign dilepton (OSD) events [7]. Furthermore, Yukawa couplings of a RHN are generically flavour-dependent and thus lead benchmark points, we carry out a Pythia-FastJet level collider simulation at the 14 TeV LHC for multilepton final states to study the prospect for detecting such lepton flavour opposite-sign dilepton final states. Similar phenomenon should appear also in multi-lepton integrated luminosity at the very early stage of LHC14. It is straightforward to follow the Br(N eW ) resulting in a similar conclusion. Let us here recall that the neutrino mass models with R-parity and lepton number violating couplings lead to similar signatures [8, 9]. In particular, there could appear an interesting connection of the electron excess assumed in this work and the neutrinoless double-beta decay caused by R-parity violation as pointed out by Allanach et al. [10, 11]. Furthermore, the supersymmetric left-right symmetric model [12] can also lead to the similar signatures except a possible W 0 appearance in the next LHC run. This paper is organized as follows. In section 2, we describe a general Z0 phenomenology by taking (...truncated)


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Priyotosh Bandyopadhyay, Eung Jin Chun. Lepton flavour violating signature in supersymmetric U(1)′ seesaw models at the LHC, Journal of High Energy Physics, 2015, pp. 45, Volume 2015, Issue 5, DOI: 10.1007/JHEP05(2015)045