Springer : Seeking for sterile neutrinos with displaced leptons at the LHC

Liu, Jia; Liu, Zhen; Wang, Lian-Tao; Wang, Xiao-Ping

doi:10.1007/JHEP07(2019)159

Article
Report number	arXiv:1904.01020 ; EFI-19-6
Title	Seeking for sterile neutrinos with displaced leptons at the LHC
Author(s)	Liu, Jia (Chicago U., EFI) ; Liu, Zhen (Maryland U.) ; Wang, Lian-Tao (Chicago U., EFI ; Chicago U., KICP) ; Wang, Xiao-Ping (Argonne)
Publication	2019-07-26
Imprint	2019-04-01
Number of pages	19
Note	22 pages, 6 figures, 1 table; v2: matched to Journal version;
In:	JHEP 1907 (2019) 159
DOI	10.1007/JHEP07(2019)159 (publication)
Subject category	hep-ph ; Particle Physics - Phenomenology
Accelerator/Facility, Experiment	CERN LHC
Abstract	We study the signal of long-lived sterile neutrino at the LHC produced through the decay of the $W$ boson. It decays into charged lepton and jets. The characteristic signature is a hard prompt lepton and a lepton from the displaced decay of the sterile neutrino, which leads to a bundle of displaced tracks with large transverse impact parameter. Different from other studies, we neither reconstruct the displaced vertex nor place requirement on its invariant mass to maintain sensitivity for low sterile neutrino masses. Instead, we focus on the displaced track from the lepton. A difficulty for low mass sterile neutrino study is that the displaced lepton is usually \textit{non-isolated}. Therefore, leptons from heavy flavor quark is the major source of background. We closely follow a search for displaced electron plus muon search at CMS and study their control regions, which is related to our signal regions, in great detail to develop a robust estimation of the background for our signals. After further optimization on the signal limiting the number of jets, low $H_T$ and large lepton displacement $d_0$ to suppress SM background, we reach an exclusion sensitivity of about $10^{-8}$ ($10^{-5}$) for the mixing angle square at 10 (2) GeV sterile neutrino mass respectively. The strategy we propose can cover the light sterile masses complimentary to beam dump and forward detector experiments.
Copyright/License	CC-BY-4.0 preprint: (License: arXiv nonexclusive-distrib 1.0)

$The decay partial widths, branching ratios and lifetime for heavy sterile neutrino $N$, under a democratic mixing assumption $\sin\theta_{\nu_e N} = \sin\theta_{\nu_\nu N} =\sin\theta_{\nu_\tau N}=\sin\theta$. In the left panel, we show the partial decay width from mixing with $\nu_e$, $\nu_\mu$, $\nu_\tau$ and total decay width respectively. In the middle panel, we show the branching ratios to leptonic channels $\nu \ell \ell'$, $\nu \nu \nu $ and semi-leptonic channels $\ell j j$ and $\nu j j$. In the right panel, we show the lifetime as a function of $m_N$ with benchmark mixing angle $\sin \theta = 10^{-4}$, $ 10^{-5}$ and $10^{-6}$ respectively.$ $The normalized distribution for the number of b-jets with $p_T^b > 30$ GeV denoted as $N_b^{30}$, the number of jets with $p_T^j > 20$ GeV denoted as $N_j^{20}$, the number of jets with $p_T^j > 50$ GeV denoted as $N_j^{50}$ and $H_T^{\rm vis}$, where $H_T^{\rm vis}$ is the scalar sum of $p_T$ for all visible objects, including hadronic jets and leptons. The two SM backgrounds $t\bar t\to b\bar b+\ell+X$ and $W+b\bar{b}, ~W\to \ell \nu$ are dot-dashed lines with black and brown color respectively. The signals are solid lines with blue, red, purple and green colors for increasing $m_N$. The dashed gray lines with arrow indicates the optimization cuts that the events to its left are retained, namely $N_b^{30} = 0$, $N_j^{20} < 2$, $N_j^{50} =0$ and $H_T^{\rm vis} < 100$ GeV. The above cuts have been applied after its distribution have been shown in figure (a), (b), (c), and (d).$ $The normalized distribution for the number of b-jets with $p_T^b > 30$ GeV denoted as $N_b^{30}$, the number of jets with $p_T^j > 20$ GeV denoted as $N_j^{20}$, the number of jets with $p_T^j > 50$ GeV denoted as $N_j^{50}$ and $H_T^{\rm vis}$, where $H_T^{\rm vis}$ is the scalar sum of $p_T$ for all visible objects, including hadronic jets and leptons. The two SM backgrounds $t\bar t\to b\bar b+\ell+X$ and $W+b\bar{b}, ~W\to \ell \nu$ are dot-dashed lines with black and brown color respectively. The signals are solid lines with blue, red, purple and green colors for increasing $m_N$. The dashed gray lines with arrow indicates the optimization cuts that the events to its left are retained, namely $N_b^{30} = 0$, $N_j^{20} < 2$, $N_j^{50} =0$ and $H_T^{\rm vis} < 100$ GeV. The above cuts have been applied after its distribution have been shown in figure (a), (b), (c), and (d).$ Show more plots

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