CMS-PAS-EXO-16-045

CMS-PAS-EXO-16-045
Search for heavy neutrinos and W bosons with right handed couplings in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
May 2017

Abstract: A search for heavy right handed neutrinos $\mathrm{N}_\ell$ ($\ell = \mathrm{e}, \mu$), and right handed $W_R$ bosons, performed by the CMS experiment is summarized here. Using the 2.6 fb$^{-1}$ of integrated luminosity recorded by the CMS experiment in 2015 at a center-of-mass energy of 13 TeV, this search seeks evidence of $W_R$ bosons and $\mathrm{N}_\ell$ neutrinos in events with two leptons and two jets. The data do not significantly exceed expected backgrounds, and are consistent with expected results given uncertainties. For Standard Model extensions with strict left-right symmetry, and assuming only one $\mathrm{N}_\ell$ flavor contributes significantly to the $W_R$ decay width, mass limits are set in the two-dimensional $(M_{W_R}, M_{\mathrm{N}_\ell})$ plane at the ninety five percent confidence level. The limits extend to $M_{W_R}$ of 3.3 TeV (3.5 TeV) in the electron (muon) channel, and span a wide range of $M_{\mathrm{N}_\ell}$ values below $M_{W_R}$.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram for the production of a ${W_R}$ boson and its decay to two charged leptons and two quarks through a heavy neutrino.
png pdf	Figure 2: Bin-by-bin ratio of ${M_{\ell \ell j j}}$ distribution and ${M_{\mathrm{ e } \mu j j}}$ from top quark background simulations, where $\ell $ is an electron (left) or a muon (right).
png	Figure 2-a: Bin-by-bin ratio of ${M_{\ell \ell j j}}$ distribution and ${M_{\mathrm{ e } \mu j j}}$ from top quark background simulations, where $\ell $ is an electron.
png	Figure 2-b: Bin-by-bin ratio of ${M_{\ell \ell j j}}$ distribution and ${M_{\mathrm{ e } \mu j j}}$ from top quark background simulations, where $\ell $ is a muon.
png pdf	Figure 3: ${M_{\ell \ell j j}}$ in the electron and muon channel signal regions. For the ${W_R}$ signal shown, $M_{ {\mathrm {N}_{\ell }} } = \frac {1}{2} {M_{\mathrm{ W } _{\mathrm {R}}}} $. Both plots use 200 GeV wide bins from 600 to 1800 GeV, then one bin spans 1800 to 2200 GeV, and finally the last bin includes all events above 2200 GeV. In addition, the bin contents are divided by the bin widths.
png	Figure 3-a: ${M_{\ell \ell j j}}$ in the electron channel signal region. For the ${W_R}$ signal shown, $M_{ {\mathrm {N}_{\ell }} } = \frac {1}{2} {M_{\mathrm{ W } _{\mathrm {R}}}} $. The plot uses 200 GeV wide bins from 600 to 1800 GeV, then one bin spans 1800 to 2200 GeV, and finally the last bin includes all events above 2200 GeV. In addition, the bin contents are divided by the bin widths.
png	Figure 3-b: ${M_{\ell \ell j j}}$ in the muon channel signal region. For the ${W_R}$ signal shown, $M_{ {\mathrm {N}_{\ell }} } = \frac {1}{2} {M_{\mathrm{ W } _{\mathrm {R}}}} $. The plot uses 200 GeV wide bins from 600 to 1800 GeV, then one bin spans 1800 to 2200 GeV, and finally the last bin includes all events above 2200 GeV. In addition, the bin contents are divided by the bin widths.
png pdf	Figure 4: ${W_R}$ cross section limits for $M_{ {\mathrm {N}_{\ell }} } = \frac {1}{2} {M_{\mathrm{ W } _{\mathrm {R}}}} $.
png	Figure 4-a: ${W_R}$ cross section limits for $M_{ {\mathrm {N}_{\ell }} } = \frac {1}{2} {M_{\mathrm{ W } _{\mathrm {R}}}} $. Electron channel.
png	Figure 4-b: ${W_R}$ cross section limits for $M_{ {\mathrm {N}_{\ell }} } = \frac {1}{2} {M_{\mathrm{ W } _{\mathrm {R}}}} $. Muon channel.
png pdf	Figure 5: 95% confidence level exclusion in the $( {M_{\mathrm{ W } _{\mathrm {R}}}} , M_{ {\mathrm {N}_{\ell }} })$ plane for $ {W_R} \to eejj$ (left) and $ {W_R} \to \mu \mu jj$ (right).
png	Figure 5-a: 95% confidence level exclusion in the $( {M_{\mathrm{ W } _{\mathrm {R}}}} , M_{ {\mathrm {N}_{\ell }} })$ plane for $ {W_R} \to eejj$.
png	Figure 5-b: 95% confidence level exclusion in the $( {M_{\mathrm{ W } _{\mathrm {R}}}} , M_{ {\mathrm {N}_{\ell }} })$ plane for $ {W_R} \to \mu \mu jj$.

Tables
png pdf	Table 1: ${M_{\ell \ell j j}}$ windows that minimize the expected ${W_R}$ cross section limit at each ${M_{\mathrm{ W } _{\mathrm {R}}}}$ point.
png pdf	Table 2: For ${W_R}$ mass hypotheses with $M_{ {\mathrm {N}_{\ell }} } = \frac {1}{2} {M_{\mathrm{ W } _{\mathrm {R}}}} $, these are the mean number of expected ${W_R}$ signal and background events, their statistical and systematic errors, and the number of observed events in data. All errors are given in number of events, and refer to the mean number of expected events. The analysis was run 3200 times to estimate the errors listed here, and the systematic errors from jet and lepton energy scale uncertainties are added in quadrature with systematic errors from luminosity, pileup and background normalization uncertainty. For the $ {M_{\mathrm{ W } _{\mathrm {R}}}} = $ 2.6 TeV hypothesis, the electron channel top quark background estimate is higher than the muon channel because the electron channel ${M_{\mathrm{ W } _{\mathrm {R}}}}$ window extends lower in ${M_{\ell \ell j j}}$ (see Table {tab:masscuts}). In the $e\mu jj$ data used to estimate the top quark background, there is a deficit of events in the region 1.8 TeV $ < {M_{\mathrm{ e } \mu j j}} < $ 2.1 TeV that, in both channels, results in little change in the top quark background going from the $ {M_{\mathrm{ W } _{\mathrm {R}}}} = $ 2.0 TeV to the $ {M_{\mathrm{ W } _{\mathrm {R}}}} = $ 2.2 TeV bin.

Summary

A search for right-handed bosons ($W_R$) and heavy right-handed neutrinos ($\mathrm{N}_\ell$) in the left-right symmetric extension of the standard model has been presented. $W_R$ boson production is excluded at the 95% confidence level up to $ M_{W_R} < $ 3.5 (3.3) TeV in the muon (electron) channel. These results are consistent with expectations based on simulations of SM processes, and extend the mass limits in the muon (electron) channel 400 GeV (400 GeV) higher in $M_{W_R}$, and 150 GeV (300 GeV) higher in $M_{\mathrm{N}_ell}$ relative to the Run I limits.

Additional Figures
png pdf	Additional Figure 1: Data compared to the Run I excess plus expected backgrounds in the electron channel $M_{LLJJ}$ distribution. For the ${W_R}$ signal shown, $M_{N_{l}} = \frac {1}{2} M_{W_{R}}$. The bins are 200 GeV wide.
png pdf	Additional Figure 2: Data compared to the Run I excess plus expected backgrounds in the electron channel $M_{LLJJ}$ distribution. For the ${W_R}$ signal shown, $M_{N_{l}} = \frac {1}{2} M_{W_{R}}$. The plot uses 200 GeV wide bins from 600 to 1800 GeV, then one bin spans 1800 to 2200 GeV, and finally the last bin includes all events above 2200 GeV. All bin contents are divided by the bin widths.

The results from 2015 data in the electron channel are compared to expected backgrounds plus the Run I electron channel excess in Additional Fig. 1 and Additional Fig. 2. The Run I excess normalization is rescaled to account for differences in integrated luminosity and $W_{R}$ cross section between 2012 and 2015, and its shape is modeled using a Gaussian. Since the Run I excess spanned 1.8 $ < M_{EEJJ} < $ 2.2 TeV, the mean of the Gaussian representing the excess was set to 2.0 TeV. Furthermore, the standard deviation of the Gaussian $\sigma_{gaus}$ was chosen so that +3$\sigma_{gaus}$ (-3$\sigma_{gaus}$) away from the mean equaled 2.2 (1.8)

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