CMS-PAS-EXO-17-008

CMS-PAS-EXO-17-008
Search for a W' boson decaying to a $\tau$ lepton and a neutrino in proton-proton collisions at $\sqrt{s}$ = 13 TeV
CMS Collaboration
June 2018

Abstract: A search for new high-mass resonances decaying to a tau lepton and a neutrino is reported. The analysis uses proton-proton collision data collected by the CMS experiment at the LHC at $\sqrt{s}= $ 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The search utilizes hadronically decaying tau leptons, $\tau_{\text{h}}$. No excess in the event yield is observed at high transverse masses of the $\tau_{\text{h}}$ and missing transverse momentum. An interpretation of results within the sequential standard model excludes W' boson masses below 4.0 TeV at 95% confidence level. Existing limits are also improved on models in which the W' boson decays preferentially to fermions of the third generation. For further reinterpretations, a model-independent limit on potential signals for various transverse mass thresholds is also presented along with limits on the coupling of the W' to standard model particles.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 792 (2019) 107. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of the expected signal $ {\mathrm {W}'} \rightarrow \tau \nu $.
png pdf	Figure 2: Branching fractions B(W'), as functions of the mixing angle $\cot\theta _{\rm E}$ for the W' boson decays in the NUGIM G(221), as calculated in Refs. [2,24,25]. For $\cot\theta _{\rm E}$ = 1 the values are the same as those in the SSM, rescaled to accommodate the WH decay channel.
png pdf	Figure 3: The ${M_\mathrm {T}}$ distribution after the final selection (left). The black points with error bars show data, while the filled histograms represent the SM backgrounds. Signal examples for SSM W' bosons with masses of 0.6, 1.0, 4.0, and 5.0 TeV are shown with the open histograms. The integral transverse mass distribution (right), where the value in each bin is equal to the number of events with transverse mass equal to or greater than the left of the bin. The lower panels show the ratio of data to prediction, and the gray band represents the systematic uncertainties.
png pdf	Figure 3-a: The ${M_\mathrm {T}}$ distribution after the final selection (left). The black points with error bars show data, while the filled histograms represent the SM backgrounds. Signal examples for SSM W' bosons with masses of 0.6, 1.0, 4.0, and 5.0 TeV are shown with the open histograms. The integral transverse mass distribution (right), where the value in each bin is equal to the number of events with transverse mass equal to or greater than the left of the bin. The lower panels show the ratio of data to prediction, and the gray band represents the systematic uncertainties.
png pdf	Figure 3-b: The ${M_\mathrm {T}}$ distribution after the final selection (left). The black points with error bars show data, while the filled histograms represent the SM backgrounds. Signal examples for SSM W' bosons with masses of 0.6, 1.0, 4.0, and 5.0 TeV are shown with the open histograms. The integral transverse mass distribution (right), where the value in each bin is equal to the number of events with transverse mass equal to or greater than the left of the bin. The lower panels show the ratio of data to prediction, and the gray band represents the systematic uncertainties.
png pdf	Figure 4: Expected (black dashed line) and observed (black solid line) 95% CL upper cross section limits for an SSM W' boson. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands. The NNLO theoretical cross section with the corresponding PDF uncertainty band is also shown.
png pdf	Figure 5: Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the ratio of couplings as a function of the W' boson mass. The values above the limit are excluded. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands.
png pdf	Figure 6: Left: The width of the W' boson as a function of $M_{{\mathrm {W}'}}$ and mixing angle $\cot\theta _{\rm E}$ in NUGIM, as obtained from Ref. [25]. Right: Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the mixing angle $\cot\theta _{\rm E}$ as a function of the W' boson mass. The region left of the solid line is excluded. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands.
png pdf	Figure 6-a: Left: The width of the W' boson as a function of $M_{{\mathrm {W}'}}$ and mixing angle $\cot\theta _{\rm E}$ in NUGIM, as obtained from Ref. [25]. Right: Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the mixing angle $\cot\theta _{\rm E}$ as a function of the W' boson mass. The region left of the solid line is excluded. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands.
png pdf	Figure 6-b: Left: The width of the W' boson as a function of $M_{{\mathrm {W}'}}$ and mixing angle $\cot\theta _{\rm E}$ in NUGIM, as obtained from Ref. [25]. Right: Expected (black dashed line) and observed (black solid line) 95% CL upper limits on the mixing angle $\cot\theta _{\rm E}$ as a function of the W' boson mass. The region left of the solid line is excluded. The shaded bands represent the one and two standard deviation (s.d.) uncertainty bands.
png pdf	Figure 7: Expected (thick black dashed line) and observed (thick black solid line) 95% CL upper cross section limits for an SSM W' boson together with shaded bands that represent the one and two standard deviation (s.d.) uncertainty regions. The LO (dotted thin line) and NNLO (solid thin line) theoretical cross sections with the corresponding PDF uncertainty band are also shown.

Tables
png pdf	Table 1: Measured event yields compared to the expected yields for the signal and background contributions, for three regions of ${M_\mathrm {T}}$. The uncertainties correspond to the total systematic uncertainties.

Summary

A search for new physics in final states with a hadronically decaying $\tau$ lepton and missing transverse momentum is performed by the CMS experiment, using proton-proton collision data at the center-of-mass energy $\sqrt{s} = $ 13 TeV with an integrated luminosity of 35.9 fb$^{-1}$. No significant excess beyond the standard model expectation is observed in the transverse mass distribution. A sequential standard model (SSM) W' boson is excluded in the mass range 0.4 $ < M_{\mathrm{W'}} < $ 4.0 TeV at 95% confidence level. Couplings that are weaker than assumed in the SSM can be excluded down to values of $7\times10^{-2}$ for $M_{\mathrm{W'}} = $ 1 TeV. Within the nonuniversal gauge interaction $SU(2) \times SU(2) \times U(1)$ model, the lower limit on the W' boson mass depends on the coupling constant $\cot\theta_{\rm E}$ and varies from 1.7 to 3.9 TeV at 95% confidence level. In addition, a model-independent limit is provided allowing other interpretations in models with the same final state and similar kinematics.

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