CMS-PAS-SUS-15-009

CMS-PAS-SUS-15-009
Search for natural supersymmetry in events with top quark pairs and photons in pp collisions at $\sqrt{s}= $ 8 TeV
CMS Collaboration
October 2016

Abstract: We present a search for a natural gauge-mediated supersymmetry breaking scenario with the top squark as the lightest squark and the gravitino as the lightest supersymmetric particle. The strong production of top squark pairs and their decays would produce events with pairs of top quarks and neutralinos, with each neutralino decaying to a photon and a gravitino. This search is performed with the CMS experiment using pp collision data at $\sqrt{s}= $ 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$, in the electron + jets and muon + jets channel, requiring one or two photons in the final state. We compare the missing transverse momentum of these events against the expected spectrum of standard model processes. No excess of events is observed beyond background predictions and the result of the search is interpreted in the context of a general model of gauge-mediated supersymmetry breaking deriving limits on the mass of top squarks up to 750 GeV.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 03 (2018) 167. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of the GMSB scenario of interest. With top squarks as the lightest squark, the pair production of stops would be the dominant production mechanism for SUSY in pp collisions at the LHC. Assuming a bino-like neutralino NLSP, each stop would decay to a top quark and a neutralino, with the neutralino decaying to a photon and gravitino. Shown above is the electron+jets or muon+jets final state of the top pair decay.
png pdf	Figure 2: Template fit results for the dilepton invariant mass in deriving $SF_{\mathrm{Z(\gamma )}}$ for the electron (left) and muon (middle) channels. The template fit result for $SF_{\mathrm{e \rightarrow \gamma }}$ in $m_{\mathrm{e\gamma }}$ for the electron channel in SR1 with the b tag requirement removed is shown on the right. The ratio of data to the total background is included at the bottom of each figure. Uncertainties shown are statistical only.
png pdf	Figure 2-a: Template fit results for the dilepton invariant mass in deriving $SF_{\mathrm{Z(\gamma )}}$ for the electron channel. The ratio of data to the total background is included at the bottom of the figure. Uncertainties shown are statistical only.
png pdf	Figure 2-b: Template fit results for the dilepton invariant mass in deriving $SF_{\mathrm{Z(\gamma )}}$ for the muon channel. The ratio of data to the total background is included at the bottom of the figure. Uncertainties shown are statistical only.
png pdf	Figure 2-c: The template fit result for $SF_{\mathrm{e \rightarrow \gamma }}$ in $m_{\mathrm{e\gamma }}$ for the electron channel in SR1 with the b tag requirement removed is shown. The ratio of data to the total background is included at the bottom of the figure. Uncertainties shown are statistical only.
png pdf	Figure 3: Comparison of data and MC in $ {E_{\mathrm {T}}^{\text {miss}}} $ for the combined (e+$\mu $) control regions. Both CR1 (left) with one misidentified photon and CR2 (right) with two misidentified photons are shown. The disagreement of 10-20% between data and MC in CR1 is taken as an additional shape-based systematic uncertainty in the signal regions. The comparitively poor agreement in CR2 is attributable to the very small number of events in data and is not taken as an additional uncertainty.
png pdf	Figure 3-a: Comparison of data and MC in $ {E_{\mathrm {T}}^{\text {miss}}} $ for the combined (e+$\mu $) control region CR1, with one misidentified photon. The disagreement of 10-20% between data and MC in CR1 is taken as an additional shape-based systematic uncertainty in the signal regions.
png pdf	Figure 3-b: Comparison of data and MC in $ {E_{\mathrm {T}}^{\text {miss}}} $ for the combined (e+$\mu $) control region CR2, with two misidentified photons. The comparitively poor agreement in CR2 is attributable to the very small number of events in data and is not taken as an additional uncertainty.
png pdf	Figure 4: Comparison of data and MC in $ {E_{\mathrm {T}}^{\text {miss}}} $ for the combined (e+$\mu $) signal regions. SR1 (left) with one reconstructed photon and SR2 (right) with two reconstructed photons are shown.
png pdf	Figure 4-a: Comparison of data and MC in $ {E_{\mathrm {T}}^{\text {miss}}} $ for the combined (e+$\mu $) signal region SR1, with one reconstructed photon.
png pdf	Figure 4-b: Comparison of data and MC in $ {E_{\mathrm {T}}^{\text {miss}}} $ for the combined (e+$\mu $) signal region SR2, with two reconstructed photons.
png pdf	Figure 5: Observed upper cross section CL$_s$ limits at 95% CL in the M$_{\text {Stop}}$ - M$_{\text {Bino}}$ plane.
png pdf	Figure 6: Observed and expected 95% CL exclusion contours for stop and bino masses.

Tables
png pdf	Table 1: Scale factors for the normalization of $\mathrm{Z(\gamma )}$+jets backgrounds and electron-to-photon misidentification rate. For the electron+jets channel, the product of the two is applied to $\mathrm{Z}$+jets and $\mathrm{W/Z}$+$\gamma $ backgrounds. For the muon+jets channel, only the first one is applied. The uncertainties displayed are statistical followed by all systematic uncertainties added in quadrature.
png pdf	Table 2: Summary of systematic uncertainties considered. The dominant rate uncertainty is the ${\mathrm{ t \bar{t} } }$+$\gamma$ and $\mathrm{V\gamma} $ normalizations, although the ${\mathrm{ t \bar{t} }}$+jets and ${\mathrm{ t \bar{t} }}$+$\gamma $ backgrounds are allowed to float freely in the upper limit determination. Check marks indicate uncertainties that affect the shape of the $ {E_{\mathrm {T}}^{\text {miss}}} $ distribution in addition to overall rates. The dominant shape uncertainties are the control region-derived uncertainties.
png pdf	Table 3: Observed data and expected background yields for the combined (e+$\mu $) signal regions. Expectations from two GMSB signal models are included, for which (460-175) refers to M$_{\text {stop}} = 460 $ GeV and M$_{\text {bino}} = 175 $ GeV and similarly for (560-325). The errors represented below are statistical followed by all systematic uncertainties added in quadrature.

Summary

We have presented a search for natural gauge-mediated supersymmetry breaking in events with a top quark pair and photons. No significant excess in the shape of the $E_{\mathrm{T}}^{\text{miss}}$ distribution is observed that would indicate the presence of new physics. Cross section upper limits are calculated for a range of stop and bino masses, and stop masses below 650-750 GeV (depending on the bino mass) are excluded. These results set the most stringest exclusion on stop masses in GMSB scenarios available to date.

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