CMS-SUS-15-009

CMS-SUS-15-009 ; CERN-EP-2017-124
Search for natural supersymmetry in events with top quark pairs and photons in pp collisions at $\sqrt{s} = $ 8 TeV
CMS Collaboration
10 July 2017
JHEP 03 (2018) 167
Abstract: Results are presented from a search for natural gauge-mediated supersymmetry (SUSY) in a scenario in which the top squark is the lightest squark, the next-to-lightest SUSY particle is a bino-like neutralino, and the lightest SUSY particle is the gravitino. The strong production of top squark pairs can produce events with pairs of top quarks and neutralinos, with each bino-like neutralino decaying to a photon and a gravitino. The search is performed using a sample of pp collision data accumulated by the CMS experiment at $\sqrt{s} = $ 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. The final state consists of a lepton (electron or muon), jets, and one or two photons. The imbalance in transverse momentum in the events is compared with the expected spectrum from standard model processes. No excess event yield is observed beyond the expected background, and the result is interpreted in the context of a general model of gauge-mediated SUSY breaking that leads to exclusion of top squark masses below 650-730 GeV.
Links: e-print arXiv:1707.03325 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The event topology used to search for low mass top squarks pairs
png pdf	Figure 2: The dilepton invariant masses used in determining $SF_{{\mathrm {Z}} (\gamma)}$ (upper pane) for the electron and (middle pane) for the muon channels. The lower pane shows the result of the fit of $m_{{\mathrm {e}} {\gamma}}$ SR1 electron data (without the b tag requirement) to determine $SF_{{\mathrm {e}}\to \gamma}$. The mass spectra are shown post-fit after the application of the derived scale factors. The ratio of data to the total background is included in the lower panel of each plot. Uncertainties include the quadratic sum of all statistical and systematic components.
png pdf	Figure 2-a: The dielectron invariant masses used in determining $SF_{{\mathrm {Z}} (\gamma)}$ (upper pane) for the electron and (middle pane) for the muon channels.
png pdf	Figure 2-b: The dimuon invariant masses used in determining $SF_{{\mathrm {Z}} (\gamma)}$ (upper pane) for the electron and (middle pane) for the muon channels.
png pdf	Figure 2-c: Result of the fit of $m_{{\mathrm {e}} {\gamma}}$ SR1 electron data (without the b tag requirement) to determine $SF_{{\mathrm {e}}\to \gamma}$. The mass spectra are shown post-fit after the application of the derived scale factors. The ratio of data to the total background is included in the lower panel. Uncertainties include the quadratic sum of all statistical and systematic components.
png pdf	Figure 3: Comparison of data and simulated events as a function of $ {{p_{\mathrm {T}}} ^\text {miss}} $ for the combined e and $\mu $ control regions is shown: (upper pane) CR1 with one fake photon, and (lower pane) CR2 with two fake photons. The content of each bin is normalized to its bin width. The ratios of data to background are shown below the two panels. The overall uncertainties are obtained from the sum in quadrature of the statistical and systematic components. Note the Diboson background includes WW, WZ, ZZ, W+gamma, and Z+gamma.
png pdf	Figure 3-a: Comparison of data and simulated events as a function of $ {{p_{\mathrm {T}}} ^\text {miss}} $ for the combined e and $\mu $ for the CR1 control region with one fake photon. The content of each bin is normalized to its bin width. The ratio of data to background is shown below. The overall uncertainties are obtained from the sum in quadrature of the statistical and systematic components. Note the Diboson background includes WW, WZ, ZZ, W+gamma, and Z+gamma.
png pdf	Figure 3-b: Comparison of data and simulated events as a function of $ {{p_{\mathrm {T}}} ^\text {miss}} $ for the combined e and $\mu $ for the CR2 control region with two fake photons. The content of each bin is normalized to its bin width. The ratio of data to background is shown below. The overall uncertainties are obtained from the sum in quadrature of the statistical and systematic components. Note the Diboson background includes WW, WZ, ZZ, W+gamma, and Z+gamma.
png pdf	Figure 4: Comparison of data and MC simulation in $ {{p_{\mathrm {T}}} ^\text {miss}} $ for the combined (e and $\mu $) signal regions: (upper pane) SR1 with one reconstructed photon and (lower pane) SR2 with two reconstructed photons. Each bin is normalized by its bin width.
png pdf	Figure 4-a: Comparison of data and MC simulation in $ {{p_{\mathrm {T}}} ^\text {miss}} $ for the combined (e and $\mu $) SR1 signal region with one reconstructed photon. Each bin is normalized by its bin width.
png pdf	Figure 4-b: Comparison of data and MC simulation in $ {{p_{\mathrm {T}}} ^\text {miss}} $ for the combined (e and $\mu $) SR2 signal region with two reconstructed photons. Each bin is normalized by its bin width.
png pdf	Figure 5: The observed (upper pane) and expected (lower pane) CLs upper limits on the cross section at 95% CL in the $m_{\tilde{\mathrm{t}}}$-$m_{\tilde{\chi}^{0} _1}$ plane.
png pdf	Figure 5-a: The observed CLs upper limits on the cross section at 95% CL in the $m_{\tilde{\mathrm{t}}}$-$m_{\tilde{\chi}^{0} _1}$ plane.
png pdf	Figure 5-b: The expected CLs upper limits on the cross section at 95% CL in the $m_{\tilde{\mathrm{t}}}$-$m_{\tilde{\chi}^{0} _1}$ plane.
png pdf	Figure 6: Observed and expected mean exclusions at the 95% CL in the top squark and bino mass plane, and their ranges of uncertainties given by the contours at the 68% CL. The region to the left of the contour for $m_{\tilde{\mathrm{t}}}-m_{\tilde{\chi}^{0} _1} < m_t$ is excluded by this analysis.

Tables
png pdf	Table 1: Software used in MC simulations of backgrounds.
png pdf	Table 2: Measured values of scale factors, $SF_{{\mathrm {Z}} (\gamma)}$ and $SF_{{\mathrm {e}}\to \gamma}$, used to correct the MC predictions for Z+jets and Z+$ {\gamma}$ backgrounds and electron-to-photon misidentification. For the electron+jets channel, the product of the two is applied to Z+jets and Z+$ {\gamma}$ backgrounds. In the muon+jets channel, only the $SF_{{\mathrm {Z}} (\gamma)}$ scale factor is relevant. The first uncertainties are statistical, obtained from uncertainties in the resultant fits. The second uncertainties correspond to differences in the resulting scale factors, added in quadrature, that were obtained by allowing each systematic uncertainty to fluctuate up and down by one standard deviation and refitting.
png pdf	Table 3: Summary of systematic uncertainties: the dominant uncertainties are extracted from the control region. In the calculation of the upper limits, the normalizations of the $ {{\mathrm {t}\overline {\mathrm {t}}}} $+jets and $ {{\mathrm {t}\overline {\mathrm {t}}}} $+$ {\gamma}$ backgrounds are allowed to float freely in the fit. Check marks indicate the uncertainties that affect the shape of $ {{p_{\mathrm {T}}} ^\text {miss}} $.
png pdf	Table 4: Observed data and expected background yields for the combined (e and $\mu $) signal regions. Expectations from two GGM signal model points are included, for which (460, 175) refers to $m_{\tilde{\mathrm{t}}} = $ 460 GeV and $m_{\tilde{\chi}^{0}_1} = $ 175 GeV, and similarly for (560, 325). The first group of uncertainties is statistical and the second is systematic.

Summary

We have presented a search for natural gauge-mediated supersymmetry breaking in events with a top quark pair and one or two photons. No significant deviation is found in the distribution of the missing transverse momentum between data and expected SM backgrounds that would indicate the presence of new physics. Upper limits on signal cross sections are calculated for a range of top squark and bino masses. Top squark masses between 650 to 730 GeV are excluded at the 95% CL corresponding to the neutralino mass range of 500 to 150 GeV, respectively. These top squark mass points are obtained using the $-1 \sigma$ theoretical excursion from the observed exclusion mean. These results set the most stringent exclusions on top squark masses in gauge-mediated supersymmetric model considered here.

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