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CMS-SUS-16-044 ; CERN-EP-2017-127

Search for higgsino pair production in pp collisions at $\sqrt{s} = $ 13 TeV in final states with large missing transverse momentum and two Higgs bosons decaying via $\mathrm{H}\to\mathrm{b\bar{b}}$

CMS Collaboration

14 September 2017

Phys. Rev. D 97 (2018) 032007

Abstract: Results are reported from a search for new physics in 13 TeV proton-proton collisions in the final state with large missing transverse momentum and two Higgs bosons decaying via $\mathrm{H}\to\mathrm{b\bar{b}}$. The search uses a data sample accumulated by the CMS experiment at the LHC in 2016, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The search is motivated by models based on gauge-mediated supersymmetry breaking, which predict the electroweak production of a pair of higgsinos, each of which can decay via a cascade process to a Higgs boson and an undetected lightest supersymmetric particle. The observed event yields in the signal regions are consistent with the standard model background expectation obtained from control regions in data. Higgsinos in the mass range 230-770 GeV are excluded at 95% confidence level in the context of a simplified model for the production and decay of approximately degenerate higgsinos.

Links: e-print arXiv:1709.04896 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Additional information on efficiencies needed for reinterpretation of these results are available here Additional technical material for CMS speakers can be found here

Figures
png pdf	Figure 1: Diagram for the gauge-mediated symmetry breaking signal model, $\tilde{\chi}^0_1 \tilde{\chi}^0_1 \to \mathrm{H} \mathrm{H} \tilde{\mathrm{G}} \tilde{\mathrm{G}} $ (TChiHH), where $ \tilde{\mathrm{G}} $ is a goldstino. The NLSPs $ \tilde{\chi}^0_1 $ are not directly pair produced, but are instead produced in the cascade decays of several different combinations of neutralinos and charginos, as described in the text.
png pdf	Figure 2: Distributions of $ {\Delta m} $, $ {\Delta R_\text {max}} $, $ { < m > } $, and $ {{p_{\mathrm {T}}} ^\text {miss}} $ for data and simulated background samples, as well as three signal benchmark points denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. All figures include baseline requirements (except on the variable being plotted in the case of $ {\Delta m} $ and $ {\Delta R_\text {max}} $). The $ {\Delta m} $, $ { < m > } $, and $ {\Delta R_\text {max}} $ distributions also include the 4b selection. The simulation is normalized to the observed data yields. The gray shading indicates the statistical uncertainty in the total simulated background. The vertical dotted lines indicate baseline requirements in the top row figures, the search region mass window in $ { < m > } $ in the bottom left figure, and the $ {{p_{\mathrm {T}}} ^\text {miss}} $ binning in the bottom right figure. The last bin includes overflow.
png pdf	Figure 2-a: Distribution of $ {\Delta m} $ for data and simulated background samples, as well as three signal benchmark points denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. All baseline requirements except for $\Delta m < $ 40 GeV are applied. The distribution also includes the 4b selection. The simulation is normalized to the observed data yields. The gray shading indicates the statistical uncertainty in the total simulated background. The vertical dotted lines indicate baseline requirements. The last bin includes overflow.
png pdf	Figure 2-b: Distribution of $ {\Delta R_\text {max}} $ for data and simulated background samples, as well as three signal benchmark points denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. All baseline requirements except for $ \Delta R_{\text{max}} < $ 2.2 are applied. The distribution also include the 4b selection. The simulation is normalized to the observed data yields. The gray shading indicates the statistical uncertainty in the total simulated background. The vertical dotted lines indicate baseline requirements. The last bin includes overflow.
png pdf	Figure 2-c: Distribution of $ { < m > } $ for data and simulated background samples, as well as three signal benchmark points denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. The figure includes baseline requirements. The distribution also include the 4b selection. The simulation is normalized to the observed data yields. The gray shading indicates the statistical uncertainty in the total simulated background. The vertical dotted lines indicate the search region mass window in $ { < m > } $. The last bin includes overflow.
png pdf	Figure 2-d: Distribution of $ {{p_{\mathrm {T}}} ^\text {miss}} $ for data and simulated background samples, as well as three signal benchmark points denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. The figure includes baseline requirements. The simulation is normalized to the observed data yields. The gray shading indicates the statistical uncertainty in the total simulated background. The vertical dotted lines indicate the $ {{p_{\mathrm {T}}} ^\text {miss}} $ binning. The last bin includes overflow.
png pdf	Figure 3: Distribution of $ { < m > } $ after the baseline selection, showing the agreement between the $ { < m > } $ shapes among the three b tag categories. The comparison is based on simulation including all backgrounds except QCD multijet production, for which the simulation suffers from large statistical uncertainties. QCD multijet events account for less than 5% of the total yield. The vertical dotted lines indicate the Higgs boson mass window.
png pdf	Figure 4: Values of the double ratios $\kappa _{3\text {b}}$ and $\kappa _{4\text {b}}$ obtained from the background simulation for each of the $ {{p_{\mathrm {T}}} ^\text {miss}} $ bins. The error bars correspond to the statistical uncertainty of the background simulation.
png pdf	Figure 5: Comparison of the distributions of $ { < m > } $ in data and simulation in the single-lepton control sample (left) and in the dilepton control sample (right), where in both cases we have integrated over $ {{p_{\mathrm {T}}} ^\text {miss}} $. The overall yields in simulation have been normalized to those observed in data. The gray shading indicates the statistical uncertainty in the total simulated background. The last bin includes overflow.
png pdf	Figure 5-a: Comparison of the distributions of $ { < m > } $ in data and simulation in the single-lepton control sample, where we have integrated over $ {{p_{\mathrm {T}}} ^\text {miss}} $. The overall yields in simulation have been normalized to those observed in data. The gray shading indicates the statistical uncertainty in the total simulated background. The last bin includes overflow.
png pdf	Figure 5-b: Comparison of the distributions of $ { < m > } $ in data and simulation in the dilepton control sample, where we have integrated over $ {{p_{\mathrm {T}}} ^\text {miss}} $. The overall yields in simulation have been normalized to those observed in data. The gray shading indicates the statistical uncertainty in the total simulated background. The last bin includes overflow.
png pdf	Figure 6: Comparison of the ${\kappa}$ values found in the single-lepton control sample, for data and simulated events, for the 3b /2b and 4b /2b ABCD tests in each $ {{p_{\mathrm {T}}} ^\text {miss}} $ bin as well as after integrating over $ {{p_{\mathrm {T}}} ^\text {miss}} $ (labeled as "Inclusive'').
png pdf	Figure 7: Comparison of the $ {\kappa} $ values found in the dilepton control sample, data and simulation, for the 1b /0b ABCD tests in bins of $ {p_{\mathrm {T}}(\ell ^{+}\ell ^{-})} $ as well as after integrating over $ {p_{\mathrm {T}}(\ell ^{+}\ell ^{-})} $ (labeled as "Inclusive'').
png pdf	Figure 8: Distributions of $ { < m > } $ in data and two signal benchmark models denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. The points with error bars show the data in the 3b (top) and 4b bins (bottom) for 150 $ < {{p_{\mathrm {T}}} ^\text {miss}} \leq $ 200 GeV (left) and $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 200 GeV (right). The histograms show the shape of the $ { < m > } $ distribution observed in the 2b bin with an overall event yield normalized to those observed in the 3b and 4b samples. The shaded areas reflect the statistical uncertainty in the $ { < m > } $ distribution in the 2b data. The vertical dashed lines denote the boundaries between the HIG and the SBD regions. The ratio plots demonstrate that the shapes are in agreement.
png pdf	Figure 8-a: Distribution of $ { < m > } $ in data and two signal benchmark models denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. The points with error bars show the data in the 3b bins for 150 $ < {{p_{\mathrm {T}}} ^\text {miss}} \leq $ 200 GeV. The histograms show the shape of the $ { < m > } $ distribution observed in the 2b bin with an overall event yield normalized to those observed in the 3b sample. The shaded areas reflect the statistical uncertainty in the $ { < m > } $ distribution in the 2b data. The vertical dashed lines denote the boundaries between the HIG and the SBD regions. The ratio plots demonstrate that the shapes are in agreement.
png pdf	Figure 8-b: Distribution of $ { < m > } $ in data and two signal benchmark models denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. The points with error bars show the data in the 3b bins for $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 200 GeV. The histograms show the shape of the $ { < m > } $ distribution observed in the 2b bin with an overall event yield normalized to those observed in the 3b sample. The shaded areas reflect the statistical uncertainty in the $ { < m > } $ distribution in the 2b data. The vertical dashed lines denote the boundaries between the HIG and the SBD regions. The ratio plots demonstrate that the shapes are in agreement.
png pdf	Figure 8-c: Distribution of $ { < m > } $ in data and two signal benchmark models denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. The points with error bars show the data in the 4b bins for 150 $ < {{p_{\mathrm {T}}} ^\text {miss}} \leq $ 200 GeV. The histograms show the shape of the $ { < m > } $ distribution observed in the 2b bin with an overall event yield normalized to those observed in the 4b sample. The shaded areas reflect the statistical uncertainty in the $ { < m > } $ distribution in the 2b data. The vertical dashed lines denote the boundaries between the HIG and the SBD regions. The ratio plots demonstrate that the shapes are in agreement.
png pdf	Figure 8-d: Distribution of $ { < m > } $ in data and two signal benchmark models denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. The points with error bars show the data in the 4b bins for $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 200 GeV. The histograms show the shape of the $ { < m > } $ distribution observed in the 2b bin with an overall event yield normalized to those observed in the 4b sample. The shaded areas reflect the statistical uncertainty in the $ { < m > } $ distribution in the 2b data. The vertical dashed lines denote the boundaries between the HIG and the SBD regions. The ratio plots demonstrate that the shapes are in agreement.
png pdf root	Figure 9: Expected (dashed black line) and observed (solid black line) excluded cross sections at 95% CL as a function of the higgsino mass. The theoretical cross section for the TChiHH simplified model is shown as the red solid line.

Tables
png pdf	Table 1: Event yields obtained from simulated event samples scaled to an integrated luminosity of 35.9 fb$^{-1}$, as the event selection criteria are applied. The category "${\mathrm{t} {}\mathrm{\bar{t}}} $+$X$'' is dominated by $ {\mathrm{t} {}\mathrm{\bar{t}}} $ (98.5%), but also includes small contributions from ${\mathrm{t} {}\mathrm{\bar{t}}} {\mathrm{t} {}\mathrm{\bar{t}}} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{W} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{Z} $, ${\mathrm{t} {}\mathrm{\bar{t}}} \mathrm{H} $, and ${\mathrm{t} {}\mathrm{\bar{t}}} \gamma $ backgrounds. The category "V+jets'' includes Z+jets and W+jets backgrounds in all their decay modes. The category "Other'' includes ZZ, WZ, WW, WH ($\to {\mathrm{b} \mathrm{\bar{b}}}$), and ZH ($\to {\mathrm{b} \mathrm{\bar{b}}}$) processes. The event selection requirements listed above the horizontal line in the middle of the table are defined as the baseline selection. The trigger efficiency is applied as an event weight and is first taken into account in the $ {{p_{\mathrm {T}}} ^\text {miss}} > $ 150 GeV row. The uncertainties in the "Total SM bkg.'' column is statistical only. The columns corresponding to the yields for three signal benchmark points are labeled by TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. The simulated samples for TChiHH(225,1), TChiHH(400,1), and TChiHH(700,1) are equivalent to 10, 100, and over 1000 times the data sample, respectively, so the statistical uncertainties in the signal yields are small.
png pdf	Table 2: Observed event yields for all control regions--(HIG,2b), (SBD,2b), (SBD,3b), and (SBD, 4b)--and the two signal regions--(HIG,3b) and (HIG, 4b)--in each of the four $ {{p_{\mathrm {T}}} ^\text {miss}} $ bins. The second column shows the results of the global fit which uses the observed yields in all control and signal regions, under the background-only hypothesis ($r=$ 0). The third column gives the predicted SM background rates in the signal regions obtained via the predictive fit which only takes as input the observed event yields in the control regions. The expected signal yields for three signal benchmark points denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV, are also shown for reference.
png pdf	Table 3: Range of values for the systematic uncertainties in the signal efficiency and acceptance across all analysis bin for three signal benchmark points denoted as TChiHH($ {m_{\tilde{\chi}^0_1}} $, $ {m_{\tilde{\mathrm{G}}}} $), with $ {m_{\tilde{\chi}^0_1}} $ and $ {m_{\tilde{\mathrm{G}}}} $ in units of GeV. Uncertainties due to a particular source are treated as fully correlated among bins, while uncertainties due to different sources are treated as uncorrelated.

Summary

A search for an excess of events is performed in proton-proton collisions in the channel with two Higgs bosons and large missing transverse momentum ($ {p_{\mathrm{T}}}^{\text{miss}} $), with each of the Higgs bosons reconstructed in its $\mathrm{H}\to\mathrm{b\bar{b}}$ decay. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$ at $\sqrt{s} =13$ TeV. Because the signal has four b quarks, while the background is dominated by $\mathrm{t\bar{t}}$ events containing only two b quarks from the $\mathrm{t}$ quark decays, the analysis is binned in the number of b-tagged jets. In each event, the mass difference between the two Higgs boson candidates is required to be small, and the average mass of the two candidates is used in conjunction with the number of observed b tags to define signal and sideband regions. The observed event yields in these regions are used to obtain estimates for the standard model background in the signal regions without input from simulated event samples. The data are also binned in regions of ${p_{\mathrm{T}}}^{\text{miss}}$ to enhance the sensitivity to the signal. The observed event yields in the signal regions are consistent with the background predictions. These results are interpreted in the context of a model in which each higgsino decays into a Higgs boson and a nearly massless lightest supersymmetric particle (LSP), which is weakly interacting. Such a scenario occurs in gauge-mediated supersymmetry breaking models, in which the LSP is a goldstino. The cross section calculation assumes that the higgsino sector is mass degenerate and sums over the cross sections for the pair production of all relevant combinations of higgsinos, but all decays are assumed to be prompt. Higgsinos with masses in the range 230 to 770 GeV are excluded at 95% confidence level. These results constitute the most stringent exclusion limits on this model to date.

Additional Figures
png pdf root	Additional Figure 1: (a) Correlation and (b) covariance matrices for the background rates obtained with the predictive fit in the 8 signal bins. "MET1'' corresponds to 150 $ < p_{\rm T}^{\rm miss}\leq $ 200 GeV, "MET2'' to 200 $ < p_{\rm T}^{\rm miss}\leq $ 300 GeV, "MET3'' to 300 $ < p_{\rm T}^{\rm miss}\leq $ 450 GeV, and "MET4'' to $p_{\rm T}^{\rm miss} > $ 450 GeV.
png pdf root	Additional Figure 1-a: Correlation matrix for the background rates obtained with the predictive fit in the 8 signal bins. "MET1'' corresponds to 150 $ < p_{\rm T}^{\rm miss}\leq $ 200 GeV, "MET2'' to 200 $ < p_{\rm T}^{\rm miss}\leq $ 300 GeV, "MET3'' to 300 $ < p_{\rm T}^{\rm miss}\leq $ 450 GeV, and "MET4'' to $p_{\rm T}^{\rm miss} > $ 450 GeV.
png pdf root	Additional Figure 1-b: Covariance matrix for the background rates obtained with the predictive fit in the 8 signal bins. "MET1'' corresponds to 150 $ < p_{\rm T}^{\rm miss}\leq $ 200 GeV, "MET2'' to 200 $ < p_{\rm T}^{\rm miss}\leq $ 300 GeV, "MET3'' to 300 $ < p_{\rm T}^{\rm miss}\leq $ 450 GeV, and "MET4'' to $p_{\rm T}^{\rm miss} > $ 450 GeV.
png pdf	Additional Figure 2: A posteriori expected discovery significance and observed significance for the TChiHH model as a function of the higgsino mass.
png pdf root	Additional Figure 3: Efficiency for the logical OR of the various $p_{\rm T}^{\rm miss}$ triggers used in the analysis as a function of $p_{\rm T}^{\rm miss}$ and $H_{\rm T}$.
png pdf root	Additional Figure 4: Efficiency to reconstruct all b-quarks from the two Higgs boson decays as individual jets, given that all four of the b-quarks are within acceptance, $p_{\mathrm{T}} > $ 30 GeV and $ | \eta | < $ 2.4. The efficiency is calculated based on the TChiHH model with all higgsino masses included.
png pdf root	Additional Figure 5: Efficiency to pass the mass requirement, $ < m > $ between 100 and 140 GeV, given that all b-quarks from the di-Higgs system are reconstructed as individual jets. The efficiency is calculated based on the TChiHH model with all higgsino masses included.

Additional Tables
png pdf	Additional Table 1: Total background and signal yields in simulation for selections based on the CSVv2 b-tagging discriminator. To be compared to the equivalent table based on the DeepCSV discriminator used in the analysis.
png pdf	Additional Table 2: Total background and signal yields in simulation for selections based on the DeepCSV b-tagging discriminator. The background is dominated by events with 2 true b quarks, while the signal has 4 b quarks. Compared to CSVv2, the high b-tagging efficiency of the DeepCSV algorithm extends the a-posteriori expected exclusion limit by approximately 150 GeV in the higgsino mass, corresponding to a cross-section that is 3 times smaller. This gain in mass reach is aided by the increasingly more favorable kinematics of the signal at higher higgsino masses and the observed yields in the signal regions.

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