CMS-PAS-SUS-19-002

CMS-PAS-SUS-19-002
Search for supersymmetry with a compressed mass spectrum in events with a soft $\tau$ lepton, a highly energetic jet, and large missing transverse momentum in proton-proton collisions at $\sqrt{s}=$ 13 TeV
CMS Collaboration
August 2019

Abstract: The first search for supersymmetry in events with one soft tau lepton, one energetic jet from initial state radiation, and transverse momentum imbalance is presented. These event signatures are consistent with those from supersymmetric models exhibiting co-annihilation between the scalar tau lepton ( $\tilde{\tau}$ ) and the lightest neutralino ( $\tilde\chi^{0}_{1}$ ) that could generate the observed relic density of dark matter. The data correspond to an integrated luminosity of 77.2 fb $^{-1}$ of proton-proton collisions at $\sqrt{s}=$ 13 TeV collected with the CMS detector at the CERN LHC in 2016 and 2017. The results are interpreted considering a supersymmetric scenario with a small mass difference ( $\Delta m$ ) between the chargino ( $\tilde{\chi}^{\pm}_{1}$ ) or the next-to-lightest neutralino ( $\tilde{\chi}^{0}_{2}$ ), and the lightest neutralino. The mass of the $\tilde{\tau}$ is defined as the average of the $\tilde\chi^{\pm}_{1}$ / $\tilde\chi^{0}_{2}$ and $\tilde{\chi}^{0}_{1}$ masses. The data are found to be consistent with the standard model background predictions. Upper limits at 95% confidence level are set on the $\tilde{\chi}^{\pm}_{1}$ , $\tilde\chi^{0}_{2}$ , and $\tilde{\tau}$ production cross sections for $\Delta{m}(\tilde{\chi}^{\pm}_{1}, \tilde\chi^{0}_{1}) =$ 50 GeV, resulting in a lower mass limit of 290 GeV on the mass of the $\tilde\chi^{\pm}_{1}$ / $\tilde\chi^{0}_{2}$ , which is the most stringent to date.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PRL 124 (2020) 041803. The superseded preliminary plots can be found here.

Figures	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The $m_{\textrm {T}} ({{p_{\mathrm {T}}} ^\text {miss}}, {\tau _\mathrm {h}})$ distribution for SR events with 2016 data (left) and 2017 data (right). On the top canvas of the figures, the solid colors correspond to the expected background processes, the black dots to the observed data, and the dashed lines to the expected signal from simulation. The bottom canvas of the figures show the ratio between the observed data and the total expected background. The shaded band correspond to the total statistical uncertainty.
png pdf	Figure 1-a: The $m_{\textrm {T}} ({{p_{\mathrm {T}}} ^\text {miss}}, {\tau _\mathrm {h}})$ distribution for SR events with 2016 data (left) and 2017 data (right). On the top canvas of the figures, the solid colors correspond to the expected background processes, the black dots to the observed data, and the dashed lines to the expected signal from simulation. The bottom canvas of the figures show the ratio between the observed data and the total expected background. The shaded band correspond to the total statistical uncertainty.
png pdf	Figure 1-b: The $m_{\textrm {T}} ({{p_{\mathrm {T}}} ^\text {miss}}, {\tau _\mathrm {h}})$ distribution for SR events with 2016 data (left) and 2017 data (right). On the top canvas of the figures, the solid colors correspond to the expected background processes, the black dots to the observed data, and the dashed lines to the expected signal from simulation. The bottom canvas of the figures show the ratio between the observed data and the total expected background. The shaded band correspond to the total statistical uncertainty.
png pdf	Figure 2: The plot on the left shows the 95% confidence level (CL) upper limits on the $\tilde{\chi}^{\pm}_{1}$ and $\tilde{\chi}^{0}_{2}$ pair production cross sections in SSM1, as function of $m(\tilde{\chi}^{\pm}_{1})$ . The solid blue line corresponds to the theoretical cross section. The observed limit is shown with the solid black line, while the expected limit is represented with the dashed black line. The yellow (green) band corresponds to the one (two) standard deviation from the central value of the expected limit. The plot on the right, top canvas, presents the ratio of the 95% CL upper limit on the direct $\tilde{\tau}$ pair production signal cross section in SSM2 to the theoretical cross section, as function of $m(\tilde{\tau})$ and $\Delta m(\tilde{\tau}, \tilde{\chi}_{1}^{0})$ . The bottom canvas presents the 95% CL upper limits on the direct $\tilde{\tau}$ pair production signal cross sections in SSM2, as function of $m(\tilde{\tau})$ for $m(\tilde{\tau}) - m(\tilde{\chi}^{0}_{1}) =$ 50 GeV. The extremely small $\tilde{\tau} \tilde{\tau}$ production cross sections make these SSM2 scenarios very challenging, especially when $\Delta m(\tilde{\tau}, \tilde{\chi}_{1}^{0}) <$ 50 GeV. For a $\tilde{\tau}$ mass of 100 GeV, the observed limit is about 10 times the theoretical cross section.
png pdf	Figure 2-a: The plot on the left shows the 95% confidence level (CL) upper limits on the $\tilde{\chi}^{\pm}_{1}$ and $\tilde{\chi}^{0}_{2}$ pair production cross sections in SSM1, as function of $m(\tilde{\chi}^{\pm}_{1})$ . The solid blue line corresponds to the theoretical cross section. The observed limit is shown with the solid black line, while the expected limit is represented with the dashed black line. The yellow (green) band corresponds to the one (two) standard deviation from the central value of the expected limit. The plot on the right, top canvas, presents the ratio of the 95% CL upper limit on the direct $\tilde{\tau}$ pair production signal cross section in SSM2 to the theoretical cross section, as function of $m(\tilde{\tau})$ and $\Delta m(\tilde{\tau}, \tilde{\chi}_{1}^{0})$ . The bottom canvas presents the 95% CL upper limits on the direct $\tilde{\tau}$ pair production signal cross sections in SSM2, as function of $m(\tilde{\tau})$ for $m(\tilde{\tau}) - m(\tilde{\chi}^{0}_{1}) =$ 50 GeV. The extremely small $\tilde{\tau} \tilde{\tau}$ production cross sections make these SSM2 scenarios very challenging, especially when $\Delta m(\tilde{\tau}, \tilde{\chi}_{1}^{0}) <$ 50 GeV. For a $\tilde{\tau}$ mass of 100 GeV, the observed limit is about 10 times the theoretical cross section.
png pdf	Figure 2-b: The plot on the left shows the 95% confidence level (CL) upper limits on the $\tilde{\chi}^{\pm}_{1}$ and $\tilde{\chi}^{0}_{2}$ pair production cross sections in SSM1, as function of $m(\tilde{\chi}^{\pm}_{1})$ . The solid blue line corresponds to the theoretical cross section. The observed limit is shown with the solid black line, while the expected limit is represented with the dashed black line. The yellow (green) band corresponds to the one (two) standard deviation from the central value of the expected limit. The plot on the right, top canvas, presents the ratio of the 95% CL upper limit on the direct $\tilde{\tau}$ pair production signal cross section in SSM2 to the theoretical cross section, as function of $m(\tilde{\tau})$ and $\Delta m(\tilde{\tau}, \tilde{\chi}_{1}^{0})$ . The bottom canvas presents the 95% CL upper limits on the direct $\tilde{\tau}$ pair production signal cross sections in SSM2, as function of $m(\tilde{\tau})$ for $m(\tilde{\tau}) - m(\tilde{\chi}^{0}_{1}) =$ 50 GeV. The extremely small $\tilde{\tau} \tilde{\tau}$ production cross sections make these SSM2 scenarios very challenging, especially when $\Delta m(\tilde{\tau}, \tilde{\chi}_{1}^{0}) <$ 50 GeV. For a $\tilde{\tau}$ mass of 100 GeV, the observed limit is about 10 times the theoretical cross section.

Summary

In summary, we have presented a search for compressed supersymmetry in the stau-neutralino (

$\tilde{\tau}-\tilde{\chi}^{0}_{1}$ ) coannihilation region. It is the first collider search with exactly one soft, hadronically-decaying tau (

$\tau$ ) lepton and a large transverse momentum imbalance (

$p_{\mathrm{T}}^{\text{miss}}$ ) resulting from the recoil effect of high transverse momentum jet from initial state radiation (ISR). The search utilizes data corresponding to an integrated luminosity of 77.2 fb

$^{-1}$ collected in 2016 and 2017 with the CMS detector in proton-proton collisions at

$\sqrt{s} =$ 13 TeV. This particular search targets compressed mass spectra where the mass difference between the chargino (

$\tilde{\chi}^{\pm}_{1}$ ), or the next-to-lightest neutralino (

$\tilde{\chi}^{0}_{2}$ ), and the neutralino (

$\tilde{\chi}^{0}_{1}$ ), denoted by (

$\Delta{m}$ ), is 50 GeV. This is motivated by models considering

$\tilde{\tau}-\tilde{\chi}^{0}_{1}$ co-annihilation aiming to maintain consistency in the estimation of the relic dark matter density between particle physics and cosmology. In the context of the minimal supersymmetric standard model (MSSM), the search considers purely electroweak production of

$\tilde{\tau}$ s via cascading decays of

$\tilde{\chi}^{\pm}_{1}$ s and

$\tilde{\chi}^{0}_{2}$ s as well as direct production of

$\tilde{\tau}$ s. The data do not reveal any evidence for new physics. The results are used to exclude a range of

$\tilde\chi^{\pm}_{1}$ masses for a mass splitting

$\Delta{m}(\tilde{\chi}^{\pm}_{1},\tilde{\chi}^{0}_{1})$ of 50 GeV. For

$\Delta{m}(\tilde{\chi}^{\pm}_{1},\tilde{\chi}^{0}_{1}) =$ 50 GeV and

$Br(\tilde{\chi}^{\pm}_{1}\rightarrow{\tilde{\tau}\nu}\rightarrow{\tau\tilde{\chi}^{0}_{1}\nu}) = 100%$ ,

$\tilde\chi^{\pm}_{1}$ masses up to 290 GeV are excluded at 95% CL. This sensitivity exceeds that of other

$\tilde\tau$ searches to date.

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