Search for bottom squark pair production in proton-proton collisions at [Formula: see text] TeV with the ATLAS detector

Abstract

The result of a search for pair production of the supersymmetric partner of the Standard Model bottom quark ([Formula: see text]) is reported. The search uses 3.2 fb[Formula: see text] of pp collisions at [Formula: see text] TeV collected by the ATLAS experiment at the Large Hadron Collider in 2015. Bottom squarks are searched for in events containing large missing transverse momentum and exactly two jets identified as originating from b-quarks. No excess above the expected Standard Model background yield is observed. Exclusion limits at 95 % confidence level on the mass of the bottom squark are derived in phenomenological supersymmetric R-parity-conserving models in which the [Formula: see text] is the lightest squark and is assumed to decay exclusively via [Formula: see text], where [Formula: see text] is the lightest neutralino. The limits significantly extend previous results; bottom squark masses up to 800 (840) GeV are excluded for the [Formula: see text] mass below 360 (100) GeV whilst differences in mass above 100 GeV between the [Formula: see text] and the [Formula: see text] are excluded up to a [Formula: see text] mass of 500 GeV.

Fig. 1

Left $m_{bb}$ distribution in CRzA before the final selection $m_{bb}>200$ GeV is applied (indicated by the arrow). Right $p_{\text{T}}$ of the leading jet in CRttB. In both distributions the MC normalization is rescaled using the results from the background-only fit, showing good agreement between data and the predicted SM shapes. The shaded band includes statistical and detector-related systematic uncertainties as detailed in Sect. 7 and the last bin includes overflows

Fig. 2

Left $m_{\mathrm{CT}}$ distribution in SRA250 with all the selection criteria applied except the $m_{\mathrm{CT}}$ threshold. Right $m_{bb}$  distribution in SRA250 with all selection criteria applied except the $m_{bb}$ requirement. The arrows indicate the final applied selection. The shaded band includes statistical and detector-related systematic uncertainties. The SM backgrounds are normalized to the values determined in the fit. For illustration the distributions expected for one signal model with bottom squark and neutralino masses of 700 and 1 GeV, respectively, are overlaid. The last bin includes overflows

Fig. 3

Left $E_{\mathrm{T}}^{\mathrm{miss}}$ distribution in SRB with all the selection criteria applied except the $E_{\mathrm{T}}^{\mathrm{miss}}$ threshold. Right leading jet $p_{\text{T}}$ distribution in SRB with all the selection criteria applied except the selection on the leading jet $p_{\text{T}}$ itself. The arrows indicate the final selection applied in SRB. The shaded band includes statistical and detector-related systematic uncertainties. The SM backgrounds are normalized to the values determined in the fit. For illustration the distributions expected for one signal model with bottom squark and neutralino masses of 400 and 300 GeV, respectively, are overlaid. The last bin includes overflows

Fig. 4

Observed and expected exclusion limits at 95 % CL, as well as $\pm 1\mathit{\sigma }$ variation of the expected limit, in the ${\tilde{b}}_1^{}$–${\tilde{\mathit{\chi }}}_1^0$ mass plane. The SR with the best expected sensitivity is adopted for each point of the parameter space. The yellow band around the expected limit (dashed line) shows the impact of the experimental and SM background theoretical uncertainties. The dotted lines show the impact on the observed limit of the variation of the nominal signal cross-section by $\pm 1\mathit{\sigma }$ of its theoretical uncertainties. The exclusion limits from the Run-1 ATLAS searches [72, 81] and from the 13 TeV monojet search [82] are also superimposed. The latter limit is only published for values of $m_{{\tilde{b}}_1^{}}-m_{{\tilde{\mathit{\chi }}}_1^0}=$ 5 and 20 GeV