Search for direct pair production of the top squark in all-hadronic final states in proton-proton collisions at √s = 8 TeV with the ATLAS detector

Abstract

The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of 20.1 fb−1 of proton-proton collision data at √s = 8 TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>t</mi><mo accent="false">¯<!-- ¯ --></mo></mover><mtext> </mtext><mo stretchy="false">→<!-- → --></mo><mi>t</mi><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mn>0</mn></msubsup></math> or <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>t</mi><mo accent="false">¯<!-- ¯ --></mo></mover><mtext> </mtext><mo stretchy="false">→<!-- → --></mo><mi>b</mi><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mrow class="MJX-TeXAtom-ORD"><mo>±<!-- ± --></mo></mrow></msubsup><mtext> </mtext><mo stretchy="false">→<!-- → --></mo><mi>b</mi><msup><mrow class="MJX-TeXAtom-ORD"><mi>W</mi></mrow><mrow class="MJX-TeXAtom-ORD"><mrow><mo>(</mo><mo>∗<!-- ∗ --></mo><mo>)</mo></mrow></mrow></msup><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mn>0</mn></msubsup></math>, where <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mn>0</mn></msubsup><mtext> </mtext><mrow><mo>(</mo><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mrow class="MJX-TeXAtom-ORD"><mo>±<!-- ± --></mo></mrow></msubsup><mo>)</mo></mrow></math> denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fullyhadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>t</mi><mo accent="false">¯<!-- ¯ --></mo></mover><mtext> </mtext><mo stretchy="false">→<!-- → --></mo><mi>t</mi><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mn>0</mn></msubsup></math>. For a branching fraction of 100%, top squark masses in the range 270–645 GeV are excluded for <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mn>0</mn></msubsup></math> masses below 30 GeV. For a branching fraction of 50% to either <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>t</mi><mo accent="false">¯<!-- ¯ --></mo></mover><mtext> </mtext><mo stretchy="false">→<!-- → --></mo><mi>t</mi><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mn>0</mn></msubsup></math> or <math xmlns="http://www.w3.org/1998/Math/MathML"><mover><mi>t</mi><mo accent="false">¯<!-- ¯ --></mo></mover><mtext> </mtext><mo stretchy="false">→<!-- → --></mo><mi>b</mi><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mrow class="MJX-TeXAtom-ORD"><mo>±<!-- ± --></mo></mrow></msubsup></math>, and assuming the <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>&#x03C7;<!-- χ --></mi><mo accent="false">&#x00AF;<!-- ¯ --></mo></mover></mrow><mn>1</mn><mrow class="MJX-TeXAtom-ORD"><mo>&#x00B1;<!-- ± --></mo></mrow></msubsup></math> mass to be twice the <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mn>0</mn></msubsup></math> mass, top squark masses in the range 250–550 GeV are excluded for<math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow class="MJX-TeXAtom-ORD"><mover><mi>χ<!-- χ --></mi><mo accent="false">¯<!-- ¯ --></mo></mover></mrow><mn>1</mn><mn>0</mn></msubsup></math> masses below 60 GeV.