Multi-lepton probes of new physics and lepton-universality in top-quark interactions

We explore the sensitivity to new physics (NP) in the associated production of top-quarks with leptons <math altimg="si1.svg"><mi>p</mi><mi>p</mi><mo stretchy="false">→</mo><mi>t</mi><mover accent="true"><mrow><mi>t</mi></mrow><mrow><mo stretchy="false">¯</mo></mrow></mover><msup><mrow><mi>ℓ</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><msup><mrow><mi>ℓ</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math>, which leads to the multi-leptons signals <math altimg="si2.svg"><mi>p</mi><mi>p</mi><mo stretchy="false">→</mo><mi>n</mi><mi>ℓ</mi><mo linebreak="goodbreak" linebreakstyle="after">+</mo><mrow><mi mathvariant="monospace">jets</mi></mrow><mo linebreak="goodbreak" linebreakstyle="after">+</mo><msub><mrow><mi>E̸</mi></mrow><mrow><mi>T</mi></mrow></msub></math>, where <math altimg="si3.svg"><mi>n</mi><mo linebreak="goodbreak" linebreakstyle="after">=</mo><mn>2</mn><mo>,</mo><mn>3</mn><mo>,</mo><mn>4</mn></math>. The NP is parameterized via 4-Fermi effective <math altimg="si4.svg"><mi>t</mi><mover accent="true"><mrow><mi>t</mi></mrow><mrow><mo stretchy="false">¯</mo></mrow></mover><msup><mrow><mi>ℓ</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><msup><mrow><mi>ℓ</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math> contact interactions of various types, which are generated by multi-TeV heavy scalar, vector or tensor exchanges in <math altimg="si5.svg"><mi>t</mi><mover accent="true"><mrow><mi>t</mi></mrow><mrow><mo stretchy="false">¯</mo></mrow></mover><mo stretchy="false">→</mo><msup><mrow><mi>ℓ</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><msup><mrow><mi>ℓ</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math>; we focus on the case of <math altimg="si6.svg"><mi>ℓ</mi><mo linebreak="goodbreak" linebreakstyle="after">=</mo><mi>e</mi><mo>,</mo><mi>μ</mi></math>. We match the 4-Fermi <math altimg="si7.svg"><mi>t</mi><mi>t</mi><mi>ℓ</mi><mi>ℓ</mi></math> terms to the SMEFT operators and also give examples of specific underlying heavy physics that can generate such terms. Analysis of the SM signals and corresponding backgrounds shows that the di-lepton and tri-lepton channels are much better probes of the effective <math altimg="si4.svg"><mi>t</mi><mover accent="true"><mrow><mi>t</mi></mrow><mrow><mo stretchy="false">¯</mo></mrow></mover><msup><mrow><mi>ℓ</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">+</mo></mrow></msup><msup><mrow><mi>ℓ</mi></mrow><mrow><mo linebreak="badbreak" linebreakstyle="after">−</mo></mrow></msup></math> 4-Fermi terms than the four-lepton one at the 13 TeV LHC. Therefore, the best sensitivity is obtained in the di- and tri-lepton channels, for which the dominant background <math altimg="si8.svg"><mi>p</mi><mi>p</mi><mo stretchy="false">→</mo><mi>t</mi><mover accent="true"><mrow><mi>t</mi></mrow><mrow><mo stretchy="false">¯</mo></mrow></mover></math> and <math altimg="si9.svg"><mi>p</mi><mi>p</mi><mo stretchy="false">→</mo><mi>W</mi><mi>Z</mi></math>, respectively, can be essentially eliminated after applying the 2ℓ and 3ℓ selections and a sufficiently high invariant mass selection for the opposite sign same flavor (OSSF) lepton-pair. We explore two cases: lepton flavor universal (LFU) NP where the <math altimg="si10.svg"><mi>t</mi><mi>t</mi><mi>e</mi><mi>e</mi></math> and <math altimg="si11.svg"><mi>t</mi><mi>t</mi><mi>μ</mi><mi>μ</mi></math> contact interactions are of same size and LFU violating (LFUV) NP, where the scale of the <math altimg="si11.svg"><mi>t</mi><mi>t</mi><mi>μ</mi><mi>μ</mi></math> terms is assumed to be much lower. We show that in both cases it is possible to obtain new 95% CL bounds on the scale of the <math altimg="si7.svg"><mi>t</mi><mi>t</mi><mi>ℓ</mi><mi>ℓ</mi></math> contact interactions at the level <math altimg="si12.svg"><mi mathvariant="normal">Λ</mi><mo>≳</mo><mn>2</mn><mo linebreak="goodbreak" linebreakstyle="after">−</mo><mn>3</mn></math> TeV, which are considerably tighter than the current bounds on these 4-Fermi terms.