Probing the low mass pseudoscalar in the flipped two-Higgs-doublet model

The phenomenology of the flipped two-Higgs-doublet model (2HDM) is relatively less explored so far, as compared to the other, commonly discussed, types. It is found that this scenario, like several others, admits of a light neutral pseudoscalar <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>A</a:mi></a:math> in the mass range 20–60 GeV, consistently with all current experimental data and theoretical constraints. However, the fact that such a pseudoscalar decays overwhelmingly into a <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>b</c:mi><c:mover accent="true"><c:mi>b</c:mi><c:mo stretchy="false">¯</c:mo></c:mover></c:math> pair makes its identification at the Large Hadron Collider (LHC) a challenging task. After identifying the region of the flipped 2HDM parameter space yielding a light pseudoscalar, we identify a useful search channel in the process <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>p</g:mi><g:mi>p</g:mi><g:mo stretchy="false">→</g:mo><g:mi>A</g:mi><g:mi>Z</g:mi><g:mo stretchy="false">(</g:mo><g:msup><g:mi>Z</g:mi><g:mo>*</g:mo></g:msup><g:mo stretchy="false">)</g:mo><g:mo stretchy="false">→</g:mo><g:mi>b</g:mi><g:mover accent="true"><g:mi>b</g:mi><g:mo stretchy="false">¯</g:mo></g:mover><g:msup><g:mo>ℓ</g:mo><g:mo>+</g:mo></g:msup><g:msup><g:mo>ℓ</g:mo><g:mo>−</g:mo></g:msup></g:math>. A cut-based analysis, followed by one based on boosted decision trees, shows that the light-<o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi>A</o:mi></o:math> scenario in flipped 2HDM should be detectable with rather high statistical significance at the high-luminosity LHC run, even after including systematic uncertainties. Furthermore, part of the parameter space, especially around <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:msub><q:mi>m</q:mi><q:mi>A</q:mi></q:msub><q:mo>=</q:mo><q:mn>30</q:mn><q:mi>–</q:mi><q:mn>40</q:mn><q:mtext> </q:mtext><q:mtext> </q:mtext><q:mi>GeV</q:mi></q:math>, is amenable to detection at the discovery level within Run-2 itself.