Leveraging on-shell interference to search for FCNCs of the top quark and the Z boson

Lucas Cremer (Fakultät für Physik, TU Dortmund, Dortmund, 44221, Germany) ; Johannes Erdmann (III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany) ; Roni Harnik (Fermilab Accelerator Laboratory, Batavia, IL, USA) ; Jan Späh (III. Physikalisches Institut A, RWTH Aachen University, Aachen, Germany) ; Emmanuel Stamou (Fakultät für Physik, TU Dortmund, Dortmund, 44221, Germany)

Flavour-changing-neutral currents (FCNCs) involving the top quark are highly suppressed within the Standard Model (SM). Hence, any signal in current or planned future collider experiments would constitute a clear manifestation of physics beyond the SM. We propose a novel, interference-based strategy to search for top-quark FCNCs involving the Z boson that has the potential to complement traditional search strategies due to a more favourable luminosity scaling. The strategy leverages on-shell interference between the FCNC and SM decay of the top quark into hadronic final states. We estimate the feasibility of the most promising case of anomalous tZc couplings using Monte Carlo simulations and a simplified detector simulation. We consider the main background processes and discriminate the signal from the background with a deep neural network that is parametrised in the value of the anomalous tZc coupling. We present sensitivity projections for the HL-LHC and the FCC-hh. We find an expected 95% CL upper limit of $${\mathcal {B}}_{\textrm{excl}}(t\rightarrow Zc) = 6.4 \times 10^{-5}$$ B excl ( t Z c ) = 6.4 × 10 - 5 for the HL-LHC. In general, we conclude that the interference-based approach has the potential to provide both competitive and complementary constraints to traditional multi-lepton searches and other strategies that have been proposed to search for tZc FCNCs.

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      "value": "Flavour-changing-neutral currents (FCNCs) involving the top quark are highly suppressed within the Standard Model (SM). Hence, any signal in current or planned future collider experiments would constitute a clear manifestation of physics beyond the SM. We propose a novel, interference-based strategy to search for top-quark FCNCs involving the Z boson that has the potential to complement traditional search strategies due to a more favourable luminosity scaling. The strategy leverages on-shell interference between the FCNC and SM decay of the top quark into hadronic final states. We estimate the feasibility of the most promising case of anomalous tZc couplings using Monte Carlo simulations and a simplified detector simulation. We consider the main background processes and discriminate the signal from the background with a deep neural network that is parametrised in the value of the anomalous tZc coupling. We present sensitivity projections for the HL-LHC and the FCC-hh. We find an expected 95% CL upper limit of  $${\\mathcal {B}}_{\\textrm{excl}}(t\\rightarrow Zc) = 6.4 \\times 10^{-5}$$  <math> <mrow> <msub> <mi>B</mi> <mtext>excl</mtext> </msub> <mrow> <mo>(</mo> <mi>t</mi> <mo>\u2192</mo> <mi>Z</mi> <mi>c</mi> <mo>)</mo> </mrow> <mo>=</mo> <mn>6.4</mn> <mo>\u00d7</mo> <msup> <mn>10</mn> <mrow> <mo>-</mo> <mn>5</mn> </mrow> </msup> </mrow> </math>   for the HL-LHC. In general, we conclude that the interference-based approach has the potential to provide both competitive and complementary constraints to traditional multi-lepton searches and other strategies that have been proposed to search for tZc FCNCs."
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Published on:
27 September 2023
Publisher:
Springer
Published in:
European Physical Journal C , Volume 83 (2023)
Issue 9
Pages 1-16
DOI:
https://doi.org/10.1140/epjc/s10052-023-11982-1
Copyrights:
The Author(s)
Licence:
CC-BY-4.0

Fulltext files: