Strategy to measure tau <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>g</mml:mi><mml:mo>−</mml:mo><mml:mn>2</mml:mn></mml:math> via photon fusion in LHC proton collisions

Measuring the tau-lepton (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>τ</a:mi></a:math>) anomalous magnetic moment <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>a</c:mi><c:mi>τ</c:mi></c:msub><c:mo>=</c:mo><c:mo stretchy="false">(</c:mo><c:msub><c:mi>g</c:mi><c:mi>τ</c:mi></c:msub><c:mo>−</c:mo><c:mn>2</c:mn><c:mo stretchy="false">)</c:mo><c:mo>/</c:mo><c:mn>2</c:mn></c:math> in photon fusion production (<g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>γ</g:mi><g:mi>γ</g:mi><g:mo stretchy="false">→</g:mo><g:mi>τ</g:mi><g:mi>τ</g:mi></g:math>) tests foundational Standard Model principles. However, <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mi>γ</j:mi><j:mi>γ</j:mi><j:mo stretchy="false">→</j:mo><j:mi>τ</j:mi><j:mi>τ</j:mi></j:math> eludes observation in LHC proton collisions (pp) despite enhanced new physics sensitivity from higher-mass reach than existing probes. We propose a strategy to measure <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>pp</m:mi><m:mo stretchy="false">→</m:mo><m:mi mathvariant="normal">p</m:mi><m:mo stretchy="false">(</m:mo><m:mi>γ</m:mi><m:mi>γ</m:mi><m:mo stretchy="false">→</m:mo><m:mi>τ</m:mi><m:mi>τ</m:mi><m:mo stretchy="false">)</m:mo><m:mi mathvariant="normal">p</m:mi></m:math> by introducing the overlooked electron-muon signature with vertex isolation for signal extraction. Applying the effective field theory of dipole moments, we estimate 95% confidence level sensitivity of <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mo>−</u:mo><u:mn>0.0092</u:mn><u:mo>&lt;</u:mo><u:msub><u:mi>a</u:mi><u:mi>τ</u:mi></u:msub><u:mo>&lt;</u:mo><u:mn>0.011</u:mn></u:math> assuming <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mn>300</w:mn><w:mtext> </w:mtext><w:mtext> </w:mtext><w:msup><w:mi>fb</w:mi><w:mrow><w:mo>−</w:mo><w:mn>1</w:mn></w:mrow></w:msup></w:math> luminosity and 5% systematics. This fourfold improvement beyond existing constraints opens a crucial path to unveiling new physics imprinted in tau-lepton dipoles. Published by the American Physical Society 2024