Study of the measurement of the τ lepton anomalous magnetic moment in high energy lead-lead collisions at the LHC

The <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>τ</a:mi></a:math> lepton 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:mfrac><c:mrow><c:msub><c:mi>g</c:mi><c:mi>τ</c:mi></c:msub><c:mo stretchy="false">−</c:mo><c:mn>2</c:mn></c:mrow><c:mn>2</c:mn></c:mfrac></c:math> was measured, so far, with a precision of only several percents, despite its high sensitivity to physics beyond the Standard Model such as compositeness or supersymmetry. A new study is presented to improve the sensitivity of the <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:msub><f:mi>a</f:mi><f:mi>τ</f:mi></f:msub></f:math> measurement with photon-photon interactions from ultraperipheral lead-lead collisions at the LHC. The theoretical approach used in this work is based on an effective Lagrangian and on a photon flux implemented in the adraph5 Monte Carlo simulation. Using a multivariate analysis to discriminate the signal from the background processes, a sensitivity to the anomalous magnetic moment <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:msub><h:mrow><h:mi>a</h:mi></h:mrow><h:mrow><h:mi>τ</h:mi></h:mrow></h:msub><h:mo>=</h:mo><h:msubsup><h:mrow><h:mn>0</h:mn></h:mrow><h:mrow><h:mo>+</h:mo><h:mn>0.015</h:mn></h:mrow><h:mrow><h:mo>−</h:mo><h:mn>0.019</h:mn></h:mrow></h:msubsup></h:mrow></h:math> is obtained at 95% confidence level with a dataset corresponding to an integrated luminosity of <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mn>2</j:mn><j:mtext> </j:mtext><j:mtext> </j:mtext><j:msup><j:mi>nb</j:mi><j:mrow><j:mo>−</j:mo><j:mn>1</j:mn></j:mrow></j:msup></j:math> of lead-lead collisions and assuming a conservative 10% systematic uncorrelated uncertainty for signal and background. The present results using multivariate analysis are compared to similar results obtained using sequential cuts, as done in previous measurements, showing an improvement of about 35% in the sensitivity to <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mrow><l:msub><l:mrow><l:mi>a</l:mi></l:mrow><l:mrow><l:mi>τ</l:mi></l:mrow></l:msub></l:mrow></l:math>.