A measurement of the high-mass $τ\barτ$ production cross-section at $\sqrt{s}=13$ TeV with the ATLAS detector and constraints on new particles and couplings

TL;DR

The paper reports the first unfolded differential measurement of high-mass $ auar{ au}$ production at $ sqrt{s}=13$ TeV with ATLAS, showing agreement with the Standard Model. It advances by performing a fit in bins of $b$-jet multiplicity to probe non-resonant new physics described by SMEFT operators, vector/scalar leptoquarks, and a $Z'$ boson that couple preferentially to the third generation, including interference effects. The analysis constrains SMEFT coefficients—some affecting the tau anomalous magnetic moment $a_ au$—and sets competitive limits on leptoquark and $Z'$ parameter spaces, improving on prior ATLAS and CMS results. The results illustrate the importance of interference and $b$-jet tagging in enhancing sensitivity to non-SM interactions in the $ auar{ au}$ channel.

Abstract

The production cross-section of high-mass $τ$-lepton pairs is measured as a function of the dilepton visible invariant mass, using 140 fb$^{-1}$ of $\sqrt{s}=13$ TeV proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider. The measurement agrees with the predictions of the Standard Model. A fit to the invariant mass distribution is performed as a function of $b$-jet multiplicity, to constrain the non-resonant production of new particles described by an effective field theory or in models containing leptoquarks or $Z'$ bosons that couple preferentially to third-generation fermions. The constraints on new particles improve on previous results, and the constraints on effective operators include those affecting the anomalous magnetic moment of the $τ$-lepton.

Figures (12)

• Figure 1: (a) The fake factor as a function of $\tau_{\textrm{had}}$ for each category of events and $\tau_{\textrm{had}}$ objects with one associated charged track and $|\eta|<1$. The overall fake factor used in the analysis (black hatched) is determined using the relative fractions of the categories obtained from the width fit. (b) The result of the width fit for $\tau_{\textrm{had}}$ objects with one associated charged track, between 70 and 90 GeV, and $|\eta|<1$. The ratio of data to prediction is shown in the bottom panel, along with the combined statistical uncertainty in the templates (hatched band). This fit is used to obtain the overall fake factor in the second bin of the left-hand plot.
• Figure 2: The distribution of the visible invariant mass of (a) two same-charge $\Pgt_{\text{had}}$ objects or (b) one $\Pgt_{\text{had}}$ object and an electron or muon with the same charge. The bottom panels show the ratio of data to the SM prediction, with the shaded band representing the total uncertainty in the prediction. The processes labelled 'Others' are the production of dibosons, Higgs bosons, and Drell--Yan pairs of electrons and muons. The events are used to validate the estimate of the misreconstructed $\Pgt_{\text{had}}$ background.
• Figure 3: The $\Pgt_{\text{had}}$$\Pgt_{\text{had}}$ visible invariant mass in region of the measurement, before background subtraction and correction for detector effects.
• Figure 4: The nominal transfer matrix, showing the probability of an event from a fiducial $m_{\tau\tau}^\mathrm{vis}$ bin to be reconstructed at detector-level in a given bin of $\Pgt_{\text{had}}\xspace\Pgt_{\text{had}}\xspace$ invariant mass.
• Figure 5: Uncertainty breakdown for the $m_{\tau\tau}^\mathrm{vis}$ differential cross-section measurement.