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LHC taupair production constraints on $a_\tau$ and $d_\tau$
by Ulrich Haisch, Luc Schnell, Joachim Weiss
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Authors (as registered SciPost users):  Ulrich Haisch 
Submission information  

Preprint Link:  scipost_202308_00018v1 (pdf) 
Date submitted:  20230814 15:42 
Submitted by:  Haisch, Ulrich 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Phenomenological 
Abstract
We point out that relevant constraints on the anomalous magnetic ($a_\tau$) and electric ($d_\tau$ ) moment of the tau lepton can be derived from taupair production measurements performed at the LHC. Our conclusion is based on the observation that the leading relative deviations from the Standard Model prediction for $pp \to \tau^+ \tau^$ due to aτ and dτ are enhanced at high energies. Less precise measurements at hadron colliders can therefore offer the same or better sensitivity to new physics with respect to highprecision lowenergy measurements performed at lepton machines. We derive bounds on aτ and dτ using the full LHC Run II data set on taupair production and compare our findings with the current best limits on the tau anomalous moments.
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Report
This article provides a nice analysis of tau pair DY production and its relevance for the tau anomalous couplings. The conclusions rest on a range of assumptions, which are, however, not entirely clear.
(i) all effects are attributed to the tau leptonsector. How does this compare to the competing coupling modifications that can be expected in other fermion interactions that DY is sensitive to?
(ii) the Z contribution is chosen to vanish. Is this a reasonable assumption? I would expect through Zphoton mixing to see correlated effects away from the Z resonance that can become relevant at large momentum transfers that are highlighted as particularly relevant by the authors.
Report
This work is precise and succinct, yet detailed enough, highlighting a key insight into probing new physics in anomalous dipole moments of the tau lepton. The authors effectively set limits on the relevant SMEFT dipoles using highmass DrellYan tails. Notably, the determined limits are the most competitive for the anomalous magnetic moment. Before I recommend this for publication, I have two areas I would like the authors to address:
1. Could the authors briefly comment on the UV models where these bounds are of interest? It is my understanding that perturbative models typically generate these effects at the oneloop level. Referring to eq. (13), this seems pertinent primarily for lighter particle mediators, where the EFT methodology might not be applicable and direct searches come into play.
2. The authors suggest that the SM target at the HLLHC will be met. Could they elaborate on the implications for the highmass tails? Specifically, which SM corrections will become more significant? Additionally, how adequate is the SM prediction at such a precision level?
Author: Ulrich Haisch on 20231114 [id 4112]
(in reply to Report 2 on 20231102)\section*{Report of the referee 2}
\begin{enumerate}
\item[(Q1)] {\it All effects are attributed to the tau leptonsector. How does this compare to the competing coupling modifications that can be expected in other fermion interactions that DY is sensitive to?}
\item[(A1)] Taupair production is a sensitive probe of various operators in the Standard Model effective field theory~(SMEFT). For instance, fourfermion operators of the form $(\bar q \hspace{0.5mm} \Gamma \hspace{0.25mm} q) (\bar \tau \hspace{0.125mm} \Gamma \hspace{0.0mm} \tau)$ with $\Gamma$ denoting a Dirac structure and $q$ a up or down quark are known to lead to visible enhancements in the highenergy tails of the $pp \to \tau^+ \tau^$ process. See for example
\begin{verbatim}
@article{Greljo:2017vvb,
author = "Greljo, Admir and Marzocca, David",
title = "{High$p_T$ dilepton tails and flavor physics}",
eprint = "1704.09015",
archivePrefix = "arXiv",
primaryClass = "hepph",
reportNumber = "ZUTH1217",
doi = "10.1140/epjc/s1005201751198",
journal = "Eur. Phys. J. C",
volume = "77",
number = "8",
pages = "548",
year = "2017"
}
\end{verbatim}
for a comprehensive discussion. Notice that the goal of our note is not to perform a global SMEFT fit using the existing ditau data but to point out that the effective interactions~(6) that give a BSM effect to $a_\tau$ and $d_\tau$ also lead to energyenhanced effects in $pp \to \tau^+ \tau^$ production. As we show in our work, this opens up the possibility to use existing LHC data on taupair production to set bounds on $a_\tau$ that are better than all other existing constraints that are based on the same assumptions.
\item[(Q2)] {\it The $Z$ contribution is chosen to vanish. Is this a reasonable assumption? I would expect through Zphoton mixing to see correlated effects away from the Z resonance that can become relevant at large momentum transfers that are highlighted as particularly relevant by the authors.}
\item[(A2)] We are not exactly sure what the referee implies with the second part of the question. Let us explain our motivation to consider only cases with $c_{\tau Z} =0$ in the analysis. Allowing for $c_{\tau Z} \neq 0$ and deriving constraints in the $c_{\tau \gamma}\hspace{0.25mm}$$\hspace{0.25mm}c_{\tau Z}$ plane using $pp \to \tau^+ \tau^$ data would be straightforward, however, we refrain from performing such an analysis in scipost\_202308\_00018v1. The reason for this is simply that the bounds~(1) and~(2) have been derived under the assumption that there is only an anomalous $\gamma \tau^+ \tau^$ coupling but no anomalous $Z \tau^+ \tau^$ coupling. The main results of our study,~i.e.~the limits~(13) and (14) that have been derived under the same assumption, can therefore be compared directly to~(1) and~(2) which would not be the case if we were to consider cases with $c_{\tau Z}\neq 0$. We have added a short explanation along these lines to the text.
\end{enumerate}
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