Measurement of $R(D^*)$ with hadronic $\tau^+$ decays at $\sqrt{s}$ = 13 TeV by the LHCb collaboration

<!--HTML--><p><span style="background-color:rgba(255,255,255,0.85);color:rgb(50,49,48);"><span style="-webkit-text-stroke-width:0px;display:inline !important;float:none;font-family:&quot;Segoe UI&quot;, Segoe, Tahoma, Helvetica, Arial, sans-serif;font-size:13px;font-style:normal;font-variant-caps:normal;font-variant-ligatures:normal;font-weight:400;letter-spacing:normal;orphans:2;text-align:start;text-decoration-color:initial;text-decoration-style:initial;text-decoration-thickness:initial;text-indent:0px;text-transform:none;white-space:normal;widows:2;word-spacing:0px;">The ratios of branching fractions $R(D^{(*)}) = {\cal B}(B^0 \to D^{(*)}\tau \nu_{\tau})/{\cal B}(B^0 \to D^{(*)} \ell \nu_{\ell})$, with $\ell = \mu, e$, allow to test lepton flavour universality in semi-leptonic $b$-decays and have been measured by BaBar, Belle and LHCb collaborations. The measurements have attracted significant attention as the difference between the experimental world-average values of $R(D^*)$ and $R(D)$ and their theoretical prediction is at the level of three standard deviations. LHCb presented results on $R(D)$ and $R(D^*)$ with muonic $\tau^+$ decays in autumn 2022 using the Run 1 data sample.</span></span><br><span style="background-color:rgba(255,255,255,0.85);color:rgb(50,49,48);"><span style="-webkit-text-stroke-width:0px;display:inline !important;float:none;font-family:&quot;Segoe UI&quot;, Segoe, Tahoma, Helvetica, Arial, sans-serif;font-size:13px;font-style:normal;font-variant-caps:normal;font-variant-ligatures:normal;font-weight:400;letter-spacing:normal;orphans:2;text-align:start;text-decoration-color:initial;text-decoration-style:initial;text-decoration-thickness:initial;text-indent:0px;text-transform:none;white-space:normal;widows:2;word-spacing:0px;">This seminar presents an updated measurement on $R(D^{*-})$ using $B^0 \to D^{*-}\tau^+ \nu_{\tau}$ decays with hadronic $\tau^+$ decay channels, based on $pp$ collision data collected by the LHCb collaboration at $\sqrt{s}$ = 13 TeV in 2015 and 2016, corresponding to an integrated luminosity of 2 fb$^{-1}$ . This new sample has about 1.5 times more $B^0$ decays than the one used in the previous Run 1 analysis. The excellent vertexing capabilities of the LHCb detector allow for the exploration of interesting features of the three-prong $\tau^+$ decays which will be discussed in this talk. This measurement provides one of the most precise single results on $R(D^*)$ to date.</span></span></p><p><span><strong>Refreshments will be served at 10:30</strong></span></p>