Speaker
Description
The pair of excited states $B_c^{(*)}(2S)^+$ of the $B_c$ meson is studied, with the aim of measuring the mass of the $B_c(2S)^+$ state and the parameter $m^*=m\big(B_c^*(2S)^+\big)-m\big(B_c^{*+}\big)+m\big(B_c^+\big)$.
$B_c$-meson is a rare and understudied particle due to its large mass of over 6 GeV, and research of its excited states lies at the forefont of high energy physics.
In this study, the excited states of the $B_c$-meson are observed through the decay to the base $B_c^{(*)+}$ state through a $\pi^+\pi^-$ pair, and then different decays of the base state: $B_c^{+} \rightarrow J_\psi\pi^+ $, $B_c^{+} \rightarrow J_\psi\pi^+\pi^+\pi^- $, $B_c^{+} \rightarrow \Psi(2S)\pi^+ $, $B_c^{+} \rightarrow \Psi(2S)\pi^+\pi^+\pi^- $. The data from the CMS detector is used; events used for the study were collected during the Run 2 (2016-2018) and Run 3 (2022-2026) of the Large Hadron Collider.
Background has been studied using similar events with the same charge in the pion pair, which should be unaffected by $B_c^{(*)}(2S)^+$ decay. Machine learning algorithms have been utilized for background suppression in the $B_c$ decay reconstruction. Monte-Carlo simulations of the studied and related processes in the CMS detector are used to train the model, as well as find the best fitting distribution for the resulting signal events.
The mass of the $B_c(2S)^+$ state is measured with the highest precision achieved to date. The parameter $m^*$ is also measured, which will help establish the mass of the other excited state $B_c^{*}(2S)^{+}$, may the related value of $m\big(B_c^{*+}\big)-m\big(B_c^+\big)$ be measured. Both values are consistent with earlier published results.
