Measurement of prompt and non-prompt ${\mathrm{D}}^{\ast +}$-meson spin alignment in pp collisions at $\sqrt{s}$ = 13.6 TeV$

Abstract

A precise measurement of prompt and non-prompt ${\mathrm{D}}^{\ast +}$-meson spin alignment with respect to the helicity and production axes in proton-proton (pp) collisions at $\sqrt{s}=13.6$ TeV is presented, using data collected with the ALICE experiment at the LHC. The spin alignment is quantified by measuring the diagonal spin density matrix element ${\rho }_{00}$ for ${\mathrm{D}}^{\ast +}$ mesons at midrapidity ($|y|<0.8$). The measurement is performed in the transverse-momentum ($p_{\mathrm{T}}$) range $3<p_{\mathrm{T}}<100$ GeV/$c$ for prompt ${\mathrm{D}}^{\ast +}$ mesons and $3<p_{\mathrm{T}}<30$ GeV/$c$ for non-prompt ${\mathrm{D}}^{\ast +}$ mesons, respectively. The measured ${\rho }_{00}$ values for prompt ${\mathrm{D}}^{\ast +}$ mesons are consistent with the unpolarised expectation of $1/3$ over the entire $p_{\mathrm{T}}$ range, indicating no evidence of spin alignment. In contrast, ${\rho }_{00}$ of non-prompt ${\mathrm{D}}^{\ast +}$ mesons exhibit a deviation from $1/3$, consistent with expectations from weak beauty-hadron decays and well described by PYTHIA 8 simulations coupled with the EvtGen decay package. These results confirm and extend previous measurements, providing a precise baseline for future studies of ${\mathrm{D}}^{\ast +}$ spin alignment in heavy-ion collisions.

arXiv comment25 pages, 7 captioned figures, 1 table, authors from page 20, submitted to Journal of High Energy Physics, figures at http://alice-publications.web.cern.ch/node/13656