论文标题
提示和非预付款$ {\ rm d^{*+}} $ vector meson旋转对齐的首次测量
First measurement of prompt and non-prompt ${\rm D^{*+}}$ vector meson spin alignment in pp collisions at $\sqrt{s} = 13$ TeV
论文作者
论文摘要
这封信报告了有关螺旋轴的首次测量，以$ {\ rm d^{*+}} $ vector Mesons及其来自Charm Quark Handronisation（提示）和Beauty-Meson腐烂（非prompt）的荷兰媒介物的$ {\ rm d^{*+}} $ vector Mesons及其电荷共轭。该测量是在爱丽丝（Alice）在中心的质子 - 普罗顿（PP）碰撞中，在横向动量（$ p _ {\ rm t} $）的函数上以横向动量（$ p _ {\ rm t} $）的函数进行测量。 $ {\ rm d^{*+}} $ mesons的对角旋转密度矩阵$ $ρ_{00} $是从$ {\ rm d^{*+}}} \的角度分布中测量的π^+$衰减产品，在$ {\ rm d^{*+}} $ REST框架中，相对于$ {\ rm d^{*+}} $动量方向在质量框架的中心中。提示$ {\ rm d^{*+}} $ Mesons的$ρ_{00} $值与$ 1/3 $一致，这意味着没有旋转对齐。但是，对于非预付款$ {\ rm d^{*+}} $ mesons $ρ_{00} $大于$ 1/3 $的证据。旋转密度元素的测量值为$ρ_{00} = 0.455 \ pm0.022（\ text {stat。}）\ pm0.035（\ $ 5 <p _ {\ p _ {\ rm t} <20 $ gev/$ c $ carlo Interclulation py car py py py py py py py car EVTGEN软件包，在$ {\ rm d^{*+}}} $ Meson的衰减中实现螺旋性保护。在非中央重型离子碰撞中，$ {\ rm d^{*+}} $梅森的自旋可能与初始角动量和磁场的方向相对。基于本信中报告的PP碰撞结果，表明非预付款$ {\ rm d^{*+}} $ Mesons是由于螺旋性保护与集体各向异性扩张相关的，可能模仿重型成分碰撞中全球旋转信号的信号。
This letter reports the first measurement of spin alignment, with respect to the helicity axis, for ${\rm D^{*+}}$ vector mesons and their charge conjugates from charm-quark hadronisation (prompt) and from beauty-meson decays (non-prompt) in hadron collisions. The measurements were performed at midrapidity ($|y|<0.8$) as a function of transverse momentum ($p_{\rm T}$) in proton-proton (pp) collisions collected by ALICE at the centre-of-mass energy $\sqrt{s} = 13$ TeV. The diagonal spin density matrix element $ρ_{00}$ of ${\rm D^{*+}}$ mesons was measured from the angular distribution of the ${\rm D^{*+}} \to {\rm D}^0 (\to {\rm K}^{-}π^{+}) π^+$ decay products, in the ${\rm D^{*+}}$ rest frame, with respect to the ${\rm D^{*+}}$ momentum direction in the pp centre of mass frame. The $ρ_{00}$ value for prompt ${\rm D^{*+}}$ mesons is consistent with $1/3$, which implies no spin alignment. However, for non-prompt ${\rm D^{*+}}$ mesons an evidence of $ρ_{00}$ larger than $1/3$ is found. The measured value of the spin density element is $ρ_{00}=0.455\pm0.022(\text{stat.})\pm0.035(\text{syst.})$ in the $5 < p_{\rm T} < 20$ GeV/$c$ interval, which is consistent with a PYTHIA 8 Monte Carlo simulation coupled with the EVTGEN package, which implements the helicity conservation in the decay of ${\rm D^{*+}}$ meson from beauty mesons. In non-central heavy-ion collisions, the spin of the ${\rm D^{*+}}$ mesons may be globally aligned with the direction of the initial angular momentum and magnetic field. Based on the results for pp collisions reported in this letter it is shown that alignment of non-prompt ${\rm D^{*+}}$ mesons due to the helicity conservation coupled to the collective anisotropic expansion may mimic the signal of global spin alignment in heavy-ion collisions.
