[論文レビュー] Polarization measurement of $Λ^+_c$ and $\overlineΛ{}^-_c$ baryons in $p$Ne collisions at $\sqrt{s_{NN}} = 68.6$ GeV

LHCb collaboration, R. Aaij+1194 more|arXiv (Cornell University)|Feb 19, 2026

Particle physics theoretical and experimental studies被引用数 0

ひとこと要約

First separate-charge polarization measurements of charm baryons in fixed-target LHCb data at $ sqrt{s_{NN}}=68.6$ GeV; yields $P_{Λ_c^+} = (24 ± 9_{stat} ± 2_{syst})\kg$ and $P_{ar{Λ}_c^-} = (-8 ± 12_{stat} ± 3_{syst})\kg$, with differential results in $p_T$ and $x_F$.

ABSTRACT

The first measurement of the polarization of charm baryons by the LHCb experiment recorded in fixed-target mode is presented. The polarization of $Λ_c$ baryons is studied in collisions of protons, at an energy of 2.51 TeV, incident on a gaseous target of neon, at a nucleon-nucleon center-of-mass energy of $68.6$ GeV. The world's first measurement of separate-charge polarizations for $Λ^+_c$ and $\overlineΛ{}^-_c$ baryons is performed, determining $$ P_{Λ^+_c} = ( 24 \pm 9 \pm 2 \, )\% , $$ $$ P_{\overlineΛ{}^-_c} = (-8 \pm 12 \pm 3 \, ) \% , $$ where the first uncertainty is statistical and the second systematic. The polarization is also measured in intervals of baryon transverse momentum and the Feynman-$x$ variable.

研究の動機と目的

• Motivate and test heavy-quark spin dynamics in charm baryon production and hadronization in a low-energy fixed-target regime.
• Measure the polarization of charmed baryons separately for $Λ_c^+$ and $\bar{Λ}_c^-$ to probe potential differences in production mechanisms.
• Develop and apply a helicity-formalism amplitude model to extract polarization from the $Λ_c^+ \rightarrow p K^- π^+$ decay.
• Investigate the dependence of polarization on transverse momentum ($p_T$) and Feynman-$x$ ($x_F$) to compare with historical hyperon and charm-baryon results.
• Validate a data-driven amplitude model by anchoring its parameters to a higher-statistics sample from semileptonic $b$-hadron decays.

提案手法

• Use a helicity-formalism based amplitude model to describe $Λ_c^+ \rightarrow p K^- π^+$ decays with consistent spin-state definitions across intermediate resonances.
• Fix the amplitude model from a larger LHCb data sample of $Λ_c^+ \rightarrow p K^- π^+$ (from semileptonic $b$-hadron decays) to enable polarization extraction.
• Perform independent maximum-likelihood fits to five-dimensional phase-space distributions (two invariant masses and three decay-orientation angles) to extract the transverse polarization component.
• Incorporate detector acceptance via simulation and background shapes via Legendre-polynomial expansions derived from mass sidebands, using TensorFlowAnalysis for the multi-dimensional fit.
• Estimate statistical uncertainties via bootstrapping pseudoexperiments and evaluate systematics from the amplitude model, background description, and efficiency calibration.

Figure 1: Distributions of the $pK^{-}\pi^{+}$ invariant mass of selected (left) ${\mathchar 28931\relax}^{+}_{c}$ and (right) ${\kern 1.79993pt\overline{\kern-1.79993pt\mathchar 28931\relax}}{}^{-}_{c}$ candidates. The results from the fit described in the text are also shown.

実験結果

リサーチクエスチョン

• RQ1 What is the transverse polarization of $Λ_c^+$ and $\bar{Λ}_c^-$ produced in fixed-target $p$ Ne collisions at $\sqrt{s_{NN}}=68.6$ GeV?
• RQ2 Are the polarizations of $Λ_c^+$ and $\bar{Λ}_c^-$ different, indicating distinct production/hadronization mechanisms?
• RQ3 How does charm-baryon polarization vary with kinematics, specifically with $p_T$ and $x_F$?
• RQ4 Does the observed polarization agree with parity conservation expectations for in-strong-interaction production (i.e., components in the production plane should be zero)?

主な発見

Bin	P_{Λ_c^+} ± stat ± syst (%)	P_{\bar{Λ}_c^-} ± stat ± syst (%)
p_T<2.8 GeV	17 ± 17 ± 4	-2 ± 20 ± 4
2.8 ≤ p_T<4 GeV	18 ± 14 ± 4	-6 ± 17 ± 5
p_T ≥ 4 GeV	45 ± 17 ± 4	-27 ± 22 ± 14
x_F < -0.15	41 ± 17 ± 5	-15 ± 21 ± 6
-0.15 ≤ x_F < -0.06	9 ± 14 ± 3	-13 ± 18 ± 3
x_F ≥ -0.06	42 ± 18 ± 6	2 ± 20 ± 8

• $Λ_c^+$ polarization is observed with a value $P_{Λ_c^+} = (24.4 ± 9.5_{stat} ± 2.2_{syst})%$ (significant at ~2.4$\sigma$).
• $\bar{Λ}_c^-$ polarization is consistent with zero within uncertainties: $P_{\bar{Λ}_c^-} = (-8 ± 12_{stat} ± 3_{syst})%$.
• Differential results show hints of increasing $P_{Λ_c^+}$ with $p_T$ in the highest bin but are statistically inconclusive; no clear kinematic trend for $P_{\bar{Λ}_c^-}$.
• The production-plane polarization components are compatible with zero, as required by parity conservation.
• An asymmetry in $Λ_c^+$ vs $\bar{Λ}_c^-$ yields is observed, reflecting proton–neon collision dynamics.
• The analysis provides the first separate-charge charm-baryon polarization measurements in fixed-target LHCb data and introduces a novel helicity-based amplitude modeling approach.

Figure 2: Comparisons of the phase-space projections between the data and the fitted model, for the ${\mathchar 28931\relax}^{+}_{c}$ dataset.