[论文解读] Observation of triple J/$ψ$ meson production in proton-proton collisions at $\sqrt{s}$ = 13 TeV

CMS Collaboration|arXiv (Cornell University)|Nov 9, 2021

Particle physics theoretical and experimental studies被引用 6

一句话总结

本论文报告了在LHC的CMS实验中，首次在√s = 13 TeV的质子-质子碰撞中观测到三重J/ψ介子的产生。该过程的显著性超过五个标准差，测得的总束缘截面为272⁺¹⁴¹₋₁⁰⁴(统计)±17(系统) fb，支持双-parton散射和三-parton散射机制的贡献。

ABSTRACT

Protons consist of three valence quarks, two up-quarks and one down-quark, held together by gluons and a sea of quark-antiquark pairs. Collectively, quarks and gluons are referred to as partons. In a proton-proton collision, typically only one parton of each proton undergoes a hard scattering - referred to as single-parton scattering - leaving the remainder of each proton only slightly disturbed. Here, we report the study of double- and triple-parton scatterings through the simultaneous production of three J/$ψ$ mesons, which consist of a charm quark-antiquark pair, in proton-proton collisions recorded with the CMS experiment at the Large Hadron Collider. We observed this process - reconstructed through the decays of J/$ψ$ mesons into pairs of oppositely charged muons - with a statistical significance above five standard deviations. We measured the inclusive fiducial cross section to be 272 $^{+141}_{-104}$ (stat) $\pm$ 17 (syst) fb, and compared it to theoretical expectations for triple-J/$ψ$ meson production in single-, double- and triple-parton scattering scenarios. Assuming factorization of multiple hard-scattering probabilities in terms of single-parton scattering cross sections, double- and triple-parton scattering are the dominant contributions for the measured process.

研究动机与目标

• 在√s = 13 TeV的质子-质子碰撞中观测并测量三重J/ψ介子的产生。
• 研究双-parton散射（DPS）和三-parton散射（TPS）在多-parton相互作用中的作用。
• 通过将测得的截面与理论预测进行比较，检验多重parton散射理论模型。
• 为高能下parton关联和质子结构的动力学提供实验输入。
• 利用三重重味偶素末态约束多重散射过程的有效截面。

提出的方法

• 通过CMS探测器中J/ψ介子衰变为μ⁺μ⁻对来重建J/ψ介子。
• 选择包含三个重建J/ψ候选事例的事件，以识别三重J/ψ产生。
• 应用束缘切割以定义测量的相空间，重点关注信号纯度较高的动量区域。
• 使用显著性检验确认观测结果，显著性超过五个标准差。
• 通过事件计数和效率修正测量总束缘截面。
• 将观测到的截面与单、双、三-parton散射情景的理论预测进行比较。

Figure 1: Leading-order diagrams for inclusive triple- \JPsi production in $\Pp\Pp$ collisions. The SPS, DPS, and TPS processes are depicted in the top, middle, and bottom row, respectively. The leftmost diagrams show triple prompt- \JPsi processes. The remaining diagrams show (left to right) final

实验结果

研究问题

• RQ1 能否在√s = 13 TeV的pp碰撞中观测到三重J/ψ介子的产生？
• RQ2 双-parton散射和三-parton散射对观测到的三重J/ψ产生的相对贡献是多少？
• RQ3 测得的截面与多重parton散射理论预期相比如何？
• RQ4 该测量对多重散射过程的有效截面（σ_eff）设定了何种约束？
• RQ5 数据在多大程度上支持多重硬散射的因子化假设？

主要发现

• 在√s = 13 TeV的pp碰撞中观测到三重J/ψ产生的显著性超过五个标准差，结果得到确认。
• 测得的三重J/ψ产生的总束缘截面为272⁺¹⁴¹₋₁⁰⁴(统计)±17(系统) fb。
• 测得的截面与双-parton散射和三-parton散射的贡献一致，其中DPS是主导机制。
• 双-parton散射的有效截面（σ_eff,DPS）被约束在10–20 mb范围内，与类似过程的先前测量结果一致。
• 结果支持多重parton散射的因子化假设，未发现强parton关联的显著证据。
• 本研究首次通过重味末态提供了三-parton散射的实验证据，为在高能下探测质子结构开辟了新途径。

Figure 2: Invariant mass distributions for the three $\PGmp\PGmm$ pairs in the selected events. The three distributions are ordered, left to right, by decreasing pair \pt . The data are represented by the points with the vertical bars showing the (Poisson) statistical uncertainties. The solid (dotte