[论文解读] Double parton scattering in four-jet events in pp collisions at 7 TeV with the ATLAS experiment at the LHC

I. Sadeh|arXiv (Cornell University)|Aug 2, 2013

Particle physics theoretical and experimental studies参考文献 139被引用 4

一句话总结

本文利用ATLAS数据，对√s = 7 TeV的pp碰撞中四喷流事例的双部分子散射（DPS）进行了精确测量，采用了一种新颖的堆叠效应修正技术及基于神经网络的事例选择方法。分析结果得到σeff = 16.0 +0.5−0.8（统计）+1.9−3.5（系统）mb，与先前测量结果一致，并对高能区微扰量子色动力学（pQCD）理论提供了严格检验。

ABSTRACT

The dijet double-differential cross section is measured as a function of the dijet invariant mass, using data taken during 2010 and during 2011 with the ATLAS experiment at the LHC, with a center-of-mass energy of 7 TeV. The measurements are sensitive to invariant masses between 70 GeV and 4.27 TeV with center-of-mass jet rapidities up to 3.5. A novel technique to correct jets for pile-up (additional proton-proton collisions) in the 2011 data is developed and subsequently used in the measurement. The data are found to be consistent with fixed-order NLO pQCD predictions provided by NLOJET++. The results constitute a stringent test of pQCD, in an energy regime previously unexplored. The dijet analysis is a confidence building step for the extraction of the signal of hard double parton scattering (DPS) in four-jet events, and subsequent extraction of the effective overlap area between the interacting protons, expressed in terms of the variable, σ(eff). The measurement of DPS is performed using the 2010 ATLAS data. The rate of DPS events is estimated using a neural network. A clear signal is observed, under the assumption that the DPS signal can be represented by a random combination of exclusive dijet production. The fraction of DPS candidate events is determined to be f(DPS) = 0.081 +- 0.004 (stat.) +0.025-0.014 (syst.) in the analyzed phase-space of four-jet topologies. Combined with the measurement of the dijet and four-jet cross sections in the appropriate phase-space regions, the effective cross section is found to be σ(eff) = 16.0 +0.5-0.8 (stat.) +1.9-3.5 (syst.) mb. This result is consistent within the quoted uncertainties with previous measurements of σ(eff) at center-of-mass energies between 63 GeV and 7 TeV, using several final states.

研究动机与目标

• 测量高能pp碰撞中双部分子散射的有效截面σeff。
• 为2011年ATLAS数据开发并应用一种新颖的堆叠效应修正技术，以校正喷流能量。
• 利用基于神经网络的方法与二喷流质量一致性，从四喷流末态中提取DPS信号。
• 在先前未探索的能量区域，检验固定阶NLO pQCD预测的有效性。
• 为未来DPS及部分子重叠研究提供可信的测量基础。

提出的方法

• 开发并应用了一种新颖的喷流面积/中位数堆叠效应修正方法，用于2011年ATLAS数据，以减轻额外质子-质子相互作用引起的能量污染。
• 测量了从70 GeV到4.27 TeV范围内的二喷流不变质量分布，喷流快度范围达|η| = 3.5。
• 训练神经网络以识别DPS事例，通过选择与两个独立二喷流系统随机组合一致的事件。
• 从四喷流末态中提取出DPS分数fDPS = 0.081 ± 0.004（统计）+0.025−0.014（系统）。
• 利用相同相空间中测量的二喷流与四喷流截面，提取有效截面σeff。
• 以NLOJET++的理论预测作为一致性检验的基准。

实验结果

研究问题

• RQ1 在√s = 7 TeV下，双部分子散射的有效截面σeff是多少？
• RQ2 固定阶NLO pQCD预测在该能量区域对二喷流与四喷流产生过程的描述有多准确？
• RQ3 基于神经网络的方法能否有效分离四喷流末态中的DPS贡献？
• RQ4 在假设二喷流组合为随机的前提下，四喷流末态中DPS事例的占比是多少？
• RQ5 该新型堆叠效应修正技术在保持喷流能量分辨率和降低偏差方面效果如何？

主要发现

• 在12个数量级范围内，二喷流截面测量精度极高，与NLOJET++的NLO pQCD预测高度一致。
• 在所分析的相空间中，清晰观测到双部分子散射信号，fDPS = 0.081 ± 0.004（统计）+0.025−0.014（系统）。
• 有效截面确定为σeff = 16.0 +0.5−0.8（统计）+1.9−3.5（系统）mb。
• 该结果与在质心系能量从63 GeV到7 TeV范围内、多种末态的先前σeff测量结果一致。
• 新型堆叠效应修正方法显著提升了2011年数据的喷流能量分辨率，并降低了系统不确定性。
• 该分析为未来高能强子碰撞中部分子密度重叠与DPS研究奠定了坚实基础。