BESIII上e^+ e^-→K_L^0 K_S^0产生截面的测量

Discrepancy was discovered recently on the muon anomalous magnetic moment between measured value and prediction by the Standard Model, which implies the existence of new physics. To understand the issue, cross sections of hadronic processes need to be measured as precisely as possible. As an essential hadronic process, e^+ e^-→K_L^0 K_S^0 need to be measured with high precision. However, measurements of it on previous experiments mostly covered the energy region lower than 2 GeV and the results were not as precise as needed. In recent years, BESIII collected more than enough data at large number of energy points. In this application, cross section of the process e^+ e^-→K_L^0 K_S^0 will be measured at BESIII. Signal selection, background rejection, radiation correction and systematic uncertainty will be studied in details. Based on the methods already existed in similar analysis, improvements on the methods will be included to get more precise results. With the high statistic data collection, the statistic uncertainty will be reduced largely. This is the first cross section measurement on e^+ e^-→K_L^0 K_S^0 at BESIII experiment, which will cover the energy region previous experiments not or improve the precision in the overlapped region. At the same time, method called missing momentum will be attempted in radiation correction to reduce the reliance on theory and Monte Carlo.

近年来，实验发现μ轻子反常磁矩与理论预言不相符，为新物理的存在给出了重要暗示。深入研究其原因需要精确测量强子的产生截面。作为必不可少的重要强子产生过程，e^+ e^-→K_L^0 K_S^0需要被高精度地测量。然而以往的实验能量大都在2 GeV以下，而且测量结果精度都不高。BESIII于近几年在2 GeV以上多个能量点上取得了大样本数据。本申请基于BESIII上获取的数据，测量e^+ e^-→K_L^0 K_S^0的产生截面。对信号选择、本底估计、辐射修正及系统误差等方面进行全面细致的研究，并在已有的分析手段上加以改进，提高测量精度。这是BESIII上第一次测量e^+ e^-→K_L^0 K_S^0的产生截面，涵盖以往实验没有达到的能量区域或在重叠区域大大提高测量精度，并尝试利用丢失动量的方法减少辐射修正对蒙特卡洛样本和理论的依赖。

正负电子对湮灭过程中K介子产生机制的研究对于理解rho, phi, omega激发态的性质具有重要意义，目前的研究热点phi(2170)在许多理论模型中被预言会衰变到K介子对。在国家自然科学基金委的支持下，申请人开展了对e^+e^- → K_S^0 K_L^0过程的研究。研究基于BESIII探测器于2015年在2.0-3.08GeV能量范围内15个质心能量点上采集的583pb^-1积分亮度的扫描数据。测量得到的这15个能量点上e^+e^- → K_S^0 K_L^0的产生截面是目前世界上这一能量范围内这一过程最精确的测量结果。将测得的15个能量点上的产生截面线形状进行展示并拟合，发现2.2GeV附近存在的共振态结构质量和宽度分别为2282.6 +- 9.6 +- 27.5 MeV/c^2 和117.9 +- 64.3 +- 48.6 MeV，其中第一个和第二个误差分别是统计和系统误差。这与e^+e^- →K^+K^-过程中发现的共振态参数一致。结合BESIII上带电K介子对的测量结果，计算了e^+e^- →K^+K^-与K_S^0 K_L^0产生截面的比值和误差，为研究K介子产生机制及2.2GeV附近的共振态和phi(2170)提供了重要的数据。除此之外，利用2.0-3.08GeV能量范围内19个能量点的扫描数据对e^+e^- →omegaK^+K^-产生截面进行测量，对其线形状的展示表明在这一能量范围内没有共振态衰变到omegaK^+K^-，测量结果对e^+e^- →omegaK^+K^-的研究仍然可以为2.2GeV附近的共振态和phi(2170)提供重要的数据。发表的会议论文总结了BESIII上强子类时形状因子测量结果。对连续区域带电和中性K介子对产生截面的研究从夸克层次上分析得到了其比值在10左右，与目前已发表的实验结果基本一致。