Direct TPE measurement via $$e^+p/e^-p$$ scattering at low $$\varepsilon $$ in Hall A
Direct TPE measurement via $$e^+p/e^-p$$ scattering at low $$\varepsilon $$ in Hall A
复制标题
通过 A 厅低 $$varepsilon $$ 处的 $$e^ p/e^-p$$ 散射直接进行 TPE 测量
DOI：
10.1140/epja/s10050-021-00597-3
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发表时间：
2021
期刊：
影响因子：
--
通讯作者：
Schmidt, Axel
中科院分区：
文献类型：
--
作者：
Cline, Ethan;Bernauer, Jan C.;Schmidt, Axel
质子弹性形状因子比可以通过在非偏振束和靶的实验中通过RosenBluth分离来测量，或者通过使用极化自由度来测量。然而，这两种方法产生的数据显示出差异，随着。对这种差异提出的解释--双光子交换--最近已经通过三个实验进行了验证。结果支持小的双光子交换效应的存在，但不能确定测量的动量转移的理论处理是有效的。在更大的动量转移中，理论仍未得到检验，如果没有进一步的数据，就不可能解决这种差异。杰斐逊实验室的正电子束使我们能够高精度地在更大范围内直接测量双光子交换。有了这一点，我们可以验证该影响是否与外形因数比测量结果一致，并测试几种理论方法，这些方法在测试范围的不同部分有效。在这个方案中，我们描述了A厅的一个测量计划，该计划结合了Super BigBite、BigBite和高分辨率光谱仪来直接测量双光子效应。虽然正电子束的有限束流将限制其运动范围，但该测量具有很小的系统不确定度，使其成为一个干净的双光子交换探头。
The proton elastic form factor ratio can be measured either via Rosenbluth separation in an experiment with unpolarized beam and target, or via the use of polarization degrees of freedom. However, data produced by these two approaches show a discrepancy, increasing with. The proposed explanation of this discrepancy – two-photon exchange – has been tested recently by three experiments. The results support the existence of a small two-photon exchange effect but cannot establish that theoretical treatments at the measured momentum transfers are valid. At larger momentum transfers, theory remains untested, and without further data, it is impossible to resolve the discrepancy. A positron beam at Jefferson Lab allows us to directly measure two-photon exchange over an extendedandrange with high precision. With this, we can validate whether the effect reconciles the form factor ratio measurements, and test several theoretical approaches, valid in different parts of the testedrange. In this proposal, we describe a measurement program in Hall A that combines the Super BigBite, BigBite, and High Resolution Spectrometers to directly measure the two-photon effect. Though the limited beam current of the positron beam will restrict the kinematic reach, this measurement will have very small systematic uncertainties, making it a clean probe of two photon exchange.