Borexino Solar Neutrino Experiment

Borexino太阳中微子实验概述Borexino是一种大型闪烁探测器，旨在观测太阳产生的低能中微子。它将包含300吨液体闪烁体，由放射性原子含量较低的材料制成，位于意大利的Gran Sasso地下实验室。将要进行的测量将考验我们对太阳如何产生能量的理解，以及我们对中微子的理解。太阳被认为从一系列核聚变反应中获得能量，这些核聚变反应将四个氢核转化为一个氦核，并释放出大量的能量(26.7 MeV)。这些反应发生在密集的太阳热核中，能量被转移到带电粒子、伽马射线和中微子上。然而，只有中微子作为信使从热核逃逸到太空中。雷·戴维斯和他的同事们的氯探测器是第一个为太阳中微子设计的实验，作为对太阳模型的测试。令人惊讶的是，它只记录了预测的中微子的大约三分之一。对于这项实验和太阳模型--预期的中微子速率是基于该模型--的怀疑依然存在。然而，在用新的探测器进行了几次测量后，很明显，戴维斯实验和计算的中微子速率都是正确的。最近，加拿大萨德伯里中微子天文台利用重水毫无疑问地证明，探测到的太阳中微子的不足是由于一种被称为“中微子振荡”的全新过程造成的。这些中微子在太阳中以电子-中微子的形式产生，是已知的三种中微子状态之一，然后在从太阳到地球的旅程中振荡到其他中微子状态。由于太阳中微子实验主要探测到电子中微子，所以观测到的速度总是比预期的要小。Mikheyev，Smirnov和Wolfenstein提出的一种过程(MSF效应)将通过中微子与太阳核心中的电子的相互作用来增强振荡。Borexino将对这个故事做出什么贡献？第一个目标是检验城市生活垃圾理论。中微子振荡性质的转变预计将在中微子能量约为2 MeV时发生。在跃迁能以上，中微子振荡以MSW振荡为主，而在该能量以下，物质效应很小，中微子应该通过“真空振荡”来振荡。Borexino将是第一个直接探测到跃迁能量以下的中微子的实验。目标是测量0.86 MeV的7Be中微子和1.44 MeV的PEP中微子的速率，对于这些中微子来说，真空振荡应该是主导的。SNO和Superk实验探测到了高能中微子，其中MSW振荡占主导地位。中微子在跃迁能以上和之下的存活几率的显著变化应该会为MSW图像提供一个独特的测试。Borexino的第二个目标是测试太阳产生的所有能量是否都来自聚变反应。这可以通过测量发生在太阳上的所有核反应的中微子速率来实现。根据观测到的中微子速率，经中微子振荡校正后，就可以确定太阳从核反应中产生的总能量。然后，将该总功率与由辐射功率、光子光度确定的功率进行比较。差异将意味着另一种能量来源，或者可能是一种非平衡状态。Borexino还具有核物理和天文学以外的应用。它是地球物理反中微子(由地球上的自然放射性产生)和超新星中微子的理想探测器。Borexino还将提供对所谓的稀有中微子和非标准中微子相互作用的灵敏搜索。该探测器是由美国国家科学基金会和欧洲科学机构提供资金建造的。这项提议的目标是在未来两年内使探测器全面投入使用，并开始数据采集；这项工作的重要部分将包括研究生和博士后研究助理，以及参与其中的教职员工。

Summary of the Borexino Solar Neutrino ExperimentBorexino is a large scintillation detector designed to observe low energy neutrinos produced by the Sun. It will contain 300 tons of liquid scintillator made of materials chosen for their low content of radioactive atoms and is located in the underground Gran Sasso laboratory in Italy. Measurements to be performed will test our understanding of how the Sun produces energy and our understanding of neutrinos. The Sun is thought to derive its energy from a series of nuclear fusion reactions that convert four hydrogen nuclei into a helium nucleus, with the release of a large amount of energy (26.7 MeV). The reactions occur in the dense hot core of the sun and the energy is transferred to charged particles, gamma rays, and neutrinos. However, only the neutrinos escape into space as messengers from the hot core.The chlorine detector of Ray Davis and colleagues was the first experiment designed for solar neutrinos as a test of the solar model. Surprisingly, it recorded only about 1/3 of the neutrinos predicted. Doubts about the experiment and the solar model, on which the expected neutrino rate is based, persisted. However, after several measurements with new detectors, it became clear that both the Davis experiment and the calculated neutrino rates were correct. More recently, the Sudbury Neutrino Observatory in Canada, using heavy water, demonstrated beyond doubt that the deficit of detected solar neutrinos was due to a fundamentally new process known as "neutrino oscillations". The neutrinos are produced in the sun as "electron-neutrinos", one of the three known neutrino states, and then oscillate to other neutrino states in their journey from the Sun to Earth. Since solar neutrino experiments detect mainly electron neutrinos, the observed rates were always smaller than expected. A process proposed by Mikheyev, Smirnov and Wolfenstein (MSF effect) would enhance oscillations by the interaction of the neutrinos with electrons in the solar core. What will Borexino contribute to the story? The first goal is to test the MSW theory. A transition in the nature of neutrino oscillations is expected to occur at a neutrino energy of approximately 2 MeV. Above the transition energy, neutrino oscillations are dominated by MSW oscillations, whereas below that energy, the matter effect is small and neutrinos should oscillate by "vacuum oscillations". Borexino will be the first experiment to directly detect neutrinos below the transition energy. The goal is to measure the rate of the 0.86 MeV 7Be neutrino and the 1.44 MeV pep neutrino, for which vacuum oscillations should dominate. The SNO and SuperK experiments detected high energy neutrinos, where MSW oscillations dominate. A significant change in the survival probability of the neutrinos above and below the transition energy should provide a unique test the MSW picture. A second goal of Borexino is to test whether all the energy produced by the Sun arises from the fusion reactions. This can be done by measuring the neutrino rates from all the nuclear reactions that occur in the Sun. From the observed neutrino rates, corrected for neutrino oscillations, one determines the total power produced by the Sun from nuclear reactions. This total power is then compared to the power determined from the radiant power, the photon luminosity. A difference would imply another source of energy, or possibly a non-equilibrium condition. Borexino also has applications beyond nuclear physics and astronomy. It is an ideal detector of geophysical anti-neutrinos (produced by natural radioactivity in the earth), and supernova neutrinos. Borexino will additionally provide a sensitive search for so-called sterile neutrinos and non-standard neutrino interactions. The detector was built with prior funding from the NSF and with funds from European science agencies. The goal of this proposal is to bring the detector into full operation within the next two years and to start data acquisition; significant parts of this effort will involve graduate students and postdoctoral research associates, in addition to the faculty members involved.