Search for Physics Beyond the Standard Model through ultra-rare Kaon decays with the NA62 Experiment at CERN

粒子物理标准模型(SM)充分描述了正常物质中的高能相互作用，就像欧洲核子研究中心在大型强子对撞机上发现类希格斯玻色子一样，但没有解释暗物质或暗能量。粒子物理学中追求的两种方法是在SM之外寻找新的高能粒子，以及研究SM精确预测的罕见衰变。欧洲核子研究中心的NA62实验旨在通过精确测量带电介子衰变为一个介子和两个中微子来追求第二种方法，SM非常精确地预测，每100亿次介子衰变到其他一切可能的情况下，才会发生一次。希望找到一些与预测不同的东西来帮助我们解释为什么SM工作得如此好，并了解如何改进它。整个NA62探测器系统长达270米，位于欧洲核子研究中心的一个地下洞穴中。普通物质中不存在介子介子，每秒大约产生5000万介子，连同另外10亿介子和介子，由SPS CERN加速器每秒提取的超过1万亿个质子与铍目标相互作用产生。NA62探测器的设计目的是识别介子，并测量它们衰变过程中可能产生的所有不同类型的粒子。关键的组成部分是：i)通过测量切伦科夫辐射特定于它们的质量和速度来识别Kaon，ii)精确地重建Kaon的衰变，iii)通过测量它们的质量和能量来分离介子、Muon和电子，以及iv)识别光子，其精度优于十万分之一。至关重要的是，任何粒子都不能逃脱探测，为此，用仪器测量向前传播的高能电子和光子的能量和位置，而光束线周围的12个铅玻璃探测器系统探测所有剩余的光子。为了尽可能地减少对带电粒子的动量和角度的扰动，轻质稻草管被建造在两对腔室中，由产生大磁场的弯曲磁铁隔开，以测量它们的位置和动量。这使我们能够利用能量守恒和动量守恒来排除许多想要的衰变的背景。已经安装了一个17米长的切伦科夫探测器，里面充满了气态霓虹，以改善带电介子和介子之间的区别，主要由一个量热计提供，该量热计由由闪烁体隔开的铁板组成，用于测量粒子静止时释放的能量。这个实验的目的是保留我们希望研究的所有稀有衰变，并以非常低的概率以一种无偏见的方式采样所有其他的Kaon衰变。重建终态粒子的关键不仅是它们与我们正在寻找的非常罕见的衰变一致，而且它们与任何其他类型的衰变都不一致。要以可靠的方式做到这一点，我们必须根据数据测量所有背景，而不是依赖于模拟。这项实验计划持续到2018年。2014年和2015年的数据应该可以让我们在标准模型的灵敏度水平上观察到几个非常罕见的介子衰变成一个介子和两个中微子的现象，而衰变几率的精确测量只有在数据采集结束时才能实现。这项测量，再加上大型强子对撞机未来同时进行的许多直接和间接搜索的结果，将有助于区分标准模型之外的机制，这些机制可能会产生一幅连贯的观测宇宙图景。

The Standard Model of particle physics (SM) fully describes high energy interactions in normal matter, as witness the discovery of the Higgs-like boson at LHC at CERN, without explaining dark matter or dark energy. Two approaches pursued in particle physics to search beyond the SM are the direct search for new particles at high energy, and the study of rare decays precisely predicted by the SM. The NA62 experiment at CERN is designed to pursue this second approach by precisely measuring the decay of the charged kaon into a pion and two neutrinos that is predicted very precisely by the SM to occur only once for every ten billion times that a kaon decays into everything else possible. The hope is to find something different from the prediction to help us to explain why the SM works so well and to understand how to improve it.The entire system of detectors for NA62 takes up 270 metres and is located at CERN in an underground cavern. Kaon mesons do not exist in ordinary matter and about fifty million of them are produced per second, together with another billion pions and muons, by more than one thousand billion protons per second extracted from the SPS CERN accelerator and interacting with a beryllium target. The NA62 detectors are designed to recognize the kaons and to measure all the different types of particles that can be produced in their decay. The key ingredients are: i) kaon identification by measuring the Cherenkov radiation specific to their mass and speed, ii) precise reconstruction of the decay of the kaon, iii) the separation between pions, muons and electrons by measuring their masses and energies, and iv) the identification of photons with an accuracy of better than one part in a hundred thousand. It is vital that no particles escape detection and to this end an instrumented volume of liquid krypton measures the energy and position of the high-energy electrons and photons travelling forwards, while a system of 12 lead-glass detectors around the beamline detects all remaining photons. In order to perturb as little as possible the momentum and angles of the charged particles, lightweight straw-tubes have been built into two pairs of chambers, separated by a bending magnet producing a large magnetic field, to measure their position and momentum. This enables us to use the conservation of energy and momentum to reject much of the background to the wanted decay. A 17-metre-long Cherenkov detector filled with gaseous Neon has been installed to improve the distinction between charged pions and muons, provided principally by a calorimeter built from iron plates separated by scintillators that measure the energy released when the particles come to rest. The experiment is designed to keep all of the rare decays we wish to study and to sample all other kaon decays in an unbiased way with very low probability. The key point in reconstructing the final state particles is not just that they are consistent with the very rare decay we are seeking, but that they are not consistent with any other type of decay. To do this in a reliable way we must measure all backgrounds from data rather than relying on simulation. The experiment is scheduled to run up to 2018. The 2014 and 2015 data should allow us to observe a few of the very rare kaon decays into a pion and two neutrinos at the level of the Standard Model sensitivity, while the precision measurement of the decay probability will be achievable only at the end of the data taking. This measurement, together with the results from the many future direct and indirect searches produced concurrently at LHC, will help to distinguish the mechanisms beyond the Standard Model that may produce a coherent picture of the observed universe.