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

粒子物理学的标准模型（SM）完全描述了正常物质中的高能相互作用，见证了在欧洲核子研究中心的大型强子对撞机上发现的希格斯玻色子，但没有解释暗物质或暗能量。在粒子物理学中，有两种方法可以超越标准模型，一种是直接寻找高能新粒子，另一种是研究标准模型精确预测的稀有衰变。欧洲核子研究中心的NA62实验旨在通过精确测量带电的K介子衰变为1个π介子和2个中微子来实现第二种方法，SM非常精确地预测，K介子衰变为其他一切可能的物质每100亿次只发生一次。希望能找到一些与预测不同的东西，以帮助我们解释为什么SM工作得这么好，并了解如何改进它。NA62的整个探测器系统占地270米，位于欧洲核子研究中心的一个地下洞穴中。K介子不存在于普通物质中，每秒约有5000万个K介子，以及另外10亿个π介子和μ介子，由SPS CERN加速器每秒提取的1000亿多个质子与铍靶相互作用产生。NA62探测器被设计用于识别K介子，并测量它们衰变中可能产生的所有不同类型的粒子。关键成分是：i）通过测量切伦科夫辐射特定的质量和速度来识别K介子，ii）精确重建K介子的衰变，iii）通过测量它们的质量和能量来分离π介子，μ介子和电子，iv）识别光子，精度优于十万分之一。至关重要的是，没有粒子逃脱检测，为此，一个装有仪器的液体氪体积测量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检测所有剩余的光子。为了尽可能少地干扰带电粒子的动量和角度，在两对腔室中建造了轻质吸管，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和动量守恒来拒绝想要的衰变的大部分背景。安装了一个17米长的切伦科夫探测器，里面装满了气态氖，以改善带电π介子和μ介子之间的区别，主要是由一个由铁板制成的量热计提供的，铁板之间由测量粒子静止时释放的能量的放热器隔开。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无偏的方式对所有其他K介子衰变进行采样。重建末态粒子的关键不仅在于它们与我们正在寻找的非常罕见的衰变相一致，而且还在于它们与任何其他类型的衰变都不一致。为了以可靠的方式做到这一点，我们必须从数据中测量所有背景，而不是依赖于模拟。该实验计划持续到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一些非常罕见的K介子衰变为1个π介子和2个中微子，而衰变概率的精确测量只有在数据采集结束时才能实现。这种测量，加上未来在LHC同时产生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个观测宇宙的连贯图像。

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.