Probing new physics through B meson mixing and decays: highly improved lattice QCD calculation of hadronic matrix elements.

基本粒子物理学的标准模型目前还不完整。目前缺乏的是一个令人满意的解释，即为什么控制夸克风味衰变的弱力（核衰变背后的潜在现象）如此微弱。我们有一个可能的解释，需要希格斯玻色子的存在，我们应该在几年内在瑞士日内瓦的大型强子对撞机（LHC）上发现它。即使我们真的找到了希格斯粒子，也还有更多悬而未决的问题。它们大多围绕着希格斯粒子的质量值：在我们的模型中引入这样一个粒子，单靠标准模型动力学就会给希格斯粒子一个难以理解的大质量。也许大型强子对撞机会产生标准模型无法描述的新粒子或新现象，并为我们指明正确的解释方向。这种“新物理学”有许多候选模型。对它们进行分类是一项复杂的任务，既需要像大型强子对撞机那样的高能实验，也需要高精度实验。特别是，这些新模型一般预测了夸克风味衰变的异常模式。研究奇怪的、迷人的、美丽的介子（K、D和B）的实验已经严格限制了这些新模型的参数。在与大型强子对撞机和其他高能实验相配合的情况下，风味物理实验和理论计算需要进一步精确，以减少新物理的候选模型。我们的建议是采用新技术来进一步减少风味衰变的理论不确定性。自由夸克从未见过；它们只存在于束缚态中，称为强子。实验学家测量强子的衰变速率，需要精确的计算来从测量中提取控制夸克衰变的参数。答案可以从牢固建立的夸克强相互作用理论QCD中得到；然而，解决QCD以从夸克相互作用中获得强子性质必须使用超级计算机进行数值计算。我们一直致力于开发和采用能够利用现有计算资源进行更好计算的方法。其中两种，称为自动晶格微扰理论和移动非相对论性QCD (mNRQCD)，现在已经准备好用于改进B介子衰变的晶格计算，特别是半光子和辐射衰变（在这些衰变中，最终状态只有一个子介子和一个W玻色子-它迅速衰变为电子或介子和中微子-或光子）。拥有一个相对于晶格静止框架运动的底夸克（或美夸克）的能力是一种新的能力，它允许我们更好地提取子介子动量的功能依赖；这就是mNRQCD的作用。其他反应可以用不同的方法来测量相同的基本参数。这些测量中的任何差异或不匹配都表明存在“新物理学”，然后需要解决这种差异。为了使其可行，需要精确的实验和理论。我们的理论计算旨在将误差降低到几个百分点的水平，以实现这一目标。在计算QCD对这些粒子反应的全部非摄动贡献之前，必须仔细调整衰变过程动力学的晶格描述。这种调优计算量很大，技术上也很复杂，只有通过使用面向对象编程实现大部分计算的自动化才能实现。其结果是一种灵活的理论方法，将与在标准模型之外寻找新物理的实验保持同步。

The Standard Model of elementary particle physics is incomplete at the moment. What is lacking is a satisfactory explanation of why the weak force governing quark flavor decays, the underlying phenomenon behind nuclear beta decay, is so weak. We have a possible explanation which requires the existence of a Higgs boson, which we should discover at the Large Hadron Collider (LHC) in Geneva, Switzerland in a few years. Even if we do find the Higgs, there are further open questions. They mostly surround the value of the Higgs mass: having introduced such a particle into our model, Standard Model dynamics alone would give the Higgs an incomprehensibly large mass. Perhaps the LHC will produce new particles or phenomena which are not described by the Standard Model and point us in the right direction toward an explanation. There are many candidate models for this 'new physics.' Sorting through them is a complicated task, requiring both high energy experiments, like those at the LHC, and high precision experiments. In particular, these new models generically predict deviant modes of quark flavor decays. Already experiments studying the strange, charmed, and beautiful mesons (K, D, and B) tightly constrain the parameters of these new models. Further precision in flavor physics experiments and theoretical calculations is needed, in concert with the LHC and other high energy experiments, to pare down the candidate models of new physics. Our proposal is to implement new techniques to further reduce theoretical uncertainties in flavor decays. Free quarks are never seen; they are only seen within bound states, called hadrons. Experimentalists measure decay rates of hadrons and need accurate calculations to extract from their measurements parameters governing quark decays. The answers can be obtained from the firmly established theory of strong interactions between quarks, QCD; however, solving QCD to get hadronic properties from quark interactions must be done numerically, using supercomputers. We have worked on developing and employing methods which will allow better calculations with present computational resources. Two of these, called automated lattice perturbation theory and moving nonrelativistic QCD (mNRQCD), are now ready to be used to improve lattice calculations of decays of the B meson, in particular semileptonic and radiative decays (in these decays the final state is only one daughter meson and either a W boson -- which quickly decays to an electron or muon and a neutrino -- or a photon). The ability to have a bottom (or beauty) quark which is moving with respect to the lattice rest frame is a new one, and it allows us to better extract functional dependences on the daughter meson's momentum; this is the role of mNRQCD. Other reactions can be used to measure the same fundamental parameters in different ways. Any discrepany, or mismatch, in these measurements signals the underlying presence of 'new physics' which is then needed to resolve the discrepancy. For this to be feasible, accurate experiment and theory are needed. Our theoretical calculations are designed to reduce errors to the level of a few percent to achieve this goal. Careful tuning of the lattice description of the dynamics of the decay processes are necessary prior to computing the full non-perturbative contribution of QCD to these particle reactions. Such tuning calculations are large and technically complex, and are made possible only by the automation of much of the calculation using object-oriented programming. The outcome is a flexible theoretical approach that will keep pace with experiment in the search for new physics beyond the Standard Model.