Discovery of Matter Anti-matter Asymmerty in the Charm Sector

对称性在我们对宇宙的理解中起着至关重要的作用。从许多动物身体的两侧对称到基本物理定律的数学对称，大自然在所有尺度上都展现出惊人的对称性。但只有不完美的对称性才能揭示出全貌。例如，在天体物理学中，宇宙微波背景辐射的各向异性让我们了解了大爆炸后大约30万年的宇宙结构。在大型强子对撞机（LHC）中，我们研究控制大爆炸后不到一秒的过程的物理类型。大型强子对撞机位于日内瓦郊外地下100米处，以地球上实验室从未达到的能量对质子进行碰撞。LHC的四个大型实验中最小的一个被称为LHCb，专门用于发现物质和反物质行为的不对称性。在我们目前的理解中，物质由12个基本粒子组成：6个夸克（u, d, s, c, b， t按质量递增顺序排列），一个电子及其较重的伙伴介子和τ以及它们的3个相关中微子。它们通过所谓玻色子的交换相互作用，玻色子是携带基本力的粒子。粒子物理学的标准模型（SM）解释了这些物质的相互作用，并且迄今为止经受住了所有的实验测试。然而，它无法解释一些基本问题，比如为什么t夸克比u夸克重5万多倍。另一个谜题是，完美的对称会导致等量的物质和反物质，以及大爆炸后不久宇宙的湮灭。像这样的问题告诉我们，SM并没有涵盖基本相互作用的所有方面，因此必须加以扩展。大自然给了我们三个专门的实验室来研究物质和反物质的相互作用：中性介子（称为K介子，B介子和d介子），由一个夸克（物质）和一个反夸克（反物质）组成的粒子。这些介子周期性地变成它们的反粒子，然后再变回来，这个过程被称为混合。对其中两个系统（K介子和B介子）的研究导致了获得诺贝尔奖的突破。我将研究第三个系统中的混合现象，称为d介子系统。特别是，我将在这个过程中寻找一个微小的不对称，物质和反物质行为之间的不对称。这种不对称性已经在K介子和b介子系统中被观察到，但在d介子系统中仍有待发现。LHCb实验至少在未来十年内是测量d介子的最佳设备。我正在寻找的d介子的不对称性在SM中被预测为很小。我们所能达到的精度允许观察到这种效应，如果它被增强到超过SM水平。对涉及d介子的几个过程的详细分析不仅可以为新的物理现象提供证据，而且可以通过与现有模型的比较来确定新物理的类型。从万维网到医学成像，粒子物理学研究不断为社会带来技术效益，但其真正目的永远是寻求对宇宙的基本理解。我的目标是进一步加深这种理解，利用物质与反物质的不对称来揭示自然之美。真正的美存在于不完美之中。

Symmetries play an essential role in our understanding of the universe. Nature exhibits a stunning level of symmetry across all scales, from the bilateral symmetry of many animal's bodies to the mathematical symmetry of the fundamental physical laws. But only the imperfections of symmetries reveal the full picture. In astrophysics for example, the anisotropy of the cosmic microwave background radiation has given insight into the structure of the universe roughly 300,000 years after the big bang.At the Large Hadron Collider (LHC) we study the type of physics which governed the processes just a fraction of a second after the big bang. The LHC, located 100m underground just outside Geneva, collides protons at energies that have never previously been reached in a laboratory on earth. The smallest of the four large experiments at the LHC is known as LHCb, and is specifically designed to discover asymmetries in the behaviour of matter and antimatter.In our current understanding, matter consists of twelve fundamental particles: the six quarks (u, d, s, c, b, t ordered by increasing mass), and an electron with its heavier partners muon and tau and their three associated neutrinos. These interact by the exchange of so-called bosons, which are fundamental force carrying particles. The standard model of particle physics (SM) explains these matter interactions and has so far held up to all experimental tests. However, it fails to give explanations to basic questions like the reason why the t-quark is over 50,000 times heavier than the u-quark. Another is the puzzle that perfect symmetry would lead to equal amounts of matter and antimatter and the annihilation of the universe shortly after the big-bang. Questions like these tell us that the SM does not cover all aspects of fundamental interactions and therefore has to be extended.Nature has given us three special laboratories to study matter and antimatter interaction: neutral mesons (called K, B and D-meson), particles consisting of one quark (matter) and one antiquark (antimatter). These mesons periodically change into their antiparticle and back, a process called mixing. The studies of two of these systems (K and B mesons) have led to Nobel-prize winning breakthroughs. I will study the mixing phenomenon in the third system, known as the D-meson system. Particularly, I will be looking for a tiny asymmetry in this process, an asymmetry between the behaviour of matter and antimatter. Such an asymmetry has been observed in the K and B-meson systems but remains to be detected in D-mesons.The LHCb experiment is the best apparatus for measuring D-mesons for at least the next decade. The asymmetry in D-mesons I am looking for is predicted to be tiny in the SM. The precision we can achieve allows the observation of this effect if it is enhanced beyond the SM level. Detailed analyses of several processes involving D-mesons cannot only give evidence for new physics phenomena but also allow to identify the type of new physics by comparison with existing models.Particle physics research continues to lead to technological benefits to society, from the World Wide Web to medical imaging, but its true purpose will always be the search for fundamental understanding of our universe. My goal is to further this understanding, using the matter-antimatter asymmetry to reveal nature's beauty. And true beauty lies in imperfections.