Search for the Higgs Boson at the ATLAS Experiment

经过一个多世纪的研究，物理学家发展出了关于物质的组成部分和它们之间的力的标准模型理论(SM)。这结合了将夸克束缚成原子核的强力、解释放射性的弱力和将电子保持在原子中的电磁力。到目前为止，它是经过验证的最准确的科学理论，验证到了十亿分之几！鉴于这一惊人的成功，你可能会奇怪，为什么科学家们会对这一理论产生怀疑。然而，有一点是缺失的：理解质量的产生。尽管SM的所有预测都是可测试的，但它无法预测任何粒子的质量(进而也无法解释你或我的重量)。这一挑战激发了各种理论，从SM的微调，到更古怪但可能是正确的理论，这些理论需要过多的新粒子，可能会解释暗物质。1964年，现为爱丁堡名誉教授的彼得·希格斯提出了一个优雅的解决方案，解决了只需要一个新粒子就能产生质量的问题。这种“玻色子”抓住了其他粒子，它抓得越牢，它们就变得越重。自那以后，与之同名的希格斯玻色子一直是基础科学的圣杯。尽管希格斯玻色子的设计初衷是产生质量，但它本身的质量是未知的。世界各地的科学家都没有成功地寻找它，排除了大质量范围的可能性，并将搜索推向了越来越高的能量。来自其他精密数据的证据现在预测，如果SM希格斯玻色子存在，它可以在欧洲粒子物理中心(CERN)的大型强子对撞机(LHC)上找到，该对撞机将在2008年以前所未有的能量在27公里的地下环中与质子相撞。在那里，我们应该找到期待已久的SM希格斯玻色子，或者可能是一个具有意想不到的性质的玻色子，它揭示了自然界中发生了更复杂的动力学，促使人们对SM进行了彻底的重新思考或扩展。如果它存在，我提议的计划将在实验中青睐的低质量范围内发现SM希格斯玻色子。如果不是，我可能会找到大自然的希格斯代理人。我的仔细研究将确定这些情景中的哪些是实现的。我将搜索来自LHC的ATLAS实验的数据，分析100亿个编目的冲突，这项任务被比作在1000个电话簿中找到一个电话号码。然而，这不是随机搜索，因为我们知道如何寻找这个‘数字’：我们可以通过在数据中寻找希格斯玻色子的已知特征来等同于找出这个人的姓氏！对玻色子的理论理解和实验技能将使它处于高能质子碰撞活动的漩涡中。我将在牛津ATLAS小组中建立并领导一个团队，寻找希格斯玻色子衰变到两个轻子或底夸克的过程。通过对数据的仔细分析，这些将提供独特的实验指纹。模拟表明，这些衰变将产生令人信服的发现的五个统计标准偏差“黄金标准”。该团队的专业知识和我在美国Tevatron对撞机上的深入经验相结合，将为团队中的每一名成员研究签名的一个元素提供坚实的基础。例如，通过定位于包含已知粒子的签名的数据集，这些粒子像希格斯玻色子一样衰减，识别算法可以得到改进。要找到tau轻子或底夸克，需要算法，我将基于我在底夸克部分进行精确测量的经验。这些研究将描述ATLAS硬件的特征，使我们能够搜索实验提供的自然电话簿。我们不能确定标准模型希格斯玻色子的存在，但我们可以肯定，寻找希格斯玻色子将是一次迷人的旅程，其目的地可能证明彼得·希格斯是正确的，或者更令人兴奋的是，大自然比他想象的更丰富、更复杂。

Over a century of study has led physicists to develop the Standard Model theory (SM) of the building blocks of matter and the forces among them. This combines the strong force which binds quarks into nuclei, the weak force which explains radioactivity and the electromagnetic force which holds electrons in atoms. To date, it is the most accurately tested scientific theory, verified to a few parts per billion! Given this amazing success you may wonder why scientists have any doubts about the theory. However, one piece is missing: understanding the generation of mass. Despite all of the SM's testable predictions, it cannot predict any particle's mass (and by extension cannot explain your or my weight). This challenge inspires theories ranging from tweaks of the SM, to more outlandish, but possibly true, theories which require a plethora of new particles, possibly accounting for dark matter. In 1964 Peter Higgs, now Edinburgh professor emeritus, proposed an elegant solution to the mass generation problem requiring only one new particle. This 'boson' grabs hold of other particles and the stronger it holds on, the heavier they become. The eponymous Higgs boson has been a holy grail of fundamental science ever since. Although the Higgs boson was devised to generate mass, its own mass is unspecified. Scientists worldwide have searched for it unsuccessfully, ruling out large mass ranges and driving the search to increasingly high energies. Evidence from other precision data now predicts that if the SM Higgs boson exists, it can be found at the Large Hadron Collider (LHC) at the European Centre for Particle Physics (CERN) which will collide protons in a 27km underground ring at unprecedented energies in 2008. There, we should find the long-awaited SM Higgs boson or, possibly, a boson with unexpected properties revealing that more complex dynamics occur in nature, prompting an entire re-think or extension of the SM. If it exists, my proposed programme will discover the SM Higgs boson in the experimentally favoured low mass range. If not, I may find nature's Higgs surrogate. My careful study will identify which of these scenarios is realised. I will search data from the ATLAS experiment at LHC, analysing 10 billion catalogued collisions, a task likened to finding one phone number in a 1000 directories. However, this is no random search since we know how to look for this 'number': we can do the equivalent of finding out the person's surname by looking for known characteristics of the Higgs boson in data! Theoretical understanding of the boson and experimental skill will pinpoint it in the midst of the maelstrom of activity in energetic proton collisions. I will build and lead a team in the Oxford ATLAS group to find the Higgs boson in its decay to two tau leptons or bottom quarks. With careful analysis of the data, these will provide distinctive experimental fingerprints. Simulations show that these decays will yield the five statistical standard deviations 'gold standard' of convincing discovery. The combination of the group's expertise and my in-depth experience from the USA's Tevatron collider will provide a firm foundation for each member of the team to study one element of the signature. For example by homing in on datasets which contain signatures of known particles which decay like the Higgs boson, the identification algorithms can be honed. Finding the tau lepton or bottom quark requires algorithms which I will base on my experience of precision measurements in the bottom quark sector. These studies will characterise the ATLAS hardware enabling us to search the phonebooks of nature which the experiment provides. We cannot be sure that the Standard Model Higgs boson exists, but we can be certain that the hunt for the Higgs will be a fascinating journey whose destination may prove Peter Higgs right, or even more excitingly, that nature is richer and more complex than he imagined.