Higgs Searches at the LHC

长期以来，粒子物理学家的目标一直是发现和理解宇宙的组成部分。多年来，对这些组成部分的理论描述已经发展起来，它描述了基本粒子以及它们如何相互作用。这个理论被称为“标准模型”。尽管这一理论非常成功——许多测量结果都非常精确地证实了它的预测——但这个谜题中仍有缺失的部分。一个特别引人注目的悬而未决的问题是质量的起源。标准模型最简单的形式不允许夸克、电子、介子、光子、胶子等基本粒子拥有质量。这是一个真正的问题，因为实验证据压倒性地支持这样的命题，即其中一些粒子是巨大的。根据希格斯、温伯格和萨拉姆等人的研究成果，对这一理论进行了修正，提出了一种被称为“自发电弱对称性破缺”的机制。也被称为希格斯机制，该理论的预测之一是存在一种称为希格斯玻色子的中性粒子。这并不是唯一的可能性，另一类流行的理论进一步扩展了标准模型，称为“超对称”，或简称SUSY，预测了许多新粒子的存在，其中包括几个希格斯玻色子。通过寻找希格斯玻色子，人们可以找到支持或反驳这些理论的证据。目前的实验结果排除了质量小于质子115倍的希格斯玻色子的存在。人们希望现有的实验能够将这个边界扩展到质子质量的130倍左右。因为我们从爱因斯坦那里知道E = mc*c，其中E是能量，m是质量，c是光速，因此，如果希望产生更重的粒子，就需要使用更高的能量。大型强子对撞机（LHC）将以相当于质子质量14000倍的能量将质子粉碎在一起。理论计算表明，这将足以产生足够的希格斯玻色子，这样我们就可以发现和测量它们，只要它们的质量在质子质量的100到1000倍之间。还有其他计算表明，如果存在希格斯玻色子，它的质量一定小于质子质量的1000倍左右。当质子在大型强子对撞机中碰撞时，它们会产生巨大的粒子喷雾。碰撞将在三个探测器内进行：CMS、ATLAS和LHC-B，它们将探测到喷雾中的一些粒子。不幸的是，并不是每一次质子碰撞都会产生有趣的结果，即使它们产生了有趣的结果，也并不总是产生我们想要的结果。由于我们无法在探测器中直接探测到希格斯粒子——它在衰变成其他更轻的粒子之前只存在短暂的片刻——我们必须尝试找到它衰变成的粒子，并通过测量它们的性质得出它们原来的粒子的性质。因此，为了从不那么有趣的“背景”事件中挑选出这些有趣的“信号”事件，有必要有好的方法来分析来自探测器的信号，以便重建在特定碰撞中实际发生的事情，看看是否有看起来像希格斯粒子的东西。这项研究的主要目的是分析在CMS探测器上收集的碰撞数据，以寻找希格斯玻色子存在的证据。

It has long been the goal of particle physicists to discover and understand the building blocks of the universe. Over the years a theoretical description of these building blocks has been developed which describes the fundamental particles and how they interact with one another. The theory is called the 'Standard Model'. Although this theory has been extremely successful - with many measurements confirming its predictions to great precision - there are missing pieces in the puzzle. One particularly compelling open question is that of the origin of mass. In its simplest form the Standard Model does not allow for the fundamental particles such as quarks, electrons, muons, photons, gluons etc to possess mass. This is a real problem since experimental evidence overwhelmingly supports the proposition that some of these particles are massive. A modification to this theory based on the work of many but most prominently, Higgs, Weinberg and Salaam, proposes a mechanism known as 'Spontaneous Electroweak Symmetry Breaking'. Also known as the Higgs mechanism, one of the predictions of this theory is the existence of a neutral particle called the Higgs Boson. This is not the only possibility, another popular class of theories which further extend the Standard Model - called 'Super-Symmetry', or SUSY for short, predict the existence of many new particles - amongst them several Higgs bosons. By searching for Higgs boson(s) one can thus find evidence to support or refute these theories. Current experimental results rule out the existence of a Higgs boson with a mass less than around 115 times that of the proton. It is hoped that existing experiments will be able to extend this boundary out to about 130 times the mass of the proton. Since we know from Einstein that E = mc*c, where E is energy, m is mass and c is the speed of light, it follows that if one wishes to produce heavier particles one needs to use higher energies. The Large Hadron Collider (LHC) will smash protons together at an energy equivalent to 14,000 times the mass of the proton. Theoretical calculations show that this will be sufficient to produce enough Higgs bosons such that we can discover and measure them so long as they have a mass anywhere between 100 and 1000 times the mass of the proton. There are other calculations which suggest that if there is a Higgs boson it must have a mass less than around 1000 times the mass of the proton. When the protons collide at the LHC they produce a huge spray of particles. The collisions will take place inside three detectors: CMS, ATLAS and LHC-B which will detect some of the particles in this spray. Unfortunately not every proton collision produces interesting results and even when they do they do not always produce those we were looking for. Since we cannot detect a Higgs directly in the detector - it exists only for fleeting moments before decaying into other lighter particles - we must try and find the particles it has decayed into and from measurements of their properties derive the properties of the particle from which they came. So in order to pick these interesting 'signal' events out from the not-so-interesting 'background' events it is necessary to have good methods for analysing the signals which come from the detector in order to reconstruct what actually happened in a particular collision and see if there was something that looked like it might be a Higgs particle. It is the primary purpose of the proposed research to analyse the data collected from collisions at the CMS detector to search for evidence of the existence of the Higgs Boson.