Disentangling QCD and new physics with charmonium resonances at LHCb

粒子物理学的标准模型(SM)描述了所有已知粒子和力(重力除外)的性质和相互作用。它告诉我们有12个基本粒子和它们的反粒子：6夸克、电子和它的较重的近亲Muon和tau以及与它们相关的三个中微子。这些粒子通过交换载力玻色子相互作用：电磁相互作用的光子；导致放射性衰变的弱相互作用的W和Z粒子；以及强相互作用的胶子，它们将夸克绑定到组成原子核的质子和中子中。2012年，日内瓦欧洲核子研究中心的大型强子对撞机(LHC)发现了最后一块拼图，希格斯玻色子给所有粒子带来了裸露的质量。尽管取得了这一惊人的成功，但我们知道SM拼图是不完整的，因为我们对宇宙的认识仍然存在一些漏洞。例如，我们如何解释引力，物质对反物质的支配地位，以及我们如何解释所有我们认为存在于宇宙中但迄今未被我们发现的暗物质？大型强子对撞机上正在进行的实验正在以前所未有的能量和精度探测SM，试图测试它的每一个角落，寻找它在哪里发生故障的迹象。我们正在寻找一些意想不到的东西，这可能是一个新理论的第一个迹象，它给出了一个更完整的宇宙图景。欧洲核子研究中心的LHCb实验最近对B介子(由反底部(B)夸克和下(D)夸克组成的粒子)的衰变进行了一些非常有趣的测量，它被设计成高精度地进行研究。令人感兴趣的衰变是罕见的，每产生10^8B介子才会发生一次，但正是因为它们很罕见，所以它们对控制衰变的量子力学过程中可能出现的潜在新物理效应很敏感。综合来看，这些测量结果暗示，非SM物理可能正在发挥作用，这在粒子物理界引起了极大的兴趣和兴奋，引发了对这些影响的来源的不同预测。然而，这些反常现象也有可能是由于量子色动力学(QCD)的某些性质造成的，QCD是SM中解释胶子和夸克如何相互作用的部分。特别是，B介子的衰变幅度可能会受到来自魅力夸克和反魅力夸克(Charmonium)共振的未知效应的影响，这可能会干扰信号过程，也许会模仿一些新物理的迹象。鉴于QCD的复杂性质，我们试图从多个方向来处理和约束这个问题是很重要的。这项拟议的研究将使用LHCb记录的(以及即将记录的)数据来进行新的测量，通过解开QCD和潜在的新物理的复杂效应来理解这些新的和有趣的异常。LHCb实验最适合进行这些研究，因为它使用高效的触发系统来选择感兴趣的B衰变，并使用出色的粒子识别来抑制背景。在与粒子理论家的合作下，我们将能够扩大我们对这些B介子衰变的理解，并根据我们的发现，为下一步的测量计划设定方向，这些测量应该在未来的大型强子对撞机的数据采集期间进行。

The Standard Model (SM) of particle physics describes the properties and interactions of all known particles and forces (except gravity). It tells us that there are 12 fundamental particles and their anti-particles: the 6 quarks, the electron and its heavier cousins the muon and tau as well as their three associated neutrinos. These particles interact with each other via the exchange of the force-carrying bosons: the photon of the electromagnetic interaction; the W and Z particles of the weak interaction that are responsible for radioactive decay; and the gluons of the strong interaction that bind the quarks into the protons and neutrons that compose atomic nuclei.The last piece of the puzzle, the Higgs boson that gives all particles their bare mass, was discovered in 2012 at the Large Hadron Collider (LHC) at CERN, Geneva. Despite this amazing success we know that the SM jigsaw is incomplete as there remain some holes in our knowledge of the Universe. For example, how do we explain gravity, the dominance of matter over anti-matter and how do we account for all of the Dark Matter that we believe exists in the Universe but has so far evaded our detection? The experiments being performed at the LHC are probing the SM at an unprecedented level of energy and precision, trying to test every corner of it to look for signs of where it breaks down. We are searching for something unexpected that may be the first signs of a new theory that gives a more complete picture of the Universe.The LHCb experiment at CERN has recently made some very interesting measurements in the decays of B mesons (particles composed of an anti-bottom (b) quark and a down (d) quark), something that it was designed to do with high precision. The decays of interest are rare, happening only once for every 10^8 B mesons produced, but is is precisely because they are rare that makes them sensitive to potential new physics effects that could appear in the quantum mechanical process that controls the decay. Viewed together, these measurements have hinted that non-SM physics could be playing a role, which has generated significant interest and excitement in the particle physics community, giving rise to different predictions for the source of these effects. However, it is also possible that these anomalies are due to some property of Quantum Chromodynamics (QCD), the part of the SM that explains how the gluons and quarks interact. In particular, B meson decay amplitudes may be modified by unknown effects from resonances of a charm and anti-charm quark (charmonium) that can interfere with the signal process, perhaps mimicking some signs of new physics.Given the complicated nature of QCD, it is important that we try to approach and constrain the problem from many directions. The proposed research will use the data recorded (and soon to be recorded) by LHCb to perform new measurements to understand these new and intriguing anomalies by disentangling the complicated effects of QCD and potential new physics. The LHCb experiment is best placed to perform these studies owing to the efficient triggering system that it uses to select the B decays of interest and the excellent particle identification that is used to suppress backgrounds. In collaboration with particle theorists, we will be able to expand our understanding of these B meson decays and, depending upon what we find, can set the direction for the next programme of measurements that should be made during future data taking periods of the LHC.