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Precision measurements of beauty decays and the W boson mass at LHCb

LHCb 的美衰变和 W 玻色子质量的精确测量

基本信息

批准号：
ST/N004892/1
负责人：
Mika Vesterinen
金额：
$ 61.92万
依托单位：
University of Oxford
依托单位国家：
英国
项目类别：
Fellowship
财政年份：
2017
资助国家：
英国
起止时间：
2017 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=ST%2FN004892%2F1
关键词：
Precision measurements beauty decays boson

项目摘要

In elementary particle physics we study the basic building blocks of Nature including those that have not naturally existed since the earliest stages of the Universe. In our Standard Model theory (SM) there are 12 fundamental matter particles, each with its antimatter twin. They interact by exchanging one of four types of "bosons". Over 40 years, this elegant theory has proven remarkably accurate in explaining what we see in our experiments. However the SM is not the final word. For example it fails to explain why the present day Universe contains any matter at all. The Big Bang should have created matter and antimatter in equal quantities, but we know that matter and antimatter "annihilate" when they meet, producing photons (one of the four types of bosons). Almost 14 billion years later, all that should remain is the annihilation photons. That is very nearly true, except that our Universe appears to possess one matter particle for every few billion photons. This seemingly insignificant imbalance is what makes up everything from the the stars and galaxies, to you and I. The SM simply cannot explain it. This is just one example of why we are certain of "new physics" beyond the SM. The Large Hadron Collider is built to search for heavy new particles that are expected to exist. It does so by smashing protons (a type of "hadron") together at high energy. Bunches of 100 billion protons collide 30 million times each second. The energy is critical because Einstein's famous equation (E=mc^2) tells us that energy can be transformed into mass. The new particles are expected to be massive (heavy), so according to the equation we need large energy to produce them. After its first run in 2010-2012, the LHC has just begun a new journey into the unknown, with a three year "Run-II" at almost double the energy.Of the four main LHC experiments, LHCb has a cunning strategy to look for new physics. Rather than search for the direct production of new particles it makes extremely precise measurements of "beauty hadrons". They have been known for decades, yet are of great interest because their behaviour can be indirectly affected by new particles. The theory of Quantum Mechanics allows particles to flicker in and out of existence in so called "loops", and the b-hadrons exhibit various phenomena that depend on them. The goal is to see the effects of loops containing new particles. We must study huge quantities of b-hadrons to discern tiny differences compared to the calculations of the SM theory. LHCb sees some million-billion of them each year. With the three years of Run-II data, I will make two measurements related to differences between matter and antimatter versions of b-hadrons. A key challenge will be to avoid being fooled by fake effects due to imperfections in the apparatus.Quantum loops also affect the masses of the force carrying bosons. The mass of the W-boson is notoriously difficult to measure. While an uncertainty of 2 parts in 10,000 might seem impressive, a further reduction could reveal a deviation from the expectation of the SM. Two of the LHC experiments have already set out on a mission to do this, but their ultimate precision will be limited by how well we understand the details of proton collisions. I have shown that LHCb, which wasn't designed for this purpose, can actually make a similarly precise measurement and its special features will crucially reduce our dependence on how well we understand proton collisions.One of the biggest challenges in all LHC experiments is to decide, within less than a second, which collisions to save for further study. More than 99.9% of them need to be discarded. This is the task of the "trigger" system and I have long been involved with the LHCb trigger and I will continue my prominent role. This is a challenging but exciting programme of research with the potential for great rewards - deviations from the SM predictions that would point us to new physics.

在基本粒子物理学中，我们研究自然界的基本构件，包括那些自宇宙最早阶段以来就没有自然存在的构件。在我们的标准模型理论(SM)中，有12个基本粒子，每个粒子都有它的反物质孪生粒子。它们通过交换四种“玻色子”中的一种来相互作用。40多年来，这一优雅的理论已经被证明非常准确地解释了我们在实验中看到的东西。然而，SM并不是最终的决定。例如，它未能解释为什么今天的宇宙包含任何物质。大爆炸本应产生等量的物质和反物质，但我们知道，当物质和反物质相遇时，它们会“湮灭”，产生光子(四种玻色子之一)。近140亿年后，应该只剩下湮灭的光子。这几乎是正确的，除了我们的宇宙似乎每几十亿个光子就有一个物质粒子。这种看似微不足道的不平衡构成了从恒星和星系到你和我的一切。SM根本无法解释它。这只是一个例子，说明了为什么我们确信SM之外的“新物理学”。大型强子对撞机的建造目的是搜索预计将存在的重新粒子。它通过将质子(一种“强子”)在高能下撞击在一起来做到这一点。1000亿个质子以每秒3000万次的速度碰撞。能量是至关重要的，因为爱因斯坦著名的方程(E=mc^2)告诉我们，能量可以转化为质量。新粒子预计是大质量(重)的，所以根据方程，我们需要很大的能量来产生它们。在2010-2012年首次运行后，大型强子对撞机刚刚开始了一段通往未知的新旅程，为期三年的“第二次运行”的能量几乎是大型强子对撞机的两倍。在四个主要的大型强子对撞机实验中，大型强子对撞机有一个寻找新物理的巧妙策略。它不是寻找直接产生新粒子的方法，而是对“美丽强子”进行极其精确的测量。人们已经知道它们几十年了，但人们对它们非常感兴趣，因为它们的行为可能会间接受到新粒子的影响。量子力学的理论允许粒子在所谓的“环”中闪进闪出，而b强子表现出依赖于它们的各种现象。目标是查看包含新粒子的循环的效果。我们必须研究大量的b强子，以辨别出与SM理论的计算相比的微小差异。LHCb每年都会看到大约数十亿个这样的人。利用三年的Run-II数据，我将对b强子的物质和反物质版本之间的差异进行两次测量。一个关键的挑战将是避免被由于仪器缺陷而产生的虚假效果所愚弄。量子环也会影响携带玻色子的力的质量。众所周知，W玻色子的质量很难测量。虽然万分之二的不确定性似乎令人印象深刻，但进一步的降低可能会揭示出与SM的预期背道而驰。其中两个大型强子对撞机实验已经开始执行这项任务，但它们的最终精度将受到我们对质子碰撞细节的理解程度的限制。我已经证明，LHCb并不是为这个目的而设计的，它实际上可以进行类似的精确测量，它的特殊功能将至关重要地减少我们对质子碰撞理解程度的依赖。所有LHC实验中最大的挑战之一是，在不到一秒的时间内决定保存哪些碰撞以供进一步研究。其中99.9%以上的垃圾需要丢弃。这是“触发器”系统的任务，我长期以来一直参与LHCb触发器，我将继续扮演我的重要角色。这是一个具有挑战性但令人兴奋的研究计划，有可能带来巨大的回报--偏离SM的预测，将把我们引向新的物理学。