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

• 批准号: ST/N004892/2
• 负责人: Mika Vesterinen
• 金额: $ 44.52万
• 依托单位: University of Warwick
• 依托单位国家: 英国
• 项目类别: Fellowship
• 财政年份: 2018
• 资助国家: 英国
• 起止时间: 2018 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=ST%2FN004892%2F2
• 关键词: Precision; measurements; beauty; decays; boson

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，我们研究自然的基本组成部分，包括那些自宇宙最初阶段以来并不自然存在的物质。在我们的标准模型理论 (SM) 中，有 12 个基本物质粒子，每个粒子都有其反物质孪生粒子。它们通过交换四种“玻色子”中的一种来相互作用。 40 多年来，这一优雅的理论在解释我们在实验中所看到的内容方面已被证明非常准确。然而SM并不是最终的决定。例如，它无法解释为什么当今的宇宙包含任何物质。大爆炸应该创造出等量的物质和反物质，但我们知道物质和反物质相遇时会“湮灭”，产生光子（四种玻色子之一）。近 140 亿年后，剩下的只有湮灭光子。这几乎是正确的，只是我们的宇宙似乎每几十亿个光子就拥有一个物质粒子。这种看似微不足道的不平衡构成了从恒星和星系到你我的一切。SM 根本无法解释它。这只是我们确信 SM 之外的“新物理学”的一个例子。大型强子对撞机的建造目的是寻找预计存在的重质新粒子。它通过以高能量将质子（一种“强子”）粉碎在一起来实现这一点。 1000 亿个质子束每秒碰撞 3000 万次。能量至关重要，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。新粒子预计质量很大（重），因此根据方程，我们需要大量能量来产生它们。在 2010-2012 年首次运行之后，LHC 刚刚开始了通往未知的新旅程，为期三年的“运行-II”几乎增加了一倍的能量。 在四个主要的 LHC 实验中，LHCb 拥有寻找新物理的狡猾策略。它不是寻找新粒子的直接产生，而是对“美丽的强子”进行极其精确的测量。它们已经为人所知数十年，但仍引起人们极大的兴趣，因为它们的行为可能会受到新粒子的间接影响。量子力学理论允许粒子在所谓的“循环”中闪烁存在和消失，并且 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，以辨别与 SM 理论计算相比的微小差异。大型强子对撞机每年观测到大约数百万个这样的粒子。利用三年的 Run-II 数据，我将进行两次与 b 强子的物质和反物质版本之间的差异相关的测量。一个关键的挑战是避免因设备缺陷而被虚假效应所愚弄。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，但进一步减少可能会揭示与 SM 预期的偏差。两个大型强子对撞机实验已经开始执行这一任务，但它们的最终精度将受到我们对质子碰撞细节的了解程度的限制。我已经证明，LHCb 并非为此目的而设计，但实际上可以进行类似的精确测量，其特殊功能将极大地减少我们对质子碰撞理解程度的依赖。所有大型强子对撞机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%以上需要丢弃。这是“触发”系统的任务，我长期以来一直参与LHCb触发，我将继续发挥我的突出作用。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——与 SM 预测的偏差将为我们指出新的物理学。

项目成果

Understanding and constraining the PDF uncertainties in a W boson mass measurement with forward muons at the LHC

• DOI: 10.1140/epjc/s10052-019-6997-8
• 发表时间: 2019-02
• 期刊: The European Physical Journal C
• 作者: S. Farry;O. Lupton;M. Pili;M. Vesterinen

A Comparison of CPU and GPU Implementations for the LHCb Experiment Run 3 Trigger

LHCb 实验运行 3 触发器的 CPU 和 GPU 实现比较

• DOI: 10.1007/s41781-021-00070-2
• 发表时间: 2021
• 期刊: Computing and Software for Big Science
• 作者: Aaij R

A comprehensive real-time analysis model at the LHCb experiment

• DOI: 10.1088/1748-0221/14/04/p04006
• 发表时间: 2019-03
• 期刊: Journal of Instrumentation
• 影响因子: 1.3
• 作者: R. Aaij;S. Benson;S. Neubert;E. Govorkova;O. Lupton;M. Vesterinen;R. Matev;C. Fitzpatrick;H. Schreiner;A. Pearce;M. Cian;A. Dziurda;S. Stahl

Simultaneously determining the $W^±$ boson mass and parton shower model parameters

同时确定$W^±$玻色子质量和部分子簇射模型参数

• 作者: Lupton Olli

A simple method to determine charge-dependent curvature biases in track reconstruction in hadron collider experiments

确定强子对撞机实验轨道重建中电荷相关曲率偏差的简单方法

• DOI: 10.1140/epjc/s10052-021-09016-9
• 发表时间: 2021
• 期刊: The European Physical Journal C
• 作者: Barter W