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

在基本粒子物理学中，我们研究自然的基本组成部分，包括那些自宇宙最早阶段以来就不自然存在的组成部分。在我们的标准模型理论（SM）中，有12个基本物质粒子，每个粒子都有它的反物质孪生子。它们通过交换四种“玻色子”中的一种来相互作用。40多年来，这个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，它无法解释为什么今天的宇宙包含任何物质。大爆炸应该创造了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）。大约140亿年后，剩下的应该是湮灭光子。这几乎是真的，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是什么构成了一切从恒星和星系，你和我。SM根本无法解释它，这只是我们为什么确信SM之外存在“新物理学”的一个例子。大型强子对撞机的建造是为了寻找预计存在的新的重粒子。它是通过在高能量下将质子（一种“强子”）撞击在一起来实现的。一千亿质子束每秒碰撞三千万次。能量是关键的，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。新的粒子预计是巨大的（重），所以根据方程，我们需要大量的能量来产生它们。在2010-2012年的首次运行之后，LHC刚刚开始了一段新的未知之旅，为期三年的“运行II”几乎是能量的两倍。在四个主要的LHC实验中，LHCb有一个狡猾的策略来寻找新的物理。它不是寻找新粒子的直接产生，而是对“美女强子”进行极其精确的测量。它们已经被发现了几十年，但仍然引起了人们的极大兴趣，因为它们的行为可以间接地受到新粒子的影响。量子力学的理论允许粒子在所谓的“环”中闪烁，而b-强子则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，才能分辨出与SM理论计算相比的微小差异。LHCb每年会看到数百亿个这样的人。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设备的不完美而产生的假效应所欺骗。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，但进一步降低可能会显示偏离SM的预期。两个大型强子对撞机实验已经开始了这样的使命，但它们的最终精度将受到我们对质子碰撞细节理解程度的限制。我已经证明了LHC b，虽然不是为这个目的而设计的，实际上可以进行类似的精确测量，它的特殊功能将极大地减少我们对质子碰撞的理解程度的依赖。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哪些碰撞可以保存下来以供进一步研究。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，有可能获得巨大的回报-偏离SM预测，这将为我们指明新的物理学。

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.