New physics searches with charm decays at LHCb

粒子物理学的目标是理解宇宙的基本组成部分和它们的运行规则。其中一些基本粒子可以在自然界中进行研究-例如，地球上的所有物质都是由两种夸克(组装成质子和中子)加上电子组成的。位于太阳中心的核熔炉喷出大量的中微子。还有一些更稀有的粒子出现在宇宙阵雨中。我们在自然界中发现的所有粒子，加上我们能够在超级对撞机中产生的其他粒子，都由一个被称为标准模型的理论来描述。这是一种古老的理论，自20世纪70年代以来一直存在，基本上没有修改过。然而，越来越多的证据表明标准模型是不完整的-它是一个很好的近似值，但缺少了一些重要的部分，就像牛顿的运动定律在大多数情况下几乎是正确的，但被爱因斯坦的相对论取代一样。标准模型没有解释宇宙的许多方面，比如为什么物质比反物质常见得多，或者暗物质和暗能量的性质(它们加起来占宇宙的95%！)，甚至引力本身。该模型还存在内部一致性问题。这些问题中的许多都指向了一些新的、令人兴奋的、在地平线上完全不被理解的东西；我们简单地称其为“新物理学”。解决这一问题的一个有效方法是寻找标准模型认为不可能的东西，但这些东西在新物理学中是可行的。然后，如果你看到了所谓的不可能的效果，你马上就会知道标准模型是错误的-更有价值的-它是如何出错的。如果你什么都没看到，那仍然是有用的信息：新物理学的模型现在必须解释为什么这种效应是不允许的，并确定它们。在这个项目中，我们将研究一种被称为魅力介子的特殊粒子。这些粒子由一个魅力夸克和一个轻反夸克组成。它们还有一个反粒子，由一个迷人的反夸克和一个轻夸克组成。这两种生物都不稳定，但可以活到足以旅行一厘米的零点几厘米。在标准模型中，两者之间有一个几乎完美的镜像对称：它们的平均寿命相同，它们以相同的方式衰变，等等。然而，有了新的物理学，这种对称性就可以被打破。这种影响非常轻微，所以我们需要研究大量的粒子才能看到它。我们将利用大型强子对撞机LHCb探测器记录的数据进行这项工作，该对撞机将于2008年开始运行。LHCb将记录数以亿计的魅力介子衰变，这是一个比我们以前接触到的样本要大得多的样本。通过筛选这些数据，我们希望找到这个微小的差异，打破标准模型，并最终发现隐藏在更远的地方的新物理。

The goal of particle physics is to understand the fundamental building blocks of the universe and the rules by which they play. Some of these fundamental particles can be studied in nature---for example, all of the matter on earth is made from two kinds of quark (assembled into protons and neutrons) plus electrons. The nuclear furnace at the heart of the sun spits out immense numbers of neutrinos. Still other, rarer particles arrive in cosmic showers. All of the particles that we have found in nature, plus others that we have been able to produce in super-colliders, are described by a theory known as the Standard Model. This is a venerable theory that has stood, largely unmodified, since the 1970s. However, more and more evidence is piling up that the Standard Model is incomplete---that it is a good approximation but is missing some vital pieces, just as Newton's laws of motion were nearly right most of the time but were superseded by Einstein's theory of relativity. There are many aspects of the universe that the Standard Model does not explain, such as why matter is so much more common than anti-matter, or the nature of dark matter and dark energy (which together make up 95% of the universe!), or even gravity itself. There are also problems of internal consistency in the model. Many of these problems point to something new, exciting, and not at all understood just over the horizon; we simply call this 'new physics'. One powerful way to grapple with this is to look for things that the Standard Model says are impossible, but which are plausible with new physics. Then, if you see the supposedly impossible effect, you learn immediately that the Standard Model is wrong and---even more valuable---how it went wrong. If you don't see anything, that is still useful information: models of new physics now have to explain why the effect is not allowed, pinning them down. In this project, we will study a specific kind of particle known as charmed mesons. These particles are made from one charm quark and one light antiquark. They also have an antiparticle, made from one charm antiquark and one light quark. Both kinds are unstable, but can live for long enough to travel a fraction of a centimeter. In the Standard Model, there is a virtually perfect mirror symmetry between the two: they live for the same amount of time on average, they decay in equivalent ways, and so on. With new physics, though, this symmetry can be broken. The effect is very slight, so we need to study enormous numbers of particles to see it. We will do this with the data recorded by the LHCb detector at the Large Hadron Collider, which will begin operation in 2008. LHCb will record hundreds of millions of charm meson decays, a vastly larger sample than we've had access to before. By sifting through this data, we hope to find this slight discrepancy, break open the Standard Model, and finally uncover the new physics that lies beyond.