# 中微子：物质的鬼魂

21 Jun 2005

The discovery that 中微子 have mass and can oscillate between different 味道s was one of the major breakthroughs in particle physics in the past decade, but there is much about these mysterious particles that we still do not understand

The world of neutrino physics has come a long way in the last 30 years. Once a ghostly afterthought of particle physicists, introduced to explain something that was missing rather than something that was there, 中微子 have proved to be every bit as fascinating as quarks, gluons and all the other fundamental particles. Indeed, they might even be able to explain one of the biggest puzzles in physics: where did the matter in the universe come from?

It is now becoming clear that the answer to this puzzle could come from a very unexpected quarter: the behaviour of 中微子. How we have come to this startling conclusion is a fascinating tale and, as is so often the case in science, the story begins with a completely different problem.

### 太阳的问题

In the 1960s, while other particle physicists were investigating all these newly discovered particles, Ray Davis at the Brookhaven National Laboratory in the US was pursuing the idea of using 中微子 as a probe. For decades astronomers had thought that the most likely power source for the Sun and other stars was thermonuclear fusion, but no direct proof was available. Davis believed he could observe the fusion reactions directly by detecting the 中微子 they produced.

In the basic fusion reactions in the Sun, four protons are converted into a helium-4 nucleus, emitting two positrons and two electron 中微子 in the process. These 中微子 have a wide range of energies and vast numbers of them escape from the Sun without interacting with anything, hurtling towards the Earth at close to the speed of light. But it is precisely this extremely low probability of interacting with matter that makes 中微子 so hard to detect.

Kamiokande实验由地下深水箱中的几千吨纯净水组成，最初是为了寻找质子衰减而建造的。然而，它的设计者意识到，该实验也许还能够检测出来自太阳的高能中微子，这些中微子通过散射反应与电子相互作用。这些电子的传播速度可能快于水中的局部光速，从而使它们发出的声音相当于声波的光学发散。–坦克周围的超灵敏光电倍增管可以检测到称为Cerenkov辐射的蓝光。

In 1989 the Kamiokande team confirmed that the flux of 中微子 from the Sun was indeed much lower than expected. But experimental particle physicists take a lot of convincing, and there was still the possibility that the 太阳中微子问题 arose not from the 中微子 but from the solar models themselves. This is because the neutrino flux measured by the Davis and the Kamiokande experiments was dominated by high-energy 中微子 from a small side reaction involving the decay of boron-8. The rate of this reaction depends critically on the core temperature of the Sun, so a small error in this temperature could explain the low neutrino fluxes seen in both experiments. We therefore had to confirm that all solar 中微子 were SUPpressed, not just those at high energies.

### 中微子振荡

If neither the solar models nor the experiments were at fault, then what was the source of the 太阳中微子问题? One solution, which was actually proposed by Pontecorvo the year before Davis had obtained his first results, was that 中微子 may change from one 味道 to another on their journey from the Sun to the Earth (see box 1 下面). Since the existing experiments were predominantly sensitive to electron 中微子, rather than muon and tau 中微子, this could explain why we only detected about a third of the solar 中微子.

The largest source of such background events are 中微子 from cosmic rays, the high-energy particles that constantly bombard the Earth’s atmosphere from sources in our galaxy and beyond. The debris of these collisions is dominated by pions, which decay into muons plus muon 中微子 in reactions such as π → μ + νbarμ, where the horizontal bar depicts a antineutrino. The muons themselves then decay into electrons and more 中微子 via the reaction μ → e + νbare + νμ.

However, for events coming from 以上, SuperK saw roughly the expected 2:1 ratio of muon to electron 中微子, while for events coming from 下面 it saw many fewer muon 中微子. This was 子sequently confirmed by the Soudan II and MACRO experiments, and demonstrated that nature really does satisfy the first condition for neutrino oscillations: that 中微子 have mass. But what about the second condition, that 中微子 change 味道?

### Solving the 太阳中微子问题: SNO and KamLAND

Demonstrating that 中微子 can change 味道 was the main purpose of the Sudbury Neutrino Observatory (SNO) in Canada, which was built by a large collaboration of Canadian, US and UK physicists (and which I have been a part of 罪ce 1988). SNO is a water Cerenkov detector like Kamiokande, but instead of using normal water it uses heavy water, D2O. The deuterons, D, in the heavy water are the most weakly bound of all nuclei, which gives SNO the chance to observe three different reactions induced by solar 中微子.

The results, announced in 2001 and 2002, confirmed beautifully the neutrino-oscillation prediction. The number of neutral-current events matched the predictions of the solar models quite precisely, showing that the total neutrino flux is actually spot on. However, the charge-current reaction rate showed that only about a third of these 中微子 are electron 中微子 by the time they reach the Earth, which proved that 中微子 change 味道 on the way.

KamLAND是在日本利用优势的老Kamiokande腔中建造的大型探测器’的核反应堆，是电子中微子的强大来源（日本约30％’s energy comes from nuclear power stations). Coincidentally, these reactors are at the right distance away for neutrino physics: close enough for their antineutrinos to be detected, but far enough away that neutrino oscillations should significantly SUPpress the number of electron antineutrinos detected.

The most recent results from KamLAND, reported last summer, clearly show not only a SUPpression of the detected flux, but also a distortion of the spectrum as a function of energy, which is precisely what the oscillation model predicts. The real clincher, however, is that the SUPpression is much less than that seen for solar 中微子 because their oscillation is modified as they pass through the dense matter of the Sun. This is exactly what is expected for a model of neutrino oscillations, but not for any of the other models, and seems to be the final piece of evidence needed to state that 中微子 really do oscillate. Furthermore, it allows us to measure Δm122 准确地将其与太阳中微子的测量结果结合在一起，可以约束中微子振荡参数的值。

### 长基线实验

If terrestrial experiments like KamLAND can observe the neutrino oscillations originally seen by solar-neutrino experiments, are there terrestrial experiments that can detect the oscillations seen in 大气的 中微子? The answer is yes, but it means we have to make our own high-energy 中微子, and this requires an accelerator.

Experiments of this latter type have a long history, but we now know they were looking in the wrong place. Guided by a theoretical prejudice that all the 混合角度 would be small and by the belief that neutrino masses should be large enough to explain the missing matter in the universe, researchers were looking for small 混合角度 and large mass differences. But we now know from the results of solar, 大气的 and reactor oscillation experiments that we should be looking for small mass differences and large 混合角度, and a new generation of experiments has been designed to do just that.

The first of these, called K2K, produced its first results in 2000. The proton beam is produced at the KEK laboratory just north of Tokyo, and the resulting muon-neutrino beam is fired 250 km under Japan to the SuperKamiokande detector. Sure enough, this experiment has seen too few muon 中微子, exactly as would be expected if the 大气的-neutrino anomaly really is caused by neutrino oscillations.

In 1996 the LSND team claimed to see evidence for the appearance of electron 中微子 in a muon-neutrino beam, which suggested a small mixing angle and a relatively large value of Δm122。尽管在英国卢瑟福·阿普尔顿实验室的另一项灵敏度类似的实验称为KARMEN，但尚未见到这种效应的证据，但仍在热切期待着名为MiniBooNE的专门实验的结果。 MiniBooNE探测器位于费米实验室，将在费米实验室产生的μ子中微子束中搜索电子’质子加速器。如果MiniBooNE看不到效果，那么除了爱丁顿之外，每个人都可以松一口气，并相信现有实验绘制的三中微子振荡图。另一方面，如果MiniBooNE确认了LSND结果，那么我们确实生活在一个非常陌生的世界中，并且混合现象将比我们目前的理解要复杂得多。

### 测量θ13：故事的下一步

Fermilab计划进行另一个名为NOvA的新实验，该实验将使用与MINOS相同的光束。该实验将比T2K实验具有更高的能量和更长的基线，这有望观察到物质效应，并使我们能够确定质量层次。

### The more distant future: the 中微子工厂

Superbeam neutrino experiments may be sensitive to values of δ that are near π/2 or 3π/2, where CP violation is the largest (see box 2 下面), but to really pin this angle 下 we need even more intense and “cleaner” neutrino beams. A feasibility study is currently taking place at CERN to find out if this can be achieved with beams of unstable nuclei, which undergo beta decay and produce pure beams of electron 中微子 or antineutrinos.

## 方框1：理论上的中微子振荡

If 中微子 have mass, then the identity of a given neutrino becomes a bit complicated. This is because in addition to the electron (νe），介子（νμ）和tau（ντ) “flavour” states that have well-defined weak interactions, 中微子 have another set of states – denoted ν1，ν2 和ν3 –具有明确定义的质量。任何特定的中微子将以ν出现e，νμ 或ντ 如果测量对弱相互作用敏感，或者作为ν1，ν2 或ν3 如果测量对质量敏感。这两个集合可能是相同的，但是通常它们是“mixed”。换句话说，一个νe 将部分为ν1，部分ν2 部分ν3，对于ν同样μ 和ντ.

Real-life reactions produce 味道 states: for instance, thermonuclear reactions in the Sun generate only νe. However, it is the mass states that propagate through space. If we take the simplest case of θ = 45°, the 以上 equation states that νe = ν1 – ν2 和νμ = ν1 + ν2。回顾粒子也可以描述为波，这意味着ν1 和ν2 在ν的情况下，波是异相振荡的e，而ν1 和ν2 与ν同相μ。如果ν1 和ν2 具有不同的质量，它们也将具有不同的波长，因此它们的相对相位将随时间或距离而变化（请参见波长图）。

$P\left({\nu }_{\mu }\to {\nu }_{\mu }\right)=1–{罪}^{2}2\theta {罪}^{2}\left(1.27\frac{\Delta {m}^{2}L}{E}\right)$

When the 中微子 are travelling through empty space, the decrease in the overall νμ 通量（以及ν的相应增加e 因此，磁通量取决于混合量θ。但是，由于e 与ν相比，物质的相互作用略有不同μ 或ντ, the oscillations can actually be enhanced when a neutrino passes through the Sun or the Earth. Real experiments also involve all three 中微子, which can lead to “振荡”在未来的实验中将具有重要意义（请参见文字）。与此处介绍的两味情形相比，三味振荡需要测量的参数更多：三个混合角（θ12θ23 和θ13），两个独立的质量差（Δm122 和Δm232) and one additional parameter, δ, which could produce differences in the oscillations of 中微子 and antineutrinos.

## 方框2：违反中微子和CP

1967年，俄罗斯物理学家安德烈·萨哈罗夫（Andre Sakharov）指出，为了从能量主导的初始状态发展到今天我们所看到的物质主导的宇宙，必须满足三个条件。首先，物理定律必须产生不同数量的物质和反物质。第二，中子和质子等重子的数量不能守恒；第三，宇宙不能处于热平衡。后两个条件似乎很容易满足，但第一个条件–也称为违反收费平价（CP）–事实证明存在更多问题。

### 进一步阅读

Q R艾哈迈德 等。 （SNO Collaboration）2002在萨德伯里中微子天文台从中性流相互作用中微子风味转变的直接证据 物理莱特牧师 89 011301

E阿留 等。 （K2K协作）2005年基于加速器的实验中的μ-中微子振荡的证据 物理莱特牧师 94 081802

T荒木 等。 （KamLAND Collaboration）2005用KamLAND测量中微子振荡：光谱失真的证据 物理莱特牧师 94 081801

Y Ashie 等。 (SuperKamiokande Collaboration) 2004 Evidence for an oscillatory signature in 大气的-neutrino oscillations 物理莱特牧师 93 101801

G Drexlin 2003 LSND和KARMEN产生的最终中微子振荡 核仁物理乙 （Proc。Suppl。）118 146–153

H Murayama 2002中微子质量的起源 物理世界 可能pp35–39 （仅印刷版）