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粒子与相互作用

粒子与相互作用

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

MiniBooNE检测器

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?

根据大爆炸模型,宇宙始于137亿年前,是一个很小的纯能量区域,它不断膨胀和冷却,形成了我们今天所看到的宇宙。但是,在该模型的许多成功中,至少存在一个明显的问题:宇宙由物质控制,几乎没有反物质。物理定律确实允许能量转化为物质,但是要求在此过程中产生几乎相等数量的反物质。

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.

中微子的诞生

中微子的概念可以追溯到1930年代,当时研究人员注意到,当一个原子核衰变为另一个原子核加一个电子时,能量似乎消失了。沃尔夫冈·保利(Wolfgang Pauli)撞上了“desperate remedy”为了解释这种情况,提出了衰变中发射的第三个粒子带走了缺失的能量。对于现代读者来说,这似乎不是特别具有革命性,但在保利’那天只有两个已知粒子–电子和质子–因此引入第三个粒子极端是极端的。

可以理解,Pauli最初并不愿意发表这个想法,甚至后来为预测一个他认为不可能检测到的粒子而道歉(如果只有现代理论家同样关注的话!)。这是因为他的“neutrinos”, as Enrico Fermi christened them, had no electric charge and would interact only weakly with other matter. Happily, Pauli was proved wrong and lived to witness the Nobel-prize-winning detection of 中微子 in 1953 by the late Fred Reines and Clyde Cowan.

但是,新兴的粒子物理学领域当然并没有就此结束。我们继续发现正电子(反电子),介子,介子和许多其他新颖的粒子。伴随着μ子,出现了另一个中微子,现在称为μon中微子,νμ,这与鲍里(ν)提出的电子中微子不同e)。然后,在1975年发现tau lepton之后,很明显,“flavour”中微子:τ中微子,ντ,最终于2000年在美国Fermilab的DONUT实验中被发现。

同时,夸克模型将过多的强相互作用粒子(如质子和介子)排序。结合少数其他可以解释基本粒子之间作用力的粒子,这给我们留下了一个相当简单但极为强大的粒子物理图景,称为标准模型。在该模型中,中微子最初被认为是严格无质量的,并且仅通过弱相互作用而相互作用,尽管交换“Z” and the “W”粒子。但是事实证明,事实要比这复杂得多。

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.

戴维斯于1946年在加拿大查尔克河实验室使用布鲁诺·庞特科沃(Bruno Pontecorvo)提出的放射化学技术解决了这一问题:组装大量目标原子,这些目标原子偶尔会受到太阳中微子触发的核反应。戴维斯选择了氯的同位素作为目标,他设法以可接受的价格获得了600,000升清洁液的形式。然后,来自太阳的中微子将与氯反应生成放射性氩原子,这可能是“gathered up”并单独计算。首先知道中微子会触发反应的可能性,然后就有可能推断出太阳中微子通量。

这项大胆的实验的第一批结果于1968年宣布,几乎让每个人都感到惊讶。戴维斯’研究小组仅检测到可用最佳太阳能模型(即John Bahcall及其同事开发的模型)预测的中微子的约30%。起初,研究人员对在如此庞大的液体中是否真的能看到少量氩原子持怀疑态度,但经过严格的测试,看来该实验没有错。然而,真正的证据表明“太阳中微子问题”直到二十年后才在这里停留,当时日本的Kamiokande实验证实戴维斯’结果。东京大学的戴维斯(Davis)和神冈坎德(Kamiokande)发言人Masatoshi Koshiba分享了2002年诺贝尔物理学奖,以表彰他们在中微子天体物理学方面的开拓性工作。

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.

这需要两个新实验,分别称为SAGE和GALLEX,它们遵循了戴维斯的基本思想’ experiment except that they used gallium instead of chlorine as the target atom. Due to the more complex chemistry involved, these experiments were more difficult to perform, but in the early 1990s we eventually got the answer: the low-energy 中微子 were missing too. The problem did not lie with the solar models –那是另一回事。

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.

但是,这种中微子振荡思想存在一个大问题:它要求中微子具有质量,而在标准模型中则没有。当时这是一个令人兴奋的前景,因为这意味着中微子可能解释了“dark matter” that is thought to dominate the universe. We now know that neutrino masses are too small to account for most of this strange, non-luminous 子stance (even so, there is roughly the same amount of mass in 中微子 as there is in all the visible matter in the universe). But these tiny neutrino masses are still of great interest because they might arise from some fundamentally different mechanism to the way the masses of other particles are generated –即希格斯机制。

中微子振荡理论包含一些基本参数:三个中微子态ν的质量1,ν2和ν3 (see box 1 下面) or rather the two independent differences between them, Δm122 和Δm232;三“mixing angles”, θ12θ23 和θ13;关键参数称为相位δ。测量这一阶段可能是回答为什么宇宙包含比反物质更多的物质之谜的关键之一。但是在物理学家探索这种可能性之前,我们仍然必须确定中微子振荡是简单的数学还是真实的物理。特别是,我们需要测量质量差异和混合角度。

在寻求中微子振荡的证据时,在寻找质子衰减的实验中一个单独的问题开始出现。质子可能会发生衰变,也可能不会发生,但是它确实非常罕见(预计质子的寿命至少为1032 年份)。因此,实验人员不得不担心可能掩盖甚至模仿质子在探测器中衰变的其他过程。

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.

因此,此过程应产生两个“atmospheric” muon-neutrino events for every electron-neutrino event. However, to the surprise of researchers working on a Kamiokande-like detector called the Irvine Michigan Brookhaven (IMB) experiment, and of the Kamiokande team itself, this ratio was not seen. Instead, the two experiments saw roughly the same number of both types of neutrino. As with the 太阳中微子问题, many physicists initially thought that this “大气中微子异常” was simply due to a problem with the experiments, or possibly the models of 大气的-neutrino generation. However, in 1998 a vastly larger version of Kamiokande called SuperKamiokande convinced almost everyone that the 大气的 anomaly must lie with the 中微子 themselves.

之所以取得突破是因为SuperK能够比较发生的事件“down”从大气中发生的事件“up” from 下面, and hence arose from interactions in the atmosphere on the other side of the planet (figure 1). The only significant difference between these two classes of neutrino is the distance they have travelled, but if 中微子 are massless this should make no difference.

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.

这些过程中的第一个是充电电流反应νe + D →  p + p + e是通过从高能反冲电子e观察切伦科夫光子来检测的. This reaction is only sensitive to electron 中微子, which is good because these are the only type produced by nuclear reactions in the Sun’s core. But what happens if these 中微子 oscillate on their way from the Sun to the Earth?

这是第二反应,中性电流反应νχ + D → p + n + νχ发挥作用。这是通过发射的中子n观察到的,与入射中微子χ的味道无关。因此,它提供了一种标准化太阳发射的中微子总通量的方法。在没有中微子振荡的情况下,从充电电流和中性电流反应推断出的通量将是相同的,而中微子振荡将降低充电电流速率,但不会降低中性电流速率。结果,SNO可以确定中微子是否会改变风味,而与太阳能模型的细节无关。第三种相互作用是两个Kamiokande探测器ν已经观察到的相同的电子散射反应χ + e → νχ + e, which has some sensitivity to all neutrino 味道s but does not allow a clean comparison between them.

建立像SNO这样的实验绝非易事。首先,您需要1000吨重水,这在您当地的五金店通常是找不到的。幸运的是,安大略水电在其核反应堆中使用大量重水,并愿意向我们提供一个反应堆’的价值,前提是我们将其归还(价值几亿美元)。然后,您需要在很深的地方挖一个巨大的空腔,以容纳检测器。再次,巨大的好运使我们来到了INCO镍矿公司,该公司对一群物理学家在其利润丰厚的矿山中做着相当奇怪的事情具有超自然的宽容度。最终,一旦您获得了数百万美元的全部资金,就到了最困难的部分:构建探测器本身。

出现困难的原因是可以模仿中微子信号的自然本底反应。普遍存在的铀和or的放射性衰变产生的伽马射线是一个特殊的问题,因为它们可以分解氘核并释放中子,这些中子无法与中性电流反应产生的中子区分开。控制此背景的唯一方法是在巨大的洁净室中使用精选材料建造探测器–在活跃且非常肮脏的矿井中地下2公里的洁净室。

一旦所有这些事情在1999年完成,乐趣就真正开始了,因为探测器不仅会说,“嘿,有个中微子–将一个添加到充电电流列表中!”问题是您不能直接测量检测器的响应来挑选出几个– about 10 per day –来自背景过程的每秒数十个事件中的太阳中微子事件。取而代之的是,我们必须使用光学和放射源以及计算机模拟,从第一原理上费力地了解探测器的行为。一旦这项工作完成,实际上适合中微子信号就相对简单了。

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.

在2004年,我们通过在重水中添加两吨盐来改进此测量方法,这使得来自中性电流事件的中子更易于检测。但是,为了绝对确定太阳中微子确实会改变风味,我们现在再次重复该实验。但是,这次,我们将能够使用一组非常敏感的中子探测器来独立于充电电流事件来检测中性电流事件。这将使检测器对电子型和介子型中微子之间的混合更加敏感(从而改善角度θ的测量12),同时还要确保我们没有被前两次测量所欺骗。

所以,超级神冈’s 大气的-neutrino results from 1998 showed that 中微子 have mass, while, a few years later, SNO showed that 中微子 can change 味道. Does this prove that neutrino oscillations occur? Well, not quite, because a number of other models have been proposed that can also explain these data, ranging from new neutrino properties to the effects of higher dimensions. Luckily, in 1994 another Japanese group had proposed a very clever experiment called KamLAND, which provided the final piece of evidence for neutrino oscillations.

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.

自从1960年代初发现μ子中微子以来,就一直存在创建纯净,准直的中微子束的想法。出发点是将一束质子射入某个目标以产生离子,然后将其衰减以产生一束μ子和μ子中微子。通过在只有少数极少数子能衰变之前停止μ子的束缚,出现了几乎仅由μ子中微子组成的束。用这种光束可以进行两种类型的实验。首先,人们可以根据能量或距离的函数,从期望值中寻找μ子中微子通量的减小。其次,人们可以在光束中寻找电子或tau中微子。

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.

许多其他这样的长基线实验正在建设中或正在计划中。其中第一个被称为MINOS,它将从芝加哥附近的费米实验室向明尼苏达州Soudan矿的一个探测器发射一束强烈的of子中微子束(距离735公里)。与水切伦科夫探测器不同,MINOS由大型铁质热量计组成,该热量计在存在磁场的情况下可以跟踪和测量带电粒子通过检测器时的动量。因此,MINOS可以更精确地测量与费米实验室中微子相互作用而产生的μ子的能谱,从而可以更精确地确定振荡最大的能量。这将使我们能够更好地测量Δm232,目前尚不确定。

另一个名为CNGS的长基线实验正在欧洲进行建设,将从中将CEU的中子微子从日内瓦发射到罗马附近的Gran Sasso国家实验室(距离730公里)。在Gran Sasso建造的两个大型探测器– OPERA and ICARUS –将对所得的粒子轨迹进行极其精确的测量。通过实际观察束中的tau中微子,这应该使CNGS团队能够确认我们当前中微子振荡模型的集中预测– that the 大气的-neutrino anomaly is caused primarily by the oscillation of muon to tau 中微子.

我相信是爱丁顿(Eddington)所说的,任何与所有现有实验数据一致的东西都一定是错误的,因为某些数据几乎肯定是错误的。因此,对于那些希望相信中微子振荡的人来说,一个实验是一种解脱– LSND at Los Alamos –不适合此处概述的中微子图。

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

那么,我们在中微子基本振动参数的测量中站在哪里呢?太阳中微子实验和KamLAND测量了角度θ12 大约32°且质量差Δm122 大约是8 x 10-5 eV2。大气中微子的振荡测量具有约束θ23 to be nearly 45° (i.e. 中微子 oscillate as much as is possible) and also measured Δm232 大约是2.5 x 10-3 eV2。剩下第三个混合角θ13.

我们还需要更多地了解中微子质量状态。对于三个不同的质量,只有两个独立的质量差,并且由于我们已经测量了两个质量差,因此您可能认为我们已经完成了。不幸的是,这不是那么简单。原因是我们不知道 先验 the ordering of the mass states, because the vacuum-neutrino oscillations we have measured depend on the square of the mass differences and are therefore independent of their sign. But matter effects, such as those present for 中微子 that escape the dense interior of the Sun, do depend on the sign of the mass difference. We therefore know from the oscillations of solar 中微子 that ν2 比ν更重1. But 罪ce we have not yet seen any matter effects in the oscillations of 大气的 中微子, because the Earth is not sufficiently dense, there are two remaining possible orderings (figure 2).

确定θ13和mass hierarchy is the target of the next generation of planned experiments. We already know from experiments on shorter baselines than KamLAND that the angle θ13与其他两个角度不同,它很小。新实验计划寻找θ13 in the so-called 子-leading oscillations that arise when the effects of all three 中微子 are taken into account. These oscillations would produce small ripples on the primary oscillation pattern, and cause the additional appearance of electron 中微子 in a terrestrial muon-neutrino beam. New reactor and accelerator oscillation experiments have been proposed that will use a near detector and a far detector in order to improve the sensitivity to tiny SUPpressions and small appearance probabilities, and hence to smaller values of θ13.

诸如MINOS之类的长期基线实验对这些二阶效应具有一定的敏感性,但要显着改善我们现有的知识,我们将需要使用甚至更强的中微子束的全新设施。这些中的第一个“superbeam”目前正在日本进行名为T2K的实验,该实验涉及从该国东海岸的JPARC设施向西的SuperKamiokande穿过295公里的岩石发射强度空前的中微子束。在所涉及的能量和距离处,太阳中微子中看到的振荡类型不会产生任何明显的影响,因此,在介子中微子束中看到的任何电子中微子将是由第三角度θ调制的振荡信号。13.

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

那么,正如所有这些话之前所承诺的,这一切与宇宙中反物质中多余物质的关系如何?线索可能在于参数δ。产生物质-反物质不对称性需要物质和反物质的物理定律不同,并且δ的非零值确实会导致中微子和反中微子振荡的差异。早期宇宙中的一种称为瘦素生成的相关效应可能会导致物质与物质的不平衡。但是,瘦素生成取决于地球上无法测量的参数,因此我们必须首先测量δ,然后信任我们的理论家以找到将δ与瘦素生成联系起来的正确模型。

基本思想很简单:首先从一束介子中微子开始,测量它们变成电子中微子的可能性,然后切换到一束介子中微子,并在相同的能量和基线下测量转变为电子中微子的可能性。如果您使反中微子穿过反物质地球,并允许它们与反物质探测器相互作用,则任何差异都表示δ不为零。不幸的是,这超出了大多数国家的科学预算,因此我们必须使用由普通物质制成的探测器来测量中微子和反中微子,并纠正由此带来的无趣的差异(这限制了实验的灵敏度)。

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.

更加雄心勃勃的计划是建立一个“neutrino factory”会产生非常纯净而强烈的中微子束,可以将其发射穿过地球数千公里,到达遥远的探测器。像传统的长基线实验一样,这种机器会产生会衰减为介子的介子。但是它将把这些μ子加速到高能,以便它们衰变时产生具有众所周知能量的准直中微子束。这将使我们能够以比任何其他计划的实验都要好几个数量级的灵敏度来测量中微子振荡,尽管这样做的难度和费用是相当大的。

一个主要的挑战是找到一种方法,以便在它们有时间衰减之前收集并加速它们。现有方法“cool” particles by reducing their angular or energy spread are too slow to be used for muons (which rapidly decay into other particles). This has led to a major international experiment called MICE (Muon Ionization Cooling Experiment) at the Rutherford Appleton Laboratory that will test an entirely new type of 凉ing (see 物理世界 April p5). Technological developments such as MICE should make it possible to build a 中微子工厂 some time soon, hopefully before I retire in about 20 years.

回报

因此,已经发现了中微子振荡。但是值得记住的是,中微子振荡的两个证实的观察都来自最初为寻找其他东西而建立的实验。实际上,当雷·戴维斯(Ray Davis)最初提出他的实验时,大多数人认为这是浪费时间(一位评论员甚至将其与试图通过站在梯子上并举起手来测量到月球的距离进行比较!)。在今天’戴维斯(David Davis)的竞争激烈的筹资体系中,它是否有可能完全获得建立该实验所需要的资源,这是非常令人怀疑的。因此,我们必须小心谨慎,以免挤压导致全新事物的冒险和好奇心。在设计实验时,没有完全找到您要寻找的东西并不能降低风险,这就是我们进行实验的原因。

中微子振荡似乎是量子力学的一个怪癖,但它们可以帮助我们更深入地了解粒子物理学。矛盾的是,中微子质量非常小,使许多理论家认为它们以比我们的加速器所能达到的能量高得多的能量为物理学提供了一个窗口。中微子的混合在原则上可以比更熟悉的夸克混合更精确地测量,这可能有助于解释为什么所有粒子都存在三种形式的难题。中微子质量在我们对整个宇宙的理解中很重要,此外,中微子可能已经产生了所有构成我们的物质。对于质子衰减实验中的一罐清洁液和一些不透明的背景来说,这还不错。

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.

如果我们以仅两个中微子状态之间的混合为例,则可以用单个混合角θ来描述这种混合(请参见等式1)。

νμνe=cosθ    θθ   cosθν1ν2

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)。

如果我们以纯ν开始μ 梁(左),ν1 和ν2 波最终将变得完全异相并显示为ne(中间),然后再变回νμ 随着光束继续传播(右)。身份变化发生的速度取决于两个波长之间的差异,取决于ν的质量平方之间的差异1 和ν2,Δm122 = m22 – m12。实际上,找到一个ν的概率μ 在最初具有能量的纯nm光束中 E 经过一段距离之后 L 由给出(见方程式2)。

Pνμνμ=122θ21.27Δm2LE

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。

实际上,1964年已经在中性钾离子及其反粒子的衰变中发现了违反CP的现象,并将解释它的机制引入了标准模型。最近,对中性B介子衰变的观察证实,这种简单而优雅的解释更普遍,这是理论物理学的胜利。但是,由标准模型产生的违反CP的数量太小了几个数量级,无法解释宇宙中观察到的物质过量,因此我们不得不得出结论,肯定还有其他违反CP的过程。

中微子物理学为此提供了更有吸引力的可能性之一,并提出了中微子可能成为所谓的马约拉纳粒子的第一个已知例子的前景。–是自己反粒子的物质粒子。这种结果的一个结果可能是一个被称为瘦发生的过程,通过该过程,早期宇宙中非常重的中微子的衰变中的CP破坏会产生反物质过多的物质。反过来,这可能与中微子振荡实验中可能观察到的违反CP的相位δ有关。

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

艾哈迈德 等。 (SNO合作)2004年总活跃人数的测量 8萨德伯里中微子天文台的B太阳中微子通量具有增强的中性电流灵敏度 物理莱特牧师 92 181301

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

福田 等。 (SuperKamiokande Collaboration) 1998 Evidence for oscillation of 大气的 中微子 物理莱特牧师 81 1562

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

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