在2012年欧洲核子研究组织（CERN）发现希格斯玻色子十年后，高能物理学正处在一个十字路口。大型强子对撞机（LHC）目前正在进行一项耗资11亿英镑的重大升级，以实现高亮度LHC（HL-LHC），但粒子物理学家面临的挑战是如何确定下一步该建造哪种机器以及在哪里建造，以便以空前的精度研究希格斯玻色子，希望揭示新的物理学。目前存在几种设计方案，其中之一是CERN的一个周长为91公里的巨大对撞机，被称为未来环形对撞机（FCC）。但新技术也为大型机器提供了诱人的替代方案，特别是μ子对撞机。今年是CERN成立70周年，迈克尔·班克斯与来自威斯康星大学麦迪逊分校的图丽卡·博斯、牛津大学的菲利普·伯罗斯和利物浦大学的塔拉·希尔斯谈论了关于希格斯玻色子的最新研究、HL-LHC可能发现什么以及下一代大型粒子对撞机的各种提议。

😊 自2012年发现希格斯玻色子以来，我们了解到它看起来像是标准模型预测的希格斯玻色子，但仍有许多问题尚未解决，例如它是否会衰变为更奇异的东西？它如何与标准模型中的所有其他粒子相互作用？尽管我们已经理解了其中一些相互作用，但仍有许多希格斯与其他粒子的相互作用我们并不完全理解。当然，还有一个关于希格斯如何与自身相互作用的重大开放性问题。它会吗？如果是，它的相互作用强度是多少？这些是我们目前正在LHC上努力回答的一些激动人心的问题。

🤔 我们尚未看到任何尚未预测到的奇异现象，这告诉我们我们需要关注不同的能量尺度。这是一种可能性——我们需要达到更高的能量。另一种可能性是，我们一直在寻找标准的地方。也许存在一些我们尚未检测到的粒子，它们与希格斯非常轻微地耦合。

🤩 LHC尚未发现希格斯以外的粒子，但这并不令人失望。仅仅是希格斯本身就是一个巨大的进步，它完善了我们对标准模型的理解，当然，前提是它是标准模型希格斯。除了希格斯，我们还学到了很多其他东西，例如了解其他粒子的行为，例如物质与反物质奇异夸克之间的差异。

🚀 HL-LHC将通过积累大量数据来寻找非常罕见的现象，从而帮助我们更深入地了解希格斯，这正是HL-LHC真正发挥作用的地方。它将使我们能够扩展对迄今为止已研究粒子的研究，并首次观察希格斯如何与较轻的粒子（如μ子）以及希格斯如何与自身相互作用。我们希望在HL-LHC上看到这一点。

💰 HL-LHC的11亿英镑升级包括更换两个大型实验ATLAS和CMS的最终聚焦系统中的磁体。这些磁体会将入射光束聚焦到大约10微米横截面的非常小的尺寸。这次升级包括安装全新的最先进的铌锡（Nb3Sn）超导聚焦磁体。

🗓️ 计划在未来十年内完成的HL-LHC升级计划，将使LHC的亮度提高10倍。升级包括关闭LHC大约三到四年，以安装高亮度升级，然后在十年末投入运行。目前的CERN时间表显示HL-LHC将运行到2041年底。因此，这台升级后的对撞机将再运行10年，谁知道会有什么激动人心的发现。

💡 在考虑成本时，需要注意的是，使用时间跨度非常大，因此它是一项对未来相当长一段时间内科学开发的投资。它也是一项潜在的衍生技术的投资。

📈 加速器的性能通常用亮度来衡量，它定义为每平方厘米每秒在这些碰撞点处交叉的粒子数量。LHC的亮度大约为10^34。然而，在高亮度升级中，我们谈论的是在未来十年左右的时间里收集的总数据样本量大约增加一个数量级。换句话说，我们目前只收集了大约10%的总数据样本。升级后，将收集另外10倍的数据，这将彻底改变可以进行的测量的统计精度以及对新物理学的敏感性和探测范围。

🎯 即使在HL-LHC结束后，也有一些事情我们无法在LHC上做到，原因有很多。其中一个原因是LHC是质子-质子机器，当您碰撞质子时，与电子和正电子之间干净的碰撞相比，您最终会得到一个相当混乱的环境，这使您能够进行某些在LHC上无法进行的测量。

🔍 希格斯工厂可以进行的测量之一是找出希格斯与电子的耦合程度。我们永远无法在HL-LHC上找出这一点，因为它太罕见了，无法测量，但在希格斯工厂，它成为一种可能性。这很重要，不是因为它是集邮，而是因为理解希格斯玻色子负责的电子的质量为何具有那个特定值，这对我们理解原子的尺寸至关重要，而原子的尺寸是化学和材料科学的基础。

🧲 虽然我们经常将这台未来的机器称为希格斯工厂，但它的用途远不止制造希格斯玻色子。例如，如果您以更高的能量运行它，您可以制造一对顶夸克和反顶夸克。我们迫切希望了解顶夸克，因为它是我们已知的最重的基本粒子——它大约比质子重180倍。您也可以以较低的能量运行希格斯工厂，并对Z和W玻色子进行更精确的测量。所以它不仅仅是一个希格斯工厂。有些人说它是“希格斯和电弱玻色子工厂”，但这听起来不像希格斯工厂那么顺口。

🆚 虽然人们似乎对希格斯工厂达成共识，但似乎还没有就建造线性机器还是圆形机器达成一致？

⚡ 目前台面上有两种主要的设计——圆形和线性。线性对撞机的动机是由于将电子和正电子送入圆形轨道的问题——它们会辐射光子。因此，当您在圆形对撞机中达到更高的能量时，电子和正电子会以同步辐射的形式辐射掉能量。在20世纪90年代后期，人们认为由于同步辐射的限制，圆形电子-正电子对撞机已经走到了尽头。但希格斯玻色子在125 GeV的发现比一些人预测的要轻。这意味着电子-正电子对撞机只需要大约250 GeV的质心能量。然后，圆形电子-正电子对撞机又重新流行起来。

⏳ 线性对撞机的缺点是，光束不像在圆形对撞机中那样循环使用。相反，您有“射击”，因此很难在线性对撞机中达到相同的数据量。然而，事实证明，这两种解决方案都具有很强的竞争力，这就是为什么它们仍然都在台面上。

🏆 是的，虽然一台圆形机器可能在环中拥有两个甚至四个主要探测器，但在线性机器中，光束只能在给定时间发送到一个探测器。因此，拥有两个探测器意味着您必须共享亮度，因此每个探测器理论上都会获得一半的数据。但打个汽车的比喻，这就像争论劳斯莱斯和宾利的优劣一样。线性对撞机和圆形对撞机都是非常棒的、令人惊叹的选择，一些对撞机在这里有铃铛和哨声，而另一些对撞机在那里有铃铛和哨声，但您...

More than a decade following the discovery of the Higgs boson at the CERN particle-physics lab near Geneva in 2012, high-energy physics stands at a crossroads. While the Large Hadron Collider (LHC) is currently undergoing a major £1.1bn upgrade towards a High-Luminosity LHC (HL-LHC), the question facing particle physicists is what machine should be built next – and where – if we are to study the Higgs boson in unprecedented detail in the hope of revealing new physics.

Several designs exist, one of which is a huge 91 km circumference collider at CERN known as the Future Circular Collider (FCC). But new technologies are also offering tantalising alternatives to such large machines, notably a muon collider. As CERN celebrates its 70th anniversary this year, Michael Banks talks to Tulika Bose from the University of Wisconsin–Madison, Philip Burrows from the University of Oxford and Tara Shears from the University of Liverpool about the latest research on the Higgs boson, what the HL-LHC might discover and the range of proposals for the next big particle collider.

What have we learnt about the Higgs boson since it was discovered in 2012?

Tulika Bose (TB): The question we have been working towards in the past decade is whether it is a “Standard Model” Higgs boson or a sister, or a cousin or a brother of that Higgs. We’ve been working really hard to pin it down by measuring its properties. All we can say at this point is that it looks like the Higgs that was predicted by the Standard Model. However, there are so many questions we still don’t know. Does it decay into something more exotic? How does it interact with all of the other particles in the Standard Model? While we’ve understood some of these interactions, there are still many more particle interactions with the Higgs that we don’t quite understand. Then of course, there is a big open question about how the Higgs interacts with itself. Does it, and if so, what is its interaction strength? These are some of the exciting questions that we are currently trying to answer at the LHC.

So the Standard Model of particle physics is alive and well?

TB: The fact that we haven’t seen anything exotic that has not been predicted yet tells us that we need to be looking at a different energy scale. That’s one possibility – we just need to go much higher energies. The other alternative is that we’ve been looking in the standard places. Maybe there are particles that we haven’t yet been able to detect that couple incredibly lightly to the Higgs.

Has it been disappointing that the LHC hasn’t discovered particles beyond the Higgs?

Tara Shears (TS): Not at all. The Higgs alone is such a huge step forward in completing our picture and understanding of the Standard Model, providing, of course, it is a Standard Model Higgs. And there’s so much more that we’ve learned aside from the Higgs, such as understanding the behaviour of other particles such as differences between matter and antimatter charm quarks.

How will the HL-LHC take our understanding of the Higgs forward?

TS: One way to understand more about the Higgs is to amass enormous amounts of data to look for very rare processes and this is where the HL-LHC is really going to come into its own. It is going to allow us to extend those investigations beyond the particles we’ve been able to study so far making our first observations of how the Higgs interacts with lighter particles such as the muon and how the Higgs interacts with itself. We hope to see that with the HL-LHC.

What is involved with the £1.1bn HL-LHC upgrade?

Philip Burrows (PB): The LHC accelerator is 27 km long and about 90% of it is not going to be affected. One of the most critical aspects of the upgrade is to replace the magnets in the final focus systems of the two large experiments, ATLAS and CMS. These magnets will take the incoming beams and then focus them down to very small sizes of the order of 10 microns in cross section. This upgrade includes the installation of brand new state-of-the-art niobium-tin (Nb₃Sn) superconducting focusing magnets.

What is the current status of the project?

PB: The schedule involves shutting down the LHC for roughly three to four years to install the high-luminosity upgrade, which will then turn on towards the end of the decade. The current CERN schedule has the HL-LHC running until the end of 2041. So there’s another 10 years plus of running this upgraded collider and who knows what exciting discoveries are going to be made.

TS: One thing to think about concerning the cost is that the timescale of use is huge and so it is an investment for a considerable part of the future in terms of scientific exploitation. It’s also an investment in terms of potential spin-out technology.

In what way will the HL-LHC be better than the LHC?

PB: The measure of the performance of the accelerator is conventionally given in terms of luminosity and it’s defined as the number of particles that cross at these collision points per square centimetre per second. That number is roughly 10³⁴ with the LHC. With the high-luminosity upgrade, however, we are talking about making roughly an order of magnitude increase in the total data sample that will be collected over the next decade or so. So in other words, we’ve only got 10% or so of the total data sample so far in the bag. After the upgrade, there’ll be another factor of 10 data that will be collected and that is a completely new ball game in terms of the statistical accuracy of the measurements that can be made and the sensitivity and reach for new physics

Looking beyond the HL-LHC, particle physicists seem to agree that the next particle collider should be a Higgs factory – but what would that involve?

TB: Even at the end of the HL-LHC, there will be certain things we won’t be able to do at the LHC and that’s for several reasons. One is that the LHC is a proton–proton machine and when you’re colliding protons, you end up with a rather messy environment in comparison to the clean collisions between electrons and positrons and this allows you to make certain measurements which will not be possible at the LHC.

So what sort of measurements could you do with a Higgs factory?

TS:  One is to find out how much the Higgs couples to the electron. There’s no way we will ever find that out with the HL-LHC, it’s just too rare a process to measure, but with a Higgs factory, it becomes a possibility. And this is important not because it’s stamp collecting, but because understanding why the mass of the electron, which the Higgs boson is responsible for, has that particular value is of huge importance to our understanding of the size of atoms, which underpins chemistry and materials science.

PB: Although we often call this future machine a Higgs factory, it has far more uses beyond making Higgs bosons. If you were to run it at higher energies, for example, you could make pairs of top quarks and anti-top quarks. And we desperately want to understand the top quark, given it is the heaviest fundamental particle that we are aware of – it’s roughly 180 times heavier than a proton. You could also run the Higgs factory at lower energies and carry out more precision measurements of the Z and W bosons. So it’s really more than a Higgs factory. Some people say it’s the “Higgs and the electroweak boson factory” but that doesn’t quite roll off the tongue in the same way.

While it seems there’s a consensus on a Higgs factory, there doesn’t appear to be one regarding building a linear or circular machine?

PB: There are two main designs on the table today – circular and linear. The motivation for linear colliders is due to the problem of sending electrons and positrons round in a circle – they radiate photons. So as you go to higher energies in a circular collider, electrons and positrons radiate that energy away in the form of synchrotron radiation. It was felt back in the late-1990s that it was the end of the road for circular electron–positron colliders because of the limitations of synchrotron radiation. But the discovery of the Higgs boson at 125 GeV was lighter than some had predicted. This meant that an electron–positron collider would only need a centre of mass energy of about 250 GeV. Circular electron–positron colliders then came back in vogue.

TS: The drawback with a linear collider is that the beams are not recirculated in the same way as they are in a circular collider. Instead, you have “shots”, so it’s difficult to reach the same volume of data in a linear collider. Yet it turns out that both of these solutions are really competitive with each other and that’s why they are still both on the table.

PB: Yes, while a circular machine may have two, or even four, main detectors in the ring, at a linear machine the beam can be sent to only one detector at a given time. So having two detectors means you have to share the luminosity, so each would get notionally half of the data. But to take an automobile analogy, it’s kind of like arguing about the merits of a Rolls-Royce versus a Bentley. Both linear and circular are absolutely superb, amazing options and some have got bells and whistles over here and others have got bells and whistles over there, but you’re really arguing about the fine details.

CERN seems to have put its weight behind the Future Circular Collider (FCC) – a huge 91 km circumference circular collider that would cost £12bn. What’s the thinking behind that?

TS: The cost is about one-and-a-half times that of the Channel Tunnel so it is really substantial infrastructure. But bear in mind it is for a facility that’s going to be used for the remainder of the century, for future physics, so you have to keep that longevity in mind when talking about the costs.

TB: I think the circular collider has become popular because it’s seen as a stepping stone towards a proton–proton machine operating at 100 TeV that would use the same infrastructure and the same large tunnel and begin operation after the Higgs factory element in the 2070s. That would allow us to really pin down the Higgs interaction with itself and it would also be the ultimate discovery machine, allowing us to discover particles at the 30–40 TeV scale, for example.

What kind of technologies will be needed for this potential proton machine?

PB: The big issue is the magnets, because you have to build very strong bending magnets to keep the protons going round on their 91 km circumference trajectory. The magnets at the LHC are 8 T but some think the magnets you would need for the proton version of the FCC would be 16–20 T. And that is really pushing the boundaries of magnet technology. Today, nobody really knows how to build such magnets. There’s a huge R&D effort going on around the world and people are constantly making progress. But that is the big technological uncertainty. Yet if we follow the model of an electron–positron collider first, followed by a proton–proton machine, then we will have several decades in which to master the magnet technology.

With regard to novel technology, the influential US Particle Physics Project Prioritization Panel, known as “P5”, called for more research into a muon collider, calling it “our muon shot”. What would that involve?

TB: Yes, I sat on the P5 panel that published a report late last year that recommended a course of action for US particle physics for the coming 20 years. One of those recommendations involves carrying out more research and development into a muon collider. As we already discussed, an electron–positron collider in a circular configuration suffers from a lot of synchrotron radiation. The question is if we can instead use a fundamental elementary particle that is more massive than the electron. In that case a muon collider could offer the best of both worlds, the advantages of an electron machine in terms of clean collisions but also reaching larger energies like a proton machine. However, the challenge is that the muon is very unstable and decays quickly. This means you are going to have to create, focus and collide them before they decay. A lot of R&D is needed in the coming decades but perhaps a decision could be taken on whether to go ahead by the 2050s.

And potentially, if built, it would need a tunnel of similar size to the existing LHC?

TB: Yes. The nice thing about the muon collider is that you don’t need a massive 90 km tunnel so it could actually fit on the existing Fermilab campus. Perhaps we need to think about this project in a global way because this has to be a big global collaborative effort. But whatever happens it is exciting times ahead.

Tulika Bose, Philip Burrows and Tara Shears were speaking on a Physics World Live panel discussion about the future of particle physics held on 26 September 2024. This Q&A is an edited version of the event, which you can watch online now

The post How a next-generation particle collider could unravel the mysteries of the Higgs boson appeared first on Physics World.