Will a muon accelerator save particle physics?

In recent years, physicists from around the world have been discussing the construction of a muon accelerator instead of other conventional accelerators based on protons or electrons. Such an accelerator has clear advantages in discovering new physics, but its construction is accompanied by many technological challenges. In this article I will review the words of Prof. Nima Arkani-Hamed from the Institute for Advanced Studies in Princeton about the need to build such an accelerator

Particle physics relies mostly on accelerator experiments that provide a direct glimpse of nature at high energies, or alternatively, at short distances. The famous accelerator in Sarn accelerates protons, positively charged particles made up of quarks and gluons. Accelerators mainly come in two forms: a linear accelerator that launches particles towards a target in a straight trajectory, or a circular accelerator that launches particle beams that move in a cyclic and opposite trajectory until a collision. Of course, protons are not the only particles that can be accelerated. Less familiar accelerators are based on electrons, (relatively) heavy ions, and positrons. In recent years, several proposals have been put forward to build a ion accelerator, but the discussion remains at a "low heat". Recently, the discussion gained momentum and in the last few weeks it was decided to dedicate a conference to the topic at the Kabali Research Institute for Theoretical Physics in Santa Barbara. As part of the event, Prof. Nima Arkani-Hamed from the Institute for Advanced Studies in Princeton, one of the famous theoreticians in the community, presented his position on the need for such an accelerator. In this article, I will review parts of his speeches that were presented as part of the conference.

Because the muon is less known to the general public, it is desirable that we give it proper exposure. Let's start with general details:

The muon is an elementary particle from the lepton family (the light particles). Just like its "brother" the electron, the muon has a negative electric charge, but it is 200 times heavier than it. In numbers, its mass is 105 megaelectron volts (in units where the speed of light is normalized to one) or, in the known units, about a millionth of a trillionth of a trillionth of a gram. The muon is an unstable particle, which means it decays into lighter particles in a relatively short time. At rest, its half-life is about a millionth of a second, and during the decay process it splits into an ion neutrino and a W boson, which itself decays into an electron and an electron anti-neutrino. At the same time, the muon is very common in nature. Every moment, about 200 muons pass through us per square meter.

on a muon accelerator

If the half-life is a millionth of a second, how is it possible to accelerate and destroy muons in accelerators? Time is relative, and according to the special theory of relativity, the period of time that passes in a body moving at a speed close to the speed of light is different from the period of time that passes in a body at rest. In fact, when particles are accelerated to high speeds, the average time to decay increases significantly, enough for us to conduct experiments on them.
What speeds are these? About 99.99999999 percent of the speed of light. The energy that will be released from the collision of muons moving at this speed is close to 10 tera electron volts. The energy released during the collision in the axis accelerator is about 14 tera electron volts, so why is the muon accelerator better? The answer to this question is hidden in the fact that the proton is not an elementary particle. The energy spent in accelerating protons is divided between the quarks and gluons that make it up. Thus, the Axial Accelerator tests the Standard Model at an energy scale of 1 tera electron volt (or by analogy with physics that occurs at distances of a billionth of a billionth of a meter). In contrast, a muon accelerator creates collisions between elementary particles, so that all the energy is translated into a single collision. In the last decade, proposals have been made to build a larger proton accelerator, which will produce an energy of 100 tera electron volts from each collision. The accelerator in question is supposed to be huge, about a hundred kilometers in circumference, and in practice will enable research in physics around 10 tera electron volts. In contrast, the muon accelerator should be much smaller (about 10 kilometers in circumference), since the energy spent on acceleration is significantly lower than a proton accelerator. A muon accelerator also saves energy indirectly because the synchrotron radiation emitted by muon acceleration is significantly smaller than that emitted by proton acceleration. When this radiation is emitted, the energy the particle carries is reduced. Unfortunately, this is a non-negligible process, but we will see later that this disadvantage will turn into an advantage in muon accelerators.

Is particle physics dead?

Accelerators are the most direct means of examining the existence of particles in nature. Although there is indirect evidence for the existence of hypothetical particles such as dark matter (a hypothetical particle that comes into contact with known particles only through gravity), there is no dispute that there is no substitute for direct measurement of particles in the laboratory. The close readers we built must be wondering is particle physics dead? This is a legitimate question we should ask! After all, the last significant discovery was a decade ago, when the Higgs boson (estimated fifty years ago) was located in Sarn and completed the construction of the standard model. Beyond that, the promises of the existence of additional dimensions, "super particles" and macroscopic black holes did not materialize. Every experiment carried out in the past few decades has reconfirmed the standard model, and if not, it caught the attention of the popular media too quickly because shortly after, it was revealed to be an anomaly in the experiment that converged to a result consistent with the theory. 

To this question Nima answers no, particle physics is not dead yet. The reason for this is that the Higgs particle itself describes new physics, one that people don't appreciate enough. In fact, the fact that in the last decade only the Higgs particle was discovered and nothing beyond that is a theoretical challenge in itself. This is why it is necessary to build more experiments to find the correct theory for the Higgs particle and its properties. The particle, which is a boson with zero spin, is considered a special particle, because there is nothing like it in nature. It is such a simple particle that apart from mass, it has no other properties. Its uniqueness also stems from the fact that it gives the other particles their valence thanks to its interaction with them. In fact, the question of why particles like electrons and massive kerosene translates into another question, and that is - what is the source of the Higgs mass? And why is its value determined that way?

What about the non-massive particles, can we explain why they are massless? And more generally, are there any particles that we know how to explain from fundamental principles (that do not arise from interactions with other particles)? The answer to that is yes. We can answer, for example, the question of why the photon does not acquire mass when it comes into contact with other particles. The reason for this is a bit complex and it stems from the mathematical structure that describes the photon. In fact, the number of degrees of freedom, i.e. the number of possible directions a photon can oscillate relative to its direction of motion forces it to be massless. Other elementary particles do not have similar constraints that a photon has and therefore they acquire mass in contact with other particles.

For this reason, the Higgs particle is considered unique, because according to the standard model its mass cannot be derived from another source. In fact, to determine its mass we must conduct experiments in accelerators. For theoretical physicists, this fact is considered a catastrophe. For them, there is no numerical value in nature that cannot be predicted through calculations. Some will say by force, and some will say by chance and elegance, physicists have developed models from which calculations can be made and the Higgs mass can be accurately determined. Unfortunately, these models are not without side effects. The supersymmetric model for example predicts the existence of many more particles in nature. If these existed, we should already have seen them in accelerators, but they have never been observed. This is an interesting theoretical surprise, forcing theorists to think more deeply to find the real source of the Higgs mass. 

Another solution that researchers have proposed assumes that the Higgs is not an elementary particle, but is composed of other elementary particles, such as the proton for example. So far, the experimental resolution suggests that the Higgs particle is an elementary particle, but this question is still open. How far are the Higgs spots? Does it have an internal structure? Not sure if we will get an unequivocal answer to this question, but future accelerators will be able to improve their estimate tenfold. Fortunately, each answer is interesting in itself and will force us to deduce new physics from it.

The benefits of building an ion accelerator

What are the advantages of the muon accelerator in Higgs research? First, as I already mentioned above, the muons are elementary particles, so all the energy invested in them is transferred directly to a single collision, unlike a proton accelerator whose energy is distributed among its various components. Furthermore, the muon emits electroweak radiation during acceleration. On the energy scale in question, it is not possible to differentiate between the electromagnetic radiation and the weak radiation emitted by the acceleration of muons. When the electro-weak radiation collides with each other, it creates Higgs particles with a relatively high probability. Estimates speak of tens of millions of Higgs particles per run. From these collisions we can deduce the size of the Higgs with an accuracy of a single percent, compared to theoretical accelerators based on electrons and positrons for example, which will estimate the size of the Higgs with an accuracy of ten percent.

What about non-higgs related physics? Can new particles appear in accelerators? Well, physicists try and fear predicting the existence of new particles, but it cannot be said that they do not try. Claims are sometimes heard that we did not predict any particle at an energy scale 10 times higher than that produced by Sarn, but this claim can be easily refuted. The wimp model for dark matter is an example of this. The basic model developed back in the eighties predicts the presence of these particles in proton accelerators that will reach energies of the order of 100 tera electron volts.

The challenges in building a ion accelerator

Well, what are the challenges in building a muon accelerator?

First, to create the muons, charged particles that strike a graphite surface are accelerated. The collision emits pi particles that instantly decay into muons. In such a process it is possible to create about a trillion muons with an energy of 200 mega electron volts. The volume they occupy in space when released from the graphite is roughly the size of an inflated balloon. The next step is to reduce the degree of dispersion of muons in space to millikov. To do this, the particles need to be cooled. The conventional method of cooling particles is based on the gasket using a collision with surfaces made of special materials. The muons are then accelerated to the desired speeds and set on a collision course, all before they decay. This is a huge challenge that has never been demonstrated before, is it crazy? In Nima's opinion, it is entirely possible. For him, this is a new accelerator, based on a new technology that has never been tried and this is a good enough reason to build the accelerator, because it won't just be "the same accelerator but bigger".

Do you have a question in physics or an idea for a topic that I should write about? Contact me at the email address - noamphysics@gmail.com

More of the topic in Hayadan:

• CERN experiments announce initial evidence of rare Higgs boson decay into muons 
• Muons continue to challenge the Standard Model
• A decade of Higgs: what have we discovered, what is hidden and why is particle physics not dead?
• For the first time, the Higgs boson was characterized by its decay into a pair of "magic" quarks
• For the first time a time-dependent asymmetry between matter and antimatter was observed