On open questions in astroparticle physics with Jakub Vícha
Where do high-energy cosmic ray particles come from and how can we even learn what they are? Can they open up an opportunity for us to discover completely new physical processes? And can cosmic rays influence the weather? These questions have not been answered yet, but research by Jakub Vícha from the Department of Astroparticle Physics at FZU is bringing us closer to unravelling these mysteries.
In Physical Review D you recently published a new method for describing showers triggered by high-energy cosmic rays that will allow us to better understand their composition. Could you describe it in more detail?
We have long observed discrepancies between measured data and model predictions of how particles interact in showers. For a long time, these discrepancies have been interpreted in terms of a single type of particles that are produced in showers. These are called muons. These are observed in much higher numbers than was predicted. It was previously thought that we only had a problem with muons generated by models of hadronic interactions that take place in cosmic ray showers, but we have shown that we have a more complex problem: cosmic ray particles penetrate much deeper into the atmosphere than the models predict. This consequently affects our determination of cosmic ray composition.
Is it related to the original particle and its velocity or trajectory?
Mostly to the type of the original particle. I came up with an idea to generalize the problem; not to focus only on the fact that we may have a mismatch in the muons, but consider the possibility that the core of the problem may also lie in the scale of the penetration of particles into the atmosphere, which is the most appropriate parameter we use to determine their composition. The results of my method were surprising showing that the original models of shower interactions provide predictions outside the observed data, even in this parameter.
What we are detecting is not the original high-energy particle, but showers of secondary particles caused by it. They are locales for a lot of phenomena that need to be accounted for together. Let's think of them as a cascade. And it reaches its maximum at a different depth and its course is different than we previously thought. As a result, the original particles detected are probably much heavier and therefore have a higher charge, which makes them more curved in the magnetic field of our galaxy, and more information is lost about the direction from which the particle came into our galaxy.
To what extent might data from accelerators facilitate better shower description?
Accelerators will help us to some extent, but they have their limitations. The energies we observe are many orders of magnitude higher, and it's a slightly different area of investigation. At synchrotron accelerators, two fast-flying particles collide against each other, whereas we have a high-energy cosmic ray particle that is virtually crashing into stationary nuclei in the atmosphere. Such collisions are not quite comparable. The collisions we are looking at will be mostly peripheral, meaning that the particles will just "scrape" the nucleus. Whereas accelerators mainly study rare collisions, where particles collide directly against each other – a different kinematic space of investigation.
When we try to interpret what particle actually arrived in the atmosphere, we have to extrapolate the accelerator measurements and imagine how subsequent collisions behave in regions we haven't mapped in detail. And higher energies lead to more uncertain extrapolations and to rarer cosmic ray particles, and significantly so. If we want a ten times more energetic particle, it's roughly a thousand times rarer. At the highest energies, we are getting into the frequency of one particle per square kilometre arriving on average about once per century.
So, when the Pierre Auger Observatory stands across 3,000 square kilometres, we can see a few such particles a year on average, right?
Yes, at the highest energies we expect maybe one or two particles a year.
To what extent do cosmic ray sources overlap with gamma-ray sources detected by, say, CTA[1] or, in the future, SWGO[2]? Can these observatories help us?
There's some overlap, but we're working on the assumption that when there are some extreme processes, they produce both gamma rays, which are photons, and cosmic rays, which are element nuclei. But a direct link between gamma and cosmic ray sources at the highest energies has not yet been demonstrated.
We try to test our assumptions, so we pick interesting sources in the gamma region and then try to see if there is any correlation with the direction of the incoming cosmic rays. Of course, that's assuming that the highest energy cosmic rays are light, that is, made up of hydrogen nuclei, protons. However, also thanks to my method, the more pessimistic possibility, i.e. that they are very heavy nuclei all the way down to iron, seem more likely. This complicates our efforts to find their direction considerably.
Are there any assumptions about how many of these particles can reach us from distant sources, e.g. active galactic nuclei?
For us, it's still a relatively close universe. There is a certain horizon of cosmic rays from where particles can come to us. It is relatively small, about 100 megaparsecs – about 326 million light years [ed. note: that's about three times the diameter of the Local Supercluster in Virgo, and there are at least thousands of galaxies]. That's still a relatively close universe. The transparency of the universe for these particles is limited by the fact that the universe is not quite so empty but contains a large number of relic photons.
We used to think that if we looked at the showers caused by cosmic rays at the highest energies and got enough statistics on where the radiation was coming from, the protons would point us nicely to a source we knew was in that region. But the situation is different. The directions of arrival are fairly isotropic across the sky. We don't see them accumulating from any given direction.
Could the still unknown dark matter play a role in this?
Some exotic scenarios of cosmic ray formation account for it. We have a big problem explaining theoretically how in some astrophysical sources the most energetic particles can arise at all. You need to have a very strong magnetic field, relatively large objects, and at the same time a small energy loss in acceleration. One option was that soon after the big bang, the universe was already filled with some superheavy particles that could produce radiation.
Something like hypothesized primordial black holes?
Something like that. Such superheavy particles could decay over time. And they would decay into cosmic ray particles, for example. And there we would expect a more isotropic distribution and a larger representation of photons.
So gamma rays?
Yes, but at quite extreme energies, ten to the twentieth electron volts. But that hypothesis has almost been disproved. It does not seem to be the case that the showers are produced mostly by photons. So, these exotic scenarios can be virtually ruled out.
What information about the sources of gamma rays and the origin of cosmic rays can be provided by the planned SWGO observatory?
It will certainly allow us to find more sources that are currently missed, as we do not yet have such a wide-angle observatory in the southern hemisphere. As one of the main benefits, we expect to find the most powerful accelerators in our galaxy. These are called PeVatrons because they are capable of accelerating particles to PeV energies of more than 1015 electron volts. These will be searched for in our galaxy, because gamma rays cannot fly infinitely far either as even for them the universe is very opaque. Another possibility is to look for dark matter signatures. Dark matter would accumulate around the most massive part of our galaxy, the centre, which would be easily observable by SWGO, and its annihilation would produce photons that we can observe.
To detect cosmic and gamma rays, we are building detectors on Earth up to thousands of square kilometres. Is there a chance to observe them with space-based observatories?
In general, we need as large an area of detector arrays as possible to collect enough data. But there is a proposal to observe the showers that take place in our atmosphere from Earth orbit. Two satellites would operate in stereo. The project is now in the design phase and we'll see if it gets funding. Once the satellites fly over an area that would not be heavily light polluted, such as the oceans, they could observe the showers due to the fluorescence produced in the air. This would give us huge statistics in a relatively short time.
Do the showers also produce radio signals?
Yes, it's a recently rediscovered method. The upcoming upgrade of the Pierre Auger Observatory also includes radio detectors, which are in fact antennas on the existing barrels. The method itself has been with us for quite a long time, but reconstructing the properties of a shower is quite a complex thing. The radio signal is no longer as unambiguous as in the case of isotropic fluorescent light, there are other phenomena, and the geomagnetic field also has an effect.
Therefore, only now, practically in the last, let's say, ten years, has this detection technique advanced to a level where we can start to use it experimentally.
Going back to Earth and the effects of cosmic rays on our atmosphere, what is also interesting are the hypotheses that cosmic rays affect the frequency and intensity of lightning or the weather in general. Can you say more about that?
It hasn't been confirmed yet. But there are studies that are trying to do so. The idea is that passing through the atmosphere, cosmic rays carry a charge. This is accompanied by various small discharges. If there were no cosmic rays, they would not contribute to the charge in our atmosphere to be so easily discharged, and as a result much stronger discharges, i.e. less frequent but more intense lightning, could occur.
Cosmic rays can also affect the condensation of nuclei in the atmosphere, which affects cloud cover. There is also research into whether we can use the behaviour of cosmic rays to predict whether an earthquake will occur. When tectonic plates are stressed, the magnetic field around our planet also changes. And the magnetic field greatly influences the penetration of cosmic rays coming towards us. It's extremely interesting, but still inconclusive.
Where do you see the greatest opportunities for breakthroughs in cosmic rays in the future? Is it understanding the sources of high-energy cosmic rays, their effect on Earth, or something else?
For us, the number one mystery and the huge driver is to figure out where these particles originate. That's the main unknown so far, but the search for sources is made more difficult by the uncertainty of what particles these actually are. Once we've uncovered that, we might also find out where they might be coming from. This is followed by a question of how the particles are created in the first place. Processes may be involved that are as yet unknown. For now, we are still in a basic research phase and may discover processes that will benefit other fields.
In addition to these astrophysical processes, we are also very interested in particle physics in cosmic ray showers. I have already mentioned the observed inconsistency between the data and the models. The question is still whether there is some new physics beyond what we can study at the accelerators. The standard model works very well for the data we have so far, but it still only describes a small part of the phase space of possible collisions. One of the tasks we have set ourselves is to figure out if the discrepancy is just due to measurement uncertainties, or if we really can't describe our data without introducing some new physics. Of course, we also want to predict what it might be. This is a question especially for colleagues who are trying to come up with models of how interactions take place.
What led you to astroparticle physics from your initial interest in astronomy as a child and then from studying nuclear physics? Was it these open questions that were the main motive?
For sure. Astronomy and astrophysics in general were something I have been interested in since I was young. And when I went to college, I was not sure whether to go into astrophysics or to focus on more fundamental things like particle physics. Over time, the opportunity arose to combine these and study astroparticle physics. By then I knew clearly that this was what I wanted to do.
If you are interested in open questions of contemporary physics, come to the panel discussion as part of the Researchers' Night this Friday, October 27 at 5 pm!
[1] Cherenkov Telescope Array
[2] Southern Wide-field Gamma-ray Observatory