James Cronin (2010) - Cosmic Rays: the Most Energetic Particles in the Universe

I want to thank the organisers of this wonderful meeting for inviting me and giving me the chance to talk about recent work I've done. It's quite different from what was involved in being awarded the Nobel Prize, but in some sense it has some similarity in the sense that going from using accelerators, I've decided to try to study natures accelerators. And in so doing one uses the same techniques that one uses in accelerator physics, but just taking and making an apparatus that looks up in the sky rather than is on a dingy accelerator floor looking at beams. And so the title of my talk is Cosmic Rays, The most energetic -and you have to inject "atomic" - particles in the Universe, because a hard thrown baseball has more energy than these cosmic rays that I'm going to study, however not that much more. And I'd like to begin by a little prelude talking about how cosmic rays were discovered. In the late 19th century Crookes and others showed that if you have a charged object in a bulb filled with air, like a gold leaf electrometer, this will gradually discharge. And people wondered how that happened. Perhaps air ionised spontaneously. Then with the discovery in about 1895 by Becquerel of radioactivity, one began to understand that everything is a bit radioactive, the ground is radioactive and that's probably the source of the discharge of an electrometer or the source of detection of radiation. Now most physicists said this is just background, let's not worry about it. But there were a small group of physicists that worried in detail, could we account for this ionisation by radioactivity. And this was about at the turn of the century and the idea was if the radioactivity is coming from the ground, then if I take my detector, whatever it is, and take it high into the atmosphere, the radiation should go down. And the first very dramatic example of this was a Jesuit priest, Theodore Wolf from Holland, Valkenburg, who took a beautiful detector that he designed and climbed up the Eiffel tower which in itself is a bit of a feat. And so he began, this is the ionisation rate 22 ion pairs per cubic centimetre per second. He goes to Paris, it's a little different, that shows that the natural radioactivity is perhaps a little different. And then he climbs up the Eiffel tower and makes several measurements, comes back to the ground and goes back home. And you can see that there's perhaps weak evidence, I think we'd be very, it would not be accepted in physical review letters, but in any case there was a small decrease in the ionisation when his detector was at the top of the Eiffel tower. Now this suggested then that you want to go higher. And so Victor Hess, proceeded by a Swiss balloonist, went up in a balloon to see if the radioactivity, if that's what it was, decreased further. And this is from his paper, his last flight in 1912, where here is the height, he had 3 detectors measuring this ionisation and as he went up in height, the one he liked the most was one up in the air, and you had to correct it and one went from about 20 to 35 ion pairs per cc per second. And this was discovered, this was essentially recognised later on by the Nobel Prize, but only in 1936. He had to wait a long time, 1912 to 1936. I think the mean time maybe between the discovery and the award is probably 12 years. Well, this was a little fluctuation on the long side. And then in 1913, 1914 another physicist, who I really admire, Werner Kolhörster, took a balloon flight up to 9000 meters. Had to breath oxygen and the balloon was filled with hydrogen to get enough lifting power. And so you can see that as one went up to 9000 meters, the effect went up by nearly a factor of 10. So there's just no question that radiation was coming from outer space. Now let´s fast forward about 90 years, and this is now what we knew maybe 10 years ago about cosmic rays. And this is the spectrum of cosmic rays in energy and you can see it spans something like 10 orders of magnitude or so, And it's a simple spectrum following not many features. There's a feature right here, which is the knee right there, and then there's some more features down here. And just to put some perspective, the beam of the LHC is about 7, 10 to the 12th electron volts, but by cleverly colliding beams you get the equivalent energy in the centre mass which is something a little above 10 to the 17th electron volt. Nevertheless there's significant number of cosmic rays well above what one can make with manmade accelerators. So this is quite interesting to study. And for example now if we take a 10 to the 20th eV proton that has 16 Joules of energy, this is about the kinetic energy of Roger Federer's second serve. So that's why we have it microscopic and macroscopic, same energy scale and there's really no known astrophysical sources that seem able to produce such enormous energies. So here's a scientific imperative. Is something worth investigating and all you can do as a physicist is measure as best you can all the attributes of these showers. And the rate however at this highest energy, which I'll show you in a moment is really where we may learn something, is 1 per square kilometre per century. But physicists, especially coming from a background of high energy physics think big. So if you make something big, say cover 3000 square kilometres with detectors, then you're going to get 30 events per year above 10 to the 20th. So this is the motivation. And nature provides us some analytical tools. Well, the first is that there are magnetic fields outside our galaxy, in our galaxy. This is just a little exercise done in 2 dimensions, what happens when you have 10 to the 18th eV protons in a random nanogas field with sort of 1 megaparsec. And you can see that what you're looking at here is a diffusive process, so that if you see a cosmic ray like here you think it´s coming from over here but of course it originated at the centre. So there's no way of looking at where cosmic rays come from, as you observe them, that is related to the source. However let´s go up, same calculation, but let's go up by a factor of 100 in energy and this is what you get. So there's some hope, if we look at the highest energy cosmic rays we may be able to see where they come from. Now there's a second analytical tool of nature which is the cosmic microwave background which has been discussed extensively here. Great discovery of 1965 and awarded again a Nobel Prize. Now, what happens is a proton of 10 to the 20th eV interacts with a microwave background photon which is 10 to the minus 3 eV, so this extreme application of special relatively, because in the centre mass of that collision you have several hundred MeV, which means that this proton interacting with the microwave background will make pions and hence lose energy. And this little diagram shows: If I have a source whose energy is 10 to the 21 eV and here we have a plot of 1, 10, 100 megaparsecs of distance, as it propagates it loses energy by interacting with microwave background. And if it started at 10 to the 22, same thing, but what you can roughly see is that if you see a cosmic ray in excess of 10 to the 20th eV, it cannot have come from very far away because it loses energy. So this is the effect of the microwave background which means, if you look at the highest energies they cannot come from very far away, that means the number of possible sources is smaller and also means having a shorter path distance, the effects of the magnetic field would be less. So it's a thought, that it might be useful to build an apparatus sensitive enough to see a significant number of cosmic rays of very high energy, because you have a chance to move from random directions to an astronomy, if you like, of cosmic rays. Now a little more detail, let´s not worry too much about this, but it turns out, when you propagate in the microwave background heavy nuclei, they photodisintegrate. And if we take protons, they interact with microwave background as I describe, iron photodisintegrates, but the lighter elements - because they have a higher Lorentz factor - are more easily photodisintegrated. So this is a plot of, if you see a particle greater than 6 x 10 to the 19th eV, it will have come from, But the intermediate elements are essentially absorbed by this factor. So it´s more than an ansatz, which cosmic ray people use, is if I have a mixed composition at the atmosphere the assumption is protons and iron, if they're coming from 10, 20 megaparsecs, is more than ansatz, it´s close to reality. So we can think in trying to understand, is the beam of cosmic rays either protons or iron, it´s not perfect but it's a good way to think and not too, too wrong. Now, how do we detect these things at this higher energy? The flux is so low you cannot send up a satellite, however when a 10 to the 20th eV proton hits here, it´s first interaction may produce 1000, 5000 particles, each of those may produce 50, each of those 10. So you have a cascade that builds up, so 1 particle of 10 to the 20th will produce on the ground something like 10 to the 10th secondary particles, and they spread out over many kilometres. So the way to detect them is to put particle detectors on the ground and measure coincidences over several tanks, and then you pick out and select the highest energy cosmic rays, and by timing one can get the direction. There's a second far more elegant technique which is air fluorescence, nitrogen fluorescence, when charged particles pass through it and although it's weak, maybe 4 ultra violet photons per metre, still if you have 10 to the 10th particles, you get quite a bit of light so you can build a segmented telescope with photo detectors to see directly the light of the cosmic rays. So these are 2 techniques that have been used. And 6 years ago the status of the cosmic spectrum at the highest energies, that's where we want to concentrate, that's where I try to explain, we have some chance of doing some astronomy, there were 2 experiments: A Japanese one in red, which saw a spectrum that continued without stopping, and a second experiment using fluorescence detectors which showed this bend over here. Now this bend over here isn't really what you'd expect from the effect of the cosmic microwave background, because it's removing a lot of sources from your beam at the highest energies, because of loss of energy. So there were these 2. Now this red curve would have been extremely exciting, because it defied normal expectation, normal laws of physics and lots of papers were written, invented to have extraordinary phenomenon to account for this extension of the spectrum. I can give you all these details this afternoon, if you come to the Altes Rathaus, because I can have ample time to talk about all of this. So with colleagues and a great deal of effort starting in 1992, we tried to organize and were successful in organizing a large international collaboration running from Portugal to Vietnam and settled on a site in Western Argentina, just up against the Andes. And just the story of getting that all done and successfully completing, it is an interesting tale to relate as well but I don't have time. So what we designed, is in a site in Argentina, an array of 1600 water detectors, which I'll describe in a moment, surrounded by 4 of these telescopes, which recognize the fluorescence of the cosmic rays. And we decided to choose and work both techniques together and the 2 in combination are much more powerful than either one alone. And here shows the 2 detectors, here is the surface detector which is a tank of water, 12 metric tons of water, and here is one of the 4 telescopes. I'll show you a little more about this in a moment. The tanks are quite large and they're stretched out and so it's very hard, if you're at one to see the other, but I think you can see 1, 2, 3, 4 in this zoomed view. So it's an immense experiment and anybody who really wants to understand this, it´s not little points on a computer screen, it's a huge thing over 100's and 100's of square kilometres. Now, just a little bit about the tank, it uses Cherenkov effect, and we put the tanks out in the field. There's no place to plug them in, there are no mains so we use solar power, we use for timing the GPS system, batteries to accumulate the solar power. There are 3 phototubes and then there's a small Yagi antenna, which sends the data of this tank, if it thinks it has something, to one of the centres which are concentrators at the 4 fluorescence detectors. This just shows a little bit about the detection. Shrink off light is very easy, water tanks are very cheap. However they're not so easy installed. At least in the site we had a lot of streams and rivers, so here's one tank being dragged across on a trailer to the other side of the stream, and there's a water truck here with a pipe to fill it with water once it gets across. They're not all like this, this is just an extreme. And then we share this land with the cows and other animals, which just is our friends in the physicists. Now, the surface detector, when you have a large shower, many of the tanks are excited and this is anarray of about 10 tanks that are hit. This is the lateral distribution where you reconstruct to get the core of the shower, and then you can see that the effects of the shower are more than 3 kilometres away from the centre, and this little red dot represents 1000 metres. And that is the number that really characterizes the energy as seen by the tank. The signals are recorded in a time recorder and spread out over several microseconds. There's a lot of information in a plot like that. And what's the strength of the signal? We calibrate the tanks with single muons that go through, which deposit a few hundred MeV, and that's what we call vertical equivalent muon. So all our signals are calibrated in terms of how many equivalent muons that went through. These are details not too important but can be explained adequately this afternoon. The other detector is a fluorescence detector. There are 4 buildings like this, and they have 6 bays looking out and the fluorescence detectors see in altitude of 30 degrees, in azimuth 30 degrees. More details: they have a spherical mirror which focuses the sky on to an array of photomultipliers, 440 of them, here are the photomultipliers, here's the entrance aperture and it´s kind of a crude Schmidt optics, with a Schmidt corrector that roughly works. You have to know the fluorescence efficiency, and if you do and a few other details, you can get absolutely the energy of the shower by measuring the amount of light. So experiments have been done, which are probably pioneering or the best in measuring the nitrogen fluorescence spectrum excited by electrons. We have to measure, characterize the atmosphere, many things we do, this is just an example of balloon flights, which measure the temperature, humidity and pressure of the atmosphere as you go up. You need this to do the fluorescence measurement. Fluorescence measurement is beautiful, there's one fault: it only works on dark moonless nights, which means that only 10% of the time is it working. But as you'll see we use the fluorescence detector to calibrate the ground detector. This is just a view of a beautiful hybrid event, where we see here the shower axis comes in here, and makes the shower hitting about 20 tanks, and then with the fluorescence detector you can see the development of the shower, it´s beginning, rises to a peak and then dies out. If you integrate the area under here and have the right constants you get directly the energy of the shower. On May of 2007 we saw our first event, all 4 fluorescence detectors saw this shower. This was a nice PR piece. And now I'm going to talk about 3 things quickly. The spectrum: you measure the spectrum by calibrating the signal in the ground detector plotted here versus the signal or the energy measured in the fluorescence detectors. And you see there's a beautiful correlation. So we use the fluorescence detector to calibrate the surface detector. And this is now the spectrum that we see just up until fairly recently. You can see there are several features, and this is now about 3.5 years of exposure, huge exposure, as these things go for events whose zenith angle is less than 60 degrees. Now this is just a plot of the number of events, but the detector is uniform in its response from 10 to the 18th to wherever the data die out. You see 3 features, you see the upper part is a steeper spectrum and then there's a point right here... The battery is dead, I can't see it on the screen... Anyway, you see that the spectrum then flattens out and is linear for a long time and then falls off quite dramatically, above 19.5. This is the evidence of the effect of the microwave background. Dramatically and beautifully demonstrated by loss of a factor of 10. There's 55 EeV, sort of a mark of where we might want to look at those events, which might have a chance to point, because that's the point where the spectrum has dropped off by a factor of 2 from its normal slope. Ok, so now let's look at those events that are greater than 55 EeV. Here they are half way through the experiment, up to now and the little red dots are a catalogue of AGN's and the little circles are the directions of our cosmic rays. This is a plot of our galaxy, this is a galactic plain along here and we found a correlation up to August of 2007 of about 80%. A correlation between the direction of cosmic rays. They have a finite size because that's an estimate of what the resolution in angle might be, because of the detector, but more probably an estimate of what the magnetic field might do. We don't know that exactly but anyway, we found correlation and we have our science... Not just see elegance, we have our beautiful cover, which you can´t really see anything, because artists insist on making it too fancy, destroying information. But the information was inside the article of course. Now, what has happened since then? Here's a plot where the red are the accumulation of these high energy events, greater than 55 EeV. And the blue is the number that correlate with the catalogue of the AGNs. This red marker right there is when we published our paper. Having done that, that forced nature to say, maybe you're not so right, for the next 200 days not a single correlation came, although the data kept mounting and finally started to recover but our correlation dropped from 80% to about 40%. And the chance correlation is 20%. So our argument that we have a correlation with AGNs, which is really what we're looking for is not as strong as it was now, but what can I do, what can we do, nature, she tells the story. And now the last thing is something called elongation rate. And roughly in words, if I have a heavy nucleus like iron, it will interact higher in the atmosphere and produce a shower which is on the average higher in the atmosphere. If it's a proton it will penetrate more deeply, and the shower will peak lower in the atmosphere. So if you do a little sort of simulation, this is what you get. Blue is an ensemble of iron showers, iron induced showers, and the red are proton showers. You see 2 things, first what I mentioned that the iron is higher in the atmosphere, plotting the atmosphere depth from bottom. But there's another thing you see is the fluctuations, the iron is much less than the fluctuations of protons and this is just if you have a small cross section, it's going to fluctuate more in the depth and a larger cross section less so. And then you can essentially plot what you would expect for the, what was called elongation rate, here protons, and as you go up in energy, the mean maximum of the shower grows with energy logarithmically. And this is protons and this is iron, and photons, if we had photons in the beam, would be even deeper. So this is a way of measuring, at least crudely, what the primary composition is. And I told you that roughly speaking it's either protons or iron. Now what we see is, these are 2 plots, the first plot on the left is the mean depth of maximum of the shower, and you can see it´s climbing a bit and then flattens off. And as we go closer to blue it means the primary energy, primary composition is getting lighter. But what's more informative is the fluctuations which get very, very narrow and this suggests that whatever you have, that either it´s iron or it´s something that interacts much more strongly than what we expected, is clearly indicated. However fluorescence detector having only 10% duty cycle, the overlap with number of events above 55 EeV is a little bit, almost none existent. Ok, so I'm ending now, so what we have at the present time is a conflict perhaps between the correlation which, with the AGNs, which really means that you don't have too much effect from magnetic field and the composition, if it gets heavy, these are highly, all the nuclei are fully ionised, it´s going to be highly effected by the magnetic field. If it´s iron 26 times as much, so any correlation would be totally wiped out if you had heavy nuclei. So this is the tension between these 2 results, that's what nature is presenting us with. And it leads to the possibility, that the proton air interaction cross section becomes much larger, above 10 to the 19th eV, which would be a magnificent result in particle physics. there's no comparison of what the LHC can do and the few modest things we can do here. Ok, so this is a summary, there are many other measurements and many neutrino limits, photon limits, all that, but there are 3 things we've done is the spectrum and shows clearly the fall of the spectrum, flux suppression of the high energy cosmic rays. Here's another correlation which is pretty impressive of another AGN catalogue with sort of weighted things. You see the cosmic rays which are black dots correlate pretty well. And finally, what I just alluded to, the puzzling composition. So we have a lot of work to do, we'd love to build another array 7 times bigger in Colorado: We're fighting with the DOE, I hope they're here to listen to me and we'll see how it goes. So that's it, thank you so much.

James Cronin (2010)

Cosmic Rays: the Most Energetic Particles in the Universe

James Cronin (2010)

Cosmic Rays: the Most Energetic Particles in the Universe

Abstract

Astrophysical objects are able to accelerate atomic nuclei to energies 10^7 times more than man made accelerators such as LHC.
Particles arrive at earth from space with energies as great as 50 joules, a macroscopic energy in a microscopic particle. It is not understood how nature can accelerate particles to such energies.

Such remarkable events occur rarely, about one per kilometer squared per century. Recently a cosmic ray observatory has been built in Argentina that covers an area of 3000 square kilometers.
This Pierre Auger Observatory is a collaboration of 16 countries.
Excellent data has been collected since January 2004. The details of the observatory will be described. The most significant results will be reported.

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