Samuel Ting (2016) - The Alpha Magnetic Spectrometer (AMS) on the International Space Station

Good morning. When the chairman mentioned I received the Nobel Prize in �76 it suddenly reminded me, first time I was here I met Heisenberg, Dirac. And so it�s been many years since my wife and I came here and met a new set of distinguished physicists. What I would like to do today is share with you a particle physics experiment on the international space station. The international space station from one end to another end, from here to here, is 110 metres, from here to here it�s 80 metres, the weight 420 tons and the construction cost is about 10^11 dollars. On the space station at this moment there is a magnetic spectrometer, a particle physics detector. It is a precision physics detector to measure charged cosmic rays and study their characteristics. Let me mention the beginning of this field is done by the previous speaker. George, you probably remember this, your first experiment on search for cosmic antimatter, published in Physical Review Letters, July �75. And that was the first time, people had the vision to study charged cosmic rays with a magnetic spectrometer. And this was a superconducting magnet, really pioneering for this field. There are 2 kinds of cosmic rays travelling through space. Neutral cosmic rays, light rays and neutrinos can be measured with satellites, large ground-based and underground detectors. Charged cosmic rays: The Earth�s atmosphere is the equivalent to 10 meters of water. AMS, in space, measures directly the original properties of charged cosmic rays. On the ground, only the energy of cosmic rays can be studied by summing up their showers. So this is a very famous, very successful experiment led by Nobel Laureate James Cronin. And that is to measure the energy of charged cosmic ray shower with Pierre Auger Laboratory, using the fact that charged cosmic ray in the atmosphere produces many showers. So you have many, many detectors to measure the showers, summing up the showers, you know, the total energy. So this is the size of the detector, 68 kilometres by 68 kilometres, and for comparison this is the size of the city of Paris. And this is located in Argentina. Another very impressive experiment was to study extremely high-energy neutrinos with IceCube. And this is their control room in the South Pole, it is 1.4 kilometres below the ice. It�s 1 kilometre by 1 kilometre by 1 kilometre and it has 5,160 detectors. So what are the physics one can learn from AMS? Let me present you a few examples. One is the search for the origin of dark matter. We all know more than 90% of the matter in the universe is not observable, because you cannot see it you call it dark matter. The galaxy as seen by the telescope will look like this. But if you could see dark matter in the galaxy the galaxy may look like this. You cannot see dark matter but collision of dark matter, that we call neutralino, produces energies which turn into ordinary matter, such as positrons or antiprotons. This excess of positrons can be accurately measured by AMS. The excess of positrons is measured, called positron fraction. Namely the number of positrons normalise the total number of electron positrons. So if you plot the positron fraction against the positron or positron electron energy you have a ray produced by collision of ordinary cosmic rays. But if a neutralino or dark matter really exists collision of a neutralino produces positrons. So you have a spectrum like this. And the characteristics depend on the mass, and because the mass is finite at high energies you have a sharp drop-off. So this is the AMS detector, it�s 5 metre by 4 metre by 3 metre and weighs 7.5 tons. It was assembled and tested with very strong CERN support. It is a trillion electron volt particle physics detector. Cosmic rays are characterised by their charge, energy, or momentum. For the top is something called transition radiation detector. Then there are 9 layers of silicon detector and the bottom electromagnetic calorimeter and 2 layers of time of flight detector and this is a magnet and this is a ring imaging Cherenkov counter. So this, this, this, this and this all measure energy and charge. So energy and charge are measured independently by 4 detectors. For experiments in space you always need to remember if something goes wrong you cannot send a graduate student to fix it. This is an international collaboration with many countries, many institutes and many physicists. We have very strong support from CERN and from the United States Department of Energy. And from space agencies in the United States and Europe, in Italy, in Germany and in China. Before it was sent to space the most important thing we did was to test the detector extensively at the CERN accelerator. So we know what the response should be once it�s in space. So it was sent to space on May 16th, 2011. In 5 years in space we have collected 80 billion cosmic rays up to the energy of 1 trillion electron volts. So let me show you examples of some physics results. One is the search for the origin of dark matter. I already mentioned collision of ordinary cosmic rays would produce a curve like this as function of energy, a collision of dark matter would produce a curve like this. So this is our first measurement, the red points are data, awfully close to collision of dark matter. So this got newspaper people very excited, from New York Times, from Wall Street Journal, in Germany and in France. Everybody says, we have seen dark matter - not so. So now we have new results on positron fraction based on 11 million cosmic rays. Before this experiment normally people talk about a few hundred, and this is 11 million. With 11 million you can examine the following. First at low energy, the red curve and the green curve must agree with each other, otherwise you don�t know what you�re doing. Second thing is the rate of the increase must agree with the model. And a curve like this must have a turning point. And the force must drop down very quickly because of conservation of energy. So the first property, the energy at which it begins to increase. So this is collision of ordinary cosmic rays. Indeed, a very low energy in our measured data agreed with the collision of ordinary cosmic rays. But at this energy it suddenly goes away. This deviation from the traditional understanding of collision of cosmic rays shows the existence of something new has happened. We don�t know what it is but something new has happened. Second: With 11 million you really see very good agreement with collision of dark matter at very high energy. But you can also produce astrophysical sources to explain this data. The third is with this positron fraction you can measure its slope, the rate of change. You see the rate of change increases, flattens out, then drops to zero. When the slope goes to zero it means you have found the turning point. The maximum has been found, from now you will go down. What has not been settled is how quickly it can go down. So we have measured up to here and it will take a few more years to go to the highest energy because the rate is quite low. If it�s a dark matter model because of conservation of energy you go down quickly. If it�s a pulsar, a light ray goes in a strong magnetic field producing the electron positron pairs. So a pulsar will show a curve like this. So it will take a few more years to resolve the last part. Let me mention there are 3 independent methods to search for dark matter. The traditional method is called scattering. You do an experiment underground, make sure only neutralinos come in, you put a nuclei, you detect a recoil. And this is the traditional way people do search for dark matter. And then at LHC you do proton, proton, produce neutralino. In Space (AMS) you do neutralino, neutralino, produce positron, electrons. These 3 are all orthogonal methods and not correlated with each other. And you can visualise this by looking at the physics of electrons and protons. Electron and protons for scattering, electron and proton scattering, leading to the discovery of partons and electroweak theory. Production from proton, proton go to electron positron pair in many laboratories leading to quarks, the fifth quark, Z-, W-, and Higgs bosons. Annihilation, the electron positron go to particles, leading to psi and tau leptons. All these were awarded the Nobel Prize. And the fact you do not see tau in protons-proton collisions, does not mean it does not exist. Therefore if you do not see one reaction in here, that does not mean it does not exist. Let me then share with you some interesting results. These are the electron and positron spectra before AMS. And this axis the electron spectrum and these are the measurements in green. And this axis is positron spectrum in red for many, many experiments. These were the best measurements over the last 100 years. The data has large errors, they are not always in agreement with each other. And because of this large errors you created many theoretical models. This is an AMS result. This is the electron spectrum. And this is the positron spectrum. Positrons are on this axis, electrons on this axis. The data clearly exhibits different behaviour between the electron and positron, not only in magnitude but most importantly functional behaviour. And then we also measure protons. Protons are the most abundant cosmic rays. And these were the many, many measurements over the last 100 years. And because the errors are so large you created many, many models. This is the measurement from AMS and the accuracy in absolute value is 1%, based on 300 million events and this is the measurement as function rigidity. Rigidity means momentum per unit charge. The flux goes up, goes down, and suddenly changes behaviour. The conclusion is, the red point with our measurement and this is the traditional assumption, the spectrum is a power law. But the data agree with the theory until this point. Then it breaks away and that means you have a new unexpected phenomenon. This is the measurement of helium before AMS and the many, many measurements. Helium are the second most abundant cosmic rays and mostly produced in supernovas. So they were the best data over the 100 years. The data has very large errors and is therefore not consistent. This is a measurement from AMS because we have 7 independent detectors and thorough calibrators. And so we know, we can measure the spectrum to accuracy of 1%. This is based on 50 million events. The data again disagrees with traditional assumption, you have an unexpected new phenomenon as a function of rigidity. The most surprising result, and this will be published soon, is the antiprotons. So this is the spectrum of elementary particles in space. Of all the elementary particles there are only 4 that travel through space. And that is electrons, positrons, protons, antiprotons for charged particles because they have an infinite lifetime. The others decay quickly. Of all 4, 3 of them, antiproton, positron and proton, have the exact same energy dependence. And this is the ray, and this is the function of rigidity. And this is the proton flux and this antiproton flux, this is a positron flux. The absolute value is different but the functional behaviour is exactly the same. And you can look, antiproton to proton, the flux basically has a zero slope. And similarly to antiproton to positron, proton to positron, they somehow behave exactly the same. This axis of antiproton over collision of ordinary cosmic ray is a very important thing because this axis can not be explained from pulsars. A photon does not produce a proton-antiproton-pair in a strong magnetic field. So some new explanation is necessary. So for all the elementary particles traveling through space, antiproton, positron, proton have exactly the same energy dependence, but the electron is different. Very curious effect, up to a trillion electron volts. Now, to search for the origin of dark matter, I mentioned the collision of dark matter produced positrons, antiprotons. And these must be detected above the background from collision of ordinary cosmic rays. So now you study the signal. The next thing you need to do is to look at the background, to look at the property of ordinary cosmic rays. So the first thing you need to do is to measure the periodic table in space. We know the periodic table on the ground. And so you better make sure your detector can map out all the periodic table to a high accuracy. So this is the measurement of lithium. The red points are the AMS data. And these are the previous measurements. And lithium again cannot agree with traditional interpretation, you have the new phenomenon. And this is the measurement of carbon compared with the previous measurement. And this is the measurement of nitrogen, previously people saw structures and we do now see that. And these are accurate to 1%. So there are 2 kind of cosmic rays: produced from the sources, called primary, interact with interstellar media, called secondary. So oxygen is primary - this is an absolute measurement. And this is nitrogen, partially primary, partially secondary, and boron, completely secondary. You see they are characteristically different from each other. So that is where we are with search for dark matter. Now, another example is search for existence of antimatter, something started by George in �75. Now, the Big Bang origin of the universe required matter and antimatter to be equally abundant at the very hot beginning. And so we want to look where is the universe made out of antimatter. In the universe we see helium, carbon or the periodic table which I have already shown. The question is, is there somewhere far away an Antimatter Universe filled with antihelium, anticarbon? The detection of Antimatter Universe: Cosmic antimatter cannot be detected on Earth because matter and antimatter will annihilate each other. We are living under 10 metres of water. Matter and antimatter have opposite charges, so we need a magnetic detector to measure the charge of antimatter. Positives go one way, negatives go the opposite way. To do that the first thing you need to do is to understand your instrument and you produce the periodic table for all of them. We have done that and we now have 80 billion events. And by the time we have 100 billion events, we will begin to look for the antimatter. So the latest AMS measurement on positron fraction, the behaviour of fluxes of electrons, positrons, protons, helium and other nuclei is providing new, precise, and unexpected results. None of our measurements will produce previous results or will have been predicted by previous models. So once it is on the space station, it stays there because there is no more space shuttle to bring it back. So it stays there forever, for the lifetime of the space station. And so we will continue very accurate measurement to look how quickly it drops down and then search for existence of antimatter and search for new phenomenon. Like the previous speaker just mentioned, cosmos is the ultimate laboratory. Cosmic rays can be observed at energies higher than any accelerator. But the most exciting objective of AMS is to probe the unknown, so far all our measurements do not reproduce previous measurements, and to search for phenomena which exist in nature that we have not yet imagined nor had the tools to discover. Now let me show you a video, how this detector is assembled. Could I please have the video? This is to reduce 16 years into 3 minutes. Going to European Space Agency to do tests under simulated space conditions in the thermal vacuum chambre. Tests in the test beam. Testing in 2000 directions. Sent to Kennedy Space Centre. This is in Kennedy Space Centre. This to make sure it fits into the space shuttle. (Laughter) This is inside the space shuttle. Sent to space. The total weight is 2000 tons at lift off to carry 7.5 tons to space. This take us off, put on the space station. Install on the space station. Ok, that�s fine, thank you. You can stop now. (Laughter) Thank you.