Carlo Rubbia (2015) - Future Accelerators for Astro-Particle Physics

Alright, I want to talk to you today about astroparticle physics. Astroparticle physics is a branch of particle physics that studies the properties of elementary particles and their relationship on one hand to astrophysics, and the other hand to cosmology. It's a relatively new field of researching emerging at the intersection of particle physics, astronomy, astrophysics, detector physics, relativity, accelerator physics, and cosmology. This field has undergone a rapid experimental development, both theoretically and experimentally, since the early 2000s. And the recent years have witnessed the remarkable discovery of the Standard Model describing the Electro-weak and Strong interactions, and its recent completion with the discovery at CERN of the Higgs particle. However, there are still very many unsolved problems beyond the standard model, for instance the existence of the dark matter, we heard people discussing it yesterday and today; dark energy, and the accelerating universe. Another question for astro-particle physicists is why there is so much matter than antimatter in the universe today. And in order to go beyond the standard model, no question, novel particle accelerators also needed. Elementary particle accelerators, later colliders have been essential in the process of unveiling experimentally with high-energy collisions the very first evolutions after the Big Bang. The dimensions and the cost of this particle accelerator have been progressively growing during the last half century, reaching today the ultimate size with the LHC inside the 27 kilometre CERN underground ring at the construction expense of as much as 10 billion dollars. Probably the most expensive program on ground. The large accelerator driven research lines have been presently focused on mainly two subjects: One was the question of the discovery of the Higgs-Boson, has crowned the successful Standard Model and it will call for further studies on the new scalar sector at higher energies; and neutrinos, which are elementary fermions whose properties are still largely unknown, and where main new discoveries will be made. The particle accelerator driven Astro-particle physics community is now made of more than 20,000 scientists worldwide. Let me show you here a picture of the tunnel of the LHC, the Large Accelerator. Let me point out to you the huge dimensions of LHC are evident from its tiny curvature. You can see barely the thing to curve. Think how long it takes, how far you go, before you go to 360 degrees. Now, the CERN complex is represented very briefly here. It's a combination of many rings that start with small machines, and this is also purpose of time; for PS was constructed first, and SPS was constructed after; LHC is the next one. This process has taken something like 60 years of elementary particle physics programs. It is a vastly cooperative research, the progressive expansion of the world-wide users' cooperation in these programs. For instance, according to inSPIRE database, compared with 2,400 concerning the Higgs boson. You can see here the behaviour of these numbers about the neutrinos at function of time. It's a really, very large activity now centred mostly on the CERN accelerator at LHC. Now, the latest Nobel-winning discovery was mentioned, was a discovery by ATLAS and CMS, which found, both of them, mass of a Higgs particle, at this very close mass; this consistent with fermionic and bosonic coupling, and searches were performed in several decay modes. However, always in the presence with very substantial backgrounds. Experimental energy resolutions have been so far much wider than any conceivable with natural, intrinsic width of Higgs particles. Now, results of this experiment also exclude any other possible Standard Model Higgs boson all the way up to 600 GeV. Now, the example of the discovery is right there. I'd like to show it to you because it's really quite nice. You can see this little peak here, presence of the situation. That's the signature of the Higgs on the CMS. And then you have a similar experiment performed at the ATLAS. You can see again a slight little change in distributions, and that is the discovery of the new scalar particle, so it's certainly, if you really want to study this thing, you'd like to see it in more accurate way, with many more additional pieces of information. There was a long discussion about the Higgs sector, because many people claimed that in fact the Higgs sector was required in the existence of so-called "no fail theorem" which implied that if Higgs was there, also something like supersymmetry was necessary to be present. Now actually it turns out that the choice of masses is such that in fact the Higgs particle nowadays, at this value is essentially close to stability. It's not unstable, it's close to stability. It's metastable. A metastability of this particle is a lifetime longer than the age of the universe. So, the conclusion is that maybe there is one standard Higgs particle, and no need for anything else, except this presence of this new object in the future to come. But new discoveries are also possible. The Higgs particle is represented here. It's a very interesting distribution, the cross sections are indicated in this Gaussian distribution. The important question, this unique characteristic of the fact that the Higgs is incredibly narrow. You can see here, the energy change. You can see that we are talking about effect of the order of part per thousand, part per million, in the mass. You see the few MeV in the distribution of the detector will heavy modify the distribution of the Higgs particle itself. The resolution is very substantial, resolution is demanded in order to be able to see clearly the Higgs signal. You need a resolution which is of .003%. This is an absolutely new situation that elementary particle physics will have to study with great care. Now, the ultimate LHC uncertainties are represented here by systematic effects, and they are quite large. You can see the LHC detectors are predicting as an ultimate result at the end of long-running periods, systematic errors. You can see that systematic errors here are on of the order of something like 5%. For these, most of the object is W, Z, b, gluon, gammas. There are invisible cross sections, and therefore, if you really want to do something on these things you need five sigmas. This is one sigma, so quite clearly, at five sigma, these effects are the order of 25-35%. So therefore, it's very unlikely that such a thing will provide ultimate results. In fact, the distribution here indicated, in order to study the Higgs particle properly, and I suppose that other people will also describe the thing in a similar situation; you need to look at the very many diagrams. One is the diagram, for instance the Higgs is a touch of Zed. Other diagram, one Higgs becomes two Higgs, or in a variety of ways, and all this requires, of course, a very large amount of spectrum of energies going from the beginning of the observation of a single Higgs all the way down to about one TeV. So you need a new machine which is capable of seeing the process of emissions of a Higgs particle all the way, probably to about one TeV in order to get the comprehensive story about scalar particles. Scalar particles are new types of particles. All the other particles we have seen so far have been vectors, and scalar particle is a new brand of particles yet to be studied totally. And for that you do need to have a reasonable window of energies in which to study the process. So the question is, how do you want to study the Higgs beyond LHC? For which there is a big need. Now the two alternatives are indicated here. One alternative is called e+e- colliders, which have a luminosity of something like 10^34, which have a huge dimension and cost, namely we are talking about a ring which is about four times LHC, but still in this case limited only to 250 GeV which is probably insufficient, which is missing most of the Higgs-strahlung and Higgs boson diagrams. We have also a Linear Collider, which is called ILC, which is a new type of detector, and two linear particle beams are hitting each other, which will go also to about 1 TeV, but will require about 50 kilometre distance. This is a major new technology that we need to be developed for the future. Now, this project will largely exceed LHC, both in cost and in time schedule. The other alternatives which I would like to propose, which I will discuss now, is much more promising and much of exciting system, represented by mu rings, which are much lower cost, much shorter time schedule, and we may easily fit within the existing CERN site, but are requiring one new concept, which is the ability to compress the phase space of muon beams, with two possible alternatives. One is, the peak of the Higgs particle, 126 GeV, in order to observe a 4 MeV wide mass without backgrounds; and the other one is a high-energy collider ring all the way up to 1 TeV to compete with the luminosity of the possible accelerators, and which can provide whole phenomenology of the Higgs scalar in all its complexity. Now, let me show you here many ideas which are now coming up. You can see here, the huge rings. These are the rings. Options for circular rings factories are becoming popular around the world. You can see here, all the institutions which have proposed to build such a ring. These rings, as I said, there's one at CERN here, there is the Japan, there is China, there's Russia, U.S., and so forth, they're all getting involved into these activities. And these rings are in fact capable of bringing the Higgs into a signal, but will not measure the mass. The various names of these proposals indicated here. For instance, this is an example proposed by the CERN management of making a new ring at CERN. This is this very large ring of LHC which I showed you a minute ago, with a very small curvature. Now, you want to make a new ring which is about four times LHC, which is up here, this is at TELAB, which will cross the lake of Geneva, and will go from France to Switzerland, this particular configuration. Similar proposal has been proposed in Japan, by, by KEK, where we have here a ring of about 25 kilometres diameter, which is arranged within somewhere near the KEK. The requirements of this technology are extremely complicated, because you have essentially a huge ring, and also you require a very small emittance. We are talking now about an emittance of about .01 microns. You know how small a micron is. Now imagine that you take one-hundredth of a micron, and you take two beams who are traveling across over a distance of 100 kilometres. They somehow meet each other in one place, and they have to meet within 10 nanometres, one-hundredth of a micron. This is a really quite remarkable project, there's no doubt. You could say that the performance of such a system is the border of feasibility. The second important question is the LEP, which was the one used in the LHC ring had the luminosity of 10^34, so you need a luminosity which is a hundred times what has been done by ordinary rings like DESY-rings and CERN rings, and rings in various places in the world. Now, the second alternative is a so-called Linear collider options, which is a new idea, in which you have two very long LINACs. Here the LINAC is 25 kilometres in length, as you can see; and there are two of them. First of all, you produce electrons and positrons. You produce these damping rings which are collecting the electron and positron, then you accelerate those particles. They hit at one point where it would make one experiment. Now, again here, sub micro beams are required to do the job, and there are many possible alternatives described in this field. Let me point out to you that this is the 500 GeV which is probably not sufficient. It's about 31 kilometres. In fact all the design, the ILC, and so forth and so on, foresee that the final centre of mass energy will be 1 TeV, and therefore you need about 22 kilometres, so the length of this accelerator will be, as I said at the beginning, about 50 kilometres: the distance between Geneva and Lausanne. And this is the second possibility. Now, the main parameters of the ILC are indicated here. The ILC essentially, that is a very major effort, is being developed by various groups, about 300 laboratories and universities. And there's a global design study. However, the estimated cost just for the 500 GeV, first option, is comparable to the one LHC, so they are 10 billion euro, which is something, which I don't see how member states will possibly consider that as a possibility in the future to come. So we are now in front of this situation in which we are very interested in developing a new physics which will suggest, enlarge even further the accelerator. Right now the big, bold step above the LHC, but this is requiring time and cost, which are such a value, that either you run four times as long as the LHC, so you run, instead of 25 years, you have run for 100 years; or utilize four times the money that we have spent so far for high-energy physics, which is certainly not something that today people will be really in interest of. So we have, there is a big need of future alternatives. And I will present to you a possible alternative which is here. And let me also point out to you, that even this beautiful machine, namely the ILC, will not really produce enough resolution in the system. You can see here, for instance, how the uncertainty in the signal level exists for LHC, which is indicated there by the 250 GeV ring, which is the 100 kilometres, which is indicated there; and by the ILC, at 500 GeV, or 1 TeV eventually, and this is the width that the standard deviation that you probably will like to have. If you need five sigma width, you can see that the effect may be still not be sufficiently accurate to be able to get this information. So, we need a new alternative. And a new alternative, the one which is the second part of my brief presentation here. The idea is to use, essentially, a cooling process and this is possible, already being done in the past indication of antiprotons, and you can see Synchrotron and electrons, sorry. Synchrotron radiation, electron cooling, are things which will be well known. Stochastic cooling, the one we used to build a collider. And the new idea which I'm bringing forth right now to the discussion is the so-called Ionization cooling, and this ionization cooling in fact, as you will see, will be fast enough to be able to compress the muon beam this time so well that surely you can make collision between muons at all the different sides. In fact, the basic idea is to go from antiproton to muons. Now, and I say here, the antiproton success is suggesting very clearly the possibility to do ionization cooling for muons, and the muon, can be done possibly. This idea has been around since almost fifty years, Budker and Skrinksy proposed that, but now this is becoming as a real possibility. And there is a lot of work being done already in the '90's by many distinguished physicists. Now, the method is called "dE/dx cooling". It closely resembles to a synchrotron compression of relativistic electrons with the multiple energy loss of absorber substituting the synchrotron light. It's main feature of this method is that it produces an extremely fast cooling compared to other traditional methods, and this is of course a necessity for muon case. Transverse betatron oscillations are cooled by a target over foil, typically a fraction of gram per centimetres, and the cooling produced by foil is replaced by an accelerator cavity which continuously replaces the momentum which is lost. Now, of course, this is a bit delicate in the case of the present, we want to do, because we have to make some chromaticity phenomena which I'll discuss very briefly in second case. Now, the transverse we are cooling is therefore balanced between one hand, one hand we have, one hand we have the cooling, which is reducing the width of the particle as a function of time. And the scattering, which is produced by the absorber, which is expanding the size. And you get an equilibrium between the two; roughly a separate value which is quite small. You can see here as a function of muon momentum MeV/c, what is a kind of invariant emittance that you obtain is an equilibrium between cooling and scattering. Longitudinal balance is also important, in which you choose the angle, the strength of the electro beam. This is done again by combining intrinsic energy losses, a wedge shape absorber, which is choosing a different thickness, according to a different momenta, and the straggling. The straggling is compensating, it was warming, it's compensating the cooling, and the two systems are combined in some kind of formula which you can see here. Essentially, the fixed absorber has to be arranged in such a way that the particles which are faster, they have to go through a thicker material in order to be cooled, while the slower particles are going through a narrower piece of material, which means that our wedge, our thin absorber has to be arranged like a wedge. This is something which is a combination between longitudinal cooling and reverse cooling. There is a famous formula which is the Robinson formula; tells you the transverse plus longitudinal cooling all the time is to be about two. Now, with cooled muons we can do two experiments. One experiment is a cooling ring in the narrow s-channel resonance, which means that we just measure purely the Higgs line, and we can measure very accurately. The second possibility is to make high-energy ring up to 1 TeV, a luminosity comparable to the collider. However, the interesting thing for the muon system is: Both rings will easily fit within the CERN site, because for instance the single 126 ring which will identify the s-channel, will only have 50 metre radius. Now think about the fact that the LHC is 25 kilometres, and look at this ring, if you're talking about, which is 50 metres radius. About one-half of the CERN PS and the resolution's very small. which is less than the SPS, with indicator resolutions. Now, both are being obtained by taking muons from a proton beam and then muons will then get cooled and then will be accelerated. And therefore, we'll have the solution to the problem. Now, as I say, the basic steps of the construction of such a ring at CERN is as follows: First of all, you produce protons with 5 megawatts power. With LINAC it's something like 5 GeV with 10^14 proton per pulse. Then, you take this ring and you compress it to a very narrow bunch, and pions are becoming muons, and then you collect them into muons at the proper energy which is quite low, 250 MeV/c, and then you have the famous cooling that you have to demonstrate. Once you've done the cooling, then the bunches are accelerated to the energy of the collisions. And with a structure, which is a recirculating LINAC by a couple hundred meters, and then the radius, about 60 meters, 1% of LHC, is the size of the ring where the Higgs are collected and recorded in each of the experiments. Okay, now, the process of cooling is represented here. We have no time to describe it precisely. Initially, we inject a muon beam, which is very large in longitudinal emittance, in transverse emittance and longitudinal emittance, and with a number of steps, we go all the way down here where in fact the muons are collected and then they are used to shoot the beam, the two beams against each other. Essentially, the key of the question, after you produce, after you cool the LINAC, if after you cooled your muons, you have to accelerate it and you accelerate it with a very simple recirculated ring, let me explain how this ring is made. Muons come along here, go around in a circle, they go back, they get more energy, then they go around back and get even more energy. So they go around about six or seven times So only with a 200 hundred meters long accelerator you can bring your beam to something like 63 GeV and therefore you have them collide in this famous ring, which I said is about 60 meters radius. And this is the idea. Now; the collision occurs in the so-called low beta point which is typical situation, the two beams are crossing here and here. The beam is focused with magnetic, with quadrupoles and lenses in order to have a very narrow image, but it is quite a normal system. Now, one major problem is caused by the muon decays which are present, and they, coming from the muons, they give background, but this can be studied, describe itself in a possible way. The conclusion here is that simple device which is 5 GeV protons, of four megawatts of power. And they can produce something like 10^14 participle pulse. They produce about 10^12 muons of each side colliding, and they are accelerated and then they get to the proper luminosity where they have a luminosity of 5x10^32, which means that every year, each one or two detector will collect 40,000 events of muons. Two of these detectors will be hundred thousand, 10 years will be one million of Higgs particles, which will be sufficient to do this job. And this is done with a very simple device. Now, let me skip this one. Now let me summarize, therefore the conclusion, I have a few more minutes and then I'll finish. I'll stay within the half an hour. The s-channel production collider which I am proposing has a lot of advantages and a few challenges. The advantages are large cross sections for s-channel and for mu/mu going to ZH values, so the cross sections are large. Has a very small footprint, as I said, the rings have few hundred meters radius, and they can be transported easily in the CERN or even Fermilab place, so you can construct in existing site. You don't have to go and ask for a new laboratory, you have already one. No synchrotron radiation, no beamstrahlung problems. Precise measurements of line shape and total decay width which you only do with muons. Exquisite measurements of all channels and tests of the Standard Model. And the cost of the facility, provided cooling is successful, is about less than one-tenth of one of the LHC, which means that you can probably do it in present budgets without risking into problems. What are the challenges? The less cost of demonstration of the cooling must be done first. Well, it's not working anymore. It's dead. Now it goes. It's not shooting. Okay, now it's shooting. Muons, of course muon cooling has to demonstrate it. Backgrounds for cost of muon decay may be a problem. And of course some significant, not very large R&D is required to the end-to-end design. So let me spend a few more, two minutes to describe the next step. In order to pave this muon cooling, you have to demonstrate that the muon cooling can be done. And that can be done with initial cooling experiments. And essentially, physical requirement and studies are already undertaken with muon cooling suggest the next step, prior to adequate, specific physics programs could be a practical realization of an appropriate cooling ring demonstrator. Indicatively this corresponds to a very small ring of between 20 and 40 meters circumference, so it would fit in this room here; in order to achieve a theoretical expected longitudinal and transverse emittance of asymptotically cooled muons. Injection of muons from pion decays can be produced from an already existing accelerator at a reasonable intensity; PS, SPS, so many of those. And the goal is to prove experimentally the full 3D cooling within a specific experiment; and then, all the other facilities, namely pion production, the cooling system, the acceleration, the accumulation in a storage ring, can be perfectly conventional, and be constructed only after success of the initial cooling experiment at the low cost. So the first step is to demonstrate cooling works is the only gap that you need. Once you solve that, the next step will be to go and construct something as indicated there. Now, the cooling ring, as I was saying to you, is represented here; it's a very small ring. As I said, about 33 meter diameter circumference and it's sufficient to measure all of the aspects of the cooling. Now, conclusions. Now, astro-particle physics accelerators have reached with LHC a construction time of a quarter of a century and investment of ten billion dollars. It's very hard to imagine that you could do better with a linear collider of 50 kilometres in length, or maybe another machine so corresponding, large magnitude. The short time approval and construction of such a program today are rather unlikely, so we need an alternative. And the alternative which I'm proposing here is the alternative of, in my view, cooling of muons, a view of the mu plus mu+/mu- ring represents a very attractive alternative which should be carefully tested with the help of the tiny and cheap Initial Cooling Experiment. It's something we are starting now. Next step is that if successful, it should permit to proceed with muon cooling at a suitable level. Suitable for the high energy muon collider both for the Higgs production at the s-state with a single conventional ring of 50 meter radius which is 1.2% of LHC and second step, enlarge this thing to something like 400 meters radius, which, still a small ring by certain standards, and luminosity comparable with the 50 kilometre long LHC, but with much easier way, much lower cost. So conclusion; provided muon cooling is demonstrated, both options can be constructed within the existing CERN or Fermilab site for a cost and timescale which are realistic and financially feasible. So in my view, I think that in order to proceed in high-energy physics, we need another accelerator. The other accelerator cannot be a monstrous machine which will cost four or five times the cost of LHC. We have to use a little bit of intelligence. My proposal here is to do it with muons instead of electrons, and I've shown to you, this is perfectly feasible program, which should be put together rather rapidly, and should give interesting results. Thank you.

Carlo Rubbia (2015)

Future Accelerators for Astro-Particle Physics

Carlo Rubbia (2015)

Future Accelerators for Astro-Particle Physics

Abstract

One of the most remarkable results of astro-particle Physics has been the success of the Standard Model, recently culminated in the discovery of the Higgs particle (Ho). However, the Ho is observable only in few channels at the LHC, in the presence of substantial backgrounds. A future phase in much cleaner conditions may also be necessary as it has been the case of the Zo, where its discovery with the hadronic p-pbar collider has been followed by very precise measurements with the e+e-‘s at LEP. The Ho decay channels, the Ho-strahlung, the Ho-fusion diagrams and the double Ho diagrams have all to be carefully studied, requiring √s ≥ 500 GeV in clean conditions and with adequate rates. Sensitivity to new physics at “5 sigma” discoveries may need 1 per-cent to sub 1-per-cent accuracies on rates. Different methods have been described using e+e-‘s, like for instance a LEP-like ring or a Linear Collider however both with huge dimensions. Alternatively, a novel “muon cooling” facility, which has to be demonstrated experimentally, may offer two comprehensive µ+µ-programmes, (1) a Ho factory collider-ring at the s-channel resonance at the Ho mass = √s = 126 GeV and (2) a ring for higher energies. In contrast with the e+e- alternatives, both muon rings (which have radii of only 60 m and 400 m) can easily fit within the CERN site and have a much smaller size and cost, although a superior performance. In addition to the Ho, a neutrino factory (NF) is an important new alternative, in which high-energy cooled muon decays produce intense beams of neutrinos and antineutrinos.
In order to demonstrate the feasibility of an adequate muon cooling, an initial Cooling Experiment of a very moderate cost is proposed.

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