2005 Late Show with Leon Lederman

Late Show with Leon Lederman

Questions and Answers

Question: With the concept of supersymmetry, is it possible that our universe is a "particle" within a supersymmetric universe?

Answer: Though there are presently no experimental indications to guide us in this question, theoretical ideas based on superstring theory do postulate the existence both of extra spatial dimensions and of supersymmetry. In many of these theories, the 3-dimensional space in which we live is a membrane-like structure, called a D-brane, that is embedded in a fundamentally higher-dimensional universe. Loosely speaking, a D-brane is where the ends of open strings are attached. Normally, the full universe then has 9 or 10 spatial dimenions, plus 1 time dimension. The object we live on, the D-brane, would be a 3-dimensional so-called hypersurface inside this space. While the D-brane is not exactly a particle, it has some properties like particles. It can have charges on it. It can have a certain tension. It can have waves on it, and it can even have a finite `thickness' in the extra dimensions. All this, of course, is pure speculation.)

Best Regards, Peter

Question: How, in the theory of inflation, does the expansion rate of the universe exceed "c"? Does the value of c increase at that time to accommodate the high rate of expansion?

Answer: You don't have to go to inflation theory to find things moving apart at speeds greater than c. Consider the present universe. It is expanding, though slower now than in the inflationary period. Still, whatever the expansion rate, if you pick two points that are far enough apart, they will appear to be separating from each other at a speed greater than c. This in no way violates relativity. The reason is, that these points, which are receding from each other faster than the speed of light, now as well as back when inflation happened, cannot communicate with each other Ð they are not in `causal' contact, as we say. They are outside each other's observable universe. Whatever happens at one point cannot affect anything at the other point, hence relativity is still in good shape.

Best Regards, Peter

Question: What can cause the Higgs field to exist in a way that it can cover the entire universe?

Answer: That's a good question. I could say: we don't know. But that's a lousy answer. In fact, it's not just the Higgs field that we believe covers all of space. ALL fields stretch infinitely far, at least according to quantum field theory, even electron, photon, and quark fields. It's just that MOST fields, all the other ones besides the Higgs field, are zero over most of space Ð hence they might as well not really be there. An electron field is peaked around the atom where it sits and is very very small everywhere else. The odd thing about the Higgs field is thus not that it fills all of space, but that it is non-zero over all of space. We believe this was not always so, and that it has to do with a symmetry that was broken in the early universe. In the beginning, the Higgs field was probably zero everywhere, or at least its average was zero (there are always quantum fluctuations around the average, and in the very early universe those can be pretty big). The Universe later underwent a phase transition (still within the first fraction of a millisecond), where the so-called electroweak symmetry was broken, and the `vacuum' decayed into a new state Ð the state that we find it in today, with a non-zero Higgs field. But as I started out by answering, we don't really know how this happened. There are many possible theories out there for explaining the underlying mechanism of electroweak symmetry breaking Ð it is something like a holy grail at the moment Ð some involve new matter and/or forces, others involve new (broken) symmetries, and yet others involve new dimensions of space-time. and perhaps we will soon be able to test whether any of these ideas make sense, in experiments at Fermilab and at CERN.

Best Regards, Peter

Question: Re Higgs: if you say that a Higgs particle is coalessing out of the Higgs field, there is no reason to assume they are all going to be the same size. In effect the Higgs particles are all the known matter particles.

Answer: That's a great question! Yes, it would be extremely appealing to explain all fundamental matter in terms of just one field. That's related to another of the holy grails of theoretical physics, the possible unification of matter and forces. However, just with the standard Higgs field it unfortunately doesn't work. You see, the Higgs field has certain charges under each of the fundamental forces. For instance, it does not interact via the strong interaction at all. So strongly interacting particles cannot be made of Higgs stuff. Also, it has no lepton or baryon number, so again, particles with such quantum numbers can then not be made of Higgs stuff, and so on. There is another fundamental distinction between elementary particles; some are fermions (they have a so-called spin equal to a half-integer number), and some are bosons (they have integer spins). Electrons are examples of fermions, and photons are examples of bosons. The Higgs field is a bosonic field, so you could not create any fermions out of it. For these reasons, you really need all the standard elementary particles, in addition to the Higgs field.

Best Regards, Peter

Question: If the limit of the Higgs boson is within 114 Gev, why it has not been discovered yet?

Answer: The experiments have excluded a Higgs boson with a mass less than 114 GeV, at least within the framework of the Standard Model of Particle Physics. That's just another way of saying that nothing has been discovered below 114 GeV. So we really believe that, if the Higgs boson exists, and if it is the way the Standard Model predicts it to be, then it must have a mass greater than 114 GeV. Such a particle would probably be discovered at the next round of accelerator experiments. In some other theories, the experimental bound is less strict, and the Higgs could be lighter, and still have avoided discovery at earlier experiments, for instance due it, in these theories, having invisible decays or just different decays which make it harder for the experiments to see a clear signal that it was there at all. That is, in many of these cases, earlier experiments actually did produce Higgs bosons, but they could not tell!!! I guess that's what you call tough luck.

Best Regards, Peter

Question: One of the topics in high-energy physics is experimental confirmation of existence of the Higgs boson. What about the case of negative verification of the existence of it? Are there some ways of saving of Standard Model?

Answer: If neither the Tevatron at Fermilab nor the upcoming LHC accelerator at CERN find evidence for the Higgs, then there must be something very strange going on Ð an interesting case! As I mentioned in the show, all our ÒbestÓ theories predict one (or more) Higgs bosons that should show up in the experiments. Of course, theoretical physics is about figuring Nature out, and there is no indication that Nature is not capable of handling her numbers Ð so if no Higgs shows up, then there has to be a theoretical description to match as well; it is then just a matter of finding it, which may not be so easy. So, the short answer is no. If we do not find the Higgs at these experiments, then we would know that something is wrong with the Standard Model. We would have to modify it in some way, and which way that would be would hopefully then be indicated by discoveries of things other than a Higgs boson.

Best Regards, Peter

Question: What are some of the criteria for a trigger to take data from CDF?

Answer: Let me give you an detailed example of how the CDF trigger can help us select an interesting event, like a Higgs boson event. In this case, we will use the trigger to identify a high-energy electron from the collision which is produced in conjunction with the Higgs boson.

For the Level-1 trigger decision, we add up all of the energy deposited in the CDF detector, and we require at least 8 GeV (this is about 8 times the mass of the proton). At the same time, we identify a series of hits in the tracking chamber which could form the electron's track. If both the energy and the track are present, we move on to Level 2. (The Level-1 decision is made with custom electronics boards.)

For the Level-2 trigger decision, we identify a cluster of energy in the detector which could be the deposit from the electron. If there is a cluster larger than 16 GeV, then we pass the event to the Level-3 trigger. (The Level-2 decision is made using FPGAs [Field Programmable Gate Array chips] which have been loaded with the decision algorithms.)

For the Level-3 trigger decision, we analyze the event using a stripped-down version of the CDF event reconstruction code. We complete the tracking and confirm the energy cluster measurement. (The Level-3 decision is made on a plain old PC, one of hundreds in the Level-3 farm.)

The main purpose of the three-level system is to filter out events which look like an electron at first glance, but turn out to be fake tracks or noise in the detector.

By the way, you have one of the world's leading experts on the trigger there in Wisconsin. Wesley Smith at UW-Madison is leading the trigger effort for the CMS experiment at the LHC. The trigger rates there are pretty amazing. You can read more about his research at http://www.hep.wisc.edu/wsmith/researchact.pdf

Best Regards, Jason

Question: What is space?

Answer:I don't think I know the answer to that question, yet alone what created it! Does space exist beyond the edge of the universe? Is there an edge to the universe? If the universe is finite and wraps back upon itself (like a donut or similar shape) what exists outside the universe? These are philosophical questions to which physics does not have good answers. You would likely get a different answer from each and every physicist. Also, it is not clear that the Big Bang "created" all the matter in the universe. The Big Bang describes how the universe evolved from the beginning to the present. It does not address what created it or how.