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    Why America's Higgs Party Won't Be a Blowout: Fermilab’s Rob Roser on the View from the Tevatron

    Written by

    Alex Pasternack

    Editor-At-Large

    Tomorrow, when the world hears a giant announcement about the Higgs boson from scientists at Europe’s CERN laboratory, they might not notice its American accomplice. On Monday, the Fermi National Laboratory, or Fermilab, which is situated in the middle of the Illinois prairie, in Batavia, announced its best evidence yet for the existence of the Higgs, the particle theorized to give matter its mass. In its particle collision experiments, a significant “bump” is hard to see against the noise of other particles, but Fermilab has found a bump in the data with an error probability of 1 in 550. It’s not a smoking gun for the Higgs — something like that will have to come out of the experiments in Europe — but, hey, it’s something.

    Fermilab’s big moment in the spotlight came in 1995. A number of years after discovering the bottom quark, scientists at the lab had zeroed in on the top quark, the the most massive of all observed elementary particles, whose existence they had carefully confirmed over a period of a year. They used the Tevatron, the largest particle accelerator of its time. As the search ramped up for the Higgs boson, the next puzzle piece in our description of the universe, the Tevatron lost its lead to the Large Hadron Collider at CERN, which, when it launched in 2009, became the largest and more powerful atom smasher ever built.

    The Tevatron, which closed in September of last year, has been instrumental in ruling out energy levels where the Higgs is not located, and its latest data, released yesterday, provides some indications about where it might be. But as the world’s nerdier eyes are on the LHC and the results that its two major experiments will release on July 4, the glory of discovery will be reserved mainly for the lab in Europe. The U.S. might have been home to the discovery of the Higgs if a larger atom smasher, the Superconducting Supercollider, hadn’t been cancelled in 1993. Instead America will be at the beach when Europe issues one of the biggest announcements in the history of physics.

    Yesterday I called physicist Robert Roser, who’s spent the past leading the Higgs hunt at Fermilab, to gauge the pre-Higgs buzz , hear some real talk about discovery, and ask what the physics frontier looks like in a cash-strapped America. Part one of our conversation is below.

    Hey, Rob. How’s it going over there?
    This has been the day from hell, because I still want to see an experiment and we’re trying to upload 2013 budgets, and because I also run the computing division here, and the LHC is about to come out with results too. So we’re juggling a lot of things today. But I‘m happy to talk to you.

    Given the announcement tomorrow, what sort of party preparations are being made right now in Batavia?
    In Batavia, none.

    Will you be watching the results out of CERN? They’re going to happen around 2 a.m. Batavia time.
    I will be at the Fermilab auditorium watching it on a live link, with perhaps 100 other scientists.

    There’s not an air of excitement over there? Or is it more an air of ‘maybe we could have been involved more in finding something, if…"
    I would put it this way: we thought five or six years ago, what could we do, with the way the accelerator ran, with some level of precision?, and we have exceeded that. So I think the Tevatron scientists are excited that they were able to accomplish what they set out to do, even if that is not to actually be able to discover a particle. The community of particle physicists is excited because we all want to know what the answer is and what lies in store, and whether we get the answer in July, we’ll know shortly, but I think we’re all pretty confident that we will get the answer in 2012, so that’s also very exciting. We have a big [CERN] CMS group, but the laboratory doesn’t have ownership the way it did with the Tevatron. So the scientists are excited, because we just want to know the answer, but the laboratory is calmer.

    How would you describe the kind of competitive spirit that’s been felt at Fermilab before? The idea that maybe the Tevatron would find something? There were moments of serious competition between the Tevatron and the LHC, especially when the LHC broke down for a while.
    I think we love it because it brings the best out of everybody. It also gets results on the table faster because you’re motivated, because if you don’t, someone else may do it before you. Competition isn’t an evil thing, it’s very good, also it’s a cross check for a future expensive experiment, and you could have had a collective bias in one experiment that threw you down the wrong path, you wouldn’t have if you can double check. So the competition is very healthy and vibrant. But we all like each other, many people run both experiments, so it’s not cutthroat the way you may imagine, it isn’t like that at all.

    No one’s slashing anybody’s tires outside in the parking lots?
    Or sabotaging data or anything like that. But there’s always a desire to be first, so there was some additional optimism at one point and if nature was really kind, if the Higgs mass was about 115 or 116 TeV, or maybe it still is, then we would have had a better chance at finding it then we do it at a higher mass. Nature is what it is, and we just get to observe what it provides to us. So at the moment with the Higgs mass, it looks like we’re all seeing something in the mid 120’s. Which is okay — it’s not a disaster for Fermilab because we still have a pretty good sensitivity there, but it’s not where our ultimate sensitivity is, which means that we could have made a better statement had it been a different mass.

    It’s much scarier when all of a sudden you actually start to see something, because then you really have to be right.

    __

    What evidence have you seen of the Higgs in the Tevatron’s data, since the machine was shut down in late September?
    In February or March of this year, we actually started to see a signal with almost a full dataset, and today we have more of a signal with the entire dataset. So it’s changing the mindset from, number one, getting to a point where you’re just trying to say, well the Higgs isn’t really here. That’s a pretty easy mindset to be in. It’s much scarier when all of a sudden you actually start to see something, because then you really have to be right. It adds another layer of scrutiny on top of what’s already a pretty difficult process. So the Tevatron had to get its wind around the fact that we’re starting to see things that were screwing up our limits, and that the things we’re seeing could in fact be a Higgs signal. So that’s where we’ve been at for the last year, just trying to optimize as best we can our sensitivity to go after that.

    What have you found?
    From the Tevatron’s full data set, if we look look at both the CDF and Dzero experiments and look at all of our channels across the entire mass region, [between particle beam energy levels of 115 to 135 GeV/c2], we see something that’s inconsistent with a background-only hypothesis at 2.5 sigma. So that’s roughly 1 in 550 times would a background fluctuate high enough to mimic the observed signal that we’re seeing. In particle physics we have a vocabulary that’s very mathematically-oriented, and so for us to start using the words, “we see evidence for a Higgs boson,” we need to get to 3 sigma. We also looked at a channel that’s unique to the Tevatron at least in these kinds of side datasets, which is looking for a Higgs decaying into two b quarks, and there we see stronger hints of a Higgs particle at a level of 2.9 sigma, so almost 3 sigma.

    When dealing with these statistical uncertainties, we use what’s called a “look elsewhere effect.” In other words, we don’t know what the mass of the Higgs Boson is, and so we look over a whole wide range. And if we start to see something, you don’t get to take credit if you see it in one place, you also have to say that we looked in all these other places and didn’t see it. So that’s called the “look elsewhere effect” and those are the numbers that I gave you.

    But if we took the LHC results in December, indicating that there is a Higgs at 125 TeV, saying okay, let’s just say they have it at that mass region, what would we get? Our significance would be 3 sigma and 3.2 sigma respectively, so that’s kind of it. We are seeing something, but we can’t quite say the word “evidence” yet.

    A symposium about the Tevatron at Fermilab in June

    So now confirming this is up to CERN, exploring energy levels with the LHC that the Tevatron wasn’t able to?
    The LHC is a higher energy machine with more modern detectors. The energy gives them the big advantage in that they make more of the particles we’re after because energy is related to mass. So they’re able to make more, and they’ve got an accelerator that is more modern than the one we had. They collide protons and protons together, not protons and antiprotons, so they have an advantage in that sense. They can look for this particle in a level of precision that we don’t have available to us because our machine is 30 years old and theirs isn’t.

    If the Tevatron had been able to survive past last year, what kind of contributions could it have made?
    It was an ending that was bound to come sooner or later, but I would give my eye teeth if I had one more year’s worth of data. We’re very close. If the Tevatron was the only machine in town, and we are capable of discovering this particle, it’s just that we need to amass a huge dataset to do that, because the thing we’re looking at looks a lot like the background of lots of standard model objects. So you need a lot of statistics to be able to separate it from everything else.

    We’ve already shown that we’ve got the analysis sensitivity to do it, we just need the statistics, so getting another year’s worth of data would have helped quite a bit. But we’re in a race, and because we’re in a race, my statement would be very self-serving. If you’re trying for global optimization, it’s not clear if you want to run longer. Because at some level, the world just wants to know the answer. I only want the answer, but I want us to have the answer. So I’m a little bit biased.

    It was an ending that was bound to come sooner or later, but I would give my eye teeth if I had one more year’s worth of data.

    _

    How do you think tomorrow’s announcement will effect operations at CERN and the operations at Fermilab?
    So first of all, the Higgs results that we put out from the Tevatron are one result. We’re still working through a number of other physics analyses with our full dataset that we will put out in the course of the next year. So the Tevatron is not done today. We still have a bunch more things to do with that data.

    At CERN, let’s say both of their experiments see evidence for that particle and later on, or even on Wednesday, discover the particle. Even if they have a lot of significance, they’re not going to say they observed the Higgs boson, because in order to that the behavior you’re observing has to act like it, and so they will want to do all the cross checks, and make sure that not only do they see it, but they see it behaving in different decay modes properly, et cetera et cetera. So finding it is the start of it, not the end of the story. It’ll be the beginning of another story.

    How is it going to compare to Fermilab’s discovery of the top quark in terms of the announcement, the analysis, the process of deliberation and checking?
    I was on the team for the top quark discovery. The difference between the top quark and the Higgs is, once the bottom quark was discovered at Fermilab in the late-’70s, everybody knew that a top quark had to exist, and it’s just a matter of finding it and understanding what its mass was. It didn’t change the way people think per say, on a very large scale.

    The Higgs boson is a hypothesis; it does or doesn’t have to be true, it’s a way to describe mass to the standard model. But it’s not the only way to do it. Either they’ll announce and tell us, this is a direction that’s true, and that other directions on how to deal with mass are not true. There could be models that have 4 or 5 Higgs bosons, or a single boson in a standard model. There’s lot of variations, so it will take a while to untangle whatever they’re seeing. But it’s larger, because it’s answering a bigger question than what the top quark answered. It’s changing the way some theorists go to work every day, whereas the top quark’s mass, I don’t think really did that.

    The Large Hadron Collider under construction, 2008

    Do you think that textbooks will need to be rewritten next year?
    At the moment, no. They might have to move a paragraph or something, but the Higgs boson is a theory. It’s easy to explain, it’s been around for a long time so it’s grown a life of its own, but it didn’t necessarily have to be correct. There could be other explanations. It’ll take a while before we understand how textbooks should be written.

    What are you looking forward to, what kind of result are you most excited about and how will it guide your work going forward?
    Well, I would like to see something that both CMS and Atlas see that’s consistent with each other and is consistent with the Tevatron, so we’re all seeing a consistent picture. So that’s first and foremost, I want to make sure that each detector is seeing the same object and then I want them to get to the point where they can say conclusively that they discovered it and then measure its properties. I think given that we now know we’re seeing strong hints, and they’ve seen hints before and whatnot, it would surely be more interesting if they don’t see it, because then they would really turn the field on its side. They’d say okay this isn’t the theory, now we’ve got to come up with another one. But the details of the theory still need to be ironed out.

    Fermilab nights

    The Higgs has reared its tiny head a number of times before. Do you think the excitement is a little premature or do you think this is it?
    It’s hard to know if it’s premature because I haven’t seen the results of those two experiments; I’ve just heard the various hoop-la and rumors, but the fact that they are willing to make a statement now tells me that probably what they’re seeing is consistent, at least at some level, with what they’ve shown before. So yeah, I think it’s an exciting time, I think 2012 is an exciting year. I don’t know if July 4th will be the exciting moment or not, but certainly 2012 should be the year of the Higgs boson. It’s pretty amazing that we’ve talked about it for so long, that this should be the year, and it actually will be the year.

    Why is CERN announcing on July 4th. It seems like poor timing …
    Well it’s a complicated thing. In the end, all of us experimenters like to present our results towards the year, but we do updates for national conferences. The major particle physics conference, ICHEP, starts on July 5th, so we all show our results just prior to that to our home laboratories, and I think they waited for the last second because they want to make sure what they present it going to be correct. So I think it was the date everybody thought it should be ready, just before the conference. Wanting as much time as possible, they waited for the last day to show it at home before the conference started.

    How big a story do you expect this will be in the US given that it’s happening in Europe?
    I would expect this would get an awful amount of attention on July 4th, because they’re all expecting them to announce evidence, so that’s already going to turn out a lot of excitement. If they don’t find it, that would generate even more excitement because it’s unexpected. But I think it will get an awful lot of attention all over the world. I’m amazed how many people ask me about it. Just amateur science buffs know about it, which is pretty remarkable.

    What’s your opinion on “God particle” as a nickname?
    It was a great way to sell a book. I think the underlying thing is we need this piece of the puzzle to help us fully explain how the universe was created, so there’s some link to it, although I tend to not play it up at all because it just gets people with certain religious biases all up in an uproar, and we don’t need to do that so I don’t use it.

    So do you think Higgs boson is always going to be the term we use for this field and this particle?
    I don’t think so. There were three papers that came out within two months of each other authored by six gentlemen, total. Certainly Peter [Higgs] has the better PR publicity person. I don’t know how the historians will do it. I suspect you’re right, but that’s a question we should ask each other twenty years from now.

    See part two of the interview.

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