Of Particular Significance

A Visit to Brandeis

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 04/18/2012

Yesterday, I was visiting Brandeis University, where I gave a colloquium on the Large Hadron Collider [LHC]. It was such a gorgeous June April day outside that I felt quite lucky to see a substantial audience, which I think reflects a sense in the wider physics community that the excitement surrounding the LHC no longer reflects merely its future, but also its present.

I also enjoyed a research talk given by a Harvard postdoc, Matt Reece.  He had some things to say about which variants of supersymmetry can now largely be excluded by LHC data, both directly from existing searches for superpartner particles, and indirectly from the search for the Standard Model Higgs particle (the simplest possible form that the Higgs particle might take), assuming the hints of a Higgs particle with a mass of 125 GeV/c2 turn out to be the real deal.  He also made a nice little back-of-the-envelope calculation in his introduction, showing how that the hierarchy (the same one for which we have a hierarchy problem) between the incredible weakness of gravity and the strengths of the other forces is required for there to be large objects (i.e. planets, stars) that are held together by gravity, but prevented from collapse into a black hole by the effect of electromagnetism.

These days, at every place I visit — and Brandeis was no exception — the question of the Higgs particle always comes up.  No surprise; it’s the hottest topic in particle physics right now.  And the quality of the evidence always gets discussed.  Dr. Reece was of course asked his opinion by someone in the room.  So I get a chance to hear a lot of viewpoints.

One of the things that I have found puzzling is that almost all of the other particle physics and string theory bloggers not only are of the opinion that the Higgs particle has definitely been found, but also claim that almost all other particle physicists think so too. I honestly just can’t understand how they can say this.  I find plenty of theorists who say things like “well, if I didn’t already have a strong theoretical reason to believe the Higgs particle exists, I wouldn’t be very confident in the evidence that is in the current data.” Meanwhile, many senior experimenters regale me with stories of past errors and biases, some of which you find in the history books, and some of which you can’t. All of this is anecdotal; I can’t tell you how opinion is really distributed. But clearly a substantial fraction of the community — maybe a minority, but not a small one — are much less confident than most of the bloggers.  It’s not a question of nay-saying — I haven’t heard anyone argue there’s no evidence at all for a Higgs at 125 — but many physicists feel that the evidence is too weak at this stage for any certainty.  Of course we all expect the uncertain situation to be resolved in 2012.

One of the other great pleasures of visiting other universities is that I always get to hear about interesting research directions being pursued that I’d not been following.  (The theorists at Brandeis were all doing neat stuff that unfortunately would take way too long to describe here. ) And then, as the day comes to a close, I always hear some good stories.  My favorite this time was of an ATLAS experimentalist describing how easy it is to get lost while crawling around, installing or fixing things, inside the vast muon system of the ATLAS detector (one of the two general purpose detectors at the LHC.)   Can you imagine getting lost inside your own experiment?! 🙂  Well, this one’s the size of an eight-story office building, but hasn’t got hallways, elevators, big EXIT signs, or even an obvious THIS WAY UP.  Meanwhile, your GPS device doesn’t work down there either!

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15 Responses

  1. Great post again,

    I’m also theoretically pretty convinced that the Higgs exists but that is a silly argument to use if you are doing experiments to see if the Higgs really exists. And (as you wrote many times) we ALWAYS have to REALLY worry about the trigger! And we have to worry MUCH MORE about the trigger than about statistics (statistics will get better over time, so much that it can practically be forgotten).

  2. “He had some things to say about which variants of supersymmetry can now largely be excluded by LHC data”

    what’s left for supersymmetry then? and what’s *natural* about them?

  3. Would it be too rude to ask for a name of Atlas experimentalist who got lost ? Hopefully it was not the spokesperson… 🙂

  4. Matt: “One of the things that I have found puzzling is that almost all of the other particle physics and string theory bloggers not only are of the opinion that the Higgs particle has definitely been found, but also claim that almost all other particle physicists think so too. I honestly just can’t understand how they can say this. “

    This is a fair and honest skepticism in terms of the available data.

    Matt: “I find plenty of theorists who say things like “well, if I didn’t already have a strong theoretical reason to believe the Higgs particle exists, I wouldn’t be very confident in the evidence that is in the current data.”

    This is the first time that I have heard that there are many theorists who didn’t have a strong theoretical reason to believe the Higgs particle exists. All SUSY and String theorists need some kinds of Higgs. I thought that I was the only one who sees the Higgs particle of all kinds are fictitious (shadows of the real mass-rising mechanism) because of the theoretical reason which of course cannot be discussed here for being out of the scope of this website.

    I am not a person of making bets. I am simply confident that there will not be any type of Higgs with theoretical reasons. If LHC announces the discovery of a SM Higgs this summer, it will eventually turn out to be another OPERA fiasco. While I must not discuss the theoretical reason here, there is one piece common sense evidence available. It is the Tevatron Run II Higgs data, http://profmattstrassler.files.wordpress.com/2012/03/tevatron10_higgs.png . The true peak of that graph is at 130 to 135 Gev. while this range was already ruled out by many tests. Even a high school kid who knows absolutely no physics will know that the excess at 125 Gev. cannot be an evidence of excess while the “higher” point on the same curve is deemed as the evidence of exclusion.

    1. Well, that’s the problem with high-school kids doing physics. They would be absolutely certain (as you are) but they would be wrong. It’s very easy to misinterpret the Tevatron plot.

      This was discussed in the comments to my post on the Tevatron result. http://profmattstrassler.com/2012/03/07/higgs-results-from-the-first-week-of-the-moriond-conference/

      Ben Kilminster (who helped do the measurement at CDF) heard this very same objection by Daniel Whiteson (also on CDF and on ATLAS) and responded with an explanation of how to interpret the graph. It’s more subtle than you realize.

      1. Matt, you are one of the most honest scholars, but I must disagree with your saying this time.

        1. Daniel Whiteson is obviously not a high-school kid while he had questions about that graph.

        2. This kind of “graph” is a very special “language” (having nothing to do with physics), one graph one point (idea or concept). If the subtlety is the point (key idea), then makes it as the “point” (key idea) for the graph, being able to read it out with a single glance, even by a high-school kid. The points on the line of a graph must represent the same variable. If there are subtle difference between some points on the same line, then make them two curves or with two graphs. Any subtlety among the points (in the definition of those points) of the same curve in the same graph must not be allowed in this graph-language.

        3. My view on physics is all mines and I should shoulder any criticism on it myself. It is not fair for the high-school kids to carry this burden for my ignorance.

        1. 🙂 I agree that we should leave the high-school kids alone.

          My point is that Ben Kilminster had a sensible answer — that a real signal at 125 would produce an excess in that plot that would be broad and extend up to 135, and would increase toward 135 when plotted in this particular way.

          You can question the analysis, certainly, but you should not interpret the graph as saying that 135 is the preferred mass for any Higgs signal that might be present. There simply isn’t a very strong preference for any particular mass in the 120-140 range.

  5. hooray for your healthy skepticism and maybe some interesting things will turn up. Maybe Veltman’s qualms are justified.

    1. I don’t think Veltman’s qualms are justified. I just know of many ways that Higgs particles can be harder to observe than a Standard Model Higgs would be.

  6. Dear Prof. Strassler, do you then think the hints mostly interpreted as a higgs at 125 GeV are rather due to something else? If so I would be very curious to know what … :-).

    The story about the experimentalist getting lost inside his experiment is indeed very funny, LOL 😀

    1. The hints, like so many hints before them, may simply involve nasty statistical flukes in the easier measurements, combined with overly optimistic interpretations occurring in the more difficult measurements. In other words, there may be nothing there at all — a pure mirage. I’m not trying to convince you of this one way or the other — I’m just pointing out that history has given us plenty of surprising flukes, and that we’re not yet at the point where the data is so convincing that I (and many of the people I talk to) are ready to say for sure what we’re seeing is a real physical effect. It may well be real. It might not be.

      1. Dear Prof. Strassler,

        I`ve just read about an alternative interpretation of the signal at about 125 GeV (if it should be real) here: http://arstechnica.com/science/news/2012/04/excuse-me-sir-your-higgs-doesnt-fit-in-my-model.ars?clicked=related_right, and here: http://arxiv.org/abs/1112.4146.

        What do You think about the idea that the “125 GeV higgs” could be rather such a higher dimensional radion? Are you and your colleagues investigaging such (warped) extra directions (pun intended 🙂 ) or considering this possibility too?

        I’ ve read that the radion`s branching ratios would better fit the observed branching ratios then a SM higgs (at present …).


        1. Look, we don’t have enough data yet to even say for certain that there’s something real there. Now you want to start analyzing whether it is or isn’t a Higgs particle?

          If you look at how data actually comes in — how statistical fluctuations move things around as one analyzes new data and reanalyzes old data — you would see that, historically, attempts to squeeze information from insufficient data generate random results. You should simply ignore all the current papers and claims about whether this is or isn’t a Higgs — they are noise, not information.

          That is not to say you should ignore papers that ask “what if?” — it is still interesting to ask “what if this is a real signal of a Standard-Model-like Higgs at 125?” “what if there were a radion or other non-Higgs particle at 125?” (A radion, by the way, is just one of a blizzard of possibilities.) But we don’t know anything from the data yet.

          Anyone who tells you that there’s enough data there to know anything at this point ought to read some history and talk to our senior scientists. Or run some simulations of how data actually builds up over time and how imperfect information easily leads you to the wrong conclusion. (I have done all of these.)

          The quickest way to tell, if this excess turns into a real signal, that it is a Higgs particle is to look at the ratio of the two-photon signal to the two lepton/anti-lepton pair signal. Once we can say with some statistical significance that this ratio is near to what is expected for a Standard Model Higgs, that already almost proves the particle is a Higgs of some type — because (unless you tune things very carefully) for any non-Higgs particle it is extremely unnatural for the two lepton/anti-lepton signal to be anywhere close to the two-photon signal. You simply would expect the number of two lepton/anti-lepton events to be tiny. Right now that ratio is basically unknown; we cannot conclude anything.

          Similarly, (see http://profmattstrassler.com/articles-and-posts/the-higgs-particle/the-standard-model-higgs/production-of-the-standard-model-higgs-particle/ ), unless it is a Higgs particle, you would expect no lepton-antilepton-neutrino-antineutrino events and no qqH production (Higgs produced along with two quark jets via two virtual W or Z “particles” though remember they’re not really particles, http://profmattstrassler.com/articles-and-posts/particle-physics-basics/virtual-particles-what-are-they/ ). And you would expect no production of a Higgs along with a W or Z particle. All of these processes will be examined in 2012.

  7. When I was (more) childish, I wanted to see a UFO (“flying saucer”) and thought that, if I wanted it intensely enough, one would come.

    Emotionally, I feel the same way about the Higgs Boson. Intellectually, I know the story is different. But I see why the popular press enjoys jumping to a conclusion.

  8. Could you expand on the relation between the hierarchy and the existence of large objects, by which I mean if you could point me where to find such calculation or give a bit more details so I could reproduce it myself?

    thank you

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