Blogging ICHEP 2010


A collective forum about the 35th edition of
the International Conference on High Energy Physics (Paris, July 2010)

Thursday, July 8, 2010

Rumors About A Light Higgs

And for once, I feel totally free to speculate without the fear of being crucified. If you have followed my past blog adventures for long enough, you know that in at least a couple of occasions my posts have created some friction.

Blogging can mean walking on a rope for particle physicists involved in large collaborations - the ways of the internet are infinite, really: you never know where trouble may come from! The chance to piss someone off forces bloggers to avoid making names even when they discuss humorous incidents; the internal rules of the experiments they participate in make bloggers wary of even discussing stuff that is approved for public distribution. A daily application of self-censoring review procedures before hitting the "submit" button must be enforced.

But not this time. I am sure of one thing: I know nothing at all, so I can certainly talk about it without violating any rule! It so happens that I have heard voices about a possible new "three-sigma" Higgs effect, and I do not even know which experiment this comes from! Surely, no single experiment can get mad at me this time if I tell you what it is about, right ?

...Right. Well, I am not totally sure, but I am willing to declare that I have the right to express myself here, to some extent at least! So let me spill my guts. They are almost empty anyways...

The Rumor

It reached my ear, from two different, possibly independent sources, that an experiment at the Tevatron is about to release some evidence of a light Higgs boson signal. Some say a three-sigma effect, others do not make explicit claims but talk of a unexpected result. That the result comes from the Tevatron is for sure, since the LHC experiments do not have nearly enough data yet to search for that elusive particle, and other particle physics experiments in the world have not nearly enough energy to produce it. However, I am unable to understand whether the rumor comes from CDF or from D0.

Lest you jump at conclusions too early, I need to explain something more: despite being a CDF author, I unfortunately do not follow actively the works of the Higgs Discovery Working Group within CDF, so a Higgs excess in CDF data could well have escaped me. In principle, if I now took on digging hard enough in the internal pages of the CDF experiment I might be able to find out if this signal is coming from there, and maybe learn more about it. But there are at least a dozen analyses to dig into! Too much work - while wild speculation is more fun!

Reasoning On It

So let us take a look at the latest Higgs boson limits, released jointly by CDF and D0 last November. The dozens of analyses combined for the global limit were based on a dataset amounting to anything between 2 and 5.4 inverse femtobarns of proton-antiproton collisions, while right now the experiments have probably in their hands over 50% more processed and analyzable data.

The graph I choose to make a point is actually not one describing the limit on the Higgs boson cross section as a function of Higgs mass. Rather, let me pick the one showing, as a function of mass, a quantity that describes more clearly whether the data are background-like or signal-plus-background-like. The hatched black and red curves in the figure below show the value of the statistical estimator LLR (not going to explain you here what it is, but ask for it in the comments thread if you are interested) that the experiments would have globally observed, on average, if the higgs were there (red) or not (black). The farther the two curves are, the more sensitive the experiments are to a Higgs signal.



Also note the green and yellow bands, drawn around the expected background curve: they denote the typical extent of one- and two-sigma fluctuations expected in the data. In other words, if the Higgs is NOT at 130 GeV, say, then the LLR is expected to be on average equal to 1, but 68% of the time we may expect to find it anywhere between -1 (lower edge of the green band at 130 GeV) and +2.6 (upper edge). This is the so-called "one-sigma" band.

Now, look at the full black line. This shows the actual LLR value of the data, after the complicated analyses that sought the Higgs decay in dozens of different possible final states is processed. You notice several things.

The first thing to note is that the curve stands more in the "signal-plus-background" region for masses below 145 GeV, then going up and following the "background-only" curve for higher values.

The second thing to note is that while at 165 GeV the two LLR expectation curves are quite far apart (meaning that a Higgs boson might have produced a 3-sigma excess there, quite easily), at 120-140 GeV the curve of signal-plus-background stays on the border of the green band: the _expected sensitivity is there at most a one-sigma effect. In other words, a Higgs boson at 130 GeV would on average produce a 1-sigma deviation from the background-only curve, in the Tevatron data analyzed until November 2009. On average, though! The actual observed data, if it contained a Higgs boson, could produce larger signals, if the experiments got lucky.

The third thing to note is that the black curve in the low-mass region stands even lower than the red hatched curve! That means that the data there is definitely more signal-like than background-only-like. But is this a significant observation ? Well, no: the curve is well-contained within the yellow band. A less-than-two-sigma effect.

So, that was the situation last November. What should we expect now ? Could the black curve fall further down, hinting at a Higgs boson in the 115-140 GeV range ?

It could. In my opinion, a further fluctuation of the data, and the addition of 50% more of it, could bring the black curve out of the yellow band, toward a three-sigma signal-like effect. Is this what the rumors are about ? I do not know, but one thing is sure: we will know soon... If you are coming to Paris for ICHEP, you are among the lucky ones who will get the information first-hand from the analyzers.


An Appendix: Why Rumor Mongering ?

Why am I doing this ? I know several "serious" physicists and colleagues who have questioned this care-free attitude of mine in the past. What good does it do to shout "Higgs" every second week ?

It does a lot of good to particle physics, in my very humble, but not quite uninformed, opinion. I have made this point other times, and will not repeat it here. Suffices to say that, in a nutshell, keeping particle physics in the press with hints of possible discoveries that later die out is more important than speaking loud and clear once in ten years, when a groundbreaking discovery is actually really made, and keeping silent the rest of the time.

And there is another reason why I find this kind of rumor-mongering entertaining: maybe some informed soul out there might comment anonymously and share some more gossip about the matter with us... ;-)

10 comments:

  1. Totally agree with your appendix. =)
    And this would be great for SUSY models, I think.
    Anyway, let's wait.

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  2. So if your rumor is valid at the 2-sigma level....

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  3. Hi Tommaso, of course one should be skeptic towards rumors circulating so close to the opening of the biggest hep conference ICHEP, it might be instrumental (and this blog is the right place where one should discuss that!). But let's assume the results will be announced at ICHEP 2010 as you say: how will it work? Will they be announced during the parallel sessions by the people who actually did the analysis or during the plenary sessions (few days later, maybe after more rumor leaking) by the big bosses? How are these things typically decided in the collaborations?

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  4. I wish you stopped entertaining yourself, Tommaso.
    You make us all look like a bunch of idiots.
    And you are definitely wrong about the usefulness of what you do. Public might be amused, but the funding agencies are not.

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  5. Dear Anon,

    if the rumor turns out to be diffused by people who know what they are talking about (those who related it to me and to Lubos and to a third person), something will be known on July 16th at a Wine and Cheese seminar at Fermilab. If not, we will have to wait for ICHEP!

    Dear Yuri,
    first of all I am not entertaining myself -no more than you are by leaving nasty comments here.
    Second, I totally disagree with your twisted perception of the matter, as well as with the fact that funding agencies may be disturbed. Remember SSC ? Too often the future of HEP rests on the decision of congressmen who only care about the public perception of research. We need to distribute our knowledge, let people understand its value, not keep it to ourselves. and we need to make it interesting.

    Best,
    T.

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  6. Dear Tommaso,

    your own words, "Too much work - while wild speculation is more fun!" tell me that you lie when you say that you are not entertaining yourself. This blog is a pinnacle of narcissism.

    After spending my time rebutting your lies, believe me, I'm not entertained, and the reason I posted is clearly stated in my message - I would like you to stop.

    I remember SSC well. I'm not sure you understand US politics (or any, for that matter) enough to speculate on congressmen motivation. In any case, it is your point of view that goes against conventional wisdom on the matter.

    hoping in vain that the press would figure you out for who you are,

    Yuri

    ReplyDelete
  7. Yuri, how many hopes. Hopes that I stop. Hopes that the press takes your view. Hopes that you understand my motivations. Hopes that you manage to bother me by saying I understand no politics.

    I only have one hope. That you walk away from here and that you go back to your important work. You are wasting my time by not understanding what is obvious to many who are not personally involved in these matters: that I have done more good to Fermilab PR with one post than their own public means in three months.

    Best,
    T.

    ReplyDelete
  8. well, not all bloggers seem to be as crazy as you are. Absolutely *LOVE* Paris Hilton analogy, hope you will to :)

    http://physicsandphysicists.blogspot.com/2010/07/no-higgs-yet.html

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  9. Anonymous SnowboarderJuly 14, 2010 at 11:46 PM

    T - While I disagree with those who feel you make posts such as these simply for your own ego, it does seem to me too that they have some validity in saying that rumors concerning a very significant event should not be widely publicized. There is truth to the statement that the public becomes desensitized over time - not just to news 'about' but also hype and rumors.

    That said, I think there can be a better middle ground whereby collaborations provide a better roadmap and more frequent updates/guidance than just a 5 sigma announcement. I think even your post here, had it been instead written as an "update" to reflect what might be seen with the additional data would have been valuable. Like sex, sometimes things are more exciting when some of the clothes are still on - word of the rumors could have been left unsaid.

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  10. Dear Snowboarder,

    from the height of my experience with science popularization ;-), I can say that my impression is the opposite. I have made the point several times. If you explain to the public what the Higgs is once, they will forget the next day. If instead you provide constant pressure, with stories of competition, of tentative discoveries, etcetera, they grow interested, and learn some science.

    Best,
    T.

    ReplyDelete

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