Reviewing the Search for the Higgs

Since we’re now approaching the time when the preliminary results from December on the search for the Higgs particle at the Large Hadron Collider (LHC) will be presented in final form, possibly with small but important adjustments, and since there will be additional results based on the fall’s data in the next few weeks, it would be good to do a little review of where things stand and where they’re going.  I won’t do this all in one post but let’s get started.

Of course there is a lot of material on this website already and I’ll point you to it.  Perhaps my most concise and least technical discussion of the search for the Higgs appeared as a guest post on Cosmic Variance (thank you to Sean Carroll for the invitation.)  In that post I emphasized that the LHC’s Higgs program has two phases (broadly speaking.)  The first phase is to search for and either find or exclude the simplest possible form of Higgs particle, known as the Standard Model Higgs particle (or SM Higgs for short).  This is a finite task, so at a certain point (presumably this year) this phase comes to an end.  The second phase, which will take a better part of a decade, and (as I’ll describe over time) begins this year, depends on whether a particle that resembles an SM Higgs is discovered or not.  To explain the basic logic, I presented a figure in the Cosmic Variance post, which I am reproducing here:

Fig. 1: From my guest post in Cosmic Variance: The basic logic of the Higgs search, showing Phase 1 in which the simplest form of Higgs particle (the Standard Model Higgs) is found or excluded, and the logical questions that will follow in Phase 2. See Figure 3 for a more complete version.

If you’ve been noticing how slowly the discovery of the Higgs takes place — first there are hints, and only months later can one know whether those hints are real or not — then it will not surprise you to learn that the line between Phase 1 and Phase 2 isn’t sharp. The scientists at the LHC experiments ATLAS and CMS will be doing both of them this year, because they can, and because they should.

  • The search for the SM Higgs in 2011 was so successful that only a few possibilities remain (see Figure 2): a Higgs between about 115 and 128 GeV/c2 or above 600 GeV/c2 (though the latter is disfavored by precision measurements.)
  • We’ve got clear-enough hints for a Higgs particle with a mass of about 125 GeV/c2 (where c is the speed of light and GeV is defined here) to take seriously the possibility that it’s been found.
  • If it’s been found, it resembles (only roughly so far, but that’s perhaps because there’s not enough data yet) an SM Higgs particle.
  • All that remains to be done in Phase 1 is to close the window between 115 and 127 GeV/c2 , to confirm or refute the hints around 125, and push up the limits at 600 up as far as they can go, perhaps 800 and beyond.

Fig. 2: Phase 1 of the Higgs search now leaves only a tiny window at small masses and a window at large masses that is disfavored by two decades of precision measurements. Notice how much progress was made in 2011 alone!

So we can start looking ahead to Phase 2 — indeed we must.    In particular, if there is indeed a new particle with a mass of 125 GeV/c2 that resembles an SM Higgs, then (from Figure 1) we now have to verify whether it is or isn’t exactly what the Standard Model predicts.  Any deviation whatsoever from the precise predictions of the Standard Model would be a historic, game-changing, and Nobel Prize-deserving discovery!  So the stakes are very high.

Now, Figure 1 was actually a simplified version of the game plan for the Higgs search.  A more complete version, which was too elaborate for the short and sweet Cosmic Variance article, is shown in Figure 3.  I’ll go through this figure carefully (which itself isn’t entirely complete) over the coming days and weeks.  But suffice it to say that 2012 will involve a lot of work on the three research programs located at the far right of the figure:

Fig. 3: As Phase 1 of the Higgs search comes to a close, Phase 2 begins, with a series of logical questions and corresponding measurements. Even while the hints of a Higgs particle at 125 GeV are being checked with more data, studies of whether this might or might not be a Standard Model Higgs particle will be underway.

As you can see from the figure, these very large research programs needs to be carried out whether or not the current hints of a Higgs turn out to be ephemeral.  In other words, the experimenters can be working already on Phase 2 no matter what the outcome of Phase 1 turns out to be!  Very convenient, of course…  but it means we need to do some planning for Phase 2 now!  [And not of all of the most urgent issues have been addressed yet, which is why I’ve been very busy… more on that later…]

17 responses to “Reviewing the Search for the Higgs

  1. Were the recent CMS and ATLAS Higgs searches done blind?

    • You know, I really want to avoid answering this question. The answer is “not really”, but a proper explanation of what that means and why this is the case is very complicated, and I don’t feel competent to do it justice. I’d really like one of my experimental colleagues to answer it, if anyone is interested to do so.

      • Thanks for being forthright, Matt. I did notice that in the slides from the December presentations, blinding/unblinding discussions seemed to be absent. But absence of words on a slide isn’t actually probative.

        The best case would be… cuts now frozen for the 2012 data analysis, or at least, cut changes carefully discussed and justified. Then the results in the new data would be unlikely to arise from ooches.

        • You can bet that the standards are extremely high — cuts get set in advance, based on studies, and they are frozen from there, and not changed post facto. And everything that has to do with the Higgs search gets an exceptional level of scrutiny within the collaborations before it is allowed into the public sphere. But still, it is important to keep in mind that ATLAS and CMS are not entirely independent of each other (especially if blogs start posting premature rumors) and within ATLAS and CMS there are risks of one measurement (say, of Higgs decaying to four leptons) affecting another (say, of Higgs decaying to two photons). Everybody knows this, of course, and I think that is one reason some experimentalists are more cautious than the data might immediately suggest they should be. The notion that these are completely independent analyses that can be safely combined without concern for bias is something that might be too optimistic. Fortunately, we have enough data coming in 2012 that, at least for the current Higgs hints, we don’t have to worry about this issue.

      • I guess the deeper issue is subconscious bias… science history is full of cases where good honest people and groups fooled themselves…. cranial volume versus race, the weak V-A theory itself (didn’t the field `prove’ it was S+T??), the Crystal Ball Higgs. An awful lot rides on whether there is a light Higgs… SUSY itself might be hanging in the balance. Given all that, I would have thought the Higgs search would have been the best blinded result in particle physics history.

        But again, the truth will out eventually, maybe even in 2012, but whatever is seen in the 2012 data will be tested with the 2013, 2014, … data

  2. This is a terrifically referenced article, and much appreciated as a summary of progress and future developments. I track this research with much interest. I’m a scientist, and know that discoveries can often lead to, well, let’s say, insider jokes…I imagine there is much excitement in the community about this, so I’ve generated the “Top 10 Remarks Overheard at Discovery of Higgs Boson, aka, “The God Particle” — hope you enjoy the levity. All the best – Tim http://scienceforfiction.com

  3. As a noob, there is any possiblity that Higgs could be part of a force math, I mean an equation mathematically force in existence, a manipulating number??? like a dummy variable to Higgs exist??

    • I’m sorry, I don’t understand the question…

      • I´m not a scientific person, I will try to explain… All nuclear physics at first glance is based on math, formulas… and later experiences can or cannot prove the formula. I read in some lectures, in math that is some fragment equation that use only to validate the mean formula. What I´m thinking if Higgs´particles is not in the same way. Higgs could never exist because is a misunderstanding formula, maybe correcting this will find other validate principle.

        ps.: I´m sorry for not clear english.

        • I am afraid that I still can’t quite figure out what you are asking… I am sorry. [We have two language barriers, one into English, and the other into physics!] But I will try to answer anyway!

          The situation with the Higgs particle is that we know there is a Higgs *field* of some sort, but we do not know there is a Higgs particle. We already know a little bit about the Higgs field from experiments. It’s existence is pretty certain, because the predictions of the theory with a Higgs field match existing experiments very well.

          But there are a number of possibilities for how the Higgs field behaves, and not all of them predict a Higgs particle. So as far as the particle’s existence, we have formulas that predict a Higgs particle, and alternative formulas that do not. We don’t know which set of formulas is right. So whether the Higgs particle exists is something only experiment can tell us for sure.

          You can read more about this in the Higgs Frequently Asked Questions (FAQ), http://profmattstrassler.com/articles-and-posts/the-higgs-particle/360-2/

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  5. DEAR DOCTOR MAT. : IF THE PARTICLE MASS IS GENERATED VIA ITS INTERACTION WITH THE HIGGS FIELD ACCORDING TO SPECIFIC INTERACTION STRENGTH FOR EACH PARTICLE , THEN WHAT MECHANISM SPECIFY A SPECIFIC STRENGTH FOR EACH SPECIFIC PARTICLE ?
    THANKS
    shami

    • We don’t know. The best we can do is say that the masses of the W and Z particles and the overall strengths of the electric and weak-nuclear force are directly related (but we don’t know what sets the strengths of the forces.) For the matter particles, we have no idea what sets these strengths. There is some hope that the LHC will shed light on this question.

  6. Dear Dr. Matthew : Are there any connection between higgs field generating particle mass and superstrings excitation modes generating particle type and specifications ? are both acting together ? what mechanism causally connects them ?
    thanks
    shami

    • We don’t know there are any such things as superstrings. If there are, then superstring excitations are much, much heavier than any of the particles we know, and the sources of their masses has no connection with the Higgs mechanism. See also my articles on extra dimensions which show yet another way that particles can have masses that has nothing to do with the Higgs mechanism. Even the Higgs particle itself doesn’t get all of its mass from the Higgs mechanism.

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