Tag Archives: searches

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…]

About the NY Times article from 8/02/11

On Tuesday, the New York Times had an article on the Higgs particle search.  Not bad, and does quote relevant people, but just a little bit thin on content.  If you want some actual content, try my article on the hints of the Higgs particle.

Also, the article falls into the common mistake of not distinguishing “The Higgs particle” from “The Standard Model Higgs particle.”   One paragraph says

The Higgs boson is the keystone and last undiscovered piece of the so-called Standard Model, a suite of equations that agrees with all the experiments physicists have been able to do so far in the laboratory.  If the Higgs boson does not exist, theorists will have to go back to their blackboards.

If you conflate the general with the specific, you will at some point down the line end up very confused about what the LHC is trying to do and what its results do and don’t mean.  It would be good for the public if the NY Times reporter would read the Higgs FAQ before phrasing a statement like this.  A more correct statement is:

The Standard Model Higgs boson is the keystone and last undiscovered piece of the so-called Standard Model, a suite of equations that agrees with all the experiments physicists have been able to do so far in the laboratory. If the Standard Model Higgs boson does not exist, this will be very exciting for particle physicists, as it will imply (as has been known for decades) that there must be other particles — perhaps multiple Higgs bosons, and/or perhaps other types of particles — that physicists have not yet discovered, but should be able to discover with the LHC.

The spin is pretty different.

And then there’s the generic paragraph about supersymmetry — more or less right, but again with some big missing points.  Well, stay tuned at this website for an article or two on supersymmetry, and what we have and haven’t learned about it from the LHC so far.

New Post on the Higgs Hints

Just finished my new article on the hints of a Higgs particle.  I hope you find it useful! I have tried to explain, in largely non-technical terms,

  • how experimentalists at the Large Hadron Collider are looking for the Higgs, using various methods;
  • what makes methods of this type easy or difficult, with analogies;
  • that the current hints of a Higgs particle rely on the more difficult techniques, making it unclear whether we should trust them;
  • that more data over the coming year will bring the simpler techniques into their own, eliminating this problem over time.

I welcome your comments, questions and advice as to how to make this article more transparent to the non-technical reader!