[Note added: It is official — as expected, at this year’s Chamonix workshop, where the Large Hadron Collider’s [LHC’s] future is planned out each year, it was decided that the LHC’s energy will be increased by 14% next year (from 3.5 TeV energy per proton and 7 TeV energy per collision in 2010-2011 to 4 TeV per proton and 8 per collision.) Also the time between collisions will remain at 50 nanoseconds. I’ll have some things to say about the pros and cons of this decision, in particular the challenges for the experiments, over the next few days.]
On Monday last week, I gave you half the explanation as to why a lightweight Higgs particle is a sensitive creature, one that is easily altered by new phenomena — by particles and/or forces that we might not yet know about. It all had to do with an analogy between a violin string (or a guitar string or a xylophone key) and the properties of the Higgs particle. Today, on the same webpage as the first half, I have provided the second half of the story. (If you have already read the first half, just look for the boldface words “The Diverse Modes of a Higgs’ Demise”, which separate last week’s prose from the new stuff.) I’ve also added, for particle physicists and for those laypersons who want to go a little deeper, a short quantitative discussion of my main points.
Also: I will have the honor to be interviewed on Wednesday at 5 p.m. Eastern time, at
which you can listen to either live or later. My interviewer, Tom Levenson, is an eminent science journalist who has written fascinating and surprising books on Einstein and on Newton, among others, won awards for his work on television (e.g. NOVA), has a great blog (and also posts here), and is a professor of science writing at MIT. In short, he’s a bright and interesting dude whom you should consider following on Twitter, or in whatever way floats your boat in the ocean of social media. For this reason I suspect that the conversation is going to be a lot deeper and more interesting than the average interview, with the interviewer making at least as many interesting comments about the topic as the interviewee.
I’ll report after reading them. There will probably be I see no major changes from December 13th, though there will be are interesting minor ones. That’s because the measurements are very sensitive and precise. [UPDATE 1: The two-photon measurement from CMS includes something new: they separate the small number of events that have two additional jets out from the rest. This subset of the data is sensitive to both the g g –> Higgs and q q –> q q Higgs production processes, and it shows an excess at 124 GeV/c2, consistent with the existence of a Standard Model-like Higgs at that mass.] [UPDATE 2: See below]
The reason that a lightweight Higgs particle is a sensitive creature is this: it … decays … slowly …
Slowly??!?? In what sense can one think of a particle that typically disappears in 0.000,000,000,000,000,000,001 seconds — less than the time it takes light to cross from one side of an atom to the other — as durable?!?
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:
Keep doing what we’ve been doing in the SM Higgs search (as described in the three articles here) but with more data. Specifically, if there is a new particle at 125 GeV/c2 we want to make sure that it is produced and that it decays as expected of an SM Higgs particle.
Look for something that shouldn’t be there if there is only an SM Higgs particle: exotic modes of production, or exotic decays, or a second Higgs particle. As I’ve not previously described the large class of searches required, I will write one or more articles about this soon.
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…]
For me, one of the great pleasures of scientific research is the small, focused “workshop”, where a few scientists (typically 20 — 40 faculty, postdoctoral researches and students) assemble for a day or two to discuss a particular topic in detail. Presentations are typically short and often quite informal, and there’s lots of time for discussion built in to the schedule. The best workshops, of course, are those where the quality of the science and the knowledge, experience and skill of the participants is exceptional. I was fortunate to enjoy such a workshop yesterday at Imperial College in London, one of Britain’s finest scientific institutions. About half the participants were experimentalists from the CMS experiment at the Large Hadron Collider (LHC) and the other half theorists like myself (one of whom is also on the ATLAS experiment at the LHC.)
Another great pleasure in doing science, at least in my field, is its international nature. The participants from yesterday’s workshop included scientists from at least seven countries and three continents, many of whom are now working in a country that is not their native land. Add to this diversity of cultures, appearances and accents some lively personalities, good senses of humor, and high quality science, and you have the ingredients for a very serious workshop that is a very serious form of fun, followed by much less serious fun when the workshop is over and everyone who can stick around goes out to a pub, a restaurant, and then another pub. (more…)
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.
Nice post went up yesterday on the ATLAS experiment’s blog [ATLAS is one of the detectors at the Large Hadron Collider] from Kyle Cranmer (Professor at New York University) ; as I posted on this blog, Kyle gave Friday’s presentation summarizing ATLAS’s results on the search for the Higgs particle. Kyle is a young star with a special gift for the statistical reasoning that is essential for combining many different measurements together so as to squeeze the maximum knowledge out. His post has some technical parts, but even if you can’t follow the details I still recommend skimming it for the non-technical insights.
Another interesting post went up on the Cosmic Variance blog, by John Conway (Professor at UC Davis [and not to be confused with the mathematician John Conway]). John mentions that ATLAS and CMS (the other big LHC experiment) have observed no sign of quarks being composite (made of other yet smaller things) and have seen no sign of supersymmetry. [I’ll explain supersymmetry on this site soon.] He then goes on to discuss the Higgs search situation (see also here for my initial take.) But I disagree quite strongly with what John has to say about the search for supersymmetry; look at comment #9 in reply to his post for my opinion. John states as a near-fact that we pretty much know now that the super-partners (heavy new particles that are predicted by supersymmetry) of the quarks are heavier than 1 TeV, but I just put a paper out yesterday with Mariangela Lisanti, Philip Schuster and Natalia Toro that contradicts that statement, arguing that the searches performed so far do not by any means cover all the territory, and suggesting that other search strategies need to be added. And I know that there is more unexplored territory that we did not even discuss in our current paper. [I’ll also give a more detailed discussion of our paper’s argument soon.] This is a key job of particle theorists; make sure all the ground gets covered by the experimentalists before they give up and move on!
Taking advantage of our day and a half off from physics talks in Grenoble, I’ve written the answers to some questions that are often asked (or should be asked) about the Higgs particle. I had planned to do a very careful job of explaining what the Higgs particle is all about — and eventually I will — but it seems as though doing something quick would be a good idea, given that the search for the Higgs particle has leapt into the news again and there’s a lot of interest in it right now. So here you go; the Higgs FAQ. I hope this is useful to you!
