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?!?
Yesterday I spent the afternoon at the Third Indian-Israeli International Meeting on String Theory, held at the Israel Institute for Advanced Studies. The subject of the meeting is “Holography and its Applications”. No, this isn’t holography as in that optical trick that allows you to create a three-dimensional image on the security strip of your credit card — this is “holography” as string theorists like to discuss it, that trick of describing gravitational or string-theoretic physics in a certain number of spatial dimensions as quantum field theory (without gravity) in a smaller number of spatial dimensions. It’s impressive, even stunning, that sometimes you can use a precise form of the holographic principle to solve some difficult string theory problems by rewriting them as easier quantum field theory problems, and solve some difficult quantum field theory problems by rewriting them as easier string theory problems.
I worked in this research area on and off for quite a while (mainly 1999-2007) so I know most of the participants in this subfield. In fact my most commonly cited paper happens to be on this subject. But ironically my role at this conference was to present, as the opening talk, a review of 2011 at the Large Hadron Collider (LHC). (more…)
I’ve mentioned before that one of the great pleasures of working in high-energy physics is its international nature. I find the diversity of people and cultures I encounter in the pursuit of knowledge exciting and refreshing. This week I am visiting the famous Weizmann Institute of Science, outside of Tel Aviv. Many of the physicists in my field who are here or at nearby institutions were in the United States as students, as postdoctoral researchers, or as visitors, at places where I myself was working at one point or another. Even one of my former students is here. So there are many old friends and former collaborators in both string theory and particle physics for me to meet with, and to learn from. I’ll also be giving a couple of scientific lectures during this visit.
Meanwhile, I wanted to mention a mildly interesting article that I happened to notice:
I have nothing negative to say about people doing research into exactly what babies do and don’t have intuition for at various ages, but there are two things I find problematic about the spin that is put on this story. (more…)
One of the things that has to be done in Phase 2 is to search for decays of the Higgs particle that are not among the decays predicted to occur in the Standard Model. [“Decay” = “a disintegration of one particle into two or more”. Click here for an introduction.] Such “exotic” decays are thought of as particularly plausible, because a lightweight Higgs (below about 150 GeV/c2 or so) is a very sensitive creature. It is very easy for new particles and/or forces to alter the Higgs’ properties, perhaps causing changes in how (or how often) it is produced, and to what (and with what probability) it may decay. As shown in a large number of papers, written by quite a variety of particle physics theorists, there are many, many types of possible exotic decays, and they can arise for many reasons. If you’re curious what kind of exotic decays might occur, I gave a few examples in my now somewhat out-of-date analysis of what the summer’s Higgs searches imply. The basic logic of how unusual Higgs decays might arise is still correct in the cases described, but there are many, many more possibilities too. I’ll have to write a long article about the options in the coming month or so. (more…)
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…]
