In the field of particle physics, the word “workshop” has a rather broad usage; some workshops are just conferences with a little bit of time for discussion or some other additional feature. But some workshops are about WORK…. typically morning-til-night work. This includes the one I just attended at the Perimeter Institute (PI) in Waterloo, Canada, which brought particle experimentalists from the CMS experiment (one of the two general-purpose experiments at the Large Hadron Collider [LHC] — the other being ATLAS) together with some particle theorists like myself. In fact, it was one of the most productive workshops I’ve ever participated in.
The workshop was organized by the PI’s young theoretical particle physics professors, Philip Schuster and Natalia Toro, along with CMS’s current spokesman Joseph Incandela and physics coordinator Greg Landsberg. (Incandela, professor at the University of California at Santa Barbara, is now famous for giving CMS’s talk July 4th announcing the observation of a Higgs-like particle; ATLAS’s talk was given by Fabiola Gianotti. Landsberg is a senior professor at Brown University.) Other participants included many of the current “conveners” from CMS — typically very experienced and skilled people who’ve been selected to help supervise segments of the research program — and a couple of dozen LHC theorists, mostly under the age of 40, who are experienced in communicating with LHC experimenters about their measurements.
The stated aim of the workshop was to try to help the CMS experiment define and sharpen the goals of its research program for the coming year or two. Obviously a group of forty to fifty people can’t hope to represent the thousands of CMS experimenters and hundreds of particle theorists that one can find around the world — so the outcome of such a workshop can only be a set of tentative suggestions for the full CMS experiment to consider in its planning. But this combination of people seemed especially well-suited for an efficient exchange of sophisticated ideas. I’m sure I’m not alone among the workshop participants in feeling that I learned a great deal from the other attendees’ individual and collective wisdom and knowledge.
I also thought that the workshop was very well-organized — with a small number of talks to lay important topics out on the table, followed by discussion sessions that split the participants into smaller focus groups. The atmosphere was intense: informal discussion continued even over lunch and coffee breaks, and often was heard at breakfast and dinner too.
It’s really for CMS to decide whether to make the conclusions public, so I’ll give you instead a broader sense of what was discussed and how it seems to me to fit together scientifically. There were (very roughly) four main topics [you may also want to read about Professor Peskin's Slogans for 2012]:
- studying the recently discovered Higgs-like particle
- searching for signs that the universe is or is not “natural”, a confusing term I’ll explain below
- making careful and detailed checks of the predictions of the Standard Model (the set of equations that describes the known particle and forces.)
- thinking carefully about exotic particles that, if they exist, would require specialized measurement techniques to identify
and there’s a fifth topic that underlies them all: triggering. I’ll describe the first four of these now, and hold the last for a later post.
The Higgs-Like Particle: obviously, everything about this particle needs to be studied. Precision measurements of how it is produced and how it decays are needed… including both production and decay modes that are expected for the simplest type of Higgs and most other types and (as I’ve discussed on this website) exotic modes of production and decay that are not expected for the simplest Higgs (and many other types.) And we also need to look for cousins that this particle might have: additional Higgs-like particles that might be harder to find than this first one.
Naturalness-inspired particles: “Naturalness”, and the related “hierarchy problem”, deserve long explanations which I can’t give here, so here’s the short and unsatisfying explanation of what this means.
We’ve just discovered what appears likely to be a Higgs particle, a particle of spin zero and a mass of 125 GeV/c2; (which means its mass-energy E = m c2, or “rest-energy”, is 125 GeV.) Everything we know about spin-zero particles from theory and past experiments tells us that a particle of this type should most “naturally” — meaning “generically” — be accompanied by additional new particles that we haven’t yet found. More specifically, for the known particles whose large mass is due, entirely or partly, to the particle’s fairly strong interaction with the Higgs field — the W, the Z, the top, and the Higgs itself (shorthand to be used below: W/Z/t/H)— there may well be “partner” particles, light enough to be accessible at the LHC. These partners are affected by the known forces in similar ways; for example, just as the top quark has electric charge that is 2/3 the charge of the proton, and is affected by the strong nuclear force, the same would be true of its partner. However, the known particles and their partners may have different spin. [For example, the partner of the top quark or of the Higgs would probably be spin-1/2 or spin-0; the partners of the W and Z would probably be spin-1/2 or spin-1.] There might be partners for other particles too, but possibly not accessible at the LHC. If the known particles have the right types of partners, then it becomes generic (i.e. natural) for the value of the Higgs field to be small but not zero, and for the mass of the Higgs particle to be small, relative to other even higher energy scales, such as the scale at which gravity’s effects on an elementary particle become strong — a thousand million million bigger than the mass-energy of the Higgs. Well, this is a long story, but the point is that if naturalness is a good guide — and it may not be — then we generally expect to find partner particles at the LHC of some type, at least for the heaviest known particles. Actually finding them can be pretty hard, however.
The best known theory with partner particles is supersymmetry, but extra dimensional theories, composite-Higgs theories, little-Higgs theories, and others have them also. (In the extra-dimensional case they are called “Kaluza-Klein partner particles”.) So experimentally, if you want to try to know whether naturalness is a feature of nature, you want to look for, and either find or exclude, the various types of W/Z/t/H partners. Maybe you’d look for other particles’ partners, but there’s no requirement that they be accessible at the LHC.
And that’s important. If the partners of the up and down quarks, and of the gluons (shorthand: u/d/g), exist and aren’t very heavy, they would be easy to produce in proton-proton collisions — because the protons contain up and down quarks (and anti-quarks) and gluons in abundance. Conversely, because the heavy W/Z/t/H are essentially absent from the proton, their partners are not so easily made in proton-proton collisions. And therefore all the searches for supersymmetry, and for similar theories, that have been carried out so far at the LHC are sensitive more to the u/d/g partners than to the partners of the W/Z/t/H. The fact that they’ve turned up nothing only tells us that the u/d/g partners, if they exist, are too heavy to make abundantly at the LHC right now; the W/Z/t/H partners might still be waiting for us to find. The problem is that the W/Z/t/H partner particles have low production rates, smaller than the rates for other ordinary processes that mimic them. On top of that, there are many different scenarios for how they might decay, and we don’t know what their masses are — so we have to look for them in many different ways… dozens, in fact.
Standard Model backgrounds: for almost any signal of a new phenomena that might show up at the LHC, including production of the Higgs and production of W/Z/t/H partner particles, there are processes in proton-proton collisions involving known particles and forces that will look very, very similar, and often are much more common. The problem of finding a new phenomenon at the LHC is therefore typically unlike looking for a needle in a haystack; rather, it’s like looking for some aluminum needles in a haystack full of tin needles. This isn’t easy, unless you really know what tin looks like and what aluminum looks like and can find ways to tell the difference. And since the tin is so prevalent, you need to understand it very, very well; the better you understand it, the more likely you are to recognize a needle that isn’t made from tin. So most discoveries we can imagine making with the 2012 data require high-precision calculations, as well as cross-checks of those calculations in real data.
Exotic new particles: When a proton-proton collision occurs at the center of the CMS detector, the particles that come flying out from the collision pass through the detector’s many electronic sensors, leaving electronic signals. These signals are then stored on computer, and converted, using software, into the best possible guess as to where the particles went and what they might have been; this process is called “reconstruction”. But some types of new particles that might be present in nature could look so weird that the software for reconstruction that the experimenters have written might partly or completely fail. However, as long as the experimenters have thought carefully about these weird particles, they can write specialized software to look in their data for signs that they might be present. Up to now there’s been limited manpower devoted to these possibilities, which made sense, because there were other things that would be easier to do and seemed less of a long-shot. But with the first phase of the LHC coming to an end, it’s important to start bringing these possibilities up a little higher on the priority list. This is especially so since exotic new particles, unrecognized in the data, could possibly have caused the searches even for the more common signals of naturalness, such as the u/d/g partners, to have failed so far. So searching for them is important in making sure that a striking new phenomenon hasn’t escaped notice.
Triggering: I’ll tell you more about this later in the week. There’s quite an interesting story here, in which I’ve had some level of involvement during the first six months of the year.