Today, Wednesday December 4th, at 8 pm Eastern/5 pm Pacific time, Sean Carroll and I will be interviewed again by Alan Boyle on “Virtually Speaking Science”. The link where you can listen in (in real time or at your leisure) is
What is “Virtually Speaking Science“? It is an online radio program that presents, according to its website:
Informal conversations hosted by science writers Alan Boyle, Tom Levenson and Jennifer Ouellette, who explore the explore the often-volatile landscape of science, politics and policy, the history and economics of science, science deniers and its relationship to democracy, and the role of women in the sciences.
For those who don’t know of him, Sean is a very fine scientist, an expert on the early and current universe, among other things, as well as a very skilled and engaging writer and speaker… and very importantly, he maintains very high standards for accuracy and clarity. I recommend him highly!
Sean and I were interviewed on a Virtually Speaking Science on-line radio show in February (you can listen to it here) and will be appearing again in early December. (By the way, I also appeared on this show when the hunt for the Higgs was still on…)
In sports, as in science, there are two very different types of heroes. There are the giants who lead the their teams and their sport, winning championships and accolades, for years, and whose fame lives on for decades: the Michael Jordans, the Peles, the Lou Gherigs, the Joe Montanas. And then there are the unlikely heroes, the ones who just happen to have a really good day at a really opportune time; the substitute player who comes on the field for an injured teammate and scores the winning goal in a championship; the fellow who never hits a home run except on the day it counts; the mediocre receiver who turns a short pass into a long touchdown during the Super Bowl. We celebrate both types, in awe of the great ones, and in amused pleasure at the inspiring stories of the unlikely ones.
In science we have giants like Newton, Darwin, Boyle, Galileo… The last few decades of particle physics brought us a few, such as Richard Feynman and Ken Wilson, and others we’ll meet today. Many of these giants received Nobel Prizes. But then we have the gentlemen behind what is commonly known as the Higgs particle — the little ripple in the Higgs field, a special field whose presence and properties assure that many of the elementary particles of nature have mass, and without which ordinary matter, and we ourselves, could not exist. Following discovery of this particle last year, and confirmation that it is indeed a Higgs particle, two of them, Francois Englert and Peter Higgs, have been awarded the 2013 Nobel Prize in physics. Had he lived to see the day, Robert Brout would have been the third.
The paper written by Brout and Englert; the two papers written by Higgs; the paper written by Gerald Guralnik, Tom Kibble and Carl Hagen; these tiny little documents, a grand total of five and one half printed pages — these were game-winning singles in the bottom of the 9th, soft goals scored with a minute to play, Hail-Mary passes by backup quarterbacks — crucial turning-point papers written by people you would not necessarily have expected to find at the center of things. Brout, Englert, Higgs, Guralnik, Kibble and Hagen are (or rather, in Brout’s case, sadly, were) very fine scientists, intelligent and creative and clever, and their papers, written in 1964 when they were young men, are imperfect but pretty gems. They were lucky: very smart but not extraordinary physicists who just happened to write the right paper at the right time. In each case, they did so
…not by studying the real world but by studying simplified, imaginary worlds, in which it was easier to learn the general lessons they wanted to extract…
…and with no suspicion that their results would instead be applied to a different real-world question: the possibility that the weak nuclear force is associated with photon-like particles, called W and Z, that have a mass.[A photon is a particle of light, has spin=1, and, unlike the W and Z particles, has no mass.]
History in general, and history of science in particular, is always vastly more complex than the simple stories we tell ourselves and our descendants. Making history understandable in a few pages always requires erasing complexities and subtleties that are crucial for making sense of the past. Today, all across the press, there are articles explaining incorrectly what Higgs and the others did and why they did it and what it meant at the time and what it means now. I am afraid I have a few over-simplified articles of my own. But today I’d like to give you a little sense of the complexities, to the extent that I, who wasn’t even alive at the time, can understand them. And also, I want to convey a few important lessons that I think the Hi(gg)story can teach both experts and non-experts. Here are a couple to think about as you read:
1. It is important for theoretical physicists, and others who make mathematical equations that might describe the world, to study and learn from imaginary worlds, especially simple ones. That is because
1a. one can often infer general lessons more easily from simple worlds than from the (often more complicated) real one, and
1b. sometimes an aspect of an imaginary world will turn out to be more real than you expected!
2. One must not assume that research motivated by a particular goal depends upon the achievement of that goal; even if the original goal proves illusory, the results of the research may prove useful or even essential in a completely different arena.
My summary today is based on a reading of the papers themselves, on comments by John Iliopoulos, and on a conversation with Englert, and on reading and hearing Higgs’ own description of the episode.
The story is incompletely but perhaps usefully illustrated in the figure below, which shows a cartoon of how four important scientific stories of the late 1950s and early 1960s came together. They are:
How do superconductors (materials that carry electricity without generating heat) really work?
How does the proton get its mass, and why are pions (the lightest hadrons) so much lighter than protons?
Why do hadrons behave the way they do; specifically, as suggested by J.J. Sakurai(who died rather young, and after whom a famous prize is named), why are there photon-like hadrons, called rho mesons, that have mass?
How does the weak nuclear force work? Specifically, as suggested by Schwinger and developed further by his student Glashow, might it involve photon-like particles (now called W and Z) with mass?
These four questions converged on a question of principle: “how can mass be given to particles?”, and the first, third and fourth were all related to the specific question of “how can mass be given to photon-like particles?’’ This is where the story really begins. [Almost everyone in the story is a giant with a Nobel Prize, indicated with a parenthetic (NPyear).]
My best attempt at a cartoon history…
In 1962, Philip Anderson (NP1977), an expert on (among other things) superconductors, responded to suggestions and questions of Julian Schwinger (NP1965) on the topic of photon-like particles with mass, pointing out that a photon actually gets a mass inside a superconductor, due to what we today would identify as a sort of “Higgs-type’’ field made from pairs of electrons. And he speculated, without showing it mathematically, that very similar ideas could apply to empty space, where Einstein’s relativity principles hold true, and that this could allow elementary photon-like particles in empty space to have mass, if in fact there were a kind of Higgs-type field in empty space.
In all its essential elements, he had the right idea. But since he didn’t put math behind his speculation, not everyone believed him. In fact, in 1964 Walter Gilbert (NP1980 for chemistry, due to work relevant in molecular biology — how’s that for a twist?) even gave a proof that Anderson’s idea couldn’t work in empty space!
But Higgs immediately responded, arguing that Gilbert’s proof had an important loophole, and that photon-like particles could indeed get a mass in empty space.
Meanwhile, about a month earlier than Higgs, and not specifically responding to Anderson and Gilbert, Brout and Englert wrote a paper showing how to get mass for photon-like particles in empty space. They showed this in several types of imaginary worlds, using techniques that were different from Higgs’ and were correct though perhaps not entirely complete.
A second paper by Higgs, written before he was aware of Brout and Englert’s work, gave a simple example, again in an imaginary world, that made all of this much easier to understand… though his example wasn’t perhaps entirely convincing, because he didn’t show much detail. His paper was followed by important theoretical clarifications from Guralnik, Hagen and Kibble that assured that the Brout-Englert and Higgs papers were actually right. The combination of these papers settled the issue, from our modern perspective.
And in the middle of this, as an afterthought added to his second paper only after it was rejected by a journal, Higgs was the first person to mention something that was, for him and the others, almost beside the point — that in the Anderson-Brout-Englert-Higgs-Guralnik-Hagen-Kibble story for how photon-like particles get a mass, there will also generally be a spin-zero particle with a mass: a ripple in the Higgs-type field, which today we call a Higgs-type particle. Not that he said very much! He noted that spin-one (i.e. photon-like) and spin-zero particles would come in unusual combinations. (You have to be an expert to even figure out why that counts as predicting a Higgs-type particle!) Also he wrote the equation that describes how and why the Higgs-type particle arises, and noted how to calculate the particle’s mass from other quantities. But that was it. There was nothing about how the particle would behave, or how to discover it in the imaginary worlds that he was considering; direct application to experiment, even in an imaginary world, wasn’t his priority in these papers.
In his second paper, Higgs considers a simple imaginary world with just a photon-like particle and a Higgs-type field. Equation 2b is the first place the Higgs-type particle explicitly appears in the context of giving photon-like particles a mass (equation 2c). From Physical Review Letters, Volume 13, page 508
About the “Higgs-type” particle, Anderson says nothing; Brout and Englert say nothing; Guralnik et al. say something very brief that’s irrelevant in any imaginable real-world application. Why the silence? Perhaps because it was too obvious to be worth mentioning? When what you’re doing is pointing out something really “important’’ — that photon-like particles can have a mass after all — the spin-zero particle’s existence is so obvious but so irrelevant to your goal that it hardly deserves comment. And that’s indeed why Higgs added it only as an afterthought, to make the paper a bit less abstract and a bit easier for a journal to publish. None of them could have imagined the hoopla and public excitement that, five decades later, would surround the attempt to discover a particle of this type, whose specific form in the real world none of them wrote down.
In the minds of these authors, any near-term application of their ideas would probably be to hadrons, perhaps specifically Sakurai’s theory of hadrons, which in 1960 predicted the “rho mesons”, which are photon-like hadrons with mass, and had been discovered in 1961. Anderson, Brout-Englert and Higgs specifically mention hadrons at certain moments. But none of them actually considered the real hadrons of nature, as they were just trying to make points of principle; and in any case, the ideas that they developed did not apply to hadrons at all. (Well, actually, that’s not quite true, but the connection is too roundabout to discuss here.) Sakurai’s ideas had an element of truth, but fundamentally led to a dead end. The rho mesons get their mass in another way.
Meanwhile, none of these people wrote down anything resembling the Higgs field which we know today — the one that is crucial for our very existence — so they certainly didn’t directly predict the Higgs particle that was discovered in 2012. It was Steven Weinberg (NP1979) in 1967, and Abdus Salam (NP1979) in 1968, who did that. (And it was Weinberg who stuck Higgs’ name on the field and particle, so that everyone else was forgotten.) These giants combined
the ideas of Higgs and the others about how to give mass to photon-like particles using a Higgs-type field, with its Higgs-type particle as a consequence…
…with the 1960 work of Sheldon Glashow (NP1979), Schwinger’s student, who like Schwinger proposed the weak nuclear force was due to photon-like particles with mass,…
…and with the 1960-1961 work of Murray Gell-Man (NP1969) and Maurice Levy and of Yoichiro Nambu (NP2008) and Giovanni Jona-Lasinio, who showed how proton-like or electron-like particles could get mass from what we’d now call Higgs-type fields.
The Hi(gg)story is populated with giants. I’m afraid my attempt to tell the story has giant holes to match. But as far as the Higgs particle that was discovered last year at the Large Hadron Collider, the unlikely heroes of the story are the relatively ordinary scientists who slipped in between the giants and actually scored the goals.
This week I’m in California, at a conference celebrating two famous professors, from whom I learned an enormous amount when I was a graduate student and postdoctoral researcher. More on this later in the week.
For today’s post, I’ve added a bit more information to the article that I’m gradually writing on “naturalness”. So far, in that article and an accompanying one, I have
Specifically, the new section of the article explains how the Higgs field’s average value, and the Higgs particle’s mass, are determined (as for any similar field) by how the energy of empty space — to which the above-mentioned quantum fluctuations are a crucial contributor — depends on the Higgs field itself.
Yes, this is a long story — but so central to current “conventional wisdom” about the universe that we’d better go through it carefully. By the next installment, we should be getting to the heart of the matter.
So I got the following questions from a high school English teacher this morning, and I thought, for fun, I’d put the answers here for you to enjoy. Here (slightly abridged) is what the teacher wrote, and my answers:
I’ve turned my classroom into a video game to increase student engagement. In my gamified classroom, the villain is experimenting with/on the Higgs field. Your article on what would happen if the Higgs field was turned off answered a lot of my questions, but … I was hoping you could answer a couple of questions for me. I am sure these questions probably don’t have “real” answers, and are completely ridiculous, but I’d love to hear from you. (more…)
Arguably the two greatest problems facing particle physicists, cosmologists, string theorists, and the like are both associated with an apparent failure of a notion called “naturalness”. Until now, I’ve mostly avoided this term on this site, because to utter the word demands an extended explanation. After all, how could nature be unnatural, by definition?
Well, the answer is that the word “natural” has multiple meanings. The one that scientists are using in this context isn’t “having to do with nature” but rather “typical” or “as expected” or “generic”, as in, “naturally the baby started screaming when she bumped her head”, or “naturally it costs more to live near the city center”, or “I hadn’t worn those glasses in months, so naturally they were dusty.” And unnatural is when the baby doesn’t scream, when the city center is cheap, and when the glasses are pristine. Usually, when something unnatural happens, there’s a good reason.
I’ve started writing an article about naturalness and unnaturalness, and how there are two great mysteries about how unnatural our universe is, one of which lies at the heart of the Large Hadron Collider‘s [LHC’s] research program. What I’ve written so far explains what naturalness means and (in part) how it applies to the Standard Model (the equations we use to describe the known elementary particles and forces). I’ll be extending the article to explain this in more detail, and to explain the scientific argument as to why it is so unnatural to have a Higgs particle that is “lonely” — with no other associated particles (beyond the ones we already know) of roughly similar mass. This in turn is why so many particle physicists have long expected the LHC to discover more than just a single Higgs particle and nothing else… more than just the Standard Model’s one and only missing piece… and why it will be a profound discovery with far-reaching implications if, during the next five years or so, the LHC experts sweep the floor clean and find nothing more in the LHC’s data than the Higgs particle that was found in 2012.
Day 3 of the SEARCH workshop (see here for an introduction and overviews of Day 1 and Day 2) opened with my own talk, entitled “On The Frontier: Where New Physics May Be Hiding”. The issue I was addressing is this:
Even though dozens of different strategies have been used by the experimenters at ATLAS and CMS (the two general purpose experiments at the Large Hadron Collider [LHC]) to look for various types of new particles, there are still many questions that haven’t been asked and many aspects of the data that haven’t been studied. My goal was to point out a few of these unasked or incompletely asked questions, ones that I think are very important for ATLAS and CMS experts to investigate… both in the existing data and also in the data that the LHC will start producing, with a higher energy per proton-proton collision, in 2015.
I covered four topics — I’ll be a bit long-winded here, so just skip over this part if it bores you.
1. Non-Standard-Model (or “exotic”) Higgs Decays: a lightweight Higgs particle, such as the one we’ve recently discovered, is very sensitive to novel effects, and can reveal them by decaying in unexpected ways. One class of possibilities, studied by a very wide range of theorists over the past decade, is that the Higgs might decay to unknown lightweight particles (possibly related in some way to dark matter). I’ve written about these possible Higgs decays a lot (here, here, here, here, here, here and here). This was a big topic of mine at the last SEARCH workshop, and is related to the issue of data parking/delaying. In recent months, a bunch of young theorists (with some limited help and advice from me) have been working to write an overview article, going systematically through the most promising non-Standard-Model decay modes of the Higgs, and studying how easy or difficult it will be to measure them. Discoveries using the 2011-2012 data are certainly possible! and at least at CMS, the parked data is going to play an important role.
2. What Variants of “Natural” Supersymmetry (And Related Models) Are Still Allowed By ATLAS and CMS Searches? A natural variant of supersymmetry (see my discussion of “naturalness”=genericity here) is one in which the Higgs particle’s mass and the Higgs field’s value (and therefore the W and Z particles’ masses) wouldn’t change drastically if you were somehow to vary the masses of superpartner particles by small amounts. Such variants tend to have the superpartner particle of the Higgs (called the “Higgsino”) relatively light (a few hundred GeV/c² or below), the superpartner of the top (the “top squark”, with which the Higgs interacts very strongly) also relatively light, and the superpartner of the gluino up in the 1-2 TeV range. If the gluino is heavier than 1.4 TeV or so, then it is too heavy to have been produced during the 2011-2012 LHC run; for variants with such a heavy gluino, we may have to wait until 2015 and beyond to discover or rule them out. But it turns out that if the gluino is light enough (generally a bit above 1 TeV/c²) it is possible to make very general arguments, without resort to the three assumptions that go into the most classic searches for supersymmetry, that almost all such natural and currently accessible variants are now ruled out. I say “almost” because there is at least one class of important exceptions where the case is clearly not yet closed, and for which the gluino mass could be well below 1 TeV/c². [Research to completely characterize the situation is still in progress; I’m working on it with Rutgers faculty member David Shih and postdocs Yevgeny Kats and Jared Evans.] What we’ve learned is applicable beyond supersymmetry to certain other classes of speculative ideas.
3. Long-Lived Particles: In most LHC studies, it is assumed that any currently unknown particles that are produced in LHC collisions will decay in microscopic times to particles we know about. But it is also possible that one or more new type of particle will decay only after traveling a measurable distance (about 1 millimeter or greater) from the collision point. Searching for such “long-lived” particles (with lifetimes longer than a trillionth of a second!) is complicated; there are many cases to consider, a non-standard search strategy is almost always required, and sometimes specialized trigger strategies are needed. Until recently, only a few studies had been carried out, many with only 2011 data. A very important advance occurred very recently, however, when CMS produced a study, using the full 2011-2012 data set, looking for a long-lived particle that decays to two jets (or to anything that looks to the detector like two jets, which is a bit more general) after traveling up to a large fraction of a meter. The specialized trigger that was used requires about 300 GeV of energy or more to be produced in the proton-proton collision in the form of jets (or things that look like jets to the triggering system.) This is too much for the search to detect a Higgs particle decaying to one or two long-lived particles, because a Higgs particle’s mass-energy [E=mc2 energy] is only 125 GeV, and it is rather rare therefore for 300 GeV of energy in jets-et-al to be observed when a Higgs is produced. But in many speculative theories with long-lived particles, this amount of energy is easily obtained. As a result, this new CMS search clearly wipes out, at one stroke, many variants of a number of speculative models. It will take theorists a little while to fully understand the impact of this new search, but it will be big. Still, it’s by no means the final word. We need to push harder, improving and broadening the use of these methods, in order that decays of the Higgs itself to long-lived particles can be searched for. This has been done already in a handful of cases (for example if the long-lived particle decays not to jets but to a muon/anti-muon pair or an electron/positron pair, or if the long-lived particle travels several meters before it decays) and in some cases it is already possible to show that at most 1 in 100 to 1000 Higgs particles produce long-lived particles of this type. For some other cases, the triggers developed for the parked data may be crucial.
4. “Soft” Signals: A frontier that has never been explored, but which theorists have been talking about for some years, is one in which a high-energy process associated with a new particle is typically accompanied by an unusually large number of very low-energy particles (typically photons or hadrons with energy below a few GeV). The high-energy process is mimicked by certain common processes that occur in the Standard Model, and consequently the signal is drowned out, like a child’s voice in a crowded room. But the haze of a large number of low-energy particles that accompanies the signal is rare in the mimicking processes, so by keeping only those collisions that show something like this haze, it becomes possible to throw out the mimicking process most of the time, making the signal stand out — as though, in trying to find the child, one could identify a way to get most of the people to leave the room, reducing the noise enough for the child’s voice to be heard. [For experts: The most classic example of this situation arises in certain types of objects called “quirks”, though perhaps there are other examples. For non-experts: I’ll explain what quirks are some other time; it’s a sophisticated story.]
I was pleased that there was lively discussion on all of these four points; that’s essential for a good workshop.
After me there were talks by ATLAS expert Erez Etzion and CMS’s Steve Wurm, surveying a large number of searches for new particles and other phenomena by the two experiments. One new result that particularly caught my eye was a set of CMS searches for new very heavy particles that decay to pairs of W and/or Z particles. The W and Z particles go flying outwards with tremendous energy, and form the kind of jet-like objects I mentioned yesterday in the context of Jesse Thaler’s talk on “jet substructure”. This and a couple of other related measurements are reflective of our moving into a new era, in which detection of jet-like W and Z particles and jet-like top quarks has become part of the standard toolbox of a particle physicist.
The workshop concluded with three hour-long panel discussions:
on the possible interplay between dark matter and LHC research (for instance: how production of “friends” of dark matter [i.e., particles that are somehow related to dark matter particles] may be easier to detect at the LHC than production of dark matter itself)
on the highest priorities for the 2013-2014 shutdown period before the LHC restarts (for instance, conversations between theorists and experimentalists about the trigger strategies that should be used in the next LHC run)
on what the opportunities of the 2015-2020 run of the LHC are likely to be, and what their implications may be (for instance, the ability to finally reach the 3 TeV/c2 mass range for the types of particles one would expect in the so-called “Randall-Sundrum” class of extra-dimensions models; the opportunities to look for very rare Higgs, top and W decays; and the potential to complete the program I outlined above of ruling out all but a very small class of natural variants of supersymmetry.)
All in all, a useful workshop — but its true value will depend on how much we all follow up on what we discussed.
The first day of the SEARCH workshop was focused on current and future measurements of the new Higgs particle discovered in 2012. A lot of the issues I’ve written about before (for instance here and here) and most of the updates were rather technical, so I won’t cover them today. But I thought it useful to take a look at what was said by Raman Sundrum and separately by Nima Arkani-Hamed, whom you’ve heard about many times (for instance, here and here), on the subject of the hierarchy problem and “naturalness”.
First, let me remind you of the issue. The hierarchy problem can be phrased in many ways. Here’s one. Here’s another: for a Standard Model Higgs (the simplest possible type of Higgs particle) to show up, without any other new particles or forces at the Large Hadron Collider, is … well, let’s say it’s completely shocking, with a caveat. Why?
Because every spin-zero particle (or particle-like object) that has ever been observed, in particle physics and in similar contexts within solids and fluids, has been accompanied by new phenomena at an energy scale comparable to the scalar’s mass-energy (E=mc2 energy).
And although we cannot calculate the mass of the Higgs particle using the Standard Model (the equations we use to predict the behavior of the known particles and forces) — the Higgs particle’s mass is something we put in to the equations, which is why we didn’t know, before the LHC, what it would be — there are many speculative theories that go beyond the Standard Model where the Higgs particle’s mass can be computed, or at least estimated. And in all of these cases, the Higgs particle is accompanied by other particles and forces that show up at scales comparable to the Higgs particle’s mass-energy.
This fact — that spin-zero particles like the Higgs are accompanied by other particles and forces at a similar energy range — isn’t a mystery. Particle physicists (and others who use quantum field theory, the type of math used in the Standard Model) understand why this should be true, and have for several decades. The jargon is that it is “natural” (not meaning “from nature”, but rather meaning “generically true”) for spin-zero particles to have other particles and forces around at comparable energy scales. (I’ll explain the argument another time.)
So to discover the Higgs particle at a mass-energy of 125 GeV, and no other new particles or phenomena below, say, 1000-2000 GeV or so, would fly in the face of what we’ve seen again and again in physics, both in past data and in calculations within speculative theories. In this sense, finding nothing except a Standard Model Higgs at the LHC would be shocking. (I say “would be” rather than “is” because the LHC is still young, and no overarching conclusions can yet be drawn from its current data.)
But — here’s the caveat — how bad is this shock? After all, somewhat surprising things do happen in nature all the time. Only astonishingly, spectacularly surprising things are very rare. Yes, it would be a very big shock if new particles and forces associated with the Higgs have a mass-energy a trillion trillion times higher than that of the Higgs. But what if they’re just a few times higher than would be natural, let’s say at 10,000 GeV — which would be out of reach of the LHC? Maybe that is a small enough shock that we shouldn’t pay it much attention. Unfortunately, this is a judgment call; there’s no sharp answer to this question.
Raman Sundrum and his three options.
As Sundrum put it, there are (crudely) three logically distinct possibilities for what lies ahead:
No shock: The hierarchy problem is resolved naturally; the associated new particles will soon be seen at the LHC.
Mild shock: The hierarchy problem is resolved in a roughly natural way; most of the associated new particles will be a bit beyond the reach of the LHC, but perhaps one or more will be lightweight enough to be discovered during the lifetime of the LHC.
Severe shock: The hierarchy problem is not resolved naturally; any associated particles may lie far out of reach, though of course other particles (associated, say, with dark matter) might still show up at the LHC.
Arkani-Hamed made a similar distinction, but addressed the third case in more detail, breaking it up into two sub-cases.
The solution to the hierarchy problem is that it results from a bias (= selection effect = a form of the “anthropic principle”) ; the universe is huge, complex and diverse, with particles and forces that differ from place to place [sometimes called a “multiverse”], and most of that universe is inhospitable to life of any sort; the reason we live in an unusual part of that universe, with a lightweight unaccompanied scalar particle, is because this happens to be the only place (or one of very few places) that life could have evolved. A key test of this argument is to show that if the particles and forces of nature were much different from what we find them in our part of the universe, then our environment would become completely inhospitable — perhaps there would be no atoms, or no stars. It is controversial whether this test has been passed; good arguments can be made on both sides.
The solution to the hierarchy problem involves a completely novel mechanism. Easy to say — but got any ideas? Arkani-Hamed gave us two examples of mechanisms which he had studied that he couldn’t make work — but perhaps someone else can do better. One is based on trying to apply notions related to self-organized criticality, but he was never able to make much progress. Another is based on an idea of Ed Witten’s that perhaps our world is best understood as one that
has two dimensions of space (not the obvious three)
is supersymmetric (which seems impossible, but in three dimensions supersymmetry and gravity together imply that particles and their superpartner particles need not have equal masses)
All of this seems completely contradictory with what we observe in our world. But! One of the important conceptual lessons from string theory [this is yet another example of something important that would not have been learned if people hadn’t actually been studying string theory] is that when forces become very strong, making the physics extremely complicated to describe, it is possible that a better description of that world becomes available — and that in some special cases, this better description has one additional dimension of space and weaker forces. In short, Witten’s idea is that our way of understanding our world, with three spatial dimensions, no apparent superymmetry and no extremely strong forces, might actually be simply an alternative and simpler description of a supersymmetric world with only two spatial dimensions with an extremely strong force. Arkani-Hamed, trying to apply this to the hierarchy problem, noticed this idea makes a prediction, but he showed that the prediction is false in the Standard Model, and it seems impossible to add any collection of particles that would make it true.
A well-dressed Nima Arkani-Hamed, waving his hands.
Greetings from Stony Brook’s Simon’s Center, and the SEARCH 2013 workshop. (I reported on the SEARCH 2012 workshop here, here, here and here.) Over the next three days, a small group (about 50) of theoretical particle physicists and experimentalists from ATLAS and CMS (two of the experiments at the Large Hadron Collider [LHC]) will be discussing the latest results from the LHC, and brainstorming about what else should be done with the existing LHC data and with future data.
The workshop was organized by three theorists, Raman Sundrum, professor at Maryland (who has opened the day with a characteristically brilliant and inspirational talk about the status of the field and the purpose of the workshop), Patrick Meade, professor at Stony Brook, and Michele Papucci, soon-to-be professor at Michigan.
Of course we’ll be discussing the newly discovered Higgs particle — that discussion will occupy most of today — but we’ll be also looking at many other types of particles, forces and other phenomena that nature might be hiding from us, and how we would be able to uncover them if they exist. There’ve been many dozens of searches done at both ATLAS and CMS, but the experimentalists certainly haven’t had time to try everything plausible — and theorists haven’t yet thought of everything they might try. Workshops like this are aimed at making sure no stones are left unturned in the existing huge pile of data from 2011-2012, and also that we’re fully prepared to deal with the new data, from higher-energy proton-proton collisions, that will start pouring in starting in 2015.
