Guest Post: Matt Strassler on Hunting for the Higgs

Perhaps you’ve heard of the Higgs boson. Perhaps you’ve heard the phrase “desperately seeking” in this context. We need it, but so far we can’t find it. This all might change soon — there are seminars scheduled at CERN by both of the big LHC collaborations, to update us on their progress in looking for the Higgs, and there are rumors they might even bring us good news. You know what they say about rumors: sometimes they’re true, and sometimes they’re false.

So we’re very happy to welcome a guest post by Matt Strassler, who is an expert particle theorist, to help explain what’s at stake and where the search for the Higgs might lead. Matt has made numerous important contributions, from phenomenology to string theory, and has recently launched the website Of Particular Significance, aimed at making modern particle physics accessible to a wide audience. Go there for a treasure trove of explanatory articles, growing at an impressive pace.

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After this year’s very successful run of the Large Hadron Collider (LHC), the world’s most powerful particle accelerator, a sense of great excitement is beginning to pervade the high-energy particle physics community. The search for the Higgs particle… or particles… or whatever appears in its place… has entered a crucial stage.

We’re now deep into Phase 1 of this search, in which the LHC experiments ATLAS and CMS are looking for the simplest possible Higgs particle. This unadorned version of the Higgs particle is usually called the Standard Model Higgs, or “SM Higgs” for short. The end of Phase 1 looks to be at most a year away, and possibly much sooner. Within that time, either the SM Higgs will show up, or it will be ruled out once and for all, forcing an experimental search for more exotic types of Higgs particles. Either way, it’s a turning point in the history of our efforts to understand nature’s elementary laws.

This moment has been a long time coming. I’ve been working as a scientist for over twenty years, and for a third decade before that I was reading layperson’s articles about particle physics, and attending public lectures by my predecessors. Even then, the Higgs particle was a profound mystery. Within the Standard Model (the equations that used at the LHC to describe all the particles and forces of nature we know about so far, along with the SM Higgs field and particle) it stood out as a bit different, a bit ad hoc, something not quite like the others. It has always been widely suspected that the full story might be more complicated. Already in the 1970s and 1980s there were speculative variants of the Standard Model’s equations containing several types of Higgs particles, and other versions with a more complicated Higgs field and no Higgs particle — with a key role of the Higgs particle being played by other new particles and forces.

But everyone also knew this: you could not simply take the equations of the Standard Model, strip the Higgs particle out, and put nothing back in its place. The resulting equations would not form a complete theory; they would be self-inconsistent. Though still effective in many contexts, they would become useless for predicting certain high-energy processes, including ones that the LHC, a few years from now, will directly study. So it was widely known, over thirty years ago, that something like a Higgs particle had to be in those equations to make them sensible. In fact, the condition is even stronger than that. The equations of the Standard Model will require significant and historic modifications unless the Standard Model Higgs particle is found with a mass below about 800 GeV/c2. (For scale, the mass of a hydrogen atom is about 1 GeV/c2.)

It’s this last point that explains why the current moment is such a critical one. Sure, previous experiments have looked for the Higgs particle too. And they were able to sweep some areas clean; we know, from these experiments, that the mass of the SM Higgs cannot lie below 115 GeV/c2. But the LHC is special; it is the first accelerator capable of finding the SM Higgs over the entire allowed mass range still remaining, from 115 up to and beyond 800 GeV/c2.

Phase 1, the search for the SM Higgs, is the easy part of the quest for the Higgs particle (or particles or whatever). What makes it easy? The Standard Model equations are so detailed and well-specified that everything about the SM Higgs particle is already known, except for one thing: its mass. More precisely, if you told me the SM Higgs particle’s mass, I could tell you how it is produced at the LHC and at what rate, and what it decays to, and how often it decays to one set of particles rather than another. This makes life relatively simple for the experimenters at ATLAS and CMS, because all they have to do is this: pick a mass in the allowed range, ask theorists to calculate the properties of an SM Higgs particle of that mass, figure out the best way to seek it in their data, and look at the data: is there evidence for or against its presence? They must then repeat this across the entire range of possible masses systematically, until they’ve covered all the allowed territory. (Actually they do all of these searches simultaneously, not sequentially.) When their coverage is complete — or when they find something — Phase 1 is over.

  • Current status of the search for the Standard Model Higgs particle, showing that the LHC's November results, using the summer's data, leave a gap from 115 to 141 GeV/c2, as well as a region above 470 GeV/c2 that is disfavored by other considerations.
  • It is useful to think of Phase 1 as three subprojects, going on all at the same time but proceeding at different rates, involving the search strategies for a lightweight, middleweight and heavyweight SM Higgs. We’re almost done with the middleweight case, the easy one, in which one looks mainly for a Higgs decaying (i.e., disintegrating) to two W particles or two Z particles. The entire range from 141 to 470 GeV/c2 is now excluded (according to the combination of the summer’s data from ATLAS and CMS that was announced a few weeks ago). The lightweight range, down to 115 GeV/c2, is dominated by a search for a Higgs particle decaying to two photons. To observe this decay, a very rare process, requires a lot of data, so exploring this range fully will take another six months to a year. But we should already learn more about the lightweight Higgs on December 13th, when CERN will be providing an update on the Higgs search.

  • A lightweight Standard Model Higgs particle must be found through the very rare process in which it disintegrates ("decays") into two particles of light (photons). Discovery of a medium- or heavyweight Standard Model Higgs particle can be made using its common decays into two W particles or two Z particles (each of which immediately decays into two other particles).
  • The heavyweight range — above 450 GeV/c2 or so — is a little more subtle. Many theorists argue this window is already closed, by indirect experimental evidence. There are processes, carefully measured over the past 20 years, that are indirectly sensitive to the mass of the SM Higgs, and that strongly suggest it should be on the lighter side… below something like 300-400 GeV/c2, though reasonable people might disagree on where exactly to set this bound. But even if you didn’t buy this powerful argument, it wouldn’t trouble the experiments. Depending upon exactly how much data the LHC takes in 2012, we should see most of the heavyweight range explored experimentally by late next year. The experimental results, combined with the theoretical arguments, should allow Phase 1 to conclude, to the satisfaction of almost all experts, once the 2012 data is fully analyzed.

    So what are the possible outcomes of Phase 1?

    1) The SM Higgs particle, already known with substantial confidence not to be in the middleweight range, might turn up in the lightweight or heavyweight range.
    2) The SM Higgs particle might be entirely excluded, from 115 up to 800 GeV/c2 or so. (Remember, though, that this would not mean there is no Higgs particle of any type — it would mean only that the simplest type is not found in nature.)
    3) A Higgs-like particle that is clearly not a Standard Model Higgs particle (because it has the wrong production rates, or the wrong decay rates, given its mass) might be found instead.
    3a) Some other great discovery at the LHC might move the SM Higgs search off the front pages for a while.

    What would be the pros and cons of these different scenarios?

    1) If the SM Higgs is found, that will be a historic discovery by the LHC, provisionally confirming the Standard Model’s simplest Higgs. That said, in some ways it will be a bit disappointing, since the Standard Model leaves many important questions in particle physics unanswered, and only by finding flaws in its equations do we have much hope of answering those questions.

    2) If the SM Higgs is excluded, that will be an even more historic discovery, implying that the Standard Model’s equations are not the complete story at the LHC. For most particle physicists, this will be a much more exciting outcome! There will be a great opportunity for the LHC to teach us something profound about nature that we may not currently even suspect, although we’ll be on tenderhooks, potentially for quite a while, wondering whether the LHC’s data will provide clear guidance as to how to modify the Standard Model, or only give us some suggestive hints.

    3) If a new non-SM Higgs particle is found, that will be the best possible outcome! Not only will we see the Standard Model’s equations fail, we’ll have a direct clue, in the form of the new particle, as to how to begin modifying them. In this case the LHC will immediately help us to start writing the new chapter in particle physics textbooks.

    3a) And if something else unexpected is found in LHC data in the meantime, no one will complain! This of course would also mean the failure of the Standard Model’s equations, and new clues into nature’s mysteries.

    You may have noticed that on this list there’s really no bad outcome. That’s right: as long as there are no technical problems at the LHC that limit the amount of data it collects in 2012, we are in a no-lose situation in the short term. This does not happen very often! And this is why there’s so much excitement right now in the field. We’re not wondering if we’ll get some historic information over the next year or so, or whether it will change the field of particle physics. We’re just wondering what it will be. (Needless to say, we’re all wishing the accelerator physicists and engineers over at the LHC, who keep the machine running efficiently, the very, very best — and while we’re at it, let’s hear a big round of applause for them right now, for what they’ve achieved in 2011!)

  • A highly simplified illustration of how Phase 2 of the Higgs search will proceed, depending on the outcome of Phase 1.
  • No matter what, there will be a very important Phase 2 to the Higgs search, which will extend for perhaps ten years beyond Phase 1. If Phase 1 finds something that looks like an SM Higgs particle, Phase 2 will be all about checking its details with high precision. The new particle may look at first as though it is just what the simplest version of the Higgs story would predict, but if even the slightest detail is out of place, it would show the Higgs is not so simple after all, which would be an exciting turn of events. If instead Phase 1 rules out the SM Higgs, then a great host of new search strategies will be brought to bear, and the experimentalists will (figuratively) fan out like a massive search party, looking for all varieties of exotic types of Higgs particles. Also — remembering that there may be no Higgs particle, but if so, the Standard Model’s equations can’t be entirely right — they’ll step up their efforts to look for the many other types of particles and forces that we might need to add to the Standard Model to make its equations sensible again, absent any type of Higgs particle in nature. And if a non-SM Higgs particle is found in Phase 1, Phase 2 will involve all of these strategies at once, producing the new Higgs particle in large quantities and studying it in detail, while looking for more clues as to what else is missing from the Standard Model’s equations.

    It has been decades since a moment in particle physics looked as bright as do the next couple of years (healthy LHC operations permitting.) We’ve seen turning points in other fields: the human genome project was guaranteed to revolutionize genetics and genomics, and the study of the cosmic microwave background radiation was guaranteed to change our understanding of the early universe, as long as the experimental methods worked. These were great historic achievements that gave new life to those subjects; but we do not yet know whether the same is in store for particle physics. Will the flash of new understanding provided by Phase 1 quickly fade, or will it brighten into a dawn of a new age? Will the Standard Model’s equations work perfectly at the LHC, giving us a sense of satisfaction but no clues for future understanding? Will the equations fail, but in an obscure fashion, leaving us uncertain as to how to fix them? Or will their failure be clear and instructive, as has been the case for so many sets of equations before them, allowing the LHC, along perhaps with other experiments, to provide us with the insights we need to proceed to a more profound understanding of nature?

    There is only one way to find out: run the experiment, and let nature speak.

    So keep your eyes on Phase 1 of the Higgs search as it progresses toward a conclusion over the coming weeks and months. If the LHC works as hoped, the year ahead will be a memorable one.

    56 Comments

    56 thoughts on “Guest Post: Matt Strassler on Hunting for the Higgs”

    1. Really like the cogency and brevity of the explanation, Matt. Thanks a lot for making this reasonably understandable. Also like the last paragraph of #25- the realization that we don’t know the absolute truth is so the truth!!!

    2. So, when you find the higgs, and i’m sure you will, because you have to have something to show for your efforts and justify all the money you’ve used to do so. Then there will be another exotic something-or-other you’ll come up with to extend your job a little longer. Why don’t you get a real job and give the taxpayer a break?

    3. prentice (27) did you use your cell phone today? I imagine that your grandfather probably told the experimentalists back then to stop wasting his taxpayer money on something as obviously useless as atomic physics and quantum mechanics. Scientific research is an investment in the future; over time it tends to pay for itself many times over.

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    11. Matt (comment 24) – my thesis would be, first, that the relative neglect of the Koide relation has been a lost opportunity. Consider the subtle chain of deductions reviewed e.g. in arXiv:hep-ph/0207124, whereby the paradigm of beyond-standard-model physics evolved from GUT to supersymmetric GUT, on the basis of what is really a very small number of hints, like the near-unification of coupling constants at high energy. The Koide relation is another such hint, a huge one, and Rivero’s extension of it strengthens the case enormously.

      Where would we be by now, if in the lore and culture of physics, it had been considered as important a clue as the GUT-scale relationships? Presumably, the sort of models that Yoshio Koide himself has assembled in his very recent papers, would have been studied perhaps in the mid-1990s (around the same time the SSC would have been doing what the LHC is doing now, if it had been built!).

      My opinion is that Rivero’s paper is a turning point, that it contains a discovery with profound implications for physics, and that under slightly different historical conditions, we could have been exploring those implications for 15 years by now. There would have been predictions to be tested; who knows how much would have been figured out already, that will instead only be explored in 2012 and beyond. Obviously it’s more important to get on with that, than to puzzle over how things might have been different. But I dissented because physics really could have come a lot further than it has, without any new data at all.

    12. Sorry, the true and fundamental/general unification of physics begins with gravity and involves the direct experience of the body. Physics necessarily takes place in and with time.
      Any true/fundamental unification of physics FUNDAMENTALLY demonstrates/incorporates instantaneity. Ultimate truth in physics is not found in what is inanimate, and nature prefers low energy. Our growth and becoming other than we are are fundamental. We all originate (and grow) at/from the center of the human body; and we CONTINUE to grow (and live, of course — ordinarily) AFTER birth. Think, we cannot outsmart nature (as it occurs naturally). Now, witness the following. Here is real progress in physics:

      Balanced attraction and repulsion that involves balanced and equivalent inertia and gravity is the requirement of fundamentally unifying gravity and electromagnetism. Both gravity and inertia must [necessarily] be at half strength/force for such a union to occur. This is required of quantum gravity as well. This can only be done by making space equally (and both) visible and invisible. Opposites must be combined, included, and balanced. Gravity enjoins and balances invisible and visible space. Space must be contracted/flattened and stretched/expanded in an equivalent and balanced fashion.

      All of this occurs in/as dream experience. Full gravity is full distance in/of space. Just look directly downward at the ground while standing.

      The problem with modern physics is that is tries to be at y and z, when it is not even at A through v. You always begin with the basics and with typical/ordinary experience.

      Mathematics cannot fundamentally and ultimately combine, include, and balance opposites. That is obvious. The ultimate understanding of physics combines, balances, and includes opposites. Dreams fundamentally combine and include opposites. Dreams generally and fundamentally unify physics.

      Gravity and inertia are both fundamental to distance in/of space. The visible AND YET INVISIBLE equivalency and balancing of inertial/gravitational space in dreams even allows for vision, as this is evident in the invisible and visible space of/inside the body/eye while waking. (Vision begins invisibly inside the body/eye.) HALF GRAVITY AND HALF INERTIA ARE EQUALLY (AND BOTH) VISIBLE AND INVISIBLE IN DREAMS in keeping with the middle distance in/of space and middle force/energy. Indeed, the space [as a whole/generally] IS semi-visible/semi-invisible in dreams. The space in dreams is equally (and it is both) visible and invisible.

      Author Frank Martin DiMeglio has generally and fundamentally unified physics for all time.

    13. I guess the large number of eager Einsteins out there is another reason why any genuine breakthrough by an unknown or an outsider may get overlooked for a long time…

    14. Hi, Matt has not told me that he was trying to thumb a hitchhike for us in this comment thread but I found it via google anyway.

      I can not speak by him, but I think that the point –besides plain pressure for people to think on the implications of the mass ladder described in my preprint– is that no enough effort has been done in the recent years to understand the structure of the standard model.

      In this goal, t seems to exist a lag since 1984 till now, when fortunately the experiments are giving again some motivation to model builders (The only exception I can think during all this time is the work of Connes, and even this case it was a mathematician, an ousider to HEP). What is amazing is that the period previous to the lag was full of highly quoted activity. There was preon models, and there was GUT groups, and Susy GUT and SuGRA. But also no-so-GUT groups, as Pati Salam for instance, and also sometimes the extension of the SM was only a technique to pinpoint some special symmetry, as B-L or as the discrete permutations that allowed to justify the empirical link betweeb Cabibbo angle and some quark masses. In particular for this last research it is amazing how many big names were involved in it, a try that nowadays should sell as nonsensical in the mainstream, and it was -jointly with preons- the initial inspiration for the models of Koide.

      And perhaps the most spectacular halt of the period is at the end, when the standard model gauge group is found (by Witten) to be one of the few possible symmetries of seven dimensional manifols, and then it is forgotten; in this case it can not even be argued that the field have been exhausted. It is not an stagment, it is a crash.

      So, probably we have left a lot of things half-baked in the pre-1984 ovens, and a lot of spirit with them. If we kept on the belief that science evolved consistently in the last 25 years, filling the holes, if we believe that we are pretty done and that all we can hope is for new experimental input, I am afraid that we will not have the enough spirit to answer to the results of the LHC, specially if they are too much consistent with the actual corpus of knowledge.

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    16. Great post and exciting times! I’ve just finished reading the book about the Higgs Boson (www.popsciencebooks.com/physics-2/massive-the-missing-particle-that-sparked-the-greatest-hunt-in-science) and am very curious to know if they have really found this particle!

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    18. In this context of our “desperate search” for the Higgs particle, please ‘take five’ to review the fundamentals here of the issue. Consider a photon of energy E and frequency f hertz, or cycles per second, moving from a source atom to a detector atom. This momentum-packing photon would come a-knocking on the detector atom, detectably at f knocks per second. Let us call this individual quantum-energy entity which effects a single knock on the detector – the RADIATON (note spelling!). In the macroscopic world, this would be analogous to a machine-gun (source) firing at a target (detector)at f shots, or bullets, each second. Each bullet, packing momentum, can thus be correlated back to the radiaton, or “per-cycle” quantum particle of the photon.
      Thus, we may define the radiaton as the per-cycle quantum particle of the photon
      According to quantum theory,
      E = hf ……………………..(1)

      and,
      E = mc^2 ……………….(2)
      Eq. (1) tells us,
      Photon energy = an absolute constant (h)x cycles/unit of time.
      Transposing the equation,
      energy/cycles = absolute constant/unit of time
      In other words,
      energy per cycle, which is energy of the radiaton = an absolute constant.
      And Eq. (2) tells us, energy and energy are equivalent and transmutable.
      Hence,
      Mass per cycle, which would be mass of the radiaton = an absolute constant.

      From the above, and as shown further in Section 2 of: http://www.sittampalam.net/Synopsis.htm,
      the radiaton is the ultimate quantum of the singular entity: MASS-ENERGY, which fundamentally (and irreducibly) constitutes all of matter and energy in our universe (and, moreover, packs the singular and fundamental clout to effect – and unify – ALL of the forces of nature!).
      Thus, say,energy, or mass, or mass-energy. of a photon would simply be its radiaton number and mass, or energy, or mass0energy, of an atom would be its radiaton number.
      Hence, we would have no need for the Higgs field, or whatever else, other than the radiaton, to give mass to matter.
      Sorry, but the desperate search here for the Higgs is simply a wild goose chase and a terrible waste of top-of-the-cream human resource, cost notwithstanding.
      Thank you all for your precious time here. Merry & Holy Christmas!

    19. Truth (and truth in/with physics) is NOT ultimately found in the inanimate. Truth is found in/with the integrated and interactive extensiveness of being, experience, and thought.

      When this is fully and properly understood, it readily becomes clear why modern physics is so lost/off.

      How can ultimate, fundamental, and extensive truth (in physics too) be found in what is inanimate and unnatural and/or in what precludes, reduces, and/or disengages/detaches (or eliminates) life, our thought, our being, space/vision, and our growth?

      Gravity and electromagnetism are united given equivalent and balanced inertia and gravity (both at half force/energy strength.)

    20. More definitive and inescapable proof as to why the Higgs search is plain wrong and a waste.

      Real unification in physics begins with gravity. Gravity, invisible and visible, is key to distance in/of space. Gravity and electromagnetism are united given equivalent and balanced inertia and gravity (both at half force/energy strength) and equivalent/balanced attraction and repulsion. Moreover, gravity and inertia and electromagnetism are all fundamental to distance in/of space. This would then demonstrate space as equally (and both) invisible and visible in conjunction with the FUNDAMENTAL (or thoroughly extensive/complete) demonstration and inclusion of instantaneity. Fundamentally and importantly, gravity enjoins AND balances invisible and visible space. This is how we can attain FUNDAMENTAL gravitational/electromagnetic/inertial equilibrium and equivalency. Gravity must be shown as reduced to the extent that inertia is increased. And there we have it all.

      We must know and identify what are the ideal/theoretical/true requirements of unification in physics.

      The best/true unification of physics fundamentally, completely, and thoroughly demonstrates and includes instantaneity.

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    22. Hopefully this question will not be completely buried under the “interesting” “theories” above and someone might consider a response. I’m a layman and have wondered the following: If the Higgs is found, what does this imply for the Equivalence Principle? It’s my understanding that the Equivalence Principle very strongly suggests that gravity and which creates inertia leave us with an inexplicable coincidence in its equivalence with gravity? Is this question misguided or am I missing something obvious?

    23. I somehow mangled that text. The questions should have read: “If the Higgs is found, what does this imply for the Equivalence Principle? It’s my understanding that the Equivalence Principle strongly suggests that gravity and inertia are essentially the same thing. If the Higgs produces inertia, doesn’t that leave us with an inexplicable coincidence in its equivalence with gravity?”

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    25. Here is why the Higgs search will never generally and fundamentally unify physics.

      Fundamentally enjoining and balancing invisible and visible space — in conjunction with inertial, gravitational, and electromagnetic equivalency and equilibrium/balancing — makes force/energy fundamentally potential and actual in keeping with the fundamental and true/extensive inclusion of instantaneity. Inertia and gravity are then (and they have to be) at half strength force/energy to fundamentally/truly unify them. (Note that gravity is reduced to the extent that inertia is increased in keeping with balanced and equivalent attraction and repulsion in dreams.) Space must be equally, and both, invisible and visible in keeping with [fundamental] instantaneity. This hits rock bottom reality in physics, as dreams are now clearly proven to fundamentally and generally unify physics.

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