Category Archives: LHC Background Info

Unexpected Decays of the Higgs Particle: What We Found

A few weeks ago, I reported on the completion of a large project, with which I’ve been personally involved, to investigate how particle physicists at the Large Hadron Collider [LHC] could be searching, not only in the future but even right now, for possible “Exotic Decays” of the newly-discovered Higgs particle .

By the term “exotic decays” (also called “non-Standard-Model [non-SM] Decays”), we mean “decays of this particle that are not expected to occur unless there’s something missing from the Standard Model (the set of equations we use to describe the known elementary particles and forces and the simplest possible type of Higgs field and its particle).”  I’ve written extensively on this website about this possibility (see herehere,  hereherehereherehere and here), though mostly in general terms. In our recent paper on Exotic Decays, we have gone into nitty-gritty detail… the sort of detail only an expert could love.  This week I’m splitting the difference, providing a detailed and semi-technical overview of the results of our work.  This includes organized lists of some of the decays we’re most likely to run across, and suggestions regarding the ones most promising to look for (which aren’t always the most common ones.)

Before I begin, let me again mention the twelve young physicists who were co-authors on this work, all of whom are pre-tenure and several of whom are still not professors yet.  [ When New Scientist reported on our work, they unfortunately didn’t even mention, much less list, my co-authors.] They are (in alphabetical order): David Curtin, Rouven Essig, Stefania Gori, Prerit Jaiswal, Andrey Katz, Tao Liu, Zhen Liu, David McKeen, Jessie Shelton, Ze’ev Surujon, Brock Tweedie, and Yi-Ming Zhong. Continue reading

Teaching at a “Winter School”

Professors at research universities engage in many different activities, and one which is little known to the public involves teaching at short and focused “schools” for graduate students. These schools, which generally last one to four weeks, and are usually (but not always) held outside the main academic year in winter or summer, allow these students to learn advanced topics in short courses that their universities wouldn’t be able to offer.

For instance, at most universities in the United States, a course focused on the theory of quarks and gluons (the set of equations known as “QCD”) would be attended by just a few students. And many universities don’t even have a professor who is truly expert on this subject. But when interested students from many universities are brought together at one of these specially organized schools, a world’s expert on QCD can teach a group of students as large as fifty or more. Not only is there economy of scale in this arrangement, it also helps to foster a future community among the students who attend. I myself went to one such school when I was a graduate student, and the faculty and students I met there include a number who are my professional peers today.

Usually, professors are not paid to teach at these schools, even though preparing a course is often a huge amount of work. There are two inducements, other than the satisfaction derived from the teaching itself. The first is that travel and lodging are free for the teacher; they are paid for by the organizers of the school, who in turn get the required funds from their university and/or government organizations. The latter (wisely, in my opinion) see such schools as having national value, in that they help assure a strong national research community in the future. The second is that the schools are often held in places where a person would not regret spending a week. The schools at which I have taught over the years have occurred in Boulder, Colorado (USA); Vancouver, British Columbia (Canada); Fermilab National Lab in Aurora, Illinois (USA); Cambridge, England; Kyoto, Japan; and Varna, Bulgaria. I’ve also taught in Italy, previously in the towns of Trieste and Erice, and this month in Florence (i.e. Firenze). For the next ten days or so, I’ll be at the Galileo Galilei Institute for Theoretical Physics (GGI), which is named, of course, after Florence’s most famous scientist.

(Several of my previous short courses are available in written or video form, and most are still sufficiently up-to-date to be useful to future experts. All of them assume, at least in large part, that a student has had a beginning course in quantum field theory. I can provide some links later this week if there is interest, though most of them easily show up in a web search.)

This is my first visit to the GGI, which is associated with the University of Florence, and is located on a hill a couple of miles from downtown Florence, not very far from where Galileo himself lived for some years. It was founded around 2006 to host focused research workshops, as well as brief schools. The theoretical particle physics graduate students at this school have already learned about dark matter from Tomer Volansky (a collaborator of mine on a trigger-related project), and about supersymmetry from David Shih (a former colleague at Rutgers and a recent collaborator on a supersymmetry/LHC project.) They’ll also be learning about the Higgs phenomenon and its generalizations from Raman Sundrum (who’s been mentioned many times on this blog, and whom I visited last month); about the physics of “flavor” — including the issue of how the six different types quarks transition from one to another via the weak nuclear force — from Gino Isidori; and about the physics of quarks and gluons from one of the world’s great experts, Stefano Catani. (You may not recognize these names, as none of them have written books for the public or developed a popular website or blog; but any expert in the theoretical particle physics knows them very well.) And last and perhaps least, they’ll be learning various bits of particle physics that one ought to know in the context of particle colliders, and particularly of the Large Hadron Collider [LHC], from me.

One corollary of this news is that I’ll be pretty busy for the next ten days, so I’m not sure how active the blog will really be. But I can promise you at least one post on string theory!

Our Survey of Exotic Decays of the Higgs is Done

After many months gestation and a difficult labor, a behemoth is born!  Yes, it’s done, finally: our 200 page tome entitled “Exotic Decays of the 125 GeV Higgs Boson“.  Written by thirteen hard-working theoretical particle physicists, this is a paper that examines a wide class of possible decays that our newly found Higgs particle might exhibit, but that would not occur if the Standard Model of particle physics (the equations we use to describe the known elementary particles and forces plus the simplest possible type of Higgs particle) were all there was to see at the Large Hadron Collider [LHC], the giant proton-proton collider outside of Geneva, Switzerland.  

[Non-experts; sorry, but this paper was written for experts, and probably has a minimum of two words of jargon per sentence. I promise you a summary soon.]

Why is looking for unusual and unexpected decays of the Higgs particle so important?  [I’ve written about the possibility of these “exotic” decays before on this website (see herehere,  hereherehereherehere and here).]  Because Higgs particles are sensitive creatures, easily altered, possibly in subtle ways, by interactions with new types of particles that we wouldn’t yet know about from the LHC or our other experiments. (This sensitivity of the Higgs was noted as far back to the early 1980s, though its generality was perhaps only emphasized in the last decade.)  The Higgs particle is very interesting not only on its own, for what it might reveal about the Higgs field (on which our very existence depends), but also as a potential opportunity for the discovery of currently unknown, lightweight particles, to which it might decay.  Such particles might be the keys to unlocking secrets of nature, such as what dark matter is, or maybe even (extreme speculation alert) the naturalness puzzle — very roughly, the puzzle of why the mass of the Higgs particle can be so small compared to the masses of the smallest possible black holes.

The goal of our paper, which is extensive in its coverage (though still not comprehensive — this is a very big subject) is to help our experimental colleagues at ATLAS and CMS, the general purpose experiments at the LHC, decide what to search for in their current (2011-2012) and future (2015-) data, and perhaps assist in their decisions on triggering strategies for the data collecting run that will begin in 2015.  (Sorry, LHCb folks, we haven’t yet looked at decays where you’d have an advantage.) And we hope it will guide theorists too, by highlighting important unanswered questions about how to look for certain types of exotic decays.  Of course the paper has to go through peer review before it is published, but we hope it will be useful to our colleagues immediately. Time is short; 2015 is not very far away.

Although our paper contains some review of the literature, a number of its results are entirely new.  I’ll tell you more about them after I’ve recovered, and probably after most people are back from break in January.  (Maybe for now, as a teaser, I’ll just say that one of the strongest limits we obtained, as an estimate based on reinterpreting published ATLAS and CMS data, is that no more than a few × 10-4 of Higgs particles decay to a pair of neutral spin-one particles with mass in the 20 – 62 GeV/c2 range… and the experimentalists themselves, by re-analyzing their data, could surely do better than we did!)  But for the moment, I’d simply like to encourage my fellow experts, both from the theory side and the experimental side, to take a look… comments are welcome.

Finally, I’d like to congratulate and thank my young colleagues, all of whom are pre-tenure and several of whom are still not professors yet, on their excellent work… it has been a pleasure to collaborate with them.  They led the way, not me.  They are (in alphabetical order): David Curtin, Rouven Essig, Stefania Gori, Prerit Jaiswal, Andrey Katz, Tao Liu, Zhen Liu, David McKeen, Jessie Shelton, Ze’ev Surujon, Brock Tweedie, and Yi-Ming Zhong. They hail from around the world, but they’ve worked together like family… a great example of how our international effort to understand nature’s deep mysteries brings unity of purpose from a diversity of origins.

Visiting the University of Maryland

Along with two senior postdocs (Andrey Katz of Harvard and Nathaniel Craig of Rutgers) I’ve been visiting the University of Maryland all week, taking advantage of end-of-academic-term slowdowns to spend a few days just thinking hard, with some very bright and creative colleagues, about the implications of what we have discovered (a Higgs particle of mass 125-126 GeV/c²) and have not discovered (any other new particles or unexpected high-energy phenomena) so far at the Large Hadron Collider [LHC].

The basic questions that face us most squarely are:

Is the naturalness puzzle

  1. resolved by a clever mechanism that adds new particles and forces to the ones we know?
  2. resolved by properly interpreting the history of the universe?
  3. nonexistent due to our somehow misreading the lessons of quantum field theory?
  4. altered dramatically by modifying the rules of quantum field theory and gravity altogether?

If (1) is true, it’s possible that a clever new “mechanism” is required.  (Old mechanisms that remove or ameliorate the naturalness puzzle include supersymmetry, little Higgs, warped extra dimensions, etc.; all of these are still possible, but if one of them is right, it’s mildly surprising we’ve seen no sign of it yet.)  Since the Maryland faculty I’m talking to (Raman Sundrum, Zakaria Chacko and Kaustubh Agashe) have all been involved in inventing clever new mechanisms in the past (with names like Randall-Sundrum [i.e. warped extra dimensions], Twin Higgs, Folded Supersymmetry, and various forms of Composite Higgs), it’s a good place to be thinking about this possibility.  There’s good reason to focus on mechanisms that, unlike most of the known ones, do not lead to new particles that are affected by the strong nuclear force. (The Twin Higgs idea that Chacko invented with Hock-Seng Goh and Roni Harnik is an example.)  The particles predicted by such scenarios could easily have escaped notice so far, and be hiding in LHC data.

Sundrum (some days anyway) thinks the most likely situation is that, just by chance, the universe has turned out to be a little bit unnatural — not a lot, but enough that the solution to the naturalness puzzle may lie at higher energies outside LHC reach.  That would be unfortunate for particle physicists who are impatient to know the answer… unless we’re lucky and a remnant from that higher-energy phenomenon accidentally has ended up at low-energy, low enough that the LHC can reach it.

But perhaps we just haven’t been creative enough yet to guess the right mechanism, or alter the ones we know of to fit the bill… and perhaps the clues are already in the LHC’s data, waiting for us to ask the right question.

I view option (2) as deeply problematic.  On the one hand, there’s a good argument that the universe might be immense, far larger than the part we can see, with different regions having very different laws of particle physics — and that the part we live in might appear very “unnatural” just because that very same unnatural appearance is required for stars, planets, and life to exist.  To be over-simplistic: if, in the parts of the universe that have no Higgs particle with mass below 700 GeV/c², the physical consequences prevent complex molecules from forming, then it’s not surprising we live in a place with a Higgs particle below that mass.   [It’s not so different from saying that the earth is a very unusual place from some points of view — rocks near stars make up a very small fraction of the universe — but that doesn’t mean it’s surprising that we find ourselves in such an unusual location, because a planet is one of the few places that life could evolve.]

Such an argument is compelling for the cosmological constant problem.  But it’s really hard to come up with an argument that a Higgs particle with a very low mass (and corresponding low non-zero masses for the other known particles) is required for life to exist.  Specifically, the mechanism of “technicolor” (in which the Higgs field is generated as a composite object through a new, strong force) seems to allow for a habitable universe, but with no naturalness puzzle — so why don’t we find ourselves in a part of the universe where it’s technicolor, not a Standard Model-like Higgs, that shows up at the LHC?  Sundrum, formerly a technicolor expert, has thought about this point (with David E. Kaplan), and he agrees this is a significant problem with option (2).

By the way, option (2) is sometimes called the “anthropic principle”.  But it’s neither a principle nor “anthro-” (human-) related… it’s simply a bias (not in the negative sense of the word, but simply in the sense of something that affects your view of a situation) from the fact that, heck, life can only evolve in places where life can evolve.

(3) is really hard for me to believe.  The naturalness argument boils down to this:

  • Quantum fields fluctuate;
  • Fluctuations carry energy, called “zero-point energy”, which can be calculated and is very large;
  • The energy of the fluctuations of a field depends on the corresponding particle’s mass;
  • The particle’s mass, for the known particles, depends on the Higgs field;
  • Therefore the energy of empty space depends strongly on the Higgs field

Unless one of these five statements is wrong (good luck finding a mistake — every one of them involves completely basic issues in quantum theory and in the Higgs mechanism for giving masses) then there’s a naturalness puzzle.  The solution may be simple from a certain point of view, but it won’t come from just waving the problem away.

(4) I’d love for this to be the real answer, and maybe it is.  If our understanding of quantum field theory and Einstein’s gravity leads us to a naturalness problem whose solution should presumably reveal itself at the LHC, and yet nature refuses to show us a solution, then maybe it’s a naive use of field theory and gravity that’s at fault. But it may take a very big leap of faith, and insight, to see how to jump off this cliff and yet land on one’s feet.  Sundrum is well-known as one of the most creative and fearless individuals in our field, especially when it comes to this kind of thing. I’ve been discussing some radical notions with him, but mostly I’ve been enjoying hearing his many past insights and ideas… and about the equations that go with them.   Anyone can speculate, but it’s the equations (and the predictions, testable at least in principle if not in practice, that you can derive from them) that transform pure speculations into something that deserves the name “theoretical physics”.

Wednesday: Sean Carroll & I Interviewed Again by Alan Boyle

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.

Sean Carroll is a Caltech physicist, astrophysicist, writer and speaker, blogger at Preposterous Universe, who recently completed an excellent and now prize-winning popular book (which I highly recommend) on the Higgs particle, entitled “The Particle at the End of the Universe“.  Our interviewer Alan Boyle is a noted science writer, author of the book “The Case for Pluto“, winner of many awards, and currently NBC News Digital’s science editor [at the blog  “Cosmic Log“].

Sean and I were interviewed in February by Alan on this program; here’s the link.  I was interviewed on Virtually Speaking Science once before, by Tom Levenson, about the Large Hadron Collider (here’s the link).  Also, my public talk “The Quest for the Higgs Particle” is posted in their website (here’s the link to the audio and to the slides).

Quantum Field Theory, String Theory, and Predictions (Part 7)

Appropriate for Advanced Non-Experts

[This is the seventh post in a series that begins here.]

In the last post in this series, I pointed out that there’s a lot about quantum field theory [the general case] that we don’t understand.  In particular there are many specific quantum field theories whose behavior we cannot calculate, and others whose existence we’re only partly sure of, since we can’t even write down equations for them. And I concluded with the remark that part of the reason we know about this last case is due to “supersymmetry”.

What’s the role of supersymmetry here? Most of the time you read about supersymmetry in the press, and on this website, it’s about the possible role of supersymmetry in addressing the naturalness problem of the Standard Model [which overlaps with and is almost identical to the hierarchy problem.] But actually (and I speak from personal experience here) one of the most powerful uses of supersymmetry has nothing to do with the naturalness problem at all.

The point is that quantum field theories that have supersymmetry are mathematically simpler than those that don’t. For certain physical questions — not all questions, by any means, but for some of the most interesting ones — it is sometimes possible to solve their equations exactly. And this makes it possible to learn far more about these quantum field theories than about their non-supersymmetric cousins.

Who cares? you might ask. Since supersymmetry isn’t part of the real world in our experiments, it seems of no use to study supersymmetric quantum field theories.

But that view would be deeply naive. It’s naive for three reasons. Continue reading

Off to Illinois’s National Labs For a Week of Presentations

I have two very different presentations to give this week, on two very similar topics. First I’m going to the LHC Physics Center [LPC], located at the Fermilab National Accelerator Laboratory, host of the now-defunct Tevatron accelerator, the predecessor to the Large Hadron Collider [LHC]. The LPC is the local hub for the United States wing of the CMS experiment, one of the two general-purpose experiments at the LHC. [CMS, along with ATLAS, is where the Higgs particle was discovered.] The meeting I’m attending is about supersymmetry, although that’s just its title, really; many of the talks will have implications that go well beyond that specific subject, exploring more generally what we have and still could search for in the LHC’s existing and future data.  I’ll be giving a talk for experts on what we do and don’t know currently about one class of supersymmetry variants, and what we should be perhaps be trying to do next to cover cases that aren’t yet well-explored.

Second, I’ll be going to Argonne National Laboratory, to give a talk for the scientists there, most of whom are not particle physicists, about what we have learned so far about nature from the LHC’s current data, and what the big puzzles and challenges are for the future.  So that will be a talk for non-expert scientists, which requires a completely different approach.

Both presentations are more-or-less new and will require quite a bit of work on my part, so don’t be surprised if posts and replies to comments are a little short on details this week…

At a CMS/Theory Workshop in Princeton

For Non-Experts Who've Read a Bit About Particle Physics

I spent yesterday, and am spending today, at Princeton University, participating in a workshop that brings together a group of experts from the CMS experiment, one of the two general purpose experiments at the Large Hadron Collider (where the Higgs particle was discovered.) They’ve invited me, along with a few other theoretical physicists, to speak to them about additional strategies they might use in searching for phenomena that are not expected to occur within the Standard Model (the equations we use to describe the known elementary particles and forces.) This sort of “consulting” is one of the roles of theorists like me. It involves combining a broad knowledge of the surprises nature might have in store for us with a comprehensive understanding of what CMS and its competitor ATLAS (as well as other experiments at and outside the LHC) have and have not searched for already.

A lot of what I’ll have to say is related to what I said in Stony Brook at the SEARCH workshop, but updated, and with certain details adjusted to match the all-CMS audience.

Yesterday afternoon’s back-and-forth between the theorists and the experimentalists was focused on signals that are very hard to detect directly, such as (still hypothetical) dark matter particles. These could perhaps be produced in the LHC’s proton-proton collisions, but could then go undetected, because (like neutrinos) they pass without hitting anything inside of CMS. But even though we can’t detect these particles directly, we can sometimes tell indirectly that they’re present, if the collision simultaneously makes something else that recoils sharply away from them. That sometime else could be a photon (i.e. a particle of light) or a jet (the spray of particles that tells you that a high-energy gluon or quark was produced) or perhaps something else. There was a lot of interesting discussion about the various possible approaches to searching for such signals more effectively, and about how the trigger strategy might need to be adjusted in 2015, when the LHC starts taking data again at higher energy per collision, so that CMS remains maximally sensitive to their presence. Clearly there is much more work to do on this problem.