Tag Archives: cms

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

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.

What’s the Status of the LHC Search for Supersymmetry?

It’s been quite a while (for good reason, as you’ll see) since I gave you a status update on the search for supersymmetry, one of several speculative ideas for what might lie beyond the known particles and forces.  Specifically, supersymmetry is one option (the most popular and most reviled, perhaps, but hardly the only one) for what might resolve the so-called “naturalness” puzzle, closely related to the “hierarchy problem” — Why is gravity so vastly weaker than the other forces? Why is the Higgs particle‘s mass so small compared to the mass of the lightest possible black hole?

Click here to read more about the current situation…

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.

Visiting the Host Lab of the Large Hadron Collider

Greetings from Geneva, and CERN, the laboratory that hosts the Large Hadron Collider [LHC], where the Higgs particle was found by the physicists at the ATLAS and CMS experiments. Between jet lag, preparing a talk for Wednesday, and talking to many experimental and theoretical particle physicists from morning til night, it will be a pretty exhausting week.

The initial purpose of this trip is to participate in a conference held by the LHCb experiment, entitled “Implications of LHCb measurements and future prospects.” Its goal is to bring theoretical particle physicists and LHCb experimenters together, to exchange information about what has been and what can be measured at LHCb.

On this website I’ve mostly written about ATLAS and CMS, partly because LHCb’s measurements are often quite subtle to explain, and partly because the Higgs particle search, the highlight of the early stage of the LHC, was really ATLAS’s and CMS’s task. But this week’s activities gives me a nice opportunity to put the focus on this very interesting experiment, which is quite different from ATLAS and CMS both in its design and in its goals, and to explain its important role.

ATLAS and CMS were built as general purpose detectors, whose first goal was to find the Higgs particle and whose second was to find (potentially rare) signs of any other high-energy processes that are not predicted by the Standard Model, the equations we use to describe all the known particles and forces of nature. Crudely speaking, ATLAS and CMS are ideal for looking for new phenomena in the 100 to 5000 GeV energy range (though we won’t reach the upper end of the range until 2015 and beyond.)

LHCb, by contrast, was built to study in great detail the bottom and charm quarks, and the hadrons (particles made from quarks, anti-quarks and gluons) that contain them. These quarks and their antiquarks are produced in enormous abundance at the LHC. They and the hadrons that contain them have masses in the 1.5 to 10 GeV/c² range… not much heavier than protons, and much lower than what ATLAS and CMS are geared to study. And this is why LHCb has been making crucial high-precision tests of the Standard Model using bottom- and charm-containing hadrons.  (Crucial, but not, despite repeated claims by the LHCb press office, capable of ruling out supersymmetry, which no single measurement can possibly do.)

Although this is the rough division of labor among these experiments, it’s too simplistic to describe the experiments this way. ATLAS and CMS can do quite a lot of physics at the low mass range, and in some measurements can compete well with LHCb.   Less well-known is that LHCb may be able to do a small but critical set of measurements involving higher energies than is their usual target.

LHCb is very different from ATLAS and CMS in many ways, and the most obvious is its shape. ATLAS and CMS look like giant barrels centered on the location of the proton-proton collisions, and are designed to measure as many particles as possible that are produced in the collision of two protons. LHCb’s shape is more like a wedge, with one end surrounding the collision point.

Left: Cut-away drawing of CMS, which is shaped like a barrel with proton-proton collisions occurring at its center.  ATLAS's shape is similar. Right: the LHCb experiment is shaped something like a wedge, with collisions occurring at one end.

Left: Cut-away drawing of CMS, which is shaped like a barrel with proton-proton collisions occurring at its center. ATLAS’s shape is similar. Right: Cut-away drawing of LHCb, which is shaped something like a wedge, with collisions occurring at one end.

This shape only allows it to measure those particle that go in the “forward” direction — close to the direction of one of the proton beams. (“Backward” would be near the other beam; the distinction between forward and backward is arbitrary, because the two proton beams have the same properties. “Central” would be far from either beam.) Unlike ATLAS and CMS, LHCb is not used to reconstruct the whole collision; many of the particles produced in the collision go into backward or central regions which LHCb can’t observe.  This has some disadvantages, and in particular put LHCb out of the running for the Higgs discovery. But a significant fraction of the bottom and charm quarks produced in proton-proton collisions go “forward” or “backward”, so a forward-looking design is fine if it’s bottom and charm quarks you’re interested in. And such a design is a lot cheaper, too. It also means that LHCb  is well positioned to make some other measurements where the forward direction is important. I’ll give you one or two examples later in the week.

To make their measurements of bottom and charm quarks, LHCb makes use of the fact that these quarks decay after about a trillionth of a second (a picosecond) [or longer if, as is commonly the case, there is significant time dilation due to Einstein's relativity effects on very fast particles].  This is long enough for them to travel a measurable distance — typically a millimeter or more. LHCb is designed to make the measurements of charged particles with terrific precision, allowing them to infer a slight difference between the proton-proton collision point, from which most low-energy charged particles will emerge, and the location where some other charged particles may have been produced in the decay of a bottom hadron or some other particle that travels a millimeter or more before decaying. The ability to do precision “tracking” of the charged particles makes LHCb sensitive to the presence of any as-yet unknown particles that might be produced and then decay after traveling a small or moderate distance. More on that later in the week.

A computer reconstruction of the tracks in a proton-proton collision measured by LHCb.  Most tracks start at the proton-proton collision point, but the two tracks drawn in purple emerge from a different point, the apparent location of the decay of a hadron containing a bottom quark.

A computer reconstruction of the tracks in a proton-proton collision, as measured by LHCb. Most tracks start at the proton-proton collision point at left, but the two tracks drawn in purple emerge from a different point about 15 millimeters away, the apparent location of the decay of a hadron, whose inferred trajectory is the blue line, and whose mass (measured from the purple tracks) indicates that it contained a bottom quark.

One other thing to know about LHCb; in order to make their precise measurements possible, and to deal with the fact that they don’t observe a whole collision, they can’t afford to have too many collisions going on at once. ATLAS and CMS have been coping with ten to twenty simultaneous proton-proton collisions; this is part of what is known as “pile-up”. But near LHCb the LHC beams are adjusted so that the number of collisions at LHCb is often limited to just one or two or three simultaneous collisions. This has the downside that the amount of data LHCb collected in 2011 was about 1/5 of what ATLAS and CMS each collected, while for 2012 the number was more like 1/10.  But LHCb can do a number of things to make up for this lower rate; in particular their trigger system is more forgiving than that of ATLAS or CMS, so there are certain things they can measure using data of a sort that ATLAS and CMS have no choice but to throw away.

Did the LHC Just Rule Out String Theory?!

Over the weekend, someone said to me, breathlessly, that they’d read that “Results from the Large Hadron Collider [LHC] have blown string theory out of the water.”

Good Heavens! I replied. Who fed you that line of rubbish?!

Well, I’m not sure how this silliness got started, but it’s completely wrong. Just in case some of you or your friends have heard the same thing, let me explain why it’s wrong.

First, a distinction — one that is rarely made, especially by the more rabid bloggers, both those who are string lovers and those that are string haters. [Both types mystify me.] String theory has several applications, and you need to keep them straight. Let me mention two.

  1. Application number 1: this is the one you’ve heard about. String theory is a candidate (and only a candidate) for a “theory of everything” — a silly term, if you ask me, for what it really means is “a theory of all of nature’s particles, forces and space-time”. It’s not a theory of genetics or a theory of cooking or a theory of how to write a good blog post. But it’s still a pretty cool thing. This is the theory (i.e. a set of consistent equations and methods that describes relativistic quantum strings) that’s supposed to explain quantum gravity and all of particle physics, and if it succeeded, that would be fantastic.
  2. Application number 2: String theory can serve as a tool. You can use its mathematics, and/or the physical insights that you can gain by thinking about and calculating how strings behave, to solve or partially solve problems in other subjects. (Here’s an example.) These subjects include quantum field theory and advanced mathematics, and if you work in these areas, you may really not care much about application number 1. Even if application number 1 were ruled out by data, we’d still continue to use string theory as a tool. Consider this: if you grew up learning that a hammer was a religious idol to be worshipped, and later you decided you didn’t believe that anymore, would you throw out all your hammers? No. They’re still useful even if you don’t worship them.

BUT: today we are talking about Application Number 1: string theory as a candidate theory of all particles, etc. Continue reading

A Discrepancy to Keep an Eye On

Today (as I sit in a waiting room for jury service) I’ll draw your attention to something that has been quite rare at the Large Hadron Collider [LHC]: a notable discrepancy between prediction and data.  (Too rare, in fact — when you make so many measurements, some of them should be discrepant; the one place we saw plenty of examples was in the search for and initial study of the Higgs particle.)  It’s not big enough to declare as a definite challenge to the Standard Model (the equations we use to describe the known particles and forces), but it’s one we’ll need to be watching… and you can bet there will be dozens of papers trying to suggest possibilities for what this discrepancy, if it is real, might be due to.

The discrepancy has arisen in the search at the CMS experiment for “multileptons”: for proton-proton collisions in which at least three charged leptons — electrons, muons and (to a degree) taus — were produced. Such events are a good place to look for new phenomena: very rare in the Standard Model, but in the context of some speculative ideas (including the possibility of additional types of Higgs particles, or of superpartner particles from supersymmetry, or new light neutral particles that decay sometimes to lepton/anti-lepton pairs, etc.) they can be produced in the decays of some unknown type of particle. Continue reading