Tag Archives: detectors

Dark Matter Debates

Last week I attended the Eighth Harvard-Smithsonian Conference on Theoretical Astrophysics, entitled “Debates on the Nature of Dark Matter”, which brought together leading figures in astronomy, astrophysics, cosmology and particle physics. Although there wasn’t much that was particularly new, it was a very useful conference for taking stock of where we are. I thought I’d bring you a few selected highlights that particularly caught my eye. Continue reading

LHC Producing 8 TeV Data

Still early days in the 2012 data-taking run, which just started a couple of weeks ago, but already the Large Hadron Collider [LHC] accelerator wizards, operating the machine at 8 TeV of energy per proton-proton collision (compared to last year’s 7 TeV) have brought the collision rates back up nearly to where they were last year.    This is very good news, in that it indicates there are no significant unexpected technical problems preventing the accelerator from operating at the high collision rates that are required this year.   And the experiments are already starting to collect useful data at 8 TeV.

The challenges for the experiments of operating at 8 TeV and at the 2012 high collision rate are significant.  One challenge is modeling. To understand how their experiments are working, well enough that they can tell the difference between a new physical phenomenon and a badly understood part of their detector, the experimenters have to run an enormous amount of computer simulation, modeling the beams, the collisions, and the detector itself.  Well, 8 TeV isn’t 7 TeV; all of last year’s modeling was fine for last year’s data, but not for this year’s.  So a lot of computers are running at full tilt right now, helping to ensure that all of the needed simulations for 8 TeV are finished before they’re needed for the first round of 2012 data analysis that will be taking place in the late spring and early summer.

Another challenge is “pile-up.”  The LHC proton beams are not continuous; they consist of up to about 1300 bunches of protons, each bunch containing something like 100,000,000,000 protons.  Collisions in each detector occur whenever two bunches pass through each other, every 50 nanoseconds (billionths of a second).  With the beam settings that were seen late in 2011 and that will continue to intensify in 2012, every time two bunches cross at the center of the big experiments ATLAS and CMS, an average of 10 to 20 proton-proton collisions occur essentially simultaneously.  That means that every proton-proton collision in which something interesting happens is doused in the debris from a dozen uninteresting ones.  Moreover, some of the debris from all these collisions hangs around for a while, creating electronic noise that obscures measurements of future collisions.  One of the questions for 2012 is how much of a nagging problem the increasing pile-up will pose for some of the more delicate measurements — especially study of Higgs particle decays, both expected ones and exotic ones, and searches for relatively light-weight new particles with low production rates, such as particles created only via the weak nuclear force (e.g. supersymmetric partners of the W, Z and Higgs particles.)

But I have a lot of confidence in my colleagues; barring a really nasty surprise, they’ll manage pretty well, as they did last year.  And so far, so good!

A Neutrino Success Story

Almost all the news on neutrinos in the mainstream press this past few months was about the OPERA experiment, and a possible violation of Einstein’s foundational theory of relativity. That the experiment turned out to be wrong didn’t surprise experts. But one of the concerns that scientists have about how this story turned out and was reported in the press is that perhaps many non-experts may get the impression that science is so full of mistakes that you can’t trust it at all. That would be a very unhappy conclusion — not just unhappy but in fact a very dangerous conclusion, at least for anyone who would like to keep their economy strong, their planet well-treated and their nation well-defended.

So it is important to balance the OPERA mini-fiasco with another hot-off-the-presses neutrino story that illustrates why, even though mistakes in individual scientific experiments are common, collective mistakes in science are rare. A discipline such as physics has intrinsic checks and balances that significantly reduce the probability of errors going unrecognized for long. In the story I’m about to relate, one can recognize how and why scientists start to come to consensus.  Though quite suspicious of any individual experiment, scientists generally take a different view of a group of experiments that buttress one another.

The context of this story, though much less revolutionary than a violation of Einstein’s speed limit, still represents a milestone in our understanding of neutrinos, which has been advancing very rapidly over the past fifteen years or so. When I was a starting graduate student in the late 1980s, almost all we knew about neutrinos was that there were at least three types and that they were much lighter than electrons, and perhaps massless. Today we know much, much more about neutrinos and how they behave. And in just the last few months and weeks and days, one of the missing entries in the Encyclopedia Neutrinica appears to have been filled in. Continue reading

Professor Peskin’s Four Slogans: Advice for the 2012 LHC

On Monday, during the concluding session of the SEARCH Workshop on Large Hadron Collider [LHC] physics (see also here for a second post), and at the start of the panel discussion involving a group of six theorists, Michael Peskin, professor of theoretical particle physics at the Stanford Linear Accelerator Center [and my Ph.D. advisor] opened the panel with a few powerpoint slides.  He entitled them: “My Advice in Four Slogans” — the advice in question being aimed at experimentalists at ATLAS and CMS (the two general-purpose experiments at the LHC) as to how they ought best to search for new phenomena at the LHC in 2012, and beyond. Since I agree strongly with his points (as I believe most LHC theory experts do), I thought I’d tell you those four slogans and explain what they mean, at least to me. [I'm told the panel discussion will be posted online soon.]

1. No Boson Left Behind

There is a tendency in the LHC experimental community to assume that the new particles that we are looking for are heavy — heavier than any we’ve ever produced before. However, it is equally possible that there are unknown particles that are rather lightweight, but have evaded detection because they interact very weakly with the particles that we already know about, and in particular very weakly with the quarks and antiquarks and gluons that make up the proton.

Peskin’s advice is thus a warning: don’t just rush ahead to look for the heavy particles; remember the lightweight but hard-to-find particles you may have missed.

The word “boson” here is a minor point, I think. All particles are either fermions or bosons; I’d personally say that Peskin’s slogan applies to certain fermions too.

2. Exclude Triangles Not Points

The meaning of this slogan is a less obscure than the slogan itself.  Its general message is this: if one is looking for signs of a new hypothetical particle which

  • is produced mostly or always in particle-antiparticle pairs, and
  • can decay in multiple ways,

one has to remember to search for collisions where the particle decays one way and the antiparticle decays a different way; the probability for this to occur can be high.  Most LHC searches have so far been aimed at those cases where both particle and anti-particle decay in the same way.  This approach can in some cases be quite inefficient.   In fact, to search efficiently, one must combine all the different search strategies.

Now what does this have to do with triangles and points?  If you’d like to know, jump to the very end of this post, where I explain the example that motivated this wording of the slogan.  For those not interested in those technical details, let’s go to the next slogan.

3. Higgs Implies Higgs in BSM

[The Standard Model is the set of equations used to predict the behavior of all the known particles and forces, along with the simplest possible type of Higgs particle (the Standard Model Higgs.) Any other phenomenon is by definition Beyond the Standard Model: BSM.]

 [And yes, one may think of the LHC as a machine for converting theorists' B(SM) speculations into (BS)M speculations.]

One of the main goals of the LHC is to find evidence of one or more types of Higgs particles that may be found in nature.  There are two main phases to this search, Phase 1 being the search for the “Standard Model Higgs”, and Phase 2 depending on the result of Phase 1.  You can read more about this here.

Peskin’s point is that the Higgs particle may itself be a beacon, signalling new phenomena not predicted by the Standard Model. It is common in many BSM theories that there are new ways of producing the Higgs particle, typically in decays of as-yet-unknown heavy particles. Some of the resulting phenomena may be quite easy to discover, if one simply remembers to look!

Think what a coup it would be to discover not only the Higgs particle but also an unexpected way of making it! Two Nobel prize-winning discoveries for the price of one!!

Another equally important way to read this slogan (and I’m not sure why Peskin didn’t mention it — maybe it was too obvious, and indeed every panel member said something about this during the following discussion) is that everything about the Higgs particle needs to be studied in very great detail. Most BSM theories predict that the Higgs particle will behave differently from what is predicted in the Standard Model, possibly in subtle ways, possibly in dramatic ways. Either its production mechanisms or its decay rates, or both, may easily be altered. So we should not assume that a Higgs particle that looks at first like a Standard Model Higgs actually is a Standard Model Higgs. (I’ve written about this here, here and here.)  Even a particle that looks very much like a Standard Model Higgs may offer, through precise measurements, the first opportunity to dethrone the Standard Model.

4. BSM Hides Beneath Top

At the Tevatron, the LHC’s predecessor,  top quark/anti-quark pairs were first discovered, but were rather rare. But the LHC has so much energy per collision that it has no trouble producing these particles. ATLAS and CMS have each witnessed about 800,000 top quark/anti-quark pairs so far.

Of course, this is great news, because the huge amount of LHC data on top quarks from 2011 allowed measurements of the top quark’s properties that are far more precise than we had previously. (I wrote about this here.) But there’s a drawback. Certain types of new phenomena that might be present in nature may be very hard to recognize, because the rare collisions that contain them look too similar to the common collisions that contain a top quark/anti-quark pair.

Peskin’s message is that the LHC experimenters need to do very precise measurements of all the data from collisions that appear to contain the debris from top quarks, just in case it’s a little bit different from what the Standard Model predicts.

A classic example of this problem involves the search for a supersymmetric partner of a top quark, the “top squark”. Unlike the t’ quark that I described a couple of slogans back, which would be produced with a fairly high rate and would be relatively easy to notice, top squarks would be produced with a rate that is several times smaller. [Technically, this has to do with the fact that the t' would have spin-1/2 and the top squark would have spin 0.] Unfortunately, if the mass of the top squark is not very different from the mass of the top quark, then collisions that produce top squarks may look very similar indeed to ones that produce top quarks, and it may be a big struggle to separate them in the data. The only way to do it is to work hard — to make very precise measurements and perhaps better calculations that can allow one to tell the subtle differences between a pile of data that contains both top quark/anti-quark pairs and top squark/anti-squark pairs, and a pile of data that contains no squarks at all.

Following up on slogan #2: An example with a triangle.

Ok, now let’s see why the second slogan has something to do with triangles.

One type of particle that has been widely hypothesized over the years is a heavy version of the top quark, often given the unimaginative name of “top-prime.” For short, top is written t, so top-prime is written t’. The t’ may decay in various possible ways. I won’t list all of them, but three important ones that show up in many speculative theories are

  • t’ → W particle + bottom quark   (t’ → Wb)
  • t’ → Z particle + top quark      (t’ → Zt)
  • t’ → Higgs particle + top quark    (t’ → ht)

But we don’t know how often t’ quarks decay to Wb, or to Zt, or to ht; that’s something we’ll have to measure. [Let's call the probability that a t' decays to Wb ``P1'', and similarly define P2 and P3 for Zt and ht].

Of course we have to look for the darn thing first; maybe there is no t’. Unfortunately, how we should look for it depends on P1, P2, and P3, which we don’t know. For instance, if P1 is much larger than P2 and P3, then we should look for collisions that show signs of producing a t’ quark and a t‘ antiquark decaying as t’ → W+ b and t‘ → W- b. Or if P2 is much larger than P1 and P3, we should look for t’ → Zt and t‘ → Z t.

Peskin's triangle for a t' quark; at each vertex the probabilty for the decay labeling the vertex is 100%, while at dead center all three decays are equally probable. One must search in a way that is sensitive to all the possibilities.

Peskin has drawn this problem of three unknown probabilities, whose sum is 1, as a triangle.  The three vertices of the triangle, labeled by Wb, Zt and ht, represent three extreme cases: P1=1 and P2=P3=0; P2=1 and P1=P3=0; and P3=1, P1=P2=0. Each point inside this triangle represents different possible non-zero values for P1, P2 and P3 (with P1+P2+P3 assumed to be 1.)  The center of the triangle is P1=P2=P3=1/3.

Peskin’s point is that if the experiments only look for collisions where both quark and antiquark decay in the same way

  • t’ → W+ b and t‘ → W- b;
  • t’ → Zt and t‘ → Z t;
  • t’ → ht and t‘ → h t;

which is what they’ve done so far, then they’ll only be sensitive to the cases for which P1 is by far the largest, P2 is by far the largest, or P3 is by far the largest — the regions near the vertices of the triangle.  But we know a number of very reasonable theories with P1=1/2 and P2=P3=1/4 — a point deep inside the triangle.  So the experimenters are not yet looking efficiently for this case.  Peskin is saying that to cover the whole triangle, one has add three more searches, for

  • t’ → W+ b and t‘ → Z t, or t’ → W-  b and t’ → Zt;
  • t’ → W+ b and t‘ → h t, or t‘ → W- b  and t’ → ht;
  • t’ → Zt and t‘ → h t, or t’ → ht or t‘ → Z t;

so as to cover that case (and more generally, the whole triangle) efficiently. Moreover, no one search is very effective; one has to combine them all six searches together.

His logic is quite general.  If you have a particle that decays in four different ways, the same logic applies but for a tetrahedron, and you need ten searches; if two different ways, it’s a line segment, and you need three searches.

The Benefits of 8 TeV Collisions Over 7 TeV.

Yesterday, a commenter asked me a very good question that I realized I hadn’t yet addressed on this site.  Answering it gives us a chance to look at real data from the Large Hadron Collider [LHC], and to see what differences will arise the machine’s energy is increased from 7 TeV to 8.

The protons that are smashed together at the LHC are made from many quarks, gluons and antiquarks. The proton-proton collisions take place at a definite energy: 7 TeV = 7000 GeV in 2011, 8 TeV = 8000 GeV  in 2012.  But what we’re mainly interested in — what can really create new physical phenomena for us to observe — are the collisions of a quark in one proton with an antiquark in the other proton, or the collision of two gluons, etc. These “mini-collisions” carry only a fraction — typically a very small fraction — of the total proton-proton collision energy. How high a fraction can they carry?  and what are the motivations for increasing the energy from 7 TeV per collision to 8 TeV?  Click here for the answer.

Exotic Decays of the Higgs: A High Priority for 2012

2012 may well turn out to be The Year of The Higgs.  Right now we have very little knowledge about this particle, but that may change dramatically over the year. As I described in my previous post, we’re coming toward the end of Phase 1 of the Higgs search (where the ATLAS and CMS experiments at the Large Hadron Collider [LHC] search for the simplest possible form of the Higgs particle, the Standard Model Higgs, or SM Higgs for short.) And we’re also starting up Phase 2 of the Higgs search. As discussed in my Cosmic Variance guest post, and in more detail in my most recent post, if a particle resembling the SM Higgs is found, Phase 2 involves checking its details and determining as well as possible whether it is or isn’t precisely what is predicted by the Standard Model. If no such particle is found, Phase 2 involves searching widely for the many other types of Higgs particles that nature might or might not possess. Fortunately, despite these apparently divergent aims, the two possible branches of Phase 2 involve asking some of the same experimental questions (see Figure 3 of the most recent post), and so we can start on Phase 2 before even finishing Phase 1. And that is happening now.

One of the things that has to be done in Phase 2 is to search for decays of the Higgs particle that are not among the decays predicted to occur in the Standard Model.  ["Decay" = "a disintegration of one particle into two or more". Click here for an introduction.]  Such “exotic” decays are thought of as particularly plausible, because a lightweight Higgs (below about 150 GeV/c2 or so) is a very sensitive creature. It is very easy for new particles and/or forces to alter the Higgs’ properties, perhaps causing changes in how (or how often) it is produced, and to what (and with what probability) it may decay.  As shown in a large number of papers, written by  quite a variety of particle physics theorists, there are many, many types of possible exotic decays, and they can arise for many reasons.  If you’re curious what kind of exotic decays might occur, I gave a few examples in my now somewhat out-of-date analysis of what the summer’s Higgs searches imply. The basic logic of how unusual Higgs decays might arise is still correct in the cases described, but there are many, many more possibilities too. I’ll have to write a long article about the options in the coming month or so. Continue reading

New Post on the Higgs Hints

Just finished my new article on the hints of a Higgs particle.  I hope you find it useful! I have tried to explain, in largely non-technical terms,

  • how experimentalists at the Large Hadron Collider are looking for the Higgs, using various methods;
  • what makes methods of this type easy or difficult, with analogies;
  • that the current hints of a Higgs particle rely on the more difficult techniques, making it unclear whether we should trust them;
  • that more data over the coming year will bring the simpler techniques into their own, eliminating this problem over time.

I welcome your comments, questions and advice as to how to make this article more transparent to the non-technical reader!