One of the strange but crucial features of our world is that every type of atom except hydrogen contains neutrons in its nucleus, even though neutrons, on their own, decay (to a proton, electron and anti-neutrino) within about 15 minutes on average. At first glance this seems puzzling. At second glance too. How can stable matter be made from unstable ingredients?
I’ve now written an article explaining why neutrons inside of nuclei can be stable, giving the example of the deuteron (one proton bound to one neutron) which is the nucleus of “heavy hydrogen”, or “deuterium”. If you understand this example, you’ll basically understand the point for other nuclei as well.
[For those of you in the New York City area: I’ll be joined by the wonderfully talented singer-songwriter-pianist Andrea Wittgens in giving a physics/music joint performance/presentation at the storied Cornelia Street Cafe, Sunday May 13th at 6 p.m., as part of their Entertaining Science series. It’s entitled Rhapsody for Piano and Universe, and intended for the general public. The place is pretty small, so get reservations in advance.]
My talk for the general public about the Large Hadron Collider [LHC] and the search for the Higgs particle, given online as part of the series of talks put on by MICA (Meta Institute for Computational Astrophysics) at Caltech, is now posted. The pdf of the slides, and the audio, are available here:
What is MICA? Here’s what they say at their website.
Meta Institute for Computational Astrophysics
The Meta Institute for Computational Astrophysics (MICA) is a professional scientific and educational, non-profit organization based in virtual worlds [VWs]. We are currently using Second Life (SL), and the Intel’s OpenSim-based world ScienceSim, and may expand to other venues as the VWs evolve.
You will sense, if you listen to the talk, that the virtual world is still a little buggy, there are some amusing moments! But on the whole, the virtual world offers many new opportunities for bringing together large but dispersed communities of people with common interests.
Since particle physicists are dispersed across the globe, in professional settings we use video and audio conferencing all the time. In fact, in just a few minutes after posting this, I’m going to listen to and watch a presentation at a conference at the CERN Laboratory (which houses the LHC) from the comfort of my office. I’ve attended conferences in Geneva while in Ontario, and attended conferences in India while in New York. I’ve even given a talk to a conference in Europe while I was just outside one of our National Parks in California! (You can either view this as letting work intrude into a vacation, or not allowing work to prevent a vacation; up to you.) And any experimentalist at the LHC probably attends at least one virtual meeting each day. So for our especially international and collaborative community, virtual experiences have been the norm for quite a while.
CBS NEWS, today: “A never-before-seen subatomic particle has popped into existence inside the world’s largest atom smasher, bringing physicists a step closer to unraveling the mystery of how matter is put together in the universe.”
Overall, this is not a bad article — but that last bit is propaganda. Maybe physicists made a nano-step (“nano” = one billionth). We knew this particle — a composite object made from known particles — would be there; we just didn’t know its details. To suggest this is a step toward a breakthrough is just silliness. Of course all new information proves useful eventually for something, but… really!
And the first part? Less exciting than it sounds (though a very nice bit of research!) Here’s the link to my post about this from Friday, explaining that the CMS experiment at the Large Hadron Collider has reported very strong evidence for a new composite object — a hadron, a member of a particular subclass called a `baryon’ — made from a single bottom (or `beauty’ — or just `b’) quark and from other known particles.
The reporter tells the usual white lie:
Baryons are particles made of three quarks (the building blocks of the protons and neutrons that populate the nuclei of atoms). Beauty baryons are baryons that contain at least one beauty quark (also known as a bottom quark). The new specimen is a particular type of excited beauty baryon called Xi(b)*, pronounced “csai-bee-star.”
A better description of baryons (including protons) is that they are made from three quarks, many gluons, and many additional pairs of quarks and anti-quarks. The three excess quarks are the ones being referred to in the above paragraph. See my article on what protons are and Friday’s post on this new one, which shows a sketch that gives better intuition about what protons and Xi(b)*s look like.
[Sometimes white lies get you into a lot of trouble later. The fact that protons have gluons and anti-quarks in them is crucial to understanding how the Large Hadron Collider does its thing.]
The reporter gets this part right:
The Xi(b)* particle had been predicted by a physics theory called quantum chromodynamics, which predicts how quarks bind together to form heavy particles, but had never before been observed.
“It was expected to be more or less where it was found,” [Vincenzo] Chiochia said. “Not all of those heavy states have been discovered, so you have to look for all those particles. It may well be that the theory is not complete. In this particular case it was expected, but we have to keep looking for things that are unexpected.”
Well. I don’t know about that, Dr. Chiochia. As a quantum field theorist, I would agree that “quantum chromodynamics” (which does more than what the reporter writes — it is the set of equations that describes and predicts all of the interactions of quarks, gluons and anti-quarks of all types) might indeed be turn out to be incomplete. But if you really want to look for places where it might fail, this is an especially bad place to look! This composite particle itself may be new, but the forces and particles required to make this object have already been very well-studied in other contexts. (Nevertheless, congratulations on nice work!)
I have written a lot about energy, but I’ve put off introducing the most important type of energy again and again. It’s the most important, because it is this type of energy that is responsible for all the structure in the universe, from galaxy clusters down to protons and everything in between. It is the most challenging to write about because it is not particularly intuitive. All the types of energy we intuitively understand, such as the energy of motion, are positive, but this type of energy, crucially, can be negative. On this website I’ll call it “interaction energy” (not the technical term, but my own, chosen to avoid misconceptions that might otherwise arise) because it is associated with the interactions among fields — including their little ripples that we call “particles”. If you’ve taken physics you’ve heard of “potential” energy; what you learned within that concept is a subset of what is included under interaction energy.
I’ve been wanting to address this for a while, because many of you have asked penetrating and central questions about the basic structure of matter, such as:
Why is the neutron stable inside of atomic nuclei, given that on its own it is unstable?
Why is the proton arguably heavier than the quarks and gluons that make it up?
And there are other equally important questions that no readers have yet stumbled upon but that I ought to address. Before I can answer any of those questions, however, I have to first describe interaction energy and the role that it plays in structure.
So — without further ado, here’s the article. This was an especially hard article to write and it may well be confusing in places — so I very much welcome your feedback, in order that I can try to make it clearer, if necessary, in later versions.
Yes, it’s true what you’ve read; the CMS experiment at the Large Hadron Collider has found a new particle. However, this isn’t one to get excited about. Or rather, it’s the particle that’s excited, not the rest of us. It’s a nice result; a neat result; but this particle is a slightly more massive version of a hadron that we already knew about, a composite object similar to a proton, built out of more fundamental particles we discovered over 30 years ago. So in the grand scheme of things, this is minor news; no big mysteries to resolve here. Nevertheless, congratulations to CMS! Finding such particles always involves reconstructing them from their decay products, and since this one decays in a very complicated way, the result represents a technical tour-de-force!
[A Heads Up: I’m giving a public lecture about the LHC on Saturday, April 28th, 1 p.m. New York time/10 a.m. Pacific, through the MICA Popular Talks series, held online at the Large Auditorium on StellaNova, Second Life; should you miss it, both audio and slides will be posted for you to look at later.]
Is supersymmetry, as a symmetry that might explain some of the puzzling aspects of particle physics at the energy scales accessible to the Large Hadron Collider [LHC], ruled out yet? If the only thing you’re interested in is the answer to precisely that question, let me not waste your time: the answer is “not yet”. But a more interesting answer is that many simple variants of supersymmetry are either ruled out or near death.
Still, the problem with supersymmetry — and indeed with any really good idea, such as extra dimensions, or a composite Higgs particle — is that such a basic idea typically can be realized in many different ways. Pizza is a great idea too, but there are a million ways to make one, so you can’t conclude that nobody makes pizza in town just because you can’t smell tomatoes. Similarly, to rule out supersymmetry as an idea, you can’t be satisfied by ruling out the most popular forms of supersymmetry that theorists have invented; you have to rule out all its possible variants. This will take a while, probably a decade.
That said, many of the simplest and popular variants of supersymmetry no longer work very well or at all. This is because of two things: (click here to read the rest of the article.)
[UPDATE March 31 2 a.m.: following study of the slides from a mini-workshop recording the results of investigations by OPERA and LVD, I now have the information to remove all the guesswork from my original post; you’ll see outdated information crossed out and newer and more precise information written in orange. I’ve also added figures from the talks.]
March 30 5:30 p.m. Two main scientists at OPERA, one leading the OPERA team as a whole and the other leading the neutrino speed measurement, resigned their leadership positions today. The suggestion from the press is that this is due to personal and scientific conflicts within the OPERA experiment, rather than due directly to the errors made in the neutrino speed experiment; but of course the way the measurement was publicized by OPERA caused serious internal conflicts at the time and are surely part of the issue. [Oh, and meanwhile, back over at the CERN lab, some good news: collisions at the Large Hadron Collider with 8 TeV of energy per collision were achieved this afternoon.]
Today I am beginning a long sequence of articles that will allow me to address some very basic and easily misunderstood concepts in physics: mass, energy, matter, force, and so on. Along the way I will be dealing with some of the questions that readers have been asking me, such as whether mass and energy are or are not fundamentally the same; why the neutron is stable inside many atomic nuclei even though it decays when it is on its own; why the proton has a mass which is larger than those of the quarks that it contains; etc. I’ve just completed the first link in this long chain, an article on the annihilation of particles with anti-particles. I’ve described some (not all!) of the rules that govern this process, which is one of the most important in all of particle physics. (You can also read a shorter and older article about anti-particles here.)
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 LHCexperimenters 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.
Posts have been a bit rare due to overwork and travel, but I have a few things to say about the search for the Standard Model Higgs particle (the simplest possible form of the Higgs) at CMS, one of the two major Large Hadron Collider [LHC] experiments. Last week I focused on the big news from the Tevatron and from ATLAS (the other major LHC experiment involved in the Higgs search), because the changes in their results were much larger than those from CMS, partly because CMS had already analyzed all their data for all of the different types of Higgs decays, and also the changes at CMS were, on the face of it, rather small.
However, the results at CMS have a significant effect on the overall picture, both negative and positive, and so deserve comment.
The main change from the December and January CMS two-photon Higgs search is that a more powerful and sophisticated technique has been applied to the same data. The results are roughly consistent with the previous search, but show important differences (which tells you how sensitive the current evidence is to how you slice the data.) Here are the consequences of the new result compared to the old, illustrated in the two figures late in the post: (more…)
