1. 2013

    Planck exposes the universe

    Yesterday, the team behind the European Space Agency’s Planck satellite released their first set of data. This was a seriously exciting moment in the world of cosmology, in the same way as the previous weeks’ Higgs updates were an exciting moment in the world of particle physics. And I have the perfect way to explain it to you:

    Go read this, this, this, this, and this.

    OK, seriously though. The preceding five blog posts do a fantastic job (individually, and even more so together) of explaining, at a reasonably abstract level why the Planck data release is important and what it means. Now, I do plan to do my usual act of digging into the science and explaining some of the details, but in this case, there’s a lot of science. The Planck collaboration released thirty papers, and I just haven’t had time to comb through them yet. So a proper Planck post will have to wait for some time later this weekend. Until then, you can get a good, just-technical-enough overview of results from Ethan Siegel’s summary post, the last link in that last paragraph.

  2. 2013

    More Higgs updates from Moriond

    Sit back, close your eyes, and think all the way back to… last week, when physicists from the LHC experiments presented their latest results on the Higgs search at the Rencontres de Moriond Electroweak session. Yes, I know, we barely had time to digest those results. But digest we must, because this week there are even more new results coming out, from the Moriond session on QCD and High Energy Interactions. And what the experiments have presented today is, rightly or wrongly, turning a lot of heads.

    The key update from today’s presentations is a measurement by ATLAS of the cross section for the Higgs decaying to two W bosons, which each then decay to a lepton and a neutrino: the \(H\to WW\to ll\nu\nu\) channel. It comes on the heels of a similar measurement presented by CMS last week. Both detectors are now reporting that they measure a strong signal for \(\ell\bar\ell\nu\bar\nu\) detection beyond the standard model (without a Higgs boson) at \(\SI{125}{GeV}\), with a significance of \(4.0\sigma\) at CMS and \(3.8\sigma\) at ATLAS. In other words, if the particles of the standard model …

  3. 2013

    Higgs updates from Moriond

    This week sees a major physics conference in Italy, the Rencontres de Moriond 2013 Electroweak session. It’s notable because the LHC experimentalists involved in the search for the Higgs boson are presenting their latest results. (There are also many other things being presented — less high-profile, but no less important!) I won’t give too many details of what has been presented, since there are plenty of other places on the web you can read about it, but certainly a quick overview is in order.

    When last we left the Higgs search, it was November, and the experimentalists had just presented the results of analyzing the data the LHC had collected in the later half of summer 2012, combined in some cases with earlier data.

    Of the various ways (channels) the standard model Higgs boson can decay, the experiments are looking most closely at these five:

    • \(H\to\gamma\gamma\) (two photons)
    • \(H\to WW\to ll\nu\nu\) (two leptons and two neutrinos)
    • \(H\to ZZ\to 4l\) (four leptons)
    • \(H\to \tlp\talp\) (two tau leptons)
    • \(H\to \btq\btaq\) (bottom and antibottom quark)

    Remember that if the particle discovered is really the standard model Higgs boson, it …

  4. 2013

    Good night LHC

    Just a quick note that the LHC stopped operation this past week and has gone into its first long shutdown. This shutdown period lasts two years, and during that time the accelerator will be upgraded to allow it to run at its full design energy of \(\SI{7}{TeV}\) per beam for proton-proton collisions.

    You can always track the status of the accelerator complex using LHC Page 1 or the LHC Dashboard, although they’re not going to have anything interesting for a while.

    In the meantime, theorists and much of the LHC experimental collaboration members are going to have their hands busy analyzing the data that came out of this first run. Of course there is the ongoing search for the Higgs boson, which is by now actually a search to determine whether the boson that was discovered is in fact the standard model Higgs. But there are all sorts of other predictions to be checked, most of which have to do with pinning down the behavior of known particles under extreme conditions, rather than discovering any new particles. The LHC ran three different types of collisions: proton-proton, proton-lead, and lead-lead, and some of the most interesting results (in …

  5. 2013

    Could you light the Superdome with cell phones?

    If you care at all about (American) football, or are trying to pretend you do, you probably saw the power go out during the Superbowl this past Sunday. Half the stadium lights, the scoreboard, and the announcers’ booth, completely out of commission. Hey, did you know there are actually people talking during the game most of the time?

    Anyway, one of my friends made an offhand comment about people holding their phones up like candles, but it got me thinking, XKCD-style: what kind of light could you get on a football field from cell phones? Enough to play? Or would you have to give everyone xenon lamps? To find out, we have to delve into the, um, murky world of photometry, the science of measuring the perception of light.

    Let’s start with something simple. Anyone who’s familiar with a bit of physics knows about power: the amount of energy per unit time. When you characterize a light bulb as a hundred-watt bulb, for instance, that’s a measure of the power it puts out when attached to the circuitry of a standard lamp. There’s a whole hierarchy of other measurements you can make that are all …

  6. 2013

    The coolest thing since absolute zero

    I’m a sucker for good (or bad) physics puns. And the latest viral physics paper (arXiv preprint) allows endless opportunities for them. It’s actually about a system with a negative temperature!

    Negative temperature sounds pretty cool, but I have to admit, at first I didn’t think this was that big of a deal to anyone except condensed matter physicists. Sure, it could pave the way for some neat technological applications, but that’s far in the future. The idea of negative temperature itself is old news among physicists; in fact, this isn’t even the first time negative temperatures have been produced in a lab. But maybe you’re not a physicist. Maybe you’ve never heard about negative temperature. Well, you’re in luck, because in this post I’m going to explain what negative temperature means and why this experiment is actually such a hot topic. ⌐■_■

    On Temperature

    To understand negative temperature, we have to go all the way back to the basics. What is temperature, anyway? Even if you’re not entirely sure of the technical definition, you certainly know it by its feel. Temperature is what distinguishes a day you can walk …

  7. 2012

    Yes, Virginia, the universe is (probably) real

    It’s been far too long since I posted anything — almost three weeks, in fact, since the end of NaBloWriMo. So I took the time to make a solid writeup of the speculation that we may be living in a computer. Consider it my Christmas present to the blogosphere. Happy holidays everyone!

    Have you ever wondered if everything you know might only exist inside some hyperintelligent alien’s computer?

    It’s a fun possibility to consider, that we might be living in a computer simulation, and we might even be able to figure out how to manipulate it. It even makes for some great science fiction (as well as some terrible science fiction :-P perhaps). But in reality, figuring out whether the universe is a simulation isn’t as easy as popping a red pill and waking up in an alternate reality with one less layer of abstraction. If we are in a simulation, the way it’s going to reveal itself is in the tiny details of physical processes that take place on sub-subatomic distance scales.

    There is actually a branch of physics, lattice QCD, that is entirely devoted to simulating reality. Right now it only works for very …

  8. 2012

    Color Glass Condensate

    Over the past few days, the news about the LHC possibly discovering (or even confirming, in some sources!) a new kind of matter has popped up on quite a few websites — blogs, science news sites, Twitter, Google+, Reddit, elsewhere on Reddit, you name it. Of course, I was writing about this before it was cool :-) I reported the discovery of the ridge presented by CMS at the High pT LHC Physics Workshop. Back then it wasn’t clear that anyone really knew what was going on, but of course that important fact got lost in the hype.

    As I posted in a more recent update, there is a lot of speculation that this ridge might have something to do with something called the color glass condensate (CGC). This is what so many sites are calling the new state of matter that the LHC has supposedly discovered. Well, I actually know something about this! Sort of. The CGC is an incredibly complex mathematical model, but it is somewhat closely related to what I work on, so I figured I could try to explain what’s going on in some more detail.

    Parton distributions

    All this starts with the question: what’s …

  9. 2012

    New type of matter?

    Phys.org has picked up on the results from the LHC pilot pA run showing the formation of The Ridge in high-multiplicity collisions. I wrote about this last month, when it was first presented at the High-pT LHC Physics Workshop in Wuhan, and at that time, the sentiment at the conference seemed to be that it wasn’t clear what could be causing the ridge.

    Now, people are starting to lean toward the color glass condensate (CGC) as an explanation. The CGC has been called a new state of matter, which probably isn’t the worst description, but I think that makes it sound like more than it is. It’s a model that predicts the behavior of gluons within a proton or nucleus, under conditions in which the gluons are so numerous that they regularly “bump into” each other and fuse, or “recombine” to use the technical term. (That’s an extreme oversimplification, of course; perhaps someday I’ll do a post explaining this in more detail.) This model predicts some correlations among gluons which might be able to explain the ridge. But it’s not at all clear yet that that is the case. The LHC didn’t …

  10. 2012

    Sudakov parameters

    Sudakov parameters are a common mathematical tool in the realm of high-energy physics — so I was surprised not to find a web page describing them at the top of a list of Google search results.

    $$k_1^\mu = \frac{1}{\sqrt{2}}(K, K, 0, 0)$$
    $$k_2^\mu = \frac{1}{\sqrt{2}}\biggl(\frac{1}{K}, -\frac{1}{K}, 0, 0\biggr)$$

    Since these are both null vectors, \(k_1^2 = k_2^2 = 0\), and they also satisfy \(k_1\cdot k_2 = 1\). They form two components of a complete basis. Any arbitrary four-vector can thus be parametrized as

    $$A^\mu = \alpha k_1^\mu + \beta k_2^\mu + A_\perp^\mu$$

    where \(A_\perp^\mu\) is a four-vector that contains only two nonzero components, the ones orthogonal to \(k_1^\mu\) and \(k_2^\mu\). This is the Sudakov parametrization of the vector.

    The Sudakov parametrization is useful because for an object moving at or near the speed of light, if the directions of the coordinate axes are appropriately chosen, only one of \(\alpha\) or \(\beta\) will be large, and the other will be nearly zero, as will both components of \(A_\perp^\mu\).

    Source: appendix B of Transverse spin physics by Vincenzo Barone and Philip …