1. 2013

    The Higgs boson, fermions, and you

    Here’s a tidbit of science that I should have written about a long time ago. Last month, there was a surge of excitement in the particle physics community when the ATLAS detector team announced that they had pretty much conclusively observed the Higgs boson decaying to tau leptons for the first time.

    Now, you might be forgiven for thinking “what’s the big deal? I thought we already found the Higgs boson?” Well yes. A year and a half later, physicists are fairly confident that what they found is the standard model Higgs boson. But back in July 2012, when the discovery was first announced, all anyone knew was that ATLAS and CMS had discovered a previously unknown particle with a mass of \(\SI{126}{GeV}\). It wasn’t clear just what kind of particle it was: the standard model Higgs boson, another kind of Higgs boson, or something else entirely. That question would be resolved by gathering more data on how the new particle interacts with other, known particles, and that’s what this most recent bit of news is about.

    The Higgs search: a year in review

    To understand all this in more detail, let’s back …

  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. 2012

    Extra! Extra! Higgs results from HCP 2012

    The Hadron Collider Physics conference is nearing its end now, and that means one thing: Higgs results! The LHC collaborations have just presented their updated measurements of the Higgs boson candidate whose discovery was announced in July, based on the \(\SI{7}{fb^{-1}}\) of new data that was collected in late summer.

    I’ll jump straight to the punch line: what they see is mostly consistent with the new particle actually being a plain old standard model Higgs boson (although it’s not absolutely confirmed yet). Which is kind of disappointing, because it indicates a lack of exciting new stuff to discover. Back in July when the discovery of the particle was originally announced, there were some slight discrepancies between the results obtained by the experiments and the predictions, and a lot of physicists were hoping that was a hint at something new and unexpected, but it’s looking more and more as though that is not the case.

    Anyway. On to the results. As in the original announcement, ATLAS and CMS are searching for the Higgs in five different channels: they’re trying to detect five different sets of particles that the standard model Higgs boson can decay …

  5. 2012

    On the 2012 Nobel Prize in Physics

    I’ve been so caught up with other things that I completely forgot to follow up on my last post about the Nobel Prize and the (presumed) Higgs discovery. But really, there’s not much to follow up on. The Higgs boson did not get the prize, which seems eminently fair because it’s not totally confirmed yet. Next year’s Nobel award will likely come at the perfect time for the Higgs results to get it. And this year’s recipients, Serge Haroche and David J. Wineland, have done some spectacular work on partial measurements of quantum states in quantum optical systems, paving the way toward construction of a quantum computer. (No, I don’t really know what they did. You should read about it, though. I will!)

  6. 2012

    Nobel Prize for the Higgs? Meh, maybe

    The physics community online is abuzz with speculation about who will be awarded this year’s Nobel Prize in Physics. And in a lot of people’s minds, the announcement by ATLAS and CMS in July of a new particle, widely expected to be the Higgs boson, is the leading candidate.

    Certainly nobody doubts that between the theoretical discovery of the Higgs mechanism and the experimental discovery of the presumed Higgs boson, there’s more than enough to deserve a Nobel Prize. And I hope and expect that some subset of the scientists involved will get it eventually. But I think making that announcement this year would be a little premature. For starters, we don’t actually know that it is the Higgs boson that was discovered at the LHC. Sure, it has a mass in the right range, and it decays to the right particles, but possibly in the wrong amounts. That could indicate that there is some extra effect that modifies the properties of the Higgs boson from what was predicted, or that it’s not the Higgs at all. The LHC will shed more light on that over the coming years, so it seems sensible to wait …

  7. 2012

    Higgs papers pass peer review! So what?

    Today the internet is abuzz with the news that the papers from ATLAS and CMS announcing their discovery of the Higgs boson have passed peer review and are officially published in Physics Letters B. That means now they’re actual science, right?

    Not really. (I’m assuming the ExtremeTech headline was a bit of a joke.) Peer review is really not as big of a deal as people outside the scientific community are often led to think. In particular, “peer-reviewed” does not mean “correct.” Peer review is just a high-level check to make sure that the paper isn’t complete nonsense and that the problem it’s addressing is relevant and interesting. Journals have limited space to publish these things, and they have to determine which of the many submissions they get are the most worthy of being put in that space. That’s what the peer review process is for.

    When something comes out of a big experimental collaboration like ATLAS or CMS, though, it has already gone through a rigorous vetting process. Doubly so for a high-profile result like this one — in fact, I’m sure the results had been double- and triple-checked by dozens of people even …

  8. 2012

    Particles gone wild: the Higgs mechanism, uncensored

    If you’ve been following the news about the discovery of the Higgs boson, you’ve probably noticed that it gets reported in two ways. There are the actual presentations with the full details of the experiments, which can only be understood by particle physicists their authors well, I’m assuming somebody knows what all of it means. Anyway, then there’s the other way. Sensationalized science journalism. “Hey, look, it’s a God Particle!!!11!” And Other Misrepresentations.

    I think some of us want a middle ground, though. Certainly I would have, not too long ago. So if you’re not actually a physicist, but you’re also not afraid to look at a little math, this post is for you. (This is adapted from a post on Physics Stack Exchange.)

    Spontaneous symmetry breaking

    In order to understand the Higgs mechanism in detail, you need to know about two concepts that are involved in quantum field theory. The first is spontaneous symmetry breaking. This is actually a pretty simple idea: suppose that the physical laws that govern a system are symmetric in some way, meaning that you can make some kind of transformation on the system without changing the …

  9. 2012

    Higgs boson: now more discovered! Or is it?

    Well, there you go… the webcast from CERN is over, the results have been announced: CMS and ATLAS have discovered the Higgs boson respectively at \(\SI{125.3 +- 0.6}{GeV}\) with a statistical significance of \(4.9\sigma\), and at \(\SI{126.5}{GeV}\) with a statistical significance of \(5.0\sigma\)!

    I totally called it :-)

    Okay, to be fair, it didn’t take much (any) imagination to predict that result a mere 12 hours before it was announced. The rumors that were flying around over the past day turned out to be basically true. But the LHC detectors actually performed better than I would have thought — whereas I was expecting them to have to combine their results to constitute a discovery by the standard particle physicists use, the two experiments were actually both able to do it individually. (So what if CMS was \(0.1\sigma\) short — that’s close enough as far as I’m concerned.)

    But what have they actually found? The standard model Higgs boson? Perhaps, but it’s too early to be sure. In order to verify what kind of particle this is, it’s going to take a lot more data collection and …

  10. 2012

    ATLAS and CMS to present Higgs search results

    A couple of weeks ago, I posted about the rumors that were flying around the internet concerning the presumed discovery of the Higgs boson. At the time, those rumors were unfounded: nobody really knew what the results of the analysis were. But no longer! The LHC experiments will be presenting those results tomorrow, July 4, at 9 AM Zurich time — that’s 3 AM on the east coast of the US. There will be a live webcast of the presentation if you want to see the results as they happen.

    So what exactly is going to be presented? Officially the results from ATLAS and CMS are being kept under wraps, but the rumors going around this time (which should be a little more reliable) suggest that the new data from both experiments support the bump at \(\SI{125}{GeV}\) that was last announced in December. It’s also been suggested that, while neither experiment individually has achieved the \(5\sigma\) significance physicists are waiting for to claim a “discovery,” when you combine the results from the two experiments together, they do exceed that \(5\sigma\) threshold.

    Despite all the excitement, I think these rumors reflect exactly what most people in …