1. 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 …

  2. 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 …

  3. 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 …

  4. 2012

    Higgs rumors: hold on for another couple weeks

    Finally, a post I can make in just a few minutes! The blogs of the physics world are abuzz with a rumor that the LHC experiments have… well, what have they (supposedly) found? If you read some of the more sensationalist physics websites out there, or the popular media (the news has even made it to the New York Times, although to their credit they’re being pretty reserved about it), you might get the idea that scientists at CMS and ATLAS have found the Higgs boson already and are just keeping the news officially under wraps until they can present it with much fanfare at a conference next month. That is not true.

    It’s important to remember that finding the Higgs boson is not like finding a lost key or something: you can’t just look at it and say “yep, there’s a Higgs boson,” even if you do have what is essentially a giant microscope. Instead, it’s a probability thing. You analyze the results of the experiment and get out a result like “90% chance that the Higgs boson exists” or “95% chance that the Higgs boson does not exist” (actually even there I’m …

  5. 2012

    Day 5: Plenary sessions (again!)

    DIS 2012 wrapped up today, and the last day of the conference was filled with another round of plenary sessions (attended by everybody). This time, though, the talks were mostly devoted to summarizing the parallel sessions which took place over the previous three days.

    The conference was divided up by topic into seven working groups: structure functions, the future of DIS, diffraction and vector mesons, electroweak and new physics searches, hadronic final states, heavy flavor, and spin physics. Each of these working groups was organized by two or three conveners, who were also responsible for putting together and presenting the summary slides. I have to recognize the impressive amount of work this must have taken: in one afternoon, the conveners went through every single presentation given in the conference, and organized and adapted the main conclusions from all of them into an experimental and a theoretical summary talk for each working group. Not to mention they had to stay awake and attentive for the entire three days of talks — much easier said than done!

    Anyway, the full summary presentations can be found on Indico, so if you’re interested, go ahead and check those out. I’ll post a more …

  6. 2011

    CP violation at the LHC

    Yesterday’s big news in the physics world: the LHCb experiment has observed a \(3.5\sigma\) asymmetry between the decays of \(D^0\) and \(\bar{D}^0\) mesons. This has already been described in detail elsewhere on the web: Sean Carroll has a nice explanation accessible to non-experts, or you can look at the presentation of the results from the HCP conference (which itself is reasonably clear and informative, if you have some experience looking at particle physics presentations).

    For those who are not inclined to click on links, here’s a quick summary of the story. CP symmetry violation is a difference between the behaviors of a particle and the mirror image of its antiparticle. The probability of a CP-violating process to occur is controlled by a complex phase parameter in the quark mixing matrix. There are two kinds of CP-violating processes that we can detect:

    • Some particles (kaons, D and B mesons) transform into their antiparticles and back as they propagate. CP violation means that the oscillation probability for going from the particle to the antiparticle is different from the probability to go from the antiparticle to the particle. Intuitively, you could imagine that the meson spends …
  7. 2011

    Getting further away from the elusive Higgs boson

    Last month I posted about the then-current results from the ATLAS and CMS detectors at the LHC hinting at a possible new particle around \(\unit{120-150}{\giga\electronvolt}\). But in light of new data presented at the 2011 Lepton-Photon conference in Mumbai, we’re not so sure about it anymore.

    Take a look at these plots from the ATLAS and CMS experiments, respectively:

    The solid line in each plot represents the observed data, and the dotted line represents the expected background, which is basically the theoretical prediction based only on the stuff we already know to exist. The yellow band shows the \(2\sigma\) confidence interval. In other words, if there is nothing left to discover within this energy range (in particular if the standard model Higgs does not exist), there’s a 95% chance that experimental data falls within the yellow band.

    When I displayed the equivalent plots from EPS HEP-2011 in my post last month, I pointed out that the interesting features were a couple of small regions where the solid line rose above the yellow band. Looking at the newer plots, you can see that that’s no longer the case. The experimental results are starting to …

  8. 2011

    Getting closer to the elusive Higgs boson?

    One of the neat things about being at the CTEQ school last week (more on that in an upcoming post, by the way) was how the representatives from ATLAS and CMS, the two major detectors at the LHC, kept hinting that they’d be releasing some really interesting results at the European Physical Association’s HEP-2011 conference conference this week. Well, it looks like the cat is out of the bag: both detectors are already reporting an excess of events at \(2-3\sigma\) significance around \(\unit{120-150}{\giga\electronvolt}\) in the \(h\to WW \to ll\nu\nu\) decay channel.

    What this means, in short, is that the number of times they detected two leptons (\(ll\)) and an amount of missing momentum that corresponds to two neutrinos (\(\nu\nu\)) exceeds the theoretical prediction when the total energy of the leptons and neutrinos is between roughly \(\unit{120}{\giga\electronvolt}\) and \(\unit{150}{\giga\electronvolt}\). This is the sort of thing we would expect to see if the Higgs boson has a mass somewhere in that range, around \(\unit{135}{\giga\electronvolt}\). Of course, it could be a fluke; that happens fairly often, because the way particles interact is essentially random …

  9. 2009

    LHC to test hyperdrive

    Wait, what?

    Just stumbled across this nifty little idea: that a particle moving at high speed can actually repel a stationary object in the other direction. I haven’t read the paper but if the summary is to be believed, this is very cool. Although it’s not actually faster-than-light propulsion, so not quite the hyperdrive of science fiction.

  10. 2009

    Sonic Black Holes

    Here’s something interesting that came up on Slashdot today: scientists at the Israel Institute of Technology report having created an “acoustic black hole”, a region from which no sound waves can escape, just as a normal black hole is a region from which no light waves can escape.

    How did they do it? Well, whenever sound travels through a medium, it does so at a characteristic speed — about \(\unit{343}{\frac{\meter}{\second}}\) in air, for example. That speed is relative to the medium, though, so if you can get the medium to move through your lab at a faster speed, the sound waves won’t be able to propagate fast enough to move against it (relative to the lab). If you had a wind tunnel blowing air to the right at \(\unit{400}{\frac{\meter}{\second}}\), the air would carry along all sound waves traveling through it, even those emitted in the leftward direction. Any sound waves produced at the right end of the tunnel would be stuck there — in effect, it’s a one-dimensional acoustic black hole, with an event horizon at the point (surface, really) where the air accelerates past the speed of sound as it …