1. 2012

    This (Past) Week in Physics

    There’s been a lot of big news from the experimental physics community over the past week or so, but unfortunately I’ve been busy with spring cleaning and making arrangements for a trip to DIS 2012 so I haven’t been able to keep on top of it. Funny how I have less free time when I’m on vacation…

    Anyway, here’s a recap of some of the major recent events in the physics world:

    Higgs boson search update

    Tevatron combined Higgs signal

    At the Moriond conference on electroweak physics, CDF and D0, the two major experiments from the (now closed) Tevatron, reported an excess of collision events between about \(\SI{115}{GeV}\) and \(\SI{140}{GeV}\), peaking at \(2.2\sigma\). This could be a very weak signal of the Higgs boson, but it wouldn’t have been much to get excited about if ATLAS and CMS hadn’t already detected similar (but stronger) signals in the same energy range.

    It’s worth keeping in mind that the Tevatron has been shut down, so these latest results aren’t based on new data (like the LHC results); they’re based on a new analysis of the same …

  2. 2011

    The Higgs boson remains ephemeral (no surprise)

    Since the big news in the physics world is this morning’s presentation of the Higgs search results from the LHC, it’s only appropriate that I comment on it here, even though every physics blog in the world will be doing the exact same thing so there will be no shortage of Higgs information out there ;-) In summary: no, they haven’t really found it, but there is a bump around \(\SI{126}{GeV}\) that could represent detection of a Higgs boson. It will take another year’s worth of data to be confident either way.

    Here are the plots that were released this morning by ATLAS and CMS, respectively:

    The quantity being plotted here is the cross section for candidate Higgs events, which is denoted \(\sigma\), relative to a theoretical prediction, \(\sigma_{SM}\). In other words, the thing on the vertical axis is related to the fraction of collisions in which something that looks like a sign of a Higgs boson comes out. (I’ll perhaps post on this in more detail later; for now see Matt Strassler’s post on Higgs production for a good explanation.) But not everything that looks like a sign of a Higgs …

  3. 2011

    Possibility of a Higgs observation

    Just to keep everyone up to date on the latest rumors circulating in the physics world: there is speculation that ATLAS and CMS have a \(3\sigma\) candidate Higgs detection at around \(\SI{125}{\giga\electronvolt}\). I’m not going to speculate on the veracity of the rumors, but there will be a press conference next week on the 13th at which they officially announce their latest results, and it’s bound to be something interesting. Stay tuned!

  4. 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 …

  5. 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 …