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 …

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 …

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 …

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 …

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 …

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 …

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 …

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!

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 …

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 …