Time for a blog post that has been far too long coming! Remember the Quest for B Meson Decay? I wrote about this several months ago: the LHCb experiment had seen one of the rarest interactions in particle physics, the decay of the \(\mathrm{B}^0_s\) meson into a muon and antimuon, for the first time after 25 years of searching.

Lots of physicists were interested in this particular decay because it’s unusually good at distinguishing between different theories. The standard model (which incorporates only known particles) predicts that a muon and antimuon should be produced in about 3.56 out of every billion \(\mathrm{B}^0_s\) decays — a number known as the branching ratio. But many other theories that involve additional, currently unknown particles, predict drastically different values. A precise measurement of the branching ratio thus has the ability to either rule out lots of theoretical predictions, or provide the first confirmation on Earth of the existence of unknown particles!

Naturally, most physicists were hoping for the latter possibility — having an unknown particle to look for makes things exciting. But so far, the outlook doesn’t look good. Last November, LHCb announced their measurement of the branching ratio …

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 …

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 …

Note: I’m posting this from the road, so it will be somewhat lacking in pictures and details. Stay tuned for an update that fills all that in!

I’ve spent a lot of time poring over the results coming out of the Hadron Collider Physics conference this week, and I’ve noticed a trend. Higgs candidate cross sections are consistent with the standard model. B meson branching ratios are consistent with the standard model. Multijet event counts are consistent with the standard model. Maybe you can see where this is going. Vector boson production rates are consistent with the standard model. CMS rediscovered an unknown particle. Meson masses are consistent with—

Wait, what?

Yeah, that happened. CMS announced the second observation ever [PDF] of a mysterious new particle which defies classification.

This mystery object, called the Y(4140) or sometimes X(4140), was first seen in 2009 by CDF, one of the experiments at the Tevatron. It wasn’t just a slight fluctuation, either; the CDF data excluded the background-only hypothesis at more than a \(5\sigma\) level, which is the threshold physics uses to define a proper discovery. Certainly much ado has been made about less strong statistical …

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 …

One unexpected perk of being in China: I woke up before 7:30 this morning. That would never happen without jet lag.

Unfortunately, even waking up at 7:30 every day hasn’t given me any time to write up a mid-conference blog post. Talks have been running from 8:30-6:30, with the rest of the time mostly taken up by meals and discussions. So I’ll just post this “teaser” of some of the more interesting results that were presented.

Of the presentations that gave new results, most of them are based the September proton-lead run at the LHC. This was just a pilot run, meant to ensure that there wouldn’t be any unexpected problems with colliding two different types of particles, so there wasn’t a lot of data collected — only 2 million collisions — but it was already enough to start shedding some light on the underlying physics.

No initial state effects

Ion-ion collisions have already been extensively studied at both RHIC and the LHC, and as you might imagine, when you smash a blob of a hundred blobs of particles into another blob of a hundred blobs of particles, what you get is a mess …

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 …

OK, actually it is kind of a big deal. Discovering a new particle is not something that happens every day, and it’s a concrete result of having a well-tuned detector. Besides, it’s just cool. So congratulations to the CMS collaboration!

In case you haven’t heard the story, late last week CMS announced that they had a statistically significant observation of the \({\Xi^*}_b^0\) baryon, a particle made up of an up quark, a strange quark, and a bottom quark. In this case, “statistically significant” means that they detected this particular decay signature 21 times, of which only \(3\pm 1.4\) of them can be attributed to random coincidences in the detector. So they’re about as sure as you can be in physics that they are seeing signs of a real particle. They’ve also managed to reconstruct various properties of this particle by examining the decay products, and everything matches up with the predicted properties of the \({\Xi^*}_b^0\).

Now, why isn’t this a bigger deal, and why didn’t I write about it right away? Well, as I just mentioned, this particle was predicted to exist. Of course, the Higgs boson …