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 detailed summary, or perhaps multiple summaries, once I have some time to go through the presentations again (perhaps in a few days when I recover high-speed internet access), but for now, I just wanted to highlight a few key developments:

• LHC data is starting to dominate the input for various sorts of theoretical calculations, including constraints on the mass of the weak boson, the coupling constants of the standard model, and parton distribution functions (which describe the structure of the proton), among other things. Several of the groups which calculate these parton distribution functions will shortly be releasing updated numbers which take the 2011 LHC data into account.
• The first collisions in the LHC at actually took place today!
• Although the Tevatron is shut down, analysis of its data is still yielding interesting results, especially with respect to the nature of parity violation in the weak force. Since the Tevatron collided two beams of different particles, it's well-placed to look for these parity violations (as opposed to the LHC, where the two beams are the same and thus most parity violations are hidden).
• The COMPASS experiment continues to produce data which is very useful for analyzing the spins of constituents of the proton.
• We've known for a while now, but the Higgs boson is excluded at (95% chance that it doesn't exist) outside of two narrow ranges, a -ish range around and a range around . (I might be a tiny bit off on the numbers, I forgot to write them down as they came up)
• Pretty much everything still agrees with the standard model (so, no big experimental surprises).
• Future plans for the LHC include two upgrades to the luminosity over the next 10 years or so, and also — if the plans are approved — the addition of an electron accelerator to form the LHeC, Large Hadron-electron Collider. This new machine will enable a whole host of new physics experiments that are impractical or impossible using the current LHC.
• In addition, there is a long-term plan to build the EIC (Electron-Ion Collider) to complement the LHeC.

Finally, although it didn't actually happen at this conference, we did find out today that the OPERA experiment has tentatively adjusted their analysis to account for the equipment errors they discovered earlier, and the new time-of-flight discrepancy is , which is perfectly consistent with relativity. I'll be writing more about this tomorrow, but for now Matt Strassler has a post on it which will surely be updated with additional details.

Yesterday's big news in the physics world: the LHCb experiment has observed a asymmetry between the decays of and 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 more time as a particle than as an antiparticle, or vice versa (in the sense of a time-averaged expectation value of the state). This is called indirect CP violation.
• Most particles decay, and each reaction by which a particle can decay has a characteristic time constant that gives the average lifetime of the particles that decay via that reaction. (whew!) If the time constant of a particular decay reaction that starts with the particle differs from the time constant of the opposite reaction that starts with the antiparticle, that is called direct CP violation.

So far we have observed both direct and indirect CP violation in kaons and B mesons, which contain strange quarks and bottom quarks respectively. The new results from LHCb, if they pan out, will add direct CP violation in D mesons (which contain charm quarks) to that list. Finding and measuring the properties of CP violation with a new quark type will go a long way toward increasing our understanding of the mechanism behind this violation. That in turn will help explain why the universe contains matter and not antimatter — which is, in a sense, the ultimate example of CP violation.

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 . 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 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 look more consistent with the hypothesis that there is no Higgs boson there. Of course, it's not a huge change, just from to about , but it does generally make sense to put more trust in the result that is based on more data.

So what does this mean for physics? For starters, it does not mean that there is no Higgs boson, and it does not mean that the standard model (which has been in development since the 1970s) is ruled out. These data only address the simplest possibility, namely that there is a single Higgs boson. If that doesn't work out, there could be a set of multiple Higgs bosons, and in that case physicists would have to go back and reanalyze the data. It's even possible that the LHC is not powerful enough to detect any of these multiple Higgs bosons. And even if we are able to completely rule out the Higgs, there are other mechanisms for creating mass that could fit into the standard model. Technicolor in particular is starting to attract attention, but personally I'm reserving judgment on any of these possibilities. Bottom line, we just don't have enough data to say anything with certainty.

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 significance around in the decay channel.

What this means, in short, is that the number of times they detected two leptons () and an amount of missing momentum that corresponds to two neutrinos () exceeds the theoretical prediction when the total energy of the leptons and neutrinos is between roughly and . This is the sort of thing we would expect to see if the Higgs boson has a mass somewhere in that range, around . Of course, it could be a fluke; that happens fairly often, because the way particles interact is essentially random, and sometimes you see more of a certain kind than you expect simply by pure chance. In this case, though the difference between the prediction and the actual result is large enough that there's only a 1% chance of seeing what we're seeing if a Higgs boson with a mass doesn't exist. So a theory which predicts that the Higgs does exist with that mass might be a better explanation of the results ATLAS and CMS presented today.

Of course, nothing cuts through the statistical mumbo-jumbo like some pretty pictures. Here are some representative plots, direct from the EPS presentations (click each to go to the original presentation PDF):

In both plots, the dots and the solid lines connecting them represent the data, and the dashed lines represent the theoretical prediction. The yellow shading represents a interval; effectively they are saying, "if the Higgs does not exist at this energy, there is a 95% chance that the data will be within the yellow region." The interesting feature is where the solid lines rise above the yellow region toward the left section of each graph, in the range . Again, it's not conclusive — we haven't found anything, yet — but it's interesting enough that the detector teams will be focusing a lot of their attention on that energy region in the coming months.

Further reading

• The original presentations from ATLAS and CMS, along with all others at the EPS meeting, are available online at the conference proceedings page
• A well-written perspective on what this result really means, from someone attending the meeting
• For more information on what is really involved in detecting a new particle, check out this question on proving the existence of the Higgs boson at Physics Stack Exchange.

In case you were wondering:

http://www.hasthelhcdestroyedtheearth.com/

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.

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 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 , 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's drawn into the tunnel.

Building a supersonic wind tunnel is no easy task, though — and even if you did it, you'd run into problems with turbulence messing up the propagation of sound waves through the air. The acoustic black hole actually reported was created in a Bose–Einstein condensate, a special low-temperature state, of rubidium. This has two advantages: first, you don't have to deal with turbulence, and also, the speed of sound is much slower, typically less than according to the paper. Although creating and stabilizing a Bose-Einstein condensate in the first place is no easy matter, it's a lot easier to control than a wind tunnel.

The reason everyone is so excited about this is that it paves the way for experimental detection of Hawking radiation. In real black holes, Hawking radiation occurs when a particle and an antiparticle spring into existence near the event horizon of a black hole; one of them falls in, and the other is "radiated" away. But in an acoustic black hole, instead of particles and antiparticles, you can get sound waves ("phonons") radiating in opposite directions, one heading into the acoustic black hole and one heading away. Here's how that works: the energy of a phonon, relative to the medium (the air or BEC), is given by

where is the momentum carried by the wave and is the speed of sound (it's not a coincidence that this looks just like the formula for the energy of a photon). But when that whole medium is moving with a speed relative to your lab, you see sound moving at the speed , so you would calculate the energy as

So if two phonons are created with the same momentum, but on opposite sides of the event horizon, the one inside the horizon has , giving it negative energy, and the other has , giving it positive energy. And for waves created in the right positions, the energies add up to zero, meaning that these waves can be created at virtually no cost to the universe.

Why is Hawking radiation so special? Nearly all physicists are very confident that it exists, but the fact remains that nobody has ever directly observed Hawking radiation, simply because nobody has been able to get up close to an event horizon to study it. So seeing the stuff for the first time will be an exciting moment in the history of physics. Plus, when you can create small, easily controllable systems that mimic black holes, you could potentially discover all sorts of other cool things that could be happening out in space.

Probably of more interest to the public is the way Hawking radiation is relevant to the whole LHC "doomsday" myth. There's been a lot of hype about the idea that the LHC could produce a black hole which could destroy the world, and Hawking radiation is the favorite explanation for why that can't happen. Simply put, any black hole that might be produced on Earth would evaporate due to Hawking radiation long before anything could fall into it. (Of course, there's also the fact that the LHC just reproduces processes that occur in the upper atmosphere. So even if Hawking radiation isn't going to save us from black holes, something else will.)