So what’s next for high-energy physics? The LHC won’t be the end. Physicists will want to go to higher energies. The question is, how? The International Linear Collider — a planned linac that will do for positrons and electrons what the LHC did for protons — is what the community wants right now, but it won’t go to any higher energies. It might be too expensive to build just to match the LHC’s capabilities. Machines like the ILC struggle to achieve accelerating gradients of 30 MeV per metre, and the radio wave power delivered to the metallic cavities would melt them were they not supercooled and superconducting.

But all is not lost. SLAC’s Mark Hogan gave a talk about all the novel accelerating technologies in the works. He presented the so-called Livingston chart, which at right shows the hard-won exponential increase in accelerator energies with time. Every few decades or so, an order of magnitude increase in energy is earned. But you can also see how each successive family of accelerator technologies – cyclotrons, synchrotrons, storage rings – leap to new heights, but taper off with time. Something has to be done. And Hogan thinks that wakefields will be the way to go — using initial pulses to create a wave in a plasma, or a fiber, that in turn allow successive electron pulses to surf on the wake.

Of course, the technology is years away — but it could offer gradients in the GeV per metre range. That would put desktop light sources within reach. Who knows — maybe there could be desktop colliders, too. The future is wide open. But my future at Lepton Photon is quite limited. I am skipping out on Friday, the last day — but have enjoyed my time in Hannover thoroughly. Thanks very much to the organizers.

So nearly all of the talks at this conference have been reviews — not surprising, given the paucity of fresh data in the field. But there is one machine still chugging along — the Tevatron — and new results were presented by each of the main experiments, CDF and DZero. The new Higgs search results weren’t all that surprising, just incremental advances on the last big rollout of results in the spring. But let’s look at the latest results for CDF in the plot here. You can see that, even without combining their data together for a joint analysis, the individual teams are getting close to excluding certain ranges of Higgs masses. In particular, it is interesting how the plot shows observed values exceeding the expected values, especially in the low mass regime around 120 GeV — precisely the region where many theorists expect to find the Higgs. Could this be the beginning of a signal? CDF spokesman Rob Roser says it’s only a one-sigma difference from expectation — barely a blip on the radar in the physics world. But nonetheless, he says it’s a motivation to his team. This is precisely the part of the energy spectrum where the LHC is the worst at detecting the Higgs — a place where the LHC will need a year or two of data to say anything. So it helps give Tevatron continued justification to run in 2011. And it means that, if the Higgs is in fact a low-mass particle, the race for priority is still on.

While most of the talks here at Lepton Photon have focused on blowing up the Standard Model at high energies — i.e., firing up the LHC — Joerg Jaeckel of the University of Durham in the UK gave an interesting talk on how you can look for new physics at energies of just an electron-volt or so. The LHC might miss particles that only rarely interact with normal matter. And so Jaeckel extolled the virtues of exquisitely sensitive experiments that probe this alternative (and cheaper) ‘low-energy’ frontier.

One classic example is the GammeV experiment at Fermilab, which basically tries to shine a light through an opaque wall. On one side of the wall you can have a source that shoots 10^20 photons per second. On the other is a detector so sensitive that it could catch a single photon per second — a full 20 orders of magnitude sensitivity. Now photons can’t go through walls. But a weakly interacting particle could, something like an axion — a hypothetical dark matter particle. And there are certain rare scenarios where photons could create axions. If that happened, the axion could pass through the wall and create a photon on the other side.

There are plenty of other low-energy experiments — some have looked for slight differences in the way light is polarized in the vacuum, and some have looked for axions in the presence of a strong magnetic field. One has even found hints of an anomaly in the standard model by looking at that way the spin of a muon deviates in a magnetic field.

But these experiments in general get little attention. I suggested to Jaeckel that maybe it’s because the LHC is guaranteed to discover new things, whereas these experiments could end up being as fruitless as the seemingly pointless effort of shining a light on a wall. Jaeckel had a nice analogy. “[The LHC] is digging a broad ditch. With a low-energy experiment, you’re digging a hole. It’s a very deep hole, but you may be drilling in the wrong spot.”

Here are the several hundred physicists that made their way to Hamburg for the conference. It’s a relatively small conference, and since there is only one plenary session at a time, with invite-only talks given, there is none of the helter-skelter feeling of something like an APS meeting. But several people complained to me that there are only about half as many people as usual — and that that reflects the growing ennui, malaise even, of a community that hasn’t pushed at the energy frontier in decades. You can see that more people made the trek out to Korea for the last Lepton Photon in 2007 in the group photo from that year.

High energy physics has been waiting for something like the LHC for 25 years, says Bob Cousins of UCLA, and a deputy spokesman on CMS. He remembers a DOE advisory committee in 1983 that outlined the need for a super-collider — some machine that would reach beyond the TeV energy scales where the Standard Model breaks down. Sure, the Tevatron has done plenty of interesting things in the interim — but there has been no upheaval to the overall picture. The community has watched as the Superconducting Super Collider got partway built and then was canceled. And it has waited patiently as the LHC worked its way slowly into existence. It really has taken a generation for the LHC, Cousins says. “The lost generation?” I asked. “Yeah, something like that — my generation,” he said. But if they can hold on a little longer, they could end up being the greatest generation.

So in all of the discussions of the status of the LHC, there has been little change to the new plan that emerged a few weeks ago: the machine will run at half-energy through most of 2010. The two general purpose experiments, Atlas and CMS, will just have to be patient. But at the end of an LHC status talk, the Atlas team’s Mel Shochet, of the University of Chicago, had a question: how will the LHC fix the problem with the splices for good? Mel had heard a rumor that the LHC would be shut down for quite a while as the welded splices were replaced with clamps — a simpler technology used effectively at Fermilab’s Tevatron.

Would LHC engineers do the clamping over the course of several planned shutdowns each winter? If so, that could put the time when the LHC could operate at full 14 TeV energies way off in the future. On the other hand, shutting down to do all the clamping at once would mean that the LHC would have to be shut down for a long time — maybe even a year — which presents a problem of its own.

In the hallways, I found Rolf-Dieter Heuer, the Gandalf-like director general of CERN, and asked him for his sage thoughts. “Clamping hasn’t been decided, but it very well could be an option,” says Heuer, who is having a bit of a homecoming here — he was the research director of DESY for many years. Heuer says the main thing is to get some data to the experiments and give engineers a feel for the machine. Deciding how to fix the bad splices will come later, he said.

Welcome to Lepton Photon 2009, the premier high-energy physics conference of the summer, this year taking place in Hamburg: Germany’s second largest city and, with its system of rivers, dammed lakes and canals, the country’s biggest port. It’s also the place where the Beatles, back in the early 60s, cut their teeth, playing night after night in the city’s famous red-light district.

Not sure where the title of the conference comes from, though I think it’s pretty sweet. As one of my colleagues said, it could be a good name for a rock band. But at least here at the beginning of the conference, we’re not going to be hearing about leptons or photons, but, rather, protons: updates on the status of the proton-smashing LHC, and its two main detector experiments, Atlas and CMS. As most know, the LHC fizzled out last year after a bad weld vaporized — and so most of the people in the room have been closely following every detail of the machine’s rehabilitation. Most of the plan is settled after a new energy schedule was set by LHC management on 6 August, so we’re not expecting any new revelations, but let’s wait and see.