Quantum Diaries

Maybe I have a bit of a contrarian streak, but some of my fellow bloggers’ exhortation to “try to be optimistic” rubs me the wrong way.  I absolutely agree with them that some recent news coverage of LHC delays has been unduly pessimistic and misleading, and I am as excited as ever about both the short- and long-term prospects for the LHC, but I think I would rather try to be realistic.  Fortunately, the two positions aren’t that different: realism still gives us mostly good news.

Of course, there is an underlying optimism in being a particle physicist: the basic assumption — perhaps belief, or hope — that the physical universe can be described by equations and laws, so that the physics we know already can point us to the physics we have yet to discover.  That idea has proven true since the days when physics was a branch of “natural philosophy,” and been very constructive indeed, and that line of reasoning tells us that the LHC is a place where great discoveries will very probably be made.  So that much optimism I’m happy to grant; it’s what I’m building my entire career on.  No high-energy particle physicist has changed his or her mind about the importance of the LHC.  Of all the ATLAS graduate students I know, only one has switched away from the ATLAS to work at the Tevatron — and that’s not based on a lack of confidence or interest in the LHC, but simply a bet that he can write a thesis and get back to the LHC in time to catch most of the exciting stuff.

It’s pure realism that refutes some of the implied doubts about the long-term stability of the LHC project.  Repairs on the machine are done except for a few tweaks, and it will be running very soon.  There are a lot of important measurements to be made at our initial run goal of “only” half energy — which, don’t forget, is more than triple the energy of any previous particle physics experiment.  It is significantly less likely, though by no means impossible, that new particles will be discovered at lower energy; but getting to the LHC’s design energy, or very close to it, is only a matter of time and effort.  And, realistically again, time and effort are things that the LHC has in abundance: CERN’s core funding and personnel are guarenteed by international treaty, and contributions to the LHC and its experiments are guaranteed by almost-equally ironclad agreements between universities and funding agencies worldwide.

I think the main difference between realism and optimism is on what we can expect in the next few months.  The start-up plan calls for the LHC to begin running in November, which happens to be exactly when I’m moving back to Berkeley.  I would love to be here at CERN, working in the trenches, as things start up — so why on earth am I doing that?  There are three reasons.  The first is practical; my group wants to keep graduate students in both Berkeley and Geneva, and a bunch of younger students are about to move out.  That means that I could stay a little longer, but not too much — so I’d have to believe that we were going to start in November and get very prompt collisions if I were going to try to stay for them.  Second, although I have no complaints about any schedule or plan produced for the LHC this year, they have all been provisional from a realistic perspective.  There’s a very good reason for this: you can say how long things will take if everything goes according to plan, but it’s very hard to guess what extra work or delays might crop up with a one-of-a-kind machine.  So I’m “betting” on a little bit more unscheduled delay, although I don’t expect it to be very long in the grand scheme of things.  (Of course, I’m not an expert on the work on the LHC or how it’s going; I’m just guessing, and making the best decision I can based on that.)  Third, it will take some time to proceed from the first running of the LHC to high-energy collisions.  How long?  Again, nobody quite knows, it’s never been done before.  But I do know that the folks who run the LHC plan to be cautious and take things step-by-step; we experimentalists waiting for collisions, no matter how eager we are, obviously support this.  So an optimist might not move back in November, but I am.  I’ll come back out for a month in the Spring once I have a better idea how things are going.

Of course, realistically, I could be wrong.

Facebook folks: I’m Seth, http://blogs.uslhc.us/?author=9

LHC Chief Steve Myers gave a very interesting talk on the LHC Status today to a packed auditorium here at CERN.  The slides and video can be found here: http://indico.cern.ch/conferenceDisplay.py?confId=62277

Here is a brief summary of what I took to be the most important lessons from the talk (although of course you’ll learn more from going back to the original):

• The LHC schedule has been delayed by about three weeks from what was forseen in February.  That’s routine schedule slippage and no big deal as far as I’m concerned; if there are no other major issues than we will certainly be running this Fall.
• The limiting factor in when and how the LHC starts are the measurements of the resistances of the welding between copper busbars at the connection between magnets, which only carry current when the superconducting wire isn’t superconducting.  Some of them have resistances that are too high, and if one of these is affected during a quench, then both the copper and the superconducting wire can melt and cause a current arc. This is what happened last September, and was part of the chain of events that caused significant damage to the LHC.  There are other busbars with resistances that are too high, and the highest one in the LHC will limit the total current in the magnets and therefore the accelerator energy.
• So far about half the resistances in the accelerator have been fully checked.  The rest are planned to be checked by early August, and if there are no significantly higher resistances are found than those seen so far, the LHC will probably be able to do an initial run with energy of about 4 TeV.
• Once all these resistances have been measured, and in particular if higher values are found, the LHC experts will have to decide what the highest safe machine energy is.  Then CERN, the LHC, and the experiments can decide together if it is better to run at that energy or wait longer in order to repair the worst resistances.
• There have been significant upgrades to a variety of systems for preventing and reducing large-scale problems caused by future current arcs or other damage to the helium cryostats in the LHC magnets, although of course the goal is to avoid these in the future.