Life as a Physicist

The LHC hasn’t delivered its first collisions yet. ATLAS and CMS haven’t taken collider data yet. But we already have to plan for the next steps. There was a recent kickoff event at CERN for people working on the upgrade to the LHC - the so-called SLHC (Super Large Hadron Collider).

That picture above is a simulation of what the CMS tracker would look like at a luminosity of 10^35 - that is 4 orders of magnitude greater than what we expect to be running at by the middle of 2009. Being able to reconstruct that many particles is going to require both experiments to replace their tracking detectors with more robust and more accurate detectors. This takes years and years to prepare for - the R&D for much of the replacement is already well underway. They are talking about installing these new detectors in 2013 - 5 years from now.

The accelerator talk is also fascinating — but my favorite talk was from MLM who was summarizing some of the physics possibilities of the SLHC:

While there is no guarantee that any deviation from the Standard Model will be found, the existence of physics beyond the Standard Model will demand and fully justify these studies: we’ll be measuring the properties, however trivial, of something which we know exists, as opposed to blindly looking for “we don’t know what” as we are unfortunately doing today!

Worth a look if you are curious about the next step!

There is an essay by Dennis Overbye in the NYTimes today that is a much better discussion of the black hole flap that occurred a week ago, generated by a real article by Dennis. My favorite (laugh) quote:

Besides, the random nature of quantum physics means that there is always a minuscule, but nonzero, chance of anything occurring, including that the new collider could spit out man-eating dragons.

And my favorite serious quote:

“As in all explorations of uncharted domains, there may be a risk,” Dr. Rees wrote, “but there is a hidden cost of saying no.”

Definitely worth a read - much more so that the actual article itself, I think.

ATLAS has just finished one of its large collaboration meeting. One of the nice things about these meetings is we get to hear a fairly detailed report on the machine status - something I don’t always hear except in rumors. In this case it was filling in some of the blanks that were in the recent press release explaining that the start up of the LHC would be at the reduced energy of 10 TeV instead of 14 TeV.

The problem is some of the dipole magnets. They have to be trained to run at full field. Full field for most magnets is 8 Tesla, which is about 133333 times stronger than the earth’s magnetic field. They have to be that strong in order to bend the very high energy 7 TeV beams of protons (magnets are to charged particles like protons what lenses are to light). The power requirements are stupendous (scientific term). In fact, they would probably melt if they were made out of regular copper wire. Instead they make them out a special wire that is superconducting when it is very cold. About -270 degrees Celsius.

The beauty about superconducting wire is that it doesn’t dissipate any energy of the current it is carrying. You know how an overloaded plug socket gets warm? That is because some of the current is converted into heat instead of being used to run your computer - a waste. When dealing with the currents in these magnets - well, it would be so hot that it would melt the magnet.

These magnets have a tendency to quench. Which is a problem. Lets say you have a bundle of wires all at -270 degrees carrying a huge amount of current. Lets say a flaw in one part of one wire causes it suddenly to loose its superconducting property. As a result the current flowing through that bit of wire starts to generate heat. That heat, of course, warms up all the wire around it, which causes it to “go normal” as well. This process rapidly cascades until the whole magnet ceases to be superconducting. This is called a quench. If not handled correctly this can be disastrous - you could melt the whole thing (and these things are expensive!). Part of the magnet design is quench protection.

Now, here is the cool thing. To get to their full field strength you have to train the magnets. This is particularly true when you are pushing the envelope of what the technology can do. You do this by slowly increasing the current in the magnet until it quenches. Once it has, you cool it down again and try again. And repeat.

This process is what will prevent the LHC from being ready to run at 14 TeV this year. The retraining of some magnets is taking too long (all the magnets were trained to full strength before they were installed - so some have become “untrained”). So their plans are to retain these magnets that are not properly trained over the first shutdown in the winter of 2008-2009.

And that, right there, tells us how long we will be running at the reduced energy of 10 TeV. If we are very lucky we will see beam in August and that will be our first run. So, probably a few months. Now, if I’m allowed to put on my old-guy hat, I’m going to guess that we won’t really get collisions until later than that and then the data coming out of our detector won’t make much sense until just around the shutdown. So it could well be this initial 10 TeV run gets almost no useful physics out - but is exactly what we need to get our brand spanking new detector into shape for the first real 14 TeV run.

BTW, I should say that the LHC has not told the experiments at what energy it will actually run yet. People think it will probably be 10 TeV, but the official word has not come from the machine division yet. Next week that should happen.

There were several other things of general note at the meeting (actually, there was a lot, but…). One thing is if you watched Peter Jenni’s talk - he gave out a few links you can go for status info. One has the current cooling status of the accelerator. I don’t think it is meant for everyone to look at, so I won’t post the link. But if you are member of ATLAS you can just look at Peter’s talk on the agenda server. The graphic is cool! I want to make it the background on my computer!

The other thing that, as a member of ATLAS, really makes this time exciting is the detection of cosmic rays. More and more detectors are getting turned on - and the first thing that is done with them is to look for cosmic rays. A few months ago people talked about the first cosmic ray having been seen. Now everyone in ATLAS is showing these things. Maybe this thing will work after all…

There is a joke going around ATLAS right now. There are various people who are obsessed by their physics studies. Each time something new about the detector is found they want to re-run their analysis. Nothing wrong with that - except that if it requires re-simulating the Monte Carlo that can take real resources (minutes per event just to reconstruct the event - we need millions of events in order to do one out of 1000’s of analyses).

Well, a big one was dropped the other day. Actually, we’ve all heard rumors this was coming, but now it is official, so I can joke about it. This is a big one because it changes everything - the production cross sections, average energy will find in our calorimeter, and other things. So, if you want to know how you are going to discover the Higgs at 10 TeV - well, better restart your simulation.

Of course what really matters is how long the accelerator remains at that energy. I don’t know details (perhaps I’ll learn them at ATLAS week next week) but the release claims that the magnets just need some re-training. If that is the case, that is no big deal and we will be at 10 TeV for less time that it will take the detectors to get themselves in shape to take physics-quality data.

At any rate. I’m sure, in some private cluster, somewhere, some 10 TeV data is being simulated as I type this!

One one of my lasts posts about computers and HEP, Kevin left a comment.

It would appear to me that the LHC has been for many years “methods development,” yet I’m assuming a couple people already have tenure on it and more than a couple Ph Ds have been awarded for developing the technology.

As far as I know, in the USA, no institutions will give a Ph.D. for an experimentalist if they have not touched data. As far as I know, no one has managed to get an experimental LHC Ph.D. in the USA by just running Monte Carlo or working on a bit of the detector. Now, parts of the LHC have taken data — i.e. the test beam.

The point of the test beam is exactly as it sounds - we put portions of the detector in the test beam to test them out. We fire known particles at known energies into bits of the detector and make sure they react (and readout) as we expect them. If they don’t, we adjust the physics models we use to simulate them or perhaps find something wrong with the detectors and fix them. It is not common to get a Ph.D. in the USA on test beam data, but it has happened. For example, D0’s initial startup (Run I) was so delayed I think a few people did this and then remained on D0 as post-docs to get their hands on real data.

Europe is different - there you can get a Ph.D. on Monte Carlo studies or on building a detector. As far as I know, it isn’t viewed as any different than getting a Ph.D. on data.

But, if you are in the USA, what do you do? This is exactly why most HEP groups maintain a foot in more than one pie. For example, I do a lot of work on ATLAS now - but I also do a lot of work on D0. D0 is a running experiment and produces real results. My tenure decision was on D0. I could have started on ATLAS when I arrived at UW 8 years ago - there was plenty of work to do - but it was correctly seen as suicide. Instead I worked on D0. I only just now have graduate students working on the LHC. I bet if you looked at the number of US graduate students on the LHC it was rather small and is now rapidly increasing. And that is because we are finally in the time frame that these students can get a Ph.D. on LHC data.

Finally, I have heard of programs that offer Ph.D. in detector physics and accelerator physics - which is very different from the work I do. I know less about them than I should, however.

However, Kevin correctly points out, once you are past the tenure bar you can just do what you want. Want to put all your marbles in the LHC basket? Go for it - no problem! Directly addressing the implied question in Kevin’s comment - presumably the person on the LHC who is making these criticisms had to go through the tenure process. And hopefully they are applying the same standards that were applied to them. Sometimes it is hard - I went through the qualifying exam as a student. Hated it, and it wasn’t clear that it offered any net gain for me or my fellow students. I passed, and now, about 15 years later, I sometimes catch myself thinking “it wasn’t that bad…” Some people carry that to an extreme. In the tenure case this is exactly why it is necessary to consult with other people in the department to understand if this is something unique criticism held by one person or is generally shared criticism.

Final installment of this 3-part series next.

UPDATE: Changed the tone of MC physics paragraph above.

Thirsting for some stunning pictures of the LHC? Check out this article in the National Geographic magazine. Make sure to look at the photo-gallery that comes along with the article. Some of the pictures — like the ALICE and CMS detector pictures are really stunning (ATLAS too, of course, but we’ve seen that one already).

BTW — the ATLAS detector is no longer all that photogenic on a grand scale - the cavern is now so full of bits of the detector it is quite difficult to get an idea of how big it is - all your site lines are blocked!

The picture is one of mine of ATLAS. The ones from ATLAS are much better!