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Archive for August, 2008

Pixeling along, 24-7

Tuesday, August 12th, 2008

The LHC startup is getting closer and closer. A few previous blog entries already informed you that there was a successful insertion of beam into the LHC. This is of course great news, but means that the testing and final preparations of the detectors has now become serious business. As the CMS pixel detector was planned to be installed as one of the final components before the first beam was delivered, we are very much under pressure to be ready in time. The initial performance of the CMS forward pixel detector You can read that as ‘continuously on shift until things are stable enough to be run by non-experts’. This also explains the lack of blog entries by me and some of the other people working closely on the detector, at the moment the pressure is really on and the detector comes first!
(more…)

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Not completely there…

Saturday, August 9th, 2008

Indeed at 21:40 (GMT+1) on Aug 8, the first shot into the LHC made it all the way to IR3 – way to go!! One can probably say that there were no major polarity problems or big magnetic errors, miraculous and thanks to all those working hard in the tunnel installing and commissioning the sector. Hope the trend continues all around.

Later around 2 am one of the aperture tests being carried had caused a minor quench (remember 450GeV & very low intensity). Some access is taking place both from the accelerator side and ALICE. Should resume early afternoon and continue injection tests. Will investigate further and update.

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Congratulations!

Friday, August 8th, 2008

Beam in the LHC! A few of us were following the developments from the ATLAS control room. The comments on this screen shot from the LHC Operation Group web page say it all:

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Are we there yet ?

Friday, August 8th, 2008

It is like a fish market in the main control room now. Today, Aug 8, 2008 at 16:40 all the safety interlocks tests in the LHC were completed and the machine was fully closed for the first beam tests. However, it hasn’t without some small hickups, a fire alarm in the pre-injector (PS) and along with some other cabling problems, small panics before the big thing…

It is about 6pm now and they have managed to put the beam through the injector chain and all the way through the transfer line just 15 m before the LHC tunnel where it is stopped. The stopper in the transfer line will be removed shortly (6-8pm). The plan is to then adjust the timing of the injection magnets to put the beam into the LHC and thread the beam thereafter the sector 2-3 before midnight. Slightly optimistic but when you wake up tomorrow could the “first beam in the LHC” already be an old story ?

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In the recent mini-wave of “What Will We Find at the LHC” posts, no-one mentioned that the first actual measurements at the LHC will certainly not be of anything as exotic as the Higgs boson, supersymmetry, or large extra dimensions. This is not for any reason as prosaic as the fact that it’ll take time to get to the design energy and luminosity, which is true. If we define the very notion of “finding” something as being “measuring a quantity that we could not predict with current tools”, then the very first measurements at the LHC will count as discoveries of great interest to those not just focussed on what typically counts as exotic phenomena.


To boil it down to something concrete, consider the number of particles produced in a typical collision at the LHC. And to make things more straightforward, only consider the number of charged particles, the ones that leave curling tracks when moving through a magnetic field.
These are neat looking events, and they happen each and every time protons collide, at every energy, since the dawn of the accelerator era (you need a few GeV to even make a bunch of pions!). Thus, these particles are the “grass” that one sees in lego plots showing two huge jets, or the steel wool amidst which the two high energy muons emerge after a Higgs particle decays. For most people, this part of the collision is a background that needs to be cut away to see the interesting physics.

Here’s the rub: while it’s moderately easy to count the number of particles in each event, no-one has ever managed to come up with a bottom-up theoretical scheme by which one can predict this number. This is mainly due to the somewhat-scandalous situation that we know what protons “do” when they get close to each other (and can propagate that information into very precise predictions for the production of high pT particles, etc — the bread and butter of the LHC), but we don’t really get “why” they do it. Thus, we don’t have a very solid means to extrapolate our current knowledge into the LHC era, even if the Tevatron is only a factor of 7 lower in energy.

Thus, I promise that the first things you will see coming out of the LHC program are a bunch of measurements pertaining to “minimum bias” events, i.e. the events 99% of the experiment want to throw away so they can look for the needle in the haystack. Some of us (which include many of us directly interested in the heavy ion program at the LHC) want to see how the grass grows when those first collisions appear. We’ll count the particles emerging near 90 degrees, turn it into a “particle density” (by restricting the angle over which we count them), and put it on a plot with the rest of the data — probably with a few curves reflecting our favorite predictions. And everyone wants the first paper from the LHC, so it’ll be a real race, and so the results will appear almost as soon as real collision data is written to tape (which may well be this fall!)

For entertainment value, here’s my take.

This is a pretty straightforward application of the ancient (and bizarre) Landau hydrodynamical model to p+p minimum bias collisions and heavy ion collisions. It assumes that the two protons dump all of their energy into the collision as they overlap, and the system expands collectively like a relativistic fluid after that (sound familiar?). It describes the multiplicity (linear with the entropy) weirdly well for heavy ions (the top curve) above 20 GeV or so, and predicts a very high density at the LHC. It is pretty scratchy for p+p (the bottom curve — which may or may not be related to trigger bias issues — we’ll have to discuss that soon, too) but at least predicts something quite a bit higher than most popular models. But if this model has anything non-trivial to say about proton-proton collisions (something suggested by Landau and Fermi in the 1950’s, but which became controversial and even “heretical” during the rise of QCD, something I wrote about a few years ago), then we may have to start to take seriously the possibility that even small systems have “medium”-like aspects similar to what people already say about the QGP at RHIC. And how fun would that be?

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LHC start-up date announced

Thursday, August 7th, 2008

It was just announced that the LHC startup date is September 10. This means

“the first attempt to circulate a beam in the Large Hadron Collider (LHC) will be made on 10 September”.

The energy of the beam will be 450 GeV. It will be an exciting day for sure, and should be followed in 1-2 months by the most exciting day, when we get the first proton-proton collisions. This will be between two beams of 450 GeV each. Then, the acceleration system will be commissioned to bring the beam energies up to 5 TeV each.

This coming weekend, protons will be injected into one section of the LHC as a test of transfering the beam from the Super Proton Synchrotron (SPS) accelerator into the LHC.

From the ATLAS side of things, we are starting 24-hour-a-day staffing of the control room just before this September date as well. We are finally entering the LHC era…

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Teaching an Old Dog New Tricks

Wednesday, August 6th, 2008

I’ve been slacking. I don’t think I have written anything for more than a month for sure. I can’t blame being busy, because that is the usual excuse. I can find time to write a short blog, I think. But it has been, well, my car was in an accident and was totaled (no one hurt, thank goodness), we’ve had several shorter versions of the CRuZeT described in earlier posts, and we’ve been off to Montreux for a day of jazz, and somehow training for a marathon (the next run is 20 miles long) in September. By the end of the day, I’m shot, and honestly, uninspired. But today, I was at least inspired.

I am reaching a point now, after working on hardware for nearly 6 years straight, that I have to actually begin to look again at what comes out of it with software tools. The problem is, the tools have changed. Not necessarily for the better, but they have changed, and this dog has a few new tricks to learn.

One of these tools is an analysis and graphing package, that I need to use to turn columns of numbers, for example, into a graph to get something I can look at use to make a decision, for example, on timing. It is called ROOT which I think stands for R(?) Object Oriented Tool or something like that. I don’t really know. I was raised on PAW (Physics Analysis Workstation), another analysis and graphing package, based on FORTRAN, and got my paws wet with that. I spent years working with PAW, and now I have to switch to ROOT. I am basically learning by reading web pages and such, but for awhile today it completely flummoxed me. But finally, I got it, and I have to admit, some things are better, like the C++ like programming. But I still have a long ways to go…

The other I am trying to figure out is our CMSSW (CMS Soft Ware) package. I can now get it to run for me and produce some output that might be useful, but I needed lots of hand-holding to do that. Slowly I am getting it, but I am not yet ready to change the base code. I’ll leave that to the Graduate Students for awhile longer. I’m liable to throw a monkey wrench into the works.

Now I am going to work on a talk with another tool that seems to have changed significantly in its new release, ugh. Thanks Mr. Gates.

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Organizing Fairness

Monday, August 4th, 2008

Once, a very long time ago, I made the personal resolution that my job would never require making or looking at charts like this

I thought. Physics, that is the complete opposite of practical. That is perfect.

Yet, even in Physics, management tasks are necessary. And I have clearly failed in this resolution. Because I have to deal with charts like this all the time.

The surprising thing is that once you start making organizational charts like this, you can really get into them. I mean, I have had hours of conversations with people about whether or not this line should be dashed versus solid. Or do we call it a ‘shift’ versus a ‘task’? Or should we name this ‘Data-Quality Validation’ or ‘Detector-Quality Assessment’? These are very important questions.

Life in high-energy physics is in many ways just like any other job. There are the things that everyone wants to do. Like discover Supersymmetry and win the Nobel Prize. And there are the things that nobody wants to do but are absolutely necessary to be done. Like spending hours and hours in the control room making sure the detector is working properly.

And in this field, we believe very strongly that every person must do his/her fair share of the dirty work.

Therefore to monitor ‘fairness’ within ATLAS, my all-time favorite acronym was created: OTSMOU.

This stands for ‘Operation Task Sharing and Maintenance and Operation Update’. If you have no idea what that means, you are not alone. No one else on ATLAS does either.

I love OTSMOU, I really do. Because it offers such an excellent anthropological insight into the inner working of physicist’s mind. The purpose of OTSMOU is to ensure that there is an even distribution of dirty work (or more politely service work) for all physicists. In other words, no one gets to eat the cake without having helped set-up for the party first. It sounds like a simple task in principle, but it has been attacked with the same statistical methods and precision as searches for new physics. Different jobs and people have different weights. We have ‘tasks’ versus ‘privileges’. We have charts (like above) to display how the different tasks fit into the ‘big picture’. We have graphs and distributions to plot all the results (by funding agency even).

Fairness, like so many other aspects in life, is one of those things which theoretically is so simple to understand and yet requires very complicated software to actually achieve.

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