Final Pieces of the CMS Puzzle

By John Conway | July 31, 2008 5:51 pm

The last and arguably highest-tech detector elements are, this week, being installed into the giant CMS experiment at CERN: the pixel detectors. After these detectors are installed, there remains only the beam conditions monitor, a small device, and then the experiment can be buttoned up in anticipation of the first circulating proton beams, hopefully in August. Nearly the entire LHC machine is cold – superconducting cold – and so at long last it seems that we may soon see the first data. Rumor has it that there can be first protons circulating by August 9 (a week from Saturday!) but I bet it will take a bit longer. There will be a many-week shakedown process before “ramping” the beams to high energy. This year, if all goes well, it it foreseen to ramp to 10 TeV total collision energy; the design energy is 14 TeV and that will happen next year. (A TeV is a trillion electron volts, an energy equivalent to about a thousand proton masses.)

The pixel detectors are the innermost devices in CMS, and are the first to record the passage of high energy charged particles which emerge from the proton-proton collisions. The central driving idea of these devices is to record tiny three-dimensional space points along charge particle paths, allowing us to measure to within 10 microns (10 millionths of a meter; a human hair is 50 microns in diameter) the trajectory of the charged particles, and thereby infer where in space they may have emanated from.

This is particularly important information. The LHC machine has many “bunches” of protons in each counter-rotating beam, and each bunch is spread out over a length of around 8 cm. Every time bunches collide (and that will eventually be every 25 nanoseconds) we will get many proton-proton collisions. In all likelihood only one of these will be of interest for later analysis; we need to identify which particles come from that collision. The pixel detectors will help us pinpoint that location in space.

But perhaps of even more importance is to know when some of the particles appear to come from somewhere other than the “primary vertex” where the collision actually happened. The presence of these “secondary vertices” tell us that some particle travelled a distance and then decayed. In the case of a high energy bottom (b) quark, it can travel several millimeters or even centimeters and then decay into several charged particles. The presence of a b quark “jet” is often a good indicator of whether there were top (t) quarks, the heaviest of them all, produced in the event. There is a ton of physics, including searches for new physics beyond the Standard Model, that relies on these abilities of the experiment.

If we could strip away all the support frames, cooling, electronics, etc. from the pixel detectors, leaving only the detectors themselves, they would have an arrangement sort of like the diagram at right.

As you can see, there is a central “barrel” portion, and two “forward disks”. The detectors themselves are rectangular, and, as the name implies, segmented into very small pixels about a tenth of a millimeter in size. That’s a lot larger than the pixel size in your digital camera. But this detector can take 40 million pictures a second, keeping the interesting ones and discarding the vast majority.

The heart of the pixel detector is the readout chip, a silicon microchip specifically designed and fabricated for this detector, in this experiment. The effort to develop the readout chip was led by Roland Horisberger of the Paul Sherer Institut in Villigen, Switzerland. Each chip has over 4000 input channels arranged in a grid; each channel is bump-bonded to a sensor channel. The sensors are also very thin silicon wafers with one surface segmented into pixels. Each pixel channel can sense when a certain minimum amount of charge has been deposited by a passing charged particle, digitizes and time-stamps it, and sends it out onto the readout bus when a trigger signal matching the time stamp is received. All the thousands of readout chips in the detector do this in parallel, ultimately sending the torrent of data out on optical fibers to data acquisition electronics modules in the service cavern adjacent to the main detector cavern.

The PSI group built the central barrel portion of the CMS pixel detector, and the forward disks, which are somewhat more complicated mechanically, were built by a consortium of US universities and Fermilab. The forward disk detectors were assembled at Fermilab and then transported to CERN for final assembly, testing, and now installation.

My own involvement in the project has been varied, but most recently focused on getting the detectors to CERN last year, and then working with engineers at Fermilab and UC Davis to design and build the fixtures and procedures to get the forward detector installed.

A postdoc in the Davis group, Ricardo Vasquez Sierra, and I hand-carried the assembled half-disks aboard commercial aircraft from Chicago to Zurich to Geneva in four separate trips last year. These incredibly delicate devices were housed in special acrylic cases so as to facilitate security inspection. (We had made special arrangements with the TSA in Chicago…Zurich was more dificult.) The acrylic cases were in turn carried inside foam-lined hard shell cases. Needless to say, we carried each one, valued at about $500k, very carefully. People thought we were crazy – there is a certain history in our field of detectors arriving damaged when shipped – but we made it there with no problems at all. My biggest fear, I think, was some idiot tearing through the terminal and hitting one of our detectors with a luggage cart.

Meanwhile we needed to design a system to perform a sort of ship-in-a-bottle feat with the forward detectors. The detectors are deep inside the CMS tracker, the central bore of which is about seven meters long. The detector half-disks are mounted on two-meter-long carbon fiber service cylinders which also support the cables and tubes feeding power and cooling to the detector, plus some of the electronics. The two service cylinders sit vertically and slide into their final position along grooves in carbon fiber beds on the top and bottom of the bore. So as to have no uninstrumented regions in the vertical plane, at the end of travel the grooves are curved so as to make the half disks mesh. Thus, the two half cylinders need to be pushed in simultaneously with millimeter precision. Later the detectors need to be removed, at which point they will have become radioactive from exposure to the intense radiation environment in the center of the CMS detector. So the system had to be simple, easy and fast to use, so as to minimize radiation exposure to personnel.

Here is a remarkable photo of one of the forward pixel half cylinders half way into position. Note the converging tracks in which the half cylinder feet ride and the vertical beam pipe support that the detector has to clear on the way into position.

I have always been mechanically minded and enjoy problems like this. It was not the most sexy part of the pixel project, but an essential piece of making the whole thing work. We did a test of the inserttion a year ago when the tracker was still in a surface building at CERN. From the lessons we learned from a that test we built the final system for installation and tested it in May, before the beam pipe installation was completed.

So, just a few hours ago the CMS pixel detector was successfully installed. I was unable to be there, due to the recent birth of my son Ian. (Gotta have your priorities straight…) My able colleagues filled in seamlessly for me. Soon, though, the LHC and the ATLAS and CMS experiments will be up and running, and this great human adventure into inner space will commence.


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