Archive for March 20, 2010

Twenty-three papers: Commissioning CMS

We were all rather down when the LHC magnet blew up in December 2008. Enough has been written about that. Quickly enough, the CMS Collaboration made the best of the situation and launched a serious campaign to commission the detector as much as possible using cosmic rays. The result is twenty-three scientific papers appearing as a special volume in the Journal of Instrumentation published by IOP Science. The link is 2010 J. Inst. 5. The editors of this journal have been very helpful and the CMS Collaboration is grateful for their cooperation.

The papers cover nearly every aspect of the detector. There is an overview paper, which describes how the data were logged and how the data acquisition and event processing were carried out. There are papers on the alignment of the tracking devices and of the muon system – the result is equivalent to tens of pb-1 of collision data. In a related effort, the magnetic field map in the muon chambers, with highly non-trivial spatial variations, was verified at the percent level. The energy deposited in the electromagnetic and hadronic calorimeters as a function of muon momentum was measured and compared to simulations. Anomalous signals (“noise”) in the calorimeters were also studied extensively, pointing the way to filters to remove them in collision data.

The event sample amounts to approximately 300 million cosmic ray muon triggers (remember that the CMS detector is installed in an underground cavern) collected over a four-week period. Essentially all of the CMS subdetectors delivered high-quality data, good enough for physics analysis. The operational efficiency was rather high, above 80%, and sustained over that four-week period. The superconducting solenoid was on for most of that period, delivering a field of 3.8 T.

The HCAL group at my home institution, Northwestern University, contributed in a major way to all three HCAL papers. For example, they studied the per-tower calibration as quantified with cosmic ray muons, deriving corrections that clearly improve the uniformity of response:
HB response in CRAFT
Here is the measured response as a function of muon energy. One can see clearly the relativistic rise for high-momentum muons, which evidently is reproduced well by the CMS detector simulation:

HCAL dE vs P

HCAL measured energy as a function of muon momentum

The muon group at Northwestern contributed to several papers as well. In fact, I was the main author of the paper on the cathode strip chambers (CSCs) and my group produced more than half of the content of this paper. (Yes I am proud of that!) We checked the detector simulation. It is good but far from perfect. We performed serious measurements of the efficiency, and found some problems which have been fixed since then. Here is an example of the efficiency of the local charged track triggers – basically, the trigger primitives generated in the on-board CSC electronics:

CSC ALCT and CLCT efficiency

Efficiencies for the ALCT and CLCT (see text)

The plot on the left shows the efficiency for the anode LCT, or ALCT, as a function of the angle. The trigger is designed to be efficient when the muons point to the interaction point, and in that region, they are well above 99% efficient. On the right, the corresponding plot for the cathode LCT, or CLCT. Again, for the region where the efficiency should be high, we measured above 99%.

We also studied the chamber resolution. Due to the nature of cathode strip chambers, the resolution is better with more charge measured than with small charge. The 1/Q trend can be seen in the plot below, up to about 300 fC (the mean charge left by a muon). The plot shows a worsening in the resolution at very high charges, due to the interference of delta ray electrons with the charge measurement.

CSC resolution vs Q

CSC resolution as a function of the measured charge

We also studied the resolution as a function of the muon impact within a strip, muon incident angle, and magnetic field.

I could write pages and pages about all the nice results obtained for all the CMS detectors – after all, there is a lot of material in twenty-three articles accepted by the referees of JINST. If you are interested (and I am sure the experts of you have specific interests), please go take a look using the link above.

The work done for these papers has been of immense value for the CMS Collaboration, placing the detectors in an unprecedented degree of preparedness – the impact of the understanding of the tracking devices for the first CMS physics paper has been tremendous.

Finally, let me point out that there is some interest in cosmic ray characteristics and one can hope that CMS will use their cosmic ray data to perform some physics measurements, too.


March 20, 2010 at 11:24 am Leave a comment

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