Quantum Diaries

In response to my last posting about the ‘6th Milestone week’, the following questions were posed by Jacques.

Once you are satisfied with the results of this test, or any subsequent test that might be decided using cosmic rays, will you “just” have to wait for the LHC to start beaming, at which time you can immediately start gathering and exploiting the data from proton collisions, or does another heavy test campaign begin then? For how long?

Outside the difference in the data frequency/volume(is this not a 1 to 1 million ratio?), are there limitations in the current ATLAS tests due to the nature of cosmic rays? Or will the most volatile particles created by the collision decay so quickly that you can only track the results of such decays for which cosmic rays are a good subrogate to calibrate the various detectors and check they deliver consistent “tracks” whenever a given particle crosses them?
How about the calibration of the Level One Trigger in this context?

For starters, ‘satisfied with the results of this test’ is a constantly changing criterion. A year ago, satisfaction was getting just two sub-systems to run together. Today satisfaction is running all sub-systems with a level-one trigger rate of 10kHz. Next month, satisfaction will be running at 100kHz (which is the level-one rate we want to have during beam running). At the start of each ‘M-week’, we have a whole list of problems that we experienced in the previous ‘M-week’. At the end of the week, we have an entirely new list of problems which are generally more complicated and therefore more difficult to solve. So I suppose forward progress is defined as finding harder and harder problems.

We will never be in a position where we ‘just wait’ for the beam. Nor when the beam is running will we just be waiting for the data to roll in. It is a cultural trait of high energy particle physics to push the system. If we are stable with a level-one trigger rate of 100kHz, someone will suggest an idea to push that rate to 120kHz. There is a saying, ‘If it was easy, it would have been discovered already’. Thus in order to make the big discovery, we have to be willing to take risks and push the detector to its design limits and if possible beyond. And pushing the limits is all the fun!

In the period before the beam, cosmic rays aren’t a great way of testing ATLAS’ limitations. But it is all we have. Using the muon trigger chambers, the cosmic ray rate is about 100Hz. The beam will be 40 MHz which is roughly a factor of a million greater. We try to push the rate during cosmic running by using a high rate ‘random’ trigger but there are no physics events associated with these triggers. Additionally most cosmic events tend to be a single muon slicing through the detector. Whereas with the beam running there will be hundreds to thousands of particles in the detector.

Cosmic muons are very useful to study tracking in the inner detector and muon chambers. For the calorimeters, we can use them as a preliminary cross-check of our energy calibration. They are also helpful to establish the relative timing between sub-systems (which needs to be known on the nanosecond level). While cosmic muons are helpful for calibrating the part of the level-one trigger that looks at muons, it doesn’t help us much with calorimeter-based level-one trigger. The reason is that the calorimeters measure energies that are typically associated with “jets” of many particles, not a single track like those from cosmic muons.

The bottom line is cosmics are all good and fun. But if given the choice of cosmics vs beam. Give us beam!