Why is there a difference in the Cms and Atlas luminosities?

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jmc2000
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Why is there a difference in the Cms and Atlas luminosities?

Post by jmc2000 » Wed Apr 18, 2012 5:30 pm

First, congratulations to the guys at Cern for getting the LHC to 1380 bunches, 0.6 beta, 1.3x10^11 protons/bunch :happy-jumpyellow:

Yesterday they did some vdm scans which I would have thought should have calibrated the luminosities to around the same value as last year whereas on the dashboard, Atlas is at 4.3/nb/s, Cms 5.5nb/s. Anyone know why the difference?

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PhilG
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Re: Why is there a difference in the Cms and Atlas luminosit

Post by PhilG » Wed Apr 18, 2012 9:21 pm

It could simply be that they have not finished analysing the scans yet
Last edited by PhilG on Thu Apr 19, 2012 1:57 pm, edited 1 time in total.

jmc2000
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Re: Why is there a difference in the Cms and Atlas luminosit

Post by jmc2000 » Wed Apr 18, 2012 9:46 pm

PhilG wrote:It could simply be that they have finished analysing the scans yet
Cms with its current max of 5.5/nb/s is closest to the right value?
Atlas with its current max of 4.5/nb/s is closest to the right value?

So where are you placing your bets, guys?

I'm going with Cms

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PhilG
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Re: Why is there a difference in the Cms and Atlas luminosit

Post by PhilG » Thu Apr 19, 2012 6:13 am

Last year they reached 3.5/nb/s with beta* = 1.0 so with beta* = 0.6 they should expect 3.5/0.6 = 5.8/nb/s assuming intensity and emittance are the same. So the CMS figure looks better.

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Re: Why is there a difference in the Cms and Atlas luminosit

Post by adam_jeff » Thu Apr 19, 2012 8:04 am

Analysing the van der Meer scans is not straightforwards and it will take some time before the results are known, and the calibration is applied to the luminosity monitors. Even then, ATLAS and CMS may not agree on the calibration.
Already last year there was considerable disagreement on the integrated luminosity. There was a luminosity workshop in February, you can find all the details in the presentations there:
http://indico.cern.ch/conferenceOtherVi ... fId=162948

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Re: Why is there a difference in the Cms and Atlas luminosit

Post by pcatom » Thu Apr 19, 2012 6:50 pm

Last year they reached 3.5/nb/s with beta* = 1.0 so with beta* = 0.6 they should expect 3.5/0.6 = 5.8/nb/s assuming intensity and emittance are the same. So the CMS figure looks better.
the VdM scans still need analysis before feeding back new calibrations. The "correct" value will lie somewhere between the present values of CMS and ATLAS.

The present luminosity is pretty good - since the intensity per bunch is still quite a bit lower than the peak values from last year. Presently running at 1.3x10+11. Last year the peak was close to 1.5x10+11. Increasing to 1.5x10+11 will give 33% more luminosity than at present! Don't expect this overnight though!

jmc2000
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Re: Why is there a difference in the Cms and Atlas luminosit

Post by jmc2000 » Thu Apr 19, 2012 10:32 pm

pcatom wrote:The present luminosity is pretty good - since the intensity per bunch is still quite a bit lower than the peak values from last year. Presently running at 1.3x10+11. Last year the peak was close to 1.5x10+11. Increasing to 1.5x10+11 will give 33% more luminosity than at present! Don't expect this overnight though!
It's amusing to see 2010's total integrated luminosity routinely captured in 2.5 hours of running today. 1fb/week will soon be upon us in the coming months.

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Re: Why is there a difference in the Cms and Atlas luminosit

Post by PhilG » Fri Apr 20, 2012 9:49 pm

Not overnight but maybe over a couple of nights. They are aiming for 1.45 E11 now!

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Re: Why is there a difference in the Cms and Atlas luminosit

Post by PhilG » Sat Apr 21, 2012 7:12 am

PhilG wrote:Not overnight but maybe over a couple of nights. They are aiming for 1.45 E11 now!
They didn't get it to work but nice try.

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Re: Why is there a difference in the Cms and Atlas luminosit

Post by jmc2000 » Sun Apr 29, 2012 4:27 pm

Looks like the Cms was out of calibration because the recorded instantaneous luminosity has been reduced from 6 to 5.6/nb/s which now matches Atlas.

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