Newbie: Lumi difference between Atlas and CMS

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jlourenc
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Newbie: Lumi difference between Atlas and CMS

Post by jlourenc » Wed Apr 27, 2011 4:56 am

Hi

I was wondering why we have such a "big" difference in instantaneous luminosity between Atlas and CMS. I thought the experiments were supposed to get the same lumi.
On the Chat, I was told the reason was the beam separation of 0.5H in CMS. But why are they doing that? Should they not be aiming towards maximal luminosity? Is this a compromise between luminosity and background?

pcatom
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Re: Newbie: Lumi difference between Atlas and CMS

Post by pcatom » Wed Apr 27, 2011 8:13 pm

It is suspected that Atlas are presently reading a little high. To calibrate the luminosity correctly the experiments need to do Van de Meer scans. This involves scanning the beams through each other over a wide range to establish the full luminous region as well as a good knowledge of the intensity etc.

A VdM scan campaign is scheduled just after the next technical stop.

At the moment there is no indication that the real luminosity delivered to the 2 experiments is significantly different.

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DCWhitworth
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Re: Newbie: Lumi difference between Atlas and CMS

Post by DCWhitworth » Wed Apr 27, 2011 10:02 pm

Sheesh, how many billions have they spent on this machine and it turns out the speedometer is faulty ! :roll:

Seriously, thanks for the insider information pcatom, it's great getting the answer to these little mysteries :D
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tomey36
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Re: Newbie: Lumi difference between Atlas and CMS

Post by tomey36 » Thu Apr 28, 2011 4:37 am

Am i crazy or did they change the font on page 1?

Kasuha
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Re: Newbie: Lumi difference between Atlas and CMS

Post by Kasuha » Thu Apr 28, 2011 8:00 am

pcatom wrote:It is suspected that Atlas are presently reading a little high. To calibrate the luminosity correctly the experiments need to do Van de Meer scans. This involves scanning the beams through each other over a wide range to establish the full luminous region as well as a good knowledge of the intensity etc.

A VdM scan campaign is scheduled just after the next technical stop.

At the moment there is no indication that the real luminosity delivered to the 2 experiments is significantly different.
I always thought WdM scan was a method to find the "sweet spot" where the luminosity is the highest - while measured luminosity itself comes simply from counting collisions...

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DCWhitworth
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Re: Newbie: Lumi difference between Atlas and CMS

Post by DCWhitworth » Thu Apr 28, 2011 8:25 am

tomey36 wrote:Am i crazy or did they change the font on page 1?
Odd, I thought that too.
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PhilG
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Re: Newbie: Lumi difference between Atlas and CMS

Post by PhilG » Mon May 02, 2011 9:13 am

The luminosities of ATLAS and CMS seem to be slowly getting closer. Now they are only about 5% different.

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DCWhitworth
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Re: Newbie: Lumi difference between Atlas and CMS

Post by DCWhitworth » Mon May 02, 2011 10:27 am

PhilG wrote:The luminosities of ATLAS and CMS seem to be slowly getting closer. Now they are only about 5% different.
Sunday's report said that ATLAS had been tuning their luminosity measurements.
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PhilG
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Re: Newbie: Lumi difference between Atlas and CMS

Post by PhilG » Mon May 02, 2011 12:11 pm

It looked like CMS were also tuning theirs up a little. Obviously its all being done very scientifically :)

Kasuha
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Re: Newbie: Lumi difference between Atlas and CMS

Post by Kasuha » Mon May 02, 2011 1:35 pm

PhilG wrote:It looked like CMS were also tuning theirs up a little. Obviously its all being done very scientifically :)
Since they're getting about 14 collisions per bunch crossing and each collision can yield 2 to more than 100 particles, I bet it's not easy to calibrate. But in the end it's probably nothing more than multiplying trigger count by an empirically enumerated coefficient.

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Bornerdogge
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Re: Newbie: Lumi difference between Atlas and CMS

Post by Bornerdogge » Thu May 19, 2011 8:05 pm

Since the new VdM scans the difference disappeared.

If I remember well the lumi in CMS was even slightliy higher than in ATLAS...

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