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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?

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.

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

Seriously, thanks for the insider information pcatom, it's great getting the answer to these little mysteries

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

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...

tomey36 wrote:Am i crazy or did they change the font on page 1?

Odd, I thought that too.

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

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.

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

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.

Since the new VdM scans the difference disappeared.

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