LHC portal

Hi,

does anybody know the reason for why the separation bump in IP2 is not collapsed completely (the CONFIG-Vistar shows a target separation of about -0.133 mm (if I remember correctly))? This (of course) reduces the delivered luminosity to ALICE drastically.

I read that is was planned to reduce the lumi in ALICE to a few 10^30 (assuming a nominal run), but the lumi they see now is quite below this threshold.

best regards,
ikarus177

A guess could be that they are having problems with pileup. I.e. problems reconstruction the tracks of particles when there are more than one collision per intersection. This would explain why it comes at the same time as the intensity per bunch is increased.
Alice is a little different from CMS and Atlas, in that their detector does not cover the entire solid angle around the beam pipe. This I guess makes it more difficult for them to reconstruct events with pileup.
But this is all speculation.

Cheers / Anders

Wait until the ion physics: then Alice will be the detector that really shines. Here the number of collisions will be low, but there will be an enormous amount of particles, and Alice is optimised for that situation.

I wonder if it is because these collisions aren't of great value to ALICE (it's not what it was built to do) so if they reduce the collisions there they also reduce the rate at which the luminosity drops a little ?

Hi,

I recently mailed a friend at CERN about this question.

He said that ALICE is mostly interested in the "original" pp-collisions, i.e. how many secondary particles are produced by a single event.
If you take two nominal colliding bunches, you will typically see 1.5 collisions per bunchcrossing. So, there might be 2 collisions will approximately the same primary vertex - this will look like a single event with much more secondary particles coming out.

So they just reduced the luminosity to avoid too much events and a lot of confusion

best regards,
ikarus177