Current Events Discussion

[jmayes]

Woot! Look at the pretty purples!

[gronos]

Why is the luminosity of Alice essentially zero, while the other detectors are above 16?

Rich

[PhilG]

ALICE is designed to study the heavy ion collisions that will run in Novemeber, so it does not need high luminosity now. They have configured the fill to give it just 8 bunch collisions per turn compared to 93 for the others, plus they have offset the beams so they only just collide.

[Kasuha]

Current beam parameters allow multiple p-p collisions in each bunch crossing. While this is what other experiments are built for, for Alice the most useful are single collisions - so beam parameters are set up so that the chance for just one collision per bunch crossing is most likely.

[gronos]

Thanks for the quick response. When the heavy ion experiments are begun in Nov., lead ions I believe, I assume the luminosity measurements will go way down since I would think it would be more difficult to get a high density of lead ions compared to protons. Plus much more energy must be needed to accelerate them compared to protons? Or am I mistaken in my understanding of how things will work?

Thanks,

Rich

[Harbles]

Another factor in the low luminosities at ALICE may be that they want to minimize radiation damage to the detector so they keep lumi down till they get Lead ion collisions.

Update...Doh!

Apparently ALICE is concentrating on low transverse momentum interaction events during the proton collision run and there is a problem with event pileup or multiple events happening at once that complicates the analysis hence low lumi to minimize this effect.

[Kasuha]

LHC is limited by its bending magnets - the speed of acceleration (i.e. the time it takes to accelerate to the maximum energy) is limited by how fast is it possible to increase current in bending magnets, and the maximum energy is limited by the maximum current it is possible to run through bending magnets. These two factors will not change for ion runs.
Of course, ions have worse ratio of charge to mass, and because amount of bending in bending magnets depends on charge/mass ratio, it will only be possible to accelerate ions to lower energy per particle.
And last but not least, ion runs will have to go through commissioning, too - so they will start with safe ion beams and increase the intensity over time.

[PhilG]

They are now on the third good run with 104 bunches, which qualifies them to go to the next step.

They seem to keep changing their mind about how long to make the bunch trains. Last week the plan was to start with 8 bunches per train, then 12 then 24. They wanted to keep the number of injections below 17. Now they plan to go to 152 bunches with trains of 8 which will mean 19 injections. They are also going to test 16 bunch trains today. We will have to wait to see if they will go all the way to 384 bunches with trains of 16 which would require 24 injections, or switch to 24 bunch trains later.

It looks like they are just trying to find the quickest way to get to 384 bunches so that they have time to run at that intensity, and possibly try one final step, such as a squeeze below 3.5m.

[tomey36]

is 384 the maximum number of bunches they are going to run this year?

[PhilG]

To put more than 384 bunches in they need to decrease the separation again. That would require more setting up and there isnt time for it. It is better to run as much as possible with 384 bunches to get as much data as possible before they have to stop.

[morgad]

Looks like they are now about to try 152 bunches,
filling scheme 150ns_152b_140_16_140_8bpi19injB

so I assume 19 injections x 8 bunches

Dave

[tomey36]

stable beams with 152 bunches! lumi near 5e31 new record!

[jmayes]

stable beams with 152 bunches! lumi near 5e31 new record!

Darn, I missed it Maybe next time

[ferar]

What does UFO #xx means? Seems it is causing beam dumps.
Thanks
Fernando.

[Kasuha]

I guess it refers to beam dumps caused by not yet fully understood events. They know which circuit tripped but they can see it's not the primary reason and they can't find what the primary reason is.