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Re: What to ask the LHC team about its operations or displays

Posted: Wed Nov 03, 2010 7:53 am
by Kasuha
amiso wrote:Now, assuming I understand the problem correctly, my question is: Do all the protons of a bunch cross the RF cavity during the same RF cycle?

I guess another way of asking the same question is: Do all the protons of one bunch reside in the same bucket?
Generally yes, one RF cycle defines one bucket and all protons of a bunch should be circulating in just one bucket, but in practice they always get certain amount of "leakage" where some protons pass the bucket boundary and start circling in satellite buckets, either before or after the main bunch. Their population is very small compared to main bunch but they still generate some off-center collisions.
They were measuring this effect some time ago and found protons leaking as far as five buckets ahead. They weren't able to measure leakage behind main bunch because their detector was oversaturated by main bunch data and wasn't able to measure them.

Re: What to ask the LHC team about its operations or displays

Posted: Wed Nov 03, 2010 5:29 pm
by amiso
Kasuha wrote:
amiso wrote:Now, assuming I understand the problem correctly, my question is: Do all the protons of a bunch cross the RF cavity during the same RF cycle?

I guess another way of asking the same question is: Do all the protons of one bunch reside in the same bucket?
Generally yes, one RF cycle defines one bucket and all protons of a bunch should be circulating in just one bucket, but in practice they always get certain amount of "leakage" where some protons pass the bucket boundary and start circling in satellite buckets, either before or after the main bunch. Their population is very small compared to main bunch but they still generate some off-center collisions.
They were measuring this effect some time ago and found protons leaking as far as five buckets ahead. They weren't able to measure leakage behind main bunch because their detector was oversaturated by main bunch data and wasn't able to measure them.
Ah, that's interresting. Those off-center collision can only upset the triggering systems of the detectors and add some noise on the measurements. I guess that they try to have as perfect bunches as possible.

Do you know if they have a procedure to "clean" those unwanted buckets?

Re: What to ask the LHC team about its operations or displays

Posted: Wed Nov 03, 2010 6:00 pm
by amiso
On the same note, I have one "dummy" question:

I understand that the beams are not colliding because there are some "collision bumps" that prevent the beams from overlapping. After the "squeeze" and "adjust" procedures, the bumps are removed so that the beams overlap at the IPs (interaction points). Now the dummy question:

Do they trigger the "overlaping" bunches "on demand" (dynamic), or instead they just align the beams one into another and then leave the overlap indefinitely, until the end of the fill.

This has a big impact on the way the detectors work: in one case, it would mean to 'arm' the detector, trigger the overlap, record, and then leave some time to download the data, recharge and start over...

In the second case, the events would happen more or less in a continuous (non-synchroneous) mode.

I guess my question is what is the data rate of the detectors and can the detector trigger the LHC, or is it the opposite ? Can the detector switch off the collisions when it is recharging?

Re: What to ask the LHC team about its operations or displays

Posted: Thu Nov 04, 2010 6:34 am
by Kasuha
Separating bumps are "bumps" on the beam trajectory. They adjust the beam trajectories so that they intersect at the proper point. They can adjust the trajectory any time so they can stop collisions after some time if they want, too. Sometimes they perform "end of fill studies" which usually involves re-applying separation bumps and switching off sensitive parts of detectors.
The fact that bunches of protons really meet at the collision point is given by precise timing of bunches which is in turn given by keeping the whole beam trajectory at precisely the correct length, too (one mm of difference in beam trajectory length over the 27 km ring may lead to the bunch position offset by 11 m in one second) - and this parameter is maintained all the time since the injection (otherwise it wouldn't be possible e.g. to accelerate the beam, because bunches must pass the RF cavity at the right time too).
Experiments cannot trigger collisions on or off. Only if there is something wrong they can trigger a beam dump.

Re: What to ask the LHC team about its operations or displays

Posted: Sun Nov 07, 2010 6:13 pm
by Oli
I don't understand why on the Page1 display, before Ion ops the E indicator said 3500GeV, now 3500 Z GeV.
Do you know why?
Thank you

Re: What to ask the LHC team about its operations or displays

Posted: Sun Nov 07, 2010 6:37 pm
by Kasuha
Because they did not update the display (yet).
The graph does not show particle energy, it shows current through bending magnets which is proportional to particle energy - but at different ration for protons and ions. So they are using the same current for ions as they were using for protons at 3.5 TeV - but for ions it actually means lower energy.

Re: What to ask the LHC team about its operations or displays

Posted: Sun Nov 07, 2010 7:21 pm
by PhilG
Z means the atomic number which is 82 for lead, so the energy per ion is 3.5 x 82 TeV

Re: What to ask the LHC team about its operations or displays

Posted: Sun Nov 07, 2010 9:09 pm
by Kasuha
It's colliding (208)Pb(82+) - 82 protons, 126 neutrons. Energy per particle (or per mass unit) is thus 3.5 x 82 / 208 = 1.38 TeV

Re: What to ask the LHC team about its operations or displays

Posted: Mon Nov 08, 2010 8:38 pm
by morgad
Are the LHCb team going to reset their fill luminosity back to zero?
It is causing problems with the integated luminosity graph

DAve

Re: What to ask the LHC team about its operations or displays

Posted: Tue Nov 09, 2010 12:51 am
by amiso
No, they never reset this. They just let it go.

Re: What to ask the LHC team about its operations or display

Posted: Fri Mar 11, 2011 10:47 am
by maliububarbosa
That's great for the status of the machine but won't show any collision info, I think. What I was one times hoping for was an overview showing how the various detectors were doing at logging collisions.