Current Events Discussion

The place to discuss the LHC. Commissioning, operation, issues, events ....
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mrgumby
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Re: Current Events Discussion

Post by mrgumby » Mon May 24, 2010 8:05 am

winding it up a little bit at a time...only way to fly!

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Bornerdogge
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Re: Current Events Discussion

Post by Bornerdogge » Mon May 24, 2010 9:15 am

They're injecting 13x13 bunches right now :)

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PhilG
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Re: Current Events Discussion

Post by PhilG » Mon May 24, 2010 11:08 am

If they can keep all those plates spinning and reach stable beams this fill should increase peak luminosities by a factor of nearly three. The increase in the number of bunches is a factor of 8/3 but intensities per bunch look a little higher than before. Let's see what they can get out of it.

I hope they use that extra plot they had up yesterday showing the luminosities over time.

The filling scheme they are using is this one: http://lpc.web.cern.ch/lpc/documents/Fi ... _8_8_8.txt
It is just four copies of the earlier 3x3 scheme with two extra non-colliding bunches. This scheme also has four displaced collisions so it is not as clean as the 6 bunch scheme used previously.

What is the purpose of the non-colliding bunches and why did they not use a better sheme without displaced collisions?

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Bornerdogge
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Re: Current Events Discussion

Post by Bornerdogge » Mon May 24, 2010 1:23 pm

There's an awesome 1.7e29 luminosity in CMS (thats a 15kHz event rate)!! But they had again some problems establishing collisions in ATLAS, and the luminosity in the other experiments is a little behind...

And apparently they lost ~5e10 protons in beam 2 during the squeeze...

edit: 1.4e29 in ATLAS and 1.5e29 in LHCb...

I think it's time to update the "luminosity record" page :character-beavisbutthead:
Last edited by Bornerdogge on Mon May 24, 2010 1:40 pm, edited 2 times in total.

Kasuha
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Re: Current Events Discussion

Post by Kasuha » Mon May 24, 2010 1:35 pm

PhilG wrote:What is the purpose of the non-colliding bunches and why did they not use a better sheme without displaced collisions?
I believe they are doing it intentionally - they can measure properties of each bunch over time and compare how different collision/passing modes affect them. Non-colliding bunches act as "control".

pcatom
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Re: Current Events Discussion

Post by pcatom » Mon May 24, 2010 2:55 pm

The collision point for LHCb is displaced with respect to the other experiments. Providing collisions in LHCb therefore means displaced collisions in the other experiments.

The filling schemes are chosen to optimize the number of collisions in all 4 experiments.

zaim
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Re: Current Events Discussion

Post by zaim » Mon May 24, 2010 5:59 pm

Now when there is no beam, it seams like a good time to give a status over the weekend + monday so far.

Fill 1117
- CMS wasn't reporting luminosity during the first part of this fill. They probably recorded a bit more than Atlas

Atlas : Integrated luminosity: 9.342e+002 ub^-1
Alice : Integrated luminosity: 9.118e+002 ub^-1
CMS : Integrated luminosity: 1.915e+002 ub^-1
LHCb : Integrated luminosity: 1.070e+003 ub^-1

Fill 1118
Atlas : Integrated luminosity: 7.356e+002 ub^-1
Alice : Integrated luminosity: 8.066e+002 ub^-1
CMS : Integrated luminosity: 9.961e+002 ub^-1
LHCb : Integrated luminosity: 9.336e+002 ub^-1

Fill 1119
Atlas : Integrated luminosity: 4.427e+002 ub^-1
Alice : Integrated luminosity: 6.322e+002 ub^-1
CMS : Integrated luminosity: 7.931e+002 ub^-1
LHCb : Integrated luminosity: 4.797e+002 ub^-1

Fill 1121
Atlas : Integrated luminosity: 1.072e+003 ub^-1
Alice : Integrated luminosity: 9.865e+002 ub^-1
CMS : Integrated luminosity: 1.259e+003 ub^-1
LHCb : Integrated luminosity: 1.108e+003 ub^-1

Total fill 1119-1121 (fill 1120 didn't reach stable beam)
Atlas : Integrated luminosity: 3.185e+003 ub^-1
Alice : Integrated luminosity: 3.337e+003 ub^-1
CMS : Integrated luminosity: 3.240e+003 ub^-1
LHCb : Integrated luminosity: 3.591e+003 ub^-1

At the status meeting on May 5th Atlas and CMS had 1.1e3 ub^-1, which they had recorded during the previous month. The same amount of luminosity was delivered during 2.5 hour monday :D

CMS would have had ~3.98 ub^-1 in total, if their lum counter wasn't broken in fill 1117.
Lets hope they will be up and running with 13x13 soon, so more good data can be put on tape.

Cheers Anders (Zaim)

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mrgumby
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Re: Current Events Discussion

Post by mrgumby » Tue May 25, 2010 8:01 am

Wow!......what else is there to say?

Kasuha
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Re: Current Events Discussion

Post by Kasuha » Tue May 25, 2010 8:52 am

Surprising how the luminosity can decrease to less than half over several hours even though there is almost no change in beam intensity. Apparently quality of the beam is really important factor.

And I like the luminosity graph on page 1.

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PhilG
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Re: Current Events Discussion

Post by PhilG » Tue May 25, 2010 9:00 am

Yes, the hump has been causing the beam to spread out which could cause a drop in luminosity even before intensity is lost. It will be interesting to see if they provide an explanation.

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Tim Bergel
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Re: Current Events Discussion

Post by Tim Bergel » Tue May 25, 2010 11:27 am

pcatom wrote:
The collision point for LHCb is displaced with respect to the other experiments. Providing collisions in LHCb therefore means displaced collisions in the other experiments.
Can any guru out there explain why that is? Surely it would be much easier if LHCb was not so displaced and a set of bunches could collide in all the experiments. So presumably there was a reason why it had to be like that, no?

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DCWhitworth
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Re: Current Events Discussion

Post by DCWhitworth » Tue May 25, 2010 11:31 am

Tim Bergel wrote:pcatom wrote:
The collision point for LHCb is displaced with respect to the other experiments. Providing collisions in LHCb therefore means displaced collisions in the other experiments.
Can any guru out there explain why that is? Surely it would be much easier if LHCb was not so displaced and a set of bunches could collide in all the experiments. So presumably there was a reason why it had to be like that, no?
I guess it's because the tunnel system was largely recycled from the LEP accelerator, some modifications were made to the experiment areas but probably moving or creating a new area was deemed too expensive.

In many ways the LHC has been built 'on the cheap' with several compromises. For instance the tunnel itself it too narrow to accomodate two independant beam pipes so they have been all built into one structure which must also complicate matters.
DC

The LHC - One ring to rule them all !

Kasuha
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Re: Current Events Discussion

Post by Kasuha » Tue May 25, 2010 1:15 pm

The cavern where LHCb is situated is placed symmetrically, i.e. the distance between the Atlas cavern and the LHCb cavern is the same as the distance between Atlas cavern and Alice cavern. But the LHCb itself is asymmetrical and to compensate it, the collision point was moved. This means the distance between Atlas collision point and LHCb collision point is different than the distance between Atlas and Alice collision points.

Something like this:

Code: Select all

        Alice           Atlas           LHCb
       [==*==]---------[==*==]---------[=*===]
equal         <------->       <------->
unequal    <-------------> <------------>
The difference is not big (11.25 m) which means that evenly spaced beam patterns which would collide in Alice and Atlas would not collide at all in LHCb, instead just pass by 11.25m from the collision point - not producing any collisions, but still affecting each other (beams are still very close to each other at that distance).

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Bornerdogge
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Re: Current Events Discussion

Post by Bornerdogge » Wed May 26, 2010 8:09 am

Apparently they had their first 3.5TeV collisions with high intensity bunches last night... Has anyone seen what luminosity it gave?

EDIT: they're trying again right now! There's a 4e28 luminosity in CMS and Atlas, with only 1 bunch, which is 4 times higher than the first run on march 30th... Though as luminosity increases with the square of the intensity, the expected theoretical value would have been 100 times higher. There's still much work to do in correcting orbits and squeezing the beams!

The plan for the night is 8x1e11 :shock:

Kasuha
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Re: Current Events Discussion

Post by Kasuha » Thu May 27, 2010 11:54 am

I think they had the beam un-squeezed due to some problems.

I wonder if they'll consider using logarithmic scale in beam intensity graph on page 1... the pilot bunch is almost invisible with nominal bunches being used...

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