The Large Hadron Collider has today reached new record luminosities of about 2.8 Megahertz per barn using 25 bunches per beam. Each experiment now sees 16 collisions per turn which is twice the number in the previous scheme that used 13 bunches per beam. With some tuning slightly higher luminosities will be possible.

The current plan confirmed by Steve Meyers at ICHEP is to run with this scheme constantly during August. This will give the collider control teams the opportunity to perfect their operations and improve stability so that they can get the maximum amount of stable beam time.

It should now be possible to sustain average luminosities of about 2 MegaHertz per barn with this scheme. If they can produce enough hours of stable beam per day during August the experiments could collect about ten times the integrated luminosity seen so far. From results presented at ICHEP - especially the dijet and dimuon observations – there is some hope that new physics at above 1 TeV could already become evident with such a data sample.

After August they will once again start to step up the number of bunches ready to collect the much higher target of 1 inverse femtobarns by the end of 2011.

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10 Responses to Record Luminosity at LHC with 25 Bunches

Dear Phil, atlas.ch says that they have collected 32 billion collisions which, I believe, is an inverse picobarn, 0.001 of inverse femtobarn scheduled by 2011. One thousandth of the annual goal – already good.

May I ask you a stupid question? Those numbers for ATLAS, assuming that they are ATLAS numbers only, lead to a strange cross section. For example, 13.5 billion collisions appeared when the integrated luminosity was claimed 0.34/pb or so.

So if you divide the figures, you get a cross section about 0.25 barn or so. But the proton-proton inelastic cross section should only be 0.1-0.13 barn, right? So they have seen twice as many collisions as expected, didn’t they? Or are the collision numbers on atlas.ch sums over ATLAS and CMS?

Or what am I missing? You may answer anywhere on TRF, it could be appreciated. Cheers, LM

I think the cross-section figure that should matter is the total inelastic cross-section which should be about 60 mb at these energies.

When they showed 32 billion collisions they had a total luminosity of about 0.5/pb. 32/0.5 = 64 mb so that looks about right.

It is not always clear at what point their collision counts are taken. Sometimes they may not be updated quickly. I think the 13.5 billion count must have been for an earlier smaller luminosity than the 0.34/pb they had at that time.

The luminosity charts at http://lpc.web.cern.ch/lpc/lumiplots.htm have just been updated to show a total of 0.55/pb in Atlas. If this corresponds to the current count of 45 billion collisions, it indicates a cross section of 81 mb which is on the high side.

This means that cross section computed from those figures has gone from 39mb to 64mb then to 81mb. I cant see how that makes sense.

Another good run is underway now with a peak luminosity of 3/ub/s. That’s their highest so far so this could make a difference if they sustain it for a while.

They have only had a few runs since the 0.34/pb figure and I estimate they are still a little short of 0.5/pb. An increase from 13.5 billion to 32 billion therefore seems unlikely. Perhaps those figures are just wrong.

No, I am confident that the figures are correct. It’s just normal for the total integrated luminosity to double or triple every week or two. It’s still in the exponential growth. Too bad I couldn’t find out what the actual total cross section is – whether 0.12 barn or 0.25 barn.

Sorry I have not been able to come back to this. I have been away. It’s an interesting question and it would be nice to get to the bottom of it.

I had been watching the physics runs very carefully. After the 0.35/pb point they had a technical stop you three days followed by a week or so before they could get it running properly again. They have now doubled the number of bunches but at the point where this thread started they had only done some shortish physics runs at that level. I worked out the integrated luminosites and it came to an additional 0.15/pb. I am certian they had not reached 1/pb in total. This makes the sharp increase in number of collisions look wrong, unless something else needs to be taken into account.

Once I have caught up on my e-mail I’ll look at the cross section numbers.

thanks but I know these numbers – except for the figure for the elastic cross section which is highly modified. Note that the total cross section of elastic Coulomb scattering is infinite so your figure had to make an arbitrary cut. ;-)

But could you please make the multiplication for me? What is the total integrated luminosity for 13.5 billion or 36 billion collisions?

I suspect that they actually collect some elastic scattering events as well, taking an arbitrary piece from the infinite cross section, so that the total cross section as given by their number of events is not 110 mb but 250 mb or so.

Otherwise, the total cross section shouldn’t depend so much on the doubling/halving the total energy. I am convinced that it is at most 20% of a differenec per doubling.

I believe the number of collisions being reported by ATLAS (36 billion and counting) represents only the ones they’ve recorded, after the filtering has been applied. Whereas the integrated luminosity is everything, so the relationship between the two is not immediately apparent.

Dear Phil, atlas.ch says that they have collected 32 billion collisions which, I believe, is an inverse picobarn, 0.001 of inverse femtobarn scheduled by 2011. One thousandth of the annual goal – already good.

May I ask you a stupid question? Those numbers for ATLAS, assuming that they are ATLAS numbers only, lead to a strange cross section. For example, 13.5 billion collisions appeared when the integrated luminosity was claimed 0.34/pb or so.

So if you divide the figures, you get a cross section about 0.25 barn or so. But the proton-proton inelastic cross section should only be 0.1-0.13 barn, right? So they have seen twice as many collisions as expected, didn’t they? Or are the collision numbers on atlas.ch sums over ATLAS and CMS?

Or what am I missing? You may answer anywhere on TRF, it could be appreciated. Cheers, LM

I think the cross-section figure that should matter is the total inelastic cross-section which should be about 60 mb at these energies.

When they showed 32 billion collisions they had a total luminosity of about 0.5/pb. 32/0.5 = 64 mb so that looks about right.

It is not always clear at what point their collision counts are taken. Sometimes they may not be updated quickly. I think the 13.5 billion count must have been for an earlier smaller luminosity than the 0.34/pb they had at that time.

The luminosity charts at http://lpc.web.cern.ch/lpc/lumiplots.htm have just been updated to show a total of 0.55/pb in Atlas. If this corresponds to the current count of 45 billion collisions, it indicates a cross section of 81 mb which is on the high side.

This means that cross section computed from those figures has gone from 39mb to 64mb then to 81mb. I cant see how that makes sense.

Another good run is underway now with a peak luminosity of 3/ub/s. That’s their highest so far so this could make a difference if they sustain it for a while.

They have only had a few runs since the 0.34/pb figure and I estimate they are still a little short of 0.5/pb. An increase from 13.5 billion to 32 billion therefore seems unlikely. Perhaps those figures are just wrong.

I suggest asking on the Atlas blog http://www.atlas.ch/blog/

I will be disconnected from internet for next few days.

No, I am confident that the figures are correct. It’s just normal for the total integrated luminosity to double or triple every week or two. It’s still in the exponential growth. Too bad I couldn’t find out what the actual total cross section is – whether 0.12 barn or 0.25 barn.

Sorry I have not been able to come back to this. I have been away. It’s an interesting question and it would be nice to get to the bottom of it.

I had been watching the physics runs very carefully. After the 0.35/pb point they had a technical stop you three days followed by a week or so before they could get it running properly again. They have now doubled the number of bunches but at the point where this thread started they had only done some shortish physics runs at that level. I worked out the integrated luminosites and it came to an additional 0.15/pb. I am certian they had not reached 1/pb in total. This makes the sharp increase in number of collisions look wrong, unless something else needs to be taken into account.

Once I have caught up on my e-mail I’ll look at the cross section numbers.

According to a CERN document, here’s the pp cross section at 7 TeV

(Not clear, but I think they mean the origjnal 7 TeV per beam)

110 mb – Total, consisting of

40 mb – Elastic

60 mb – Inelastic

12 mb – Diffraction

Dear Bill K,

thanks but I know these numbers – except for the figure for the elastic cross section which is highly modified. Note that the total cross section of elastic Coulomb scattering is infinite so your figure had to make an arbitrary cut. ;-)

But could you please make the multiplication for me? What is the total integrated luminosity for 13.5 billion or 36 billion collisions?

I suspect that they actually collect some elastic scattering events as well, taking an arbitrary piece from the infinite cross section, so that the total cross section as given by their number of events is not 110 mb but 250 mb or so.

Thanks,

Lubos

Otherwise, the total cross section shouldn’t depend so much on the doubling/halving the total energy. I am convinced that it is at most 20% of a differenec per doubling.

I believe the number of collisions being reported by ATLAS (36 billion and counting) represents only the ones they’ve recorded, after the filtering has been applied. Whereas the integrated luminosity is everything, so the relationship between the two is not immediately apparent.