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Ken Bloom | USLHC | USA

View Blog | Read Bio

A great fill!

Yesterday and last night (US time), the LHC had a really amazing fill. CERN DG Rolf Heuer just sent a message about it, and it says it so well I’m just going to copy it here:

A long period of machine development paid dividends last night with a game-changing fill in the LHC. As I write this, the fill, which started colliding at 19:00 yesterday evening, has just wound down. Both ATLAS and CMS have posted integrated luminosities of over 680 inverse nanobarns, and the initial luminosity for the fill doubles the previous record at 2 x 1031cm-2s-1.

But it’s not the records that are important this time – it’s normal that in the start-up phase of a new machine, records will fall like autumn leaves – what’s significant here is that the LHC’s performance this fill significantly exceeded some crucial design parameters, opening up the path to much better still to come.

Last night’s fill was the first with 56 bunches arranged in trains of eight bunches per train. The significance of bunch train running is that we can configure the orbits such that more bunches collide in the experiments, so even though the number of bunches may not be much higher, the collision rate is. For example, last night’s 56-bunch fill had 47 bunches colliding at ATLAS, CMS and LHCb, with 16 colliding in ALICE, whose needs are lower. This compares to a maximum of 36 colliding bunches out of 48 total before we introduced bunch trains.

A big jump in luminosity was clearly expected in moving to bunch trains and colliding more bunches. What came as a pleasant surprise is that it was accompanied by an exceptional beam lifetime of 40 hours, and less disruption to the beams caused by packing more protons into a smaller space (in technical terms, the beam-beam tune shift was much less destructive to the beams than anticipated). This result means that the LHC operators have more leeway in operational parameters in the quest for higher luminosity.

The plan for today and the weekend is to run for one more fill with 56 bunches in trains of eight before moving on to 104 bunches in 13 trains of eight, with 93 bunches colliding in ATLAS and CMS. Ultimately, the LHC will run with 2808 bunches in each beam, so there’s still a long way to go. We’ll get there slowly but surely by adding bunches to each train until the trains meet in a single machine-filling train. That will take time, but for the moment, last night’s fill puts us well on the way to achieving the main objective for 2010: a luminosity of 1032 cm-2s-1.

To put this in perspective: the much-heralded LHC results at the July ICHEP conference were based on about 250 inverse nanobarns of data, drawn from about 350 inverse nanobarns delivered by the LHC from April through mid-June. Yesterday, in less than one day, the LHC delivered almost double that entire amount! The state of play will be changing quite rapidly on all LHC experiments. Stay tuned!


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  • Thats Great…Can’t Wait To See What LHC Gonna Show ^_^

  • hans

    Hi Ken,
    As I’m no physisist, I’not sure if this is much of a question, but it’s someting that puzzeled me this night while driving home in my car, after noticing a Doppler sound effect.
    I was wondering if, when protons in a bunch pass through a bunch coming “their” way, would the proton bunches then experience something like a Doppler effect? Maybe not through the high vacuum in the tube, but through someting like a “proton medium” that perhaps occurs in each bunch. And if this bunch-doppler would exist, does it disrupt the bunch parameters (like diameter)?

  • Ken Bloom

    Hi Hans,

    There is in fact an effect that is something like you describe, although it has nothing to do with the Doppler effect. As two protons collide, one can in fact do something much like “focusing” the other. This is known as “diffraction”. I don’t know much about the topic, but we can in fact see diffractive events in the detector; since the protons keep going after diffracting, most of the energy in the collision is seen along the beamline, i.e. in the most forward part of the detector.

    As I write at this very minute, the LHC seems to have broken another luminosity record, thanks to filling the machine with 248 bunches. Whole lot of diffraction going on. Best wishes.