Baby Steps for the LHC

By John Conway | April 28, 2010 2:33 pm

Since March 30, when the LHC at CERN first collided protons at an unprecedented total energy of 7 TeV (7 trillion electron volts) the machine has been steadily moving from crawling to walking. Last Saturday, I’d say it took its first steps, and like any toddler, will soon be running.

The plot shows what we call “integrated luminosity” which is simply a measure of the number of collisions of protons in the interaction regions at the four experiments. In this case, it’s my own experiment, CMS, the Compact Muon Solenoid experiment. CMS and ATLAS are the two large general-purpose detectors, each with thousands of physicists eager for real physics data.


As you can see, the vertical axis of the plot is labelled in units of “nb-1” or inverse nanobarns. The unit “barn” is a unit of area, a kind of joke from Enrico Fermi and friends who, despite the tiny size of a nucleus, said it was “as big as a barn” even though in cross sectional area it’s on the order of 10-28 m2 (which is in fact the definition of one barn). If we think about the cross sectional area of the protons colliding in the LHC, they have a cross sectional area (or simply a total collision cross section) of about 0.12 barns.

So what’s an inverse nanobarn? Well, if we try to collide lots of protons, we might ask “how many collisions per barn or cross sectional area did we make?” It’s like throwing little paint blobs at a wall, one at a time. Eventually the wall is covered, and then covered again, and then covered many times over. We can ask “how many paint blobs per unit area of the wall did we cover?” The nano in nanobarns means one billionth of a barn, and so, now, the LHC has managed to produce its first inverse nanobarn: one collision per every billionth of a barn of cross section.

It’s just a unit – all that matters is “how many collision events of my favorite kind should have been produced?” To get this, you multiply the number of inverse nanobarns by the production cross section for that kind of event, and also by the probability that you actually detect it. So for Z boson production, for example, the cross section is about 30 nanobarns, so we should have a few by now. (I am not at liberty to say whether we do or not…)

The plot has stair steps – the horizontal axis is real time, and the LHC machine is filled with protons, then brought to full energy, then collimators put in, then the experiment turns on and records data for some time until the accelerator folks decide to dump the beam out and refill. As you can see this cycle has been going like clockwork, with fill after fill of the machine. And the experiment has been recording a very large fraction of the delivered collisions, the losses being quite normal and due to end effects and the occasional glitch.

But then came the LHC baby’s first real step last weekend: squeezing the beam. By raising the quadrupole beam focusing magnets to high field, the transverse size of proton bunches in the machine shrinks down and the probability of collisions goes up. In this case, the luminosity went up by an order of magnitude – it was a stunning success. Any imperfection in the focusing fields can send the beam right out of the machine, and, clearly, that did not happen.

The goal in the next year is to get to one inverse femtobarn – a million times more data. In the next week or so the plan, if all goes well, is to achieve another couple orders of magnitude in luminosity. Shit’s about to get real, folks…

CATEGORIZED UNDER: News, Science, Technology
  • Adam

    YAHOO! Cool stuff & great news, John. Go splatter some more protons and make nice meson flowers in those detectors for us all to see.

  • Mandeep

    And this is the *good* kind of Shit. (so very unlike the Shitty deals of Goldman-Sachs — cf. Senate hearings of yesterday..).

    The *very* good kind.

    Exciting times ahead soon..?..!!!

  • Lab Lemming

    So does the flat bit from 20/4 to 24/4 mean that it took them 4 days to steer the beams into each other?

  • OXO

    So they dump the beam and refill because it has become polluted with other types of particles.. or what?

  • Chris

    Lab Lemming — I believe that from 20/4 to 24/4, the beam team worked on “squeezing” the bunches of protons that make up the beam. The smaller the bunch, the more collisions occur when bunches hit each other. They had to stop beam collisions to do this work, but when they began collisions again, on 24/4, the integrated luminosity began to increase much more quickly as a result of the squeeze.

  • nobody

    “So for Z boson production, for example, the cross section is about 30 nanobarns, so we should have a few by now. (I am not at liberty to say whether we do or not…)”

    I’ve heard some rumors by certain people that the LHC teams have seen something weird going on with the the Z bosons. So, this means that the rumors are true????

  • Ali

    I’m sort of surprised the luminosity is still so low! The Tevatron is getting, what, 50,000 inverse nanobarns per week? And the LHC’s design luminosity is over an order of magnitude higher, right?

    Yet even now, it looks like they’re getting, what, ~7 inverse nanobarns a week? A factor of 10^4 below the Tevatron? I know they’ve got plenty to do and the Tevatron’s been running a long time, but still, that seems like an surprisingly low level. Are they just not injecting many particles, or is the beam focus still just that bad?

  • Sili

    Ah. This baby is far more adorable than the one I just had dinner with.

  • John

    The flat stretches between stores are when there is no beam in the machine, and they are getting ready to put beam back in, or fixing something, or doing tests without beam.

    Nothing weird about the Z’s so far…we’ve made tons of them at LEP and the Tevatron. And we’ll make hundreds of them next week at the LHC.

    Indeed, Ali, we need a factor of a million more data from the LHC in the next year – so far we are on track to do that! This is how it starts.

  • Stephen Brooks

    OXO — The beam doesn’t become “polluted” (not that we know of, anyway!) but they do gradually lose protons from the ring over the course of hours and days. I think it’s just random motion within the beam and a few hitting the sides of the machine. Also protons within the same bunch can sometimes hit each other, or some residual gas due to imperfect vacuum in the beam pipe and scatter out.

    Ali — Yes, a slow commissioning period, there will be several more orders of magnitude in intensity to go because only 3 of the ~2000 LHC bunch locations are currently filled. Also as the (collision) energy is 7TeV compared to the Tevatron’s 1.96TeV, the cross-sections themselves are higher so you get more events for a given luminosity, see

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