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Regina Caputo | USLHC | USA

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First pb^-1, courtesy of the LHC

Over the weekend the LHC was able to deliver our first pb^-1 of data! Milestones keep rolling on by and the data keeps rolling in. This is a big first step in getting what will hopefully be lots and lots of data. I’ve included a link to the ATLAS luminosity plot for your viewing pleasure. (CMS has one too… but I’m on ATLAS :))

To anyone who isn’t a particle physicist an inverse picobarn (pb^-1) is a pretty bizarre unit. I’ll start out with the base unit: the barn (b). It’s a measurement of area, proportional to m^2 or cm^2. The barn unit comes from when nuclear physics was in its infancy and refers to a uranium nucleus which is as big as a barn (1 barn = 10^-24 cm^2). (I still think physicists should hire writers to come up with this stuff… anywho, back to the post).

An inverse barn (or b^-1) in the particle physics world is a measure of collision events in an area of a barn. Throw in a metric prefix (pico which is 10^-12*base unit) and now you’re all caught up to speed. But what does that mean really? Fermilab has over an inverse femtobarn (fb^-1, which means 1000x an inverse picobarn) of data but of course they’ve been running their collider for over a decade. We’ll still need much more data to do searches for things like the Higgs, but very early searches are definitely underway – not to mention all the Standard Model physics and calibration that’s going on too.

So cheers to the first pb-1 of data… I can’t wait to start analyzing.



6 Responses to “First pb^-1, courtesy of the LHC”

  1. Bob says:

    Thanks for explaining that – the area definition is clear but its still not clear to me how many collisions you need in an area of a pico barn (equal to an area of 10^-36cm^2?) to say you have collected an inverse pico barn of data?

  2. anon says:

    Fermilab has much more than _one_ inverse femtobarn. Okay not quite yet an order of magnitude higher, but ~8 fb^-1 is much more than 1!

  3. Regina says:

    @Bob – Your unit conversion is spot on and… That is a good question. The total number of events expected (N) is equal to the luminosity*cross section. I found that the cross section of proton-proton collisions at 7 TeV is about 0.1 barns.
    With 1 pb^-1 of data that would yield 1*10^11 collisions (if I did the math right).

    @anon – thanks for the clarification. In the interest of full disclosure, I also said that the machine had run for a decade (silly truncation of orders of magnitude). In actuality it’s been running (with the Main Injector) since 1994 – which is 16 years. I was at the 1 fb^-1 party as a summer intern in the summer of 2005 – which was and still is an amazing accomplishment.

  4. jochen says:

    what is the reason of the low luminosity of alice? can you explain it?

  5. mark says:

    An open question to all the bloggers. I have just finished reading Michio Kaku’s Physics of the Impossible and in it he mentions “sparticles” and the fact that the LHC will be looking for it. In the posts I have read here I have never seen it mentioned (possibly I missed it :) ), just the search for the Higg’s Boson. Is it a focus of the work of the LHC? and when, or if, is evidience to prove, or otherwise disprove, likely to be found? Thank you all for the excellent posts.

  6. Seth Zenz says:

    Hi Mark,

    “Sparticles” is short for “supersymmetric particles,” and yes, Supersymmetry is one of the things we might find at the LHC. I’m sure we’ve mentioned it at once point or another!

    But from my perspective, whether we mention Supersymmetry, the Higgs Boson, or something else, it’s really only examples. We pick a few common ideas from our toolkit to explain how we might look for things, but the truth is that we don’t know what we’ll find, and our detectors our built to see anything that’s there.


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