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Insights LHC Part 3: Protons as Large as a Barn - Comments

  1. May 25, 2015 #1


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  2. jcsd
  3. May 25, 2015 #2


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    So, how big is the side of an actual barn in barns?

    I remember having an "aha" moment after playing around with some dimensional analysis on extinction coefficients and determining that M-1 cm-1 is equivalent to an area, so extinction coefficients are just molar absorption cross sections.

    I've enjoyed reading your LHC series, and am looking forward to the next installment.
  4. May 25, 2015 #3


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    A yet interesting 3rd part about this topic...Thank you!

    " 75% of the design value has been achieved in 2012, the plan for 2015 is about 150%."

    What does this practically mean?
  5. May 25, 2015 #4
    1x10^(-24) cm^2 per barn and an average barn is 2 stories and about 75' in length.
    Putting it all together we get about 5.5x10^28 barns/barn.

    Or we could get really complicated on this and go with the atomic make-up of wood and figure out the 'real' barns/barn!
  6. May 25, 2015 #5


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    An African or European barn?
    Thanks :)

    What do you mean with practically? 0.77*1034/(cm2*s) was the record in 2012.
    In terms of collision rate, about 750 million per second in 2012, 1.5 billion per second in 2015. The bunch spacing goes down from 50 to 25 nanoseconds (40 instead of 20 million bunch crossings per second), so the number of collisions per bunch crossing will be similar, about 30-40 at peak luminosity. A bit lower if you consider the inelastic collisions only, as the elastic collisions are usually too close to the beam pipe to be visible (the small TOTEM experiment can see them).
  7. May 27, 2015 #6
    Interesting series of articles, thanks.

    One thing that puzzles me is how timing is handled.

    With so many collisions going on there could be some exotic quark-gluon plasma being formed that has a certain lifespan and brakes up into new particles emerging a fraction later ... all this with bunches traveling only a couple of meters behind each other with a dozen of nano seconds between them ... how is this measured when some collisions happening at the front of the 100 mb and their product could be mixed with things that happen at the end of the 100 mb long bunch, also taking in account that it are two bunches colliding extending the collision zone ... how long do collisions last ... mini black hole evaporates very quickly ... but on the other end of the spectrum a new proton being created would last forever ... how is it taken into account what lies in-between?
  8. May 27, 2015 #7


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    All those processes happen at timescales of the strong and electromagnetic interaction, about 10-22 seconds or shorter. The ratio to the time between bunch crossings (25 ns - so far most of the time the LHC used 50 nanoseconds but we had 25 ns and it is planned for the future - I'll use 25 here) is much more than 10 orders of magnitude.

    Produced particles are extremely short-living (as above) and/or ultrarelativistic, which means they or their decay products fly away at nearly the speed of light. At the time the next bunch crossing occurs, they are 7.5 meters away from the collision point. They are still in the detector at that time - the inner detectors start seeing new collisions while the products from previous collisions are still flying through the outer detector, but well separated by those 7.5 meters. Actually, if you take the length of ATLAS (46 m), products from three to four bunch crossings are in the detector at the same time. This is taken into account for the readout and data processing, of course.

    What do you mean with "front of the 100 mb"? A cross-section is not a distance.
    The bunches are a few centimeters long, all collisions in a bunch crossing (up to 30-40, see above) happen within a fraction of a nanosecond. The detectors record all their products together. Their separation along the beam axis is used to figure out which track came from which collision.

    Their could be new unstable particles that live relatively long compared to 25 nanoseconds. If they are charged, they would probably get stuck in the calorimeters and decay there at a random time, not synchronized with bunch crossings. There are dedicated searches for those particles as well. Examples: ATLAS, CMS
    If they live a bit shorter, they might look similar to heavy mesons, and produce a secondary vertex in the tracker.

    Finding unknown stable particles is much harder, because there is no decay to see. They would have to be so heavy that they move slower than other particles at the same energy, or get detected by kinematics (energy/momentum conservation).
  9. May 28, 2015 #8
    Great reply, thanks! This could have been 'PF Insights post' on it's own.

    I can imagine that a higher production rate is more helpful to produce larger statistics which makes it easier to spot specific types of particles such as the Higgs boson, but doesn't it add 'noise' and hinders finding more subtle particles with a longer lifespan?

    I mentioned it more vaguely in the sense like a Centuria going into battle ... the ones clashing in the front ... but thanks for clearing it precisely out.
  10. May 28, 2015 #9


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    The better statistics is a stronger argument. Models with long-living heavy particles are very exotic, you have to tune the interactions in the right way to suppress decays by orders of magnitude. And you would gain only for lifetimes between 10 and 30 nanoseconds, a tiny range in ~20 orders of magnitude of known particles lifetimes. The focus is on possible new short-living particles and studies of the known particles.

    There is a different negative effect: not all detectors are fast enough. The large LHCb tracker, for example, includes drift tubes that need up to ~100 nanoseconds to see their full signal. With 25 nanoseconds, they will get the signals from four bunch crossings combined.

    To get the scale right, you have about one man per light second (300 000 km) of your "front line".
    Last edited: May 28, 2015
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