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CERN | Geneva | Switzerland

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What’s in a bunch?

CERN’s recent tweets have been cramming as much excitement as you can squeeze into 140 characters about the increasing number of bunches in the LHC beams, culminating with the record intensity for a hadron collider that was set with 480 bunches per beam last night. Time, then, to explain what that’s all about.

A beam in the LHC is not a continuous string of particles, but is divided into chunks a few centimetres long squeezed down to the size of a human hair at the collision point. Elsewhere in the ring the beam size varies but is normally less than a millimetre.

These chunks are what we call bunches. Each bunch contains about a hundred billion protons, and it’s a measure of just how small protons are that if you were to scale each one up to the size of a marble, the bunch length would be as far as the distance from Earth to Uranus and the width of the bunch would be something like the distance between the Earth and the Moon. Neighbouring marble sized protons would be as far apart as Geneva and Hamburg. So it’s not surprising that when bunches collide in the LHC, only a handful of proton-proton collisions happen.

Discovery in particle physics is a statistical process, so increasing the number of bunches is important. It increases the number of collisions, or the statistics as physicists put it.  The LHC is designed to run with 2808 bunches per beam, separated by a gap of just 25 nanoseconds. Since this is still early days in LHC running, we’re still at relatively low numbers and the bunch spacing is 50 nanoseconds. Nevertheless, building the number of bunches steadily this year towards last night’s record-breaking 480 per beam and beyond means that LHC experiments have already collected far more data so far this year that they collected in all of 2010.

Increasing the number of bunches in the beam is a stepwise process, since although each proton only has the energy of a mosquito in flight, by the time you multiply that by hundreds of billions, you have a large amount of energy stored in the beams. The operators need to be sure that the systems designed to protect the machine from damage are all ready before increasing the number of bunches. When the LHC reaches its full design potential, the beams will carry the energy of a 20,000 tonne aircraft carrier travelling at 12 knots. With 480 bunches per beam at half the LHC’s design energy, the energy stored in the beams equates roughly to the same aircraft carrier travelling at the rather more sedate pace of a little under 3 knots. Not quite so impressive, but a significant amount of energy nevertheless.

James Gillies and Mike Lamont

  • Pingback: From Quantum Diaries/CERN: “What’s in a bunch?” « Science Springs()

  • This is exciting news. Keep going!

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  • walid

    “if you were to scale each one up to the size of a marble, the bunch length would be as far as the distance from Earth to Uranus and the width of the bunch would be something like the distance between the Earth and the Moon.”

    waaaaaaaw O,o

  • Frank Tufaro

    Thanks for these explanations. It makes the news coming from LHC more relatable and engaging.

  • Peter

    It took a minute for me to grasp the enormity of the power of these beams. My first impression (shamefully!) was “that’s not too impressive” which changed to “I can’t believe it, that is truly mind-blowing, utterly incredible!”

    Thank you so much for these updates and explanations! Keep them coming please!

  • Amin

    Amazing!! I’ve been there and had the honor stepping on the ground where the LHC is burried underneath. Waiting with excitment for a new physics!

  • John Teagardin

    Do you bump up the total bunch count in multiples of 2? Would that then increase the energy by 2?

  • The HUBBLE Space Telescope had a bumpy start. The LHC had a bumpy start. HST has proven itself an outstanding tool to study the cosmos. The LHC is starting to prove its potential at the smallest scales. I believe many electrifying discoveries are just around the corner. Fascinating !

  • John D. Chato

    What is gravity a particle or ???

  • Initially we bumped the bunch count up by a 50 a go, now we’re stepping up by 144 bunches – so look out for 624 bunches soon. Doubling the number would indeed double the total energy stored in the beam.

  • The postulated carrier of gravity is a particle called a graviton – but no one’s seen one as yet.

  • Meto

    Super Amazing News Keep it Up! (BRAVO)

  • fluidic

    while 20,000 Ton aircraft does not yet exit, and travelling at 3knots is also infeasible unless it is moving or being dragged on airport runway, let look at this energy from eV viewpoint. If we make the calculations the compounded energy is equivalent to say 20,000-ton cargo coal train (200 cars) travelling at 3 knots = 47,637, 555.555J X 1.602 X 10(pow19) eV = 76.315 X 10(pow25) eV. If this is only little impressive! then what is impressive?

    It is highly impressive.
    am alam

  • Incredible! Thanks for the explanation and keep up the good work.

  • Renee de Villiers

    Thanks for the analogy of a proton size! It’s mind blowing what you do. South African best wishes x

  • fluidic says: “while 20,000 Ton aircraft does not yet exit, and travelling at 3knots is also infeasible unless it is moving or being dragged on airport runway”

    They are talking about an aircraft CARRIER! Read the post again.

  • Bon

    I wonder why modern physicists are obsessed with making scale comparisons when trying to communicate with non professionals. I was watching a Brian Cox show recently, and he spent half the program making tiresome physical analogies. I don’t find this technique compelling.

  • Cool, keep smashing them!

  • Sean Williams

    Love these explanations – they make all this stuff understandable for a layman like me.
    Once you have the beam storing the energy of a 20,000 ton aircraft carrier, how is it dissipated at the end of the Run/Experiment, without causing large amounts of damage!


  • Good question about how we dissipate the energy at the end of an LHC fill. The LHC beam can be safely extracted in under 0.3 milliseconds. The beam is extracted a bit at a time and steered towards a beam stop block whose core is made of graphite. On its way there, the beam is made as diffuse as possible so that the energy is spread across the beam stop block.

  • Michel Doidic

    It would seem that stopping the equivalent of a 20,000 ton aircraft carrier in 0.3 millisecond should release a huge amount of heat. How does the graphite block stop deal with that?

  • Quantum Diaries

    @Michel: There’s an excellent article from our friends at symmetry Magazine about how beam dumps work. The short answer is that all that energy is diffused through a graphite block that is eight meters long and one meter in diameter.

  • Michel Doidic

    Very good article indeed. Thanks for taking the time to explain your work.

  • Andrew Cliffe

    A fascinating appetizer. Where can we go for more?

  • Ashley

    Been following CERN and the LHC since I learned a bit about what you do there back in school. love seeing progress like this! Thanks for explaining.