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accelerator centers worldwide

accelerator centers worldwide. the world’s largest accelerators. cross sections vary over many orders of magnitude inelastic: 10 9 Hz W -> l n : 100 Hz tt: 10 Hz Higgs (100 GeV): 0.1 Hz Higgs (600 GeV): 0.01 Hz required selectivity

brittania-sellers
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accelerator centers worldwide

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  1. accelerator centers worldwide

  2. the world’s largest accelerators

  3. cross sections vary over • many orders of magnitude • inelastic: 109 Hz • W ->ln: 100 Hz • tt: 10 Hz • Higgs (100 GeV): 0.1 Hz • Higgs (600 GeV): 0.01 Hz • required selectivity • 1 : 10 10- 11 • trigger - cross sections and rates

  4. projectiles • SPS and Fermilab used proton-antiproton collisions • LHC uses proton-proton collisions • why?

  5. proton-antiproton proton-proton

  6. how big is a proton? • roughly 1 fm (10-15 m) • “femtometer” or “fermi” • 1 barn is the area of a 10 fm × 10 fm square • big unit • derived from uranium nucleus • physicists joked: “that cross section is as big as a barn” • proton-proton cross section at LHC energies: 70 mbarn • = 7 fm2 • r ~ 1.5 fm

  7. luminosity • (instant) luminosity is rate per cross section • usualunits: cm-2 s-1 • e.g., 1030 cm-2 s-1 corresponds, for a reaction cross section of 10-30 cm-2 ( = 1 μbarn), to a rate of 1 event per second • for a collider, the luminosity can be calculated as follows:

  8. integrated luminosity • number of events collected divided by the cross section • usual units: nb-1 (“inverse nanobarn”), pb-1 (“inverse picobarn”) etc. • an integrated luminosity of 1 fb-1 means that for a process with a cross section of 1 fb, 1 event (on average) should have been collected • or 1000 events for a cross section of 1 nb, etc. • so, 1 inverse femtobarn = 1000 inverse picobarns : • 1 fb-1 = 1000 pb-1 • physicists are now looking for very rare events, so it is vital to reach not only high energies (so that heavy particles can be produced) but also high luminosities • handling the resulting data rates is a challenge also for the detectors, trigger systems, and readout electronics

  9. Instantaneous luminosity • Nearly all the parameters are variable (and not independent) • Number of bunches per beam kb • Number of particles per bunch  • Normalized emittance n • Relativistic factor (E/m0)  • Beta function at the IP  * • Crossing angle factor F • Full crossing angle c • Bunch length z • Transverse beam size at the IP * Total Intensity Beam Brightness Energy Interaction Region

  10. LHC proton-proton circumference: 27 km bunches: 3564 + 3564 protons / bunch: 1011 beam energy: 2 x 3.5 (7) TeV luminosity: 1033-1034 cm-2s-1 bunch spacing: 25 ns collision rate: 108 - 109 Hz dipole field: 8.4 T number of dipoles: ~ 1200 heavy ions (Pb-Pb) beam energy: 2.8 (5.5) TeV / nucleon pair luminosity: 1027 cm-2s-1

  11. how to hit a proton • p ~ 1 fm • beam ~ 10 - 100 μm = 1010 - 1011 fm • ratio of area: 1020 • 10-20 chance to hit one proton • 1011 protons per beam • typical distance between protons: 10-10 m = 100’000 fm • rate: 1011 × 1011 × 10-20 = 102 • nominal LHC: ~ 20 interactions per bunch crossing (“pileup”) • achieved now: ~ 8

  12. beam sizes around Atlas

  13. layout of the LHC storage ring (built into the former LEP tunnel)

  14. I don’t want to fall into a black hooooolee...  !!! • some (few) physicists believe that at LHC energies we could already produce “mini black holes” • they would disappear very quickly • but what if they don’t ? • could they engulf the Earth? • eat up Cern, Geneva, Switzerland, Europa ... and then Siberia and Lake Baikal with the nice seals  ?? • are those scientists crazy ???? • don’t worry, be happy! • there are convincing experimental arguments that we are safe

  15. I don’t want to fall into a black hooooolee...  !!! • physicist: those black holes will evaporate much too quickly – we know that from calculations • concerned citizen: and what if those calculations are wrong (as usual)?? • physicist: the Earth has been bombarded by cosmic rays of much higher energy for the last 5 billion years and we are still here! • concerned citizen: but maybe then they are so fast they just whiz through the Earth and have no chance to stop and grow?

  16. I don’t want to fall into a black hooooolee...  !!! • physicist: at least some of them would be charged and would be slowed down by the Earth • concerned citizen: but maybe due to who knows why they are all neutral? Then they would fly through and we wouldn’t notice • physicist: through Earth, yes – but there are neutron stars and they are so dense that there the black holes would stop! And my astronomer friends tell me there are lots of neutron stars out there, so they (and we) are in no danger! • concerned citizen: you are right, Socrates! • oops ... the last answer must have crept in from one of Platon’s dialogues

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