the large hadron collider - technische universität...
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The Large Hadron Collider
...with its fresh Pb-Pb collisions in ALICE
...more at http://aliceinfo.cern.ch/Public/Welcome.html
Another Pb-Pb event in ALICE
...see also http://atlas.ch/, http://cms.web.cern.ch/
Basics on colliders
(...where charged particles are accelerated, guided and confined by electromagnetic fields)
bending: dipole magnets; (de)focusing: quadrupoles; acceleration: RF cavities
in a synchrotron (LHC) they are ramped synchronously to match the beam energy
• Energy: Bρ = p/q, B=bending field, ρ=radius, p=momentum, q=charge
• Luminosity: L × σX = NX/second, σX=cross section for process X
L =N2nbfrev4πσxσy
F , N=nr. of particles per bunch, nb=nr. of bunches,
frev=revolution frequency, σx,y=colliding beam sizes, F=geometric factor
F = 1/
√
1 +(
θcσz
2σ∗
)2
, θc=crossing angle
L ∼ 1/β∗ beam size: σx(s) =√
ǫβx(s) + [Dx(s)δ]2
ǫ=transverse emittance; δ = (∆p/p)rms=bunch energy spread
βx(s)=beta function; Dx(s)=dispersion ...”lattice” properties
• Integral luminosity [cm−2]: Lint =∫
Ldt ...[1 fb−1=1000 pb−1=106 nb−1 (1 b = 10−24 cm2)]
for Lint = N fb−1 will get N events of X if σX=1 fb
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The CERN accelerator complex
• p: LINAC2: 50 MeV; Booster (synchrotron): 1.4 GeV;
Pb: LINAC3 + LEIR (with electron cooling)
• Proton Synchrotron: 26 GeV (generates the bunch pattern)
• Super Proton Synchrotron: 450 GeV p; 157 AGeV Pb
• Large Hadron Collider: 14 TeV pp; 5.5 ATeV PbPb
The Large Hadron Collider
http://lhc.web.cern.ch/lhc/
The LHC in numbers (design values)
...”a behemoth” (O. Bruning, P. Collier, Nature 448 (2007) 285)
...probably humanity’s most complex project
• 27 km long, 8 arcs (∼3 km), each 46×(1 quadrupole+3 dipole megnets)
8 straight sections: RF cavities (IP4) + beam cleaning (IP3,7), dump (IP6)
• 1232 superconducting dipoles (+3700 correctors); 392 quadrupoles (+2500 correctors);
8 RF cavities/beam (400 MHz); 108 collimators and absorbers
• cooled with 120 tons of (superfluid) He at 1.9 K; 8.33 T (1.5-2 kA/mm2)
• 2808 bunches per ring, each 1.15×1011 protons (8 min filling time, 12 “ribbons”)
592 bunches per ring, each 7×107 Pb ions
• transverse beamsize: σx,y=16 µm (β*=0.55 m); bunch length: σz=7.6 cm
• beam kinetic energy: 362 MJ per beam (1 MJ melts 2 kg copper)
• total stored electromagnetic energy: 8.5 GJ (dipole magnets only)
The LHC dipole magnet
...admittedly, the mostinnovative component ofLHC ...“2-in-1 design”
• Length: 15 m
• Diameter: ∼1 m
• Weight: 30 tons
• Rings aperture: ∼ cm
transverse beam size:
below 1.3 mm
The life of the beams
circulation / acceleration / squeeze ...
alternating gradient lattice (“strong focusing”, arrangement of focus-defocus)
...and then collidevertically in CMS, LHCbhorizontally in ATLAS, ALICE
...and after some hours are dumped
...on to the beam dump: the place where the beam ends it’s “life”
-
• 7 m graphite
• 1 m Al
• 2 m Fe
• 1000 tons of concrete shielding
...and here is the dumped beam
...10 bunches (1.6 mm rms xy)
LHC “earthly” facts
• project approved in 1995 ...produced collisions in 2009 (70-40 m underground)
• the distance between bunches is 7.5 m, or 25 ns travel time
...there are also empty buckets (bunch slots) → mean colliding rate: 31.6 MHz
• the beams circles the 27 km ring 11245 times per second
• it takes ∼20 minutes to accelerate to full energy
• power consumption: 120 MW (experiments: 22 MW; cryogenics: 27 MW)
• total cost: 3 billion euros (LHC) + ∼1 billion euros (experiments)
• the superconducting cables (NbTi) are made of 6300 filaments, each 6 µm thick(total length: 70 light-minutes, or five times to Sun and back)
• the kinetic energy of 1 beam is like the ICE (200 tons) at 200 km/h
• 36800 tons are cooled down to -271.3 ◦C in 2 weeks (8 circuits)
...using 10800 tons of liquid nitrogen down to -193 ◦C
• 6500 m3 are pumped down to 10−13 atm (ultra-high vacuum)
The LHC in 2010
• Beam energy: p: 3.5 TeV; Pb: 208×3.5× 82208=287 TeV
• Bunch intensity: p: 1.2×1011; Pb: 7×107 (nominal)
• Maximum bunches: p: 368 (348 colliding in ATLAS & CMS; 16 in ALICE);
Pb: 121 (114 colliding in ALICE) ...as of 23.11
• Beam sizes: p: σx,y=40 µm, σz ≃7 cm; Pb: similar
bunch separation: p: 150 ns (50 ns tried); Pb: 500 ns
• Peak luminosity: pp: 2×1032 cm−2s−1; PbPb: 3×1025
• Delivered luminosity: pp: Lint=50 pb−1 (β*=3.5 m); PbPb: ongoing...
• Stored energy: pp: 28 MJ (25.2 MJ in stable beams)
...and plans for 2011:
• 4 TeV; 936 bunches (75 ns separation); β*=2.5 m
• run from end Feb. to 12 Dec. (with 4 weeks of Pb)
• L = 6.4×1032 cm−2s−1; Lint=2.2 fb−1; stored energy: 72 MJ [email protected]