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 +(

θcσz2σ∗

)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

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The Large Hadron Collider

http://lhc.web.cern.ch/lhc/

The LHC in numbers (design values)

...”a behemoth” (O. Brüning, 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)

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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

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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

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...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)

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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)

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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 A.Andronic@GSI.de