Why the Large Hadron Collider (LHC) ?

1. Why the Large Hadron Collider (LHC) ? • - 27 km ring, 100 m below the ground • 2 proton beams in opposite directions • - operation starts in Summer 2007 • pp collision energy : 14 TeV • (a football with the same energy would be moving • at 90000 Km/hour) 1 TeV = 10-7 Joule

2. Protons of such a high E (7 TeV) are bent inside the ring by 1232 high-tech superconducting magnets, providing an (unprecedented) magnetic field of 8.3 T. They work at 1.9 0K (-271 degrees).

3. 25 ns In the experiments: 109 pp interactions per second ~ 1500 particles (p,n, ) produced in the detectors at each bunch-crossing

4. ATLAS and CMS experiments • Multi-purpose : able to detect all • know particles • Fast response : ~50 ns • 108 electronic channels • Radiation hard (up to 106 Gy in the • hottest regions after 10 years of • operation)

5. A few more numbers ….. Number of ring turns made by protons in one second: 11000 Machine temperature : -271 degrees (largest cryogenic system in the world, cooler than the Universe) ATLAS size : length = 46 m, height=25 m Weight of CMS experiment: ~ 13000 tons (30% more than the Tour Eiffel) ATLAS and CMS equipped with 108 electronic channels Amount of cables used in ATLAS : ~ 3000 km Data collected by CMS in 1 second: equivalent to 10000 Encyclopedia Britannica Data collected by experiments in 1 year: ~ 30 km of CD ROM Number of physicists involved : > 4000 Etc. etc. etc.

6. WHY ???

7. Matter particles : fermions, spin =1/2 e  e   uc t d s b q= -1 q= +2/3 q= 0 q= -1/3 + anti-particles quarks u,d electrons The Standard Model of the elementary particles and their interactions Predicts 3 families of elementary “matter” particles Note : -- our world is made mainly of 1st family … -- m(e-) ~ 0.5 MeV, m(top)~ 175 GeV !

8. These “matter” particles interact via the EM, strong and weak forces. These forces are transmitted through the exchange of other elementary particles e e g relative to strong e e Force carriers : bosons, spin=1 Particle Force Coupling (E~100 GeV) Mass Intensity  EM0 ~ 10-1 (charged particles) e+ q g  W, Zweak~ 100 GeV ~ 10-5 (q, l, W, Z) q e- 8 gstrong0 1 (q, g) e e- W-

9. 1994 : top quark discovered at Fermilab pp Collider: m ~ 175 GeV Heaviest elementary particle ! 1983 : W, Z discovered at CERN pp Collider : m ~ 100 GeV as predicted by theory UA2 qq  Z e+e-  e+ Jet 4  Why do we like the Standard Model ? All the SM predictions (but one …), in terms of particles and features of their interactions, have been verified by many experiments at many machines Production and decay of a top-quark pair from CDF data 1989-2000: LEP e+e- collider at CERN: many precise measurements of Z particle  agreement theory-data at the permil level !

10. Mass of quark top (heaviest elementary particle observed)  mass of Gold atom Electron mass is 1 000 000 times smaller than top-quark mass Photon mass = 0 P.W. Higgs, Phys. Lett. 12 (1964) 132 Why we don’t like the Standard Model ? Still many open questions … first of all What is the origin of the particle masses ? The mass mystery could be solved by the “Higgs mechanism”, which predicts the existence of a new elementary particle : the Higgs particle This particle has been searched for 20 years at accelerators all over the world and has not been observed yet. The Higgs particle has a mass between 2 times the mass of an Iron atom and 4 times the mass of an Uranium atom. The LHC has sufficient energy to produce it. If the Higgs particle is not found at the LHC, the Higgs mechanism is wrong and we will have to find another solution to the mass problem

11. How would a Higgs particle appear in the experiments ? One example …

12. And many other mysteries … -- Why 3 lepton/quark families ? Why is the first family privileged ? -- Are there additional (heavy) quarks ? -- Are quarks and leptons really elementary ? -- What is the origin of matter / anti-matter asymmetry in the universe ? -- What is the nature of the Universe Dark Matter ? • Recent astrophysical measurements • (instruments on satellites) indicate • that the Universe is made of: • -- 5% of known matter • -- 25 % of “Dark Matter” • (no SM particle can explain it) • -- 70% of “Dark Energy” • today we understand only 5% of the Universe composition

13. A more fundamental theory of which SM is low-E approximation New Physics need a machine to explore the ~ TeV energy range CERN Large Hadron Collider (LHC) That’s why we believe today that there must be Best candidates : Supersymmetry Extra-dimensions Technicolour To solve the SM problems, all predict New Physics at  TeV energy

14. The best motivated theory beyond the Standard Model today is Supersymmetry Supersymmetry predicts a new spectrum of (heavy) elementary particles (in addition to those that we know), not yet observed. Among them the neutralino, which is our present best candidate to be the constituent of the Universe dark matter (its predicted features are in agreement with astrophysics observations and cosmological predictions) It is expected to be light enough to be abundantly produced at the LHC in the decay of heavier supersymmetric particles.

15. Universe cools down and energy density decreases with time • LHC energy corresponds to the Universe • energy ~10-10 s after the Big Bang • we expect to observe/reproduce in the lab similar phenomena as at that time …  SURPRISES ??? And more …. back in time toward the Universe origins …..