The Large Hadron Collider: fundamental particles and big bangs Dr. James Monk Prof. Jonathan Butterworth Physics & Astronomy Department University College London
13/06/2011 Mont Blanc The Large Hadron Collider at CERN
Proton collisions at 14,000,000,000,000 eV*
Lake Geneva (eventually – this year 7,000,000,000,000 eV)
ATLAS Two “general purpose” experiments: ATLAS and CMS will detect the products of the collisions. CMS *dropping the proton 10km in the Earth’s gravitational field ~1eV A Toroidal Lhc ApparatuS
Small distance means high energy.
High energy wave means small wavelength
Low energy wave means long wavelength High Energy means near the big bang!
High energy density
Lower energy density
THEN NOW
Feynman diagrams Feynman diagrams are cartoons of the actual maths behind particle interactions.
Forces carried by 'virtual' particles – transient existence. An Equation
E = mc2 Electron - Positron annihilation
For the photon, E = 35 GeV, M=??? Virtual particles
Forces carried by 'virtual' particles – transient existence.
Don't have to have the 'correct' mass (“uncertainty principle”)
Probability of an exchange is proportional to 1/(Q2-M2)2 Electron - Positron annihilation at LEP
For the photon, E = 91 GeV, M=??? Electron - Positron annihilation at LEP
For the Z, E = 91 GeV, M=91 GeV !! Electron - Positron annihilation Quark-antiquark annihilation The weak force • Carried by the W and Z – W, Z have a high mass – Virtual W and Z are rarer at low energies than virtual photon – (remember exchange probability ~1/(Q2-M2)2) – This is the reason the weak force is weak.. • At high energies, therefore, the strengths should be the same.
Jon Butterworth, UCL 18 The weak force • Carried by the W and Z – W, Z have a high mass – Virtual W and Z are rarer at low energies than virtual photon – (remember exchange probability ~1/(Q2-M2)2) – This is the reason the weak force is weak.. • At high energies, therefore, the strengths should be the same. • And they are!
Jon Butterworth, UCL 19 Electroweak Symmetry Breaking • So the mass of the W and Z breaks the symmetry between the forces – But why are the W and Z so heavy when the photon is light? • In the theory, we are not at liberty to just invent stuff. – The Standard Model hangs together very beautifully. – Adding an arbitrary mass breaks the symmetries which make the SM work at all. – Adding a mass inserts a new scale into the theory. The Standard Model is supposed to be a fundamental model - true for all energy scales (remember how the strength of the forces changes with scale). Such a scale-invariant theory has more predictive power than one that isn’t. – Any new theory must “contain” the Standard Model, like relativity “contains” Newtonian mechanics.
Jon Butterworth, UCL 20 Electroweak Symmetry Breaking
• In the standard model, the “Higgs mechanism” gives the mass in a way which works. – “Spontaneous” symmetry breaking • Is it right? – If it is, ATLAS will see the Higgs (in fact Tevatron might even see it first). – If it’s not, the Standard Model is history, and ATLAS will give us at least a clue what the future is. • To find out, need an experiment with (more than) enough energy to study the region above unification – i.e. the Large Hadron Collider.
Jon Butterworth, UCL 21 How the Standard Model Higgs might appear at ATLAS
q “mono”-Jet Z mH Z b H _ q q H W υ
l
JMB, Davison, Salam, Rubin Jon Butterworth, UCL 22 How the Standard Model Higgs might appear at ATLAS
JMB, Davison, Salam, Rubin Jon Butterworth, UCL 23 At the LHC, what we see in this (and similar) interactions will tell us about
• Why the W and Z have mass and the photon is massless • Where mass comes from • Possible new particles and forces (supersymmetry?) • Possible new dimensions of space • Mini black holes/quantum gravity • Physics in a fundamentally new regime
A lot happening, and a lot to look forward to over the next few years...!