The Standard Model and Beyond

1. The Standard Model and Beyond [Secs 17.1 Dunlap]

2. Electro-Weak Unification Steven Weinberg Sheldon Glashow Abdus Salam The 1979 Nobel Prize went to Glashow, Salam and Weinberg for: “for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles” It is arguably one of the most important theoretical achievements of the 20th century. They predicted the W and Z particles

3. The Universal Fermi Interaction (UFI) Not but n g p W+ Around 1935 Fermi had postulated that particles carried a “WEAK CHARGE” – just as particles carried a “EM Charge” – it should be the SAME IN ALL WEAK processes – but no: By 1963 Cabbibo mixing of quarks had brought the UFI back into line. g p→n 7% slower than expected from muon decay. g W+ g

4. W0 Electro-Weak Unification By the mid 1960s physicists started applying the principle of “gauge invariance” to Weak interactions. Application of “gauge invariance” to the EM field had led to the knowledge of the boson being force mediator. Application of “gauge invariance” to the Weak field led to another triplet set W-, W+, W0 of bosons being force mediators W+ W -

5. Electro-Weak Unification g+ W0 W0 W± g - W± W± SU2 (Special Unitary 2) symmetry group – similar to pion (ud) and nucleon (NN) symmetry W0 The neutrino was to the electron what the up quark was to the down – members of a charge doublet – but a weak charge doublet

6. Electro-Weak Unification It was first proposed that the electro-weak force was governed by the triplet of new bosons plus a singlet (another case of 2+2=3+1, 2 electric charge states – 2 weak charge states). W- W0 W+ Triplet Singlet B0 This theory required that (i) the masses of the Ws should be zero and (ii) that the neutral W0 currents (force) be as strong as that of the W± . But the Ws had to have mass to make the weak force weaker than the electric force.Moreover the W0 force was found to be experimentally weaker than the W±. Something was wrong!

7. Electro-Weak Unification W- W0 W+ (Triplet) B0 (Singlet) (Higgs scalar field) H0 Z0 W- W+ (Quartet) It was discovered that the W0 is not the observed particle eigenstate, but that a Higgs Field Particle H0 was mixing things up to make a Z0 and a ! The H0also gave the W and Z mass.

8. Electro-Weak Unification (Higgs scalar field) H0 Z0 W- W+ (Quartet) The observed states and Z0 are mixtures of more the more fundamental bosons W0 and a B0 Where θW is the Weinberg angle ~28°

9. Electro-Weak Unification Observed Strength GF of the Weak Interaction (as seen for example in beta decay) relates to the electric force The theory tells us that the observed force is much less than the electric force by ~ (MW.sin θW)2. [θw=Weinberg angle ~ 28°] Measurement of GF and sin2θW (as obtained from weak neutral currents) was thus was able to predict the value of MW. Predicted masses of W and Z are 78 GeV/c2 and 89GeV/c2 which are close to the observed values. MZ MW

10. Spontaneous Symmetry Breaking Maxwell’s equations are spatially symmetric – defining no special direction – yet a set of magnets (i.e. Fe atoms) tends to line up in some arbitrary direction. There is a spontaneous breaking of the symmetry of the EM laws. In the same way the Higgs field breaks the massless symmetry of the weak massless W fields. This causes the differentiation of the EM force from the weak force at low energies < 100,000MeV (T= 1012K) Most physicists believe that at the highest energy the universe has a single symmetry – that has been broken down into the 4 forces.

11. The discovery of the W and Z Simon Van-der-Meer Carlo Rubbia The 1984 Nobel Prize in physics was awarded to Rubbia and Van-der-Meer who led the CERN team in finding the W and Z particles.

12. Searching for the Higgs particle Theoretician Peter Higgs postulated the existence of the particle that bears his name in 1964. No one has yet discovered it – but the hunt is on. It is expected to be produced in the high energy interaction of quarks, but no one really knows how heavy it is. It is known to be heavier than 60 GeV/c2. LEP gave some evidence at 115GeV/c2 (1999). The Large Hadron Collider (LHC) at CERN – aimed at achieving 14TeV (14,000GeV/c2) should be able to find it – if it is there!

13. The Standard Model c2000 LEPTONS (E-W) HADRONS (S+E-W) Quarks Leptons Baryons FERMIONS Mesons BOSONS Gauge Bosons gi

14. The Standard Model c2000 LEPTONS (E-W) HADRONS (S+E-W) FERMIONS Quarks Leptons g (1-8) Gauge Bosons BOSONS H0 The Higgs “H0” is expected to be responsible for the masses of all the Fermions and Quarks as well as the W and Z.

15. Grand Unified Theories EM STRONG WEAK • The EM interaction gets stronger the closer one gets to the particle, because surrounding the particle is a cloud of e+e- pairs from virtual photons that screen the central charge. • The Strong interaction – the color charge gets extended in space. Shown is a red quark that has emitted RB gluons. Penetrating particles see less Red charge. • The Weak interaction – same as the strong – an electron emits W- particles that spread out the weak charge. A penetrating particle sees less Weak charge.

16. Grand Unified Theories Mass -Energy Strength of Interaction One thing we know about the Strong, Weak and EM interactions is that their strength converges at energies ~ 1015 GeV

17. GUT predicts decay of proton Through processes such as these the proton is expected to decay: i.e. to a positron plus a neutral pion. The SU5 theory predicts that the proton’s half life should be between 1030 and 1033 years. But Super-Kamiokande data put the half life as more than 1035 years. Simple SU5 now seems unlikely to be true. [The life-time of the universe is only 1.5 x 1011years]