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The Standard Model and Beyond [Secs 17.1 Dunlap].

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1 The Standard Model and Beyond [Secs 17.1 Dunlap]

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

3 The Universal Fermi Interaction (UFI) 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. p n W+W+ W+W+ g g g g p→n 7% slower than expected from muon decay. Notbut

4 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 +, W 0 of bosons being force mediators W0W0 W - W+W+

5 Electro-Weak Unification W±W± W±W± W0W0 W0W0 W0W0 W±W± g+g+ g - The neutrino was to the electron what the up quark was to the down – members of a charge doublet – but a weak charge doublet SU2 (Special Unitary 2) symmetry group – similar to pion (ud) and nucleon (NN) symmetry

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 - W 0 W + But the Ws had to have mass to make the weak force weaker than the electric force. This theory required that (i) the masses of the Ws should be zero and (ii) that the neutral W 0 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 W 0 force was found to be experimentally weaker than the W ±. Something was wrong! Triplet Singlet B0B0

7 Electro-Weak Unification W - W 0 W + W - W + Z0Z0 (Triplet) (Singlet) (Quartet) It was discovered that the W 0 is not the observed particle eigenstate, but that a Higgs Field Particle H 0 was mixing things up to make a Z 0 and a ! The H 0 also gave the W and Z mass. H0H0 (Higgs scalar field) B0B0

8 Electro-Weak Unification W - W + Z0Z0 (Quartet) (Higgs scalar field) H0H0 The observed states and Z 0 are mixtures of more the more fundamental bosons W 0 and a B 0 Where θ W is the Weinberg angle ~28°

9 Electro-Weak Unification Observed Strength G F 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 ~ (M W.sin θ W ) 2. [θ w =Weinberg angle ~ 28°] Measurement of G F and sin 2 θ W (as obtained from weak neutral currents) was thus was able to predict the value of M W. Predicted masses of W and Z are 78 GeV/c 2 and 89GeV/c 2 which are close to the observed values. MZMZ MWMW

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= K) 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 Carlo Rubbia Simon Van-der-Meer 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 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/c 2. LEP gave some evidence at 115GeV/c 2 (1999). The Large Hadron Collider (LHC) at CERN – aimed at achieving 14TeV (14,000GeV/c 2 ) should be able to find it – if it is there!

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

14 The Standard Model c2000 Gauge Bosons FERMIONS BOSONS Quarks Leptons HADRONS (S+E-W)LEPTONS (E-W) g (1-8) H0H0 The Higgs “H 0 ” 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 EMSTRONG WEAK (i)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. (ii)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. (iii)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 One thing we know about the Strong, Weak and EM interactions is that their strength converges at energies ~ GeV Strength of Interaction Mass - Energy

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 and years. But Super-Kamiokande data put the half life as more than years. Simple SU5 now seems unlikely to be true. [The life-time of the universe is only 1.5 x years]


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