TESTING THE FUNDAMENTAL LAWS OF NATURE AT THE ENERGY FRONTIER Roberto Contino Scuola Normale Superiore, Pisa INFN, Pisa Physics Colloquium - Università degli studi di Pavia - 26 March, 2020
Much of the progress in Physics has been driven by the quest of simplicity (reductionism) Several layers of structure in the microscopic description of matter have been uncovered at different length scales that are more and more fundamentals length 1 cm 1-100 μ m cells 2
Much of the progress in Physics has been driven by the quest of simplicity (reductionism) Several layers of structure in the microscopic description of matter have been uncovered at different length scales that are more and more fundamentals length 1 cm 1-100 μ m 10 -8 m cells molecules 3
Much of the progress in Physics has been driven by the quest of simplicity (reductionism) Several layers of structure in the microscopic description of matter have been uncovered at different length scales that are more and more fundamentals length 1 cm 1-100 μ m 10 -8 m 10 -10 m 10 -14 m 10 -15 m cells molecules atoms nuclei proton neutron 4
Much of the progress in Physics has been driven by the quest of simplicity (reductionism) Several layers of structure in the microscopic description of matter have been uncovered at different length scales that are more and more fundamentals Quarks and leptons appear point-like (i.e. fundamental) at the shortest scales probed so far (1 billionth of billionth of billionth of centimeter) length m 1 cm 1-100 μ m 10 -8 m 10 -10 m 10 -14 m 10 -15 m 1 0 - 2 0 10 -35 m Planck cells molecules atoms nuclei proton neutron length 5
Much of the progress in Physics has been driven by the quest of simplicity (reductionism) Reductionism in modern terms: • Theory with the fewest possible fundamental constituents (elementary particles) • All (but one) length/energy scales dynamically generated length m 1 cm 1-100 μ m 10 -8 m 10 -10 m 10 -14 m 10 -15 m 1 0 - 2 0 10 -35 m Planck cells molecules atoms nuclei proton neutron length 6
Particle Colliders: our most powerful microscopes λ Exploring small distances λ = h requires probes with short p wavelength, i.e. high momentum d To study their internal From the collision, new structure, particles are particles are created accelerated and made to collide 7
The Large Hadron Collider (LHC): the Lord of the collider rings protons collide with 13TeV center-of-mass energy in four interaction points circumference = 27km protons accelerated by up to 99.999999% of the speed of light 8
Experimental landscape in the late 1970s Energy A zoo of particles described in terms of a 5 GeV B few building blocks: quarks and leptons τ 1 GeV p, n The dynamics of quarks and leptons obeys ρ the laws of QED+QCD, a quantum field K theory based on SU (3) c × U (1) em π µ 1 MeV e ≈ 1 meV(?) ν 9
Experimental landscape in the late 1970s Energy A zoo of particles described in terms of a 5 GeV B few building blocks: quarks and leptons τ 1 GeV p, n The dynamics of quarks and leptons obeys ρ the laws of QED+QCD, a quantum field K theory based on SU (3) c × U (1) em π µ Hadronic mass scale explained dynamically 1 MeV e by QCD but key properties of spectrum rely on arbitrary quark and lepton masses ≈ 1 meV(?) ν 9
Experimental landscape in the late 1970s Energy A zoo of particles described in terms of a 5 GeV B few building blocks: quarks and leptons τ 1 GeV p, n Hadrons The dynamics of quarks and leptons obeys mass ∼ Λ QCD ρ the laws of QED+QCD, a quantum field K theory based on SU (3) c × U (1) em π µ Hadronic mass scale explained dynamically 1 MeV e by QCD but key properties of spectrum rely on arbitrary quark and lepton masses ≈ 1 meV(?) ν 9
Experimental landscape in the late 1970s Energy A zoo of particles described in terms of a 5 GeV B few building blocks: quarks and leptons τ 1 GeV p, n Hadrons The dynamics of quarks and leptons obeys mass ∼ Λ QCD ρ the laws of QED+QCD, a quantum field K theory based on SU (3) c × U (1) em Light Hadrons p π m q Λ QCD mass ∼ µ Hadronic mass scale explained dynamically 1 MeV e by QCD but key properties of spectrum rely on arbitrary quark and lepton masses ≈ 1 meV(?) ν 9
Experimental landscape in the late 1970s Energy A zoo of particles described in terms of a Heavy Hadrons 5 GeV B few building blocks: quarks and leptons mass ∼ m q τ 1 GeV p, n Hadrons The dynamics of quarks and leptons obeys mass ∼ Λ QCD ρ the laws of QED+QCD, a quantum field K theory based on SU (3) c × U (1) em Light Hadrons p π m q Λ QCD mass ∼ µ Hadronic mass scale explained dynamically 1 MeV e by QCD but key properties of spectrum rely on arbitrary quark and lepton masses ≈ 1 meV(?) ν 9
Experimental landscape in the late 1970s Energy A zoo of particles described in terms of a Heavy Hadrons 5 GeV B few building blocks: quarks and leptons mass ∼ m q τ 1 GeV p, n Hadrons The dynamics of quarks and leptons obeys mass ∼ Λ QCD ρ the laws of QED+QCD, a quantum field K theory based on SU (3) c × U (1) em Light Hadrons p π m q Λ QCD mass ∼ µ Hadronic mass scale explained dynamically 1 MeV e by QCD but key properties of spectrum rely on arbitrary quark and lepton masses Leptons ≈ mass = m ` 1 meV(?) ν 9
Experimental landscape in the late 1970s Energy A zoo of particles described in terms of a Heavy Hadrons 5 GeV B few building blocks: quarks and leptons mass ∼ m q τ 1 GeV p, n Hadrons The dynamics of quarks and leptons obeys mass ∼ Λ QCD ρ the laws of QED+QCD, a quantum field K theory based on SU (3) c × U (1) em Light Hadrons p π m q Λ QCD mass ∼ µ Hadronic mass scale explained dynamically 1 MeV e by QCD but key properties of spectrum rely on arbitrary quark and lepton masses Leptons ≈ mass = m ` Q: Can the whole spectrum be explained in 1 meV(?) terms of more fundamental scales ? ν 9
A new symmetry and a new force emerging at high energies p ¯ ν e e − In 1934 Fermi formulated a theory of weak interactions to explain nuclear beta decays W − n 10
A new symmetry and a new force emerging at high energies p ¯ ν e e − In 1934 Fermi formulated a theory of weak interactions to explain nuclear beta decays W − n By 1968 the electromagnetic and weak interactions were unified and incorporated into a complete theory based on SU (2) L × U (1) Y by Glashow, Salam and Weinberg 10
A new symmetry and a new force emerging at high energies p ¯ ν e e − In 1934 Fermi formulated a theory of weak interactions to explain nuclear beta decays W − n By 1968 the electromagnetic and weak interactions were unified and incorporated into a complete theory based on SU (2) L × U (1) Y by Glashow, Salam and Weinberg The carriers of the electroweak force, the W and Z bosons, were discovered at CERN in 1983 by an experimental collaboration led by C. Rubbia 10
� Electroweak symmetry “hidden” at distances larger than 1/ m W γ At large distances the weak force appears much weaker than W Z the electromagnetic one since W,Z bosons are massive, while the photon is massless 11
� Electroweak symmetry “hidden” at distances larger than 1/ m W γ At large distances the weak force appears much weaker than W Z the electromagnetic one since W,Z bosons are massive, while the photon is massless f > f critical � Example of spontaneous symmetry breaking: ☞ i) Equations of motions are symmetric ii) Their solutions (including the vacuum) are not 11
� Electroweak symmetry “hidden” at distances larger than 1/ m W γ At large distances the weak force appears much weaker than W Z the electromagnetic one since W,Z bosons are massive, while the photon is massless f > f critical � Example of spontaneous symmetry breaking: ☞ i) Equations of motions are symmetric ii) Their solutions (including the vacuum) are not � The theoretical formulation of SSB of a gauge symmetry was given in a series of papers by Brout and Englert, by Higgs and by Guralnik, Hagen and Kibble in 1964. 11
Quarks and leptons are both charged under the symmetry SU (2) L × U (1) Y SU (3) c SU (2) L U (1) Y q +1 / 6 ⇤ ⇤ ¯ u c 1 − 2 / 3 ⇤ ¯ d c 1 +1 / 3 ⇤ ` 1 − 1 / 2 ⇤ e c 1 1 +1 (1 family) 12
Quarks and leptons are both charged under the symmetry SU (2) L × U (1) Y Chiral SU (3) c SU (2) L U (1) Y SU (3) c SU (2) L U (1) Y Representations ¯ ¯ q +1 / 6 − 1 / 6 ⇤ ⇤ ⇤ ⇤ ¯ u c 1 − 2 / 3 ⇤ 1 +2 / 3 ⇤ ¯ d c 1 +1 / 3 ⇤ 1 − 1 / 3 ⇤ charge ¯ ` 1 − 1 / 2 1 +1 / 2 ⇤ ⇤ conjugation e c 1 1 +1 1 1 − 1 (1 family) Not the same ! 12
Quarks and leptons are both charged under the symmetry SU (2) L × U (1) Y Chiral SU (3) c SU (2) L U (1) Y SU (3) c SU (2) L U (1) Y Representations ¯ ¯ q +1 / 6 − 1 / 6 ⇤ ⇤ ⇤ ⇤ ¯ u c 1 − 2 / 3 ⇤ 1 +2 / 3 ⇤ ¯ d c 1 +1 / 3 ⇤ 1 − 1 / 3 ⇤ charge ¯ ` 1 − 1 / 2 1 +1 / 2 ⇤ ⇤ conjugation e c 1 1 +1 1 1 − 1 (1 family) Not the same ! � Bare masses not allowed (not gauge invariant) for chiral representations … … but, due to the spontaneous symmetry breaking, quarks and leptons propagate in the vacuum as massive fields 12
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