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Prelude A.H. Chamseddine, A. Connes, Clifford Structures in Noncommutative Geometry The Spectral Action Principle , and The Standard Model of Particle Physics Commun. Math. Phys. 186 (1997), 731750. The noncommutative geometry approach to


slide-1
SLIDE 1

Clifford Structures in Noncommutative Geometry and The Standard Model of Particle Physics

Francesco D’Andrea 19/09/2017

Geometry and Physics Seminar, Penn State University, 19 September 2017

Prelude

A.H. Chamseddine, A. Connes, The Spectral Action Principle,

  • Commun. Math. Phys. 186 (1997), 731–750.

The noncommutative geometry approach to particle physics: algebraic reformulation of (quantum) field theory that works for spaces described by noncommutative algebras too. Following the “philosophical” or “meta-mathematical” point of view that:

  • C∗-algebras are a generalization of (locally compact, Hausdorff) topological spaces;
  • C∗-algebras with additional structure are a generalization of manifolds (smooth,

Riemannian, spin, etc.)

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Prelude

“A theory is a black box that we can shake to make predictions of physical observables.”

[ particlephd.wordpress.com ]

G , H , L G , H , L A , H , D / A , H , D / L L

◮ Classical Yang-Mills Theory: G = Lie group,

H = Hilbert space, L = Lagrangian

◮ Noncommutative Geometry: A = C∗-algebra, H = Hilbert space, D

/ = Dirac operator

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Punchline

A , H , D /

Two main goals:

  • derive the Standard Model (the complicated Lagrangian) from simple geometric data;
  • get some clues on unification with gravity.

Advantages:

  • The Lagrangian is not postulated but derived from the theory;
  • One gets for free the Higgs field (in the Standard Model case). . .
  • . . . and a theory coupled with (classical) gravity.

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slide-2
SLIDE 2

The Lagrangian of the Standard Model

LSM = −1 2∂νga

µ∂νga µ − gsfabc∂µga νgb µgc ν − 1

4g2

sfabcfadegb µgc νgd µge ν − ∂νW+ µ ∂νW− µ − M2W+ µ W− µ − 1

2∂νZ0

µ∂νZ0 µ −

1 2c2

w

M2Z0

µZ0 µ − 1

2∂µAν∂µAν − igcw(∂νZ0

µ(W+ µ W− ν − W+ ν W− µ )

− Z0

ν(W+ µ ∂νW− µ − W− µ ∂νW+ µ ) + Z0 µ(W+ ν ∂νW− µ − W− ν ∂νW+ µ )) − igsw(∂νAµ(W+ µ W− ν − W+ ν W− µ ) − Aν(W+ µ ∂νW− µ − W− µ ∂νW+ µ ) + Aµ(W+ ν ∂νW− µ − W− ν ∂νW+ µ )) − 1

2g2W+

µ W− µ W+ ν W− ν

+ 1 2g2W+

µ W− ν W+ µ W− ν + g2c2 w(Z0 µW+ µ Z0 νW− ν − Z0 µZ0 µW+ ν W− ν ) + g2s2 w(AµW+ µ AνW− ν − AµAµW+ ν W− ν ) + g2swcw(AµZ0 ν(W+ µ W− ν − W+ ν W− µ ) − 2AµZ0 µW+ ν W− ν ) − 1

2∂µH∂µH − 2M2αhH2 − ∂µφ+∂µφ− − 1 2∂µφ0∂µφ0 − βh 2M2 g2 + 2M g H + 1 2(H2 + φ0φ0 + 2φ+φ−)

  • + 2M4

g2 αh − gαhM

  • H3 + Hφ0φ0 + 2Hφ+φ−

− 1 8g2αh

  • H4 + (φ0)4 + 4(φ+φ−)2

+4(φ0)2φ+φ− + 4H2φ+φ− + 2(φ0)2H2 − gMW+

µ W− µ H − 1

2g M c2

w

Z0

µZ0 µH − 1

2ig

  • W+

µ (φ0∂µφ− − φ−∂µφ0) − W− µ (φ0∂µφ+ − φ+∂µφ0)

  • + 1

2g

  • W+

µ (H∂µφ− − φ−∂µH)

+W−

µ (H∂µφ+ − φ+∂µH)

  • + 1

2g 1 cw (Z0

µ(H∂µφ0 − φ0∂µH) + M ( 1

cw Z0

µ∂µφ0 + W+ µ ∂µφ− + W− µ ∂µφ+) − igs2 w

cw MZ0

µ(W+ µ φ− − W− µ φ+) + igswMAµ(W+ µ φ− − W− µ φ+)

− ig1 − 2c2

w

2cw Z0

µ(φ+∂µφ− − φ−∂µφ+) + igswAµ(φ+∂µφ− − φ−∂µφ+) − 1

4g2W+

µ W− µ

  • H2 + (φ0)2 + 2φ+φ−

− 1 8g2 1 c2

w

Z0

µZ0 µ

  • H2 + (φ0)2 + 2(2s2

w − 1)2φ+φ−

− 1 2g2 s2

w

cw Z0

µφ0(W+ µ φ− + W− µ φ+) − 1

2ig2 s2

w

cw Z0

µH(W+ µ φ− − W− µ φ+) + 1

2g2swAµφ0(W+

µ φ− + W− µ φ+) + 1

2ig2swAµH(W+

µ φ− − W− µ φ+) − g2 sw

cw (2c2

w − 1)Z0 µAµφ+φ−

− g2s2

wAµAµφ+φ− + 1

2igs λa

ij(¯

i γµqσ j )ga µ − ¯

eλ(γ∂ + mλ

e)eλ − ¯

νλ(γ∂ + mλ

ν)νλ − ¯

j (γ∂ + mλ u)uλ j − ¯

j (γ∂ + mλ d)dλ j + igswAµ

  • −(¯

eλγµeλ) + 2 3(¯ uλ

j γµuλ j ) − 1

3(¯ dλ

j γµdλ j )

  • + ig

4cw Z0

µ{(¯

νλγµ(1 + γ5)νλ) + (¯ eλγµ(4s2

w − 1 − γ5)eλ) + (¯

j γµ(4

3s2

w − 1 − γ5)dλ j ) + (¯

j γµ(1 − 8

3s2

w + γ5)uλ j )} + ig

2 √ 2W+

µ

νλγµ(1 + γ5)Ulep

λκeκ) + (¯

j γµ(1 + γ5)Cλκdκ j )

  • + ig

2 √ 2W−

µ

eκUlep†

κλγµ(1 + γ5)νλ) + (¯

j C† κλγµ(1 + γ5)uλ j )

  • +

ig 2M √ 2φ+ −mκ

e(¯

νλUlep

λκ(1 − γ5)eκ) + mλ ν(¯

νλUlep

λκ(1 + γ5)eκ

+ ig 2M √ 2φ− mλ

e(¯

eλUlep†

λκ(1 + γ5)νκ)

−mκ

ν(¯

eλUlep†

λκ(1 − γ5)νκ

− g 2 mλ

ν

M H(¯ νλνλ) − g 2 mλ

e

M H(¯ eλeλ) + ig 2 mλ

ν

M φ0(¯ νλγ5νλ) − ig 2 mλ

e

M φ0(¯ eλγ5eλ) − 1 4 ¯ νλ MR

λκ (1 − γ5)ˆ

νκ − 1 4 ¯ νλ MR

λκ (1 − γ5)ˆ

νκ + ig 2M √ 2φ+ −mκ

d(¯

j Cλκ(1 − γ5)dκ j ) + mλ u(¯

j Cλκ(1 + γ5)dκ j

  • +

ig 2M √ 2φ− mλ

d(¯

j C† λκ(1 + γ5)uκ j ) − mκ u(¯

j C† λκ(1 − γ5)uκ j

  • − g

2 mλ

u

M H(¯ uλ

j uλ j ) − g

2 mλ

d

M H(¯ dλ

j dλ j )

+ ig 2 mλ

u

M φ0(¯ uλ

j γ5uλ j ) − ig

2 mλ

d

M φ0(¯ dλ

j γ5dλ j )

Lagrangian of the Standard Model with neutrino mixing and Majorana mass terms (Minkowski space, Feynman gauge fixing). M. Veltman, Diagrammatica: the path to Feynman diagrams, Cambridge Univ. Press, 1994.

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Instructions: how to shake the red box

Mathematical Physics Studies

Walter D. van Suijlekom

Noncommutative Geometry and Particle Physics

( Chapter 12 → Phenomenology )

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Outline of the Talk

1

Toolkit for Yang-Mills Theory

2

“Historical” background:

◮ Grand Unified Theory (GUT) [ by Howard Georgi and Sheldon Glashow – 1974 ] ◮ KK-theory

[ Theodor Kaluza – 1921, Oskar Klein – 1926 ]

◮ Particle models and noncommutative geometry

[ Alain Connes – 1996: “Gravity coupled with matter and foundation of non-commutative geometry”, with John Lott – 1991, with Ali Chamseddine – since 1996 ]

3

Clifford Structures in Noncommutative Geometry and Morita Equivalence [ joint with Ludwik Dabrowski and Andrzej Sitarz ]

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Toolkit for Yang-Mills Theory

Classical (i.e. before 2nd quantization) Euclidean gauge theory:

1 A complex Hilbert space H, typically:

H = ψ ∈ L2(M, E)

  • sections of

cpx vector bundle E→M

⊗ v ∈ V 

  • finite
  • dim. v.s.

Usual QM interpretation: ψ = probability amplitude; v = internal degrees of freedom (spin, charge, etc.)

Remark: in the Standard Model, rk(E) = 4 (spinor bundle on a 4-dim. manifold). The spin degree of freedom is counted twice (in E and in V), cf. fermion doubling: F . Lizzi, G. Mangano, G. Miele, G. Sparano, Phys. Rev. D 55 (1997), 6357–6366. Another doubling is typical of Euclidean field theories and is cured after Wick rotation, cf. F . D’Andrea, M. Kurkov and F . Lizzi, Phys. Rev. D 94, 025030 (2016).

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SLIDE 3

1 A complex Hilbert space H , typically: H = L2(M, E) ⊗ V 2 A unitary representation on H of a compact Lie group G , the (global) gauge group. 3 A function L on H, the Lagrangian, that is G-invariant.

On H, there is a natural unitary representation of the local gauge group: G := Fun(M → G)

  • say C0 or C∞

One can make L invariant under G by introducing additional degrees of freedom (fields): gauge bosons = connections on E ⊗ (M × V) ↓ M

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Some “History”

In the Standard Model of elementary particles, G = U(1) × SU(2) × SU(3) and the representation π is dictaded by the experiments. One has: V = C32 ⊗ Cn

  • n generations

(From now on n = 1, to simplify the discussion.) The representation of G is highly non-trivial. Where does it come from? Consider the inclusion (in block form): ı : G = U(1) × SU(2) × SU(3) → SU(5) , (x, y, z) →

  • 2×2

3×3

↓ x3 y x2 z

  • It turns out that π is the restriction of the natural representation of SU(5) on ∧•C5

dim=32

.

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Some “History”

Thus:

  • H = L2(M) ⊗ ∧•C5 or more generally H = L2(M, E) ⊗ ∧•C5
  • G ⊂ SU(5) with its natural representation on H.

Questions:

  • What if we replace G by SU(5) ?

(SU(5) GUT: in its original form disagrees with experiments.)

  • ∧•C5 is the fiber of ∧•T ∗

CM if M a 5 dimensional manifold.

What if we replace H by differential forms on a 5-dim. manifold M5? The second idea was developed, in a different form, by Kaluza and Klein:

  • Gravity theory on M4 × S1 = gravity + Yang-Mills on M4.

problems: unobserved extra-dimension (compactification?), tower of unobserved particles. What if, instead of M5, we use M4 × F with F a 0-dim. nc-space? (No extra dimension.) What if we replace H by nc-differential forms? [ joint with L. Dabrowski and A. Sitarz ]

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Nc manifold = C∗-algebra + generalized Dirac operator

◮ Consider the triple: A = C(S1), H = L2(S1) , D = i d

dθ . It motivates the definition:

Definition

A unital spectral triple (A, H, D) is the datum of: (i) a (real or complex) unital C∗-algebra A of bounded operators on a (separable) complex Hilbert space H, (ii) a selfadjoint operator D on H with compact resolvent, such that the set (unital ∗-subalgebra) of Lipschitz elements:

  • a ∈ A : a · Dom(D) ⊂ DomD and [D, a] ∈ B(H)
  • is dense in A.

Example (finite nc spaces)

Take any finite-dimensional H, any A ⊂ B(H) and D ∈ B(H).

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SLIDE 4

Examples of spectral triples

Let: (M, g) = compact oriented Riemannian manifold without boundary, E → M herm. vector bundle equipped with a unitary Clifford action c : C∞(M, T ∗

CM ⊗ E) → C∞(M, E)

and a connection ∇E compatible with g. Then: A = C(M) H = L2(M, E) D = c ◦ ∇E is a spectral triple. Two main examples belonging to this class are:

◮ the Hodge operator D = d + d∗ on E = • T ∗

CM;

◮ the Dirac operator D = D

/ on the spinor bundle E (if M is a spin manifold).

Remarks

In both examples, H carries commuting representations of C(M) and Cℓ(M, g). In the former, it carries two commuting representation of Cℓ(M, g).

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Where is the gauge group?

Consider the following example: A = C(M → AF) with AF finite-dim. real or complex C∗-algebra. Then: Inn(A) = C(M → G) with G = Inn(AF) = U(AF)/U(Z(AF)) Example (Standard Model) AF = C ⊕ H ⊕ M3(C) = ⇒ U(AF) ≈ U(1) × SU(2) × SU(3)

(cf. unimodularity)

To get the correct quantum numbers, we have to use the adjoint representation of U(AF).

  • Definition. A spectral triple (A, H, D) is real if ∃ an antilinear isometry J on H s.t. J2 = ±1,

JD = ±DJ and, ∀ a, b ∈ A: [a, JbJ−1] = 0 (reality) [[D, a], JbJ−1] = 0 (1st order) The reality axiom ensures that the map: U(A) → B(H) , u → u JuJ−1, is a representation of U(A) (green and blue commute). The gauge group of the spectral triple is defined as: G(A, J) :=

  • uJuJ−1 : u ∈ U(A)
  • 12 / 21

Algebraic characterization of Dirac spinors

From the example of Hodge-Dirac operator, we learn: Real spectral triple ← → oriented Riemannian manifold Take M = R4: ψ ∈ Ω•(M) has 16 components. A Dirac spinor ψ ∈ L2(M, S) has 4 components. Both carry a rep. of C0(M) and Cℓ4,0(R), but only the latter satisfies the following: If a bounded operator commutes with C0(M) and all γµ’s, then it is a function. This completely characterizes Dirac spinors.

Theorem

  • 1. A closed oriented Riem. manifold M admits a spinc structure iff ∃ a Morita equivalence

C(M)-Cℓ(M) bimodule Σ, with Cℓ(M) the algebra of sections of the Clifford bundle.

  • 2. Σ = C0 sections of the spinor bundle S → M (Dirac spinors in the conventional sense).

Once we have S, we can canonically introduce the Dirac operator D of the spinc structure:

  • 3. M is a spin manifold iff ∃ a real structure J on L2(M, S).

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What is a noncommutative spin manifold?

A paradigm shift:

  • on a (spin) manifold one constructs the Dirac operator from the Clifford algebra;
  • on a nc-manifold we get the Clifford algebra from the Dirac operator.

Definition (1-forms)

If (A, H, D) is a spectral triple, we define Ω1

D ⊆ B(H) as:

Ω1

D := Span

  • a[D, b] : a, b ∈ A
  • Definition (Clifford algebra)

[≈ Lord, Rennie & V´ arilly, J.Geom.Phys. 2012]

We call CℓD(A) ⊆ B(H) the algebra generated by A, Ω1

D.

Let A◦ :=

  • JaJ−1 : a ∈ A
  • . The reality and 1st order cond. are equivalent to the statement

A◦ ⊆ CℓD(A)′ :=

  • b ∈ B(H) : [b, ξ] = 0 ∀ ξ ∈ CℓD(A)
  • .

(⋆)

Definition (Dirac spinors)

Elements of H are “Dirac spinors” iff (⋆) is an equality: A◦ = CℓD(A)′.

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slide-5
SLIDE 5

Time’s up?

I II III IV V VI VII VIII IX X XI XII

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On the Hodge operator

In the geometric examples (slide 6), [D, f] = c(df). In the Hodge example: H = Ω•(M)

L2

≃ Cℓ(M, g)

L2

B := CℓD(A) = Cℓ(M, g) Representation of B: by Clifford multiplication on Ω•(M), or by left multiplication on itself. Real structure: J(ω) = ω∗ . The algebra B◦ = JBJ−1 acts by right multiplication on H, that up to completion is a self-Morita equivalence B-bimodule.

Definition (2nd order condition)

[S. Farnsworth & L. Boyle, New J. Phys. 2014]

(A, H, D, J) satisfies the 2nd order condition if CℓD(A)◦ := J CℓD(A) J−1 ⊆ CℓD(A)′ (⋆⋆)

Definition (Hodge property)

(dim H < ∞) Elements of H are “Hodge spinors” if (⋆⋆) is an equality: CℓD(A)◦ = CℓD(A)′.

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The Standard Model

[Picture from www.texample.net/tikz ]

1st 2nd 3rd

generation

R / G / B 2/3 1/2

2.3 MeV

up

u

R / G / B −1/3 1/2

4.8 MeV

down

d

−1 1/2

511 keV

electron

e

1/2

< 2 eV

e neutrino

νe

R / G / B 2/3 1/2

1.28 GeV

charm

c

R / G / B −1/3 1/2

95 MeV

strange

s

−1 1/2

105.7 MeV

muon

µ

1/2

< 190 keV

µ neutrino

νµ

R / G / B 2/3 1/2

173.2 GeV

top

t

R / G / B −1/3 1/2

4.7 GeV

bottom

b

−1 1/2

1.777 GeV

tau

τ

1/2

< 18.2 MeV

τ neutrino

ντ

±1 1

80.4 GeV

1

91.2 GeV

Z

1

photon

γ

c

  • l
  • r

1

gluon

g

125.1 GeV

Higgs

H

strong nuclear force (color) electromagnetic force (charge) weak nuclear force (weak isospin) charge colors mass spin

Quarks

(+ 6 anti-quarks × 3 colors)

Leptons

(+ 6 anti-leptons)

Fermions

increasing mass →

Gauge Bosons

standard matter unstable matter force carriers Goldstone bosons

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Particles in a box. . .

We arrange particles in a 4 × 4 matrix:      νR u1

R

u2

R

u3

R

eR d1

R

d2

R

d3

R

νL u1

L

u2

L

u3

L

eL d1

L

d2

L

d3

L

     So, for example the unit vectors:      1      and 1 √ 2      1 1      represent a right-handed electron a mix right-handed neutrino/left-handed electron. Internal degrees of freedom are encoded in the Hilbert space V = M4(C)

(particles)

⊕ M4(C)

(antiparticles)

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SLIDE 6

The Standard Model spectral triple

The underlying space is

M × F

(spin manifold) (finite nc space)

with finite-dim. spectral triple (AF, HF, DF, γF, JF) given by:

◮ HF ≃ C32n internal degrees of freedom of the elementary fermions. Total nr:

2 × 4 × 2 × 2 × n = 32n

(weak isospin) (lepton + quark (L,R chirality) (particle or (generations) in 3 colors) antiparticle)

◮ γF = chirality operator ◮ AF = C ⊕ H ⊕ M3(C)

JF = charge conjugation

  • G(AF, JF) ≈ U(1) × SU(2) × SU(3)

◮ DF encodes the free parameters of the theory.

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On free parameters & the Higgs mass

◮ Matrix algebras have less representations than groups.

Despite this, similarly to what happens with SU(5) GUT, from a “simple” representation

  • f A with obtain the correct representation of the gauge group of the Standard Model.

◮ Nevertheless: too many free parameters (lot of arbitrarity in the choice of DF)

A problem with the Higgs mass:

◮ In the Standard Model: 19 parameters, whose numerical values are established by

  • experiments. One of these is the Higgs mass: mH ≈ 126 GeV.

◮ In Chamseddine-Connes’ original spectral triple, mH is not a free parameter. It was

predicted mH ≈ 170 GeV, a value ruled out by Tevatron in 2008. Solution:

◮ add one extra scalar field; ◮ this also cures the problem of instability (or “metastability”) of the SM vacuum.

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Can we use the condition(s) Dirac Hodge CℓD(A)′ = A◦ CℓD(A)′ = CℓD(A)◦ to “select” good Dirac operators? Let’s focus on the 1st one (today I will not talk about the 2nd one).

Theorem

(see arxiv:1501.00156 for the precise statement)

In order to satisfy the 1st condition, one must modify the operator studied by A. Connes and

  • A. Chamseddine. As a byproduct one gets:

→ a new scalar field σ; → a field coupling leptons with quarks. Physical implications are under investigation (in collaboration with M. Kurkov and F . Lizzi).

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