Flavour physics : Lecture 1theory.tifr.res.in/~amol/talks/technical/2017/susy...Flavour physics :...

Flavour physics : Lecture 1History, CP violation, Neutral meson mixing

Amol DigheDepartment of Theoretical Physics, TIFR

SUSY 2017 pre-school,TIFR Mumbai, Dec 4-8, 2017

Why Flavour physics

Role of flavour physics in building up the Standard Modelτ − θ puzzle⇒ Parity violationCabibbo angle⇒

weak coupling universality ⊕ quark mixingGIM mechanism⇒ no FCNC at tree level, charmCKM paradigm⇒ (at least) three quark familiesLarge B–B mixing⇒ heavy top quarkRate of radiative B decay⇒ top quark mass

The standard model looks complete now,with no confirmed signals of BSM physics at the LHC yet !Flavour physics may give some indirect hints....

Why Flavour physics

Flavour structure of the Standard Model

Three families ofquarks and leptonsQuark masses (GeV):

up charmed top0.005 1.5 175

down strange bottom0.01 0.1 5

Mixing between families:Strong within the same family,Weak across families

The meson nomenclature

Pions:π+ ≡ ud , π− ≡ du , π0 ≡ (uu − dd)/

√2 (∼ 0.15 GeV)

Strange mesons (Kaons):K + ≡ us ,K− ≡ su ,K 0 ≡ ds ,K 0 ≡ sd (∼ 0.5 GeV)Charmed mesons:D+ ≡ cd ,D+

s ≡ cs ,D0 ≡ cu(And antiparticles: ∼ 1.5− 1.8 GeV)B mesons:B+ ≡ ub ,B0 ≡ db ,Bs ≡ sb ,Bc ≡ cb(And antiparticles: ∼ 5− 6.5 GeV)Quarkonia:φ ≡ ss (∼ 0.7 GeV)J/ψ ≡ cc (∼ 3.1 GeV)Υ ≡ bb (∼ 10 GeV)

√2 (∼ 0.15 GeV)

Contents

1 Some historical perspective

2 CP violation: discovery and interpretation

3 The Kobayashi-Maskawa mechanism

4 Neutral meson mixing and oscillations

5 Flavour tests of the CKM paradigm

Outline

Discovery of parity violation: 1956-57

τ − θ puzzle: Particles with thesame mass and lifetime decayto ππ and πππTheoretical possibility thatmirror world does not behavethe same as the real worldT.D.Lee and C.N.Yang,Phys. Rev. 104, 254 (1956)Experiments: 1957

Wu (60Co)Friedman-Telegdi(π+ → µ+ → e+)

Nobel prize 1957: Lee–Yang

The particle zoo (1960’s)

p ≡ uud

n ≡ udd

π+ ≡ ud

π− ≡ du

π0 ≡ (uu − dd)/√

K + ≡ us

K− ≡ su

K 0 ≡ ds

K 0 ≡ sd

Universality of weak interactions: Cabibbo angleInterrelated coupling constants:

(i) muon decay: geµµ− → νµe−νe

(ii) neutron decay : gudn→ pe−νe (d → ue−νe)(ii) kaon decay: gusK− → π0e−νe (s → ue−νe)

|geµ|2 = |gud |2 + |gus|2

Universality:

There is only one coupling constant, g = geµ

u quark couples to only one combination of d and s:d ′ ≡ cos θc · d + sin θc · s

Cabibbo angle θc : the first quark mixing angle

N. Cabibbo, “Unitary Symmetry and Leptonic Decays,”Phys. Rev. Lett. 10, 531 (1963)

Universality of weak interactions: Cabibbo angleInterrelated coupling constants:

(i) muon decay: geµµ− → νµe−νe

(ii) neutron decay : gudn→ pe−νe (d → ue−νe)(ii) kaon decay: gusK− → π0e−νe (s → ue−νe)

|geµ|2 = |gud |2 + |gus|2

Universality:

There is only one coupling constant, g = geµ

u quark couples to only one combination of d and s:d ′ ≡ cos θc · d + sin θc · s

Cabibbo angle θc : the first quark mixing angle

N. Cabibbo, “Unitary Symmetry and Leptonic Decays,”Phys. Rev. Lett. 10, 531 (1963)

Suppression of flavor-changing neutral currents

Cabibbo angle unable to explain whyΓ(KL → µ+µ−) << Γ(K + → µ+νµ)

Possible explanation via another “c” quark:charge +2/3, couples to

s′ ≡ − sin θc · d + cos θc · sThe s → u → d and s → c → d contribution cancel,leading to the suppression of FCNC s → dGIM mechanism: existence of the “charmed” quark.

S. L. Glashow, J. Iliopoulos and L. Maiani,“Weak Interactions with Lepton-Hadron Symmetry,”Phys. Rev. D 2, 1285 (1970)

Outline

Can Charge ⊕ Parity may be conserved ?

Parity: left landed↔ right handed

Neutrinos violate parity: they are only left-handedBut antineutrinos are right-handed !Does that mean C and P violations cancel each other togive CP conservation ?

Prediction of CP violation in K decay

K 0 ≡ ds K 0 ≡ sd

CP eigenstates:K1 ≡ (K 0 + K )/

√2 (CP even)

K2 ≡ (K 0 − K )/√

2 (CP odd)

CP even decay channel: ππCP odd decay channel: πππCP conservation⇒

K1 → ππ short-lived, KShortK2 → πππ long-lived, KLong

Original K 0 = (KShort + KLong)/√

K 0 ≡ ds K 0 ≡ sd

√2 (CP even)

K2 ≡ (K 0 − K )/√

2 (CP odd)

K 0 ≡ ds K 0 ≡ sd

√2 (CP even)

K2 ≡ (K 0 − K )/√

2 (CP odd)

Discovery of CP violation: 1964

Cronin-Fitch experiment

Nobel prize 1980

Charge-parity violated slightly

“Day and Night”, M.C.Escher

Questions raised by the discovery of CP violation

Is it small or large ? Is CP an approximate symmetry ?

Is the symmetry breaking spontaneous ?

Where does it come from ? Are there extra interactions ?

CP violation and complex coupling

CP violation: A(X → Y ) 6= A(X → Y )

If all amplitudes are real, |A(X → Y )|2 = |A(X → Y )|2

CP violation possible if complex numbers involved in

|A(X → Y )|2 = |A(X → w → Y ) + A(X → z → Y )|2

CP violation⇒ Amplitudes complex⇒ Couplings complex

Outline

The paper and the authors

Two generations of quarks are not enough

The mixing matrix between up-type and down-type quarkshas to be a 2× 2 unitary matrix

L ∝ (uL, cL)

(cos θeiφ1 sin θeiφ2

− sin θeiφ3 cos θei(φ2+φ3−φ1)

)(dLsL

)Can change three relative phases of quarks to get rid of allthree complex phases φ1, φ2, φ3

L ∝ (uL, cLei(φ3−φ1))

(cos θ sin θ− sin θ cos θ

)(dLeiφ1

sLeiφ2

)Mixing matrix real⇒ no CP violation

Two generations of quarks are not enough

The mixing matrix between up-type and down-type quarkshas to be a 2× 2 unitary matrix

L ∝ (uL, cL)

(cos θeiφ1 sin θeiφ2

− sin θeiφ3 cos θei(φ2+φ3−φ1)

)(dLsL

)Can change three relative phases of quarks to get rid of allthree complex phases φ1, φ2, φ3

L ∝ (uL, cLei(φ3−φ1))

(cos θ sin θ− sin θ cos θ

)(dLeiφ1

sLeiφ2

)Mixing matrix real⇒ no CP violation

Paremeter counting for two generations

2× 2 complex matrix⇒ 4 real + 4 imaginary quantities

Unitarity U†U = I: 3 real and 1 imaginary conditions1 real and 3 imaginary parameters left

Can choose the 3 relative phases between quarks to getrid of the 3 imaginary parametersThe mixing matrix is completely real

Three generations work !

3× 3 matrix⇒ 9 real + 9 imaginary quantitiesUnitarity U†U = I: 6 real and 3 imaginary conditions3 real and 6 imaginary parameters leftCan choose the 5 relative phases between quarks to getrid of 5 imaginary parametersIn addition to 3 real parameters (Euler angles of rotation),one imaginary quantity is unavoidableMixing matrix complex⇒ CP violation may be present c1 −s1c3 −s1s3

s1c2 c1c2c3 − s2s3eiδ c1c2s3 + s2c3eiδ

s1s2 c1s2c3 + c2s3eiδ c1s2s3 − c2c3eiδ

Three families needed for the complex nature

Experimental discovery of the third generation

Discovery of τ : 1976Υ,B,Bs, λb contain b quarkTop quark: 1995The last element, ντ , discovered in 2000.

CKM paradigm in modern language

Flavor basis vs. mass basis:

U ′ ≡

, D′ ≡

Charged current in the basis of flavor eigenstates:

LCC = g√2U ′Lγ

µD′LW +µ + h.c.

Charged current in the basis of mass eigenstates:

LCC = g√2ULγ

µ(V †ULVDL)DLW +µ + H.c.

VUL,VDL: unitary matrices that change the basis

Cabibbo-Kobayashi-Maskawa (CKM) matrix

Coupling between UL and DL: (g/√

2)VCKM

VCKM ≡ V †ULVDL

U ′ ≡

, D′ ≡

LCC = g√2U ′Lγ

µD′LW +µ + h.c.

LCC = g√2ULγ

2)VCKM

VCKM ≡ V †ULVDL

U ′ ≡

, D′ ≡

LCC = g√2U ′Lγ

µD′LW +µ + h.c.

LCC = g√2ULγ

2)VCKM

VCKM ≡ V †ULVDL

Wolfenstein parametrization of the CKM matrix

VCKM =

Vud Vus VubVcd Vcs VcbVtd Vts Vtb

1− λ2/2 λ Aλ3(ρ− iη)−λ 1− λ2/2 Aλ2

Aλ3(1− ρ− iη) −Aλ2 1

+O(λ4)

λ ≈ 0.2: Cabibbo angleη: the imaginary component of VCKM

η/ρ large⇒ CP violation is large, not approximateAll CP violation in terms of a single number:Jarlskog invariant J ≡ s1s2s3c2

1c2c3sδ = A2λ6η

VCKM =

1− λ2/2 λ Aλ3(ρ− iη)−λ 1− λ2/2 Aλ2

Aλ3(1− ρ− iη) −Aλ2 1

+O(λ4)

1c2c3sδ = A2λ6η

VCKM =

1− λ2/2 λ Aλ3(ρ− iη)−λ 1− λ2/2 Aλ2

Aλ3(1− ρ− iη) −Aλ2 1

+O(λ4)

1c2c3sδ = A2λ6η

VCKM =

1− λ2/2 λ Aλ3(ρ− iη)−λ 1− λ2/2 Aλ2

Aλ3(1− ρ− iη) −Aλ2 1

+O(λ4)

1c2c3sδ = A2λ6η

VCKM =

1− λ2/2 λ Aλ3(ρ− iη)−λ 1− λ2/2 Aλ2

Aλ3(1− ρ− iη) −Aλ2 1

+O(λ4)

1c2c3sδ = A2λ6η

Unitarity relations and triangles

Unitarity relation VudV ∗ub + VcdV ∗cb + VtdV ∗tb = 0

α ≡ Arg(−

VtdV ∗tb

VudV ∗ub

), β ≡ Arg

VcdV ∗cb

VtdV ∗tb

), γ ≡ Arg

VudV ∗ub

VcdV ∗cb

More unitarity triangles

All triangles have the same area, J/2

χ ≡ βs ≡ φs ≡ Arg(−VcsV∗

cbVtsV∗

)∼ λ2 ,

χ′ ≡ βK ≡ Arg(−Vcd V∗

csVcd V∗

)∼ λ4 ,

Outline

Bq–Bq mixing: parametrization

Oscillation and decay of a|Bq〉+ b|Bq〉 :

(M− i

M ≡(

M11 M12M21 M22

), Γ ≡

(Γ11 Γ12Γ21 Γ22

)CP|Bq〉 = eiϕ|Bq〉, CP|Bq〉 = e−iϕ|Bq〉

CPT invariance : M11 = M22, Γ11 = Γ22

Hermiticity : M21 = M∗12, Γ21 = Γ∗12

Mass difference and lifetime difference

Mass eigenstates:|BL,H〉 = p|Bq〉 ± q|Bq〉 (|q|2 + |p|2 = 1)

Mass difference Lifetime difference∆m = MH −ML ∆Γ = ΓL − ΓH

(→ ∆m > 0,∆Γ > 0 in SM)Eigenvalue equations:

(∆m)2 − 14

(∆Γ)2 = (4|M12|2 − |Γ12|2)

∆m∆Γ = −4Re(M∗12Γ12).

∆m = 2|M12|+ O(m4b/m

∆Γ = −2Re(M∗12Γ12)

|M12|+ O(m4

b/m4t ).

(∆m)2 − 14

(∆Γ)2 = (4|M12|2 − |Γ12|2)

∆m∆Γ = −4Re(M∗12Γ12).

∆m = 2|M12|+ O(m4b/m

∆Γ = −2Re(M∗12Γ12)

|M12|+ O(m4

b/m4t ).

(∆m)2 − 14

(∆Γ)2 = (4|M12|2 − |Γ12|2)

∆m∆Γ = −4Re(M∗12Γ12).

∆m = 2|M12|+ O(m4b/m

∆Γ = −2Re(M∗12Γ12)

|M12|+ O(m4

b/m4t ).

Time evolution of a tagged Bq or Bq decay

Af ≡ 〈f |Bq〉, Af ≡ 〈f |Bq〉 , λf ≡qp

(λf independent of the unphysical phase ϕ)

Γ(Bq(t)→ f ) = Nf |Af |21 + |λf |2

2e−Γt ×[

cosh∆Γq t

2+Adir

CP cos(∆m t) + A∆Γ sinh∆Γq t

2+Amix

CP sin (∆m t)],

Γ(Bq(t)→ f ) = Nf |Af |21 + |λf |2

2e−Γt ×[

cosh∆Γq t

2−Adir

CP cos(∆m t) + A∆Γ sinh∆Γq t

2−Amix

CP sin(∆m t)].

AdirCP =

1− |λf |2

1 + |λf |2, Amix

CP = − 2 Imλf

1 + |λf |2A∆Γ = − 2 Reλf

1 + |λf |2,

Outline

B factories: B ≡ bu, bd , bs, bc, B ≡ ub, db, sb, cb

Babar (SLAC, USA) BELLE (KEK, Japan)

e+e− → BB → decayproducts

More and more stringent tests of the CKM mechanism

Semileptonic decay B → D`ν

Semileptonic decay B → D`ν“Charmed” decays B → DK

Semileptonic decay B → D`ν“Charmed” decays B → DKCP violation in K mesons

Semileptonic decay B → D`ν“Charmed” decays B → DKCP violation in K mesons∆M in Bd–Bd system

Semileptonic decay B → D`ν“Charmed” decays B → DKCP violation in K mesons∆M in Bd–Bd system∆M in Bs–Bs system

Semileptonic decay B → D`ν“Charmed” decays B → DKCP violation in K mesons∆M in Bd–Bd system∆M in Bs–Bs systemDecays to π and K

Semileptonic decay B → D`ν“Charmed” decays B → DKCP violation in K mesons∆M in Bd–Bd system∆M in Bs–Bs systemDecays to π and KCP asymmetry in B → J/ψKS

Concluding remarks: end of Lecture 1

No deviation from the CKM predictions has been observed⇒ Constrains many new physics models

CP violation required for baryon asymmetry, but the CKMis not enough, so there will be life beyond CKM

Physics beyond the Standard Model must exist !

Where to look for it ?How to identify and quantify it ?⇒ Lecture 2

Flavour physics : Lecture 1theory.tifr.res.in/~amol/talks/technical/2017/susy...Flavour physics :...

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