FDR & pittau/talk_ ¢  FDR Interpretation Bottom¢â‚¬â€œup Top¢â‚¬â€œdown Summary I show that FDR is

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  • FDR Interpretation Bottom–up Top–down Summary

    The FDR way to renormalizable

    and non−renormalizable QFTs

    Roberto Pittau

    (University of Granada)

    Torino, December 13th 2013

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    I show that FDR is more convenient than DR because

    1 four-dimensional (good for numerical methods & SUSY)

    2 order-by-order renormalization is avoided

    3 a finite renormalization is only required to fix the parameters of the theory in terms of experimental observables

    4 ℓ-loop integrals are directly re-usable in (ℓ+1)-loop calculations, with no need of further expanding in ǫ

    5 infrared and collinear divergences can be dealt with within the same four-dimensional framework used to cope with the ultraviolet infinities

    6 it allows a novel interpretation of non-renormalizable theories in which predictivity is restored

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    Outline

    1 The FDR idea

    2 Physical interpretation

    3 Bottom-up: Tests (renormalizable QFTs)

    4 Top-down: Non-renormalizable QFTs

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    The Four Dimensional Regularization/Renormalization approach (FDR)

    Subtraction of UV infinities encoded in the definition of loop integral

    R. P., arXiv:1208.5457 (first paper)

    A. M. Donati and R. P., arXiv:1302.5668 (1-loop EW)

    R. P., arXiv:1305.0419 (effective theories)

    R. P., arXiv:1307.0705 (massless QCD)

    A. M. Donati and R. P., arXiv:1311.5500 (2-loop)

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    Take the integrand of a ℓ-loop function

    J(q1, . . . , qℓ) = JINF(q1, . . . , qℓ) + JF,ℓ(q1, . . . , qℓ)

    To avoid the occurrence of infrared divergences

    +i0 = −µ2

    and µ → 0 outside integration

    The loop integrands in JINF(q1, . . . , qℓ) allowed to depend on µ, but not on physical scales ⇒ physics in JF,ℓ(q1, . . . , qℓ)

    The FDR integral over J(q1, . . . , qℓ) defined as

    ∫ [d4q1] . . . [d

    4qℓ] J(q1, . . . , qℓ) ≡ lim µ→0

    ∫ d4q1 . . . d

    4qℓ JF,ℓ(q1, . . . qℓ)

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    2-loop example

    Jαβ(q1, q2) = qα1 q

    β 1

    D̄31D̄2D̄12

    D̄1 = q̄ 2 1 −m21 D̄2 = q̄22 −m22 D̄12 = q̄212 −m212

    q12 = q1 + q2 q̄ 2 j = q

    2 j − µ2

    Needed denominator expansion (FDR defining expansion) with

    1

    D̄j =

    1

    q̄2j +

    m2j q̄2j D̄j

    1

    q̄212 =

    1

    q̄22 − q

    2 1 + 2(q1 · q2)

    q̄22 q̄ 2 12

    Jαβ(q1, q2) = q α 1 q

    β 1

    {[ 1

    q̄6 1 q̄2 2 q̄2 12

    ] +

    ( 1

    D̄3 1

    − 1 q̄6 1

    )([ 1

    q̄4 2

    ] − q

    2 1 + 2(q1 · q2)

    q̄4 2 q̄2 12

    )

    + 1

    D̄3 1 q̄2 2 D̄12

    ( m22 D̄2

    + m212 q̄2 12

    )}

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    Then

    JαβINF(q1, q2) = q α 1 q

    β 1

    {[ 1

    q̄61 q̄ 2 2 q̄

    2 12

    ] +

    ( 1

    D̄31 − 1

    q̄61

    )[ 1

    q̄42

    ]}

    JαβF,2(q1, q2) = q α 1 q

    β 1

    { 1

    D̄31 q̄ 2 2D̄12

    ( m22 D̄2

    + m212 q̄212

    )

    − (

    1

    D̄31 − 1

    q̄61

    ) q21 + 2(q1 · q2)

    q̄42 q̄ 2 12

    }

    And

    ∫ [d4q1][d

    4q2] qα1 q

    β 1

    D̄31D̄2D̄12 = lim

    µ→0

    ∫ d4q1d

    4q2 q α 1 q

    β 1

    { · · ·

    }

    ︸ ︷︷ ︸ J αβ

    F,2 (q1,q2)

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    Formal properties of the FDR integration

    i) invariance under shift of any integration variable

    ii) simplifications among numerators and denominators

    i) + ii) guarantee Gauge Invariance: usual manipulations hold at the integrand level

    i)

    FDR integrals as finite differences of UV divergent integrals

    ∫ [d4q1] . . . [d

    4qℓ]J({qi}) = lim µ→0

    ∫ dnq1 . . . d

    nqℓ

    ( J({qi})− JINF({qi})

    )

    r.h.s. regulated in dimensional regularization (just one option,

    since the dependence on any UV regulator drops in the difference) Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    ii)

    By construction, provided any q2i generated by Feynman rules is considered as q̄2i = q

    2 i − µ2i . For example

    ∫ [d4q1][d

    4q2] q̄21

    D̄31D̄2D̄12 =

    ∫ [d4q1][d

    4q2] 1

    D̄21D̄2D̄12

    + m21

    ∫ [d4q1][d

    4q2] 1

    D̄31D̄2D̄12

    Only one kind of µ2 exists:

    ∫ [d4q1][d

    4q2] µ21

    D̄31D̄2D̄12 = lim

    µ→0 µ2

    ∫ d4q1d

    4q2

    { · · ·

    }

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    For consistency, irreducible tensor decomposition works as in the following example

    ∫ [d4q1][d

    4q2] qα1 q

    β 1

    D̄31D̄2D̄12 =

    gαβ

    4

    ∫ [d4q1][d

    4q2] q21

    D̄31D̄2D̄12

    = gαβ

    4

    ∫ [d4q1][d

    4q2] q̄21 + µ

    2 1

    D̄31D̄2D̄12

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    Dependence on µ

    ∫ [d4q1] . . . [d

    4qℓ]J({qi}) = lim µ→0

    ∫ dnq1 . . . d

    nqℓ

    ( J({qi})− JINF({qi})

    )

    1 First term in r.h.s. does not depend on µ, because limµ→0 can be moved inside integration

    2 Any polynomially divergent integral in JINF cannot contribute either, being proportional to positive powers of µ

    3 µ dependence of the l.h.s. entirely due to powers of ln(µ/µR) generated by the logarithmically divergent subtracted integrals

    a) FDR integrals depend on µ logarithmically b) if all powers of ln(µ/µR) are moved to the l.h.s. limµ→0

    formally taken by trading ln(µ) for ln(µr)

    FDR integrals do not depend on any cut off but only on the renormalization scale µR

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    1-loop example (with a cutoff regulator)

    J(q) = 1

    (q̄2 −m20)((q + p)2 −m21 − µ2) =

    [ 1

    q̄4

    ] + JF,1(q)

    lim µ→0

    Λ d4q

    [ 1

    q̄4

    ] = lim

    µ→0 2iπ2

    (∫ µR 0

    dq +

    ∫ Λ

    µR

    dq

    ) q3

    (q2 + µ2)2

    −iπ2 ( 1 + ln

    µ2

    µ2 R

    )

    µR can also be thought as an arbitrary separation scale from the UV regime

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    ∫ [d4q]J(q) = −iπ2

    ∫ 1

    0 dx ln

    ( m20x+m

    2 1(1− x)− p2x(1− x)

    µ2R

    )

    is cutoff independent

    In summary, the symbol

    ∫ [d4q] means

    1 Use partial fraction to move all divergences in vacuum integrands treating q̄2 globally

    2 Drop all divergent vacuum terms from the integrand

    3 Integrate over d4q

    4 Take µ → 0 until a logarithmic dependence on µ is reached 5 Compute the result in µ = µR (µ → µR in [d4q] definition)

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    Physical Interpretation

    QFTs vs UV cutoff (I)

    ΛUV

    QFT1 QFT2 . . . QFTN

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    QFTs vs UV cutoff (II)

    ΛUV

    QFT1 QFT2 . . . QFTN

    lnj(ΛUV) and Λ j UV

    generated by loops

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    QFTs vs UV cutoff (III)

    ΛUV

    QFT1 QFT2 . . . QFTN

    lnj(ΛUV) and Λ j UV

    generated by loops

    Unphysical if ΛUV → ∞ Modern version of aether?

    One just ignore them

    Roberto Pittau, U. of Granada FDR & QFTs

  • FDR Interpretation Bottom–up Top–down Summary

    What is the cost of throwing away infinities?

    No cost for polynomially divergent infinities (decoupling)

    Only logarithmic infinities influence the physical spectrum (µR pops up in physical observables when separating them)

    Physics at ΛUV scale manifests itself only logarithmically at lower energies

    ln(MHiggs/GeV) ∼ 5 ln(MPlank/GeV) ∼ 44

    Hierarchy problem with no BSM particles?

    Roberto Pittau, U. of Granada FDR &