QCD Workshop on Chirality, Vorticity and Magnetic Field in Heavy Ion Collisions @UCLA, Jan. 21-23, 2015

Photon Propagation in Magnetic Fields in Strong Magnetic Fields

KH, K. Itakura (KEK), Ann. Phys. 330 (2013); Ann. Phys. 334 (2013)

Koichi HattoriRIKEN-BNL Research Center

RHIC@BNL

LHC@CERN

Phase diagram of QCD matter

Asymptotic freedomQuark-gluon plasma

Light-meson spectra in B-fields Hidaka and A.Yamamoto

Quark and gluon condensates at zero and finite temperatures Bali et al.

Results from lattice QCD in magnetic fields

Extremely strong magnetic fieldsUrHIC NS/Magnetar

Lienard-Wiechert potential

Z = 79(Au), 82(Pb)

Lighthouse in the sky

PSR0329+54

c.f.) Possible mechanism of strong B from “chiral instability”: the conversion of μ5

generated in electron captures.

Ohnishi and Yamamoto, Akamatsu and Yamamoto

Strong magnetic fields in nature and laboratories

Magnet in Lab.

Magnetar

Heavy ion collisions

Polarization 1Polarization 2

Incident light“Calcite” (方解石 )

“Birefringence”: Polarization-dependent refractive indices.

Response of electrons to incident lights Anisotropic responses of electrons result in polarization-dependent and anisotropic photon spectra.

Photon propagations in substances

+ Lorentz & Gauge symmetries n ≠ 1 in general

+ Oriented response of the Dirac sea ``Vacuum birefringence”

How about the vacuum with external magnetic fields ?- The Landau-levels

B

Modifications of photon propagations in strong B-fields- Old but unsolved problems

Quantum effects in magnetic fields

Photon vacuum polarization tensor:

Modified Maxwell eq. :

Dressed propagators in Furry’s picture

・・・

・・・

Should be suppressed in the ordinary perturbation theory, but not in strong B-fields.

eBeB eB

Break-down of naïve perturbation in strong B-fields

Naïve perturbation breaks down when B > Bc

Need to take into account all-order diagrams

Critical field strengthBc = me

2 / e

Dressed fermion propagator in Furry’s picture

Resummation w.r.t. external legs by “proper-time method“ Schwinger

Nonlinear to strong external fields

Photon propagation in a constant external magnetic field

Vanishing B limit:

θ: angle btw B-field and photon propagation

BGauge symmetries lead to a tensor structure,

Schwinger, Adler, Shabad, Urrutia, Tsai and Eber, Dittrich and Gies

Exponentiated trig-functions generate strongly oscillating behavior witharbitrarily high frequency.

Integrands with strong oscillations

Summary of relevant scales and preceding calculations

Strong field limit: the lowest-Landau-level approximation(Tsai and Eber, Shabad, Fukushima )

Numerical computation below the first threshold(Kohri and Yamada) Weak field & soft photon limit

(Adler)

?Untouched so far

General analytic expression

Euler-Heisenberg LagrangianIn soft photon limit

2nd step: Getting Laguerre polynomials

Associated Laguerre polynomial

Decomposing exponential factors

Linear w.r.t. τ in exp.

1st step: “Partial wave decomposition”

Linear w.r.t. τ in exp.

Linear w.r.t. τ in exp.

By the decomposition of the integrand, any term reduces to either of three elementary integrals.

Transverse dynamics: Wave functions for the Landau levels given by the associated Laguerre polynomials

UrHIC

Prompt photon ~ GeV2

Thermal photon ~ 3002 MeV2 ~ 105 MeV2

Untouched so far

Strong field limit (LLL approx.)(Tsai and Eber, Shabad, Fukushima )

Soft photon & weak field limit(Adler)

Numerical integration(Kohri, Yamada)

External photon momentum

Analytic result of integrals- An infinite number of the Landau levels

KH, K.Itakura (I)

⇔Polarization tensor acquires an imaginary part when

Lowest Landau level

Narrowly spaced Landau levels

Complex refractive indices

Solutions of Maxwell eq. with the vacuum polarization tensor

The Lowest Landau Level (ℓ=n=0)

Refractive indices at the LLL

Polarization excites only along the magnetic field``Vacuum birefringence’’

KH, K. Itakura (II)

Self-consistent solutions of the modified Maxwell Eq.

Photon dispersion relation is strongly modified when strongly coupled to excitations (cf: exciton-polariton, etc)

cf: air n = 1.0003, water n = 1.333

𝜔2/4𝑚2

≈ Magnetar << UrHIC

𝜔2/4𝑚2

Angle dependence of the refractive indexReal part

No imaginary part

Imaginary part

“Mean-free-path” of photons in B-fields

λ (fm)

When the refractive index has an imaginary part,

Summary for Photon propagation+ We obtained an analytic form of the polarization tensor in magnetic fields as the summation w.r.t. the Landau levels.

+ We obtained the complex refractive indices (photon dispersions) by solving the modified Maxwell Eq. self-consistently. Photons decay within a microscopic spatial scale.

Neutron stars = Pulsars Possibly “QED cascade” in strong B-fields

What is the mechanism of radiation?

We got precise descriptions of vertices: Dependences on magnitudes of B-fields, photon energy, propagation angle and polarizations.

Prospects: Microscopic mechanism of the pulsation

“Photon Splitting”Softening of photons

Analytical modeling of colliding nuclei, Kharzeev, McLerran, Warringa, NPA (2008)

Pre-equilibrium

QGP

Strong magnetic fields induced by UrHIC

Dimesionless variables Schwinger, Adler, Shabad, Urrutia, Tsai and Eber, Dittrich and Gies

1st step: Use a series expansion known as partial wave decomposition Baier and Katkov

Exponentiated trig-functions generate strongly oscillating behavior by arbitrarily high frequency.

After 1st step:

2nd step:

Associated Laguerre polynomial

Then, any term reduces to either of three elementary integrals.

1st step: Use a series expansion known as partial wave decomposition Baier and Katkov

2nd step:

Associated Laguerre polynomial

Then, any term reduces to either of three elementary integrals.

Close look at the integrals

What dynamics is encoded in the scalar functions ?

An imaginary part representing a real photon decay

⇔

⇔Invariant mass of a fermion-pair in the Landau levels

Analytic results of integrals without any approximation

Polarization tensor acquires an imaginary part above

Every term results in either of three simple integrals.

A double infinite sum

KH, K. Itakura (I)

Renormalization

+= ・・・+ +

Log divergence

Term-by-term subtraction

Ishikawa, Kimura, Shigaki, Tsuji (2013)

Taken from Ishikawa, et al. (2013)

Finite

Re Im

Br = (50,100,500,1000,5000,10000, 50000)

Real part of n on stable branch

Imaginary part of n on unstable branch

Real part of n on unstable branch

Relation btw real and imaginary partson unstable branch

Br = (50,100,500,1000,5000,10000, 50000)

Real part of ε on stable branch

Imaginary part of ε on unstable branch

Real part of ε on unstable branch

Relation btw real and imaginary partson unstable branch