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Nonexistence of Majorana fermions in Kerr-Newman type spacetimes with nontrivial charge

  • We show that the Dirac equation is separated into four differential equations for time-periodic Majorana fermions in Kerr-Newman and Kerr-Newman-(A)dS spacetimes. Although they cannot be transformed into radial and angular equations, the four differential equations yield two algebraic identities. When the electric or magnetic charge is nonzero, they conclude that there is no differentiable time-periodic Majorana fermions outside the event horizon in Kerr-Newman and Kerr-Newman-AdS spacetimes, or between the event horizon and the cosmological horizon in Kerr-Newman-dS spacetime.
  • The Dirac equation in black hole spacetimes plays a significant role in the study of general relativity and quantum cosmology. A Dirac fermion is a spinor Ψ in spacetimes satisfying the Dirac equation

    (D+iλ)Ψ=0,

    (1)

    where λ is a certain real number. In 1968, Kinnersley introduced the null basis to study the Petrov type D metric [1]. In [2], Chandrasekhar separated the Dirac equation in Kerr spacetime when Dirac fermions are time-periodic and given by

    Ψ=S1ψ,ψ=ei(ωt+(k+12)ϕ)(R(r)Θ(θ)R+(r)Θ+(θ)R+(r)Θ(θ)R(r)Θ+(θ)),

    (2)

    where S is a diagonal matrix,

    S=Δ 14rdiag((r+iacosθ)12I2×2,(riacosθ)12I2×2),

    and Page extended his method to Kerr-Newman spacetime [3]. Since then, various studies have been conducted to investigate Hawking radiation and the numerical solutions of Dirac fermions in various spacetime backgrounds (for examples, see [47]. In [8], Finster, Kamran, Smoller, and Yau applied Chandrasekhar's separation to prove the nonexistence of the L2 integrable, time-periodic solutions of the Dirac equation in non-extreme Kerr-Newman spacetime. This indicates that the normalizable time-periodic Dirac fermions must either disappear into the black hole or escape to infinity. In [9, 10], Belgiorno and Cacciatori applied the spectral properties to prove the non-existence of the L2 integrable, time-periodic solutions of the Dirac equation with mass greater than 12|Λ|3 in non-extreme Kerr-Newman-(A)dS spacetimes, where Λ is the cosmological constant. In [11], Wang and Zhang applied Chandrasekhar's separation to prove the nonexistence of the Lp integrable, time-periodic solutions of the Dirac equation with arbitrary mass and 0<p43, or with mass greater than qΛ3 and 43<p432q, 0<q<32 in non-extreme Kerr-Newman-AdS spacetime. In non-extreme Kerr-Newman-dS spacetime, the nonexistence of Lp integrable, time-periodic Dirac fermions hold true for arbitrary mass and p2 [12]. In particular, taking p=2, they confirmed Belgiorno and Cacciatori's results in which normalizable time-periodic Dirac fermions with mass greater than 12Λ3 must either disappear into the black hole or escape to infinity.

    For Chandrasekhar's separation, the Dirac equation can be transformed into radial and angular equations. In [13], Kraniotis observed that the radial and angular equations could be reduced to generalized Heun's equations in Kerr-Newman spacetime, which provide local time-periodic solutions in terms of holomorphic functions, whose power series coefficients are determined by a four-term recurrence relation. Using the four-term recursion formula, he also proved that there is no time-periodic solution with a fermion energy strictly less than its mass in Kerr-Newman spacetime.

    In the recent search for neutrinos, which are one of the most mysterious particles in the universe, we are interested in discovering whether they are Majorana fermions. The most promising method to date is through double beta decay [14]. Various approaches have been studied to distinguish between Majorana and Dirac fermions (for examples, see [1522]).

    A Majorana fermion is a Dirac fermion whose antiparticle is itself. To define a Majorana fermion precisely, let us introduce the 4-component charge conjugate operator

    C=(ϵβαϵ˙β˙α)

    with the Pauli matrix σ2 and antisymmetric operator on spin indices, where

    ϵ˙α˙β=ϵαβ=iσ2=(11).

    The charge conjugation of the Dirac fermion Ψ is defined by

    ΨC=CˉΨT.

    Therefore, Majorana fermions are given by

    ΨMaj=(ΨWeyliσ2ΨWeyl)

    (3)

    and satisfy the Dirac equation [2325], where ΨWeyl is the Weyl spinor, and ΨWeyl is its complex conjugate. Time-period Majorana fermions can be given by

    Ψ=S1Eψ,ψ=(R(r)Θ(θ)R+(r)Θ+(θ)ˉR+(r)ˉΘ+(θ)ˉR(r)ˉΘ(θ)),

    (4)

    where S and E are the diagonal matrices

    S=Δ 14rdiag((r+iacosθ)12I2×2,(riacosθ)12I2×2),E=diag(ei(ωt+(k+12)ϕ)I2×2,ei(ωt+(k+12)ϕ)I2×2).

    In this short paper, we show that the Dirac equation is separated into four differential equations for time-periodic Majorana fermions given by (4) in Kerr-Newman and Kerr-Newman-(A)dS spacetimes. Although they cannot be transformed into radial and angular equations, the four differential equations yield two algebraic identities. When the electric or magnetic charge is nonzero, they conclude that there are no differentiable time-periodic Majorana fermions outside the event horizon in Kerr-Newman and Kerr-Newman-AdS spacetimes, or between the event horizon and the cosmological horizon in Kerr-Newman-dS spacetime.

    We remark that Dirac fermions taking form (2) are not consistent with Majorana condition (3). Thus, previous results on the existence or non-existence of time-periodic Dirac fermions cannot be applied to the current situation for time-periodic Majorana fermions.

    For convenience of discussion, we unify the Kerr-Newman and Kerr-Newman-(A)dS metrics

    ds2KNType=ΔrU(dtasin2θΞdϕ)2+UΔrdr2+UΔθdθ2+Δθsin2θU(adtr2+a2Ξdϕ)2,

    (5)

    by taking κ as zero, pure imaginary, and real, where Λ=3κ2 is the cosmological constant, and

    Δr=(r2+a2)(1+κ2r2)2mr+P2+Q2,Δθ=1κ2a2cos2θ,U=r2+a2cos2θ,Ξ=1κ2a2>0.

    The metric (5) solves the Einstein-Maxwell field equations with the electromagnetic potential

    A=QrU(dtasin2θΞdφ)PcosθU(adtr2+a2Ξdφ),

    (6)

    where P and Q are real numbers representing the magnetic and electric charges, respectively.

    In the following, we let 0μ,ν3, and 1i,j3. On a 4-dimensional Lorentzian manifold, we choose the frame {eμ} such that e0 is timelike and ei are spacelike. We denote {eα} as the dual coframe. The Cartan structure equations are

    deμ=ωμ νeν,ωμν=gμγωγ ν=ωνμ.

    If it is spin, we use the cotangent bundle to define the Clifford multiplication, spin connection, and Dirac operator [11, 12]. We fix the Clifford multiplications as the following Weyl representation:

    e0(I2×2I2×2),ei(σiσi),

    (7)

    where σi are Pauli matrices,

    σ1=(11), σ2=(ii), σ3=(11).

    We fix our discussion in the region Δr>0. The coframe is

    e0=ΔrU(dtasin2θΞdϕ) ,e1=UΔθdθe2=ΔθUsinθ(adtr2+a2Ξdϕ),e3=UΔrdr.

    with the dual frame

    e0=r2+a2UΔr(t+aΞr2+a2ϕ) , e1=ΔrUr,e2=ΔθUθ , e3=1UΔθ(asinθt+Ξsinθϕ).

    With respect to the above coframe, we obtain

    de0=C010e1e0+C030e3e0+C012e1e2,de1=C131e3e1,de2=C230e3e0+C232e3e2+C212e1e2,de3=C331e3e1,

    where

    C010=a2UΔθUsinθcosθ,C030=rΔrU,C012=2aUΔrUcosθ,C131=rUΔrU,C212=1sinθθ(ΔθUsinθ),C232=rUΔrU,C230=2arUΔθUsinθ,C331=a2UΔθUsinθcosθ.

    Thus, the connection 1-forms are

    ω0 1=ω01=C010e0+12C012e2,ω0 2=ω02=12C230e312C012e1,ω0 3=ω03=C030e012C230e2,ω1 2=ω12=12C012e0C212e2,ω1 3=ω13=C331e3+C131e1,ω2 3=ω23=12C230e0+C232e2.

    (8)

    The spin connection is defined as

    ˜XΨ=X(Ψ)14ωμν(X)eμeνΨ,

    where X is a vector, Φ is a spinor, and eμ is the Clifford multiplication. Therefore, using (8), we obtain

    ˜e0Ψ=e0Ψ12ω01(e0)e0e1Ψ12ω03(e0)e0e3Ψ12ω12(e0)e1e2Ψ12ω23(e0)e2e3Ψ,˜e1Ψ=e1Ψ12ω02(e1)e0e2Ψ12ω12(e1)e1e2Ψ,˜e2Ψ=e2Ψ12ω01(e2)e0e1Ψ12ω03(e2)e0e3Ψ12ω12(e2)e1e2Ψ12ω23(e2)e2e3Ψ,˜e3Ψ=e3Ψ12ω02(e3)e0e2Ψ12ω13(e3)e1e3Ψ.

    Using Clifford multiplication (7), these can be written as the matrix forms

    ˜eμΨ=eμΨ+EμΨ,Eμ=12(ϵμ00ϵhμ),

    (9)

    where ϵhμ is the Hermitian conjugate of ϵμ and

    ϵ0=(C010i2C230)σ1+(C030i2C012)σ3,ϵ1=(12C012+iC131)σ2,ϵ2=(12C012iC232)σ1(12C230iC212)σ3,ϵ3=(12C230+iC331)σ2.

    In this section, we prove the nonexistence of time-periodic Majorana fermions in Kerr-Newman type spacetime when the electric or magnetic charge is nonzero.

    First, we simplify the Dirac equation (1) on metric (5) when Ψ is given by (4). The Dirac operator with electromagnetic potential A is

    D=eμ(˜eμ+iA(eμ)).

    (10)

    We denote J=diag(I2×2,I2×2). In terms of (6) and (9), we obtain

    eμeμ(Ψ)=ΔrUe3S1Erψ+ΔθUe1S1Eθψir2+a2UΔr(ω+aΞr2+a2(k+12))e0JS1Eψ+12U32e3(Sr(UΔr)S1)Eψ+asinθ2U32ΔθΔre1(iJS+2acosθΔrS1)Eψ+iUΔθ(aωsinθ+Ξsinθ(k+12))e2S1JEψ,eμEμΨ=2ΔθΞ2UΔθcotθe1S1Eψia2ΔrΔθUsinθe1JS3Eψ+rΔr2Ue3S1Eψ+Δr2Ue3S3Eψ,eμ(iA(eμ))Ψ=iQrUΔre0S1EψiPcotθUΔθe2S1Eψ.

    Note that

    eμJ=Jeμ,eμE=E1eμ,eμE1=Eeμ,eμS1=1UΔrSeμ,eμS=UΔrS1eμ.

    Substituting the above formulas into (10), we obtain

    DΨ=1UΔrSE1(ΔrDrΔθLθ)ψ

    (11)

    with

    Dr=e3r+iΔr(ω(r2+a2)+aΞ(k+12))Je0iQrΔre0,Lθ=e1θ+iΔθ(ωasinθ+Ξsinθ(k+12))Je2(1Ξ2Δθ)cotθe1+iPΔθcotθe2.

    We denote λωk=λe2i(ωt+(k+12)ϕ). Using (11), we can reduce Dirac equation (1) to

    Drψ=Lθψ,ψ=(R(r)Θ(θ)R+(r)Θ+(θ)ˉR+(r)ˉΘ+(θ)ˉR(r)ˉΘ(θ))

    (12)

    where

    Dr=(iλωkrΔrDr.00iλωkrΔrDr,01ΔrDr,11i¯λωkrΔrDr,10i¯λωkr),Lθ=(aλωkcosθΔθLθ,00aλωkcosθΔθLθ,01ΔθLθ,11a¯λωkcosθΔθLθ,10a¯λωkcosθ)

    and for l, m=0, 1,

    Dr,lm=(1)mr+(1)liΔr(ω(r2+a2)+(k+12)Ξa)iQrΔr,Lθ,lm=(1)lθ+(1)l+mΔθ(ωasinθ+Ξsinθ(k+12)+(1)lPcotθ(1)m(ΔθΞ2)cotθ).

    Writing each row of (12), we obtain

    iλωkrRΘ+ΔrDr,00ˉR+ˉΘ+=aλωkcosθRΘΔθLθ,00ˉRˉΘ,

    (13)

    iλωkrR+Θ+ΔrDr,01ˉRˉΘ=aλωkcosθR+Θ++ΔθLθ,01ˉR+ˉΘ+,

    (14)

    i¯λωkrˉR+ˉΘ++ΔrDr,11RΘ=a¯λωkcosθˉR+ˉΘ++ΔθLθ,11R+Θ+,

    (15)

    i¯λωkrˉRˉΘ+ΔrDr,10R+Θ+=a¯λωkcosθˉRˉΘ+ΔθLθ,10RΘ.

    (16)

    These equations cannot be separated into radial and angular equations. However,

    ¯Dr,lm=Dr,lˉm,¯Lθ,lm=Lθ,lm

    and

    Dr,lm+Dr,ˉlˉm=2iQrΔr,Lθ,ˉlm+Lθ,lm=2(1)mPcotθΔθ,

    where ˉl=(l+1)mod2 and ˉm=(m+1)mod2. Thus, by subtracting the complex conjugation of (13) from (16) and adding the complex conjugation of (14) to (15), we obtain the two algebraic identities

    iα(r)RΘβ(θ)R+Θ+=0,β(θ)RΘ+iα(r)R+Θ+=0,

    where

    α(r)=QrΔr,β(θ)=PcotθΔθ.

    Therefore,

    (α(r)2β(θ)2)R+Θ+=(α(r)2β(θ)2)RΘ=0.

    If R+Θ+ or RΘ is nontrival, it must hold that α(r)=±β(θ). As α(r) depends only on r>0 (outside the event horizon), and β(θ) depends only on θ, both are constant. Therefore, three cases occur: (i) P=Q=0, (ii) P0, Q=0, and θ=π/2, and (iii) P0, Q0, and r=r0 is a positive constant, θ=θ0 is a constant. However, R+Θ+=RΘ=0 outside the hypersurface θ=π/2 is equipped with the metric

    ds23=Δrr2(dtaΞdϕ)2+r2Δrdr2+1r2(adtr2+a2Ξdϕ)2

    in case (ii), and outside the 2-surface is equipped with the metric

    ds22=Δr(r0)U(r0,θ0)(dtasin2θ0Ξdϕ)2+Δθ(θ0)sin2θ0U(r0,θ0)(adtr20+a2Ξdϕ)2

    in case (iii). This indicates that Majorana fermions are not differentiable in cases (ii) and (iii). Therefore, if P0 or Q0, we conclude that there is no differentiable time-periodic Majorana fermions in Kerr-Newman-type spacetimes.

    We note that Chandrasekhar's separation for time-periodic Dirac fermions is not consistent with the condition for Majorana fermions and introduce a new separation for time-periodic Majorana fermions. With this separation, the Dirac equation cannot be transformed into radial and angular equations, as is done in Chandrasekhar's separation. Instead, it is separated into four differential equations, which yield two algebraic identities. When the electric or magnetic charge is nonzero, they conclude that there is no differentiable time-periodic Majorana fermions outside the event horizon in Kerr-Newman and Kerr-Newman-AdS spacetimes, or between the event horizon and the cosmological horizon in Kerr-Newman-dS spacetime. This conclusion plays a role in searching for free Majorana fermions when considering the gravitational effect.

    The authors are grateful to the referees for many valuable suggestions.

    [1] W. Kinnersley, Journal of Mathematical Physics 10, 1969 (1195) doi: 10.1063/1.1664958
    [2] S. Chandrasekhar, Proceedings of the Royal Society, Series A 349, 571 (1976) doi: 10.1098/rspa.1976.0090
    [3] D. N. Page, Phys. Rev. D 14, 1509 (1976) doi: 10.1103/PhysRevD.14.1509
    [4] Z. Zhao, Y. X. Guei, and L. Liu, Chinese Astronomy and Astrophysics 5, 365 (1981) doi: 10.1016/0275-1062(81)90060-6
    [5] F. Belgiorno and M. Martellini, Phys. Lett. B 453, 17 (1999) doi: 10.1016/S0370-2693(99)00313-5
    [6] S. K. Chakrabarti and B. Mukhopadhyay, Monthly Notices of the Royal Astronomical Society 317, 979 (2000) doi: 10.1046/j.1365-8711.2000.03726.x
    [7] M. Angheben, M. Nadalini, L. Vanzo et al., Journal of High Energy Physics 05, 014 (2005) doi: 10.1088/1126-6708/2005/05/014
    [8] F. Finster, N. Kamran, J. Smoller et al., Communications on Pure and Applied Mathematics 53, 90 (2000) doi: 10.1002/(SICI)1097-0312(200007)53:7<902::AID-CPA4>3.0.CO;2-4
    [9] F. Belgiorno and S. L. Cacciatori, Journal of Physics A: Mathematical and Theoretical 42(13), 135207 (2009) doi: 10.1088/1751-8113/42/13/135207
    [10] F. Belgiorno and S. L. Cacciatori, Journal of Mathematical Physics 51, 033517 (2010) doi: 10.1063/1.3300401
    [11] Y. Wang and X. Zhang, Nonexistence of time-periodic solutions of the Dirac equation in non-extreme Kerr-Newman-AdS spacetime, Science China Mathematics, 61 , 73 (2018)
    [12] M. Fan, Y. Wang, and X. Zhang, Nonexistence of time-periodic solutions of the Dirac equation in Kerr-Newman-(A)dS spacetime, (2024), arXiv: 2404.13255v1[gr-qc]
    [13] G.V. Kraniotis, Journal of Physics: Communications 3, 035026 (2019) doi: 10.1088/2399-6528/ab1046
    [14] W. Rodejohann, International Journal of Modern Physics E 20, 1833 (2011) doi: 10.1142/S0218301311020186
    [15] S. A. Alavi and A. Abbasnezhad, Gravitation and Cosmology 22, 288 (2016) doi: 10.1134/S0202289316030038
    [16] T. P. Cheng and L. F. Li, Phys. Rev. Lett. 45, 1980 (1908)
    [17] T. Garavaglia, Phys. Rev. D 29, 387 (1984) doi: 10.1103/PhysRevD.29.387
    [18] J. Nieves and P. Pal, Phys. Rev. D 36, 315 (1987) doi: 10.1103/PhysRevD.36.315
    [19] S. P. Rosen, Phys. Rev. Lett. 48, 842 (1982) doi: 10.1103/PhysRevLett.48.842
    [20] R. E. Shrock, Nucl. Phys. B 206, 359 (1982) doi: 10.1016/0550-3213(82)90273-5
    [21] D. Singh, N. Mobed, and G. Papini, Phys. Rev. Lett. 97, 041101 (2006) doi: 10.1103/PhysRevLett.97.041101
    [22] C. S. Kim, M. V. N. Murthy et al., Phys. Rev. D 105, 113006 (2022) doi: 10.1103/PhysRevD.105.113006
    [23] E. Majorana, Nuovo Cimento C-Colloquia and Communications in Physics 14, 171 (1937)
    [24] T. Prokopec and V. H. Unnithan, Majorana propagator on de Sitter space, European Physical Journal C, 82 ,1015 (2022)
    [25] C-L. Hsieh, V. Memari and M. Halilsoy, Dirac and Majorana fermions in the anti-de Sitter spacetime with tachyonic approaches, (2024), arXiv: 2403.13810v1[hep-th]
  • [1] W. Kinnersley, Journal of Mathematical Physics 10, 1969 (1195) doi: 10.1063/1.1664958
    [2] S. Chandrasekhar, Proceedings of the Royal Society, Series A 349, 571 (1976) doi: 10.1098/rspa.1976.0090
    [3] D. N. Page, Phys. Rev. D 14, 1509 (1976) doi: 10.1103/PhysRevD.14.1509
    [4] Z. Zhao, Y. X. Guei, and L. Liu, Chinese Astronomy and Astrophysics 5, 365 (1981) doi: 10.1016/0275-1062(81)90060-6
    [5] F. Belgiorno and M. Martellini, Phys. Lett. B 453, 17 (1999) doi: 10.1016/S0370-2693(99)00313-5
    [6] S. K. Chakrabarti and B. Mukhopadhyay, Monthly Notices of the Royal Astronomical Society 317, 979 (2000) doi: 10.1046/j.1365-8711.2000.03726.x
    [7] M. Angheben, M. Nadalini, L. Vanzo et al., Journal of High Energy Physics 05, 014 (2005) doi: 10.1088/1126-6708/2005/05/014
    [8] F. Finster, N. Kamran, J. Smoller et al., Communications on Pure and Applied Mathematics 53, 90 (2000) doi: 10.1002/(SICI)1097-0312(200007)53:7<902::AID-CPA4>3.0.CO;2-4
    [9] F. Belgiorno and S. L. Cacciatori, Journal of Physics A: Mathematical and Theoretical 42(13), 135207 (2009) doi: 10.1088/1751-8113/42/13/135207
    [10] F. Belgiorno and S. L. Cacciatori, Journal of Mathematical Physics 51, 033517 (2010) doi: 10.1063/1.3300401
    [11] Y. Wang and X. Zhang, Nonexistence of time-periodic solutions of the Dirac equation in non-extreme Kerr-Newman-AdS spacetime, Science China Mathematics, 61 , 73 (2018)
    [12] M. Fan, Y. Wang, and X. Zhang, Nonexistence of time-periodic solutions of the Dirac equation in Kerr-Newman-(A)dS spacetime, (2024), arXiv: 2404.13255v1[gr-qc]
    [13] G.V. Kraniotis, Journal of Physics: Communications 3, 035026 (2019) doi: 10.1088/2399-6528/ab1046
    [14] W. Rodejohann, International Journal of Modern Physics E 20, 1833 (2011) doi: 10.1142/S0218301311020186
    [15] S. A. Alavi and A. Abbasnezhad, Gravitation and Cosmology 22, 288 (2016) doi: 10.1134/S0202289316030038
    [16] T. P. Cheng and L. F. Li, Phys. Rev. Lett. 45, 1980 (1908)
    [17] T. Garavaglia, Phys. Rev. D 29, 387 (1984) doi: 10.1103/PhysRevD.29.387
    [18] J. Nieves and P. Pal, Phys. Rev. D 36, 315 (1987) doi: 10.1103/PhysRevD.36.315
    [19] S. P. Rosen, Phys. Rev. Lett. 48, 842 (1982) doi: 10.1103/PhysRevLett.48.842
    [20] R. E. Shrock, Nucl. Phys. B 206, 359 (1982) doi: 10.1016/0550-3213(82)90273-5
    [21] D. Singh, N. Mobed, and G. Papini, Phys. Rev. Lett. 97, 041101 (2006) doi: 10.1103/PhysRevLett.97.041101
    [22] C. S. Kim, M. V. N. Murthy et al., Phys. Rev. D 105, 113006 (2022) doi: 10.1103/PhysRevD.105.113006
    [23] E. Majorana, Nuovo Cimento C-Colloquia and Communications in Physics 14, 171 (1937)
    [24] T. Prokopec and V. H. Unnithan, Majorana propagator on de Sitter space, European Physical Journal C, 82 ,1015 (2022)
    [25] C-L. Hsieh, V. Memari and M. Halilsoy, Dirac and Majorana fermions in the anti-de Sitter spacetime with tachyonic approaches, (2024), arXiv: 2403.13810v1[hep-th]
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HE-QUN ZHANG and XIAO ZHANG. Nonexistence of Majorana fermions in Kerr-Newman type spacetimes with nontrivial charge[J]. Chinese Physics C. doi: 10.1088/1674-1137/ad709e
HE-QUN ZHANG and XIAO ZHANG. Nonexistence of Majorana fermions in Kerr-Newman type spacetimes with nontrivial charge[J]. Chinese Physics C.  doi: 10.1088/1674-1137/ad709e shu
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Nonexistence of Majorana fermions in Kerr-Newman type spacetimes with nontrivial charge

  • 1. Academy of Mathematics and Systems Science, Chinese Academy of Sciences, Beijing 100190, China
  • 2. School of Mathematical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: We show that the Dirac equation is separated into four differential equations for time-periodic Majorana fermions in Kerr-Newman and Kerr-Newman-(A)dS spacetimes. Although they cannot be transformed into radial and angular equations, the four differential equations yield two algebraic identities. When the electric or magnetic charge is nonzero, they conclude that there is no differentiable time-periodic Majorana fermions outside the event horizon in Kerr-Newman and Kerr-Newman-AdS spacetimes, or between the event horizon and the cosmological horizon in Kerr-Newman-dS spacetime.

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    I.   INTRODUCTION
    • The Dirac equation in black hole spacetimes plays a significant role in the study of general relativity and quantum cosmology. A Dirac fermion is a spinor Ψ in spacetimes satisfying the Dirac equation

      (D+iλ)Ψ=0,

      (1)

      where λ is a certain real number. In 1968, Kinnersley introduced the null basis to study the Petrov type D metric [1]. In [2], Chandrasekhar separated the Dirac equation in Kerr spacetime when Dirac fermions are time-periodic and given by

      Ψ=S1ψ,ψ=ei(ωt+(k+12)ϕ)(R(r)Θ(θ)R+(r)Θ+(θ)R+(r)Θ(θ)R(r)Θ+(θ)),

      (2)

      where S is a diagonal matrix,

      S=Δ 14rdiag((r+iacosθ)12I2×2,(riacosθ)12I2×2),

      and Page extended his method to Kerr-Newman spacetime [3]. Since then, various studies have been conducted to investigate Hawking radiation and the numerical solutions of Dirac fermions in various spacetime backgrounds (for examples, see [47]. In [8], Finster, Kamran, Smoller, and Yau applied Chandrasekhar's separation to prove the nonexistence of the L2 integrable, time-periodic solutions of the Dirac equation in non-extreme Kerr-Newman spacetime. This indicates that the normalizable time-periodic Dirac fermions must either disappear into the black hole or escape to infinity. In [9, 10], Belgiorno and Cacciatori applied the spectral properties to prove the non-existence of the L2 integrable, time-periodic solutions of the Dirac equation with mass greater than 12|Λ|3 in non-extreme Kerr-Newman-(A)dS spacetimes, where Λ is the cosmological constant. In [11], Wang and Zhang applied Chandrasekhar's separation to prove the nonexistence of the Lp integrable, time-periodic solutions of the Dirac equation with arbitrary mass and 0<p43, or with mass greater than qΛ3 and 43<p432q, 0<q<32 in non-extreme Kerr-Newman-AdS spacetime. In non-extreme Kerr-Newman-dS spacetime, the nonexistence of Lp integrable, time-periodic Dirac fermions hold true for arbitrary mass and p2 [12]. In particular, taking p=2, they confirmed Belgiorno and Cacciatori's results in which normalizable time-periodic Dirac fermions with mass greater than 12Λ3 must either disappear into the black hole or escape to infinity.

      For Chandrasekhar's separation, the Dirac equation can be transformed into radial and angular equations. In [13], Kraniotis observed that the radial and angular equations could be reduced to generalized Heun's equations in Kerr-Newman spacetime, which provide local time-periodic solutions in terms of holomorphic functions, whose power series coefficients are determined by a four-term recurrence relation. Using the four-term recursion formula, he also proved that there is no time-periodic solution with a fermion energy strictly less than its mass in Kerr-Newman spacetime.

      In the recent search for neutrinos, which are one of the most mysterious particles in the universe, we are interested in discovering whether they are Majorana fermions. The most promising method to date is through double beta decay [14]. Various approaches have been studied to distinguish between Majorana and Dirac fermions (for examples, see [1522]).

      A Majorana fermion is a Dirac fermion whose antiparticle is itself. To define a Majorana fermion precisely, let us introduce the 4-component charge conjugate operator

      C=(ϵβαϵ˙β˙α)

      with the Pauli matrix σ2 and antisymmetric operator on spin indices, where

      ϵ˙α˙β=ϵαβ=iσ2=(11).

      The charge conjugation of the Dirac fermion Ψ is defined by

      ΨC=CˉΨT.

      Therefore, Majorana fermions are given by

      ΨMaj=(ΨWeyliσ2ΨWeyl)

      (3)

      and satisfy the Dirac equation [2325], where ΨWeyl is the Weyl spinor, and ΨWeyl is its complex conjugate. Time-period Majorana fermions can be given by

      Ψ=S1Eψ,ψ=(R(r)Θ(θ)R+(r)Θ+(θ)ˉR+(r)ˉΘ+(θ)ˉR(r)ˉΘ(θ)),

      (4)

      where S and E are the diagonal matrices

      S=Δ 14rdiag((r+iacosθ)12I2×2,(riacosθ)12I2×2),E=diag(ei(ωt+(k+12)ϕ)I2×2,ei(ωt+(k+12)ϕ)I2×2).

      In this short paper, we show that the Dirac equation is separated into four differential equations for time-periodic Majorana fermions given by (4) in Kerr-Newman and Kerr-Newman-(A)dS spacetimes. Although they cannot be transformed into radial and angular equations, the four differential equations yield two algebraic identities. When the electric or magnetic charge is nonzero, they conclude that there are no differentiable time-periodic Majorana fermions outside the event horizon in Kerr-Newman and Kerr-Newman-AdS spacetimes, or between the event horizon and the cosmological horizon in Kerr-Newman-dS spacetime.

      We remark that Dirac fermions taking form (2) are not consistent with Majorana condition (3). Thus, previous results on the existence or non-existence of time-periodic Dirac fermions cannot be applied to the current situation for time-periodic Majorana fermions.

    II.   GEOMETRY OF KERR-NEWMAN-TYPE SPACETIMES
    • For convenience of discussion, we unify the Kerr-Newman and Kerr-Newman-(A)dS metrics

      ds2KNType=ΔrU(dtasin2θΞdϕ)2+UΔrdr2+UΔθdθ2+Δθsin2θU(adtr2+a2Ξdϕ)2,

      (5)

      by taking κ as zero, pure imaginary, and real, where Λ=3κ2 is the cosmological constant, and

      Δr=(r2+a2)(1+κ2r2)2mr+P2+Q2,Δθ=1κ2a2cos2θ,U=r2+a2cos2θ,Ξ=1κ2a2>0.

      The metric (5) solves the Einstein-Maxwell field equations with the electromagnetic potential

      A=QrU(dtasin2θΞdφ)PcosθU(adtr2+a2Ξdφ),

      (6)

      where P and Q are real numbers representing the magnetic and electric charges, respectively.

      In the following, we let 0μ,ν3, and 1i,j3. On a 4-dimensional Lorentzian manifold, we choose the frame {eμ} such that e0 is timelike and ei are spacelike. We denote {eα} as the dual coframe. The Cartan structure equations are

      deμ=ωμ νeν,ωμν=gμγωγ ν=ωνμ.

      If it is spin, we use the cotangent bundle to define the Clifford multiplication, spin connection, and Dirac operator [11, 12]. We fix the Clifford multiplications as the following Weyl representation:

      e0(I2×2I2×2),ei(σiσi),

      (7)

      where σi are Pauli matrices,

      σ1=(11), σ2=(ii), σ3=(11).

      We fix our discussion in the region Δr>0. The coframe is

      e0=ΔrU(dtasin2θΞdϕ) ,e1=UΔθdθe2=ΔθUsinθ(adtr2+a2Ξdϕ),e3=UΔrdr.

      with the dual frame

      e0=r2+a2UΔr(t+aΞr2+a2ϕ) , e1=ΔrUr,e2=ΔθUθ , e3=1UΔθ(asinθt+Ξsinθϕ).

      With respect to the above coframe, we obtain

      de0=C010e1e0+C030e3e0+C012e1e2,de1=C131e3e1,de2=C230e3e0+C232e3e2+C212e1e2,de3=C331e3e1,

      where

      C010=a2UΔθUsinθcosθ,C030=rΔrU,C012=2aUΔrUcosθ,C131=rUΔrU,C212=1sinθθ(ΔθUsinθ),C232=rUΔrU,C230=2arUΔθUsinθ,C331=a2UΔθUsinθcosθ.

      Thus, the connection 1-forms are

      ω0 1=ω01=C010e0+12C012e2,ω0 2=ω02=12C230e312C012e1,ω0 3=ω03=C030e012C230e2,ω1 2=ω12=12C012e0C212e2,ω1 3=ω13=C331e3+C131e1,ω2 3=ω23=12C230e0+C232e2.

      (8)

      The spin connection is defined as

      ˜XΨ=X(Ψ)14ωμν(X)eμeνΨ,

      where X is a vector, Φ is a spinor, and eμ is the Clifford multiplication. Therefore, using (8), we obtain

      ˜e0Ψ=e0Ψ12ω01(e0)e0e1Ψ12ω03(e0)e0e3Ψ12ω12(e0)e1e2Ψ12ω23(e0)e2e3Ψ,˜e1Ψ=e1Ψ12ω02(e1)e0e2Ψ12ω12(e1)e1e2Ψ,˜e2Ψ=e2Ψ12ω01(e2)e0e1Ψ12ω03(e2)e0e3Ψ12ω12(e2)e1e2Ψ12ω23(e2)e2e3Ψ,˜e3Ψ=e3Ψ12ω02(e3)e0e2Ψ12ω13(e3)e1e3Ψ.

      Using Clifford multiplication (7), these can be written as the matrix forms

      ˜eμΨ=eμΨ+EμΨ,Eμ=12(ϵμ00ϵhμ),

      (9)

      where ϵhμ is the Hermitian conjugate of ϵμ and

      ϵ0=(C010i2C230)σ1+(C030i2C012)σ3,ϵ1=(12C012+iC131)σ2,ϵ2=(12C012iC232)σ1(12C230iC212)σ3,ϵ3=(12C230+iC331)σ2.

    III.   TIME-PERIODIC MAJORANA FERMIONS
    • In this section, we prove the nonexistence of time-periodic Majorana fermions in Kerr-Newman type spacetime when the electric or magnetic charge is nonzero.

      First, we simplify the Dirac equation (1) on metric (5) when Ψ is given by (4). The Dirac operator with electromagnetic potential A is

      D=eμ(˜eμ+iA(eμ)).

      (10)

      We denote J=diag(I2×2,I2×2). In terms of (6) and (9), we obtain

      eμeμ(Ψ)=ΔrUe3S1Erψ+ΔθUe1S1Eθψir2+a2UΔr(ω+aΞr2+a2(k+12))e0JS1Eψ+12U32e3(Sr(UΔr)S1)Eψ+asinθ2U32ΔθΔre1(iJS+2acosθΔrS1)Eψ+iUΔθ(aωsinθ+Ξsinθ(k+12))e2S1JEψ,eμEμΨ=2ΔθΞ2UΔθcotθe1S1Eψia2ΔrΔθUsinθe1JS3Eψ+rΔr2Ue3S1Eψ+Δr2Ue3S3Eψ,eμ(iA(eμ))Ψ=iQrUΔre0S1EψiPcotθUΔθe2S1Eψ.

      Note that

      eμJ=Jeμ,eμE=E1eμ,eμE1=Eeμ,eμS1=1UΔrSeμ,eμS=UΔrS1eμ.

      Substituting the above formulas into (10), we obtain

      DΨ=1UΔrSE1(ΔrDrΔθLθ)ψ

      (11)

      with

      Dr=e3r+iΔr(ω(r2+a2)+aΞ(k+12))Je0iQrΔre0,Lθ=e1θ+iΔθ(ωasinθ+Ξsinθ(k+12))Je2(1Ξ2Δθ)cotθe1+iPΔθcotθe2.

      We denote λωk=λe2i(ωt+(k+12)ϕ). Using (11), we can reduce Dirac equation (1) to

      Drψ=Lθψ,ψ=(R(r)Θ(θ)R+(r)Θ+(θ)ˉR+(r)ˉΘ+(θ)ˉR(r)ˉΘ(θ))

      (12)

      where

      Dr=(iλωkrΔrDr.00iλωkrΔrDr,01ΔrDr,11i¯λωkrΔrDr,10i¯λωkr),Lθ=(aλωkcosθΔθLθ,00aλωkcosθΔθLθ,01ΔθLθ,11a¯λωkcosθΔθLθ,10a¯λωkcosθ)

      and for l, m=0, 1,

      Dr,lm=(1)mr+(1)liΔr(ω(r2+a2)+(k+12)Ξa)iQrΔr,Lθ,lm=(1)lθ+(1)l+mΔθ(ωasinθ+Ξsinθ(k+12)+(1)lPcotθ(1)m(ΔθΞ2)cotθ).

      Writing each row of (12), we obtain

      iλωkrRΘ+ΔrDr,00ˉR+ˉΘ+=aλωkcosθRΘΔθLθ,00ˉRˉΘ,

      (13)

      iλωkrR+Θ+ΔrDr,01ˉRˉΘ=aλωkcosθR+Θ++ΔθLθ,01ˉR+ˉΘ+,

      (14)

      i¯λωkrˉR+ˉΘ++ΔrDr,11RΘ=a¯λωkcosθˉR+ˉΘ++ΔθLθ,11R+Θ+,

      (15)

      i¯λωkrˉRˉΘ+ΔrDr,10R+Θ+=a¯λωkcosθˉRˉΘ+ΔθLθ,10RΘ.

      (16)

      These equations cannot be separated into radial and angular equations. However,

      ¯Dr,lm=Dr,lˉm,¯Lθ,lm=Lθ,lm

      and

      Dr,lm+Dr,ˉlˉm=2iQrΔr,Lθ,ˉlm+Lθ,lm=2(1)mPcotθΔθ,

      where ˉl=(l+1)mod2 and ˉm=(m+1)mod2. Thus, by subtracting the complex conjugation of (13) from (16) and adding the complex conjugation of (14) to (15), we obtain the two algebraic identities

      iα(r)RΘβ(θ)R+Θ+=0,β(θ)RΘ+iα(r)R+Θ+=0,

      where

      α(r)=QrΔr,β(θ)=PcotθΔθ.

      Therefore,

      (α(r)2β(θ)2)R+Θ+=(α(r)2β(θ)2)RΘ=0.

      If R+Θ+ or RΘ is nontrival, it must hold that α(r)=±β(θ). As α(r) depends only on r>0 (outside the event horizon), and β(θ) depends only on θ, both are constant. Therefore, three cases occur: (i) P=Q=0, (ii) P0, Q=0, and θ=π/2, and (iii) P0, Q0, and r=r0 is a positive constant, θ=θ0 is a constant. However, R+Θ+=RΘ=0 outside the hypersurface θ=π/2 is equipped with the metric

      ds23=Δrr2(dtaΞdϕ)2+r2Δrdr2+1r2(adtr2+a2Ξdϕ)2

      in case (ii), and outside the 2-surface is equipped with the metric

      ds22=Δr(r0)U(r0,θ0)(dtasin2θ0Ξdϕ)2+Δθ(θ0)sin2θ0U(r0,θ0)(adtr20+a2Ξdϕ)2

      in case (iii). This indicates that Majorana fermions are not differentiable in cases (ii) and (iii). Therefore, if P0 or Q0, we conclude that there is no differentiable time-periodic Majorana fermions in Kerr-Newman-type spacetimes.

    IV.   CONCLUSION
    • We note that Chandrasekhar's separation for time-periodic Dirac fermions is not consistent with the condition for Majorana fermions and introduce a new separation for time-periodic Majorana fermions. With this separation, the Dirac equation cannot be transformed into radial and angular equations, as is done in Chandrasekhar's separation. Instead, it is separated into four differential equations, which yield two algebraic identities. When the electric or magnetic charge is nonzero, they conclude that there is no differentiable time-periodic Majorana fermions outside the event horizon in Kerr-Newman and Kerr-Newman-AdS spacetimes, or between the event horizon and the cosmological horizon in Kerr-Newman-dS spacetime. This conclusion plays a role in searching for free Majorana fermions when considering the gravitational effect.

    ACKNOWLEDGEMENT
    • The authors are grateful to the referees for many valuable suggestions.

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