Searching for electric dipole moment of electrons Key words : CP violation, electron dipole moment (EDM), ACME collaboration 2018.11.21 KAIST Department of Physics Geun-Hee Lee KAIST Physics/November 21st, 2018 1
Contents • Introduction • CP violation and electric dipole moment (EDM) • Theory • CP violation in standard model (SM) and possibility of new CP symmetry breaking channels in beyond standard model (BSM) • Experiment • Effort for finding electron EDM : ACME collaboration • Summary KAIST Physics/November 21st, 2018 2
Introduction : CP violation and electric dipole moment (EDM) KAIST Physics/November 21st, 2018 3
Symmetries and CP violation • Symmetry : Operation that leaves physical situation Symmetry Action Explanation invariant. Inversion of • Broken symmetry (or violation) means symmetric P 𝑠 → −Ԧ Ԧ 𝑠 spatial operation is no longer symmetric. coordinate Inversion of • Two kinds of symmetries : continuous or discrete T 𝑢 → −𝑢 time • CPT theorem : successive operation of charge coordinate conjugation, parity and time reversal is symmetry, Inversion of internal under the Lorentz invariant theory & Lorentz invariant 𝑟 → −𝑟 C quantum vacuum. number • One implication : CP violation iff T symmetry is broken. Kinds of discrete symmetries KAIST Physics/November 21st, 2018 4
Symmetries and EDM • By the virtue of Wigner-Eckart theorem, EDM of spin ½ particles should be collinear with their spin direction. • EDM exists only if both P and T symmetries are broken. • P-violation : 𝛾 -decay of 60 𝐷𝑝 • CP-violation (= T-violation) : Neutral Kaon mixing • So possibility of finding EDM of fundamental particles! (New source of CP violation) • But electron EDM has not been measured yet. 60 𝐷𝑝 − 𝛾 𝑒𝑓𝑑𝑏𝑧 P and T violation in the (https://galileospendulum.org/tag/symmetry/) presence of EDM (Wikipedia, neutron electric dipole moment) • Related topics : Strong CP problem & baryogenesis CP-violating QCD Hamiltonian KAIST Physics/November 21st, 2018 5
Theory : CP violation in and beyond Standard Model (SM) KAIST Physics/November 21st, 2018 6
EDM in SM • In standard model, the only observed CP violating mechanism is quark mixing: From complex Yukawa Yukawa interaction in SM interaction • Cabbibo-Kobayashi-Maskawa (CKM) quark mixing matrix involves CP-violating phase.( 𝜀 𝐿𝑁 ) CKM matrix with conventional notation • However, it is hard to make electron EDM only using this mechanism: Loop should involve quarks, All effects from three-loop diagrams are vanished. Order estimation of electron EDM yields 𝑒 𝑓 < 10 −37 𝑓 ⋅ 𝑑𝑛 . • • Note : Possibility of lepton mixing, and CP-violation during that. Resultant EDM is negligible, even if comparing with Three loop diagram generating electron EDM quark mixing. KAIST Physics/November 21st, 2018 7
EDM in BSM • Feynman diagram should involve quark loop to generate EDM: Small electron EDM • BSMs introduce new particles which can make CP-violation in one-loop diagram. • Supersymmetric(SUSY) model : Complex superpotential, Generic one-loop diagram generating EDM, F and B refer fermions and bosons spontaneous SUSY breaking • Left-Right symmetric model : Complex vacuum expectation value of Higgs multiplet and their contribution on mass of weak bosons. • Etc.. • If we found relatively large electron EDM, then we reasonably consider another channels of CP violation. (Suggested by models other than SM) One loop contributions to EDM of neutralinos, charginos, and RH neutrinos Generous assumptions yield 𝑒 𝑓 ≈ 10 −27 𝑓 ⋅ 𝑑𝑛 • KAIST Physics/November 21st, 2018 8
Experiment : ACME collaboration KAIST Physics/November 21st, 2018 9
General experiment scheme • How can we measure electron EDM? • Effective Hamiltonian for spin is 𝐼 = − 𝑒 𝑓 𝐹 + 𝑒 𝑛 𝐼 ⋅ Ԧ 𝜏 where • 𝑒 𝑓 , 𝑒 𝑛 are electric and magnetic dipole moment, http://www.electronedm.org/ 𝜏 are electric, magnetic and spin, respectively . • 𝐹, 𝐼, Ԧ • Hence the coupling between electric field and electron EDM appears in terms of spin precession: initial state preparation and control of magnetic field are very en.wikipedia.org/wiki/Electron_electric_dipole_momnet important. KAIST Physics/November 21st, 2018 10
ACME collaboration • A dvanced C old M olecule E lectron EDM (ACME) collaboration is experiment group to find electron EDM using cold molecules. • They have used thorium monoxides • Relatively high intramolecular electric field : 𝐹 𝑓𝑔𝑔 ~84𝐻𝑊/𝑑𝑛 • Cold molecules (single quantum state) with high flux H-state of ThO : Red dot = magnetic field coupling, Green dot = electric field coupling (intensive signal) • Metastable H state (long coherence time 𝜐 𝑑 ~2𝑛𝑡 ) • Direction of intra-molecular field is controlled by Spin precession described by wave function external electric field. KAIST Physics/November 21st, 2018 11
Ultrafast Spin Dynamics Lab. ACME collaboration Thermalization of 𝑈ℎ𝑃 J-cooling & M-remixing (designed to make larger J=1 population) KAIST Physics/November 21st, 2018 12
ACME collaboration KAIST Physics/November 21st, 2018 13
ACME collaboration • Systematic errors (caused by stray field, B- Experiment 𝒆 𝒇 values with error 90% confidence level gradient, spin orbit coupling, etc.) and statistical 𝑒 𝑓 errors are comparable to measured value: 𝑒 𝑓 < 8.7 × 10 −29 𝑓 ⋅ 𝑑𝑛 ACME 1 = −2.1 ± 3.7 𝑡𝑢𝑏𝑢 ± 2.5 𝑡𝑧𝑡𝑢 × 10 −29 𝑓 ⋅ 𝑑𝑛 • Authors have limited energy scale of new CP- 𝑒 𝑓 ACME 2 𝑒 𝑓 < 1.1 × 10 −29 𝑓 ⋅ 𝑑𝑛 = 4.3 ± 3.1 𝑡𝑢𝑏𝑢 ± 2.6 𝑡𝑧𝑡𝑢 × 10 −30 𝑓 ⋅ 𝑑𝑛 violating physics using simple order estimation. Result of ACME experiments • Λ ≈ 3~30 TeV scale, Λ is rest-mass energy of new particle mediating CP-violation • Another experiments are currently designing. • e.g. John Doyle (Harvard Univ, ACME collaborator) proposed experiments on YbOH molecular trap, and expected PeV scale energy regime. KAIST Physics/November 21st, 2018 14
Ultrafast Spin Dynamics Lab. Summary • CP-violation (equivalent to T-violation under CPT theorem) with P-violation makes EDM of fundamental particles. • In SM, the only known source of CP-violation is quark mixing, and SM expects that it would generate very small electron EDM (Order of 10 −38 𝑓 ⋅ 𝑑𝑛 ) • Hence, discovery of large electron EDM opens the way of CP-symmetry breaking with new channels through new particles. • Experimentally, physicists found 𝑒 𝑓 < 1.1 × 10 −29 𝑓 ⋅ 𝑑𝑛 using polar molecule ThO. This implies there could be no particle mediating CP-violation up to tens of TeV level. • They currently design new experiments searching PeV CP-violation regime. KAIST Physics/November 21st, 2018 15
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