Measurement of permanent electric dipole moments of charged hadrons in storage rings
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Measurement of permanent electric dipole moments of charged hadrons in storage rings Jörg Pretz on behalf of the JEDI collaboration
Published online: 1 February 2013 © Springer Science+Business Media Dordrecht 2013
Abstract Permanent Electric Dipole Moments (EDMs) of elementary particles violate two fundamental symmetries: time reversal invariance (T ) and parity (P ). Assuming the CPT theorem this implies CP violation. The CP violation of the Standard Model is orders of magnitude too small to be observed experimentally in EDMs in the foreseeable future. It is also way too small to explain the asymmetry in abundance of matter and anti-matter in our universe. Hence, other mechanisms of CP violation outside the realm of the Standard Model are searched for and could result in measurable EDMs. Up to now most of the EDM measurements were done with neutral particles. With new techniques it is now possible to perform dedicated EDM experiments with charged hadrons at storage rings where polarized particles are exposed to an electric field. If an EDM exists the spin vector will experience a torque resulting in change of the original spin direction which can be determined with the help of a polarimeter. Although the principle of the measurement is simple, the smallness of the expected effect makes this a challenging experiment requiring new developments in various experimental areas. Complementary efforts to measure EDMs of proton, deuteron and light nuclei are pursued at Brookhaven National Laboratory and at Forschungszentrum Jülich with an ultimate goal to reach a sensitivity of 10−29 e·cm. Keywords Electric dipole moment · Standard Model · CP violation · Matter–anti-matter asymmetry
The 5th International Symposium on Symmetries in Subatomic Physics (SSP 2012), Groningen, The Netherlands, 18–22 June 2012. J. Pretz (B) III. Physikalisches Institut, Physikzentrum 26C 212, RWTH Aachen, 52056 Aachen, Germany e-mail: [email protected]
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Table 1 Current limits of hadron EDMs
Particle/atom
Current limit/e·cm
Reference
Neutron 199 Hg → Proton Deuteron 3 He
< 3 · 10−26 (90 % CL) < 3.1 · 10−29 (95 % CL) < 7.9 · 10−25 – –
[14] [15]
1 Introduction & motivation One of the great mysteries in particle physics is the dominance of matter over antimatter in our Universe. The net baryon number is [1] n B − n B¯ −10 . = 6.1+0.3 −0.2 · 10 nγ In the Standard Model (SM) this ratio is expected to be on the order of 10−18 . In 1967 Sakharov [2–5] formulated three prerequisites for baryogenesis. One of these is the combined violation of the charge and parity, CP , symmetry. New CP violating sources outside the realm of the SM are clearly needed to explain this discrepancy of eight orders of magnitude. For non-degenerate systems like for example elementary particles, including hadrons, an electric dipole moment is only possible if the two fundamental symmetries, parity P and time reversal invariance T are violated. Using the CPT theorem a violation of T is equivalent to CP violation. CP violation is present in th
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