A CMB Millikan experiment with cosmic axiverse strings
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Springer
Received: February 28, 2020 Accepted: July 2, 2020 Published: July 21, 2020
A CMB Millikan experiment with cosmic axiverse strings
a
Jefferson Physical Laboratory, Harvard University, 17 Oxford Street, Cambridge, MA 02138, U.S.A. b Maryland Center for Fundamental Physics, University of Maryland, College Park, MD 20742, U.S.A. c Perimeter Institute for Theoretical Physics, 31 Caroline St. N., Waterloo, Ontario N2L 2Y5, Canada
E-mail: [email protected], [email protected], [email protected] Abstract: We study axion strings of hyperlight axions coupled to photons. Hyperlight axions — axions lighter than Hubble at recombination — are a generic prediction of the string axiverse. These axions strings produce a distinct quantized polarization rotation of CMB photons which is O(αem ). As the CMB light passes many strings, this polarization rotation converts E-modes to B-modes and adds up like a random walk. Using numerical simulations we show that the expected size of the final result is well within the reach of current and future CMB experiments through the measurement of correlations of CMB B-modes with E- and T-modes. The quantized polarization rotation angle is topological in nature and can be seen as a geometric phase. Its value depends only on the anomaly coefficient and is independent of other details such as the axion decay constant. Measurement of the anomaly coefficient by measuring this rotation will provide information about the UV theory, such as the quantization of electric charge and the value of the fundamental unit of charge. The presence of axion strings in the universe relies only on a phase transition in the early universe after inflation, after which the string network rapidly approaches an attractor scaling solution. If there are additional stable topological objects such as domain walls, axions as heavy as 10−15 eV would be accessible. The existence of these strings could also be probed by measuring the relative polarization rotation angle between different images in gravitationally lensed quasar systems. Keywords: Strings and branes phenomenology ArXiv ePrint: 1912.02823
c The Authors. Open Access, Article funded by SCOAP3 .
https://doi.org/10.1007/JHEP07(2020)138
JHEP07(2020)138
Prateek Agrawal,a Anson Hookb and Junwu Huangc
Contents 1
2 Photon polarization rotation by a single axion string
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3 Quantized anomaly coefficient and its implications 3.1 Model-independent implications 3.2 Model-dependent implications 3.3 Kinetic mixing with a dark photon
6 7 9 9
4 Polarization rotation by a string network 4.1 Properties of the string network 4.2 Analytical approximation of the two-point function 4.3 A toy numerical simulation 4.4 Method comparison and limitations
10 10 12 16 19
5 CMB observables 5.1 Polarization rotation angle power spectrum 5.2 Distinguishing strings from other sources of B-modes 5.3 Edge detection
21 21 24 24
6 Other observational signature of axion strings 6.1 Quasar lensing 6.2 Gravitational and gravitational wave signatures
26 26 28
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