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New Search for Mirror Neutrons at HFIRL. J. Broussarda, K. M. Baileya, W. B. Baileya, J. L. Barrowb, B. Chanceb,C. Crawfordd, L. Crowa, L. Debeer-Schmitta, N. Fominb, M. Frosta,b,A. Galindo-Uribarria, F. X. Gallmeiera, L. Heilbronnb, E. B. Iversona,Y. Kamyshkovb, C.-Y. Liuc, I. Novikove, S. I. Penttil ̈aa, A. Rugglesb,B. Ryboltb, M. Snowc, L. Townsendb, L. J. Varrianob, S. Vavrab, A. R. YoungfaOak Ridge National Laboratory, Oak Ridge, TN 37831 USAbUniversity of Tennessee, Knoxville, TN 37996 USAcIndiana University, Bloomington, IN 47405 USAdUniversity of Kentucky, Lexington, KY 40506 USAeWestern Kentucky University, Bowling Green, KY 42101 USAfNorth Carolina State University, Raleigh, NC 27695 USA
Abstract
The theory of mirror matter predicts a hidden sector made up of a copy of theStandard Model particles and interactions but with opposite parity. If mirrormatter interacts with ordinary matter, there could be experimentally accessi-ble implications in the form of neutral particle oscillations. Direct searches forneutron oscillations into mirror neutrons in a controlled magnetic field havepreviously been performed using ultracold neutrons in storage/disappearancemeasurements, with some inconclusive results consistent with characteristic os-cillation time ofτ∼10 s. Here we describe a proposed disappearance and regen-eration experiment in which the neutron oscillates to and from a mirror neutronstate. An experiment performed using the existing General Purpose-Small An-gle Neutron Scattering instrument at the High Flux Isotope Reactor at OakRidge National Laboratory could have the sensitivity to exclude up toτ<15 sin 1 week of beamtime and at low cost.1. IntroductionA strong foundation of evidence has been built over the past several decadeswhich indicates that 85% of the matter in the universe is unknown to us
[1].The evidence for Dark Matter comes from many diverse astronomical sourcesincluding rotational curves of galaxies, weak and strong lensing, galaxy collisionssuch as the Bullet Cluster, and the cosmic microwave background; however, todate it is based solely on astrophysical signatures of its gravitational influence.Determining the particle nature of dark matter has for years been one of thehighest priorities of the particle physics community, but there remains no con-clusive evidence to its origin. A significant focus of these searches has been thewell-motivated Weakly Interacting Massive Particles [2, 3], but the diminishingparameter space above the neutrino background provides strong motivation to1arXiv:1710.00767v2 [hep-ex] 25 Oct 2017
consider alternate theories.
The 2014 Report of the Particle Physics ProjectPrioritization Panel (P5) stressed the importance of considering “every feasibleavenue,” culminating in a recent community workshop to explore the sciencecase for small-scale projects which can explore beyond this parameter space [4].The possibility of mirror matter as a type of hidden sector dark matter can-didate has been considered for decades [5–9] (see also reviews [10–12]). Mirrormatter manifests as a perfect copy of Standard Model particles and interac-tions such that parity and time reversal are exact symmetries, and interactsvery weakly with our known universe, primarily gravitationally. It is a type ofasymmetric, self-interacting, baryonic dark matter. To meet constraints fromBig Bang nucleosynthesis and the effective number of light neutrino species, themirror universe could not be identical in terms of cosmological evolution, andshould instead have a lower temperature, and therefore be helium dominated,with a larger baryon asymmetry [13].
Like ordinary matter, mirror mattershould dissipate energy at too high a rate for halos to form; the formation ofdisk galaxies may be avoided by a more rapid stellar evolution [14], or the energydissipation rate may be compensated via an energy injection from supernovaedue to kinetic mixing between the two sectors [15].2. Neutron OscillationsIf the mirror universe only interacts gravitationally with the ordinary uni-verse, it is not of much interest as there are no testable consequences that couldbe performed by particle physicists. Instead we consider the possibility of mixingbetween the two universes which could manifest as neutral particle oscillations.Photons, neutrinos, neutral pions and kaons, and neutrons are good candidatesfor consideration. Neutron oscillations (n→n′) in particular are interestingdue to the implications of a new baryon number violating process. The phe-nomenology of mirror neutron oscillations was considered with the realizationthat strong limits on the possibility of rather fast oscillation times did not yetexist [16], and a more detailed treatment followed including the consideration ofa small but nonzero mirror magnetic fields, which could originate from mirrormatter accumulated in the Earth
[17].In this model, the Hamiltonian of the free neutron in the presence of nonzeroordinary and mirror magnetic fieldsBandB′respectively is of the formH=(μ~B·~σ τ−1τ−1μ′~B′·~σ′)(1)where~σis the neutron spin, andτis the characteristicn→n′oscillation time,μis the neutron magnetic moment, andμ=μ′as a consistent assumption ofthe mirror matter model. Then→n′oscillation probability versus free flight2
timetfor unpolarized neutrons is [18]P(t) =sin2[(ω−ω′)t]2τ2(ω−ω′)2+sin2[(ω+ω′)t]2τ2(ω+ω′)2+ (cosβ)(sin2[(ω−ω′)t]2τ2(ω−ω′)2−sin2[(ω+ω′)t]2τ2(ω+ω′)2)(2)whereω=12|μB|andω′=12|μ′B′|andβrepresents the angle between~Band~B′. Then→n′oscillation probability scales with the free neutron flight timeas∼t2τ2when~B≈~B′and exhibits a resonance behavior when the difference inthe magnitudes of the magnetic fields is small. The dependence on the magneticfield direction is contained incosβ: the probability is maximal when the fieldsare aligned, but the components with the resonance condition are cancelledwhen the fields are anti-aligned.Previous published limits onn→n′oscillations [1] have used ultracoldneutrons: neutrons which have energy<∼300 neV such that they can be totallyinternally reflected by the Fermi potential of some materials and thus be storedin material bottles. The most stringent limit available,τ <414 s (90 % C.L.),was obtained in an experiment which examined the storage time of ultracoldneutrons in a material bottle with and without the presence of an externalmagnetic field [19]; however, this limit assumes~B′= 0, or no mirror magneticfield present at the Earth. When reanalyzed to consider the possibility~B′6= 0,an anomalous signal consistent withτ∼10 s andB′∼100 mG was observedwith 5σsignificance [18]. Another ultracold neutron storage experiment alsoscanned the magnetic field up to±125 mG and excludedτ <12 s (95% C.L.)for this range
[20]. The sensitivity of this experiment was limited primarily bythe step size in the magnetic field scan, 25 mG, which was much larger than theresonance width.Potential systematic effects which can induce unmonitored changes in thestorage time of ultracold neutrons could include wall-loss probabilities whichdepend on magnetic field or spectral evolution of the ultracold neutron popula-tion on a timescale shorter than the field scan time. An independent approachwith different systematic considerations is needed to resolve these controversialresults. A cold neutron disappearance and regeneration experiment could givea clear, unambiguous indication of mirror neutron oscillation.
3. Cold Neutron Regeneration
continued..https://arxiv.org/pdf/1710.00767.pdf
New Search for Mirror Neutrons at HFIRL. J. Broussarda, K. M. Baileya, W. B. Baileya, J. L. Barrowb, B. Chanceb,C. Crawfordd, L. Crowa, L. Debeer-Schmitta, N. Fominb, M. Frosta,b,A. Galindo-Uribarria, F. X. Gallmeiera, L. Heilbronnb, E. B. Iversona,Y. Kamyshkovb, C.-Y. Liuc, I. Novikove, S. I. Penttil ̈aa, A. Rugglesb,B. Ryboltb, M. Snowc, L. Townsendb, L. J. Varrianob, S. Vavrab, A. R. YoungfaOak Ridge National Laboratory, Oak Ridge, TN 37831 USAbUniversity of Tennessee, Knoxville, TN 37996 USAcIndiana University, Bloomington, IN 47405 USAdUniversity of Kentucky, Lexington, KY 40506 USAeWestern Kentucky University, Bowling Green, KY 42101 USAfNorth Carolina State University, Raleigh, NC 27695 USA
Abstract
The theory of mirror matter predicts a hidden sector made up of a copy of theStandard Model particles and interactions but with opposite parity. If mirrormatter interacts with ordinary matter, there could be experimentally accessi-ble implications in the form of neutral particle oscillations. Direct searches forneutron oscillations into mirror neutrons in a controlled magnetic field havepreviously been performed using ultracold neutrons in storage/disappearancemeasurements, with some inconclusive results consistent with characteristic os-cillation time ofτ∼10 s. Here we describe a proposed disappearance and regen-eration experiment in which the neutron oscillates to and from a mirror neutronstate. An experiment performed using the existing General Purpose-Small An-gle Neutron Scattering instrument at the High Flux Isotope Reactor at OakRidge National Laboratory could have the sensitivity to exclude up toτ<15 sin 1 week of beamtime and at low cost.1. IntroductionA strong foundation of evidence has been built over the past several decadeswhich indicates that 85% of the matter in the universe is unknown to us
[1].The evidence for Dark Matter comes from many diverse astronomical sourcesincluding rotational curves of galaxies, weak and strong lensing, galaxy collisionssuch as the Bullet Cluster, and the cosmic microwave background; however, todate it is based solely on astrophysical signatures of its gravitational influence.Determining the particle nature of dark matter has for years been one of thehighest priorities of the particle physics community, but there remains no con-clusive evidence to its origin. A significant focus of these searches has been thewell-motivated Weakly Interacting Massive Particles [2, 3], but the diminishingparameter space above the neutrino background provides strong motivation to1arXiv:1710.00767v2 [hep-ex] 25 Oct 2017
consider alternate theories.
The 2014 Report of the Particle Physics ProjectPrioritization Panel (P5) stressed the importance of considering “every feasibleavenue,” culminating in a recent community workshop to explore the sciencecase for small-scale projects which can explore beyond this parameter space [4].The possibility of mirror matter as a type of hidden sector dark matter can-didate has been considered for decades [5–9] (see also reviews [10–12]). Mirrormatter manifests as a perfect copy of Standard Model particles and interac-tions such that parity and time reversal are exact symmetries, and interactsvery weakly with our known universe, primarily gravitationally. It is a type ofasymmetric, self-interacting, baryonic dark matter. To meet constraints fromBig Bang nucleosynthesis and the effective number of light neutrino species, themirror universe could not be identical in terms of cosmological evolution, andshould instead have a lower temperature, and therefore be helium dominated,with a larger baryon asymmetry [13].
Like ordinary matter, mirror mattershould dissipate energy at too high a rate for halos to form; the formation ofdisk galaxies may be avoided by a more rapid stellar evolution [14], or the energydissipation rate may be compensated via an energy injection from supernovaedue to kinetic mixing between the two sectors [15].2. Neutron OscillationsIf the mirror universe only interacts gravitationally with the ordinary uni-verse, it is not of much interest as there are no testable consequences that couldbe performed by particle physicists. Instead we consider the possibility of mixingbetween the two universes which could manifest as neutral particle oscillations.Photons, neutrinos, neutral pions and kaons, and neutrons are good candidatesfor consideration. Neutron oscillations (n→n′) in particular are interestingdue to the implications of a new baryon number violating process. The phe-nomenology of mirror neutron oscillations was considered with the realizationthat strong limits on the possibility of rather fast oscillation times did not yetexist [16], and a more detailed treatment followed including the consideration ofa small but nonzero mirror magnetic fields, which could originate from mirrormatter accumulated in the Earth
[17].In this model, the Hamiltonian of the free neutron in the presence of nonzeroordinary and mirror magnetic fieldsBandB′respectively is of the formH=(μ~B·~σ τ−1τ−1μ′~B′·~σ′)(1)where~σis the neutron spin, andτis the characteristicn→n′oscillation time,μis the neutron magnetic moment, andμ=μ′as a consistent assumption ofthe mirror matter model. Then→n′oscillation probability versus free flight2
timetfor unpolarized neutrons is [18]P(t) =sin2[(ω−ω′)t]2τ2(ω−ω′)2+sin2[(ω+ω′)t]2τ2(ω+ω′)2+ (cosβ)(sin2[(ω−ω′)t]2τ2(ω−ω′)2−sin2[(ω+ω′)t]2τ2(ω+ω′)2)(2)whereω=12|μB|andω′=12|μ′B′|andβrepresents the angle between~Band~B′. Then→n′oscillation probability scales with the free neutron flight timeas∼t2τ2when~B≈~B′and exhibits a resonance behavior when the difference inthe magnitudes of the magnetic fields is small. The dependence on the magneticfield direction is contained incosβ: the probability is maximal when the fieldsare aligned, but the components with the resonance condition are cancelledwhen the fields are anti-aligned.Previous published limits onn→n′oscillations [1] have used ultracoldneutrons: neutrons which have energy<∼300 neV such that they can be totallyinternally reflected by the Fermi potential of some materials and thus be storedin material bottles. The most stringent limit available,τ <414 s (90 % C.L.),was obtained in an experiment which examined the storage time of ultracoldneutrons in a material bottle with and without the presence of an externalmagnetic field [19]; however, this limit assumes~B′= 0, or no mirror magneticfield present at the Earth. When reanalyzed to consider the possibility~B′6= 0,an anomalous signal consistent withτ∼10 s andB′∼100 mG was observedwith 5σsignificance [18]. Another ultracold neutron storage experiment alsoscanned the magnetic field up to±125 mG and excludedτ <12 s (95% C.L.)for this range
[20]. The sensitivity of this experiment was limited primarily bythe step size in the magnetic field scan, 25 mG, which was much larger than theresonance width.Potential systematic effects which can induce unmonitored changes in thestorage time of ultracold neutrons could include wall-loss probabilities whichdepend on magnetic field or spectral evolution of the ultracold neutron popula-tion on a timescale shorter than the field scan time. An independent approachwith different systematic considerations is needed to resolve these controversialresults. A cold neutron disappearance and regeneration experiment could givea clear, unambiguous indication of mirror neutron oscillation.
3. Cold Neutron Regeneration
continued..https://arxiv.org/pdf/1710.00767.pdf
