Electromagnetic neutrino properties: new constraints and new effects
aa r X i v : . [ h e p - ph ] F e b Electromagnetic neutrino properties: new constraints andnew eο¬ects
Alexander Studenikin β π Department of Theoretical Physics, Moscow State University,119991 Moscow, Russia π Joint Institute for Nuclear Research,141980 Dubna, Moscow Region, Russia
E-mail: [email protected]
The electromagnetic properties of neutrinos have attracted considerable attention from researchersfor many decades (see [1] for a review). However, until recently, there was no indication infavour of nonzero electromagnetic properties of neutrinos either from laboratory experimentswith ground-based neutrino sources or from observations of astrophysical neutrino ο¬uxes. Thesituation changed after the XENON collaboration reported [2] results of the search for newphysics with low-energy electronic recoil data recorded with the XENON1T detector. The resultsshow an excess of events over the known backgrounds in the recoil energy which, as one ofthe possible explanations, admit the presence of a sizable neutrino magnetic moment, the valueof which is of the order of the existing laboratory limitations. In these short notes we givea brief introduction to neutrino electromagnetic properties and focus on the most importantconstraints on neutrino magnetic moments, charge radii and millicharges from the terrestrialexperiments and astrophysical considerations. The promising new possibilities for constrainingneutrino electromagnetic properties in future experiments are also discussed. β Speaker Β© Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ lectromagnetic neutrino properties: new constraints and new eο¬ects
Alexander Studenikin
Introduction.
The most general form of the neutrino electromagnetic vertex function [1]is given by Ξ π ππ ( π ) = (cid:0) πΎ π β π π / π / π (cid:1) h π π ππ ( π ) + π π ππ΄ ( π ) π πΎ i β ππ ππ π π h π π ππ ( π ) + π π π ππΈ ( π ) πΎ i ,where Ξ π ( π ) and form factors π π,π΄,π ,πΈ ( π ) are 3 Γ π =
0) form factors provide four sets of neutrinoelectromagnetic characteristics: 1) the dipole magnetic moments π π π = π π ππ ( ) , 2) the dipoleelectric moments π π π = π π ππΈ ( ) , 3) the millicharges π π π = π π ππ ( ) and 4) the anapole moments π π π = π π ππ΄ ( ) . Neutrino dipole magnetic moments.
The most well understood and studied among neutrinoelectromagnetic characteristics are the neutrino magnetic moments. In the Standard Model withmassless neutrinos magnetic moments of neutrinos are zero. Therefore, it is believed that thestudies of neutrino electromagnetic properties open a window to new physics [1β4]. In a minimalextension of the Standard Model the diagonal magnetic moment of a Dirac neutrino is given [5]by π π·ππ = ππΊ πΉ π π β π β . Γ β (cid:16) π π (cid:17) π π΅ , π π΅ is the Bohr magneton. The Majorana neutrinos canhave only transition (oο¬-diagonal) magnetic moments π ππ β π . The same is valid also for the ο¬avourneutrinos in the case of the Majorana mass states.The most stringent constraints on the neutrino magnetic moments are obtained with the reactorantineutrinos (GEMMA Collaboration [6]): π π π < . Γ β π π΅ , and solar neutrinos (BorexinoCollaboration [7]): π π π ππ < . Γ β π π΅ . The last limit can be translated to the upper limits forο¬avour neutrinos: ( π π π , π π π,π ) βΌ ( , ) Γ β π π΅ .Note that in general in the scattering experiments the neutrino is created at some distance fromthe detector as a ο¬avor neutrino, which is a superposition of massive neutrinos. Therefore, themagnetic and electric moments that are measured in these experiments are not that of a massiveneutrino, but there are eο¬ective moments that take into account the neutrino mixing and oscillationsduring the propagation between the source and detector [8, 9]. For the recent and detailed study ofthe neutrino electromagnetic characteristics dependence on neutrino mixing see [10].A new phase of the GEMMA project for measuring the neutrino magnetic moment is nowunderway at the Kalinin Power Plant in Russia. The discussed next experiment [11] called GEMMA-3 / π GEN is aimed at the further increase in sensitivity to the neutrino magnetic moment and willreach the level of π π π βΌ ( β ) Γ β π π΅ . To reach the claimed limit on the neutrino magneticmoment the π GEN experiment setup reasonably improves characteristics in respect to those ofthe previous editions of the GEMMA project. The most important are the following [12]: 1) afactor of 2 increase in the total neutrino ο¬ux at the detector because of much closer location of thedetector to the reactor core, 2) a factor of 3.7 increase in the total mass of the detector, 3) the energythreshold would be improved from 2 . πππ to 200 ππ . Furthermore, the π GEN experimental setupis located in the new room at the Kalinin Power Plat with much better (by an order of magnitude)gamma-background conditions and on a moveable platform. The later gives an opportunity to varyonline the neutrino ο¬ux and thus suppress systematic errors.The observation of coherent elastic neutrino-nucleus scattering reported for the ο¬rst time [13]by the COHERENT experiment at the Spallation Neutron Source can be also used for constrainingneutrino electromagnetic properties. For the case of neutrino magnetic moments, however, as itwas shown in [14] and then conο¬rmed in recent studies (see, for instance, [15] ) the bounds for theο¬avour neutrino magnetic moments are of the order π π , π π βΌ β π π΅ .2 lectromagnetic neutrino properties: new constraints and new eο¬ects Alexander StudenikinIn the recent studies [16] it is shown that the puzzling results of the XENON1T collaboration[2] at few keV electronic recoils could be due to the scattering of solar neutrinos endowed with ο¬niteMajorana transition magnetic moments of the strengths lie within the limits set by the Borexinoexperiment with solar neutrinos [7]. The comprehensive analysis of the existing and new extendedmechanisms for enhancing neutrino transition magnetic moments to the level appropriate for theinterpretation of the XENON1T data and leaving neutrino masses within acceptable values isprovided in [17].In the most recent paper [18] we have proposed an experimental setup to observe coherentelastic neutrino-atom scattering using electron antineutrinos from tritium decay and a liquid heliumtarget. In this scattering process with the whole atom, that has not beeen observed so far, theelectrons tend to screen the weak charge of the nucleus as seen by the electron antineutrino probe.Finally, we study the sensitivity of this apparatus to a possible electron neutrino magnetic momentand we ο¬nd that it is possible to set an upper limit of about π π < Γ β π π΅ , that is more thanone order of magnitude smaller than the current experimental limits from GEMMA and Borexino.An astrophysical bound on an eο¬ective neutrino magnetic moment (valid for both cases of Diracand Majorana neutrinos) is provided [19β21] by observations of the properties of globular clusterstars: (cid:16) Γ π, π (cid:12)(cid:12) π π π (cid:12)(cid:12) (cid:17) / β€ ( . β . ) Γ β π π΅ . There is also a statement [22], that observations ofsupernova ο¬uxes in the future largevoluem experiments like JUNO, DUNE and Hyper-Kamiokande( see for instance [23β25]) may reveal the eο¬ect of collective spin-ο¬avour oscillations due to theMajorana neutrino transition moment π ππ βΌ β π π΅ . Other new possibilities for neutrino magneticmoment visualization in extreme astrophysical environments are considered recently in [26, 27].A general and termed model-independent upper bound on the Dirac neutrino magnetic moment,that can be generated by an eο¬ective theory beyond a minimal extension of the Standard Model, hasbeen derived in [28]: π π β€ β π π΅ . The corresponding limit for transition moments of Majorananeutrinos is much weaker [29]. Neutrino dipole electric moments.
In the theoretical framework with
πΆ π violation a neutrinocan have nonzero electric moments π π π . In the laboratory neutrino scattering experiments forsearching π π (for instance, in the GEMMA experiment) the electric moment π π π contributionsinterfere with those due to π π π . Thus, these kind of experiments also provide constraints on π π π .The astrophysical bounds on π π π are also applicable for constraining π π π (see [19β21] and [30]). Neutrino electric millicharge.
There are extensions of the Standard Model that allow fornonzero neutrino electric millicharges. This option can be provided by not excluded experimentallypossibilities for hypercharhge dequantization or another new physics related with an additional π ( ) symmetry peculiar for extended theoretical frameworks. Note that neutrino millicharges arestrongly constrained on the level π π βΌ β π ( π is the value of an electron charge) from neutralityof the hydrogen atom.A nonzero neutrino millicharge π π would contribute to the neutrino electron scattering in theterrestrial experiments. Therefore, it is possible to get bounds on π π in the reactor antineutrinoexperiments. The most stringent reactor antineutrino constraint π π < . Γ β π is obtained in[31] within the free-electron approximation using the GEMMA experimental data [6]. This limit iscited by the Particle Data Group since 2016 (see also [32]). A certain increase in the cross section isexpected in the case when instead of the free-electron approximation one accounts for the so called3 lectromagnetic neutrino properties: new constraints and new eο¬ects Alexander Studenikinatomic ionization eο¬ect [33], and the obtained corresponding limit on the neutrino millicharge is π π < Γ β π .The expected increasing sensitivity to the neutrino-electron scattering of the future π GENexperiment that is aimed to reach a new limit for the magnetic moment would provide a possibility[31] to check the neutrino millicharge at the scale of π π βΌ β π .As it has been already mentioned above, the coherent elastic neutrino-nucleus scattering [13]is a new powerful tool to probe the electromagnetic neutrino properties [14]. In the ο¬avour basisneutrinos can have diagonal π π π ( π = π , π, π = π, π, π ) and transition π π π ( π β π ) electric charges(see, for instance, [1] and [10]). Such possibilities are not excluded by theories beyond the StandardModel. Recently [34] from the analysis of the COHERENT data new constraints for all neutrinocharges on the level of βΌ β π are obtained. It follows, that the bounds for involving theelectron neutrino ο¬avour charges π ππ , π ππ and π ππ are not competitive with respect to constraints βΌ β π obtained for the eο¬ective electron neutrino charge π π π π = q π ππ + π ππ + π ππ from thereactor antineutrino scattering experiments [31, 33]. Note, that the bounds for π ππ and π ππ from alaboratory data are obtained in [34] for the ο¬rst time.The most recent and one of the most detailed statistical studies [35] of experimental datafrom the elastic neutrino-electron and coherent neutrino-nucleus scattering show that the combinedinclusion of diο¬erent experimental data can lead to stronger constraints on π π than those based onindividual analysis of diο¬erent experiments.A neutrino millicharge would have speciο¬c phenomenological consequences in astrophysicsbecause of new electromagnetic processes are opened due to a nonzero charge (see [1, 36, 37]).Following this line, the most stringent astrophysical constraint on neutrino millicharges π π < . Γ β π was obtained in [37]. This bound follows from the impact of the neutrino star turning mechanism ( πππ ) [37] that can be considered as a new physics phenomenon end up with a pulsarrotation frequency shift engendered by the motion of escaping from the star neutrinos along curvedtrajectories due to millicharge interaction with a constant magnetic ο¬eld of the star. The existedother astrophysical constraints on the neutrino millicharge, however less restrictive than that of [37],are discussed in [1, 35]. Neutrino cherge radius and anapole moment.
Even if a neutrino millicharge is vanishing, theelectric form factor π π ππ ( π ) can still contain nontrivial information about neutrino electromagneticproperties. The corresponding electromagnetic characteristics is determined by the derivative of π π ππ ( π ) over π at π = h π π π i = β π π π ππ ( π ) ππ | π = (see [1] forthe detailed discussions). Note that for a massless neutrino the neutrino charge radius is the onlyelectromagnetic characteristic that can have nonzero value. In the Standard Model the neutrinocharge radius and the anapole moment are not deο¬ned separately, and there is a relation betweenthese two values: π = β h π i .A neutrino charge radius contributes to the neutrino scattering cross section on electrons andthus can be constrained by the corresponding laboratory experiments [38]. In all but one previousstudies it was claimed that the eο¬ect of the neutrino charge radius can be included just as a shift ofthe vector coupling constant π π in the weak contribution to the cross section. However, as it hasbeen recently demonstrated in [10] within the direct calculations of the elastic neutrino-electronscattering cross section accounting for all possible neutrino electromagnetic characteristics and4 lectromagnetic neutrino properties: new constraints and new eο¬ects Alexander Studenikinneutrino mixing, this is not the fact. The neutrino charge radius dependence of the cross section ismore complicated and there are, in particular, the dependence on the interference terms of the type π π h π π π i and also on the neutrino mixing. The current constraints on the ο¬avour neutrino chargeradius h π π,π,π i β€ β β β ππ from the scattering experiments diο¬er only by 1 to 2 orders ofmagnitude from the values h π π,π,π i β€ β ππ calculated within the minimally extended StandardModel with right-handed neutrinos [38]. This indicates that the minimally extended Standard Modelneutrino charge radii could be experimentally tested in the near future.Note that there is a need to re-estimate experimental constraints on h π π,π,π i from the scatteringexperiments following new derivation of the cross section [10] that properly accounts for theinterference of the weak and charge radius electromagnetic interactions and also for the neutrinomixing.Recently constraints on charged radii have been obtained [39] from the analysis of the data oncoherent elastic neutrino-nucleus scattering obtained in the COHERENT experiment [13, 40].In addition to the customary diagonal charge radii h π π,π,π i also the neutrino transition (oο¬-diagonal) charge radii have been constrained in [39] for the ο¬rst time: (cid:16) |h π π ππ i| , |h π π ππ i| , |h π π ππ i| (cid:17) < ( , , ) Γ β cm . Since 2018 these limits are included by the Particle Data Group to Reviewof Particle Properties (see also [32]) and also were noted by the Editorsβ Suggestion as the mostimportant results (PRD Highlights 2018) published in the journal.The work is supported by the Russian Foundation for Basic Research under grant No. 20-52-53022-GFEN-a. References [1] C.Giunti, A.Studenikin, Rev. Mod. Phys. (2015) 531.[2] E.Aprile et al., [XENON Collaboration], Phys. Rev. D (2020) 072004 .[3] A.Studenikin, Nucl. Phys. Proc. Suppl. (2009) 220.[4] A.Studenikin, PoS EPS-HEP2017 (2017) 137.[5] K.Fujikawa, R.Shrock, Phys. Rev. Lett. (1980) 963.[6] A.Beda, V.Brudanin, V.Egorov et al. , Adv. High Energy Phys. (2012) 350150.[7] M.Agostini et al. , [Borexino Collaboration], Phys. Rev. D (2017) 091103.[8] W.Grimus, P.Stockinger, Phys. Rev. D (1998) 1762 .[9] J.Beacom, P.Vogel, Phys. Rev. Lett. (1999) 5222 .[10] K.Kouzakov, A.Studenikin, Phys. Rev. D (2017) 055013.[11] V.Belov et al. , JINST (2015) no.12, P12011.[12] A. Lubashevskiy, private communications , 2020.[13] D.Akimov et al. , [COHERENT Collaboration], Science (2017) no. 6356, 1123.5 lectromagnetic neutrino properties: new constraints and new eο¬ects Alexander Studenikin[14] D.Papoulias, T.Kosmas, Phys. Rev. D (2018) 033003 .[15] O.Miranda et al. , JHEP (2020) 130 .[16] O.Miranda, D.Papoulias, M.TΓ³rtola, J.W.F.Valle, Phys. Lett. B (2020) 135685 .[17] K.Babu, S.Jana, M.Lindner, JHEP (2020) 040 .[18] M.Cadeddu, F.Dordei, C.Giunti, K.Kouzakov, E.Picciau, A.Studenikin, Phys. Rev. D (2019) 073014.[19] G.Raο¬elt, Phys. Rev. Lett. (1990) 2856.[20] N.Viaux, M.Catelan, P.Stetson, G.Raο¬elt et al. , Astron. & Astrophys. (2013) A12.[21] S.Arceo-DΓaz, K.SchrΓΆder, K.Zuber, D.Jack, Astropart. Phys. (2015) 1.[22] A. de Gouvea, S.Shalgar, JCAP (2012) 027; JCAP (2013) 018.[23] F. An et al. , [JUNO Collaboration], J. Phys. G (2016) 030401.[24] C.Giunti, K.Kouzakov, Y.F.Li, A.Lokhov, A.Studenikin, S.Zhou, Ann.Phys. (2016) 198 .[25] J.S.Lu, Y.F.Li, S.Zhou, Phys. Rev. D (2016) 023006.[26] A.Grigoriev, A.Lokhov, A.Studenikin, A.Ternov, JCAP (2017) 024.[27] P.Kurashvili, K.Kouzakov, L.Chotorlishvili, A.Studenikin, Phys. Rev. D (2017) 103017.[28] N.Bell, V.Cirigliano, M.Ramsey-Musolf et al. , Phys. Rev. Lett. (2005) 151802.[29] N.Bell, M.Gorchtein, M.Ramsey-Musolf, P.Vogel, P.Wang, Phys. Lett. B (2006) 377.[30] G.Raο¬elt, Phys. Rept. (2000) 593.[31] A.Studenikin, Europhys.Lett. (2014) 21001.[32] P.Zyla et al. , [Particle Data Group], PTEP (2020) no.8, 083C01.[33] J.Chen et al. , Phys. Rev. D (2014) 011301.[34] M.Cadeddu, F.Dordei, C.Giunti, Y.F.Li, Y.Y.Zhang, Phys. Rev. D (2020) 033004.[35] A.Parada, arXiv:1907.04942 [hep-ph].[36] G.Raο¬elt, Stars as laboratories for fundamental physics : The astrophysics of neutrinos,axions, and other weakly interacting particles , Chicago, USA: Univ. Pr. (1996) 664 p.[37] A. Studenikin and I. Tokarev, Nucl. Phys. B (2014) 396.[38] J.Bernabeu, J.Papavassiliou, D.Binosi, Nucl. Phys. B (2005) 352.[39] M.Caddedu, C.Giunti, K.Kouzakov, Y.F.Li, A.Studenikin, Y.Zhang, Phys. Rev. D (2018)113010 .[40] D.Akimov et al.et al.