Absolute neutrino mass update

The determination of absolute neutrino masses is crucial for the understanding of theories underlying the standard model, such as SUSY. We review the experimental prospects to determine absolute neutrino masses and the correlations among approaches, using the ∆m2’s inferred from neutrino oscillation experiments and assuming a three neutrino Universe. 1 Neutrinos and new physics The most pending puzzles in particle and astroparticle physics concern the origin of mass, the unification of interactions, the nature of the dark matter in the universe, the existence of hidden extra dimensions, the origin of the highest energy cosmic rays and the explanation of the matter-antimatter excess. The investigation of the unknown absolute neutrino mass scale is situated at a crossing point of these tasks: • The most elegant explanation for light neutrino masses is the see-saw mechanism, in which a large Majorana mass scale MR drives the light neutrino masses down to or below the sub-eV scale, mν = m 2 D/MR, (1) where the Dirac neutrino masses are typically of the order of the weak scale. A combination of information aboutmD from charged lepton flavor violation mediated by sleptons (see e.g. [2]) and mν may allow to probe the scale MR not far from the GUT scale. • An alternative mechanism generates neutrino masses radiatively at the SUSY scale, with R-parity violating couplings λ ), fermions f and squarks or sleptons in the loop, mν ∝ λ(′)λ(′)m2f/(16π2MSUSY ). (2) In this case information about the strength of couplings and the masses of SUSY partners can be obtained from absolute neutrino masses (see e.g. [3]). Talk presented by H. Päs. PA 2+3: Low Energies, Flavors, and CP 1039 • In theories with large extra dimensions small neutrino masses may be generated by volume-suppressed couplings to right-handed neutrinos which can propagate in the bulk, by the breaking of lepton number on a distant brane, or by the curvature of the extra dimension. Thus neutrino masses can provide information about the volume or the geometry of the large extra dimensions (see e.g. [4]). • A simple and elegant explanation of the matter-antimatter excess in the universe is given by the out-of-equilibrium decay of heavy Majorana neutrinos in leptogenesis scenarios. To avoid strong washout processes of the generated lepton number asymmetry light neutrino masses with mν < 0.2 eV are required [5]. In fact it is a true experimental challenge to determine an absolute neutrino mass below 1 eV. Three approaches have the potential to accomplish the task, namely larger versions of the tritium end-point distortion measurements, limits from the evaluation of the large scale structure in the universe, and next-generation neutrinoless double beta decay (0νββ) experiments. In addition there is a fourth possibility: the extreme-energy cosmic-ray experiments in the context of the recently emphasized Z-burst model. For discussions of the sensitivity in time of flight measurements of supernova (O(1 eV)) or gamma ray burst neutrinos (O(10−3 eV), assuming complete understanding of GRB’s and large enough rates), see [11]. 2 Tritium beta decay In tritium decay, the larger the mass states comprising ν̄e, the smaller is the Q-value of the decay. The manifestation of neutrino mass is a reduction of phase space for the produced electron at the high energy end of its spectrum. An expansion of the decay rate formula about mνe leads to the end point sensitive factor m2νe ≡ ∑ j |Uej |2 mj , (3) where the sum is over mass states mi which can kinematically alter the end-point spectrum. If the neutrino masses are nearly degenerate, then unitarity of the mixing matrix U leads immediately to a bound on √ mνe = m3. A larger tritium decay experiment (KATRIN) to reduce the present 2.2 eV mνe bound is planned to start taking data in 2007; direct mass limits as low as 0.4 eV, or even 0.2 eV, may be possible in this type of experiment [6]. 3 Cosmological limits In the currently favored ΛDM cosmology, there is scant room left for the neutrino component. The power spectrum of early-Universe density perturbations is processed by gravitational instabilities. However, the free-streaming relativistic neutrinos suppress the growth of fluctuations on scales below the horizon (approximately the Hubble size c/H(z)) until they become nonrelativistic at z ∼ mj/3T0 ∼ 1000 (mj/eV) (for an overview see [7]). 1040 Parallel Sessions A recent limit [8] derived from the 2dF Galaxy Redshift Survey power spectrum constrains the fractional contribution of massive neutrinos to the total mass density to be less than 0.13, translating into a bound on the sum of neutrino mass eigenvalues, ∑ j mj < 1.8 eV (for a total matter mass density 0.1 < Ωm < 0.5 and a scalar spectral index n = 1). A limit from gravitational lensing by dwarf satellite galaxies reveals sufficient structure to limit ∑ j mj < 0.74 eV, under some reasonable but unproven assumptions [9]. In ref. [10] it has been shown, that a combination of Planck satellite CMB data with the SDSS sky survey will improve the sensitivity down to ∑ j mj = 0.12 eV. A future sky survey with an order of magnitude larger survey volume would allow to reach even ∑ j mj = 0.03− 0.05 eV. Some caution is warranted in the cosmological approach to neutrino mass, in that the many cosmological parameters may conspire in various combinations to yield nearly identical CMB and large scale structure data. An assortment of very detailed data may be needed to resolve the possible “cosmic ambiguities”. 4 Neutrinoless double beta decay The 0νββ rate is a sensitive tool for the measurement of the absolute mass-scale for Majorana neutrinos [12]. The observable measured in the amplitude of 0νββ is the ee element of the neutrino mass-matrix in the flavor basis. Expressed in terms of the mass eigenvalues and neutrino mixing-matrix elements, it is mee = | ∑ i U eimi| . (4) A reach as low as mee ∼ 0.01 eV seems possible with double beta decay projects under preparation such as GENIUSI, MAJORANA, EXO, XMASS or MOON. This provides a substantial improvement over the current bound from the IGEX experiment, mee < 0.4 eV [13]. A recent evidence claim [14] by the Heidelberg-Moscow experiment reports a best fit value of mee = 0.4 eV, but is subject to possible systematic uncertainties. For masses in the interesting range >∼ 0.01 eV, the two light mass eigenstates are nearly degenerate and so the approximation m1 = m2 is justified. Due to the restrictive CHOOZ bound, |Ue3| < 0.025, the contribution of the third mass eigenstate is nearly decoupled from mee and so U 2 e3 m3 may be neglected in the 0νββ formula. We label by φ12 the relative phase between U e1 m1 and U 2 e2 m2. Then, employing the above approximations, we arrive at a very simplified expression for mee: mee = [ 1− sin(2θsun) sin ( φ12 2 )] m1 . (5) The two CP-conserving values of φ12 are 0 and π. These same two values give maximal constructive and destructive interference of the two dominant terms in eq. (4), which leads to upper and lower bounds for the observable mee in terms of a fixed value of m1, cos(2θsun) m1 ≤ mee ≤ m1 with cos(2θsun) >∼ 0.1 weakly bounded for the LMA solution [15]. This uncertainty disfavors 0νββ in comparison to direct measurements if a specific value of m1 has to be determined, while 0νββ is more sensitive as long as bounds on m1 are aimed at. Knowing the value of θsun better will improve the estimate of the inherent PA 2+3: Low Energies, Flavors, and CP 1041