CC BY
0Available online at www.sciencedirect.com
DIRECT "
Physics Letters B 613 (2005) 61-66
www.elsevier.com/locate/physletb
Neutrino physics from new SNO and KamLAND data and future
prospects
A.B. Balantekina, V. Bargera, D. Marfatiabc, S. Pakvasad, H. Yüksela
a Department of Physics, University of Wisconsin, Madison, WI53706, USA b Department of Physics, Boston University, Boston, MA 02215, USA c Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA d Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822, USA
Received 6 May 2004; received in revised form 9 March 2005; accepted 21 March 2005
Available online 29 March 2005
Editor: M. Cvetic
Abstract
We analyze the cumulative data from the SNO, KamLAND and other solar neutrino experiments in the standard scenario of three oscillating active neutrinos. We determine the solar neutrino oscillation parameters and obtain new bounds on 0x. We also place constraints on the fraction of oscillating solar neutrinos that transform to sterile neutrinos with the 8B flux normalization left free. Concomitantly, we assess the sensitivity of future data from the SNO and KamLAND experiments to 0x and to the sterile neutrino content of the solar flux. © 2005 Elsevier B.V. All rights reserved.
The SNO [1] and KamLAND [2] experiments have been crucial in selecting the large mixing angle (LMA) solution [3], thereby solving the long-standing solar neutrino problem. Additional KamLAND data [4] have narrowed the two-neutrino oscillation parameter space even further [4,5]. We perform a more general three-neutrino analysis of KamLAND and solar neutrino data including the cumulative salt-phase SNO data announced recently [6]. We refine the existing up-
per bound on 0x .1 We also explore if future data from KamLAND and SNO can play an important role in the study of neutrino physics beyond the determination of the primary solar oscillation parameters.
One of the main goals of ongoing and planned neutrino experiments is a measurement of 0x, and if it is large enough, to determine if CP is violated in the neutrino sector [7]. Today, we know from
E-mail address: marfatia@buphy.bu.edu (D. Marfatia).
1 We use the notation of Ref. [7] in which Sma and Sm2 are the
atmospheric and solar mass-squared differences, and 8a, 9s and 9x are the mixing angles conventionally denoted by $23, $12 and $13, respectively.
0370-2693/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physletb.2005.03.057
the CHOOZ [8] and Palo Verde [9] experiments that sin2 20x < 0.19 at the 90% C.L. for Sm2a = 0.002 eV2; our analysis below yields sin220x < 0.17. Data from the K2K experiment have established an independent and consistent bound, sin220x < 0.45 for the same Sm2a [10]2; further support that 0x is small is obtained from Super-Kamiokande (SuperK) atmospheric data [11]. Long-baseline experiments such as MINOS [12] and the CERN to Gran Sasso (CNGS) experiments, ICARUS [13] and OPERA [14], will begin the hunt for vM ^ ve transitions resulting from a nonzero 0x in the near future. Within five years of running they could have compelling evidence for such transformations or they will strengthen the CHOOZ bound. In the meantime, however, there is a possibility that additional solar neutrino data may provide guidance on the size of 0x .A constraint from solar neutrino data is independent of &m2a so long as it is much larger than Sm^. This is especially important because the values of Sm2a from the SuperK Collaboration's analyses have shifted with additional data and refinements in the analyses (in quite a narrow range which, however, sensitively affects conclusions about the size of 0x); compare the results from a zenith-angle analysis [15] and from an L/E analysis [16]. If Sm2 turns out to be smaller than 0.001 eV2, then the CHOOZ bound will be inoperable, and solar data will provide the most stringent bound on 0x; even MINOS and the CNGS experiments will not do better. Although we have no reason to believe that this will be the case, we mention this as a hypothetical possibility under which solar/KamLAND data provide the best bound on 0x. After all, the K2K experiment confirms the Sma values from SuperK at the 2a C.L. [17].
More realistically, we investigate if future solar data can improve on the CHOOZ bound for the Sm2a values that are consistent with SuperK and K2K.
Another unresolved issue is whether solar neutrinos oscillate into sterile species [18]. We know from solar data that the possibility that solar neutrinos oscillate exclusively to sterile states is excluded at 7.6a [7]. However, it is easily conceivable that solar ve oscillate into both active and sterile neutrinos. The latter scenario is not satisfactorily constrained at present, and
significant improvement in this direction is unlikely in the near future [19]. We evaluate how future SNO and KamLAND data may confirm and somewhat improve existing bounds on a sterile fraction in the solar flux with minimal dependence on the standard solar model (SSM) and without resort to involved global analyses of strongly correlated datasets from many experiments.
All the 3He proportional counter tubes or neutral current detectors are installed and are taking data for the third phase of the SNO experiment. The future NC measurement is expected to have an overall uncertainty (statistical and systematic uncertainties combined) of about 6.4%. At the same time an improved CC integrated flux measurement will be made with an expected overall uncertainty of about 5.5%. To a good approximation, these measurements will be uncorrected with previous measurements and with each other. We use these expectations in our analyses.
In the analysis of the latest KamLAND data we take into account the fact that some of the reactors were nonoperational by using the expected number of nonoscillated events given in Fig. 1 ofRef. [4].
We employ the SSM [20] in our analyses, but treat the 8B flux normalization as a free parameter throughout.
1. Sensitivity to 0x
For the ve survival probability in the three-neutrino framework, we use the standard modification of the two-neutrino survival probability as derived in Ref. [21].
The regions of parameter space allowed by existing CHOOZ, KamLAND and solar data are shown in Fig. 1.
The effect of how future data from the SNO experiment will impact our knowledge of 0x is comprehensively represented in Fig. 2. The figure clearly suggests that future SNO data will not have a significant impact on existing bounds, especially for Sm2a values relevant to atmospheric neutrino oscillations.
2. Sensitivity to sterile neutrinos
2 The aforementioned limits are quoted for two degrees of free-
In a scenario in which oscillations to sterile neutrinos are allowed, the fraction of oscillating neutrinos
Fig. 1. The 90% C.L., 2a, 99% C.L. and 3a allowed regions from a combined three-neutrino fit to CHOOZ, KamLAND and solar neutrino data. The best-fit point SmS: = 8 x 10-5 eV2, tan2 9s = 0.45 and sin2 2$x = 0 is marked with an "x". In the analysis, the 8B flux was a free parameter.
Fig. 2. Estimates of how future SNO data will affect bounds on 9x. The shaded curved bands depict the effect of future SNO 6.4% NC and 5.5% CC measurements (whose central values lie within their current la values) on bounds from all existing CHOOZ, KamLAND and solar neutrino data. The thick (thin) solid curves are the 90% C.L. (3a) bounds from current CHOOZ, KamLAND and solar data. The dotted curves are the corresponding CHOOZ bounds. The horizontal shaded regions encompass the values of Smm^ favored by SuperK atmospheric data at the 90% and 99% C.L. [15]. The 8B flux normalization is a free parameter in our analyses.
. 2 sin a
Fig. 3. Ax2 vs sin2 a from analyses of all solar and KamLAND data (solid), and only SNO and KamLAND data (dashed), with the 8B flux free in both analyses. From bottom to top, the horizontal dotted lines indicate the Ax2 values corresponding to 1a, 2a and 3a.
that transform to active neutrinos is (in terms of quantities measured by SNO) [19]
^nc — ^CC
sin a =
^8B — @cc
The current constraints on sin2 a are shown in Fig. 3. The most stringent bound from all available solar and KamLAND data is sin2a > 0.91 (0.65) at 1a (3a). Our estimates are conservative since the 8B flux normalization is left free in the analyses.
Our knowledge of sin2 a can be refined if we can observationally infer the 8B flux produced in the Sun. We now describe such a method.
The KamLAND experiment which detects i>e from surrounding nuclear reactors will determine the solar oscillation parameters to 10% precision independently of solar physics. These parameters can be used as inputs in analyses of SNO data to extract the average ve survival probability measured by SNO. The solar flux can be obtained via
= $CC/Pe,
where Pee is the average survival probability of ve at SNO. It has been shown in Ref. [22] that with a few
years of KamLAND data, Pee should be known to about 7% for parameters in the LMA region obtained from solar data. Although matter effects in the Sun depend on the active-sterile admixture, for the oscillation parameters and sterile fraction allowed by current data, they have little effect on Pee.
The dotted lines in Fig. 4 are iso-sin2 a lines and the solid lines are iso-a^ a/sin2 a lines, or lines with the same fractional uncertainty in the content at 1a. Although sin2 a > 1 values are unphysical, they are experimentally obtainable since $nc could be measured to be higher than ^ssm. The figure should be interpreted as follows: each point marks the central values of the $nc and @cc measurements with 6.4% and 5.5% uncertainties, respectively. The solid line passing through each point gives the corresponding asin2 a/ sin2 a. Since the expected uncertainties on @nc and @cc are incorporated in the solid lines, one should not plot the measurements with their uncertainties to read-off the envelope of a^ a/ sin2 a.
In Fig. 4, from left to right, we show our expectations for a - 2 / sin2 a for Pee = 0.28, 0.33 and
sin a'
0.38, all with 7% uncertainties. Since both the solid and dotted lines have slopes higher than 2.5, both
Fig. 4. Iso-asin2 a/sin2a (solid) lines and iso-sin2 a (dotted) lines for future 6.4% #nc and 5.5% #cc measurements from SNO. #8b is obtained from Eq. (2) and Pee is determined by KamLAND with 7% uncertainty. From left to right, the three panels are for three possible measurements, Pee = 0.28, 0.33 and 0.38, respectively. The "+" signs mark the current central values of #nc (= 4.94) and #cc (= 1.68) measured by SNO.
asin2 / sin2 a and sin2 a will have greater sensitivity to the value of @cc than to the value of We conclude that 0^2 a/ sin2 a will be known to 1617%. These projections are comparable with existing bounds as represented by the dashed line of Fig. 3.
Since these expectations are based only on future SNO and KamLAND data, they are conservative. Further improvement can be achieved by combining with other solar data. Joint analyses of solar data are dictated by the paucity of the data. With the future availability of larger datasets it will be worthwhile to perform more definitive analyses of data from experiments which do not have correlations with each other (such as SNO and KamLAND).
3. Conclusions
In a three-neutrino framework, our analysis of all existing KamLAND, CHOOZ and solar neutrino data yields
Sm2s = 8.0+0:6 x 1°-5 eV2, tan2 es = 0.45+0.12,
where the uncertainties are at the 2a C.L. Current bounds on Qx are significantly improved for lower values of Sm\ favored by SuperK. For the SuperK best-fit Sm2a = 0.002 eV2, the CHOOZ upper limit is slightly improved by KamLAND and solar data to
sin2 2$x < 0.13 (0.20)
at the 90% C.L. (3a).
The fraction of solar neutrinos oscillating into active neutrinos is greater than (0.91) 0.65 at 1a (3a) from all existing solar and KamLAND data.
A substantially improved constraint on 9x from future SNO data should not be anticipated unless Sma is at the lower edge of what SuperK atmospheric data prefer (in which case, the CHOOZ data are not very constraining).
With future SNO and KamLAND data alone, it will be possible to know the fraction of solar neutrinos transforming to active species to a precision of 1617% at 1a. This will be an important confirmation of existing bounds because the SNO and KamLAND datasets are completely uncorrelated with each other.
A nonnegligible sterile neutrino component in the solar flux incident on the earth will remain a possibility.
Acknowledgements
We thank A. McDonald for sparking our interest and for useful discussions and comments on the manuscript. This research was supported by the US DOE under Grants No. DE-FG02-95ER40896, No. DE-FG02-91ER40676 and No. DE-FG03-94ER40833, by the US NSF under Grant No. PHY-0244384, and by the Wisconsin Alumni Research Foundation.
References
[1] SNO Collaboration, Q.R. Ahmad, et al., Phys. Rev. Lett. 89 (2002) 011301, nucl-ex/0204008;
SNO Collaboration, Q.R. Ahmad, et al., Phys. Rev. Lett. 89 (2002) 011302, nucl-ex/0204009; S.N. Ahmed, et al., nucl-ex/0309004.
[2] KamLAND Collaboration, K. Eguchi, et al., Phys. Rev. Lett. 90 (2003) 021802, hep-ex/0212021.
[3] V. Barger, D. Marfatia, Phys. Lett. B 555 (2003) 144, hep-ph/0212126;
A.B. Balantekin, H. Yuksel, J. Phys. G 29 (2003) 665, hep-ph/0301072;
A.B. Balantekin, H. Yuksel, Phys. Rev. D 68 (2003) 113002, hep-ph/0309079.
[4] KamLAND Collaboration, T. Araki, et al., hep-ex/0406035.
[5] P. Aliani, V. Antonelli, R. Ferrari, M. Picariello, E. Torrente-Lujan, hep-ph/0406182;
J.N. Bahcall, M.C. Gonzalez-Garcia, C. Pena-Garay, JHEP 0408 (2004) 016, hep-ph/0406294; M. Maltoni, T. Schwetz, M.A. Tortola, J.W.F. Valle, New J. Phys. 6 (2004) 122, hep-ph/0405172.
[6] SNO Collaboration, B. Aharmim, et al., nucl-ex/0502021.
[7] For a recent review see: V. Barger, D. Marfatia, K. Whisnant, Int. J. Mod. Phys. E 12 (2003) 569, hep-ph/0308123.
[8] M. Apollonio, et al., Eur. Phys. J. C 27 (2003) 331, hep-ex/0301017.
[9] F. Boehm, et al., Phys. Rev. D 64 (2001) 112001, hep-ex/0107009.
[10] K2K Collaboration, M.H. Ahn, et al., hep-ex/0402017.
[11] Super-Kamiokande Collaboration, T. Nakaya, eConf C020620 (2002) SAAT01, hep-ex/0209036.
[12] MINOS Collaboration, Fermilab Report No. NuMI-L-375 (1998).
[13] ICARUS Collaboration, A. Rubbia, talk at Skandinavian NeutrinO Workshop (SNOW) Uppsala, Sweden, February 2001, Phys. Scr. T93 (2001)70.
[14] OPERA Collaboration, CERN/SPSC 2000-028, SPSC/P318, LNGS P25/2000, July 2000.
[15] Super-Kamiokande Collaboration, Y. Ashie, et al., hep-ex/0501064.
[16] Super-Kamiokande Collaboration, Y. Ashie, et al., Phys. Rev. Lett. 93 (2004) 101801, hep-ex/0404034.
[17] K2K Collaboration, S.H. Ahn, et al., Phys. Lett. B 511 (2001) 178, hep-ex/0103001;
M.H. Ahn, et al., Phys. Rev. Lett. 90 (2003) 041801, hep-ex/0212007.
[18] For a recent review of oscillations into sterile neutrinos see: M. Cirelli, G. Marandella, A. Strumia, F. Vissani, hep-ph/0403158.
[19] V.D. Barger, D. Marfatia, K. Whisnant, Phys. Rev. Lett. 88 (2002) 011302, hep-ph/0106207;
V. Barger, D. Marfatia, K. Whisnant, B.P. Wood, Phys. Lett. B 537 (2002) 179, hep-ph/0204253.
[20] J.N. Bahcall, M.H. Pinsonneault, astro-ph/0402114;
J.N. Bahcall, A.M. Serenelli, S. Basu, Astrophys. J. 621 (2005) L85, astro-ph/0412440.
[21] T.K. Kuo, J. Pantaleone, Rev. Mod. Phys. 61 (1989) 937; G.L. Fogli, E. Lisi, D. Montanino, A. Palazzo, Phys. Rev. D 62 (2000) 113004, hep-ph/0005261.
[22] J.N. Bahcall, M.C. Gonzalez-Garcia, C. Pena-Garay, Phys. Rev. C 66 (2002) 035802, hep-ph/0204194.
