These proceedings summarize the first MiniBooNE electron antineutrino appearance search results, corresponding to a data sample collected for 3.39×1020 protons on target (POT). The search serves as a direct test of the LSND

oscillation signature, and provides complementary information which can be used in studies addressing the MiniBooNE neutrino-mode low-energy excess

Karagiorgi, G.

English

arXiv

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DOI 10.1393/ncc/i2010-10525-1Colloquia: LaThuile09IL NUOVO CIMENTO Vol. 32 C, N. 5-6 Settembre-Dicembre 2009New results from MiniBooNE: A search for electron antineutrinoappearance at ∼ 1 eV2G. Karagiorgi for the MiniBooNE CollaborationMassachusetts Institute of Technology - Cambridge, MA 02139, USA(ricevuto il 10 Novembre 2009; pubblicato online il 15 Gennaio 2010)Summary. — These proceedings summarize the first MiniBooNE electron an-tineutrino appearance search results, corresponding to a data sample collected for3.39×1020 protons on target (POT). The search serves as a direct test of the LSNDoscillation signature, and provides complementary information which can be usedin studies addressing the MiniBooNE neutrino-mode low-energy excess.PACS 14.60.Lm – Ordinary neutrinos.PACS 14.60.Pq – Neutrino mass and mixing.PACS 14.60.St – Non-standard-model neutrinos, right-handed neutrinos, etc.1. – The MiniBooNE and LSND anomaliesThe MiniBooNE experiment has performed a search for ν¯μ → ν¯e oscillations at largeΔm2 [1], an oscillation signature that had been observed by the LSND experiment, with3.8σ significance [2]. This oscillation interpretation relies on the existence of a fourth,sterile neutrino mass eigenstate, with Δm2 ∼ 0.1–100 eV2. Mixing via this fourth masseigenstate can lead to a small probability amplitude for ν¯μ → ν¯e and νμ → νe oscillationsat L[m]/E[MeV] ∼ 1. MiniBooNE has previously searched for this type of oscillationusing a neutrino beam [3], and, under the assumption of CPT conservation, has excludedthe LSND interpretation 98% confidence level (CL) [3]. At the same time, the searchrevealed an excess of νe events at low energy [4], which is inconsistent with the LSNDexcess under the a single sterile neutrino oscillation hypothesis; however, extensions ofthis model [5] offer the possibility of reconsiling the MiniBooNE neutrino results withthe LSND antineutrino result. These models involve large CP violation which leads todifferent probabilities for νμ → νe as opposed to ν¯μ → ν¯e oscillations. Other models [6]have also been suggested as explanations, some of which offer predictions for antineutrinorunning at MiniBooNE. In order to provide another handle on the low-energy excess,MiniBooNE was approved in 2007 for extended antineutrino running [7], which alsoenabled MiniBooNE to perform a direct test of the LSND oscillation interpretation,using antineutrinos.c© Societa` Italiana di Fisica / INFN - Laboratori Nazionali di Frascati 7172 G. KARAGIORGI for the MINIBOONE COLLABORATIONFig. 1. – Monte Carlo background prediction and observed data as a function of reconstructedneutrino energy for the present antineutrino data sample corresponding to 3.39× 1020 POT.2. – The MiniBooNE electron antineutrino appearance searchThe MiniBooNE experiment uses 8GeV protons incident on a beryllium target inorder to produce mesons which subsequently decay to generate the neutrino beam. Amagnetic field is used at the target to focus positively charged mesons in the forwarddirection, and defocus negatively charged mesons, to produce a neutrino beam. Revers-ing the polarity of the magnetic field allows MiniBooNE to switch from a neutrino toan antineutrino beam. The flux [8] consists primarily of ν¯μ and νμ. The low νe and ν¯econtent of the beam minimizes the background to the oscillation search, ensuring sensi-tivity to small-amplitude (of order 10−3) oscillations. The ν¯μ flux has a mean energy of∼ 600MeV. The MiniBooNE detector [9] is located at L = 541m from the proton tar-get. This establishes an L/E similar to LSND, and therefore sensitivity to Δm2 ∼ 1 eV2.The detector is a spherical tank, 12 meters in diameter, filled with mineral oil and linedwith photomultiplier tubes (PMTs). The particle detection and identification methodrelies on the detection of Cherenkov and scintillation light emitted by outgoing chargedparticles which are produced in neutrino interactions.The antineutrino oscillation analysis [1] employs the same analysis chain that wasimplemented in neutrino mode [4]. The analysis relies on differentiation between a ma-jority of ν¯μ charged-current quasi-elastic (CCQE) events, and ν¯e CCQE events, whichare the signal. A track-based event reconstruction is implemented, which uses PMT hittopology and timing to identify electron-like or muon-like Cherenkov rings from the cor-responding CCQE interactions. The ν¯e and ν¯μ CCQE spectra are fitted simultaneouslyas a function of reconstructed antineutrino energy, EQEν , and the oscillation parametersΔm2 and sin2 2θ are extracted. The ν¯μ CCQE prediction is used in the fit in order toprovide a constraint to the ν¯e CCQE prediction, as both spectra are correlated throughflux and cross-section systematics.The ν¯e CCQE background prediction, for 3.39 × 1020 POT, is shown in fig. 1. Thebackground is dominated at low energy by mis-identified ν¯μ events, such as neutral-current (NC) π0 interactions. That is because MiniBooNE cannot differentiate between asingle photon or a single electron produced in the detector. At high energy, the dominantbackground is CCQE interactions of intrinsic ν¯e from the beam, produced in K andπ → μ decays. The sensitivity to ν¯μ → ν¯e oscillations is shown in fig. 2. The MiniBooNENEW RESULTS FROM MINIBOONE: A SEARCH FOR ELECTRON ETC. 73Fig. 2. – The antineutrino sensitivity and limit to LSND-allowed ν¯μ → ν¯e oscillations, from fitsto 200 < EQEν < 3000MeV. The MiniBooNE antineutrino dataset corresponds to 3.39 × 1020POT. Also shown is the limit from KARMEN [10].sensitivity provides substantial coverage of the lower Δm2 region allowed at 90% CL bya joint analysis of LSND and KARMEN [10] data [1].3. – ResultsThe reconstructed energy spectrum of ν¯e CCQE data is shown in fig. 1, overlaidon the predicted ν¯e CCQE background. At energies above 475MeV, the data agreewith the background prediction within systematic and statistical uncertainties. The475–675MeV energy region shows a 2.8σ data fluctuation above background prediction.This fluctuation forces the MiniBooNE limit, shown in fig. 2, to be significantly worsethan the sensitivity at lower Δm2. The MiniBooNE best oscillation fit corresponds to(Δm2 = 4.4 eV2, sin2 2θ = 0.004).Interestingly, the low-energy region (200–475MeV) shows no evidence of an excess.In this range, MiniBooNE observes 61 events, compared to a background expectationof 61.5 ± 11.7 (sys+stat) events. Table I shows the probability (from a two-parameterfit to the data) that each of the following hypotheses explains the observed number oflow-energy neutrino and antineutrino events: 1) Same σ: Same NC cross-section forneutrinos and antineutrinos. 2) π0 scaled: Scaled to the number of NC π0 events. 3)POT scaled: Scaled to number of POT. 4) BKGD scaled: Scaled to the total number ofbackground events. 5) CC scaled: Scaled to the number of CC events. 6) Kaon scaled:Scaled to the number of low-energy K events. 7) Neutrino scaled: Scaled to the numberof neutrino events. The same σ, POT scaled, and Kaon scaled hypotheses are disfavoredas explanations of the MiniBooNE low-energy excess. The most preferred model is thatwhere the low-energy excess is contributed from only neutrinos in the beam.74 G. KARAGIORGI for the MINIBOONE COLLABORATIONTable I. – The χ2-probability that each hypothesis explains the observed number of low-energyneutrino and antineutrino events, assuming only statistical, fully correlated systematic, and fullyuncorrelated systematic errors. A proper treatment of systematic correlations is in progress.Hypothesis stat.-only stat. and correlated sys. stat. and uncorrelated sys.Same σ 0.1% 0.1% 6.7%π0 scaled 3.6% 6.4% 21.5%POT scaled 0.0% 0.0% 1.8%BKGD scaled 2.7% 4.7% 19.2%CC scaled 2.9% 5.2% 19.9%Kaon scaled 0.1% 0.1% 5.9%ν scaled 38.4% 51.4% 58.0%4. – ConclusionMiniBooNE has performed a blind analysis for ν¯μ → ν¯e oscillations. The ν¯e data isfound in agreement with the background prediction as a function of EQEν . No strongevidence for oscillations in antineutrino mode has been found, given the current statis-tics. Interestingly, there is no evidence of significant excess at low energy in antineutrinomode. This has already placed constraints to various suggested low-energy excess in-terpretations. MiniBooNE is currently collecting more antineutrino data, for a total of5.0×1020 POT, and has been approved for further running, to collect a total of 10.×1020POT. This will improve sensitivity to oscillations, and allow further investigation of theneutrino-mode low-energy excess. Additional information will be provided by the NuMI-beam neutrinos detected at MiniBooNE [11].REFERENCES[1] Aguilar-Arevalo A. A. et al., preprint arXiv:0904.1958 [hep-ex].[2] Athanassopoulos C. et al., Phys. Rev. Lett., 75 (1995) 2650; 77 (1996) 3082; 81 (1998)1774; Aguilar-Arevalo A. A. et al., Phys. Rev. D, 64 (2001) 112007.[3] Aguilar-Arevalo A. A. et al., Phys. Rev. Lett., 98 (2007) 231801.[4] Aguilar-Arevalo A. A. et al., Phys. Rev. Lett., 102 (2009) 101802.[5] Sorel M. et al., Phys. Rev. D, 70 (2004) 073004; Maltoni M. and Schwetz T., Phys.Rev. D, 76 (2007) 093005; Karagiorgi G., AIP Conf. Proc., 981 (2008) 210.[6] Harvey J. A. et al., Phys. Rev. Lett., 99 (2007) 261601; Phys. Rev. D, 77 (2008) 085017;Pas H. et al., Phys. Rev. D, 72 (2005) 095017; Goldman T. et al., Phys. Rev. D, 75 (2007)091301; Nelson A. E. and Walsh J., Phys. Rev. D, 77 (2008) 033001; KosteleckyV. A. and Mewes M., Phys. Rev. D, 69 (2004) 016005; Katori T. et al., Phys. Rev. D,74 (2006) 105009; Giunti C. and Laveder M., Phys. Rev. D, 77 (2008) 093002.[7] Aguilar-Arevalo A. A. et al., Addendum to the MiniBooNE Run Plan: MiniBooNEPhysics in 2006 (2006).[8] Aguilar-Arevalo A. A. et al., Phys. Rev. D, 79 (2009) 072002.[9] Aguilar-Arevalo A. A. et al., Nucl. Instrum. Methods A, 599 (2009) 28.[10] Armbruster B. et al., Phys. Rev. D, 65 (2002) 112001.[11] Adamson P. et al., Phys. Rev. Lett., 102 (2008) 211801.

New results from MiniBooNE: A search for electron antineutrino appearance at ∼ 1 eV2

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