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Search for Muon Neutrino Disappearance in a Short-Baseline

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Search for Muon Neutrino Disappearance in a Short-Baseline Proceedings of the XXIX PHYSICS IN COLLISION 1 Search for Muon Neutrino Disappearance in a Short-Baseline Accelerator Neutrino Beam Yasuhiro Nakajima, for the SciBooNE Collaboration Kyoto University Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan Abstract 2.2. SciBooNE Detector We report a search for muon neutrino disappearance in The SciBooNE detector [5] is located 100 m down- the ∆m2 region of 0.5−40 eV2 using data from both Sci- stream from the beryllium target. BooNE and MiniBooNE experiments. SciBooNE data The detector complex consists of three sub-detectors: provides a constraint on the neutrino flux, so that the a fully active fine grained scintillator tracking detec- sensitivity to νµ disappearance with both detectors is tor (SciBar), an electromagnetic calorimeter (EC) and better than with just MiniBooNE alone. The prelim- a muon range detector (MRD). inary sensitivity for a joint νµ disappearance search is The SciBar detector consists of 14,336 extruded plastic presented. scintillator strips (CH), each with dimension of 1.3 × 2.5 × 300 cm3. The scintillators are arranged vertically 1. Introduction × × 3 Neutrino oscillations have been observed and con- and horizontally to construct a 3 3 1.7 m detector. firmed at mass splitting (∆m2) of ∼ 10−5 eV2 and The detector itself is the neutrino target and its fiducial ∼ −3 2 volume is 10.6 tons. 10 eV , called the “solar” and “atmospheric” re- The EC is installed downstream of the SciBar, and is gions, respectively. The observed mixing is consistent made of scintillating fibers embedded in lead foil. with three generations of neutrinos. The MRD is located downstream of the EC in order to However, the LSND experiment observed an excess of measure the momentum of muons up to 1.2 GeV/c using νe in a νµ beam, indicating a possible oscillation in the the muon range. It consists of 12 layers of 2”-thick iron ∆m2 ∼ 1 eV2 region [1]. To explain LSND with oscilla- plates sandwiched between layers of 6 mm-thick plastic tions requires more than three generations of neutrinos scintillator planes. or other exotic physics beyond the Standard Model. The SciBooNE experiment ran from June 2007 until To test the oscillation at ∆m2 ∼ 1 eV2, the Mini- August 2008, collecting a total of 2.52 × 1020 Protons 20 BooNE experiment recently made searches for both νe on Target (POT) for physics analysis; 0.99 × 10 POT appearance [2, 3] and νµ disappearance [4] in this pa- in neutrino mode and 1.53 × 1020 POT in antineutrino rameter region. The experiment observed no signifi- mode. cant νe appearance signal and ruled out as being due to 2-neutrino oscillations. However, the sensitivity of MiniBooNE-only νµ disappearance search was limited 2.3. MiniBooNE Detector by the large flux and neutrino interaction cross-section uncertainties. The MiniBooNE detector [7] is located 440 m down- Here, we discuss an improved search for ν disappear- stream from the SciBooNE detector. The detector is a 12 µ m diameter spherical tank filled with 800 tons of min- ance using data from both the SciBooNE [5] and the eral oil (CH2). The MiniBooNE experiment has been MiniBooNE experiments, where SciBooNE detector is taking beam data since 2002, including the SciBooNE used to constrain flux and cross-section uncertainties. and MiniBooNE joint-run period. The collected number 2. Experimental Setup of POT after data quality cut in the neutrino mode is 5.579 × 1020 in addition to the data from the joint-run 2.1. Fermilab Booster Neutrino Beam period. arXiv:1010.5721v1 [hep-ex] 27 Oct 2010 3. νµ Disappearance Analysis 3.1. Analysis Overview In this paper, we report only the neutrino data (νµ → νx) disappearance analysis. We search for muon neu- trino disappearance by comparing neutrino fluxes at Sci- BooNE and MiniBooNE detectors. Fig. 1. The setup of SciBooNE and MiniBooNE experiments. The analysis is performed in the following three steps: (1) Neutrino flux measurement at SciBooNE, (2) Flux extrapolation to MiniBooNE, and (3) Oscillation fit. The experiments use the Booster Neutrino Beam At each step, systematic errors are estimated and (BNB) at Fermilab [6]. The primary proton beam, ex- propagated to the final prediction. The majority of the tracted with a kinetic energy of 8 GeV, strikes a 71 cm flux and cross-section uncertainties cancels since the neu- long, 1 cm diameter beryllium target. The mesons, pri- + trino interaction target in both detectors is effectively marily π , generated by the p−Be interactions are fo- carbon, and the two detectors are on the same beam cused with a magnetic horn and decay in the following line. 50 m decay volume, producing an intense neutrino beam We describe these steps in detail in the following sec- with the peak energy of ∼0.7 GeV. When the horn po- tions. larity is reversed, π− are focused and hence a predomi- nantly antineutrino beam is created. c 2009 by Universal Academy Press, Inc. 2 Sat Oct 17 15:44:00 2009 3.2. Neutrino Flux Measurement at SciBooNE Recostructed Muon Momentum Entries 20227 1600 Preliminary Data Charged Current Event Selection 1400 Other For the spectrum analysis at SciBooNE, we use inclu- 1200 Dirt NC sive νµ charged current (CC) interactions, whose signa- 1000 CC other 800 ture is long muon tracks. First, we identify muons by se- CC coherent π lecting the longest track with energy deposit consistent 600 CC resonance π with a minimum-ionizing particle. Second, we require 400 CCQE the vertex of the track to be within the SciBar fiducial 200 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 volume. The events are further divided into two sub- µ samples based on the location of the muon track end P (GeV) points: a “SciBar-stopped” sample containing muons Recostructed Muon Angle Sat Oct 17 15:44:01 2009 2500 Entries 20227 that have stopped inside the SciBar detector and a Preliminary Data Other “MRD-stopped” sample with muons that have stopped 2000 in the MRD. These two samples each contain approxi- Dirt 1500 NC mately 14k and 20k events with mean energies of 0.8 and CC other π 1.1 GeV, respectively. 1000 CC coherent CC resonance π 500 CCQE Spectrum Fitting 0 0 20 40 60 80 100 120 140 160 180 The neutrino spectrum at SciBooNE is extracted by µθ (deg) fitting muon momentum (Pµ) and muon angle (θµ) dis- tributions from each sample. Fig. 3. Distribution of reconstructed muon momentum (top) and muon angle (bottom) for the MRD-stopped sample. The We prepare MC templates for Pµ and θµ distributions dots show the data, and histograms show the MC prediction for several true neutrino energy (Eν ) regions. The Eν with the contributions from neutrino interaction modes. The regions are divided by 250 MeV up to 1.25 GeV, and a MC distributions are tuned by the Eν scale factors obtained single region is assigned for Eν > 1.25 GeV. Then, the by the spectrum fit. scale factors for each Eν region are determined to mini- mize the χ2 between data and MC. Figure 2. shows the fit result. The systematic errors from SciBooNE detector 2 − 2 response and neutrino cross-section models are estimated where ∆M = Mn Mp ; M indicate the muon, proton, and shown in the plot. or neutron mass with appropriate subscripts; EB is the Figure 3. is the Pµ and θµ distributions of SciBooNE’s nucleon binding energy; Eµ is the reconstructed muon MRD-stopped sample, after applying scale factors ob- energy. tained by the spectrum fitting. We confirm the MC dis- tributions agrees well to data after fitting. Rec MiniBooNE Eν prediction Rec To predict the Eν distribution at MiniBooNE, we 2 extrapolate the measured SciBooNE flux to MiniBooNE in two steps. 1.8 Preliminary First, we apply MiniBooNE/SciBooNE flux ratio to 1.6 make a prediction of the true neutrino energy distribu- 1.4 tion at MiniBooNE. Then, we smear the true neutrino energy prediction to the reconstructed neutrino energy. 1.2 Systematic uncertainties for the flux ratio is estimated 1 by varying the cross-section and flux models. Addition- 0.8 ally, the uncertainties of the smearing function, which Rec 0.6 convert true Eν to Eν , is estimated by varying the cross-section models. 0.4 Finally, we add MiniBooNE detector response error to Rec 0.2 the Eν prediction. 0 The predicted MiniBooNE reconstructed neutrino en- 0 0.5 1 1.5 2 2.5 Eν (GeV) ergy distribution and its systematic uncertainties are shown in the Figure 4.. Fig. 2. Scale factors obtained by SciBooNE spectrum fitting. The error bars show the sum of SciBooNE statistical and systematic uncertainties. 3.4. Oscillation Fit and Sensitivity Fit Method We test the oscillation hypothesis assuming the mixing 3.3. Flux Extrapolation to MiniBooNE between 2 neutrino flavors; νµ and νx. The νµ → νx disappearance probability is given as MiniBooNE Event Selection We select events in MiniBooNE by requiring single P (ν → ν ) = sin2 2θ sin2(1.27∆m2L/E), muon and its decay electron. Neutrino energy is recon- µ x structed from muon kinematics by assuming CC Quasi − 2 2 Elastic (CCQE) interaction (νµn → µ p): where θ is the mixing angle, ∆m [eV ] is the mass split- ting between 2 flavors, L[km] is the distance traveled and − − 2 − 2 E[GeV] is the neutrino energy. Rec 2(Mn EB )Eµ (EB 2MnEB + ∆M + Mµ) Eν = , 2[(Mn − EB) − Eµ + pµ cos θµ] 3 ] 2 Total err. [eV 25000 2 Preliminary m Flux + X-sec.
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