The MINOS experiment uses the intense NuMI beam created at

Fermilab and two magnetized tracking calorimeters, one located at Fermilab and one located 735km away at the Soudan Mine in Minnesota, to study lepton-flavor

violation in the neutrino sector. We present results of the precise measurement of the atmospheric neutrino oscillation parameters, from the search for sterile neutrinos and from the search for the θ13 mixing angle by searching for νe appearance in the νμ beam. Future prospects for measurements by MINOS will also be discussed

Paley, Jonathan M.

English

arXiv

The MINOS experiment uses the intense NuMI beam created at Fermilab and two
magnetized tracking calorimeters, one located at Fermilab and one located 735
km away at the Soudan Mine in Minnesota, to make precise measurements of
$\nu_\mu$ disappearance oscillation parameters. We present recent results from
the first two years of NuMI beam operations, including the precise measurement
of the atmospheric neutrino oscillation parameters and the search for sterile
neutrinos. Future prospects for MINOS will also be discussed, including an
improved limit on the $\theta_{13}$ mixing angle by searching for $\nu_e$
appearance in the $\nu_\mu$ beam.Comment: PANIC 2008 Proceedings, 3 pages, 4 figure

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Recent Results and Future Prospects from MINOS

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DOI 10.1393/ncc/i2011-10723-3Colloquia: LaThuile10IL NUOVO CIMENTO Vol. 33 C, N. 5 Settembre-Ottobre 2010Recent results and future prospects for the MINOS experimentJonathan M. Paley for the MINOS CollaborationArgonne National Laboratory - Argonne, IL, USA(ricevuto il 14 Settembre 2010; pubblicato online l’11 Gennaio 2011)Summary. — The MINOS experiment uses the intense NuMI beam created atFermilab and two magnetized tracking calorimeters, one located at Fermilab andone located 735 km away at the Soudan Mine in Minnesota, to study lepton-flavorviolation in the neutrino sector. We present results of the precise measurement ofthe atmospheric neutrino oscillation parameters, from the search for sterile neutrinosand from the search for the θ13 mixing angle by searching for νe appearance in theνμ beam. Future prospects for measurements by MINOS will also be discussed.PACS 14.60.Pq – Neutrino mass and mixing.PACS 14.60.St – Non-standard-model neutrinos, right-handed neutrinos, etc.1. – The MINOS experimentThe Main Injector Neutrino Oscillation Search (MINOS) experiment was designedto primarily confirm the phenomenon of neutrino oscillations via a precise measurementof the atmospheric neutrino oscillation parameters Δm2 and sin2(2θ). MINOS data arealso used to search for sterile neutrinos and the yet-unobserved θ13 mixing angle viaνμ → νe oscillations. Futhermore, MINOS is capable of conducting tests of CPT andatmospheric neutrino and cosmic ray studies, however these analyses will not be coveredin this presentation.Neutrino oscillation measurements in MINOS primarily utilize the Neutrinos at theMain Injector (NuMI) beam at Fermi National Laboratory. NuMI is a very pure andintense beam of muon neutrinos aimed at an underground laboratory 735 km from theNuMI production target in Soudan, MN. A 5.4 kton magnetizied iron tracking calorimeter(the Far Detector, FD) located at the Soudan laboratory is used to detect the neutri-nos [1]. A functionally identical 0.98 kton Near Detector (ND) is located approximately1 km downstream from the NuMI target. The neutrino energy spectrum is measuredusing both detectors. The shapes of the two energy spectra are compared and used tomeasure and constrain neutrino oscillation parameters.In the analyses of MINOS data presented here, neutrino interactions in the MINOSdetectors are characterized by an event energy and interaction type. There are threetypes of neutrino interactions of interest: νμ charged-current (CC), neutral-current (NC)c© Societa` Italiana di Fisica 107108 JONATHAN M. PALEY for the MINOS COLLABORATIONand νe CC events. CC events are characterized by having the corresponding lepton inthe final state; furthermore, all of the incident neutrino energy is contained in the event.Therefore, νμ CC events are characterized by a long muon track with a shower at thevertex, whereas νe CC events are shorter and have an electromagnetic shower profile. NCevents, on the other hand, are “flavor-blind” and the final state consists of a hadronicshower and a neutrino which carries away at least some of the energy of the incidentneutrino; these events are short, but diffuse. Each analysis discussed here uses one ofthese types of events as the signal and the other events are backgrounds for the sameanalysis. The energy of the event is the sum of the reconstructed shower energy and thereconstructed energy of the muon (if one is found). Muon energies are determined froma combination of curvature and range; shower energies are determined from Monte Carlo(MC) simulations tuned to external data.2. – Measurement of Δm2 and sin2 2θThe determination of the atmospheric neutrino oscillation parameters Δm2 andsin2(2θ) is accomplished by measuring the probability that a muon neutrino of energyE and traveling a distance L is observed in the muon neutrino weak eigenstate. Thisneutrino oscillation “survival probability” may be written asP (νμ → νμ) = 1− sin2(2θ) sin2(1.27Δm2LE),(1)where θ  θ23, Δm2  Δm232 is in units of eV2, L is in km and E in GeV. A comparisonof the measured muon neutrino energy spectra in the ND and FD yields the parameterssin2(2θ) and Δm2.The dominant background to νμ CC events is NC events that are reconstructed aslow-energy νμ CC interactions. NC events are separated from νμ CC events based ontopological characteristics that indicate a muon in the final state: track length, meanpulse height, fluctuation in pulse height and transverse track profile. A CC/NC separa-tion parameter cut is determined that maximizes the CC event selection efficiency andminimizes the NC background [2]. A comparison of the selection parameter distributionin the ND data and MC shows excellent agreement above the final CC/NC parametercut.The energy spectrum for νμ CC events is measured in the ND and extrapolated to theFD. The extrapolation is non-trivial because the neutrino events seen in the ND appear tobe coming from a line source, whereas events in the FD appear to be coming from a pointsource. MC simulations are therefore used to determine energy smearing and acceptancecorrections for the expected FD energy spectrum. The dominant systematic uncertaintiesin the predicted FD spectrum are a 4% normalization uncertainty, a 10.3% hadronicenergy calibration uncertainty and a 50% uncertainty on the NC background that pass theCC cut selection criteria. The normalization and hadronic energy uncertainties primarilyeffect the measurement of Δm2 at the level of ±0.081×10−3 eV2 and ±0.052×10−3 eV2,respectively. The NC background is the largest effect on the measurement of sin2(2θ) atthe level of ±0.016.Figure 1 shows the measured FD energy spectrum, the expected spectrum with nooscillations, the best oscillation fit to the data, and the NC background (significant only inthe lowest energy bin). A total of 848 events are observed in the FD, with 1065±60 eventsexpected under the no-oscillation hypothesis for a data set based on 3.36× 1020 protonsRECENT RESULTS AND FUTURE PROSPECTS FOR THE MINOS EXPERIMENT 109Fig. 1. – (Colour on-line) The Far Detector νμ energy spectrum determined from CC events.The data are shown as black points with error bars; the NC background is shown in shaded grayand is only significant in the first bin. The predicted spectrum for the no-oscillation hypothesisis shown in red. The black line represents the best-fit spectrum for the oscillation hypothesis.on target (POT). Fitting the observed energy spectrum to the survival probability, andconstraining the fit parameters to their physically meaningful values, we find Δm2 =2.43 ± 0.13 × 10−3 and sin2(2θ) > 0.90 (90% CL), with a χ2/ndof = 90/97. Figure 2shows the oscillation parameter phase space allowed by the latest MINOS measurement,with comparisons to other experiments. MINOS has the most precise measurement ofΔm2 to-date. Alternative hypotheses of neutrino decay [3] and neutrino decoherence [4]have also been tested with the MINOS data, and these are disfavored with respect to theosciallation hypothesis by 3.7σ and 5.7σ, respectively.Fig. 2. – 68% and 90% contour lines for the oscillation fit parameters Δm2 and sin2(2θ) comparedto other experimental measurements.110 JONATHAN M. PALEY for the MINOS COLLABORATIONFig. 3. – (Colour on-line) Visibile energy spectrum of NC events in the FD. Data are shown inblack points. The predicted spectrum obtained from the ND NC spectrum is shown is red forθ13 = 0 (dashed blue for θ13 at the CHOOZ limit).3. – The search for sterile neutrinosSince the NC event rate is independent of neutrino flavor, and thus unaffected byoscillations between the three active neutrino flavor states, a deficit in the NC rate inthe FD would indicate the existence of at least one additional neutrino that does notinteract via the weak force (“sterile neutrino”, νs). The dominant background in thismeasurement are low-energy νμ CC events that do not have a clear muon track. NCevents are selected and their visible energy determined from reconstructed “shower-like”events. The ND NC spectrum is extrapolated to the FD, and several sterile neutrinomodels are tested against these two spectra.The quantity fs = (Pνμ→νs)/(1 − Pνμ→νμ) describes the fraction of νμ that haveoscillated to νs in a simple four-flavor model where oscillations to sterile neutrinos occurat the same mass splitting as the νμ disappearance measured from the CC interactions.The search for sterile neutrinos using MINOS data is sensitive to two sterile neutrinomodels: the case where m4 = m1 and the case where m4  m3.Figure 3 shows the measured visible energy spectrum in the FD, based on 3.18 ×1020 POT, along with the expected spectra for θ13=0 (solid red) and θ13 at the CHOOZlimit [5] (dashed blue) and the predicted νμ CC event background (hashed black). Inthe case where m4 = m1 and θ13=0, the 90% CL limit on fs is 0.51 (0.55 for θ13 = 12◦).In the case where m4  m3, the limit on fs is 0.52 (0.55). A detailed description andresults of the search for sterile neutrinos in MINOS may be found [6].4. – The search for θ13 via νμ → νeFor the search of νe appearance in the MINOS detector we again make use of CCevents; in this case, we are looking for an excess of events with an electron in the finalstate in the MINOS FD compared to the expected number from measurements in theMINOS ND. The MINOS detectors were optimized for muon identification in the finalRECENT RESULTS AND FUTURE PROSPECTS FOR THE MINOS EXPERIMENT 111Fig. 4. – (Colour on-line) Top: the Far Detector νe energy spectrum determined from CC events.The data are shows as black points with error bars; backgrounds are shown as solid, coloredhistograms. Bottom: data—predicted background as a function of energy. The data excess isrepresented by the black points, the best fit to the signal is represented by the solid histogram.state; therefore the search for electron-neutrino appearance in MINOS is very challengingbecause of the very large background of NC events (low-energy νμ CC events also con-tribute a non-negligible amount to the background). However as mentioned previously, νeCC events do differ topologically from NC events and are tagged by selecting events thathave electromagnetic-like shower profiles. Using topological cuts and a neural-networkto classify events, the signal-to-background ratio is reduced from 1:55 to 1:4.Fig. 5. – (Colour on-line) Best-fit value of sin2(2θ13) as a function of the CP -violating phaseδCP . 68% and 90% contour lines for sin2(2θ13) are shown as solid red and blue lines. TheCHOOZ limit is drawn as a dashed line for comparison.112 JONATHAN M. PALEY for the MINOS COLLABORATIONThe top of the left plot of fig. 4 shows the measured νe CC spectrum for an exposureof 3.14×1020 POT in the MINOS FD (solid black points) with the backgrounds overlaid.The dominant NC background is in blue, the νμ CC background in red, and τ and beamνe CC events are in green and magenta, respectively. The bottom of the left plot showsthe amount of data excess (black points) as a function of energy, with the best fit for theνμ → νe oscillation hypothesis plotted in solid purple. A total of 35 events are observedin the MINOS FD with a predicted background of 27 ± 5(stat) ± 2(syst) events, whichcorresponds to a 1.5 σ excess. The observed excess is comparable to the CHOOZ [5] limitfor |Δm2| = 2.43 × 10−3 eV2 and sin2(2θ23) = 1.0. With these results, sin2(2θ13) = 0 isincluded at the 92% confidence level. The contours on the right of fig. 5 show the best-fitvalues of sin2(2θ13) as a function of the CP -violating phase δCP , with 68% (blue) and90% (red) CL intervals for the normal (top) and inverted (bottom) hierarchies. Furtherdetails of this analysis may be found in [7].5. – Future outlookAt the time of this conference, the MINOS Collaboration had completed and pub-lished analyses based on approximately 3 × 1020 POT exposure but had collected morethan twice that amount of data (∼ 7 × 1020 POT). All analyses were being redone toincorporate not only the additional data, but also improvements in reconstruction, parti-cle identification algorithms and background reductions. In some cases, the backgroundshave been reduced by a factor of two. MINOS expects to produce new results for all ofthese analyses in the summer of 2010 that will incorporate all of these improvements.Furthermore, MINOS has collected ∼ 1.5× 1020 POT of reverse-horn-current data; thisconfiguration results in a ν¯μ-dominated beam. These data will be analyzed and a mea-surement of the atmospheric oscillation parameters Δm¯2 and sin2(2θ¯) will be forth-coming from MINOS in the summer of 2010 with obviously limited statistics. MINOSplans to continue collecting data through 2011, but it is not clear yet under which beamconfiguration.REFERENCES[1] Michael D. G. et al., Nucl. Instrum. Methods A, 596 (2008) 190.[2] Adamson P. et al., Phys. Rev. Lett., 101 (2008) 131802.[3] Barget V. et al., Phys. Rev. Lett., 82 (1999) 2640.[4] Fogli G. L. et al., Phys. Rev. D, 67 (2003) 093006.[5] Apollonio M. et al., Eur. Phys. J. C, 27 (2003) 331.[6] Adamson P. et al., Phys. Rev. D, 81 (2010) 052004.[7] Adamson P. et al., Phys. Rev. Lett., 103 (2009) 261802.

Recent results and future prospects for the MINOS experiment

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Recent results and future prospects for the MINOS experiment

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