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First Observations of Separated Atmospheric and Events in the MINOS Detector. A. S. T. Blake* (for the MINOS collaboration) *Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge, CB3 0HE, United Kingdom. down-going fit up-going fit ABOVE travel length 10~30 km p, He p, He BELOW travel length up to 13,000 km Studying Atmospheric Neutrinos at MINOS cosmic muons PCUP PCDN FC Overview Selecting Atmospheric Neutrinos (a) Fully Contained or Down-Going Partially Contained Events. (b) Up-Going Partially Contained Events. Results of Event Selection Oscillation Analysis Charge Ratio Separating Neutrinos and Anti-Neutrinos Example Events: The complete MINOS far detector has been collecting atmospheric neutrino data since August 2003. The detector is a 5.4 kT steel- scintillator calorimeter located at a depth of 700m (2100 mwe) in the Soudan Underground Laboratory, Minnesota. It is the first massive underground detector to possess a magnetic field, making the separation of atmospheric and possible for the first time. This poster presents the first MINOS observations of separated and charged current neutrino interactions based on a detector exposure of 418 days (6.18 kT-Yrs). The ratio of to is calculated and compared to the Monte Carlo expectation. An extended maximum likelihood analysis of the observed L/E distribution is used to fit the neutrino oscillation parameters m 23 2 and sin 2 2 23 . High energy cosmic ray interactions at the top of the atmosphere produce an intense flux of neutrinos that can be detected on the earth. In recent years, the deficit observed in the atmospheric flux has become firmly established, with the favoured interpretation being → oscillations. oscillations. The MINOS far detector can be used to study atmospheric neutrino oscillations. Its deep location 700m underground provides shielding against the high flux of cosmic rays incident on the surface of the earth, and its large 5.4 kT mass ensures that a high rate of atmospheric neutrinos can be observed in the detector. space The MINOS far detector, located in the Soudan mine, is a sampling calorimeter composed of many planes of steel and plastic scintillator. The detector is magnetized by a current-carrying coil running through its centre. This allows the charge of muons produced in neutrino interactions to be identified, enabling the separation of neutrinos and anti- neutrinos. A scintillator veto shield is constructed above the main detector to tag cosmic muons entering the detector. space Atmospheric neutrinos are incident on the MINOS detector from all directions. The signature for → oscillations is a deficit in the observed flux of up- going events, since these events correspond to large neutrino path lengths and therefore large oscillation probabilities. In order to perform an accurate measurement of these oscillations, a clean sample of CC events must be selected. The signature used to identify these events is a muon track whose interaction vertex is contained inside the fiducial volume of the detector. space The contained vertex CC events are divided into three classes: The selection of contained atmospheric events is hampered by the large background of cosmic muon events appearing contained or up- going. This background must be reduced by a factor of ~10 6 in order to obtain a clean neutrino signal. space Events are reconstructed using software specially designed for this analysis. A pre-selection is applied requiring a clean muon track. The events are then divided into the FC/PCDN and PCUP classes and separate selection criteria are applied. The planar structure of the detector allows cosmic muons to enter the detector between planes and appear contained. These events tend to travel nearly parallel to the planes or to contain large energy deposits at the track vertex. A series of topology cuts are applied to the FC and PCDN events to separate the signal and background. space The following topology cuts are applied: The dominant background in the PCUP selection arises from cosmic muons reconstructed as up-going rather than down-going. Up-going events are identified using timing information. After careful calibration a single-hit timing resolution of 2.3 ns is achieved. Selection cuts are applied to the timing information to ensure that the events are up-going. space The event direction is determined by applying fits to the measured times along the muon track, assuming that the muon is travelling upwards and then downwards at the speed of light. The RMS of each fit is calculated, and a cut is placed on the quantities RMS up , RMS down , and RMS up – RMS down to select a clean sample of up-going events. The MINOS detector is magnetized with a mean field of 1.3T. This allows the charge sign of muons to be determined from the curvature of their tracks. A track fitting algorithm is used to determine the best fit value of Q/p and its error Q/p , where Q is the muon charge and p is the muon momentum. An event is classified as having a well-measured charge sign if (Q/p)/( Q/p ) < 50%. A total of 52 events are found to have a well-measured charge, with 34 and 18 events. The - ratio is: From 418 days detector exposure, a total of 107 candidate events are selected from the data. This compares with an expectation of 127±13 events assuming no oscillations, and 96±10 events assuming m 23 2 =0.0024eV 2 and sin 2 2 23 =1.0. The background contribution from cosmic muons is measured to be 4.4±0.5 events using the shield. space A total of 77 events are found to have a well-measured direction, with 49 down-going and 28 up-going events. The up-down double-ratio is: This is ~2 standard deviations away from unity. The MINOS Far Detector • Fully Contained Events (FC). • Downward-Going Partially Contained Events (PCDN). • Upward-Going Partially Contained Events (PCUP). _ • The reconstructed track is first extrapolated back to the outside of the detector and a cut placed on the distance traversed perpendicular to the planes. • For events travelling parallel to the planes a cut is placed on the energy deposited at the track vertex. • The background is tagged using the veto shield. cosmic muon background left: illustration of cosmic muon background event; middle: distribution of perpendicular distance between track vertex and edge of detector; right: energy deposited at track vertex vs. reconstructed direction for MC signal and background events, showing the cut regions. .) sys ( 02 . 0 .) stat ( 62 . 0 R / R 19 . 0 14 . 0 MC down / up data down / up above: The reconstructed zenith angle distribution for the 77 events with well-measured direction, compared with the MC expectation. above: The reconstructed log 10 L/E distribution for the 77 events with well-measured direction, compared with the MC expectation. reference: Phys. Rev. D73, 072002 (2006) _ _ .) sys ( 03 . 0 .) stat ( 53 . 0 R 21 . 0 15 . 0 data / FC up-going PC down-going PC up-going _ _ An extended maximum likelihood analysis of the observed L/E distribution is used to fit the parameters m 23 2 and sin 2 2 23 assuming two-flavour → oscillations. The sensitivity of the L/E distribution to neutrino oscillations depends on the L/E resolution. Therefore a Bayesian method is used to calculate a log(L/E) probability density function for each event and the resolution log(L/E) of each event is given by the rms of this distribution. space The data is binned in terms of log(L/E) and log(L/E) and a likelihood fit to the oscillations is performed. The maximum likelihood occurs at: m 23 2 = 0.0013 eV 2 , sin 2 2 23 = 0.90 The 68% and 90% confidence limits are determined using the Feldman-Cousins method. The data are consistent with a wide range of oscillation parameters, but disfavour the null oscillation hypothesis at the 98% confidence level. above: the reconstructed log(L/E) resolution binned in regions of log(L/E) resolution (). right: confidence limits calculated for the oscillation parameters m 23 2 and sin 2 2 23 . The ratio of to events in the data compared to the MC expectation (which assumes the same oscillation parameters for both neutrinos and anti- neutrinos) is calculated to be: Although the current statistics are limited, the data are found to be consistent with the hypothesis that neutrinos and anti-neutrinos oscillate with the same parameters. .) sys ( 15 . 0 .) stat ( 96 . 0 R / R 38 . 0 27 . 0 MC / data / above: the distribution of (Q/p)/ (Q/p) for selected neutrino events. The two broad peaks correspond to neutrinos and anti-neutrinos. _ above: the up-down distributions of selected neutrino and anti-neutrino events, showing the MC expectations for three different neutrino oscillation scenarios. Summary _ _ _ The MINOS detector has been taking atmospheric neutrino data since August 2003, and an analysis of 418 days (6.18 kT-Yrs) data is presented. A total of 107 atmospheric neutrino events are observed compared with an expectation of 127±13 events. The background is estimated to be 4.4±0.5 events. space The and events are separated using the curvature of muons in the MINOS magnetic field. From the events containing a well-measured muon, a total of 34 and 18 events are observed. This represents the first observation of separated atmospheric neutrino and anti-neutrino interactions. _ _ primary cosmic rays /κ e e ~20 km p/He
