GFP-SAΔAnk
D) Región carboxilo-terminal
3.3. Estudio del exporte nuclear de DGKz I La localización nuclear de una proteína
One of the ways in which to reconcile the LSND result (section 3.2.7) with other experiments is to introduce one or more neutrino(s) that do not couple to the Z0 and are thus called sterile neutrino(s). The SNO experiment has shown that the total flux of active neutrinos from the Sun agrees with the solar model [80], which limits the extent to which the sterile neutrino(s) can interact with the first or second mass eigenstates. Furthermore, Super-K limits the atmospheric oscillation to be predominantly νµ → ντ [79]. However, this does not rule out νµ → νs as a subdominant oscillation. The MINOS neutral current analysis [115] investigates νµoscillation with a sterile state as well as decay into a sterile state. In both these cases a depletion of NC events in the FD would be observed.
The NC analysis must select events that are well understood beam events, and must ensure that all of the event is accounted for. After the pre-selection events are classified into NC and CC events by a event length cut and assign-ment by a PID. The predicted FD spectrum is formed by correcting the FD MC in each energy bin by comparing the differences between data and MC at the ND. The main systematic uncertainties for this analysis come from: absolute en-ergy scale; relative calibration of hadronic enen-ergy in the two detectors; relative flux normalisation between the two detectors; charged-current contamination of the NC selected events. The results from this analysis are consistent with νµ not oscillating, or decaying to a sterile neutrino.
6.2.1 Pre-Selection of Neutral Current Events
The NC analysis is different to the CC νµ analysis in that most of the visible energy of the neutrino is in the shower. Rather than following the beam centre in the ND the fiducial volume follows the detector outline. For events to be selected the vertex needs to be 50 cm away from the nearest edge of the partial plane.
This enables good containment and reduces contamination from events outside
6.2. THE MINOS NEUTRAL CURRENT ANALYSIS 97 the detector. A cut of 1.7 m < z < 4.7 m means that the events interact in the fully instrumented section of the ND reduced by 10 planes on either side. Also, as a result of looking for showers, rather than tracks, the NC analysis is more susceptible to: split events; leakage events; incomplete events.
Split events are those where a single neutrino is reconstructed as two or more events. This leads to double counting and reduces the energy of the recon-structed event. If an event is split, two reconrecon-structed events will appear close in time and space. In order to reduce the number of split events, a requirement of
∆t > 40 ns is applied. If the separation between events is 40 ns < ∆t < 120 ns, a requirement of ∆z > 1 m is applied.
One type of leakage events are the vertexing failures, which are events that occur outside the fiducial volume, and are reconstructed inside the fiducial vol-ume. These events are typically cosmic ray events, which have a steep shower.
They are removed with a cut on the steepness (S) of the event; this is defined as the ratio of number of strips per plane to the total number of planes in the event.
An S of less than one is accepted in the analysis. Another type of leakage event is from secondary particles from interactions outside the fiducial volume migrating into the fiducial volume; these events enter the detector laterally due to sparse instrumentation at the sides. Although the initial event is not reconstructed it will cause extra activity at the edge of the detector, which can be used to veto events within a time window. For events that are less than 5 GeV in energy are selected:
if there are less than four strips in the veto region active at the same time as the event; or the energy deposited in the veto is less than 1000 in calibrated pulse height units.
One mode of incomplete events are events where not all the strips hit in an event are assigned to the shower. This is caused by either large gaps in the shower or if the shower is generally sparse. A requirement of an event to be made up of more than four strips cuts these events out.
6.2.2 Event Classification
To get a as pure as possible a selection of NC events, any event that crosses more than 60 planes are classed as CC. If an event crosses less than 60 planes
but has a track that crosses at least five planes more than the shower, the event is passed to the PID as used in the 2006 νµ-CC [116]1. Events that pass the selector are classed as CC; otherwise they are thrown out. All other events i.e., those that have a track that cross less than four planes more than the shower, are classified as NC. The FD prediction was made by extrapolating the ND data to the FD by the “Far over Near (F/N)” method. This corrects the FD MC to make a FD prediction by correcting the ND MC to the ND data and then applying similar shifts to the FD MC on a bin by bin basis. The extrapolation is applied separately to five different classes of event: NC interactions; νµCC interactions; ντ CC interactions;
νeinteractions from νµoscillation; νeinteraction from the beam. The νµ-CC events are oscillated with the best fit parameters from the νµ-CC analysis[82]. These different extrapolations are combined in the final FD predicted spectrum.
6.2.3 Neutral Currents Systematic Errors
The same systematic errors affect the NC analysis as the CC, with the follow-ing exceptions: Absolute hadronic response is 12 %, NC contamination is re-placed with CC background and set to 15 %, and muon momentum is rere-placed with ND selection efficiency which is 15.2 % for Ereco < 0.5 GeV, 2.9 % for 0.5 < Ereco < 1.5 GeV and negligible for higher energies. CC cross-section (vi) and Beam uncertainties (vii) cancel in the F/N method.
The CC background was estimated by comparing the number of events in the LE beam running (NLE) to number of events in an alternative beam (Nalt). The number of events in each beam is described by:
NLE = NCLE+CCLE, (6.2)
Nalt = raltNC· NCLE+raltCC· CCLE, (6.3)
where raltN C is determined by MC. NCLE is the number of neutral current events in the low energy beam configuration and CCLE is the number of charged current events in the low energy beam configuration, Equations 6.2 and 6.3 have the
1also used for the main νµanalysis with a different cut value and other cuts
6.2. THE MINOS NEUTRAL CURRENT ANALYSIS 99 solution
CCLE = (Nalt− raltCCNCLE)/(raltCC− raltNC) (6.4) NCLE = (Nalt− raltNCNCLE)/(raltNC− raltCC). (6.5) The final estimate of CCLE background is found by weighting the result from the three alternative beam configurations. The final uncertainty in the CC background was calculated from the double ratio of data over MC for CCLE over CCalt.
6.2.4 Neutral Current Result
The results of the NC analysis are presented with the yet unmeasured θ13set to 0◦ and also at the CHOOZ limit θ13 = 12◦, with the CP parameter δ = 3π/2. The agreement between data and prediction is given by
R ≡ NData− BCC
SNC
where BCC is the extrapolated charged current background from all flavours, SNC
is the number of neutral current events predicted from the extrapolation of ND data and NData is the number of events found in the data after cuts. As most νµ disappearance is below 6 GeV the data is split into two groups: low energy, Ereco < 3 GeV, and high energy, 3 GeV < Ereco < 120 GeV. The median ν true energy of the low energy group is 3.1 GeV and that of the high energy group is 7.6 GeV. As can be seen in table6.2and figure6.3, R agrees with three active flavours to within errors. To account for three active neutrinos and one sterile neutrino the PMNS matrix needs to be expanded to a 4 × 4 matrix, where the fourth mass can be either degenerate with the first mass state, or much more massive than the third2. The allowed parameter space can be seen in figure6.4.
Although pure neutrino decay has been ruled out by other analysis, oscillation with decay has not been ruled out. In this analysis this is consistent with zero (fig-ure6.5with α = 0.00+0.90× 10−3GeV/ km and a neutrino lifetime mτ3
3 > 2.1 × 10−12s/ eV.
αis the mass of the neutrino over lifetime.
2It could also be degenerate with the third mass state. However, this would mean that there would be no oscillation between active and sterile neutrinos, as the SNO result indicates no oscillation between the first two mass states and the fourth.
Figure 6.3: The FD data energy spectrum compared to the prediction for three active neutrino flavours with θ13 = 0 (red) and θ13 = 12◦ (blue). Also shown is the νµ-CC background.