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Energy measurement in quasi-elastics Unfolding detector and physics effects. Alain Blondel Mario Campanelli Maximilien Fechner. Introduction. Quasi-elastic events, dominant at low energy (< 1 GeV) easier to reconstruct than DIS.

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## Energy measurement in quasi-elastics Unfolding detector and physics effects

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**Energy measurement in quasi-elasticsUnfolding detector and**physics effects Alain Blondel Mario Campanelli Maximilien Fechner A. Blondel, M.Campanelli, M.Fechner**Introduction**Quasi-elastic events, dominant at low energy (< 1 GeV) easier to reconstruct than DIS. Very important for low-energy Super-Beams, in particular for Cerenkov detectors. For a target nucleon at rest, the neutrino energy can be reconstructed exactly from lepton information only (proton below Cerenkov threshold): A. Blondel, M.Campanelli, M.Fechner**Experiment/Physics effects**In order to use this formula in a real experiment, we have to account for: • Resolution on lepton measurement • Nuclear effects (Fermi motion, Pauli blocking) • efficiencies All these effects lead to a widening of neutrino energy resolution, and to a clear bias on the reconstructed energy We consider the case of a large water Cerenkov detector, illuminated by a low energy super-beam (CERN SPL-> Frejus), for a 200 kton*year exposure. However, most of the following considerations have general application. A. Blondel, M.Campanelli, M.Fechner**Event simulation**Standard SPL+UNO event rates considered Lepton energy and angle resolution are taken from SuperKamiokande: s(Ee)/Ee=0.5%+2.5%/E s(Eμ)/Eμ=3% s(q)=3o • Fermi motion: neutrons are assumed having momentum uniformly distributed on a sphere with kF=225 MeV • Pauli blocking: to have a quasi-elastic event, we need the produced proton to be outside the Fermi sphere p n ν kF μ A. Blondel, M.Campanelli, M.Fechner**Energy reconstruction from leptons**Perfect detector Using the above formula, for e,at SPL energies: Lepton resolution Nuclear effects only Erec Erec All effects included Egen Egen A. Blondel, M.Campanelli, M.Fechner**Biases and Energy resolution**resolution Erec-Egen 20% average resolution, with 5% bias A. Blondel, M.Campanelli, M.Fechner**Effects on neutrino oscillations**The dip due to neutrino oscillations almost disappears after smearing effects are considered dip no dip A general bias towards higher energies is observed A. Blondel, M.Campanelli, M.Fechner**Fitting for oscillation parameter in presence of distorting**effects Classical problem in HEP; here we follow the approach of the MonteCarlo re-weighting with box method (used eg for W physics at LEP) Basic idea: produce a large MonteCarlo correspondence table between the real quantity (Eνgen) and measured one (Eνrec), and consider for each data event all those with reconstructed energy sufficiently close to the data event. Since normally the MC sample is produced with a given set of parameters θ0, events are reweighted according to the ratio of oscillation probabilities A. Blondel, M.Campanelli, M.Fechner**Box!**Box reweighting at work Reconstructed MC distribution Data event MC events in the box reconstructed weights generated Image of the box A. Blondel, M.Campanelli, M.Fechner**Fits with reweighting**The final fit is performed from a likelihood containing two terms, one for the shape (box method) and one describing the Poisson probability for the number of events: box counting The two parts of the likelihood can be studied separately to isolate contributions from the spectrum and from the simple counting of the number of events A. Blondel, M.Campanelli, M.Fechner**Fits to Δm2**The algorithm was tested for the whole relevant range of Dm2 and showed good linearity and precision A. Blondel, M.Campanelli, M.Fechner**2d results**Using this method, we fit several quantities, using the following oscillation parameters: Dm23=2.5 10-3 Dm12=5.44 10-5 tan2q12=0.4 sin22q23=0.95 sin2 2q13=0.02 d=0 Counting only As expected, Dm2 determination benefits much from spectrum, while q13 is basically counting events spectral information also A. Blondel, M.Campanelli, M.Fechner**Atmospheric parameters**Even more spectacular is the improvement on the fit of both atmospheric parameters, where the lack of spectral information results in large correlations and a “banana-shaped” contour. Counting only spectral information also A. Blondel, M.Campanelli, M.Fechner**Conclusions**• Spectral information is essential to fully exploit the capabilities of a Super-beam oscillation experiment • At low energy, detector and especially nuclear effects introduce large spectral distortions, to be corrected for • The MC re-weighting is one of the most powerful methods to deal with such situations • Very good precision on the main oscillation parameters obtained; no systematics yet; applicability to CP violation under study A. Blondel, M.Campanelli, M.Fechner

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