detection of the mode: nomad - harvard physicsusers.physics.harvard.edu/~feldman/lecture_2.pdf ·...
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Gary Feldman SLAC Summer Institute 14 - 25 August 2000 43
NOMAD: τ Detection by Kinematics
l Detection of the mode: τ νν→ e
Gary Feldman SLAC Summer Institute 14 - 25 August 2000 44
NOMAD: τ Detection by Kinematics
l Detection of the τ → h ν + n (π0) modes:
Gary Feldman SLAC Summer Institute 14 - 25 August 2000 45
NOMAD: Detector
l ÒAn electronic bubble chamberÓ
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NOMAD: νµ CC Event
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NOMAD: νe CC Event
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NOMAD: Analysis
l Extensive use of likelihood functions
5+ independentkinematic variables.Using likelihoodfunctions yieldsoptimum signal tobackground ratios.
Example fromtheanalysis
τ νν→ e
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NOMAD: Analysis
l Data simulatorsl The Monte Carlo simulations did not simulate either the
details of the reconstruction or the physics precisely.
l Therefore, whenever possible, we used data simulators:The muon from observed νµ CC events was eliminated
and replaced by a simulated particle to form a fakeNC event , νe CC event, or ντ CC event.
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NOMAD: Analysis
l Data simulators (continued)l The data simulator is itself simulated by a Monte Carlo to
produce a ÒMonte Carlo simulatorÓ (MCS).
l Efficiencies and backgrounds are then calculated fromthe following formula:
l To first order, all the deficiencies of both the datasimulator and the Monte Carlo cancel in this ratio.
l No data simulator was possible for themode, so it was not used to avoid uncontrolledsystematic errors.
ε ε ε
ε= MC DS
MCS
τ µνν→
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NOMAD: Analysis
l Elimination of self-reference biasl An independent set of data or simulations must be used
to set cuts and form likelihood functions from those used
to evaluate efficiencies and backgrounds.l This can be done in such a way that statistics are not
lost.
l The bias from not eliminating self-reference can be easilyevaluated.
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NOMAD: Analysis
l Blind analysesl Early results with non-blind analyses indicated biases, so
blind analyses were instituted:
l A Òsignal boxÓ was defined near the start of the analysisand it was not permitted to examine data in the box.
l Analyses had to come to their final form and showconsistency with data outside the box and Òwrong signÓdata within the box before permission to open the box wasgiven.
l Everyone in NOMAD becameconvinced that this was the onlyway to do reliable analysis. Mostcollaborations now use blindanalyses for sensitive results.
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NOMAD: Results
l The analysis was done by combining 31 modes and bins.l If every νµ had oscillated to a ντ, we would have observed
14,900 signal events (≡Nτ).
l We actually saw 58 signal events with 55 ± 5 events expectedfrom backgrounds.
l However, 75% of the sensitivity came from low backgroundbins: Nτ = 7,600; 1 signal event observed with eventsexpected from backgrounds.
l At 90% c.l., sin2(2θ) < 4.1 10-4 (sensitivity < 5.2 10-4).
1 3 0 21 6. ..
−+
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NOMAD: νµ → ντExclusion Plot
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NOMAD: νe → ντExclusion Plot
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LSND Beam
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LSND Detector
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LSND Technique
l π+ produced in the target come to rest in the beamdump and decay to muons, which also decay:
l This produces but not Thus, onecan look for oscillations.
π µ ν
ν νµ
µ
+ +
+
→
→ e e
ν ν νµ µ, , ,e νe .
ν νµ → e
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LSND Technique
l But what about the π-s? DonÕt they produce
l Yes, but they are highly suppressed:l 8 times more π+ produced than π-.
l Only 5% of π- decay (in flight). The rest are captured by
the strong interactions and do not produce neutrinos.
l 88% of µ- get captured from atomic orbits producing anm, but not a
l Thus the total suppression is (0.125)(0.05)(0.12) =7.5 10-4.
νes?
νe .
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LSND Technique
l But how does one detect a few in an intensebeam of νes?
l The technique relies on the fact that only caninteract with protons to produce both a neutronand an electron with energy > 20 MeV.
l The signal is a coincidence between an electron(positron) with E > 20 MeV followed by a photonfrom the neutron capture reaction n + p → d + γ.
l Define a likelihood ratio R which measures thecoincidence of the γ to the e+ in position and time.
νes
νes
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LSND: R Distribution
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Neutrino Energy Spectrum
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LSND E Distribution
83 events total-33.7 beam off-16.6 beam on= 32.7 ± 9.2 signal
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LSND Allowed Region
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LSND Decay in Flight Analysis
l 3.4% of π+ decay in flight between the target andthe beam dump, yielding high-energy νµs.
l Signal is a singleelectron with60 < E < 200 MeV.
l 40 events observedwith 21.9 backgroundexpected ⇒18.1 excess events.
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LSND DIF Allowed Region
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KARMEN Experiment
l The KARMEN experiment, located at the ISISneutron spallation source at the Rutherfordlaboratory, is similar to LSND, except that thedetector is only 18 m from the beam stop and thebeam is pulsed at 50 Hz:
Gary Feldman SLAC Summer Institute 14 - 25 August 2000 68
KARMEN Detector
512 modulesof liquid scintiIlator with Gd2O3 for neutron capture
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KARMENð Event Distributions
11 events observed for12.3 ± 0.6backgroundevents expected
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KARMEN Exclusion Plot
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KARMEN Exclusion Plot
Exclusion based ona maximum likelihood fit
1993-95 LSND signal regioncompletely excludedby KARMEN
KARMEN will run 1 more year
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MiniBooNE Layout
l Experiment to check LSND result by detectingνµ → νe oscillations.
l Technique and backgrounds different from LSND.
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MiniBooNE Energy Spectrum
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MiniBooNE Detector
800 T mineral oil445 T fiducial
1280 PMTs signal 240 PMTs veto
Detect Cerenkovrings
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MiniBooNE Sensitivity
MiniBooNEcalculatedsensitivityafter 1 year
Plan to startrunning inDec 2001
MiniBooNE
LSND
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MiniBooNE:How Easy Will This Be?
l To rule out LSND, MiniBooNE needs to havesensitivity << 10-3.
l Backgrounds will be at the level of 5 10-3:l νe contamination from muons 2.3 10-3
l νe contamination from kaons 0.7 10-3
l misidentified νµ CC events 1.0 10-3
l misidentified NC events 1.0 10-3
l 2-length decay pipe only helps with the first ofthese -- the easiest one.
l To be believable, must have a blind analysis.
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CERN I-216 Proposal
l CERN proposal for a two-detector experiment tocheck the LSND result
Same beamline as old CDHS/CHARM experiments.
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CERN I-216 Detector
l Each modulel Fine-grained scintillator-streamer-tube calorimeter
l 20 plate 1-cm Fe tail catcher
l 10 plate 10-cm Fe muon catcher
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CERN I-216 Detector Simulation
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CERN I-216 Sensitivity
Proposedsensitivityafter two years
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CERN I-216 Proposal
l This proposal was rejected by CERN
l Why?
l Apparently CERN is betting that LSND is wrong
l ÒLSND is an American problemÓ
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K2K Layout
l K2K is the first of the long-baseline experiments tostudy the region of the atmospheric oscillationsl KEK is a 12 GeV p synchrotron
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K2K Near Detector
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K2K Energy Spectrum
Muon energymeasured bythe nearscintillatingfiber detector
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K2K Event Classification
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K2K Results
l Through June 2000, K2K sees 43 FC events ofwhich 27 are in the fiducial volume, plus 23 ODevents.
l Results based only on the 27 FC fiducial events.
l No oscillation expectation is 40.3 ± 4.7 events(error is systematic, due mainly to volumeuncertainties and near-far extrapolation error).
l Thus, oscillations observed at 2 σ level.
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Unauthorized K2K Allowed Region
10
15
20
25
30
35
40
45
0 1 2 3 4 5 6 7 8
90% c.l. for sin2(2θ) = 1
Eve
nts
∆ m2 (eV/c2)2
x 10-3
DonÕt blameK2K anddonÕt taketoo seriously
However,almost anexact overlapwith SKallowed region
Gary Feldman SLAC Summer Institute 14 - 25 August 2000 88
K2K Expected Sensitivity
K2K now has about 1/4 of their total expected data
Therefore, expect about 100 FC events
Expect E spectrumanalysis
Enlarged fiducialregion?