Detection of the mode: NOMAD - Harvard Physicsusers.physics.harvard.edu/~feldman/Lecture_2.pdf ·...

Gary Feldman SLAC Summer Institute 14 - 25 August 2000 43

NOMAD: τ Detection by Kinematics

l Detection of the mode: τ νν→ e


NOMAD: τ Detection by Kinematics

l Detection of the τ → h ν + n (π0) modes:


NOMAD: Detector

l ÒAn electronic bubble chamberÓ


NOMAD: νµ CC Event


NOMAD: νe CC Event


NOMAD: Analysis

l Extensive use of likelihood functions

5+ independentkinematic variables.Using likelihoodfunctions yieldsoptimum signal tobackground ratios.

Example fromtheanalysis

τ νν→ e


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.


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→


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.


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.


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. ..

−+


NOMAD: νµ → ντExclusion Plot


NOMAD: νe → ντExclusion Plot


LSND Beam


LSND Detector


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


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 .


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


LSND: R Distribution


Neutrino Energy Spectrum


LSND E Distribution

83 events total-33.7 beam off-16.6 beam on= 32.7 ± 9.2 signal


LSND Allowed Region


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.


LSND DIF Allowed Region


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:


KARMEN Detector

512 modulesof liquid scintiIlator with Gd2O3 for neutron capture


KARMENð Event Distributions

11 events observed for12.3 ± 0.6backgroundevents expected


KARMEN Exclusion Plot


KARMEN Exclusion Plot

Exclusion based ona maximum likelihood fit

1993-95 LSND signal regioncompletely excludedby KARMEN

KARMEN will run 1 more year


MiniBooNE Layout

l Experiment to check LSND result by detectingνµ → νe oscillations.

l Technique and backgrounds different from LSND.


MiniBooNE Energy Spectrum


MiniBooNE Detector

800 T mineral oil445 T fiducial

1280 PMTs signal 240 PMTs veto

Detect Cerenkovrings


MiniBooNE Sensitivity

MiniBooNEcalculatedsensitivityafter 1 year

Plan to startrunning inDec 2001

MiniBooNE

LSND


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.


CERN I-216 Proposal

l CERN proposal for a two-detector experiment tocheck the LSND result

Same beamline as old CDHS/CHARM experiments.


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


CERN I-216 Detector Simulation


CERN I-216 Sensitivity

Proposedsensitivityafter two years


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


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


K2K Near Detector


K2K Energy Spectrum

Muon energymeasured bythe nearscintillatingfiber detector


K2K Event Classification


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.


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


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?

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