Neutrino Experiments: Lecture 3 M. Shaevitz Columbia University

Transcription

1 Neutrino Experiments: Lecture 3 M. Shaevitz Columbia University 1

2 Outline 2 Lecture 1: Experimental Neutrino Physics Neutrino Physics and Interactions Neutrino Mass Experiments Neutrino Sources/Beams and Detectors for Osc. Exp s Lecture 2: The Current Oscillation Results Solar and Kamland Neutrino Results Atmospheric and Accelerator Neutrino Results Global Oscillation Fits Lecture 3: Present and Future Oscillation Experiments The Fly in the Ointment: LSND and MiniBooNE Searches for θ 13 / Mass Hierarchy / CP Violation Current Hints Reactor Experiments Longbaseline experiments Combining Experiments Future Plans for Oscillation Experiments

3 Current Oscillation Summary 3 Ruled out by MiniBooNE (almost) ( ν running; low-e excess)

4 The Fly in the Ointment The LSND Anomaly 4 π + µ + ν µ e + ν e ν µ Oscillations? ν e LSND took data from ,000 Coulombs of protons - L = 30m and 20 < E ν < 53 MeV Saw an excess of: 87.9 ± 22.4 ± 6.0 events. With an oscillation probability of (0.264 ± ± 0.045)%. 3.8 σ evidence for oscillation.

5 LSND Interpretations 5 LSND observed a (~3.8σ) excess of ν e events in a pure ν µ beam: 87.9 ± 22.4 ± 6.0 events Oscillation Probability: ( ν ν ) = (0.264 ± ± 0.045)% P µ e LSND in conjunction with the atmospheric and solar oscillation results needs more than 3 ν s Models developed with 2 sterile ν s or Other new physics models 3+2 models m 5 (Sorel, Conrad, and Shaevitz, PRD 70(2004) (hep-ph/ ) Karagiorgi et al., PRD75(2007) (hep-ph/ )

6 The MiniBooNE Experiment at Fermilab 6 LMC 8GeV Booster magnetic horn and target K + µ+ ν µ π + decay pipe 25 or 50 m absorber? ν µ ν e 450 m dirt detector Proposed in summer 1997,operating since 2002 Goal to confirm or exclude the LSND result - Similar L/E as LSND Different systematics: event signatures and backgrounds different from LSND High statistics: ~ x5 LSND Since August 2002 have collected data: POT ν POT ν

7 ν µ ν e Appearance Search in LSND Region 7 Method: Search for an excess of ν e events over expectation Knowing expectation is key Use observed ν µ events to constrain ν e physics and background In analysis region between 475 < Eν < 3000 MeV, no evidence for oscillation in LSND region Simple 2ν osc excluded at 98% CL Unexpected excess of events at low energy < 475 MeV Phys. Rev. Lett. 98, (2007), arxiv: [hep-ex] Also: Unexplained Excess of Electron-Like Events from a 1 GeV ν Beam, PRL 102, (2009)

8 New ν µ ν e Appearance Results 8 The antineutrino search important because Provides direct tests of LSND ν appearance More information on low-energy excess The backgrounds at low-energy are almost the same for the neutrino and antineutrino data samples. Antineutrino analysis is the same as the neutrino analysis. First antineutrino result has low statistics POT giving about 100K event Inconclusive wrt LSND No indication of ν data-mc excess: MeV: -0.5 ± 11.7 events MeV: 3.2 ± 10.0 events (arxiv: )

9 Low Energy Excess Models 9 Few standard model explanations and many new physics ideas Many models have equal effects in neutrinos and antineutrinos These models are disfavored by absence of ν e excess.

10 Future Plans and Prospects 10 Will triple the MiniBooNE ν data over the next 2 years Allow better comparison of low-energy excess New MicroBooNE Experiment approved at Fermilab Liquid Argon TPC detector which can address the low-energy excess: Reduced background levels Is excess due to single electron or photon events? Approximately 70-ton fiducial volume detector, located near MiniBooNE (initial data ~2011)

11 The Search for the Little Mixing Angle (θ 13 ), CP Violation, and the Mass Hierarchy 11

12 Oscillations Parameterized by 3x3 Unitary Mixing Matrix 12 ν ν ν e µ τ Flavor Eigenstate = U U U = e1 µ 1 τ 1 U U U e 2 µ 2 τ 2 U e 3 U U e µ 3 τ 3 iδ ν ν ν ( Mixing Matrix ) Mass Eigenstate 3-mixing angles Three mass splittings: m = m m, m = m m, m = m m But only two are independent since only three masses ( µ e ) ( ) If δ 0, then have CP violation P ν ν P ν µ ν e Current Measurements: solar atmospheric m = 8 10 ev, m m = ev iδcp cosθ12 sinθ12 0 cosθ13 0 e sinθ U = sinθ12 cosθ cosθ23 sinθ23 iδcp e sinθ13 0 cosθ 13 0 sinθ23 cosθ 23 Solar: θ 12 ~ 33 Little mixing angle, θ 13 sin 2 2θ 13 < 0.2 at 90% CL Atmospheric: θ 23 ~ 45 (or θ 13 < 13 ) and δ =??

13 CP Violation in Neutrino Oscillations 13 Disappearance measurements cannot see CP violation effect Very, very hard to see CP violation effects in exclusive (appearance) measurements. Only can see CP violation effects if an experiment is sensitive to oscillations involving at least three types of neutrinos. P * * ( ν ) P( ν µ ν e ) = 4 Im( U U U U ) ν µ e µ 1 e1 µ 3 where s ij = P sin ( ν ν ) = P( ν µ µ ) µ µ ν ( δm L 2E) and δm = m m ij All the terms (s 12, s 13, s 23 ) must not be <<1 or effectively becomes only two component oscillation For example, if s 31 0 then s 12 s 23 s 12 + s 31 + s 23 0 To see CP violation must be sensitive to all three neutrino oscillations i.e. the hardest is usually the lowest (solar neutrino) m 2 12 = ev 2 ij i e3 ( s j 12 + s 23 + s 31 )

14 Current Global Fits to Solar, Atmospheric, Accelerator, and Reactor Data 14 θ 13 mixing angle limits

15 Big Questions in Neutrino Oscillations 15 Still missing some information 1. What is ν e component in the ν 3 mass eigenstate? The size of the little mixing angle, θ 13? Only know θ 13 <13 0 θ Is the µ - τ mixing maximal? 35 0 < θ 23 < What is the mass hierarchy? Is the solar pair the most massive or not? 4. What is the absolute mass scale for neutrinos? We only know m 2 values 8 5. Do neutrinos exhibit CP violation, i.e. is δ 0? Normal Hierarchy Inverted Hierarchy

16 What sin 2 2θ 13 Sensitivity Is Needed? 16 Theoretical / Phenomenology Really no solid information or constraints. U ν U CKM Data driven not theory driven field sin 2 2θ 13 could be very small if associated with some symmetry. Models: Simple models do not fit current oscillation data Put in small? perturbations θ 13 = m solar2 / m atmos2 or (..) or (m e /m µ ) (i.e. Altarelli,Feruglio, hep-ph/ )?? sin 2 2θ 13 very small to CHOOZ limit?? Practical / Political Information for next step Need sin 2 2θ 13 > 0.01 to measure neutrino mass hierarchy and CP violation with longbaseline exp s Probably will not embark on expensive (~500M$) project without a clear measurement of sin 2 2θ 13 Competition and Complementarity Proposed experiments have sensitivity in the >sin 2 2θ region Combination of appearance and disappearance may be powerful if comparable sensitivity

17 Predicted Values of θ 13 for Various Models Remember sin 2 2θ 13 = 4 sin 2 θ 13 Carl H. Albright, arxiv: v1 [hep-ph] 28 Mar 2008

18 MINOS ν e Appearance: Hint of θ 13? 18 Best Fit osc signal

19 Other Hints of Non-zero θ ν analysis Atmospheric region from Super-K, K2K, and Chooz Small excess of sub-gev electron-like events if solar δm 2 included in the fit 1 sigma effect Solar and Kamland prefer a different value of θ 12 unless θ 13 >0 1.2 to 1.5 sigma effect MINOS sees a 1.5 sigma excess in ν e appearance search Solar (red) vs Kamland (blue) Be careful, these are regions for sin 2 θ ij!! sin 2 θ 13 = 0.02 sin 2 2θ 13 = 0.08 Cheat Sheet: U 2 e3 = sin 2 θ 13 ~ ½ sin 2 θ µe ~1/4 sin 2 2θ 13 Fogli et al. hep-ph

20 Experimental Methods to Measure the Little Mixing Angle, θ Long-Baseline Accelerators: Appearance (ν µ ν e ) at m ev 2 Look for appearance of ν e in a pure ν µ beam vs. L and E Use near detector to measure background ν e 's (beam and misid) NOνA: <E ν > = 2.3 GeV L = 810 km T2K: <E ν > = 0.7 GeV L = 295 km Reactors: Disappearance ( ν e ν e ) at m ev 2 Look for a change in ν e flux as a function of L and E Look for a non- 1/r 2 behavior of the ν e rate Use near detector to measure the un-oscillated flux Double Chooz: <E ν > = 3.5 MeV L = 1100 m

21 Long-Baseline Accelerator Appearance Experiments 21 Oscillation probability complicated and dependent not only on θ 13 but also: 1. CP violation parameter (δ) 2. Mass hierarchy (sign of m 312 ) 3. Size of sin 2 θ 23 These extra dependencies are both a curse and a blessing Reactor Disappearance Experiments Reactor disappearance measurements provide a straight forward method to measure θ 13 with no dependence on matter effects and CP violation m13l P( νe νe) = 1 sin 2θ13 sin + 4E small terms

22 Reactor Neutrino Experiments 22

23 Reactor Measurements of θ Nuclear reactors are very intense sources of ν e with a well understood spectrum 3 GW ν e /s 700 events / yr / ton at 1500 m away Reactor spectrum peaks at ~3.7 MeV Oscillation Max. for m 2 = ev 2 at L near 1500 m Arbitrary From Bemporad, Gratta and Vogel Observable ν Spectrum Flux Cross Section Observed Events m m 2 = ev 2 Full Mixing E ν (MeV) No Osc E ν (MeV) Disappearance Measurement: Look for small rate deviation from 1/r 2 measured at near and far baselines Counting Experiment Compare events in near and far detector Energy Shape Experiment Compare energy spectrum in near and far detector

24 Reactor Disappearance Oscillation Probability 24 A reactor disappearance experiment provides a straight forward method to measure sin 2 2θ 13 with no dependence on matter effect and CP violation Only complication is associated with the atmospheric and solar m 2 interference terms which is small. Measure θ 13 Measure m 2 12 : Kamland

25 Reactor Measurements of P( ν ν ) e e 25 Past measurements: 2 2 m atm m solar Next: Search for small oscillations at km distance (corresponding to m atm ) m13l 2 2 m12l P( νe νe) 1 sin 2θ13 sin sin 2θ12 sin 4E 4E P ee m = sin 2 13 = 0.04 E ν θ = 3.5MeV ev Distance to reactor (m)

26 Reactor Neutrino Detection 26 Inverse Beta Decay (IBD) Signal Correlated Background Neutrons from cosmic ray muon interactions in rock Fake signal 1) Scattered proton looks like positron 2) Neutron then gets captured

27 Backgrounds for Reactor Disappearance Exp s 27 Backgrounds to the e + - n coincidence signal Uncorrelated Backgrounds ambient radioactivity accidentals cosmogenic neutrons Correlated Backgrounds cosmic rays induce neutrons in the surrounding rock and buffer region of the detector cosmogenic radioactive nuclei that emit delayed neutrons in the detector eg. 8 He (T1/2=119ms) 9 Li (T1/2=178ms)

28 Previous CHOOZ Reactor Experiment 28 CHOOZ Experiment probed this region One detector experiments Major systematic associated with reactor flux (flux) Detectors used liquid scintillator with gadolinium and buffer zones for background reduction Shielding: CHOOZ: 300 mwe Fiducial mass: CHOOZ: 5 1km, 5.7 GW ~2.2 evts/day/ton with bkgnd evts/day/ton ~3600 ν events

29 Experimental Cuts to Isolate IBD Signal CHOOZ Data and Predictions Data Compared to Expectation 29

30 Current Limits on sin 2 θ Best current limit from: CHOOZ (single detector experiment) sin 2 (2θ 13 )<0.2 (sin 2 (θ 13 )<0.05)

31 Upcoming Multi-Detector Reactor Experiments 31

32 Precision Reactor Disappearance Exp. Are Difficult 32 Looking for a small change in the expected rate and/or shape of the observed event Past reactor measurements: 2 2 m atm m solar Chooz How to do better than previous reactor experiments? Reduce systematic uncertainties due to reactor flux and detector Larger detectors Reduce and control backgrounds Use Near/Far Detectors Kamland

33 Two Detector Reactor Experiment 33 ν e ν e ν e Well understood, isotropic source of electron anti-neutrinos neutrinos E ν 8 MeV Oscillations observed as a deficit of ν e ν e ν e ν e 1.0 Probability ν e Unoscillated flux observed here Survival Probability P( νe νe) = 1 sin 2θ13 sin (1.27 m13l / Eν ) sin 2 2θ 13 Distance 1200 to 1800 meters

34 Example Measurement (Double Chooz 3 yrs) 34 Near Far Near Far (norm 1/R 2 ) Far/Near Ratio (norm 1/R 2 )

35 Detector Design Basics Homogenous Volume 35 Viewed by PMT s Coverage of 10% or better Gadolinium Loaded, Liquid Scintillator Target Enhances neutron capture Extra scintillator region to capture gammas that might leak out from Gd target region Multi-layer, layer, high efficiency veto system Pure Mineral Oil Buffer To shield the scintillator from radioactivity in the PMT glass.

36 Fast neutrons Veto µ s s and shield neutrons μ μ capture Veto Background Events Veto μ 9 Li and 8 He Produced by a few cosmic ray muons through spallation Large fraction decay giving a correlated β+n 36 Recoil p n from µ capture Gd KamLAND Data Gd n capture on Gd Recoil p Spallation fast neutron A few second veto after every muon that deposits more than 2 GeV in the detector may be able to reduce this rate.

37 Reactor θ 13 Projects 37 Double Chooz RENO Daya Bay

38 Double Chooz Reactor Experiment 38 in Ardennes, France

39 Systematic uncertainties 39 Chooz Double-Chooz ν flux and σ 1.9 % <0.1 % Reactorinduced Reactor power 0.7 % <0.1 % Two identical detectors, Low bkg Energy per fission 0.6 % <0.1 % Solid angle 0.3 % <0.1 % Distance 10 cm + monitor core barycenter Volume 0.3 % 0.2 % Precise control of detector filling Detector - induced Density H/C ratio & Gd concentration 0.3 % 1.2 % <0.1 % <0.1 % Accurate T control (near/far) Same scintillator batch + Stability Spatial effects 1.0 % <0.1 % Identical detectors and monitoring Live time % Special electronic systems and monitoring Analysis From 7 to 3 cuts 1.5 % % Simplified cuts due to detector design Total 2.7 % < 0.6 %

40 Daya Bay Experiment 40

41 Reactor Experiment for Neutrino Oscillations at YoungGwang in Korea 41

42 Expected Sensitivities 42

43 Sensitivity Estimates for θ 13 vs Time 43 From Mauro Mezzetto NT 2009

44 Longbaseline ν e Appearance Experiments 44

45 Long-Baseline Accelerator Appearance 45 Oscillation probability dependent not only on mixing angles but also: 1. CP violation parameter (δ) 2. Mass hierarchy (sign of m 312 ) 3. Size of sin 2 θ 23 (as opposed to the measured sin 2 2θ 23 ) These are both complications and an opportunity to measure these parameters Use information from other oscillation measurements: reactors, solar/atmospheric/accelerator disappearance Use combinations of appearance measurements for neutrinos and antinuetrinos at different baselines to determine CP δ and mass hierarchy

46 Ambiguities and Correlations in Appearance Measurements 46 Mass Hierarchy Expansion to second order in α and CP Violation ~cosδ Ambiguities due to: 1± 1 sin 2θ Need sin =, not sin 2 2 Sign of m Overall shifts θ23 θ ~sinδ Correlations: CP violation phase δ Ellipse Regions Interference with subdominant m terms 2 12 sin 2 2θ 13 Minakata and Nunokawa, hep-ph/

47 The Curse and the Blessing 47 ( m 2 = 2.5x10-3 ev 2, sin 2 2θ 13 = 0.05) Oscillation probability vs δ CP for T2K and Nova Blue: normal hierarchy Red: inverted hierarchy Solid: neutrino Dashed: antineutrino

48 Upcoming Longbaseline Experiment: T2K and Nova 48 Improved Beams and Near/Far Detectors Much Higher Intensity

49 Use Near Detectors to Measure Beam Flux and Backgrounds 49 T2K Near Detector NOνA Near Detector

50 T2K Experiment 50

51 NOvA Experiment in Minnesota 51

52 Main Backgrounds For Appearance Experiments 52

53 Expected Sensitivity to θ T2K NOνA 5yrs 750 kw Experiments sensitive to sin 2 (2θ 13 ) > 0.008

54 Better Measurements of θ 23 and m Current Measurements: m 2 = 2.43 ± ev 2 sin 2 (2θ 23 ) = 1.00 ± 0.03 T2K Improvements by x3 to x5 NOνA Stat 5yrsOnly 750 kw

55 NOvA + T2K Has Some Sensitivity to Mass Hierarchy (sign m 2 23) 55 Normal Inverted

56 And If One Is Lucky. 56 There are some values of the CP parameter δ that are easier to isolate and measure, i.e. If θ 13 is big enough Mass hierarchy is normal ( m 2 > 0) δ around 3π/2 Then Can observe a hint of CP violation at the 1 sigma level. Need Much Larger Experiments For Measuring CP Violation Super-Beam Exps

57 Future Longbaseline Experiments 57

58 Hyper-K Experiment 58

59 Hyper-K CP Violation Sensitivities kton Liquid Argon Detector

60 Long Baseline Neutrino Experiment at DUSEL 60 Beam Requirements: Large neutrino flux covering 1 st and 2 nd oscillation max points (0.8 and 2.4 GeV) High purity ν µ flux with little ν e contamination Minimize flux with energy above 5 GeV that causes background Run at reduced energy 90 ± 30 GeV but then less flux

61 DUSEL LBNE Experiment and Expectations 61 Baseline experiment: Three 100 kton fiducial water Cherenkov detectors (Each 5 times Super-K) 1 MegaWatt (2.3 MW) 120 GeV beam with plug to reduce high E ν 3 yrs ν + 3 yrs ν of data

62 On-axis Beam May Be Better for DUSEL Exp 62 On-axis beam spans large energy region that allows one to measure the oscillation probability at both the first and second maximum (sin 2 (1.27 m 2 L/E) 1 st Maximum : Gives the neutrino mass hierarchy 2 nd Maximum : Sensitive to CP Violation effects

63 Fermilab to DUSEL Sensitivities 63 NOvA - NOvA+5ktLAr - NOvA+5ktLAr+PX - NOvA+100kt LAr +PX 100ktLAr (OR 500kt WC) +New WBB+PX at DUSEL

64 Final Comments 64 Reactor and longbaseline experiments will be soon providing new information on θ 13 θ 13 is a important physics parameter for modeling ν mixing θ 13 is key for planning future long-baseline experiments to measure CP violation and the mass hierarchy If sin 2 2θ 13 is > ~0.03, T2K and Nova can make important measurements If sin 2 2θ 13 is < ~0.01, need other techniques to access the physics (1 st,2 nd max. measurements; Superbeam exps, Neutrino Factory.) Longbaseline experiments are more complicated but have the promise to give information on the mass hierarchy and CP violation T2K and Nova could give some early hints of these parameters Next generation superbeams will be necessary to make quantitative measurements There is a strong ongoing program of oscillation experiments and serious plans for taking the next step to superbeams Bright future for energetic young physicists to make all this happen

65 Hallelujah! 65 Maybe it was the ν s!