The Current Excitement in Neutrino Physics

Transcription

1 The Current Excitement in Neutrino Physics Rob Plunkett Fermi National Accelerator Laboratory Lectures Presented to CTEQ 2007 Part 1

2 Objectives of these Lectures Neutrinos in historical context Modern phenomenology of neutrinos The pioneers solar and atmospheric Terrestrial long-baseline, especially MINOS and NoVA Other special types of Experiments Speculation and future initiatives

3 In the Infancy of Particle Physics Electron - Direct Discovery 1897 J. J. Thomson Nucleus - backscattering 1911 Rutherford CRT Neutron - programmatic experimentation 1932 Chadwick, Joliot-Curies α Au α Geiger Be Wax Neutrino - inferred solution to phenomenology crisis 1930 Pauli Direct discovery only in Reines Indirect, lengthly β decay?

4 The Loneliness of the Neutrino Other particles in the Standard Model of particle physics either have mass or have a symmetry that keeps them massless. Photon is the main example N.B Only 3 neutrinos allowed LEP Data There is no such known symmetry for the neutrinos. In fact, since they are neutral, there are 2 frameworks to acquire mass ( Dirac and Majorana ) - Majorana particles are their own antiparticles. Massless neutrinos were always odd, and are now excluded by experiments.

5 Neutrino participates only in weak interactions Typical cross-sections: Impressive mean free path ~ 10 A.U. of concrete! σ pp ~ 50 mb ~ cm 2 σ top ~ 50 pb ~ cm 2 σ(νn; 1 GeV)~10-38 cm 2 Certainly not unmeasurably small for accelerator, decay neutrinos. Neutrino cross-sections usually decrease at least as fast ~ E ν. Many solar, cosmological relic neutrinos much more difficult. D0 Top Event Neutrino Still often detected simply by its absence!

6 Basics of Neutrino Interactions Charged current (CC) µ d u hadron shower Long events ν W + µ Neutral current (NC) u u hadron shower Short events ν Z 0 e Detector s-eye View NC/CC constant for E ν > 1GeV

7 Neutrino Oscillations I - Two Bases Suppose neutrino mass is diagonal in a basis rotated w.r.t the lepton number states: ν = ν e cos θ ν µ sin θ (Ignore τ for now) 1 + ν = ν µ sin θ + ν cos 2 e θ Similar rotation is well known in particle physics -- for example, neutral K s... K S 1 = ( K 0 K K L = ( K 0 + K 0 2 ) ) Oscillations require at least one massive partner!

8 Complications of 3 families ν l = U ν n, where (c ij cos θ ij, s ij sin θ ij ) c 13 0 s 13 e iδ c 12 s 12 0 U= 0 c 23 s s 12 c s 23 c 23 s 13 e iδ 0 c Atmospheric ν µ ν τ Atmospheric ν e ν µ, ν τ Solar

9 Mixing Matrix very different from standard model quarks

10 Neutrino Oscillations II - Optical Analogy Birefringent crystal - different index of refraction (light speed) for different polarizations Polarization rotates because of differential phase advance ik x E Y R = E n 1 e e ik 1 2 x E X n 2 For neutrinos giving ω ω k = n = β m γ = E c c ν 2 1 m m i i k x R = e = e 2E 2 2 x

11 Rotation of Polarization with Distance ν τ ν 2 ν µ θ = π/4 ν 1 L 2 E = π 2 m ν τ ν µ θ = π/4 ν 1 Amount of rotation determined by admixture in initial state, e.g mixing angle θ P( 2 ν µ ) = 1 sin (2θ )sin k ( x )

12 How to Interpret Oscillation Results Controlled by sin 2 2θ Monte Carlo 1 Controlled by m 2 2 Width determined by statistics, beam detector energy resolution Width largely determined by statistics

13 Neutrino Masses and Cosmology WMAP Data for angular correlations of CMB radiation as function of angle. A large neutrino mass would cause modifications to the shape of galaxy correlation spectrum and its characteristic fluctuations Analysis is done by combining WMAP with galaxy surveys, fit comparing to a simpler model Result for 3 degenerate ν s: m ν < 0.23 ev (95% C.L.) Courtesy WMAP: astro-ph/

14 Direct Mass Limits Experimental Challenge Tritium β-decay Kurie plot K(E) log count rate E 0 -m ν m v > 0 m v = 0 resolution E 0 2 x decays in last ev KATRIN experiment at Karlsruhe (2009) Sensitivity (discovery) 0.35 ev Electron energy ~24 m Main spectrometer tank Uses 40 g of 3 H daily (recycled)

15 Chirality,helicity and pion decay Fermions have chirality (handedness): f L (left), f R (right) For neutrinos, only left-handed ν L participate in standard model: µ - ν L Helicity is spin projection along P r Mass couples f L and f R Seen in π + decay at rest ν ( ) ν L Helicity suppression of S W + For massless ν helicity = chirality S f (+) f ( ) f ) ( ) = fl + ( m / E f R π + + e R + π + π has only (m/e) + e ν + µ ν + e ( )

16 Neutral Particles can be their own antiparticles Example o π = 1 2 ( uu + dd ) Majorana Not its own antiparticle: K o = su K o = su Associated: for fermions, two way to get mass Dirac: m D f R f L f L f R Linear superposition ~m/e for wrong sign Majorana (only for neutral): m M f R f L ν L ν R Note: Majorana mass conventionally and formally written as: m f C f M L L This is rigorous from field theory point of view, but provides little physical insight.

17 Sidebar: Majorana Mass allows special Double beta-decay with no neutrino. e e p ν R m ν E ν L p Reactions: n n p + e + ν + n p + e ν L n R Positive-helicity has O(m/E) left-chirality component Required for second interaction, suppresses rate Add in U s ; get: A mu 2 ~ i ei i Implcations for experiments from: a) Mass Hierarchy b) Absolute value of lowest mass

18 Majorana Mass and the See-Saw Why is m ν so small, e.g. compared to m u or m d? Assume both L and R neutrinos and summarize their mass matrix as M = µ m D m D M ~ operating on R-L Dirac R-R Majorana L R m heavy Eigenvalues if m = 0 are simple (GUT scale) λ M M ± ( 2 2 M 4 ) 2 m D m 1/ 2 light (EW/GUT scale) Heavy makes light ( See-Saw ) Plausible in GUT context m M 2 D

19 Status of our Knowledge Know to some extent Don t know O. Mena and S. Parke, hepph/

20 Hints of Oscillations #1 - Neutrinos from the Sun Our sun is a plentiful source of low energy electron neutrinos. Principal reactions are + p + p d + e Be + e Li +ν B 8 Be e +ν v E max Rel. Flux (error) 0.42 MeV 1.0 (1%) 0.86 MeV 0.1 (10%) MeV 10-4 (20%) Fluxes are constrained with solar data - Especially pp, using solar sound velocities

21 SNO- Sudbury, Ont. SNO and KamLand 1 kton of D 2 O 2100 m below surface Charged (CC) and Neutral Current (NC ν ν e e + d + d p+ n+ p+ e p+ ν e KamLand - Japan 1 kton of liquid scintillator looks at total Japanese reactor flux ν e + p n + e +

22 Solar Oscillations are due to interactions within the sun e W e Matter effects from ν ν e W-exchange reaction ν = ν 2 > Inside sun Outside sun Neutrino follows adiabatic trajectory on levelcrossing diagram. Exits sun as mass eigenstate, no more oscillation.

23 Solar Oscillations - Result from Fitting all Data sin 2 2θ

24 Oscillations #2 - Atmospheric Neutrinos Source is primary cosmic radiation plus basic hadronic decay chain. Expect ν µ /ν e ~ 2 Prefer to look at ratio of ratios to control small model-dependencies R double = N data ν µ sim ν µ N N N data ν e sim ν e

25 Super Kamiokande - a Second Generation Underground Experiment 50 kiloton water interaction volume (22.5 fiducial) More than 11, PMT s detect Cerenkov light from recoil electrons and muons. Neutrino interaction rate - about 1/90 minutes.

26 Super K Results no-oscillation expectation No big effect for electrons Zenith angle effect SEEN Interpretation: Oscillations with ν µ > ν τ dominant.

27 Interlude - the physics of GeV Neutrino Interactions NUANCE MINOS, NuMI K2K, MiniBooNE, NOvA T2K Super-K atmospheric ν Complex physics modeled as a combination of low-multiplicity processes

28 Pions are produced via intermediate resonant states Resonances as observed in πn scattering experiments (Figure from Perkins, Introduction to High Energy Physics

29 Different models have underlying physics in common Quasi-elastic Resonant π production A π + A Coherent production DIS X Different models combine channels differently. e.g. NUANCE - coherent addition of resonances NEUGEN - incoherent addition

30 Existing data not strongly constraining Coherent, ν bar data basically nonexistent in region of interest.

31 Parameters in the models are tuned to exclusive channel cross sections. σ i tot = σ QE + ( σ res + i f σ i i DIS ) Charged Current Single Pion Production σ (10-38 cm 2 ) A ν µ p µ p π E ν (GeV) σ (10-38 cm 2 ) B σ (10-38 cm 2 ) 0.8 ν µ n µ n π + Cν µ n µ p π E ν (GeV) E ν (GeV) Examples from NEUGEN courtesy H. Gallagher

32 Worldwide effort to improve knowledge NOMAD, 12 C (R. Petti, NuInt05) (K2K, hep-ex/ ) K2K, coherent π+ MiniBoone single π (Monroe,Wasco) Plots courtesy G. Zeller, NOvE-06

33 MINERvA, a fine-grained neutrino scattering experiment EM, hadronic calorimetry Active Scintillator Nuclear Targets Precision study of ν - nucleus scattering. Important for minimizing systematic errors of neutrino oscillation experiments To be located just upstream of MINOS Near Near Detector High-granularity, fully-active (~6T) scintillator strip based design. ~1 T of nuclear targets (C,Fe,Pb) form first detector section.

34 Example of MINERνA s Analysis Potential Coherent Pion Production Some points are NC data rescaled to CC

35 MINOS Long-Baseline Experiment Study the ν µ ν τ oscillation hypothesis, by measuring precisely m 2 32 and sin 2 2θ 23 Search for ν µ ν e oscillations Constrain contributions of exotic phenomena e.g Sterile ν or ν decay Compare ν, ν oscillations Test of CPT violation Atmospheric neutrino oscillations Phys. Rev. D73, (2006) Far Detector: 5400 tons Near Detector: 980 tons Detector 2 2 Detector km

36 The MINOS Collaboration Athens Cambridge College de France Oxford Rutherford Sussex UCL Argonne Benedictine Brookhaven Caltech Fermilab Harvard IIT Indiana Minnesota Minnesota-Duluth Pittsburgh South Carolina Stanford Texas-Austin Texas A&M Tufts UNICAMP/Sao Paolo William & Mary Wisconsin 24 Universities, 3 National Laboratories, 5 Countries

37 Two-detector Disappearance Experiment Study atmospheric scale ν µ ν τ m 2 and sin 2 2θ 23 Greatly improve existing measurement; excellent test against alternative hypotheses Sensitivity to ν e appearance discussed later

38 Real Elements of NuMI Beamline decay region FODO Quadrupole pair Bending magnets NuMI Pretarget final transport

39 Completed NuMI Decay Tunnel 6m diameter excavated via Tunnel Boring Machine Descends at 3.3 degrees 6% Now filled with decay pipe, shielding, and access passageway.

40 Secondary Particle Production π and K production in the target are the ultimate source of neutrino flux Knowledge and understanding of this is an important systematic for oscillation experiments. Two types of modeling (using experimental data as input): -Hadronic cascade Monte Carlos FLUKA, MARS, GEANT Tend to be black boxes Hard to factorize errors Parametrized Simulations Example for these lectures: BMPT Provide the experimenter with functions, errors

41 Experimental data compared to NuMI data range Note poorly sampled tail region. LE10/185 ka Beam HE Beam Atherton 400 GeV/c p-be Barton 100 GeV/c p-c SPY 450 GeV/c p-be Data with two NuMI beam settings Notes: in parametrizations, data may use scaling variables xf 2 p * L / x R E * / E * max s These two very similar, especially for π s x p / lab lab p incident

42 Example of BMPT parametrization - K s Invariant cross-section under p L > γp L E d d σ = p 3 3 BMPT parametrization A(1 x R ) α x β R G( x R, p T ) e a ( x R ) p T Notes: Limited p T! Cutoff at high x Enhancement at low x

43 Example of Secondary Focusing: Hyperbolic Horn Old technology - form pulsed sheets of current in a cylindrical geometry. 2 L r return conductor I = Current B 1/ r Bdl r between conductors lens! r Many variations on this shape have been made, with different momentum acceptances HIGH CURRENTS - e.g. NuMI horn uses 180 ka for ~2 ms.

44 NuMI Horns Outer Conductor Stripline π + Inner Conductor Β Ι Focus π + toward decay pipe Horn 2 inner conductors Horn installation

45 NuMI Beamline Layout 120 GeV primary Main Injector beam Target readily movable in beam direction 2-horn beam adjusts for variable energy ranges 675 meter decay pipe for π decay

46 Neutrino intensity depends on beam power Counter-intuitive because fewer incident particles! NuMI simulation As primary beam energy increases, more low momentum particles are brought within the focussing zone

47 Soudan Underground Operated by U. of Minn. and Minnesota Dept. of Natural Resources Soudan Mine -tourist attraction during summer months 1 elevator shaft limits loads to 1m x 2m x 9m Laboratory

48 The MINOS Far Detector 8m Octagonal Tracking Calorimeter 486 layers of 2.54cm magnetized Fe plates 2 sections, each 15m long 4.1cm wide solid scintillator strips with WLS fiber readout (both ends). Hamamatsu M16 multi-anode PMT readout Veto shield against entering cosmic ray muons

49 MINOS Near Detector - slightly different B 280 single steel plates, shorter modules Calorimeter (1st 3/7 - logically Veto, Target, Hadron Absorber is partially instrumented except for 1/5 of planes with full coverage Muon Spectrometer section has only every 5 th plane instrumented Magnet coil provides <B> ~ 1.3 T Near electronics optimized for high occupancy (~20) during 10 µs spill

50 Large event rate in near detector A spill (10 µs) in near detector Individual Events Beam arrives in 10 µs batches Multiple events separated by timing, topology. Relative timing greatly simplifies event identification at far detector

51 MINOS Reconstructed Track Angle Beam going down Near Detector y z x Beam going up Far Detector, using longer events to get best angular resolution.

52 Minos Far Detector Events Contained CC event Expected rate ~3/day Far Detector triggers on spill time (50 µs window), also activity triggers Further require fiducial, angle cuts Estimate ~9% NC background, <1 cosmic event

53 Predicting the Spectrum Events/GeV/10 16 POT Reconstructed E ν (GeV) Event Classification Parameter MINOS No oscillations Best fit Near Detector Spectrum vs. simulation phe/200ka Likelihood based on event length, pulse height in track Use near detector data, beam kinematics to predict far spectrum

54 MINOS Results FD neutrino spectrum and ratios MINOS MINOS NC Subtracted m 2 32 = sin 2θ = (stat + syst) (stat + syst) 3 ev 2 Data Sample FD Data Expected (Matrix Method; Unoscillated) Data/MC (Matrix Method) Sample Observed Expected Ratio ν µ (<30 GeV) ± ±0.05 ν µ (<10 GeV) ± ±0.05 FD Event totals ν µ (<5 GeV) ± ±0.06

55 MINOS Allowed Region m sin θ = = ev 2

56 Migliozzi-NUFACT 05 CNGS Program at CERN Beam commisioning Higher Energy for τ appearance 4.5 x pot/year For m 2 =2.4x10-3 and maximal mixing expect 16 ν τ CC/kton/year at Gran Sasso

57 Migliozzi-NUFACT 05 CNGS ν τ appearance detectors em shower ICARUS liquid Ar T Tons built so far About 15 events in 5 years. 434 cm OPERA hybrid emulsion detector Fiducial Target Mass ντ signal ( m 2 = 2.4 x 10-3 ev 2 ) Background 1.8 kton 12.8 events 0.8 events Petronzio- NO-VE 06

58 BACKUP SLIDES

59 Beam Energy Variability Best for low m 2 ν µ CC Events in MINOS 5kt detector (2.5 x POT/yr) Low Medium High Example spectra from varying horn positions (same effect from moving target) ~ 1600/yr ~ 4300/yr ~ 9250/yr

60 Detector Assembly at Soudan Steel Welded and modules placed. Plane lifted to vertical 6-8 Planes per week Crane carries plane down the hall for installation! Completed 2003

61 Completion of Buildings, Shafts

62 Status of our Knowledge Know to some extent Don t know O. Mena and S. Parke, hepph/

63 Neutrino Beam 101 (from Sacha Kopp)

64 Near Detector Assembly at Fermilab Scintillator modules Planes assembled above ground, then moved to final location.

65 Groups of 20 or 28 strips are assembled into modules Both ends read out to increase light yield. Detector Technology 4.1 cm x 1 cm Scintillator strips are extruded polystyrene (Itasca Plastic) 1.2 mm Kuraray wavelength shifting fiber fits into groove