Neutrinos From The Sky and Through the Earth

Transcription

1 Neutrinos From The Sky and Through the Earth Kate Scholberg, Duke University DNP Meeting, October 2016

2 Neutrino Oscillation Nobel Prize! The fourth Nobel for neutrinos: 1988: neutrino flavor 1995: discovery of the neutrino 2002: solar and supernova neutrinos 2015: neutrino oscillations (and mass)

3 And also: the Breakthrough Prize Recognized also 1300 scientists from 6 collaborations!

4 A mystery in the early part of last century: Radioactive beta decay A nucleus apparently splits into two pieces... it spits off an electron

5 It's actually a neutron inside the nucleus decaying: n p + e Because it's a decay into two particles, if momentum is conserved, the p and e - have specific energies

6 But that's not what's observed! Instead, electrons have many different energies, all less than expected Energy is missing!!

7 Wolfgang Pauli, 1930: "Dear Radioactive Ladies and Gentlemen,...I have hit upon a desperate remedy..."

8 Pauli's solution: invisible particle makes off with the missing energy! Explains the observed electron energies perfectly

9 1933 Enrico Fermi named the NEUTRINO 'Little Neutral One' Zero charge, very small (zero?) mass, interacts weakly Very hard to detect directly!

10 Neutrinos do interact with matter to make a charged particle... but very rarely First neutrinos (from a nuclear reactor) detected in 1956 by Reines & Cowan

11 We now know much more: ~3 ~ ,000 MeV/c 2 Quarks u d c s t b Leptons l Spin 1/2 l Zero charge l 3 flavors (families) ~6 ~100 ~4200 MeV/c MeV/c 2 e µ τ ν ν ν e µ τ l Interact only via weak interaction (& gravity) l Tiny mass (< 1 ev) In the Standard Model of particle physics, neutral partners to the charged leptons

12 Why do neutrinos matter? fundamental particles and interactions astrophysical systems nuclear physics cosmology Neutrinos make up a ~few % of dark matter, but are important in understanding of history of structure formation

13 And in particular: understanding of neutrino parameters may give insight into the origin of MATTER-ANTIMATTER ASYMMETRY = ( b b) = B Mechanism of asymmetry generation not known... charge parity But knowledge of ν properties essential for understanding! CP violation is likely involved: a difference in behavior between a particle and its mirror-inverted antiparticle, observed so far in quarks but not leptons

14 Direct Tests for Neutrino Mass No. of counts m ν = 0 m ν 0 maximum electron energy Energy shared by 3 particles E= mc 2 m ν Electron energy Missing energy at the endpoint of the beta decay spectrum non-zero neutrino mass So far nothing found! Best upper limits: m ν < 1/250,000 m e

15 There's another way of getting at the question of neutrino mass... NEUTRINO OSCILLATIONS Use the wave-like nature of all particles, including neutrinos Quantum mechanics massive neutrinos (as waves) would propagate with different frequencies according to their masses

16 A neutrino may be made of different "mass states" The different mass frequencies can interfere

17 Neutrino Mass and Oscillations How can we learn about neutrino mass? Flavor states related to mass states by a unitary mixing matrix e µ = U e1 U e2 U e3 U µ1 U µ2 U µ3 U 1 U 2 U participate in weak interactions unitary mixing matrix eigenstates of free Hamiltonian f = N i=1 U fi i If mixing matrix is not diagonal, get flavor oscillations as neutrinos propagate (essentially, interference between mass states)

18 Neutrino Interactions with Matter Neutrinos are aloof but not completely unsociable Charged Current (CC) Neutral Current (NC) d u d d W + ν l l - Z 0 ν l + N l ± + N' ν x ν x Produces lepton with flavor corresponding to neutrino flavor Flavor-blind (must have enough energy to make lepton)

19 Two-flavor case f = cos 1 + sin 2 g = sin 1 + cos 2 Propagate a distance L: i (t) = e ie it i (0) e im2 i L/2p i (0) Probability of detecting flavor g at L: P ( f g ) = sin 2 2 sin m 2 L Parameters of nature to measure: θ, Δm 2 =m 12 -m 2 2 E E in GeV L in km Δm 2 in ev 2

20 P ( f g ) = sin 2 2 sin m 2 L E Δm 2 =m 12 -m 2 2 If flavor oscillations are observed, then there must be at least one non-zero mass state * Note: oscillation depends on mass differences, not absolute masses

21 For 2 flavors: P ( f g ) = sin 2 2 sin m 2 L amplitude E wavelength= πe/(1.27δm 2 ) P ( f f ) P ( f g ) Δm 2, sin 2 2θ are the parameters of nature; L, E depend on the experimental setup Distance traveled

22 The Experimental Game l Start with some neutrinos (wild or tame) l Measure (or calculate) flavor composition and energy spectrum l Let them propagate l Measure flavor and energies again Have the flavors and energies changed? If so, does the change follow P ( f g ) = sin 2 2 sin m 2 L E Disappearance: ν's oscillate into 'invisible' flavor e.g. ν e ν µ at ~MeV energies Appearance: directly see new flavor e.g. ν µ ν τ at ~GeV energies?

23 Neutrino oscillation parameter space P ( f g ) = sin 2 2 sin m 2 amplitude L E wavelength= πe/(1.27δm 2 ) allowed region change L/E need experimental statistics

24 Sources of wild neutrinos The Big Bang The Atmosphere (cosmic rays) Super novae AGN's, GRB's mev ev kev MeV GeV TeV PeV EeV Radioactive decay in the Earth The Sun

25 Sources of 'tame' neutrinos Proton accelerators Beta beams Nuclear reactors ev kev MeV GeV TeV Artificial radioactive sources Stopped pion sources Muon storage rings Usually (but not always) better understood...

26 We now have strong evidence for flavor oscillations: In each case, first measurement with wild ν s was confirmed and improved with tame ones SOLAR NEUTRINOS Electron neutrinos from the Sun are disappearing... e µ, e x... now confirmed by a reactor experiment Described by θ 12, Δm 2 12 ATMOSPHERIC NEUTRINOS Muon neutrinos created in cosmic ray showers are disappearing on their way through the Earth µ...now confirmed by beam experiments Described by θ 23, Δm 2 23 P ( f g ) = sin 2 2 sin m 2 L E

27 Atmospheric Neutrinos cosmic ray (p) E~ GeV L~ km π + µ + e + ν µ ν µ νe Absolute flux known to ~15%, but flavor ratio known to ~5% By geometry, expect flux with up-down symmetry above ~1 GeV (no geomagnetic effects)

28 Detecting Neutrinos with Cherenkov Light Charged particles produced in neutrino interactions emit Cherenkov radiation if β>1/n E th = Thresholds (MeV) m 1 1/n 2 e 0.73 µ 150 π 200 p 1350 Angle: cos C = θ C = 42 0 for relativistic particle in water 1 n No. of photons energy loss

29 Water Cherenkov ν Detectors Photons photoelectrons PMT pulses digitize charge, time reconstruct energy, direction, vertex

30 Super-Kamiokande Outer detector: 1889 outwardlooking PMTs Water Cherenkov detector in Mozumi, Japan 32 kton of ultrapure water Inner detector: 11,146 inward-looking PMTs 1 km underground to keep away from cosmic rays

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33 Atmospheric ν's Experimental Strategy: High energy interactions of ν's with nucleons d u W + ν l l - ν e + n e - + p ν e + p e + + n ν µ + n µ - + p ν µ + p µ + + n Tag neutrino flavor by flavor of outgoing lepton ν l + N l ± + N' CHARGED LEPTON Cherenkov light cone

34 Get different patterns in Cherenkov light for e and µ (sim. for other detector types) From Cherenkov cone get angle, infer pathlength

35 Zenith angle distribution 1489 days of SK data e-like µ-like up-going down-going Deficit of ν µ from below (long pathlength)

36 Allowed Parameters P ( f g ) = sin 2 2 sin m 2 L E Δm 2 23, θ 23 2 Δ χ decoherence = ( ) 4.8σ 2 Δ χ decay = ( ) 5.3σ Disappearance consistent with ν µ ν τ Parameters describing disappearance inside this region at 90% C. L.

37 m ev 2 ) at least one neutrino state has mass of at least 0.2 ev electron even tiny mass has profound impact on particle physics, astrophysics, cosmology

38 But there s more! So far have been talking about oscillations of two flavors of neutrinos, which describes atmospheric neutrinos well µ = cos sin sin cos But in fact there are three flavors, and three mass states 2 3 Prob of observing flavor µ Distance traveled e µ 1 A = U e1 U e2 U e3 U µ1 U µ2 U µ3 U 1 U 2 U A Neutrino flavors and masses are mixed and all three interfere... Particle Zoo

39 With three flavors, get more complicated wiggles, of superposed short and long wavelengths: Prob of observing flavor e µ Distance traveled Governed by three mixing angle parameters, θ 12, θ 13, θ 23 and mass differences

40 In the past ~15 years, we ve been teasing out the hums of 3 neutrinos atmospheric solar f = N U fi i i=1 U= c 23 s 23 0 s 23 c 23 c 13 0 s 13 e i s 13 e i 0 c 13 c 12 s 12 0 s 12 c beams reactor

41 And there are more questions, and experiments to address them: What is the absolute mass scale of the neutrino? What is the mass pattern? Do neutrinos violate CP symmetry? Will this help us understand the matter-antimatter asymmetry? Are there new neutrino states? Are neutrinos their own antiparticles?

42 Next-generation long-baseline experiment in the U.S. Going after mass hierarchy, CP violation (+ wild neutrinos, pdk) Deep Underground Neutrino Experiment (DUNE) 40 kton LArTPC in 4850 ft 1300 km baseline New 1.2 MW beam strength is precision event reconstruction

43 Hyper-Kamiokande 379 kton fiducial volume in 2 tanks Beam from J-PARC 295 km away CP violation, atmospheric neutrinos, supernova neutrinos, proton decay,..

44 The past two decades have filled in much of in the three-flavor picture... But still unknowns! mass pattern, CP violation... More to ν physics than oscillations... are neutrinos and antineutrinos really the same particle? What is the absolute mass scale? And how does it all fit in?? beyond the Standard Model, matter-antimatter asymmetry, cosmology... (neutrinos are weird! why so light?) Symmetry magazine Still many interesting years lie ahead!

45 On top of the Super-K tank in 1999 Prof T. Kajita

46 Extras/Backups

47 In fifteen years parameters have been shrunk down many orders of magnitude! atmospheric/ beam neutrinos Described by θ 23, Δm 2 23 solar/reactor neutrinos Described by θ 12, Δm 2 12

48 But there s more! In the standard picture, we have three flavors N3 f = U fi i i=1 Parameterize mixing matrix U as: U= c 23 s 23 0 s 23 c 23 c 13 0 s 13 e i s 13 e i 0 c 13 c 12 s 12 0 s 12 c s ij sin ij,c ij cos ij

49 So where do we stand in the three-flavor picture? f = N U fi i U = c 23 s 23 0 s 23 c 23 c 13 0 s 13 e i s 13 e i 0 c 13 Parameters of Nature 3 masses m 1,m 2,m 3 (2 mass di erences + absolute scale) 3 mixing angles 23, 12, 13 1 CP phase (2 Majorana phases) 1, 2 i=1 c 12 s 12 0 s 12 c e i 1/ e i 2/ s ij sin ij,c ij cos ij signs of the mass differences matter

50 Oscillation probability can be computed straightforwardly: f = N i=1 U fi i m 2 ij m 2 i m 2 j (L in km, E in GeV, m in ev) P ( f! g )= fg 4 X i>j <(U fiu gi U fj U gj)sin 2 (1.27 m 2 ijl/e) ± 2 X i>j =(U fiu gi U fj U gj)sin(2.54 m 2 ijl/e) oscillatory behavior in L and E m 2 23 >> m 2 12 è two frequency scales For appropriate L/E (and U ij ), oscillations decouple, and probability can be described the two-flavor expression P ( f g ) = sin 2 2 sin m 2 L E

51 Long-baseline approach for going after MH and CP Measure transition probabilities for and Change of sign for antineutrinos µ e µ e A. Cervera et al., Nucl. Phys. B 579 (2000) through matter 2 B L sin c 2 23 sin sin 2 AL A 2 P e µ ( e µ) = s 2 23 sin B + J 12 A are small 13 B sin AL 2 sin J c 13 sin 2 12 sin 2 23 sin 2 13 ij m2 ij 13, 12L, 12/ 13 B L 2 cos ± 13 L 2 2E, B A 13, A= 2G F N e Different probabilities as a function of L& E for neutrinos and antineutrinos, depending on: - CP δ - matter density (Earth has electrons, not positrons)

52 A first hint from T2K Joint ν µ, ν e three-flavor fit, overlap region favored including reactor constraint on θ 13 C. Walter, Neutrino 2014 Mild preference for δ ~ -π/2