The Neutron Lifetime, the Big Bang and the Spallation Neutron Source. Geoff Greene University of Tennessee / Oak Ridge National Laboratory

Transcription

1 The Neutron Lifetime, the Bi Ban and the Spallation Neutron Source Geoff Greene University of Tennessee / Oak Ride National Laboratory 1

2 Neutron Decay and Bi Ban Nucleosynthesis Measurin the Neutron Lifetime The Spallation Neutron Source (SNS) 2

3 The neutron Lifetime and the Standard Model 3

4 4

5 Theoretical Framework for the Theory of Neutron Decay 5

6 Theoretical Framework for the Theory of Neutron Decay Question: How do we accommodate V-A (parity violation) 6

7 Includin V-A ( Parity Violation ) A Handed Interaction Requires: A VECTOR and An Axial Vector the Push the Twist 7

8 Includin V-A ( Parity Violation ) A Handed Interaction Requires: A VECTOR and An Axial Vector the Push the Twist The relative sin of the vector and axial-vector determines the handedness 8

9 V-A in the Ideal Quark-Lepton Interaction vector operator axial-vector operator H weak = Gweak νe γ µ γ µ γ5 e d γ µ γ µ γ5 u } V-A Vector minus Axial Vector Means a Left-Handed Interaction 9

10 Neutron Decay is a Bit More Complicated The stronly interactin quarks within the neutron modify the relative size of the vector and axial vector couplins But since the stron interaction conserves parity, this does not chane the relative phase (handedness), It only chanes the pitch of the screw H weak = Gweak νe γ µ γ µ γ5 e d V γ µ Aγ µ γ5 u If Neutron Decay is Purely Left-Handed, only two parameters completely describe it 10

11 Theory Summary Our interest in fundamental symmetry violation led us to examine neutron beta decay as a model system If the Standard Model for the Electro-Weak Interaction is correct, only two parameters, A and V, are required. ( A / V can be thouht of as the pitch of the screw) The experimental challene is to determine whether or not all the phenomenoloy in neutron beta decay can consistently be explained by two just parameters. If two parameters are not enouh, somethin is wron. This be an indication of somethin new perhaps an indication of incomplete symmetry breakin! 11

12 Phenomenoloy of Neutron Beta Decay Momentum Must Be Conserved! ν r σ n p v ν e p v e n p v p p 12

13 Phenomenoloy of Neutron Beta Decay Momentum Must Be Conserved! r σ n ν p v ν V-A says that neutrinos are purely Left-Handed with r σ p r = 1 e p v e n p v p p Conservation of linear and anular momentum implies that there are stron correlations between the initial neutron spin and decay particle momenta. 13

14 14 Correlations in Neutron Decay Parity violation implies a rich phenomenoloy in neutron decay. V-A implies that All experimental Quantities can be related to the axial and vector couplin constants A and V υ υ υ υ σ σ τ E B E A E m b E E a 1 ) E F( 1 dw n e e n e e e e e n p p p p = V A 2 V A a 0 b = 2 V A V A 2 V A A + = 2 V A V A 2 V A 1 2 B = V A 2 V 2 A ) 3 ( + Neutron beta decay measurements ive: ) 3 1 /( 2 V 2 A n + τ 0 b = 2 V A V A 2 V A 1 2 B = = V A 2 V A a ) 3 1 /( 2 V 2 A n + τ 2 V A V A 2 V A A + =

15 The FREE neutron is unstable aainst beta-decay τ n 890 s 15

16 Some Theoretical Implications the Neutron Lifetime Cosmoloy: The neutron lifetime sets the time scale over which nucleosynthesis occurs durin the Bi-Ban. The comparison of the neutron lifetime, the cosmoloical He/H (or D/H) ratio, and the number of neutrino species provides a prediction for the Universal Baryon Density. This is a critical component of the Dark Matter Problem. Particle Physics: A comparison between the neutron lifetime and neutron decay correlations provides an important test of the standard model, and provides a an insiht into the oriin of parity violation. Astrophysics: The reaction which provides the dominant source of enery in the Sun (pp fusion) is overned by the same matrix element as neutron decay. The neutron lifetime is a key parameter of the solar models which are involved in the Solar Neutrino Problem 16

17 Important Processes with the same Feynman Diaram as Neutron Decay After D.Dubbers 17

18 Review of Bi-Ban Nucleosynthesis 18

19 The Time Scale for the Bi Ban (1) Time Since Bi Ban T (K) ρ (/cm -3 ) <10-43 s ~10-43 s ~10-35 s ~10-35 s s K K Quantum era Universe consists of soup of leptons & quarks Grand Unification Era Gravity separates from other Grand Unified Forces End of Grand Unification Stron Force breaks symmetry w/ ElectroWeak Force. Inflationary epoch Universe inflates by a factor of or more ( observable Universe expands from size of an atomic nucleus to size of a pea). 19

20 The Time Scale for the Bi Ban (2) Time Since Bi Ban T (K) ρ (/cm -3 ) ~10-12 s ~10-6 s K K Particle Era Electromanetic force and Weak Force break symmetry. Quark Hadron transition. Protons and neutrons (plus antiprotons and anti neutrons) are formed from quarks - at this time the matter particles have an excess of ~one in a billion over antimatter particles. 0.01s K 4 x 10 9 The Universe is expandin rapidly, scale is doublin every 0.02s. As Universe expands it cools, T ~ 1/R. Althouh the temperature is too low for Protons and neutrons to be created from the thermal enery of the early universe reactions such as: ν e + n p + + e - And vice-versa, maintain an equal number of protons and neutrons. As the temperature decreases proton/neutron balance shifts in favor of less massive protons. 1s K 4 x 10 5 Weakly interactin neutrinos decouple from the rest of the Universe 20

21 The Time Scale for the Bi Ban (3) Time Since Bi Ban T (K) ρ (/cm -3 ) 15s 3 min 3 1/2 min 5 x 10 5 yr 10 9 yr 3 x 10 9 K 10 9 K 10 8 K 4000K 4 x Temperature is below threshold for creation of electron/positron pairs e + /e - annihilate: e + + e - γ + γ The Universe is reheated about 35% by annihilation. Era of Nucleosynthesis Nuclei can bein to hold toether, e.. p + n d + γ At this time the baryons are divided into about 87% protons 13% neutrons. End of Nuclear Reactions Neutrons have been used-up formin 4 He Universe is now 80% H nuclei (p + ) & 20% He nuclei Era of Recombination nuclei & electrons recombine to form atoms Universe becomes transparent Era of Galaxy Formation 21

22 p He He He p He He D n He T D p T D D n He D D He D p d n p e p n γ γ ν Some of the reactions in Bi Ban Nucleosynthesis

23 ν s decouple here Temperature (10 9 K) 23

24 The Cosmic He/H Ratio Depends upon three quantities: 1) The Coolin rate of the Universe Given by the heat capacity of the Universe Determined mainly by the number of liht particles (m 1 MeV ) Includes photons, electrons (positrons), neutrinos (x3) 2) The Rate at which Neutrons are decayin The neutron lifetime 3) The rate at which nuclear interactions occur Determined by the the loarithm of the density of nucleons (baryons) 24

25 The Parameters of Bi Ban Nucleosynthesis Y = lo η N ν ( τ n p ) Cosmic Helium Abundance Neutron Lifetime in Minutes Number of Neutrino Flavors Cosmic Baryon Density 25

26 We can invert this line of reasonin. If we measure the Helium Abundance, the Neutron Lifetime, and if we know the Number of liht Neutrinos is 3, We can determine the density of ordinary matter in the universe. lo [ Y ( )]/ η Cosmic Baryon Density 10 = P τ n Cosmic Helium Abundance Neutron Lifetime in Minutes 26

27 The Cosmic Helium Abundance Y P vs the Baryon Density Density of Normal Matter (protons & neutrons) 27

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29 A Brief Diression on the Mass of the Universe From Bi Ban Nucleosynthesis, we conclude that, averaed over the entire universe, there are a few protons per cubic meter. Is this a lot, or this it a little? The only sensible way to answer this question is to respond: Compared to What? 29

30 A Scale for the Density of the Universe From red-shift observations we know that the Universe is in a state of (nearly) uniform expansion. If the density of the Universe is sufficiently hih, this expansion will come to a stop and a universal collapse will ensue. If the density of the Universe is sufficiently low, it will expand forever. The critical density of the universe is iven by the Hubble constant H, and the Gravitational constant G We define: ρ = critical 3 H 8π G Ω = ρ / ρ critical 2 30

31 An Upper Limit for Ω If Ω were sufficiently lare, the universe could NOT have attained its apparent ae (1/H) without collapsin. Very simple aruments based on the fact that the universe is here and still expandin today indicate that: Ω total <

32 A Lower Limit for Ω By simply countin the number of stars and alaxies we can et Ω total 32

33 Ω is NOT necessarily constant over time If Ω >1 at any time, Then Ω will continue to row larer with time If Ω <1 at any time, Then Ω will tend toward zero with time Only if Ω = 1 EXACTLY, will it stay constant for all time We observe that Ω is NOW not too far from 1 (0.01 < Ω < 2.5). Thus: Ω was EXTREMELY close to 1 early in the bi ban ( Ω ) This raises the Fine Tunin question: If Ω was so very nearly equal to 1, Isn t it reasonable that it equals 1? For a many compellin reasons (fine tunin, inflation, CMB, ), We stronly believe that Ω = 1 EXACTLY 33

34 The Preposterous Universe ~3% Ordinary Matter ~30% Dark Matter ~67% Dark Enery 34

35 Neutron lifetime provides a measurement of the Cosmic Baryon Density: ρbaryon Ω B ρ critical Ω B = ( 3.3 ± 0.7)% BBN This can be compared with the determination from the Cosmic Microwave Backround: Ω B = ( 2.3 ± 0.1)% CMB The larest uncertainty to the nuclear theory of BBN is the experimental value of the neutron lifetime. 35

36 Determination of the Neutron Lifetime 36

37 Measurement of the Neutron Lifetime Bottle Method An ensemble of ultracold neutrons is confined in a material or manetic bottle. The population in the bottle decreases as: N = N 0 e t τ n Beam Method A detector counts neutron decay products from a well defined volume traversed by a neutron beam with known neutron density. The decay rate in the volume will be: dn dt N = τ n 37

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40 The NIST Pennin Trap Lifetime Experiment 40

41 The NIST Pennin Trap Lifetime Experiment Openin the Trap 41

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43 Error Budet (all entries in seconds) 43

44 Ultracold Neutrons and Neutron Bottles 44

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46 At low eneries S-wave scatterin dominates, phase shift is iven by cot( δ ) b coh b coh critical rane for b coh <0 = 1 kb coh For random well depths and random well size, it is unlikely to obtain a positive coherent scatterin lenth: n = 1 N λ coh 2 b π Index of refraction is therefore <1 for most nuclei * *In the vicinity of A~50 (V,Ti,Mn) nuclear sizes are such that b coh <1 and thus n>1 46

47 Neutron Index of Refraction n 2 2 λ Nbcoh = 1 cosϕ crit = n 2π For sufficiently lare neutron wavelenth, λ, n=0 and cosφ crit =90 This implies that neutrons will be reflected at ALL ANGLES! 47

48 Cold Neutrons Characterized by a thermal velocity comparable to T~20K v 500 m/s E k 5 mev λ 5 Å Cold Neutrons Characterized by a thermal velocity comparable to T~20K v 5 m/s E k 100 nev λ 500 Å Material Bottle V eff 200 nev for Ni, Be, Cu, Manetic Bottle mv 2 2µ n B for B ~ 1 Tesla Gravitational Well mv 2 mh for h ~ 1 meter 48

49 PNPI Rotatin Spherical Bottle τ = ± 3.1 n s [V. Nesvizhevsky, A. Serebrov et al., JETP 75 (1992) 405] 49

50 The Systematic Concern with UCN Bottles Loss rate in Bottle with material walls: 1 τ bottle 1 = τ decay 1 + τ absorb + τ 1 up scatter 1 + τ holes Neutrons can be lost to absorption on walls, up-scatterin on walls, escape throuh aps in the wall. These effects are velocity dependent, not well understood, and depend stronly on trace contamination on walls 50

51 MAMBO Ultracold Neutron Bottle τ = ± 3 n s [W. Mampe et al., NIM A284 (1989) 111] 51

52 Double Bottle with Neutron Loss Monitors τ = ± 0.9 ± 0.4 n stat syst s [S. Arzumanov, L. Bondarenko et al., NIM A 440 (2000) 511] 52

53 Measurements of the Neutron Lifetime 53

54 Measurements of the Neutron Lifetime prior to

55 Current Situation 55

56 What Next 56

57 Improvin the error on the NIST Beam Experiment 57

58 Neutron Detection Usin Absolute Calorimetry 58

59 Improvin the error on the NIST Beam Experiment 59

60 Current Situation Reduce this x2-3 60

61 Manetic Trappin of Ultra Cold Neutrons 61

62 Manetic Trappin of Ultracold Neutrons [P. Huffman, et. al, Nature, 403, no.6765, 2000] S. N. Dzhosyuk, C. E. H. Mattoni, D. N. McKinsey, L. Yan and J. M. Doyle Harvard University P. R. Huffman and A. K. Thompson NIST, Gaithersbur R. Golub HMI, Berlin S. K. Lamoreaux, G. Greene Los Alamos National Laboratory K. J. Coakley NIST, Boulder 62

63 Ioffe-Type Manetic Trap r (cm) Z (cm) Z (cm) fiure courtesy J. Doyle 63

64 Wonderful Property #2 Production of Ultra Cold Neutrons in Superfluid Helium p 2 2m Neutrons of enery E 0.95 mev (11 k or 0.89 nm) can scatter in liquid helium to near rest by emission of a sinle phonon. Upscatterin by absorption of an 11 k phonon is a UCN loss mechanism. But population of 11 K phonons is suppressed by a lare Boltzman Factor: ~ e 11/T where T~200 mk Elementary Excitations in Liquid Helium p Momentum Q ( in nm -1 ) h Golub and Pendlebury (1977) 64

65 Detection of Trapped Neutrons He 2* liquid helium e γ 80nm 430nm TPB Recoil electron creates an ionization track in the helium. Helium ions form excited He 2 * molecules (ns time scale) in both sinlet and triplet states. He 2 * sinlet molecules decay, producin a lare prompt (< 20 ns) emission of extreme ultraviolet (EUV) liht. EUV liht (80 nm) is converted to blue usin the oranic fluor TPB (tetraphenyl butadiene). 65

66 Manet form Racetrack coil Cupronickel tube Trappin reion Solenoid Acrylic lihtuide Beam stop TPB-coated acrylic tube Neutron shieldin Collimator fiure courtesy J.Doyle 66

67 Recent Results From The Harvard/NIST/LANL/HMI Neutron Lifetime Expt. Fiure Courtesy of Paul Huffman 67