Transcript LANL Master
Parity Violation in Neutron Spin Rotation Experiments W. M. Snow Physics Department Indiana University/IUCF PAVI06 What is the weak NN interaction? Why measure it? What is the phenomenon of PV spin rotation? Example: spin rotation experiment in n+4He Other possible spin rotation measurements (n+p, n+D) The Weak NN Interaction: What is it? ~1 fm outside=QCD vacuum |N>=|qqq>+|qqqqq>+…=valence+sea quarks+gluons+… interacts through strong NN force, mediated by mesons |m>=|qq>+… Interactions have long (~1 fm) range, QCD conserves parity ~1/100 fm range weak If the quarks are close, the weak interaction can act, which violates parity. Relative weak/strong amplitudes: ~[e2/m2W]/[g2/m2]~10-6 Quark-quark weak interaction induces NN weak interaction Visible using parity violation q-q weak interaction: an “inside-out” probe of strong QCD SM Structure of Low-Energy qq Weak Interaction At low energies Hweak takes a current-current form with charged and neutral weak currents Hweak~GF(Jc Jc + Jn Jn),where Jc ~u(1+ 5)d´ ; Jn ~u(1+ 5)u-d(1+ 5)d-s(1+ 5)s -4sin2w JEM Isospin structure: ∆I = 0, 1, 2 Hweak ∆I = 2~ Jc I = 1 Jc I = 1 , charged currents Hweak ∆I = 1~ Jc I = 1/2 Jc I = 1/2+ Jn I = 0 Jn I = 1 neutral current will dominate the ∆I = 1 channel [unless strange sea quarks contribute]. Hweak ∆I = 0~ Jc I = 0 Jc I = 0+ Jn I = 0 Jn I = 0 + Jn I = 1 Jn I = 1, both charged and neutral currents Hweak is known,can be used to probe QCD Weak qq-> Weak NN: what can we learn? s=1 nonleptonic weak interactions [I=1/2 rule, hyperon decays not understood] Question: is this problem specific to the strange quark, or is it a general feature in the nonleptonic weak interactions of light quarks? To answer, we must look at s=0 nonleptonic weak interactions (u,d quarks) Any such process is dominated by strong interaction->must measure ~1E-7 PV effects Weak NN interaction is one of the few experimentally feasible systems Recent Development: perturbation theory for weak NN interaction [Zhu, Holstein, Ramsey-Musolf, et al,Nucl. Phys. A748 (2005) incorporates chiral symmetry of QCD, Ramsey-Musolf and Page review(hep-ph/0601127): relates coefficients in perturbation theory to experimental weak NN observables in NN and few body systems ] Longer-term goal: calculate coefficients of weak NN operators from QCD on lattice [Bean & Savage] NN PT coefficients and observables (Musolf/Page) EFT coupling (partial wave mixing) np A np P nD A n mt (3S1- 3P1) -0.11 0 -3.56 -2.68 0 -1.07 mt (3S1- 1P1) 0 0.63 -1.39 1.34 0 -0.54 ms0 (1S0- 3P0) 0 -0.16 -0.95 1.8 4k/mN -0.72 ms1 (1S0- 3P0) 0 0 -0.24 -1.2 4k/mN -0.48 ms2 (1S0- 3P0) 0 0.32 1.18 0 4k/mN 0 experiment 0.6 ±2.1 1.8 ±1.8 42 ±38 8 ±14 -0.93 /-1.5 ±0.21 (10-7 ) np pp Az p Az -3.3 ±0.9 Column gives relation between PV observable and weak couplings (A=-0.11mt ) assumes calculations of PV in few body systems are reliable (some need to be done [nD ] or redone/) Weak NN: Relations to Other Areas PV in atomic physics: nuclear anapole moment [seen in 133Cs, searched in Tl, plans in Fr, Yb,…] PV in p-shell and light s-d shell nuclei with parity doublets [lots of good measurements (14N, 18F, 19F, 21Ne), should be calculable given weak NN] PV in heavy nuclei [TRIPLE measurements +theoretical analysis with chaotic nuclear wavefunctions gives prediction for effective weak couplings in heavy nuclei] PV in electron scattering [use Z exchange to extract s portion of proton EM form factors, but need to avoid contamination from q-q weak] Neutrinoless double beta decay [test of EFT treatment for matrix elements of 4-quark operators similar to weak NN] Weak NN: What can be learned with Low Energy (meV) Neutrons? Low energy neutrons: s-wave and p-wave elastic scattering, (n,) For elastic scattering, Az->0 as k ->0 Hard to flip spin quickly for large polarized targets MeV gamma polarimeters are inefficient Easy to flip neutron spin -> 2 classes of experiments: PV spin rotation [~Re(f)] and reactions with inelastic channels [gamma capture] Possible experiments: PV spin rotation in n-p and n-4He (and just maybe n-D?), PV gamma asymmetry in n-p and n-D (next talk D. Bowman) νβ n 2MeV 235U 235U 300K Nuclear reactor ν γ γ β 30K n 235U 3 10 T30K T293K 2 10 1 10 0 10 W(En) n 0.1eV “Slow” Neutrons: MeV to neV n -1 10 -2 10 -3 10 -4 10 UCN -5 10 Very cold Cold ThermEpitherm -6 10 -8 10 -7 10 -6 10 -5 10 -4 10 En [eV] -3 10 -2 10 -1 10 0 10 NIST Cold Neutron Guide Hall Reactor makes neutrons, cooled to ~20K by liquid hydrogen Neutron mirrors (“guides”) conduct the neutrons ~100 meters with small losses. Neutron Optical Potential |k> |K=nk> ei|k> s-wave fscatt=b L <V>=neutron optical potential ~spatially-averaged version of nN potential matter Phase shift =(n-1)kL Index of refraction n=|K|/|k|=√[(Ek-<V>)/Ek] For Ek-<V> negative, neutron reflects from the optical potential c c=√[b/] critical angle Parity Violation in Neutron Spin Rotation Transverse Polarization Medium with Parity Violation φ y z Helicity Components Optical Rotation transversely polarized neutrons corkscrew due to weak NN interaction [opposite helicity components of |>y=1/√2(| >z+ |>z) accumulate different phases from sn·kn term in forward scattering amplitude] PV rotation angle per unit length d/dx related to parity-odd amplitude [=(n-1)kx, n-1=2f/k2, fweak=gk ->d/dx~g] d/dz ~1x10-6 rad/m expected on dimensional grounds Spin rotation in neutron optics n Incoming neutron with spin along y = ŷ : 1 2 z z medium f (0) = f PC f PV ( k ) Coherent forward scattering amplitude for low-energy neutrons: neutron spin k neutron wave vector helicity k Neutron optical phase shift: is different for opposite helicity states PV phase shift is opposite for |+z> and |-z> The PV rotation of the angle of transverse spin is the accumulated phase difference: l 2 = kl(1 2 ( f PC f PV ( k ))) k = PC PV PC = kl1 2 f PC ) 2 k PV = 2lf PV 1 i (PC PV ) e z e i (PC PV ) z 2 PV = = 2 PV = 4lf PV n N-4He Spin Rotation Collaboration C.D. Bass1, B.E. Crawford2, J.M. Dawkins1, K. Gan6, B.R. Heckel5, J.Horton1, C. Huffer1, P.R. Huffman4 ,D. Luo1, D.M. Markoff4, A.M. Micherdzinska1, H.P. Mumm3, J.S. Nico3,A.K. Opper6, E.Sharapov7 ,M.G. Sarsour1, W.M. Snow1, H.E. Swanson5, V. Zhumabekova8 Indiana University / IUCF2 Gettysburg College2 National Institute of Standards and Technology (NIST)3 North Carolina Central University / TUNL4 University of Washington5 George Washington University6 Joint Institute for Nuclear Research, Dubna, Russia7 Al-Farabi Khazakstan National University8 NSF PHY-0457219 n The Background: Magnetic Fields sn pn polarizer sn Bext sn pn PC PV x sn l y z analyzer Parity Violating d/dz: of order 1E-6 rad/meter d/dz of low energy neutron due to Larmour precession in external B field of the Earth: ~1 rad/meter! design of experiment is completely dominated by need to reduce systematic effects from magnetic fields Conceptual Design of Spin Rotation Apparatus n Neutron Spin Rotation apparatus (top view) n Neutron Spectrum and Flux Neutron flux: ~1E+9 neutrons/cm2 sec over 5 cm x 5 cm guide How can neutrons be polarized? B gradients (Stern-Gerlach, sextupole magnets) electromagnetic F=()B B B L Reflection from magnetic mirror: electromagnetic+ strong f=a(strong) +/- a(EM) with | a(strong)|=| a(EM)| f+=2a, f-=0 Transmission through polarized nuclei: strong ≠ - T ≠ T Spin Filter:T=exp[-L] n “Supermirror” Neutron Polarizer Magnet Box 28 cm White Neutron Beam polarized Neutron Beam B Permanent magnet box Plate Curvature Radius ~ 10m Neutrons are polarized through spin-dependent scattering from magnetized mirrors Polarization: ~98% transmission: ~25% n Target/Cryostat Magnetic Shielding n Suppressing the magnetic field ~2nT field in the target region still not good enough (causes rotation ~100 times bigger than pv), and still need to oscillate PV signal ->Target Design n Nonmagnetic Helium Cryostat n Target design: Oscillation of PV Signal A B 3He n-detector mag - PV mag + PV B-F) - PV B-F)+ PV -F - PV -F Analyzer Target Chamber – back position • – Coil F+ PV Target Chamber – front position Polarizer • F z • x Cold Neutron Beam • y • B – A = 2PV Cold Neutron Beam n Target design: Noise Suppression A B 3He n-detector Analyzer Target Chamber – back position – Coil • Target Chamber – front position Polarizer • Cold Neutron Beam B – A = 2PV x̂ Output Coil and Polarization Analyzer x̂ ŷ B B ẑ sin = N+-NN++N- Output coil adiabatically rotates neutron spin by +/- /2, conserving component of neutron spin along B, flipped at 1 Hz n Segmented 3He ionization chamber signal plates full voltage plates HV plate window half voltage rings 3He and Ar gas mixture Neutrons detected through n+3He g 3H+1H High voltage and grounded charge-collecting plates produce a current proportional to the neutron flux 4 Detection Regions along beam axis velocity separation (1/v absorption) S.D.Penn et al. [NIM A457 332-37 (2001)] charge collection plates are divided into 4 quadrants (3" diam) separated L/R and U/D beam Systematic Effects in PV Spin Rotation Associated with residual longitudinal B fields (~ 10 nT ) effect – He diamagnetism – He optical potential – small angle/inelastic – scattering – Internal fields in pi-coil estimated size (B~10 nT) (BHe ≠ Bvac) (VHe ≠ Vvac) B1 d 1 B2 d 2 v1 v2 ~10-8 rad/m ~10-8 rad/m ~10-8 rad/m PV spin rotation independent of neutron energy, B rotation depends on neutron energy At NIST we will amplify systematic effects from target scattering by increasing B to~1 T in separate measurements. The Spallation Neutron Source at ORNL $1.4B--1GeV protons at 2MW, ready in ~2007 (first neutrons recently! Short (~1 usec) proton pulse– mainly for high TOF resolution Will be brightest spallation neutron source (also JSNS@JPARC) SNS Fundamental Neutron Physics Beam 30-50 m ballistic guide n Summary and plans for PV n spin rotation 1. Apparatus at NIST now, beam/apparatus tests 2. Previous attempt in 1996 ( PV(n,) = ( 8.0 ±14 (stat) ± 2.2 (syst) ) 10-7 rad/m, D. Markoff thesis) 3. NIST goal: PV=310-7 rad/m (3 months) + ~1 month of systematic study. This sensitivity will improve our knowledge of NN weak interaction 4. Letter of Intent approved to perform PV neutron spin rotation in LHe at SNS (sensitivity goal PV=110-7 rad/m in 12 months) 5. n-p spin rotation possible at SNS in a ~20 cm liquid parahydrogen target 6. n-D spin rotation possible at ILL/FRM in a few cm liquid orthodeuterium target Status, Near-Term Goals for ∆I = 0, 1 Weak NN n-4He spin rotation in terms of weak couplings (10-6rad/m, Dmitriev n-4He orthogonal to 133Cs, p-4He n+p->D+ asymmetry determines f = f Desplanques) plot: =3E-7 rad/m, =5E-9 n Neutron Spin Rotation apparatus- Filters 1) Small wavelength cut-off filer: Be-Bi Polycrystalline Beryllium => when neutron =< 2dmax (dmax – maximum interplanar spacing in the Be), neutrons will be scattered out of beam by diffraction. Neutrons with > 2dmax do not diffract, will pass through. Bismuth crystal – stops gammas. Both filters needs to be cooled to LN temperature. (We have 4” of Be and 4” of Bi) 1) Long wavelength cut-off filter: stack of Ni/Ti supermirrors deposited on~100 Si wafers. c = 3* c(Ni); c(Ni) = 21 mrad/nm and oriented at a small angle Supermirrors Incident beam Transmitted beam (<) (2< < 2 Absorber (>) Reflected beam Flight of the neutron Top view: MAGNETIC SHIELDING PI-COIL L R SM FRONT TARGET BACK TARGET FLIPPING COIL Side view: L R Supermirror polarizer passes neutrons with vertical “up” spin Typical polarization 98% SM Flight of the neutron Top view: MAGNETIC SHIELDING PI-COIL L R SM FRONT TARGET BACK TARGET y FLIPPING COIL SM L R Side View x Both neutrons experience Larmour precession about residual B-fields Right neutron experiences additional rotation due to weak interaction with LHe Top view: Flight of the neutron MAGNETIC SHIELDING PI-COIL L R SM FRONT TARGET BACK TARGET FLIPPING COIL y L R Side view: x -Coil Larmour precesses the neutron spin 180-degrees about the vertical axis SM Flight of the neutron Top view: MAGNETIC SHIELDING PI-COIL L R SM FRONT TARGET BACK TARGET FLIPPING COIL SM y R L Side view: x Both neutrons experience Larmour precession about residual B-fields Left neutron experiences additional rotation due to weak interaction with LHe Top view: Flight of the neutron MAGNETIC SHIELDING PI-COIL L R SM FRONT TARGET BACK TARGET FLIPPING COIL y Side view: or: x 1. 2. Transverse component of spin rotated into a horizontal B-field Spins then rotated into a vertical direction (same as ASM) (the direction of rotation into vertical reverses at 1Hz rate) SM Top view: Flight of the neutron MAGNETIC SHIELDING PI-COIL L R SM FRONT TARGET BACK TARGET FLIPPING COIL SM Neutron spins are either parallel or antiparallel to the ASM Parallel spins pass through ASM and enter 3He Ion Chamber detector Asymmetry of count rate for flipping coil states & target states yields spin rotation P A sin = N N N N n NIST Center for Neutron Research (NCNR) Eight cold neutron guides, two for fundamental physics (NG-6, NG-7) Reactor 20 MW (fission neutrons) Moderation D2O Thermal neutrons Moderation LH Cold neutrons E<5 meV; T=20K wavelength ( ~ few A Cold neutrons are conducted by neutron mirrors (guides) over 80 – 100m with small losses to experimental areas. Signal Modulation via Target Motion Plus Magnetic Field Precession (Top View) BKG PNC BKG + PNC n-detectors Target is split into 2 sections A,B along neutron beam with Analyzer “ coil” between 3He PNC PNC • Target Chamber Front full: +PNC in A, Back Position + -> - in coil, B - Coil empty Target Chamber Back full: 0 in A, 0 in Front Position coil, +PNC in B PNC • Cold Neutron Beam Polarizer Difference=2 PNC Theory needed at nucleon and quark levels Nucleon level: need new calculations of relationship between NN P-odd observables and NN weak couplings, especially for n+D->3H+g asymmetry, n+4He spin rotation, n+D spin rotation This should be doable: add VPNC as perturbation to strong calculation in potential models, EFT approaches,… Quark level: need to calculate weak couplings in QCD models, use measured couplings to discriminate, or lattice+chiral extrapolation Models will need to treat q-q correlations in nucleon correctly in strong regime to get the physics right n Cold Neutron Guide Hall at NCNR n Scientific Impact of Measurement of Weak NN Couplings Parts of the weak NN interaction needed for nuclear and atomic systems will essentially be determined (hr2 missed) New probe of nuclear structure using existing PV measurements in medium and heavy nuclei Resolve present inconsistencies in data from previous experiments Test internal consistency of meson exchange model and/or cPT Ambitious but foreseeable goal for a SM+QCD calculation (lattice gauge theory+chiral extrapolation) Sensitivity to aspects of QCD dynamics (qq correlations) which cannot be “faked” by single-particle model approaches Executive Committee of SNS Instrument Development Team (IDT) David Bowman, LANL Vince Cianciolo, ORNL Martin Cooper, LANL John Doyle, Harvard Chris Gould, NC State/TUNL Geoff Greene, Tennessee/ORNL Paul Huffman, NC State/TUNL Mike Snow, Indiana/IUCF, chair SNS: open user facility, peer review, scheduling Join the IDT! www.phy.ornl.gov/nuclear/ neutrons QCD vs Electroweak: which is more “fundamental” ? LQCD=-1/4 F F+q(iD-m)q (QCD=0) Strong Lagrangian LEW= LV + LF+ LHiggs+ Lint LV = =-1/4 F F+mW2(W+2+ W-2)/2 +mZ2 Z2/2 LF= q(i∂-m)q Lhiggs= ∂µ ∂µ -mH22 Electroweak Lint= LVF + LHV+ LHF+ LHH Lagrangian LVF =g/2(W+µ J+µ +W-µ J-µ) +e Aµ Jµ,EM+g/cosW ZµJµ,neut LHV=[mW2(W+2+ W-2)/2+mZ2 Z2/2] /(1+ /2) LHF= qMq / LHH=- 3/6 - 4/24 3He ionization chamber neutron detector signal plates full voltage plates HV plate window half voltage rings Neutrons detected through the following reaction: n+3He g 3H+1H Charged reaction-products ionize the gas mixture High voltage and grounded charge-collecting plates produce a current proportional to the neutron flux S.D.Penn et al. [NIM A457 332-37 (2001)] n What We Have Done Before Segmented Ionization Chamber Detector for n-4He ORIGINAL DESIGN Ionization Chamber 3He and Ar gas mixture 4 Detection Regions along beam axis velocity separation (1/v absorption) Gas pressure so that transverse range of the proton < 0.3 cm Note region size increases for approximately equal countrates: 30% of beam in regions 1, 2, 3+4 n What We Have Done Before Segmented Ionization Chamber Detector for n-4He ORIGINAL DESIGN Ionization Chamber n + 3He → p + t Collect charged proton and triton on charge collection plates. Divide charge collection plates into 4 quadrants (3" diam) separated L/R and U/D beam n What We Have Done Before Segmented Ionization Chamber Detector for n-4He Penn et al. NIM 457, 332 (2001) (Includes DM Markoff in author list.) 0.5 atm 3He, 3 atm Ar gas mixture 4 detection regions along axis 4 quadrants per region 16 channels with coarse position sensitivity and large energy bins Count rate: 107 n/sec – current mode ~ 7×105 n/sec/channel (1996 digital picture shows 4-region, quadrant detector) (Allows measurement of rotations from magnetic fields ~ 40 G) qq weak processes hidden in the weak NN vertex “factorization” amplitude [<N| Hweak|Nm> ~<0|qq|m><N|q 5q|N>] P-odd admixture into N Valence quarks DDH sea quarks +… Non-negligible amplitudes from sea quarks possible Sign cancellations among different contributions Renormalization group calculations evolve from weak scale to strong scale What might the Weak NN Interaction do for QCD? QCD |vacuum>: 2 (distinct?) phenomena: Chiral symmetry breaking +quark confinement s p mq~few MeV p s <>=0 mqeff~300 MeV =helicity-flip process Chiral symmetry breaking seems to dominate ground-state dynamics of light hadrons such as protons and neutrons Mechanism of chiral symmetry breaking in QCD |vacuum> is not (yet?) understood (instantons?) but it can induce q-q correlations For strong QCD physics we need to understand q-q correlations) weak qq interaction range~1/100 size of nucleon-> sensitive to shortrange part of q-q correlations, an “inside-out” probe Meson Exchange Model (DDH) and other QCD Models assumes , , and exchange dominate the low energy PNC NN potential as they do for strong NN Weak meson-nucleon couplings f , h0, h1, h2, h0, h1 to be determined by experiment N PV N PC Barton’s theorem [CP invariance forbids coupling between S=0 neutral mesons and on-shell nucleons] -> pp parity violation blind to weak pion exchange [need np system to probe Hweak ∆I = 1] fnow calculated to be ~3E-7 with QCD sum rules (Hwang+Henley, Lobov) and SU(3) soliton model (Meissner+Weigel), calculation in chiral quark model in progress (Lee et al).