in the Neutron Decay Direct Search for Scalar and Tensor Couplings - - PowerPoint PPT Presentation

Transverse Electron Polarization in the Neutron Decay

Direct Search for Scalar and Tensor Couplings in Weak Interaction

Kazimierz Bodek for nTRV Collaboration

Institute of Physics, Jagiellonian University, Cracow, Poland

• K. Bodek, PANIC11

2

e pe p Pp Jn

Electron polarization in -decay

n e e e e n e e e e n

( | ) 1 ...

e e

E dE d dE d G E R J N E                                   J J p p

 R-coefficient can be obtained from the transverse electron polarization component perpendicular to the plane spanned by the neutron polarization and electron momentum  N-coefficient can be deduced from the transverse electron polarization component contained in the plane parallel to the parent polarization  G-coefficient can be deduced from the longitudinal electron polarization component

J.D. Jackson et al., Phys. Rev. 106, 517 (1957)

• K. Bodek, PANIC11

3

Electron polarization in -decay

 EM contributions are driven by CV and CA couplings and scale with the decay asymmetry parameter A:

' ' 0.2175 Re 0.3350 Re

S S T T SM e V A

m C C C C N A E C C                     

 0.05

' ' 0.2175 Im 0.3350 Im

S S T T SM e V A

m C C C C R A p C C                      

 0.001

2 2 2

1 ' ' 1 Im Im 1 3 1 3

S S T T e V e A

m C C m C C G p C p C                             

P-odd, T-even

   

2 2 2

2 1 2 1 ' ' Re Re 1 3 1 3 1 3

S S T T e V A

m C C C C N E C C                                 

P-even, T-even

   

2 2 2

2 1 2 1 ' ' Im Im 1 3 1 3 1 3

S S T T e V A

m C C C C R p C C                                  

P-odd, T-odd

 Assuming maximal parity violation CV = C’V = Re CV = 1, C’A = CA = Re CA =  and neglecting terms quadratic in CS and CT :

• K. Bodek, PANIC11

4

Scalar and tensor couplings

 SM contributions:

 Mixing phase CKM gives contribution which is 2nd-order in weak interaction: < 10-10  -term contributes through induced NN PVTV interactions: < 10-9

 Candidate models for scalar couplings (at tree-level):

 Charged Higgs exchange  Slepton exchange (R-parity violating super symmetric models)  Vector and scalar leptoquark exchange

 The only candidate model for tree-level tensor contribution (in renormalizable gauge theories) is:

 Scalar leptoquark exchange

• K. Bodek, PANIC11

5

Mott scattering

 Mott scattering:

– Analyzing power caused by spin-orbit force – Parity and time reversal conserving (electromagnetic process) – Sensitive exclusively to the transversal polarization

• K. Bodek, PANIC11

6

FUNSPIN – Polarized Cold Neutron Facility at PSI

10 1 n n

10 s 80 I P %



 

• K. Bodek, PANIC11

7

Mott polarimeter

 Challenges:

• Weak and diffuse decay source
• Electron depolarization in multiple Coulomb scattering
• Low energy electrons (<783 keV)
• High background (n-capture)

MWPC scintillator scintillator Pb-foil Pb-foil

50 cm

 Solutions:

• Tracking of electrons in low-

mass, low-Z MWPCs

• Identification of Mott-

scattering vertex (“V-track”)

• Frequent neutron spin flipping
• “foil-in” and “foil-out”

measurements

• K. Bodek, PANIC11

8

Neutron polarization from decay asymmetry

Pn = 0.776 ± 0.003 2/dof = 1.15

E() cos

• Average neutron polarization

from decay rate asymmetry (“single-track” events)

• Averaging super-mirror

polarimeter data: Pn = 0.87 ± 0.01

• Offset in data well understood

and corrected for (spin flipper related deadtime)

An = 0.1173

              

n n n n n n

( , ) ( , ) ( ) cos ( , ) ( , ) N P N P P A N P N P E

Pn An

• K. Bodek, PANIC11

9

Two-parameter fit

( , ) ( , ) ( ) ( , ) ( , ) ( ) ( ) ( ) ( ) ( ) ( ) n P n P n P n P R N S AP P                            A

F G H

N R A

N = 0.062 ± 0.012 R = 0.004 ± 0.012

A()-AP()F()

 

ˆ ˆ ˆ ˆ ˆ ˆ ˆ ( ) , ( ) , ( )

e e

J p n J n J p          

F G H

– normal to Mott scattering plane

ˆ n

(2)

Single parameter fit

• K. Bodek, PANIC11

10

 

               

       

, , , , , , , , n P n P n P n P n P n P n P n P AP R PS                                    U F H

R  

               

   

 

2 2 2 2

, , , , , , , , ( ) 1 ( ) n P n P n P n P n P n P n P n P PS N A P                                     S G F

N

(3) (4)

Final results

• K. Bodek, PANIC11

11

NSM = (68 ±1)×103 RSM = 0.5×103

Limits on S and T contributions

• K. Bodek, PANIC11

12

• K. Bodek, PANIC11

13

RPV MSSM contributions to n-decay

From 0+0+ From R-coefficient

100 GeV

L

e

m 

R-parity violating super-symmetric contributions to the neutron beta decay Nodoka Yamanaka et al 2010 J. Phys. G: Nucl. Part. Phys. 37 055104

R = (1)2j+3B+L.

• K. Bodek, PANIC11

14

What next?

• K. Bodek, PANIC11

15

Transverse electron polarization a b A B D H L N R S U V

^

SM () FSI ()@ ReS ReT ImS ImT a 0.104793 0.171405† 0.171405† 0.000727 +0.001171 b +0.171405 +0.828595 A 0.117233 0.000923 +0.00142 B +0.987560 0.126422 +0.194539 D +0.000923 0.000923 H +0.060888 0.171405 +0.276198 L 0.000444 +0.171405 0.276198 N +0.068116 0.217582 +0.334815 R +0.000497 0.217582 +0.334815 S 0.001845 +0.217582 0.217582 U 0.217582 +0.217582 V 0.217582 +0.217582

Sensitivity factors for scalar and tensor couplings

† (|CS|2+|C’S|2)/2 instead of ReS and (|CT|2+|C’T|2)/2 instead of ReT, respectively

* Kinematical factor averaged over Ek = (200,783) keV

16

• K. Bodek, PANIC11

@ 1st order, static field, point charge, no recoil

Impact of H, L, N, R, S, U, V measurement with anticipated accuracy of 510-4

 Constraints on real scalar contributions dominated by: – Super-allowed 0+0+ – Correlations in mirror transitions  n-decay correlations could join the game !

17

• K. Bodek, PANIC11

• K. Bodek, PANIC11

18

2nd-generation experiment with CN beam

 General features of the experimental setup:

• Axial polarimeter geometry (instead of planar)
• 2.5 m long beam acceptance
• Multi Wire Drift Chambers (instead of MWPC):
• Hexagonal cell geometry
• x-,y-coordinates from drift time (x = y = 0.5 mm)
• z-coordinate from charge division (z = 1.0 mm)
• Reduced pressure (0.2-0.3 bar)

WORKS !

• K. Lojek at al., NIMA 611 (2009) 284
• Additional background suppression and higher polarization:
• Pulsed beam (?)
• 3He spin filter (?)

 Overall gain factor in the rate of reconstructed V-track events: 20 – 30 (as compared to the present setup)  Expected sensitivity for all coefficients: 5 × 10-4

• K. Bodek, PANIC11

19 0.0 0.5 1.0 m

MWDC

(He+isobutane, 0.2-0.3 bar)

Pb-foil He, 0.2-0.3 bar scintillator CN beam

2nd-generation experiment with CN beam

With detection of electrons and recoil protons…

Mott scattering foil Plastic scintillator MWDC, hexagonal, 5 layers Grounded vacuum window: 6 µm Mylar, reinforced with Kevlar fibers p-e conversion foil

LiF (20nm) + Al (10nm) + 6F6F(100nm),

• 25 kV

Grounded grid e p+ Longitudinal neutron polarization, Axial guiding field B = 0.10.5 mT [S. Hoedl et al., J. Appl. Phys. 99, 084904 (2006)] MWPC, 1 layer

20

• K. Bodek, PANIC11

Electron-proton kinematics

e-p

pe pp p

21

• K. Bodek, PANIC11

Electron-proton kinematics

700 ns 700 ns

100 keV (200 keV for Mott scat.) 100 eV

 Measured electron energy, reconstructed proton flight path and measured proton time-of-flight must match !  Constraints from 3-body kinematics will considerably reduce coincidence time  With 105 decays per second: single rate (per wire) < 1 kHz

22

• K. Bodek, PANIC11

SCINTILLATOR

(TRIGGER)

MWDC MWPC Time-of-Flight dt1 dt2 dt3 dt4 dt5 Ee 1 µs

Quantity

• Exp. Information

Electron momentum Scintillation light & electron track* Proton momentum Time-of-flight & hit position

DAQ

Electron track reconstructed with drift times (x, y) and charge division (z)

*

23

• K. Bodek, PANIC11

Conclusions

 Within 3 months long data taking:

– 3×108 Mott scattered electrons (N, R) – 108 protons in coincidence with Mott scattered electrons (H, L, S, U, V) – 1012 single electrons (A) – 3×1011 e-p coincidences (a, B, D)

 Advantages:

– Complete set of competitive constrains for scalar and tensor contributions from neutron decay alone – In further perspective, a, b, A, B, D correlations with different systematic effects – Systematic study of FSI effects in neutron decay

 Theory challenges:

– Recoil order and electromagnetic corrections for electron spin related correlation coefficients on (at least) 10-4 accuracy level [V. Gudkow et al., PRC 73 035501 (2006);

J.L. Garcia-Luna et al., J. Phys. G: Nucl. Part. Phys. 32 (2006) 333–344]

– Deeper analysis of electron spin related correlation coefficients (e.g. LRSM)

 Experimental challenges:

– Intensive, parallel and highly polarized CN beam (with well known phase space) – p-e conversion foil (2 m2 !) [S. Hoedl et al., J. Appl. Phys. 99, 084904 (2006)] – Vacuum window (3 m2 !) – Low pressure MWDC

 Possible locations:

– NIST, ILL, …

24

• K. Bodek, PANIC11

Backup slides

25

• K. Bodek, PANIC11

• K. Bodek, PANIC11

26

Measurements of the transverse electron polarization in n-decay provide direct, i.e. first-order access to the exotic scalar and tensor coupling constants In order to simultaneously access REAL and IMAGINARY parts of the exotic couplings - measure both components of the transverse polarization of electrons emitted in neutron decay

Guidelines:

• K. Bodek, PANIC11

27

Systematic errors (2007)

 Decay origin “from/off beam” (mm):

x1max, y1max, z1max

 Electron energy “from/off neutron decay” (keV):

ELmin, ELmax, EHmin, EHmax

 Mott vertex position (mm):

Xmin, Xmax, Ymax, Zmax

x ,

Background subtraction related Decay asymmetry correction

RPV MSSM contributions to n-decay

• K. Bodek, PANIC11

28

R-parity violating super-symmetric contributions to the neutron beta decay Nodoka Yamanaka et al 2010 J. Phys. G: Nucl. Part. Phys. 37 055104

R = (1)2j+3B+L.

• K. Bodek, PANIC11

29

Experimental setup

• K. Bodek, PANIC11

30

MWPCs, scintillators and electronics

• K. Bodek, PANIC11

31

“V-track” events – on-line display

• K. Bodek, PANIC11

32

Energy resolution

 Conversion electrons from 207Bi

Hodoscope 1 Counts

• K. Bodek, PANIC11

33

Background subtraction

Absorption threshold Electronic threshold

 Observation:

Spectral distribution of background depends weakly on the electron origin

• K. Bodek, PANIC11

34

Mott scattering vertex distribution

Coinciding vertices in V and H projections Vertex in V projection

• nly

Vertex in H projection

• nly

• K. Bodek, PANIC11

35

Single-track events (no Mott scattering) Double V-track events (Mott scattering)

Electron energy distribution

• K. Bodek, PANIC11

36

Neutron beam “tomography”

S V-V V H

• K. Bodek, PANIC11

37

Projection of vertices onto XY-plane

x y z

Scint. Scint. MWPC Pb-foil Pb-foil

• K. Bodek, PANIC11

38

Projection of vertices onto Pb-foil planes

• K. Bodek, PANIC11

39

2nd-generation experiment with CN beam

Item Gain factor 1. Beam intensity 10 2. Beam fiducial volume 5 3. Detector acceptance 10 4. Beam polarization 1.2 5 Analyzing power 1.3 Total 780 Item Reduction factor 1. Background subtraction 10 2. Average neutron polarization 5 3. Analyzing power 3

 Statistics  Systematical uncertainty

Reconstruction of momenta

pe pp p

assigned weight is proportional to the decay density Actual position of the decay vertex is not known

But:

It must be located on the electron trajectory segment coincident with the beam Neutron decay density distribution in the beam is known

Finally:

In extraction of correlation coefficients we sum over momenta – ambiguity in vertex position is not essential

40

• K. Bodek, PANIC11

Electron-proton kinematics

41

• K. Bodek, PANIC11

Figure-of-Merit for Mott scattering

Electron energy threshold

42

• K. Bodek, PANIC11

• K. Bodek, PANIC11

43

Ambient air Gas mixture

MWDC operation in lowered gas pressure

20 mm 8 mm

Current nt preampl plifie ifier (for readout t from both wir ire ends) s)

• K. Bodek, PANIC11

44

1 2 2 1 1 2 2 1

q q R R z q q R R      

1

q

2

q z

Gas amplification ion cloud

1

R

2

R

Achieved performance

• K. Bodek, PANIC11

45

Gas mixture: He (90%) + isobutane (10%), P = 300 mbar Wire resistance = 3 Ohm; Preamplifier input impedance = 10 Ohm From drift time From charge division  < 0.5 mm  < 1.5 mm

• K. Bodek, PANIC11

46

m = (Jpe)/|Jpe| n = (peps)/|peps| k = (mn)/|mn| k  pe (x,y,z) – LAB frame  – decay angle  – Mott scattering angle  – event projection angle J – neutron polarization pe – incident electron momentum ps – scattered electron momentum J pe ps   m n n x y z k 