P and t violating correlation in muon and beta decays
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P and T violating correlation in muon and beta decays. Oscar Naviliat-Cuncic LPC-Caen (IN2P3/CNRS-ENSI) and Université de Caen Basse-Normandie. context and plan. describe some searches for signatures of NP in weak decays, specifically sensitive to P and/or T violation.

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P and T violating correlation in muon and beta decays

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P and T violating correlationin muon and beta decays

Oscar Naviliat-Cuncic

LPC-Caen (IN2P3/CNRS-ENSI) and

Université de Caen Basse-Normandie

context and plan

describe some searches for signatures of NP in weak decays, specifically sensitive to P and/or T violation

  • CP-even, P-odd correlation (muon decay)

  • CP-odd, P-odd correlation (neutron decay)

1. The longitudinal e+ polarizationin polarized muon decay

(initially motivated by the sensitivity of P -/P + measurements in nuclear beta decay)






  • beyond SM: include all Lorentz invariants

  • decay amplitude parametrized by

  • in the SM (all others are zero)

m-decay phenomenology

4-fermions point-like interaction:

(W.Fetscher PLB 173(86)102)

g = S,V,T interactions

e,m = L,R helicity of charged fermions

parameterization of observables

Observables commonly expressed in terms of Michel parameters:


the parameter x’’ enters the angular/energy dependence of PL by the combination:

(present PDG value)






longitudinal e+ polarization

sensitivity vs x and q for Pm = 0.95

x = Ee /E0 reduced positron energy

→ measure near q = p and x = 1

relation to exotic couplings

relative measurement

compare e+ polarizations between polarized and unpolarized muons:

avoids the precise determination of the polarimeter analyzing power

sensitivity coefficient:

r -, r 0: rates of positrons incident on the polarimeter

for polarized and unpolarized muon decays (measured)


  • produce polarized muons

  • maintain/destroy the muon polarization

  • select backward emitted positrons near the end point

  • measure the longitudinal polarization


pE3 area

muons are naturally produced 100%

polarized from pion decay at rest

“surface muons”

(contamineted by “cloud muons”)

28.9 MeV/c m+

> 44 MeV/c e+



Wien filter

(B < 0.0004 T)



Al or S


experimental setup

e+/m+ velocity separation

N(e+)/N(m+) < 0.12

m stop rate: 3x107 s-1 at 1.6 mA

muon polarization

Hanle method

(with two plastic telescopes)

polarization vs muon momentum

“surface muons”

at 28.5 MeV/c: Pm = 0.94(1)

use two muon stop targets:

Al: Pm = 0.94 “polarized”

S: Pm = 0.10 “unpolarized”

1.9T 2.7T 0.8T

energy and angular selection

selector tracker refocus

pe = 50 MeV/c

60% transmission

for Ee > 44 MeV

12 < q < 13

dump of beam positons

pe = 29 MeV/c











silicon tracker

determine momentum from helix geometry

redundancy constraint!

- 3 planes with 4 detectors each

- 60x60 double SSD 1 mm pitch 300 mm thick

resolution Dpe = 1.15(1) MeV/c






Vacoflux foils with opposite


positron polarimeter

use two processes:

- Bhabha scattering

- Annihilation in flight

(have analyzing powers with opposite signs)

BHA: Aeff ~ 0.037

AIF: Aeff ~ -0.015

(incl. fraction of polarized electrons and foil orientation)

but similar FOM (luminosity)

CoNiFe foils: 75x15x0.1 cm3 with 0.75mm active region to reduce Brehmstralung events

hodoscope and calorimeter

  • 7x7 plastic scintillators (x and y)

  • 90x7 cm2 with single PM readout

  • 127 BGO crystals, 55mm diam, 20cm long

  • external magnetic shielding

  • temperture stabilized DT = ±2°

  • resolution DE/E = 10% at 42 MeV

measuring conditions

  • two magnetized foils with opposite magnetizations (simultaneous)

  • two directions of magnetization for each foil

  • two orientations of the polarimeter: +45°, -45°

  • two analyzing processes with opposite analyzing powers (BHA, AIF)

  • two stop targets for polarized and unpolarized muons

  • measure energy dependence of the longitudinal polarization

typical trigger rates:

  • total 15 kHz

  • AIF 60/s

  • BHA250/s

(consistent with MC simulations)

cluster recognition, event reconstruction,cuts, ratios, asymmetries…

  • triggers defined from scintillators and BGO

  • SSD readout is slave

  • event type sorted from hits in MWPCs, Hodoscope and BGO

preliminary results

(X. Morelle, PhD)

asymmetries of ratios under inversion of foils magnetization


x” = 1.020 ± (0.062)stat ± (…)sys

(factor of 6 improvement if sys remains negligible)


  • experiment did not acquired the planned statistics

  • no present plans to make a new run

people and institutions

J.Egger, N.Danneberg, J.Deusch, W.Fetscher, F.Foroughi, J.Govaerts, M.Hadri, Ch.Hilbes, K.Kirch, P.Knowles, K.Koehler, A.Kozela, J.Lang, M.Markiewicz, R.Medve, X.Morelle, O.Naviliat-Cuncic, A.Ninane, R.Prieels, L.Simons, J.Sromicki and P.Van Hove

  • Institut de Physique Nucléaire, UCL, Louvain-la-Neuve, Belgium

  • Institut für Teilchenphysik, ETH, Zurich, Switzerland

  • Paul Scherrer Institut, PSI, Villigen, Switzerland

  • Département de Physique, Uni-Fribourg, Switzerland

  • Laboratoire de Physique Corpusculaire, Caen, France

(thanks to R.Prieels)

related projects

TRIUMF Weak Interaction Symmetry Test: “TWIST”


tracking of e+ from polarized muon decay

goal: detemine r, d, Pmx with a relative precision at the 10-4 level

prelim. results expected in 2004

2. Measurement of the R parameterin neutron decay






the R coefficient

decay rate function:

  • the coefficients A, R, N, … probe the dynamics

  • JTW parameterization: Ci , C’i(i = S, V, A, T )

R and D in neutron decay

D: P-even T-odd R: P-odd T-odd

DFSI = 1.310-5 (10-6) RFSI = 910-4 (510-6)






measuring principle

highest sensitivity to R for mutually perpendicular vectors

Mott scattering

experimental concept

(J. Sromicki NIM A 440 (2000) 609)

  • produce polarized cold neutron beam

  • observe neutron decay in flight

  • track low energy decay electrons

  • use large angle Mott scattering

  • trigger with “opposite” scintillator


  • Spallation neutron source SINQ

  • target: “zircalloy” (Zr-Pb)

  • D2O moderator

  • liquid D2 vessel cold source


polarized cold neutron beam

  • A. Schebetov et al.

  • NIM A 497 (2003) 479

  • J. Zejma et al.

  • to be submitted to NIM A

  • flux: 2×108 /(cm2·s·mA)

  • ‹P› ≈ 97 %


scheme of the experimental setup

photo gallery




results from commissioning run

  • Energy spectra of electrons from single track events

S/B > 3:1

Mott scattering analysis

cross section and Sherman function in Au

… Pb is better (higher Z) and cheaper!

- 1mm evaporated Pb layer on 2mm mylar

- 2 or 3 bands for 50x50cm2 active surface

a “golden event”

vertex reconstruction

(A. Kozela)

positions of Pb scattering foils

vertex signals

  • after vertex reconstruction with VETO condition on

  • the hodoscope at the vertex side S/B > 10:1

  • inclusion of hot spots of the setup geometry is in

  • progress to further improve S/B


2. precision goal:R =510-3

1. polarimetry control

  • “up-down” asymmetry probes R

  • “left-right” asymmetry probes N

the simultaneous measurement of N ≠ 0 provides a control of the polarimeter

Exclusion plot on S and T frombeta decay experiments (±1s)


P. Herczeg 2004: constraints on |Im(aLS)| from Rp are hard to beat with Rn

Rn at 10-2 would require fine-tuned cancellations

→ make life less exciting…

… important considerations for new generation experiments

status and outlook

  • The Mott polarimeter for the measurement of R in neutron decay

  • is complete.

  • The commissioning run in autumn 2003 was successful.

  • The first data taking run starts next month.

  • The measurement of R with an accuracy of 10-2 is expected to be achieved within one year.

  • The sensitivity goal of 510-3 seems feasible;

  • will be checked after first complete data analysis.

people, institutions, support

G.Ban, M.Beck, A.Bialek, K.Bodek, T.Brys, A.Czarnecki, W.Fetscher, P.Gorel, K.Kirch, St.Kistryn, A.Kozela, A.Lindroth, O.Naviliat-Cuncic, J.Pulut, A.Serebrov, N.Severijns, E.Stephan and J.Zejma

  • Laboratoire de Physique Corpusculaire, Caen

  • Institute of Physics, Jegellonian University, Cracow

  • Institute of Nuclear Physics, Cracow

  • University of Alberta, Edmonton

  • St Petersburg Nuclear Physics Institute, Gatchina

  • University of Silesia, Katowice

  • Catholic University Leuven, Leuven

  • Paul Scherrer Institute, Villigen

  • Institute of Particle Physics, ETH Zurich

support for PhD students

(thanks to K. Bodek and P. Gorel)

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