pibeta: a new precise measurement of rare pion and muon …v cd v cs v cb v td v ts v tb ......
TRANSCRIPT
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PIBETA: A new precise measurement of rarepion and muon decays
Dinko Pocanic,
University of Virginia
• motivation
• about the experimental method
• our working results
◦ pion beta decay: π+ → π0e+ν
◦ radiative pion decay: π+ → e+νγ
• further plans
JLAB, 2 May 2003
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PIBETA Collaboration—Active Members
M. Bychkov, E. Frlez, W. Li, R.C. Minehart, D. Pocanic,
L.C. Smith, W.A. Stephens, B.A. Vandevender, Y. Wang,
University of Virginia, Charlottesville, VA 22904, USA
W. Bertl, J. Crawford, Ch. Bronnimann, M. Daum,
R. Horisberger, S. Ritt, R. Schnyder, H.-P. Wirtz
Paul Scherrer Institut, CH-5232 Villigen, Switzerland
T. Kozlowski
Institute for Nuclear Studies, PL-05-400 Swierk, Poland
B. G. Ritchie
Arizona State University, Tempe, AZ 85287, USA
V.A. Baranov, V.V. Karpukhin, N.V. Khomutov,
A.S. Korenchenko, S.M. Korenchenko, N.P. Kravchuk,
N.A. Kuchinsky, A.M. Rozhdestvensky, V.V. Sidorkin
Joint Institute for Nuclear Research, Dubna, Russia
D. Mzhavia, Z. Tsamalaidze
Instit. for High Energy Physics, Tbilisi St. Univ., Georgia
I. Supek
Rudjer Boskovic Institute, Zagreb, Croatia
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PIBETA Experiment: MOTIVATION
Provide precision tests of Standard Model and QCD
predictions:
• π+ → π0e+νe – main goal
◦ SM tests from CKM unitarity
• π+ → e+νeγ(ee)
◦ FA/FV , π polarizability (χPT prediction)
◦ tensor coupling besides V −A
• µ+ → e+νeνµγ(ee)
◦ departures from V −A in Lweak
• π+ → e+νe – 2nd phase
◦ e-µ universality
◦ pseudoscalar coupling besides V −A
◦ massive neutrino, Majoran, ...
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VECTOR CURRENT CONSERVATION
(a) Leptonic current (as in µ decay):
lα(x) = e(x)γα(1− γ5)νe(x)
〈e|lα|νe〉 → ueγα(1− γ5)uν (V −A form)
(b) Hadronic current (as in n → peν):
〈p|hα|n〉 → upγα(GV −GAγ5)uν
with GV ' 1 and GA ' 1. Puzzling!!
Large strong correction expected!
1956 Gerstein + Zeldovich formulated the
1958 Feynman + Gell-Mann CVC hypothesis
Analogy with electric charge: Q(p) ≡ Q(e), i.e.,
Q(bare p) ≡ Q(screened p)
due to exact conservation of electric charge.
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Cabibbo Universality
Cabibbo (1963):
GV = Gµ cos θC(= GµVud) cos θC ' 0.97
weak quark mixing: d’ = cos θC d + sin θC s.
With 3 generations, Cabibbo–Kobayashi–Masakawa
matrix (1973):
(u
d
)(c
s
)(t
b
)−→
Vud Vus Vub
Vcd Vcs Vcb
Vtd Vts Vtb
CKM unitarity:
|Vud|2 + |Vus|2 + |Vub|2?= 1 .
Experimental limits on Vud have come from:
(a) superallowed Fermi nuclear β decays,
(b) neutron β decay.
These have disagreed in the past.
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Pure vector weak (Fermi β) decays
A(Jπ) → B(Jπ) + e + ν
with J(e + ν) = 0, i.e., ~S(e) = −~S(ν).
Guaranteed when Jπ(A) = Jπ(B) = 0+ or 0−.
In nuclei 0+ → 0+ are called superallowed β
transitions.
No direct axial vector contribution, only from “loop
corrections”.
Neutron decay: n( 12
+) → p( 12
+) + e− + νe
has both V and A transition amplitudes.
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Status of CKM Unitarity
|Vus| ' 0.2196± 0.0026 from Ke3 decays.
|Vub| ' 0.0036± 0.0007 from B decays.
(a) Superallowed Fermi nuclear β decays
Longstanding discrepancies between Ormand & Brown
and Towner & Hardy ft values “reconciled” in 1990:
|Vud|2 + |Vus|2 + |Vub|2 = 0.9962± 0.0016,
or 1− 2.4σ.
(b) Neutron β decay has varied considerably:
3∑i=1
|Vui|2 = 1.0096± 0.0044, or 1 + 2.3σ,
[after Erozolimskiı et al. (1990); since retracted]
3∑i=1
|Vui|2 = 0.9917± 0.0028, or 1− 3.0σ.
[Abele et al (2002); at odds w/world data set]
(c) Pion β decay – this work
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Pion Beta Decay:
π± → π0e±ν BR ' 1× 10−8
Pure vector transition: 0− → 0−
Theoretical decay rate at tree level:
1τ0
=G2
F |Vud|2
30π3
(1− ∆
2M+
)3
∆5f(ε,∆)
= 0.40692 (22)|Vud|2 (s−1) .
With radiative and loop corrections we get:
1τ
=1τ0
(1 + δ) ,
so that the branching ratio is
BR(πβ) =τ+
τ0(1 + δ)
= 1.0593 (6)× 10−8(1 + δ)|Vud|2 .
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Recent calculations of pion beta decayradiative corrections
(1) In the light-front quark model
W. Jaus, Phys. Rev. D 63 (2001) 053009.
• total RC for pion beta decay:
δ = (3.230± 0.002)× 10−2 .
(2) In chiral perturbation theory
V. Cirigliano, M. Knecht, H. Neufeld and H. Pichl,
hep-ph/0209226
• χPT with e-m terms up to O(e2p2)
• theoretical uncertainty of 5× 10−4 in extracting
|Vud| from BR(πe3(γ)
).
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Experimental accuracy of thepion beta decay rate
Best result published to date:
BR(π+ → π0e+ν) = (1.026± 0.039)× 10−8 ,
i.e., about 4 % [McFarlane et al. (1985)].
Accuracy Constraints or cross-checks on
≤ 1 % CVC and radiative corrections
∼ 0.5 % Add to SAF & nβ input to Vud
< 0.3 % Failure of CKM unitarity (?):
◦ 4th generation coupling
◦ mZ′
◦ Λ of compositeness
◦ SUSY viol. of q-l universality
◦ signal of a smaller GF (ν osc.)
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Design of the Experiment
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Pion and Muon Decaysin the PIBETA Experiment
π+ → µ+ν (1.0),
µ+νγ (∼ 2.0× 10−4)
e+ν (∼ 1.2× 10−4) normalize to
e+νγ (∼ 5.6× 10−8) measure
π0e+ν (∼ 1.0× 10−8) measure
π0 → γγ (∼ 0.9880) measure
e+e−γ (∼ 1.2× 10−2) measure
e+e−e+e− (∼ 3.1× 10−5)
e+e− (∼ 6.2× 10−8)
µ+ → e+νν (1.0) normalize to
e+ννγ (∼ 0.014) measure
e+ννe+e− (∼ 3× 10−5) measure
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PIBETA Experiment:
◦ stopped π+ beam◦ segmented active tgt.◦ 240-elem. CsI(p) calo.◦ central tracking◦ digitized PMT readout◦ cosmic µ antihouse◦ stable temp./humidity
AT
MWPC1
MWPC2
PV
AD
AC1
AC2BC
CsIpure
π+beam
10 cm
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CENTRAL DETECTOR REGION
Active TGT
MWPC-1
MWPC-2 PVarray
(beam is perpendicular at center)
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Experimental Method: Summary
• Detect π+ decays at rest (during a delayed
180 ns gate).
gate
π stop
beam veto
π
-30 0 150 ns
• Use prescaled π+ → e+ν (πe2) decays for
normalization.
• Accept every πβ trigger – unbiased (γγ
coincidences above Michel endpoint)
1τπβ
=1
τπ+· BReνfpresc
BRπ0→γγ· Aeν
Aπβ· Nπβ
Neν
Aπβ , Aeν are acceptances for the πβ and πe2
decay modes, respectively.
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Experimental method (cont’d.)
Aπβ , Aeν influenced by:
◦ geometry (including beam spot, alignment,
cracks),
◦ e-m shower leak-through, backsplash, etc.,
◦ e-m interactions in TGT and tracking detectors,
◦ volume uniformity of CsI light response,
photoelectron statistics,
◦ gain stability of CsI detector modules,
◦ photonuclear and e knockout react’s in CsI.
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Key acceptances compared
GEANT calculated acceptances for the πβ and πe2
decays as a function of cos θ.
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PIBETA Detector Assembly (1998)
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Detector Performance
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Online “πβ” Energy Spectrum:
True πβ events buried deep under overwhelming
background!
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Preliminary Results: π+ → π0e+ν
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Preliminary Results: π+ → π0e+ν
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Domino Sampling Chip (DSC) Analysis
π+ → π0e+ν
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Preliminary Results: π+ → e+ν
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Angular acceptance: π+ → e+ν
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Angular acceptance: π+ → π0e+ν
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Preliminary Results: π+ → e+νγ (S/B)
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Preliminary Results: π+ → e+νγ (t)
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Preliminary Results: π+ → e+νγ (mπ)
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Experiment Systematics
Main sources of uncertainty:
• Acceptance: beam geometry
• Timing: pion decay fraction
• External: BR(π → eν)
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Central detector region revisited
Active TGT
MWPC-1
MWPC-2 PVarray
(beam is perpendicular at center)
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Beam geometry: TGT rate deconvolution
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TGT tomography: MWPC resolution
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TGT tomography: track reconstruction
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Beam geometry: π+ stopping distribution
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Beam stability
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Pion stop timing
∆tpeak ≈ 4 ps
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Preliminary Results
π+ → π0e+ν
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Branching Ratio for πβ Decay
Our Preliminary Interim Result:
PRELIMINARYBR ' 1.044 ± 0.007 ± 0.009 ×10−8
(stat.) (syst.)
McFarlane et al. (Phys. Rev. D 1985):
BR ' 1.026± 0.039× 10−8
SM Prediction (PDG, 2002):
BR = 1.038− 1.041× 10−8 (90% C.L.)
(1.005− 1.008× 10−8 excl. rad. corr.)
PDG 2002: Vud = 0.9734(8)
PIBETA prelim: Vud = 0.9765(56).
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Summary of πβ Uncertainties
at end of analysis phase: current final
(%) (%)
external
pion lifetime 0.019 0.019
BR(π → eν) 0.33 ∼ 0.1?
BR(π0 → γγ) 0.032 0.032
internal
A(πβ)/A(eν) 0.5 < 0.3
∆t(γ − e) 0.03 0.03
E threshold < 0.1 < 0.1
statistical: 0.7 ∼ 0.4
total: ∼ 0.9 . 0.5
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Radiative Pion Decay
π+ → e+νγ
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Anything beside V–A in RPD?
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AVAILABLE DATA on Pion Form Factors
|FV |cvc=
1α
√2~
πτπ0mπ0= 0.0259(5) .
FA × 104 reference
106± 60 Bolotov et al. (1990)
135± 16 Bay et al. (1986)
60± 30 Piilonen et al. (1986)
110± 30 Stetz et al. (1979)
116± 16 world average (PDG 2002)
Poblaguev (1990, ’92) found:
(a) inconsistencies in data set (part. istra data),
(b) need to include T q-l coupling:
FT ∼ −0.0056± 0.0017 .
Herczeg (1994) confirmed there is room (barely) for
FT of the above order of magnitude. Only plausible
explanation—leptoquarks at tree level.
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Previous π → e+νγ experiments
Bay et al., PSI 1986
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Previous π → e+νγ experiments
(reconstructed π mass)
Bolotov et al., IHEP, Protvino, 1990
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PRELIMINARY RESULTS: π+ → e+νγ
Fixed Free
. . . . FCVCV , FT = 0 FA
- - - - FT = 0 FA, FV
FCVCV FA, FT
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A Glimpse at Pion Form Factors
◦ based on analysis of regions A and B only,
◦ 2-dim fits exclude border regions in acceptance
Fit Fixed parameters Fit parameters χ2
FV : 0.0170(80)
‘PDG’ FA: 0.0116(16) none 9.2
FT : 0
‘SM’ FV : 0.0259(5) FA: −0.0028(1) 5.6
FT : 0
all free FT : 0 FV : 0.0136(13) 2.3
no T FA: 0.0136(13)
CVC FV : 0.0259(5) FA: 0.0067(6) 1.1
+T FT : −0.0017(1)
In the meantime we:
• have reduced region C background,
• have refined aspects of MC simulation,
• are pursuing additional global fit strategies.
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A Closer Look at Energy Calibration
effect of a 1% gain variation:
Michel edge and πe2 peak:
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More on Energy Calibration
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A Look at Muon Decays
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More on Radiative Muon Decays
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What We’ve Learned
◦ Very high precision experiments are needed in
order to study truly rare processes.
◦ There appear to be no surprises in the pion beta
decay channel.
◦ The accepted description of the π → eνγ
process is inadequate, and will require revision.
Plans for the Future
• Bring analysis up to the proposed level of
uncertainty.
• Prepare new proposal and collaboration for a
precise π → eν measurement.
• We are actively considering a new search for the
π0 → γγγ decay using the PIBETA apparatus.
http://pibeta.phys.virginia.edu/
http://pibeta.web.psi.ch/