october 27, 021d. sillou positronium cs du 15/02/2001

October 27, 02 1D. Sillou

Positronium

CS du 15/02/2001


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Measurement of the

OrthoPositronium Lifetime

Proposal


PositroniumE= EH/2 = 6.8eVRadius = 2 rB = 10-8 cmQuickTime™ and a

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Parapositronium

S=0C = (-1)L+S = (-1)n

Ground state L=0

n = 2, 4, 6….

N(4)/N(2) ~ 10-6

Lifetime 0.125 ns




Orthopositronium

S=1C = (-1)L+S = (-1)n

Ground state L=0

n = 3, 5….

N(5)/N(3) ~ 10-6

Lifetime ~ 142 ns


QED and positronium Most experimental results are in excellent

agreement with QED: At least 2 problems related to positronium have to

be clarified: Lifetime of o_Ps in vacuum (5.5 from th.) Hyperfine splitting of Ps levels (3.5 from th.)

And also:g-2 of muon 2.7 (E821 Brookhaven 8/02/2001)


History

1934Mohorovicic e- e+1945Ruark Name positronium1946Wheeler Th. Proof bound state ofPs and Ps-1951Deutsch Experimental proof e- e+1959Mc GerveyS. de Benedetti Ps1969PaulinAmbrosino Discovery of formation ofPs in powder targets.Positronium physicsbegins19721958Canter & al.Cherry MgO moderator(slow e+ beam)1981Mills Existence Ps-


Experimental situation

Experiment in vacuum differs from theory by 5.5 Experiment in powder agrees with theory.Butmatter related corrections are ~ 10 times the discrepancy itself.

Group Result(µs-1) Δ(µs-1) comment1987AnnArbor(Michigan) Gas 7.0516.011661989 Gas 7.0514.011461990 Vacuum7.0482.00830

BeforegasMichigandidexperimentsinpowder1994TokyoUnivPowder7.0348-.00511995 Powder7.0398-.0013 NewresultsexpectedsoonTheoryuptoα2 7.039934


Theory

•Computation of B (2nd order corrections) = hard task•Expected B ~ 40 but larger value was not excluded though about 10 times greater value was necessary to explain the discrepancy between vacuum measurements and o-Ps computed with B=0.

•Recently (2000) B = 44.52 ==> 0-ps = 7.039934±.00001 s-1

The computation was motivated by the existing discrepancy. This discrepancy persists.

0 − Ps

= 2 α6

mc2π

2

− 9

9 π h

1 −A α

π

−1

3

α2

log α− 1

+ Bα

π

⎛

⎝ ⎜

⎞

⎠ ⎟

2

−3 α

3

2 π

log α( )2

+ ...

⎡

⎣

⎢

⎢

⎤

⎦

⎥

⎥


Why?

•The discrepancy at the level of 10-3 with QED is unacceptable.•Order α2 is now computed•The positronium is the prototype model for bound states treatment in QCD [charmonium etc…]


Experimental problems:

In principle the experiment in vacuum is not very difficult: Orthopositronium should be:

** Formed. Beginning of history. You cut!**Confined, on walls pickoff seen as 511 keV

2. Should be measured and substracted as

in Tokyo. decay from vacuum seen as 3 .**Detection should not depend on positronium age. Cavity should be spherical.

In summary: we take the best of previous experiments and add our improvements.


Glossary:

Pickoff:

Annihilation of the e+ from the positronium with the wrong electronIn 2 g. Contribution from this processDepends on the positronium kineticEnergy ant the number of collision(nombre collision ).


Our experiment:Basic idea: 3 from o_Ps in vacuum 2 from the walls.

Measure directly pickoff (as Tokyo) but 2 much smaller in vacuum. a 10% precision on the computed efficiency will be enough.

•>Ann Arbor was right and there is a discrepancy•>Tokyo was right and there is no more any discrepancy•>Both are right but matter in Tokyo experiment suppress coherent effects “ a la Glashow”


Technical aspects:•Vacuum < 10-9 Torr•Efficient e+ moderator•Well focused beam (1mm)•Efficient e+ tagging•e+ Intensity ≥ 103/s

Source 22Na +

Continuous spectrum 0-0.54MeV

ModeratorMonochromatic

~1 eVBeamtransportAcceleration

< 1keV

Tagging

Beam

START


cavity MgO

Positroniumformation

Pickoff = 2 511 keV

Setup

STOP

STARTt ≤ 1ns

Final state in vacuum:3, 5 … In walls only:2, 4, …

5 /3 ~ 4 /2 ~ 10-6


What we will get

-200 0 200 400 600 800 1000120014000.0001

0.001

0.01

0.1

1

10

100

Time (ns)

Formation region

Vacuum +

pickoff

Cut o-Ps formationHistory.

B

440 460 480 500 520 540 560 580

Pickoff line


Summary

•Measurement in vacuum

•Direct pickoff measurement

•Small beam size ~mm.

•High beam rate 103 Hz

•Spherical cavity

•cavity surface


Answer to Committee I

•”Tokyo experiment takes place in a more HOMOGENEOUS medium”:

The cavity includes SiO2 powder but also 22Na source and scintillators!!! Also escape from target not excluded.

Look at the setup (reference of their paper on previous transparency).


Answer to Committee II

•”Si la différence entre experiences et QED persiste, il est peu probable que cela mette en évidence l’effondrement de QED”:(S. Glashow) A negative result would put aside many lingering doubts. A positive result (a vacuum lifetime disagreeing with qed) would be a truly momentous discovery.These days, spending so little for such a discovery (unlikely as it may seem) is a terrific bargain. I would approve the experiment immediately, and congratulate the collaboration in its willingness to engage is such a difficult task.


Answer to Committee IIIThe committee has joined to its report a mail from Tokyo group. We have some additional comments about it:•Tokyo experiment takes place in a very inhomogeneous medium as stated before.•Their claim about the precision of their pickoff correction seems to us very optimistic.•In the annex to the committee’s report they announce new results: 142.05±.03 ns : Δth-exp) = (0.13 ± 2.9) 10-3 s-1 published 1995Δth-exp) = (0.15 ± 1.5) 10-3 s-1 (referee report)


3 phases: Phase I: Tuning and debugging

O_Ps formation in low density target Study of Ge response and MC Fitting procedures First measurements of decay

Phase II: Slow Positrons beam Detector construction Vacuum intensity, brightness, focusing… Shutter, tagging

Phase III: Final setup Technical runs Start measurements


What do we need ?

4 laboratories: ETHZ LAPP INFN INR

Total budget 4370 kFF / 4 years (subj to approval)ETHZ 770 kFFIN2P3 2840 kFFINFN 260 kFFINR 500 kFF

Technical support will be:40% ETH Z+ INR


What we need II ?

Laboratory: ~30 m2Phase I (2001)

Installation of lab 30 daysMechanics 120 days [(30%)design)]Electronics 60 days

Phase II (2002)Vacuum 150 daysMechanics 75 days

Phase III (2003)Vacuum, Mechanics 150 days


Who?

•After approval at LAPP:

•Precise with IN2P3 conditions for collaboration with INR (~Dubna IN2P3 in NOMAD).

•Refine needs at LAPP.•Investigate more closely e+ beam interest in the

region.

J-P Mendiburu, P. Nédélec, J-P Peigneux, D. Sillou


Perpectives I:

•Fundamental physic with positronium.

•ortho or parapositronium properties (levels, hyperfine splitting…)

•interactions of positron/positronium with matter.


Perpectives II:

•Applied Physics

Positron beam takes more and more importance in materials studies due to:•Positron sensitive to electronic density and positive vacancies•positron studies offer a background free signal.•Size of defects attainable•Concentration of defects•Doppler and collinearity technics which offer the possibility to measure the motion of electrons


Figure 1. Positron annihilation spectroscopy fills a special niche in the group of techniques for general vacancy defect analysis. Shown are regions accessible to various standard techniques: optical microscopy (OM), neutron scattering (nS), transmission electron microscopy (TEM), scanning tunneling microscopy (STM), atomic force microscopy (AFM), and x-ray scattering (XRS). Positron techniques are both highly sensitive and can resolve the size of atomic vacancies at any depth in a sample. The solid green line outlines the range of interest for studies of fine lines used as electronic interconnects on semiconductor chips.

Livermore


Perpectives III:

•University, students, positions

Experiments at this scale will allow to perform a lot of studies in collaboration with university with relatively short terms results.A positron beam would be one of the first in France (after the CERI in Orleans 06/2000) and would be appreciated by materials physicists and chimists in the region.


Conclusions:

The question of the orthopositronium lifetime is still open.•Vacuum experiments is not in agreement with QED•Powder experiment is in agreement with QEDbut to which extent can this result be interpreted as free space annihilation?The experiments presents a lot of interesting aspects:

fundamental physic at LAPP, technical implications, and opportunities of developping collaborations with university and industry and definitely has discovery potential.

october 27, 021d. sillou positronium cs du 15/02/2001

Documents

vacuum measurements

computedthe positronium

positronium age

deutschexperimental

sillou1934mohorovicice

existing discrepancy

sillouthe discrepancy

positronium kineticenergy