Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 1

The First Direct Observation of Positronium Hyperfine Splitting

(Ps-HFS)Taikan Suehara (ICEPP, U. Tokyo, Presenter)

T. Yamazaki, A. Miyazaki (U. Tokyo)

with contributions fromG. Akimoto, A. Ishida, T. Namba, S. Asai, T. Kobayashi, H. Saito

(U. Tokyo)T. Idehara, I. Ogawa, Y. Urushizaki, S. Sabchevski

(U. Fukui & BAS)

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 2

Positronium (Ps)

positron (e+)electron(e-)

−+

Ps is the bound state of e- and e+

• The lightest Hydrogen-like atom• A good target for the precise QED test

• Free from hadronic interaction• Simplest particle-antiparticle system

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 3

Continuous energyspectrum, long lifetime of 142 ns

Spin states of Ps (o-Ps, p-Ps)

e+ e-

• ortho-positronium (o-Ps) : 75%

e+ e-

o-Ps → 3γ (, 5γ, …)α3 process

o-Ps

p-Ps

spin triplet

• para-positronium (p-Ps) : 25%spin singlet

p-Ps → 2γ (, 4γ, …)α2 process

Two back-to-back 511keV monochromatic γ rays,short lifetime of 0.125 ns

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 4

Hyperfine splitting: o-Ps – p-Ps

Ground state HFS:203.388 GHz(λ=1.5mm, mm wave)Could be examinedby stimulated transition

Much larger than 1H(1.3 GHz)

Good target forprecise bound-stateQED test

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 5

Inconsistency of Ps-HFS

Experimental results give 3.5 σ (15 ppm) smallerHFS value! -> need to investigate…

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 6

Theoretical interpretations: new particle?Unknown light particle coupled to electron or photoncan shift HFS value from QED calculation (as muon g-2)

o-Ps energy level can beaffected by X via o-Ps – γ*oscillation (p-Ps is not affected)

ex.1) particle Xcoupled to photon

ex.2) particle a0

coupled to electron p-Ps (or o-Ps) energy level can be affected by a0 via Ps – a0 oscillation(depending on a0 spin)

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 7

Unknown systematic effectsmay affect the experimentalresults

All past experiments used‘indirect’ method

Measure ∆mix (Zeeman splittingwith static B) instead of ∆HFSand calculate HFS with ∆mixe

Controlling B in ppm level isvery difficult and it may causesome unknownsystematic errors…

…or experimental mis-consideration?Energy level of Ps

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 8

• HFS value is inconsistent between experiment & theory• All former experiments employed indirect method

Our method: direct observation

Direct measurement !

The largest issue: high density radiation in 203 GHz

• o-Ps -> p-Ps transition is M1: lifetime is > 107 sec• o-Ps decay: lifetime is 142 nsec (109 times shorter)

-> very strong radiation is needed for stimulated emission(impossible in 80’s)

But recent evolutions of THz technology enable it

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 9

• Accumulation of high-power 203 GHz radiation at the positronium forming area1. High power sub-THz source: gyrotron2. Gaussian mode converter3. Fabry-Perot resonator

• Optimizing the positron source, detectors and shieldings for good signal selection

Keys for the direct observation:

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 10

1. High power sub-THz gyrotron

Gyrotron FU CW IV@ Fukui

2m

Electron gun

7.4T solenoidCavity at center

Output window

Collectorof electrons

Gyrotron utilizes cyclotron motion of electrons to resonate a cavity inside the solenoid[ characteristics ]• 100 GHz – 1 THz• High power

(used as heatersfor nuclear fusion)

• Continuous / pulse• Possibly frequency

tunable/sweep203GHz, 300W long pulse(60ms / 5Hz, duty 30%)gyrotron was developed for this study

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 11

2. Gaussian mode converterGyrotron outputs TE03

waveguide mode→ Need to convert to

TEM00 Gaussian mode to becoupled to optical resonator

Gyrotron output(quasi-) TE03

After converterTEM00

Efficiency: ~30%

550

mm

Step-cutwaveguide

Mainparabolic

mirror

Long-focusparabolicmirror 1

Long-focusparabolicmirror 2

Beamsplitter

Toresonator

Power monitors(input/reflection)

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 12

3. Fabry-Perot resonatorOne-dimensional resonator(optics)

• High power density (optical focusing)• Freely changing resonant frequency

Gold thinfilm mesh (1µm thick,200µm width, 360µm period)

is depleted on the quartz99% reflection, ~0.7% transmission

With a piezo stage, cavity length isprecisely controlled (~100nm)to maintain maximum resonance

With 130 mm cavity length,6-7 kW power accumulation

has been confirmed(Finesse: ~600)

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 13

Setup of source/detectorGyrotron power 22Na positron source(1MBq)

β-tag plasticscintillator

Lead shield

Copper mirror1.5 x 2 in. LaBr3 crystal scintillators

Resonator(1.9 atm N20.1atm iso-C4H10)

100 µm thickStart time tagging

~4% resolution @ 511 keV, fastEnergy & stop time tagging

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 14

OverviewSource / holder

Chamber / resonator

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 15

Timing spectrum

time [ns]0 200 400 600 800 1000

coun

ts /

0.5n

s / s

ec

-210

-110

1

10

210 With powerWithout power

Prompt event (p-Ps, annihilation)

o-Ps decay + HFS transitionlifetimer 135nsec (fit result)

Pileup events

No difference between power on/off events

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 16

Pileup rejection

short gatelong gate

short gatelong gate

The most significant bkg.: pileup

Cutting pileup eventsby comparing QDC value

with short/long gate

Without pileup rejectionWith pileup rejection

Pileup is reducedby a factor of 3

(power off)

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 17

Energy cut & resultLivetime: 14 hours (power on)

Power on – power off

With powerWithout power

7σ deviation is observed between on/off eventsPreliminary transition rate: 38.7 ± 5.6 mHz (Stat. error only)

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 18

• Good agreement between Data & MC (not perfect)• Transition rate is consistent between Data & MC

Comparing to MCPower off Power on

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 19

• HFS measurement needs freq scan• Frequency-tunable gyrotron (gyro-

BWO) is now under testing-> First direct HFS measurement

O(100 ppm) in 1-2 years• With positron beam, O(10 ppm)

measurement is possible (3-5 years)• Frequency-tunable gyrotron can also

be used for exotic light particle searches (axions, paraphotons…)

Next step: frequency-tunable gyrotron

Assembled gyro-BWO

Tran

sitio

n pr

obab

ility

Now!

HFS freq.

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 20

• Positronium HFS is inconsistent between QED calculation and experiment - need to be examined

• We are performing direct transition experiment• Indication of HFS transition is seen in latest data

- now confirming• Frequency-tunable gyrotron can lead to precise

measurement of ~100 ppm (ultimately 10 ppm)

Summary

Taikan Suehara et al., PANIC11 @ MIT, 2011/7/25 page 21

backup

Gyrotron

OurTarget

22

Gyrotron• Electrons emitted from the

electron gun are accelerated and move in a circle in the magnetic field and go into the cavity

• If we tune the magnetic field such that the electron cyclotron frequency Ω = eB/mγ is slightly smaller than the cavity resonant frequency

SC magnet

Resonant cavity

EM wave

Electrongun

22

+

=

Ll

Rmn

cπχω

, the energy of the cyclotron motion is converted to the energy of the EM wave in the resonant cavity 23

Gyrotron• We are developing a frequency-tunable gyrotron (reflective

gyro-BWO, Backward -Wave Oscillator). Inside this type of gyrotron, BW reflected at the entrance of the cavity interacts with electrons and resonant condition is changed as

γωωβω

meB

cz =Ω=−+ 22

• can change output frequency continuously by tuning magnetic field strength.

ω : output EM wave frequencyωc : cavity resonant frequencyΩ : cycrotron frequencyβz : axial velocity of electrons

24

GyrotronGyrotron

Gyro-BWO

Reflectivegyro-BWO

e-

e-

e-

Backward Wave is reflected at the entrance of the cavity

Blue : Forward WaveRed : Backward Wave

cavity

Outputradiation

Outputradiation

Outputradiation

High power

Frequency-tunable

High power &Frequency-tunable

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Mode Conversion

Gaussian beam (calculation)TE03 (far-field, calculation)

• Gyrotron output = TE03 waveguide mode• Fabry-Pérot cavity mode = Gaussian beam (= TEM00)• have to convert gyrotron output to Gaussian beam in order to couple the sub-THz radiation with the Fabry-Pérot cavity

26

Mode Converter• Mode conversion is based on geometric optics. Step-cut

waveguide and the first parabolic mirror are key components.

O

R

Pf

light path

Top View

P’

R’Step-cut waveguide

Parabolic Mirror

ジャイロトロン

Parabolic Mirror

Step-cut waveguide

• The first parabolic mirror converts TE03 wave to polarized plain wave because OPR = OP’R’ from the definition of parabola.

27

polarizationwave front

Piezoelectric Stage

- NANO CONTROL TS102- Long stroke : 15mm- resolution : 10nm- withstand load : 2kg- size : 60mm x 60mm x 24mm

28

Pyroelectric Detector

- SPECTRUM DETECTOR SPC-9H, SPH-49- Lithium Tantalate (LiTaO3) crystal, which generates electricity when heated (pyroelectricity)- diameter : 9mm- Max average power : 500mW- size : 60mm x 60mm x 24mm

29

• Coupling of the input beam to the cavity can be estimated from decrease of the reflection power

• The number of round-trips inside the cavity is inversely proportional to the sharpness of the resonance

• Approximate cavity gain is

Transmission power

Reflection powerFabry-Pérot resonant cavity

• Power accumulated in the Fabry-Pérot cavity is

67% decrease

Γ = 1.1μm(FWHM)

PVC (PolyVinyl Chloride)

International Journal of Infrared and Millimeter Waves 28, 363 (2007)

- Small refractive index and large absorption coefficientn = 1.65, α = 1～2 cm-1

31

Absolute Accumulated Power• PVC measurement measures temperature increase [K].• have to calibrate to obtain power [W]• Water is a total absorption calorimeter and its heat capacity is

well known.

..[sec] [K] [cc] 2.4 [W]

RDtTVP

×∆××

=

• We obtained calibration constant for PVC by comparing PVC measurement [K] with water mesurement [W]

Absolute Accumulated Power• monitored transmission power of the Fabry-Pérot cavity during

DAQ• Calibration constant from output of the pyroelectric detector to

accumulated power in the cavity was obtained as followsPVC

Cu mirror

pyroelectric detector

Ptr [V]

hole φ0.6 mm

Pin [W]

• Calibration constant : C [W/V] = Pin[W] / Ptr [V]

Gas (N2:i-C4H10 = 1.9atm:0.1atm)• i-C4H10 has 3 good points and 2 bad points• 3 good points

- kill slow e+ (at least 0.1atm of nitrogen is necessary)- high stopping power- high Ps formation probability

• 2 bad points- high pickoff probability- absorb 203GHz radiation

34

LaBr3(Ce)• LaBr3 (Ce) crystal scintillator (diameter 1.5 inch, thickness 2 inch)

– Good energy resolution (FWHM=4%@511 keV) → distinguish 2γ and 3γ

– short-time constant 16 ns → high statistics– Good time resolution (FWHM=200 ps@511 keV)– La (Z = 57), Br (Z = 35)

35energy [keV]

100 150 200 250 300 350 400 450 500 550 600

coun

ts /

keV

0

10000

20000

30000

40000

50000

60000

FWHM 4% @ 511 keV