A DPG Frühjahrstagung Aachen 13.03.03 C. Regenfus Uni-Zürich 1 ATHENA - Cold antihydrogen...

DPG Frühjahrstagung Aachen 13.03.03 C. Regenfus Uni-Zürich

1A

ATHENA - Cold antihydrogen production

Production of cold antihydrogen atoms in large quantities

• Introduction• The ATHENA experiment +• New results• Summary • Outlook

On behalf of the ATHENA collaboration

C. Regenfus

University of Zürich

H detector

Antihydrogen candidate (real data, 4-prong event)

Sept. 02: > 50k cold antiatoms produced


2A


Motivation

Antihydrogen: The simplest antimatter counterpart to matter

for testing fundamental physic principles

• CPT symmetry (Theoretical underpinning of field theories)• Gravitational acceleration (Equivalence principle)

A very high precision can be achieved by comparing antihydrogen to hydrogen


3A


Future: high resolution laser spectroscopy

Atomic 1S - 2S transition by two-photon excitation (first order Doppler-free)

Lyman E = 10.2 eV = 2.5 x 1015 Hz = 122 nm UV 2 x 243 nm photons (mW)Lifetime of 2S state: 122 ms => precision ~10-16

Cesar et al. (1996)(Laser 3kHz, 150µK)

Need: Cold antihydrogen ( T < mK )Capture in neutral trapHydrogen reference cell

243 nm LASER

Mirror

H

H spectroscopy

Gravitation: atomic fountain / interferometry


4A


Present physics menu

Plasma studies: new kind of plasma imaging

• Particle losses in trap• (Re)combination mechanism• Production of cold antihydrogen in larger quantities

Investigations

• Antihydrogen energy distribution (+ inner states)• Laser spectroscopy on non trapped atoms• Trapping H and/or creation of a H beam


5A


The ATHENA collaboration

Particle traps + control:INFN, Sez. di Genova, and Dipartimento di Fisica, Università di Genova, Italy

EP Division, CERN, Geneva, SwitzerlandDepartment of Physics, University of Tokyo, Japan

Precision lasers:Department of Physics and Astronomy, University of Aarhus, Denmark

Instituto de Fisica, Rio de Janeiro, Centro de Educação Tecnologica do Ceara, Brazil

Positron plasma:Department of Physics, University of Wales Swansea, UK

Detector + Analysis:Physik-Institut, Zürich University, Switzerland

INFN, Sez. di Pavia, and Dipartimento di Fisica Nucleare e Teorica, Università di Pavia, ItalyDipartimento di Chimica e Fisica per l'Ingegneria e per iMateriali, Università di Brescia, Italy


6A


Experimental overview

Positron Accumulator

AntihydrogenDetector(T= 140 K)

0 1 m

0 10 cm

Na-22Source

AntiprotonCapture Trap Mixing Trap

CsI crystals

Si stripdetectors

Cryostat

e+3 T superconducting solenoid

15 K , 10-11 mbar

Main ATHENA features: Open access system (no sealed vacuum)Powerful e+ accumulation Plasma diagnosis and control High granularity imaging detector

Scint. Scint.Scint.


7A


ATHENA Photo


8A


Penning traps

Trapped electron at B = 3 T, E = 1 eV, U ~ 10 V

• Cyclotron motion (perpendicular to B)f = 84 GHz, r ~ 1 µmEmission of synchrotron radiation (cooling)t cool ~ 0.3 s

• Axial motion (along B)f ~ 7 MHz, d ~µm … cm

• E x B drift (‘magnetron’) (cooling over coupling)f ~ kHz, r ~ mm

Single particle <=> Plasma Coulomb coupling parameter: Ecoul/Etherm

Electrical screening distance: Debye length

ATHENA:Multi-ring Penning trap (choose Vz as you like )


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Antiproton decelerator (CERN)


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Antiproton capture and cooling with electrons

p

5 MeV50 cm

Antiproton Capture Trap

Solenoid (3 T)Degrader

Degrading

Trapping

Cooling

Vacuume-

TrappingPotential

5 KV

Cold electron cloud(Cooled by Synchrotron Radiation,

τ=0.4sτ3T)

Univ.ofGenov/IT

(τ=0)

(τ=200ns)

(τ=20-30s)

• Capture dynamics

• Capture trap (50 cm)

10 000 p / AD shot


11A


Positron accumulation

Coldhead

300 Gauss guiding fields

T = 6 K50 mCi 22NaSolid neon moderator

Segmented electrodefor Rotating Wall

Beam strength:6 million e+ per second

e+Energy loss through collisions

e+

Accumulation rate: 106 e+/s

150 million e+ / 5 min

After transfer: 75 x 106 in mixing trap

Positron plasma : r~2mm, l~32mm, n~2.5 x 108 / cm3

Lifetime: ~hours


12A


Non destructive positron plasma diagnostics

read

heat

drive

Complete model of plasma mode excitation

(based on ‘Cold Fluid Theory’ * )

PLASMA SHAPE, LENGTH, DENSITY

Plasma temperature change* D. Dubin, PRL 66, 2076 (1991)

kΔT =mzp2

5 ω2h( )2 −ω2( )2

[ ] 3−ωp2α2

2ω22d2 f(α)dα2

⎡ ⎣ ⎢

⎤ ⎦ ⎥

−1

~ 30 MHz


13A


Detection principle of antihydrogen annihilations

• H atom dissociates to p and e+

by contact with the trap wall or rest gas atoms• pN -> charged and neutral pions

• e+ e- -> 511keV photons (back to back)

Good spatial resolution (< 1 cm ) of chargedvertex ( at least 2 prong events)Time coincidence (~ 1 µs)High rate capability (self triggering)

511 keV opening angle

Monte Carlo

Measure 1MeV on background of 2GeV


14A


Detector development

Much effort into R&D

• Low temperature (~ 140 K)

• High magnetic field (3 T)

• Low power consumption

• Light yield of pure-CsI crystals ?

• CTE matching (Kapton, silicon, ceramics)

• Electronic components

Full detector installed: August 2001

All photodiodes replaced with APDs: Spring 2002

• Compact design (radial thickness 3 cm)

• High granularity (8K strips, 192 crystals)

• Large solid angle (>75 %)

Workshop Zürich , J. Rochet

Silicon micro strip layer

Mechanics for 77K


15A


Pure-CsI crystals + Avalanche Photo Diodes

22NaT = 150 K

0

2000

4000

6000

8000511 keV

back scatter

1275 keV

10000

FWHM = 18%

12000

Pulse height [keV] cos()

4000

3000

2000

1000

-1.0 -0.5 00 60 500 1000 1500 0.5 1.0

e+e−

γ(511keV)

γ(511keV)

• Read out close up • Crystal APD unit

• Crystal detector performance

~16 times higher light yield @ 80K

C. Amsler, et al. :Temperature dependence of pure-CsI, scintillation light yield and decay time. NIM A 480, 494–500 (2002).

Pure-CsI


16A


Full GEANT Monte Carlo simulations

E&M cascades, Hadronic Showers (GEISHA) (> 10 keV)

Geometry from AutoCAD

Module-by-module (in)efficiency taken into account

Same analysis routine for MC and data

Electrode (r = 1.25 cm)

Radial vertex position


17A


Antiproton annihilations

Antiproton annihilation on the trap wall (real data, 3-prong event) • strip hits (inner + outer layer) => p vertex• crystals hit (matched to charged tracks)

• vertex resolution, ~ 4 mm (curvature not resolved)

Electrode position (r = 1.25 cm)


18A


Plasma imaging (antiprotons only)

-10 -5 0 5 100

10

20

30

40

50

60

70

Longitudinal Position [cm]-10 -5 0 5 10

0

10

20

30

40

50

60

70

Longitudinal Position [cm]

Antiprotons and electrons

Early times

Late times

Potential

Potential

Powerful plasma and loss diagnostics !

p vertex evolution in time


19A


Mixing trap (nested penning trap*)

In one mixing cycle (5 min) we mix ~104 antiprotons with ~108 positrons* G. Gabrielse et al., Phys. Lett. A129, 38 (1988)

0 2 4 6 8 10 12-50

-100

-75

-125

Length (cm)

antiprotons

108


20A


Cooling of antiprotons by 75 million positrons

• Rapid cooling (< 20 ms)• Decreasing energy of antiprotons• Increasing separation of plasmas


21A


Antiprotons in the positron plasma

Energy loss by dE/dx and thermalization

e+ cloud (108/cm3)

T = 10K ….. 10000K(by RF heating)

Incoming antiproton

1 2 3 4 5

5

10

15

20

25

30

Te=300 K

Te=30 K

Te=3000 K

ne = 2.5 108 cm-3

Le = 3.2 cm

Antiproton kinetic energy [eV]


22A


Antihydrogen production

1. Fill positron well in mixing region with 75·106 positrons;

allow them to cool to ambient temperature (~15 K)

2. Launch 104 antiprotons into mixing region

3. Mixing time 190 s - continuous monitoring by detector (charged trigger)

4. Repeat cycle every 5 minutes (data for 165 cycles)

For comparison:

“hot” mixing = continuous RF heating of positron cloud

(suppression of antihydrogen production)

0 2 4 6 8 10 12-50

-100

-75

-125

Length (cm)

antiprotons


23A


Antiproton annihilation rate (charged trigger rate)

Background trigger rate ~ 0.5 Hz

High initial rate ~ 100 Hz


24A


Analysis Procedure

• Reconstruct annihilation vertex (103 k)

• Search for ‘clean’ 511 keV-photons:exclude crystals hit by charged particles+ its 8 nearest neighbours

• ‘511 keV’ candidate =400… 620 keVno hits in any adjacent crystals

• Select events with two ‘511 keV’ photons

• Reconstruction efficiency ~ 0.25 % = “golden” events !

Antihydrogen candidate (real data, 4-prong event)

Event reconstruction (165 mixing cycles ~ 2 days)


25A


Antihydrogen Signal (“golden” events)

Opening angle between two 511 keV photons(seen from charged particle vertex)

-1 -0.5 0 0.5 1020

40

60

80

100

120

140

160

180

200

Mixing with cold positrons

Mixing with hot positrons

131± 22 events

cos(Θγγ)

> 50,000 produced antiatoms (conservative estimate)Background: mixing with hot positrons

-1 -0.75 -0.502040

60

80

100120

140

160

180

200

cosΘγγ

AnγulrresoluτionfromMCsimulτion(normlisedτopekheiγhτ)

Bck-τo-bck511keVpekfromnτihydroγennnihilτion

Comparison with Monte Carlo

M. Amoretti et al., Nature 419, 456 (2002)


26A


Background measurements

Histogram:

Antiproton-only data (99,610 vertices, 5,658 clean 2-photon events plotted).

Dots:

Antiproton + cold positrons, but analyzed using an energy window displaced upward so as not to include the 511 keV photo-peak

-1 -0.5 0 0.5 10

20

40

60

80

100

120

140

160

180

200Antiprotons onlyCold mixing (displaced E window)γ

cos(Θγγ)

Opening angle between two 511 keV photons (seen from charged particle vertex)

M. Amoretti et al., Nature 419, 456 (2002)

Can antiproton annihilations on electrode fake back-to-back signal?

No !

1) Secondary e+ within 10 mm ~ 0.1 %

2) Monte Carlo - no peak

3) Measurement - no peak


27A


Antihydrogen = main source of annihilations

Hot

Time distribution of golden events and all annihilations Cold

X-Y vertex distribution


28A


Physics of antihydrogen production

ANTIHYDROGEN VERSUS BACKGROUND

ABSOLUTE PRODUCTION RATES

DEPENDENCE ON TEMPERATURE

ANGULAR DISTRIBUTION

PRELIMINARY


29A


Opening angle fit

Fit result: ~ 2/3 of the events are antihydrogen

Fit ResultFit Input MC

Hbar

Background

cos(γγ)

cos(γγ)

Data

Fit

Background

PRELIMINARY


30A


Vertex spatial distribution fit

=>

Antihydrogen on trap electrode Antihydrogen on trapped ions or rest gas Compare to cold mix data

Average fraction of antihydrogen65 ± 10 % during mixing !

In 2002, ATHENA produced0.7 ±± 0.3 Million antihydrogen atomsPRELIMINARY


31A


Rate of antihydrogen production

High Initial Rate (> 100 Hz)

High S/B (~ 10:1) in first seconds

Analysis:

• 65 ± 10 % antihydrogen

• ~ 50 % vertex / annihilation

PRELIMINARY


32A


Pulsed antihydrogen production

Mixing time

secsec

Vert

ex Z

pos

ition

Heat OnHeat On

Vert

ex C

ount

s

Mixing time ->

Heat OnHeat On

Switch positron heating Off/On resp. On / Off We observe:

Annihilation rate

Vertex distribution along z

Rise time ~ 0.4 s(Positron cooling time)

PRELIMINARY


33A


Antihydrogen Production - T dependence

Radiative Three-body

s(T) dependence T-0.5 T-4.5

Final state n < 10 n >> 100

Stability (re-ionization) high low

Expected rates ~ Hz ?


34A


Summary

First production and detection of cold antihydrogen- high positron accumulation rate = fast duty cycle

- sensitive detector = observe clear signals

High rate production- initial rate > 100 Hz, average rate ~ 10 Hz

Antihydrogen dominates annihilation signal (~ 2/3)

Pulsed antihydrogen production

Temperature dependence measured

Antihydrogen production at room temperature


35A


Outlook

More …

Increase formation rate

More antiprotons

Laser induced recombination

Trapping and cooling ...Anti-Hydrogen at E < 0.05 meV ?

Dense plasmas in magnetic multipole fields ?

Laser cooling? Collisions with ultra-cold hydrogen atoms?

Spectroscopy

High precision comparison 1S-2S

Hyperfine structure

Gravitational effectsE ~ 0.000 1 meV

Atom interferometry

Study …

Formation process

Next steps - technology Next steps - physics

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A DPG Frühjahrstagung Aachen 13.03.03 C. Regenfus Uni-Zürich 1 ATHENA - Cold antihydrogen...

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