Antiparticle Trapping for Antihydrogen Physics
Mike Charlton, Physics
Swansea University
UK
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Summary of the Talk
Motivation for Antihydrogen Experiments
Spectroscopic comparisons with H; test of CPT
Measurement of gravitational interaction of antimatter with matter
Antiproton Production, Collection and Manipulation
Moderation, capture, cooling (I), compression and cooling (II)
Positron Production, Collection and Manipulation
Moderation, accumulation, compression (I), transfer, compression (II) and
diagnostics
Antihydrogen Production
Selected results from ATHENA
ALPHA – an Antihydrogen Trapping Experiment
Recent progress
Concluding Remarks
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Motivation for Antihydrogen Experiments
| Antihydrogen | = | Hydrogen | ?
CPT Theorem (Based upon Lorentz Invariance, spin-statistics and locality )
Some of the
most precise
tests of CPT
Relative precision
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Motivation for Antihydrogen Experiments
| Antihydrogen | = | Hydrogen | ?
Gravity
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antiprotons: CERN’s “Accelerators”
The Antiproton Decelerator
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antiprotons: the AD, Antiproton Decelerator
From PS:
1.5x1013 protons/bunch, 26 GeV/c
2 Injection at 3.5 GeV/c
1 Antiproton
Production
4 Extraction
( 2x107 in 200 ns)
g
ATRAP
ool
in
3 Deceleration and
ASACUSA
Sto
cha
st
ic C
Cooling
(3.5 - 0.1 GeV/c)
0
10
20 m
Electron Cooling
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
ATHENA
ALPHA
Antiprotons: Capture and Cooling
The trap walls were cooled to 15 K
ATHENA
Antiproton Capture Trap
Similar apparatus used currently in ALPHA;
method originally devised by Gabrielse and
co-workers (PRL, 63, 1360 (1989))
To (or close to) the
trap temperature
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antiprotons: Capture in ALPHA
Andresen et al., ALPHA collaboration
~ 25k pbars
N.B. magnetic field difference
ALPHA will routinely stack up to 8 shots from the AD or
~ 2 x 105 pbars into mixing
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antiprotons: Compression in ALPHA
G. Andresen et al., Phys. Rev. Lett.
101 (2008) 203401
Tailor size of electron plasma
to maximise the fraction of
cooled antiprotons – tuned
empirically as it depends upon
the AD output and other
experimental parameters
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antiprotons: ALPHA-Sympathetic Compression using Electrons
Sympathetic
compression of an
antiproton cloud
by electrons
G. Andresen et al., Phys. Rev. Lett.
101 (2008) 203401
Typically use a fixed frequency
rotating wall technique at 10 MHz
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antiprotons: ALPHA – Evaporative Cooling
Recently published; Andresen et al. PRL (2010) 105 013003
23 K
9K
19 K
325 K
57 K
1040 K
Typically (9 ± 4) K is lowest achievable at the
lowest well available at which (6 ± 1) % of the
initial antiprotons remain
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antiprotons: ALPHA – Evaporative Cooling
Recently published; Andresen et al. PRL (2010) 105 013003
Rate equation model
Small annihilation term
Evaporation time
constant
Heating term due to
plasma expansion
α is related to the ratio of the barrier height and the
total (KE+PE) energy of the particle in the well. It is
around eV/2kT = η for our situation.
τev ~ τcolηeη with τcol the antiproton-antiproton
collision time
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antiprotons: So far …
Antiprotons into the AD at ~ 3.5 GeV (~3x107 from 1.5x1013 protons at 26 GeV)
~ 100 s of cooling in the AD to 5.3 MeV; ejection in a 100 ns burst
Capture and electron cooling in a Penning-Malmberg trap for ~ 20 s (ε ~ 10-3)
Stacking of up to 8 AD shots. Takes ~ 1000 s for ~ 2 x105 cold antiprotons
Shuffle to 1 T region. Recool and sympathetic radial compression for about 60 s
Evaporative cooling if desired to very low temperatures. Takes ~ 10 s
… Now ready for mixing with positrons …
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Positrons: Accumulation – ATHENA and ALPHA
Segmented electrode
for Rotating Wall
300 Gauss guiding fields
Coldhead
T=6K
50 mCi 22Na
Solid neon moderator
Trap electrode voltages
Beam strength:
6 million e+ per second
Energy loss through collisions
e+
Based upon the industry standard …
{Solid-Ne moderator
+
UCSD Penning Malmberg buffer gas trap}
e+
Distance along the trap
e  ( Ei )  N 2  e  ( E f )  N 2*
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Positrons: Compression – ALPHA
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Positrons: Accumulation – ATHENA and ALPHA
Accumulated positrons / millions
200
Open circles:
no rotating electric field
150
Closed circles:
rotating field applied
100
50
0
0
200
400
600
These data using the lazy
approach – fixed rotating
wall frequency during
accumulation – N2 gas only
Accumulation time / sec.
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Positrons: Compression – ALPHA
Rotating Wall (Dipolar) @ 600 kHz
for entire accumulation cycle
No RW
N2 only
Data are a projection of the
rotationally symmetric distribution
N2 with CO2 cooling gas
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Positrons: Transfer – ATHENA and ALPHA
ATHENA positron transfer protocol –
time variation of the electrode voltages
Dynamic recapture – positron
bounces
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Positrons: Transfer and Compression II – ATHENA and
ALPHA
Rotating Wall: Frequency: 15 MHz, Amplitude: 3 V, mθ = 1
Amplifier saturation limit!
Positron stacking study in ATHENA
Rotating Wall compression:
n = 2.6 x 1010 cm-3 achieved
Amoretti et al., PRL 91 55001 (2003) and Phys. Plasmas, 10 3056 (2003) and
Funakoshi et al., PRA 76 012713 (2007)
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Positron manipulation in the single particle regime
Approximate position of
the positron cloud
Swansea 2-stage positron accumulator;
uses nitrogen buffer gas and SF6 as a
cooling gas. Incorporates 4-way
segmented electrode for rotating wall
compression
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Positron manipulation in the single particle regime
Radius of the positron cloud versus time after switching on the rotating wall
At a fixed frequency: fit
an exponential plus a
constant to derive the
compression rate.
Related to similar work by Greaves and Moxom, Phys. Plasmas, 15 072304 (2008) where the
underlying mechanism for the transport was attributed (tentatively) to “bounce resonance transport”
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Positron manipulation in the single particle regime
Curve is the theory for
a harmonic trap with
an asymmetric
oscillating dipole with
buffer gas cooling
modelled by a Stokestype term.
[Developed by CA
Isaac (Swansea)]
Frequency response around the axial frequency
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antihydrogen Production: ATHENA
Fill positron well in mixing region with 75·106 positrons;
1.
allow them to cool to ambient temperature (15 K)
2.
Launch 104 antiprotons into mixing region
3.
Mixing time 190 sec - continuous monitoring by detector
4.
Repeat cycle every 5 minutes
For comparison:
-125
“hot” mixing = continuous
RF heating of positron cloud
antiprotons
-100
(suppression of formation of
antihydrogen)
-75
-50
0
2
4
6
8
Length (cm)
10
12
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antihydrogen Detection: ATHENA
• Charged tracks to reconstruct antiproton annihilation vertex.
• Identify 511 keV photons from e+-e- annihilations.
• Identify space and time coincidence of the two.
511 keV 

Silicon micro
strips

CsI
crystals

511 keV

Two annihilation events
from antihydrogen which
strikes the wall of the
charged particle traps
• Compact (3 cm thick)
• Solid angle > 70%
• High granularity
• Operation at 140K, 3 T
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antihydrogen Production: ATHENA
200
180
160
Cold Mixing :
103270 vertices,
7125 2x511keV events
Cold mixing
Hot mixing
131± 22 events
(or about 50,000 antihydrogen atoms made)
140
120
Antihydrogen suppressed
100
80
No peak
60
Hot Mixing :
Scaled (x1.6) to 165 mixing
cycles.
40
20
0
-1
-0.5
0
cos()
0.5
1
Amoretti et al., Nature 419 456 (2002)
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antihydrogen: Selected results from ATHENA
ATHENA Vertex Z Distribution
Madsen et al., PRL 94 033403 (2005)
-125
antiprotons
-100
-75
-50
0
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
2
4
6
8
Length (cm)
10
12
Antihydrogen: Selected results from ATHENA
Behaviour of antihydrogen annihilations versus time
Example of use of plasma modes to monitor change
in plasma temperature
See Fujiwara et al., PRL 101 053401 (2008)
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antihydrogen: Selected results from ATHENA
P is important T-scaling parameter for Hbar formation
See Fujiwara et al., PRL 101 053401 (2008)
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antihydrogen Production: Formation Processes
+
e  p  H  h

Radiative
Three-body
e  e  p  H  e
Radiative
Three-body
Rate T dependence
T-0.6
T-4.5
Final state
n < 10
n >> 10
Stability (re-ionization)
high
low
Expected rates
~10s Hz
fast ???
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Antihydrogen Production: Formation Processes
The TBR is a quasi-elastic encounter of 2 positrons in the
vicinity of an antiproton. Energy exchange ~ kBTe, which will
be the same order of the binding energies.
Thus, these are very weakly bound states which are strongly
influenced by the ambient fields
Electric and magnetic fields of the Penning trap
AND
The plasma self electric field
Er (r )  ne er / 2 0
The combination of Er and Bz results in a tangential drift speed, which to 2nd
order is given by:
v d  E (r ) / B  mE(r ) 2 / eB 3 r
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
ALPHA Collaboration – 2009-10
University of Aarhus: G.B. Andresen, P.D. Bowe, J.S. Hangst
Antihydrogen
Laser PHysics Apparatus
Auburn University: F. Robicheaux
University of British Columbia: W.N. Hardy, S. Seif El Nasr
University of Calgary: T. Friesen, R. Hydomako, R.I. Thompson
University of California, Berkeley: M. Baquero-Ruiz, C. Bray, S. Chapman, J. Fajans, A. Povilus, C. So, J.S.
Wurtele
University of Liverpool: P. Nolan, P. Pusa
NRCN, Negev: E. Sarid
Riken: D. M. Silveira, Y. Yamazaki
Federal University of Rio de Janeiro: C.L. Cesar, R. Lambo
Simon Fraser University : M.D. Ashkezari, M.E. Hayden
York University, Toronto : S. Menary
Swansea University: W. Bertsche, E. Butler, M. Charlton, A. Humphries, S. Jonsell *, L. V. Jørgensen, S.J.
Kerrigan, N. Madsen, D.P. van der Werf, D. Wilding (* and Fysikum, University of Stockholm)
University of Tokyo: R.S. Hayano
TRIUMF: M. C. Fujiwara, D.R. Gill, L. Kurchaninov, K. Olchanski, A. Olin, J.W. Storey
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
ALPHA: An Antihydrogen Trapping Experiment
Main Aim
To superimpose a magnetic well neutral trap onto an
antihydrogen production and detection apparatus. Thus, to trap
antihydrogen to promote spectroscopic comparisons with
hydrogen.
Complexities are many including;
Effect of neutral trap fields on stability of charged particle clouds
Detection involves pion trajectory detection and vertex
reconstruction …
Cryogenic traps …
Laser access …
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
ALPHA: An Antihydrogen Trapping Experiment
Ioffe-Pritchard Geometry
B
quadrupole winding
Solenoid field
is the minimum
in B
mirror coils
 
U    B
N.B.
Well depth ~ 0.7 K/T
Based on Berkeley/Swansea results: standard quadrupole
not a good idea …
field gradient across charged plasmas is too great;
see Fajans et al., Phys. Rev. Lett. 95 155001 (2005)
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Plasma lifetimes
drastically
reduced in the
presence of
quadrupolar field
ALPHA: An Antihydrogen Trapping Experiment
1
0.8
Quadrupole
Magnetic field
normalised to value
at electrode wall
B/Bw
0.6
Octupole
0.4
0.2
0
-0.2
0
0.2
0.4
0.6
r/rt
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
0.8
1
Radius in trap
ALPHA: An Antihydrogen Trapping Experiment
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
ALPHA: An Antihydrogen Trapping Experiment
Correlation of the field ionized signal
with that from annihilation on the
wall of the trap
Physical cause of drop unclear, but it is caused by the presence of the octupolar field
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
ALPHA: An Antihydrogen Trapping Experiment
With the neutral trap off
With the neutral trap on
Image of the
Penning trap
electrode
Projections along
the magnetic field
axis
N.B. extra features
with the trap on
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
ALPHA: An Antihydrogen Trapping Experiment
Searching for trapped antihydrogen
Shut off magnetic minimum trap (1/e time ~ 9 ms)
Interrogate output of vertex detector in 30 ms time window
after the shut off
Apply cuts to data to reject cosmic ray events
a) Antiproton
annihilation
b) Cosmic
ray
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
ALPHA: An Antihydrogen Trapping Experiment
We have observed a few events like this in
the 30 ms time window, and distributed in
the apparatus in the way in which
released antihydrogen would be expected
to behave.
Work is ongoing to assess
systematic effects …
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Concluding Remarks
Trapping antihydrogen is a lot harder than making it!
Great care has to be taken manipulating plasmas to keep them
cold enough.
Many manipulations on both the positrons and antiprotons must
be done to prepare them for mixing.
Our apparatus seems to work well, and work is progressing ….
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010
Acknowledgements
Members of the ATHENA collaboration
Members of the ALPHA collaboration
Colleagues at Swansea
UK financial support from EPSRC
AD staff and all support from CERN
Antiparticle Trapping for Antihydrogen Physics
ECTI, Co. Durham – September 19-24 2010