Reaction cross section measurements for the positive antihydrogen ion production in the GBAR...

Reaction cross section measurements for the positive antihydrogen ion

production in the GBAR experiment

L. LiszkayIRFU CEA Saclay, France

GBAR collaboration

L. Liszkay, P. Comini*, P. Debu, P. Pérez, J.M. Rey, Y. Sacquin, B. Vallage, D. P. van der Werf**IRFU CEA Saclay (France)A. Maia-Leite, Liam Dodd (PhD students)* present address: ETHZ (Switzerland)** permanent address: Univ. Swansea (UK)

François Nez, Pierre CladéLKB (Laboratoire Kastler-Brossel, Paris)

David LunneyAudric Husson (PhD student)CSNSM (Centre de Sciences Nucléaires et de Sciences de la Matière, Orsay)

Paul-Antoine Hervieux, Giovanni ManfrediIPCMS (Institut de Physique et Chimie des Matériaux de Strasbourg)

Participants (France, ANTION project)

The GBAR collaboration

Outline

• Introduction - the GBAR project• Antihydrogen ion creation for GBAR• Reaction cross section - past experiments• Reaction cross sections - theoretical results• Preparations for new cross section measurements

– Reaction zone– Detection– Positron pulse– Laser– Proton source– Antiproton beamline

• Summary and outlook

The GBAR experiment at CERN

• Direct observation of the gravitational free fall of antihydrogen (two experiments: AEGIS and GBAR)

• Requires very cold antihydrogen (10 µK)

• Distinctive idea: cool down posively charged antihydrogen ion, then photodetach the extra positron

• Antiprotons from CERN AD + ELENA (100 keV)

• Positrons from a linac-based source + high field trap (5T) (GBAR was accepted by CERN

research Board in 2012)

Scheme of GBAR at CERN

Positron source at CEA Saclay Laszlo Liszkay, SLOPOS-13, 19 Sept.2013 6

ELENA Decelerator

Linac W target Moderator Buncher e+ trap

100 keV

𝑝 𝑝

1 keV

e-

9 MeVe+

~1 MeVe+

eV

e+

keV

Posi

tron

ium

ta

rget

clo

ud

e+

keV

AD𝑝

5.3 MeV

Cooling

Lase

r

Experimental chamber with detectors

Lase

r

𝐻+¿1 keV

𝐻+¿20 µK

- e+𝐻

20 µKLa

ser

sCapture

1eV𝐻+¿

Positronium target cloud for the GBAR experiment

eHPsH

eHPspReactions in the cloud

Target cavity

107/pulse(~110 s)~3 keV

Positron-positronium converter (mesoporous SiO2)

eHPsH

eHPsp

eHPsH

eHPsp

Cross section measurements: aims

Measurement of the cross section of the following reactions of positronium (Ps, positron-electron bound system):

1. Hydrogen and negative hydrogen ion production

2. Antihydrogen and antihydrogen ion production

With positronium (Ps) is in the fundamental, 3D or 2P stateProton and antiproton in the 1-10 keV kinetic energy range

Earlier measurement

eHPsp

Merrison et al,, Phys. Rev. Lett. 78, 2828(1997)

Method: positron detection

• 10-16 keV proton energy

• Ground state Ps only

New theoretical calculations: proton reaction

P. Comini and P. –A. Hervieux, N. J. of Phys. 15, 095022 (2013)

eHPsp• Theoretical calculations by one of the partners (IPCMS)

• 2p & 3d states are good candidates

• Optimal energy depends on the state

New theoretical calculations: second reaction

P. Comini and P. –A. Hervieux, N. J. of Phys. 15, 095022 (2013)

eHPsH• Calculation for ground state H

• 4-body problem, quantitative results may be unreliable

• Cross section is much lower at excited state of H

• Max. at lowest energy (above threshold)

• 1s, 2p, 3d states OK

New theoretical calculations: two reactions in the Ps cloud

P. Comini and P. –A. Hervieux, N. J. of Phys. 15, 095022 (2013)

eHPsH

eHPsp

• Advantage of excited Ps state is not clear

• Measurements are needed to optimize the energy & ps state

• H* relaxation to ground state is important

New theoretical calculations: comparison with experiment

P. Comini and P. –A. Hervieux, N. J. of Phys. 15, 095022 (2013) Merrison et al,, Phys. Rev. Lett. 78, 2828(1997)

eHPsp

• Calculation agrees well with measurement for the proton reaction

• No test yet for the four-body reaction

Cross section meas.: target chamber and detector

(anti)proton beam (anti)atoms

protons

(anti)ions

grid

MCP

Positron pulse

Electricquadrupole

CCDcamera

Fast phosphor screen

Viewport

Faraday cup

Ion detection

• Camera: fast shutter (1µs)• MCP: switched (~400 ns)

Positronium target cloud

Detection of keV atoms with an MCP

Detection efficiency corresponds to the sensitive area of the MCP above ~1 keV (hydrogen)

• Gated MCP + fast phosphor screen• fast ( 1 µs) CCD camera

• Low background detection needed:• Suppress direct annihilation

gamma background• Suppress MCP noise• Suppress camera dark noise• Separate charged particles

Penning trap for e+ (RIKEN)

Slow e+ rate 3x106 s-1

e- linac(4.3 MeV)

e+/e- magnetic separator

W target+ W mesh moderator

The slow positron beam at Saclay (CEA/IRFU)

Slow positron drift tube (~10 eV)

Beam switch/user port (materials science)

Multiring trap: positron cooling by trapped electrons

5 T field

e+ beam

• Electron cooling• Switched entry (synchronized with linac pulses)• Bunched exit (short, intense pulse)

~10 mm diam.4-5 keV

1 x 10 mm ellipse

Positron optics after the trap

~100 Gauss (?)

• Electrostatic focussing• Exit from magnetic field with ~4 keV energy

Optical excitation of Ps

3D - 410 nm Doppler-free(in construction, GBAR)

2P - 243 nm frequency comb

Proton source

• Penning-discharge source with electrostatic focussing• The same source is used to develop the antiproton decelerator

Experiments at IRFU (Saclay) and CERN

• The positron beam intensity is sufficient only to measure the proton reaction

• Energy dependence will be measured• Reaction with Ps in 3D and 2P states will be measured

• The measurement will be continued at the stronger source at CERN

• When antiprotons are available, the antiproton and antihydrogen reaction will be performand

eHPsp

eHPsH

eHPsp

eHPsH

Cross section of the (anti)hydrogen-Ps reaction

• At the moment we have no tool to generate hydrogen atoms in the relevant energy range

• The cross section will be deduced from the quantity of ions generated in the two reactions in one Ps target cloud

• It is sufficient for GBAR but limits the precision of the cross section measurement

Second phase: measurements at CERN

• New positron source --> measurement of the double reaction (CEA source is too weak)

• Antiproton beam (AD+ELENA+GBAR decelerator) --> cross section of the reaction with antiproton and antihydrogen

Upgrade - GBAR at CERN

300 mm

4.3 MeV (magnetron)200 Hz, 2.5 µs pulse120 mA peak current70 µA average current

3x106 e+/s

9 MeV (Klystron)300 Hz 300 mA peak current200 µA average current

1x108 e+/s

~900 mmElectron target + moderator: same construction

Antiproton deceleration after ELENA

• Antiprotons at ~1-10 keV energy

• Switched decelerator

Summary

• Preparations for cross section measurements at IRFU• Continuation at CERN (ion creation, antiproton and

antihydrogen reaction)• Essential information for GBAR (optimal Ps state, proton

energy)

• See also talk of Sebastian Wolf (next talk)

The work is supported by the Agence National de la Recherche, project number ANR-14-CE33-0008-01