Measuring g with a beam of antihydrogen (AEgIS)

C.CANALi a lstituto Nazionale di Fi.sica Nucleare and Department of Physics, University of Genoa, via Dodecaneso

33, 16146 Genova, Italy

The gravitational interaction between matter and antimatter has never been tested experi­mentally. According to some attempts to unify gravity with the other forces, the possibility that g(p) # g(p) cannot be excluded 1 . The AEGIS experiment 2 intends to measure for the first time the gravitational acceleration of antimatter using cold antihydrogen atoms. Antihydrogen atoms will be obtained tro�h a charge exchange process between Rydberg positronium atoms and antiprotons. Once H are accelerated to form a horizontal beam, they travel through a Moire deflectometer, able to measure the vertical displacement of atoms due to gravity. Knowing the velocity of the antiatoms from the time of flight measurement and the length of the flight path allows to estimate the gravity acceleration g for antihydrogen. With this setup an initial precision on the measure of g of 13 is expected.

1 Introduction

In recent years few experiments at CERN have demonstrated the feasibility of producing large amounts of antiatoms at low temperature 4 , 5 , 6 . This result opens a very interesting scenario of studies on fundamental symmetries between matter and antimatter such as the CPT invariance (through high precision spectroscopy) and direct measurements of the validity of the equivalence principle for antimatter (through ballistic experiments) .

A precise test of CPT could arise from measurements on gross structure, fine structure, Lamb shifts and hyperfine structures in antihydrogen to be compared with analougue measurements done on hydrogen. The CPT theory predicts that all these properties are identical for matter and antimatter systems. This kind of measurements, in principle, could reach a very high precision: a comparison of the 1S-2S frequency for hydrogen 7 and antihydrogen with a precision of 10-15 or higher will be the most accurate CPT tests for baryons regardless of any theoretical model.

While CPT test based on antihydrogen spectroscopy could give very precise results, at the same time antihydrogen can be used to perform for the first time a direct measurement of gravity on an antimatter system.

Such a kind of measurement could be in principle performed using charged particles (for example positrons) , bo.t a huge experimental trouble arises because the gravitational force is much weaker than the Coulomb force, and is virtually impossible to reduce electric fields to a negligible level (an electric field of only 6 - 10-1 1 V/m gives to a positron an acceleration equal to that of gravity) . This make H a simple (and neutral) system with which WEP can be directly tested.

The primary scientific goal of the AEGIS experiment is the direct measurement of the Earth

a on behalf of AEGIS collaboration

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c) Off-axis trap d) Porous e) Ps (n=l � n=20) Laser pulses

(''----.-1l'-tl Ps•0+:-l\ \ rn . L . ... .. o, ,, •. • • I m. •

\\LJ _____

.b,,,� ... �-

o-ld_? __ ii_ ... -._/ __ · ___ �_,�ii beam

-----+ B = 1T g) Stark acceleration

Figure 1: Formation of an horizzontal travelling beam of antihydrogen in AEgIS. Particles (e+ and overlinep are stored in cylindrical penning traps. The 7i production occur in few steps: positrons are sent with several keV energy on a porous target here they form positronium that is excited to Rydberg state with a double laser pulse. Rydberg positronium atoms (Ps* ) cross the cold p cloud producing H via charge exchange reaction: Ps* + p -+ H* + e - (see text for detailed explanation of each step) . A proper electric field accelerate horizzontally

Ji* atoms to form the beam.

's gravitational acceleration g on antihydrogen. CPT spectroscopy is included in the long term scientific goal of the experiment.

2 The AEglS experiment (Antimatter Experiment: Gravity, Interferometry, Spectre

2. 1 H beam formation

The AEgIS experiment is under construction at CERN, in the AD (Antiproton Decelerator) hall.

In Fig.I a scheme of the core of the AEgIS apparatus, with a sketch of the operations leading to the antihydrogen beam formation, is shown. Particles (e+ and p) are manipulated inside several cylindrical Penning traps: here an uniform axial magnetic field (B=I-5 T) ensure the radial confinement of charged particles while proper configuration of potentials applyed to the various segments of the trap provides to axial trapping of particles.

The Anti proton Decelerator delivers anti protons with a kinetic energy of 51VI e V in bunches of 2.5 · 107 particles within 100ns. In typical operations a bunch of p is delivered every c:: 200s. Antiprotons will be captured in a dedicated trap inside a 5T superconducting magnet: the use of fast high voltage pulses applyed to the entrance electrode of the trap will allow to capture more than 104 p at each cycle of AD. Once captured, antiprotons will be transferred in a second trap with a lower magnetic field (IT) where antihydrogen is produced and the beam is formed, as it will be discussed in the following.

A positron plasma (Ne+ c:: 108, density Ne+ c:: 108cm-3 ) is stored in the first penning trap (a, in Fig.I ) after being transfereed in this region from a Surko-type accumulator 3 .

At the same time in a second trap the anti protons cloud (b) i s cooled to lOOmK using electron cooling tecniques and a resistive tuned circuit. The cooling of antiprotons is a key­point of the whole experiment since this temperature determines the quality of the antihydrogen

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a)

Antihydrogen beam entrance

G 1 I

G2 I

I

I I I I 1 . I

_I

I

PSD counts b)

Figure 2: a) The antihydrogen beam travel trought two gratings (Gl and G2) and reach a position sensitive detector (PSD) where the annihilation point is detected. An interference pattern is shaped on the detector on the right. b) The interference pattern can be binned (modulo the grating period) . Lowering the beam velocity causes the pattern to shift down along the z-axis. Realistic values for the gratings system are L = 40crn, grating

period a = 80µ, grating size 20cm.

beam: obtaining cold p means having antihydrogen atoms with a velocity low enough to allow the gravity measurement: once p will be cooled at the same temperature of the ambient (lOOmK), they will have a velocity of few tens of m/s.

At this point positrons are moved off axis with a diocotron excitation 1 1 , they travel trought an off-axis trap ( c) where they are accelerated to several ke V , bunched and sent in direction of a target of porous material (d) . When the positrons hits this target with keV energy they penetrate inside the nanornetric-size channels of the target, it cools by collisions with the pore walls and form positroniurn (Ps ) : the long-life ortopositronium drifts outside the target and is excited by a double laser pulse (e) from ground state to Rydberg state (np5 > 20) just before it start crossing the cold antiprotons cloud. Cold antiprotons and Rydberg positronium react via charge exchange 13 :

Ps* + p -+ H* + e- ( 1 )

the cross section o f this reaction scales with ex nj,s and the produced H* i s i n its turn produced in excited Rydberg level. It's important to underline again that the temperature of produced antihydrogen is basically the same of antiprotons stored in the trap just before the interaction with Ps* . Immediatelly after their formation, Rydberg antihydrogen atoms will be accelerated via Stark effect (g) up to a velocity of several hundreds m/s to form an horizontally travelling beam. This tecqnique has been already demonstrated to work with hydrogen 1 4 .

2.2 Measure of the gravity acceleration g of H atoms

The antihydrogen beam will travel horizzontally along a path about lm long with a velocity of several hundreds m / s.

During its flight H fall in the gravitational field produced by the Earth. Assuming g = 9.Sm/s and a horizzontal velocity of 500m/s, the vertical deflection is too small (� lOµm) to be measured directly since a poor beam collimation must be taken into account. Nevertheless a moire deflectometer will make still possible to perform this measurement 1 2 •

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The device (Fig.2.a), sligtly modified respect to standard moire deflectometers, consists of two gratings (G 1 and G2) and a position sensitive detector (PSD) separated by a distance L '="' 40cm. Both gratings have a size of 20x20cm2 and a period of '="' 80µm. The PSD is a silicon microstrip detector with an active area of about 20x20cm2 , and a resolution of about lOµm working at cryogenic temperatures.

Since the width of the slits is much larger than the De Broglie wavelength of the antihydrogen, diffraction can be neglected and all effects will be purely classically, so the PSD just records a shadow pattern corresponding to the positions of antiatom annihilations. The velocity of the beam can be tuned changing the parameters of the Stark acceleration, so it is possible to measure the vertical deflection of the shadow path for several values of velocities. The precise velocity of H can be desumed from the time of flight measurement.

The position of anthydrogen will be detected reconstructing the annihilation point of each antiatom on the position sensitive detector. The detected positions of annihilations can be binned modulo the grating period as plotted in Fig.2.b. Here it's shown from Montecarlo results how the verical shift of the shadow pattern increase with lower horizzontal velocity of the beam, assuming g = 9.8m/s.

The use of this method allow to measure the gravity acceleration g of antihydrogen with a precision of 13 detecting 105 antihydrogen atoms: it will be the first direct measurement of gravity acceleration on an antimatter system.

3 Conclusion

Antihydrogen will be used in next years to investigate CPT validity and equivalence principle. Related to this latter topic, AEglS will use antiprotons delivered from AD (the antiproton decelerator at C'ERN) to produce an horizzontal beam of antihydrogen to measure the gravity acceleration g of antiatoms. The initial precision on the measured g is expected to be 13, and long and medium terms goals intends to improve noticeably this precision.

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