A moiré deflectometer for antimatter
S. Aghion et al. (AEgIS Collaboration), Nature Communications 5 (2014) 4538
The AEgIS Collaboration at CERN (an international consortium of 25 institutions1) has observed
the shadow ("moiré") pattern produced by antiprotons striking a pair of gratings made of narrow
equally spaced parallel slits. This result was obtained with part of the equipment foreseen for the
future study of antihydrogen properties.
The principle of the deflectometer is shown in the fig (a). A beam of antiprotons - the antimatter
counterparts of the proton - from CERN's Antiproton Decelerator, is slowed down by thin foils
before traversing a several meters long vacuum tube placed in a strong londitudi magnetic field.
Any obstacle on the antiproton path leads to its annihilation into secondary particles, such as pions.
A modulation of the antiproton intensity ("moiré" pattern) is observed by the annihilation detector
when obstructing the antiproton beam by a pair of gratings. The slits are 12 microns wide,
separated by 40 microns, etched in silicon wafers. The distance d separating the two gratings (25
mm) is equal to that between the second grating and the detector, made of emulsions, namely
photographic films especially developed for applications in nuclear and particle physics. Fig. (b)
shows the tracks left in the emulsions by the annihilation products from one antiproton, observed
with an automatic optical microscope. The “fat” tracks are due to nuclear fragments flying apart,
the faint tracks to pions. Emulsion detectors measure the annihilation point with an unbeatable
precision of 1 - 2 micron.
If one assumes that a force acts on the antiprotons traveling through the deflectometer, a
perpendicular component will lead to a displacement of the shadow pattern. A precise alignment
of the components is achieved by repeating the experiment using visible light of a suitable
wavelength, which is not subject to the disturbing force, and observing the interference pattern
with the emulsion detector. Fig. (c) shows that such a force is actually present. Its magnitude is
very tiny, about 500 aN (attonewtons, or 5 x 10-16 newton, 10 newton being the weight of a 1 kg
mass) leading to a displacement of the shadow fringes by about 10 microns. The force is either due
to a slight electric field gradient of 33 V/cm or to a small transverse magnetic field component of
about 7 gauss, encountered by the antiprotons along their 5 cm long path through the
deflectometer.
This result, achieved by combining standard techniques employed in the fields of atomic and
particle physics, constitutes the first observation of moiré patterns with antimatter. This paves the
way to detailed comparisons of the fundamental properties of antimatter with those of normal
matter, such as its behavior under the influence of gravity.
Figure caption:
a) Principle of the moiré deflectometer. The antiprotons enter from the left, traverse the grating
slits and annihilate in the emulsion detector.
b) Typical antiproton annihilation observed by the emulsion detector.
c) Displacement of the Moiré pattern under the influence of a disturbing force (from 145
annihilations).
a)
b)
c)
1
AEgIS Collaboration: Politecnico di Milano, Milan, Italy Istituto Nazionale di Fisica Nucleare, Sez. di Milano, Milan,Italy European Organization for Nuclear Research, Geneva, Switzerland Albert Einstein Center for Fundamental Physics, Laboratory for High Energy Physics, U. of Bern, Switzerland Institute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia University of Brescia, Brescia, Italy Istituto Nazionale di Fisica Nucleare, Sez. di Pavia, Pavia, Italy Kirchhoff Institute for Physics, University of Heidelber, Heidelberg, Germany Department of Physics, University of Trento and INFN-­‐TIFPA, Trento, Italy Laboratoire Aimé Cotton, CNRS, University of Paris-­‐Sud, Orsay, France University of Zurich, Physics Institute, Zurich, Switzerland University of Milan, Department of Physics, Milan, Italy Max Planck Institute for Nuclear Physics, Heidelberg, Germany Istituto Nazionale di Fisica Nucleare, Sez. di Genova, Genoa, Italy University of Bergen, Institute of Physics and Technology, Bergen, Norway University of Oslo, Department of Physics, Oslo, Norway University of Genoa, Department of Physics, Genoa, Italy Stefan Meyer Institute for Subatomic Physics, Austrian Academy of Sciences, Vienna, Austria University of Pavia, Department of Physics, Pavia, Italy Joint Institute for Nuclear Research, Dubna, Russia Istituto Nazionale di Fisica Nucleare, Sez. di Padova, Padua, Italy Claude Bernard University Lyon 1, Institute of Nuclear Physics of Lyon, Villeurbanne, France Czech Technical University in Prague, FNSPE, Prague 1, Czech Republic University of Bologna, Department of Physics and INFN Bologna, Bologna, Italy University of Napoli Federico II, Department of Physics, Naples, Italy