ALPHA EXPERIMENT OBSERVES THE LIGHT SPECTRUM OF ANTIMATTER FOR THE FIRST TIME

COMUNICADO DO CERN SOBRE LUZ VINDA DA ANTIMATÉRIA:

Geneva, 19 December 2016. In a paper published today in the journal
"Nature", the ALPHA collaboration reports the first ever measurement on
the optical spectrum of an antimatter atom. This achievement features
technological developments that open up a completely new era in
high-precision antimatter research. It is the result of over 20 years of
work by the CERN antimatter community.

_“Using a laser to observe a transition in antihydrogen and comparing
it to hydrogen to see if they obey the same laws of physics has always
been a key goal of antimatter research,”_ said Jeffrey Hangst,
Spokesperson of the ALPHA collaboration.

Atoms consist of electrons orbiting a nucleus. When the electrons move
from one orbit to another they absorb or emit light at specific
wavelengths, forming the atom's spectrum. Each element has a unique
spectrum. As a result, spectroscopy is a commonly used tool in many
areas of physics, astronomy and chemistry. It helps to characterise
atoms and molecules and their internal states. For example, in
astrophysics, analysing the light spectrum of remote stars allows
scientists to determine their composition.

With its single proton and single electron, hydrogen is the most
abundant, simple and well-understood atom in the Universe. Its spectrum
has been measured to very high precision. Antihydrogen atoms, on the
other hand are poorly understood. Because the Universe appears to
consist entirely of matter, the constituents of antihydrogen atoms –
antiprotons and positrons – have to be produced and assembled into
atoms before the antihydrogen spectrum can be measured. It’s a
painstaking process, but well worth the effort since any measurable
difference between the spectra of hydrogen and antihydrogen would break
basic principles of physics and possibly help understand the puzzle of
the matter-antimatter imbalance in the Universe.

Today’s ALPHA result is the first observation of a spectral line in an
antihydrogen atom, allowing the light spectrum of matter and antimatter
to be compared for the first time. Within experimental limits, the
result shows no difference compared to the equivalent spectral line in
hydrogen. This is consistent with the Standard Model of particle
physics, the theory that best describes particles and the forces at work
between them, which predicts that hydrogen and antihydrogen should have
identical spectroscopic characteristics.

The ALPHA collaboration expects to improve the precision of its
measurements in the future. Measuring the antihydrogen spectrum with
high-precision offers an extraordinary new tool to test whether matter
behaves differently from antimatter and thus to further test the
robustness of the Standard Model.

ALPHA is a unique experiment at CERN’s Antiproton Decelerator
facility, able to produce antihydrogen atoms and hold them in a
specially-designed magnetic trap, manipulating antiatoms a few at a
time. Trapping antihydrogen atoms allows them to be studied using lasers
or other radiation sources.

_“Moving and trapping antiprotons or positrons is easy because they
are charged particles,”_ said Hangst. _“But when you combine the two
you get neutral antihydrogen, which is far more difficult to trap, so we
have designed a very special magnetic trap that relies on the fact that
antihydrogen is a little bit magnetic.”

Antihydrogen is made by mixing plasmas of about 90 000 antiprotons from
the Antiproton Decelerator with positrons, resulting in the production
of about 25 000 antihydrogen atoms per attempt. Antihydrogen atoms can
be trapped if they are moving slowly enough when they are created.
Using a new technique in which the collaboration stacks anti-atoms
resulting from two successive mixing cycles, it is possible to trap on
average 14 anti-atoms per trial, compared to just 1.2 with earlier
methods. By illuminating the trapped atoms with a laser beam at a
precisely tuned frequency, scientists can observe the interaction of the
beam with the internal states of antihydrogen. The measurement was done
by observing the so-called 1S-2S transition. The 2S state in atomic
hydrogen is long-lived, leading to a narrow natural line width, so it is
particularly suitable for precision measurement.

The current result, along with recent limits on the ratio of the
antiproton-electron mass established by the ASACUSA collaboration, and
antiproton charge-to-mass ratio determined by the BASE collaboration,
demonstrate that tests of fundamental symmetries with antimatter at CERN
are maturing rapidly.