New road from CERN for matter-antimatter comparison

New road from CERN for matter-antimatter comparison

In Geneva, the BASE team carried out the most precise differential measurement ever of the charge and mass of protons and antiprotons

In 1928 the British physicist Paul Dirac hypothesized that for every particle there exists a corresponding antiparticle, exactly equal to the particle, but with opposite charge: for example, for the electron there should exist an "antielectron", or "positron", identical in all respects throughout, but with a positive electric charge. Intuition opened up the possibility of entire galaxies and universes made of antimatter, but when matter and antimatter come into contact, they annihilate, disappearing in a flash of energy. (Photo: CERN)
In 1928 the British physicist Paul Dirac hypothesized that for every particle there was a corresponding antiparticle, exactly equal to the particle, but with opposite charge: for example, for the electron there should exist an "antielectron", or "positron", identical in all respects throughout, but with a positive electric charge. Intuition opened up the possibility of entire galaxies and universes made of antimatter, but when matter and antimatter come into contact, they annihilate, disappearing in a flash of energy.
(Photo: CERN)

Analyzing the measurements of protons and antiprotons taken over the course of a year and a half of work at the “antimatter factory" of the CERN, a unique facility for the production and analysis of the complex of systems consisting of one or more antiparticles, the team of The Baryon Antibaryon Symmetry Experiment measured the electric charge/mass ratios of the proton and antiproton with record accuracy.
A Geneva the results found these to be identical within an experimental uncertainty of 16 parts per trillion.
“This result represents the most precise direct test of a fundamental symmetry between matter and antimatter, performed with particles composed of three quarks, known as baryons, and their antiparticles”, says the spokesperson for BASIS Stefan Ulmer.
According to the Standard model, which represents the best current theory of physicists about particles and their interactions, the elements of matter and antimatter may differ, for example in the way they transform into other particles, but most of their properties, including their masses, they should be identical.
Finding a small difference between the masses of gods protons and to the antiprotons, or between reports of their own electric charge and their mass, would break a fundamental symmetry of the Standard model, called "CPT symmetry”, and would indicate new physical phenomena beyond the Model.
The "CPT symmetry" is the fundamental symmetry of physical laws under transformations involving the simultaneous reversals of charge, parity and time.

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The BASE experiment (The Baryon Antibaryon Symmetry Experimen) at CERN in Geneva: the electronic cabinets or racks, the horizontal magnet of the Penning Trap and the orange cryostats are visible (Photo: CERN)
The BASE experiment (The Baryon Antibaryon Symmetry Experimen) at CERN in Geneva: the electronic cabinets or racks, the horizontal magnet of the Penning Trap and the orange cryostats are visible
(Photo: CERN)

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Such a difference could also shed some light on the why the universe is composed almost entirely of matter, even if in the Big Bang they should have create equal amounts of antimatter.
The differences between the particles of matter and antimatter which are consistent with the Standard model they are smaller by orders of magnitude such as to be able to explain this cosmic imbalance, experimentally observed by man for centuries.
To carry out these measurements of protons and antiprotons, the collaboration of The Baryon Antibaryon Symmetry Experiment  has confined the antiprotons and negative hydrogen ions, which are the negative particles that replace protons in this experiment, in a high-performance particle trap called Penning trap.

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An illustration of matter particles and antimatter antiparticles
An illustration of matter particles and antimatter antiparticles

December 2017-May 2019: 24.000 frequency comparisons

In this device, a particle follows a cyclical trajectory with a frequency, close to the cyclotron frequency, that varies with the strength of the trap's magnetic field and the charge-to-mass ratio of the particle.
Feeding alternately antiprotons and hydrogen ions negatively charged one at a time in the “systematic trap: every time”, the team of BASE measured, under the same conditions, the cyclotron frequencies of these two types of particles, making it possible to compare their charge-mass ratios.
Performed in four campaigns of experiments between December 2017 and May 2019, these measures have led to more than 24.000 comparisons of cyclotron frequency, each lasting 260 seconds, between the charge-to-mass ratios of the antiprotons and negatively charged hydrogen ions.

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The logo of the European Organization for Nuclear Research
The logo of the European Organization for Nuclear Research

Equal charge-to-mass ratios within the limit of 16 parts per trillion

From these comparisons, and after accounting for the difference between a proton and a negatively charged hydrogen ion, the researchers of BASE they found that reports charge-mass of protons and antiprotons are equal to within 16 parts per trillion.
“This result is four times more accurate than the previous best comparison of these ratios, and the charge-to-mass ratio is now the most precisely measured property of the antiproton”Says Stefan Ulmer.
“To achieve this precision, we made significant upgrades to the experiment and took the measurements when the antimatter factory was closed, using our antiproton tank, which can store antiprotons for years”.
Carry out measurements of frequency of the cyclotron when the antimatter factory is not in operation it is ideal, because the measurements are not affected by disturbances in the magnetic field of the experiment.
In addition to compare protons and antiprotons with unprecedented accuracyThe team BASE used his own measurements to place strict limits on models beyond the Standard model which violate theCPT symmetry”, as well as to test a fundamental physical law known as Weak Equivalence Principle.
According to this principle, different bodies in the same gravitational field undergo the same acceleration in the absence of frictional forces.

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The CERN low-energy antiproton accelerator in Geneva
The CERN low-energy antiproton accelerator in Geneva

The verification in the gravitational field on the earth's surface

Since the experiment BASE is placed on the surface of the Terra, its measures of cyclotron frequency of protons and antiprotons were carried out in the gravitational field on the earth's surface.
Any difference between the gravitational interaction of protons and antiprotons would result in a difference between the cyclotron frequencies of protons and antiprotons.
By sampling the changing gravitational field of the Terra as the planet orbits the Sun, the scientists of BASE they found no such difference and set a maximum value on this differential measure of three parts out of 100.
“This limit is comparable to the initial accuracy goals of experiments aiming to drop anti-hydrogen into the Earth's gravitational field”, followed Ulmer.
"BASE did not directly drop antimatter into Earth's gravitational field, but our measure of the influence of gravity on a baryonic antimatter particle is conceptually very similar, indicating no anomalous interaction between antimatter and gravity at the level of uncertainty reached."

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Bird's-eye view of The Baryon Antibaryon Symmetry Experiment at the Antimatter Factory of the European Organization for Nuclear Research in Geneva (Photo: CERN)
Bird's-eye view of The Baryon Antibaryon Symmetry Experiment at the Antimatter Factory of the European Organization for Nuclear Research in Geneva
(Photo: CERN)