The ALICE experiment at CERN makes precise comparison of light nuclei and antinuclei

The ALICE experiment at CERN makes precise comparison of light nuclei and antinuclei
Measurements of energy loss in the time-projection chamber enable the ALICE experiment to identify antinuclei (upper curves on the left) and nuclei (upper curves on the right) produced in the lead-ion collisions at the LHC.

The ALICE experiment at the Large Hadron Collider (LHC) at CERN has made a precise measurement of the difference between ratios of the mass and electric charge of light nuclei and antinuclei. The result, published today in Nature Physics, confirms a fundamental symmetry of nature to an unprecedented precision for light nuclei. The measurements are based on the ALICE experiment's abilities to track and identify particles produced in high-energy heavy-ion collisions at the LHC.

The ALICE collaboration has measured the difference between mass-to-charge ratios for deuterons (a proton, or hydrogen nucleus, with an additional neutron) and antideuterons, as well as for helium-3 nuclei (two protons plus a neutron) and antihelium-3 nuclei. Measurements at CERN, most recently by the BASE experiment, have already compared the same properties of protons and antiprotons to high precision. The study by ALICE takes this research further as it probes the possibility of subtle differences between the way that protons and neutrons bind together in nuclei compared with how their antiparticle counterparts form antinuclei.

"The by ALICE and by BASE have taken place at the highest and lowest energies available at CERN, at the LHC and the Antiproton Decelerator, respectively," said CERN Director-General Rolf Heuer. "This is a perfect illustration of the diversity in the laboratory's research programme."

The measurement by ALICE comparing the mass-to-charge ratios in deuterons/antideuterons and in helium-3/antihelium-3 confirms the fundamental symmetry known as CPT in these light nuclei. This symmetry of nature implies that all of the laws of physics are the same under the simultaneous reversal of charges (charge conjugation C), reflection of spatial coordinates (parity transformation P) and time inversion (T). The new result, which comes exactly 50 years after the discovery of the antideuteron at CERN and in the US, improves on existing measurements by a factor of 10-100.

The ALICE experiment records high-energy collisions of lead ions at the LHC, enabling it to study matter at extremely high temperatures and densities. The lead-ion collisions provide a copious source of particles and antiparticles, and nuclei and the corresponding antinuclei are produced at nearly equal rates. This allows ALICE to make a detailed comparison of the properties of the nuclei and antinuclei that are most abundantly produced. The experiment makes precise measurements of the curvature of particle tracks in the detector's magnetic field and of the particles' time of flight, and uses this information to determine the mass-to-charge ratios for the nuclei and antinuclei.

"The high precision of our time-of-flight detector, which determines the arrival time of particles and antiparticles with a resolution of 80 picoseconds, associated with the energy-loss measurement provided by our time-projection chamber, allows us to measure a clear signal for deuterons/antideuterons and helium-3/antihelium-3 over a wide range of momentum", said ALICE spokesperson Paolo Giubellino.

The measured differences in the mass-to-charge ratios are compatible with zero within the estimated uncertainties, in agreement with expectations for CPT symmetry. These measurements, as well as those that compare protons with antiprotons, may further constrain theories that go beyond the existing Standard Model of particles and the forces through which they interact.

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More information: Precision measurement of the mass difference between light nuclei and anti-nuclei, Nature Physics (2015) DOI: 10.1038/nphys3432
Journal information: Nature Physics

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Citation: The ALICE experiment at CERN makes precise comparison of light nuclei and antinuclei (2015, August 17) retrieved 24 June 2019 from
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Aug 18, 2015
The more closely we can constrain things like this, the better our physics gets. This isn't very sexy, but it's important to constraining just how different matter and antimatter are from one another. Ultimately, when we determine the cause of the matter/antimatter imbalance in the universe, we'll be more certain we've got it right because of lots of unsexy measurements like this that came out just the way we expected.

Aug 18, 2015
These measurements, ... may further constrain theories that go beyond the existing Standard Model

It would be interesting to know how this affects such theories.

Aug 18, 2015
@vp: Re matte/antimatter asymmetry or "baryogenesis":

"The three necessary "Sakharov conditions" are:

Baryon number B violation.
C-symmetry and CP-symmetry violation.
Interactions out of thermal equilibrium." [ https://en.wikipe...nditions ]

Having CPT symmetry as per the article confirms that you need to see C/CP violation instead. (The Hot Big Bang is a phase change of the universe that takes it out of equilibrium, so we are OK there. And baryon number violation is what we see - assuming AM was created as much - so hopefully it is possible in theory as well. =D)

The current sum of CP violations with baryons (standard particles) is too small I think. Maybe dark matter helps (since it too must have some form of M/AM symmetry breaking). But to know that we need some expert stepping in and explaining the constraints...

Aug 18, 2015
As far as the matter/antimatter imbalance in the universe that's only what we observe. I assume the energy content of matter and antimatter come from the same source, for example the dark energy, with a higher density of energy locked into matter and a lower density into antimatter. Regions of matter then displace the dark energy, causing the pressure of dark energy in these regions to be lower. The gradient of pressure between different densities of dark energy is gravity. If you think in terms of entropy then I understand this to be equivalent to Verlinde's theory of emergent gravity. Anyway for antimatter the gradient is reversed and antimatter is repulsive to itself, meaning it doesn't normally accrete. As a matter of fact its lowest energy state is when every antiparticle is separated as far away from every other antiparticle as possible. Extremely difficult to detect, but yes it's out there. Think of it this way: if you're traveling by space there will be ample fuel out there.

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