The pear shape is special because it means the neutrons and protons, which compose the nucleus, are in slightly different places along an internal axis according to the physicists who discovered the first direct evidence of pear-shaped nuclei in exotic atoms that may help explain why the "Big Bang" created more matter than antimatter and the measurement of the effects of gravity on antimatter. IMAGE: The small small pear-shaped Rose apple (aka Plum Rose, Makopa or Malay Apple) is THE actual falling apple that led Sir Isaac Newton to discover the law of gravity. (thesantosrepublic.com)

May 15, 2013 (TSR) – An international team of physicists have shown that some atomic nuclei can assume the shape of a pear which contributes to our understanding of nuclear structure and the underlying fundamental interactions.

Most nuclei that exist naturally are not spherical but have the shape of a rugby ball.  While state-of-the-art theories are able to predict this, the same theories have predicted that for some particular combinations of protons and neutrons, nuclei can also assume very asymmetric shapes, like a pear where there is more mass at one end of the nucleus than the other.

The findings could advance the search for a new fundamental force in nature that could explain why the Big Bang created more matter than antimatter—a pivotal imbalance in the history of everything.

“If equal amounts of matter and antimatter were created at the Big Bang, everything would have annihilated, and there would be no galaxies, stars, planets or people,” said Tim Chupp, a University of Michigan professor of physics and biomedical engineering and co-author of a paper on the work published in the May 9 issue of Nature.

Peter Butler: “Our findings contradict some nuclear theories and will help refine others”. (Photo: University of Liverpool/thesantosrepublic.com)
Peter Butler: “Our findings contradict some nuclear theories and will help refine others”. (Photo: University of Liverpool/thesantosrepublic.com)

Antimatter particles have the same mass but opposite charge from their matter counterparts. Antimatter is rare in the known universe, flitting briefly in and out of existence in cosmic rays, solar flares, and particle accelerators like CERN’s Large Hadron Collider, for example.

When they find each other, matter and antimatter particles mutually destruct or annihilate.

Confirmed: Pear shape is Special

The shape of 224Ra deduced from the CERN measurements. (Photo: CERN/thesantosrepublic.com)
The shape of 224Ra deduced from the CERN measurements. (Photo: CERN/thesantosrepublic.com)

What caused the matter/antimatter imbalance is one of physics’ great mysteries. It’s not predicted by the Standard Model—the overarching theory that describes the laws of nature and the nature of matter.

The Standard Model describes four fundamental forces or interactions that govern how matter behaves: Gravity attracts massive bodies to one another. The electromagnetic interaction gives rise to forces on electrically charged bodies. And the strong and weak forces operate in the cores of atoms, binding together neutrons and protons or causing those particles to decay.

Physicists have been searching for signs of a new force or interaction that might explain the matter-antimatter discrepancy. The evidence of its existence would be revealed by measuring how the axis of nuclei of the radioactive elements radon and radium line up with the spin.

Researchers confirmed that the cores of these atoms are shaped like pears, rather than the more typical spherical orange or elliptical watermelon profiles. The pear shape makes the effects of the new interaction much stronger and easier to detect.

“The pear shape is special,” Chupp says. “It means the neutrons and protons, which compose the nucleus, are in slightly different places along an internal axis.”

The pear-shaped nuclei are lopsided because positive protons are pushed away from the center of the nucleus by nuclear forces, which are fundamentally different from spherically symmetric forces like gravity.

“The new interaction, whose effects we are studying does two things,” Chupp says. “It produces the matter/antimatter asymmetry in the early universe and it aligns the direction of the spin and the charge axis in these pear-shaped nuclei.”

Electromagnetic impulse and Electric dipole moments (EDM)

The experimental observation of nuclear pear shapes is important for understanding the theory of nuclear structure and for helping with experimental searches for electric dipole moments (EDM) in atoms.

The Standard Model of particle physics predicts that the value of the EDM is so small that it lies well below the current observational limit.  However, many theories that try to refine this model predict EDMs that should be measurable.

In order to test these theories the EDM searches have to be improved and the most sensitive method is to use exotic atoms whose nucleus is pear-shaped.  Quantifying this shape will therefore help with experimental programmes searching for atomic EDMs.

Most nuclear isotopes predicted to have pear shapes have been out of reach of experimental techniques to measure them.

With beams of very heavy, radioactive nuclei can be produced only in high-energy proton collisions with a uranium carbide target. Only when the atom beams are accelerated and smashed into targets of nickel, cadmium and tin can you measure them, but due to the repulsive force between the positively charged nuclei, nuclear reactions were not possible.

To determine the shape of the nuclei, the physicists produced beams of exotic—short-lived—radium and radon atoms at CERN’s Isotope Separator facility ISOLDE. They selectively extracted using their chemical and physical properties before being accelerated to 8% of the speed of light and allowed to impinge on a target foil of isotopically pure nickel, cadmium or tin. The nuclei were then excited to higher energy levels, producing gamma rays that flew out in a specific pattern that revealed the pear shape of the nucleus.

When this happens the relative motion of the heavy accelerated nucleus and the target nucleus creates an electromagnetic impulse that excites the nuclei. By studying the details of this excitation process it is possible to understand the nuclear shape.

This method has been used successfully to study the shape of short-lived isotopes 220Rn and 224Ra.  The data show that while 224Ra is pear-shaped, 220Rn does not assume the fixed shape of a pear but rather vibrates about this shape.

Co-author Peter Butler, from the University of Liverpool’s Department of Physics who carried out the measurements, said: “Our findings contradict some nuclear theories and will help refine others. The measurements will also help direct the searches for atomic EDMs currently being carried out in North America and in Europe, where new techniques are being developed to exploit the special properties of radon and radium isotopes.”

“Our expectation is that the data from our nuclear physics experiments can be combined with the results from atomic trapping experiments measuring EDMs to make the most stringent tests of the Standard Model, the best theory we have for understanding the nature of the building blocks of the universe,” he added.

CERN reveals new matter – antimatter difference: Behavior

In April, the LHCb collaboration at CERN has made its first observations of matter – antimatter asymmetry in the decays of the particle known as the B0s.

Matter and antimatter are thought to have existed in equal amounts at the beginning of the universe, but today the universe appears to be composed essentially of matter.

By studying subtle differences in the behavior of particle and antiparticles, experiments at the LHC who are seeking to cast light on this dominance of matter over antimatter has observed a preference for matter over antimatter known as CP-violation in the decay of neutral B0s particles – only the fourth subatomic particle known to exhibit such behavior.

Scientists at the University of Liverpool have played a significant role in this, by contributing to the findings through the construction of the detectors inside the experiment.

The VELO sub-detector is key to selecting Bs mesons from all other particles produced inside LHCb. It has 42 modules containing half-moon-shaped silicon detectors, which were designed, assembled and tested at the University of Liverpool.

The detectors can locate particles to within a hundredth of a millimeter, within millionths of a second. It is this precision that allows physicists to reconstruct the very short flight distance characteristic of a Bs meson, on a timescale that allows this signature to be recognised in real time, so that the data is recorded to make the measurement.

Understanding the nature of antimatter is still very much an open and important question, this new discovery revealed another layer in our search to understand what makes antimatter that little bit different to normal matter.

The discovery reaffirmed that our understanding of how matter and antimatter behave is remarkably consistent – but also that the differences we’ve seen between them are too small to explain why we live in a universe dominated by matter.

The effect of gravity on antimatter and matter is the same

The study found antihydrogen and hydrogen had the same gravity. (Photo: University of Liverpool/thesantosrepublic.com)
The study found antihydrogen and hydrogen had the same gravity. (Photo: University of Liverpool/thesantosrepublic.com)

Scientists at the University of Liverpool have also measured this month for the first time the effect of gravity on antihydrogen – the antimatter counterpart of hydrogen – marking an important step in understanding how antimatter behaves. The work is published in Nature Communications.

Whilst scientific evidence led scientists to assume that antihydrogen had exactly the same properties as hydrogen, it had not been proven.

The researchers were part of CERN’s international ALPHA experiment which performed measurements on antihydrogen.  Supported by the Engineering and Physical Sciences Research Council (EPSRC), the University of Liverpool’s Semiconductor Centre group was responsible for building, maintaining and operating the silicon vertex detector which detects the presence of the antimatter particle of antihydrogen and allows gravity measurements to be made on antimatter particles.

The scientists trapped and then released antihydrogen atoms in order to measure its freefall gravity which enabled them to determine the ratio of antihydrogen’s gravitational mass to its inertial mass.

If it was exactly one to one then antimatter and matter responded to gravity in the same way and therefore had the same properties.

However, due to the uncertainty in making this measurement, the team were able to put an upper limit on this ratio of lower than one to 75, in the absence of systematic errors.  They also obtained a similar limit for the behaviour under antigravity but this still meant they responded in the same way.

“We studied antihydrogen and made detailed comparisons with ordinary hydrogen and found that they had the same gravity,” Professor Paul Nolan, from the Department of Physics who led the Liverpool team, said.

“Although the data does not allow a more accurate limit to be determined yet, future developments to the apparatus and methodology will allow the researchers to improve these measurements and test more accurately for possible deviations,” he further added.

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