New insights into the structure of the neutron

Date:
November 8, 2021
Source:
Johannes Gutenberg Universitaet Mainz
Summary:
An international research team has measured neutron form factors
with previously unattained precision.



FULL STORY ==========================================================================
All known atomic nuclei and therefore almost all visible matter consists
of protons and neutrons, yet many of the properties of these omnipresent natural building blocks remain unknown. As an uncharged particle, the
neutron in particular resists many types of measurement and 90 years
after its discovery there are still many unanswered questions regarding
its size and lifetime, among other things. The neutron consists of three
quarks which whirl around inside it, held together by gluons. Physicists
use electromagnetic form factors to describe this dynamic inner structure
of the neutron. These form factors represent an average distribution of electric charge and magnetization within the neutron and can be determined
by means of experimentation.


========================================================================== Blank space on the form factor map filled with precise data "A single form factor, measured at a certain energy level, does not say much at first," explained Professor Frank Maas, a researcher at the PRISMA+ Cluster
of Excellence in Mainz, the Helmholtz Institute Mainz (HIM), and GSI Helmholtzzentrum fu"r Schwerionenforschung Darmstadt. "Measurements of the
form factors at various energies are needed in order to draw conclusions
on the structure of the neutron." In certain energy ranges, which are accessible using standard electron-proton scattering experiments, form
factors can be determined fairly accurately. However, so far this has
not been the case with other ranges for which so-called annihilation
techniques are needed that involve matter and antimatter mutually
destroying each other.

In the BESIII Experiment being undertaken in China, it has recently
proved possible to precisely determine the corresponding data in the
energy range of 2 to 3.8 gigaelectronvolts. As pointed out in an article published by the partnership in the current issue of Nature Physics, this
is over 60 times more accurate compared to previous measurements. "With
this new data, we have, so to speak, filled a blank space on the neutron
form factor 'map', which until now was unknown territory," Professor
Frank Maas pointed out. "This data is now as precise as that obtained
in corresponding scattering experiments. As a result, our knowledge
of the form factors of the neutron will change dramatically and as
such we will get a far more comprehensive picture of this important
building block of nature." Truly pioneering work in a difficult field of research To make inroads into completing the required fields of the form
factor 'map', the physicists needed antiparticles. The international partnership therefore used the Beijing Electron-Positron Collider II
for its measurements. Here, electrons and their positive antiparticles,
i.e., positrons, are allowed to collide in an accelerator and destroy
each other, creating other new particle pairs -- a process known as 'annihilation' in physics. Using the BESIII detector, the researchers
observed and analyzed the outcome, in which the electrons and positrons
form neutrons and anti-neutrons. "Annihilation experiments like
these are nowhere near as well-established as the standard scattering experiments," added Maas. "Substantial development work was needed to
carry out the current experiment -- the intensity of the accelerator had
to be improved and the detection method for the elusive neutron had to be practically reinvented in the analysis of the experimental data. This was
by no means straightforward. Our partnership has done truly pioneering
work here." Other interesting phenomena As if this was not enough,
the measurements showed the physicists that the results for the form
factor do not produce a consistent slope relative to the energy level,
but rather an oscillating pattern in which fluctuations become smaller as
the energy level increases. They observed similar surprising behavior in
the case of the proton -- here, however, the fluctuations were mirrored,
i.e., phase-shifted. "This new finding indicates first and foremost that nucleons do not have a simple structure," Professor Frank Maas explained.

"Now our colleagues on the theoretical side have been asked to develop
models to account for this extraordinary behavior." Finally, on the
basis of their measurements, the BESIII partnership has modified how
the relative ratio of the neutron to proton form factors needs to be
viewed. Many years ago, the result produced in the FENICE experiment
was a ratio greater than one, which means that the neutron must have
a consistently larger form factor than the proton. "But as the proton
is charged, you would expect it to be completely the other way round,"
Maas asserted. "And that's just what we see when we compare our neutron
data with the proton data we've recently acquired through BESIII. So here
we've rectified how we need to perceive the very smallest particles."
From the micro- to the macrocosm According to Maas, the new findings
are especially important because they are so fundamental. "They provide
new perspectives on the basic properties of the neutron. What's more, by looking at the smallest building blocks of matter we can also understand phenomena that occur in the largest dimensions -- such as the fusion of
two neutron stars. This physics of extremes is already very fascinating." ========================================================================== Story Source: Materials provided by
Johannes_Gutenberg_Universitaet_Mainz. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. M. Ablikim et al. Oscillating features in the electromagnetic
structure
of the neutron. Nature Physics, 2021; 17 (11): 1200 DOI:
10.1038/s41567- 021-01345-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211108130845.htm

--- up 9 weeks, 4 days, 9 hours, 25 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)