2019-11-23 New Scientist

23 November 2019 | New Scientist | 11

A NEW clue to the existence of
a fifth fundamental force of
nature may have emerged.
It comes from Attila
Krasznahorkay at the Atomki
Institute of Nuclear Research,
Hungary, and his team. They
spent years studying the decay
of beryllium-8, a radioisotope,
and, in 2016, published details
of an odd finding, suggesting
it was caused by an unknown
particle. This was later proposed
to be a hint of a fifth force.
Now, the same researchers
have found another anomaly,
this time in an energy transition
involving an excited state of the
helium nucleus, and say that it
points to the same particle.
They calculate that the new
particle has a mass of around
17 megaelectronvolts, or about
33 times that of an electron, and
have called it X17 as a result.
The particle appears to carry
energy away from an atomic
nucleus, and then decay into an
electron-positron pair, which the
team’s experiment can detect.
Normal physics predicts
that the electron and positron
should be emitted in roughly
the same direction. But in the
beryllium-8 experiment, the
two particles diverged at an
angle of 135 degrees, while
the new helium result gives
a similar detection bump at
an angle of 115 degrees
(arxiv.org/abs/1910.10459).
This difference is crucial
because the helium transitions
take place at a higher energy to
the beryllium one. The higher
the energy, the smaller the
expected angle. “If the bump is
produced by a new particle, it
has to move in exactly this way,”
says Jonathan Feng, a theoretical
particle physicist at the
University of California, Irvine.
He investigated the original

claim in 2016. At the time,

he proposed that the X

particle must only be capable

of interacting with neutrons,

otherwise it would have been

detected already. But for this

to be true, the particle can’t be

using any known force of nature.

Instead, it must be interacting

through a force unrelated to

gravity, electromagnetism or the

two nuclear forces we know of.

Feng says that the latest

result corroborates the first

and has got people excited.

“I sent a one-line email to my

collaborators with the link and

one word: ‘Wow’,” he says.

For his part, Krasznahorkay

is looking to other, independent

groups to confirm whether the

particle really exists. “If the

existence is settled, that would

open up a new avenue in particle

physics,” he says. X17 could

be the link between ordinary

matter and dark matter.

The Positron Annihilation into

Dark Matter Experiment (PADME)

in Frascati, Italy, is also looking

for X17. Its researchers collected

data this year and will run their

experiment again next year with

improved performance. PADME

spokesperson Mauro Raggi says

to expect the analysis around

spring 2021.

“The beryllium-8 anomaly

is a result published by a very

qualified journal and the helium

[result] will soon have the same

destiny,” says Raggi. “We need

to seriously consider these two

results. The question is whether

it is connected to a new particle

or not and this is what PADME

may help to clarify.” ❚

Particle physics

Stuart Clark

A TRANSPARENT battery could

replace glass in windows. The

battery still has a low output, but

may eventually add extra energy

storage to cars or smart glasses.

Most battery research focuses

on increasing power output or

energy density, a measure of how

much energy can be stored in a

certain volume. But Hironobu

Minowa and his colleagues at

Japanese communications giant

NTT sought instead to make a

battery that was as inconspicuous

as possible.

To do this, the team had to

create new versions of battery

components to lower overall light

absorption and reflection. The

result is a battery the size of an

A4 or a US letter sheet of paper that

is as see-through as window glass.

The battery lacks the capacity

of a conventional one: it holds

only 1 milliamp hour compared

with more than 1000 in an

AA battery. But it lets 69 per cent

of light pass through it, which is

at the lower end of what window

glass achieves.

Its output is still enough to

power an LED or a digital clock,

says Minowa, who demonstrated

the battery at NTT’s R&D Forum

in Tokyo this month.

Transparent battery

looks like glass

Technology

Edd Gent

NT

T^ D

AT

A

As homes fill up with sensors

and small smart devices, used

for everything from monitoring

temperature to optimising

lighting, transparent batteries

like this could help clear up the

associated mess of wires and

battery packs.

Since building the prototype

battery last year, the team has

already doubled its output and

tripled its transparency, says

Minowa. Although the team

can’t yet make batteries any

larger than an A4 sheet, it is

possible to link multiple panes

together to boost capacity.

The ability to turn a

window into a battery could

be revolutionary, says Kevin

Curran at Ulster University,

UK. Transparent batteries

could also have applications

in smart glasses, in vehicles and

even in sensor-packed contact

lenses, he says.

“No one desires ugly cables,”

says Curran. Having transparent

batteries would open up the

possibility of having much more

beautiful devices, he says. ❚

Hironobu Minowa sought

to create a battery that

was inconspicuous

X

The name of a hypothetical new

particle that is exciting physicists

Another surprise

hint of a fifth

force of nature