ATOM & COSMOS
New results once again hold up
physicists’ standard model
ATOM & COSMOS
Astronomers find a wimpy supernova
Weak explosion sheds light on the origins of neutron star pairs
BY LISA GROSSMAN
Electrons are almost perfectly round,
a new measurement shows. The result
stymies the search for new physics
because a more squished shape could
hint at the presence of never-before-seen subatomic particles.
The electron gets its shape from the
way that positive and negative charges
are distributed inside the particle. The
best theory for how particles behave,
known as the standard model of particle
physics, holds that the electron should
keep its rotund figure almost perfectly.
But some theories suggest that an
entourage of hypothetical subatomic particles outside the electron could create a
BY LISA GROSSMAN
A faint, fleeting supernova may be key to
learning how neutron star duos are born.
Astronomers have spotted what seems
to be an ultrastripped supernova: a massive star in its death throes after its outer
layers of gas have been siphoned off
slowly by a compact companion such as
a neutron star or black hole.
“This is the first of its kind: The first
ultrastripped supernova that has been
observed,” says astronomer Kishalay
De of Caltech. Similar supernovas could
lead to binary neutron stars like the pair
that was caught colliding in 2017, he and
A spiral galaxy about 930 million light-years from Earth is shown before, during and after (left to
right) an explosion of a faint supernova called iP TF 14gqr (inside the dotted circle, middle).
slight separation between the positive
and negative charges, giving the electron
a pear shape. That charge separation
is called an electric dipole moment, or
EDM. Searching for an electron EDM can
reveal if particles not accounted for in the
standard model are hanging around.
Now, the Advanced Cold Molecule
Electron Electric Dipole Moment, or
ACME, search, based at Harvard, has
probed the electron’s EDM with the
most precision yet. The results show no
sign of smooshing, researchers report in
the Oct. 18 Nature.
The finding improves the team’s last
best measurement (SN Online: 12/19/13)
by a factor of 10 to find an EDM of 10–29
electron charge centimeters. That’s as
round as if the electron were a sphere
the size of Earth, and you shaved less
than two nanometers off the North Pole
and pasted it onto the South Pole, says
Yale University physicist David DeMille,
a member of the ACME team.
DeMille’s group tried to make electrons
colleagues report in the Oct. 12 Science.
The supernova was spotted in 2014 in a
galaxy about 930 million light-years from
Earth by an automated sky survey called
the intermediate Palomar Transient
Factory and was named iPTF 14gqr. Most
supernovas detonate when a star more
than eight times as massive as the sun has
burned through all its fuel and can no lon-
ger hold itself up against gravity. The star’s
core collapses, leaving a dense neutron
star behind. Meanwhile, a rebounding
explosion ejects the remaining outer lay-
ers of gas outward as a bright flare that
usually lasts for 17 to 20 days.
in a thorium monoxide molecule flop over
in an electric field, the way a pear tips over
due to gravity, to test for the presence of
new particles. None toppled. The group
calculated that any new particles that
could distort the electron’s shape must
carry more than three terraelectron volts.
That’s twice the energy of particles
created in collisions at the Large Hadron
Collider, located at CERN near Geneva.
That result implies that any undiscov-
ered particles may be beyond the LHC’s
reach. “We’ve now surpassed what the
LHC will be able to see,” DeMille says.
The proposed successor to the LHC,
the Future Circular Collider, could
reach such high energies if it’s built. But
smaller, cheaper experiments may beat
the collider to the punch, says physi-
cist Brent Graner of the University of
Washington in Seattle. “The real virtue
in doing EDM experiments at all is, if
you do see something at the level of what
we can detect at the moment, it’s a real,
unambiguous sign of new physics.” s
But iPTF 14gqr’s light faded after
just seven days. The supernova also
emitted unusually little energy. That
wimpy burst suggests that the star,
about 10 times the sun’s mass to begin
with, ejected only one-fifth of the sun’s
mass when it exploded. The star must
have lost much of its material sometime
before it died, the researchers suspect.
That’s expected for an ultrastripped
supernova. An extremely massive com-
panion, like a neutron star or black hole,
could have stolen the matter. Whatever
remained could still explode, but faintly.
Astrophysicist Thomas Tauris of
Aarhus University in Denmark predicted
such strange supernovas in 2013. “It is
really fantastic that observers can now
verify their existence in detail,” he says.
The result of this type of explosion
would be a close pair of compact stellar
corpses, such as two neutron stars or a
neutron star and black hole. Scientists
knew that neutron star duos exist, but
weren’t sure how the stars get so close.
The new discovery suggests that they are
born close and only get closer. s