BBC Focus

According toparticlephysics,therearefour
fundamental forces:gravity, electromagnetism,
theweakforceandthestrongforce.
Electromagnetism, the weak and the strongforces
haveparticlesassociatedwiththemcalled
bosons, which are the force-carriers. Gravityis
yet to be integrated into the Standard Model, but
isthoughttohaveaforce-carryingparticle
known as thegraviton.
Lastyear,agroupof Hungarian researchers
spotted an unusual radioactive decay signal
whilesearchingforevidenceof‘darkphotons’
–ahypotheticalparticleproposed to be the
electromagnetic force carrier for dark matter.
Though details were unclear, the signal appeared
tohavecomefromanunknownparticle just 30
times themass ofanelectron.
“If true, it’s revolutionary,” sa id r ese a r c her
Jonathan Feng. “For decades, we’ve known of four
fundamental forces:gravitation,
electromagnetism,andthestrongand weak
nuclear forces. If confirmed byfurther
experiments, this discovery of a possible fifth
force would completely change our
understanding of the Universe, with
consequences for the unification of forces and
dark matter.”
After combingthroughthedataandruling
out matter particles anddarkphotons as
candidates, theUCIteamhave proposedthe
signalmayhave comefrom a ‘protophobic X
boson’, a new type offorce-carrying particle,
so-calledas it interacts onlywithelectrons and
neutrons over an extremelylimitedrange, rather
than withelectrons andprotonslikethephotons
dofor theelectromagneticforce.
Iftheyarecorrect,thefinding wouldmean that
thereisafifthfundamentalforce operating
alongsidetheelectromagnetic andstrong and
weaknuclearforces, andpossiblyevenaseparate
darksector withits own matter andforces.
“It’s possiblethat thesetwosectorstalkto each
other andinteract withone another through
somewhat veiledbutfundamentalinteractions,”
saidFeng. “Thisdarksectorforce may manifest
itselfas this protophobicforce we’re seeing as a
resultofthe Hungarian experiment. In abroader
sense, itfitsinwithour originalresearchto
understandthe nature ofdark matter.”

ABOVE:The teamdiscovered

thepotential fifthforce

while huntingfor theforce

carrierofdark matter,

visualised above

DISCOVERIES

PH

OT

OS

:SC

IEN

CEPH

OT

O LIBRARY, FLP

A

The four known

fundamental forces

“APOSSIBLE FIFTH FORCE

WOULD COMPLETELY CHANGE

OURUNDERSTA N D I NGOF

THE UNIVERSE”

FORCE EXPERIENBYCED

FORCE

CARRIER

PARTICLE

RANGE STRENGTHRELATIVE

Gravity

acts between

objects with

mass

All particles

with mass

Graviton

(not yet

observed)

Infinity

Weakforce

governs

particle

decay

Quarks and

leptons

W+,W-,Z^0

(w and z)

, , Short

range

Electromagnetism

acts between

electrically

charged particles

Electrically

charged particles Infinity

Strongforce

binds quarks

together

Quarks and

gluons

Short

range

Much

weaker

Much

stronger

y

(photon)

y

(gluon)gg