The collaborations have set stringent new bounds on the fraction of
Higgs bosons transforming into invisible particles.
The Higgs boson lives for an extremely short time before it transforms, or
“decays”, into other particles. It is through the detection of some of these
decay products that the unique particle has first been – and continues to be
– spotted in particle collisions at the Large Hadron Collider (LHC).
But what if the Higgs boson also decayed into unexpected, new particles that
were invisible to the LHC detectors, such as the particles that may
constitute the dark matter permeating the universe? The ATLAS and CMS
collaborations at the LHC have explored this possibility in two recent
studies, setting stringent new upper bounds on the fraction of Higgs bosons
decaying into invisible particles.
According to the Standard Model of particle physics, the Higgs boson decays
indirectly into known invisible particles – almost massless particles called
neutrinos – only 0.1% of the time. However, if dark matter is made up of
particles interacting too weakly to be detected, as suspected by many
physicists, the dark-matter particle could interact with the Higgs boson
and, if not too massive, allow the Higgs boson to decay into it, increasing
the fraction of invisible Higgs-boson decays.
In their latest independent investigations, the ATLAS and CMS collaborations
searched for invisible Higgs-boson decays in proton–proton collision data
collected during the second run of the LHC. Both teams looked for a
particular type of collision event, in which a Higgs boson is produced by a
process known as vector-boson fusion and then decays into invisible
particles.
These vector-boson-fusion events contain additional sprays, or “jets”, of
particles emitted towards either end of the particle detectors, making this
mode of Higgs-boson production easier to spot than the other modes. Together
with the “missing energy” in the collision products that the invisible
particles would carry away, these jets and their properties provide
distinctive signatures of such invisible Higgs-boson events.
The ATLAS and CMS searches revealed no instances of these invisible
Higgs-boson events that would exceed the expected number of background
events mimicking the desired events. However, they showed that the Higgs
boson cannot decay into invisible particles more often than a certain
percentage of time: 15% for ATLAS and 18% for CMS, compared to an expected
percentage, based on Standard Model computer simulations, of 10% for both
ATLAS and CMS.
These bounds align well with one another and, when interpreted in the
context of dark-matter models, they translate into bounds on the interaction
strength of dark-matter particles with atomic nuclei that complement those
obtained from non-collider experiments searching for dark matter.
With the LHC set to restart later this year and deliver more data, ATLAS and
CMS will no doubt continue to chase the invisible with the Higgs boson.
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