Fermilab’s DZero experiment observes rare ZZ diboson production
Batavia, Ill.— Scientists of the DZero collaboration at the US
Department of Energy’s Fermilab have announced the observation of pairs
of Z bosons, force-carrying particles produced in proton-antiproton
collisions at the Tevatron, the world’s highest-energy particle
accelerator. The properties of the ZZ diboson make its discovery an
essential prelude to finding or excluding the Higgs boson at the
Tevatron.
The observation of the ZZ, announced at a Fermilab seminar on July
25, connects to the search for the Higgs boson in several ways. The
process of producing the ZZ is very rare and hence difficult to detect.
The rarest diboson processes after ZZ are those involving the Higgs
boson, so seeing ZZ is an essential step in demonstrating the ability
of the experimenters to see the Higgs. The signature for pairs of Z
bosons can also mimic the Higgs signature for large values of the Higgs
mass. For lower Higgs masses, the production of a Z boson and a Higgs
boson together, a ZH, makes a major contribution to Higgs search
sensitivity, and the ZZ shares important characteristics and signatures
with ZH.
The ZZ is the latest in a series of observations of pairs of the
so-called gauge bosons, or force-carrying particles, by DZero and its
sister Tevatron experiment, CDF. The series began with the study of the
already rare production of W bosons plus photons; then Z bosons plus
photons; then observation of W pairs; then WZ. The ZZ is the most
massive combination and has the lowest predicted likelihood of
production in the Standard Model. Earlier this year, CDF found evidence
for ZZ production; the DZero results presented on Friday for the first
time showed sufficient significance, well above five standard
deviations, to rank as a discovery of ZZ production.
“Final analysis of the data for this discovery was done by a
thoroughly international team of researchers including scientists of
American, Belgian, British, Georgian, Italian and Russian
nationalities,” said DZero cospokesperson Darien Wood. “They worked
closely and productively together to achieve this challenging and
exciting experimental result.”
DZero searched for ZZ production in nearly 200 trillion
proton-antiproton collisions delivered by the Tevatron. Scientists used
two analyses that look for Z decays into different combinations of
secondary particles. One analysis looked for one Z decaying into
electrons or muons, the other decaying into “invisible” neutrinos. The
neutrino signature is challenging experimentally, but worthwhile
because it is more plentiful. In the even rarer mode, both Z bosons
decay to either electrons or muons. Just three events were observed in
this mode, but the signature is remarkably distinctive, with an
expected background of only two tenths of one event.
Sourced from Fermilab Press Room
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