Dark Matter Search Results from 4.2     Tonne − Years of Exposure of the LUX-ZEPLIN (LZ) Experiment

We report results of a search for nuclear recoils induced by weakly interacting massive particle (WIMP) dark matter using the LUX-ZEPLIN (LZ) two-phase xenon time projection chamber. This analysis uses a total exposure of

4.2
±
0.1

tonne-years from 280 live days of LZ operation, of which

3.3
±
0.1

tonne-years and 220 live days are new. A technique to actively tag background electronic recoils from




Pb




214





β

decays is featured for the first time. Enhanced electron-ion recombination is observed in two-neutrino double electron capture decays of




Xe




124




, representing a noteworthy new background. After removal of artificial signal-like events injected into the dataset to mitigate analyzer bias, we find no evidence for an excess over expected backgrounds. World-leading constraints are placed on spin-independent (SI) and spin-dependent WIMP-nucleon cross sections for masses


≥
9
 
 
GeV
/


c


2




. The strongest SI exclusion set is


2.2
×


10


−
48


 
 


cm


2




at the 90% confidence level and the best SI median sensitivity achieved is

5.1
×

10

−
48


 
 


cm


2



, both for a mass of

40
 
 
GeV
/

c
2


.