The NIST Reference on Constants, Units and Uncertainty

Fundamental Physical Constants
Constants
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Constants
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Constants,
Units &
Uncertainty
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alpha particle mass
 Value  6.644 655 98 x 10-27 kg
 Standard uncertainty  0.000 000 52 x 10-27 kg
  Relative standard uncertainty   7.9 x 10-8
 Concise form  6.644 655 98(52) x 10-27 kg    
 
Click here for correlation coefficient of this constant with other constants
Source: 1998 CODATA     
recommended values
 
Definition of
uncertainty
 
     Correlation coefficient with
any other constant