ARGUS distribution
In physics, the ARGUS distribution, named after the particle physics experiment ARGUS, is the probability distribution of the reconstructed invariant mass of a decayed particle candidate in continuum background. Definition 
The probability density function (pdf) of the ARGUS distribution is:

for . Here  and  are parameters of the distribution and

where  and  are the cumulative distribution and probability density functions of the standard normal distribution, respectively. Cumulative distribution function 
The cumulative distribution function (cdf) of the ARGUS distribution is
. Parameter estimation 
Parameter c is assumed to be known (the kinematic limit of the invariant mass distribution), whereas χ can be estimated from the sample X1, …, Xn using the maximum likelihood approach. The estimator is a function of sample second moment, and is given as a solution to the non-linear equation
. The solution exists and is unique, provided that the right-hand side is greater than 0.4; the resulting estimator  is consistent and asymptotically normal. Generalized ARGUS distribution 
Sometimes a more general form is used to describe a more peaking-like distribution:

where Γ(·) is the gamma function, and Γ(·,·) is the upper incomplete gamma function. Here parameters c, χ, p represent the cutoff, curvature, and power respectively. The mode is:

The mean is:

where M(·,·,·) is the Kummer's confluent hypergeometric function. The variance is:

p = 0.5 gives a regular ARGUS, listed above. References

Further reading

Experimental particle physics
Continuous distributions