Line break in expression()?

You can easily use line breaks in regular paste, but this is plotmath paste (actually a different function also with no 'sep' argument) and the (long) ?plotmath page specifically tells you it cannot be done. So what's the work-around? Using the plotmath function atop is one simple option:

expression(atop("Histogram of "*hat(mu), Bootstrap~samples*','~Allianz))

This will break at the comma and center the plotmath expressions. More complicated options are available.

This illustrates plotting to a graphics file. Ironically, the first effort gave me a display that did have your problem with the 'hat' (are those circumflexes?) being cut off and this shows how to increase the margins. The top margin is probably the third number so c(3,3,8,0) might suit you better:

 pdf("test.pdf") ;  par(mar=c(10,10,10,10))
 hist(1:10,cex.main=2,cex.axis=1.2,cex.lab=1.2,
 main=expression(atop("Histogram of "*hat(mu), 
                       Bootstrap~samples * ',' ~Allianz)))
 dev.off() # don't need to restore;  this 'par' only applies to pdf()

You are going to need to use something else. I was directed to use mtext and bquote when I was stuck on a similar problem.

alpha = rnorm(1e3)
hist(alpha,cex.main=2,cex.axis=1.2,cex.lab=1.2,main=NULL )

title <- list( bquote( paste( "Histogram of " , hat(mu) ) ) ,
               bquote( paste( "Bootstrap samples, Allianz" ) ) )


mtext(do.call(expression, title ),side=3, line = c(1,-1) , cex = 2 )

In the above example, title (thanks to @hadley) can be simplified to

title <- as.list(expression(paste("Histogram of " , hat(mu)), "Bootstrap samples, Allianz"))