Escape parentheses in aggregate function

I want to copy query results to a text file and there's an aggregate
function in the SELECT expression. One of the aggregate function's
parentheses seems to end the \copy() function and I don't know how best to
write the statement. A minimal example:

\copy(select count_value, sum(count_value)
from table_name) to 'output_file.txt';

Do I write sumE'('count_valueE')', sum\(count_value\), or something else?

TIA,

Rich