Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: First Estimate of the Primary Cosmic Ray Energy Spectrum above 3 EeV
  from the Pierre Auger Observatory   Measurements of air showers are accumulating at an increasing rate while
construction proceeds at the Pierre Auger Observatory. Although the southern
site is only half complete, the cumulative exposure is already similar to those
achieved by the largest forerunner experiments. A measurement of the cosmic ray
energy spectrum in the southern sky is reported here. The methods are simple
and robust, exploiting the combination of fluorescence detector (FD) and
surface detector (SD). The methods do not rely on detailed numerical simulation
or any assumption about the chemical composition.

36
The categories of this paper are: cmp-lg
28
The categories of this paper are: alg-geom
3
The categories of this paper are: physics
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
20
The categories of this paper are: acc-phys
12
The categories of this paper are: nucl-th
35
The categories of this paper are: mtrl-th
25
The categories of this paper are: patt-sol
1
The categories of this paper are: cs
26
The categories of this paper are: bayes-an
22
The categories of this paper are: chao-dyn
17
The categories of this paper are: q-fin
7
The categories of this paper are: hep-th
5
The categories of this paper are: gr-qc
16
The categories of this paper are: stat