Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Search for a new gauge boson in the $A'$ Experiment (APEX)   We present a search at Jefferson Laboratory for new forces mediated by
sub-GeV vector bosons with weak coupling $\alpha'$ to electrons. Such a
particle $A'$ can be produced in electron-nucleus fixed-target scattering and
then decay to an $e^+e^-$ pair, producing a narrow resonance in the QED trident
spectrum. Using APEX test run data, we searched in the mass range 175--250 MeV,
found no evidence for an $A'\to e^+e^-$ reaction, and set an upper limit of
$\alpha'/\alpha \simeq 10^{-6}$. Our findings demonstrate that fixed-target
searches can explore a new, wide, and important range of masses and couplings
for sub-GeV forces.

37
The categories of this paper are: supr-con
17
The categories of this paper are: q-fin
9
The categories of this paper are: nlin
36
The categories of this paper are: cmp-lg
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
12
The categories of this paper are: nucl-th
34
The categories of this paper are: chem-ph
14
The categories of this paper are: math-ph
23
The categories of this paper are: solv-int
1
The categories of this paper are: cs
4
The categories of this paper are: cond-mat
32
The categories of this paper are: atom-ph
6
The categories of this paper are: astro-ph
25
The categories of this paper are: patt-sol
22
The categories of this paper are: chao-dyn