Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Sub-Doppler frequency metrology in HD for test of fundamental physics   Weak transitions in the (2,0) overtone band of the HD molecule at $\lambda =
1.38 \, \mu$m were measured in saturated absorption using the technique of
noise-immune cavity-enhanced optical heterodyne molecular spectroscopy. Narrow
Doppler-free lines were interrogated with a spectroscopy laser locked to a
frequency comb laser referenced to an atomic clock to yield transition
frequencies [R(1) = $217\,105\,181\,895\,(20)$ kHz; R(2) =
$219\,042\,856\,621\,(28)$ kHz; R(3) = $220\,704\,304\,951\,(28)$ kHz] at three
orders of magnitude improved accuracy. These benchmark values provide a test of
QED in the smallest neutral molecule, and open up an avenue to resolve the
proton radius puzzle, as well as constrain putative fifth forces and extra
dimensions.

10
The categories of this paper are: q-bio
37
The categories of this paper are: supr-con
25
The categories of this paper are: patt-sol
23
The categories of this paper are: solv-int
27
The categories of this paper are: comp-gas
32
The categories of this paper are: atom-ph
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
2
The categories of this paper are: math
33
The categories of this paper are: plasm-ph
13
The categories of this paper are: quant-ph
26
The categories of this paper are: bayes-an
1
The categories of this paper are: cs
15
The categories of this paper are: nucl-ex
9
The categories of this paper are: nlin
8
The categories of this paper are: hep-ex