Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Measurement of diffractive photoproduction of vector mesons at large
  momentum transfer at HERA   Elastic and proton-dissociative photoproduction of $\rho^0$, $\phi$ and
$J/\psi$ vector mesons ($\gamma p\to Vp$, $\gamma p\to VN$, respectively) have
been measured in e^+p interactions at HERA up to -t=3 GeV$^2$, where t is the
four-momentum transfer squared at the photon-vector meson vertex. The analysis
is based on a data sample in which photoproduction reactions were tagged by
detection of the scattered positron in a special-purpose calorimeter. This
limits the photon virtuality, Q^2, to values less than 0.01 GeV$^2$, and
selects a $\gamma p$ average center-of-mass energy of <W> = 94 GeV. Results for
the differential cross sections, d$\sigma$/d$t$, for $\rho^0$, $\phi$ and
$J/\psi$ mesons are presented and compared to the results of recent QCD
calculations. Results are also presented for the t-dependence of the pion-pair
invariant-mass distribution in the $\rho^0$ mass region and of the spin-density
matrix elements determined from the decay-angle distributions. The Pomeron
trajectory has been derived from measurements of the W dependence of the
elastic differential cross sections d$\sigma$/d$t$ for both $\rho^0$ and $\phi$
mesons.

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The categories of this paper are: stat
10
The categories of this paper are: q-bio
27
The categories of this paper are: comp-gas
4
The categories of this paper are: cond-mat
24
The categories of this paper are: dg-ga
15
The categories of this paper are: nucl-ex
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
29
The categories of this paper are: funct-an
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The categories of this paper are: econ
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The categories of this paper are: q-fin
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The categories of this paper are: nucl-th
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The categories of this paper are: atom-ph
30
The categories of this paper are: q-alg
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The categories of this paper are: supr-con
33
The categories of this paper are: plasm-ph