Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Accessing the real part of the forward $J/\psi$-p scattering amplitude
  from $J/\psi$ photo-production on protons around threshold   We provide an updated analysis of the forward $J/\psi$-p scattering
amplitude, relating its imaginary part to $\gamma p \to J/\psi p$ and $\gamma p
\to c \bar c X$ cross section data, and calculating its real part through a
once-subtracted dispersion relation. From a global fit to both differential and
total cross section data, we extract a value for the spin-averaged $J/\psi$-p
s-wave scattering length $a_{\psi p} = 0.046 \pm 0.005$ fm, which can be
translated into a $J/\psi$ binding energy in nuclear matter of $B_\psi = 2.7
\pm 0.3$ MeV. We estimate the forward-backward asymmetry to the $\gamma p \to
e^- e^+ p$ process around the $J/\psi$ resonance, which results from
interchanging the leptons in the interference between the $J/\psi$ production
and the Bethe-Heitler mechanisms. We show that to good approximation this
asymmetry depends linearly on $a_{\psi p}$, and can reach values around -25%
for forthcoming $J/\psi$ threshold production experiments at Jefferson Lab. Its
measurement can thus provide a very sensitive observable for a refined
extraction of $a_{\psi p}$.

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The categories of this paper are: comp-gas
2
The categories of this paper are: math
24
The categories of this paper are: dg-ga
31
The categories of this paper are: ao-sci
8
The categories of this paper are: hep-ex
6
The categories of this paper are: astro-ph
17
The categories of this paper are: q-fin
28
The categories of this paper are: alg-geom
19
The categories of this paper are: eess
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The categories of this paper are: chao-dyn
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The categories of this paper are: solv-int
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The categories of this paper are: nucl-th
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The categories of this paper are: acc-phys
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The categories of this paper are: supr-con
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The categories of this paper are: plasm-ph
16
The categories of this paper are: stat