Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Freeze-Out Time in Ultrarelativistic Heavy Ion Collisions from Coulomb
  Effects in Transverse Pion Spectra   The influence of the nuclear Coulomb field on transverse spectra of $\pi^+$
and $\pi^-$ measured in $Pb+Pb$ reactions at 158 A GeV has been investigated.
Pion trajectories are calculated in the field of an expanding fireball. The
observed enhancement of the $\pi^-/\pi^+$ ratio at small momenta depends on the
temperature and transverse expansion velocity of the source, the rapidity
distribution of the net positive charge, and mainly the time of the freeze-out.

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The categories of this paper are: patt-sol
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The categories of this paper are: hep-ex
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The categories of this paper are: hep-lat
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The categories of this paper are: alg-geom
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The categories of this paper are: gr-qc
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The categories of this paper are: cond-mat
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The categories of this paper are: acc-phys
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The categories of this paper are: math-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: q-fin
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The categories of this paper are: hep-th
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The categories of this paper are: plasm-ph
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The categories of this paper are: solv-int
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The categories of this paper are: eess
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The categories of this paper are: adap-org
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The categories of this paper are: astro-ph