Description
We shall discuss a possibility to study
the $\gamma\gamma\to\pi\pi$ processes
in ultrarelativistic ultraperipheral heavy ion collisions.
The $\gamma\gamma\to\pi\pi$ reactions are
more complicated than commonly believed. We shall
present an approach which takes into account many
mechanisms not included so far in the literature.
We shall also present how our approach describes
world $\gamma\gamma\to\pi\pi$ data for the first time both
for the total cross section and angular distributions
for $\gamma \gamma \to \pi^+ \pi^-$
and $\gamma \gamma \to \pi^0 \pi^0$ reactions
from kinematical threshold up to maximal accessible experimentally
energy $\sqrt{s_{\gamma\gamma}}=6$ GeV.
Several reaction mechanisms are identified.
We include dipion continuum due to $\rho^\pm$ exchange for $\pi^0\pi^0$
and Born approximation for $\pi^+\pi^-$
as well as pronounced dipion $\sigma(600)$, $f_0(980)$ and
$f_2(1270)$ s-channel resonances.
We shall discuss also possible contribution of less pronounced scalar resonances
$f_0(1500)$ and $f_0(1710)$ being glueball candidates
as well as tensor resonances $f'_2(1525)$, $f_2(1565)$ and $f_2(1950)$.
We find that the inclusion of high-spin $f_4(2050)$ resonant state
improves the situation at $\sqrt{s_{\gamma\gamma}} \approx$ 2 GeV.
At higher energies the pQCD inspired Brodsky-Lepage
and hand bag mechanisms are included in addition.
We shall present nuclear cross sections
for exclusive production of two neutral and two charged pions
in peripheral ultrarelativistic lead-lead collisions
at $\sqrt{s_{NN}}=5.5$ TeV which could be measured
e.g. by the ALICE collaboration at the LHC.
The cross section for this process is calculated
in the impact parameter space Equivalent Photon Approximation (EPA)
with realistic charge form factor of $^{208}Pb$ nuclei
being Fourier transform of realistic charge distribution.
Finally, we evaluate first differential distributions for two $\rho^0(770)$
mesons (four charged pions) in the double scattering (photon-pomeron) process.
The results are compared with the contribution of two-photon mechanism discussed
previously in the literature. The double scattering mechanism populate large dimeson invariant
masses and large rapidity distance between mesons. The resulting distributions for four
pions are presented and compared to the STAR data. We discuss a possibility
of identifying the double scattering mechanism at the LHC.
[1] M. K{\l}usek-Gawenda and A. Szczurek,
Phys. Lett. {\bf B700} (2011) 322.
[2] M. K{\l}usek-Gawenda and A. Szczurek,
arXiv:1302.4204 [nucl-th].
[3] M. K{\l}usek-Gawenda and A. Szczurek,
a paper in preparation.
Author
Mrs
Mariola Klusek-Gawenda
(Institute of Nuclear Physics, Krakow)
Co-author
Prof.
Antoni Szczurek
(Institute of Nuclear Physics, Krakow and Rzeszow University, Rzeszow)
