Running the CERN LHCf experiment and validating nuclear interaction models at ultra high energies

LHCf is a dedicated experiment to observe the spectrum of photons and neutrons produced in the very forward region of collisions at CERN LHC of which energy is comparable to the ultra-high energy cosmic rays(UHECR)。The purpose is to validate nuclear interaction models used in the Monte-Carlo(MC)simulations for cosmic-ray experiments，and thus to help clarify the mystery of UHECR.LHCf successfully observed the 450 GeV450 GeV collisions in the end of 2009 and also 3.5 TeV3.5 TeV collisions in 2010。The energy is equivalent to the laboratory energy of 4.3·10^&lt；14&gt；eV and2.6·10^&lt；16&gt；eV，respectively。We examined MC models，DPMJET(v3.04)，PYTHIA(v8.145)，QGSJET II(V03)，SIBYLL(v2.1)and EPOS(v1.99)。The spectrum in this completely unexplored region was found not completely far way from our expectation，but these models cannot fit the experimental data in a satisfactory way。The photon spectrum is softer than many MC models while hadrons show a harder spectrum.If we consider the pseudo-rapidity(η)distribution in the central region，obtained by other LHC experiments(ATLAS And CMS)，together with our data，we may say all interaction models breakdown in the LHC energy region。DPMJET is a very good model in the low energy region but its photon spectrum is much harder than the LHCf data and theηdistribution suddenly breakdowns at LHC energies.PYTHIA，though tuned to fit the LHCηdistribution，shows a similar photon spectrum as DPMJET.These facts indicate that we need to explore the highest energy LHC collisions expected in a few years to clarify the breakdown tendency and to validate the models，and also that it is important to construct a new interaction model。

LHCf is a dedicated experiment to observe the spectrum of photons and neutrons produced in the very forward region of collisions at CERN LHC of which energy is comparable to the ultra-high energy cosmic rays (UHECR). The purpose is to validate nuclear interaction models used in the Monte-Carlo (MC) simulations for cosmic-ray experiments, and thus to help clarify the mystery of UHECR.LHCf successfully observed the 450 GeV+450 GeV collisions in the end of 2009 and also 3.5 TeV+3.5 TeV collisions in 2010. The energy is equivalent to the laboratory energy of 4.3・10^<14> eV and 2.6・10^<16> eV, respectively. We examined MC models, DPMJET(v3.04), PYTHIA(v8.145), QGSJET II(v03), SIBYLL(v2.1) and EPOS(v1.99). The spectrum in this completely unexplored region was found not completely far way from our expectation, but these models cannot fit the experimental data in a satisfactory way. The photon spectrum is softer than many MC models while hadrons show a harder spectrum. If we consider the pseudo-rapidity (η) distribution in the central region, obtained by other LHC experiments (ATLAS and CMS), together with our data, we may say all interaction models breakdown in the LHC energy region. DPMJET is a very good model in the low energy region but its photon spectrum is much harder than the LHCf data and theηdistribution suddenly breakdowns at LHC energies. PYTHIA, though tuned to fit the LHCηdistribution, shows a similar photon spectrum as DPMJET.These facts indicate that we need to explore the highest energy LHC collisions expected in a few years to clarify the breakdown tendency and to validate the models, and also that it is important to construct a new interaction model.