Neutral Pion Production in Proton+Proton Collisions at $\sqrt{s}=8$ TeV and Insight into Hadron Production Mechanism

High transverse momentum particles play an important role for investigation of Quark-Gluon Plasma (QGP) induced by nucleus-nucleus collisions. When high transverse momentum partons traverse the QGP, they lose their energy. Consequently, the yield of high transverse momentum hadron is suppressed relative to proton+proton collisions scaled by the number of binary nucleon-nucleon collisions (jet quenching). The phenomenon is expected to give us much information about properties of QGP. Thus far, almost all models consider only parton energy loss during passing through. However, the perturbative Quantum Chromo Dynamics (QCD) predicts the direct hadron production (the higher-twist effect). If hadrons produced by this mechanism exist, the hadron energy loss should be considered additionally. For checking if there is the higher-twist effect, investigation of hadron production in proton+proton collisions is suitable. The invariant differential cross sections for inclusive neutral pion at midrapidity are measured in proton+proton collisions at $\sqrt{s}=8$ TeV using the ALICE detector at LHC. The neutral pion is identified from the invariant mass of photon pairs detected by the PHOS detector covering $260^\circ<\phi<320^\circ$ and $|\eta|<0.12$. To cover a wide transverse momentum range, two different triggers are used. One is a minimum-bias trigger to measure from low to middle transverse momentum range. The other is the high energy photon trigger deployed in PHOS detector to enhance high transverse momentum statistics. To calculate the neutral pion invariant cross-section, the LHC beam luminosity in proton+proton collisions at $\sqrt{s}=8$ TeV is measured by using van der Meer scan technique. The luminosity is determined by the ALICE minimum-bias trigger which requires at least one hit on both sides of V0 detector with a pseudorapidity coverage of $2.8<\eta<5.1$ and $-3.7<\eta<\-1.7$. The cross-section of the trigger is found to be $55.8\pm1.2$ mb$^{-1}$. The value is a baseline for not only the neutral pion measurement but also other measurements in ALICE. By using the $x_{\rm T}$ scaling property, the particle production mechanisms at RHIC and LHC collision energies are investigated. The result reveals that both higher-twist and jet fragmentation should be taken into account for RHIC collision energies, but on the other hand at LHC energies, the higher-twist contribution is found to be negligible below about 20 GeV/$c$. The result suggests that the high transverse momentum particle suppression is caused by the parton energy loss for the LHC energies, but both parton and hadron energy loss should be taken into account at RHIC. The phenomena which were not expected have been measured in proton+proton collisions with the high multiplicity event at LHC. To study the phenomena, the production rate of neutral pion is measured as a function of event multiplicity. Two transverse momentum ranges, from 1.0 GeV/$c$ to 1.5 GeV/$c$ and from 5.5 GeV/$c$ to 16 GeV/$c$, are compared with the Color-Reconnection (CR) model. For both ranges, the model describes the multiplicity dependence of the neutral pion yield.