Forward Calorimetry in ALICE and high multiplicity physics in proton-proton collisions at the Large Hadron Collider.

The domain of small Bjorken-x regime, dominated by gluon dynamics and unrevealed so far, can be explored at LHC energies specifically at forward rapidity region. The limited capability in forward rapidity with LHC experiments like ALICE has an opportunity for a calorimeter which can measure high energy (up to 200 GeV) photons from different sources. Calorimeter, being a unique technique to measure Heat (Energy in the wider sense), can be used in detecting and characterizing particles of different nature (mass, charge, structure) over a wide range of incident energy. Growing complexities with an increase in energy of physical processes, the technique of particle detection and characterization have reached a state of art technological advancement in the field of the calorimeter. Silicon, as detecting medium, has lots of advantages like very good Signal to Noise ratio, insensitiveness to magnetic fields, enriched technologies to achieve very high granularity and etc. A calorimeter with finely segmented detector layers should be considered to detect every individual particle produced in pp and p-Pb collision at LHC energies and which get manifold for heavy-ion collision, especially in forward rapidity. An optimized sampling type (both in longitudinal and transverse direction) silicon (detecting medium) – Tungsten (convertor/absorbing medium) calorimeter (Si-W) can be considered as an upgrade proposal for ALICE experiment to widen its physics interest starting from proton-proton to proton-Lead to Lead-Lead collision at LHC energies. An extensive study for designing a calorimeter followed by fabrication and test both in the laboratory and with CERN beam facility could help for a feasible and practically acceptable configuration for the proposed calorimeter. On the other hand, the study both with simulated (MC) and/or published available data for the proton-proton collision at LHC energies can give a complementing physics motivation behind building a new calorimeter. Usually, particle production in proton-proton collision used is explained as fundamental interaction between partons, resulted in only a few particles in the final state and is indeed true at energies before the LHC era. With the increase in collision energies, proton-proton collision found to produce a special class of events with comparatively high multiplicity (~102) which need “off the track” explanations incorporating phenomena like initial state effects, multi-partonic interactions, medium formation and etc. In the present scenario, the formation of medium (mostly partonic!!) draws lots of attention as a strong reason for high multiplicity in a proton-proton collision. An attempt to explain particle production in pp collision in the framework of Negative Binomial Distribution (NBD) can describe the existent of another mechanism of particle production apart from hard scattering which let pp collision to be used more than just as a reference for more complex systems. Moreover, a phenomenological model like “Blastwave description” can be successfully used to understand these high multiplicity pp events assuming the creation of a miniature version of the medium produced in heavy-ion collisions. The results found are quite consistent with that of theoretical predictions. The satisfactory description of medium formation in pp collision is encouraging to pursue the study about nature (partonic or something else) of the system. An interesting study about how the Degrees of Freedom (DOF) change with mean transverse momentum (representation of the temperature of the system produced) can indicate a possible existence of a partonic medium in pp collision.