Charged-Particle Multiplicity Distributions over Wide Pseudorapidity Range in Proton-Proton and Proton-Lead Collisions with ALICE

The charged–particle distribution (P (Nch) as a function ofNch), produced in high energy collisions between protons (pp) and between protons and heavy nucleus (pPb), depends on the fundamental processes, which lead to the formation of the observed particles. In particular, the so–called multiplicity distribution is sensitive to the number of collisions between quarks and gluons contained in the colliding systems. In this thesis, data using the Forward Multiplicity Detector and the Silicon Pixel Detector of ALICE at CERN’s Large Hadron Collider (LHC) are presented, for pp and pPb collisions. For the first time the multiplicity distributions are performed over such a wide kinematic range at the LHC (pseudorapidity coverage of −3.4 < η < +5.1) and at the highest energies ever, i.e. all available energies at the LHC’s first run: at √ s =0.9, 2.76, 7 and 8 TeV for pp collisions and at √ s =5.02 TeV for pPb and Pbp collisions. The results are compared, where possible, with the results of other LHC experiments and with theoretical Monte Carlo simulations (including PYTHIA, PHOJET and DMPJET) and with the IP–Glasma model. Moreover, data are compared using the Koba–Nielsen–Olesen (KNO) model, and it appears that this scaling is broken at energies from 0.9 GeV for pp collisions. Results for pPb collisions suggest that current models which include Color Glass Condensate effects, i.e. models based on the assumption of the saturation density of gluons, cannot reproduce the data. Resumé (in Danish) Fordelingen af antallet af elektrisk ladede partikler (P (Nch) som funktion af Nch), der produceres i energirige kollisioner mellem protoner (pp) og mellem protoner og tunge atomkerner (pPb), afhænger af de fundamentale processer der fører til dannelsen af observerbare partikler. I særdeleshed er den såkaldte multiplicitetsfordeling følsom overfor antallet af kollisioner mellem quarker og gluoner indeholdt i de kolliderende systemer. I denne afhandling præsenteres data indsamlet med brug af Forward Multiplicity Detector og Silicon Pixel Detector i ALICE ved CERN’s Large Hadron Collider (LHC) for pp og pPb kollisoner. For første gang er sådanne målinger udført over et bredt kinematisk område ved LHC (pseudorapiditet dækning −3.4 < η < +5.1) og ved de højeste energier nogen sinde, dvs. alle tilgængelige energier ved LHC’s første forsøgsrunde: ved √ s =0.9, 2.76, 7 og 8 TeV for pp kollisioner og ved √ sNN =5.02 TeV for pPb og Pbp kollisioner. Resultaterne sammenlignes, hvor det er muligt, med resultaterne fra andre LHC eksperimenter, samt gængse teoretiske Monte Carlo simuleringer (herunder PYTHIA, PHOJET and DMPJET) og IP–Glasma model. Desuden sammenlignes data med Koba–Nielsen–Olesen (KNO) modellen, og det vises, at denne skalering er brudt ved energier fra 0.9 GeV for pp kollisioner. Resultater for pPb kollisioner antyder at gængse modeller der inkluderer Color Glass Condensate effekter, dvs. modeller der bygger på antagelsen af en mætningstæthed af gluoner, ikke kan reproducere data. I am among those who think that science has great beauty. A scientist in his laboratory is not only a technician: he is also a child placed before natural phenomena which impress him like a fairy tale. We should not allow it to be believed that all scientific progress can be reduced to mechanisms, machines, gearings, even though such machinery also has its beauty. Neither do I believe that the spirit of adventure runs any risk of disappearing in our world. If I see anything vital around me, it is precisely that spirit of adventure, which seems indestructible and is akin to curiosity.