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Identified Hadron Production as a Function of Event Multiplicity and Transverse Spherocity in pp Collisions at √s = 7 and 13 TeV with the ALICE Detector

Vislavicius, Vytautas LU (2018)
Abstract (Swedish)
Över 30 år av forskning inom tungjonsfysik, Quantum ChromoDymanics (QCD) och kosmologi har etablerat vår nuvarande syn på universums tidigare stadie: kort efter Big Bang var universum fyllt av ett mycket hett och kompakt medium bestående av fria kvarkar och gluoner, den så kallade Kvarkgluon plasman (Quark--Gluon Plasma, QGP). Detta medium expanderade snabbt, och kyldes ner tills beståndsdelarnas växelverkan blev så stark att de individuella kvarkarna bands samman till hadroner. Efter ytterligare expansion kombinerades hadronerna till elektriskt laddade atomkärnor, vilka så småningom formade atomer genom att koppla ihop sig med elektroner. Idag lever vi i en värld där den förhistoriska kvarkgluonplasman har hadroniserats.

Att... (More)
Över 30 år av forskning inom tungjonsfysik, Quantum ChromoDymanics (QCD) och kosmologi har etablerat vår nuvarande syn på universums tidigare stadie: kort efter Big Bang var universum fyllt av ett mycket hett och kompakt medium bestående av fria kvarkar och gluoner, den så kallade Kvarkgluon plasman (Quark--Gluon Plasma, QGP). Detta medium expanderade snabbt, och kyldes ner tills beståndsdelarnas växelverkan blev så stark att de individuella kvarkarna bands samman till hadroner. Efter ytterligare expansion kombinerades hadronerna till elektriskt laddade atomkärnor, vilka så småningom formade atomer genom att koppla ihop sig med elektroner. Idag lever vi i en värld där den förhistoriska kvarkgluonplasman har hadroniserats.

Att studera kvarkgluonplasmans utveckling är ett viktigt steg i att förstå universum. Den enda nu förväntade naturligt förekommande kvark--gluonplasman finns djupt inne kärnan hos neutronstjärnor, och de är inte åtkomliga med dagens verktyg. Vi kan istället skapa droppar av QGP i små bangs genom att kollidera tunga atomkärnor vid relativistiska energier. Detta har gjorts i flera decennier, och resulterat i ett flertal etablerade teorimodeller -- även om hela teorin om allt som sker i kollisioner mellan tunga joner är långt ifrån fullständig.

Hittills har man trott att energitätheten som krävs för att producera QGP endast kan möjliggöras genom tungjonskollisioner. Detta paradigm har ifrågasatts efter att nyligen ha observerat QGP-liknande signaturer i kollisioner av proton-bly-kärnor (p-Pb). Upptäckten av de nya signalerna i det mindre kollisionssystemet leder till den naturliga frågan om ännu mindre system, såsom proton-proton-kollisioner (pp), också genererar likartade QGP-karaktäristiska signaler. I den här avhandlingen studeras spektrat av identifierade hadroner, uppmätt i pp-kollisioner vid två olika kollisionsenergier. Den partikeldynamik och hadrokemi som resultaten visar, implicerar vagt en förekomst av ett expanderande medium även i detta kollisionessystem, och indikerar således att Pb-Pb-kollisioner skulle kunna tolkas som en förlängning av pp-kollisioner. Dessa implikationer är banbrytande, och skulle kräva en omarbetning av vår nuvarande förståelse av både kvantkromodynamik och tungjonsfysik. (Less)
Abstract
This study reports on identified hadron production as a function of event multiplicity (dNch/dη) and transverse spherocity (SO) in proton-proton collisions at √s = 7 and 13 TeV measured with the ALICE detector at the LHC. The particle spectra and their ratios measured in high-multiplicity events show signatures of an expanding medium. Integrated particle yields as a function of multiplicity measured in pp collisions at √s = 7 and 13 TeV are compared to those measured in p-Pb and Pb-Pb collisions. Hadrochemical composition of particles are found to be similar in different colliding systems under different √s, provided similar multiplicities are compared. This suggests that... (More)
This study reports on identified hadron production as a function of event multiplicity (dNch/dη) and transverse spherocity (SO) in proton-proton collisions at √s = 7 and 13 TeV measured with the ALICE detector at the LHC. The particle spectra and their ratios measured in high-multiplicity events show signatures of an expanding medium. Integrated particle yields as a function of multiplicity measured in pp collisions at √s = 7 and 13 TeV are compared to those measured in p-Pb and Pb-Pb collisions. Hadrochemical composition of particles are found to be similar in different colliding systems under different √s, provided similar multiplicities are compared. This suggests that hadron yields are dominantly driven by dNch/dη , and not the colliding system or center-of-mass energy. On the other hand, particle spectra measured in pp collisions at √s = 13 TeV is harder than that at 7 TeV, when similar dNch/dη are compared. In addition, hadron production as a function of multiplicity is studied in the context of statistical, hydrodynamical and pQCD-inspired models. In order to disentangle the soft QCD component from the hard, high-multiplicity pp collisions at √s = 13 TeV are studied as a function of transverse spherocity. It is found that the amount of flow-like effects in the data sample can be controlled using event shape observables. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Hemmick, Thomas, Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY, USA
organization
publishing date
type
Thesis
publication status
published
subject
keywords
LHC, ALICE, small systems, QGP, collectivity, multiplicity, spherocity, event shapes, pp collisions
pages
219 pages
publisher
Lund University, Faculty of Science, Department of Physics
defense location
Rydberg lecture hall, Department of Physics, Sölvegatan 14A, Lund
defense date
2018-03-23 13:00
ISBN
978-91-7753-597-3
978-91-7753-596-6
language
English
LU publication?
yes
id
7b62f1df-a4b8-4a53-ab82-d274ef08d23b
date added to LUP
2018-02-22 15:18:00
date last changed
2018-05-29 11:25:33
@phdthesis{7b62f1df-a4b8-4a53-ab82-d274ef08d23b,
  abstract     = {This study reports on identified hadron production as a function of event multiplicity (d<i>N</i><sub>ch</sub>/d<i>η</i>) and transverse spherocity (<i>S</i><sub>O</sub>) in proton-proton collisions at √<i>s</i> = 7 and 13 TeV measured with the ALICE detector at the LHC. The particle spectra and their ratios measured in high-multiplicity events show signatures of an expanding medium. Integrated particle yields as a function of multiplicity measured in pp collisions at √<i>s</i> = 7 and 13 TeV are compared to those measured in p-Pb and Pb-Pb collisions. Hadrochemical composition of particles are found to be similar in different colliding systems under different √<i>s</i>, provided similar multiplicities are compared. This suggests that hadron yields are dominantly driven by d<i>N</i><sub>ch</sub>/d<i>η</i> , and not the colliding system or center-of-mass energy. On the other hand, particle spectra measured in pp collisions at √<i>s</i> = 13 TeV is harder than that at 7 TeV, when similar d<i>N</i><sub>ch</sub>/d<i>η</i> are compared. In addition, hadron production as a function of multiplicity is studied in the context of statistical, hydrodynamical and pQCD-inspired models. In order to disentangle the soft QCD component from the hard, high-multiplicity pp collisions at √<i>s</i> = 13 TeV are studied as a function of transverse spherocity. It is found that the amount of flow-like effects in the data sample can be controlled using event shape observables.},
  author       = {Vislavicius, Vytautas},
  isbn         = {978-91-7753-597-3},
  keyword      = {LHC,ALICE,small systems,QGP,collectivity,multiplicity,spherocity,event shapes,pp collisions},
  language     = {eng},
  pages        = {219},
  publisher    = {Lund University, Faculty of Science, Department of Physics},
  school       = {Lund University},
  title        = {Identified Hadron Production as a Function of Event Multiplicity and Transverse Spherocity in pp Collisions at √<i>s</i> = 7 and 13 TeV with the ALICE Detector},
  year         = {2018},
}