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Mesure des corrélations gamma-hadrons et
hadrons-hadrons dans les collisions pp à 7 TeV pour l’étude de la fragmentation des jets avec l’expérience
ALICE du LHC
Yaxian Mao
To cite this version:
Yaxian Mao. Mesure des corrélations gamma-hadrons et hadrons-hadrons dans les collisions pp à 7 TeV pour l’étude de la fragmentation des jets avec l’expérience ALICE du LHC. Physique des Hautes Energies - Expérience [hep-ex]. Université de Grenoble; East China normal university (Shanghai), 2011. Français. �NNT : 2011GRENY023�. �tel-00622562�
TH ` ESE
Pour obtenir le grade de
DOCTEUR DE L’UNIVERSIT ´ E DE GRENOBLE
Sp ´ecialit ´e : Physique/Physique subatomique & Astroparticules
Arr ˆet ´e minist ´erial :
Pr ´esent ´ee par
Yaxian Mao
Th `ese dirig ´ee parPr. Christophe Furget et codirig ´ee parPr. Daicui Zhou
pr ´epar ´ee au sein du Laboratoire de Physique Subatomique et de Cosmologie
Mesures des corr ´elations γ -hadron dans les collisions p-p avec l’exp ´erience ALICE au LHC
Th `ese soutenue publiquement le3 juin 2011, devant le jury compos ´e de :
.
Pr Yugang Ma, pr ´esident du jury, SINAP Shanghai (China) Pr. Nu Xu, rapporteur, LBNL Berkeley (USA)
Pr. Xu Cai, examinateur, CCNU Wuhan (China)
Pr. Daicui Zhou, co-directeur de th `ese, CCNU Wuhan (China) Dr. Christelle Roy, rapporteur, IPHC Strasbourg (France) Dr. Yves Schutz, examinateur, CERN Geneva (Suisse) Dr. Serge Kox, examinateur, LPSC Grenoble (France)
, Christophe Furget
20115
Two Particle Correlations with Photon Triggers to Study Hot QCD Medium in ALICE at LHC
A Dissertation Presented
BY
Yaxian Mao
to
The Doctoral School in partial fulfillment of the requirements
For the Degree of Doctor of Philosophy in Physics
of
Huazhong Normal University
and
Universit ´ e de Grenoble
Supervisors: Daicui Zhou, Christophe Furget, Yves Schutz
Academic Titles: Professor, Professor, Directeur de Recherches
May 2011
R´ esum´ e
Depuis le d´emarrage du LHC (the Large Hadron Collider), le complexe acc´el´erateurs du CERN (Organisation Europ´eenne pour la Recherche Nucl´eaire) r´ealise des collisions entre protons et entre ions lourds `a des ´energies jamais atteintes auparavant. ALICE (A Large Ion Collider Experiment), l’une des exp´eriences principales install´ees aupr`es du LHC, est d´edi´ee `a l’´etude de la mati`ere nucl´eaire soumise `a des conditions extrˆemes de temp´erature et d’´energie. L’objectif de l’exp´erience est de v´erifier l’existence d’un nouvel ´etat de la mati`ere, le QGP (Quark Gluon Plasma) et d’en
´etudier les propri´et´es. Cette ´etude permettra d’explorer les aspects fondamentaux de l’interaction forte, l’une de 4 interactions fondamentales de l’univers responsable de la coh´esion de la mati`ere ainsi que du vide qui lui est associ´e.
Pour mener `a bien cet ambitieux programme scientifique, il est essentiel de choisir des observ- ables pertinentes porteuses d’informations utiles pour la compr´ehension de la nature de la mati`ere cr´e´ee dans les collisions d’ions lourds aux ´energies ultra relativistes. Les informations extraites de nombreuses observables permettront, `a partir de modelisations des principes fondamentaux mis en jeu, de concevoir une interpr´etation coh´erente des ph´enom`enes observ´es. Apr`es une mise en contexte de ce programme de recherche, les principaux aspects des collisions entre ions-lourds et un bref ´etat des lieux des r´esultats obtenus `a ce jour dans ce domaine sont pr´esent´es dans une premi`ere partie de ce document.
Parmi les observables, la production de jets de hadrons est particuli`erement int´eressante. Les jets r´esultent du processus de hadronisation de partons de grande impulsion transverse et apparais- sent dans les d´etecteurs comme un faisceau collimat´e de hadrons. Les partons de grande impulsion transverse quant `a eux sont cr´e´es dans les interactions dures entre partons (de type 2→2) consti- tuant les projectiles en collision. La mesure de la structure des jets, telle la distribution des hadrons en fonction de la fraction d’´energie du parton initial emport´e par chacun d’eux, est l’observable de pr´edilection. En effet, dans les collisions entre ions lourds les partons de grande impulsion transverse sont cr´e´es simultan´ement avec le milieu chaud et dense, objet de notre ´etude, et voient leurs propri´et´es cin´ematiques modifi´ees lorsqu’ils traversent ce mˆeme milieu. Lors du processus de hadronisation, les hadrons gardent la m´emoire des modifications subies par le parton de fa¸con
`a modifier la structure du jet. Ces modifications sont r´ev´el´ees en comparant la structure du jet lorsque le parton de grande impulsion transverse est produit dans des collisions proton–proton (on parle de structure du jet dans le vide) avec la structure du jet lorsque le parton est produit dans
des collisions noyaux-noyaux (on parle de structure du jet dans le milieu). Reste un probl`eme technique: alors que les jets sont ais´ement identifi´es et mesur´es dans les collisions proton–proton, dans les collisions entre ions lourds, le fond sous jacent de hadrons dans l’´etat final rend la mesure des jets particuli`erement ardue. De plus il est impossible de connaˆıtre quelle ´etait l’impulsion du parton lors de sa cr´eation, seule l’impulsion finale apr`es interaction avec le milieu est mesurable, ce qui complique l’interpr´etation de la mesure.
Pour pallier `a ces probl`emes techniques, j’ai choisi d’´etudier un type de processus dur particulier, celui qui met un en jeu dans l’´etat final un photon (il s’agit des photons directs par opposition aux photons de d´ecroissance des m´esons neutres). L’impulsion (p~γ) du photon, qui n’interagit pas fortement avec le milieu, permet de calibrer l’impulsion du parton (p~p =−p~γ) tel qu’il a ´et´e cr´e´e dans le processus dur. Ainsi l’impulsion du photon nous donne l’impulsion du parton avant interaction avec le milieu et la mesure de l’impulsion du jet nous donne l’impulsion du parton apr`es interaction avec le milieu. De plus la mesure du photon permet de s’affranchir de l’identification des jets, puisqu’il suffira de mesurer la corr´elation azimutale entre le photon et l’ensemble des hadrons g´en´er´es dans la collision. Cependant, les faibles section efficace du processus photon–jet rend cette mesure relativement difficile. Les ´el´ements n´ecessaires `a l’´etude des corr´elations et les
´equipements exp´erimentaux sont d´ecrits dans la deuxi`eme partie de ce document.
La strat´egie d’un telle ´etude commence par la validation de la mesure, qui consiste `a ´etudier `a l’aide de simulations Monte Carlo d’une part quelle est la sensisibilit´e de l’observable choisie pour r´ev´eler le ph´enom`ene recherch´e et d’en quantifier les effets et d’autre part si le signal est mesurable avec les syst`emes de d´etection de l’exp´erience. Cette ´etude est d´ecrite dans la troisi`eme partie de ce document. Je d´ecris les performances attendues pour l’´etude des corr´elations azimutales entre photons et hadrons charg´es mesur´es avec l’exp´erience ALICE. Deux quantit´e sont extraites de cette ´etude `a partir des simulations de collisions proton–proton: la valeur moyenne du moment transverse total au niveau partonique (< kT >) et la distribution de la fraction (xE) d’´energie du jet emport´ee par les hadrons produits en coincidence avec un photon (fonction de corr´elation). Les mˆemes quantit´es sont extraites `a partir de simulations de collisions d’ions lourds et les modifications subies par le mileu sont analys´ees: la distribution des valeurs dekT est ´elargie d’une quantit´e qui peut ˆetre directement reli´ee au coefficient de transport du milieu et la fonction de corr´elation est modifi´ee de fa¸con `a supprimer les hadrons de grande valeur dexE (jet quenching) et `a augmenter le nombre de hadrons `a petites valeurs de xE (production radiative de gluons). L’amplitude de cette derni`ere modification est proportionnelle au coefficient de transport et `a la distance parcourue
dans le milieu. Je termine cette partie consacr´ee aux simulations (et `a laquelle j’ai consacr´e la plus grande partie de mon temps) par une ´etude d´etaill´ee qui devrait permettre, th´eoriquement, de r´ealiser une ´etude tomographique du milieu form´e dans les collisions d’ions lourds. La proc´edure repose sur une id´ee sugg´er´ee par X.N Wang et consiste `a localiser le processus dur dans le milieu (grande valeur de xE pour une production du photon et du jet en surface et petite valeur dexE
pour une production profondeur) et ainsi sonder le milieu de la partie la plus dense jusqu’`a la partie la moins dense en surface. Je conclus que la mesure rel`eve du d´efi exp´erimental !
La quatri`eme partie de ce document est consacr´ee `a l’analyse des donn´ees collect´ees en 2010 pour des collisions proton–proton `a une ´energie dans le centre de masse de √
s = 7 TeV. Pour cette premi`ere longue campagne de mesure au LHC, l’exp´erience ALICE n’´etait pas encore dans les conditions optimales pour la mesure des corr´elations photon–jet. En effet, les calorim`etres
´electromagn´etiques n’offraient qu’une acceptance r´eduite et l’exp´erience ne disposait pas encore d’un d´eclenchement s´electif pour enrichir les donn´ees enregistr´ees en ´ev´enements photon – jet. Il en a r´esult´e un nombre d’´ev´enements insuffisant pour une ´etude concluante. En particulier la statis- tique disponible a limit´e l’´etude au domaine d’´energie inf´erieur `a 10 GeV, domaine tr`es d´efavorable pour l’identification des photons directs du fait de leur raret´e dans le bruit de fond pr´edominant des photons de d´ecroissance. S’est ajout´e `a ce handicap, le peu de temps disponible pour compl´eter une analyse tr`es d´elicate. Des r´esultats pr´eliminaires sont pr´esent´es pour les corr´elations entre photons inclusifs et hadrons charg´es et photons isol´es et hadrons charg´es: les structures 2 jets et mono jet sont bien identifi´ees, une valeur de hkTia ´et´e d´etermin´ee et les distributions enxE ont
´et´e construites.
Cette premi`ere analyse est un premier pas vers une analyse compl`ete de l’observable photon – jet `a partir des donn´ees plus riches (calorim`etre complet, et d´eclenchement s´electif) qui seront collect´ees en 2011 pour les collisions proton–proton et Pb–Pb.
Dans la derni`ere partie de ce document, je d´ecris ma contribution au contrˆole qualit´e des calorim`etres ´electromagn´etiques pendant la prise de donn´ees.
᪈ ᪈ ᪈ 㾱 㾱 㾱
൘⧠ԓ㠚❦、ᆖ⹄ウѝˈа亩ᶱާᡈᙗⲴ⹄ウⴞḷᱟ൘ᴰሿቪᓖк᧒㍒⢙䍘ⲴᶴᡀDŽ а⿽
᧿䘠สᵜ㋂ᆀо⭘࣋Ⲵ⨶䇪–ḷ߶⁑රˈٷᇊ⢙䍘Ⲵสᵜ㔃ᶴᱟ⭡ཨݻ઼䖫ᆀ䙊䗷㿴㤳㋂ᆀⲴ
ӂ⭘㘼㓴ᡀⲴˈ䇔Ѫ㓴ᡀ⢙䍘㔃ᶴⲴᴰሿঅݳᱟཨݻ˄u, d˗c, s; t, b)ǃ䖫ᆀ(e, νe; µ, νµ; τ, ντ)ˈԕ৺Ր䙂ӂ⭘࣋ⲴჂӻᆀˈণ⧫㢢ᆀ(W±઼Z0)ǃ㜦ᆀ(g)઼ݹᆀ(γ)DŽสᵜ⭘࣋ṩᦞ 㿴㤳㋂ᆀⲴн਼ࡂ࠶Ѫ⭥⻱ӂ⭘ˈᕡӂ⭘઼ᕪӂ⭘DŽ ↓ᱟ䘉Ӌสᵜ㋂ᆀ઼ӂ⭘
࣋ᶴᔪҶӊᆀц⭼DŽ❦㘼ˈ䘉њ⨶䇪ᒦнॱ࠶ᆼ༷ˈഐѪᆳ❦ᰐ⌅䀓䟺ḀӋสᵜⲴ䰞仈ˈ∄
ྲ˖สᵜ㋂ᆀ䍘䟿Ⲵ䎧ⓀǃᆷᇉⲴⵏᇎⵏオǃԕ৺ᆷᇉѝ⢙䍘䘌∄৽⢙䍘ཊⲴ䰞仈...DŽ
ӪԜᲞ䙽䇔Ѫˈᕪӂ⭘⭡䟿ᆀ㢢ࣘ࣋ᆖ(Quantum Chromodynamics, QCD)᧿䘠DŽ䟿ᆀ 㢢ࣘ࣋ᆖⲴ䟽㾱⢩ᙗᱟ⑀䘁㠚⭡ᙗ䍘ˈণ䲿⵰ӂ⭘Ⲵࣘ䟿ḷᓖⲴਈབྷˈᕪӂ⭘Ⲵ㙖ਸ ᑨᮠ(αs)䎻Ҿ䴦DŽ ൘⑀䘁㠚⭡⣦ᘱлˈ䜘࠶ᆀ䰤Ⲵӂ⭘ᱟᗞᢠⲴDŽ QCD亴䀰ˈ൘ሩ䇪 䟽ᆀ⻠ᫎлˈ䜘࠶ᆀਟԕ䗮ࡠ䘉⿽⣦ᘱˈᖒᡀཨݻ㜦ᆀㅹᆀփ(QGP)DŽ 㖾ഭᐳ励⎧᮷ഭᇦ ᇎ傼ᇔ(BNL)࡙⭘ሩ䇪䟽ᆀሩᫎᵪ(RHIC)ᶕራ䘉⿽ᯠⲴ⢙䍘ᖒᘱDŽ սҾ⅗⍢Ṩᆀ⹄ウѝ
ᗳ(CERN)Ⲵབྷරᕪᆀሩᫎᵪ(LHC)Ҿ2009ᒤ↓ᔿࣘ䘀㹼ˈᇎ⧠ҶࡽᡰᵚᴹⲴTeV㜭४Ⲵ儈㜭䟽
ᆀ⻠ᫎˈLHCкⲴབྷර䟽ᆀሩᫎᵪᇎ傼(ALICE)ਸ㓴у䰘㠤࣋Ҿ儈儈㜭䟿ᇶᓖⲴᶱㄟᶑ Ԧлᕪӂ⭘Ⲵสᵜᙗ䍘৺ަ⢙⨶ⵏオⲴ⹄ウDŽ
สҾ䟿ᆀ㢢ࣘ࣋ᆖ(QCD)ˈ࡙⭘Ṭ⛩䇑㇇ᯩ⌅ˈӪԜᡀ࣏ൠ亴䀰ҶཨݻӾ⾱䰝ⲴᲞ䙊ᕪᆀ
⢙䍘ѝ䘰⾱䰝ࡠQGPⲴѤ⭼ᓖDŽ ᇎ傼кˈ䈕Ѥ⭼ᓖ俆ݸ൘CERN√sN N = 17.2 GeVл ⲴSPS䟽ᆀ⻠ᫎᇎ傼ѝ䗮ࡠDŽ䘋㘼൘㖾ഭᐳ励⎧᮷ഭᇦᇎ傼ᇔ(BNL)䘀㹼Ⲵ䍘ᗳ㜭䟿√sN N =
60−200 GeVлⲴሩ䇪䟽ᆀሩᫎᵪ(RHIC)ᇎ傼ѝˈ䇱ᇎҶӾᕪᆀ⢙䍘ࡠཨݻ⢙䍘Ⲵਈ㜭
൘儈儈ᇶᶑԦлਁ⭏DŽ սҾ⅗⍢Ṩᆀ⹄ウѝᗳ(CERN)ⲴབྷරᕪᆀሩᫎᵪLHCᇎ⧠Ҷ䍘ᗳ㜭
䟿√sN N = 2.76 TeVлⲴ䫵-䫵⻠ᫎˈ䈕㜭䟿儈ҾRHIC⻠ᫎ㜭䟿Ⲵ∼14ؽˈ֯ަ൘ᴤ儈ᓖлᖒ
ᡀⲴ✝ᇶQCD⢙䍘ᤱ㔝ᴤ䮯ᰦ䰤DŽ ਖཆˈLHC㜭४Ⲵ䟽ᆀ⻠ᫎ䈡ሬӗ⭏Ⲵ⺜ᮓሴ䗷〻ᴤ࣐Ѡ ᇼˈ䘉Ӌ䭀Ⲵ⺜᧒䪸ሶᨀTeV㜭४л㓶㠤⹄ウQGPᙗ䍘ⲴᵪՊDŽ∄ྲˈ൘Ṩ–Ṩѝᗳ⻠ᫎѝ儈
⁚ࣘ䟿ᕪᆀӗ仍Ⲵվ઼㛼ሩ㛼௧⌘ѻ䰤Ⲵޣ㚄⎸ཡDŽ ALICE᧒⍻ಘቡᱟѪҶᴤ࣐㌫㔏Ⲵ⍻䟿 Ṩ–Ṩ⻠ᫎѝӗ⭏Ⲵᵛᘱ㋂ᆀˈ⭡↔৽᱐⻠ᫎࡍᵏࠪ⧠Ⲵ✝ᇶ⢙䍘Ⲵᙗ䍘DŽ⢩࡛ᱟˈ䙊䗷⍻䟿
オ䰤ѝᕪᆀ䉡Ⲵ᭩ਈˈਟԕ⹄ウ⺜ᮓሴ䜘࠶ᆀ൘ク䎺ᇶᓖ⢙䍘ᰦⲴ㜭䟿ᦏཡ᭸ᓄ৺⺾㻲࠭ᮠᙗ 䍘DŽ ൘䟽ᆀ⻠ᫎѝˈ⭡Ҿ௧⌘␜⚛᭸ᓄˈ⺜ᮓሴ䗷〻ӗ⭏Ⲵབྷ⁚ࣘ䟿䜘࠶ᆀク䎺ᇶᓖ⢙䍘ᰦՊ ഐѪཊ䟽ᮓሴ㘼ᦏཡ㜭䟿ˈሬ㠤儈⁚ࣘ䟿Ⲵᕪᆀӗ仍߿ቁˈվࣘ䟿ᕪᆀᮠⴞՊ໎ཊˈᕅ䎧ᕪᆀ䉡
൘オ䰤ѝⲴ䟽ᯠ࠶ᐳDŽ ഐ↔ˈ䙊䗷⍻䟿Ṩ⧟ຳл䜘࠶ᆀⲴ⺾㻲࠭ᮠ(FF)ˈণ⍻䟿䜘࠶ᆀ௧⌘
ӗ⭏Ⲵᕪᆀᩪᑖ௧⌘ࣘ䟿ԭ仍z =pTh/ETjetⲴ࠶ᐳ࠭ᮠˈሶⵏオѝⲴ⺾㻲࠭ᮠ(䍘ᆀ–䍘ᆀ⻠ᫎᡆ Ṩ–Ṩ䗩⋯⻠ᫎѝⲴ⺾㻲࠭ᮠ)о㻛ӻ䍘؞↓ਾⲴ⺾㻲࠭ᮠ(Ṩ–Ṩѝᗳ⻠ᫎлⲴ⺾㻲࠭ᮠ)࠶ᐳ䘋㹼
∄䖳ˈቡਟԕ᧘ᯝṨ–Ṩ⻠ᫎѝⲴᇶᓖ⢙䍘৺ަᙗ䍘DŽ
ᵜ䇪᮷ѫ㾱ⴞⲴᱟ࡙⭘ݹᆀ–ᕪᆀⲴޣ㚄⍻䟿ᶕ⹄ウ䜘࠶ᆀⲴ⺾㻲࠭ᮠˈণ䙊䗷⍻䟿儈㜭ⴤ᧕ ݹᆀоᮓሴࡠݹᆀ㛼䶒ᕪᆀѻ䰤Ⲵޣ㚄ᶕ⹄ウ௧⌘Ⲵ⺾㻲࠭ᮠDŽ 儈㜭ⴤ᧕ݹᆀ൘⻠ᫎࡍᵏⲴ⺜
ᮓሴ䗷〻ѝⴤ᧕ӗ⭏ˈѫ㾱ᶕ㠚ᓧᲞ亯ᮓሴ(qg→γq)઼⒞⚝䗷〻(q¯q→γg)DŽ ⭡ҾݹᆀⲴᒣ൷㠚
⭡〻ᖸབྷˈ൘⻠ᫎ४ฏоަԆ㋂ᆀਚᴹ⭥⻱ӂ⭘ˈഐ↔ݹᆀᩪᑖҶᆳԜࡍӗ⭏ᰦⲴؑ
DŽ 㘼о䈕ݹᆀӗ⭏Ҿ਼а⺜ᮓሴ䗷〻Ⲵ䜘࠶ᆀՊ⺾㻲Ѫᵛᘱᕪᆀˈ䙊䗷ሩᕪᆀⲴ⍻䟿ˈቡ㜭㧧 ᗇ䜘࠶ᆀ൘ク䎺ᇶᓖ⢙䍘ਾⲴؑDŽ ሶӗ⭏Ҿ਼аһԦѝⲴݹᆀ઼ᕪᆀޣ㚄ˈᡁԜቡ㜭ᗇࡠᇶᓖ
⢙䍘⭘Ҿ䜘࠶ᆀ௧⌘кⲴ᭸ᓄDŽ ᡁ䘹ᤙҶ儈⁚ࣘ䟿Ⲵ௧⌘Ѫሩӻ䍘ᯝቲᢛ᧿Ⲵ᧒䪸˖㿲⍻
䟿ᱟ儈ࣘ䟿ⴤ᧕ݹᆀо൘ަ৽ᯩੁкⲴᕪᆀⲴޣ㚄DŽ൘䘉а⍻䟿ѝˈⴤ᧕ݹᆀḷ䇠Ҷ਼ṧѪ
൘ࡍ⻠ᫎѝ⺜䗷〻ӗ⭏Ⲵ䜘࠶ᆀⲴࡍ㜭䟿ˈ㘼ᕪᆀˈѪ䈕䜘࠶ᆀ⺾㻲ࠪⲴᵛᘱ㋂ᆀˈ᧿䘠 Ҷク䎺ӻ䍘Ⲵ䜘࠶ᆀⲴᙗDŽ 䙊䗷∄䖳䘉є⿽⻠ᫎ㌫㔏лⲴݹᆀ–ᕪᆀޣ㚄⍻䟿ˈᡁԜਟԕᗇࡠ ӻ䍘᭸ᓄሩᵛᘱ䘀ࣘᆖ઼௧⌘⺾㻲Ⲵ؞↓DŽ ❦㘼ˈ൘⻠ᫎѝᶕ㠚ᕪᆀ઼ѝᙗӻᆀ(ѫ㾱ᱟπ0઼η)Ⲵ 㺠ਈݹᆀᱟⴤ᧕ݹᆀؑਧᨀਆ䗷〻ѝⲴѫ㾱㛼Ჟˈഐ↔ݹᆀؑਧⲴᨀਆ൘ᵜᐕѝᱮᗇ㠣ޣ䟽 㾱DŽ
ᵜ䇪᮷⵰䟽ᔰኅྲлєᯩ䶒ⲴᐕDŽㅜа䜘࠶ᱟޣҾݹᆀ⍻䟿ˈ䢤࡛᭸⦷ԕ৺ⴤ᧕ݹᆀⲴᨀਆ
⹄ウˈՠ䇑ᶕ㠚㺠ਈݹᆀ䈟ࡔᕅ䎧Ⲵ㌫㔏䈟ᐞˈ ᒦ⹄ウ࡙⭘ݹᆀˉᕪᆀޣ㚄৺ᕪᆀˉᕪᆀޣ㚄ᯩ
⌅൘ALICEᇎ傼ѝ⍻䟿Ⲵਟ㹼ᙗDŽ ࡙⭘Monte-Carlo⁑ᤏᮠᦞˈᡁԜ⹄ウҶALICEᇎ傼ѝ⭥⻱䟿
㜭ಘ(PHOS઼EMCAL)൘᧒⍻઼䢤࡛ݹᆀᰦⲴᙗ㜭㺘⧠DŽ ⭡Ҿᴹ䲀Ⲵ᧕ਇᓖˈ൘⻠ᫎ䗷〻ѝӗ⭏
ⲴḀӋݹᆀ㩭൘᧒⍻ಘ䗩㕈֯ᗇ⊹〟൘ᲦփкⲴ㜭䟿н㜭㧧ᗇᆼᮤⲴ䟽ᔪˈ䘉Ӌݹᆀሶн㜭㻛↓
⺞᧒⍻ࡠDŽ਼ᰦˈݹᆀ൘ク䎺սҾ䟿㜭ಘѻࡽⲴަᆳ᧒⍻ಘᰦо᧒⍻ಘ⢙䍘ਁ⭏ӂ⭘㘼֯ݹ ᆀ䖜ᦒᡀ↓䍏⭥ᆀሩ(γ→e+e−)DŽ ⁑ᤏ㔃᷌㺘᰾ˈн਼᧒⍻ಘሩݹᆀ᧒⍻᭸⦷Ⲵᖡ૽〻ᓖна ṧDŽֻྲˈݹᆀ൘ᰦ䰤ᣅᖡᇔ(TPC)઼䜘ᖴ䘩㌫㔏(ITS)Ⲵ䖜ᦒࠐ⦷㓖Ѫ10%ˈ㘼൘䏳䗱䗀ሴ᧒
⍻ಘ(TRD)઼ᰦ䰤伎㹼᧒⍻ಘ(TOF)ѝⲴ䖜ᦒࠐ⦷᧕䘁20%DŽ ᵜ᮷俆ݸᣕҶ൘LHC㜭४ѝⲴ䍘
ᆀ–䍘ᆀ઼䫵–䫵⻠ᫎ䗷〻ѝ࡙⭘ALICEᇎ傼᧒⍻ಘ䙊䗷ⴤ᧕ݹᆀ–ᕪᆀޣ㚄⍻䟿Ⲵᯩ⌅⹄ウ௧⌘⺾
㻲࠭ᮠ઼ӻ䍘᭸ᓄⲴਟ㹼ᙗDŽ ݹᆀ–ᕪᆀޣ㚄⍻䟿൘䍘ᆀ–䍘ᆀ⻠ᫎѝᱟᗵ享ⲴˈഐѪ䈕⍻䟿㜭ཏ ѪᡁԜᨀᴹޣ䟽ᆀ⻠ᫎлⲴޣ㚄⍻䟿Ⲵส㓯৲㘳DŽ ᡁ⹄ウҶӻ䍘ਟ㜭ᕅ䎧Ⲵа㌫ࡇ᭸ᓄDŽሩ ҾㅜањޣⲴ৲ᮠቡᱟkTˈণ䜘࠶ᆀቲ⅑кⲴᴹ᭸⁚ࣘ䟿DŽ สҾഭ䱵кᐢᴹⲴᇎ傼ᮠᦞ⍻䟿
٬ˈᡁԜሶަᔦըཆ᧘ࡠLHC㜭४ˈӾ㘼亴⍻LHC㜭४кⲴkT٬DŽ ᵜ᮷ՠ䇑Ҷݹᆀ-ᕪᆀޣ㚄⍻
䟿ሩҾ儈✝ᇶ⢙䍘Ⲵᝏᓖ৺ݹᆀ–ᕪᆀޣ㚄࠭ᮠⲴ䟽ᯠ࠶ᐳˈӾ㘼ᨀਆ✝ᇶ⢙䍘Ⲵᙗ䍘DŽ ਖ ཆˈᡁ䘈䙊䗷ݹᆀ-௧⌘ӗ⭏ᵪࡦ⹄ウҶ௧⌘൘ク䎺✝ᇶ⢙䍘䟼ӗ⭏Ⲵ䗷〻䘋㹼ቲ᷀㔃ᶴ࠶᷀DŽ
䈕ᐕⲴㅜҼ䜘࠶ˈѫ㾱㚊❖൘LHCҾ2010ᒤ俆⅑䘀㹼Ⲵ䍘ᆀ-䍘ᆀ⻠ᫎл䍘ᗳ㜭䟿√ s= 7 TeVлⲴALICEᮠᦞ࠶᷀ᐕDŽ 䈕ᐕӾⵏᇎᮠᦞޕˈ䟷⭘ᡁԜ൘ࡽ䶒ᐢ㓿ᔪ・Ⲵਟ㹼ᙗ⹄
ウᯩ⌅ˈᰘ൘⍻䟿䍘ᆀ-䍘ᆀ⻠ᫎлⲴ௧⌘⺾㻲࠭ᮠˈѪ䫵-䫵ᮠᦞ࠶᷀ᨀส㓯⍻䟿DŽ 䢤Ҿᖃ
ࡽALICEᴹ䲀Ⲵ㔏䇑䟿઼нᆼᮤⲴ⭥⻱䟿㜭ಘ᧕᭦ᓖˈᵜᐕⲴ⍻䟿䳶ѝ൘ሿҾ20GeV/cⲴ⁚ࣘ
䟿४ฏ䘋㹼ˈ⭡Ҿ൘䈕⁚ࣘ䟿㤳തབྷ䟿ᶕ㠚㺠ਈݹᆀⲴ㛼Ჟ䍑⥞֯ᗇ䈕⍻䟿൘↔४䰤⢩࡛ާᴹ 䳮ᓖ઼ᡈᙗDŽ ⭡Ҿᖃࡽ᧒⍻ಘⲴṑ߶ᐕ❦൘䘋㹼ѻѝˈᡁᰐ⌅࡙⭘㋂ᆀ䢤࡛Ⲵᯩ⌅ᶕ४
࠶ݹᆀ઼π0Ѫ䀖ਁ㋂ᆀˈ㘼ᱟⴤ᧕࡙⭘൘䟿㜭ಘкᖒᡀⲴ⭥⻱㈷ሴഒ㈷઼࡙⭘нਈ䍘䟿ᯩ⌅䟽 ᔪࠪⲴπ0Ѫ䀖ਁ㋂ᆀ䘋㹼є㋂ᆀޣ㚄DŽ㘼ਾሩ䀖ਁ㋂ᆀ䘋㹼ᆔ・ᡚᯝⲴࡔ䘹ˈӾ㘼ᨀ儈ⴤ᧕ⷜ
ᰦݹᆀᮠᦞṧᵜⲴᵪ⦷DŽ ৼ௧⌘㔃ᶴоঅݹᆀ/π0–௧⌘Ⲵ䘀ࣘᆖ㔃ᶴ䙊䗷ԕк⹄ウ㿲⍻ࡠDŽ਼
ᰦˈ࡙⭘ᆔ・ݹᆀ–ᕪᆀⲴᯩս䀂ޣ㚄ᡁԜᨀਆҶ䜘࠶ᆀቲ⅑кⲴ⁚ࣘ䟿kT٬ˈ䈕⍻䟿٬оᡁѻ
ࡽ࡙⭘㫉⢩㖇ᮠᦞ亴䀰ⲴkT㔃᷌а㠤ˈᴤ䘋а↕ˈᡁቍ䈅Ҷᶴᔪ20GeVԕлⲴ௧⌘൘䍘ᆀ-䍘 ᆀ√
s= 7TeVлⲴ⺾㻲࠭ᮠDŽ նኅᔰᇎ䱵ᮠᦞ࠶᷀Ⲵᰦ䰤ѻ⸝䇙ᡁᰐ⌅ሩ䈕亩⹄ウ䘋ޕࡠᴤ␡Ⲵ
ൠ↕DŽ
ሩҾ൘2010ᒤᓅLHC俆⅑䘀㹼䫵-䫵⻠ᫎлⲴ䍘ᗳ㜭䟿√sN N = 2.76 TeVлⲴᮠᦞˈᡁڊҶ਼
ṧⲴᮠᦞ࠶᷀ˈնᰦ䰤ѻԃ׳нݱ䇨ᡁሶ䈕ᮠᦞ࠶᷀䜘࠶㓣ޕ䈕䇪᮷ѻѝDŽ 䢤Ҿ⭥⻱䟿㜭ಘ㾶ⴆ 㤳തⲴ໎࣐ԕ৺儈⁚ࣘ䟿ݹᆀ䀖ਁࡔ䘹㜭࣋Ⲵᇎ⧠ˈ2011ˈ2012ᒤLHC䘀㹼㧧ਆⲴᮠᦞᰐ⯁Պᨀ
ᴤབྷⲴ㔏䇑䟿ˈ䇙䈕亩⹄ウⲴᴹ᭸⍻䟿㤳ത䗮ࡠᴤ儈ˈӾ㘼㧧ᗇᴤཊᴹ䏓Ⲵ⢙⨶⧠䊑઼⢙⨶㔃
᷌DŽ
ᴰਾˈ ѪҶ䇪᮷ⲴᆼᮤᙗˈሩҾᡁ൘ঊ༛ᵏ䰤ሩҾᮤњALICEᇎ傼ਸ㓴ѝⲴѫ㾱䍑⥞ㆰ㾱 ᾲᤜྲл˖
• ࡙⭘ݹᆀ–ᕪᆀޣ㚄⍻䟿Ⲵਟ㹼ᙗ⹄ウDŽ䈕亩ਟ㹼ᙗ⹄ウᱟᵜӪ⺅༛䇪᮷Ⲵᔦ㔝ˈᡁ൘⺅༛
䇪᮷⹄ウࠪⲴ⍻䟿઼࠶᷀ᯩ⌅สѻкˈਁኅ઼ᔦըҶ䈕㿲⍻䟿ሩҾṨṨ⻠ᫎлᖒᡀⲴ✝ᇶ
⢙䍘ⲴᝏᓖDŽ䙊䗷亩㓶㠤⹄ウˈᙫ㔃࡙ࠪ⭘ݹᆀ–ᕪᆀޣ㚄ᯩ⌅⍻䟿ӻ䍘᭸ᓄᱟᝏ㘼 фᴹ᭸ⲴˈᡁԜ⭊㠣ਟԕ䙊䗷䈕ޣ㚄⍻䟿ᨀਆ䜘࠶ᆀቲ⅑кⲴ䘀ࣘᆖ䟿kTˈ ᒦሩᖒᡀⲴ✝
ᇶ⢙䍘ӻ䍘ᵜ䓛䘋㹼ቲ᷀ᢛ᧿DŽ 䘉Ӌ⹄ウ㔃᷌൷൘ALICEഭ䱵ਸ㓴ѝᒯ⌋䇘䇪ᒦᗇࡠ䇔
਼ˈᒦ൘བྷරഭ䱵ᆖᵟՊ䇞кᣕDŽ
• EMCal᧒⍻ಘṑ߶ᐕDŽѪҶ㧧ᗇањ䖳ྭⲴ㜭䟿઼ս㖞࠶䗘⦷ˈᡁ৲࣐Ҷ൘Ṭं䈪ᐳቄ࡙
⭘ᆷᇉሴ㓯઼൘CERN࡙⭘儈㜭⭥ᆀᶏ⍱ሩEMCal᧒⍻ಘ䘋㹼ṑ߶ⲴᐕDŽ
• EMCal൘㓯ᮠᦞ䍘䟿ⴁ᧗(DQM)DŽ ѪҶỰ⍻൘ᮠᦞ㧧ਆ䗷〻ѝⲴ᧒⍻ಘ䍘䟿ˈ㠚LHC߶
༷䘀㹼ԕᶕˈᡁᣵԫҶEMCal᧒⍻ಘᮠᦞ䍘䟿Ự⍻Ⲵԫ࣑ˈᒦᔰਁ઼ᨀҶᮠᦞỰ⍻Ⲵ䖟 Ԧᐕާᒦф؍ᤱ㔤ᣔ઼ᴤᯠDŽ䘈ᔰਁҶᮠᦞⴁ᧗Ⲵ㇇⌅઼䍘䟿Ự⍻Ⲵḷ߶ˈᒦሶަ㺕ݵ
ࡠALICEᮠᦞ㧧ਆ㌫㔏Ⲵ൘㓯ᮠᦞⴁ⍻ṶᷦѻлDŽ
• ⭥⻱䟿㜭ಘEMCal઼PHOSⲴ㓯ᮠᦞⴁ⍻઼䍘䟿䇔䇱(QA)DŽᡁ䍏䍓䘀㹼⇿ઘӾLHCкਆ
ᗇⲴALICEᮠᦞˈሩ䟿㜭ಘⲴ㓯ᮠᦞ䍘䟿Ự傼ˈᒦሶ䍘䟿Ự傼㔃᷌൘⇿ઘALICEᮠᦞ䍘
䟿Ự傼઼ᮠᦞ࠶᷀Պ䇞кᣕˈѪᇎ傼ਸ㓴Ⲵ䟿㜭ಘᮠᦞ⢙⨶࠶᷀ᨀᴹ᭸ᮠᦞᓃDŽ
• ѝᙗӻᆀ઼ݹᆀ–ᕪᆀޣ㚄Ⲵᮠᦞ࠶᷀ᐕDŽ㠚2009ᒤLHCᔰ䟷䳶ᮠᦞԕᶕˈᡁаⴤ㠤࣋
Ҿ᧒⍻ಘⲴṑ߶઼ᇎ傼ᮠᦞⲴ࠶᷀ᐕ(ᮠᦞ࠶᷀⧠൘❦䘋㹼ѻѝ)DŽަѝवᤜπ0Ⲵ䟽ᔪˈ ԕ৺ݹᆀ–ᕪᆀⲴޣ㚄⍻䟿DŽᡁ㻛ᓄⲴALICE⢙⨶ᐕ㓴᧘䘹Ѫݹᆀ/π0-ᕪᆀޣ㚄⍻䟿Ⲵ ᮠᦞ࠶᷀ॿ䈳Ӫˈ⇿ઘਜ䳶є⅑䈮仈䇘䇪Պˈᒦᇊᵏ൘ALICEਸ㓴ᣕᐕ䘋ኅDŽ
ޣޣޣ䭞䭞䭞䇽䇽䇽˖˖˖བྷර䟽ᆀሩᫎᇎ傼(ALICE)ˈཨݻ㜦ᆀㅹᆀփ(QGP)ˈ௧⌘ˈⷜᰦݹᆀˈᆔ・
ᡚᯝˈޣ㚄⍻䟿ˈ⺾㻲࠭ᮠˈ䜘࠶ᆀkT,ቲ᷀㔃ᶴ
Abstract
With the advent of the Large Hadron Collider (LHC)at the end of 2009, the new accelerator at CERN collides protons and heavy-ions at unprecedented high energies. ALICE , one of the major experiment installed at LHC, is dedicated to the study of nuclear matter under extreme conditions of energy density with the opportunity of creating a partonic medium called the Quark- Gluon-Plasma (QGP). This new experimental facility opens new avenues for the understanding of fundamental properties of the strong interaction and its vacuum.
To reach the objectives of this scientific program, it is required to select a set of appropriate probes carrying relevant information on the properties of the medium created in ultra-relativistic heavy-ion collisions. Based on the information delivered by all the observables and guided by modelization of the fundamental principles in action, a coherent picture will emerge to interpret the observed phenomena. In the first part of the present document I describe the context of the scientific program, the general concepts involved in heavy-ion collisions at ultra relativistic energies, and the main results obtained so far in the field.
Among the observables of interest, the production of hadrons jets is particularly attractive. Jets are the result of the hadronisation process of high transverse momentum partons and are observed in the detectors as a beam of collimated hadrons. High transverse momentum partons are created by hard scattering of partons (2 → 2 type of processes) constituting the colliding projectiles.
The jet structure measured, for example as the distribution of the factional jet energy carried by the individual hadrons inside the jet, is the observable of choice. In heavy ion collisions, high transverse momentum partons are created concurrently with the hot and dense medium of interest and their kinematical properties are modified as they traverse the medium. This modification, imprinted in the jet structure, is observed by comparing the jet structure measured in heavy-ion collisions (in medium jet structure) with the jet structure measured in proton-proton collisions (vacuum jet structure). Such a measurement faces however a technical difficulty: whereas jets can be easily identified and measured in proton-proton collisions, in heavy-ion collisions the large hadronic background from the underlying event (the underlying event is everything except the two hard scattered jets and is generated by the beams particle break up and by initial and final state radiation) makes the jet identification measurement quite challenging. In addition the initial momentum of the hard scattered parton is unknown since only the final jet momentum can be measured i.e. the momentum of the parton as it emerges from the medium. This complicates the
interpretation of the measurement.
To overcome these difficulties, I have selected a particular 2 → 2 process which creates a direct photon (direct photon at variance with decay photon) in the final state together with a high transverse momentum parton. The momentum of the photon (p~γ), since it does not interact strongly with the medium, calibrates the momentum of the parton (p~p = −p~γ). Therefore the photon momentum is a measure of the parton momentum when created and the jet momentum the momentum of the parton after it has traversed the medium. In addition since the photon momentum (energy and direction) defines also the jet momentum, jet reconstruction algorithms are not required anymore. Instead of studying photon – jet correlations (where the jet is fully reconstructed), it is sufficient to study photon – hadron correlations from all the hadrons in the event. However, the relatively small cross section for the production of these particular hard scattering processes makes the measurement quite challenging. An introduction to 2 particle correlations is given in the second part of this document followed by a description of the ALICE detection systems used for this measurement.
The strategy I have followed for this study starts with a validation of the measurement. It consists first in studying with the help of Monte Carlo simulations the accuracy of the selected ob- servable in revealing and quantifying the phenomenon under study. Second, it consists in verifying the ability to measure the observable and its robustness with the detectors setup of the ALICE experiment. The validation procedure and results are discussed in the third part of this document.
I have particularly studied the possibility to extract two quantities from the 2 particle azimuthal correlation measured in proton-proton collisions: (i) the average total transverse momentum (hkTi) generated at the partonic level by the Fermi motion and initial and final state radiation, and (ii) the per trigger yield of jet hadrons as a function of the fractional jet energy (xE) they each carry (correlation function). The same quantities have been studied from simulated heavy-ion collisions with the objective to analyze the effects due to the presence of highly dense color medium. The distribution ofkT values becomes broader in a way that can be directly related to the transport properties of the medium and the correlation function is modified so that the number of highxE
hadrons are suppressed (jet quenching) and the number of lowxE hadrons is increased (radiative gluon production) with an amplitude proportional to the transport coefficient and to distance tra- versed inside the medium. To finish this part of the document dedicated to Monte Carlo studies (on which I have spent most of my time as a PhD student) with another detailed study the possi- bility to exploit the photon – jet observable as a tomographic tool (following a suggestion by X.N.
Wang). The idea is to localize the hard scattering well inside the medium (by selecting hadrons with low xE values) or at the surface of the medium (by selecting hadrons with large xE values).
One would therefore choose the distance in the medium through which the hard scattered parton travels and probe the medium from its densest part (center) to its less dense part (surface). I found that such a measurement will be quite challenging.
In the fourth part of the present document, I address the analysis of the data collected in 2010 for proton-proton collisions at √
s = 7 TeV. During this first long data taking period at LHC, the ALICE detection system was not yet complete. In particular, the incomplete coverage of the electromagnetic calorimeters and the absence of a selective photon trigger was a severe handicap for the photon – jet measurement. The resulting event statistics available for the measurement of this observable was limited to the photon energy range below 10 GeV. This low energy domain is not well suited for the identification of direct photons because of their scarcity in the overwhelming background generated by decay photons. On top of that, the time between the availability of the data and the scheduled time for my defense was too short to perform an in-depth analysis. Most of the results presented from this analysis in the present document must therefore be considered as very preliminary, but the key features are there. The results concern the 2 jet and mono jet structure observed in the photon – jet azimuthal correlation, the measured value of hkTiand the xE distributions.
This very preliminary analysis of the first data collected at LHC and presented in this document is the first only toward a comprehensive study of the photon – jet observable. Since the writing of the document, the analysis has progressed and provided a few results which were considered ripe by the collaboration to be presented at the Quark Matter conference in May 2011. The data which will be collected in 2011 in proton-proton and Pb-Pb collisions will be much richer in photon – jet events thanks to the complete coverage of the ALICE electromagnetic calorimeter and thanks to a very high energy photon trigger provided by the calorimeter as well.
For completeness, I finish the present document with the description of my contribution, as being the main person in charge, to the quality assurance and monitoring tasks for the two ALICE electromagnetic calorimeters during data taking.
Keywords: ALICE, Quark-Gluon Plasma (QGP), jet, prompt photon, isolation cut, kT, fragmentation function, tomorgraphy
Acknowledgements
The writing of this thesis would not have been possible without the help and encouragement of many professors and teachers, friends and relatives. It is my great pleasure to thank all these people, though it is difficult to express my gratitude to all of them individually.
First of all, I am deeply indebted to my advisors Prof. SCHUTZ Yves, Prof. ZHOU Daicui and Prof. FURGET Christophe, who led me to the fascinating world of high energy particle physics. I feel fortunate to work under their kind guidances and supports and I will never forget the exciting moments we have enjoyed in our discussions. All of my supervisors have spent a significant effort and time in teaching me how to be a good scientist. The remarkable scope of insight with clarity, profound knowledge with breadth, together with the careful judgement, vitality, precision and enthusiasm of Prof. SCHUTZ, inspire me to work harder to live up to their expectation.
I would like to thank Andreas Morsch, Terry Awes, Gustavo Conesa Balbastre, David Silvermyr, Yuri. V. Kharlov, and Hisayuki Torii for their advices and all their work that made possible I could advance and obtain my results. Also I would like to thank Prof. WANG Xin-Nian and Francois Arleo for their bright theoretical ideas and useful discussions.
I would like also to thank Prof. XU Nu, Prof. CAI Xu, Prof. YANG Yadong, Prof. WANG Enke, Prof. LIU Feng, Prof. YANG Chunbin, Prof. LIU Fuming, Prof. HOU Defu, Prof. WU Yuanfang, Prof. ZHANG Beiwei and associate Prof. ZHOU Daimei for their teaching and helps during my studies. In particular, I would like to thank associate Prof. YIN Zhongbao and associate Prof. WANG Yaping for their discussions in our work group.
I am grateful to all colleagues of ALICE collaboration especially for Physics Work Group 4 (PWG4), PHOS and EMCal group collaboration like Jan Rak, Peter Jacobs, Mateusz Andrzej Ploskon, Christian Klei-Boesing and Francesco Blanco for their instructive suggestions and enthu- siastic discussions.
For sure I would never forget the great supports from China Scholarship Council (CSC), French Embassy in Beijing (CNOUS) and French-China Particle Physics Laboratory (FCPPL), without their continous final support, I would have no chance to stay abroad for my PhD. Thanks to all the people that I have known or shared my time at CERN and in Grenoble, specially Ulla Tihinen, Canita Hervet, Rucy Renshall, Luciano Musa and Paolo Giubellino for their patient assistance and supports when I was in trouble. I appreciate the help of Prof. KOX Serge, Prof.ROY Christelle, Julien Faivre, Rachid Guernane, Nicolas Arbor and Renaud Vernet to make my life in France more
smoothly.
It is my pleasure to thank all members from the Institute of Particle Physics (IOPP) and Laboratoire de Physique Subatomique & Cosmologie (LPSC) for their effort to make our institute and laboratory warm and enjoyable, just like a big family. I appreciate the help of Teacher Gao Yanmin, Teacher Wang Jianping, my colleagues like Zhu Xiangrong, Wang Mengliang, Zhu Hong- sheng, Huang Meidana, Yuan Xianbao, Zhang Xiaoming, Wan Renzhuo and Yin Xuan, specially the friendship of Wu Pingping, Li Nana, Jin Dan, Xiong Juan, Zhu Lilin, Chen Jiayun and Zhang Fan.
And of course to all people that I have met, appreciates and know me and who have accompanied me physically and psychologically. Finally, last but least in my thoughts, I want to express my heartfelt gratitude to my parents and my elder sisters for their love. And I am always inspired by their belief that I can succeed in doing anything which I want to do.
To all of you,
THANK YOU !
䉒䉒!
Merci Beaucoup !
Contents
1 Introduction 1
2 Heavy Ion Physics Program 11
2.1 Relativistic Heavy Ion Collisions . . . . 11
2.1.1 Centrality . . . . 16
2.2 Particle Production . . . . 17
2.2.1 Soft Particles Production . . . . 17
2.2.2 Hard Particles Production . . . . 19
2.2.3 Nuclear Effects Modifying Particles Production . . . . 22
2.3 Probes of the QGP . . . . 25
2.3.1 Global observables characterizing the collision . . . . 25
2.3.2 Observables characterizing the medium . . . . 27
3 Jet and Particle Correlations 35 3.1 Two-Particle Correlation . . . . 36
3.2 Jet Properties . . . . 40
3.3 γ+ Jet . . . . 43
4 Experimental Facility 55 4.1 The Large hadron Collider: LHC . . . . 55
4.2 ALICE: A Large Ion Collider Experiment . . . . 57
4.2.1 Central Tracking System . . . . 60
4.2.2 Electromagnetic Calorimeters . . . . 65
4.3 ALICE Offline Computing . . . . 69
5 Validation of γ -hadron observable: A Monte-Carlo Study 73
5.1 Monte Carlo Event Generator . . . . 73
5.2 Direct photon identification . . . . 76
5.2.1 Invariant Mass Analysis: IMA . . . . 77
5.2.2 Shower Shape Analysis: SSA . . . . 79
5.2.3 Isolation Cut: IC . . . . 80
5.3 Correlation distributions in pp collisions . . . . 83
5.4 kT smearing . . . . 88
5.5 Measurements of Nuclear Effect via Photon-Hadron Correlations . . . . 89
5.5.1 Energy Loss via γ+ Jets . . . . 91
5.5.2 Tomography withγ+ Jet events . . . . 96
6 Analysis of ALICE Data 103 6.1 Data Sample . . . 103
6.2 Calorimeter Calibration . . . 106
6.2.1 Cosmic muons calibration . . . 111
6.2.2 π0mass calibration . . . 111
6.3 Photon andπ0 measurements . . . 113
6.4 Two particle correlations with photon andπ0triggers . . . 118
6.4.1 Azimuthal Correlation . . . 119
6.4.2 kT extraction . . . 125
6.4.3 Per-trigger conditional yield . . . 131
7 Summary and Outlook 141 Bibliography 147 Publications 155 Appendix 158 Appendix A: 2-Dimension kinematical quantities . . . 158
Appendix B: kT dependence on transverse momentum study . . . 159
Appendix C: EMCal Data Quality Monitoring (DQM) . . . 164
List of Figures
1.1 The evolution of the energy density/T4 as a function of temperature scaled by the critical temperature TC resulting from Lattice QCD calculation [9]. The arrows on the right side indicating the values for the Stefan-Boltzmann limit. . . . . 5 1.2 The phase diagram of QCD [10]. . . . . 6 1.3 The hard scattering ofγ-jet process. . . . . 8 2.1 Schematic view of two colliding nuclei in the geometrical participant-spectator model.
The distance between the centers of the two Lorentz contracted nuclei is the impact parameter b. . . . . 12 2.2 Nuclear stopping scenarios. Th particle rapidity are given before the collision and
after the collision in the case of a full stopping (Landau) and complete transparency (Bjorken). . . . . 13 2.3 The space-time evolution of the system in center-of momentum frame in relativistic
heavy ion collisions according to Bjorken [21]. . . . . 14 2.4 Particle ratios [25] in central collisions at √sN N = 200 GeV at mid-rapidity with
thermal model prediction [26] compared with PHENIX, PHOBOS, BRAHMS, and STAR. . . . . 18 2.5 The diagram of calculation for hard scattering. . . . . 20 2.6 Parton distribution function by the CTEQ group as a function of x at Q= 2GeV
(left) and 100 GeV (right) [30]. . . . . 21 2.7 Probability that a given parton will fragment into a π0 as a function of the mo-
mentum fraction z of the π0. These fragmentation functions were evaluated at Q2= 10 GeV2 [31]. . . . . 21
2.8 The invariant differential cross section for inclusiveπ0 production (points) and the results from NLO pQCD calculations in √s = 7 TeV pp collisions. The relative difference between the data and the theory using KKP [33] and CTEQ5M [29] sets of fragmentation functions with scales ofpT/2,pT and 2pT respectively. . . . . 22 2.9 A phenomenological curve and some experimental data for nuclear effect on structure
function,F2A/F2D, from [36]. . . . . 24 2.10 Charged particle pseudorapidity density per participant pair for central nucleus-
nucleus [43]-[47] and non-single diffractive pp (p¯p) collisions [48]-[49], as a function of √sN N. The solid lines ∝ s0.15N N and ∝ s0.11N N are superimposed on the heavy-ion and pp (p¯p) data, respectively [42] . . . . 26 2.11 The decoupling time extracted from Rlong [52]. The ALICE result is compared to
those obtained for central gold and lead collisions at lower energy at the AGS [53], SPS [54]-[55] and RHIC [56]-[57]. . . . . 27 2.12 The ellipsoidal shape of participant in non-central high energy nucleus-nucleus col-
lisions. . . . . 28 2.13 Measured charged particle ν2 as a function of pT in Pb+Pb collisions at √sN N =
2.76TeV [59]. a)ν2{2}for the centrality bin 40–50% from the 2- and 4-particle cu- mulant methods for ALICE measurement and for Au–Au collisions at√sN N = 200GeV in RHIC measurements. b)ν2{4} for various centralities compared to STAR mea- surements. The data points in the 20–30% centrality bin are shifted inpT for visibility. 29 2.14 ν2 as a function of pT (left) and K ET (right) scaled by the number of constituent
quarks in minimum bias Au+Au collisions at√sN N = 200GeV [62]. . . . . 30 2.15 The energy dependence ofK+/π+ andK /π ratio at mid-rapidity [63]. . . . . . 31 2.16 Comparison of RAA in central (0–5%) Pb+Pb collisions at √sN N = 2.76 TeV at
LHC to measurements at √sN N = 200GeV by the PHENIX [65] and STAR [66]
experiments at RHIC. The vertical bars around RAA= 1 show the pT independent scaling uncertainty withhNcolli [64]. . . . . 32 2.17 TheRAA of direct photon,π0 andη meson as a function ofpT in central Au + Au
collisions at √sN N = 200GeV [65]. . . . . 33 3.1 Cartoon illustrating a measurement of two-particle correlations from jet. . . . . . 37 3.2 Two particle azimuthal correlation distributions for minimum bias in proton-proton
collision at√
s= 7 TeV. . . . . 37
3.3 (a) Two particle azimuthal correlation distributions for minimum bias and central d + Au collisions. (b) Comparison of two particle correlations for central Au + Au collisions to those seen in ppand central d + Au collisions [67]. . . . . 38 3.4 The scheme of jet quenching in heavy-ion collisions. . . . . 39 3.5 Azimuthal correlations of high-pT charged hadron pairs in central Au + Au collisions
for variousptrigT andpassocT ranges. AllpT values in this figure have GeV/c unit [68]. 39 3.6 Schematic view of a hard scattering event in the plane perpendicular to the beam. . 41 3.7 ppairT as a function of collision energy√
sfrom various particle correlation measure- ments [72]. . . . . 42 3.8 Feynman diagrams of the main production processes for direct photons in initial hard
scatterings as well as in a thermalized quark-gluon plasma phase: (a) quark-gluon Compton scattering of order αs; (b) quark-antiquark annihilation of order αs; (c) fragmentation photons of order α2s. . . . . 44 3.9 Error band, including systematic uncertainties, for the ratio of all photons to decay
photons as a function of pT [77]. . . . . 45 3.10 The photon spectrum for central P b+P b collisions at the SPS including recent
interferometry results from WA98 [78]. For comparison, results from theoretical calculation are included [79]. . . . . 46 3.11 Direct photon spectra for different centralities in Au+Aucollisions at RHIC [80]. 47 3.12 xE distributions for several ranges of ptrigT compared to calculations using quark
(solid) and gluon (dashed) fragmentation functions as parameterized by the LEP data [71]. . . . . 49 3.13 Azimuthal correlation distribution of charged hadrons with various trigger particle
types with 7 GeV /c < ptrigT <9 GeV /cand3 GeV /c < passocT <5 GeV /cforppand central Au + Au (0-20%) collisions. Top: Inclusive, decay and direct jet function inppcollisions. Middle: Inclusive, decay and direct jet function is central Au + Au collisions. Bottom: π0,η,γ(π0)andγ(η)associated yields in pp. [85]. . . . . 50 3.14 The direct γ h correlation distribution for pp collisions multiplied by a factor of
10 and Au + Au [86]. . . . . 51 3.15 R(+/ )for isolated photons (blue) andπ0 (red) triggers as a function of associated
particles passocT for several selections of ptrigT . The lines are calculated using DSS fragmentation functions [85]. . . . . 52
3.16 Transverse distributions of the initial γ jet production vertices that contribute to the final observedγ hadron pairs along a given direction (arrows) with xE ≈0.9 (left plot) andxE ≈0.3 (right plot) [87]. . . . . 53 4.1 The LHC injector complex [88]. . . . . 56 4.2 Longitudinal view of the ALICE detector [89]. . . . . 58 4.3 Physical track-finding efficiency for different combinations of the tracking detectors
in central Pb+Pb collisions (dNch/dη = 6000) (left) and proton–proton collisions (right) [77]. . . . . 62 4.4 Transverse-momentum resolution for different combinations of the tracking detectors
in central Pb+Pb collisions (dNch/dη = 6000) (left) and proton–proton collisions (right) [77]. . . . . 62 4.5 dE/dx measurements performed by TPC and ITS with ALICE in proton-proton
collisions at √s= 7 TeV. . . . . 63 4.6 Identified particle transverse momentum distribution measured by ALICE detectors
in proton-proton collisions at√
s= 7TeV. . . . . 64 4.7 Energy resolution in EMCal (left) and PHOS (right) for electrons as a function of
incident beam momentum. The continuous line represents the result of the fit of Eq. (4.1) to the experimental data [77]. . . . . 68 4.8 Schematic view of the AliRoot framework [97]. . . . . 69 5.1 The ratio of NLO contribution to LO contribution for direct photon production. . . 75 5.2 Monte Carlo (PYTHIA)γ–jet photon spectrum (left) and jet–jet photon spectrum
(right) pT spectra in proton–proton collisions at √s = 14TeV. γ–jet photons are produced in qg→γq andqq¯→γg hard processes and jet–jet photons in final state radiation during the parton shower evolution or in the decay of neutral pions frag- menting jets from2→2 hard processes. Simulation bins inphardT normalized to the cross section are also displayed. . . . . 76 5.3 Example of the two decay photon clusters from π0. . . . . 78 5.4 Example of a shower profile and its principal axese1 ande2 [93]. . . . . 79 5.5 The scheme of isolation criteria. . . . . 81
5.6 Number of photons from PYTHIA generated (left) γ-jet and jet-jet events with jet energy Ejet > 5 GeV and the isolated photons after applying the isolation cut pa- rameters of R = 0.4, ε = 0.1 and pthresT = 1 GeV/c on the photon clusters from generated ones (right). . . . . 82 5.7 Relative azimuthal angle distribution∆φ=φγ φhadronin proton–proton collisions
at√s= 14TeV for isolated photons withpTγ>20GeV/c: from Monte Carlo events (dashed line) and from events reconstructed by ALICE (solid line) forγ–jet events (left) and jet–jet events (right). In jet–jet events the π0 trigger is not necessarily the leading particle.. . . . 84 5.8 The ratio of reconstructed signal hadrons from jet fragmentation to background
hadrons from underlying events in γ–jet events. The solid line is a fit to the data points. . . . . 85 5.9 Photon-charged hadron correlation spectrum and the contribution from underlying
event using photons identified by EMCal in pp collisions at√s = 5.5 TeV [111]. 86 5.10 Photon–charged hadrons correlation distribution with pTγ > 20 GeV/c, pTh >
2 GeV/c calculated in proton–proton collisions at √
s = 14 TeV from PYTHIA generated Monte-Carlo data. The data points with error bars represents the total spectrum (γ–jet plus jet–jet events) as it will be measured, the errors bars repre- sent the statistical errors of the measurement and the area around the data points represents the systematic errors due to the jet–jet photons not properly identified.
The contribution from jet–jet photons (closed circles) and from the underlying event (downwards triangles) are displayed as well. The ideal jet fragmentation function are calculated from the PYTHIA generated Monte-Carloγ–jet events is also shown (green line) [17]. . . . . 87 5.11 kT reproduced by PYTHIA generatedγ-jet events and extrapolated to LHC energies. 89 5.12 Comparison of fragmentation function with imbalance distribution from Monte Carlo
generated γ-jet events with jet energy larger than 20GeV in pp@10TeV by tuned parameters in PYTHIA, the uncertainty ofkT from the estimation are shown as well. 90 5.13 Ratio of fragmentation function (left) and correlation function (right) distribution
when ISR/FSR radiation on and off from Monte Carlo generated γ-jet events with jet energy larger than 20GeV in pp@10TeV by PYTHIA.. . . . 90
5.14 Comparison of hadrons transverse momentum spectrum with and without quench- ing in jet fragmentation function and correlation distribution from Monte Carlo generated γ-jet events with jet energy larger than 20GeV in pp@5.5TeV by tuned parameters in PYTHIA. . . . . 92 5.15 γ-hadrons correlation distribution in heavy ion collisions, the underlying events dis-
tribution is correlated photons with the hadrons at the same side of photons. On the right is theγ-hadrons correlation distribution after the underlying subtraction.. . . 93 5.16 The nuclear modification factorIAA forγ-hadrons correlation distribution in central
and perpgeral Pb+Pb collisions at LHC energy. . . . . 93 5.17 The nuclear modification factor RAA for charged hadrons in γ-jet events at LHC
energy, different transport coefficientqˆare used for different production. . . . . 94 5.18 The nuclear modification factor IAA forγ-hadrons correlation distribution at LHC
energy, different transport coefficientqˆare used for different γ-jet production. . . . 94 5.19 Relative azimuthal angle distribution ∆φ = φγ φhadron for γ-jet events in pp
collisions at √
s = 5.5 TeV. Photons with pT,γ > 30 GeV/c are measured and isolated in EMCAL. Associated hadrons have pT,hadron > 2 GeV/c. Full circles indicate quenched events and empty circles indicate unquenched events by PYTHIA generator. Distributions are normalized to the total number of isolated photons. . 95 5.20 Left: γ-hadron correlation distributions in quenched and unquenched PYTHIA
events as a function of ξ. Right: The ratio of the photon-triggered conditional hadron yieldsRCY for PYTHIA and qPYTHIA events. . . . 96 5.21 The ratio of the leading particlespT over the photonspT with and without quenching
effect is taken into account. . . . . 97 5.22 The correlation between the leading particle ratio and the∆φdistribution of leading
particles and photons, with (low) and without (up) quenching effect is taken into account respectively. The left panel is selecting leading particles inside all the gen- erated particles (ideal case), the right one just took final state particles which could be measured in the experiment. . . . . 98
