Prompt D meson production in pp, p-Pb and Pb-Pb collisions at LHC with ALICE

201 Pages·2017·10.49 MB·English

Universita’ degli Studi di Torino
Scuola di Dottorato in Scienze della Natura
e Tecnologie Innovative
Indirizzo in Fisica ed Astrofisica
XXX Ciclo
Prompt D+ meson production in pp,
s
p-Pb and Pb-Pb collisions at LHC with
ALICE
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H8 Author: Supervisors:
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N-/2 Anastasia Barbano Prof. Stefania Beol`e
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E/ Dr. Francesco Prino
C23
Examiners:
Dr. Ralf Averbeck
Dr. Patrick Robbe
March 21, 2018

v
Abstract
Prompt D+ meson production in pp, p-Pb and Pb-Pb collisions at LHC
s
with ALICE
The aim of this thesis is the study of the D+-meson production in pp collisions at the
√ s √
centre-of-mass energy s = 7 TeV and in p-Pb and Pb-Pb collisions at s = 5.02
NN
TeV, with the ALICE detector at the CERN Large Hadron Collider. Heavy quarks
provide an excellent way to investigate the properties of the Quark-Gluon Plasma
(QGP) created in high-energy nucleus-nucleus collisions via the measurements of the
nuclear modification factor (R ) and azimuthal anisotropy of hadrons originating
AA
from their hadronisation. At low and intermediate transverse momentum p , the
T
study of the D+ meson should also reveal information about the charm-quark hadro-
s
nisation mechanism. If charm quarks hadronise by recombining with lighter quarks
from the medium, the relative abundance of D+ mesons with respect to non-strange
s
D mesons is expected to be larger in Pb-Pb than in pp collisions, at low and inter-
mediate p , due to the large abundance of strange quarks in the QGP. The analyses
T
presented in this thesis are based on the full reconstruction of the D+ → K+K−π+
s
decay,exploitingthedisplacementofthedecayvertexfromtheinteractionpoint. The
p -differentialcross-sectionofD+ mesonsandtherelativeyieldsofD+ tonon-strange
T s s
D mesons were measured in pp and Pb-Pb collisions. In particular, the nuclear mod-
ification factors and, for the first time at LHC, the elliptic flow of D+ mesons were
s
measured in Pb-Pb collisions for central, semi-peripheral and peripheral collisions.
The observed increase of strange particle yield relative to pion yield with increas-
ing particle multiplicity in pp and p-Pb collisions suggested to investigate whether
a similar enhancement is observed in the relative yield of D+ mesons with respect
s
to non-strange D mesons in high multiplicity p-Pb collisions. To this purpose, the
relative ratios of D+-meson and D+-meson yields were measured in p-Pb collisions as
s
a function of the event particle multiplicity in different p intervals.
T
In the first Chapter, the physics of heavy-ion collisions is introduced. The second
Chapter is entirely dedicated to the role of heavy-flavour observables in the investi-
gation of the QGP. A description of the main features of the ALICE apparatus and
the detectors used in the analysis is illustrated in the third Chapter. The fourth,
fifth and sixth Chapters present the work of this thesis, focusing on the strategy used
in ALICE to reconstruct the D+ mesons and discussing the results obtained in pp,
s
Pb-Pb and p-Pb collisions, respectively.
The measurements presented in this thesis were approved by the ALICE Collabo-
ration and presented in various conferences. The proton-proton analysis has already
been published; the analysis of D+ v in Pb-Pb collisions is available on arXiv, wait-
s 2
ing for a publication on a peer reviewed journal. The proton-nucleus and the other
nucleus-nucleus results are still preliminary and will be published soon.

vii
Abstract
Prompt D+ meson production in pp, p-Pb and Pb-Pb collisions at LHC
s
with ALICE
Il lavoro di questa tesi ha riguardato lo studio della produzione del mesone D+ in
√ s
collisioni pp alla energia del centro di massa di s = 7 TeV e in collisioni p-Pb e
√
Pb-Pb a s = 5.02 TeV, attraverso il rivelatore ALICE al Large Hadron Collider
NN
del CERN. I quark pesanti costituiscono una sonda eccellente per le propriet`a del
Plasma di Quark e Gluoni (QGP) che si crea in collisioni nucleo-nucleo ad alta ener-
gia,attraversolamisuradelfattoredimodificazionenucleare(R )edell’anisotropia
AA
azimutale degli adroni che originano dalla loro adronizzazione. A intermedio e basso
momentotrasversop ,lostudiodelmesoneD+ dovrebbeinoltrerivelareinformazioni
T s
riguardo il meccanismo di adronizzazione del quark charm all’interno del mezzo. Se
il quark charm adronizza per coalescenza con i quark piu` leggeri presenti nel mezzo,
l’abbondanza relativa di mesoni D+ rispetto ai mesoni D non contenenti stranezza
s
`e attesa essere maggiore in collisioni Pb-Pb rispetto a collisioni pp, nella regione
ad intermedio-basso p , grazie all’abbondanza di quark strani nel QGP. Le analisi
T
presentate in questa tesi sono basate sulla ricostruzione completa della topologia di
decadimento del canale D+ → K+K−π+, sfruttando la distanza del vertice di decadi-
s
mento dal vertice di interazione primario. La sezione d’urto differenziale in p per
T
la produzione del mesone D+ e i rapporti di produzione del mesone D+ rispetto ai
s s
mesoni D non contenti stranezza sono stati misurati in collisioni pp e Pb-Pb. In
particolare, l’analisi delle collisioni Pb-Pb ha portato alla misura del fattore di mo-
dificazione nucleare e, per la prima volta a LHC, del flow ellittico del mesone D+ per
s
collisioni centrali, semi periferiche e periferiche. L’osservazione dell’aumento della
produzione di particelle contenenti stranezza rispetto alla produzione di pioni con
l’aumentare della molteplicit`a di particelle prodotte in collisioni pp e p-Pb ha sug-
gerito di investigare la presenza di un simile incremento nella produzione relativa del
mesone D+ rispetto ai mesoni D non contenenti stranezza in collisioni p-Pb ad alta
s
molteplicit`a. A questo proposito, la misura del rapporto di produzione D+/D+ `e
s
stata effettuata in funzione della molteplicit`a di particelle in collisioni p-Pb in diversi
intervalli di p .
T
Nel primo Capitolo verr`a introdotta la fisica degli ioni pesanti. Il secondo Capi-
tolo `e invece interamente dedicato al ruolo delle osservabili per lo studio degli adroni
pesanti nell’investigazione del QGP. Una descrizioni delle principali caratteristiche
dell’esperimento ALICE e dei rivelatori utilizzati nelle analisi si pu`o trovare nel terzo
Capitolo. Nel quarto, quinto e sesto capitolo verr`a presentato il lavoro di questa tesi,
introducendo la strategia usata in ALICE per la ricostruzione del mesone D+ e pas-
s
sando poi alla discussione dei risultati in collisioni pp, p-Pb e Pb-Pb rispettivamente.
Le misure presentate in questa tesi sono state approvate dalla Collaborazione
ALICE e presentate in numerose conferenze. L’analisi dei dati in collisioni pp `e stata
pubblicata. L’analisi del v del mesone D+ disponibile su arXiv, con l’obiettivo di
2 s
una rivista peer review. I risultati delle analisi in collisioni p-Pb e il resto dei risultati
delle analisi in collisioni Pb-Pb sono ancora preliminari e saranno pubblicati a breve.

ix
Contents
1 Quark-Gluon Plasma 1
1.1 Strongly interacting matter . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Micro bang vs big bang: timescales of expansion, baryonic number . . 3
1.3 What theory tells us . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Collision Geometry and the Glauber Model . . . . . . . . . . . . . . . 7
1.5 Heavy-ion physics at the LHC . . . . . . . . . . . . . . . . . . . . . . . 9
1.5.1 Particle multiplicity and energy density . . . . . . . . . . . . . 10
1.5.2 Hadron multiplicities and chemical freeze-out . . . . . . . . . . 11
1.5.3 Strangeness enhancement . . . . . . . . . . . . . . . . . . . . . 13
1.5.4 Collective flow and kinetic freeze-out . . . . . . . . . . . . . . . 14
Radial flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Anisotropic flow . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.5.5 Chiral symmetry restoration . . . . . . . . . . . . . . . . . . . 18
1.5.6 Jet quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
High-p hadrons . . . . . . . . . . . . . . . . . . . . . . . . . . 21
T
Jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.5.7 Quarkonium production . . . . . . . . . . . . . . . . . . . . . . 24
1.5.8 Latest discoveries in small systems . . . . . . . . . . . . . . . . 27
2 Heavy flavours 31
2.1 The importance of being heavy . . . . . . . . . . . . . . . . . . . . . . 31
2.2 Heavy-quark production in pp collisions . . . . . . . . . . . . . . . . . 31
2.3 Heavy quarks in p-A collisions . . . . . . . . . . . . . . . . . . . . . . 34
2.3.1 Cold nuclear matter effects . . . . . . . . . . . . . . . . . . . . 34
2.3.2 Nuclear Parton Distribution Functions . . . . . . . . . . . . . . 35
2.3.3 Experimental results in p-A collisions . . . . . . . . . . . . . . 38
2.4 Heavy quarks in A-A collisions . . . . . . . . . . . . . . . . . . . . . . 40
2.4.1 Collisional processes . . . . . . . . . . . . . . . . . . . . . . . . 41
2.4.2 Gluon-radiation processes . . . . . . . . . . . . . . . . . . . . . 42
2.4.3 Heavy flavour hadronisation in the medium . . . . . . . . . . . 43
2.4.4 Experimental results in A-A collisions . . . . . . . . . . . . . . 46
3 The ALICE experiment at the LHC 53
3.1 The Large Hadron Collider . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2 The ALICE experiment . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.2.1 Magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.2.2 Inner Tracking System (ITS) . . . . . . . . . . . . . . . . . . . 55
3.2.3 Time Projection Chamber (TPC) . . . . . . . . . . . . . . . . . 56
3.2.4 Time-Of-Flight (TOF) . . . . . . . . . . . . . . . . . . . . . . . 58
3.2.5 V0 Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.2.6 Zero Degree Calorimeter (ZDC) Detector . . . . . . . . . . . . 60
3.3 The ALICE Trigger System and Data Aquisition . . . . . . . . . . . . 60
3.3.1 The Central Trigger Processor (CTP) . . . . . . . . . . . . . . 61
x
3.3.2 The Data AcQuisition System (DAQ) . . . . . . . . . . . . . . 62
3.3.3 The High Level Trigger (HLT) . . . . . . . . . . . . . . . . . . 62
3.4 Machine-induced background . . . . . . . . . . . . . . . . . . . . . . . 62
3.5 Electromagnetic interactions . . . . . . . . . . . . . . . . . . . . . . . . 63
3.6 Track and vertex reconstruction . . . . . . . . . . . . . . . . . . . . . . 64
3.7 Secondary vertex reconstruction. . . . . . . . . . . . . . . . . . . . . . 67
3.8 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.9 Centrality determination . . . . . . . . . . . . . . . . . . . . . . . . . . 68
3.10 The ALICE Offline Software Framework . . . . . . . . . . . . . . . . . 70
3.10.1 The AliRoot Framework . . . . . . . . . . . . . . . . . . . . . . 70
√
4 D+ production in pp collisions at s = 7 TeV 73
s
4.1 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.2 D+ reconstruction and strategy . . . . . . . . . . . . . . . . . . . . . . 74
s
4.3 Single-track selections . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.4 Decay-chain and topology selection . . . . . . . . . . . . . . . . . . . . 76
4.5 Particle identification. . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.6 Invariant mass spectra, cut optimisation and signal extraction . . . . . 80
4.7 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.7.1 Reconstruction and selection efficiency . . . . . . . . . . . . . . 84
4.7.2 Beauty feed-down subtraction . . . . . . . . . . . . . . . . . . . 84
4.8 Systematic uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.8.1 Raw yield extraction . . . . . . . . . . . . . . . . . . . . . . . . 85
4.8.2 Selection efficiency . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.8.3 PID efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.8.4 Track reconstruction efficiency . . . . . . . . . . . . . . . . . . 90
Variation of track selections . . . . . . . . . . . . . . . . . . . . 90
ITS-TPC matching efficiency . . . . . . . . . . . . . . . . . . . 91
4.8.5 B feed-down. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.8.6 Generated p shape . . . . . . . . . . . . . . . . . . . . . . . . 103
T
4.9 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
4.9.1 D+ p -differential cross section . . . . . . . . . . . . . . . . . . 104
s T
4.9.2 p -differential D-meson ratios . . . . . . . . . . . . . . . . . . . 105
T
4.9.3 p -integrated D+ cross section . . . . . . . . . . . . . . . . . . 106
T s
4.9.4 p -integrated D-meson ratios . . . . . . . . . . . . . . . . . . . 107
T
√
5 D+ production in Pb-Pb collisions at s = 5.02 TeV 109
s
5.1 D+ p -differential yields . . . . . . . . . . . . . . . . . . . . . . . . . . 109
s T
5.1.1 Signal extraction . . . . . . . . . . . . . . . . . . . . . . . . . . 109
5.1.2 Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.2 Systematic uncertainty on corrected dN/dp . . . . . . . . . . . . . . 118
T
5.2.1 Yield extraction systematics . . . . . . . . . . . . . . . . . . . . 118
5.2.2 Selection efficiency . . . . . . . . . . . . . . . . . . . . . . . . . 120
5.2.3 PID selection efficiency . . . . . . . . . . . . . . . . . . . . . . 121
5.2.4 Generated p shape . . . . . . . . . . . . . . . . . . . . . . . . 122
T
5.2.5 Feed-down subtraction . . . . . . . . . . . . . . . . . . . . . . . 123
5.2.6 Track reconstruction efficiency . . . . . . . . . . . . . . . . . . 126
5.3 Nuclear modification factor in Pb-Pb collisions . . . . . . . . . . . . . 127
5.4 Systematic uncertainty on the R . . . . . . . . . . . . . . . . . . . . 127
AA
5.4.1 Proton-proton reference . . . . . . . . . . . . . . . . . . . . . . 129
5.4.2 Normalisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130