Hyperon production in p-Pb collisions with ALICE at the LHC

191 Pages·2016·8.25 MB·English

by  Didier Alexandre

Hyperon production in p–Pb collisions
with ALICE at the LHC.
Didier Alexandre
Thesis submitted for the degree of
Doctor of Philosophy
Nuclear Physics Group
School of Physics and Astronomy
University of Birmingham
June 2016
University of Birmingham Research Archive

e-theses repository

This unpublished thesis/dissertation is copyright of the author and/or third parties.
The intellectual property rights of the author or third parties in respect of this work
are as defined by The Copyright Designs and Patents Act 1988 or as modified by
any successor legislation.

Any use made of information contained in this thesis/dissertation must be in
accordance with that legislation and must be properly acknowledged. Further
distribution or reproduction in any format is prohibited without the permission of
the copyright holder.
Abstract
This thesis discusses the production of the multi-strange, charged Ξ and Ω baryons
in proton–lead (p–Pb) collisions at a centre-of-mass energy of 5.02 TeV. The trans-
verse momentum, p , distributions are analysed as a function of event multiplic-
T
ity. A hydrodynamical model based on statistical physics reproduces the shapes of
the multi-strange p spectra, in conjunction with the spectra of lighter hadrons, in
T
high multiplicity data. The good agreement is an indication of collective behaviour
by all particles inside a system in thermal equilibrium, consistent with the picture
of the build–up of a radially outward expansion due to an initially dense medium.
These results are reminiscent of the observations made in lead–lead (Pb–Pb) colli-
sions, which are explained by the formation of a Quark-Gluon Plasma. In addition,
the p –integrated yields of the hyperons are reported on, revealing a steady increase
T
as a function of multiplicity. An enhancement with respect to non-strange hadrons
is observed, and the Ξ/π and Ω/π ratios in high multiplicity p–Pb data approach
those measured in central Pb–Pb collisions. The Ξ/π ratio is comparable with the
calculations from a thermal model for strangeness saturation, whereas the Ω/π ratio
deviates from that value by 2σ.
i
ii
Author’s contributions
During the course of my degree, I have been a member of the ALICE collaboration.
Therefore, my results cannot be solely attributed to my work. Instead, I benefited
from the input of several other members. I made use of existing code written by
David Dobrigkeit Chinellato, Michele Floris and Roberto Preghenella.
I started my physics analysis work on strangeness, by working on the production
rates of Λ and K0. I made use of a macro written by David, in order to read Event
S
Summary Data (ESD) files and extract the p -distributions of these particles. Later,
T
I adapted the existing code into one for the analysis of the doubly-strange Ξ hadrons,
and then extended it to work for the triply-strange Ω, too. These particles became
the focus of my main analysis project. David independently developed a code for the
measurement of the multi-strange particles himself, which was very helpful to per-
form cross-checks on my results. I performed the analysis that led to the final decision
on the selection cuts and analysis settings for the multi-strange hadrons, extracted
all p -spectra and calculated their uncertainties. My results were approved by the
T
collaboration to be presented in the Quark Matter conference in Darmstadt, in May
2014. I gave the corresponding talk in a parallel session and wrote the proceedings
on my contribution [1]. In addition, I led the efforts towards the publication of the
results in the Physics Letters B journal, shown in Appendix C [2], by taking the role
of chair of the paper committee. I am grateful for all the discussions and advice pro-
vided to me by my supervisor Roman Lietava, by Lee Barnby and by David during
this long process. I also have to thank Michele for introducing me to the thermal
model code, with which I performed several fits and extracted model descriptions for
the discussion of the results in the paper and in this thesis.
During my stay at CERN, I was also involved in several projects with the Central
Trigger Processor (CTP) of ALICE. My main project consisted in performing tests
on the firmware of the Local Trigger Unit (LTU), the interface between the CTP
and the ALICE sub-detectors. I developed a C-written programme to run tests on
iii
the trigger protocol. This work is the subject of the last section of Chapter 3. I
also helped in running trigger simulations with the LTU and test, together with the
experts of several sub-detectors, that these were received correctly by their systems.
In addition, I wrote a Python script to manage the trigger configuration file. All this
work was supervised by Anton Jusko and Roman. During the beginning of the Run
2 of the LHC, I also did several CTP on-call shifts. Finally, I completed a course on
the operation of the Experiment Control System, and served 72 hours in shifts in the
ALICE run control room.
iv
To Marlene
v
vi
Acknowledgements
I owe the completion of this degree to many people, who in their own way, helped me
bring it to its end. I am especially grateful to my supervisor Roman Lietava, who has
always been available to discuss my work, give good advice, but most importantly,
to teach me. I am also grateful to David Evans for the great opportunity to do
this degree with the ALICE group of Birmingham, and for all his help during it.
Special thanks go also to Lee Barnby for the many invaluable interactions we had,
in particular, but not only, in the paper committe we were part of. I also thank all
other members of our group, with whom it was always a pleasure to work: Katie
Graham, Luke Hanratty, Peter Jones, Anton Jusko, Marian Krivda, Graham Lee,
Arvinder Palaha, Plamen Petrov, Patrick Scott, Orlando Villalobos-Baillie and Nima
Zardoshti.
From the ALICE collaboration, I thank especially my friend David D. Chinellato. It
was a privilege to be able to collaborate with him for the greater part of my degree.
I am indebted to him for all that he taught me. I also thank him for the opportunity
to work with him in Campinas, for a period of three months, during which I enjoyed
all the discussions we had during the process of writing our paper. I also extend
my gratitude to his colleagues Jun Takahashi and Giorgio Torrieri. I thank other
members of the ALICE collaboration with whom I had the chance to have several
helpful exchanges on the topic of strangeness, especially Domenico Colella, Michele
Floris and Roberto Preghenella.
At this stage, it feels important to remember the beginning of this journey. I am
grateful to Fabrizio Salvatore for introducing me to the field of High Energy Physics
six years ago.
I enjoyed the company of my friends Pete Griffith, Luca Pescatore and Mark Levy,
during our time spent at CERN, as well as during the summer schools we attended.
I thank them for all the fun we had. I can also not forget the friends I made outside
the field of physics during this time. There are several, however special thanks go to
Deni, Fernanda and Luis from Brazil, for teaching me many things that cannot be
expressed in equations.
vii
On a more personal level, I thank my father, who over the last eight years must have
broken all records in phone calls to me. I also thank my late mother, who contributed
a lot to who I am today.
Finally, I thank Marlene, for her unconditional love and support through all these
years.
viii