ETD system

Electronic theses and dissertations repository


Tesi etd-06012017-171533

Thesis type
Tesi di laurea magistrale
email address
Towards a better understanding of cosmic showers: design study of a spectrometer to measure forward charm production at the LHC
Corso di studi
relatore Prof. Cavoto, Gianluca
relatore Prof. Bellettini, Giorgio
Parole chiave
  • LHC
  • charm production
  • cosmic ray
  • forward physics at LHC
Data inizio appello
Riassunto analitico
The investigation of astrophysical phenomena has always been of primary interest within the scientific community. There is a consolidated belief that the study of such phenomena could allow a significant step forward in understanding the origin and evolution of our Universe.
This general assumption is mainly due to two peculiar aspects of most astrophysical observations:
1. they are signals traveling from the past, coming from regions very far away from the Earth,
2. they often come witnessing events that, if occurring in the vicinity of the Earth, would certainly lead to an unlivable environment for the humankind.
For these reasons, the study of the matter (cosmic rays and neutrinos) and the radiation (gamma-rays) ejected during these extraordinary events seems to represent our unique opportunity to have an open window into the rest of the Universe.
As a prominent example, Ultra-High-Energy-Cosmic-Rays (UHECRs) are cosmic rays of the highest energies (~ 10^{20}eV) and their detection is of great interest to answer cosmological questions but also to give information on possible new physics, exploring a center-of-mass (CM) energy of sqrt{s} ~ 400TeV, impossible to reach at colliders.
However, the very low measured flux of these incoming objects in high-energy ranges let us conclude that only ~km^{3} detectors can collect an amount of data statistically sufficient to give significant information in a relatively-short period of time.
Therefore, the impossibility of exploiting detectors outside the Earth’s atmosphere implies that telescopes face a big challenge in identifying the background produced by the impact of these objects with air nuclei.
Neutrinos and gamma-rays produce a very few particles in this impact; for the same weakly-interacting behavior, neutrinos in particular are suited to be direct messengers of the astrophysical events of interest. On the other hand there are cosmic rays which are primarily composed by protons and heavier
nuclei: since protons are part of a family of composite objects called hadrons, bound together by the strong force, in high-energy impacts they disintegrate
and generate big hadronic showers (mostly π/K and charmed hadrons, both of which later decay into neutrinos and charged leptons), that need to be
accurately reconstructed.
It is then obvious that a poor knowledge of the set of processes occurring in such collisions can seriously limit the relevant astroparticle physics information
that can actually be extracted on the cosmic primaries, for instance for galactic propagation parameters or indirect dark matter searches.

In the search for astrophysical neutrinos, the IceCube detector at the South Pole observes an excess, in the high-energy region, that can be accommodated
by a new astrophysical source with some difficulty, therefore remarking the fact that evaluation of the (background) atmospheric production could be somehow misunderstood.
Specifically, there are reasons to believe that the π/K component (commonly referred to as conventional) of this neutrino production is well-understood: it is experimentally observed and has identifying marks. On the other hand, the charm component (called prompt) has never been measured so far and its characteristics well emulate those of neutrinos from astrophysical origins.
To estimate hadronic processes, several methods making use of the parton model of hadrons and the theory of strong interactions (QCD) have been developed, but their reliability lies in the direct measurement of observables (such as cross-sections) at particle accelerators.
However, the region currently covered by detectors at the Large Hadron Collider (LHC) is mostly limited, in terms of the production angles production that secondaries form with the beam axis, to the central production, characterized by large production around the interaction point.
In the forward region at small angles (as opposite to the central), where data are not available, extrapolations based on theory are possible but very often affected by large uncertainties, since, when applied in a non-perturbative regime of QCD, they suffer from the intrinsic property of the strong interactions
called infrared slavery.

Within this framework, with the goal of curing this lack of experimental information, this thesis presents a study meant to show how the production of charm quark could be measured at extremely small angles around the LHC beams.
In general, the development of a small angle spectrometer is of paramount importance for a full experimental knowledge of hadron collisions. With reference
to our specific problem, the interest in such a detector is strengthened by the observation that the CM energy of the LHC (\sqrt{s} = 13TeV) is now in the range where anomalies are appearing in the IceCube data, that is 10^{16}-10^{17}eV, measured in the Earth’s frame.

The work has been divided into four chapters, as described below.
Chapter 1 gives an overview of the astrophysical studies showing the importance of building a device aiming at fully exploring hadronic interactions. In particular, we follow a two-fold approach:
– discuss the very general outlines of cosmic ray physics with a particular focus on an open question (the problem of UHECRs);
– show the most recent experimental observations made by Ice- Cube and analyze the excess they find in their high-energy data, illustrating why a question is raised, specifically, on the charm (prompt) component of the atmospheric background.
Chapter 2 recalls briefly the main features of the parton model of hadronic interactions, as it was motivated by the historical results of Deep Inelastic Scattering experiments at SLAC and, also, its weakness deriving from the lack of experimental data in the forward region (large-\eta) at colliders, such as the LHC. This weakness introduces large uncertainties in extrapolating information where data are not available and results in what we call poor knowledge of the atmospheric processes.
Chapter 3 makes use of a simulating tool for particle transport in matter (MARS) to extract information about the trajectory of the secondaries, guided by the set of magnets already present around an interaction point of the LHC.
Chapter 4 is the core of the thesis. Based on the results of Chapter 3, we use a standard event generator such as PYTHIA 8.2 to design a muon detector able to see the leptonic and semi-leptonic decay products of the charmed hadrons in the very forward region at LHC, with the best possible acceptance. As anticipated, the purpose of this experiment would be to measure the production cross-section of the charm quark. Several requirements will be applied, in order to reject background muons from π/K decay. Also, an auxiliary measurement of the \mu^{+}\mu^{-} decay channel of light unflavored resonances is proposed, to improve the signal/background ratio.
Finally, a last part drives some conclusions and mentions proposals for additional future studies.