A postdoctoral researcher at Yale University, Eliane is working on an analysis involving hard scattering events that produce direct photons and has recently done her first shift as Run Manager for ALICE.
When she started studying physics in Stuttgart, her hometown, Eliane Epple was already passionate about particle physics. But since it was not possible to specialize in this field at her university, after two years she moved to Munich and attended the Technical University Munich (TUM). Here, she followed courses for two more years before joining a research project led by Prof. Laura Fabbietti, who had just received a big grant and was starting her research group. The subject of Eliane’s Diploma thesis was the study of the interactions of kaons - and other particles containing strange quarks – with nuclear matter (protons and neutrons). More in detail, for her Diploma she analyzed the decay products of a resonance called Λ(1405), which is by some theories treated as a molecular bound state of an anti-kaon and a nucleon (K-N). Its is in this sense a pre-stage of a kaonic nuclear cluster (K-NN) that she later studied during her PhD, still working with Prof. Fabbietti.
In particular, Eliane and colleagues were investigating the possible existence of anti-kaonic bound-states formed by, for example, two nucleons and one anti-kaon. Besides Fabbietti’s team, other groups all over the world were working on this topic, since a number of theoretical physicists had hypothesized that the attraction between nucleons and anti-kaons should be strong enough to give rise to this bound state, at least for a short time. “I analyzed data from the High Acceptance Di-Electron Spectrometer (HADES) at GSI. In particular, I looked for particles produced in p+p collisions that could originate
bfrom the decay of this anti-kaon-nucleon bound state,” explains Eliane. “It was a very controversial topic at the time, because there were groups that, analyzing a certain set of data, could see a signal compatible with the detection of such bound state, while others couldn’t. I didn’t find any signal proving this hypothesis, but at the same time my results set un upper limit for the existence of this bound state at the beam energy of 3.5 GeV.”
“In order to set a limit,” Eliane continues, “you compare the result of your data analysis with the outcome of a simulation, performed assuming the hypothesis that the signal you are looking for but didn't see exists. In other words, you develop a model for this case and study how much signal you can introduce and still keep consistency with your data. You proceed to add more and more signal strength to your model in little steps, until you reach a threshold: if you overcome it, the model doesn’t fit anymore with the data. This threshold is an upper limit.”
She also combined her results with data from other experiments and showed that it was very unlikely that the signal seen by some other groups could be due to an anti-kaon-nucleon bound state. “Actually, I think that this signal exists because there are many compelling reasons from our theory colleagues, but it is very challenging to see, first of all because the production cross section of this state is probably very small, which means that it occurs rarely, so we need to take a lot of data. In addition, it might be a very broad state, so we are not going to find a narrow peak. As a consequence, understanding the background well is essential.”
When she completed her PhD in 2014, she decided to change field. “In that situation, you have two possible choices,” explains Eliane, “either you stay on the same topic and become an expert in a very specific field, or you change and broaden your horizon. In this second case, you do not become a specialist of one topic but rather increase your ‘portfolio’. I preferred to go for this second option and do something completely new. This way is much harder because you basically start from the beginning but I think it benefits a researcher in the long term to look at a field, in this case QCD, from many perspectives. I thus also encourage some young researchers to give low energy QCD research a chance and see what people do beyond the TeV scale.” Therefore, she joined the research group led by John Harris and Helen Caines at Yale University, in New Haven (US), where she has been working for two and a half years now, and entered the ALICE collaboration.
Her present research activities focus on hard probes in high-energy collisions. “The proton is a very fascinating object, there is a lot going on in it,” Eliane comments. “When you scatter two protons at low energy (an energy range where I have previously been working on), you see how the ‘entire’ proton behaves, you are not able to distinguish its internal structure. On the contrary, at the high energies of LHC, when you collide two protons you start seeing what happens inside, you can observe how partons collide with each other.”
In these hard scattering events, particles with a high transverse momentum are present in the final state. Eliane is analyzing Pb-Pb events in which a parton and a photon (a gamma) are produced. Photons do not interact with strongly-interacting matter, hence, when the Quark Gluon Plasma (QGP) is created in ALICE by smashing lead nuclei, a photon produced in the collision can traverse this medium and get out unaffected. In the opposite direction, a parton moves away from the collision vertex and fragments into a particle shower. The sum of the momenta of the particles in this shower have to balance the momentum of the photon (combining these fragments with the gamma on the other side is called gamma-hadron correlation), and altogether they carry the total momentum of the mother parton.
The objective of this research is measuring the fragmentation function, which describes the correlation between the momentum of the mother and those of each particle in the shower. Normally, most of the daughter particles carry a small fraction (less than 20%*) of the momentum of the mother, whereas very few of them have a high fraction of this momentum. “By studying the behaviour of the particle shower in Pb-Pb collisions, in comparison with pp and p-Pb collisions, we can understand how the QGP medium modifies it,” explains Eliane. “We may have, for example, fewer of these very high momentum fragments and therefore more of the low momentum ones, or the shower might be broader
. This study gives information about the properties of the medium that is created.”
There are measurements of gamma-hadron correlations performed in PHENIX, at 200 GeV which show that in gold-gold collisions the fragmentation function changes, giving fewer particles with high momentum fractions and many more particles having a low momentum fraction. ALICE is investigating what happens at higher energies.
Eliane is now working in collaboration with a graduate student at her institute and other colleagues in Berkeley. “We are performing a very complex analysis. In our events, we have to identify gammas on one side and the hadron showers on the other. But gammas can also be decay products of other particles, such as pions and other mesons. Thus, it is important to avoid this background signal and take into consideration only events in which the gamma is produced in the primary vertex. This is not easy and requires a number of following steps.”
Eliane will continue working at Yale for some time. Then, she will either look for another post-doctoral position in ALICE or will directly apply for some grants, most likely in Europe. “There are various opportunities in Germany to get research funding to start your own research group.”
Even though she likes her present topic of analysis, in the future she might change for something more basic: the substructure and dynamic of the proton. “The proton is a very complex and fascinating object in its own right, we still do not know much about its internal dynamics,” she highlights. In any case, the most important thing for her is to settle on a research topic that will give her deeper insight into QCD properties — something she is very intrigued by.
In addition to doing data analysis, Eliane coordinates the activities of the EMCal calibration group and EMCal photon object group and, lately, has been Run Manager for the data taking. With so much work and a four-year-old daughter, there is not much time left. Nevertheless, when she can, she attends classes of modern dance to de-stress and relax.