by Cecilia Tosciri

Hey there!

My name is Cecilia Tosciri, I am the ESR of the AMVA4NewPhysics network selected for the position at the University of Oxford. Since you will hear from me and my work for a while, let me introduce myself and explain how I ended up in this exciting project.

I graduated one month ago at the University of Pisa (Italy), the city where I was born and raised. I’ve always been a very curious person. When I was a child I started to understand that the human brain is the most powerful tool in our hands and I decided to use it in the best way possible. I started to think a lot about life in a philosophical, religious and sentimental way but it seemed to me that nothing could give comprehensive and consistent answers to the deep questions I wanted to understand.

Only during the latest years of my classical studies in High School I begun to deal with Physics and became aware of the great potential of this discipline. This is why I chose Particle Physics for my academic studies. Eventually, this choice appears to feed both my passionate side and the rational one. Often, these two sides are not in agreement in human beings but, again, what I’ve understood is that you can control everything with your brain and let prevail your favorite side yourself!

But don’t wander off, Cecilia! Ok, let me focus on the main subject. The most interesting part of my academic career is actually quite recent and related to my master degree. In early 2015 I was selected for an internship program to work on a research project with the CDF experiment. I spent eight months at the Fermi National Accelerator Laboratories (FNAL or Fermilab), located near Chicago (USA), working on a refined measurement of the top quark mass in the lepton+jets channel.

Since the top quark mass is a fundamental parameter of the Standard Model, in fact, the CDF Collaboration has decided to make a major effort in order to produce its most precise measurement as a “legacy” of the experiment. Since the top quark discovery in 1995, different approaches have been followed by the Tevatron experiments to determine the top quark mass.

In our measurement we exploited the Matrix Element (ME) method, a very powerful technique which determines the likelihood of observing an event under both ttbar and background hypotheses. The hypotheses are determined from the entire kinematic information associated to every single event by integrating the matrix element of the process over the multidimensional phase space describing the final state.

For a given sample of selected events, the parameters to be measured are then determined as those values that maximize the overall likelihood. The superior statistical sensitivity of this method, with respect to other methods based on distribution-fitting, is due to the completeness of the information exploited in each event.

schematic_ME
Schematic representation of the calculation of the likelihood to obtain a given observed lepton+jets event at a proton-antiproton collider. From http://arxiv.org/pdf/1003.1316v2.pdf

Our analysis uses the ME method to calculate a ttbar likelihood as a bi-dimensional function of the assumed top mass mt and ΔJES, where ΔJES parametrizes the uncertainty in our knowledge of the jet energy scale. It is a shift applied to all jet energies in units of the jet-dependent systematic error.

By introducing this parameter into the likelihood, we can use as a constraint the known W mass to determine the optimal ΔJES and thereby reduce the final systematic error on the measured top quark mass. For the first time in CDF analyses, we included the background ME modeling in the likelihood integration, with an expected significant reduction of the systematic error of the final result.

The massive calculations required by the ME method imposed to develop an unconventional and less time-consuming integration method over the phase space of the events’ kinematics. In order to evaluate the multidimensional integrals, we employed a “Quasi-Monte Carlo” technique, based on deterministic sequences generated by choosing points approximately equally spaced in the integration space, such that equal phase space volumes contain an approximately equal number of points.

The work is reported in my master thesis which represents the unique official document on the analysis. Unfortunately, 8 months were not enough to complete the entire analysis and the measurement is currently in progress.

The visit at Fermilab has been a very rewarding experience for me. I had the opportunity to meet and exchange ideas with leading scientists of the CDF experiment, and to work with a group of top physics experts which supported me in my growth as a physicist.

WH
Wilson Hall, the iconic high-rise building at the heart of the Fermilab grounds. 

Beyond the professional perspective, living at Fermilab has been a really amazing experience. The site is indeed located in a natural area where the ecosystems are rich in biodiversity, and which represents the sanctuary of the local wildlife populations, like the famous herd of American bisons. In my free time I used to take care of a little vegetable garden and, as a member of the horse club, sometimes I went riding. I came back to Italy at the end of 2015 with a great deal of love for the Nature, in all of its aspects.

IMG_1972
Two examples of Fermilab’s bisons. The bison herd was established by the laboratory’s first director, Robert Wilson, in 1969. 
IMG_1893
Myself working on my vegetable garden at Fermilab. 

When I was about to finish my experience at Fermilab, I saw the announcement of the PhD position in Oxford related to the AMVA4NewPhysics network and I thought that it would have been the perfect continuation of my career, given my academic path and the most recent experience on data analysis and matrix element method. It seems that I had a good intuition, and after the final sprint in Pisa, devoted to the completion of my master studies and the writing of my thesis, here I am!

I moved to Oxford a couple of days ago, after a visit to CERN and a summer school in Lund (Sweden) on Machine Learning in High Energy Physics. The latter has been very interesting and, even if not really a basic level course, it has given me many starting points to understand and appreciate the subject. During my movements I also managed to meet some other ESRs of the project. But I think I need other posts to tell you about all of this stuff!

I am very enthusiastic to begin this new adventure, I love traveling and meeting people from all over the world and I’m sure that there will be plenty of opportunities to satisfy my ambitions. Well, that’s it, for now! I’ll keep you up to date!