Diego Casadei - Academia.edu (original) (raw)

Papers by Diego Casadei

The Astrophysical Journal, 2010

Measurements of normalized differential cross-sections for top-quark pair production are presente... more Measurements of normalized differential cross-sections for top-quark pair production are presented as a function of the top-quark transverse momentum, and of the mass, transverse momentum, and
rapidity of the tt system, in proton–proton collisions at a center-of-mass energy of √s = 7 TeV. The dataset corresponds to an integrated luminosity of 4.6 fb−1, recorded in 2011 with the ATLAS detector
at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one lepton and at least four jets with at least one of the jets tagged as originating from a b-quark.
The measured spectra are corrected for detector efficiency and resolution effects and are compared to several Monte Carlo simulations and theory calculations. The results are in fair agreement with the predictions in a wide kinematic range. Nevertheless, data distributions are softer than predicted for higher values of the mass of the tt system and of the top-quark transverse momentum. The
measurements can also discriminate among different sets of parton distribution functions.

The measurement of the efficiency of an event selection is always an important part of the analys... more The measurement of the efficiency of an event selection is always an important part of the analysis of experimental data. The statistical techniques which are needed to determine the efficiency and its uncertainty are reviewed. Frequentist and Bayesian approaches are illustrated, and the problem of choosing a meaningful prior is explicitly addressed. Several practical use cases are considered, from the problem of combining different samples to complex situations in which non-unit weights or non-independent selections have been used. The Bayesian approach allows to find analytical expressions which solve even the most complicate problems, which make use of the family of Beta distributions, the conjugate priors for the binomial sampling.

This article proposes a way to improve the presentation of histograms where data are compared to ... more This article proposes a way to improve the presentation of histograms where data are compared to expectation. Sometimes, it is difficult to judge by eye whether the difference between the bin content and the theoretical expectation (provided by either a fitting function or another histogram) is just due to statistical fluctuations. More importantly, there could be statistically significant deviations which are completely invisible in the plot. We propose to add a small inset at the bottom of the plot, in which the statistical significance of the deviation observed in each bin is shown. Even though the numerical routines which we developed have only illustration purposes, it comes out that they are based on formulae which could be used to perform statistical inference in a proper way. An implementation of our computation is available at http://svn.cern.ch/guest/psde/

The objective Bayesian treatment of a model representing two independent Poisson processes, label... more The objective Bayesian treatment of a model representing two independent Poisson processes, labelled as "signal" and "background" and both contributing additively to the total number of counted events, is considered. It is shown that the reference prior for the parameter of interest (the signal intensity) can be well approximated by the widely (ab)used flat prior only when the expected background is very high. On the other hand, a very simple approximation (the limiting form of the reference prior for perfect prior background knowledge) can be safely used over a large portion of the background parameters space. The resulting approximate reference posterior is a Gamma density whose parameters are related to the observed counts. This limiting form is simpler than the result obtained with a flat prior, with the additional advantage of representing a much closer approximation to the reference posterior in all cases. Hence such limiting prior should be considered a better default or conventional prior than the uniform prior. On the computing side, it is shown that a 2-parameter fitting function is able to reproduce extremely well the reference prior for any background prior. Thus, it can be useful in applications requiring the evaluation of the reference prior for a very large number of times. [The published version JINST 9 (2014) T10006 has a typo in the normalization N of eq.(2.6) that is fixed here.]

The model representing two independent Poisson processes, labelled as "signal" and "background" a... more The model representing two independent Poisson processes, labelled as "signal" and "background" and both contributing at the same time to the total number of counted events, is considered from a Bayesian point of view. This is a widely used model for the searches of rare or exotic events in presence of some background source, as for example in the searches performed by the high-energy physics experiments. In the assumption of some prior knowledge about the background yield, a reference prior is obtained for the signal alone and its properties are studied. Finally, the properties of the full solution, the marginal reference posterior, are illustrated with few examples.

The statistical methods used by the ATLAS Collaboration for setting upper limits or establishing ... more The statistical methods used by the ATLAS Collaboration for setting upper limits or establishing a discovery are reviewed, as they are fundamental ingredients in the search for new phenomena. The analyses published so far adopted different approaches, choosing a frequentist or a Bayesian or a hybrid frequentist-Bayesian method to perform a search for new physics and set upper limits. In this note, after the introduction of the necessary basic concepts of statistical hypothesis testing, a few recommendations are made about the preferred approaches to be followed in future analyses.

The Alpha Magnetic Spectrometer (AMS) is a cosmic ray (CR) experiment that will operate on the In... more The Alpha Magnetic Spectrometer (AMS) is a cosmic ray (CR) experiment that will operate on the International Space Station for three years, measuring the particle spectra in the rigidity range from 0.2 GV to 2 TV. The AMS-02 detector will provide measurements with unprecedented statistics of the hadronic and leptonic cosmic rays, allowing for a better study of the Earth magnetosphere, of the solar system environment, of the solar system neighborhood, and of the galactic interstellar medium.

The Astrophysical Journal, 2014

The Astrophysical Journal, 2004

The direct measurements of electrons and positrons over the last 30 years, corrected for the sola... more The direct measurements of electrons and positrons over the last 30 years, corrected for the solar effect in the force-field approximation, are considered. The resulting overall electron spectrum may be fitted with a single power law above few GeV with spectral index ($\gamma_{-} = 3.41 \pm 0.02$), consistent with the spectral index of the positron spectrum ($\gamma_{+} = 3.40 \pm 0.06$), therefore suggesting a common acceleration process for both species. We propose that the engine was a shock wave originating from the last supernova explosion among those that formed the local bubble. In addition, at low energy, the electron spectrum measured during the last A+A+A+ solar phase is damped, whereas the positron spectrum is well represented by a single power law down to the lowest inferred local interstellar energy (0.8 GeV). We suggest that this difference arises from a time- and charge-dependent effect of the solar modulation that is not taken into account by the force-field approximation.

Journal of High Energy Physics, 2012

Journal of High Energy Physics, 2012

Journal of High Energy Physics, 2012

Drafts by Diego Casadei

https://arxiv.org/abs/1701.02788, 2017

Neutrons travel along straight lines in free space, but only survive for a distance which depends... more Neutrons travel along straight lines in free space, but only survive for a distance which depends on their energy. Thus, detecting neutrons in space in principle provides directional and distance information. Apart from secondary neutrons produced by cosmic-ray interactions in the Earth atmosphere, which are the dominant background, direct neutron emission is caused by solar flares, with clear time correlation with X-rays, which can be measured from few tens MeV up to few GeV. There is no detectable astrophysical source up to the PeV scale, when neutrons coming from supernova remnants may reach the Earth before decaying. In addition, ultra high energy neutrons are the most plausible explanation for the measured anisotropy of cosmic-ray showers produced in the atmosphere above 10 18 eV. From the GeV to the PeV scale, the expected neutron flux is very low and not too different from the antiproton flux, as the same cosmic-ray collisions with the interstellar medium which can produce antiprotons can also produce neutrons and antineutrons. This background flux of cosmic-ray neutrons is very low and has not yet been detected. Measuring the neutron energy spectrum in space is a very effective way of searching for decays of exotic particles. For example, dark matter could consist of Weakly Interacting Massive Particles (WIMPs), which may annihilate into final states with particle and antiparticle pairs. Consequently, a number of indirect WIMP searches are being carried on, focusing on positron and antiproton spectra. However, no experiment is presently foreseen to look for "bumps" in the neutron energy spectrum, which is virtually background free. Here we consider the implications of a measurement of the neutron energy spectrum in astronomy and astrophysics and list the interesting energy regions in the search for WIMPs. PACS numbers: 95.30.Cq, 95.35.+d, 95.55.-n, 95.55.Vj

The Astrophysical Journal, 2010

Measurements of normalized differential cross-sections for top-quark pair production are presente... more Measurements of normalized differential cross-sections for top-quark pair production are presented as a function of the top-quark transverse momentum, and of the mass, transverse momentum, and
rapidity of the tt system, in proton–proton collisions at a center-of-mass energy of √s = 7 TeV. The dataset corresponds to an integrated luminosity of 4.6 fb−1, recorded in 2011 with the ATLAS detector
at the CERN Large Hadron Collider. Events are selected in the lepton+jets channel, requiring exactly one lepton and at least four jets with at least one of the jets tagged as originating from a b-quark.
The measured spectra are corrected for detector efficiency and resolution effects and are compared to several Monte Carlo simulations and theory calculations. The results are in fair agreement with the predictions in a wide kinematic range. Nevertheless, data distributions are softer than predicted for higher values of the mass of the tt system and of the top-quark transverse momentum. The
measurements can also discriminate among different sets of parton distribution functions.

The measurement of the efficiency of an event selection is always an important part of the analys... more The measurement of the efficiency of an event selection is always an important part of the analysis of experimental data. The statistical techniques which are needed to determine the efficiency and its uncertainty are reviewed. Frequentist and Bayesian approaches are illustrated, and the problem of choosing a meaningful prior is explicitly addressed. Several practical use cases are considered, from the problem of combining different samples to complex situations in which non-unit weights or non-independent selections have been used. The Bayesian approach allows to find analytical expressions which solve even the most complicate problems, which make use of the family of Beta distributions, the conjugate priors for the binomial sampling.

This article proposes a way to improve the presentation of histograms where data are compared to ... more This article proposes a way to improve the presentation of histograms where data are compared to expectation. Sometimes, it is difficult to judge by eye whether the difference between the bin content and the theoretical expectation (provided by either a fitting function or another histogram) is just due to statistical fluctuations. More importantly, there could be statistically significant deviations which are completely invisible in the plot. We propose to add a small inset at the bottom of the plot, in which the statistical significance of the deviation observed in each bin is shown. Even though the numerical routines which we developed have only illustration purposes, it comes out that they are based on formulae which could be used to perform statistical inference in a proper way. An implementation of our computation is available at http://svn.cern.ch/guest/psde/

The objective Bayesian treatment of a model representing two independent Poisson processes, label... more The objective Bayesian treatment of a model representing two independent Poisson processes, labelled as "signal" and "background" and both contributing additively to the total number of counted events, is considered. It is shown that the reference prior for the parameter of interest (the signal intensity) can be well approximated by the widely (ab)used flat prior only when the expected background is very high. On the other hand, a very simple approximation (the limiting form of the reference prior for perfect prior background knowledge) can be safely used over a large portion of the background parameters space. The resulting approximate reference posterior is a Gamma density whose parameters are related to the observed counts. This limiting form is simpler than the result obtained with a flat prior, with the additional advantage of representing a much closer approximation to the reference posterior in all cases. Hence such limiting prior should be considered a better default or conventional prior than the uniform prior. On the computing side, it is shown that a 2-parameter fitting function is able to reproduce extremely well the reference prior for any background prior. Thus, it can be useful in applications requiring the evaluation of the reference prior for a very large number of times. [The published version JINST 9 (2014) T10006 has a typo in the normalization N of eq.(2.6) that is fixed here.]

The model representing two independent Poisson processes, labelled as "signal" and "background" a... more The model representing two independent Poisson processes, labelled as "signal" and "background" and both contributing at the same time to the total number of counted events, is considered from a Bayesian point of view. This is a widely used model for the searches of rare or exotic events in presence of some background source, as for example in the searches performed by the high-energy physics experiments. In the assumption of some prior knowledge about the background yield, a reference prior is obtained for the signal alone and its properties are studied. Finally, the properties of the full solution, the marginal reference posterior, are illustrated with few examples.

The statistical methods used by the ATLAS Collaboration for setting upper limits or establishing ... more The statistical methods used by the ATLAS Collaboration for setting upper limits or establishing a discovery are reviewed, as they are fundamental ingredients in the search for new phenomena. The analyses published so far adopted different approaches, choosing a frequentist or a Bayesian or a hybrid frequentist-Bayesian method to perform a search for new physics and set upper limits. In this note, after the introduction of the necessary basic concepts of statistical hypothesis testing, a few recommendations are made about the preferred approaches to be followed in future analyses.

The Alpha Magnetic Spectrometer (AMS) is a cosmic ray (CR) experiment that will operate on the In... more The Alpha Magnetic Spectrometer (AMS) is a cosmic ray (CR) experiment that will operate on the International Space Station for three years, measuring the particle spectra in the rigidity range from 0.2 GV to 2 TV. The AMS-02 detector will provide measurements with unprecedented statistics of the hadronic and leptonic cosmic rays, allowing for a better study of the Earth magnetosphere, of the solar system environment, of the solar system neighborhood, and of the galactic interstellar medium.

The Astrophysical Journal, 2014

The Astrophysical Journal, 2004

The direct measurements of electrons and positrons over the last 30 years, corrected for the sola... more The direct measurements of electrons and positrons over the last 30 years, corrected for the solar effect in the force-field approximation, are considered. The resulting overall electron spectrum may be fitted with a single power law above few GeV with spectral index ($\gamma_{-} = 3.41 \pm 0.02$), consistent with the spectral index of the positron spectrum ($\gamma_{+} = 3.40 \pm 0.06$), therefore suggesting a common acceleration process for both species. We propose that the engine was a shock wave originating from the last supernova explosion among those that formed the local bubble. In addition, at low energy, the electron spectrum measured during the last A+A+A+ solar phase is damped, whereas the positron spectrum is well represented by a single power law down to the lowest inferred local interstellar energy (0.8 GeV). We suggest that this difference arises from a time- and charge-dependent effect of the solar modulation that is not taken into account by the force-field approximation.

Journal of High Energy Physics, 2012

Journal of High Energy Physics, 2012

Journal of High Energy Physics, 2012

https://arxiv.org/abs/1701.02788, 2017

Neutrons travel along straight lines in free space, but only survive for a distance which depends... more Neutrons travel along straight lines in free space, but only survive for a distance which depends on their energy. Thus, detecting neutrons in space in principle provides directional and distance information. Apart from secondary neutrons produced by cosmic-ray interactions in the Earth atmosphere, which are the dominant background, direct neutron emission is caused by solar flares, with clear time correlation with X-rays, which can be measured from few tens MeV up to few GeV. There is no detectable astrophysical source up to the PeV scale, when neutrons coming from supernova remnants may reach the Earth before decaying. In addition, ultra high energy neutrons are the most plausible explanation for the measured anisotropy of cosmic-ray showers produced in the atmosphere above 10 18 eV. From the GeV to the PeV scale, the expected neutron flux is very low and not too different from the antiproton flux, as the same cosmic-ray collisions with the interstellar medium which can produce antiprotons can also produce neutrons and antineutrons. This background flux of cosmic-ray neutrons is very low and has not yet been detected. Measuring the neutron energy spectrum in space is a very effective way of searching for decays of exotic particles. For example, dark matter could consist of Weakly Interacting Massive Particles (WIMPs), which may annihilate into final states with particle and antiparticle pairs. Consequently, a number of indirect WIMP searches are being carried on, focusing on positron and antiproton spectra. However, no experiment is presently foreseen to look for "bumps" in the neutron energy spectrum, which is virtually background free. Here we consider the implications of a measurement of the neutron energy spectrum in astronomy and astrophysics and list the interesting energy regions in the search for WIMPs. PACS numbers: 95.30.Cq, 95.35.+d, 95.55.-n, 95.55.Vj