arXiv subjects

R. Trotta

Publications and source records attributed to R. Trotta.

At least 55 records · Page 3Linked to original sources

arXiv2011

Constraints on cosmic-ray propagation models from a global Bayesian analysis

Research in many areas of modern physics such as, e.g., indirect searches for dark matter and particle acceleration in SNR shocks, rely heavily on studies of cosmic rays (CRs) and associated diffuse emissions (radio, microwave, X-rays, gamma rays). While very detailed numerical models of CR propagation exist, a quantitative statistical analysis of such models has been so far hampered by the large computational effort that those models require. Although statistical analyses have been carried out before using semi-analytical models (where the computation is much faster), the evaluation of the results obtained from such models is difficult, as they necessarily suffer from many simplifying assumptions, The main objective of this paper is to present a working method for a full Bayesian parameter estimation for a numerical CR propagation model. For this study, we use the GALPROP code, the most advanced of its kind, that uses astrophysical information, nuclear and particle data as input to self-consistently predict CRs, gamma rays, synchrotron and other observables. We demonstrate that a full Bayesian analysis is possible using nested sampling and Markov Chain Monte Carlo methods (implemented in the SuperBayeS code) despite the heavy computational demands of a numerical propagation code. The best-fit values of parameters found in this analysis are in agreement with previous, significantly simpler, studies also based on GALPROP.

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arXiv2010

Global fits of the Non-Universal Higgs Model

We carry out global fits to the Non-Universal Higgs Model (NUHM), applying all relevant present-day constraints. We present global probability maps for the NUHM parameters and observables (including collider signatures, direct and indirect detection quantities), both in terms of posterior probabilities and in terms of profile likelihood maps. We identify regions of the parameter space where the neutralino dark matter in the model is either bino-like, or else higgsino-like with mass close to 1 TeV and spin-independent scattering cross section ~ 10^{-9}-10^{-8} pb. We trace the occurrence of the higgsino-like region to be a consequence of a mild focusing effect in the running of one of the Higgs masses, the existence of which in the NUHM we identify in our analysis. Although the usual bino-like neutralino is more prominent, higgsino-like dark matter cannot be excluded, however its significance strongly depends on the prior and statistics used to assess it. We note that, despite experimental constraints often favoring different regions of parameter space to the Constrained MSSM, most observational consequences appear fairly similar, which will make it challenging to distinguish the two models experimentally

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arXiv2010

Efficient reconstruction of CMSSM parameters from LHC data - A case study

We present an efficient method of reconstructing the parameters of the Constrained MSSM from assumed future LHC data, applied both on their own right and in combination with the cosmological determination of the relic dark matter abundance. Focusing on the ATLAS SU3 benchmark point, we demonstrate that our simple Gaussian approximation can recover the values of its parameters remarkably well. We examine two popular non-informative priors and obtain very similar results, although when we use an informative, naturalness-motivated prior, we find some sizeable differences. We show that a further strong improvement in reconstructing the SU3 parameters can by achieved by applying additional information about the relic abundance at the level of WMAP accuracy, although the expected data from Planck will have only a very limited additional impact. Further external data may be required to break some remaining degeneracies. We argue that the method presented here is applicable to a wide class of low-energy effective supersymmetric models, as it does not require to deal with purely experimental issues, eg, detector performance, and has the additional advantages of computational efficiency. Furthermore, our approach allows one to distinguish the effect of the model's internal structure and of the external data on the final parameters constraints.

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arXiv2010

Should we doubt the cosmological constant?

While Bayesian model selection is a useful tool to discriminate between competing cosmological models, it only gives a relative rather than an absolute measure of how good a model is. Bayesian doubt introduces an unknown benchmark model against which the known models are compared, thereby obtaining an absolute measure of model performance in a Bayesian framework. We apply this new methodology to the problem of the dark energy equation of state, comparing an absolute upper bound on the Bayesian evidence for a presently unknown dark energy model against a collection of known models including a flat LambdaCDM scenario. We find a strong absolute upper bound to the Bayes factor B between the unknown model and LambdaCDM, giving B < 3. The posterior probability for doubt is found to be less than 6% (with a 1% prior doubt) while the probability for LambdaCDM rises from an initial 25% to just over 50% in light of the data. We conclude that LambdaCDM remains a sufficient phenomenological description of currently available observations and that there is little statistical room for model improvement.

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arXiv2010

Comment on "Bayesian evidence: can we beat MultiNest using traditional MCMC methods", by Rutger van Haasteren (arXiv:0911.2150)

In arXiv:0911.2150, Rutger van Haasteren seeks to criticize the nested sampling algorithm for Bayesian data analysis in general and its MultiNest implementation in particular. He introduces a new method for evidence evaluation based on the idea of Voronoi tessellation and requiring samples from the posterior distribution obtained through MCMC based methods. He compares its accuracy and efficiency with MultiNest, concluding that it outperforms MultiNest in several cases. This comparison is completely unfair since the proposed method can not perform the complete Bayesian data analysis including posterior exploration and evidence evaluation on its own while MultiNest allows one to perform Bayesian data analysis end to end. Furthermore, their criticism of nested sampling (and in turn MultiNest) is based on a few conceptual misunderstandings of the algorithm. Here we seek to set the record straight.

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arXiv2009

Prospects for dark matter detection with IceCube in the context of the CMSSM

We study in detail the ability of the nominal configuration of the IceCube neutrino telescope (with 80 strings) to probe the parameter space of the Constrained MSSM (CMSSM) favoured by current collider and cosmological data. Adopting conservative assumptions about the galactic halo model and the expected experiment performance, we find that IceCube has a probability between 2% and 12% of achieving a 5sigma detection of dark matter annihilation in the Sun, depending on the choice of priors for the scalar and gaugino masses and on the astrophysical assumptions. We identify the most important annihilation channels in the CMSSM parameter space favoured by current constraints, and we demonstrate that assuming that the signal is dominated by a single annihilation channel canlead to large systematic errors in the inferred WIMP annihilation cross section. We demonstrate that ~ 66% of the CMSSM parameter space violates the equilibrium condition between capture and annihilation in the center of the Sun. By cross-correlating our predictions with direct detection methods, we conclude that if IceCube does detect a neutrino flux from the Sun at high significance while direct detection experiments do not find a signal above a spin-independent cross section sigma_SI^p larger than 5x10^{-9} pb, the CMSSM will be strongly disfavoured, given standard astrophysical assumptions for the WIMP distribution. This result is robust with respect to a change of priors. We argue that the proposed low-energy DeepCore extension of IceCube will be an ideal instrument to focus on relevant CMSSM areas of parameter space.

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arXiv2009

Are BR(b->s gamma) and (g-2)_muon consistent within the Constrained MSSM?

We employ two different statistical tests to examine whether, in the framework of the Constrained MSSM, the experimentally determined values of BR(b->s gamma) and the anomalous magnetic moment of the muon, (g-2), are consistent with each other. Our tests are designed to compare the theoretical predictions of the CMSSM in data space with the actual measurements, once all of the CMSSM free parameters have been integrated out and constrained using all other available data. We investigate the value of (g-2) as obtained by using e+e- data alone (which shows a ~3sigma discrepancy with the SM prediction) and as obtained based on tau decay data (which shows a much milder, 1sigma discrepancy). We find that one of our tests returns either a statistically inconclusive result or shows weak evidence of tension between BR(b->s gamma) and the e+e- data based value of (g-2). On the other hand, our second test, which is more stringent in this application, reveals that the joint observations of BR(b->s gamma) and (g-2) from e+e- data alone are incompatible within the CMSSM at the ~2sigma level. On the other hand, for both tests we find no significant tension between BR(b->s gamma) and the value of (g-2) evaluated using tau decay data. These results are only weakly dependent on the three different priors that we employ in the analysis. We conclude that, if the discrepancy between the Standard Model and the experimental determinations of (g-2) is confirmed at the 3sigma level, this could be interpreted as strong evidence against the CMSSM.

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arXiv2008

The impact of priors and observables on parameter inferences in the Constrained MSSM

We use a newly released version of the SuperBayeS code to analyze the impact of the choice of priors and the influence of various constraints on the statistical conclusions for the preferred values of the parameters of the Constrained MSSM. We assess the effect in a Bayesian framework and compare it with an alternative likelihood-based measure of a profile likelihood. We employ a new scanning algorithm (MultiNest) which increases the computational efficiency by a factor ~200 with respect to previously used techniques. We demonstrate that the currently available data are not yet sufficiently constraining to allow one to determine the preferred values of CMSSM parameters in a way that is completely independent of the choice of priors and statistical measures. While b->s gamma generally favors large m_0, this is in some contrast with the preference for low values of m_0 and m_1/2 that is almost entirely a consequence of a combination of prior effects and a single constraint coming from the anomalous magnetic moment of the muon, which remains somewhat controversial. Using an information-theoretical measure, we find that the cosmological dark matter abundance determination provides at least 80% of the total constraining power of all available observables. Despite the remaining uncertainties, prospects for direct detection in the CMSSM remain excellent, with the spin-independent neutralino-proton cross section almost guaranteed above sigma_SI ~ 10^{-10} pb, independently of the choice of priors or statistics. Likewise, gluino and lightest Higgs discovery at the LHC remain highly encouraging. While in this work we have used the CMSSM as particle physics model, our formalism and scanning technique can be readily applied to a wider class of models with several free parameters.

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arXiv2008

Bayesian Selection of sign(mu) within mSUGRA in Global Fits Including WMAP5 Results

We study the properties of the constrained minimal supersymmetric standard model (mSUGRA) by performing fits to updated indirect data, including the relic density of dark matter inferred from WMAP5. In order to find the extent to which mu < 0 is disfavoured compared to mu > 0, we compare the Bayesian evidence values for these models, which we obtain straightforwardly and with good precision from the recently developed multi-modal nested sampling ('MultiNest') technique. We find weak to moderate evidence for the mu > 0 branch of mSUGRA over mu < 0 and estimate the ratio of probabilities to be P(mu > 0)/P(mu < 0) = 6-61 depending on the prior measure and range used. There is thus positive (but not overwhelming) evidence that mu > 0 in mSUGRA. The MultiNest technique also delivers probability distributions of parameters and other relevant quantities such as superpartner masses. We explore the dependence of our results on the choice of the prior measure used. We also use the Bayesian evidence to quantify the consistency between the mSUGRA parameter inferences coming from the constraints that have the largest effects: (g-2)_mu, BR(b -> s gamma) and cold dark matter (DM) relic density Omega_{DM}h^2.

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arXiv2007

Flat Tree-level Inflationary Potentials in Light of CMB and LSS Data

We use cosmic microwave background and large scale structure data to test a broad and physically well-motivated class of inflationary models: those with flat tree-level potentials (typical in supersymmetry). The non-trivial features of the potential arise from radiative corrections which give a simple logarithmic dependence on the inflaton field, making the models very predictive. We also consider a modified scenario with new physics beyond a certain high-energy cut-off showing up as non-renormalizable operators (NRO) in the inflaton field. We find that both kinds of models fit remarkably well CMB and LSS data, with very few free parameters. Besides, a large part of these models naturally predict a reasonable number of e-folds. A robust feature of these scenarios is the smallness of tensor perturbations (r < 10^{-3}). The NRO case can give a sizeable running of the spectral index while achieving a sufficient number of e-folds. We use Bayesian model comparison tools to assess the relative performance of the models. We believe that these scenarios can be considered as a standard physical class of inflationary models, on a similar footing with monomial potentials.

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arXiv2007

Reconstructing the history of dark energy using maximum entropy

We present a Bayesian technique based on a maximum entropy method to reconstruct the dark energy equation of state $w(z)$ in a non--parametric way. This MaxEnt technique allows to incorporate relevant prior information while adjusting the degree of smoothing of the reconstruction in response to the structure present in the data. After demonstrating the method on synthetic data, we apply it to current cosmological data, separately analysing type Ia supernovae measurement from the HST/GOODS program and the first year Supernovae Legacy Survey (SNLS), complemented by cosmic microwave background and baryonic acoustic oscillations data. We find that the SNLS data are compatible with $w(z) = -1$ at all redshifts $0 \leq z \lsim 1100$, with errorbars of order 20% for the most constraining choice of priors. The HST/GOODS data exhibit a slight (about $1σ$ significance) preference for $w>-1$ at $z\sim 0.5$ and a drift towards $w>-1$ at larger redshifts, which however is not robust with respect to changes in our prior specifications. We employ both a constant equation of state prior model and a slowly varying $w(z)$ and find that our conclusions are only mildly dependent on this choice at high redshifts. Our method highlights the danger of employing parametric fits for the unknown equation of state, that can potentially miss or underestimate real structure in the data.

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arXiv2004

Constraints on a mixed inflaton and curvaton scenario for the generation of the curvature perturbation

We consider a supersymmetric grand unified model which naturally solves the strong CP and mu problems via a Peccei-Quinn symmetry and leads to the standard realization of hybrid inflation. We show that the Peccei-Quinn field of this model can act as curvaton. In contrast to the standard curvaton hypothesis, both the inflaton and the curvaton contribute to the total curvature perturbation. The model predicts an isocurvature perturbation too which has mixed correlation with the adiabatic one. The cold dark matter of the universe is mostly constituted by axions plus a small amount of lightest sparticles. The predictions of the model are confronted with the Wilkinson microwave anisotropy probe and other cosmic microwave background radiation data. We analyze two representative choices of parameters and derive bounds on the curvaton contribution to the adiabatic perturbation. We find that, for the choice which provides the best fitting of the data, the curvaton contribution to the adiabatic amplitude must be smaller than about 67% (at 95% confidence level). The best-fit power spectra are dominated by the adiabatic part of the inflaton contribution. We use Bayesian model comparison to show that this choice of parameters is disfavored with respect to the pure inflaton scale-invariant case with odds of 50 to 1. For the second choice of parameters, the adiabatic mode is dominated by the curvaton, but this choice is strongly disfavored relative to the pure inflaton scale-invariant case (with odds of 10^7 to 1). We conclude that in the present framework the perturbations must be dominated by the adiabatic component from the inflaton.

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arXiv2004

Testing the paradigm of adiabaticity

We introduce the concepts of adiabatic (curvature) and isocurvature (entropy) cosmological perturbations and present their relevance for parameter estimation from cosmic microwave background anisotropies data. We emphasize that, while present-day data are in excellent agreement with pure adiabaticity, subdominant isocurvature contributions cannot be ruled out. We discuss model independent constraints on the isocurvature contribution. Finally, we argue that the Planck satellite will be able to do precision cosmology even if the assumption of adiabaticity is relaxed.

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arXiv2003

WMAP Constraints on varying $α$ and the Promise of Reionization

We present up-to-date constraints on the value of the fine-structure constant at the epoch of decoupling from the recent observations made by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite. In the framework of models we considered, a positive (negative) variation of the value of $α$ at decoupling with respect to the present-day value is now bounded to be smaller than 2% (6%) at 95% confidence level. We point out that the existence of an early reionization epoch as suggested by the above measurements will, when more accurate cosmic microwave background polarization data is available, lead to considerably tighter constraints. The so-called `reionization bump', in particular, will be extremely useful for this purpose. We find that constraints on $α$ can in principle only be as small as $0.1 %$ using CMB data alone--tighter constraints will require further (non-CMB) priors.

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arXiv2003

Measuring $α$ in the Early Universe: CMB Polarization, Reionization and the Fisher Matrix Analysis

We present a detailed analysis of present and future Cosmic Microwave Background constraints of the value of the fine-structure constant, $α$. We carry out a more detailed analysis of the WMAP first-year data, deriving state-of-the-art constraints on $α$ and discussing various other issues, such as the possible hints for the running of the spectral index. We find, at 95% C.L. that $0.95 < α_{\text{dec}} / α_0 < 1.02$. Setting $dn_S /dlnk=0$, yields $0.94< α_{\text{dec}} / α_0 < 1.01$ as previously reported. We find that a lower value of $α/ α_0$ makes a value of $d n_S /dlnk = 0$ more compatible with the data. We also perform a thorough Fisher Matrix Analysis (including both temperature and polarization, as well as $α$ and the optical depth $τ$), in order to estimate how future CMB experiments will be able to constrain $α$ and other cosmological parameters. We find that Planck data alone can constrain $τ$ with a accuracy of the order 4% and that this constraint can be as small as 1.7% for an ideal cosmic variance limited experiment. Constraints on $α$ are of the order 0.3% for Planck and can in principle be as small as 0.1% using CMB data alone - tighter constraints will require further (non-CMB) priors.

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arXiv2003

New constraints on varying $α$

We present a summary of recent constraints on the value of the fine-structure constant at the epoch of decoupling from the recent observations made by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite. Within the set of models considered, a variation of the value of $α$ at decoupling with respect to the present-day value is now bounded to be smaller than 2% (6%) at 95% confidence level. We point out that the existence of an early reionization epoch as suggested by the above measurements will, when more accurate cosmic microwave background polarization data is available, lead to considerably tighter constraints. We find that the tightest possible constraint on $α$ is about $0.1 %$ using CMB data alone--tighter constraints will require further (non-CMB) priors.

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arXiv2002

The cosmological constant and general isocurvature initial conditions

We investigate in detail the question whether a non-vanishing cosmological constant is required by present-day cosmic microwave background and large scale structure data when general isocurvature initial conditions are allowed for. We also discuss differences between the usual Bayesian and the frequentist approaches in data analysis. We show that the COBE-normalized matter power spectrum is dominated by the adiabatic mode and therefore breaks the degeneracy between initial conditions which is present in the cosmic microwave background anisotropies. We find that in a flat universe the Bayesian analysis requires Ω_Λ\neq 0 to more than 3 σ, while in the frequentist approach Ω_Λ= 0 is still within 3 σfor a value of h < 0.48. Both conclusions hold regardless of initial conditions.

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arXiv2002

Measuring $α$ in the Early Universe: CMB Temperature, Large-Scale Structure and Fisher Matrix Analysis

We extend our recent work on the effects of a time-varying fine-structure constant $α$ in the cosmic microwave background, by providing a thorough analysis of the degeneracies between $α$ and the other cosmological parameters, and discussing ways to break these with both existing and/or forthcoming data. In particular, we present the state-of-the-art CMB constraints on $α$, through a combined analysis of the BOOMERanG, MAXIMA and DASI datasets. We also present a novel discussion of the constraints on $α$ coming from large-scale structure observations, focusing in particular on the power spectrum from the 2dF survey. Our results are consistent with no variation in $α$ from the epoch of recombination to the present day, and restrict any such (relative) variation to be less than about 4%. We show that the forthcoming MAP and (particularly) Planck experiments will be able to break most of the currently existing degeneracies between $α$ and other parameters, and measure $α$ to better than percent accuracy.

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