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8 pages, 2 tables, 7 figures.--PACS nrs.: 21.60.-k; 05.45.Mt; 24.60.Lz.--Available on line on Nov 13, 2006., Sequences of energy levels in nuclei are often plagued with missing levels whose number and position are unknown. It is also quite usual that all the quantum numbers of certain levels cannot be experimentally determined, and thus levels of different symmetries are mixed in the same sequence. The analysis of these imperfect spectra (from the point of view of spectral statistics) is unavoidable if one wants to extract some statistical information. The power spectrum of the δq statistic has emerged in recent years as an important tool for the study of quantum chaos and spectral statistics. We derive analytical expressions for the observed power spectrum in terms of the fraction of observed levels and the number of mixed sequences. These expressions are tested with large shell model spectra simulating realistic experimental situations. A good estimation of the number of mixed symmetries and the fraction of missing levels is obtained by means of a least-squares fit in a wide set of different situations., This work is supported in part by Spanish Government grants BFM2003-04147-C02 and FTN2003-08337-C04-04.

4 pages, 2 figures.-- PACS numbers: 02.10.De, 05.45.Mt.-- ArXiv pre-print available at: http://arxiv.org/abs/0805.4079, The number N(E) of complex zeros of the Riemann zeta function with positive imaginary part less than E is the sum of a `smooth' function Ñ(E) and a 'fluctuation'. Berry and Keating have shown that the asymptotic expansion of Ñ(E) counts states of positive energy less than E in a 'regularized' semi-classical model with classical Hamiltonian H=xp. For a different regularization, Connes has shown that it counts states 'missing' from a continuum. Here we show how the 'absorption spectrum' model of Connes emerges as the lowest Landau level limit of a specific quantum mechanical model for a charged particle on a planar surface in an electric potential and uniform magnetic field. We suggest a role for the higher Landau levels in the fluctuation part of N(E)., This work was supported by the CICYT project FIS2004-04885 (G.S.) and by the EPSRC (P.K.T.). GS also acknowledges ESF Science Programme INSTANS 2005-2010.

11 pages, 3 figures.-- PACS nrs.: 05.45.Mt; 02.10.Yn; 24.60.-k.-- PMID: 18517325 [PubMed]., The continuous Gaussian ensemble, also known as the v-Gaussian or v-Hermite ensemble, is a natural extension of the classical Gaussian ensembles of real (v=1), complex (v=2), or quaternion (v=4) matrices, where v is allowed to take any positive value. From a physical point of view, this ensemble may be useful to describe transitions between different symmetries or to describe the terrace-width distributions of vicinal surfaces. Moreover, its simple form allows one to speed up and increase the efficiency of numerical simulations dealing with large matrix dimensions. We analyze the long-range spectral correlations of this ensemble by means of the δn statistic. We derive an analytical expression for the average power spectrum of this statistic, [overline Pk[sup δ]], based on approximated forms for the two-point cluster function and the spectral form factor. We find that the power spectrum of δn evolves from [overline Pk[sup δ]] proportional to 1/k at v=1 to [overline Pk[sup δ]] proportional to 1/k^2 at v=0. Relevantly, the transition is not homogeneous with a 1/fα noise at all scales, but heterogeneous with coexisting 1/f and 1/f^2 noises. There exists a critical frequency kc proportional to v that separates both behaviors: below kc, [overline Pk[sup δ]] follows a 1/f power law, while beyond kc, it transits abruptly to a 1/f^2 power law. For v>1 the 1/f noise dominates through the whole frequency range, unveiling that the 1/f correlation structure remains constant as we increase the level repulsion and reduce to zero the amplitude of the spectral fluctuations. All these results are confirmed by stringent numerical calculations involving matrices with dimensions up to 10^5., This work was supported in part by Spanish Government Grants Nos. FIS2006-12783-C03-01 and FIS2006-12783-C03-02 and by Comunidad de Madrid–CSIC Grant No. 200650M012. A.R. was supported by the Spanish program "Juan de la Cierva".

4 pages, 1 table, 3 figures.--PACS nrs.: 05.45.Mt, 03.65.Xp, 02.10.Yn, It has been shown that the spectral fluctuations of different quantum systems are characterized by 1=f noise, with 1 ≤ ≤ 2, in the transition from integrability to chaos. This result is not well understood. We show that chaos-assisted tunneling gives rise to this power-law behavior. We develop a random matrix model for intermediate quantum systems, based on chaos-assisted tunneling, and we discuss under which conditions it displays 1=f noise in the transition from integrability to chaos. We conclude that the variance of the elements that connect regular with chaotic states must decay with the difference of energy between them. We compare the characteristics of the transition modeled in this way with what is obtained for the Robnik billiard., This work was supported by Grants No. FIS2006-12783-C03-01 from Ministerio de Educación y Ciencia of Spain, and No. CCG07-CSIC/ESP-1962 from Comunidad de Madrid and CSIC.

9 pages, 2 tables, 9 figures.-- PACS nrs.: 05.45.Mt; 03.65.Yz; 02.30.Ik; 75.10.Jm., We investigate the decoherence properties of a central system composed of two spins 1/2 in contact with a spin bath. The dynamical regime of the bath ranges from a fully integrable limit to complete chaoticity. We show that the dynamical regime of the bath determines the efficiency of the decoherence process. For perturbative regimes, the integrable limit provides stronger decoherence, while in the strong coupling regime the chaotic limit becomes more efficient. We also show that the decoherence time behaves in a similar way. On the contrary, the rate of decay of magnitudes like linear entropy or fidelity does not depend on the dynamical regime of the bath. We interpret the latter results as due to a comparable complexity of the Hamiltonian for both the integrable and the fully chaotic limits., This work was supported by grants FIS2006-12783-C03-01 from Ministerio de Educación y Ciencia of Spain,and 200650M012 from Comunidad de Madrid and CSIC. A.R. is supported by the Spanish program “Juan de la Cierva.” R.A.M. is supported by the I3P program funded by the European Social Fund.

4 pages, 1 table, 1 figure.--PACS nrs.: 14.20.-c; 05.45.Mt; 12.39.Ki.--ArXiv pre-print available at: http://arxiv.org/abs/hep-ph/0701111v1, We study the low-lying baryon spectrum (up to 2.2 GeV) provided by experiments and different quark models using statistical tools which allow us to postulate the existence of missing levels in spectra. We confirm that the experimental spectrum is compatible with random matrix theory, the paradigmatic model of quantum chaos, and we find that the quark models are more similar to a Poisson distribution, which is not compatible with what should be expected in a correlated spectrum. From our analysis it stems that the spectral fluctuation properties of quark-model spectra are incompatible with experimental data. This result can be used to enlighten the problem of missing resonances., This work has been partly supported under contracts of Ministerio de Educación y Ciencia (Spain) No. FTN2003-08337-C04-04 and No. FIS2005-00640.

6 pages, 1 table, 4 figures.--PACS nrs.: 05.45.Mt; 05.45.Tp; 05.40.-a, It was recently pointed out that the spectral fluctuations of quantum systems are formally analogous to discrete time series, and therefore their structure can be characterized by the power spectrum of the signal. Moreover, it is found that the power spectrum of chaotic spectra displays a 1/f behavior, while that of regular systems follows a 1/f2 law. This analogy provides a link between the concepts of spectral rigidity and antipersistence. Trying to get a deeper understanding of this relationship, we have studied the correlation structure of spectra with high spectral rigidity. Using an appropriate family of random Hamiltonians, we increase the spectral rigidity up to hindering completely the spectral fluctuations. Analyzing the long range correlation structure a neat power law 1/f has been found for all the spectra, along the whole process. Therefore, 1/f noise is the characteristic fingerprint of a transition that, preserving the scale-free correlation structure, hinders completely the fluctuations of the spectrum., This work is supported in part by Spanish Government Grants No. BFM2003-04147 and No. FTN2003-08337-C04-04.

14 pages, 3 tables, 13 figures.--PACS nrs.:05.45.Mt; 05.45.Pq; 24.60.-k, It is widely accepted that the statistical properties of energy level spectra provide an essential characterization of quantum chaos. Indeed, the spectral fluctuations of many different systems like quantum billiards, atoms, or atomic nuclei have been studied. However, noninteracting many-body systems have received little attention, since it is assumed that they must exhibit Poisson-like fluctuations. Apart from a heuristic argument of Bloch, there are neither systematic numerical calculations nor a rigorous derivation of this fact. Here we present a rigorous study of the spectral fluctuations of noninteracting identical particles moving freely in a mean field emphasizing the evolution with the number of particles N as well as with the energy. Our results are conclusive. For N2 the spectra of these systems exhibit Poisson fluctuations provided that we consider sufficiently high excitation energies. Nevertheless, when the mean field is chaotic there exists a critical energy scale Lc; beyond this scale, the fluctuations deviate from the Poisson statistics as a reminiscence of the statistical properties of the mean field., This work is supported in part by Spanish Government Grants No. BFM2003-04147 and No. FTN2003-08337-C04-04.