The Physics of Communicability in Complex Networks

A fundamental problem in the study of complex networks is to provide quantitative measures of correlation and information flow between different parts of a system. To this end, several notions of communicability have been introduced and applied to a wide variety of real-world networks in recent years. Several such communicability functions are reviewed in this paper. It is emphasized that communication and correlation in networks can take place through many more routes than the shortest paths, a fact that may not have been sufficiently appreciated in previously proposed correlation measures. In contrast to these, the communicability measures reviewed in this paper are defined by taking into account all possible routes between two nodes, assigning smaller weights to longer ones. This point of view naturally leads to the definition of communicability in terms of matrix functions, such as the exponential, resolvent, and hyperbolic functions, in which the matrix argument is either the adjacency matrix or the graph Laplacian associated with the network. Considerable insight on communicability can be gained by modeling a network as a system of oscillators and deriving physical interpretations, both classical and quantum-mechanical, of various communicability functions. Applications of communicability measures to the analysis of complex systems are illustrated on a variety of biological, physical and social networks. The last part of the paper is devoted to a review of the notion of locality in complex networks and to computational aspects that by exploiting sparsity can greatly reduce the computational efforts for the calculation of communicability functions for large networks.Comment: Review Article. 90 pages, 14 figures. Contents: Introduction; Communicability in Networks; Physical Analogies; Comparing Communicability Functions; Communicability and the Analysis of Networks; Communicability and Localization in Complex Networks; Computability of Communicability Functions; Conclusions and Prespective

Elliptic flow from two- and four-particle correlations in Au + Au collisions at sqrt{s_{NN}} = 130 GeV

Elliptic flow holds much promise for studying the early-time thermalization attained in ultrarelativistic nuclear collisions. Flow measurements also provide a means of distinguishing between hydrodynamic models and calculations which approach the low density (dilute gas) limit. Among the effects that can complicate the interpretation of elliptic flow measurements are azimuthal correlations that are unrelated to the reaction plane (non-flow correlations). Using data for Au + Au collisions at sqrt{s_{NN}} = 130 GeV from the STAR TPC, it is found that four-particle correlation analyses can reliably separate flow and non-flow correlation signals. The latter account for on average about 15% of the observed second-harmonic azimuthal correlation, with the largest relative contribution for the most peripheral and the most central collisions. The results are also corrected for the effect of flow variations within centrality bins. This effect is negligible for all but the most central bin, where the correction to the elliptic flow is about a factor of two. A simple new method for two-particle flow analysis based on scalar products is described. An analysis based on the distribution of the magnitude of the flow vector is also described.Comment: minor text change

Charged-to-neutral correlation at forward rapidity in Au+Au collisions at sNN\sqrt{s_{NN}}=200 GeV

Event-by-event fluctuations of the ratio of inclusive charged to photon multiplicities at forward rapidity in Au+Au collision at $\sqrt{s_{NN}}$=200 GeV have been studied. Dominant contribution to such fluctuations is expected to come from correlated production of charged and neutral pions. We search for evidences of dynamical fluctuations of different physical origins. Observables constructed out of moments of multiplicities are used as measures of fluctuations. Mixed events and model calculations are used as baselines. Results are compared to the dynamical net-charge fluctuations measured in the same acceptance. A non-zero statistically significant signal of dynamical fluctuations is observed in excess to the model prediction when charged particles and photons are measured in the same acceptance. We find that, unlike dynamical net-charge fluctuation, charge-neutral fluctuation is not dominated by correlation due to particle decay. Results are compared to the expectations based on the generic production mechanism of pions due to isospin symmetry, for which no significant (<1%) deviation is observed.Comment: 14 pages, 6 figure

Is the analysis of flow at the CERN SPS reliable?

Several heavy ion experiments at SPS have measured azimuthal distributions of particles with respect to the reaction plane. These distributions are deduced from two-particle azimuthal correlations under the assumption that they result solely from correlations with the reaction plane. In this paper, we investigate other sources of azimuthal correlations: transverse momentum conservation, which produces back-to-back correlations, resonance decays, HBT correlations and final state interactions. These correlations increase with impact parameter: most of them vary with the multiplicity N like 1/N. When they are taken into account, the experimental results of the NA49 collaboration at SPS are significantly modified. These correlations might also explain an important fraction of the pion directed flow observed by WA98. Data should be reanalyzed taking into account carefully these non--flow correlations.Comment: Revised version (minor corrections), 13 pages, LaTeX, 6 Postscript figures included. Submitted to Physical Review

Elliptic flow from two- and four-particle correlations in Au+Au collisions at sqrt[sNN]=130 GeV

Elliptic flow holds much promise for studying the early-time thermalization attained in ultrarelativistic nuclear collisions. Flow measurements also provide a means of distinguishing between hydrodynamic models and calculations which approach the low density (dilute gas) limit. Among the effects that can complicate the interpretation of elliptic flow measurements are azimuthal correlations that are unrelated to the reaction plane (nonflow correlations). Using data for Au + Au collisions at sqrt[sNN]=130 GeV from the STAR time projection chamber, it is found that four-particle correlation analyses can reliably separate flow and nonflow correlation signals. The latter account for on average about 15% of the observed second-harmonic azimuthal correlation, with the largest relative contribution for the most peripheral and the most central collisions. The results are also corrected for the effect of flow variations within centrality bins. This effect is negligible for all but the most central bin, where the correction to the elliptic flow is about a factor of 2. A simple new method for two-particle flow analysis based on scalar products is described. An analysis based on the distribution of the magnitude of the flow vector is also described.alle Autoren: C. Adler11, Z. Ahammed23, C. Allgower12, J. Amonett14, B. D. Anderson14, M. Anderson5, G. S. Averichev9, J. Balewski12, O. Barannikova9,23, L. S. Barnby14, J. Baudot13, S. Bekele20, V. V. Belaga9, R. Bellwied31, J. Berger11, H. Bichsel30, A. Billmeier31, L. C. Bland2, C. O. Blyth3, B. E. Bonner24, A. Boucham26, A. Brandin18, A. Bravar2, R. V. Cadman1, H. Caines33, M. Calderón de la Barca Sánchez2, A. Cardenas23, J. Carroll15, J. Castillo26, M. Castro31, D. Cebra5, P. Chaloupka20, S. Chattopadhyay31, Y. Chen6, S. P. Chernenko9, M. Cherney8, A. Chikanian33, B. Choi28, W. Christie2, J. P. Coffin13, T. M. Cormier31, J. G. Cramer30, H. J. Crawford4, W. S. Deng2, A. A. Derevschikov22, L. Didenko2, T. Dietel11, J. E. Draper5, V. B. Dunin9, J. C. Dunlop33, V. Eckardt16, L. G. Efimov9, V. Emelianov18, J. Engelage4, G. Eppley24, B. Erazmus26, P. Fachini2, V. Faine2, K. Filimonov15, E. Finch33, Y. Fisyak2, D. Flierl11, K. J. Foley2, J. Fu15,32, C. A. Gagliardi27, N. Gagunashvili9, J. Gans33, L. Gaudichet26, M. Germain13, F. Geurts24, V. Ghazikhanian6, O. Grachov31, V. Grigoriev18, M. Guedon13, E. Gushin18, T. J. Hallman2, D. Hardtke15, J. W. Harris33, T. W. Henry27, S. Heppelmann21, T. Herston23, B. Hippolyte13, A. Hirsch23, E. Hjort15, G. W. Hoffmann28, M. Horsley33, H. Z. Huang6, T. J. Humanic20, G. Igo6, A. Ishihara28, Yu. I. Ivanshin10, P. Jacobs15, W. W. Jacobs12, M. Janik29, I. Johnson15, P. G. Jones3, E. G. Judd4, M. Kaneta15, M. Kaplan7, D. Keane14, J. Kiryluk6, A. Kisiel29, J. Klay15, S. R. Klein15, A. Klyachko12, A. S. Konstantinov22, M. Kopytine14, L. Kotchenda18, A. D. Kovalenko9, M. Kramer19, P. Kravtsov18, K. Krueger1, C. Kuhn13, A. I. Kulikov9, G. J. Kunde33, C. L. Kunz7, R. Kh. Kutuev10, A. A. Kuznetsov9, L. Lakehal-Ayat26, M. A. C. Lamont3, J. M. Landgraf2, S. Lange11, C. P. Lansdell28, B. Lasiuk33, F. Laue2, A. Lebedev2, R. Lednický9, V. M. Leontiev22, M. J. LeVine2, Q. Li31, S. J. Lindenbaum19, M. A. Lisa20, F. Liu32, L. Liu32, Z. Liu32, Q. J. Liu30, T. Ljubicic2, W. J. Llope24, G. LoCurto16, H. Long6, R. S. Longacre2, M. Lopez-Noriega20, W. A. Love2, T. Ludlam2, D. Lynn2, J. Ma6, R. Majka33, S. Margetis14, C. Markert33, L. Martin26, J. Marx15, H. S. Matis15, Yu. A. Matulenko22, T. S. McShane8, F. Meissner15, Yu. Melnick22, A. Meschanin22, M. Messer2, M. L. Miller33, Z. Milosevich7, N. G. Minaev22, J. Mitchell24, V. A. Moiseenko10, C. F. Moore28, V. Morozov15, M. M. de Moura31, M. G. Munhoz25, J. M. Nelson3, P. Nevski2, V. A. Nikitin10, L. V. Nogach22, B. Norman14, S. B. Nurushev22, G. Odyniec15, A. Ogawa21, V. Okorokov18, M. Oldenburg16, D. Olson15, G. Paic20, S. U. Pandey31, Y. Panebratsev9, S. Y. Panitkin2, A. I. Pavlinov31, T. Pawlak29, V. Perevoztchikov2, W. Peryt29, V. A. Petrov10, M. Planinic12, J. Pluta29, N. Porile23, J. Porter2, A. M. Poskanzer15, E. Potrebenikova9, D. Prindle30, C. Pruneau31, J. Putschke16, G. Rai15, G. Rakness12, O. Ravel26, R. L. Ray28, S. V. Razin9,12, D. Reichhold8, J. G. Reid30, G. Renault26, F. Retiere15, A. Ridiger18, H. G. Ritter15, J. B. Roberts24, O. V. Rogachevski9, J. L. Romero5, A. Rose31, C. Roy26, V. Rykov31, I. Sakrejda15, S. Salur33, J. Sandweiss33, A. C. Saulys2, I. Savin10, J. Schambach28, R. P. Scharenberg23, N. Schmitz16, L. S. Schroeder15, A. Schüttauf16, K. Schweda15, J. Seger8, D. Seliverstov18, P. Seyboth16, E. Shahaliev9, K. E. Shestermanov22, S. S. Shimanskii9, V. S. Shvetcov10, G. Skoro9, N. Smirnov33, R. Snellings15, P. Sorensen6, J. Sowinski12, H. M. Spinka1, B. Srivastava23, E. J. Stephenson12, R. Stock11, A. Stolpovsky31, M. Strikhanov18, B. Stringfellow23, C. Struck11, A. A. P. Suaide31, E. Sugarbaker20, C. Suire2, M. Sumbera20, B. Surrow2, T. J. M. Symons15, A. Szanto de Toledo25, P. Szarwas29, A. Tai6, J. Takahashi25, A. H. Tang14, J. H. Thomas15, M. Thompson3, V. Tikhomirov18, M. Tokarev9, M. B. Tonjes17, T. A. Trainor30, S. Trentalange6, R. E. Tribble27, V. Trofimov18, O. Tsai6, T. Ullrich2, D. G. Underwood1, G. Van Buren2, A. M. VanderMolen17, I. M. Vasilevski10, A. N. Vasiliev22, S. E. Vigdor12, S. A. Voloshin31, F. Wang23, H. Ward28, J. W. Watson14, R. Wells20, G. D. Westfall17, C. Whitten, Jr.6, H. Wieman15, R. Willson20, S. W. Wissink12, R. Witt33, J. Wood6, N. Xu15, Z. Xu2, A. E. Yakutin22, E. Yamamoto15, J. Yang6, P. Yepes24, V. I. Yurevich9, Y. V. Zanevski9, I. Zborovský9, H. Zhang33, W. M. Zhang14, R. Zoulkarneev10, and A. N. Zubarev

Flow with PMD: Past and Future

Measurements of azimuthal distribution of inclusive photons using the fine granularity preshower photon multiplicity detector (PMD) at CERN SPS are used to obtain anisotropy in the azimuthal distributions. These results are used to estimate the anisotropy in the neutral pion distributions. The results are compared with results of charged particle data, both for first order and second order anisotropy. Assuming the same anisotropy for charged and neutral pions, the anisotropy in photons is estimated and compared with the measured anisotropy. The effect of neutral pion decay on the correlation between the first order and the second order event plane is also discussed. Data from PMD can also be used to estimate the reaction plane for studying any anisotropy in particle emission characteristics in the ALICE experiment at the Large Hadron Collider. In particular, we show that using the event plane from the PMD, it will be possible to measure the anisotropy in Jpsi absorption (if any) in the ALICE experiment.Comment: Invited talk in the Fourth International Conference on the Physics and Astrophysics of Quark Gluon Plasma, 26-30 Nov.2001, Jaipur, Indi

Network-based ranking in social systems: three challenges

Ranking algorithms are pervasive in our increasingly digitized societies, with important real-world applications including recommender systems, search engines, and influencer marketing practices. From a network science perspective, network-based ranking algorithms solve fundamental problems related to the identification of vital nodes for the stability and dynamics of a complex system. Despite the ubiquitous and successful applications of these algorithms, we argue that our understanding of their performance and their applications to real-world problems face three fundamental challenges: (i) Rankings might be biased by various factors; (2) their effectiveness might be limited to specific problems; and (3) agents' decisions driven by rankings might result in potentially vicious feedback mechanisms and unhealthy systemic consequences. Methods rooted in network science and agent-based modeling can help us to understand and overcome these challenges.Comment: Perspective article. 9 pages, 3 figure

Empirical analysis of the ship-transport network of China

Structural properties of the ship-transport network of China (STNC) are studied in the light of recent investigations of complex networks. STNC is composed of a set of routes and ports located along the sea or river. Network properties including the degree distribution, degree correlations, clustering, shortest path length, centrality and betweenness are studied in different definition of network topology. It is found that geographical constraint plays an important role in the network topology of STNC. We also study the traffic flow of STNC based on the weighted network representation, and demonstrate the weight distribution can be described by power law or exponential function depending on the assumed definition of network topology. Other features related to STNC are also investigated.Comment: 20 pages, 7 figures, 1 tabl

Baryon femtoscopy in sqrt(sNN) = 2.76 TeV Pb-Pb collisions at ALICE

We present femtoscopic results for proton and Lambda correlation functions measured by ALICE in sqrt(sNN) = 2.76 TeV Pb-Pb collisions. Femtoscopic radii are extracted from pp, pbarpbar, and ppbar pairs. Comparisons of these radii with those from pion and kaon analyses reveal an approximate transverse mass scaling that is consistent with explanations of radial flow. Inelastic final state interactions are explored in baryon-antibaryon correlations functions to investigate their relationship with reduced proton yields at LHC energies.Comment: 5 pages, 9 figures, proceedings for the 29th Winter Workshop on Nuclear Dynamics, Squaw Valley, CA, February 3-10, 201