Archive ouverte UNIGE | last documents for author 'Riccardo Sturani'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Riccardo Sturani'engEffective field theory analysis of the self-interacting chameleonhttps://archive-ouverte.unige.ch/unige:115910https://archive-ouverte.unige.ch/unige:115910We analyse the phenomenology of a self-interacting scalar field in the context of the chameleon scenario originally proposed by Khoury and Weltman. In the absence of self-interactions, this type of scalar field can mediate long range interactions and simultaneously evade constraints from violation of the weak equivalence principle. By applying to such a scalar field the effective field theory method proposed for Einstein gravity by Goldberger and Rothstein, we give a thorough perturbative evaluation of the importance of non-derivative self-interactions in determining the strength of the chameleon mediated force in the case of orbital motion. The self-interactions are potentially dangerous as they can change the long range behaviour of the field. Nevertheless, we show that they do not lead to any dramatic phenomenological consequence with respect to the linear case and solar system constraints are fulfilled.Tue, 09 Apr 2019 11:17:19 +0200Results of the IGEC-2 search for gravitational wave bursts during 2005https://archive-ouverte.unige.ch/unige:35684https://archive-ouverte.unige.ch/unige:35684The network of resonant bar detectors of gravitational waves resumed coordinated observations within the International Gravitational Event Collaboration (IGEC-2). Four detectors are taking part in this Collaboration: ALLEGRO, AURIGA, EXPLORER and NAUTILUS. We present here the results of the search for gravitational wave bursts over 6 months during 2005, when IGEC-2 was the only gravitational wave observatory in operation. The implemented network data analysis is based on a time coincidence search among AURIGA, EXPLORER and NAUTILUS; ALLEGRO data was reserved for follow-up studies. The network amplitude sensitivity to bursts improved by a factor ≈3 over the 1997-2000 IGEC observations; the wider sensitive band also allowed the analysis to be tuned over a larger class of waveforms. Given the higher single-detector duty factors, the analysis was based on threefold coincidence, to ensure the identification of any single candidate of gravitational waves with high statistical confidence. The false detection rate was as low as 1 per century. No candidates were found.Mon, 14 Apr 2014 16:32:01 +0200The 2003 run of the EXPLORER–NAUTILUS gravitational wave experimenthttps://archive-ouverte.unige.ch/unige:35683https://archive-ouverte.unige.ch/unige:35683We report here the preliminary results of the search for gravitational wave bursts from the 2003 run of the EXPLORER–NAUTILUS experiment. The total common measuring time was 149 days. The detectors had a typical noise spectral amplitude of about 2 × 10−21 Hz−1/2, a bandwidth of the order of 10 Hz and a very good stability. We derive a new upper limit for the GW burst rate, of the order of 0.02 events/day for hRSS ≥ 2 × 10−19, and discuss the implication of this result with respect to the results obtained with the 2001 runMon, 14 Apr 2014 16:31:09 +0200Status report on the EXPLORER and NAUTILUS detectors and the present science runhttps://archive-ouverte.unige.ch/unige:35682https://archive-ouverte.unige.ch/unige:35682We report on the present scientific run (04–05) of the two detectors EXPLORER and NAUTILUS. The 04–05 run of the two detectors started in March 2004. The strain sensitivity is about 7 × 10−22 Hz−1/2 and the bandwidth is about 5 Hertz. The sensitivity for 1 ms bursts is h = 3 × 10−19Mon, 14 Apr 2014 16:30:20 +0200IGEC2: A 17-month search for gravitational wave bursts in 2005–2007https://archive-ouverte.unige.ch/unige:35673https://archive-ouverte.unige.ch/unige:35673We present here the results of a 515 day search for short bursts of gravitational waves by the IGEC2 observatory. This network included 4 cryogenic resonant-bar detectors: AURIGA, EXPLORER, and NAUTILUS in Europe, and ALLEGRO in America. These results cover the time period from November 6th 2005 until April 15th 2007, partly overlapping the first long term observations by the LIGO interferometric detectors. The observatory operated with high duty cycle, namely, 57% for fourfold coincident observations, and 94% for threefold observations. The sensitivity was the best ever obtained by a bar network: we could detect, with an efficiency >50%, impulsive events with a burst strain amplitude hrss≲1×10−19 Hz−1/2. The network data analysis was based on time coincidence searches over at least three detectors, used a blind search technique, and was tuned to achieve a false alarm rate of 1/century. When the blinding was removed, no gravitational wave candidate was found.Mon, 14 Apr 2014 16:00:42 +0200Extracting the three- and four-graviton vertices from binary pulsars and coalescing binarieshttps://archive-ouverte.unige.ch/unige:35672https://archive-ouverte.unige.ch/unige:35672Using a formulation of the post-Newtonian expansion in terms of Feynman graphs, we discuss how various tests of general relativity (GR) can be translated into measurement of the three- and four-graviton vertices. In problems involving only the conservative dynamics of a system, a deviation of the three-graviton vertex from the GR prediction is equivalent, to lowest order, to the introduction of the parameter βPPN in the parametrized post-Newtonian formalism, and its strongest bound comes from lunar laser ranging, which measures it at the 0.02% level. Deviation of the three-graviton vertex from the GR prediction, however, also affects the radiative sector of the theory. We show that the timing of the Hulse-Taylor binary pulsar provides a bound on the deviation of the three-graviton vertex from the GR prediction at the 0.1% level. For coalescing binaries at interferometers we find that, because of degeneracies with other parameters in the template such as mass and spin, the effects of modified three- and four-graviton vertices is just to induce an error in the determination of these parameters and, at least in the restricted PN approximation, it is not possible to use coalescing binaries for constraining deviations of the vertices from the GR predictionMon, 14 Apr 2014 15:59:55 +0200EXPLORER and NAUTILUS gravitational wave detectors: a status reporthttps://archive-ouverte.unige.ch/unige:35669https://archive-ouverte.unige.ch/unige:35669The two cryogenic resonant bar detectors of the ROG Collaboration, EXPLORER and NAUTILUS, have been taking data continuously with a high duty cycle for several years. We report here on the status of recent analysis of the data and in particular on the results of the burst searches in the year 2004Mon, 14 Apr 2014 15:57:13 +0200All-sky incoherent search for periodic signals with Explorer 2005 datahttps://archive-ouverte.unige.ch/unige:35666https://archive-ouverte.unige.ch/unige:35666The data collected during 2005 by the resonant bar Explorer are divided into segments and incoherently summed in order to perform an all-sky search for periodic gravitational wave signals. The parameter space of the search spanned about 40 Hz in frequency, over 23 927 positions in the sky. Neither source orbital corrections nor spindown parameters have been included, with the result that the search was sensitive to isolated neutron stars with a frequency drift less than 6 × 10−11 Hz s−1. No gravitational wave candidates have been found by means of the present analysis, which led to a best upper limit of 3.1 × 10−23 for the dimensionless strain amplitude.Mon, 14 Apr 2014 15:55:20 +0200Multichannel matched filtering for spherical gravitational wave antennashttps://archive-ouverte.unige.ch/unige:2218https://archive-ouverte.unige.ch/unige:2218We study the performance of a multidimensional matched filter as a follow-up module of the coherent method recently developed by two of us for the detection of gravitational wave bursts by spherical resonant detectors. We have tested this strategy on the same set of injections used for the coherent method and found that the matched filter sensibly improves the determination of relevant parameters as the arrival time, amplitude, central frequency and arrival direction of the signal. The matched filter also improves the false alarm rate, reducing it roughly by a factor of 3. The hierarchical structure of the whole analysis pipeline allows to obtain these results without a significant increase of the computation time.Fri, 10 Jul 2009 15:15:12 +0200Supersymmetric vacuum configurations in string cosmologyhttps://archive-ouverte.unige.ch/unige:2216https://archive-ouverte.unige.ch/unige:2216We examine in a cosmological context the conditions for unbroken supersymmetry in N=1 supergravity in D=10 dimensions. We show that the cosmological solutions of the equations of motion obtained considering only the bosonic sector correspond to vacuum states with spontaneous supersymmetry breaking. With a non vanishing gravitino-dilatino condensate we find a solution of the equations of motion that satisfies necessary conditions for unbroken supersymmetry and that smoothly interpolates between Minkowski space and DeSitter space with a linearly growing dilaton, thus providing a possible example of a supersymmetric and non-singular pre-big-bang cosmology.Thu, 09 Jul 2009 10:52:52 +0200Loop corrections and graceful exit in string cosmologyhttps://archive-ouverte.unige.ch/unige:2212https://archive-ouverte.unige.ch/unige:2212We examine the effect of perturbative string loops on the cosmological pre-big-bang evolution. We study loop corrections derived from heterotic string theory compactified on a $Z_N$ orbifold and we consider the effect of the all-order loop corrections to the Kahler potential and of the corrections to gravitational couplings, including both threshold corrections and corrections due to the mixed Kahler-gravitational anomaly. We find that string loops can drive the evolution into the region of the parameter space where a graceful exit is in principle possible, and we find solutions that, in the string frame, connect smoothly the superinflationary pre-big-bang evolution to a phase where the curvature and the derivative of the dilaton are decreasing. We also find that at a critical coupling the loop corrections to the Kahler potential induce a ghost-like instability, i.e. the kinetic term of the dilaton vanishes. This is similar to what happens in Seiberg-Witten theory and signals the transition to a new regime where the light modes in the effective action are different and are related to the original ones by S-duality. In a string context, this means that we enter a D-brane dominated phase.Thu, 09 Jul 2009 10:11:57 +0200Generalized second law in string cosmologyhttps://archive-ouverte.unige.ch/unige:2211https://archive-ouverte.unige.ch/unige:2211A generalized second law in string cosmology accounts for geometric and quantum entropy in addition to ordinary sources of entropy. The proposed generalized second law forbids singular string cosmologies, under certain conditions, and forces a graceful exit transition from dilaton-driven inflation by bounding curvature and dilaton kinetic energy.Thu, 09 Jul 2009 10:09:49 +0200Sensitivity of a small matter-wave interferometer to gravitational waveshttps://archive-ouverte.unige.ch/unige:2202https://archive-ouverte.unige.ch/unige:2202We study the possibility of using matter wave interferometry techniques to build a gravitational wave detector. We derive the response function and find that it contains a term proportional to the derivative of the gravitational wave, a point which has been disputed recently. We then study in detail the sensitivity that can be reached by such a detector and find that, if it is operated near resonance, it can reach potentially interesting values in the high frequency regime. The correlation between two or more of such devices can further improve the sensitivity for a stochastic signal.Tue, 07 Jul 2009 13:41:47 +0200Experimental signatures of gravitational wave burstershttps://archive-ouverte.unige.ch/unige:2201https://archive-ouverte.unige.ch/unige:2201Gravitational wave bursters are sources which emit repeatedly bursts of gravitational waves, and have been recently suggested as potentially interesting candidates for gravitational wave (GW) detectors. Mechanisms that could give rise to a GW burster can be found for instance in highly magnetized neutron stars (the “magnetars” which explain the phenomenon of soft gamma repeaters), in accreting neutron stars and in hybrid stars with a quark core. We point out that these sources have very distinctive experimental signatures. In particular, as already observed in the -ray bursts from soft gamma repeaters, the energy spectrum of the events is a power-law, dN ∼ E− dE with ≃ 1.6, and they have a distribution of waiting times (the times between one outburst and the next) significantly different from the distribution of uncorrelated events. We discuss possible detection strategies that could be used to search for these events in existing gravitational wave detectors.Tue, 07 Jul 2009 13:38:40 +0200Event trigger generator for resonant spherical detectors of gravitational waveshttps://archive-ouverte.unige.ch/unige:2200https://archive-ouverte.unige.ch/unige:2200We have set up and tested a pipeline for processing the data from a spherical gravitational wave detector with six transducers. The algorithm exploits the multichannel capability of the system and provides a list of candidate events with their arrival direction. The analysis starts with the conversion of the six detector outputs into the scalar and the five quadrupolar modes of the sphere, which are proportional to the corresponding gravitational wave spherical components. Event triggers are then generated by an adaptation of the WaveBurst algorithm. Event validation and direction reconstruction are made by cross-checking two methods of different inspiration: geometrical (lowest eigenvalue) and probabilistic (maximum likelihood). The combination of the two methods is able to keep substantially unaltered the efficiency and can reduce drastically the detections of fake events (to less than ten per cent). We show a quantitative test of these ideas by simulating the operation of the resonant spherical detector miniGRAIL, whose planned sensitivity in its frequency band (few hundred Hertz's around 3kHz) is comparable with the present LIGO one.Tue, 07 Jul 2009 13:33:43 +0200All-sky search of NAUTILUS datahttps://archive-ouverte.unige.ch/unige:2059https://archive-ouverte.unige.ch/unige:2059A search for periodic gravitational-wave signals from isolated neutron stars in the NAUTILUS detector data is presented. We have analyzed half a year of data over the frequency band lang922.2; 923.2rang Hz, the spindown range lang − 1.463 × 10−8; 0rang Hz/s and over the entire sky. We have divided the data into two day stretches and we have analyzed each stretch coherently using matched filtering. We have imposed a low threshold for the optimal detection statistic to obtain a set of candidates that are further examined for coincidences among various data stretches. For some candidates we have also investigated the change of the signal-to-noise ratio when we increase the observation time from 2 to 4 days. Our analysis has not revealed any gravitational-wave signals. Therefore we have imposed upper limits on the dimensionless gravitational-wave amplitude over the parameter space that we have searched. Depending on frequency, our upper limit ranges from 3.4 × 10−23 to 1.3 × 10−22. We have attempted a statistical verification of the hypotheses leading to our conclusions. We estimate that our upper limit is accurate to within 18%.Tue, 16 Jun 2009 16:13:51 +0200Coherent detection method of gravitational wave bursts for spherical antennashttps://archive-ouverte.unige.ch/unige:2058https://archive-ouverte.unige.ch/unige:2058We provide a comprehensive theoretical framework and a quantitative test of the method we recently proposed for processing data from a spherical detector with five or six transducers. Our algorithm is a trigger event generator performing a coherent analysis of the sphere channels. In order to test our pipeline we first built a detailed numerical model of the detector, including deviations from the ideal case such as quadrupole modes splitting, and non-identical transducer readout chains. This model, coupled with a Gaussian noise generator, has then been used to produce six time series, corresponding to the outputs of the six transducers attached to the sphere. We finally injected gravitational wave burst signals into the data stream, as well as bursts of non-gravitational origin in order to mimic the presence of non-Gaussian noise, and then processed the mock data. We report quantitative results for the detection efficiency versus the false alarm rate and for the affordability of the reconstruction of the direction of arrival. In particular, the combination of the two direction reconstruction methods can reduce by a factor of 10 the number of false alarms due to the non-Gaussian noise.Tue, 16 Jun 2009 15:34:55 +0200Frequency of gravitational waveshttps://archive-ouverte.unige.ch/unige:952https://archive-ouverte.unige.ch/unige:952We show that there are physically relevant situations where gravitational waves do not inherit the frequency spectrum of their source but its wavenumber spectrum.Thu, 26 Feb 2009 17:59:27 +0100