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Bernabeu, J., Espriu, D., & Puigdomenech, D. (2011). Gravitational waves in the presence of a cosmological constant. Phys. Rev. D, 84(6), 063523–13pp.
Abstract: We derive the effects of a nonzero cosmological constant Lambda on gravitational wave propagation in the linearized approximation of general relativity. In this approximation, we consider the situation where the metric can be written as g(mu nu) = eta(mu nu) + h(mu nu)(Lambda) + h(mu nu)(W), h(mu nu)(Lambda,W) << 1, where h(mu nu)(Lambda) is the background perturbation and h(mu nu)(W) is a modification interpretable as a gravitational wave. For Lambda not equal 0, this linearization of Einstein equations is self-consistent only in certain coordinate systems. The cosmological Friedmann-Robertson-Walker coordinates do not belong to this class and the derived linearized solutions have to be reinterpreted in a coordinate system that is homogeneous and isotropic to make contact with observations. Plane waves in the linear theory acquire modifications of order root Lambda, both in the amplitude and the phase, when considered in Friedmann-Robertson-Walker coordinates. In the linearization process for h(mu nu), we have also included terms of order O(Lambda h(mu nu)). For the background perturbation h(mu nu)(Lambda), the difference is very small, but when the term h(mu nu)(W)Lambda is retained the equations of motion can be interpreted as describing massive spin-2 particles. However, the extra degrees of freedom can be approximately gauged away, coupling to matter sources with a strength proportional to the cosmological constant itself. Finally, we discuss the viability of detecting the modifications caused by the cosmological constant on the amplitude and phase of gravitational waves. In some cases, the distortion with respect to gravitational waves propagating in Minkowski space-time is considerable. The effect of Lambda could have a detectable impact on pulsar timing arrays.
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Bernabeu, J., Espinoza, C., & Mavromatos, N. E. (2010). Cosmological constant and local gravity. Phys. Rev. D, 81(8), 084002–7pp.
Abstract: We discuss the linearization of Einstein equations in the presence of a cosmological constant, by expanding the solution for the metric around a flat Minkowski space-time. We demonstrate that one can find consistent solutions to the linearized set of equations for the metric perturbations, in the Lorentz gauge, which are not spherically symmetric, but they rather exhibit a cylindrical symmetry. We find that the components of the gravitational field satisfying the appropriate Poisson equations have the property of ensuring that a scalar potential can be constructed, in which both contributions, from ordinary matter and Lambda > 0, are attractive. In addition, there is a novel tensor potential, induced by the pressure density, in which the effect of the cosmological constant is repulsive. We also linearize the Schwarzschild-de Sitter exact solution of Einstein's equations ( due to a generalization of Birkhoff's theorem) in the domain between the two horizons. We manage to transform it first to a gauge in which the 3-space metric is conformally flat and, then, make an additional coordinate transformation leading to the Lorentz gauge conditions. We compare our non-spherically symmetric solution with the linearized Schwarzschild-de Sitter metric, when the latter is transformed to the Lorentz gauge, and we find agreement. The resulting metric, however, does not acquire a proper Newtonian form in terms of the unique scalar potential that solves the corresponding Poisson equation. Nevertheless, our solution is stable, in the sense that the physical energy density is positive.
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Aguilar-Saavedra, J. A., Bernabeu, J., Mitsou, V. A., & Segarra, A. (2017). The Z boson spin observables as messengers of new physics. Eur. Phys. J. C, 77(4), 234–6pp.
Abstract: We demonstrate that the eight multipole parameters describing the spin state of the Z boson are able to disentangle known Z production mechanisms and signals from new physics at the LHC. They can be extracted from appropriate asymmetries in the angular distribution of lepton pairs from the Z boson decay. The power of this analysis is illustrated by (1) the production of Z boson plus jets; (2) Z boson plus missing transverse energy; (3) W and Z bosons originating from the two-body decay of a heavy resonance.
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Bernabeu, J., Botella, F. J., Mavromatos, N. E., & Nebot, M. (2017). The signal of ill-defined CPT weakening entanglement in the B-d system. Eur. Phys. J. C, 77(12), 865–10pp.
Abstract: In the presence of quantum-gravity fluctuations (space-time foam), the CPT operator may be ill-defined. Its perturbative treatment leads to a modification of the Einstein-Podolsky- Rosen correlation of the neutral meson system by adding an entanglement-weakening term of the wrong exchange symmetry, the omega-effect. In the current paper we identify how to probe the complex omega in the entangled B-d system using the flavour (f)-CP(g) eigenstate decay channels: the connection between the intensities for the two timeordered decays (f, g) and (g, f) is lost. Appropriate observables are constructed allowing independent experimental determinations of Re(omega) and Im(omega), disentangled from CPT violation in the evolution Hamiltonian Re(theta) and Im(theta). 2 sigma tensions for both Re(theta) and Im(omega) are shown to be uncorrelated.
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Barenboim, G., Bernabeu, J., Mitsou, V. A., Romero Adam, E., & Vives, O. (2016). METing SUSY on the Z peak. Eur. Phys. J. C, 76(2), 57–13pp.
Abstract: Recently the ATLAS experiment announced a 3 sigma excess at the Z-peak consisting of 29 pairs of leptons together with two or more jets, E-T(miss) > 225 GeV and H-T > 600 GeV, to be compared with 10.6 +/- 3.2 expected lepton pairs in the Standard Model. No excess outside the Z-peak was observed. By trying to explain this signal with SUSY we find that only relatively light gluinos, m((g) over bar) less than or similar to 1.2 TeV, together with a heavy neutralino NLSP of m((chi) over bar) greater than or similar to 400 GeV decaying predominantly to Z-boson plus a light gravitino, such that nearly every gluino produces at least one Z-boson in its decay chain, could reproduce the excess. We construct an explicit general gauge mediation model able to reproduce the observed signal overcoming all the experimental limits. Needless to say, more sophisticated models could also reproduce the signal, however, any model would have to exhibit the following features: light gluinos, or heavy particles with a strong production cross section, producing at least one Z-boson in its decay chain. The implications of our findings for the Run II at LHC with the scaling on the Z peak, as well as for the direct search of gluinos and other SUSY particles, are pointed out.
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Bevan, A. J. et al, Martinez-Vidal, F., Pich, A., Azzolini, V., Bernabeu, J., Lopez-March, N., et al. (2014). The Physics of the B Factories. Eur. Phys. J. C, 74(11), 3026–916pp.
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Amelino-Camelia, G. et al, Bernabeu, J., & Passemar, E. (2010). Physics with the KLOE-2 experiment at the upgraded DA Phi NE. Eur. Phys. J. C, 68(3-4), 619–681.
Abstract: Investigation at a f-factory can shed light on several debated issues in particle physics. We discuss: (i) recent theoretical development and experimental progress in kaon physics relevant for the Standard Model tests in the flavor sector, (ii) the sensitivity we can reach in probing CPT and Quantum Mechanics from time evolution of entangled-kaon states, (iii) the interest for improving on the present measurements of non-leptonic and radiative decays of kaons and eta/eta' mesons, (iv) the contribution to understand the nature of light scalar mesons, and (v) the opportunity to search for narrow di-lepton resonances suggested by recent models proposing a hidden dark-matter sector. We also report on the e(+)e(-) physics in the continuum with the measurements of (multi) hadronic cross sections and the study of gamma gamma processes.
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Borja-Lloret, M., Barrientos, L., Bernabeu, J., Lacasta, C., Muñoz, E., Ros, A., et al. (2023). Influence of the background in Compton camera images for proton therapy treatment monitoring. Phys. Med. Biol., 68(14), 144001–16pp.
Abstract: Objective. Background events are one of the most relevant contributions to image degradation in Compton camera imaging for hadron therapy treatment monitoring. A study of the background and its contribution to image degradation is important to define future strategies to reduce the background in the system. Approach. In this simulation study, the percentage of different kinds of events and their contribution to the reconstructed image in a two-layer Compton camera have been evaluated. To this end, GATE v8.2 simulations of a proton beam impinging on a PMMA phantom have been carried out, for different proton beam energies and at different beam intensities. Main results. For a simulated Compton camera made of Lanthanum (III) Bromide monolithic crystals, coincidences caused by neutrons arriving from the phantom are the most common type of background produced by secondary radiations in the Compton camera, causing between 13% and 33% of the detected coincidences, depending on the beam energy. Results also show that random coincidences are a significant cause of image degradation at high beam intensities, and their influence in the reconstructed images is studied for values of the time coincidence windows from 500 ps to 100 ns. Significance. Results indicate the timing capabilities required to retrieve the fall-off position with good precision. Still, the noise observed in the image when no randoms are considered make us consider further background rejection methods.
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Roser, J., Barrientos, L., Bernabeu, J., Borja-Lloret, M., Muñoz, E., Ros, A., et al. (2022). Joint image reconstruction algorithm in Compton cameras. Phys. Med. Biol., 67(15), 155009–15pp.
Abstract: Objective. To demonstrate the benefits of using an joint image reconstruction algorithm based on the List Mode Maximum Likelihood Expectation Maximization that combines events measured in different channels of information of a Compton camera. Approach. Both simulations and experimental data are employed to show the algorithm performance. Main results. The obtained joint images present improved image quality and yield better estimates of displacements of high-energy gamma-ray emitting sources. The algorithm also provides images that are more stable than any individual channel against the noisy convergence that characterizes Maximum Likelihood based algorithms. Significance. The joint reconstruction algorithm can improve the quality and robustness of Compton camera images. It also has high versatility, as it can be easily adapted to any Compton camera geometry. It is thus expected to represent an important step in the optimization of Compton camera imaging.
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Bernabeu, J., & Martinez-Vidal, F. (2015). Time-Reversal Violation (Vol. 65). Annual Reviews.
Abstract: The violation of CP symmetry between matter and antimatter in the neutral K and B meson systems is well established, with a high degree of consistency between all available experimental measurements and with the Standard Model of particle physics. On the basis of the up-to-now-unbroken CPT symmetry, the violation of CP symmetry strongly suggests that the behavior of these particles under weak interactions must also be asymmetric under time reversal T. Many searches for T violation have been performed and proposed using different observables and experimental approaches. These include T-odd observables, such as triple products in weak decays, and genuine observables, such as permanent electric dipole moments of nondegenerate stationary states and the breaking of the reciprocity relation. We discuss the conceptual basis of the required exchange of initial and final states with unstable particles, using quantum entanglement and the decay as a filtering measurement, for the case of neutral B and K mesons. Using this method, the BaBar experiment at SLAC has clearly observed T violation in B mesons.
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