arXiv subjects
Publications and source records attributed to Luis A. Anchordoqui.
The LHC program will include the identification of events with single high-k_T photons as probes of new physics. We show that this channel is uniquely suited to search for experimental evidence of TeV-scale open string theory.
The mass scale M_s of superstring theory is an arbitrary parameter that can be as low as few TeVs if the Universe contains large extra dimensions. We propose a search for the effects of Regge excitations of fundamental strings at LHC, in the process p p \to γjet. The underlying parton process is dominantly the single photon production in gluon fusion, g g \to γg, with open string states propagating in intermediate channels. If the photon mixes with the gauge boson of the baryon number, which is a common feature of D-brane quivers, the amplitude appears already at the string disk level. It is completely determined by the mixing parameter -- and it is otherwise model-(compactification-) independent. Even for relatively small mixing, 100 fb^{-1} of LHC data could probe deviations from standard model physics, at a 5σsignificance, for M_s as large as 3.3 TeV.
Ongoing experimental efforts to detect cosmic sources of high energy neutrinos are guided by the expectation that astrophysical accelerators of cosmic ray protons would also generate neutrinos through interactions with ambient matter and/or photons. However there will be a reduction in the predicted neutrino flux if cosmic ray sources accelerate not only protons but also significant number of heavier nuclei, as is indicated by recent air shower data. We consider plausible extragalactic sources such as active galactic nuclei, gamma-ray bursts and starburst galaxies and demand consistency with the observed cosmic ray composition and energy spectrum at Earth after allowing for propagation through intergalactic radiation fields. This allows us to calculate the expected neutrino fluxes from the sources, normalised to the observed cosmic ray spectrum. We find that the likely signals are still within reach of next generation neutrino telescopes such as IceCube.
Eventual signals of split sypersymmetry in cosmic ray physics are analyzed in detail. The study focusses particularly on quasi-stable colorless R-hadrons originating through confinement of long-lived gluinos (with quarks, anti-quarks, and gluons) produced in pp collisions at astrophysical sources. Because of parton density requirements, the gluino has a momentum which is considerable smaller than the energy of the primary proton, and so production of heavy (mass ~ 500 GeV) R-hadrons requires powerful cosmic ray engines able to accelerate particles up to extreme energies, somewhat above 10^{13.6} GeV. Using a realistic Monte Carlo simulation with the AIRES engine, we study the main characteristics of the air showers triggered when one of these exotic hadrons impinges on a stationary nucleon of the Earth atmosphere. We show that R-hadron air showers present clear differences with respect to those initiated by standard particles. We use this shower characteristics to construct observables which may be used to distinguish long-lived gluinos at the Pierre Auger Observatory.
The Pierre Auger Observatory (PAO) has measured the spectrum and composition of the ultrahigh energy cosmic rays with unprecedented precision. We use these measurements to constrain their spectrum and composition as injected from their sources and, in turn, use these results to estimate the spectrum of cosmogenic neutrinos generated in their propagation through intergalactic space. We find that the PAO measurements can be well fit if the injected cosmic rays consist entirely of nuclei with masses in the intermediate (C, N, O) to heavy (Fe, Si) range. A mixture of protons and heavier species is also acceptable but (on the basis of existing hadronic interaction models) injection of pure light nuclei (p, He) results in unacceptable fits to the new elongation rate data. The expected spectrum of cosmogenic neutrinos can vary considerably, depending on the precise spectrum and chemical composition injected from the cosmic ray sources. In the models where heavy nuclei dominate the cosmic ray spectrum and few dissociated protons exceed GZK energies, the cosmogenic neutrino flux can be suppressed by up to two orders of magnitude relative to the all-proton prediction, making its detection beyond the reach of current and planned neutrino telescopes. Other models consistent with the data, however, are proton-dominated with only a small (1-10%) admixture of heavy nuclei and predict an associated cosmogenic flux within the reach of upcoming experiments. Thus a detection or non-detection of cosmogenic neutrinos can assist in discriminating between these possibilities.
TeV gamma-rays can result from the photo-de-excitation of PeV cosmic ray nuclei after their parents have undergone photo-disintegration in an environment of ultraviolet photons. This process is proposed as a candidate explanation of the recently discovered HESS source at the edge of Westerlund 2. The UV background is provided by Lyman-alpha emission within the rich O and B stellar environment. The HESS flux results if there is efficient acceleration at the source of lower energy nuclei. The requirement that the Lorentz-boosted ultraviolet photons reach the Giant Dipole resonant energy (~ 20 MeV) implies a strong suppression of the gamma-ray spectrum compared to an E_γ^{-2} behavior at energies below about 1 TeV. This suppression is not apparent in the lowest-energy Westerlund 2 datum, but will be probed by the upcoming GLAST mission.
TeV gamma-rays may provide significant information about high energy astrophysical accelerators. Such gamma-rays can result from the photo-de-excitation of PeV nuclei after their parents have undergone photo-disintegration in an environment of ultraviolet photons. This process is proposed as a candidate explanation of the recently discovered HEGRA source at the edge of the Cygnus OB2 association. The Lyman-alpha background is provided by the rich O and B stellar environment. It is found that (1) the HEGRA flux can be obtained if there is efficient acceleration at the source of lower energy nuclei; (2) the requirement that the Lorentz-boosted ultraviolet photons can excite the Giant Dipole resonance implies a strong suppression of the gamma-ray spectrum compared to an E_γ^{-2} behavior at energies \alt 1 TeV (some of these energies will be probed by the upcoming GLAST mission); (3) a TeV neutrino counterpart from neutron decay following helium photo-disintegration will be observed at IceCube only if a major proportion of the kinetic energy budget of the Cygnus OB2 association is expended in accelerating nuclei.
It is commonly assumed that high-energy gamma-rays are made via either purely electromagnetic processes or the hadronic process of pion production, followed by decay. We investigate astrophysical contexts where a third process (A*) may dominate, namely the photo-disintegration of highly boosted nuclei followed by daughter de-excitation. Starbust regions such as Cygnus OB2 appear to be promising sites for TeV gamma-ray emission via this mechanism. A unique feature of the A* process is a sharp energy minimum ~ 10 TeV/(T/eV) for gamma-ray emission from a thermal region of temperature T. We also check that a diffuse gamma-ray component resulting from the interaction of a possible extreme-energy cosmic-ray nuclei with background radiation is well below the observed EGRET data. The A* mechanism described herein offers an important contribution to gamma-ray astronomy in the era of intense observational activity.
If the fundamental Planck scale is about a TeV and the cosmic neutrino flux is at the Waxman-Bahcall level, quantum black holes are created daily in the Antarctic ice-cap. We re-examine the prospects for observing such black holes with the IceCube neutrino-detection experiment. To this end, we first revise the black hole production rate by incorporating the effects of inelasticty, i.e., the energy radiated in gravitational waves by the multipole moments of the incoming shock waves. After that we study in detail the process of Hawking evaporation accounting for the black hole's large momentum in the lab system. We derive the energy spectrum of the Planckian cloud which is swept forward with a large, O (10^6), Lorentz factor. (It is noteworthy that the boosted thermal spectrum is also relevant for the study of near-extremal supersymmetric black holes, which could be copiously produced at the LHC.) In the semiclassical regime, we estimate the average energy of the boosted particles to be less than 20% the energy of the neutrino-progenitor. Armed with such a constraint, we determine the discovery reach of IceCube by tagging on "soft" (relative to what one would expect from charged current standard model processes) muons escaping the electromagnetic shower bubble produced by the black hole's light descendants. The statistically significant 5-sigma excess extends up to a quantum gravity scale ~ 1.3 TeV.
We emphasize the inelasticity distribution of events detected at the IceCube neutrino telescope as an important tool for revealing new physics. This is possible because the unique energy resolution at this facility allows to separately assign the energy fractions for emergent muons and taus in neutrino interactions. As a particular example, we explore the possibility of probing second and third generation leptoquark parameter space (coupling and mass). We show that production of leptoquarks with masses \agt 250 GeV and diagonal generation couplings of O(1) can be directly tested if the cosmic neutrino flux is at the Waxman-Bahcall level.
Motivated by recent interest in TeV-scale gravity and especially by the possibility of fast baryon decay mediated by virtual black holes, we study another dangerous aspect of spacetime foam interactions: lepton flavor violation. We correlate existing limits on gravity-induced decoherence in the neutrino sector with a lower bound on the scale of quantum gravity, and find that if spacetime foam interactions do not allow an S-matrix description the UV cutoff is well beyond the electroweak scale. This suggests that string theory provides the appropriate framework for description of quantum gravity at the TeV-scale.
The sources of the observed ultra-high energy cosmic rays must also generate ultra-high energy neutrinos. Deep inelastic scattering of these neutrinos with nucleons on Earth probe center-of-mass energies $\sqrt{s} \sim 100$ TeV, well beyond those attainable at terrestrial colliders. By comparing the rates for two classes of observable events, any departure from the benchmark (unscreened perturbative QCD) neutrino-nucleon cross-section can be constrained. Using the projected sensitivity of the Pierre Auger Observatory to quasi-horizontal showers and Earth-skimming tau neutrinos, we show that a `Super-Auger' detector can thus provide an unique probe of strong interaction dynamics.
The recent detection of TeV gamma-rays from the microquasar LS 5039 by HESS is one of the most exciting discoveries of observational gamma-ray astronomy in the very high energy regime. This result clearly demonstrates that X-ray binaries with relativistic jets (microquasars) are sites of effective acceleration of particles (electrons and/or protons) to multi-TeV energies. Whether the gamma-rays are of hadronic or leptonic origin is a key issue related to the origin of Galactic Cosmic Rays. We discuss different possible scenarios for the production of gamma-rays, and argue in favor of hadronic origin of TeV photons, especially if they are produced within the binary system. If so, the detected gamma-rays should be accompanied by a flux of high energy neutrinos emerging from the decays of π^\pm mesons produced at pp and/or p γinteractions. The flux of TeV neutrinos, which can be estimated on the basis of the detected TeV gamma-ray flux, taking into account the internal γγ\to e^+e^- absorption, depends significantly on the location of gamma-ray production region(s). The minimum neutrino flux above 1 TeV is expected to be at the level of 10^{-12} cm^{-2} s^{-1}; however, it could be up to a factor of 100 larger. The detectability of the signal of multi-TeV neutrinos significantly depends on the high energy cutoff in the spectrum of parent protons; if the spectrum of accelerated protons continues to 1 PeV and beyond, the predicted neutrino fluxes can be probed by the planned km^3-scale neutrino detector.
Ultra-high energy cosmic rays and neutrinos probe energies far above the weak scale. Their usefulness might appear to be limited by astrophysical uncertainties; however, by simultaneously considering up- and down-going events, one may disentangle particle physics from astrophysics. We show that present data from the AMANDA experiment in the South Pole ice already imply an upper bound on neutrino cross sections at energy scales that will likely never be probed at man-made accelerators. The existing data also place an upper limit on the neutrino flux valid for any neutrino cross section. In the future, similar analyses of IceCube data will constrain neutrino properties and fluxes at the O(10%) level.
Neutrino oscillations can be affected by decoherence induced e.g. by Planck scale suppressed interactions with the space-time foam predicted in some approaches to quantum gravity. We study the prospects for observing such effects at IceCube, using the likely flux of TeV antineutrinos from the Cygnus spiral arm. We formulate the statistical analysis for evaluating the sensitivity to quantum decoherence in the presence of the background from atmospheric neutrinos, as well as from plausible cosmic neutrino sources. We demonstrate that IceCube will improve the sensitivity to decoherence effects of ${\cal O}(E^2/M_{\rm Pl})$ by 17 orders of magnitude over present limits and, moreover, that it can probe decoherence effects of ${\cal O}(E^3/M_{\rm Pl}^2)$ which are well beyond the reach of other experiments.
Motivated by a recent change in viewing the onset of the extra-galactic component in the cosmic ray spectrum, we have fitted the observed data down to $10^{8.6}$ GeV and have obtained the corresponding power emissivity. This transition energy is well below the threshold for resonant $pγ$ absorption on the cosmic microwave background, and thus source evolution is an essential ingredient in the fitting procedure. Two-parameter fits in the spectral and redshift evolution indices show that a standard Fermi $E_i^{-2}$ source spectrum is excluded at larger than 95% confidence level (CL). Armed with the primordial emissivity, we follow Waxman and Bahcall to derive the associated neutrino flux on the basis of optically thin sources. For $pp$ interactions as the generating mechanism, the neutrino flux exceeds the AMANDA-B10 90%CL upper limits. In the case of $pγ$ dominance, the flux is marginally consistent with AMANDA-B10 data. In the new scenario the source neutrino flux dominates over the cosmogenic flux at all energies. Thus, should data from AMANDA-II prove consistent with the model, we show that IceCube can measure the characteristic power law of the neutrino spectrum, and thus provide a window on the source dynamics.
This paper presents a brief review of the current status of ultrahigh energy cosmic ray observations and discusses nearby starburst-like galaxies as their possible origin.
A leading candidate for the extragalactic source of high energy cosmic rays is the Fermi engine mechanism, in which protons confined by magnetic fields are accelerated to very high energy through repeated scattering by plasma shock fronts. In the process of acceleration, collisions of trapped protons with the ambient plasma produce pions which decay to electromagnetic energy and neutrinos. For optically thin sources, a strong connection between the emerging cosmic rays and secondary neutrinos can be established. In this context, we show the feasibility of using the Glashow resonance as a discriminator between the $pp$ and $pγ$ interactions in Fermi engines as sources of neutrinos. In particular, we demonstrate how three years of observation at the km$^3$ IceCube facility can serve as a filter for the dominance of the $pp$ interaction at the source.