TeV-scale Gravity Research Articles

Centauros and/or Chirons as evaporating mini black holes

It is argued that the signals expected from the evaporation of mini black holes - predicted in TeV-scale gravity models with large extra dimensions, and possibly produced in ultra high energy collisions in the atmosphere - have characteristics quite similar to those of the Centauro events, an old mystery of cosmic-ray physics.

Bulk versus brane in the absorption and emission: 5d rotating black hole case

The absorption and emission spectra for the minimally-coupled brane and bulk scalar fields are numerically computed when the spacetime is a 5d rotating black hole carrying the two different angular momentum parameters a and b. The effect of the superradiant scattering in the spectra is carefully examined. It is shown that the low-energy limit of the total absorption cross section always equal to the area of the non-spherically symmetric horizon, i.e., 4 π ( r H 2 + a 2 ) for the brane scalar and 2 π 2 ( r H 2 + a 2 ) ( r H 2 + b 2 ) / r H for the bulk scalar where r H is an horizon radius. The energy amplification for the bulk scalar is roughly order of 10 −9% while that for the brane scalar is order of unity. This indicates that the effect of the superradiance is negligible for the case of the bulk scalar. Thus the standard claim that black holes radiate mainly on the brane is not changed although the effect of the superradiance is taken into account. The physical implication of this fact is discussed in the context of TeV-scale gravity.

Close-limit analysis for head-on collision of two black holes in higher dimensions: Brill-Lindquist initial data

Motivated by the TeV-scale gravity scenarios, we study gravitational radiation in the head-on collision of two black holes in higher dimensional spacetimes using a close-limit approximation. We prepare time-symmetric initial data sets for two black holes (the so-called Brill-Lindquist initial data) and numerically evolve the spacetime in terms of a gauge invariant formulation for the perturbation around the higher-dimensional Schwarzschild black holes. The waveform and radiated energy of gravitational waves emitted in the head-on collision are clarified. Also, the complex frequencies of fundamental quasinormal modes of higher-dimensional Schwarzschild black holes, which have not been accurately derived so far, are determined.

Escape of Black Holes from the Brane

TeV-scale gravity theories allow the possibility of producing small black holes at energies that soon will be explored at the CERN LHC or at the Auger observatory. One of the expected signatures is the detection of Hawking radiation that might eventually terminate if the black hole, once perturbed, leaves the brane. Here, we study how the "black hole plus brane" system evolves once the black hole is given an initial velocity that mimics, for instance, the recoil due to the emission of a graviton. The results of our dynamical analysis show that the brane bends around the black hole, suggesting that the black hole eventually escapes into the extra dimensions once two portions of the brane come in contact and reconnect. This gives a dynamical mechanism for the creation of baby branes.

Cosmic ray signals from mini black holes in models with extra dimensions: an analytical/Monte Carlo study

We present a study of the multiplicities, of the lateral distributions and of the ratio of the electromagnetic to the hadronic components in the air showers, generated by the collision in the atmosphere of an incoming high energy cosmic ray and mediated by the formation of a mini black hole, predicted in TeV scale gravity models with large extra dimensions. The analysis is performed via a large scale simulation of the resulting cascades over the entire range ($10^{15}-10^{19}$eV) of ultra high initial energies, for several values of the number of large extra dimensions, for a variety of altitudes of the initial interaction and with the energy losses in the bulk taken into account. The results are compared with a representative of the standard events, namely the shower due to the collision of a primary proton with a nucleon in the atmosphere. Both the multiplicities and the lateral distribution of the showers show important differences between the two cases and, consequently, may be useful for the observational characterization of the events. The electromagnetic/hadronic ratio is strongly fluctuating and, thus, less decisive for the altitudes considered.

Improved analysis of black hole formation in high-energy particle collisions

We investigate formation of an apparent horizon (AH) in high-energy particle collisions in four- and higher-dimensional general relativity, motivated by TeV-scale gravity scenarios. The goal is to estimate the prefactor in the geometric cross section formula for the black hole production. We numerically construct AHs on the future light cone of the collision plane. Since this slice lies to the future of the slice used previously by Eardley and Giddings [Phys. Rev. D 66, 044011 (2002)] and by one of us and Nambu [Phys. Rev. D 67, 024009 (2003)], we are able to improve the prefactor estimates. The black hole production cross section increases by 40%--70% in the higher-dimensional cases, indicating larger black hole production rates in future-planned accelerators than previously estimated. We also determine the mass and the angular momentum of the final black hole state, as allowed by the area theorem.

Next-to-leading order QCD corrections to the Drell–Yan cross section in models of TeV-scale gravity

The first results on next-to-leading order QCD corrections to graviton-induced processes in hadron collisions in models of TeV-scale gravity are presented focusing on the case of dilepton pair production in p ¯ p and pp collisions. Distributions in the invariant mass Q, the longitudinal fraction x F , the rapidity Y and the forward–backward asymmetry of the lepton pair are studied. The quantitative impact of the QCD corrections for searches of extra dimensions at hadron colliders is investigated. It turns out that at the LHC ( S = 14 TeV ) the K-factor is rather large ( K = 1.6 ) for large invariant mass Q of the lepton pair, indicating the importance of accounting for these QCD corrections in the experimental search for TeV-scale gravity. At the Tevatron, the K-factor does not substantially deviate from the Standard Model value. However, its inclusion is necessitated to make the cross section stable with respect to scale variations.

Distinguishing Between Small Arkani-Hamed–Dimopoulos–Dvali and Randall-Sundrum Accelerator-Generated Black Holes

In models with extra dimensions that accommodate a TeV-scale gravity, small black holes could be produced in near future accelerator experiments. Such small black holes, whose gravitational radius is much smaller than the characteristic size of extra dimensions can be very well described by asymptotically flat solutions, thus losing the information about the global geometry of the extra manifold. One might conclude that such small black holes would be indistinguishable in different scenarios. We argue that important differences still exist, especially regarding experimental signature in colliders, which may help us distinguish between the various extra dimensional scenarios. The main differences come from the fact that most of the models with the warped extra dimension have an additional discrete Z2 symmetry that makes the brane behave as if it were an infinite tension brane.

Uncertainties in limits on TeV-gravity from neutrino-induced showers

In models with TeV-scale gravity, ultrahigh energy cosmic rays can generate microscopic black holes in the collision with atmospheric and terrestrial nuclei. It has been proposed that stringent bounds on TeV-scale gravity can be obtained from the absence of neutrino cosmic ray showers mediated by black holes. However, uncertainties in the cross section of black hole formation and, most importantly, large uncertainties in the neutrino flux affects these bounds. As long as the cosmic neutrino flux remains unknown, the non-observation of neutrino induced showers implies less stringent limits than present collider limits.

Proof of universality for the absorption of massive scalars by the higher-dimensional Reissner–Nordström black holes

Motivated by black hole experiments as a consequence of the TeV-scale gravity arising from modern brane-world scenarios, we study the absorption problem for the massive scalars when the spacetime background is a ( 4 + n ) -dimensional Reissner–Nordström black hole. For analytic computation we adopt the near-extreme condition in the spacetime background. It is shown that the low-energy absorption cross section for the s-wave case holds an universality, i.e., the absorption cross section equals to the area of the black hole horizon divided by a velocity parameter.

High energy physics in the atmosphere: phenomenology of cosmic ray air showers

The properties of cosmic rays with energies above 10 6 GeV have to be deduced from the spacetime structure and particle content of the air showers which they initiate. In this review we summarize the phenomenology of these giant air showers. We describe the hadronic interaction models used to extrapolate results from collider data to ultra high energies, and discuss the prospects for insights into forward physics at the LHC. We also describe the main electromagnetic processes that govern the longitudinal shower evolution, as well as the lateral spread of particles. Armed with these two principal shower ingredients and motivation from the underlying physics, we provide an overview of some of the different methods proposed to distinguish primary species. The properties of neutrino interactions and the potential of forthcoming experiments to isolate deeply penetrating showers from baryonic cascades are also discussed. We finally venture into a terra incognita endowed with TeV-scale gravity and explore anomalous neutrino-induced showers.

NLO-QCD corrections toe e hadrons in models of TeV-scale gravity

We present results on NLO-QCD corrections to the process e+e−→ hadrons via photon-, Z- and graviton-exchange in the context of TeV-scale gravity models. The quantitative impact of these QCD corrections for searches of extra dimensions at a Linear Collider is briefly discussed.

Black hole production in particle collisions and higher curvature gravity

The problem of black hole production in trans-Planckian particle collisions is revisited, in the context of large extra dimensions scenarios of TeV-scale gravity. The validity of the standard description of this process (two colliding Aichelburg--Sexl shock waves in classical Einstein gravity) is questioned. It is observed that the classical spacetime has large curvature along the transverse collision plane, as signaled by the curvature invariant ${(R}_{\ensuremath{\mu}\ensuremath{\nu}\ensuremath{\lambda}\ensuremath{\sigma}}{)}^{2}.$ Thus quantum gravity effects, and in particular higher curvature corrections to the Einstein gravity, cannot be ignored. To give a specific example of what may happen, the collision is reanalyzed in the Einstein--Lanczos--Lovelock gravity theory, which modifies the Einstein--Hilbert Lagrangian by adding a particular ``Gauss--Bonnet'' combination of curvature squared terms. The analysis uses a series of approximations, which reduce the field equations to a tractable second order nonlinear PDE of the Monge--Amp\`ere type. It is found that the resulting spacetime is significantly different from the pure Einstein case in the future of the transverse collision plane. These considerations cast serious doubts on the geometric cross section estimate, which is based on the classical Einstein gravity description of the black hole production process.

Transplanckian collisions in TeV scale gravity

Collisions at transplanckian energies offer model independent tests of TeV scale gravity. One spectacular signal is given by black-hole production, though a full calculation of the corresponding cross-section is not yet available. Another signal is given by gravitational elastic scattering, which may be less spectacular but which can be nicely computed in the forward region using the eikonal approximation. In this talk I discuss the distinctive signatures of eikonalized scattering at future accelerators.

Updated limits on TeV-scale gravity from the absence of neutrino cosmic ray showers mediated by black holes

We revise existing limits on the D-dimensional Planck scale M_D from the nonobservation of microscopic black holes produced by high energy cosmic neutrinos in scenarios with D=4+n large extra dimensions. Previous studies have neglected the energy radiated in gravitational waves by the multipole moments of the incoming shock waves. We include the effects of energy loss, as well as form factors for black hole production and recent null results from cosmic ray detectors. For n>4, we obtain M_D > 1.0 - 1.4 TeV. These bounds are among the most stringent and conservative to date.

Electromagnetic radiation from collisions at almost the speed of light: An extremely relativistic charged particle falling into a Schwarzschild black hole

We investigate the electromagnetic radiation released during the high energy collision of a charged point particle with a four-dimensional Schwarzschild black hole. We show that the spectra is flat, and well described by a classical calculation. We also compare the total electromagnetic and gravitational energies emitted, and find that the former is suppressed in relation to the latter for very high energies. These results could apply to the astrophysical world in the case that charged stars and small charged black holes are out there colliding into large black holes, and to a very high energy collision experiment in a four-dimensional world. In this latter scenario the calculation is to be used for the moments just after black hole formation, when the collision of charged debris with the newly formed black hole is certainly expected. Since the calculation is four dimensional, it does not directly apply to TeV-scale gravity black holes, as these inhabit a world of six to eleven dimensions, although our results should qualitatively hold when extrapolated with some care to higher dimensions.

Search for ‘invisible’ Higgs signals at LHC via associated production with gauge bosons

A light Higgs boson with substantial branching ratio into invisible channels can occur in a variety of models with: light neutralinos, spontaneously broken lepton number, radiatively generated neutrino masses, additional singlet scalar(s) and/or right handed neutrinos in the extra dimensions of TeV scale gravity. We study the observability of the WH and ZH modes at LHC with H decaying invisibly, by carrying out a detailed simulation with two event generators (HERWIG and PYTHIA) and realistic detector simulations (GETJET and CMSJET). We find that the signal with ‘single lepton plus missing ET’ resulting from WH production suffers from a very large background due to the (off-shell) W∗ production via the Drell–Yan process. In contrast, the ZH mode provides a clean signal in the ‘dilepton plus missing ET’ channel. By exploiting this second signature, we show that invisible branching ratios of Higgs bosons, BRinv, larger than ∼0.42 (0.70) can be probed at 5σ level for MH=120 (160) GeV, respectively, assuming an accumulated luminosity of L=100 fb−1.

ULTRAHIGH ENERGY COSMIC RAYS: THE STATE OF THE ART BEFORE THE AUGER OBSERVATORY

In this review we discuss the important progress made in recent years towards understanding the experimental data on cosmic rays with energies ≳ 1019eV. We begin with a brief survey of the available data, including a description of the energy spectrum, mass composition, and arrival directions. At this point we also give a short overview of experimental techniques. After that, we introduce the fundamentals of acceleration and propagation in order to discuss the conjectured nearby cosmic ray sources. We then turn to theoretical notions of physics beyond the Standard Model where we consider both exotic primaries and exotic physical laws. Particular attention is given to the role that TeV-scale gravity could play in addressing the origin of the highest energy cosmic rays. In the final part of the review we discuss the potential of future cosmic ray experiments for the discovery of tiny black holes that should be produced in the earth's atmosphere if TeV-scale gravity is realized in Nature.

BLACK HOLE AND BRANE PRODUCTION IN TEV GRAVITY: A REVIEW

In models with large extra dimensions, particle collisions with center-of-mass energy larger than the fundamental gravitational scale can generate nonperturbative gravitational objects such as black holes and branes. The formation and the subsequent decay of these super-Planckian objects would be detectable in particle colliders and high energy cosmic ray detectors, and have interesting implications in cosmology and astrophysics. In this paper we present a review of black hole and brane production in TeV-scale gravity.

Rotating black holes at future colliders: Greybody factors for brane fields

We study theoretical aspects of rotating black hole production and evaporation in extra dimension scenarios with TeV scale gravity, within the mass range in which the higher dimensional Kerr solution provides a good description. We evaluate the production cross section of black holes, taking their angular momenta into account. We find that it becomes larger than the Schwarzschild radius squared, which is conventionally utilized in the literature, and our result nicely agrees with the recent numerical study by Yoshino and Nambu within a few percent error for the higher dimensional case. In the same approximation used to obtain the above result, we find that the production cross section becomes larger for a black hole with larger angular momentum. Second, we derive the generalized Teukolsky equation for spin 0, 1/2, and 1 brane fields in higher dimensional Kerr geometry and explicitly show that it is separable in any dimensions. For a five-dimensional (Randall-Sundrum) black hole, we obtain analytic formulas for the greybody factors in the low frequency expansion and we present the power spectra of the Hawking radiation as well as their angular dependence. The phenomenological implications of our results are briefly sketched.