arXiv subjects

Tomasz R. Taylor

Publications and source records attributed to Tomasz R. Taylor.

At least 55 records · Page 3Linked to original sources

arXiv2014

Graviton as a Pair of Collinear Gauge Bosons

We show that the mixed gravitational/gauge superstring amplitudes describing decays of massless closed strings - gravitons or dilatons - into a number of gauge bosons, can be written at the tree (disk) level as linear combinations of pure open string amplitudes in which the graviton (or dilaton) is replaced by a pair of collinear gauge bosons. Each of the constituent gauge bosons carry exactly one half of the original closed string momentum, while their +/- 1 helicities add up to +/- 2 for the graviton or to 0 for the dilaton.

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arXiv2014

Closed String Amplitudes as Single-Valued Open String Amplitudes

We show that the single trace heterotic N-point tree-level gauge amplitude A_HET can be obtained from the corresponding type I amplitude A_I by the single-valued (sv) projection: A_HET=sv(A_I). This projection maps multiple zeta values to single-valued multiple zeta values. The latter represent a subclass of multiple zeta values originating from single-valued multiple polylogarithms at unity. Similar relations between open and closed string amplitudes or amplitudes of different string vacua can be established. As a consequence the alpha'-expansion of a closed string amplitude is dictated by that of the corresponding open string amplitude. The combination of single-valued projections, Kawai-Lewellen-Tye relations and Mellin correspondence reveal a unity of all tree-level open and closed superstring amplitudes together with the maximally supersymmetric Yang-Mills and supergravity theories.

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arXiv2013

Limits on Stringy Signals at the LHC

We explain what features of string theory can be tested at the LHC. We review the present bounds on the string mass and on extra gauge bosons as well as the prospects for the upcoming experiments.

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arXiv2013

Superstring/Supergravity Mellin Correspondence in Grassmannian Formulation

We extend the recently established Mellin correspondence of supergravity and superstring amplitudes to the case of arbitrary helicity configurations. The amplitudes are discussed in the framework of Grassmannian varieties. We generalize Hodges' determinant to a function of two sets of independent coordinates and show that tree-level supergravity amplitudes can be obtained by contour integrations of both sets in separate Grassmannians while in superstring theory, one set of coordinates is identified with string vertex positions at the disk boundary and Mellin transformed into generalized hypergeometric functions of Mandelstam invariants.

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arXiv2013

Superstring Amplitudes as a Mellin Transform of Supergravity

At the tree level, the maximally helicity violating amplitudes of N gauge bosons in open superstring theory and of N gravitons in supergravity are known to have simple representations in terms of tree graphs. For superstrings, the graphs encode integral representations of certain generalized Gaussian hypergeometric functions of kinematic invariants while for supergravity, they represent specific kinematic expressions constructed from spinor-helicity variables. We establish a superstring/supergravity correspondence for this class of amplitudes, by constructing a mapping between the positions of gauge boson vertices at the disk boundary and the helicity spinors associated to gravitons. After replacing vertex positions by a larger set of N(N-3)/2 coordinates, the superstring amplitudes become (multiple) Mellin transforms of supergravity amplitudes, from the projective space into the dual Mellin space of N(N-3)/2 kinematic invariants. Similarly, inverse Mellin transforms transmute open superstrings into supergravity. We elaborate on the properties of multiple Mellin and inverse Mellin transforms in the framework of superstring/supergravity correspondence.

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arXiv2013

Vacuum Stability of Standard Model^{++}

The latest results of the ATLAS and CMS experiments point to a preferred narrow Higgs mass range (m_h \simeq 124 - 126 GeV) in which the effective potential of the Standard Model (SM) develops a vacuum instability at a scale 10^{9} -10^{11} GeV, with the precise scale depending on the precise value of the top quark mass and the strong coupling constant. Motivated by this experimental situation, we present here a detailed investigation about the stability of the SM^{++} vacuum, which is characterized by a simple extension of the SM obtained by adding to the scalar sector a complex SU(2) singlet that has the quantum numbers of the right-handed neutrino, H", and to the gauge sector an U(1) that is broken by the vacuum expectation value of H". We derive the complete set of renormalization group equations at one loop. We then pursue a numerical study of the system to determine the triviality and vacuum stability bounds, using a scan of 10^4 random set of points to fix the initial conditions. We show that, if there is no mixing in the scalar sector, the top Yukawa coupling drives the quartic Higgs coupling to negative values in the ultraviolet and, as for the SM, the effective potential develops an instability below the Planck scale. However, for a mixing angle -0.35 \alt α\alt -0.02 or 0.01 \alt α\alt 0.35, with the new scalar mass in the range 500 GeV \alt m_{h"} \alt 8 TeV, the SM^{++} ground state can be absolutely stable up to the Planck scale. These results are largely independent of TeV-scale free parameters in the model: the mass of the non-anomalous U(1) gauge boson and its branching fractions.

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arXiv2012

LHC Phenomenology and Cosmology of String-Inspired Intersecting D-Brane Models

We discuss the phenomenology and cosmology of a Standard-like Model inspired by string theory, in which the gauge fields are localized on D-branes wrapping certain compact cycles on an underlying geometry, whose intersection can give rise to chiral fermions. The energy scale associated with string physics is assumed to be near the Planck mass. To develop our program in the simplest way, we work within the construct of a minimal model with gauge-extended sector U (3)_B \times Sp (1)_L \times U (1)_{I_R} \times U (1)_L. The resulting U (1) content gauges the baryon number B, the lepton number L, and a third additional abelian charge I_R which acts as the third isospin component of an SU(2)_R. All mixing angles and gauge couplings are fixed by rotation of the U(1) gauge fields to a basis diagonal in hypercharge Y and in an anomaly free linear combination of I_R and B-L. The anomalous $Z'$ gauge boson obtains a string scale Stückelberg mass via a 4D version of the Green-Schwarz mechanism. To keep the realization of the Higgs mechanism minimal, we add an extra SU(2) singlet complex scalar, which acquires a VEV and gives a TeV-scale mass to the non-anomalous gauge boson Z". The model is fully predictive and can be confronted with dijet and dilepton data from LHC8 and, eventually, LHC14. We show that M_{Z"} \approx 3 - 4 TeV saturates current limits from the CMS and ATLAS collaborations. We also show that for M_{Z"} \alt 5 TeV, LHC14 will reach discovery sensitivity \agt 5σ. After that, we demostrate in all generality that Z" milli-weak interactions could play an important role in observational cosmology. Finally, we examine some phenomenological aspects of the supersymmetric extension of the D-brane construct.

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arXiv2012

Maximally Helicity Violating Disk Amplitudes, Twistors and Transcendental Integrals

We obtain simple expressions for tree-level maximally helicity violating amplitudes of N gauge bosons from disk world-sheets of open superstrings. The amplitudes are written in terms of (N-3)! hypergeometric integrals depending on kinematic parameters, weighted by certain kinematic factors. The integrals are transcendental in a strict sense defined in this work. The respective kinematic factors can be succinctly written in terms of "dual" momentum twistors. The amplitudes are computed by using the prescription proposed by Berkovits and Maldacena.

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arXiv2012

Z'-gauge Bosons as Harbingers of Low Mass Strings

Massive Z'-gauge bosons act as excellent harbingers for string compactifications with a low string scale. In D-brane models they are associated to U(1) gauge symmetries that are either anomalous in four dimensions or exhibit a hidden higher dimensional anomaly. We discuss the possible signals of massive Z'-gauge bosons at hadron collider machines (Tevatron, LHC) in a minimal D-brane model consisting out of four stacks of D-branes. In this construction, there are two massive gauge bosons, which can be naturally associated with baryon number B and B-L (L being lepton number). Here baryon number is always anomalous in four dimensions, whereas the presence of a four-dimensional B-L anomaly depends on the U(1)-charges of the right handed neutrinos. In case B-L is anomaly free, a mass hierarchy between the two associated Z'-gauge bosons can be explained. In our phenomenological discussion about the possible discovery of massive Z'-gauge bosons, we take as a benchmark scenario the dijet plus W signal, recently observed by the CDF Collaboration at Tevatron. It reveals an excess in the dijet mass range 150 GeV/c^2, 4.1σbeyond SM expectations. We show that in the context of low-mass string theory this excess can be associated with the production and decay of a leptophobic Z', a singlet partner of SU(3) gluons coupled primarily to baryon number. Even if the CDF signal disappears, as indicated by the more recent D0 results, our analysis can still serve as the basis for future experimental search for massive Z'-gauge bosons in low string scale models. We provide the relevant cross sections for the production of Z'-gauge bosons in the TeV region, leading to predictions that are within reach of the present or the next LHC run.

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arXiv2011

Higher Level String Resonances in Four Dimensions

We study higher level Regge resonances of open superstrings, focusing on the universal part of the Neveu-Schwarz sector common to all D-brane realizations of the standard model. For Regge states with masses far above the fundamental string scale, we discuss the spin-dependence of their decay rates into massless gauge bosons. Extending our previous work on lowest level string excitations, we study the second mass level at which spins range from 0 to 3. We construct the respective vertex operators and compute the amplitudes involving one massive particle and two or three gauge bosons. To illustrate the use of BCFW recursion relations in superstring theory, we build the four gluon amplitude from on-shell amplitudes involving string resonances and gauge bosons.

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arXiv2011

Stringy origin of Tevatron Wjj anomaly

The invariant mass distribution of dijets produced in association with W bosons, recently observed by the CDF Collaboration at Tevatron, reveals an excess in the dijet mass range 120-160 GeV/c^2, 3σbeyond Standard Model expectations. We show that such an excess is a generic feature of low mass string theory, due to the production and decay of a leptophobic Z', a singlet partner of SU(3) gluons coupled primarily to the U(1) baryon number. In this framework, U(1) and SU(3) appear as subgroups of U(3) associated with open strings ending on a stack of 3 D-branes. In addition, a minimal model contains two other stacks to accommodate the electro-weak SU(2) \in U(2) and the hypercharge U(1). Of the three U(1) gauge bosons, the two heavy Z' and Z" receive masses through the Green-Schwarz mechanism. For a given Z' mass, the model is quite constrained. Fine tuning three of its free parameters is just sufficient to simultaneously ensure: a small Z-Z' mixing in accord with the stringent LEP data on the $Z$ mass; very small (less than 1%) branching ratio into leptons; and a large hierarchy between Z" and Z' masses. The heavier neutral gauge boson Z" is within the reach of LHC.

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arXiv2011

Searching for string resonances in e^+e^- and γγcollisions

If the fundamental mass scale of superstring theory is as low as few TeVs, the massive modes of vibrating strings, Regge excitations, will be copiously produced at the Large Hadron Collider (LHC). We discuss the complementary signals of low mass superstrings at the proposed electron-positron facility (CLIC), in e^+e^- and γγcollisions. We examine all relevant four-particle amplitudes evaluated at the center of mass energies near the mass of lightest Regge excitations and extract the corresponding pole terms. The Regge poles of all four-point amplitudes, in particular the spin content of the resonances, are completely model independent, universal properties of the entire landscape of string compactifications. We show that γγ\to e^+ e^- scattering proceeds only through a spin-2 Regge state. We estimate that for this particular channel, string scales as high as 4 TeV can be discovered at the 11σlevel with the first fb^{-1} of data collected at a center-of-mass energy \approx 5 TeV. We also show that for e^+e^- annihilation into fermion-antifermion pairs, string theory predicts the precise value, equal 1/3, of the relative weight of spin 2 and spin 1 contributions. This yields a dimuon angular distribution with a pronounced forward-backward asymmetry, which will help distinguishing between low mass strings and other beyond the standard model scenarios.

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arXiv2010

LHC Phenomenology of Lowest Massive Regge Recurrences in the Randall-Sundrum Orbifold

We consider string realizations of the Randall-Sundrum effective theory for electroweak symmetry breaking and explore the search for the lowest massive Regge excitation of the gluon and of the extra (color singlet) gauge boson inherent of D-brane constructions. In these curved backgrounds, the higher-spin Regge recurrences of Standard Model fields localized near the IR brane are warped down to close to the TeV range and hence can be produced at collider experiments. Assuming that the theory is weakly coupled, we make use of four gauge boson amplitudes evaluated near the first Regge pole to determine the discovery potential of LHC. We study the inclusive dijet mass spectrum in the central rapidity region |y_{jet}| < 1.0 for dijet masses M \geq 2.5 TeV. We find that with an integrated luminosity of 100 fb^{-1}, the 5σdiscovery reach can be as high as 4.7 TeV. Observations of resonant structures in pp \rightarrow direct γ+ jet can provide interesting corroboration for string physics up to 3.0 TeV. We also study the ratio of dijet mass spectra at small and large scattering angles. We show that with the first fb^{-1} such a ratio can probe lowest-lying Regge states for masses \sim 2.5 TeV.

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arXiv2010

Direct Production of Lightest Regge Resonances

We discuss direct production of Regge excitations in the collisions of massless four-dimensional superstring states, focusing on the first excited level of open strings ending on D-branes extending into higher dimensions. We construct covariant vertex operators and identify ``universal'' Regge states with the internal parts either trivial or determined by the world-sheet SCFT describing superstrings propagating on an arbitrary Calabi-Yau manifold. We evaluate the amplitudes involving one such massive state and up to three massless ones and express them in the helicity basis. The most important phenomenological applications of our results are in the context of low-mass string (and large extra dimensions) scenarios in which excited string states are expected to be produced at the LHC as soon as the string mass threshold is reached in the center-of-mass energies of the colliding partons. In order to facilitate the use of partonic cross sections, we evaluate them and tabulate for all production processes: gluon fusion, quark absorbing a gluon, quark-antiquark annihilation and quark-quark scattering.

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arXiv2010

LHC Phenomenology for String Hunters

We consider extensions of the standard model based on open strings ending on D-branes, with gauge bosons due to strings attached to stacks of D-branes and chiral matter due to strings stretching between intersecting D-branes. Assuming that the fundamental string mass scale is in the TeV range and the theory is weakly coupled, we discuss possible signals of string physics at the Large Hadron Collider (LHC). In previous works, direct channel excitations of Regge recurrences in parton-parton scattering supplied the outstanding new signature. The present work considers the deviation from standard model expectations for the 4-fermion processes qq\to qq and qq' \to qq', in which the s-channel excitation of string resonances is absent. In this case, we find that Kaluza-Klein recurrences at masses somewhat less than the string scale generate effective 4-fermion contact terms which can significantly enhance the dijet R ratio above its QCD value of about 0.6. The simultaneous observation of a nearby resonant structure in the dijet mass spectrum would provide a "smoking gun" for TeV scale string theory. In this work, we also show that (1) for M_{string}<3.5 TeV, the rates for various topologies arising from the pp \to Z^0 + jet channel could deviate significantly from standard model predictions and (2) that the sizeable cross sections for Regge recurrences can allow a 6σdiscovery for string scales as large as 3 TeV after about 1 year of LHC operation at \sqrt{s} =10 TeV and \int L dt ~ 100 pb^{-1}.

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arXiv2010

Dijet signals for low mass strings at the LHC

We consider extensions of the standard model based on open strings ending on D-branes, with gauge bosons due to strings attached to stacks of D-branes and chiral matter due to strings stretching between intersecting D-branes. Assuming that the fundamental string mass scale is in the TeV range and the theory is weakly coupled, we discuss possible signals of string physics at the Large Hadron Collider (LHC). In such D-brane constructions, the dominant contributions to full-fledged string amplitudes for all the common QCD parton subprocesses leading to dijets are completely independent of the details of compactification, and can be evaluated in a parameter-free manner. We make use of these amplitudes evaluated near the first resonant pole to determine the discovery potential of LHC for the first Regge excitations of the quark and gluon. Remarkably, the reach of LHC after a few years of running can be as high as 6.8 TeV. Even after the first 100 pb^{-1} of integrated luminosity, string scales as high as 4.0 TeV can be discovered. For string scales as high as 5.0 TeV, observations of resonant structures in pp \to {\rm direct} γ+ jet can provide interesting corroboration for string physics at the TeV-scale.

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arXiv2010

String Phenomenology at the LHC

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arXiv2009

Neutralino dark matter annihilation to monoenergetic gamma rays as a signal of low mass superstrings

We consider extensions of the standard model based on open strings ending on D-branes, in which gauge bosons and their associated gauginos exist as strings attached to stacks of D-branes, and chiral matter exists as strings stretching between intersecting D-branes. Under the assumptions that the fundamental string scale is in the TeV range and the theory is weakly coupled, we study models of supersymmetry for which signals of annihilating neutralino dark matter are observable. In particular, we construct a model with a supersymmetric R-symmetry violating (but R-parity conserving) effective Lagrangian that allows for the s-wave annihilation of neutralinos, once gauginos acquire mass through an unspecified mechanism. The model yields bino-like neutralinos (with the measured relic abundance) that annihilate to a gamma-gamma final state with a substantial branching fraction (~ 10%) that is orders of magnitude larger than in the minimal supersymmetric standard model. A very bright gamma-ray spectral line could be observed by gamma-ray telescopes.

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