Massive String States Research Articles

Revisiting higher-spin gyromagnetic couplings

We analyze Bosonic, Heterotic, and Type II string theories compactified on a generic torus having constant moduli. By computing the hamiltonian giving the interaction between massive string excitations and U(1) gauge fields arising from the graviton and Kalb-Ramond field upon compactification, we derive a general formula for such couplings that turns out to be universal in all these theories. We also confirm our result by explicitly evaluating the relevant string three-point amplitudes. From this expression, we determine the gyromagnetic ratio g of massive string states coupled to both gauge-fields. For a generic mixed symmetry state, there is one gyromagnetic coupling associated with each row of the corresponding Young Tableau diagram. For all the states having zero Kaluza Klein or Winding charges, the value of g turns out to be 1. We also explicitly consider totally symmetric and mixed symmetry states (having two rows in the Young diagram) associated with the first Regge-trajectory and obtain their corresponding g value.

Crossing Symmetric Dispersion Relations in Quantum Field Theories.

For 2-2 scattering in quantum field theories, the usual fixed t dispersion relation exhibits only two-channel symmetry. This Letter considers a crossing symmetric dispersion relation, reviving certain old ideas from the 1970s. Rather than the fixed t dispersion relation, this needs a dispersion relation in a different variable z, which is related to the Mandelstam invariants s, t, u via a parametric cubic relation making the crossing symmetry in the complex z plane a geometric rotation. The resulting dispersion is manifestly three-channel crossing symmetric. We give simple derivations of certain known positivity conditions for effective field theories, including the null constraints, which lead to two sided bounds and derive a general set of new nonperturbative inequalities. We show how these inequalities enable us to locate the first massive string state from a low energy expansion of the four dilaton amplitude in type II string theory. We also show how a generalized (numerical) Froissart bound, valid for all energies, is obtained from this approach.

Symmetry of String Scattering Amplitudes

We explore the hidden symmetry in string theory by studying string scattering amplitudes. We calculate 4-point open string scattering amplitudes with three tachyons and a massive higher spin string state. The result can be expressed as Type D Lauricella functions which are generalization of Gaussian Hypergeometric functions. In various high energy limits, the string amplitudes reduced to the expected results that we obtained previously. We find exact SL (K + 3, ℂ) symmetry for the string amplitudes at general energy.

Yukawa's of light stringy states

Light massive string states can appear at D‐brane intersections with small angles. We compute tri‐linear Yukawa couplings of such open‐string states to massless ones and to one another. Due to ambiguities in the normalisation of the vertex operators, that involve twist fields, we proceed via factorization of appropriate scattering amplitudes. Some peculiar features are observed that may lead to interesting signatures at colliders in the future.

Chiral heterotic strings with positive cosmological constant

We present explicit examples of semi-realistic heterotic models with spontaneously broken supersymmetry, which dynamically lead to breaking scales much smaller than MPlanck and exponentially small positive values for the cosmological constant. Contrary to field theoretic intuition, we find that the global structure of the effective potential is significantly affected by contributions of massive and non-level matched string states and we investigate the conditions that dynamically ensure a number of desired properties.

Revisiting light stringy states in view of the 750 GeV diphoton excess

We investigate light massive string states that appear at brane intersections. They replicate the massless spectrum in a richer fashion and may be parametrically lighter than standard Regge excitations. We identify the first few physical states and determine their BRST invariant vertex operators. In the supersymmetric case we reconstruct the super-multiplet structure. We then compute some simple interactions, such as the decay rate of a massive scalar or vector into two massless fermions. Finally we suggest an alternative interpretation of the 750 GeV diphoton excess at LHC in terms of a light massive string state, a replica of the Standard Model Higgs.

Towards the one-loop Kähler metric of Calabi-Yau orientifolds

We evaluate string one-loop contributions to the Kahler metric of closed string moduli in toroidal minimally supersymmetric (Calabi-Yau) orientifolds with D-branes. We focus on the poorly understood N=1 sectors that receive contributions from all massive string states.

String resonances at hadron colliders

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 ${M}_{s}$ is in the TeV range and that the theory is weakly coupled, we discuss possible signals of string physics at the upcoming HL-LHC run (integrated $\mathrm{luminosity}=3000\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$) with a center-of-mass energy of $\sqrt{s}=14\text{ }\text{ }\mathrm{TeV}$ and at potential future $pp$ colliders, HE-LHC and VLHC, operating at $\sqrt{s}=33$ and 100 TeV, respectively (with the same integrated luminosity). In such D-brane constructions, the dominant contributions to full-fledged string amplitudes for all the common QCD parton subprocesses leading to dijets and $\ensuremath{\gamma}+\mathrm{jet}$ 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 $(n=1)$ and second $(n=2)$ resonant poles to determine the discovery potential for Regge excitations of the quark, the gluon, and the color singlet living on the QCD stack. We show that for string scales as large as 7.1 TeV (6.1 TeV) lowest massive Regge excitations are open to discovery at the $\ensuremath{\ge}5\ensuremath{\sigma}$ in dijet ($\ensuremath{\gamma}+\mathrm{jet}$) HL-LHC data. We also show that for $n=1$ the dijet discovery potential at HE-LHC and VLHC exceedingly improves: up to 15 TeV and 41 TeV, respectively. To compute the signal-to-noise ratio for $n=2$ resonances, we first carry out a complete calculation of all relevant decay widths of the second massive level string states (including decays into massless particles and a massive $n=1$ and a massless particle), where we rely on factorization and conformal field theory techniques. Helicity wave functions of arbitrary higher spin massive bosons are also constructed. We demonstrate that for string scales ${M}_{s}\ensuremath{\lesssim}10.5\text{ }\text{ }\mathrm{TeV}$ (${M}_{s}\ensuremath{\lesssim}28\text{ }\text{ }\mathrm{TeV}$) detection of $n=2$ Regge recurrences at HE-LHC (VLHC) would become the smoking gun for D-brane string compactifications. Our calculations have been performed using a semianalytic parton model approach which is cross checked against an original software package. The string event generator interfaces with HERWIG and Pythia through BlackMax. The source code is publicly available in the hepforge repository.

String Phenomenology: Past, Present and Future Perspectives

The observation of a scalar resonance at the Large Hadron Collider (LHC), compatible with perturbative electroweak symmetry breaking, reinforces the Standard Model (SM) parameterisation of all subatomic data. The logarithmic evolution of the SM gauge and matter parameters suggests that this parameterisation remains viable up to the Planck scale, where gravitational effects are of comparable strength. String theory provides a perturbatively consistent scheme to explore how the parameters of the Standard Model may be determined from a theory of quantum gravity. The free fermionic heterotic string models provide concrete examples of exact string solutions that reproduce the spectrum of the Minimal Supersymmetric Standard Model. Contemporary studies entail the development of methods to classify large classes of models. This led to the discovery of exophobic heterotic-string vacua and the observation of spinor-vector duality, which provides an insight to the global structure of the space of (2,0) heterotic-string vacua. Future directions entail the study of the role of the massive string states in these models and their incorporation in cosmological scenarios. A complementary direction is the formulation of quantum gravity from the principle of manifest phase space duality and the equivalence postulate of quantum mechanics, which suggest that space is compact. The compactness of space, which implies intrinsic regularisation, may be tightly related to the intrinsic finite length scale, implied by string phenomenology.

Correlators of massive string states with conserved currents

We calculate correlation functions of the R-current or the stress-energy tensor T_{\mu\nu} with two non-protected operators dual to generic massive string states with rotation in S^5, in the context of the AdS/CFT correspondence. Field theory Ward identities make predictions about the all-loop behaviour of these correlators. In particular, they restrict the fusion coefficient to be proportional to the R-charge of the operators or to their dimension, respectively, with certain coefficients of proportionality. We reproduce these predictions, at strong coupling, using string theory. Furthermore, we point out that the recently observed strong coupling factorisation of 4-point correlators is consistent with conformal symmetry and puts constraints on the strong coupling expressions of 4-point correlators involving R-currents or the stress-energy tensor.

COSMOLOGICAL BACKREACTION OF HEAVY STRING STATES

We propose a mechanism to have a smooth transition from a pre-Big Bang phase to a standard cosmological phase. Such transition is driven by gravitational production of heavy massive string states that backreact on the geometry to stop the growth of the curvature. Close to the string scale, particle creation can become effective because the string phase space compensate the exponential suppression of the particle production. Numerical solutions for the evolution of the Universe with this source are presented.

Semiclassical computation of three-point functions of closed string vertex operators inAdS5×S5

We consider the leading large string tension correction to correlation functions of three vertex operators of particular massive string states in AdS_5 x S^5 string theory. We assume that two of these states are "heavy" carrying large spins (of order string tension) and thus can be treated semiclassically while the third state is "light" having fixed quantum numbers. We study several examples. In the case when the "heavy" states are described by a folded string with large spin in AdS_5 the 3-point function scales as a semiclassical spin parameter of the "heavy" state in power of the string level of the "light" massive string state. We observe similar behavior in the case of "heavy" states which admit a small angular momentum limit which may thus represent correlators of three quantum massive string states.

Mesons as Open Strings in a Holographic Dual of QCD

We study meson spectrum obtained from massive open string modes in a holographic dual of QCD constructed on the basis of a D4/D8-brane configuration in type IIA string theory. The spectrum includes mesons with spin higher than one. Taking into account the effect of curved background perturbatively, we obtain a mass formula for these mesons, which exhibits the linear Regge behavior at the leading order with subleading non-linear corrections. We argue that the string spectrum captures some features of the observed meson spectrum. For example, a_2(1320), b_1(1235), \pi(1300), a_0(1450), etc., are identified as the first excited massive open string states and \rho_3(1690), \pi_2(1670), etc., are identified as the second excited states.

Self-interacting fundamental strings and black holes

We study the size distribution of very massive close string states and the typical string configuration as one slowly increases the string coupling, both in the case of zero and of non-zero Neveu–Schwarz charges. The computations are performed rigorously in string theory, starting from quantities that are well-defined in the theory and therefore clarify previous works on the subject which were based on various approximation techniques. We find that, starting from a value of the coupling in agreement with the one predicted by the black hole correspondence principle, the string ensemble is dominated in any dimensions by compact states whose size is within the correspondent black hole horizon radius, which is of the order of the string scale at the black hole/string transition point.

String mass shifts

We study closed string one-loop amplitudes in string theory, in particular the average mass shift for states at given mass and Neveu–Schwarz charges. Our analysis is based only on well-defined string amplitudes and the exploitation of symmetries and unitarity properties of the torus amplitudes. We obtain the result Δ M 2 = − g s 2 M 2 + 3 − D 2 in D space–time dimensions for the average closed string mas-shift ( Δ M 2 = − g s 2 ( M 2 − Q 2 ) 1 + 3 − D 4 for states with non-zero Neveu–Schwarz charges Q). An interesting picture of one-loop corrections for the string in non-supersymmetric configurations comes out: the dominant interactions responsible for these corrections are of long-range type (namely, gravitational) and it appears that perturbations theory is generally reliable on the spectrum of massive string states.

Deep inelastic scattering from gauge string duality in the soft wall model

Deep inelastic structure functions have been calculated by Polchinski and Strassler in gauge/string duality introducing a hard infrared (IR) cut off in AdS space. Here we investigate this problem using a soft IR cut off that leads to linear Regge trajectories for mesons. We calculate the structure functions for scalar particles in the large x regime where supergravity approximation holds and the small x regime where massive string states contribute. We also propose a hybrid model to calculate structure functions for fermions in the supergravity approximation. In the deep inelastic limit our results are in agreement with those obtained using a hard cut off.

Non-perturbative gravity, the Hagedorn bounce and the cosmic microwave background

In Biswas et al (2006 J. Cosmol. Astropart. Phys. JCAP03(2006)009 [hep-th/0508194]) it wasshown how non-perturbative corrections to gravity can resolve the big bang singularity,leading to a bouncing universe. Depending on the scale of the non-perturbative corrections,the temperature at the bounce may be close to or higher than the Hagedorn temperature.If matter is made up of strings, then massive string states will be excited near the bounce,and the bounce will occur inside (or at the onset of) the Hagedorn phase forstring matter. As we discuss in this paper, in this case cosmological fluctuationscan be generated via the string gas mechanism recently proposed in Nayeri et al (2005 Preprint hep-th/0511140). In fact, the model discussed here demonstratesexplicitly that it is possible to realize the assumptions made in Nayeri et al (2005Preprint hep-th/0511140) in the context of a concrete set of dynamical backgroundequations. We also calculate the spectral tilt of thermodynamic stringy fluctuationsgenerated in the Hagedorn regime in this bouncing universe scenario. Generally wefind a scale-invariant spectrum with a red tilt which is very small but does notvanish.

Stability of strings binding heavy-quark mesons

We investigate the stability against small deformations of strings dangling into AdS_5-Schwarzschild from a moving heavy quark-anti-quark pair. We speculate that emission of massive string states may be an important part of the evolution of certain unstable configurations.

Scatterings of massive string states from D-brane and their linear relations at high energies

We study scatterings of bosonic massive closed string states at arbitrary mass levels from D-brane. We discover that all the scattering amplitudes can be expressed in terms of the generalized hypergeometric function F 2 3 with special arguments, which terminates to a finite sum and, as a result, the whole scattering amplitudes consistently reduce to the usual beta function. For the simple case of D-particle, we explicitly calculate high-energy limits of a series of the above scattering amplitudes for arbitrary mass levels, and derive infinite linear relations among them for each fixed mass level. The ratios of these high-energy scattering amplitudes are found to be consistent with the decoupling of high-energy zero-norm states of our previous works.

Decays of near BPS heterotic strings

The decay of highly excited massive string states in compactified heterotic string theories is discussed. We calculate the decay rate and spectrum of states carrying momentum and winding in the compactified direction. The longest lived states in the spectrum are near Bogomol'nyi-Prasad-Sommerfield (BPS) states whose decay is dominated by a single decay channel of massless radiation which brings the state closer to being BPS.