Claudio Coriano

STABLE SUPERSTRING RELICS

We investigate the cosmological constraints on exotic stable matter states which arise in realistic free fermionic superstring models. These states appear in the superstring models due to a “Wilsonline” breaking of the unifying non-abelian gauge symmetry. In the models that we consider the unifying SO(10) gauge symmetry is broken at the string level to SO(6) x SO(4), SU(5) x U(1) or SU(3) x SU(2) x U(1)2. The exotic matter states are classified according to the patterns of the SO(10) symmetry breaking. In SO(6) x SO(4) and SU(5) x U(1) type models one obtains fractionally charged states with Qe.m. = ±12. In SU (3) x SU (2) x U(1)2 type models one also obtains states with the regular charges under the Standard Model gauge group but with “fractional” charges under the U(1)z′ symmetry. These states include down-like color triplets and electroweak doublets, as well as states which are Standard Model singlets. By analyzing the renormalizable and nonrenormalizable terms of the superpotential in a specific superstring model, we show that these exotic states can be stable. We investigate the cosmological constraints on the masses and relic density of the exotic states. We propose that, while the abundance and the masses of the fractionally charged states are highly constrained, the Standard Model-like states, and in particular the Standard Model singlet, are good dark matter candidates.

Stable superstring relics and ultrahigh-energy cosmic rays

One of the most intriguing experimental results of recent years is the observation of Ultrahigh Energy Cosmic Rays (UHECRs) above the GZK cutoff. Plausible candidates for the UHECR primaries are the decay products of a meta–stable matter state with mass of order O(10 12−15 GeV), which simultaneously is a good cold dark matter candidate. We study possible meta–stable matter states that arise from Wilson line breaking of GUT symmetries in semirealistic heterotic string models. In the models that we study the exotic matter states can be classified according to patterns of SO(10) symmetry breaking. We show that cryptons, which are states that carry fractional electric charge ±1/2, and are confined by a hidden gauge group cannot produce viable dark matter. This is due to the fact that, in addition to the lightest neutral bound state, cryptons give rise to meta–stable charged bound states. However, these states may still account for the UHECR events. We argue that the uniton, which is

Conformal anomalies and the gravitational effective action: The TJJ correlator for a Dirac fermion

We compute in linearized gravity all the contributions to the gravitational effective action due to a virtual Dirac fermion, related to the conformal anomaly. This requires, in perturbation theory, the identification of the gauge-gauge-graviton vertex off mass shell, involving the correlator of the energy-momentum tensor and two vector currents (TJJ), which is responsible for the generation of the gauge contributions to the conformal anomaly in gravity. We also present the anomalous effective action expanded in the inverse mass of the fermion as in the Euler-Heisenberg case.

Windows over a New Low Energy Axion

Abstract We outline some general features of possible extensions of the Standard Model that include anomalous U ( 1 ) gauge symmetries, a certain number of axions and their mixings with the CP-odd Higgs sector. As previously shown, after the mixing one of the axions becomes a physical pseudoscalar (the axi-Higgs) that can take the role of a modified QCD axion. It can be driven to be very light by the same non-perturbative effects that are held responsible for the solution of the strong CP-problem. At the same time the axi-Higgs has a sizeable gauge interaction, which is not allowed to the Peccei–Quinn axion, possibly explaining the PVLAS results. We point out that the Wess–Zumino term, typical of these models, can be both interpreted as an anomaly inflow from higher dimensional theories (second window) but also as a result of partial decoupling of an extra Higgs sector (and of a fermion) that leaves behind an effective anomalous Abelian theory (first window) in a broken Stuckelberg phase. The possibility that the axi-Higgs can be heavy, of the order of the Higgs mass or larger, however, cannot be excluded. The potentialities for the discovery of this particle and of anomaly effects in the neutral current sector at the LHC are briefly discussed in the context of a superstring inspired model (second window), but with results that remain valid also if any of the two possibilities is realized in Nature.

Trilinear anomalous gauge interactions from intersecting branes and the neutral currents sector

We present a study of the trilinear gauge interactions in extensions of the Standard Model (SM) with several anomalous extra U(1)s, identified in various constructions, from special vacua of string theory to large extra dimensions. In these models an axion and generalized Chern-Simons interactions for anomalies cancellation are present. We derive generalized Ward identities for these vertices and discuss their structure in the Stuckelberg and Higgs-Stuckelberg phases. We give their explicit expressions in all the relevant cases, which can be used for explicit phenomenological studies of these models at the LHC.

Stückelberg axions and the effective action of anomalous Abelian models. A SU(3)C×SU(2)W×U(1)Y×U(1)B model and its signature at the LHC

We elaborate on an extension of the Standard Model with a gauge structure enlarged by a single anomalous U(1)U(1), where the presence of a Wess–Zumino term is motivated by the Green–Schwarz mechanism of string theory. The additional gauge interaction is anomalous and requires an axion for anomaly cancelation. The pseudoscalar implements the Stuckelberg mechanism and undergoes mixing with the standard Higgs sector to render the additional U(1)U(1) massive. We consider a 2-Higgs doublet model. We show that the anomalous effective vertices involving neutral currents are potentially observable. We clarify their role in the case of simple processes such as Z∗→γγZ∗→γγ, which are at variance with respect to the Standard Model. A brief discussion of the implications of these studies for the LHC is included.

Graviton vertices and the mapping of anomalous correlators to momentum space for a general conformal field theory

A bstractWe investigate the mapping of conformal correlators and of their anomalies from configuration to momentum space for general dimensions, focusing on the anomalous correlators TOO, TVV — involving the energy-momentum tensor (T) with a vector (V) or a scalar operator (O) — and the 3-graviton vertex TTT. We compute the TOO, TVV and TTT one-loop vertex functions in dimensional regularization for free field theories involving conformal scalar, fermion and vector fields. Since there are only one or two independent tensor structures solving all the conformal Ward identities for the TOO or TVV vertex functions respectively, and three independent tensor structures for the TTT vertex, and the coefficients of these tensors are known for free fields, it is possible to identify the corresponding tensors in momentum space from the computation of the correlators for free fields. This works in general d dimensions for TOO and TVV correlators, but only in 4 dimensions for TTT, since vector fields are conformal only in d = 4. In this way the general solution of the Ward identities including anomalous ones for these correlators in (Euclidean) position space, found by Osborn and Petkou is mapped to the ordinary diagrammatic one in momentum space. We give simplified expressions of all these correlators in configuration space which are explicitly Fourier integrable and provide a diagrammatic interpretation of all the contact terms arising when two or more of the points coincide. We discuss how the anomalies arise in each approach. We then outline a general algorithm for mapping correlators from position to momentum space, and illustrate its application in the case of the VVV and TOO vertices. The method implements an intermediate regularization — similar to differential regularization — for the identification of the integrands in momentum space, and one extra regulator. The relation between the ordinary Feynman expansion and the logarithmic one generated by this approach are briefly discussed.

An anomalous extra Z prime from intersecting branes with Drell–Yan and direct photons at the LHC

Abstract We quantify the impact of gauge anomalies at the Large Hadron Collider by studying the invariant mass distributions in Drell–Yan and in double prompt photon, using an extension of the Standard Model characterized by an additional anomalous U ( 1 ) derived from intersecting branes. The approach is rather general and applies to any anomalous Abelian gauge current. Anomalies are cancelled using either the Wess–Zumino mechanism with suitable Peccei–Quinn-like interactions and a Stuckelberg axion, or by the Green–Schwarz mechanism. We compare predictions for the corresponding extra Z prime to anomaly-free realizations such as those involving U ( 1 ) B − L . We identify the leading anomalous corrections to both channels, which contribute at higher orders, and compare them against the next-to-next-to-leading order (NNLO) QCD background. Anomalous effects in these inclusive observables are found to be small, below the size of the typical QCD corrections quantified by NNLO K -factors. Measurements at the LHC on the Z resonance in leptoproduction will be able to exclude a class of these models for variations of the cross section at the level of 4%, which is obtained at larger values of the anomalous coupling ( g B > 1 ). However, If they are not excluded, their anomalous nature is unlikely to be resolved with an inclusive NNLO analysis performed in these key processes.

Precision studies of the NNLO DGLAP evolution at the LHC with Candia

Abstract We summarize the theoretical approach to the solution of the NNLO DGLAP equations using methods based on the logarithmic expansions in x-space and their implementation into the C program Candia 1.0 . 1 We present the various options implemented in the program and discuss the different solutions. The user can choose the order of the evolution, the type of the solution, which can be either exact or truncated, and the evolution either with a fixed or a varying flavor number, implemented in the varying-flavor-number scheme (VFNS). The renormalization and factorization scale dependencies are treated separately. In the non-singlet sector the program implements an exact NNLO solution. Program summary Program title: CANDIA Catalogue identifier: AEBK_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEBK_v1_0.html Program obtainable from: CPC Program Library, Queens University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 101 376 No. of bytes in distributed program, including test data, etc.: 5 865 234 Distribution format: tar.gz Programming language: C and Fortran Computer: All Operating system: Linux RAM: In the given examples, it ranges from 4 to 490 MB Classification: 11.1, 11.5 Nature of problem: The program provided here solves the DGLAP evolution equations for the parton distribution functions up to NNLO. Solution method: The algorithm implemented is based on the theory of the logarithmic expansions in Bjorken x-space. Additional comments: To be sure of getting the latest version of the program, the authors suggest downloading the code from their official CANDIA website ( http://www.le.infn.it/candia ). Running time: In the given examples, it ranges from 1 to 40 minutes. The jobs have been executed on an Intel Core 2 Duo T7250 CPU at 2 GHz with a 64 bit Linux kernel. The test run script included in the package contains 5 sample runs and may take a number of hours to process, depending on the speed of the processor used and the size of the available RAM.

Vacuum stability in U(1)-prime extensions of the Standard Model with TeV scale right handed neutrinos

Abstract We investigate a minimal U ( 1 ) ′ extension of the Standard Model with one extra complex scalar and generic gauge charge assignments. We use a type-I seesaw mechanism with three heavy right handed neutrinos to illustrate the constraints on the charges, on their mass and on the mixing angle of the two scalars, derived by requiring the vacuum stability of the scalar potential. We focus our study on a scenario which could be accessible at the LHC, by selecting a vacuum expectation value of the extra Higgs in the TeV range and determining the constraints that emerge in the parameter space. To illustrate the generality of the approach, specific gauge choices corresponding to U ( 1 ) B − L , U ( 1 ) R and U ( 1 ) χ are separately analyzed. Our results are based on a modified expression of one of the β functions of the quartic couplings of the scalar potential compared to the previous literature. This is due to a change in the coefficient of the Yukawa term of the right handed neutrinos. Differently from previous analysis, we show that this coupling may destabilize the vacuum.