arXiv subjects

J. Santiago

Publications and source records attributed to J. Santiago.

At least 37 records · Page 2Linked to original sources

arXiv2004

A highly collimated, extremely high velocity outflow in Taurus

We present the first case of a highly collimated, extremely high velocity bipolar outflow in Taurus. It is powered by the low-luminosity (0.4 L_sun) source IRAS 04166+2706 and contains gas accelerated up to 50 km/s with respect to the ambient cloud both toward the blue and the red (uncorrected for projection). At the highest velocities, the outflow collimation factor exceeds 20, and the gas displays a very high degree of spatial symmetry. This very fast gas presents multiple maxima, and most likely arises from the acceleration of ambient material by a time-variable jet-like stellar wind. When scaled for luminosity, the outflow parameters of IRAS 04166 are comparable to those of other extremely high velocity outflows like L1448, indicating that even the very quiescent star-formation mode of Taurus can produce objects powering very high energy flows (L_mec/L_* > 0.15).

astro-ph↗

arXiv2004

Flavour in Intersecting Brane Models and Bounds on the String Scale

We study flavour issues in nonsupersymmetric intersecting brane models. Specifically, the purpose of the present paper is twofold: (i) to determine whether realistic flavour structures can be obtained in these models, and (ii) to establish whether the non-supersymmetric models address the gauge hierarchy problem. To this end, we find that realistic flavour structures, although absent at tree level, can arise even in the simplest models after effects of 4 fermion instanton--induced operators and radiative corrections have been taken into account. On the other hand, our analysis of flavour changing neutral currents (FCNC), electric dipole moments (EDM), supernova SN1987A and other constraints shows that the string scale has to be rather high, 10^4 TeV. This implies that non-supersymmetric intersecting brane models face a severe finetuning problem. Finally, we comment on how non--trivial flavour structures can arise in supersymmetric models.

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arXiv2004

Constraining the string scale: from Planck to Weak and back again

String and field theory ideas have greatly influenced each other since the so called second string revolution. We review this interrelation paying particular attention to its phenomenological implications. Our guiding principle is the radical shift in the way that we think about the fundamental scale, in particular the way in which string models have been able to accommodate values from the Planck $M_\mathrm{Pl}\sim 10^{18}$ GeV down to the electroweak scale $M_{EW}\sim $ TeV.

hep-ph↗

arXiv2004

L1521E: the first starless core with no molecular depletion

L1521E seems unique among starless cores. It stands out in a distribution of a ratio (R) that we define to asses core evolution, and which compares the emission of the easily-depleted C18O molecule with that of the hard to deplete, late-time species N2H+. While all cores we have studied so far have R ratio lower than 1, L1521E has an R value of 3.4, which is 8 times the mean of the other cores. To understand this difference, we have modeled the C18O and N2H+ abundance profiles in L1521E using a density distribution derived from 1.2mm continuum data. Our model shows that the C18O emission in this core is consistent with constant abundance, and this makes L1521E the first core with no C18O depletion. Our model also derives an unusually low N2H+ abundance. These two chemical peculiarities suggest that L1521E has contracted to its present density very recently, and it is therefore an extremely young starless core. Comparing our derived abundances with a chemical model, we estimate a tentative age of 1.5 x 10^5 yr, which is too short for ambipolar diffusion models.

astro-ph↗

arXiv2003

Physics of Brane Kinetic Terms

Models with extra dimensions may give new effects visible at future experiments. In these models, bulk fields can develop localized corrections to their kinetic terms which can modify the phenomenological predictions in a sizeable way. We review the case in which both gauge bosons and fermions propagate in the bulk, and discuss the limits on the parameter space arising from electroweak precision data.

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arXiv2003

Bulk fields with brane terms

In theories with branes, bulk fields get in general divergent corrections localized on these defects. Hence, the corresponding brane terms are renormalized and should be included in the effective theory from the very beginning. We review the phenomenology associated to brane kinetic terms for different spins and backgrounds, and point out that renormalization is required already at the classical level.

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arXiv2003

Flavour Changing Neutral Currents in Intersecting Brane Models

Intersecting D-brane models provide an attractive explanation of family replication in the context of string theory. We show, however, that the localization of fermion families at different brane intersections in the extra dimensions introduces flavour changing neutral currents mediated by the Kaluza-Klein excitations of the gauge fields. This is a generic feature in these models, and it implies stringent bounds on the mass of the lightest Kaluza-Klein modes (becoming severe when the compactification radii are larger than the string length). We present the full string calculation of four-fermion interactions in models with intersecting D-branes, recovering the field theory result. This reveals other stringy sources of flavour violation, which give bounds that are complementary to the KK bounds (i.e. they become severe when the compactification radii are comparable to the string length). Taken together these bounds imply that the string scale is larger than $M_s\gtrsim 10^2$ TeV, implying that non-supersymmetric cases are phenomenologically disfavoured.

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arXiv2003

Some consequences of brane kinetic terms for bulk fermions

In theories with extra dimensions there are generically brane kinetic terms for fields living in the bulk. These modify the masses and wave functions of the Kaluza-Klein expansions, and then the effective four dimensional gauge and Yukawa couplings of the corresponding modes. Here, we discuss some phenomenological consequences of fermion brane kinetic terms, emphasizing their implications for models with low compactification scales, whose agreement with experiment can be improved, and observable effects at collider energies, like the production of new fermions.

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arXiv2002

Low energy constraints on orbifold models

We review the low energy limits on Kaluza-Klein excitations in orbifold models. New vector-like quarks, as well as new Z' gauge bosons, can be accommodated with masses observable at large colliders.

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arXiv2002

Extra Dimensions, Isosinglet Charged Leptons and Neutrino Factories

Isosinglet fermions naturally arise in a variety of extensions of the Standard Model, in particular in models with extra dimensions. In this paper, we study the effect of the addition of a new isosinglet charged lepton to the standard spectrum, with special emphasis on implications for neutrino asymmetries to be measured at future neutrino factories. Lepton flavour violation in neutral current and lepton universality constraints are extensively discussed. We show that new physics effects in $ν_e-ν_μ$ CP asymmetries are significantly enhanced due to leptonic maximal mixings but still too small to give a signature at future neutrino factories. A signal for CP asymmetries in $ν_μ-ν_τ$ channel due to new physics could be observed at $1-3 σ$ if lepton flavour violating $τ$ decays are seen in a very close future in B-factories like BELLE experiment.

hep-ph↗

arXiv2002

Signals from extra dimensions decoupled from the compactification scale

Multilocalization provides a simple way of decoupling the mass scale of new physics from the compactification scale of extra dimensions. It naturally appears, for example, when localization of fermion zero modes is used to explain the observed fermion spectrum, leaving low energy remnants of the geometrical origin of the fermion mass hierarchy. We study the phenomenology of the simplest five dimensional model with order one Yukawa couplings reproducing the standard fermion masses and mixing angles and with a light Kaluza-Klein quark Q_{2/3} saturating experimental limits on V_{tb} and m_Q, and then with observable new effects at Tevatron.

hep-ph↗

arXiv2001

Improved Calculation of F2 in Electroproduction and xF3 in Neutrino Scattering to NNLO and Determination of alpha_s

We improve the existing calculations of deep inelastic scattering to next to next to leading order in the following manner. First, we use the recently calculated values of the anomalous dimensions for moments with index n=10,12 in ep scattering. Second, we use also recently calculated anomalous dimensions for n=1,3,...,13 for xF3 in neutrino N scattering, to extend the calculation to this process. Lastly, we use the determinantal constraints on the moments that follow from positivity of the structure functions to stabilize the fits. We find substantial improvement over previous results, getting the following numbers: Lambda(n_f=4, 3 loops)= 274 \pm 20 MeV, alpha_s^{3 loops}(M_Z^2)=0.1166 \pm 0.0013 and chi^2/dof= 92/(153-18) from ep and, from νN, Lambda(n_f=4, 3 loops)=255 \pm 72 MeV, alpha_s^{3 loops}(M_Z^2)=0.1153 \pm 0.0063 and chi^2/dof=0.335/(64-7). The errors take into account experimental errors and higher twist effects among other estimated theoretical errors. We can also extrapolate the fit, made originally for the range 3.5 < Q^2 < 230 GeV^2 to higher momenta, obtaining very good reproduction of experimental data up to Q^2=5000 GeV^2. From this, in particular, we can exclude supersymmetric partners (squarks or gluinos) for masses up to some 60 GeV. The fitted value of the coupling is now, including the data at high Q^2, alpha_s^{3 loops}(M_Z^2)=0.1163 \pm 0.0014.

hep-ph↗

arXiv2000

Large top mixing from extra dimensions

Fermion mixing is conveniently described using the effective Lagrangian formalism. We apply this approach to study top mixing in models with an infinite tower of Kaluza-Klein fermion excitations. In the Randall-Sundrum background with a boundary Higgs and phenomenologically viable values of the model parameters, the only effect eventually observable is the loss of universality of the top couplings. Their deviation from the SM predictions can be up to 4 % if the five-dimensional Yukawa couplings times the AdS_5 curvature scale are < 10.

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arXiv2000

Top mixing in effective theories

We review how top mixing with light quarks constrains new physics beyond the Standard Model using the effective Lagrangian approach.

hep-ph↗

arXiv2000

Universality limits on bulk fermions

The Kaluza-Klein fermion excitations induce mixing between the Standard Model fermions and loss of universality. The flavour mixing not present in the Standard Model can be made to vanish aligning the Yukawa couplings and the Dirac masses of the heavy modes, but universality is only recovered when these masses go to infinity. This implies a bound on the lightest new heavy quark, M1 \~ 3-5 TeV, which together with the electroweak precision data limits will allow the Large Hadron Collider to provide a crucial test of the Randall-Sundrum ansatz for solving the gauge hierarchy.

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arXiv2000

Observable contributions of new exotic quarks to quark mixing

Models with new vector-like quarks can produce observable quark mixing effects which are forbidden in the Standard Model. We classify all such models and write down the effective Lagrangian that results from integrating out the new quarks. We study the relations between neutral and charged currents and discuss how to distinguish among the different possibilities.

hep-ph↗

arXiv2000

Effective description of quark mixing

We use the effective Lagrangian formalism to describe quark mixing. The new W, Z and H couplings generalizing the CKM matrix and the GIM mechanism fulfil relations and inequalities which allow to discriminate among different SM extensions. As a by-product we give a useful parametrization of the generalized CKM matrix. We also show that the largest possible departures from the SM predictions result from heavy exotic fermions, which can induce, for example, top FCNC large enough to be observable at future colliders.

hep-ph↗

arXiv2000

A Three three-brane Universe:New Phenomenology for the New Millennium?

We consider an extension of the Randall-Sundrum model with three parallel 3-branes in a 5-dimensional spacetime. This new construction, apart from providing a solution to the Planck hierarchy problem, has the advantage that the SM fields are confined on a positive tension brane. The study of the phenomenology of this model reveals an anomalous first KK state which is generally much lighter than the remaining tower and also much more strongly coupled to matter. Bounds on the parameter space of the model can be placed by comparison of specific processes with the SM background as well as by the latest Cavendish experiments. The model suggests a further exotic possibility if one drops the requirement of solving the hierarchy problem. In this case gravity may result from the exchange of the ordinary graviton plus an ultralight KK state and modifications of gravity may occur at both small and extremely large scales.

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