arXiv subjects

Joerg Jaeckel

Publications and source records attributed to Joerg Jaeckel.

155 records · Page 9Linked to original sources

arXiv2006

Telltale Traces of U(1) Fields in Noncommutative Standard Model Extensions

Restrictions imposed by gauge invariance in noncommutative spaces together with the effects of ultraviolet/infrared mixing lead to strong constraints on possible candidates for a noncommutative extension of the Standard Model. In this paper, we study a general class of 4-dimensional noncommutative models consistent with these restrictions. Specifically we consider models based upon a gauge theory with the gauge group U(N_1)\times U(N_2) \times ... \times U(N_m) coupled to matter fields transforming in the (anti)-fundamental, bi-fundamental and adjoint representations. Noncommutativity is introduced using the Weyl-Moyal star-product approach on a continuous space-time. We pay particular attention to overall trace-U(1) factors of the gauge group which are affected by the ultraviolet/infrared mixing. We show that, in general, these trace-U(1) gauge fields do not decouple sufficiently fast in the infrared, and lead to sizable Lorentz symmetry violating effects in the low-energy effective theory. Making these effects unobservable in the class of models we consider would require pushing the constraint on the noncommutativity mass scale far beyond the Planck mass (M_{NC}\gtrsim 10^{100} M_{P}) and severely limits the phenomenological prospects of such models.

hep-ph↗

arXiv2006

Chiral phase structure of QCD with many flavors

We investigate QCD with a large number of massless flavors with the aid of renormalization group flow equations. We determine the critical number of flavors separating the phases with and without chiral symmetry breaking in SU(Nc) gauge theory with many fermion flavors. Our analysis includes all possible fermionic interaction channels in the pointlike four-fermion limit. Constraints from gauge invariance are resolved explicitly and regulator-scheme dependencies are studied. Our findings confirm the existence of an Nf window where the system is asymptotically free in the ultraviolet, but remains massless and chirally invariant on all scales, approaching a conformal fixed point in the infrared. Our prediction for the critical number of flavors of the zero-temperature chiral phase transition in SU(3) is Nf^{cr}=10.0\pm 0.29(fermion)[+1.55;-0.63](gluon), with the errors arising from approximations in the fermionic and gluonic sectors, respectively.

hep-ph↗

arXiv2005

Noncommutativity, Extra Dimensions, and Power Law Running in the Infrared

We investigate the running gauge couplings of U(N) noncommutative gauge theories with compact extra dimensions. Power law running of the trace-U(1) gauge coupling in the ultraviolet is communicated to the infrared by ultraviolet/infrared mixing, whereas the SU(N) factors run exactly as in the commutative theory. This results in theories where the experimentally excluded trace-U(1) factors decouple with a power law running of the momentum in the extreme infrared, effectively hiding them from detection.

hep-ph↗

arXiv2005

Testing Dark Energy and Light Particles via Black Hole Evaporation at Colliders

We show that collider experiments have the potential to exclude a light scalar field as well as generic models of modified gravity as dark energy candidates. Our mechanism uses the spectrum radiated by black holes and can equally well be applied to determine the number of light degrees of freedom. We obtain the grey body factors for massive scalar particles and calculate the total emissivity. While the Large Hadron Collider (LHC) may not get to the desired accuracy, the measurement is within reach of next generation colliders.

astro-ph↗

arXiv2004

Renormalization flow of QED

We investigate textbook QED in the framework of the exact renormalization group. In the strong-coupling region, we study the influence of fluctuation-induced photonic and fermionic self-interactions on the nonperturbative running of the gauge coupling. Our findings confirm the triviality hypothesis of complete charge screening if the ultraviolet cutoff is sent to infinity. Though the Landau pole does not belong to the physical coupling domain owing to spontaneous chiral symmetry breaking (chiSB), the theory predicts a scale of maximal UV extension of the same order as the Landau pole scale. In addition, we verify that the chiSB phase of the theory which is characterized by a light fermion and a Goldstone boson also has a trivial Yukawa coupling.

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arXiv2003

Towards a renormalizable standard model without fundamental Higgs scalar

We investigate the possibility of constructing a renormalizable standard model with purely fermionic matter content. The Higgs scalar is replaced by point-like fermionic self-interactions with couplings growing large at the Fermi scale. An analysis of the UV behavior in the point-like approximation reveals a variety of non-Gaussian fixed points for the fermion couplings. If real, such fixed points would imply nonperturbative renormalizability and evade triviality of the Higgs sector. For point-like fermionic self-interactions and weak gauge couplings, one encounters a hierarchy problem similar to the one for a fundamental Higgs scalar.

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arXiv2003

Effective Actions for Strongly Interacting Fermionic Systems

We compare different non-perturbative methods for calculating the effective action for fermionic systems featuring bosonic bound states (BBS) and spontaneous symmetry breaking (SSB). In a purely fermionic language proceeding into the SSB phase requires techniques beyond perturbation theory and renormalization group equations. Improvement comes from a description with BBS fields and elementary fields treated on equal footing. Yet, ``partial bosonization'' introduces an arbitrariness as the choice for the composite fields is usually not completely determined by the classical action. Results of approximate calculations, e.g. mean field theory, may depend strongly on this choice, thus limiting their quantitative reliability. Using the Nambu--Jona-Lasinio model as an example we demonstrate how this dependence can be reduced, sometimes even be eliminated by suitably chosen approximations. Schwinger-Dyson equations (SDE) allow for a description of SSB without auxiliary fields. The 2PI effective action enables us to compare different solutions of the SDE and find the stable one. We apply this method to a six-fermion interaction resembling the three-flavor instanton interaction in QCD. We find a first order chiral phase transition but no stable phase with broken color symmetry. The existence of an elementary scalar boson in the Standard Model -- the Higgs -- raises several questions. The smallness of its mass compared to some fundamental scale ($\sim M_{\textrm{GUT}}$) requires an extreme amount of fine-tuning. Moreover, its $ϕ^4$-potential may not be renormalizable in a strict sense. In view of this we discuss the possibility of a Higgs as BBS of fermions.

hep-ph↗

arXiv2003

First Order Chiral Phase Transition from a Six-fermion ''Instanton''-Interaction

We compute the first order chiral phase transition for an instanton motivated quark model with a local six-quark interaction. In order to compare different solutions of the gap equation we compute the bosonic effective action -- a two particle irreducible free energy functional. We find that the first order transition ends for a critical current quark mass, with continuous crossover for larger quark masses. Furthermore, we investigate different possible order parameters, including a color octet condensate. We also compare our formalism with mean field theory.

hep-ph↗

arXiv2003

Flow Equations without Mean Field Ambiguity

We compare different methods used for non-perturbative calculations in strongly interacting fermionic systems. Mean field theory often shows a basic ambiguity related to the possibility to perform Fierz transformations. The results may then depend strongly on an unphysical parameter which reflects the choice of the mean field, thus limiting the reliability. This ambiguity is absent for Schwinger-Dyson equations or fermionic renormalization group equations. Also renormalization group equations in a partially bosonized setting can overcome the Fierz ambiguity if the truncation is chosen appropriately. This is reassuring since the partially bosonized renormalization group approach constitutes a very promising basis for the explicit treatment of condensates and spontaneous symmetry breaking even for situations where the bosonic correlation length is large.

hep-ph↗

arXiv2002

Understanding the Fierz Ambiguity of Partially Bosonized Theories

A useful tool in non perturbative studies of fermionic theories is partial bosonization. However, partial bosonization is often connected to an ambiguity due to Fierz rearrangement in the original theory. We discuss two different approximations for the calculation of the effective action $Γ$ with respect to a spurious dependence on the choice of Fierz transformation: Mean field theory and the truncated flow of an exact renormalization group equation for the effective average action.

hep-ph↗

arXiv2002

Loop Corrections to Scalar Quintessence Potentials

The stability of scalar quintessence potentials under quantum fluctuations is investigated both for uncoupled models and models with a coupling to fermions. We find that uncoupled models are usually stable in the late universe. However, a coupling to fermions is severely restricted. We check whether a graviton induced fermion-quintessence coupling is compatible with this restriction.

astro-ph↗