7 research outputs found
Neutron electric dipole moment from gauge/string duality
We compute the electric dipole moment of nucleons in the large QCD
model by Witten, Sakai and Sugimoto with degenerate massive flavors.
Baryons in the model are instantonic solitons of an effective five-dimensional
action describing the whole tower of mesonic fields. We find that the dipole
electromagnetic form factor of the nucleons, induced by a finite topological
angle, exhibits complete vector meson dominance. We are able to
evaluate the contribution of each vector meson to the final result - a small
number of modes are relevant to obtain an accurate estimate. Extrapolating the
model parameters to real QCD data, the neutron electric dipole moment is
evaluated to be . The
electric dipole moment of the proton is exactly the opposite.Comment: Latex, 4 pages; v2: minor corrections, few comments adde
Theta dependence in holographic QCD
We study the effects of the CP-breaking topological -term in the
large QCD model by Witten, Sakai and Sugimoto with degenerate light
flavors. We first compute the ground state energy density, the topological
susceptibility and the masses of the lowest lying mesons, finding agreement
with expectations from the QCD chiral effective action. Then, focusing on the
case, we consider the baryonic sector and determine, to leading order
in the small regime, the related holographic instantonic soliton
solutions. We find that while the baryon spectrum does not receive corrections, this is not the case for observables like the
electromagnetic form factor of the nucleons. In particular, it exhibits a
dipole term, which turns out to be vector-meson dominated. The resulting
neutron electric dipole moment, which is exactly the opposite as that of the
proton, is of the same order of magnitude of previous estimates in the
literature. Finally, we compute the CP-violating pion-nucleon coupling constant
, finding that it is zero to leading order in the large
limit.Comment: Latex, 61 pages, 5 figures; v2: Section 8.5 with a novel analysis of
the electric dipole form factor has been added; further comments and
references added; typos correcte
On zero-point energy, stability and Hagedorn behavior of Type IIB strings on pp-waves
Type IIB strings on many pp-wave backgrounds, supported either by 5-form or
3-form fluxes, have negative light-cone zero-point energy. This raises the
question of their stability and poses possible problems in the definition of
their thermodynamic properties. After having pointed out the correct way of
calculating the zero-point energy, an issue not fully discussed in literature,
we show that these Type IIB strings are classically stable and have well
defined thermal properties, exhibiting a Hagedorn behavior.Comment: Latex, 13 pages. v2: regularization/renormalization prescription
clarified, refs. adde
Non-critical holography and four-dimensional CFT's with fundamentals
We find non-critical string backgrounds in five and eight dimensions,
holographically related to four-dimensional conformal field theories with N=0
and N=1 supersymmetries. In the five-dimensional case we find an AdS_5
background metric for a string model related to non-supersymmetric, conformal
QCD with large number of colors and flavors and discuss the conjectured
existence of a conformal window from the point of view of our solution. In the
eight-dimensional string theory, we build a family of solutions of the form
AdS_5 x \tilde{S}^3 with \tilde{S}^3 a squashed three-sphere. For a special
value of the ratio N_f/N_c, the background can be interpreted as the
supersymmetric near-horizon limit of a system of color and flavor branes on
R^{1,3} times a known four-dimensional generalization of the cigar. The N=1
dual theory with fundamental matter should have an IR fixed point only for a
fixed ratio N_f/N_c. General features of the string/gauge theory correspondence
for theories with fundamental flavors are also addressed.Comment: 38 pages, 3 figures; JHEP class. Minor corrections, references adde