Search

We study meson decays using instanton methods in two string models. The first model is the old string model in flat space which combines strings and massive particles and the second is the holographic, Sakai-Sugimoto model. Using the the old string model, we reproduce the QCD formula for the probability of splitting of the QCD flux tube derived by Casher-Neuberger-Nussinov (CNN). In the holographic model we construct a string worldsheet instanton which interpolates between a single and double string configuration, which determines the decay from one to two dual mesonic particles. The resulting probability for meson decay incorporates both the effects of finite meson size as well as back-reaction of the produced quarks on the QCD flux tube. In the limit of very large strings the probability for a split reduces to the CNN formula. A byproduct of our analysis is the analysis of the moduli space of a generic double concentric Wilson loop with circles which are separated in the holographic direction of the confining background., 26 pages, 17 figures

Holographic QCD at finite baryon number density and zero temperature is studied within the five-dimensional Sakai-Sugimoto model. We introduce a new approximation that models a smeared crystal of solitonic baryons by assuming spatial homogeneity to obtain an effective kink theory in the holographic direction. The kink theory correctly reproduces a first order phase transition to lightly bound nuclear matter. As the density is further increased the kink splits into a pair of half-kink constituents, providing a concrete realization of the previously suggested dyonic salt phase, where the bulk soliton splits into constituents at high density. The kink model also captures the phenomenon of baryonic popcorn, in which a first order phase transition generates an additional soliton layer in the holographic direction. We find that this popcorn transition takes place at a density below the dyonic salt phase, making the latter energetically unfavourable. However, the kink model predicts only one pop, rather than the sequence of pops suggested by previous approximations. In the kink model the two layers produced by the single pop form the surface of a soliton bag that increases in size as the baryon chemical potential is increased. The interior of the bag is filled with abelian electric potential and the instanton charge density is localized on the surface of the bag. The soliton bag may provide a holographic description of a quarkyonic phase., 18 pages, 5 figures

We investigate the effect of a magnetic field on the vacuum of low-temperature QCD at large-N_c in the presence of a chiral chemical potential, using the holographic Sakai-Sugimoto model. Above some critical chemical potential we find an instability, which triggers a decay of the homogeneous vacuum to a non-homogeneous configuration with a spiral form, which we construct explicitly. We find that this decay is suppressed for sufficiently large magnetic field, and determine the critical strength. We find that the new vacuum does not exhibit the chiral magnetic effect. This is related to the way the chiral chemical potential is introduced. We discuss an alternative way of introducing the chiral chemical potential that leads to a nonzero chiral magnetic effect., 40 pages; v2: references added, minor typos corrected

We investigate N=2 super-Yang-Mills theory on a three sphere in the presence of an isospin chemical potential at strong coupling using the AdS/CFT correspondence. This system exhibits an instability for sufficiently large values of the chemical potential. In contrast to other related models, the first excitation to condense is not a vector meson but rather a scalar charged under the global SO(4) symmetry group. Furthermore, the spectrum of fluctuations exhibits an interesting cross-over behaviour as a function of the dimensionless temperature. We construct the new ground state of the non-linear theory., Comment: 29 pages

We study the instability of the low-temperature QCD vacuum at large-N_c due to an axial chemical potential, using the holographic Sakai-Sugimoto model. We explicitly construct the ground state of the theory, which corresponds to a translationally non-invariant configuration., Comment: 23 pages, 12 figures; v2: references added

We analyse the effect of velocity and acceleration on the temperature felt by particles and strings in backgrounds relevant in holographic models. First, we compare accelerated strings and strings at finite temperature. We find that for fixed Unruh temperature felt by the string endpoints, the screening length is smaller for the accelerated Wilson loop than for the static one in a thermal background of the same temperature; hence acceleration provides a "more efficient" mechanism for melting of mesons. Secondly, we show that the velocity-dependence of the screening length of the colour force, previously obtained from a moving Wilson loop in a finite temperature background, is not specific for the string, but is a consequence of the generic fact that an observer which moves with constant velocity in a black hole background measures a velocity-dependent temperature. Finally, we analyse accelerated particles and strings in the AdS black hole background, and show that these feel a temperature which increases as a function of time. As a byproduct of our analysis we find a global Minkowski embedding for the planar AdS black hole., Comment: 27 pages, 9 figures; v2: references added; v3: various points clarified, published version

We analyze the effect of an isospin chemical potential \mu_I in the Sakai-Sugimoto model, which is the string dual of a confining gauge theory related to large N_c QCD, at temperatures below the chiral symmetry restoration temperature. For small chemical potentials we show that the results agree with expectations from the low-energy chiral Lagrangian, and the charged pion condenses. When the chemical potential reaches a critical value \mu_I = \mu_{crit} ~ 1.7 m_{\rho}, the lowest vector meson (the "rho meson") becomes massless, and it condenses (in addition to the pion condensate) for \mu_I > \mu_{crit}. This spontaneously breaks the rotational symmetry, as well as a residual U(1) flavor symmetry. We numerically construct the resulting new ground state for \mu_I > \mu_{crit}., Comment: 33 pages, 7 figures; v2: references added, minor corrections; v3: added footnote about other possible condensate

Ideas about a duality between gauge fields and strings have been around for many decades. During the last ten years, these ideas have taken a much more concrete mathematical form. String descriptions of the strongly coupled dynamics of semi-realistic gauge theories, exhibiting confinement and chiral symmetry breaking, are now available. These provide remarkably simple ways to compute properties of the strongly coupled quark-gluon fluid phase, and also shed new light on various phenomenological models of hadron fragmentation. We present a review and highlight some exciting recent developments., Comment: Lectures presented by KP at the 45th Winter School on Theoretical Physics, February 2007, Schladming, Austria; v3: minor corrections, typos fixed, references added

We show that mesons, described using rotating relativistic strings in a holographic setup, undergo dissociation when their acceleration 'a' exceeds a value which scales with the angular momentum 'J' as a_max ~ \sqrt{T_s/J}, where 'T_s' is the string tension., Comment: 6 pages, 3 figures

We analyse mesons at finite temperature in a chiral, confining string dual. The temperature dependence of low-spin as well as high-spin meson masses is shown to exhibit a pattern familiar from the lattice. Furthermore, we find the dissociation temperature of mesons as a function of their spin, showing that at a fixed quark mass, mesons with larger spins dissociate at lower temperatures. The Goldstone bosons associated with chiral symmetry breaking are shown to disappear above the chiral symmetry restoration temperature. Finally, we show that holographic consideration imply that large-spin mesons do not experience drag effects when moving through the quark gluon plasma. They do, however, have a maximum velocity for fixed spin, beyond which they dissociate., Comment: 25 pages, 23 figures; v2: emphasised dependence of interquark distance on velocity, added plot, added references; v3: error in section V corrected, conclusions unchanged

We study the decay process of large-spin mesons in the context of the gauge/string duality, using generic properties of confining backgrounds and systems with flavour branes. In the string picture, meson decay corresponds to the quantum-mechanical process in which a string rotating on the IR "wall" fluctuates, touches a flavour brane and splits into two smaller strings. This process automatically encodes flavour conservation as well as the Zweig rule. We show that the decay width computed in the string picture is in remarkable agreement with the decay width obtained using the phenomenological Lund model., Comment: 40 pages, 16 figures; v2: some clarifications added, typos corrected; v3: references added