2,162 research outputs found
Lattice-Renormalization of the Polyakov Loop
No full textZantow F. Lattice-Renormalization of the Polyakov Loop. Bielefeld: Fakultät für Physik; 2003
GL(1) charged states in twistor string theory
No full textWe discuss the appearance of the GL(1) charged physical operators in the twistor string theory. These operators are shown to be BRST-invariant and non-trivial and some of their correlators and conformal β-functions are computed. Remarkably, the non-conservation of the GL(1) charge in interactions involving these operators, is related to the anomalous term in the Kac-Moody current algebra. While these operators play no role in the maximum helicity violating (MHV) amplitudes, they are shown to contribute non-trivially to the non-MHV correlators in the presence of the worldsheet instantons. We argue that these operators describe the non-perturbative dynamics of solitons in conformal supergravity. The exact form of such solitonic solutions is yet to be determined. © 2005 Published by Elsevier B.V.Azcarraga J., 1989, PHYS REV LETT, V3, P2443; Berkovits N., 2004, JHEP, V0404; Berkovits N., 2004, JHEP, V0408, P009; CACHAZO F, 2004, JHEP, V410; Gubser SS, 1998, PHYS LETT B, V428, P105, DOI 10.1016-S0370-2693(98)00377-3; Maldacena J M, 1998, ADV THEOR MATH PHYS, V2, P231, DOI DOI 10.1023-A:1026654312961; POLYAKOV D, HEPTH0406079; POLYAKOV D, IN PRESS INT J MOD P; Witten E., 1998, ADV THEOR MATH PHYS, V2, P253, DOI DOI 10.1016-S0370-1573(99)00083-6; Witten E., HEPTH9112056; Witten E, 2004, COMMUN MATH PHYS, V252, P189, DOI 10.1007-s00220-004-1187-344
Improved hadronic measurements and spectral sums on the lattice
No full textIn this thesis we address several topics of lattice QCD.
As a first project we perform calculations of ground and excited states of light mesons. Especially the extraction of the excited states turns out be very difficult. Therefore we utilize the variational method, which requires the construction of a rich basis of different interpolating fields, i.e., lattice discretized operators which have the same quantum number as the desired states. To build such a basis we consider quarks with different spatial wavefunctions, including some that mimic orbital excitations.
In the second project, we develop a new improvement scheme to compute estimates of all-to-all quark propagators, i.e., quark propagators which connect each point on the lattice to all others. For that purpose we decompose the lattice in two disjoint regions or domains which allows to significantly reduce the amount of random variables used in the estimation process. As a first major application of this improvement scheme, we compute the spectrum of heavy-light hadrons, i.e., hadrons containing one very heavy quark (bottom) and one or more light quarks (up, down, strange). To reduce the computational costs for the heavy quark, we describe it by means of the lowest order of heavy quark effective theory and thus treat it as infinitely heavy. From our calculations we are able to extract several ground and excited meson states and even a number of baryon ground states.
In the last project, we study two very important features of QCD: Confinement and spontaneous breaking of chiral symmetry. Both of them are temperature dependent: As the temperature is increased above a critical value, the theory becomes deconfined and chiral symmetry is restored. The temperature, at which these phase transitions take place, is approximately the same at least for zero baryon density. To study a possible connection between these phenomena, we try to relate the order parameters of the phase transitions.
In pure Yang-Mills theory the order parameter for the confinement-deconfinement transition is the Polyakov loop. The order parameter for the spontaneous breaking of chiral symmetry is given by the chiral condensate, which is related to the eigenvalues of the Dirac operator. It has been shown that also the Polyakov loop can be expressed in terms of these eigenvalues, as a spectral sum. In a first step we perform a numerical study of these sums. We find that the ordinary Polyakov loop is governed by the ultraviolet modes of the Dirac operator. Therefore, in a second step we define a new order parameter for confinement, the "dressed Polyakov loop". This quantity has two advantages: It can be expressed as a spectral sum which has a better convergence behavior (infrared dominated) and it is closely related to the quark condensate via a Fourier transformation. Also the spectral sums for the dressed Polyakov loop are investigated numerically
Polyakov loop correlations in Landau gauge and the heavy quark potential
No full textAttig N, Karsch F, Petersson B, Satz H, Wolff M. Polyakov loop correlations in Landau gauge and the heavy quark potential. Physics Letters, B. 1988;209(1):65-70.We calculate Polyakov loop correlation functions in SU(3) gauge theory on a 12 3 × 4 lattice. We determine from this the colour average heavy quark potential and compare it with the corresponding colour singlet potential in Landau gauge. A comparison with finite temperature perturbation theory shows that perturbative relations are at best recovered for very high temperatures
Heavy quark free energies, potentials and the renormalized Polyakov loop
No full textKaczmarek O, Karsch F, Petreczky P, Zantow F. Heavy quark free energies, potentials and the renormalized Polyakov loop. In: Nuclear Physics B - Proceedings Supplements. Nucl.Phys.Proc.Suppl. Vol 129. Elsevier BV; 2004: 560-562.We discuss the renormalized free energy of a heavy quark anti-quark pair in the color singlet channel for quenched and full QCD at finite temperature. The temperature and mass dependence, as well as its short distance behavior is analyzed. Using the free energies we calculate the heavy quark potential and entropy in quenched QCD. The asymptotic large distance behavior of the free energy is used to define the non-perturbatively renormalized Polyakov loop which is well behaved in the continuum limit. String breaking is studied in the color singlet channel in 2-flavor QCD
Geração radiativa de repulsão vetorial para quarks leves
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas, Programa de Pós-Graduação em Física, Florianópolis, 2014Abstract: We apply a non-perturbative analytical method, known as the Optimized PerturbationTheory (OPT), to the Polyakov-Nambu-Jona-Lasinio (PNJL) model in order to investigate physical quantities associated with the QCD phase transitions. We consider the Taylor expansion of the pressure in powers of µ/T obtaining the second cumulant (c2) which is associated to the quark number susceptibility. We discuss how the OPT nite Nc radiative (quantum) corrections induce a contribution to the pressure which behaves as a vector repulsion even when such a channel is absent in the original classical potential. Our results are then compared with the ones furnished by lattice QCD simulations and by the large-Nc approximation showing that, physically, the OPT results resemble those furnished by the latter approximation when a repulsive vector channel is explicitly included in the classical potential. In this case, both approximations fail to correctly describe the Stefan-Boltzmann limit at high temperatures. We discuss how this problem can be circumvented by taking the couplings to be temperature dependent so as to simulate the phenomenon of asymptotic freedom. Since this is the first time the OPT is applied to the PNJL we also discuss many technicalities associated with the evaluation of two loop (exchange) diagrams.Neste trabalho o método analítico não perturbativo conhecido como Teoria de Perturbação Otimizada (OPT) é aplicada ao modelo de Polyakov-Nambu-Jona-Lasinio (PNJL) para que quantidades físicas, associadas com as transições de fase da QCD, possam ser calculadas. A expansão da pressão em potências de µ/T é considerada para obter o segundo cumulante (c2) que é uma quantidade relacionada com a susceptibilidade do número de quarks. Primeiramente discutimos como as correções radiativas de Nc finito geradas pela OPT produzem uma contribuição que se comporta como um termo vetorial repulsivo mesmo quando este tipo de canal está ausente no potencial clássico original. Em seguida, nossos resultados são comparados com aqueles fornecidos pelas simulações na rede e também pela aproximação de Nc grande(LN). Fisicamente, os resultados da OPT são similares aqueles fornecidos pela aproximação LN quando um canal vetorial repulsivo é explicitamente incluido no potencial clássico. Neste caso, nenhuma destas aproximações analíticas produz corretamente o limite de Stefan Boltzmann para altas temperaturas. Contudo, nossos resultados sugerem como estes problemas podem ser contornados tomando-se as constantes de acoplamento como sendo dependentes da temperatura, de maneira que o fenômeno da liberdade assimptótica possa ser simulado. Esta é a primeira vez que a OPT é aplicada ao modelo de PNJL e por isto vários aspectos técnicos relacionados com o cálculo de diagramas de dois laços são também aqui apresentados
Screening of the QCD heavy quark potential at finite temperature
No full textKarsch F, Wyld HW. Screening of the QCD heavy quark potential at finite temperature. Physics Letters, B. 1988;213(4):505-510
Gittersimulation einer zentrumssymmetrischen dreidimensionalen effektiven Theorie für SU(2) Yang-Mills
No full textWe present lattice simulations of a center symmetric dimensionally reduced effective field theory for SU(2) Yang Mills which employ thermal Wilson lines and three-dimensional magnetic fields as fundamental degrees of freedom. The action is composed of a gauge invariant kinetic term, spatial gauge fields and a potential for the Wilson line which includes a "fuzzy" bag term to generate non-perturbative fluctuations between Z(2) degenerate ground states. The model is studied in the limit where the gauge fields are set to zero as well as the full model with gauge fields. We confirm that, at moderately weak coupling, the "fuzzy" bag term leads to eigenvalue repulsion in a finite region above the deconfining phase transition which shrinks in the extreme weak-coupling limit. A non-trivial Z(N) symmetric vacuum arises in the confined phase. The effective potential for the Polyakov loop in the theory with gauge fields is extracted from the simulations including all modes of the loop as well as for cooled configurations where the hard modes have been averaged out. The former is found to exhibit a non-analytic contribution while the latter can be described by a mean-field like ansatz with quadratic and quartic terms, plus a Vandermonde potential which depends upon the location within the phase diagram. Other results include the exact location of the phase boundary in the plane spanned by the coupling parameters, correlation lengths of several operators in the magnetic and electric sectors and the spatial string tension. We also present results from simulations of the full 4D Yang-Mills theory and attempt to make a qualitative comparison to the 3D effective theory.Diese Arbeit präsentiert die Ergebnisse der Gittersimulation einer dreidimensionalen effektiven Theorie für eine SU(2) Eichtheorie bei endlichen Temperaturen. Die Theorie verwendet thermische Wilson Linien und den dreidimensionalen magnetischen Sektor als elementare Freiheitsgrade. Die Wirkung der effektiven Theorie respektiert alle Symmetrien der Eichtheorie in vier Dimensionen. Sie ist invariant unter lokalen SU(2) Eichtransformationen und zusätzlich invariant unter globalen Z(2) Transformationen, welche dem Abel'schem Zentrum der SU(2) Gruppe entsprechen. Sie enthält das Quadrat des dreidimensionalen Feldstärketensors, einen effektiven kinetischen Term der die Eichfelder an die Wilson Linien koppelt und das störungstheoretische Potential der Wilson Linien bis zur ersten Ordnung. Zusätzlich wird ein "fuzzy bag'' Term addiert, welcher einen Phasenübergang erzeugt und nicht-störungstheoretische Fluktuationen zwischen Z(2) symmetrischen Grundzuständen generiert. Wir untersuchen die Theorie sowohl im Grenzfall verschwindender Magnetfelder, als auch die volle Theorie in welcher dynamische Magnetfelder auftreten. Wir zeigen, dass die Theorie eine Phase mit spontan gebrochener Z(2) Symmetrie besitzt in welcher der Polyakov Loop einen nicht-verschwindenden Wert hat. In Analogie zur Quantenchromodynamik entspricht dies einer Aufhebung des Quark-Confinement. Wir bestimmen das Phasendiagramm der Theorie und ermitteln die Ordnung des Phasenübergangs an verschiedenen Stellen der Phasengrenze mittels Korrelationsfunktionen und des Skalenverhaltens der Suszeptibilität. Wir zeigen dass im Bereich mittel schwacher Kopplung nahe der Phasengrenze in der Symmetrie-gebrochenen Phase der "fuzzy bag'' Term Eigenwert Repulsion erzeugt, welche im Grenzfall extrem schwacher Kopplung verschwindet. Wir zeigen dass ein nicht-triviales Z(2) symmetrisches Vakuum existiert. Für die volle Theorie mit Eichfeldern untersuchen wir die räumliche String Spannung und zeigen dass magnetische Observablen vom elektrischen Sektor nur schwach beeinflusst werden. Desweiteren bestimmen wir das effektive Potential des Polyakov Loop, sowohl für den Fall dass alle Fourier Moden betrachtet werden, als auch für "gekühlte'' Konfigurationen ("Block Spins'') in welchen das Polyakov Loop Feld über kleine Volumina gemittelt wird, wodurch kurzreichweitige Fluktuationen unterdrückt werden. Qualitative Vergleiche mit der vierdimensionalen Eichtheorie zeigen dass das Verhalten des Block Spin Potentials in der dreidimensionalen Theorie mit der vierdimensionalen Theorie übereinstimmt
On the Polyakov-Mellin bootstrap
No full textWe elaborate on some general aspects of the crossing symmetric approach of Polyakov to the conformal bootstrap, as recently formulated in Mellin space. This approach uses, as building blocks, Witten diagrams in AdS. We show the necessity for having contact Witten diagrams, in addition to the exchange ones, in two different contexts: a) the large c expansion of the holographic bootstrap b) in the E expansion at subleading orders to the ones studied already. In doing so, we use alternate simplified representations of the Witten diagrams in Mellin space. This enables us to also obtain compact, explicit expressions (in terms of a F-7(6) hypergeometric function!) for the analogue of the crossing kernel for Witten diagrams i.e., the decomposition into s-channel partial waves of crossed channel exchange diagrams
Measurement of the branching fraction
No full textThe B
0
s
→ J/ψK
0
S
branching fraction is measured in a data sample corresponding to 0.41 fb−1
of integrated luminosity collected with the LHCb detector at the LHC. This channel is sensitive to
the penguin contributions affecting the sin 2β measurement from B
0
→ J/ψK
0
S
. The time-integrated
branching fraction is measured to be B(B
0
s
→ J/ψK
0
S
) = (1.83±0.28)×10−5
. This is the most precise
measurement to date
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