arXiv subjects
Publications and source records attributed to S. M. H. Wong.
We calculate the order α_s finite temperature correction to dilepton production in quark-gluon plasma arising from the two-loop photon self-energy diagrams for high invariant mass M >> T.
In the last years, the need of the colour octet state in inclusive P-wave charmonium decay has been firmly established. However, the implications of this in the corresponding exclusive reactions have not been fully recognized. We argue for the necessity of the colour octet in P- and higher-wave quarkonium decay. Using a set of phenomenologically constructed baryon wavefunctions, we consider the Chi_J decay into octet and decuplet baryon-antibaryon pair. By doing so, we subject the wavefunctions to a test of applicability. We show that the colour singlet component alone is insufficient to account for experimental measurements and only by including the colour octet contribution can the partial theoretical decay widths be brought into range of the data. By the present and earlier applications of the set of wavefunctions, they show themselves to be reasonable model wavefunctions at around the scale Q^2 \sim 10-20 GeV^2.
We propose a numerical definition for baryon stopping in relativistic heavy ion collisions that is obtainable from final hadron rapidity distributions as well as from bremsstrahlung measurements. Thus a new channel of communication is opened between the two methods.
We calculate the change in the effective mass and width of a Z boson in the environment of a quark-gluon plasma under the conditions expected in Pb-Pb collisions at the LHC. The change in width is predicted to be only about 1 MeV at a temperature of 1 GeV, compared to the natural width of 2490$\pm$7 MeV. The mass shift is even smaller. Hence no observable effects are to be expected.
In inclusive P-wave charmonium decay, to cancel infrared divergence in the colour singlet contribution requires the inclusion of the colour octet which becomes degenerate with the former in the infrared limit. In the corresponding exclusive decay, such an infrared divergence does not exist. On this ground, it becomes doubtful whether the colour octet is needed in exclusive reactions. A contradiction in the underlying picture, however, would result once all strong decay channels are summed. We provide an answer to this question in support of the colour octet with theoretical arguments as well as an explicit calculation of Chi_J ---> p \bar p.
We examine the bremsstrahlung photons emitted in the central collisions of two gold nuclei at the Relativistic Heavy Ion Collider. While the measurements of the final hadrons can reveal only the amount of stopping, they tell us very little about the space-time evolution of the bulk matter in the collisions. By using two extreme collision scenarios, we argue that the low and medium energy bremsstrahlung photons can, not only reveal the amount of stopping, but also the finer details of the space-time evolution of the charges.
We examine the effects of color screening on the transition of a parton plasma into a hadron gas at RHIC energies. It is found that as expected, color screening posed itself as a significant barrier for hadronization. Parton-hadron conversion would therefore be delayed and prolonged when compared to that occuring in a vacuum. Due to the on-going expansion, the resulting hadron densities are lowered. Parton equilibration is also shown to be seriously disrupted in the process.
We consider a four-point process, associated with a wide-angle elastic scattering of two off-mass-shell spin-1/2 matter particles, in a non-abelian gauge theory. On the basis of a worldline approach, which reverts the functional to a path-integral description of the system, we factorize an eikonal (``soft'') subsector of the full theory and calculate the Sudakov-like suppression factor for the four-point function as a whole, once we have extracted the associated anomalous dimensions and taken into account the renormalization-group controlled evolution.
The bremsstrahlung produced when heavy nuclei collide is estimated for central collisions at the Relativistic Heavy Ion Collider. Bremsstrahlung photons with energies below 100 to 200 MeV are sufficient to discern the gross features of the space-time evolution of electric charge, if they can be separated from other sources of photons experimentally. This is illustrated explicitly by considering two very different models, one Bjorken-like, the other Landau-like, both of which are constructed to give the same final charge rapidity distribution.
The approach of a parton plasma at future heavy ion colliders towards kinetic and chemical equilibrium is considered. A plasma with a self-consistent evolving parton-parton interaction strength is shown to equilibrate better and faster than the usual but inconsistent one with a fixed strength. We explain why as a consequence of this, a parton plasma is a unique kind of many-body system. Because our time evolution scheme does not require the plasma to be in either kind of equilibrium from the outset, out-of-equilibrium effect on particle productions can be revealed. We show this on photon production and discuss the implications on photon as a signal to detect the quark-gluon plasma.
We show that although there is no infrared divergence in exclusive P-wave quarkonium decays, the colour octet contribution is no less important than in the inclusive decay. Results from more proper calculations with only colour singlet contribution are not sufficient to account for the measured partial decay widths and so the next higher Fock state must be included. Using the $\c_J$ decay into $N\bar N$ as an example, a scheme is devised to yield estimates of the decay widths using both contributions, the results are comparable with experimental measurements.
We examine the effects of the new equilibration scenario of the increasingly strongly interacting parton plasma and of the non-equilibrium environment have on the production of open charm, photon and dilepton at LHC and at RHIC energies. We show that an out-of-equilibrium effect, not shown before, changes significantly the relative yield of the two main partonic contributions to photon production in the higher $p_T$ range, and higher orders for electromagnetic emissions have increased significance in the new scenario especially at RHIC energies. We argue that the effects of the new scenario are not restricted just to the parton phase but will continue through to the later hadron gas phase with potentially positive implications for the detection of the quark-gluon plasma.
We report on a new equilibration scenario in relativistic heavy ion collisions, the scenario of the Increasingly Strongly Interacting Parton Plasma, and the effects of this scenario on equilibration and open charm, photon and dilepton production. The parton plasma is shown to be a very special kind of many-body system, which contains new physics concerning the approach towards equilibrium. This is likely to be unique to the parton plasma.
In high energy nuclear collisions, most calculations on the early equilibration of the parton plasma showed that the system does not come close to full equilibrium, especially for the fermion components. However, since the system is constantly evolving, the cooling effect due to expansion will lead to a decrease of the average parton energies. So the interactions should be enhanced by a corresponding increase of the running coupling. We show that this leads to a faster and improved equilibration. This improvement is more important for the fermions compensating partially for their weaker interactions and slower equilibration.
It is argued that colour octet contribution in exclusive charmonium decays is very important even though the same infrared divergence found in P-wave colour singlet inclusive decay is absent. This comes about because of the suppression at the level of the wavefunction of P-wave quarkonia. Using the modified hard scattering approach of Brodsky, Lepage, Botts, Li and Sterman, we use charmonium decay into baryon-antibaryon pair as an example and show that the colour singlet contribution alone is clearly insufficient to explain the experimental decay widths.
The uncertainty in the equilibration in heavy ion collisions due to the choice of the value of the coupling is examined. The results of the equilibration are indeed affected by this choice. In particular, a variation of $\a_s$ from 0.3 to 0.5 reduces the parton phase of the plasma by as much as 4.0 fm/c and increasing coupling causes a reduction in the generated entropy. As far as equilibration is concerned, larger coupling results in faster equilibration both chemically and kinetically but improvements only happen to the fermions. These are accompanied by more rapid cooling and therefore shortened lifetime. In the light of these results, to choose $\a_s$ is almost equivalent to choosing the results. Also because of consistency, any fixed $\a_s$ is incompatible with an evolving plasma. The best choice is therefore not to choose at all but let the system makes its own decision. We show a simple recipe how this can be done. With an evolving coupling, equilibration is accelerated with better results at the expense of the duration of the deconfined phase.
We study the equilibration of a parton plasma in terms of its parton compositions and its state of thermalization. In studying the evolution of the plasma, one has to assume a small value of the strong coupling constant. This value is by no means fixed. By varying this only parameter in our calculation, we show the dependence of equilibration on its magnitude. It is shown that both kinetic and parton equilibration are faster with increasing coupling but the plasma cools much more rapidly resulting in shortened lifetime. The degree of equilibration improves significantly for quarks and antiquarks but not so for gluons and the total generated entropy is reduced. With a coupling depending on the average parton energy, there is additional acceleration in the equilibration during the evolution.
The dependence of the equilibration of the parton plasma on the value of the strong coupling is studied in Au+Au collisions at LHC and at RHIC energies. With increasing coupling, the following are found to happen: 1) both thermal and chemical equilibration speed up, 2) in the final degree of equilibration, only quarks and antiquarks show obvious improvements but not gluons and 3) the plasma cools much more rapidly. The deconfinement phase transition will therefore takes place sooner and it naturally results in the shortening of the parton phase of the plasma. The exact duration of this phase is however sensitive to the value of the coupling. A change from $\a_s=0.3$ to $\a_s=0.5$, for example, reduces the lifetime of the parton phase at LHC by as much as 4.0 fm/c. The total generated entropy is another sensitive quantity to the coupling. Larger values of $\a_s$ will lead to entropy reduction and therefore reduction both in the duration of the mixed phase, as well as in the final pion multiplicity. It is shown that the common choice of $\a_s=0.3$ is not a good value for the entire duration of the evolution given that the system undergoes substantial changes from the beginning to the time that the deconfinement phase transition is about to take place. Instead, by using a more consistent simple recipe, the system is allowed to decide its own strength of the interactions which evolves with the system as it should. With this approach, $\a_s$ increases with time and this leads to acceleration in the equilibration even as equilibrium is near. This is opposite to the behavior of the equilibration of a molecular gas or ordinary many-body system where the interaction strength is fixed.