What does HTL lose? Hui Liu Jinan University, China

Hui Liu, SQM2008 Beijing, October HTL approximation Hard thermal loop (HTL) –Widely used approximation to self-energy –Advantages Simple and convenient for further calculations Gauge invariant –Restrictions Requires weak coupling Equivalent to high temperature approximation

Hui Liu, SQM2008 Beijing, October Why not HTL Some facts in RHIC experiment Lattice result, hep-lat/ v1 Strongly coupled? Does HTL still work? ―What information does HTL approximation lose compared to the complete loop? STAR collaboration, PRL 89 (2002)

Hui Liu, SQM2008 Beijing, October Dispersion Relations Dispersion relation –Fundamental property of a many-body system –Energy-momentum relation determined by the pole of effective propagator. Π L : longitudinal component of self-energy Π T : transverse component Equation of dispersion

Hui Liu, SQM2008 Beijing, October Comparison Self-energy of toy model QED –HTL –Complete one loop (C1L)

Hui Liu, SQM2008 Beijing, October Dispersion relations –Solve the equation of dispersion and find out the relation between ω and q The main difference between the two curves is the appearance of a threshold frequency on the C1L curve. HTL C1L Above the plasma frequency, q is real and q i =0. While below it, q is complex. We plot ω-q i relation in the left area. q=iq i

Hui Liu, SQM2008 Beijing, October Dynamical screening Dynamical screening regime –Below the plasma frequency, the modes are complex, which can be signaled by the screening of external charges. Above, is consis- tent zero. While below it has values, which indicates an expectant change in physics pro- perties. q=q r +iq i

Hui Liu, SQM2008 Beijing, October Oscillatory potential Static limit –HTL: Purely imaginary modes –C1L: Complex modes Static screening potential where — Debye screening — Oscillatory screening ~~~~~~~~~~~~~~

Hui Liu, SQM2008 Beijing, October Radial Distribution Function RDF –Probability of finding two particles at a distance r –Density-density autocorrelation function –Typical RDFs of different states of matter solidliquidgas

Hui Liu, SQM2008 Beijing, October RDF of a liquid? Which potential can result in a liquid? RDF and the potential of “ mean force ” –Short range order –Indicate a liquid state? Gelman, Shuryak, and Zahed, PRC 74, (06) Molecular dynamical simulation

Hui Liu, SQM2008 Beijing, October Conclusion Physics concealed in the C1L dispersion relation might not be revealed by the HTL Comparing the dispersion relations we found the existence of a threshold frequency in the dynamical screening regime of the C1L –Below the threshold frequency the modes contain both real and imaginary parts –In the static limit, the complex mode leads to an oscillatory screening potential, which is contrast to the Debye-like potential in the HTL case The oscillatory potential could result in a liquid-like RDF, which might indicate the liquid QGP

Hui Liu, SQM2008 Beijing, October RDF in hot QGP Gluon polarization RDF of QGP –Short range order. Very similar to the typical shape of liquid. Footprint of liquid QGP!? –Enhanced oscillations at lower temperatures

Hui Liu, SQM2008 Beijing, October Non-zero frequencies Frequency-dependent screening For HTL, the potential is always Debye-like in the whole range of frequencies below the plasma frequency. For C1L, the potential can be either Debye-like or oscillatory. –Above the threshold frequency the screening potential is Debye-like –below that frequency, the potential is oscillating.

Hui Liu, SQM2008 Beijing, October Screening of a moving particle? Current-current correlation Static case – density correlation –Static screening potential Non-static case –Frequency dependent screening potential