Therese Renstrøm
-
Upload
damon-hansen -
Category
Documents
-
view
28 -
download
2
description
Transcript of Therese Renstrøm
Strangeness enhancement as a signal of Quark-Gluon Plasma
anda signal of the onset of
deconfinement
Therese Renstrøm
• Confinement• Deconfinement, asymptotic freedom• Quark-Gluon Plasma
Fig.1 The effective coupling constant in QCD, dependence of momentum exchange between the interacting hadrons
• How to get so hot and dense?• Answer: Ultrarelativistic heavy ion collisions
Fig. 2 An incredibly simplified picture of the creation of Quark-Gluon Plasma
QGP
Signatures of Quark-Gluon Plasma
• Electromagnetic probes (direct photons and dileptons)
• J/psi-suppression• High pT suppression• Jet modification and correlations• Elliptic flow• Strangeness enhancement
Strangeness Content: for matter at thermal and chemical equlibrium
Schwinger model for particle production gives production probability
In a nucleon-nucleon collision the ratio of strange to non-strange pairs is ca 0.1
Strangeness in hadron gas Counting the valence quarks of kaons and pions, gives the relation
In nucleon-nucleon collision this ratio is very small (p-Be, ratio=0.05)
• What about nucleus-nucleus collisions?• Produced hadrons: mainly pions and kaons• What is the ratio of strange to non-strange pairs if it is allowed to
reach thermal and chemical equlibrium?• Obtain strangeness content fraction by treating the system of
pions and kaons as an electrically neutral(!) boson gas in thermal and chemical equlibrium
• Use of Bose-Einstein statistics, and setting the chemical potential to zero is justified.
Strangeness in hadron gas at thermal and chemical equilibrium
Now we can express the density one type of meson as
The above intergral leads to the result
Where K2 is the Bessel function of order 2
At temperature T=200MeV
And following the ratio of strange to non-strange particles:
Notice that the ratio is considerably enhanced! (0.05 for p-Be)
Notice that the summands of the two sums converge rapidly Considering only the first summand in both sums, corresponds to using the Maxwell-Boltzman distribution
• Strangeness content in QGP is governed by the dynamical state of the plasma
• Thermal equilibrium- momentum distribution of particles do not change
• Chemical equilibrium- densities of particles reach steady state
• What are the densities of the different kinds of quarks if the plasma has a lifetime long enough to establish thermal and chemical equlibrium?
Strangeness in QGP in thermal and chemical equlibrium
Quarks = fermions, using Fermi-Dirac statistics
The presence of an antiquark corresponds to the absence of a quark in a negative energy state, so
Current mass, since deconfinement
We have the following number density of one type of quark
Strongly suggests strangeness enhancement as a signature of QGP!
Looking at chemical potential equals zero, we see that the predicted number density of s quarks equals that of u and d quarks. Gives a strange/nonstrange ratio of 1/2!
Approaching chemical equilibrium in QGM
So what are the mechanisms of strange pair production in QGP?
Fig. Lowest order Feynman diagrams for strange anti-strange production from quark antiquark annihilation (a) and gluon fusion (b),(c),(d).
So, the final expressions for the rate of production per unit space-time are:
Notice the similarites of the these two integrals.The cross sections are of the same order of magnitude,The difference between the Fermi-Dirac and the Bose-Einsteindistribution functions are negible at high temperatures,
The main difference comes from the degeneracy factors
We can conclude that the main source of strangenessproduction in QGP comes from gluon fusionThis was theoretically predicted by J.Rafelski and B.Mullerin 1982
The equalibration time of the strangeness production
The knowledge of the equilibrium strange quark denstity and the rate of change gives us an estimate of the equilibration time
• Three characteristics were predicted to exist if the fasetransition from the hadron gas to QGP was of first order
• The theory was tested experimentally of the NA49-collaboration
• The experimental results confirmed the theory
The kink, horn and step