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Quark recombination in high energy collisions for different energies

Quark recombination in high energy collisions for different energies. Steven Rose Worcester Polytechnic Institute Mentor: Dr. Rainer Fries Texas A&M University. Motivations. Understand the mechanisms that allow for particle creation in high energy collisions

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Quark recombination in high energy collisions for different energies

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  1. Quark recombination in high energy collisions for different energies Steven Rose Worcester Polytechnic Institute Mentor: Dr. Rainer Fries Texas A&M University

  2. Motivations • Understand the mechanisms that allow for particle creation in high energy collisions • Understand QCD (strong force interactions) at high temperatures and densities • Quark-Gluon Plasma is such a system

  3. Quarks/Partons • Quark- elementary particle that carries a color charge • There are three color charges and their opposites • Quarks also have one of six ‘flavors’ • Strong interactions conserve color and flavor • Gluons are the strong force carriers • Both quarks and gluons make up hadrons

  4. Hadrons • Hadrons are particles constructed of quarks • (Anti)-Baryons have three (anti)-quarks • Mesons have a quark-anti-quark pair • All hadrons are color neutral due to confinement

  5. Sea Quarks and Virtuality • Quantum Mechanics allows for qqbar pairs to be created by violating energy conservation for short periods of time • These pairs are always opposite in color and flavor • Violation of CoE is an attribute of virtuality

  6. The Collision – What Happens? • Impact- Temperature and pressure are raised and cause a phase transition. • QGP- Hadrons “melt” as quarks become relevant degrees of freedom • System expands, reaches a thermal freeze out and hadrons are recreated, but how?

  7. The Collision – Characteristic Quanitities

  8. Jets

  9. Fragmentation • Partons may escape the QGP before freeze out, but confinement must hold true. • The ‘freed’ quark is virtual, but it loses it’s own energy to create many qqbar pairs that form hadrons. • Each qqbar pair brings the quarks collectively closer to the mass shell, until there is no virtuality.

  10. Diagrams for Fragmentation Feynman diagram model describes fragmentation with a perturbative approach The gluon-string model gives a better insight as to how confinement plays a role

  11. Recombination • Fragmentation built on the idea of a single quark in a vacuum, doesn’t consider many quarks • Recombination describes hadronization of many quarks • Applicable in QGP • Recombination argues that only quarks close in phase space will be able to form hadrons

  12. Hadron Ratio - Evidence • P+P Collisions have nearly constant, and small ratios • Large nuclei exhibit a growth in the same ratio

  13. Fragmentation and Recombination • Fragmentation is dominant in p+p and electron-positron annhilations for pt > 1 GeV/c • Fails at intermediate pt (1..6 GeV/c) for heavy ions • Fragmentation has to win for high pt • Recombination wins at intermediate pt, if phase space is densely populated

  14. Methodology- Fragmentation • Perform perturbative calculations to create jet spectra for various collisions/energies/nuclei • Many integrals, best speed with FORTRAN • Calculation is Leading Order, so fits the shape well, but not the size- scale by an appropriate “k-factor” • Simple least squares fit, done easily with Mathematica • Used KKP fragmentation functions

  15. Methodology- Nuclear Effects • Experimental data has no control over impact parameter, but generalizes ‘centrality bins’ • This determines fireball geometry for calculated jet path length • With path length, we allow interactions to drain energy from the jet, changing apparent momentum • Gluons lose more energy than quarks!

  16. Methodology- Recombination • We assume thermal quark spectra (fq = distribution) with temperature T and radial flow vt • Example: A meson in terms of recombination

  17. Resulting pt spectra Au+Au 62.4 GeV Central Au+Au 200 GeV

  18. More pt spectra Au+Au 62.4 GeV Peripheral Cu+Cu 22.5 GeV

  19. Other Observables – P/Pi, RAA

  20. Conclusions • In high energy, massive nuclei collisions, • Recombination is a critical mechanism for hadron production in the range of 1 – 6 GeV/c. • Fragmentation is the dominant process for hadron production above 6 Gev/c • Recombination contributes less to smaller collisions (low A, large b)

  21. Always under construction • Need better fragmentation functions • Experimental data on mid- to light-ion collisions • Systematic study of parameters and comparison to hydrodynamics

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