Quark recombination in high energy collisions for different energies Steven Rose Worcester...
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![Page 1: Quark recombination in high energy collisions for different energies Steven Rose Worcester Polytechnic Institute Mentor: Dr. Rainer Fries Texas A&M University.](https://reader035.fdocuments.us/reader035/viewer/2022062714/56649d3e5503460f94a1748b/html5/thumbnails/1.jpg)
Quark recombination in high energy collisions for different energies
Steven RoseWorcester Polytechnic InstituteMentor: Dr. Rainer FriesTexas A&M University
![Page 2: Quark recombination in high energy collisions for different energies Steven Rose Worcester Polytechnic Institute Mentor: Dr. Rainer Fries Texas A&M University.](https://reader035.fdocuments.us/reader035/viewer/2022062714/56649d3e5503460f94a1748b/html5/thumbnails/2.jpg)
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
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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
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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
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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
tE222 mpE
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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?
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The Collision – Characteristic Quanitities
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Jets
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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.
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Diagrams for Fragmentation
Feynman diagram modeldescribes fragmentation witha perturbative approach
The gluon-string model givesa better insight as to howconfinement plays a role
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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
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Hadron Ratio - Evidence
•P+P Collisions have nearly constant, and small ratios•Large nuclei exhibit a growth in the same ratio
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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
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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
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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!
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Methodology- Recombination
We assume thermal quark spectra (fq = distribution) with temperature T and radial flow vt
Example: A meson in terms of recombination
23 )1()( xxqfxfqpd
dNE meson
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Resulting pt spectraAu+Au 200 GeV
Au+Au 62.4 GeV Central
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More pt spectraAu+Au 62.4 GeV Peripheral
Cu+Cu 22.5 GeV
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Other Observables – P/Pi, RAA
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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)
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Always under construction
Need better fragmentation functions Experimental data on mid- to light-ion
collisions Systematic study of parameters and
comparison to hydrodynamics