Transcript Quark recombination in high energy collisions for different energies Steven Rose

Quark recombination in high
energy collisions for different
energies
Steven Rose
Worcester Polytechnic Institute
Mentor: Dr. Rainer Fries
Texas A&M University
Motivations
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Understand the mechanisms that allow for
particle creation in high energy collisions
Understand QCD (strong force interactions)
at high temperatures and densities
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Quark-Gluon Plasma is such a system
Quarks/Partons
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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
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Both quarks and gluons make up hadrons
Hadrons
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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
Sea Quarks and Virtuality
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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
E t  
E  p m
2
2
2
The Collision – What Happens?
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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?
The Collision – Characteristic Quanitities
Jets
Fragmentation
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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.
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
Recombination
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Fragmentation built on the idea of a single
quark in a vacuum, doesn’t consider many
quarks
Recombination describes hadronization of
many quarks
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Applicable in QGP
Recombination argues that only quarks close
in phase space will be able to form hadrons
Hadron Ratio - Evidence
•P+P Collisions have nearly constant, and small ratios
•Large nuclei exhibit a growth in the same ratio
Fragmentation and Recombination
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Fragmentation is dominant in p+p and electronpositron 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
Methodology- Fragmentation
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Perform perturbative calculations to create jet
spectra for various collisions/energies/nuclei
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Calculation is Leading Order, so fits the
shape well, but not the size- scale by an
appropriate “k-factor”
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Many integrals, best speed with FORTRAN
Simple least squares fit, done easily with
Mathematica
Used KKP fragmentation functions
Methodology- Nuclear Effects
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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
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Gluons lose more energy than quarks!
Methodology- Recombination
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We assume thermal quark spectra (fq =
distribution) with temperature T and radial
flow vt
Example: A meson in terms of recombination
E dN
3
d p
 fq( x) f q (1  x)  meson x 
2
Resulting pt spectra
Au+Au 200 GeV
Au+Au 62.4 GeV
Central
More pt spectra
Au+Au 62.4 GeV
Peripheral
Cu+Cu 22.5 GeV
Other Observables – P/Pi, RAA
Conclusions
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In high energy, massive nuclei collisions,
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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)
Always under construction
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Need better fragmentation functions
Experimental data on mid- to light-ion
collisions
Systematic study of parameters and
comparison to hydrodynamics