Space-time picture of hadronic and nuclear collisions
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SPACE-TIME PICTURE OF HADRONIC AND NUCLEAR COLLISIONS From low to high energies
description
From low to high energies. Space-time picture of hadronic and nuclear collisions. collision @ alice. collision @ alice. It is essential to understand hadronic interactions at LHC! the structure of the underlying event behaviour of σ tot, σ el and σ diff with energy... - PowerPoint PPT Presentation
Transcript of Space-time picture of hadronic and nuclear collisions
SPACE-TIME PICTURE OF HADRONIC AND NUCLEAR COLLISIONS
From low to high energies
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COLLISION @ ALICE16.05.2008
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COLLISION @ ALICE16.05.2008
Higgs?
junk?
It is essential to understand hadronic interactions at LHC!
the structure of the underlying event
behaviour of σtot, σel and σdiff with energy...
99,9999% of event!! pp interactions at LHC are
small AA systems?
...or else, no discovery!
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OVERVIEW What is a hadron/nucleus? What are
the relevant degrees of freedom? How does a hadron interact with
another hadron? What about nuclei? What is the difference of these
interactions going from low to high energies?
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hadrons, nuclei
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COMPLEX BUILDING BLOCKS
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WHAT IS A HADRON? strongly interacting
composite subatomic particle consists of quarks
baryons: 3 quarks, fermion mesons: quark – anti-quark
pair, boson excited states - resonances at high enough temperatures
they dissolve!
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u
d
u
du
u d
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STRANGE AND STRONG INTERACTION16.05.2008
q q
qq qq
Quantum ChromoDynamics (QCD) – fundamentalo calculable for short distance processeso calculable on lattice for static situationso useless for large distance physics, dynamics...
need approximations, models, ideas!
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NUCLEAR PHYSICS AT HIGH ENERGIES
”macroscopic” volumes conditions ~early Universe free quarks and gluons? Quark-Gluon Plasma (QGP)
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u
du
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HYDRODYNAMICAL PICTURE
need to assume infinitesimally small cells are locally thermalized large rate of interaction
can define temperature, energy density and pressure
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Lev Landau
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THERMALIZATION
usually: particles start with an arbitrary velocity distribution
”equilibrate” over time, maximization of entropy take the continuum limit – loosing the concept
of ”particles”
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0 T
pguupT
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BJORKEN INITIAL CONDITION
boost invariant initial condition, adiabatical expansion
the longitudinal motion is uniform: vz=z/t
gives rise to a central plateau – height independent of energy!
initial energy density
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dydE
RT
Bj0
2
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PICTURE OF A FAST HADRON
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FEYNMANS (NAIVE) PARTON MODEL
• in a frame where the hadron is moving infinitely fast, it consists of infinitely many partons
• the partons carry a fraction of the total momentum of the hadron each
• the partons are free!
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ud
u
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HOW CAN THIS BE?
uncertainty principle! the constituent quarks can
interact via gluon exchange fluctuate into a quark-antiquark pair
when boosting the proton, the timescales related to these fluctuations are Lorentz dilated interactions of quarks now take place over much
larger timescales..
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DISTRIBUTION OF QUARKS AND GLUONS
the probatility of finding a parton (quark, gluon, heavy quark...) with momentum fraction x of the total proton
non-perturbative quantity but dependence on
”resolution” given by QCD
universal quantity measured in ep
collisions can predict νp, pp
etc...
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heavy-ion collisions at RHIC!
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PICTURE OF A FAST HADRON16.05.2008
fast partons
slow partons
target
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12222
mM2ppmpM
ΔE1τ
nxxx ...1 10
1t 2t 3t< <
The lifetime of the fluctuation can be quite large!
Strong ordering from fast to slow partons:
Only correlation between ”nearest neighbours”...
V.N. Gribov
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TRANSVERSE SPACE PICTURE
random walk in the transverse plane each step ~1/μ # of steps ~ln(E)
interaction range
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R
initial parton
target
22 )ln(
ER
“OCTOPUS” PICTURE
yybP y
tsf ee'41),( '42
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PICTURE OF A FAST HADRON16.05.2008
p
Rlog
2|| pR
fast partons- localized in a contracted disc
slow partons- spread out on long itudinal distances
xpplL0
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soft part of the fast hadron is not Lorentz contracted!AT HIGH ENERGY THE PROTON CAN BECOME LARGER THAN A NUCLEUS!
mostly gluons at low-x:
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FROM A NUCLEUS’ POINT OF VIEW...16.05.2008
overlap of nucleons in the nucleus huge density of gluons
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hadron-hadron, hadron-nucleus, nucleus-nucleus
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COLLISIONS
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COMPLICATED PROBLEM
in principle an infinite series need to find the most important terms
single-Pomeron diagram describes existing data quite OK! problems with the theory at high energies,
eg. LHC! need multi-Pomeron exchanges
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...++ ...+
Donnachie, Landshof
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MULTIPERIPHERAL MODEL
the reggeon can be seen as an exchanged ladder of particles in the t-channel
the partons take a fraction ε of the initial energy
#of particles in the fluctuation: n~ln(E) Reggeon is a highly non-local object
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Amati, Fubini, StanghelliniGribov
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IN THE CENTER OF MASS FRAME16.05.2008
single-ladder exchange a large amount of additional particles are created! Feynman-plateau
projectile
target
yy
”central plateu”FIN
AL STATE
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MULTIPLE SCATTERING
1st correction to the single-ladder exchange
”classical” rescattering picture low and intermediate energies –
potential scattering
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THE GLAUBER MODEL16.05.2008
heavy-ion collisions in each rescattering there is a
certain probability for particle production
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THE GLAUBER MODEL16.05.2008
2RA
2RA/γ
# of collisions
xxnxn
xnxddsd
collpart
pp 2)1(
2
3
entropy density related to number of interactions
”initial condition”
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BUT IS THIS PICTURE VALID AT HIGH ENERGIES?
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MULTIPLE LADDER STRUCTURE
at high energies, multiple ladders can start to evolve simultaneously
gives rise to novel physical phenomena
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NON-PLANAR GRAPHS
planar diagram vanishes at high energies ladders require a long time to develop critical energy E0=mNμRA
the projectile goes into a fluctuation long before the collision takes place ladders develop at the same time
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MandelstamGribov
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COHERENT INTERACTION16.05.2008
the projectile becomes large compared to the target
interacts simultaneously with the whole system effectively less interaction - shadowing dramatic change of space-time picture
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CHANGE OF SPACE-TIME PICTURE16.05.2008
the diagrams corresponding to ”classical” rescatterings are suppressed at high energies!
Gribov trick: Glauber is OK after all! Almost... have to take into account diffractive
intermediate states!
E1
(2)totσ + + . . .totσ
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planar diagrams
non-planar
SMOOTH BEHAVIOUR OF OBSERVABLES
the observables don’t feel the change of space-time picture
at high energy: nuclear shadowing are there any observables that are
sensitive to this transition?
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E
|Δσ2|
AN RmE 0
totalσdiffractive
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A1/3Enhanced in AA collisions
CASCADING – ENHANCED DIAGRAMS16.05.2008
k
All particles with can interact. Ο(R)k z
=Triple-Pomeron coupling:quite small or quite large?important at high energiesparton saturation!
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WHAT HAPPENS AT LHC?
interesting physics! probing the strongest force in new
domain! evolution of partons resembles
growth of bacteria colony reaction-diffusion process
striking experimental features!
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OUTLOOK16.05.2008
will large interaction of partons early on prepare a thermalized system in AA? in pp?
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COHERENCE CORRECTIONS
σtot=σ(hp)+σ(hn)=2(σinel+σel)=2Sξ+Sξ2/2
impulse approximation result
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Corrections: loss of flux in the second interaction -Sξ2
contrib to double multiplicity event -Sξ2
contrib to double multiplicity event Sξ2
enhancement of elastic cross section Sξ2/2 12-4
AGK ratios
Bartels, Ryskin Z. Phys. C 76, 241
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e)0(1'21
yigxy
L
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mpln)ln( AmR
#par
ticle
s
y
hadron targetnucleus target
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absorption
shadowingE
correction factor
AN RmE 0
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STAGES OF A HEAVY-ION COLLISION16.05.2008
Nuclear GeometryParton distributionsNuclear shadowing
Parton production& reinteraction
Chemical Freeze out &Quark Recombination
Jet FragmentationFunctions
Hadron Rescattering
Thermal Freeze out &Hadron decays
0 fm/c
2 fm/c
7 fm/c
>7 fm/c
