ALICE@LHC: Recreating primordial matter in the laboratory
Slides prepared by Trine Tveter
Primordial matter existing at the birth of the universe, until
~ 10-6 s after the Big Bang, was a quark-gluon-plasma (QGP).
In collisions between heavy atomic nuclei at the LHC,
this state of matter is recreated. The mission of
the ALICE experiment is the study and quantitative
characterization of primordial matter.
FThe phase diagram of nuclear matter
Fased
Big Bang
LHC
RHIC
SPS
Quark-gluon-plasma primordial matter
FAIR/CBM
Neutron stars?
The phase transition between hadron gas and QGP
takes place at a critical energy density εc ~ 1 GeV/fm3.
Experimental energy density is
given by the formula:
(measured transverse energy / estimated initial volume)
Energy densities ε measured in heavy-ion collisions at LHC
and RHIC are well beyond critical value εc!
Constituents of colliding
nuclei: Valence quarks,
sea quarks and gluons,
commonly called partons,
all with strong colour
charge.
Effective number of
partons increases with
collision energy.
Stages of an ultrarelativistic heavy-ion collision
Initial state for
colliding nuclei
Gluons dominating
before equilibrium
Quark-gluon
plasma
Hadron gas
- Earliest stage: Hard parton collisions. Quarkantiquark pairs materializing from gluon fields.
- An extremely hot and dense QGP is formed
and approaches thermal / chemical equilibrium
through multiple partonic interactions. Explosive
expansion and cooling.
- Hadronization when the plasma reaches a
critical temperature of ~ 170 MeV. Hadrons
form through parton fragmentation and quark
coalescence.
- Chemical and finally kinetic freeze-out of
hadrons. Only signals surviving to this stage
can be observed experimentally.
Experimental signatures of the quark-gluon plasma
Elliptic flow
- Liquid-like behaviour: Pressure gradients
In non-central collisions and initlal-state
fluctuations lead to an anisotropic expansion ->
“anisotropic flow” with low viscosity
- Partons, which carry colour charge, will lose
energy when traversing medium with free
colour charges -> “jet quenching”
- QGP in thermal equilibrium will emit a
thermal photon spectrum
-> Information on the properties of the plasma,
like density and temperature.
Photon
sources
Jet quenching
Exploring the quark-gluon plasma
with the ALICE detector at LHC
ALICE subdetectors
of particular interest:
Central tracking system
for charged particles:
ITS (Inner Tracking
System),
TPC (Time Projection
Chamber),
TRD (Transition
Radiation Detector).
Detectors for neutral
particles:
PHOS (Photon
Spectrometer),
EMCAL (Electromagnetic
calorimeter)
ALICE focuses on the exploration of the properties of
the QGP and its dynamic evolution in heavy-ion collisions.
Important signals from the early stages are collective anisotropic expansion and
selected tomographic probes – photons, jets and hadrons of different flavours
with high transverse momenta, and their interactions with the plasma.
An intelligent trigger system (HLT) is used for selection of rare, interesting signals.
November 2010: First heavy-ion collisions at LHC
Pb+Pb at s1/2 NN = 2.76 TeV
Pb+Pb collision, HLT event display, 8 November 2010
Finally: The first heavy-ion
collisions at the LHC!
The density of produced
particles exceeds most
model predictions. More
than doubled relative
to RHIC energies. QGP
with extremely high
energy density created!
Particle production vs s1/2NN
ALICE spokesman:
“With nuclear collisions,
the LHC has become a
fantastic Big Bang machine.
In some respects, the
quark-gluon matter looks
familiar, still the ideal
liquid seen at RHIC, but
we're also starting to see
glimpses of something
new.”
Detailed characterization of the quark-gluon plasma
Main objective: Explore the LHC quark-gluon plasma
both through bulk observables (energy density, collective
anisotropic flow) and through modification of specific
tomographic probes in the medium.
Flavour-dependent hierarchy in medium-induced energy
loss: Gluons > light quarks > heavy quarks > photons
Measurements as functions of system size, centrality,
emission angle vs event plane -> path length in QGP.
Different probes
and their interaction
with QCD medium
Anisotropic (pathlength dependent)
suppression /
enhancement of
particles from
different production
processes
Hydrodynamic bulk properties: Elliptic flow
QGP formed in heavy-ion collisions expands
anisotropically! Fourier decomposition of
emission pattern relative to reaction plane:
where v2 represents quadrupole component elliptic flow. Differential studies of v2 gives
information on medium properties (equation of
state, viscosity) and dynamic evolution.
Elliptic flow:
Anisotropic pT distribution
ALICE
v2 for identified particles
in ALICE
v2 as a function
of collision energy
Hydrodynamic bulk properties:
Higher Fourier components
True reaction plane
Initial-state density
fluctuations lead to a very
complex, anisotropic emission
pattern for produced particles.
Can be studied through
2-particle correlations.
Quadrupole event plane Octupole event plane
Recent discovery: Fourier decomposition of 2-particle correlations
reveals a rich spectrum of higher multipole components.
Strong octupole (triangular) flow, v3 !
Sensitive to viscosity – higher components more strongly represented
at low viscosities (less smoothening of droplet shape).
2-particle
correlations
ALICE
Fourier vn
spectrum
Tomographic probes:
Fast partons losing energy in the QGP
Partons, carrying colour charge, will lose energy when traversing a medium with free colour
charges -> modification of pT-spectra in heavy-ion collisions (“jet quenching”). Nuclear
modification factor RAA extracted by comparing spectra from p+p and heavy-ion collisions.
RAA for identified particles reflecting energy loss hierarchy:
g – no energy loss, RAA ~ 1
p0 (from gluons) – large energy loss, RAA << 1
g
Heavy ion
collisions ->
medium
p
0
RAA for identified particles
“Elementary” p+p collisions
RAA for charged particles
Tomographic probes: Dijet modification
Trigger
Away-side
jet vanishing
2-particle
correlations
Dijets, showers of
produced particles from
quarks and gluons:
Oppositely directed
(Df = 180o) with same pT.
Characteristic dijet correlations seen in p+p collisions.
In heavy-ion collisions: Partons traversing the QGP
lose energy to the medium-> path length and
density dependent modification of the dijet pattern.
Central heavy-ion collisions at LHC: Only one jet
survives.
Flavour-dependent energy loss -> study dijets with
different, identified triggers?
2-particle correlations
Fully reconstructed
dijet in ALICE
Direct photon sources in heavy-ion collisions
Thermal photon spectra:
High T -> more
photons at short l,
high energy
Prompt photon production
Au+Au
p+p
PHENIX g-spectra
Different sources
dominate in
different energy
ranges, prompt
photons hardest.
Photon spectra from
Au+Au collisions at
RHIC: Thermal
g-spectrum in addition
to prompt component.
Slope of exponential
thermal spectrum
corresponds to kT ~
300 – 600 MeV.
Heavy ion collisions: Many
photon sources.
- Prompt photons from hard
partonic collisions (also in p+p)
- Thermal photon spectrum
from hot quark-gluon plasma
(and from cooler hadron gas)
- Photons from jet-plasma
interactions?
- Huge background from meson
decays, mostly p0 (also in p+p).
Prompt
Thermal
Observation of a centrality-­‐dependent dijet asymmetry in lead-­‐lead collisions at √sNN = 2.76 TeV with the ATLAS detector at the LHC http://prl.aps.org/abstract/PRL/v105/i25/e252303 5/28/12
F. Ould-Saada
13
http://dx.doi.org/10.1016/
j.physletb.2011.02.006
Measurement of the centrality dependence of J/Psi yields and observation of Z production in lead-lead
collisions
at the LHC
5/28/12 with the ATLAS detector
F. Ould-Saada
14
5/28/12
F. Ould-Saada
15