ALICE (A Large Ion Collider Experiment) is
one of seven detector experiments at the Large
Hadron Collider at CERN. The other six are:
ATLAS, CMS, TOTEM, LHCb, LHCf and MoEDAL.
== Introduction ==
ALICE is optimized to study heavy-ion (Pb-Pb
nuclei) collisions at a centre of mass energy
of 2.76 TeV per nucleon pair. The resulting
temperature and energy density are expected
to be high enough to produce quark–gluon
plasma, a state of matter wherein quarks and
gluons are freed. Similar conditions are believed
to have existed a fraction of the second after
the Big Bang before quarks and gluons bound
together to form hadrons and heavier particles.ALICE
is focusing on the physics of strongly interacting
matter at extreme energy densities. The existence
of the quark–gluon plasma and its properties
are key issues in quantum chromodynamics for
understanding color confinement and chiral
symmetry restoration. Recreating this primordial
form of matter and understanding how it evolves
is expected to shed light on questions about
how matter is organized, the mechanism that
confines quarks and gluons and the nature
of strong interactions and how they result
in generating the bulk of the mass of ordinary
matter.
Quantum chromodynamics (QCD) predicts that
at sufficiently high energy densities there
will be a phase transition from conventional
hadronic matter, where quarks are locked inside
nuclear particles, to a plasma of deconfined
quarks and gluons. The reverse of this transition
is believed to have taken place when the universe
was just 10−6 s old, and may still play
a role today in the hearts of collapsing neutron
stars or other astrophysical objects.
== History ==
The idea of building a dedicated heavy-ion
detector for the LHC was first aired at the
historic Evian meeting "Towards the LHC experimental
Programme" in March 1992. From the ideas presented
there, the ALICE collaboration was formed
and in 1993, a LoI was submitted.ALICE was
first proposed as a central detector in 1993
and later complemented by an additional forward
muon spectrometer designed in 1995. In 1997,
ALICE received the green light from the LHC
Committee to proceed towards final design
and construction.The first ten years were
spent on design and an extensive R&D effort.
Like for all other LHC experiments, it became
clear from the outset that also the challenges
of heavy ion physics at LHC could not be really
met (nor paid for) with existing technology.
Significant advances, and in some cases a
technological break-through, would be required
to build on the ground what physicists had
dreamed up on paper for their experiments.
The initially very broad and later more focused,
well organised and well supported R&D effort,
which was sustained over most of the 1990s,
has led to many evolutionary and some revolutionary
advances in detectors, electronics and computing.
Designing a dedicated heavy-ion experiment
in the early '90s for use at the LHC some
15 years later posed some daunting challenges.
The detector had to be general purpose - able
to measure most signals of potential interest,
even if their relevance may only become apparent
later - and flexible, allowing additions and
modifications along the way as new avenues
of investigation would open up. In both respects
ALICE did quite well, as it included a number
of observables in its initial menu whose importance
only became clear later. Various major detection
system were added, from the muon spectrometer
in 1995, the transition radiation detectors
in 1999 to a large jet calorimeter added in
2007.
ALICE recorded data from the first lead-lead
collisions at the LHC in 2010. Data sets taken
during heavy-ion periods in 2010 and 2011
as well as proton-lead data from 2013 have
provided an excellent basis for an in-depth
look at the physics of quark–gluon plasma.
As of 2014 After more than three years of
successful operation, the ALICE detector is
about to undergo a major programme of consolidation
and upgrade during the long shutdown [LS1]
of CERN's accelerator complex. A new subdetector
called the dijet calorimeter (DCAL) will be
installed, and all 18 of the existing ALICE
subdetectors will be upgraded. There will
also be major renovation work on the ALICE
infrastructure, including the electrical and
cooling systems. The wealth of published scientific
results and the very intense upgrade programme
of ALICE have attracted numerous institutes
and scientists from all over the world. Today
the ALICE Collaboration has more than 1800
members coming from 176 institutes in 41 countries
== Heavy-ion collisions at the LHC ==
Searches for Quark Gluon plasma and a deeper
understanding of the QCD started at CERN and
Brookhaven with lighter ions in the 1980s.
Today's programme at these laboratories has
moved on to ultrarelativistic collisions of
heavy ions, and is just reaching the energy
threshold at which the phase transition is
expected to occur. The LHC, with a centre-of-mass
energy around 5.5 TeV/nucleon, will push the
energy reach even further.
During head-on collisions of lead ions at
the LHC, hundreds of protons and neutrons
smash into one another at energies of upwards
of a few TeVs. Lead ions are accelerated to
more than 99.9999% of the speed of light and
collisions at the LHC are 100 times more energetic
than those of protons - heating up matter
in the interaction point to a temperature
almost 100,000 times higher than the temperature
in the core of the sun.
When the two lead nuclei slam into each other,
matter undergoes a transition to form for
a brief instant a droplet of primordial matter,
the so-called quark–gluon plasma which is
believed to have filled the universe a few
microseconds after the Big Bang.
The quark–gluon plasma is formed as protons
and neutrons "melt" into their elementary
constituents, quarks and gluons become asymptotically
free. The droplet of QGP instantly cools,
and the individual quarks and gluons (collectively
called partons) recombine into a blizzard
of ordinary matter that speeds away in all
directions. The debris contains particles
such as pions and kaons, which are made of
a quark and an antiquark; protons and neutrons,
made of three quarks; and even copious antiprotons
and antineutrons, which may combine to form
the nuclei of antiatoms as heavy as helium.
Much can be learned by studying the distribution
and energy of this debris.
=== First lead-lead collisions ===
The Large Hadron Collider smashed its first
lead ions in 2010, on 7 November at around
12:30 a.m. CET.The first collisions in the
center of the ALICE, ATLAS and CMS detectors
took place less than 72 hours after the LHC
ended its first run of protons and switched
to accelerating lead-ion beams. Each lead
nucleus contains 82 protons, and the LHC accelerates
each proton to an energy of 3.5 TeV, thus
resulting in an energy of 287 TeV per beam,
or a total collision energy of 574 TeV.
Up to 3,000 charged particles were emitted
from each collision, shown here as lines radiating
from the collision point. The colors of the
lines indicate how much energy each particle
carried away from the collision.
=== Proton-lead collisions at the LHC ===
In 2013, the LHC collided protons with lead
ions for the LHC's first physics beams of
2013. The experiment was conducted by counter-rotating
beams of protons and lead ions, and begun
with centred orbits with different revolution
frequencies, and then separately ramped to
the accelerator's maximum collision energy.The
first lead-proton run at the LHC lasted for
one month and data help ALICE physicists to
decouple the effects of the plasma from effects
that stem from cold nuclear matter effects
and shed more light on the study of the Quark-Gluon
plasma.
In the case of lead-lead collisions, the configurations
of the quarks and gluons that make up the
protons and neutrons of the incoming lead
nucleus can be somewhat different of those
in the incoming protons. In order to study
if part of the effects we see when comparing
lead-lead and proton-proton collisions is
due to this configuration difference rather
than the formation of the plasma. Proton-lead
collisions are an ideal tool for this study.
== The ALICE detectors ==
A key design consideration of ALICE is the
ability to study QCD and quark (de)confinement
under these extreme conditions. This is done
by using particles, created inside the hot
volume as it expands and cools down, that
live long enough to reach the sensitive detector
layers situated around the interaction region.
ALICE's physics programme relies on being
able to identify all of them, i.e. to determine
if they are electrons, photons, pions, etc.
and to determine their charge. This involves
making the most of the (sometimes slightly)
different ways that particles interact with
matter.In a "traditional" experiment, particles
are identified or at least assigned to families
(charged or neutral hadrons), by the characteristic
signatures they leave in the detector. The
experiment is divided into a few main components
and each component tests a specific set of
particle properties. These components are
stacked in layers and the particles go through
the layers sequentially from the collision
point outwards: first a tracking system, then
an electromagnetic (EM) and a hadronic calorimeter
and finally a muon system. The detectors are
embedded in a magnetic field in order to bend
the tracks of charged particles for momentum
and charge determination. This method for
particle identification works well only for
certain particles, and is used for example
by the large LHC experiments ATLAS and CMS.
However, this technique is not suitable for
hadron identification as it doesn't allow
distinguishing the different charged hadrons
that are produced in Pb-Pb collisions.
In order to identify all the particles that
are coming out of the system of the QGP ALICE
is using a set of 18 detectors that give information
about the mass, the velocity and the electrical
sign of the particles.
=== Barrel tracking ===
An ensemble of cylindrical barrel detectors
that surround the nominal interaction point
is used to track all the particles that fly
out of the hot, dense medium. The Inner Tracking
System(ITS) (consisting of three layers of
detectors: Silicon Pixel Detector(SPD), Silicon
Drift Detector(SDD), Silicon Strip Detector(SSD)),
the Time Projection Chamber(TPC) and the Transition
Radiation Detector(TRD) measure at many points
the passage of each particle carrying an electric
charge and give precise information about
the particle's trajectory. The ALICE barrel
tracking detectors are embedded in a magnetic
field of 0.5 Tesla produced by a huge magnetic
solenoid bending the trajectories of the particles.
From the curvature of the tracks one can derive
their momentum. The ITS is so precise that
particles which are generated by the decay
of other particles with a long(~.1 mm before
decay) life time can be identified by seeing
that they do not originate from the point
where the interaction has taken place (the
"vertex" of the event) but rather from a point
at a distance of as small as a tenth of a
millimeter. This allows us to measure, for
example, bottom quarks which decay into a
relatively long-lived B-meson through "topological"
cuts.
==== Inner Tracking System ====
The short-living heavy particles cover a very
small distance before decaying. This system
aims at identifying these phenomena of decay
by measuring the location where it occurs
with a precision of a tenth of millimetre.The
Inner Tracking System (ITS) consists of six
cylindrical layers of silicon detectors. The
layers surround the collision point and measure
the properties of the particles emerging from
the collisions, pin-pointing their position
of passage to a fraction of a millimetre.
With the help of the ITS, particles containing
heavy quarks (charm and beauty) can be identified
by reconstructing the coordinates at which
they decay.
ITS layers (counting from the interaction
point):
2 layers of SPD (Silicon Pixel Detector),
2 layers of SDD (Silicon Drift Detector),
2 layers of SSD (Silicon Strip Detector).The
ITS was inserted at the heart of the ALICE
experiment in March 2007 following a large
phase of R&D. Using the smallest amounts of
the lightest material, the ITS has been made
as lightweight and delicate as possible. With
almost 5 m2 of double-sided silicon strip
detectors and more than 1 m2 of silicon
drift detectors, it is the largest system
using both types of silicon detector.
ALICE has recently presented plans for an
upgraded Inner Tracking System, mainly based
on building a new silicon tracker with greatly
improved features in terms of determination
of the impact parameter (d0) to the primary
vertex, tracking efficiency at low pT and
readout rate capabilities. The upgraded ITS
will open new channels in the study of the
Quark Gluon Plasma formed at LHC which are
necessary in order to understand the dynamics
of this condensed phase of the QCD.
It will allow the study of the process of
thermalization of heavy quarks in the medium
by measuring heavy flavour charmed and beauty
baryons and extending these measurements down
to very low pT for the first time. It will
also give a better understanding of the quark
mass dependence of in-medium energy loss and
offer a unique capability of measuring the
beauty quarks while also improving the beauty
decay vertex reconstruction. Finally, the
upgraded ITS will give us the chance to characterize
the thermal radiation coming from the QGP
and the in-medium modification of hadronic
spectral functions as related to chiral symmetry
restoration.
The upgrade project requires an extensive
R&D effort by our researchers and collaborators
all over the world on cutting-edge technologies:
silicon sensors, low-power electronics, interconnection
and packaging technologies, ultra-light mechanical
structures and cooling units.
==== Time Projection Chamber ====
The ALICE Time Projection Chamber (TPC) is
a large volume filled with a gas as detection
medium and is the main particle tracking device
in ALICE.Charged particles crossing the gas
of the TPC ionize the gas atoms along their
path, liberating electrons that drift towards
the end plates of the detector. The characteristics
of the ionization process caused by fast charged
particles passing through a medium can be
used for particle identification. The velocity
dependence of the ionization strength is connected
to the well-known Bethe-Bloch formula , which
describes the average energy loss of charged
particles through inelastic Coulomb collisions
with the atomic electrons of the medium.
Multiwire proportional counters or solid-state
counters are often used as detection medium,
because they provide signals with pulse heights
proportional to the ionization strength. An
avalanche effect in the vicinity of the anode
wires strung in the readout chambers, gives
the necessary signal amplification. The positive
ions created in the avalanche induce a positive
current signal on the pad plane. The readout
is performed by the 557 568 pads that form
the cathode plane of the multi-wire proportional
chambers (MWPC) located at the end plates.
This gives the radial distance to the beam
and the azimuth. The last coordinate, z along
the beam direction, is given by the drift
time. Since energy-loss fluctuations can be
considerable, in general many pulse-height
measurements are performed along the particle
track in order to optimize the resolution
of the ionization measurement.
Almost all of the TPC's volume is sensitive
to the traversing charged particles, but it
features a minimum material budget. The straightforward
pattern recognition (continuous tracks) make
TPCs the perfect choice for high-multiplicity
environments, such as in heavy-ion collisions,
where thousands of particles have to be tracked
simultaneously. Inside the ALICE TPC, the
ionization strength of all tracks is sampled
up to 159 times, resulting in a resolution
of the ionization measurement as good as 5%.
==== Transition radiation detector ====
Electrons and positrons can be discriminated
from other charged particles using the emission
of transition radiation, X-rays emitted when
the particles cross many layers of thin materials.
The identification of electrons and positrons
is achieved using a transition radiation detector
(TRD). In a similar manner to the muon spectrometer,
this system enables detailed studies of the
production of vector-meson resonances, but
with extended coverage down to the light vector-meson
ρ and in a different rapidity region. Below
1 GeV/c, electrons can be identified via a
combination of particle identification detector
(PID) measurements in the TPC and time of
flight (TOF). In the momentum range 1–10
GeV/c, the fact that electrons may create
TR when travelling through a dedicated "radiator"
can be exploited. Inside such a radiator,
fast charged particles cross the boundaries
between materials with different dielectric
constants, which can lead to the emission
of TR photons with energies in the X-ray range.
The effect is tiny and the radiator has to
provide many hundreds of material boundaries
to achieve a high enough probability to produce
at least one photon. In the ALICE TRD, the
TR photons are detected just behind the radiator
using MWPCs filled with a xenon-based gas
mixture, where they deposit their energy on
top of the ionization signals from the particle's
track.
The ALICE TRD was designed to derive a fast
trigger for charged particles with high momentum
and can significantly enhance the recorded
yields of vector mesons. For this purpose,
250,000 CPUs are installed right on the detector
to identify candidates for high-momentum tracks
and analyse the energy deposition associated
with them as quickly as possible (while the
signals are still being created in the detector).
This information is sent to a global tracking
unit, which combines all of the information
to search for electron–positron track pairs
within only 6 μs.
To develop such a Transition Radiation Detector
(TRD) for ALICE many detector prototypes were
tested in mixed beams of pions and electrons.
=== Particle identification with ALICE ===
ALICE also wants to know the identity of each
particle, whether it is an electron, or a
proton, a kaon or a pion.
Charged hadrons (in fact, all stable charged
particles) are unambiguously identified if
their mass and charge are determined. The
mass can be deduced from measurements of the
momentum and of the velocity. Momentum and
the sign of the charge are obtained by measuring
the curvature of the particle's track in a
magnetic field. To obtain the particle velocity
there exist four methods based on measurements
of time-of-flight and ionization, and on detection
of transition radiation and Cherenkov radiation.
Each of these methods works well in different
momentum ranges or for specific types of particle.
In ALICE all of these methods may be combined
in order to measure, for instance, particle
spectra.
In addition to the information given by ITS
and TPC, more specialized detectors are needed:
the TOF measures, with a precision better
than a tenth of a billionth of a second, the
time that each particle takes to travel from
the vertex to reach it, so that one can measure
its speed. The high momentum particle identification
detector (HMPID) measures the faint light
patterns generated by fast particles and the
TRD measures the special radiation very fast
particles emit when crossing different materials,
thus allowing to identify electrons. Muons
are measured by exploiting the fact that they
penetrate matter more easily than most other
particles: in the forward region a very thick
and complex absorber stops all other particles
and muons are measured by a dedicated set
of detectors: the muon spectrometer.
==== Time of Flight ====
Charged particles are identified in ALICE
by Time-Of-Flight (TOF). TOF measurements
yield the velocity of a charged particle by
measuring the flight time over a given distance
along the track trajectory. Using the tracking
information from other detectors every track
firing a sensor is identified. Provided the
momentum is also known, the mass of the particle
can then be derived from these measurements.
The ALICE TOF detector is a large-area detector
based on multigap resistive plate chambers
(MRPCs) that cover a cylindrical surface of
141 m2, with an inner radius of 3.7 metres
(12 ft). There are approximately 160 000 MRPC
pads with time resolution of about 100 ps
distributed over the large surface of 150
m2.
The MRPCs are parallel-plate detectors built
of thin sheets of standard window glass to
create narrow gas gaps with high electric
fields. These plates are separated using fishing
lines to provide the desired spacing; 10 gas
gaps per MRPC are needed to arrive at a detection
efficiency close to 100%.
The simplicity of the construction allows
a large system to be built with an overall
TOF resolution of 80 ps at a relatively low
cost (CERN Courier November 2011 p8). This
performance allows the separation of kaons,
pions and protons up to momenta of a few GeV/c.
Combining such a measurement with the PID
information from the ALICE TPC has proved
useful in improving the separation between
the different particle types, as figure 3
shows for a particular momentum range.
==== High Momentum Particle Identification
Detector ====
The High Momentum Particle Identification
Detector (HMPID) is a RICH detector to determine
the speed of particles beyond the momentum
range available through energy loss (in ITS
and TPC, p = 600 MeV) and through time-of-flight
measurements (in TOF, p = 1.2–1.4 GeV).
Cherenkov radiation is a shock wave resulting
from charged particles moving through a material
faster than the velocity of light in that
material. The radiation propagates with a
characteristic angle with respect to the particle
track, which depends on the particle velocity.
Cherenkov detectors make use of this effect
and in general consist of two main elements:
a radiator in which Cherenkov radiation is
produced and a photon detector. Ring imaging
Cherenkov (RICH) detectors resolve the ring-shaped
image of the focused Cherenkov radiation,
enabling a measurement of the Cherenkov angle
and thus the particle velocity. This in turn
is sufficient to determine the mass of the
charged particle.
If a dense medium (large refractive index)
is used, only a thin radiator layer of the
order of a few centimetres is required to
emit a sufficient number of Cherenkov photons.
The photon detector is then located at some
distance (usually about 10 cm) behind the
radiator, allowing the cone of light to expand
and form the characteristic ring-shaped image.
Such a proximity-focusing RICH is installed
in the ALICE experiment.
ALICE HMPID's momentum range is up to 3 GeV
for pion/kaon discrimination and up to 5 GeV
for kaon/proton discrimination. It is the
world's largest caesium iodide RICH detector,
with an active area of 11 m². A prototype
was successfully tested at CERN in 1997 and
currently takes data at the Relativistic Heavy
Ion Collider at the Brookhaven National Laboratory
in the US.
=== Calorimeters ===
Calorimeters measure the energy of particles,
and determine whether they have electromagnetic
or hadronic interactions. Particle Identification
in a calorimeter is a destructive measurement.
All particles except muons and neutrinos deposit
all their energy in the calorimeter system
by production of electromagnetic or hadronic
showers. Photons, electrons and positrons
deposit all their energy in an electromagnetic
calorimeter. Their showers are indistinguishable,
but a photon can be identified by the non-existence
of a track in the tracking system that is
associated to the shower.
The photons (particles of light), like the
light emitted from a hot object, tell us about
the temperature of the system. To measure
them, special detectors are necessary: the
crystals of the PHOS, which are as dense as
lead and as transparent as glass, will measure
them with fantastic precision in a limited
region, while the PMD and in particular the
EMCal will measure them over a very wide area.
The EMCal will also measure groups of close
particles (called "jets") which have a memory
of the early phases of the event.
==== Photon spectrometer ====
PHOS is a high-resolution electromagnetic
calorimeter installed in ALICE to provide
data to test the thermal and dynamical properties
of the initial phase of the collision. This
is done by measuring photons emerging directly
from the collision. PHOS covers a limited
acceptance domain at central rapidity. It
is made of lead tungstate crystals, similar
to the ones used by CMS, read out using Avalanche
Photodiodes (APD).
When high energy photons strike lead tungstate,
they make it glow, or scintillate, and this
glow can be measured. Lead tungstate is extremely
dense (denser than iron), stopping most photons
that reach it. The crystals are kept at a
temperature of 248 K, which helps to minimize
the deterioration of the energy resolution
due to noise and to optimize the response
for low energies.
==== Electro-Magnetic Calorimeter ====
The EMCal is a lead-scintillator sampling
calorimeter comprising almost 13,000 individual
towers that are grouped into ten super-modules.
The towers are read out by wavelength-shifting
optical fibers in a shashlik geometry coupled
to an avalanche photodiode. The complete EMCal
will contain 100,000 individual scintillator
tiles and 185 kilometers of optical fiber,
weighing in total about 100 tons.
The EMCal covers almost the full length of
the ALICE Time Projection Chamber and central
detector, and a third of its azimuth placed
back-to-back with the ALICE Photon Spectrometer
– a smaller, highly granular lead-tungstate
calorimeter.
The super-modules are inserted into an independent
support frame situated within the ALICE magnet,
between the time-of-flight counters and the
magnet coil. The support frame itself is a
complex structure: it weighs 20 tons and must
support five times its own weight, with a
maximum deflection between being empty and
being fully loaded of only a couple of centimeters.
Installation of the eight-ton super-modules
requires a system of rails with a sophisticated
insertion device to bridge across to the support
structure.
The Electro-Magnetic Calorimeter (EM-Cal)
will add greatly to the high momentum particle
measurement capabilities of ALICE. It will
extend ALICE's reach to study jets and other
hard processes.
==== Photon Multiplicity Detector ====
The Photon Multiplicity Detector (PMD) is
a Particle shower detector which measures
the multiplicity and spatial distribution
of photons produced in the collisions. It
utilizes as a first layer a veto detector
to reject charged particles. Photons on the
other hand pass through a converter, initiating
an electromagnetic shower in a second detector
layer where they produce large signals on
several cells of its sensitive volume. Hadrons
on the other hand normally affect only one
cell and produce a signal representing minimum-ionizing
particles.
==== Forward Multiplicity Detector ====
The Forward Multiplicity Detector (FMD) extends
the coverage for multiplicity of charge particles
into the forward regions - giving ALICE the
widest coverage of the 4 LHC experiments for
these measurements.The FMD consist of 5 large
silicon discs with each 10 240 individual
detector channels to measure the charged particles
emitted at small angles relative to the beam.
FMD provides an independent measurement of
the orientation of the collisions in the vertical
plane, which can be used with measurements
from the barrel detector to investigate flow,
jets, etc.
==== Muon spectrometer ====
The ALICE forward muon spectrometer studies
the complete spectrum of heavy quarkonia (J/Ψ,
Ψ′, ϒ, ϒ′, ϒ′′) via their decay
in the μ+μ– channel. Heavy quarkonium
states, provide an essential tool to study
the early and hot stage of heavy-ion collisions.
In particular they are expected to be sensitive
to Quark-Gluon Plasma formation. In the presence
of a deconfined medium (i.e. QGP) with high
enough energy density, quarkonium states are
dissociated because of colour screening. This
leads to a suppression of their production
rates. At the high LHC collision energy, both
the charmonium states (J/Ψ and Ψ′) as
well as the bottomonium states (ϒ, ϒ′
and ϒ′′) can be studied. The Dimuon spectrometer
is optimized for the detection of these heavy
quark resonances.
Muons may be identified using the just described
technique by using the fact that they are
the only charged particles able to pass almost
undisturbed through any material. This behaviour
is connected to the fact that muons with momenta
below a few hundred GeV/c do not suffer from
radiative energy losses and so do not produce
electromagnetic showers. Also, because they
are leptons, they are not subject to strong
interactions with the nuclei of the material
they traverse. This behaviour is exploited
in muon spectrometers in high-energy physics
experiments by installing muon detectors behind
the calorimeter systems or behind thick absorber
materials. All charged particles other than
muons are completely stopped, producing electromagnetic
(and hadronic) showers.
The muon spectrometer in the forward region
of ALICE features a very thick and complex
front absorber and an additional muon filter
consisting of an iron wall 1.2 m thick. Muon
candidates selected from tracks penetrating
these absorbers are measured precisely in
a dedicated set of tracking detectors. Pairs
of muons are used to collect the spectrum
of heavy-quark vector-meson resonances (J/Psi).
Their production rates can be analysed as
a function of transverse momentum and collision
centrality in order to investigate dissociation
due to colour screening. The acceptance of
the ALICE Muon Spectrometer covers the pseudorapidity
interval 2.5 ≤ η ≤ 4 and the resonances
can be detected down to zero transverse momentum.
=== Characterization of the collision ===
Finally, we need to know how powerful the
collision was: this is done by measuring the
remnants of the colliding nuclei in detectors
made of high density materials located about
110 meters on both sides of ALICE (the ZDCs)
and by measuring with the FMD, V0 and T0 the
number of particles produced in the collision
and their spatial distribution. T0 also measures
with high precision the time when the event
takes place.
==== Zero Degree Calorimeter ====
The ZDCs are calorimeters which detect the
energy of the spectator nucleons in order
to determine the overlap region of the two
colliding nuclei. It is composed of four calorimeters,
two to detect protons (ZP) and two to detect
neutrons (ZN). They are located 115 meters
away from the interaction point on both sides,
exactly along the beam line. The ZN is placed
at zero degree with respect to the LHC beam
axis, between the two beam pipes. That is
why we call them Zero Degree Calorimeters
(ZDC).The ZP is positioned externally to the
outgoing beam pipe. The spectator protons
are separated from the ion beams by means
of the dipole magnet D1.
The ZDCs are "spaghetti calorimeters", made
by a stack of heavy metal plates grooved to
allocate a matrix of quartz fibres. Their
principle of operation is based on the detection
of Cherenkov light produced by the charged
particles of the shower in the fibers.
==== V0 detector ====
V0 is made of two arrays of scintillator counters
set on both sides of the ALICE interaction
point, and called V0-A and V0-C. The V0-C
counter is located upstream of the dimuon
arm absorber and cover the spectrometer acceptance
while the V0-A counter will be located at
around 3.5 m away from the collision vertex,
on the other side.
It is used to estimate the centrality of the
collision by summing up the energy deposited
in the two disks of V0. This observable scales
directly with the number of primary particles
generated in the collision and therefore to
the centrality.
V0 is also used as reference in Van Der Meer
scans that give the size and shape of colliding
beams and therefore the luminosity delivered
to the experiment.
==== T0 detector ====
ALICE T0 serves as a start, trigger and luminosity
detector for ALICE. The accurate interaction
time (START) serves as the reference signal
for the Time-of-Flight detector that is used
for particle identification. T0 supplies five
different trigger signals to the Central Trigger
Processor. The most important of these is
the T0 vertex providing prompt and accurate
confirmation of the location of the primary
interaction point along the beam axis within
the set boundaries. The detector is also used
for online luminosity monitoring providing
fast feedback to the accelerator team.
The T0 detector consists of two arrays of
Cherenkov counters (T0-C and T0-A) positioned
at the opposite sides of the interaction point
(IP). Each array has 12 cylindrical counters
equipped with a quartz radiator and a photomultiplier
tube.
=== ALICE Cosmic Rays Detector (ACORDE) ===
The ALICE underground cavern provides an ideal
place for the detection of high energy atmospheric
muons coming from cosmic ray showers. ACORDE
detects cosmic ray showers by triggering the
arrival of muons to the top of the ALICE magnet.
The ALICE cosmic ray trigger is made of 60
scintillator modules distributed on the 3
upper faces of the ALICE magnet yoke. The
array can be configured to trigger on single
or multi-muon events, from 2-fold coincidences
up to the whole array if desired. ACORDE's
high luminosity allows the recording of cosmic
events with very high multiplicity of parallel
muon tracks, the so-called muon bundles.
With ACORDE, the ALICE Experiment has been
able to detect muon bundles with the highest
multiplicity ever registered as well as to
indirectly measure very high energy primary
cosmic rays.
== Data acquisition ==
ALICE had to design a data acquisition system
that operates efficiently in two widely different
running modes: the very frequent but small
events, with few produced particles encountered
during proton-proton collisions and the relatively
rare, but extremely large events, with tens
of thousands of new particles produced in
lead-lead collisions at the LHC (L = 1027
cm−2 s−1 in Pb-Pb with 100 ns bunch crossings
and L = 1030-1031 cm−2 s−1 in pp with
25 ns bunch crossings).The ALICE data acquisition
system needs to balance its capacity to record
the steady stream of very large events resulting
from central collisions, with an ability to
select and record rare cross-section processes.
These requirements result in an aggregate
event building bandwidth of up to 2.5 GByte/s
and a storage capability of up to 1.25 GByte/s,
giving a total of more than 1 PByte of data
every year. As shown in the figure, ALICE
needs a data storage capacity that by far
exceeds that of the current generation of
experiments. This data rate is equivalent
to six times the contents of the Encyclopædia
Britannica every second.
The hardware of the ALICE DAQ system is largely
based on commodity components: PC's running
Linux and standard Ethernet switches for the
eventbuilding network. The required performances
are achieved by the interconnection of hundreds
of these PC's into a large DAQ fabric. The
software framework of the ALICE DAQ is called
DATE (ALICE Data Acquisition and Test Environment).
DATE is already in use today, during the construction
and testing phase of the experiment, while
evolving gradually towards the final production
system. Moreover, AFFAIR (A Flexible Fabric
and Application Information Recorder) is the
performance monitoring software developed
by the ALICE Data Acquisition project. AFFAIR
is largely based on open source code and is
composed of the following components: data
gathering, inter-node communication employing
DIM, fast and temporary round robin database
storage, and permanent storage and plot generation
using ROOT.
Finally. the ALICE experiment Mass Storage
System (MSS) combines a very high bandwidth
(1.25 GByte/s) and every year stores huge
amounts of data, more than 1 Pbytes. The mass
storage system is made of: a) Global Data
Storage (GDS) performing the temporary storage
of data at the experimental pit; b) Permanent
Data Storage (PDS) for long-term archive of
data in the CERN Computing Center and finally
from The Mass Storage System software managing
the creation, the access and the archive of
data.
== Results ==
The physics programme of ALICE includes the
following main topics: i) the study of the
thermalization of partons in the QGP with
focus on the massive charming beauty quarks
and understanding the behaviour of these heavy
quarks in relation to the stroungly-coupled
medium of QGP, ii) the study of the mechanisms
of energy loss that occur in the medium and
the dependencies of energy loss on the parton
species, iii) the dissociation of quarkonium
states which can be a probe of deconfinement
and of the temperature of the medium and finally
the production of thermal photons and low-mass
dileptons emitted by the QGP which is about
assessing the initial temperature and degrees
of freedom of the systems as well as the chiral
nature of the phase transition.
The ALICE collaboration presented its first
results from LHC proton collisions at a centre-of-mass
energy of 7 TeV in March 2010. The results
confirmed that the charged-particle multiplicity
is rising with energy faster than expected
while the shape of the multiplicity distribution
is not reproduced well by standard simulations.
The results were based on the analysis of
a sample of 300,000 proton–proton collisions
the ALICE experiment collected during the
first runs of the LHC with stable beams at
a centre-of-mass energy, √s, of 7 TeV,
In 2011, the ALICE Collaboration measured
the size of the system created in Pb-Pb collisions
at a centre-of-mass energy of 2.76 TeV per
nucleon pair. ALICE confirmed that the QCD
matter created in Pb-Pb collisions behaves
like a fluid, with strong collective motions
that are well described by hydrodynamic equations.
The fireball formed in nuclear collisions
at the LHC is hotter, lives longer and expands
to a larger size than the medium that was
formed in heavy-ion collisions at RHIC. Multiplicity
measurements by the ALICE experiment show
that the system initially has much higher
energy density and is at least 30% hotter
than at RHIC, resulting in about double the
particle multiplicity for each colliding nucleon
pair (Aamodt et al. 2010a). Further analyses,
in particular including the full dependence
of these observables on centrality, will provide
more insights into the properties of the system
– such as initial velocities, the equation
of state and the fluid viscosity – and strongly
constrain the theoretical modelling of heavy-ion
collisions.
=== A perfect liquid at the LHC ===
Off-centre nuclear collisions, with a finite
impact parameter, create a strongly asymmetric
"almond-shaped" fireball. However, experiments
cannot measure the spatial dimensions of the
interaction (except in special cases, for
example in the production of pions, see).
Instead, they measure the momentum distributions
of the emitted particles. A correlation between
the measured azimuthal momentum distribution
of particles emitted from the decaying fireball
and the initial spatial asymmetry can arise
only from multiple interactions between the
constituents of the created matter; in other
words it tells us about how the matter flows,
which is related to its equation of state
and its thermodynamic transport properties.The
measured azimuthal distribution of particles
in momentum space can be decomposed into Fourier
coefficients. The second Fourier coefficient
(v2), called elliptic flow, is particularly
sensitive to the internal friction or viscosity
of the fluid, or more precisely, η/s, the
ratio of the shear viscosity (η) to entropy
(s) of the system. For a good fluid such as
water, the η/s ratio is small. A "thick"
liquid, such as honey, has large values of
η/s.
In heavy-ion collisions at the LHC, the ALICE
collaboration found that the hot matter created
in the collision behaves like a fluid with
little friction, with η/s close to its lower
limit (almost zero viscosity). With these
measurements, ALICE has just begun to explore
the temperature dependence of η/s and we
anticipate many more in-depth flow-related
measurements at the LHC that will constrain
the hydrodynamic features of the QGP even
further.
=== Measuring the highest temperature on Earth
===
In August 2012 ALICE scientists announced
that their experiments produced quark–gluon
plasma with temperature at around 5.5 trillion
kelvins, the highest temperature mass achieved
in any physical experiments thus far. This
temperature is about 38% higher than the previous
record of about 4 trillion kelvins, achieved
in the 2010 experiments at the Brookhaven
National Laboratory.The ALICE results were
announced at the August 13 Quark Matter 2012
conference in Washington, D.C.. The quark–gluon
plasma produced by these experiments approximates
the conditions in the universe that existed
microseconds after the Big Bang, before the
matter coalesced into atoms.
=== Energy loss ===
A basic process in QCD is the energy loss
of a fast parton in a medium composed of colour
charges. This phenomenon, "jet quenching",
is especially useful in the study of the QGP,
using the naturally occurring products (jets)
of the hard scattering of quarks and gluons
from the incoming nuclei. A highly energetic
parton (a colour charge) probes the coloured
medium rather like an X-ray probes ordinary
matter. The production of these partonic probes
in hadronic collisions is well understood
within perturbative QCD. The theory also shows
that a parton traversing the medium will lose
a fraction of its energy in emitting many
soft (low energy) gluons. The amount of the
radiated energy is proportional to the density
of the medium and to the square of the path
length travelled by the parton in the medium.
Theory also predicts that the energy loss
depends on the flavour of the parton.
Jet quenching was first observed at RHIC by
measuring the yields of hadrons with high
transverse momentum. These particles are produced
via fragmentation of energetic partons. The
yields of these high-pT particles in central
nucleus–nucleus collisions were found to
be a factor of five lower than expected from
the measurements in proton–proton reactions.
ALICE has recently published the measurement
of charged particles in central heavy-ion
collisions at the LHC. As at RHIC, the production
of high-pT hadrons at the LHC is strongly
suppressed. However, the observations at the
LHC show qualitatively new features. The observation
from ALICE is consistent with reports from
the ATLAS and CMS collaborations on direct
evidence for parton energy loss within heavy-ion
collisions using fully reconstructed back-to-back
jets of particles associated with hard parton
scatterings. The latter two experiments have
shown a strong energy imbalance between the
jet and its recoiling partner (G Aad et al.
2010 and CMS collaboration 2011). This imbalance
is thought to arise because one of the jets
traversed the hot and dense matter, transferring
a substantial fraction of its energy to the
medium in a way that is not recovered by the
reconstruction of the jets.
=== Studying quarkonium hadroproduction ===
Quarkonia are bound states of heavy flavour
quarks (charm or bottom) and their antiquarks.
Two types of quarkonia have been extensively
studied: charmonia, which consist of a charm
quark and an anti-charm, and bottomonia made
of a bottom and an anti-bottom quark. Charm
and anticharm quarks in the presence of the
Quark Gluon Plasma, in which there are many
free colour charges, are not able to see each
other any more and therefore they cannot form
bound states. The "melting" of quarkonia into
the QGP manifests itself in the suppression
of the quarkonium yields compared to the production
without the presence of the QGP. The search
for quarkonia suppression as a QGP signature
started 25 years ago. The first ALICE results
for charm hadrons in PbPb collisions at a
centre-of-mass energy √sNN = 2.76 TeV indicate
strong in-medium energy loss for charm and
strange quarks that is an indication of the
formation of the hot medium of QGP.As the
temperature increases so does the colour screening
resulting in greater suppression of the quarkonium
states as it is more difficult for charm – anticharm
or bottom – antibottom to form new bound
states. At very high temperatures no quarkonium
states are expected to survive; they melt
in the QGP. Quarkonium sequential suppression
is therefore considered as a QGP thermometer,
as states with different masses have different
sizes and are expected to be screened and
dissociated at different temperatures. However
- as the collision energy increases - so does
the number of charm-anticharm quarks that
can form bound states, and a balancing mechanism
of recombination of quarkonia may appear as
we move to higher energies.
The results from the first ALICE run are rather
striking, when compared with the observations
from lower energies. While a similar suppression
is observed at LHC energies for peripheral
collisions, when moving towards more head-on
collisions – as quantified by the increasing
number of nucleons in the lead nuclei participating
in the interaction – the suppression no
longer increases. Therefore, despite the higher
temperatures attained in the nuclear collisions
at the LHC, more J/ψ mesons are detected
by the ALICE experiment in Pb–Pb with respect
to p–p. Such an effect is likely to be related
to a regeneration process occurring at the
temperature boundary between the QGP and a
hot gas of hadrons.
The suppression of charmonium states was also
observed in proton-lead collisions at the
LHC, in which Quark Gluon Plasma is not formed.
This suggests that the observed suppression
in proton-nucleus collisions (pA) is due to
cold nuclear matter effects. Grasping the
wealth of experimental results requires understanding
the medium modification of quarkonia and disentangling
hot and cold-matter effects. Today there is
a large amount of data available from RHIC
and LHC on charmonium and bottomonium suppression
and ALICE tries to distinguish between effects
due to the formation of the QGP and those
from cold nuclear matter effects.
=== Double-ridge structure in p-Pb collisions
===
The analysis of the data from the p-Pb collisions
at the LHC revealed a completely unexpected
double-ridge structure with so far unknown
origin. The proton–lead (pPb) collisions
in 2013, two years after its heavy-ion collisions
opened a new chapter in exploration of the
properties of the deconfined, chirally symmetrical
state of the QGP. A surprising near-side,
long-range (elongated in pseudorapidity) correlation,
forming a ridge-like structure observed in
high-multiplicity pp collisions, was also
found in high-multiplicity pPb collisions,
but with a much larger amplitude (). However,
the biggest surprise came from the observation
that this near-side ridge is accompanied by
an essentially symmetrical away-side ridge,
opposite in azimuth (CERN Courier March 2013
p6). This double ridge was revealed after
the short-range correlations arising from
jet fragmentation and resonance decays were
suppressed by subtracting the correlation
distribution measured for low-multiplicity
events from the one for high-multiplicity
events.
Similar long-range structures in heavy-ion
collisions have been attributed to the collective
flow of particles emitted from a thermalized
system undergoing a collective hydrodynamic
expansion. This anisotropy can be characterized
by means of the vn (n = 2, 3, ...) coefficients
of a Fourier decomposition of the single-particle
azimuthal distribution. To test the possible
presence of collective phenomena further,
the ALICE collaboration has extended the two-particle
correlation analysis to identified particles,
checking for a potential mass ordering of
the v2 harmonic coefficients. Such an ordering
in mass was observed in heavy-ion collisions,
where it was interpreted to arise from a common
radial boost – the so-called radial flow
– coupled to the anisotropy in momentum
space. Continuing the surprises, a clear particle-mass
ordering, similar to the one observed in mid-central
PbPb collisions (CERN Courier, September 2013),
has been measured in high-multiplicity pPb
collisions.
The final surprise, so far, comes from the
charmonium states. Whereas J/ψ production
does not reveal any unexpected behaviour,
the production of the heavier and less-bound
(2S) state indicates a strong suppression
(0.5–0.7) with respect to J/ψ, when compared
with pp collisions. Is this a hint of effects
of the medium? Indeed, in heavy-ion collisions,
such a suppression has been interpreted as
a sequential melting of quarkonia states,
depending on their binding energy and the
temperature of the QGP created in these collisions.
The first pPb measurement campaign, expected
results were widely accompanied by unanticipated
observations. Among the expected results is
the confirmation that proton–nucleus collisions
provide an appropriate tool to study the partonic
structure of cold nuclear matter in detail.
The surprises have come from the similarity
of several observables between pPb and PbPb
collisions, which hint at the existence of
collective phenomena in pPb collisions with
high particle multiplicity and, eventually,
the formation of QGP.
== Upgrades and future plans ==
=== 
Long Shutdown 1 ===
The main upgrade activity on ALICE during
LHC's Long Shutdown 1 was the installation
of the dijet calorimeter (DCAL), an extension
of the existing EMCAL system that adds 60°
of azimuthal acceptance opposite the existing
120° of the EMCAL's acceptance. This new
subdetector will be installed on the bottom
of the solenoid magnet, which currently houses
three modules of the photon spectrometer (PHOS).
Moreover, an entirely new rail system and
cradle will be installed to support the three
PHOS modules and eight DCAL modules, which
together weigh more than 100 tones. The installation
of five modules of the TRD will follow and
so complete this complex detector system,
which consists of 18 units,
In addition to these mainstream detector activities,
all of the 18 ALICE subdetectors underwent
major improvements during LS1 while the computers
and discs of the online systems are replaced,
followed by upgrades of the operating systems
and online software.
All of these efforts are to ensure that ALICE
is in good shape for the three-year LHC running
period after LS1, when the collaboration looks
forward to heavy-ion collisions at the top
LHC energy of 5.5 TeV/nucleon at luminosities
in excess of 1027 Hz/cm2.
=== Long shutdown 2 (2018) ===
The ALICE collaboration has plans for a major
upgrade during the next long shutdown, LS2,
currently scheduled for 2018. Then the entire
silicon tracker will be replaced by a monolithic-pixel
tracker system built from ALPIDE chips; the
time-projection chamber will be upgraded with
gaseous electron-multiplier (GEM) detectors
for continuous read-out and the use of new
microelectronics; and all of the other subdetectors
and the online systems will prepare for a
100-fold increase in the number of events
written to tape
