# Recent results from ALICE

Yuri Kharlov, for the ALICE collaboration  Institute for High Energy Physics,
Protvino, 142281 Russia

###### Abstract

The ALICE experiment at the LHC has collected wealthy data in proton-proton
and lead-lead collisions. An overview of recent ALICE results is given in this
paper. Hadron spectra measured in pp collisions at $\sqrt{s}=0.9$, 2.76 and 7
TeV are discussed. Properties of hot nuclear matter produced in Pb-Pb
collisions at $\sqrt{s_{{}_{NN}}}=2.76$ TeV, revealed via many observables
measured with the ALICE experiment, are shown.

##  1 Introduction

The ALICE experiment was designed to study interactions of heavy ions at the
LHC. This goal determines the unique performance of the ALICE detectors to
reconstruct events with very high multiplicity and to measure spectra of
identified hadrons, electrons, photons, muons in a wide energy range.

The data collected with the ALICE experiment with pp collisions at
$\sqrt{s}=7$ TeV in 2010–2011, consist of a minimum-bias sample with
integrated luminosity $\int{\cal L}dT=16\leavevmode\nobreak\ \mbox{nb}^{-1}$
and a sample recorded with rare-event triggers with integrated luminosity
$\int{\cal L}dT=4.9\leavevmode\nobreak\ \mbox{pb}^{-1}$. Rare-event triggers
implemented pp collisions were based in EMCAL, PHOS and MUON detectors.
Limited data samples with the proton beams at collision energies
$\sqrt{s}=0.9$ and 2.76 TeV have been also recorded with integrated
luminosities $\int{\cal L}dT=0.14\mbox{\leavevmode\nobreak\
and\leavevmode\nobreak\ }1.3\leavevmode\nobreak\ \mbox{nb}^{-1}$ respectively.

The LHC has delivered heavy-ion collisions at the center-of-mass energy
$\sqrt{s_{{}_{NN}}}=2.76$ TeV to the ALICE experiment in 2010 with integrated
luminosity $\int{\cal L}dT=10\leavevmode\nobreak\ \mu\mbox{b}^{-1}$, and the
data set of 2011 exceeded the previous one by an order of magnitude. Data
taking of heavy ion collisions recorded in 2010 was dominated by the minimum
bias trigger. In 2011, a fraction of minimum bias events was suppressed in
favor of the triggers on the most central and semi-central events, as well as
rare events which selected events with high-energy clusters in the
electromagnetic calorimeters, muon tracks in the muon spectrometer, ultra-
peripheral collisions.

##  2 QCD tests in proton-proton collisions

Properties of hot nuclear matter produced in heavy ion collisions are studied
via a comprehensive set of observables. As a reference, similar observables
are measured in proton-proton collisions. ALICE is performing detailed studies
of hadron production spectra in pp collisions at all center-mass energies
provided by the LHC. Apart of being a reference for heavy ion collisions, pp
collisions is considered as a powerful tool for QCD studying. Advance particle
identification capabilities Aamodt:2008zz  and a moderate magnetic field
($B=0.5$ T) allow to measure a variety of hadron spectra in a wide momentum
range.

Identified charged hadron production in mid-rapidity are measured by the
central tracking system consisting of the Inner Tracking System detector
(ITS), Time Projection Chamber (TPC), Time-of-Flight detector (TOF) and a
Cherenkov High-Momentum Particle Identification detector (HMPID). Each of
these detectors provide particle identification in different complimentary
momentum ranges, which allows to measure the spectra in a wide $p_{\rm t}$
range. ALICE has already published production spectra of $\pi^{\pm}$,
$K^{\pm}$, p, $\bar{\mbox{p}}$ in pp collisions at $\sqrt{s}=0.9$ TeV
PIDhadron900GeV , and has reported preliminary results on those spectra in pp
collisions at $\sqrt{s}=7$ TeV PIDhadron7TeV . Similar to charged hadrons,
ALICE is able to measure neutral meson spectra by complimentary and redundant
methods which ensures the result validity. Neutral pion and $\eta$-meson
spectra were measured in pp collisions at all three LHC energies by the Photon
Spectrometer (PHOS) which detected real photons and by the central tracking
system which identifies photons converted to $e^{+}e^{-}$ pairs on the medium
of the inner ALICE detectors pp-pi0 . Combined analysis of all ALICE detectors
allowed to measure spectra of resonance production, in particular strange and
charmed hadrons.

Results obtained by the ALICE on hadron production in pp collisions show a
gradual increase of the mean transverse momentum with collision energy.
Observed ratio between antiprotons and protons suggests that baryon-antibaryon
asymmetry is restoring at high energies. Comparison of all measured spectra
with Monte Carlo event generators and with next-to-leading perturbative QCD
calculations demonstrates that no model can correctly describe spectra of
hadron production at LHC energies.

##  3 Global event properties in Pb-Pb collisions

Centrality of the collision, directly related to the impact parameter and to
the number of nucleons $N_{\rm part}$ participating in the collision, allows
to study particle production versus density of the colliding system. Various
ALICE detectors measure collision centrality, among them the best accuracy is
achieved with the scintillator hodoscope VZERO covering pseudorapidity ranges
$2.8<\eta<5.1$ and $-3.7<\eta<-1.7$. Distribution of the sum of amplitudes in
VZERO in minimum bias Pb-Pb collisions is shown in Fig.1 bib:PbPb-dNdy .
Centrality classes were defined by the Glauber model, and the fit of the
Glauber model to the data is shown by a solid line in this plot.

Figure 1: Centrality determination in ALICE. Glauber model fit to the VZERO
amplitude with the inset of a zoom of the most peripheral region.

Charged particle multiplicity in Pb-Pb collisions at $\sqrt{s_{{}_{NN}}}=2.76$
TeV and its dependence on the collision centrality was measured with the
Silicon Pixel Detector (SPD), two innermost layers of the barrel tracking
system covering the pseudorapidity range $|\eta|<1.4$. The charged particle
density, normalized to the average number of participants in a given
centrality class, $dN_{\rm ch}/d\eta/\left(\langle N_{\rm
part}\rangle\right)$, measured by ALICE, was compared with similar
measurements at lower energies at RHIC and SPS (Fig.2) bib:PbPb-dNdy_central .

Figure 2: Charged track density $dN/d\eta$ in pp and AA collisions vs
collision energy.

In the most central events (centrality $0-5\%$) at LHC energy the charged
particle density was found to be $dN_{\rm ch}/d\eta=1601\pm 60$ which is,
being normalized to the number of participants, is 2.1 times larger than the
charged particle density measured at RHIC at $\sqrt{s_{{}_{NN}}}=200$ GeV and
1.9 times larger than that in pp collisions at $\sqrt{s}=2.36$ TeV.

##  4 Collective phenomena in heavy ion collisions

The initial anisotropy of nuclei non-central collisions leads to anisotropic
distribution on initial matter in the overlapping reagion. During evolution of
the matter, the spatial asymmetry of initial state is converted to an
anisotropic momentum distribution. The azimuthal distribution of the particle
yield can be described by a Fourier series of a function of the angle between
the particle direction $\varphi$ and the reaction place $\Psi_{\rm RP}$. The
second coefficient of this series, $v_{2}$, is referred to as elliptic flow.
Theoretical models, based on relativistic hydrodynamics bib:hydro-v2_Kestin ;
bib:hydro-v2_Niemi , successfully described the elliptic flow observed at RHIC
and predict its increase at LHC energies from 10% to 30%.

The first measurements of elliptic flow of charged particles in Pb-Pb
collisions at $\sqrt{s_{{}_{NN}}}=2.76$ TeV were reported by ALICE in
bib:ALICE-v2 . Charged tracks were detected and reconstructed in the central
barrel tracking system, consisting of ITS and TPC. Comparison of elliptic flow
integrated over $p_{\rm t}$ measured by the ALICE and lower-energy experiments
is shown in Fig.3.

Figure 3: Azimuthal flow $v_{2}$ of charged particles measured by the ALICE in
Pb-Pb collisions at $\sqrt{s_{{}_{NN}}}=2.76$ TeV in comparison with the
lower-energy experiment results.

The observed trend of $v_{2}$ vs $\sqrt{s_{{}_{NN}}}$ confirms model
expectations that the value of $v_{2}$ in Pb-Pb collisions at
$\sqrt{s_{{}_{NN}}}=2.76$ TeV increases by about 30% with respect to $v_{2}$
in Au-Au collisions at $\sqrt{s_{{}_{NN}}}=0.2$ TeV.

The results of global event properties and collective expantion studied by
ALICE, studied via the azimuthal anisotropy and intensity interferometry of
identical particles bib:HBT , indicate that the fireball formed in nuclear
collisions at the LHC is hotter, lives longer, and expands to a larger size at
freeze-out as compared to lower energies.

##  5 Strangeness production in heavy ion collisions

Strange particle production has been considered as a probe of strongly
interacting matter by heavy-ion experiments at AGS, SPS and RHIC. We have
already demonstrated that ALICE, due to its powerful particle identification
technique, has measured strange particle spectra in pp collisions. Similar
analysis was performed on the Pb-Pb data collected in 2010. Comparison of
strange meson and baryon production is illustrated by the $\Lambda/K^{0}_{S}$
ratio measured by ALICE in different centrality classes (Fig.4, left). This
ratio in peripheral Pb-Pb collision is similar to that one measured in pp
collisions, but it grows with centrality, increasing the value of 1.5 in the
most central collisions. The qualitative behaviour of this ratio on $p_{\rm
t}$ at the LHC collision energy is similar to the ratio measured at RHIC by
the STAR experiment (Fig.4, right). An enhancement of strange and multi-
strange baryons ($\Omega^{-}$, $\bar{\Omega}^{+}$,
$\Sigma^{-}$,$\bar{\Sigma}^{+}$ ) was obsevred in heavy-ion collisions by
experiments at lower energies, and was confirmed by ALICE at LHC energy ALICE-
Hippolyte . It was also shown that multi-strange baryon enhancement scales
with the number of participants $N_{\rm part}$ and decreases with the
collision energy.

The large yield of strange, and especially multi-strange baryons in heavy-ion
collision was observed earlier at SPS and RHIC. This effect supports
predictions of quark-gluon plasma formation which assumed that strange
antiquarks are as abundant as light antiquarks in quark matter. The strange
quark phase space becomes fully equilibrated, and therefore all strange
hadrons are produced more abundantly. An overview of strangeness production in
heavy ion collisions can be found in bib:Muller2011 .

Figure 4: Ratio $\Lambda/K^{0}_{S}$ in Pb-Pb collisions at
$\sqrt{s_{{}_{NN}}}=2.76$ TeV in different centralities (left) and comparison
of this ratio at LHC and RHIC in centralities $0-5\%$ and $60-80\%$ (right).

##  6 Parton energy loss in medium

Final-state partons produced at the initial stage of nucleus-nucleus
collision, pass through medium with multiple secondary interactions. Energy
loss by partons depends on density and temperature of the QCD medium. Hadrons
produced in fragmentation of these partons should be suppressed compared to
expectations from an independent superposition of nucleon-nucleon collisions.
The strength of suppression of a hadron $h$ is expressed by the nuclear
modification factor $R_{AA}$, defined as a ratio of the particle spectrum in
heavy-ion collision to that in pp, scaled by the number of binary nucleon-
nucleon collisions $N_{\rm coll}$:

$R_{AA}(p_{\rm t})=\frac{(1/N_{AA})d^{2}N_{h}^{AA}/dp_{\rm t}d\eta}{N_{\rm
coll}(1/N_{pp})d^{2}N_{h}^{pp}/dp_{\rm t}d\eta}.$ (1)

Experiments at RHIC reported that hadron production at high transverse
momentum in central Au-Au collisions at a center-of-mass energy per nucleon
pair $\sqrt{s_{{}_{NN}}}=200$ GeV is suppressed by a factor $4-5$ with respect
to pp collisions. At the larger LHC energy, the density of the medium is
expected to be higher than at RHIC, leading to a larger energy loss of
high-$p_{\rm t}$ partons. However, the hadron production spectra are less
steeply falling with $p_{\rm t}$ at LHC than at RHIC which would reduce the
value of $R_{AA}$ for a given value of the parton energy loss.

ALICE has measured the nuclear modification factor $R_{AA}$ for many
particles. All charged particles, detected in the ALICE central tracking
system (ITS and TPC), show a spectrum suppression Otwinowski:2011gq  which is
qualitatively similar to that observed at RHIC (Fig.5).

Figure 5: Nuclear modification factor $R_{AA}$ of charged particles.

However, quantitative comparison with RHIC demonstrates that the suppression
at LHC energy is stronger which can be interpreted by a denser medium.
Benefiting from particle identification which has been already mention earlier
in this paper, ALICE has measured suppression of various identified hadrons,
which provides experimental data for studying the flavor and mass dependence
of the spectra suppression.

A nuclear modification factor $R_{AA}$ of charged pion production in mid-
rapidity (Fig.6) has lower values in the range of moderate transverse momenta
($3<p_{\rm t}<7-10$ GeV/$c$) than that of unidentified charged particles, but
at higher $p_{\rm t}$ it coincides with all charged particles.

Figure 6: Nuclear modification factor $R_{AA}$ of charged pions.

To the contrary to charged pions, strange hadrons ($K^{0}_{S}$, $\Lambda$) are
less suppressed in the most central collisions compared to all charged
particles (Fig.7). This is explained by the fact that strange quark production
is enhanced in a hot nuclear medium, and this strangeness enhancement
partially compensates energy loss of strange quarks, such that the overall
$R_{AA}$ value becomes larger than for pions. Lambda hyperons have no
suppression at $p_{\rm t}<3-4$ GeV/$c$, which is interpreted by an additional
baryon enhancement in central heavy-ion collisions.

ALICE has reported also the first measurements of $D$ meson suppression
bib:PbPb-Dmesons  in Pb-Pb collisions in two centrality classes, $0-20\%$ and
$40-80\%$, shown in Fig.7. It was shown that the $R_{AA}$ values for $D^{0}$,
$D^{+}$ and $D^{*+}$ are consistent with each other within the statistical and
systematical uncertainties. Although the statistics of the ALICE run 2010 is
marginal for $D$ meson measurement, the obtained result shows a hint that the
$D$ mesons are less suppressed than charged pions.

Figure 7: Nuclear modification factor $R_{AA}$ of charged particles, $K^{0}$,
$\Lambda$, $\pi^{\pm}$, $D^{+}$, $D^{0}$, $D^{*+}$ in central (left) and
peripheral (right) collisions.

Nuclear modification factor $R_{AA}$ is $J/\psi$ production in Pb-Pb
collisions was measured by the ALICE in two kinematic regions: in the forward
rapidity with the muon spectrometer and in mid-rapidity deploying the central
tracking system bib:PbPb-Jpsi . The result of $R_{AA}$ measurement in forward
rapidity shows almost no dependence on collision centrality with the average
value $R_{AA}=0.545\pm 0.032\leavevmode\nobreak\ \mbox{(stat.)}\pm
0.084\mbox{(syst.)}$ which is significantly different from the RHIC results.

##  7 Conclusion

The ALICE collaboration is performing QCD studies via hadron production
measurements in proton-proton collisions. Obtained results in pp collisions at
$\sqrt{s}=0.9,2.76$ and 7 TeV show statistically significant deviations from
models which well described lower-energy results. Therefore new experimental
results from pp collision allow to tune various phenomenological models and
pQCD calculations.

Comprehensive studies of heavy-ion collisions, performed by the ALICE
experiment show that the properties of strongly interacting nuclear matter
produced at the LHC energy, qualitatively similar to those observed at RHIC
and reveal smooth evolution with collision energy. The matter produced at LHC
has about 3 times larger energy density, twice larger volume of homogeneity
and about 20% larger lifetime. Like at RHIC, the matter at LHC reveals the
properties on an almost perfect liquid. Particle suppression appeared to be
stronger at LHC than at RHIC which is also an evidence of denser medium
produced at LHC.

This work was parially supported by the RFBR grant 10-02-91052.

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