The present: measurements performed so far in p-p, p-Pb, Pb-Pb
First-day observables in p-p and Pb-Pb with ALICE
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Transcript of First-day observables in p-p and Pb-Pb with ALICE
Francesco PrinoINFN – Sezione di Torino
INFN, Commissione III, Genova, September 22nd 2009
First-day observables in p-p First-day observables in p-p and Pb-Pb with ALICEand Pb-Pb with ALICE
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9 years ago: first data at RHIC9 years ago: first data at RHICResults published in the first year after RHIC startup: Multiplicity of unidentified particles at midrapidity
PHOBOS, sent to PRL on July 19th 2000 PHENIX, sent to PRL on Dec 21th 2000
Elliptic flow of unidentified particles STAR, sent to PRL on Sept 13th 2000
Particle to anti-particle ratios STAR, sent to PRL on Apr 13th 2001 PHOBOS, sent to PRL on Apr 17th 2001 BRAHMS, sent to PRL on Apr 28th 2001
Transverse energy distributions PHENIX, sent to PRL on April 18th 2001
Pseudorapidity distributions of charged particles PHOBOS, sent to PRL on June 6th 2001 BRAHMS, sent to Phys Lett B on Aug 6th 2001
Elliptic flow of identified particles STAR, sent to PRL July 5th 2001
… then came the high pT particle suppression from PHENIX (sent to PRL on Sept 9th 2008)
First 10k-20k events, fast analysis
statistics<≈100k events,longer analysis time due to the need of PID, detector calibration, combination of different detectors
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OutlineOutlineThree examples of “first day” observables Multiplicities of unidentified particles
First-day analysis from the first 10-20 k events both in p-p and Pb-Pb
Abundances and pT spectra of identified hadrons (, K, p)
Small statistics needed both in p-p and Pb-Pb, longer analysis time
Elliptic flow First-day analysis from the first 20 k Pb-Pb events
For each observable Physics motivation (in p-p and Pb-Pb) What do we need? The tools
Interaction vertex reconstruction, centrality determination, tracking, PID ...
Analysis algorithms, corrections and systematics
Where we are? Analysis readiness
First tool: ALICE at the LHCFirst tool: ALICE at the LHC
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Second tool: the GridSecond tool: the GridProductions Several production
dedicated to p-p first physics in 2009 4 M events generated,
reconstructed and analyzed specifically for first physics
Plus 108 min. bias p-p events
142 k Pb-Pb events
Analysis Organized as analysis
tasks (wagons of a common analysis train) running on the grid on ESD/AOD
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Multiplicity of unidentified Multiplicity of unidentified particlesparticles
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Nominal LHC
energy
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Physics motivationPhysics motivationp-p @ √s=900 GeV First measurement at the LHC Comparison with existing
measurements
p-p @ √s=7-14 TeV Test (soft) particle production
models in a new energy regime In hadronic and nuclear collisions
particle production is dominated by (non-perturbative) processes with small momentum transfer. Many models, but understanding of multiplicities based on first principles is missing.
Multiplicity in Pb-Pb contains information about: Energy density of the system (via Bjorken formula) Geometry (centrality) of the collision
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Factorized dependence of dNch/dmax on centrality and s reproduced by models based on gluon density saturation at small values of Bjorken x
RHIC results and modelingRHIC results and modeling
increasing s – decreasing x
Armesto Salgado Wiedemann, PRL 94 (2005) 022002
Kharzeev, Nardi, PLB 507 (2001) 121.
3
1
0
0
][2
partch
part
NGeVsNd
dN
N
Pocket formula:
and from ep and eA data
N0 only free parameter
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Towards the LHC (I)Towards the LHC (I)
Models prior to RHIC
Extrapolation of dN/dln s
5500
Saturation modelArmesto Salgado Wiedemann, PRL 94 (2005) 022002
16502.82/
/
00
d
dN
N
ddN ch
part
ch
Central collisions
Extrapolation of dNch/dmax vs s Fit to dN/d ln s Saturation model (dN/d s with =0.288) Clearly distinguishable with the first 10k events at the LHC
11005.52/
/
00
d
dN
N
ddN ch
part
ch
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Towards the LHC (II)Towards the LHC (II)Extrapolation of limiting fragmentation behavior Persistence of extended longitudinal scaling implies that
dN/d grows at most logarithmically with s difficult to reconcile with saturation models
Log extrapolationdN/d ≈ 1100
Saturation modeldN/d ≈ 1600
Borghini Wiedemann, J. Phys G35 (2008) 023001
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ALICE: figure of meritALICE: figure of meritWide angular coverage about 9 units in pseudorapidity
Different detection techniques Tracks in central barrel (ITS+TPC) Tracklets in SPD Occupancy in FMD
Tools: trigger and tagging of Tools: trigger and tagging of diffractive events in p-pdiffractive events in p-p
Minimum Bias trigger: SPDFastOr or V0A or V0C Also ZDCs and ZEM can provide a p-p MB
trigger (ZPA or ZNA or ZPC or ZNC or ZEM) Trigger efficiency (from Pythia @ 3.5+3.5 TeV) =91% Trigger efficiency independent of multiplicity in central barrel
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ZP
ZN
M
Single Diffraction(SD) ≈10mb
-10 -5 0 5 10 η
Φ
-10 -5 0 5 10 η
Φ
-10 -5 0 5 10 η
Φ
Double Diffraction(DD) ≈7 mb
Non-diffractive inelastic (ND) ≈65 mb
From 50k PYTHIA p-p @ 7 TeV (LHC09b12)SD trigger efficiency: 52%SD trigger purity: 50%ND events in MB sample: 68%ND events tagged as SD: 5.2%
From 50k PYTHIA p-p @ 7 TeV (LHC09b12)SD trigger efficiency: 52%SD trigger purity: 50%ND events in MB sample: 68%ND events tagged as SD: 5.2%
Tagging of diffractive events: based on signal only on one
side Signal in ZNC or ZPC No signal in ZNA and ZPA and ZEM
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Tools: centrality determination in Pb-PbTools: centrality determination in Pb-PbCentrality measurement from EZDC (deposited energy in ZDC) vs. EZEM (=deposited energy in ZEM) correlation Centrality classes defined by selecting events from the correlation
corresponding to certain fractions of the inelastic cross section
EZDC vs. EZEM b Nparticipants
Glauber model
Nparticipants
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Tools: Vertex reconstruction (I)Tools: Vertex reconstruction (I)
SPD RecPoints
Good (crossing the beam pipe, small DCA) tracklets
Fake (rejected by the vertexing algo) tracklets
Primary vertex
Reconstruction from SPD tracklets Tracklets = pairs of associated reconstructed points in the two
innermost ITS layers
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Tools: Vertex reconstruction (II)Tools: Vertex reconstruction (II)Reconstruction from SPD tracklets Available before tracking, used to seed the Kalman filter OK for multiplicity analyses (high efficiency, sufficient
resolution) For 80% of triggered events reconstruction in 3D available, for 15% (low
multiplicity) of triggered events only Z coordinate
Multiplicity from trackletsMultiplicity from tracklets
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Features: Large and pT acceptance
Less stringent calibration needs Suitable for the very first data
First measurement that ALICE will be able to perform in p-p and Pb-Pb
Several corrections needed Background from secondaries Algorithm efficiency Detector efficiency+acceptance Vertexing efficiency Trigger efficiency
p-p @ 7 TeV (Pythia) - LHC09b12
Multiplicity and pMultiplicity and pTT spectra of spectra of
identified particles identified particles
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Physics motivation: spectraPhysics motivation: spectrap-p @ 900 GeV Comparison with existing measurements
p-p @ 7/10/14 TeV Test for particle production models that combine perturbative QCD
for the description of hard partonic interaction and phenomenological approaches for the soft component of the spectrum
Reference for pT spectra in Pb-Pb
Pb-Pb: slope of pT spectra in the soft-pT region (< 1 GeV/c) sensitive to temperature at thermal freeze-out and radial flow Flow = collective motion superposed on top of
the thermal motion Due to large pressures arising from compressing and
heating the nuclear matter
Test of hydrodynamics models
x
y
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Physics motivation: abundancesPhysics motivation: abundancesHadron abundances: Small s (< 5 GeV):
fireball dominated by stopped particles
High baryonic content
Importance of isospin and quarks “stopped” from colliding nuclei
Large s (> 20 GeV):Fireball dominated by
produces particles
Low baryonic content
Mass hierarchy ( N > NK > Np )
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Statistical hadronization modelsStatistical hadronization modelsFit measured particle abundances (or ratios) with hadron densites from grand canonical partition function Temperature T and chemical potential B are free parameters
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22
)1(
2
)ln(1),(
k
ii
ki
ki
i
GCi
ii T
kmKm
k
TgZT
VTn
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Towards the LHCTowards the LHCExtrapolations to LHC of T and B trend vs. √s TLHC = 161±4 MeV B
LHC=0.8 MeV
A. Andronic et al. in arXiv:0711.0974 [hep-ph]
MC simulations: p-p
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Tools: tracking - ITS+TPC+TRDTools: tracking - ITS+TPC+TRDTrack reconstruction: Start from TPC signals in the outer pads + SPD vertex -> move
inward Match TPC tracks to points in outer ITS layer -> follow the track until
the innermost ITS layer Back propagate to outer TPC radius and attach TRD points
Extrapolate to outer detectors (TOF, PHOS, HMPID, EMCAL)
Refit the track inward (TRD, TPC, ITS) and propagate to SPD vertex
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Tools: tracking - ITS standaloneTools: tracking - ITS standaloneGroup clusters in , windows on the 6 layers Starting point (seed): SPD vertex + a cluster in
one of the inner ITS layers (1, 2 or 3) Extrapolation to next layer taking into account
trajectory curvature N iterations increasing at each step the ,
window sizeTrack fitted with Kalman filterGoals: Recover tracks missed by the TPC Extend low-pT reach w.r.t. TPC+ITS tracks
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Tools: PIDTools: PIDHadron identification in ALICE barrel based on: Momentum from track parameters Velocity related information (dE/dx, time of flight, Čerenkov
light...) specific for each detector
Different systems are efficient in different momentum ranges and for different particles
EMCAL +
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Particle identification with TOFParticle identification with TOF
K p
i
i
specieofparticles generated
identifiedcorrectlyspecieofparticlesEff
i
i
specieas identifiedparticles
specieas identifiedwrongly particlesCont
Features: Large acceptance (surface = 140 m2) High efficiency (>95%) Excellent time resolution (<100 ps)
Nominal resolution including all possible contributions = 80 ps
High granularity (105 channels)
5 modules in z
18 modules in
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Hadron spectra with PID in TOFHadron spectra with PID in TOF
Efficiency x acceptance for , K , p including: Tracking (ITS+TPC+TRD)
efficiency Track-TOF matching efficiency
≈80% for with 1.75<pT<2 GeV/c (including dead regions of TOF)
Identification efficiency
Spectra from few 106 p-p MB events (first day of data taking) Good accuracy up to pT
2.5 GeV/c For pT > 2.5 GeV/c
correction for contamination in PID needed
Elliptic flowElliptic flow
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Anisotropic transverse flowAnisotropic transverse flowIn heavy ion collisions with b≠0 the impact parameter selects a preferred direction in the transverse plane The fireball shows an initial geometrical anisotropy with respect to
the reaction plane Re-scatterings among produced particles convert this initial
geometrical anisotropy into an observable momentum anisotropy
Anisotropic transverse flow is a collective motion giving rise to a correlation between the azimuth [=tan-1 (py/px)] of the produced particles and the impact parameter (reaction plane) The initial particle momentum
distribution is isotropic Pressure gradients in the
transverse plane are anisotropic (= dependent)
Larger pressure gradient in the x,z plane (along impact parameter) that along y
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x
yz
Reaction plane
Elliptic flow = 2nd harmonic in Fourier expansion of particle distributions
At time = 0: Geometrical anisotropy Isotropic distribution of momentaInteraction among constituents Transform initial spatial anisotropy into a momentum anisotropy Hydrodynamics to describe the system evolution from equilibration
time until thermal freeze-outThe mechanism is self quenching The driving force dominate at early times
Elliptic flow Elliptic flow
RPv 2cos2
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Towards the LHC (I)Towards the LHC (I)Ideal hydro reproduces central collisions at RHIC Fluid created in Au-Au at RHIC has exceptionally low viscosity But also hints for incomplete equilibration / non zero viscosity
E.g. no hint for saturation in v2 vs. dN/dy
0.3
40 45 50
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Towards the LHC (II)Towards the LHC (II)Extended longitudinal scaling of v2 vs Naturally accounted in a low-density limit scenario (with
v2dN/d) Extrapolations of ideal hydrodynamics from RHIC to LHC
predict values not exceeding v2=0.06 at =0
The first 20,000 Pb-Pb events at LHC will bring new pieces of evidence to understand the picture
Tools: estimate the reaction planeTools: estimate the reaction planeReaction plane estimated from the (second harmonic) anisotropy of reconstructed tracks in ITS+TPC+TRD Event plane = estimator of the unknown reaction plane
Event plane resolution depends on v2 of produced particles Event multiplicity
Correct v2 for event plane resolution:
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ii
ii
w
w
2cos
2sintan
2
1 12
RP
v
2
22 2cos
2cos
GEANT-based simulation
Centroid resolution vs Neutron Multiplicity <cos(φZN-
φRP)> vs centrality
Tools: reaction plane from ZDCTools: reaction plane from ZDCReaction plane estimated by measuring the bounce-off of the spectator neutrons in ZDC Independent estimate, reduced non-flow correlations Allow to study v1 and the sign of v2
Resolution on ZDC event plane depends on: v1 of spectator neutrons Neutron multiplicity (on a lesser extent) 33
V1=20%
Elliptic flow: analysis methodsElliptic flow: analysis methodsComparison between three different analysis methods implemented in ALICE analysis framework and applied to 28000 Pb-Pb like events (GeVSim) Methods based on multiparticle correlation (LYZ, v2{4}) less
biased by non-flow correlations (jets, particle decays)
If non flow correlations are not included in simulations all methods correctly estimate flow
In presence of two-particle non-flow, method based on two-particle correlations (v2{2}) give biased results
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ConclusionsConclusionsSuccessful commissioning of detectors involved in “first day” observables ITS, TPC and TOF took cosmics since August 17th till September
13th. Data being analyzed for calibration and alignment. More cosmics in the next weeks.
Analysis tools ready for analysis of “first day” observables Analysis code ready and tuned on the Monte Carlo samples
produced on the Grid Acceptance/efficiency corrections extracted from the Monte
Carlo samples produced on the Grid Study of systematics on-going and in good shape
Everything ready for first p-p collisions at LHC
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Thanks to …Thanks to …
Nora De Marco, Grazia Luparello, Chiara Oppedisano, Francesco Noferini, Mariella Nicassio, Luciano Ramello For providing me a significant fraction of the material shown in
this presentation
Paolo Giubellino, Massimo Masera and Luciano Ramello For suggetions/discussions/criticism on the topics and the
analyses to be presented