First identified particle studies at the CERN-LHC
with the ALICE experiment
Panos Christakogloua,b
for the ALICE Collaboration
a NIKHEF
b
26.03.2010
Utrecht University
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
1
Outline
 The ALICE experiment at LHC
o Experimental setup
o PID design performance
 Commissioning of the PID detectors
o Results from the ITS
o Results from the TPC
o Results from the TOF
 First physics measurements
o Identified particle spectra
o Baryon number transport
 Conclusion
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
2
Experimental setup
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
3
PID in ALICE
 PID for hadrons in the central barrel (|η| < 0.9)
o
o
o
o
ITS: Excellent (3σ) separation from ~50 MeV/c for π
TPC: π from 200 MeV/c, Κ from 300 MeV/c
TOF: K up to 2.5 GeV/c, p up to 3.5 GeV/c
HMPID: intermediate to high momenta
 Photons with the dedicated spectrometer (PHOS)
 Electrons in the central barrel:
o ITS + TPC for low momentum
o TRD from P ~ 1GeV/c (beauty measurements feasible even @1GeV/c!)
 Muons reconstructed in the forward region (muon arm: -4 < η < -2.4)
PID – momentum reach
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
4
ITS commissioning
 Design goals
o
Optimal resolution for primary vertex and
track impact parameter

Minimize distance of innermost layer from
beam axis (<r>≈ 3.9 cm) and material budget
Maximum occupancy (central PbPb) < few %
dE/dx information in the 4 outermost layers
for particle ID in 1/β2 region
Installed in 2007.
Successfully commissioned during the cosmics
runs in 2008 and 2009.
o
o


Layer
Det.
Type
Radius
(cm)
Length
(cm)
1
SPD
3.9
2
SPD
3
PbPb dN/dy=6000
Resolution (mm)
rf
Z
Part./cm2
Occupancy (%)
28.2
12
100
35
2.1
7.6
28.2
12
100
12
0.6
SDD
15.0
44.4
35
25
3
2.5
4
SDD
23.9
59.4
35
25
1.5
1.0
5
SSD
38.0
86.2
20
830
0.6
4.0
6
SSD
43.0
97.8
20
830
0.45
3.3
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
5
ITS alignment
 First order alignment with cosmics using the SPD trigger
o
Coincidence between top outer SPD layer and bottom outer SPD layer (rate: 0.18 Hz)
ALICE collaboration 2010 JINST 5 P03003

Preliminary results show:
o
o

Outlook:
o
o
26.03.2010
Space point resolution for the
SPD of ~14 μm in rφ (matching
between upper and lower part
of a track).
Residual misalignment spread
of the SSD modules on the
ladders of ~5μm
Inclusion of the SDD
Alignment with magnetic field
with beams (pt dependence).
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
6
ITS calibration and PID (SDD + SSD)
 Charge distribution measured on all the
SDD modules
o
~95% of the detector included in the
readout during data taking
 Charge distribution measured on all the
SSD modules
o
o
o
90% of the detector active in 2009
<1.5% bad strips
S/N ~40
dE/dx resolution ~ 13%
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
7
TPC commissioning
 Main TPC characteristics
o Cylindrical in shape, with an active
volume that has an inner radius of about
85 cm, an outer radius of about 250 cm,
and an overall length along the beam
direction of 500 cm (largest TPC ever
built)
o Cylindrical field cage filled with ~90 m3
of Ne/CO2/N2 gas mixture (90:10:5)
o Drift time of ~90 μs
o Many (>90) 3D points (+dE/dx) per track
 Since August 2009 more than 750
million events (cosmics, krypton, and
laser) recorded, with and without Bfield.
o First round of calibrations (dE/dx,
momentum, alignment, gain) completed
before collisions.
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
8
TPC particle identification
September 2009
December 2009
J. Alme et al. (ALICE TPC), arXiv:1001.1950 (subm. to NIM)
dE/dx resolution ~ 5%
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
9
TOF
 18 sectors, 2π covering in |η| < 0.9
 Granularity: 2.5x3.5 cm2 at ∼3.7 m
from the primary vertex;
 Resolution at present ∼130 ps
(calibration only from cosmic rays)
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
10
Analysis details
Event Selection
Track Selection – Quality cuts
Online Trigger
Yes
Number of TPC clusters
80
Offline Trigger
Yes
Number of TPC clusters (dE/dx)
80
|Vz|
<1
0
x2/NTPCclusters
3.5
Number of ITS clusters
2
|Vx|
<1
Hit on SPD1 || SPD2
Yes
|Vy|
<1
Track Selection – Primary like cuts
dca
0.2 - 2.0 cm
Analysis of the p+p events collected
during the first LHC run in December
Accepted events: 250K
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
11
Hadron spectra – π, Κ, p
ITS
TPC
TOF
 Subtract measured signal (dE/dx or tof) from the expected one for each pt bin
 Yields extracted from a fit with a convolution of three Gaussians
 Compare results with a simple 3σ cut
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
12
Hadron spectra - π, Κ, p (cont).
 ITS points to be added: new reconstruction pass based
on the latest calibration constants (SDD)
 Excellent agreement between TPC and TOF in the overlap
regions: max. difference of 6%
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
13
K± from Kinks
K  m
63.43%
K   0
21.13%
K  e 0
4.87%
K  m 0
3.27%
K   0 0
1.73%
π- kinks
  m   (99%)
 Preliminary results in good
agreement with the K± spectra
obtained from the dE/dx and
TOF PID
M. Spyropoulou-Stassinaki, P. Ganoti,
A. Belogianni, F. Roukoutakis
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
14
Baryon number transport – Motivation

Who is the carrier of the baryon number?
o
o

Valence quarks: G.C. Rossi and G. Veneziano, Nucl. Phys B123 (1977) 507.
Gluonic field: B.Z. Kopeliovich and B. Zakharov, Z. Phys. C43 (1989) 241.
Different approaches based on theory:
o QGSM considers the baryon as a bound quark-diquark state. BN transport implies breaking
the diquark pair.
o Gluonic mechanism aka String Junction (SJ). BN transport implies the stopping of the SJ.
Conventional approach - QGSM
Within QGSM one expects an asymmetry ~0 at
LHC energies
 No BN transported at mid-rapidity from the
fragmentation region
d
u
u
String Junction
BN transport even at large rapidity gaps (large
energies).
 Veneziano: Probability exponentially suppressed
(aJ: SJ intercept – model dependent)
 Kopeliovich: Probability constant with rapidity
P(y) ~ ea J y P(y)  const
d
u
u
u

LHC’s large rapidity gap between the incoming protons (yp =
±9.6) will give us the opportunity to distinguish between the
different models.
26.03.2010

d
SJ
u
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
15
Baryon number transport – Experimental results
Lessons from RHIC
STAR: Phys.Rev.Lett.86 (2001) 4778
Lessons from HERA
BRAHMS: Phys. Lett. B607 (2005) 42
A. Falkiewicz: DIS2008, London
p+p - Au+Au @√s = 200GeV
p+p
Au+Au 20% central
PYTHIA
HIJING-B




26.03.2010
Proton excess at midrapidity can be attributed
to the BN transport from
the beam at RHIC energies
No sign of any rapidity or
pt dependence
PYTHIA systematically
overestimates the proton
ratio
HIJING-B describes the
experimental data

Initially reported an
asymmetry of ~8% for Δy
~ 7.
o

Results never published
due to large systematic
uncertainties caused by
the high beam-gas rate.
Results reported at the
DIS2008 are compatible
with an asymmetry for Λs
~0
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
16

Baryon number transport – Corrections (I)
 Apply corrections that don’t cancel out in the ratio:
o Absorption
o Cross section in transport codes
o Secondaries (mainly protons)
N 
N 
p
p
 

N 

N 

 p corrected  p raw
C
(y, pt )  Ccrosssec tion p (y, pt ) 
absorption  p
 


C
 
C
 Csec ondaries p (y, pt )
(y,
p
)
(y,
p
)
 absorption  p
t   crosssec tion p
t 
Absorption
 Extract corrections based on MC
sample
 Rely on the proper description of the
material in the simulation
 Rely on the interaction cross-section
used in the transport code (GEANT3)
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
17
Baryon number transport – Corrections (II)
Detailed study published in 2008: ALICE-INT-2008-010.v1
Bendiscioli and Kharzeev, Riv.Nuovo Cim.17N6, 1-142 (1994)
 Scaling factor for the absorption
corrections defined as εGEANT/εFLUKA
 ~2.5% decrease of the absorption
correction
( pt ) 
26.03.2010
( pt )
C( pt )
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
18
Baryon number transport – Corrections (III)
 Fit the dca distributions of primary antiprotons which are background free.
 Use the same function and fit the primary protons scaled by the ratio (free
parameter).
 Fit the dca distributions of secondaries (protons coming from the interaction of
particles with the material).
 Use the same fitting functions to fit the dca distributions from real data.
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
19
Baryon number transport – Results (I)
Fully corrected ratio: stat. error – lines, syst. error – shaded are around data points


Preliminary results show no sign of any rapidity or pt dependence.
Experimental points are compared with different model predictions that include variation of
BN transport mechanisms:
o
o
o
HIJING-B clearly underestimates the y dependence, whereas it predicts a decrease of the ratio with
increasing pt not seen in p+p collisions.
QGSM’s (only y information) version with aSJ ≠ 0 is systematically below the data points
The different PYTHIA tunes describe the data well.
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
20
Baryon number transport – Results (II)
 Mid-rapidity ratio
for ALICE
extracted by
fitting the y
dependence plot at
|y| < 0.5.
 Preliminary results
show still a small
excess of baryons
over antibaryons
(asymmetry of
~1.9%) at √s = 900
GeV/c.
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
21
ALICE is up and running!!!
THANK YOU FOR YOUR ATTENTION
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
22
BACKUP
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
23
Strangeness (Ks0)
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
24
Strangeness (Λ)
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
25
Cascades (Ξ)
26.03.2010
Panos.Christakoglou@cern.ch - XXVIII Workshop on Recent Advances in Particle Physics and
Cosmology,Thessaloniki
26