1. First ALICE Results from Proton-Proton Collisions
EPJC65:111 (2010)
First ALICE Results from Proton-Proton Collisions
Jan Fiete Grosse-Oetringhaus, CERN/PH
for the ALICE collaboration
Rencontres de Moriond
14. – 19. March 2010, La Thuile, Italy

2. A Large Ion Collider Experiment
T0/V0
Trigger
full
ACORDE
Cosmic trigger
full
EMCAL
γ, π0, jets
36%
L3 Magnet
TRD
Electron ID (TR)
39%
HMPID
PID high pT
full
TOF
PID
full
PMD
γ multiplicity
full
FMD
Charged
multiplicity
full
ITS
Low pT tracking
PID + Vertexing
full
Dipole
TPC
Tracking, PID (dE/dx)
full
MUON
μ-pairs
full
PHOS
γ, π0, jets
60%
Not shown: ZDC (at ±116m)
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

3. Detectors V0 scintillator array Silicon Pixel Detector (SPD) Trigger
Detector Acceptance
Detectors
V0 scintillator array
Trigger
2.8 < h < 5.1 and -1.7 < h < -3.7
Silicon Pixel Detector (SPD)
Two innermost layers of the Inner Tracking System (ITS)
Radii of 3.9/7.6 cm (|h| < 2.0/1.4)
Trigger & Tracking
|h| < 1.4; pT > 50 MeV/c
9.8 M channels
Tracklet: 2 points + vertex
During 2009 data-taking 80% active h x y
SPD Layer 1
SPD Layer 2
ALICE Performance
work in progress
Efficiency
SPD physics
efficiency for
primaries
(2009 configuration
80% active)
pT (GeV/c)
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

4. Trigger for MB Physics
ALICE measures MB properties for all inelastic (INEL) and non single-diffractive (NSD) events
Inclusive trigger ("MB1") for INEL: central pixel hit (SPD) or forward scintillator (V0)
One particle in 8 h units
(Trigger-)sensitive to 95-97% of the inelastic x-section
Two-arm trigger ("V0AND") for NSD
Both forward scintillators
One particle in 2.8 < h < 5.1 and one in -1.7 < h < -3.7
Trigger efficiency
ALICE Performance
work in progress
Pythia
0.9 TeV
Generated multiplicity in |h| < 1
900 GeV: LHC10a8, LHC10a12 ( )
0.9 TeV (in %) ND SD DD
Pythia
MB1
100 77 92
V0AND 98 29 49
Phojet 86 34 66
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

5. Data Sample Analysis based on
Events
Analysis based on
900 GeV: 186k collision events out of 430k total
2.36 TeV: 41k collision events
MC adapted to mean vertex position and spread
ALICE Performance
work in progress
z vertex distribution (cm)
inefficient sector
 no significant effect on result
ALICE Performance
work in progress
Data MC MC
Data
Tracklet Df
Entries
ALICE Performance
work in progress
Ratio Data / MC
900 GeV: 92k in run (accepted MB1); 94k in run ,90,92
Cluster yield
second layer
yield in first and second layer compatible within 0.5%
ALICE Performance
work in progress
first layer
SPD module
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

6. dNch/dh Measurement all events all events all events
Basically if the detector was perfect
But… there is
Detector acceptance, tracking efficiency
Decay, conversions, stopping, etc.
Vertex reconstruction efficiency/bias, trigger efficiency/bias
Low momentum cut-off
Three corrections needed
Track-to-particle correction
Vertex reconstruction correction
Trigger bias correction
Primary particles = charged particles produced in the collision and their decay products excluding weak decays from strange particles
all events no
vertex
trigger
all events no
vertex
trigger
all events no
vertex
trigger
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

7. dNch/dh Measurement (2)
Beam-induced
background
Use the number of triggered events without reconstructed vertex to constrain the "0 bin"
Assess background from control triggers (single bunch passing, no bunch passing)
Background with activity < 0.02%
At 2.36 TeV no control triggers
Use 0 bin from MC  larger syst. error
Average correction factors
Tracking: 1.53
Events (INEL): 1.05
Events (NSD): [+8.3% (NSD) – 6.8% (SD)]
Subtraction of SD tracks for NSD: 3%
ALICE Performance
work in progress
dNch/dh h
Main systematic uncertainties (%)
at 900 GeV
INEL
NSD
Fractions SD/DD/ND
0.6
2.8
MC dependence
2.0
1.0
Detector efficiency
1.5
run ,90,92
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

8. dNch/dh
NB: CMS data points do not include charged leptons  ~1.5 % difference
dNch/dh
ALICE, CMS: stat. and syst.
uncertainty added quadratically
dNch/dh
ALICE, CMS: stat. and syst.
uncertainty added quadratically h h
dNch/dh in |h| < 0.5
ALICE, CMS: stat. and syst.
uncertainty added quadratically
0.9 TeV
2.36 TeV
INEL
NSD
ALICE preliminary
3.02 ± 0.07
3.58 ± 0.11
3.77 ± 0.23
4.44 ± 0.16
ALICE EPJC65:111 (2010)
3.10 ± 0.26
3.51 ± 0.29
CMS JHEP 02 (2010) 041
3.48 ± 0.13
4.47 ± 0.16
UA5 Z. Phys. C33 1 (1986)
3.09 ± 0.05*
3.43 ± 0.05*
* only stat. error
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

9. dNch/dh (2)
Larger increase from 0.9 to 2.36 TeV at mid-rapidity as in MC generators
in %
INEL
NSD
ALICE prel.
CMS
28.4 ± 3.0
Pythi a
D6T
19.7
18.7
ATLAS CSC
19.2
18.3
Perugia-0
19.6
18.5
Phojet
17.5
14.5
points at the same
energy slightly shifted
Stat. uncertainty on MC values < 0.1%
D6T/Atlas: Pythia Perugia-0: Pythia Phojet 1.12 with Pythia
ALICE, CMS: stat. and syst. uncertainty added quadratically
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

10. Multiplicity Distributions
Efficiency, acceptance
Resolution vs. bin size  bin flow
Correction by unfolding
Detector response
Probability that a collision with the true multiplicity t is measured as an event with the multiplicity m
Vertex reconstruction, trigger bias correction
Like for dNch/dh, but in unfolded variables (true multiplicity) because it is applied after unfolding
|h| < 1
ALICE Performance
work in progress
measured multiplicity
LHC10a8_run104867_8
true multiplicity
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

11. Unfolding using c2-Minimization
Regularizations
One free parameters per bin for unfolded spectrum Ut
Regularization
Prefer constant locally
Prefer linear function locally
Use number of triggered events without vertex to estimate vertex efficiency in “0 bin” from data
Systematic uncertainties evaluated by using modified response matrices
Iterative Bayesian unfolding as cross-check
Blobel, CERN Comp.School 1984:0088
Nucl.Instrum.Meth.A362: ,1995
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

12. Multiplicity Distributions
Distributions in limited h-regions
Vertex range reduced a region in which every event has full h-acceptance
Consistent with UA5 results for NSD at 0.9 TeV
UA5: ZP C43,357
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

13. Multiplicity Distributions
Phojet provides a good description at 900 GeV, but fails at 2.36 TeV
Distribution at 2.36 TeV closer to Pythia with Atlas CSC tune
Qualitatively same conclusions for NSD
MC generators do not describe the energy dependence of the tail of the distribution correctly
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus

14. Summary
The pseudorapidity density and multiplicity distributions at 0.9 and 2.36 TeV have been presented
Consistent with UA5 and CMS
MC generators do not describe the energy-dependence of the average multiplicity as well as the tail of the distributions correctly
Many other analyses on the way, including
Charged particle pT, identified particles pT (π, K, p), strangeness production (K0, Λ, Ξ, Φ), baryon-antibaryon asymmetry, Bose-Einstein correlations, π0 spectra, event structure, azimuthal correlations, -φ correlations
Other opinions
on MB physics:
National Geographic News (4th Dec)
‘….a machine called ALICE....
found that a
proton-proton collision
recorded
on November 23rd
created the precise ratio of matter and antimatter particles
predicted from theory..’
See talks by Jacek Otwinowski, Francesco Noferini
First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus