ALICE First Physics Results Andrea Dainese (INFN Padova) on behalf of the ALICE Collaboration Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 1
Jan 14, 2016
ALICE First Physics ResultsAndrea Dainese(INFN Padova)
on behalf of the ALICE Collaboration
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 1
Outline Particle multiplicity
dNch/d and dN/dNch measurements at 0.9, 2.36, and 7 TeV
Momentum spectra Charged particle pt spectra at 0.9 TeV
Baryon production Anti-proton/proton ratio at 0.9 and 7 TeV
Particle emitting source Femtoscopy (HBT) at 0.9 TeV
Momentum spectra, outlook Identified charged pt spectra at 0.9 TeV
Strangeness at 0.9 and 7 TeV 0 and jet reconstruction at 7 TeV
Event topology Event shape and underlying event at 7 TeV
Heavy Flavour production Charm and J/ production at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 2
published
preliminary
in preparation
ITS, TPC, TOF, HMPID, MUON, V0, T0, FMD, PMD, ZDC (100%) TRD (7/18) EMCAL (4/12) PHOS (3/5)
Detector configuration 2009/2010
3Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese
J.Schukraft
3
“Minimum bias”, based on interaction trigger: SPD or V0-A or V0-C
o at least one charged particle in 8 units
read out all ALICE single-muon trigger:
forward muon in coincidence with Min Bias read out MUON, SPD, V0, FMD, ZDC
Activated in coincidence with the BPTX beam pickups
Trigger & Data samples
4Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 4
2009 (0.9 and 2.36 TeV)~10.3 µb-1 (5×105 min bias)
2010, to June 8th (0.9 and 7 TeV)~2.7 nb-1 (2×108 min bias)
Only 7 TeV data = 72 mb
Outline Particle multiplicity
dNch/d and dN/dNch measurements at 0.9, 2.36, and 7 TeV
Momentum spectra Charged particle pt spectra at 0.9 TeV
Baryon production Anti-proton/proton ratio at 0.9 and 7 TeV
Particle emitting source Femtoscopy (HBT) at 0.9 TeV
Momentum spectra, outlook Identified charged pt spectra at 0.9 TeV
Strangeness at 0.9 and 7 TeV 0 and jet reconstruction at 7 TeV
Event topology Event shape and underlying event at 7 TeV
Heavy Flavour production Charm and J/ production at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 5
published
preliminary
in preparation
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 6
Multiplicity measurements
Multiplicity measured using tracklets in the two pixel layers (R ~ 4 and 7 cm)
ALICE has published the pseudorapidity density and multiplicity distribution at 0.9, 2.36, and 7 TeV First LHC paper First 7 TeV paper All three papers accepted by
EPJC
arXiv:1004.3514
EPJ C: Vol. 65, 1 (2010)
P.Hristov
284 events, Nov 23rd 2009
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 7
Event Classes0.9 and 2.36 TeV
INEL and NSD Use measured cross sections
for diffractive processes Change MC generator fractions
(SD/INEL, DD/INEL) such that they match these fractions
Use Pythia and Phojet to assess effect of different kinematics of diffractive processes
7 TeV Diffraction is quite unknown Hadron-level definition of events
(similar to ATLAS first paper) All events that have at least one
charged primary particle in || < 1: “INEL>0”
Minimizes model dependence
INEL: MBOR (SPD OR VZEROA OR VZEROC) AND offline background suppression
NSD: MBAND (VZEROA AND VZEROC) AND offline background suppressionINEL>0: INEL AND at least one charged primary particle in || < 1
UA5: only stat. uncertainty shown
arXiv:1004.3034
arXiv:1004.3034
dNch/d vs. other experiments
Consistent with UA5 (only 900 GeV)
Consistent with CMS (only NSD) does not include charged
leptons ~1.5 % difference
0.9 TeV
2.36 TeV
8Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 8
dNch/d vs. Monte Carlo Pythia D6T and Perugia-0 match
neither INEL, NSD or INEL>0 at any energy
Pythia ATLAS-CSC and Phojet reasonably close with some deviations at 0.9 and 2.36 TeV
Only ATLAS-CSC close at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 9
2.36 TeV
0.9 TeV 7 TeV
dNch/d vs. s
points at the same energy slightly shifted
arXiv:1004.3514
arXiv:1004.3514
Significantly larger increase from 0.9 to 7 TeV than in MCs
Increase in dNch/dh
in || < 1 for INEL > 0
arXiv:1004.3514
s ALICE (%) MCs (%)
0.9 2.36 TeV 23.3 ± 0.4 -0.7+1.1 15 – 18
0.9 7 TeV 57.6 ± 0.4 -1.8+3.6 33 – 48
Power law dependence fits well
10Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 10
fits ~ s0.1
Multiplicity Distributions Distributions in three -regions Consistent with UA5 (||<0.5) Fits with one negative binomial work well in limited -regions
clan-based model of production production Difference between INEL and NSD in low-multiplicity region
arXiv:1004.3034arXiv:1004.3034
11Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese
0.9 TeV: ALICE vs. UA5 0.9 TeV: INEL vs. NSD
11
dN/dNch: vs. Monte Carlo Phojet
provides a good description at 900 GeV fails at 2.36 and 7 TeV
Pythia Atlas CSC fails at 0.9 TeV reasonably close at 2.36 and 7 TeV but deviations around 10-20
Pythia D6T and Perugia-0 far from the distribution at all energies
0.9 TeV 7 TeV2.36 TeVarXiv:1004.3034 arXiv:1004.3514
1212Physics at the LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 12
arXiv:1004.3034
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 1212
Outline Particle multiplicity
dNch/d and dN/dNch measurements at 0.9, 2.36, and 7 TeV
Momentum spectra Charged particle pt spectra at 0.9 TeV
Baryon production Anti-proton/proton ratio at 0.9 and 7 TeV
Particle emitting source Femtoscopy (HBT) at 0.9 TeV
Momentum spectra, outlook Identified charged pt spectra at 0.9 TeV
Strangeness at 0.9 and 7 TeV 0 and jet reconstruction at 7 TeV
Event topology Event shape and underlying event at 7 TeV
Heavy Flavour production Charm and J/ production at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 13
published
preliminary
in preparation
dNch/dpt at 0.9 TeV
14Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese
Paper ready for submission: dNch/dpt and <pt> vs. Nch
n
T
T
ch
T nT
E
dpd
Nd
p
1
1 2
Not described by MCs
PHOJET (at 900 GeV) and PYTHIAATLAS-CSC tune (at 2.36 and 7 TeV)gave best description of the multiplicitydistributions
nT
T
TT
TT
pnT
E
T
E
nT
E
mpmE
:~
exp:~
1
1
22
14
FIT
dNch/dpt at 0.9 TeV
15Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese
Paper ready for submission: dNch/dpt and <pt> vs. Nch
ALICE measures harder spectrum than CMS, ATLAS, UA1 (narrower window at central rapidity)
15
16Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese
“Power-law”-like tail becomes harder at high multiplicity
<pt> vs multiplicity (0.9 TeV)
<pt> vs Nch from fit function, with 3 pt cutoffs (> 0, 150, 500 MeV/c)
16
<pt> vs multiplicity (0.9 TeV)
17Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese
Perugia-0 (which fails for multiplicity) describes well <pt>, but only for pt>500 MeV/c (ATLAS found agreement for pt > 500 MeV/c)
Phojet (which describes multiplicity) fails for <pt>
17
Comparison to MCs: summary
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 18
MC << data MC >> data MC ≈ data
MC/TUNE
D6T Perugia0
CSC PHOJET
-20% -17% +3% -2%
NchNch>10 Nch>5 Nch>15 Nch>10
ptpt>4Ge
vpt>1GeV pt>1Ge
V
<pt>
-24% -21% -2% -8%
NchNch>10 Nch>5 Nch >20 Nch>15
-27% -24% -4% -17%
NchNch >30
√s =
0.9
TeV
√s =
2.3
6 T
eV√s
= 7
TeV
0.9 TeV: PHOJET better for Nch, Perugia-0 for pt
2.36, 7 TeV: CSC better for Nch
Outline Particle multiplicity
dNch/d and dN/dNch measurements at 0.9, 2.36, and 7 TeV
Momentum spectra Charged particle pt spectra at 0.9 TeV
Baryon production Anti-proton/proton ratio at 0.9 and 7 TeV
Particle emitting source Femtoscopy (HBT) at 0.9 TeV
Momentum spectra, outlook Identified charged pt spectra at 0.9 TeV
Strangeness at 0.9 and 7 TeV 0 and jet reconstruction at 7 TeV
Event topology Event shape and underlying event at 7 TeV
Heavy Flavour production Charm and J/ production at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 19
published
preliminary
in preparation
p/p measurement at mid-rapidity Proton identification with TPC dE/dx
Special care for secondaries and absorption corrections pbar/p at |y| < 0.5 and 0.45 < pt < 1.05 GeV/c
Baryon-stopping at low
y=ybeam-yCM
Vanishes at high LHC energy
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 20
M.Broz
p/p measurement vs. MCs
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 21
Baryon number transport is usually explained by a String Junction transfer (gluon field)
What is the intercept of the corresponding Regge trajectory
(SJ = ½ or 1?)
Data described well by PYTHIA tunes
Other models (HIJING-B, QGSM with αSJ ~ 1) underestimate the data
Data show suppression baryon transport over large rapidity gaps in pp collisions
0.9 TeV
7 TeV
No pt dependence
du
u
SJ
Outline Particle multiplicity
dNch/d and dN/dNch measurements at 0.9, 2.36, and 7 TeV
Momentum spectra Charged particle pt spectra at 0.9 TeV
Baryon production Anti-proton/proton ratio at 0.9 and 7 TeV
Particle emitting source Femtoscopy (HBT) at 0.9 TeV
Momentum spectra, outlook Identified charged pt spectra at 0.9 TeV
Strangeness at 0.9 and 7 TeV 0 and jet reconstruction at 7 TeV
Event topology Event shape and underlying event at 7 TeV
Heavy Flavour production Charm and J/ production at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 22
published
preliminary
in preparation
Femtoscopy:particle emitting source
Assess the space-time evolution of the system that emits particles in pp collisions
Measure the Bose-Einstein enhancement for pairs of pions (identical bosons) at low momentum difference qinv=|p1-p2|, vs. event multiplicity and pair kt = |pt1+pt2|/2
Fit with a Gaussian
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 23
0.9 TeV
D.Miskowiec
Femtoscopy results at 0.9 TeV Radius grows with
dNch/dconsistent with other data and expectations (larger correlation volume at large multiplicities)
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 24
No visible kt dependence
Outline Particle multiplicity
dNch/d and dN/dNch measurements at 0.9, 2.36, and 7 TeV
Momentum spectra Charged particle pt spectra at 0.9 TeV
Baryon production Anti-proton/proton ratio at 0.9 and 7 TeV
Particle emitting source Femtoscopy (HBT) at 0.9 TeV
Momentum spectra, outlook Identified charged pt spectra at 0.9 TeV
Strangeness at 0.9 and 7 TeV 0 and jet reconstruction at 7 TeV
Event topology Event shape and underlying event at 7 TeV
Heavy Flavour production Charm and J/ production at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 25
published
preliminary
in preparation
Identified spectra:one of ALICE’s specials
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 26
ITS
TPC
TOF
Identified spectra at 0.9 TeV
Analysis in progress (spectra not fully corrected yet) Good agreement between the 3 detectors (ITS, TPC, TOF) Shows that detectors’ calibration/understanding is OK
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 27
positives negatives
Strangeness at 0.9 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 28
A.Maire
The three ways to kaons K± TPC+TOF PID K0
S vertex reconstruction
K±± kink reconstruction
Spectra not fully corrected Good internal constency
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 29
and K*0 at 0.9 and 7 TeV
0.7 < pt < 1.0 GeV/c
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 30
TPC+TOF PID
K+K- K*0
K+K-
TOF PID
K*0
TOF PID
pp 0.9 TeV pp 0.9 TeV
pp 7 TeV pp 7 TeV
Multi-strange baryons at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 31
A.Maire
Prospects for 0: conversions
Electron ID in TPC TRD to join soon
Conversion reconstruction in TPC+ITS also very important for material
budget scan For 0 and : double conversion
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 32
0.9 TeV
7 TeV
backgroundfrom mixed events
backgroundfrom mixed events
Prospects for 0: conversions
Electron ID in TPC TRD to join soon
Conversion reconstruction in TPC+ITS also very important for material
budget scan For 0 and : double conversion
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 33
ALICE Performance31/05/2010
Raw 0 dN/dpt
Prospects for 0: EM calorimeters
Two EM calorimeters (back-to-back): EMCAL, PHOS Calibration ongoing, but already nice 0 signals
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 34
EMCAL PHOS
1 GeV/c1 GeV/c
5 GeV/c5 GeV/c
2 GeV/c2 GeV/c
5 GeV/c5 GeV/c
Jet reconstruction Charged-track jets raw
spectra 0.9 and 7 TeV ||<0.5 Four jets algos compared uncorrected
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 35
7 TeV0.9 TeV
uncorrected
uncorrected
Outline Particle multiplicity
dNch/d and dN/dNch measurements at 0.9, 2.36, and 7 TeV
Momentum spectra Charged particle pt spectra at 0.9 TeV
Baryon production Anti-proton/proton ratio at 0.9 and 7 TeV
Particle emitting source Femtoscopy (HBT) at 0.9 TeV
Momentum spectra, outlook Identified charged pt spectra at 0.9 TeV
Strangeness at 0.9 and 7 TeV 0 and jet reconstruction at 7 TeV
Event topology Event shape and underlying event at 7 TeV
Heavy Flavour production Charm and J/ production at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 36
published
preliminary
in preparation
Event shape studies Event shapes are sensitive to underlying event properties
multiple interactions mechanism tuning of MC generators
Can be used to classify events as “soft” or “hard” Look for unusual topologies Transverse thrust (hard scattering frame moves longitudinally)
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 37
measures the transverse sphericity of the event
= 1-2/
Event shape: Thrust
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 38
( = <1-T> ) vs. multiplicity at 0.9 and 7 TeV Thrust spectrum is unfolded based on MC (2 minimization)
data more “spherical” (less back-to-back-ish) than MCs
not fully corrected not fully corrected
Event structure: Underlying Event
Event-by-event analysis: identify leading hadron define transverse regions pt in the two regions
Region with larger energy (MAX) sensitive to QCD final-state radiation
Region wth smaller energy (MIN) sensitive to soft component (multiple interactions)
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 39
Underlying Event 7 TeV: first look at
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 40
Start by looking at inclusive correlations wrt leading track Data not corrected, compared to MCs (geant + recon) Leading pt<10 GeV/c data less back-to-back-ish than MCs
0.5 GeV/c < pT,lt < 2.5 GeV/c
2.5 GeV/c < pT,lt < 5 GeV/c
5 GeV/c < pT,lt < 10 GeV/c
10 GeV/c < pT,lt < 15 GeV/c
15 GeV/c < pT,lt < 20 GeV/c
ALICE Performance
Outline Particle multiplicity
dNch/d and dN/dNch measurements at 0.9, 2.36, and 7 TeV
Momentum spectra Charged particle pt spectra at 0.9 TeV
Baryon production Anti-proton/proton ratio at 0.9 and 7 TeV
Particle emitting source Femtoscopy (HBT) at 0.9 TeV
Momentum spectra, outlook Identified charged pt spectra at 0.9 TeV
Strangeness at 0.9 and 7 TeV 0 and jet reconstruction at 7 TeV
Event topology Event shape and underlying event at 7 TeV
Heavy Flavour production Charm and J/ production at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 41
published
preliminary
in preparation
J/ee, ||<0.9 e PID from TPC
TRD and EMCAL calib. ongoing
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 42
100M 7 TeV events
acceptance to pt=0
+- e+e-
Forward J/
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 43
J/, -4<<-2.5
acceptance to pt=0
+- e+e-
31 M events
177±30 J/
Charm: D meson reconstruction
Main selection: displaced-vertex topology Example: D0K-+
good pointing of reconstructed D momentum to the primary vertex ppair of opposite-charge tracks with large impact parameters
K ID in TPC+TOF helps rejecting background at low pt
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 44
Impact parameter resolution close to target
M.Lunardon
Charm: D0, D+, D*+
1.25×108 pp at 7 TeV Signals in 4 pt bins in 2-10 GeV/c
expect to cover 0.5-15 GeV/c with 109
compare to pQCD (FONLL) at 7 TeV
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 45
Heavy flavour from single leptons c and b production in semi-leptonic channels in preparation
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 46
Electrons ||<0.9:TPC dE/dx after K and p rejection with TOF. TRD and EMCAL will join soon
e±
±
raw spectrum
+ displacement selection beauty
Heavy flavour from single leptons c and b production in semi-leptonic channels in preparation
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 47
Muons -4<<-2.5: Light quark contribution subtracted with PYTHIA (normalized to data at low pt).Not corrected.
c (low pt) & b to be separated by fitting based on pQCD shapes, in progress ...
First Physics Results from ALICE
Particle multiplicity increase from 0.9 to 7 TeV significantly larger (>20%) than predicted
Momentum spectra <pt> VS Nch not described by any of the MCs
Anti-proton/proton ratio at midrapidity p/p goes to 1 at 7 TeV baryon number transfer suppressed over
large y Femtoscopic measurement at 0.9 TeV
particle emitting source “size” increases with multiplicity Event topology
lower “jettiness” than expected in LHC collisions Promising performance for ID spectra, strangeness, charm, charmonium
ALICE is ready to deliver many more Physics Results
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 48
Charged particle pt distribution
Baryon-antibaryon asymmetry Bose-Einstein correlations Strangeness production (K0, Λ, Ξ, Φ) Identified particles pT (π, K, p)
Outlook
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 49
dN/dpt at 7 TeV
π0, η
c and b production
J/ production
high multiplicity
jet correlations
event shape
underlying events
reconstructed jets
b-tagged jets
…
Papers in the pipeline: Analyses in progress:
November 2010: Pb-Pb collisions in ALICE!H.Torii
EXTRA SLIDES
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 50
First LHC physics paper
dNch/dη for |η| < 0.5 dNch/dη vs η
K. Aamodt et al. (ALICE), Eur. Phys. J C 65 (2010) 111
data collected 23 Nov, paper submitted 28 Nov 284 events (~ 3.7 authors per event)
51Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 52
Negative Binomial Distributions
Bernoulli experiment Probability for n failures and k
success in any order, but the last trial is a success
Physical interpretation Cascade production (clan model,
Giovannini, Z. Phys. C30 391 (1986))o Ancestor particle are produced
independently (Poisson)o Existing particle can produce
additional one with some probability p
ISR, 30 GeV
UA5, 900 GeVSuccessful for NSD events•up to 540 GeV in full phase space (ISR, UA5)•central intervals up to 1.8 TeV (UA5, CDF)
arXiv:0912.0023
arXiv:0912.0023
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese
Data Sample
Analysis based on 900 GeV: 186k collision events
out of ~430k total 2.36 TeV: 41k collision events 7 TeV: 240 k collision events
MC adapted to mean vertex position and spread
Eve
nts
z vertex distribution (cm)
53
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 54
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 (VS , zVtx) Vertex reconstruction correction (VS mult, zVtx) Trigger bias correction (VS mult, zVtx) all events
novertex
notrigger
all events
novertex novertex
notrigger
all events
novertex novertex
notrigger
notrigger
dNch/d Measurement
Events
Tracks
ddNch
Primary particles = charged particles produced in the collision and their decay products excluding weak decays from strange particles
Average correction factors (900 GeV)Tracking: 1.53Events (INEL): 1.05Events (NSD): 1.02 [+8.3% (NSD) – 6.8% (SD)]Subtraction of SD tracks for NSD: 3%
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 55
Systematic Uncertainties
Influence of systematic effects on result evaluated
E.g. fractions ND/DD/SD Change SD/DD by
measurement error E.g. SD (15.3 ± 2.3)%
Larger uncertainty for NSD than for INEL
Systematic uncertainties at 900 GeV in %
INEL NSD
Fractions ND/DD/SD 0.7 2.8
MC dependence +1.7 -0.5
Detector efficiency 1.5
Particle composition* 0.5 - 1.0
Material budget negl.
pT spectrum 0.5
SPD triggering efficiency negl.
V0 triggering efficiency negl. 0.5
Background negl.
Total -1.8
+2.5
-3.3
+3.3* -dependence
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 56
Systematic Uncertainties dNch/d
Systematic uncertainties in % 900 GeV 2.36 TeV 7 TeV
Fractions ND/DD/SD* 0.5 0.3 1.0
MC dependence +0.8 +1.5 +2.8
Detector efficiency ±1.5
Particle composition** ±(0.5 - 1.0)
Material budget negl.
pT spectrum ±0.5
SPD triggering efficiency negl.
V0 triggering efficiency negl.
Background negl.
* Fractions changed at 0.9 and 2.36 TeV like in paper 2; at 7 TeV by 50% ** -dependence
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 57
Regularizations
Unfolding using 2-Minimization
One free parameter per bin for unfolded spectrum Ut
Regularization Prefer constant locally Prefer linear function locally
Weight parameter needs to be tuned 2/ndf not larger than 1 Keep bias low
t
1-tt
t
't
tU
– U U
U
U a
t
1tt1t
t
''t
tU
U2UU
U
Ua
βR(U) URM
(U)χ
2
mm
t tmtm2
e t
2t )a( R(U)
V. Blobel, Yellow report, 1984
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 58
Why INEL>0 at 7 TeV?
7 TeV Pythia Perugia-0 Phojet
INEL=0 INEL>0 INEL=0 INEL>0
All 18.3% 81.7% 10% 90%
Vertex with |z| < 5.5
1 tracklet in || < 1
0.19% 77.3% 0.14% 87%
•A correction for the events in the INEL=0 bin is not needed (by definition)•Contamination is very low•Only the events that are lost in the INEL>0 bin are corrected by MC
minimal MC dependence•The INEL>0 requirement selects 80% of the MB1 events in 7 TeV data
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 59
X² - constant regularizationX² - linear regularizationBayesian unfolding
stat. + syst error > 50%
ALICE Performancework in progress
A structure?
Independent of Regularization scheme Unfolding method Data sample
Unfolded distribution changed to exponential Residuals show structure Slope change already in raw
data But… errors in unfolded
spectrum correlated Moving all points by 2
structure disappears Might just be a fluctuation
ALICE preliminary
measured multiplicity
|| < 1E
ven
tsN
orm
aliz
edre
sid
ua
ls
Unidentified pt spectrum Track reconstruction in TPC (≤160 hits) +ITS (≤6 hits)
pt measurement from TPC only (ITS-TPC alignment not final)
o (pt)/pt ≈ (0.7 + 0.7 pt)%
Track selection: pt > 150 MeV/c, || < 0.8
nhitsTPC > 70, 2/hits<4 in TPC
at least 2 matching hits in ITSo at least 1 in SPDo 4.7 on average
cut on transverse impact
parameter (7)
From MC, cross-checked
with data: Efficiency 50-80% Secondary cont. 9-1%
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 60
PythiaPerugia-0
PythiaPerugia-0
Baryon number at midrapidity? Basic question: who is the carrier of the baryon number nB?
Valence quarks: Rossi and Veneziano, NPB123 (1977) 507 Gluonic field: Kopeliovich and Zakharov, ZPC43 (1989) 241
QGSM considers the baryon as a bound quark-diquark state. nB transport implies breaking the diquark pair.
Gluonic mechanism aka String Junction (SJ), as in Pythia. nB transport implies the stopping of the SJ.
Physics at LHC 2010, DESY-Hamburg, 09.06.10 Andrea Dainese 61
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Conventional approach - QGSM
Within QGSM one expects an asymmetry ~0 at LHC energies No BN transported at mid-rapidity from the fragmentation region
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SJ
String Junction
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P(Δy) ~ e−a J Δy
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P(Δy) = const
BN transport even at large rapidity gaps (large energies). Veneziano: Probability exponentially suppressed (aJ: SJ intercept – model dependent) Kopeliovich: Probability constant with rapidity