First ALICE Results from Proton-Proton Collisions

First ALICE Results from Proton-Proton Collisions

Jan Fiete Grosse-Oetringhaus, CERN/PHfor the ALICE collaboration

Rencontres de Moriond14. – 19. March 2010, La Thuile, Italy

EPJC65:111 (2010)

First ALICE Results from Proton-Proton Collisions - Jan Fiete Grosse-Oetringhaus 2

Not shown: ZDC (at ±116m)

L3 Magnet

A Large Ion Collider ExperimentT0/V0Trigger

full

TPCTracking, PID (dE/dx) full ITS

Low pT trackingPID + Vertexing

full MUON μ-pairs

full

TOFPID full

TRDElectron ID (TR)

39%

HMPIDPID (RICH) @ high pT

full

PHOSγ, π0, jets

60%

PMDγ multiplicity

full

ACORDECosmic trigger

full

FMDCharged multiplicity

full

Dipole

EMCALγ, π0, jets

36%


Detectors

• V0 scintillator array– Trigger– 2.8 < < 5.1 and -1.7 < < -3.7

• Silicon Pixel Detector (SPD)– Two innermost layers of the

Inner Tracking System (ITS)– Radii of 3.9/7.6 cm (|| < 2.0/1.4) – Trigger & Tracking

– || < 1.4; pT > 50 MeV/c

– 9.8 M channels– Tracklet: 2 points + vertex– During 2009 data-taking 80%

active

Detector Acceptance

x

y

SPD Layer 1

SPD Layer 2

SPD physics efficiency for primaries(2009 configuration80% active)

pT (GeV/c)

Eff

icie

ncy

ALICE Performancework in progress


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 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 < < 5.1

and one in -1.7 < < -3.7

0.9 TeV (in %) ND SD DD

Pythia MB1 100 77 92

V0AND 98 29 49

Phojet MB1 100 86 98

V0AND 98 34 66

Pythia0.9 TeV

Generated multiplicity in || < 1

Tri

gg

er e

ffic

ien

cy



Data Sample

• 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

MCData

Tracklet

En

trie

s


SPD module

Clu

ster

yie

ld

DataMC



Eve

nts

Rat

io D

ata

/ M

C

first layer

second layer

yield in first and second layer compatible within 0.5%

inefficient sector no significant effect on result


z vertex distribution (cm)


• 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

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


dNch/d Measurement (2)

• 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): 1.02

[+8.3% (NSD) – 6.8% (SD)]

– Subtraction of SD tracks for NSD: 3%

dN

ch/d

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


Beam-inducedbackground


dNch/d

dNch/d in || < 0.5ALICE, CMS: stat. and syst.

uncertainty added quadratically

0.9 TeV 2.36 TeV

INEL NSD 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*

NB: CMS data points do not include charged leptons ~1.5 % difference

ALICE, CMS: stat. and syst. uncertainty added quadratically


dN

ch/d

dN

ch/d

* only stat. error


dNch/d (2)

• Larger increase from 0.9 to 2.36 TeV at mid-rapidity as in MC generators

in % INEL NSD

ALICE prel.

24.8 +6.1-3.0 24.0 +3.9

-1.3

CMS 28.4 ± 3.0Pythia

D6T 19.7 18.7

ATLAS CSC

19.2 18.3

Perugia-0 19.6 18.5

Phojet 17.5 14.5


6.13.0 24.8 -

3.91.3 24.0 -

points at the same energy slightly shifted


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/d, but in unfolded

variables (true multiplicity) because it is applied after unfolding

events All

Nty multiplici with Events)P(N ch

ch

true multiplicity

mea

sure

d m

ult

ipli

city || < 1

RTM



Regularizations

Unfolding using 2-Minimization

• 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

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)

Blobel, CERN Comp.School 1984:0088Nucl.Instrum.Meth.A362:487-498,1995



• Distributions in limited -regions– Vertex range reduced a region

in which every event has full -acceptance

• Consistent with UA5 results for NSD at 0.9 TeV

UA5: ZP C43,357



• 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


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

See talks by Jacek Otwinowski, Francesco Noferini

National Geographic News (4th Dec)

‘….a machine called ALICE....

found that aproton-proton collision

recorded on November 23rd created the precise ratio of matter and

antimatter particles predicted from

theory..’

Other opinions on MB physics:

Documents

First ALICE Results from Proton-Proton Collisions