first identified particle studies at the cern-lhc with the alice experiment panos christakoglou a,b...

First identified particle studies at the CERN-LHC with the ALICE experiment

Panos Christakogloua,b

for the ALICE Collaboration

[email protected] - XXVIII Workshop on Recent Advances in Particle Physics

and Cosmology,Thessaloniki1

a NIKHEFb Utrecht University

Outline

The ALICE experiment at LHCo Experimental setupo PID design performance

Commissioning of the PID detectorso Results from the ITSo Results from the TPCo Results from the TOF

First physics measurementso Identified particle spectrao Baryon number transport

Conclusion



Experimental setup



PID in ALICE

PID for hadrons in the central barrel (|η| < 0.9)o ITS: Excellent (3σ) separation from ~50 MeV/c for πo TPC: π from 200 MeV/c, Κ from 300 MeV/co TOF: K up to 2.5 GeV/c, p up to 3.5 GeV/co HMPID: intermediate to high momenta

Photons with the dedicated spectrometer (PHOS) Electrons in the central barrel:

o ITS + TPC for low momentumo 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

ITS commissioning



Design goalso Optimal resolution for primary vertex

and track impact parameter Minimize distance of innermost layer from

beam axis (<r>≈ 3.9 cm) and material budget

o Maximum occupancy (central PbPb) < few %

o 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.

Layer Det. Type

Radius (cm)

Length (cm)

Resolution (m) PbPb dN/dy=6000

r Z Part./cm2 Occupancy (%)

1 SPD 3.9 28.2 12 100 35 2.1

2 SPD 7.6 28.2 12 100 12 0.6

3 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

ITS alignment



First order alignment with cosmics using the SPD triggero Coincidence between top outer SPD layer and bottom outer SPD layer (rate: 0.18 Hz)

Preliminary results show:o Space point resolution for the

SPD of ~14 μm in rφ (matching between upper and lower part of a track).

o Residual misalignment spread of the SSD modules on the ladders of ~5μm

Outlook:o Inclusion of the SDDo Alignment with magnetic

field with beams (pt dependence).

ALICE collaboration 2010 JINST 5 P03003

ITS calibration and PID (SDD + SSD)



Charge distribution measured on all the SDD moduleso ~95% of the detector included in the

readout during data taking Charge distribution measured on all

the SSD moduleso 90% of the detector active in 2009o <1.5% bad stripso S/N ~40

dE/dx resolution ~ 13%

Main TPC characteristicso 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 μso Many (>90) 3D points (+dE/dx) per

track Since August 2009 more than 750

million events (cosmics, krypton, and laser) recorded, with and without B-field.o First round of calibrations (dE/dx,

momentum, alignment, gain) completed before collisions.

TPC commissioning





J. Alme et al. (ALICE TPC), arXiv:1001.1950 (subm. to NIM)

TPC particle identification

dE/dx resolution ~ 5%

September 2009

December 2009

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)

Analysis details



Event Selection

Online Trigger Yes

Offline Trigger Yes

|Vz| <10

|Vx| <1

|Vy| <1

Track Selection – Primary like cuts

dca 0.2 - 2.0 cm

Track Selection – Quality cuts

Number of TPC clusters 80

Number of TPC clusters (dE/dx) 80

x2/NTPCclusters 3.5

Number of ITS clusters 2

Hit on SPD1 || SPD2 Yes

Analysis of the p+p events collected during the first LHC run in DecemberAccepted events: 250K

Hadron spectra – π, Κ, p



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

ITS

TPC

TOF

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%

K± from Kinks



%73.1

%27.3

%87.4

%13.21

%43.63

00

0

0

0

K

K

eK

K

K

π- kinks

M. Spyropoulou-Stassinaki, P. Ganoti, A. Belogianni, F. Roukoutakis

Preliminary results in good agreement with the K± spectra obtained from the dE/dx and TOF PID

(99%)

Baryon number transport – Motivation



Who is the carrier of the baryon number?o Valence quarks: G.C. Rossi and G. Veneziano, Nucl. Phys B123 (1977) 507.o 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.

u

u

d u

u

d

Conventional approach - QGSM

Within QGSM one expects an asymmetry ~0 at LHC energies No BN transported at mid-rapidity from the fragmentation region

du

u

SJ

String Junction

P(y) ~ e a J y

P(y)const

BN transport even at large rapidity gaps (large energies). Veneziano: Probability exponentially suppressed (aJ: SJ intercept – model dependent) Kopeliovich: Probability constant with rapidity

LHC’s large rapidity gap between the incoming protons (yp = ±9.6) will give us the opportunity to distinguish between the different models.

Baryon number transport – Experimental results



BRAHMS: Phys. Lett. B607 (2005) 42

p+p - Au+Au @√s = 200GeV

p+p Au+Au 20% central PYTHIAHIJING-B

STAR: Phys.Rev.Lett.86 (2001) 4778

Lessons from RHIC

A. Falkiewicz: DIS2008, London

Lessons from HERA

Proton excess at mid-rapidity 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

Baryon number transport – Corrections (I)



Apply corrections that don’t cancel out in the ratio:o Absorptiono Cross section in transport codeso Secondaries (mainly protons)

Np

N p

corrected

Np

N p

raw

Cabsorption p

(y, pt )

Cabsorption p (y, pt )

Ccross sec tion p

(y, pt )

Ccross sec tion p (y, pt )

Csecondaries p (y, pt )

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)

Baryon number transport – Corrections (II)



Bendiscioli and Kharzeev, Riv.Nuovo Cim.17N6, 1-142 (1994)

Detailed study published in 2008: ALICE-INT-2008-010.v1

Scaling factor for the absorption corrections defined as εGEANT/εFLUKA

~2.5% decrease of the absorption correction

(pt )(pt )C(pt )

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.

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 HIJING-B clearly underestimates the y dependence, whereas it predicts a decrease of the ratio

with increasing pt not seen in p+p collisions.

o QGSM’s (only y information) version with aSJ ≠ 0 is systematically below the data points

o The different PYTHIA tunes describe the data well.

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.

ALICE is up and running!!!



THANK YOU FOR YOUR ATTENTION

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first identified particle studies at the cern-lhc with the alice experiment panos christakoglou a,b...

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