Resonance results by the ALICE detector in pp and in Pb-Pb collisions at LHC energies

Resonance results by the ALICE detector in pp and in Pb-Pb collisions at LHC energies

A.Badalà-EPIC-Bari(Italy)- 6-8 July 2011 1

A. Badalà - INFN Sezione di Catania

for the ALICE collaboration

•Introduction – Resonances in medium

•ALICE performances

•Results in p-p @ 7 TeV (about 25-155X 106 of MB events)

K*(892)0, (1020), Σ*(1385)±, (1232)++, (1530)0

•Results for (1020) in Pb-Pb collisions @ 2.76 ATeV (about 6x106 MB events)

•Conclusions and Prospects

t~10 fm/c

Resonances lifetimes of about a few fm/c resonance ~ fireball

Resonances may give information on the nuclear matter dynamics and chiral properties

Resonance in heavy-ion collisions

Regeneration and rescattering effectsTimescale chemical-kinetic freeze-out

Modification of width, mass and branching ratio

Space-time evolution of ultrarelativistic heavy ion collision


Mass (MeV/c2)

Width (MeV/c2)

cτ(fm) Decay

K*(892)0 896 50 4 K π

φ(1020) 1019 4 46 K K

Σ(1385)± 1385 33 6 Λ π

Ξ(1530)0 1530 9 22 Ξ π

Resonances in p-p Collisions

In general, resonance production in p-p collisions helps in

•establishing the underlying event structure and the baseline for heavy-ion collisions

•constraining QCD-inspired models (PYTHIA, PHOJET,etc…)

•understanding hadronic production processes

3

Decay of resonances large fraction of the final-state particles early step in understanding p-p collisions at LHC, i.e. opportunity to test QCD in a new energy domain

A.Badalà-EPIC-Bari(Italy)- 6-8 July 2011

4

Detector:

Size: 16 x 26 meters

Weight: 10,000 tons

ALICEALICE

material budget

Particle identificationBasic identification strategy: fit response function in pT bin

For resonances, V0 and cascade analysis PID by a cut (3-5 ) on the relative difference between:

:

5

TOF signal and integrated time

TPC signal and Bethe-Bloch function

TOF ≈ 90 ps

TPC dE/dx ≈5-6%


TPC TOF

π0.2 ÷ 0.7

0.5 ÷ 2.0

K0.3 ÷ 0.6

0.5 ÷ 2.0

p0.5 ÷ 1.0

0.5 ÷ 2.5

pT range of separation within 3σ (GeV/c)

Resonance Reconstruction

K+

-

(1232)++ p++

(1520)0 p + K-

(1385)± 0 + p±

(1530)0 - + p+

Reconstruction based on primary tracks or particles (cut on DCA)

K*

, K, p identified via TPC and TOF detectors

identified by secondary tracks, with opposite charges, within a fiducial volume, + “V0 topology”

identified by three secondary tracks, within a fiducial volume,+ “Cascade topology”


(1020)

0-

7

pT bin (GeV/c) 0.7 – 0.8 1.2 – 1.6

PDG Mass (MeV/c2)

1019.46 1531.8

PDG Γ (MeV/c2)

4.26 9.1

Fit Mass (MeV/c2)

1019.30 ± 0.10

1531.5 ± 0.4

Fit Γ (MeV/c2)

4.52 ± 0.01 Fixed to PDG

Fit σ (MeV/c2)

Fixed to 1.2 2.0 ± 0.5

(|y| ≤ 0.5) Ξ* (|y| ≤ 0.8) Fit: poly + Voigtian (i.e. Breit-

Wigner convoluted with Gaussian)

– take into account inv. mass resolution (for estimated by MC ~ 1 MeV/c2)

Raw counts: Voigtian full integral

Signal extraction: and *


Signal extraction: K*


Subtract background estimated by like-sign (2√N+

+N--)

Fit: Breit-Wigner + straight lineRaw counts: BW full integral

Rapidity range: |y| ≤ 0.5

pT bin (GeV/c) 2.0 – 2.5

PDG Mass (MeV/c2) 895.9

PDG Γ (MeV/c2) 48.7

Fit Mass (MeV/c2) 893.4 ± 0.5

Fit Γ (MeV/c2) 54.0 ± 2.0

Signal extraction: ++


Subtract background estimated by event mixingFit: relativistic p wave Breit Wigner*phase space factor+linear backgroundRaw counts: BW full integral

Rapidity range: |y| ≤ 0.5

pT bin (GeV/c) 1.2– 1.4

PDG Mass (MeV/c2) 1232

PDG Γ (MeV/c2) 116

Fit Mass (MeV/c2) 1196 ± 0.002

Fit Γ (MeV/c2) 0.108± 0.008

Invariant mass shifted to low values, alredy observed at RHIC energies. (STAR

coll. Phys. Rev. C78(2008)044906) This could be originated from N interactions with medium

Co

un

ts/(

0.0

2 G

eV/c

2)

Fit: Gaussian + negative power lawSubtract background functionRaw counts: bin count - 3σ around peak center.Rapidity range: |y| ≤ 0.8

pT bin (GeV/c) 0.7 – 0.8

PDG Mass (MeV/c2) 1382.8 (Σ*+) 1387.2 (Σ*-)

PDG Γ (MeV/c2) 35.8 (Σ*+) 39.4 (Σ*-)

Fit Mass (MeV/c2) 1383 ± 1

Fit Γ (MeV/c2) 33 ± 3

Signal extraction: *


Branching ratio limits

Ξ* efficiency includes also reconstruction efficiency for previous decay steps (Ξ decay reconstruction)

Reconstruction efficiency of K* , and *


Systematics• PID cuts• topological

selection (Ξ*)• background

subtraction• material budget• track selection• normalization

Fit: Levy / Tsallis function

Corrected spectra of K* , and *


φ @ 900 GeV : ALICE Collaboration, Eur. Phys. Journal C 71 (2011), 1594

Charged multiplicity @ 900 GeV and 7 TeV: ALICE Collaboration, Eur. Phys. Journal C 68 (2010), 345

dNφ/dy increases proportionally to dNch/dy from 900 GeV to 7 TeV

√s <pT> dN/dy T n

900 GeV 1.00 ± 0.24 0.021 ± 0.005 164 ± 91 4.2 ± 2.5

7 TeV 1.112 ± 0.02 0.0334 ± 0.0008 286 ± 14 7.0 ± 0.6

uncert = √(stat2+syst2)

resonance : comparison with 900 GeV


KK and μμ


Slope KK consistent with slope μμ (2.5<y<4.1, 1<pT<5 GeV/c)

Preliminary LHCb results, KK (2.44<y<4.06, 0.8<pT<5 GeV/c) LHCb-CONF-2010-014

pT ≤ 2 GeV/c: good agreement with PYTHIA D6T

pT ≥ 2 GeV/c: good agreement with PHOJET

Yellow band:data

systematics

Comparison with MC : resonance


Yellow band:data

systematics

Comparison with MC : K* resonance


All pT: good agreement with PYTHIA D6T

Yellow band:data

systematics

Comparison with MC : * resonance


All pT: underestimated

Particle ratios


pT bin (GeV/c) 1.2 – 1.6

PDG Mass (MeV/c2) 1019.46

PDG Γ (MeV/c2) 4.26

Fit Mass (MeV/c2) 1019.03 ± 0.02

Fit Γ (MeV/c2) 4.75 ± 0.06

• Smaller signal / background– tighter Particle ID cuts with TPC

and TOF– require TOF for tracks with pT >

600 MeV/c– subtract like-sign background

• Fit: Voigtian + polynomial• Raw counts: Voigtian full integral

Co

un

ts /

1 M

eV

/c2

Extracted φ signal in 14 bins for 0.5 ≤ pT ≤ 5 GeV/c

in Pb-Pb collisions


Conclusions and Prospects

In p-p collisions at 7 TeV performances for K*(892)0, (1020), (1385)± and (1530)0, (1232) resonance signals have been shown. Good agreement of their characteristics (mass, width) with PDG value or previous measurements at lower energies.

Measured dN/dpT for φ, K* and Ξ* at mid-rapidity in pp collisions at √s = 7 TeV.

spectrum compared with the one measured at 900 GeV and with the one measured in forward direction by the μμ decay channel. Good agreement was found.

Measured spectra have been compared with MCφ well reproduced by PYTHIA D6T below 2 GeV/c, by PHOJET aboveK* well reproduced by PYTHIA D6TΞ* underestimated by a factor ~5 in all cases

Particle ratios as /K* or K*/K don't increase with respect to lower collision energy values, even φ /π saturates.

>>> Extracted φ raw counts spectrum for different centrality in Pb-Pb collisions at √sNN = 2.76 TeV. Yield will be soon available.


Backup slides

2222

Ratios : THERMUS PredictionsRatios : THERMUS Predictions

Ratio proportional with the TRatio inversely proportional with the mass difference

Ratio proportional with the TRatio inversely proportional with the mass difference

ΔS=0ΔJ=1ΔB=0

ΔS=0ΔJ=1ΔB=0

S. Wheaton, J. Cleymans and M. Hauer, Comput. Phys. Commun. 180, 84 (2009)

Resonance results by the ALICE detector in pp and in Pb-Pb collisions at LHC energies

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