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Production of charged pions, kaons and (anti-)protons at high p T in s NN =2.76 TeV Pb-Pb collisions measured with ALICE Antonio Ortiz Velasquez on behalf of the ALICE Collaboration August 16 th , 2012 Outline Motivation Particle

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Production of charged pions, kaons and (anti-)protons at high pT in √sNN=2.76 TeV Pb-Pb collisions measured with ALICE

Antonio Ortiz Velasquez

  • n behalf of the ALICE Collaboration

August 16th, 2012

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August 16, 2012 Antonio Ortiz (Lund University) 2

Outline

➢ Motivation ➢ Particle identification in ALICE ➢ Measurement of the π/K/p spectra at high p

T

➢ Particle ratios ➢ R

AA for identified charged hadrons

➢ Conclusions

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August 16, 2012 Antonio Ortiz (Lund University) 3

Motivation: does the medium affect the high pT production and /or fragmentation?

Possible medium-induced modification

  • f jet hadrochemistry:

➢ Propagation: flavor and baryon number

exchange between medium and projectile.

➢ Energy loss: color transfer effects. Is there an

interplay between energy loss and fragmentation?

➢Fragmentation in medium?: recombination of

partons from jet and medium.

  • e. g., significant differences in jet hadrochemistry are found if only medium modification of

the parton shower is considered even in the absence of the medium-effects at or after hadronization. (S. Sapeta and U. A. Wiedemann, Eur. Phys. J. C. 55,293-302 (2008)). It produces a significant difference of the hadron ratios for medium modified and unmodified

  • jets. It derives a prediction for high pT (>7 GeV/c): RAA

K>RAA π and RAA p>RAA π.

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August 16, 2012 Antonio Ortiz (Lund University) 4

Charged particle identification in ALICE

Light charged hadrons (π±, K±, (anti-)p): ~150 MeV/c < pT < 20 GeV/c

TPC

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August 16, 2012 Antonio Ortiz (Lund University) 5

ITS

Charged particle identification in ALICE

Light charged hadrons (π±, K±, (anti-)p): ~150 MeV/c < pT < 20 GeV/c

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August 16, 2012 Antonio Ortiz (Lund University) 6

TOF

Charged particle identification in ALICE

Light charged hadrons (π±, K±, (anti-)p): ~150 MeV/c < pT < 20 GeV/c

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August 16, 2012 Antonio Ortiz (Lund University) 7

HMPID

Charged particle identification in ALICE

Light charged hadrons (π±, K±, (anti-)p): ~150 MeV/c < pT < 20 GeV/c

PID in ALICE. Talk: Marian Ivanov, 14/08 Session: Plenary 2A

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August 16, 2012 Antonio Ortiz (Lund University) 8

TPC-dE/dx in the relativistic rise

➢ At relativistic rise the particle species are not well separated, so statistical PID is

needed.

➢Are extracted from

data using PID: V0s daughters + pions and electrons from TOF.

&

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August 16, 2012 Antonio Ortiz (Lund University) 9

TPC-dE/dx in the relativistic rise Low multiplicity, pp

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August 16, 2012 Antonio Ortiz (Lund University) 10

TPC-dE/dx in the relativistic rise High multiplicity, Pb-Pb

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August 16, 2012 Antonio Ortiz (Lund University) 11

Extracting the particle fractions

Δπ is the quantity which was used to extract the particle fractions:

Spectra for identified charged particles:

( )

arXiv:1208.2711v1 [hep-ex]

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August 16, 2012 Antonio Ortiz (Lund University) 12

Particle fractions

pp

3 < pT < 3.2 GeV/c 8 < pT < 9 GeV/c

Peripheral Central

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August 16, 2012 Antonio Ortiz (Lund University) 13

π/K/p spectra

Pions: pT >2 GeV/c, Protons and Kaons: pT> 3 GeV/c come from the relativistic rise analysis.

Results on Low pT. Talk: Leonardo Milano, 16/08 Session: Parallel 5A

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August 16, 2012 Antonio Ortiz (Lund University) 14

Particle Ratios in pp

No energy dependence observed within the systematic uncertainties.

HMPID used for protons up to 6 GeV/c.

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August 16, 2012 Antonio Ortiz (Lund University) 15

Kaon/Pion (Pb-Pb, I)

In the intermediate pT region: 2-7 GeV/c, the ratio exhibits an evolution from the most central to the most peripheral Pb-Pb collisions.

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August 16, 2012 Antonio Ortiz (Lund University) 16

Kaon/Pion (Pb-Pb, II)

Same dependence is observed if we consider K

S

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Intermediate pT: 2-7 GeV/c, enhancement of the baryon to meson ratio.

Proton/Pion (Pb-Pb)

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August 16, 2012 Antonio Ortiz (Lund University) 18

Proton/Pion (ALICE vs models)

EPOS: Klaus Werner, arXiv:1204.1394, arXiv:1205.3379. AMPT: Jun Xu and Che Ming Ko, Phys. Rev. C 83, 034904 (2011). Recombination: R. J. Fries et al., Phys. Rev. Lett. 90, 202303 (2003) and private communication

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August 16, 2012 Antonio Ortiz (Lund University) 19

Is it a bulk or jet effect?

The enhancement seems to be a bulk effect (Misha Veldhoen, arXiv:1207.7195).

Particle production in jets is not affected by medium. Results on p/π at bulk: Talk: Jason G. Ulery, 15/08 Session: Parallel 4B

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August 16, 2012 Antonio Ortiz (Lund University) 20

Smoothing the particle fractions to prevent fluctuations on RAA at high pT

0-5% pp

Systematic error from smoothing was included in the RAA results.

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August 16, 2012 Antonio Ortiz (Lund University) 21

RAA for charged pions

pT< 7 GeV/c, RAA for charged pions < RAA for inclusive charged particles. pT > 7 GeV/c, RAA for charged pions ~ RAA for inclusive charged particles.

0-5% 5-10% 10-20% 20-40% 40-60% 60-80%

Results for inclusive charged particles: arXiv:1208.2711v1 [hep-ex]

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August 16, 2012 Antonio Ortiz (Lund University) 22

RAA for charged kaons

RAA for charged kaons compatible with RAA for inclusive charged particles.

0-5% 5-10% 10-20% 20-40% 40-60% 60-80%

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August 16, 2012 Antonio Ortiz (Lund University) 23

RAA for (anti-)protons

pT< 7 GeV/c, RAA for (anti-)protons > RAA for charged pions. pT > 7 GeV/c, RAA for (anti-)protons compatible with RAA for inclusive charged particles.

0-5% 5-10% 10-20% 20-40% 40-60% 60-80%

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August 16, 2012 Antonio Ortiz (Lund University) 24

RAA for π/K/p

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August 16, 2012 Antonio Ortiz (Lund University) 25

Conclusions

pp data:

➢ Particle ratios seem to be energy independent. ➢ NLO calculations and Pythia6 (Perugia2011) do not reproduce the

particle ratios at high pT.

Pb-Pb data:

➢ In the intermediate pT region (2-7 GeV/c):

The ratio K/π evolves with centrality. Same dependence is observed for K0s. The enhancement of the baryon to meson ratio observed at RHIC, is also present at LHC energies. The enhancement seems to be a bulk effect.

➢ At high pT (>7 GeV/c) the RAA for π/K/p are compatible, this

suggests that the medium does not affect the fragmentation.

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Backup

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Extending the PID using HMPID

The identification is obtained by means of the Cherenkov angle θ measured by HMPID With this technique we can identify protons with 3σ separation up to pT~5 GeV/c, and with ~2σ separation up to 6 GeV/c

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Particle ratios inside jets.

Significant difference of hadron ratios for medium modified and unmodified jets at high momenta.

~50 – 100% medium effect

If we assume that at high pT hadrons come from jets, then we have a prediction for RAA:

Predictions for RAA at high pT for identified charged hadrons

(Eur. Phys. J. C. 55,293-302 (2008)).

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August 16, 2012 Antonio Ortiz (Lund University) 29

PID at high pT, TPC-dE/dx

All particle species are described by just two curves:

This is quite important for: kaons and the high pT region where no PID can be used.

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Proton/Pion (ALICE vs STAR)

Protons from STAR not feed- down corrected.

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RAA for charged pions

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RAA for charged kaons

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RAA for (anti-)protons