Massimo Venaruzzo for the ALICE Collaboration INFN and University of Trieste

Transcription

1 Massimo Venaruzzo for the ALICE Collaboration INFN and University of Trieste 2 nd European Nuclear Physics Conference (EuNPC 2012) Bucharest (17-21 September 2012)

2 Outline Resonances in pp collisions: motivations Experimental setup and resonance reconstruction Invariant mass spectra for f, K*, S*, X*, L* in pp collisions at s = 7 TeV will be discussed together with the methods for raw yields extraction Comparison of p T -spectra with QCD-based models (PYTHIA and PHOJET) Comparison of p T -spectra at different pp collision energies ( s = 900 GeV and s = 7 TeV) Particle ratios (f/p, f/k, f/k*, K*/K, (Ω+Ω)/f, S*/p, S*/K, S*/X ) 2

3 Resonances at the LHC with ALICE ALICE is the LHC experiment mainly devoted to the study of hot and high energy density nuclear matter created in heavy-ion collisions ALICE has also developed a detailed proton-proton physics programme, aimed at: setting the baseline for the heavy-ion data investigating the novel energy regime made available by LHC (decay of resonances large fraction of the final-state particles early step in understanding pp collisions at the LHC, i.e. opportunity to test QCD in a new energy domain) The study of resonances in pp contributes to: a proper tuning of the QCD-inspired particle production models (PYTHIA, PHOJET, etc ) a better understanding of the underlying event a baseline for Pb-Pb studies understanding hadronic production processes 3

4 Resonances at LHC with ALICE Analysis of f, K*, S*, X*, L* in pp collisions at s = 7 TeV (using ~ M events from 2010) pp@900 GeV pp@900 GeV, pp@2.36 TeV, pp@7 TeV, Pb-Pb@2.76 ATeV /2013 pp@2.76 TeV, pp@7 TeV, Pb-Pb@2.76 ATeV pp@8 TeV, p-pb@4 TeV ATeV f(1020) K + + K - K*(892) 0 p ± + K L(1520) p + K - S(1385) ± L + p ± X(1530) 0 X - + p + (1232) ++(--) N + p ρ(770) π + π - ± 4

5 Resonances reconstruction K* K + S *+ p + p p - p - Short lifetimes (few fm/c)!! Decay products not distinguishable from primary particles! (unlike L, X, W) -> no topological reconstruction is possible Invariant mass analysis for signal extraction Combinatorial background + correlated background from resonances with misidentified daughter(s) 5

6 The ALICE Experiment For this analysis: Global tracking using ITS and TPC Particle ID with TPC and TOF 0.9 K. Aamodt et al. (ALICE Collaboration) J. Instrum. 3 (2008) S08002 K. Aamodt et al. (ALICE Collaboration) J. Instrum. 5 (2010) P

7 Signal Extraction f and K * y 0.5 y y Combinatorial background (estimated via Event- Mixing) subtracted Fit: polynomial + Voigtian p T bin (GeV/c) PDG Mass (MeV/c 2 ) PDG Γ (MeV/c 2 ) 4.26 Fit Mass (MeV/c 2 ) ± 0.05 Fit Γ (MeV/c 2 ) Fixed at 4.26 Fit Sigma (MeV/c 2 ) 1.20 ± 0.08 Alternative methods: like-sign and polynomial background implemented! Combinatorial background (estimated by like-sign) subtracted Fit: Breit-Wigner + polynomial for the residual background p T bin (GeV/c) PDG Mass (MeV/c 2 ) PDG Γ (MeV/c 2 ) 48.7 Fit Mass (MeV/c 2 ) ± 0.5 Fit Γ (MeV/c 2 ) 54.0 ± 2.0 For f and K*@ 7 TeV see B. Abelev et al., arxiv: v1 7

8 Signal Extraction S * and X * y 0.8 Combinatorial background (estimated via Event- Mixing) subtracted Fit: 3 rd degree polynomial + Breit-Wigner p T bin (GeV/c) Integral PDG Mass (MeV/c 2 ) PDG Γ (MeV/c 2 ) 35.8 Fit Mass (MeV/c 2 ) ± 0.7 Fit Γ (MeV/c 2 ) 36.2 ± 1.4 Alternative method: side bands implemented! Combinatorial background (estimated via Event mixing) subtracted Fit: 3 rd degree polynomial + Voigtian p T bin (GeV/c) Integral PDG Mass (MeV/c 2 ) PDG Γ (MeV/c 2 ) 9.1 Fit Mass (MeV/c 2 ) ~ 1533 Fit Γ (MeV/c 2 ) Fixed to PDG Fit σ (MeV/c 2 ) 2.5 Alternative methods: like sign and polynomial background implemented! 8

9 Signal Extraction Λ * y 0.8 Combinatorial background (estimated via Event- Mixing) subtracted Fit: polynomial residual background + Breit-Wigner p T bin (GeV/c) PDG Mass (MeV/c 2 ) PDG Γ (MeV/c 2 ) 15.6 Fit Mass (MeV/c 2 ) ± 0.4 Fit Γ (MeV/c 2 ) 18 ± 1 9

10 Comparison with models: K * and f K *0 + K *0 2 f (1020) Light blue band: data systematics Best agreement with Perugia 2011 p T 4 GeV/c: good agreement with PHOJET p T 2 GeV/c: good agreement with PYTHIA D6T For f and K*@ 7 TeV see B. Abelev et al., arxiv: v1 p T 2 GeV/c: good agreement with PHOJET p T 3 GeV/c: good agreement with Perugia

11 Comparison with models: S * p T 2 GeV/c: good agreement with ATLAS-CSC PYTHIA tunes describe better the (1020) than S(1385) resonance 11

12 For GeV see K. Aamodt et al., Eur. Phys. J. C. (2011) 71 Yields (dn/dy at midrapidity) scale with the multiplicity of the collision f spectrum and <p T > For f and K*@ 7 TeV see B. Abelev et al., arxiv: v1 A rise in the mean p T with collision energy is observed. Data well above ISR parameterization. 12

13 Particle ratios: K * Both the ratios seem independent of the energy and the collision system (with the exception of the RHIC A-A data). For f and K*@ 7 TeV see B. Abelev et al., arxiv: v1 13

14 Ratio resonance/stable particle: f The f/p ratio increases with energy both in heavy-ion and in pp collisions. This trend seems to change at the energy of LHC in pp collisions The f/k ratio does not seem to increase with energy For f and K*@ 7 TeV see B. Abelev et al., arxiv: v1 14

15 (W+anti-W)/ f ratio at 7 TeV HIJING (K=2 GeV/fm) rescale factor W W W W 7TeV 5.5TeV 1.1 For f and K*@ 7 TeV see B. Abelev et al., arxiv: v1 For 7 TeV see B. Abelev et al., Phys. Lett. B 712 (2012) 309 Pythia Perugia 2011 is a factor below data. It underpredicts multistrange baryon yields. Predictions of HIJING/BB with a Strong Color Field modeled by an increased string tension are in agreement with the data. 15

16 Ratio resonance/stable particle: S * S*/π - energyindependent as predicted by the model (grand-canonical) S*/K - in agreement within errors with STAR S */X - decreases with energy <-> increased multistrange production Thermal model = statistical hadronization model with T = 170 MeV and S =0.6 Beccatini et al., J.Phys.G38:025002,

17 Conclusions Results for f, K*, S*, X*, L* in pp collisions at s = 7 TeV (~150/200 M events collected in 2010 by ALICE) have been shown. The raw signals for all the resonances have been extracted in several pt-bins and good agreement observed for the masses and the widths wrt the PDG values. pt-spectra have been compared to QCD-based models (PYTHIA and PHOJET) indicating some disagreements in the description of the data. pt-spectra at s = 900 GeV and s = 7 TeV have been shown for the f, indicating a rise in the mean pt with collision energy, and at s = 7 TeV for K* and S*. The f/p ratio increases with energy both in heavy-ion and in pp collisions. This trend seems to change at the energy of LHC in pp collisions with a possible saturation. The f/k, f/k*, K*/K ratio does not seem to increase with energy. The(Ω+Ω)/f shows a saturation for pt > ~ 2.5 GeV/c. S*/π- and S*/K - seems energy-independent as predicted by the model (grandcanonical). S*/X- decreases with energy indicating an increased multistrange hadron production. Energy dependence of resonance production: study will be performed soon. 17

18 BackUp Slides 18

19 Results from STAR Previous results from STAR at 0.2 TeV B.I. Abelev et al., PRC 78, (2008) B.I Abelev et al., PRL 97, (2006) 19

20 Tracking and PID Tracking Tracking via Kalman filter algorithm Requirement of at least 70/159 TPC clusters Requirement of one cluster in the innermost ITS detector (SPD) to reduce the background DCA to the primary vertex used to discriminate daughter tracks and background TPC de/dx resolution ~6.5% Difference between measured energy loss and the one expected with the mass hypothesis Particle Identification: TPC p and K up to p ~ 0.7 GeV/c proton/anti-proton and p/k up to p ~ 1 GeV/c TOF K and p up to p ~ 2 GeV/c proton/anti-proton and p/k up to p ~ 2.5 GeV/c Time resolution ~160 ps Difference between measured time of flight and the one expected with the mass hypothesis 20

21 Signal Extraction f and S * Alternative method : polynomial background y 0.5 Alternative method : polynomial background y 0.8 Fit: polynomial background + Voigtian p T bin (GeV/c) PDG Mass (MeV/c 2 ) PDG Γ (MeV/c 2 ) 4.26 Fit Mass (MeV/c 2 ) ± 0.10 Fit Γ (MeV/c 2 ) 4.52 Fit σ (MeV/c 2 ) Fixed to 1.2 Fit: polynomial (Taylor + Laurent) background + Gaussian p T bin (GeV/c) PDG Mass(MeV/c 2 ) (Σ* + ) (Σ* - ) PDG Γ (MeV/c 2 ) 35.8 (Σ* + ) 39.4 (Σ* - ) Fit Mass (MeV/c 2 ) 1383 ± 1 Fit Γ (MeV/c 2 ) 33 ± 3 21

22 Signal Extraction X * Alternative method : like-sign y 0.8 Alternative method : polynomial background y 0.8 Combinatorial background (estimated via Like- Sign) subtracted Fit: 3 rd degree polynomial + Breit-Wigner p T bin (GeV/c) Integral PDG Mass (MeV/c 2 ) PDG Γ (MeV/c 2 ) 9.1 Fit Mass (MeV/c 2 ) ± 0.4 Fit Γ (MeV/c 2 ) Fixed to PDG Fit σ (MeV/c 2 ) ~ 2.5 Fit: 5th order pol. background + Voigtian p T bin (GeV/c) PDG Mass (MeV/c 2 ) PDG Γ (MeV/c 2 ) 9.1 Fit Mass (MeV/c 2 ) ± 0.4 Fit Γ (MeV/c 2 ) Fixed to PDG Fit σ (MeV/c 2 ) 2.0 ±

23 Mass and Width vs p T : S * Good agreement with PDG values 23