Multiplicity dependence of identified particle production in pp collisions with ALICE

Transcription

1 Multiplicity dependence of identified particle production in pp collisions with ALICE (for the ALICE Collaboration) NISER, INDIA Outline Introduction Results from pp collisions at s = 7 TeV and 3 TeV Collectivity in small systems Strangeness production Summary

2 A Large Ion Collider Experiment (ALICE) ALICE is the LHC experiment whi is optimised for heavy-ion physics: it aims to study the formation and properties of the quark-gluon plasma (QGP) Excellent PID capabilities allow the measurement of identified hadron production in a wide momentum range down to very low-pt (~0 MeV) in pp, p-pb, and Pb-Pb collisions ALICE can provide an important contribution to the LHC pp physics program investigation of the soft (non-perturbative) and hard (perturbative) regimes of QCD probe QCD in a wide range of particle densities pp, s = 7 TeV dn/dη =.3±0.6 (0-0.95%) = 3.9±0.4 (48-68%) p-pb, snn = 5.0 TeV dn/dη = 45± (0-5%) = 9.8±0. (60-80%) Pb-Pb, snn =.76 TeV dn/dη = 60±60 (0-5%) = 35± (70-80%)

3 Particle Identification in ALICE Inner Tracking System (ITS) : ( η < 0.9) 6 layers of silicon detector Tracking, vertex, PID (de/dx) Time Projection Chamber (TPC): ( η < 0.9) Volume of 90 m3 filled with Ne-CO Tracking, vertex, PID (de/dx) Time of Flight (TOF) ( η < 0.9) multi-gap resistive plate amber tenology PID Int. J. Mod. Phys. A (04) LA THUILE, ITALY, 5th Mar-st April, 07 3

4 Introduction: heavy-ion physics 4 In ultra-realtivistic heavy-ion (HI) collisions, a Quark Gluon Plasma (QGP) is formed After the collision, QGP fireball expands, develops collective flow and cools down Inelastic processes cease and emical composition is frozen at Tem (Pseudo)elastic interactions continue until the kinetic freeze-out (Tkin) Thermal model: Particles in HI collisions are produced in apparent emical equilibrium ØTheoretical description based on thermal-statistical models in whi (in first order) particle abundances exp(-m/tem). At LHC: Tem ~ 56 MeV Recently, several similarities between heavy-ion and (high multiplicity) pp were observed, whi were related to observables defined at Tkin (flow-like patterns as the double-ridge) What about the particle emistry (i.e. the pt-integrated particle abundances)?

5 + π + ) - ) / (π + + Ω - (Ω ALI-PUB-358 Introduction: strangeness enhancement GSI-Heidelberg model = 56 MeV T THERMUS.3 model = 55 MeV T MB Phys. LeC. B 758, (06) ALICE p-pb s NN = 5.0 TeV V0A mult. evt. classes (Pb-side) pp Pb-Pb s = 7 TeV s NN dn =.76 TeV /dη pp p-pb Pb-Pb 3 lab η < 0.5 lab Strangeness enhancement in Pb-Pb collisions w.r.t to min-bias pp collisions is a long known effect. Historically, it was proposed as one of the signatures of QGP formation Can we also observe it in high multiplicity pp? (J. Rafelski et al, Phys. Rev. Lett. 48,66 (98)) Thermal-statistical models (GSI- Heidelberg and THERMUS) well describe the particle ratios in Pb-Pb (Comput.Phys.Commun. 80, 84 6 (009), Andronic et al, Phys. LeC. B (009)) A smooth evolution of integrated particle yields from p-pb to Pb-Pb as a function of multiplicity has been observed What about pp collisions vs multiplicity? pp collisions have the additional advantage that they allow comparisons to a full set of MC generators that aim to model soft-qcd part 5

6 - (GeV/c) N /dydp d /N evt T pt-spectra in pp collisions at s = 7 TeV 4 - π + +π p+ p ALI PREL K +K Λ+Λ KS - + Ξ +Ξ ALICE Preliminary pp at s=7 TeV V0M Mult. Evt. Classes p T K*+K* - + Ω +Ω (GeV/c) π + +π, K +K, p+p KS, Λ+Λ, Ξ +Ξ : 0 X (x ) IX (x ) VIII (x ) 3 VII (x ) 4 VI (x ) 5 V (x ) 6 IV (x ) 7 III (x ) 8 II (x ) 9 I (x ) K*+K*: 0 X (x ) IX (x ) VIII (x ) 3 VII (x ) 4 VI (x ) IV + V (x 6 III (x ) 7 II (x ) 8 I (x ) - + : 5 ) Ω +Ω 0 IX + X (x ) VII + VIII (x V + VI (x ) 3 III + IV (x ) 4 I + II (x ) ) 6 V0M MulKplicity Classes: [ dn/dη MB 6.0] { I dn/dη 3.5 dn/dη MB X dn /dη 0.4 dn/dη MB

7 Collectivity in small systems: kinetic observables 7 ) - (p + p) / (π + + π ALICE Preliminary pp s = 7 TeV V0M Class I, dn /dη =.3 V0M Class X, dn /dη =.3 (V0M Multiplicity Classes) pp ALICE p-pb s NN 0-5%, dn /dη 60-80%, dn = 5.0 TeV = 45. = 9.8 /dη (V0A Mult. Classes - Pb side) p-pb ALICE Pb-Pb s NN 0-5%, dn /dη 60-80%, dn Pb-Pb =.76 TeV /dη = 60.0 = ALI-PREL-79 (GeV/c) p T QualitaKvely similar mulkplicity dependence of baryon-to-meson rako (p/π) as funckon of p T but different magnitude in the three colliding systems (pp p-pb Pb-Pb) Enhancement at intermediate-p T and deplekon at low-p T is observed in all systems In Pb-Pb: acributed to radial flow or recombinakon. (R. Fries et al, Phys. Rev. C68, (003)) In pp: hint of colleckve behaviour?

8 Collectivity in small systems: kinetic observables 8 ) - (p + p) / (π + + π ALICE Preliminary pp s = 7 TeV V0M Class I, dn /dη =.3 V0M Class X, dn /dη =.3 (V0M Multiplicity Classes) pp ALICE p-pb s NN 0-5%, dn /dη 60-80%, dn = 5.0 TeV = 45. = 9.8 /dη (V0A Mult. Classes - Pb side) p-pb ALICE Pb-Pb s NN 0-5%, dn /dη 60-80%, dn Pb-Pb =.76 TeV /dη = 60.0 = ALI-PREL-79 (GeV/c) p T QualitaKvely similar mulkplicity dependence of baryon-to-meson rako (p/π) as funckon of p T but different magnitude in the three colliding systems (pp p-pb Pb-Pb) Enhancement at intermediate-p T and deplekon at low-p T is observed in all systems In Pb-Pb: acributed to radial flow or recombinakon. (R. Fries et al, Phys. Rev. C68, (003)) In pp: hint of colleckve behaviour?

9 Collectivity in small systems: kinetic observables 9 Low-pT Mid-pT High-pT ALI-PREL-95 QualitaKvely similar features observed for parkcle produckon in pp, p-pb and Pb-Pb For a fixed p T, a smooth evolukon of p/π rako with mulkplicity density across different systems is observed Do the QCD-inspired event generators reproduce this trend for pp collisions?

10 Collectivity in small systems: kinetic observables Pythia with CR whi works becer in describing the trend colour reconneckon in pp collisions can mimic radial flow Low-pT Mid-pT Let s look at parkcle composikon of the system to inveskgate if the system also shows similarikes in the emical parkcle composikon ALI-PREL-97

11 Strangeness enhancement in pp collisions Smooth evolukon of p T -integrated parkcle rakos across different colliding systems as a funckon of mulkplicity MCs are not saksfactorily describing the observakons ØPythia8 does not describe the strangeness enhancement (with and without colour reconneckon) ØEPOS LHC (CollecKve hadronizakon + colleckve flow) shows enhancement qualitakvely but not quanktakvely ØDIPSY (colour ropes) increases mainly baryon content, not strangeness enhancement In pp : strange parkcle rako to π increases Is the enhancement due to (a) mass (b) baryon/meson nature of the parkcle or (c) strangeness content? Can we factor out these effects?

12 Strangeness enhancement in pp collisions In p/π and Λ/K 0 s the only effect could be baryon/meson or mass, because there is no net-strangeness difference the ratios are flat

13 Strangeness enhancement in pp collisions 3 S=0 S= S= S=3 S= Double-raKo in pp collisions (and ppb, also shown) evolves smoothly with mulkplicity density Proton (S=0) is consistent with unity up to highest dn /dη probed Hyperon produckon increases from low to high mulkplicity in pp and p-pb The larger the valence strange quark content, the steeper the slope the effect is due to strangeness ALI-PUB-695 arxiv: (06) MB (dashed lines to guide the eye) Is the same enhancement present at higher energy (3 TeV)? Is it collision-energy dependent or multiplicity driven?

14 Strangeness production at s = 7 TeV, 3 TeV 4 ALI-PREL-694 ALI-PREL-698 Similar scaling is observed for strangeness produckon with dn /dη in pp collisions at s = 7 TeV and 3 TeV Strange hadron produckon is collision energy independent at similar mulkplicity ALI-PREL-6306

15 Strangeness production at s = 7 TeV, 3 TeV 5 ALI-PREL-06 ALI-PREL-046 p T of K 0 s at 3 TeV p T of K 0 s at 7 TeV at similar arged parkcle mulkplicity densikes Λ and Ω: p T are similar within systemakc uncertainkes ALI-PREL-054

16 Summary 6 ALICE has a unique possibility of probing QCD in a wide range of particle densities and soft/hard regime by measuring identified particle production We have presented a comprehensive study of identified particles (π, K, p, K 0 s, Λ, Ξ, Ω, K *0 ) done by the ALICE Collaboration at s = 7 TeV and 3 TeV Similarities among different systems are observed that hint for the presence of collectivity in small systems whose origin and phenomenology is under investigation Enhancement of strangeness production observed from low to high-multiplicity pp events at s = 7 TeV MC generators need more tuning or new features to describe the data? Measurements at different energies as a function of multiplicity seem to indicate that the hadroemistry is driven by event activity regardless of collision energy Thank you!

17 Backup Baryon to meson ratio (Λ/K 0 s) 7 ALI-PREL-56

18 Strangeness production at s = 7 TeV, 3 TeV 8

19 - (GeV/c) T N/dydp d /N evt ALICE Preliminary, pp s = 7 TeV Blast-Wave fit to π, K, p Blast-Wave prediction V0M Class I+II π 0 K 50 p 0 KS Λ 0. Ξ 0.05 Ω K* fit range: GeV/c GeV/c GeV/c π K p 0 KS.5 K* (GeV/c) ALI PREL 96 Collectivity in small systems p T Λ Ξ Ω Blast-Wave A simplified hydrodynamic model describes spectra evolukon quite well with three parameters: n, β T and T kin (β T Radial flow velocity, T kin KineKc freeze-out temperature) (E. Snedermann, et al, Phys. Rev. C 48 46, (993)) Combined Blast-Wave fit to π, K and p p T -spectra (highest mulkplicity class) Extracted β T and T kin parameters for ea mulkplicity class The fit parameters of combined Blast-Wave model with π, K and p are used to predict the hyperons p T -spectra strange and mulk-strange parkcles follow a common flow with lighter hadrons The form of Blast-Wave function is, p T dn dp T / R R 0 rdrm T I 0 ( p T sinh T kin ) K ( m T cosh T kin ) Velocity profile( ) =tanh T = tan [( r R )n s] (Phys. Rev. C48 46, (993)) 9

20 dn /dη η < 0.5 Collectivity in small systems 0 (GeV) T kin Global Blast-Wave fit to π (0.5- GeV/c), K (0.-.5 GeV/c), p ( GeV/c) ALICE Preliminary, pp, s = 7 TeV s = 5.0 TeV β T ALI PREL 8839 ALICE, p-pb, ALICE, Pb-Pb, s =.76 TeV High mulkplicity pp collisions also show increasing colleckvity ( β T ~ 0.5) similar to mid central Pb-Pb and p-pb collisions In high mulkplicity pp, hint of colleckve effects? p-pb: Phys. Lett B760, 70 (06)

21 Collectivity in small systems Low-pT Mid-pT ALI-PREL-939