Jet fragmentation study in vacuum and in medium in the ALICE experiment at the LHC

Transcription

1 Jet fragmentation study in vacuum and in medium in the ALICE experiment at the LHC Magali Estienne for the ALICE Collaboration WISH 2010 Catania, Italia, September

2 Outline 2 Motivations for jet fragmentation study in vacuum and in medium ALICE jet reconstruction performances with EMCal Discussion on the internal structure of a jet Conclusion and perspectives

3 Color coherence and angular ordering QED: the «Chudakov» effect QCD: angular ordering 3 Y. Dokshitzer et al. Y. Dokshitzer et al. QCD Basics of pqcd QCD Basics of pqcd 1 t form = k 2 e T e d T e+e- e+e- t form T e + e+e- e -,k e e+e- e For large angle photon emission, e >> e+e- => d e+e- T << T The does not resolve the pair and only probes the global charge of the pair emission possible only if e < e+e- The presence of the 2 charges constrains the emission angle of the g q qq q g,k 1 gq 2 gq Same effect as in QED but due to color charge interference Different from QED: the emitted gluon carries a color charge: gq 2 gq 1 << qq Angular Angularordering orderingisisthe the result resultof of color colorcoherence effects effects :: they theysuppress suppresssoft soft radiations radiations at atlarge angles! angles!

4 Stopping scale of the parton shower 4 A minimum emission angle of the showering: R: typical size of hadronization (1/R hadron mass). 1 T = < R => k T k gq 1 R 1 kr Y. Dokshitzer et al. Y. Dokshitzer et al. QCD Basics of pqcd QCD Basics of pqcd Color Colorcoherence effects effects also alsoconstrain constrainminimum emission emissionangle! Hadron Hadron mass mass dependence of of the the minimum minimum emission emission angle angle Conflicting tendencies for a soft particle emission: Constrained to be emitted at smaller angle in the parton shower Forced out at large emission angle 1 kr gq former emission

5 The hump-backed plateau: direct consequence of color coherence 5 MLLA + LPHD predictions = log(e Tjet /p Th ) Motivations in p+p collisions: Previous measurements from OPAL, TASSO, CDF. At LHC, pqcd constrains at «intermediate» jet energy for ALICE: challenge Jet hadrochemistry of interest to test the mass dependence of the cutoff Hadronization processes Baseline for Pb+Pb studies e + e - OPAL and TASSO Modified Leading Logarithmic Approximation (MLLA) : A running strong coupling parameter defined by a QCD scale A cut-off scale, Q 0 (mass dependence) Local Parton Hadron Duality (LPHD) : #partons K.#hadrons

6 In Pb+Pb collisions: jet structure modification 6 Medium effects introduced at parton splitting Decrease of the particles at high z (low ) [energy loss] Increase of the particles at low z (high ) [radiated energy] Jet broadening & out of cone radiations increase => reduction of jet rate Increase of di-jet energy inbalance and acoplanarity z = p Thad /p Tjet = ln(1/z) Pb q q K g Pb

7 In Pb+Pb collisions: jet structure modification 6 Medium effects introduced at parton splitting Decrease of the particles at high z (low ) [energy loss] Increase of the particles at low z (high ) [radiated energy] Jet broadening & out of cone radiations increase => reduction of jet rate Increase of di-jet energy inbalance and acoplanarity Fragmentation strongly modified at p Thadron ~1-5 GeV/c even for the highest energy jets z = p Thad /p Tjet = ln(1/z) Pb N. Borghini & U. Wiedemann Hep-ph/ q q K g Pb =log(e TJet /p Th ) ALICE should be well dedicated to test this range thanks to tracking down to 100 MeV/c and really good PID!!! Question: to which extent E Tjet can be evaluated accurately?

8 Jet hadrochemistry in Pb+Pb collisions 7 Fragmentation in vacuum Projectile gluon Medium-modified fragmentation Projectile gluon S. Sapeta & U. Wiedemann S. Sapeta & U. Wiedemann Eur. Phys. J. C55, 293 (2008) Eur. Phys. J. C55, 293 (2008) K +- / +- p(p)/ +- Parton target The interaction of a parent parton with a QCD medium transfers color between projectile and target Color flow changed in the shower Hadronization affected in the final state Medium-induced modification of the hadrochemical composition of jets Motivations in Pb+Pb collisions: Measurement of parton energy loss and QCD medium properties Jet hadrochemistry of interest to test the medium of heavy ion collisions Better understanding of hadronization processes Modification of the parton splitting function relative to p+p E jet = 200 GeV E jet = 100 GeV E jet = 50 GeV

9 Excellent tracking in a high density environment! ALICE central barrel RICH TOF 8 Central barrel: < 0.9 Optimized for high multiplicity (8000 particles) Tracking down to 100 MeV/c, O( QCD ): essential to minimize out-of-cone fluctuations and unmeasured low p T hadrons. Improves the measurement of particles radiated from soft gluons. Excellent particle ID High p T charged hadrons ID up to 100 GeV/c p T resolution better than 10% until 100 GeV/c 7 ITS TPC TRD PHOS Improve E Tjet range and resolution! Less smearing of the jet spectrum Pb scintillator sampling calorimeter - r M ~ 2cm, 22.1 X 0 - Acc: 80 < < 190, < 0.7 Shashlik geometry 11 SM - ~13000 towers (x = x 0.014) - EMCal year one geometry: 4 SM E /E ~11%/E(GeV) 0 / discrimination to ~ 30 GeV/c Trigger capabilities (essential for p+p and reference collisions study)

10 «Instrumental» jet energy resolution 9 Instrumental response: detector level minus particle simulation: Experimental cuts, undetected neutral particles, tracking efficiency, tracking and calorimeter resolution Particle level: particle distribution produced by PYTHIA event generator without instrumental effects Detector level: generated event filtered through detailed GEANT simulation of the ALICE apparatus Mean energy deficit and instrumental resolution: ALICE ALICE EMCAL EMCAL PPR PPR arxiv: arxiv: [nucl-ex] [nucl-ex] 11-20% 18-22%

11 «Full» resolution: : out-of of-cone fluctuation bias 10 JETAN: Charged + neutral p+p R = 0.4 JETAN: Charged + neutral p+p R = 1 JETAN: Charged p+p R = 0.4 Full simulation of 50, 75 & 100 GeV jets in p+p 14 TeV in EMCal acceptance

12 «Full» resolution: : out-of of-cone fluctuation bias «Charged» jets R= 0.2 «Charged + neutral» jets R= JETAN: Charged + neutral p+p R = 0.4 JETAN: Charged + neutral p+p R = 1 JETAN: Charged p+p R = 0.4 Full simulation of 50, 75 & 100 GeV jets in p+p 14 TeV in EMCal acceptance EMCal first year, R=0.2!!! Mean reconstructed energy strongly biased Expected resolution charged only ~ 50 % Improved to 40 % with EMCal but still strong bias to be corrected. Jet reconstruction with first year EMCal difficult however interesting to estimate charged to neutral fluctuation corrections

13 First year jet rates in p+p collisions at 7 TeV with EMCal 11 PYTHIA fast simulation of p+p collisions at 7 TeV: 1.e6 evts generated per p T bins: 17-20, 20-24, 24-29, 29-35, hard 35-42, 42-50, 50-60, 60-72, 72-86, , , , , and GeV/c Luminosity = 2.e29cm -2 s -1 1 month data taking Comfortable statistics for fragmentation function study with jets reconstructed with charged particles only up to 80 GeV but strong bias on to be corrected) Jet reconstruction with EMCal first year configuration difficult (low statistics and bad mean and resolution) Loss of Xsection and statistics with decreasing acceptance However, a measurement is welcome: Test jet reconstruction code with calorimetry in the analysis train First estimation of charged to neutral fluctuations and correction factor to be applied to jet spectrum

14 Hump-backed plateau (not corrected) «charged» jets 12 PYTHIA full simulation: p+p collisions at 10 TeV Cone finder: UA1 Seed = 2 GeV/c η jet < 0.5 Reconstruction with good statistics of the non corrected hump-backed plateau for different energy bins. Expected behaviour with increasing energy: max peak position increases with jet energy. Caveats : Number of events : 1.1 M Number of jet events : 600 K Number of jets : GeV : 163K GeV : 85K GeV : 57K GeV : 75.6K Charged particle efficiency correction under study. Jet energy not yet corrected. As the initial parton energy is not known, we don t measure yet the fragmentation function. No background subtraction yet (can strongly affect ξmax position).

15 particle distribution inside jets 13 θ = 3D angle between jet axis and particle momentum. PYTHIA full simulation: p+p collisions at 10 TeV p jet p had Cone finder: UA1 Seed = 2 GeV/c η jet < 0.5 More particles close to jet axis with jet energy increasing. Softer emission at larger angles. With jet energy increasing, more particles at small angles over the full p T range? Caveat: no background subtraction and track efficiency correction

16 particle distribution inside jets 13 θ = 3D angle between jet axis and particle momentum. PYTHIA full simulation: p+p collisions at 10 TeV p jet p had Cone finder: UA1 Seed = 2 GeV/c η jet < 0.5 More particles close to jet axis with jet energy increasing. Softer emission at larger angles. With jet energy increasing, more particles at small angles over the full p T range? Caveat: no background subtraction and track efficiency correction <> for high momentum jets below <> of low momentum jets. Support the idea of collimation?

17 p T distribution of particles inside jets 14 PYTHIA full simulation: p+p collisions at 10 TeV The distribution of charged hadron inside jets is jet energy biased => remove the jet energy bin dependency. Leading particles are recovered at all angles : they are near the jet axis More low p T particles recovered at large angles 2 effects: Low p T particle emission at smaller angles More background enters the cone Statistical errors only Background subtraction essential here!

18 Conclusion and perspectives 15 Effects of color coherence give angular ordering in the parton shower. These effects influence the hadron distribution in jets and also jet hadrochemistry. From simulated data, we observe the jet collimation which increases with the jet energy Several distributions of interest to study intrajet radiations which need the knowledge of the corrected jet energy, track efficiency correction and background subtraction. EMCal in its actual configuration too small for accurate jet reconstruction. However, «charged +neutral» jet already of interest for several purposes: Test the reconstruction code in the official analysis train. Estimate charged to neutral fluctuation effect on jet spectrum Correction factors Use these«small»jets as «trigger jets»for correlation studies with charged particles in the opposite side. Jet reconstruction with calorimetry in progress Particle efficiency correction, jet energy correction, needed => under study Background contribution needs to be estimated and subtracted from the presented distributions.

19 Backup

20 Comparison of MLLA predictions with experimental data 1 pp 1.8 TeV CDF e + e - OPAL and TASSO pp 200 GeV STAR FERMILAB-Pub-02/096-E Jets reconstructed with mid-point cone algorithm M. Heinz, Eur. Phys. J. C61:

21 Jet hadrochemistry and modification in medium 2

22 Experimental view of jets 3 Missing Hadronic calorimeter => not covered by ALICE: no K0L, n Electromagnetic calorimeter => covered by ALICE but limited acceptance => EMCal => deal with detector dead zones, calo signal definition, electronic noise Tracker => covered by ALICE at mid-rapidity ( + ITS+TPC+TRD ) => Central barrel => deal with pile-up, UE, detector inefficiency Parton jet Particle jet Calorimeter jet HC EMC p hadrons? e K g q q p

23 Inherent biases to jet energy reconstruction 4 Several jet finders available in the JETAN module in ALICE: UA1, Fast k T, anti-k T, SISCone, DA, CDF Cone, Cone (energy flux) and k T types (parton shower) Jet energy Cone energy from MC R=1 R=0.4 No cuts Cuts Incomplete jet reconstruction leads to Signal / out-of-cone fluctuations: - Limited cone size, cuts, - Charged jets: dominated by charged-to-neutral fluctuations - Charged + neutral jets: K O L and neutrons missed - Detector effects (limited acceptance, dead zones, ) The background of heavy ion collisions and its fluctuations biases the jet reconstruction log(dn/de) Background or Signal fluctuates down Cone energy from full simulation and reconstruction Jet input spectrum Bias in the reconstructed jet energy smearing of the jet spectrum Background or Signal fluctuates up See C. Klein Bösing s talk Bias towards higher Bg log(e/gev)

24 Full resolution in Pb+Pb collisions with EMCal 5 Minbias ~ p+p Central p+p: cuts on p T and radius biases the reconstructed energy which is under-estimated = out-of-cone fluctuations dominate From Pb+Pb Minbias to Central over-estimation of the reconstructed energy = background fluctuations dominate

25 Full resolution in Pb+Pb collisions with EMCal 5 Caveat: Caveat: Jet Jet reconstruction reconstruction in in full full EMCal EMCal acceptance acceptance => => Not Not optimal optimal resolution resolution!! JETAN R = 0.4 p Tcut = 1 GeV/c Minbias ~ p+p Central Energy p+p: cuts on p T and radius biases the reconstructed energy which is under-estimated = out-of-cone fluctuations dominate From Pb+Pb Minbias to Central over-estimation of the reconstructed energy = background fluctuations dominate Direction

26 j T, a combination of momentum and angle 6 PYTHIA full simulation: p+p collisions at 10 TeV p j j T c High j T reached for both bins in jet energy: GeV: dominated by large θ emissions? p ch j T = sin. p ch GeV: dominated by large p ch hadron emissions? More hadrons in high energy jets: more splitting. Caveat: no background subtraction and track efficiency correction Statistical errors only