1 / 24

Jet Fragmentation in STAR going from pp to Au+Au

Jet Fragmentation in STAR going from pp to Au+Au. Elena Bruna, for the STAR Collaboration Yale University. Winter Workshop on Nuclear Dynamics, Big Sky Feb. 1-8 2009. Going from pp…. Topological Jets in pp at RHIC energies p T spectrum up to 50 GeV What we know:

ashby
Download Presentation

Jet Fragmentation in STAR going from pp to Au+Au

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Jet Fragmentation in STAR going from pp to Au+Au Elena Bruna, for the STAR Collaboration Yale University Winter Workshop on Nuclear Dynamics, Big Sky Feb. 1-8 2009

  2. Going from pp… • Topological Jets in pp at RHIC energies • pT spectrum up to 50 GeV • What we know: • Good agreement of jet spectrum with NLO/PYTHIA at RHIC energies • Jet spectra and fragmentation functions measured at Tevatron  good agreement with theory • Perspectives in STAR: • Measure jet Fragmentation Functions at RHIC energies • Compare conceptually different jet algorithms (kT, anti-kT, cone) • Energy resolution using PYTHIA simulation • Understand the Trigger Bias • Goal: • compare Fragmentation functions in pp and Au+Au Elena Bruna

  3. ...to Au+Au • Topological jets measured in Au+Au in STAR at RHIC energies! • What we want to measure: • Jet pT spectrum • Jet Fragmentation Functions • What we expect with an unbiased jet population: • pT spectrum Nbin scaling (jet production is a hard process) • Fragmentation functions  modification Elena Bruna

  4. Jets in high-energy collisions • High-pT partons produced in hard scatterings hadrons • Full jet (spray of collimated hadrons) after parton fragmentation gives access to: • partonic kinematics • jet cross sections • TOOL: Jet-Finding algorithms c, xc a, xa b, xb σab d, xd hadrons Elena Bruna

  5. seed Jet Reconstruction • Seed Cone: • ‘seed’ (E>Ethreshold) • iterative approach • Seedless Cone (SIS cone): • all the particles used as seeds • Splitting/Merging applied fragmentation tracks or towers outgoing parton Cone Algorithms Rcone seed R=√(Δφ2+Δη2) [Cacciari, Soyez, arXiv:0704.0292] • Seedless, not bound to a circular structure • kT: starts from merging low pT particles close in the phase-space • Anti-kT: starts from merging high pT particles close in the phase-space Recombination Algorithms Elena Bruna [Cacciari, Salam, Soyez, arXiv:0802.1189]

  6. Experimental setup for pp and Au+Au • charged particle pT (TPC) • neutral tower Et 0.05x0.05 (ηxϕ) (EMC) • corrected for hadronic energy. • Electron correction for double counting • EMC provides fast trigger. • Two trigger setups with the EMC: • Jet Patch Trigger (JP): • 1x1(ηxϕ), Et>8 GeV • High Tower Trigger (HT): • tower 0.05x0.05 (ηxϕ) Et> 5.4 GeV (pp) • cluster 0.1x0.1 (ηxϕ) Et> 7.5 GeV (AuAu) h=-1 h=0 h=+1 f : 120 towers in Df=0-2p EMC h : 40 towers Elena Bruna

  7. Going from p+p … • Analyzed STAR data-sets: • p+p (2006) High-Tower (HT) trigger (single tower Et>5.4 GeV) • p+p (2006) Jet-Patch (JP) trigger (ηxϕ=1x1 with sum Et>8 GeV) Elena Bruna

  8. Fragmentation functions for charged hadrons • Definition: x=ln (Ejet/phadr) • We use: x=ln (pT,jet/pT,hadr) : no assumptions on the particle mass 30< pT,jet<40 GeV STAR Preliminary • Fragmentation function in qualitative agreement for all Jet-Finders • Statistical errors only • Systematic studies ongoing p+p √s=200 GeV JP trigger Elena Bruna

  9. Jet-Patch vs High Tower triggers 10< pT,jet<15 GeV 20< pT,jet<30 GeV STAR Preliminary STAR preliminary STAR Preliminary STAR preliminary p+p √s=200 GeV p+p √s=200 GeV kT R=0.7 Uncorrected 30< pT,jet<40 GeV pT,jet>40 GeV STAR Preliminary STAR preliminary STAR Preliminary p+p √s=200 GeV p+p √s=200 GeV STAR preliminary • Low pTjet: Multiplicity of charged particles influenced by Neutral Energy Fraction and high z fragmenting jets  stronger trigger BIAS in the HT sample • High pTjet: JP and HT do not show difference due to a smaller bias of different trigger selections  JP and HT fragmentation functions similar for higher jet energy

  10. x for different jet energies (1) 20< pT,jet<30 GeV 10< pT,jet<15 GeV p+p √s=200 GeV JP trigger p+p √s=200 GeV JP trigger JP trigger R=0.4 pT,jet>40 GeV 30< pT,jet<40 GeV p+p √s=200 GeV JP trigger p+p √s=200 GeV JP trigger • Uncorrected spectra • Different Jet Finders show similar performance for a given R Elena Bruna

  11. x for different jet energies (2) 20< pT,jet<30 GeV 10< pT,jet<15 GeV p+p √s=200 GeV JP trigger p+p √s=200 GeV JP trigger pT>=1 GeV/c pT>=1 GeV/c JP trigger R=0.7 pT,jet>40 GeV 30< pT,jet<40 GeV p+p √s=200 GeV JP trigger p+p √s=200 GeV JP trigger pT>=1 GeV/c pT>=1 GeV/c • Conceptually different Jet Finders  Similar performance for different cone radii and for different jet pT  suggests no significant NLO effects at RHIC energies

  12. p+p Fragmentation function vs. PYTHIA JP trigger R=0.7 |ηjet|<0.3 Increasing Jet Energy 40<Ereco<50 20<Ereco<30 GeV 30<Ereco<40 GeV R<0.4 R<0.7 Increasing Cone R Good agreement with PYTHIA especially at low R Elena Bruna

  13. pp: reference for Au+Au • Jets in pp are a safe baseline for Au+Au • Good agreement with NLO/PYTHIA • Free choice of jet algorithms in pp • Systematic studies ongoing (e- identification, hadronic shower, trigger bias) … to Au+Au • Analyzed STAR data-sets: • Au+Au (2007) High-Tower (HT) trigger (cluster ET>7.5 GeV) • Au+Au (2007) Minimum-Bias (MB) trigger Elena Bruna

  14. CDF preliminary ~ 21 GeV STAR preliminary pt per grid cell [GeV] η ϕ Jet Finding in Heavy-Ion collisions • GOAL: Fully reconstruct the jet in high-multiplicity environment • How to suppress background: • Reduce the jet area • Apply a pTcut on tracks and towers Jet energy fraction outside cone R=0.3 Reconstructed Jet • Background estimation: • Mean energy in out-of-cone areas Out-of-cone area Elena Bruna

  15. Au+Au 0-20% Rc=0.4, no pt cut, out-of-cone area <pt,Bkg> [Gev] Reference multiplicity (~centrality) Background in Au+Au 0-20% • Event-by-event basis: • <pT,Bkg>=mean pT in out-of-cone area • Bkg for Fragmentation Functions = mean FF in out-of-cone area for a given pTjet • Background energy in R=0.4 ~ 45 GeV (no pT cut) • Substantial region-to-region background fluctuations ⇒ significantly reduced by applying a pT cut STAR Preliminary STAR Preliminary Background fluctuations [Gev] Elena Bruna Rc

  16. Simulation • 1) Jet Finder on PYTHIA events • 2) a. PYTHIA event embedded in Au+Au real event b. Jet Finder on PYTHIA + AuAu • 3) Compare (1) and (2) • detector effects neglected Elena Bruna

  17. STAR preliminary Consequences of background fluctuation for FF measurements Simulation STAR Preliminary • To determine the jet energy on an event-by-event basis a ptcut> 2 GeVis necessary to reduce the effect of background fluctuations in Au+Au collisions; • Pythia+Au+Au and Pythia select similar jet population.

  18. Simulation: ξ distribution for 30 GeV (pt,cut>2 GeV) Charged particle FF: Rc(FF)=0.7 and pt>0 GeV 30 GeV mono-jet embedded in 0-20% central Au+Au STAR event Jet energy determination: Rc=0.4 and pt>1(2)GeV STAR preliminary • ξ background subtraction method and jet energy resolution in Au+Au 0-20% causes deviations < 10-20% for ξ<2-2.5 using Pythia fragmentation • Systematic deviations in the ξ shape ratio at low ξ are caused by jet-energy resolution • Only statistical errors STAR Preliminary STAR preliminary

  19. Data: ξ distribution for jet energies > 30 GeV in Au+Au Au+Au HT Et>7.5 GeV STAR preliminary stat. errors only LOCone FastJet kt pthadron~10 GeV Cone and kt algorithm give similar fragmentation-function measurements for ptcut > 2 GeV and reconstructed jet pt above 30 GeV in Au+Au 0-10% Elena Bruna

  20. Fragmentation Function in Au+Au 0-20% and p+p for 30 GeV jets  AuAu jet energies should correspond to 30 GeV pp jets STAR Preliminary • 1) pt,jetrec.(pp) > 30 GeV • 2) pt,jetrec.(Au+Au)>31 GeV for ptcut>2 GeV • 3) pt,jetrec.(Au+Au)>35 GeV for ptcut>1 GeV dominated by uncertainties due to background subtraction for pthadron<2 GeV STAR Preliminary Statistical errors only No apparent modification in the fragmentation function with respect to p+p Elena Bruna

  21. Au+Au results: • Simulation  jet Finding works in high multiplicity environment!  background reasonably under control • Data: • SPECTRA(MinBias data): Binary scaling observed with no pT cut (S. Salur talk) • PYTHIA fragmentation assumed • FRAGMENTATION FUNCTIONS (HT trigger data): -- no apparent modification w.r.t. pp • Corrections for energy resolution (bkg fluctuations) • Corrections based on PYTHIA fragmentation -- we would expect a modification (high-pT hadron suppression) • Measurements consistent: why?

  22. What’s happening! • Effect A:Biased sample of jets due to the High-Tower Trigger: • the HT trigger favors “surface” jets that are not modified by the medium • Ejet (AuAu) = Ejet (pp)  FF unmodified • If this is true  HT jets should not binary scale even without pTcut! • Effect B: Biased sample of jets due to energy loss and pTcut • The jet softens in the medium • Its energy is not recovered with pTcutAND assuming PYTHIA fragmentation • Its energy is UNDERESTIMATED  ξ=ln(ptjet/pt) should be larger • If this is true  Quenching models could address this issue Au+Au p+p dN/dξ ξ Au+Au p+p dN/dξ ξ

  23. Conclusions • Full jet reconstruction feasible at RHIC • Jet Fragmentation Functions: • Measured and under control in pp • Powerful tool to study medium effects in Au+Au • We do not see modification of the Fragmentation Functions • Surface effect on HT events or Softening of jets in the medium or both? Elena Bruna

  24. What’s next • How to address this issue: • High-Tower jet-pT spectrum • di-jets in Min Bias and HT events • Quenching models will help • Systematic studies (detector effect,…) to have more control on our understanding of jet finder in Au+Au • Jets are a precious tool to explore the medium: exciting physics is coming! Thank you ! Elena Bruna

More Related