ALICE Physics Week - Erice, Andrea Dainese 1 Andrea Dainese Padova – Università e INFN D 0 meson reconstruction with ALICE 1 st ALICE Physics Week (PPR Volume 2, 6.6.4)

ALICE Physics Week - Erice, Andrea Dainese 2 Contents ALICE baseline for heavy-quark production Experimental details Selection strategy for D 0  K  Track impact parameter measurement Feed-down from B decays D 0 cross section measurement in pp, pPb, Pb-Pb (central and peripheral) Charm measurement in initial pp run? Perspective for v 2 measurement

ALICE Physics Week - Erice, Andrea Dainese 3 ALICE baseline: N.Carrer and A.D., ALICE-INT , hep-ph/ ; included in PPR2 (ALICE) Baseline for cc / bb production pQCD calculation NLO (e.g. MNR program) ALICE baseline: cross sections from NLO pQCD with “best guess” set of parameters kinematics from PYTHIA tuned to reproduce NLO p t distributions Uncertanties: large uncertainty (more than factor 2) due to choice of c and b masses and of scales (  R,  F ) however, robust  s scaling: 14 TeV (pp)  5.5 TeV (Pb-Pb) more details later … MNR: M.L.Mangano, P.Nason and G.Ridolfi, Nucl. Phys. B373 (1992) 295. system :  s : Pb-Pb (0-5% centr.) 5.5 TeV pPb (min. bias) 8.8 TeV pp 14 TeV 4.3 / / / / / / 0.007

ALICE Physics Week - Erice, Andrea Dainese 4 D 0  K -  + in ALICE Exclusive reconstruction direct measurement of the p t distribution Weak decay with mean proper length c  = 124  m Large combinatorial background  (dN ch /dy) 2 STRATEGY: invariant-mass analysis of fully-reconstructed topologies originating from (displaced) secondary vertices Measurement of Impact Parameters (ITS+TPC) Measurement of Momenta (ITS+TPC) Particle identification (TOF) to tag the two decay products

ALICE Physics Week - Erice, Andrea Dainese 5 Track impact parameter D 0 daughters mean d 0 : PIXEL CELL z: 425  m r  : 50  m Two layers: r = 4 cm r = 7 cm  9.8 M d 0 measurement in ALICE mainly provided by the two SPD layers Systematic study of track impact parameter resolution: A.D. and R.Turrisi, ALICE-INT

ALICE Physics Week - Erice, Andrea Dainese 6 Interaction vertex reconstruction in pp collisions Vertex diamond stable for few hours; if beam focused, precise vertex position from average over many events “Nominal” size of the beam:  = 15  m in Pb-Pb  = 15  m in pp ( L ~ cm -2 s -1 )   150  m in pp (if L is reduced at ALICE IP to ~ cm -2 s -1 ) In pp the E-by-E vertex position has to be reconstructed in 3D using tracks Finding/fitting algorithm developed and optimized A.D. and M.Masera, ALICE-INT as in Pb-Pb pp with charm pp min. bias

ALICE Physics Week - Erice, Andrea Dainese 7 D 0  K -  + : Selection of D 0 candidates increase S/B by factor ~10 3 !

ALICE Physics Week - Erice, Andrea Dainese 8 Time-of-flight PID TOF Pb-Pb, dN ch /dy=6000 Optimization for hadronic charm decays was studied: minimize probability to tag K as  K id. required for D 0 p t < 2 GeV/c

ALICE Physics Week - Erice, Andrea Dainese 9 S/B initial (M  3  ) S/B final (M  1  ) Significance S/  S+B (M  1  ) Pb-Pb central 5  % ~35 (for 10 7 evts, ~1 month) pPb min. bias 2  % ~30 (for 10 8 evts, ~1 month) pp 2  % ~40 (for 10 9 evts, ~7 months) D 0  K -  + : Results Note: with dN ch /dy = 3000 in Pb-Pb, S/B larger by  4 and significance larger by  2 central Pb-Pb

ALICE Physics Week - Erice, Andrea Dainese 10 Feed-down from Beauty At LHC energies Fraction of D 0 from chain b  B 0 /B +  D 0 not negligible “Standard” NLO pQCD + Pythia fragmentation + PDG: Selections increase this ratio to ~10% Results have to be eventually corrected for feed-down estimate systematic error from uncertainty on b cross section

ALICE Physics Week - Erice, Andrea Dainese 11 Estimate of the errors Statistical error on the selected signal = 1/Significance Main systematic errors considered: correction for feed-down from beauty (B.R. B  D 0 is 65%!): error of ~8% assuming present uncertainty (~80%) LHC Monte Carlo corrections: ~10% B.R. D 0  K  : 2.4% extrapolation from N(D 0 )/event to d  (D 0 )/dy:  pp: error on (~5%, will be measured by TOTEM)  Pb-Pb: error on centrality selection (~8%) + error on T AB (~5%)

ALICE Physics Week - Erice, Andrea Dainese 12 Statistical Errors (R.Grosso)

ALICE Physics Week - Erice, Andrea Dainese 13 D 0 cross section measurement inner bars: stat. errors outer bars: stat.  p t -dep. syst. errors normalization error not shown: 9% Pb-Pb and p-Pb, 5% pp

ALICE Physics Week - Erice, Andrea Dainese 14 Extrapolated performance for semi-peripheral Pb-Pb (30-60%) class14 b (fm) %  Pb-Pb 0-3.6% % Nevents (10 7 min.bias) 3.6x x10 6 (fm) HIJING N cc /ev N bb /ev Larger p t bins than for 0-5% have to be used

ALICE Physics Week - Erice, Andrea Dainese 15 Charm measurement with low statistics / low lumi pp run? Simple scaling of statistical errors according to  N evts No dedicated cuts study 30% 2-10 GeV/c coverage with 5  10 7 evts low stats high lumi (>>10 30 ), bad for d 0 30% 2-7 GeV/c coverage with 2  10 7 evts low stats low lumi (~10 30 ), good for d 0

ALICE Physics Week - Erice, Andrea Dainese 16 First look at D 0 v 2 For p t < 5 GeV/c, D 0 candidates are signal + background v 2 by performing inv. mass analysis in  bins dedicated study needed For p t > 5 GeV/c, background is negligible get v 2 from all D 0 candidates (only signal) First estimate, for p t > 5 GeV/c: assuming v 2 ~  v2 (statistical) Signalp T bin (GeV/c) 33-58% 2.5x10 6 evts 0.123x x x x745-7  v2 (statistical) Signalp T bin (GeV/c) 20-60% 4x10 6 evts x x x x745-7  v2 (statistical) Signalp T bin (GeV/c) 20-60% 4x10 7 evts see Francesco’s talk

ALICE Physics Week - Erice, Andrea Dainese 17 Status of code in AliRoot $ALICE_ROOT/ANALYSIS/AliD0toKpiAnalysis.cxx Core loop: opposite-sign track pairs (only tracks TPC+ITS, with 6 ITS points) secondary vertex finding, using standard V0 vertexer How to deal with the large combinatorial background: preselection based on single-track cuts (p t > 0.5 GeV and |d 0 | > 50  m) preselection on pairs, before secondary vertex finding, based on product of impact parameters preselection on final variables, before writing candidate to file Class being now tested and optimised in Padova (Julien) Still missing: background subtraction (event mixing?) code for invariant mass analysis

ALICE Physics Week - Erice, Andrea Dainese 18 Summary Feasibility study for D 0 reconstruction in pp, Pb-Pb and pPb carried out strong simulation requirements: fast simulation employed many critical items addressed: PID, primary vertex in pp, feed-down from beauty … Results are very promising p t coverage in the range 1—18 GeV/c some physics “applications” identified and studied (see quenching talk later on…) 1 st charm cross section measurement in pp at LHC some 10 7 events sufficient to cover up to p t ~ 7 GeV/c Analysis code skeleton prepared

ALICE Physics Week - Erice, Andrea Dainese 19 EXTRA SLIDES

ALICE Physics Week - Erice, Andrea Dainese 20 Nuclear Shadowing and Charm Pb nucleus ~ 10 5 partons (mainly gluons) ‘they are so close that they fuse’ Shadowing: suppression of small-x PDFs Reduces c&b yield at low p t Look at low-p t D in p-Pb! xaxa xbxb xa+xbxa+xb f g Pb (x)/f g p (x) For charm at p t  0:  x 1 ~x 2 ~ at y = 0  min(x 1,x 2 ) ~ at y =  3 a b c  see talk by R. Grosso Bjorken

ALICE Physics Week - Erice, Andrea Dainese 21 RHIC Au-Au 200 GeV Estimates of R AA for D and B Ingredients: BDMPS quenching weights for heavy quarks + Glauber-based medium geometry + LHC medium density extrapolated on the basis of hadron suppression at RHIC RHIC analysis: A.D., C.Loizides and G.Paic, Eur. Phys. J. C to appear, hep-ph/ N.Armesto, A.D., C.A.Salgado and U.A.Wiedemann, in preparation. LHC

ALICE Physics Week - Erice, Andrea Dainese 22 3D reconstruction with tracks Track reconstruction in TPC+ITS track seeding uses the position of the primary vertex:  (x, y) from beam position (resolution ~ 150  m)  z from pixels information (resolution ~ 150  m) Vertex reconstruction in 2 steps: VERTEX FINDING: using DCA for track pairs VERTEX FITTING:  give optimal estimate of the position of the vertex  give vertex covariance matrix  give a  

ALICE Physics Week - Erice, Andrea Dainese 23 Expected resolutions Average # rec. tracks = 7 (average on events with # > 1) Average p T of rec. tracks = 0.6 GeV/c Resolutions of track position 0.6 GeV/c:  (d0(r  ))  100  m [ d0(r  ) is  to the track! ]  (d0(z))  240  m

ALICE Physics Week - Erice, Andrea Dainese 24 Vertex Finding Algorithm Aim: get a first estimate of the vertex position in (x,y) to be used as a starting point for vertex fitter independent of beam size improved w.r.t. beam size (hopefully) Method: 1.propagate tracks to vertex nominal position 2.calculate DCA (in space) for each possible pair of tracks (using straight line approximation) 3.get estimate of x vtx and y vtx from mean of results from all pairs

ALICE Physics Week - Erice, Andrea Dainese 25 Vertex Fitting Algorithm Tracks are propagated to the point given by the vertex finder (at the moment nominal position used) A  2 is written as the sum of the single track  2 s w.r.t. a generic vertex position r vtx : where W i is track “covariance matrix in global ref. frame” The solution that minimizes this  2 is analytic: vertex covariance matrix

ALICE Physics Week - Erice, Andrea Dainese 26 Tuning of the algorithm Criterion used to reject mismeasured and secondary tracks from the fit: cut on the maximum contribution to the  2  i 2 <   max if   max is too low too many tracks are rejected and we loose resolution if   max is too high bad or secondary tracks enter the fit and we loose resolution This cut is tuned, as a function of event multiplicity, in order to optimize the resolution

ALICE Physics Week - Erice, Andrea Dainese 27 Results: resolutions  (x) = 55  m  (y) = 55  m  (z) = 90  m

ALICE Physics Week - Erice, Andrea Dainese 28 Results: pulls The covariance matrix of the vertex describes correctly the resolutions

ALICE Physics Week - Erice, Andrea Dainese 29 Resolutions VS multiplicity

ALICE Physics Week - Erice, Andrea Dainese 30 TPC tracking parameterization: N.Carrer and A.Dainese, ALICE-INT Fast Simulation Techniques The selection strategy of D 0 mesons has to be optimized using events for Pb-Pb and for pp 1 Fully simulated Pb-Pb event takes 1.2 GB and ~12 h! A number of fast simulation techniques were used in order to achieve these statistics fast response of the ITS (“fast points”, reduce ITS-time by factor 25) TPC tracking parameterization, specifically developed for charm and beauty studies (reduces total time by factor 40) Checks were done that these approximations don’t affect track resolutions (efficiencies were corrected)

ALICE Physics Week - Erice, Andrea Dainese 31 What if multiplicity in Pb-Pb is lower? We used dN ch /dy = 6000, which is a pessimistic estimate Recent analyses of RHIC results seem to suggest as a more realistic value dN ch /dy = 3000 (or less) Charm production cross section: estimate from NLO pQCD (only primary production, no collective effects) average of theoretical uncertainties (choice of: m c,  F,  R, PDF) BKG proportional to (dN ch /dy) 2 We can scale the results to the case of dN ch /dy = 3000: S/B = 44 % SGNC = 74 (this only from scaling, obviously better with retuning of cuts)

ALICE Physics Week - Erice, Andrea Dainese 32 Feed-down from Beauty (2) Selection on pointing angle expected to suppress D 0 from b (they point to the decay vertex of the B and not to the primary vertex) Resolution on pointing angle is worse for D from c pointing angle enhances D from b D D B V1V1 V1V1 K K   2<p t <3 GeV/c

ALICE Physics Week - Erice, Andrea Dainese 33 Feed-down from Beauty (3) Also selection based on impact parameter enhances beauty 20% ! D 0 from b / D 0 from c = 10 % Systematic error from b = uncertainty on b cross section  fraction of D 0 from b

ALICE Physics Week - Erice, Andrea Dainese 34 Study shadowing in p-Pb at 8.8 TeV? p-Pbpp R.Grosso, PhD thesis (2004)

ALICE Physics Week - Erice, Andrea Dainese 35 Charm and low-x gluon dynamics pp

ALICE Physics Week - Erice, Andrea Dainese 36 Charm and low-x gluon dynamics Global fits of HERA data improved by adding nonlinear corrections to standard DGLAP partonic-evolution equations Arise from gluon recombination at small x (~10 -3 ) and, at LO, imply larger gluon density (w.r.t. to DGLAP) at small x and Q 2 Charm production in pp at LHC might be enhanced w.r.t. what expected using only DGLAP terms K.J.Eskola et al., Nucl. Phys. B660 (2003) 211 [hep-ph/ ]. K.J.Eskola, V.Kolhinen and R.Vogt, Phys. Lett. B582 (2004) 157 [hep-ph/ ]. For charm at p t  0:  x 1 ~x 2 ~ at y = 0  min(x 1,x 2 ) ~ at y =  3 pp

ALICE Physics Week - Erice, Andrea Dainese 37 Gluons: DGLAP nonlinear x = Q = 2 m c = 2.4 GeV nonlinear (quadratic) correction has “–” sign Q 2 evolution is slower low Q 2 and x has to be larger than in DGLAP

ALICE Physics Week - Erice, Andrea Dainese 38 Charm: nonlinear/DGLAP vs p t Strongly dependent on choice of mass and scale* (Q 2 ) Varies from  1.5 to  5 “Enhancement” limited to p t < 2 GeV/c ALICE could address it with D 0 at low p t Caveat: effect currently calculated only at LO *  R =  F = Q pp

ALICE Physics Week - Erice, Andrea Dainese 39 How to detect the enhancement due to nonlinear effects? The idea is that the effect (enh. only at very low p t ) cannot be mimicked by NLO pQCD In practice: consider ratio “Data/Theory” for all reasonable choices of theory parameters A.D. et al., hep-ph/ Data: m c = 1.2 GeV, Q 2 = 4m T 2 and enhancement Data: m c = 1.3 GeV, Q 2 = m T 2 and enhancement pp

ALICE Physics Week - Erice, Andrea Dainese 40 TPC tracking parameterization: N.Carrer and A.Dainese, ALICE-INT Fast Simulation Techniques The selection strategy of D 0 mesons has to be optimized using 10 4 events for Pb-Pb and 10 7 for pp 1 Fully simulated Pb-Pb event takes 1.2 GB and ~12 h! A number of fast simulation techniques were used in order to achieve these statistics fast response of the ITS detectors (position resolution) TPC tracking parameterization, specifically developed for charm and beauty studies (reduces total time by factor 40) Parameterization of PID in TOF detector Checks were done that these approximations don’t affect detector performance (p t & d 0 resol., efficiencies)