Andrea Dainese

1
D0-meson reconstruction in pp and Pb-Pb
collisions with ALICE
I Convegno Italiano sulla Fisica di ALICE

• Andrea Dainese
• Padova Università e INFN

2
Contents

• Heavy-quark production at the LHC
• Exclusive reconstruction of D0 ? Kp decays in pp
  and Pb-Pb collisions in ALICE
• track impact parameter measurement
• selection and results
• feed-down from B decays
• Physics potential of the measurement

A.D. PhD Thesis 2003, nucl-ex/0311004
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(ALICE) Baseline for cc / bb production

• pQCD calculation available _at_ 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 pt
  distributions
• Uncertanties
• large uncertainty (more than factor 2) due to
  choice of c and b masses and of scales (mR, mF)
• however, robust ?s scaling 14 TeV (pp)

system ?s Pb-Pb (0-5 centr.) 5.5 TeV p-Pb (min. bias) 8.8 TeV pp 14 TeV
4.3 / 0.2 7.2 / 0.3 11.2 / 0.5
115 / 4.6 0.8 / 0.03 0.16 / 0.007
MNR M.L.Mangano, P.Nason and G.Ridolfi, Nucl.
Phys. B373 (1992) 295.
ALICE baseline N.Carrer and A.D.,
ALICE-INT-2003-019, hep-ph/0311225.
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D0? K-p in ALICE

• Exclusive channel direct measurement of
  the pt distribution ideal tool to
  study RAA
• Weak decay with mean proper length ct 124 mm
• Large combinatorial background (dNch/dy6000 in
  central Pb-Pb!)
• STRATEGY invariant-mass analysis of
  fully-reconstructed topologies originating from
  (displaced) secondary vertices
• Measurement of Impact Parameters (ITSTPC)
• Measurement of Momenta (ITSTPC)
• Particle identification (TOF) to tag the two
  decay products

5
Impact parameter resolution

• Mainly provided by the 2 layers of silicon
  pixels in the ITS

? 9.8 M
Systematic study of track impact parameter
resolution A.D. and R.Turrisi,
ALICE-INT-2003-028.
6
Interaction vertex reconstruction in pp collisions

• Position of the beam in (x,y) given by the
  machine with very high precision (stable for a
  long time)
• Nominal size of the beam
• s 15 mm in Pb-Pb
• s 15 mm in pp (L 1031 cm-2 s-1)
• s ? 150 mm in pp (if L is reduced at ALICE IP to
  1029 cm-2 s-1)
• In pp the vertex position has to be reconstructed
  in 3D using tracks
• Algorithm developed and optimized with M. Masera

A.D. and M.Masera, ALICE-INT-2003-027.
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D0? K-p Selection of D0 candidates
increase S/B by factor 103!
8
Fast Simulation Techniques

• The selection strategy of D0 mesons has to be
  optimized using 104 events for Pb-Pb and 107 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 dont
  affect detector performance (pt d0 resol.,
  efficiencies)

TPC tracking parameterization N.Carrer and
A.Dainese, ALICE-INT-2003-011.
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D0? K-p Results
S/B initial (M?3s) S/evt final (M?1s) S/B final (M?1s) Significance S/?SB (M?1s)
Pb-Pb 5 ? 10-6 1.3 ? 10-3 11 37 (for 107 evts, 1 month)
pp 2 ? 10-3 1.9 ? 10-5 11 44 (for 109 evts, 7 months)
Note with dNch/dy 3000, S/B larger by ? 4 and
significance larger by ? 2
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Feed-down from Beauty

• At LHC energies
• Fraction of D0 from chain b ? B0/B ? D0 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

0.64
0.0017
0.0196
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D0? K-p d2s(D0)/dptdy and ds(D0)/dy
ds(D0)/dy for y lt 1 and pt gt 1 GeV/c (65
of s(pt gt 0)) statistical error 7
systematic error 19
from b 9 MC
correction 10 B.R.
2.4 from AA to NN 13
12
Sensitivity to NLO pQCD params
pp
?s 14 TeV
13
Charm Quenching
A.D. Eur.Phys.J. C33 (2004) 495 nucl-ex/0312005
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Summary Outlook

• Feasibility study for D0 reconstruction in Pb-Pb
  and pp (p-Pb ? R.Grosso) carried out
• strong simulation requirements fast simulation
  employed
• many (most) critical items addressed PID,
  primary vertex in pp, feed-down from beauty
• results are very promising
• several physics applications identified and
  studied
• analysis code skeleton prepared (in
  ALICE_ROOT/ANALYSIS/)
• Outlook
• test software on Data Challenge production
• finalize analysis procedure
• define minimal/optimal running scenario (
  events, centralities, trigger)
• how many pp events (vs experimental conditions)
  are needed for 1st charm cross section
  measurement at LHC?

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EXTRA SLIDES
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Novelties at LHC (1) Hard Probes

• Hard partons and their energy loss probe the
  medium
• LHC large hard cross sections!
• Our set of tools (probes) becomes richer

quantitatively

• qualitatively
• heavy quarks
• g jet correlations
• Z0 jet correlations

LO pQCD by I. Vitev, hep-ph/0212109
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Novelties at LHC (2) Small x

• Unexplored low-x region will be probed in pp, pA
  and AA collisions
• down to x 10-4 already at central y

y 0 _at_ LHC

• Window on the rich phenomenology of high-density
  PDFs
• shadowing / saturation effects / CGC (see talk by
  F. Gelis)
• recombination effects in gluon evol. even in pp
  coll. (more later)

18
Heavy ions at ?sNN 5.5 TeV

• LHC factor 30 jump in ?sNN w.r.t. RHIC
• hotter, bigger, longer-living drops of
  QGP
• very large initial temperature
• small
  baryonic content (large ybeam)

SPS 17 GeV RHIC 200 GeV LHC 5.5 TeV
initial T 200 MeV 300 MeV gt 600 MeV
volume 103 fm3 104 fm3 105 fm3
life-time lt 2 fm/c 2-4 fm/c gt 10 fm/c
beam rapidity 2.9 5.3 8.6

• ? closer to ideal QGP
• easier comp. with theory
• (lattice)

Deep de-confinement
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Heavy-quark production
pp collisions high-virtuality partonic
scattering
pQCD collinear factorization
(pA) AA binary scaling for hard yields
broken by initial-state effects (PDF shadowing,
kt broadening ) and final-state effects
(energy loss, in-medium hadronization )
? V. Greco
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Heavy-quarks MC simulation

• NLO programs are not event generators
• Use PYTHIA LO some NLO topologies in parton
  shower

Pair Creation (PC)
Flavour Excitation (FE)
Gluon Splitting (GS)
Recently, MC _at_ NL0 (Frixione-Weber) combined
full NLO M.E. with HERWIG parton shower
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Nuclear Shadowing and Charm

• For charm at pt ? 0
• x1x2 10-4 at y 0
• min(x1,x2) 10-5 at y ?3

Bjorken

• Pb nucleus 105 partons (mainly gluons)
• they are so close that they fuse
• Shadowing suppression of small-x PDFs
• Reduces cb yield at low pt
• Look at low-pt D in p-Pb!

22
Final-state effects in nucleus-nucleus Parton
Energy Loss

• Partons travel 4 fm in the high colour-density
  medium
• Energy loss by gluon radiation

(BDMPS)
Casimir coupling factor 4/3 for quarks 3 for
gluons
Medium transport coefficient ? gluon density and
momenta
Probe the medium
R.Baier, Yu.L.Dokshitzer, A.H.Mueller, S.Peigne'
and D.Schiff, (BDMPS), Nucl. Phys. B483 (1997)
291.
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Lower E loss for heavy quarks?

• In vacuum, gluon radiation suppressed at q lt
  mQ/EQ
  
• ? dead cone effect
• Dead cone implies lower energy loss
  (Dokshitzer-Kharzeev, 2001)
• Detailed calculation confirms this qualitative
  feature, although effect is small and
  uncertainties significant (Armesto-Salgado-Wiedema
  nn, 2003)
• Exploit abundant massive probes at LHC
  study the effect by measuring the nuclear
  modification factor for D and B

Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett.
B519 (2001) 199 hep-ph/0106202. N.Armesto,
C.A.Salgado and U.A.Wiedemann, Phys. Rev. D69
(2004) 114003 hep-ph/0312106.
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Estimates of RAA 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/0406201. N.Armesto,
A.D., C.A.Salgado and U.A.Wiedemann, in
preparation.
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Background multiplicity in Pb-Pb

• What is the background to hadronic D-meson
  decays?
• combinatorial background given by pairs of
  uncorrelated tracks with large impact parameter

in central Pb-Pb at LHC
Simulations performed using
huge combinatorial background!
need excellent detector response and good
selection strategy
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D0? K-p Signal and background

• Signal
• charm cross section from NLO pQCD (MNR program),
  average of results given by MRS98 and CTEQ5M PDFs
  (with EKS98 in Pb-Pb)
• signal generated using PYTHIA, tuned to reproduce
  pt distr. given by NLO pQCD
• contribution from b?B?D0 (5) also included
• Background
• Pb-Pb HIJING (dNch/dy6000 ! we expect 2500 !)
  pp PYTHIA

system shadowing
pp 14 TeV 11.2 1 0.16 0.0007
Pb-Pb 5.5 TeV (5 cent) 6.6 0.65 115 0.5
MNR Program M.L.Mangano, P.Nason and G.Ridolfi,
Nucl. Phys. B373 (1992) 295.
27
The ALICE Detector
h lt 0.9 TPC silicon tracker g, e, p, K, p
identification
2.5 lt h lt 4 muons
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3D reconstruction with tracks

• Track reconstruction in TPCITS
• track seeding uses the position of the primary
  vertex
• (x, y) from beam position (resolution 150 mm)
• z from pixels information (resolution 150 mm)
• 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 c2

29
Expected resolutions

• Average rec. tracks 7 (average on events
  with gt 1)
• Average pT of rec. tracks 0.6 GeV/c
• Resolutions of track position parameters _at_ 0.6
  GeV/c
• s(d0(rf)) ? 100 mm d0(rf) is ? to the
  track!
• s(d0(z)) ? 240 mm

30
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
• propagate tracks to vertex nominal position
• calculate DCA (in space) for each possible pair
  of tracks (using straight line approximation)
• get estimate of xvtx and yvtx from mean of
  results from all pairs

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Vertex Fitting Algorithm

• Tracks are propagated to the point given by the
  vertex finder (at the moment nominal position
  used)
• A c2 is written as the sum of the single track
  c2s w.r.t. a generic vertex position rvtx
• where Wi is track covariance matrix in
  global ref. frame
• The solution that minimizes this c2 is analytic

covariance matrix
vertex
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Tuning of the algorithm

• Criterion used to reject mismeasured and
  secondary tracks from the fit cut on the maximum
  contribution to the c2
• ci2 lt c2max
• if c2max is too low too many tracks are rejected
  and we loose resolution
• if c2max 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
  

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Results resolutions
s(y) 55 mm
s(x) 55 mm
s(z) 90 mm
34
Results pulls
The covariance matrix of the vertex describes
correctly the resolutions
35
Resolutions VS multiplicity
36
Fast Simulation Techniques

• The selection strategy of D0 mesons has to be
  optimized using 104-105 events for Pb-Pb and
  106-107 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 dont
  affect track resolutions (efficiencies were
  corrected)

TPC tracking parameterization N.Carrer and
A.Dainese, ALICE-INT-2003-011.
37
Time-of-flight PID
TOF
Pb-Pb, dNch/dy6000
Optimization for hadronic charm decays was
studied minimize probability to tag K as p
38
What if multiplicity in Pb-Pb is lower?

• We used dNch/dy 6000, which is a pessimistic
  estimate
• Recent analyses of RHIC results seem to suggest
  as a more realistic value dNch/dy 3000 (or
  less)
• Charm production cross section
• estimate from NLO pQCD (only primary production,
  no collective effects)
• average of theoretical uncertainties (choice of
  mc, mF, mR, PDF)
• BKG proportional to (dNch/dy)2
• We can scale the results to the case of dNch/dy
  3000
• S/B 44
• SGNC 74
• (this only from scaling,
  obviously better with retuning of cuts)

39
Feed-down from Beauty (2)
Selection on pointing angle expected to suppress
D0 from b (they point to the decay vertex of the
B and not to the primary vertex)
2ltptlt3 GeV/c
40
Feed-down from Beauty (3)
Also selection based on impact parameter
enhances beauty
41
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 ? D0
  is 65!)
• error of 8 assuming present uncertainty
  (80) on _at_ LHC
• Monte Carlo corrections 10
• B.R. D0? Kp 2.4
• extrapolation from N(D0)/event to ds(D0)/dy
• pp error on (5, will be measured by
  TOTEM)
• Pb-Pb error on centrality selection (8)
  error on TAB (10)

42
Study shadowing in p-Pb at 8.8 TeV?
R.Grosso, PhD thesis (2004)
43
Charm and low-x gluon dynamics
pp
44
Charm and low-x gluon dynamics
pp

• 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 Q2
• Charm production in pp at LHC might
  be enhanced
• w.r.t. what expected using only DGLAP
  terms

• For charm at pt ? 0
• x1x2 10-4 at y 0
• min(x1,x2) 10-5 at y ?3

K.J.Eskola et al., Nucl. Phys. B660 (2003) 211
hep-ph/0211239. K.J.Eskola, V.Kolhinen and
R.Vogt, Phys. Lett. B582 (2004) 157
hep-ph/0310111.
45
Gluons DGLAP nonlinear
nonlinear (quadratic) correction has sign
Q2 evolution is slower low Q2 and x has to
be larger than in DGLAP
x 10-3
Q 2 mc 2.4 GeV
46
Charm nonlinear/DGLAP vs pt
pp

• Strongly dependent on choice of mass and scale
  (Q2)
• Varies from ?1.5 to ?5
• Enhancement limited to pt lt 2 GeV/c
• ALICE could address it with D0 at low pt
• Caveat effect currently calculated only at LO

mR mF Q
47
How to detect the enhancement due to nonlinear
effects?
pp

• The idea is that the effect (enh. only at very
  low pt) cannot be mimicked by NLO pQCD
• In practice consider ratio Data/Theory for all
  reasonable choices of theory parameters