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ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 1
Andrea DaineseAndrea Dainese
Padova – UniversitPadova – Universitàà e INFN e INFN
DD00 meson reconstruction meson reconstruction with ALICEwith ALICE
11stst ALICE Physics Week ALICE Physics Week
(PPR Volume 2, 6.6.4)
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 2
Contents
ALICE baseline for heavy-quark production
Experimental detailsSelection strategy for D0 KTrack impact parameter measurement
Feed-down from B decays
D0 cross section measurement in pp, pPb, Pb-Pb (central and peripheral)
Charm measurement in initial pp run?
Perspective for v2 measurement
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 3
ALICE baseline: N.Carrer and A.D., ALICE-INT-2003-019, hep-ph/0311225; included in PPR2
[mb] QQNNQQ
totN
(ALICE) Baseline for cc / bb productionpQCD calculation available @ 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 (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 / 0.2 7.2 / 0.3 11.2 / 0.5
115 / 4.6 0.8 / 0.03 0.16 / 0.007
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 4
D0 K-+ in ALICEExclusive reconstruction
direct measurement of the pt distribution
Weak decay with mean proper length c = 124 m
Large combinatorial background (dNch/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, 06.12.2005 Andrea Dainese 5
Track impact parameter
22
,0
/1
0
pm
cd
DK
D0 daughters mean d0:
PIXEL CELL
z: 425 m
r: 50 m
Two layers:r = 4 cmr = 7 cm
9.8 M
d0 measurement in ALICEmainly provided by the twoSPD layers
Systematic study of track impact parameter resolution:A.D. and R.Turrisi, ALICE-INT-2003-028.
sin
5311][
t
rp
m
ALICE Physics Week - Erice, 06.12.2005 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 ~ 1031 cm-2 s-1)
150 m in pp (if L is reduced at ALICE IP to ~ 1029 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
vertextrackrd )(0
A.D. and M.Masera, ALICE-INT-2003-027.
as in Pb-Pbpp with charmpp min. bias
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 7
D0 K-+: Selection of D0 candidates
increase S/Bby factor ~103!
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 8
Time-of-flight PID
TOF
Pb-Pb, dNch/dy=6000
Optimization for hadronic charmdecays was studied:minimize probability to tag K as
K id. required for D0 pt < 2 GeV/c
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 9
S/B
initial
(M3)
S/B
final
(M1)
Significance
S/S+B
(M1)
Pb-Pbcentral
5 10-6 10%~35
(for 107 evts,
~1 month)
pPbmin. bias
2 10-3 5%~30
(for 108 evts,
~1 month)
pp 2 10-3 10%~40
(for 109 evts,
~7 months)
D0 K-+: Results
Note: with dNch/dy = 3000 in Pb-Pb, S/B larger by 4 and significance larger by 2
central Pb-Pb
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 10
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
%5~/ ccbb
%5.5)(/
)/()/(/
)(/
)(/0
000
0
0
DcdydN
XDBBBRBBbdydN
DcdydN
DbdydN0.640.0017
0.0196
ALICE Physics Week - Erice, 06.12.2005 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 D0 is 65%!):
error of ~8% assuming present uncertainty (~80%) on @ LHC
Monte Carlo corrections: ~10%
B.R. D0 K: 2.4%
extrapolation from N(D0)/event to d(D0)/dy: pp: error on (~5%, will be measured by TOTEM) Pb-Pb: error on centrality selection (~8%) + error on TAB (~5%)
bb
inel
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 12
Statistical Errors
(R.Grosso)
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 13
D0 cross section measurement
inner bars: stat. errorsouter bars: stat. pt-dep. syst. errorsnormalization error not shown: 9% Pb-Pb and p-Pb, 5% pp
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 14
Extrapolated performance for semi-peripheral Pb-Pb (30-60%)
class 1 4
b (fm) 0-3 9-12
%Pb-Pb 0-3.6% 32.6-58%
Nevents(107 min.bias)
3.6x105 2.5x106
<b> (fm) 1.9 10.6
<Npart> 385 70
<Ncoll> 1644 156
<dNch/dy> HIJING
6200 600
Ncc/ev 118 12.5
Nbb/ev 4.8 0.5
Larger pt bins than for 0-5% have to be used
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 15
Charm measurement with low statistics / low lumi pp run?
Simple scaling of statistical errors according to Nevts
No dedicated cuts study
30%
2-10 GeV/c coveragewith 5107 evts
low statshigh lumi (>>1030), bad for d0
30%
2-7 GeV/c coveragewith 2107 evts
low statslow lumi (~1030), good for d0
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 16
First look at D0 v2
))(2cos( ))](2cos(21[ 220 RPRP vvNd
dN
For pt < 5 GeV/c, D0 candidates are signal + background
v2 by performing inv. mass analysis in bins
dedicated study needed
For pt > 5 GeV/c, background is negligible
get v2 from all D0 candidates (only signal)
First estimate, for pt > 5 GeV/c: assuming v2 ~ 0.10-0.20
0.201210-14
0.061205-14
0.12347-10
0.08745-7
v2 (statistical)SignalpT bin (GeV/c)
33-58%2.5x106 evts
0.123x1210-14
0.043x1205-14
0.073x347-10
0.053x745-7
v2 (statistical)SignalpT bin (GeV/c)
20-60% 4x106 evts
0.0430x1210-14
0.0130x1205-14
0.0230x347-10
0.0130x745-7
v2 (statistical)SignalpT bin (GeV/c)
20-60% 4x107 evts
see Francesco’s talk
ALICE Physics Week - Erice, 06.12.2005 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 (pt > 0.5 GeV and |d0| > 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, 06.12.2005 Andrea Dainese 18
SummaryFeasibility study for D0 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 promisingpt coverage in the range 1—18 GeV/c
some physics “applications” identified and studied (see quenching talk later on…)
1st charm cross section measurement in pp at LHC
some 107 events sufficient to cover up to pt ~ 7 GeV/c
Analysis code skeleton prepared
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 19
EXTRA SLIDES
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 20
Nuclear Shadowing and Charm
Pb nucleus ~ 105 partons (mainly gluons)‘they are so close that they fuse’
Shadowing: suppression of small-x PDFs
Reduces c&b yield at low pt
Look at low-pt D in p-Pb!
xa
xb
xa+xb
fgPb(x)/fg
p(x)
QQyQQ es
Mx
2,1
For charm at pt 0:
x1~x2 ~ 10-4 at y = 0
min(x1,x2) ~ 10-5 at y = 3
tDpp
tDpA
collt
DpA dpdN
dpdN
NpR
/
/1)(
ab
c
see talk by R. Grosso
Bjorken
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 21
RHIC Au-Au 200 GeV
Estimates of RAA for D and BIngredients: 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.
LHCLHC
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 22
3D reconstruction with tracks
Track reconstruction in TPC+ITStrack 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 vertexgive vertex covariance matrix
give a
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 23
Expected resolutions
Average # rec. tracks = 7 (average on events with # > 1)
Average pT of rec. tracks = 0.6 GeV/c
Resolutions of track position parameters @ 0.6 GeV/c:(d0(r)) 100 m [ d0(r) is to the track! ]
(d0(z)) 240 m
mm
z zdvtx
907
240
tracksof#
)GeV/c 6.0()( )(0
mm
yx rdvtxvtx
555.3
100
2/ tracksof#
)GeV/c 6.0(),( )(0
ALICE Physics Week - Erice, 06.12.2005 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 xvtx and yvtx from mean of results from all pairs
ALICE Physics Week - Erice, 06.12.2005 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 2s w.r.t. a generic vertex position rvtx:
where Wi is track “covariance matrix in global ref. frame”
The solution that minimizes this 2 is analytic:
Tivtxi
iivtx
ivtxi )rr( W)rr()r( 122
ii
ii
ivtx rWWr1
1
WV
iivertex
covariancematrix
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 26
Tuning of the algorithmCriterion used to reject mismeasured and secondary tracks from the fit: cut on the maximum contribution to the 2
i2 <
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, 06.12.2005 Andrea Dainese 27
Results: resolutions
(x) = 55 m (y) = 55 m (z) = 90 m
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 28
Results: pulls
The covariance matrix of the vertex describes correctly the resolutions
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 29
Resolutions VS multiplicity
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 30
TPC tracking parameterization: N.Carrer and A.Dainese, ALICE-INT-2003-011.
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 don’t affect track resolutions (efficiencies were corrected)
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 31
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, F, R, 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)
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 32
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)
Resolution on pointing angle is worse for D from c pointing angle enhances D from b
D
D
B
V1 V1
K
K
2<pt<3 GeV/c
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 33
Feed-down from Beauty (3)
Also selection based on impact parameter enhances beauty
20% !
D0 from b / D0 from c = 10 %
Systematic error from b =uncertainty on b cross section fraction of D0 from b
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 34
Study shadowing in p-Pb at 8.8 TeV?p-Pb
pp
R.Grosso, PhD thesis (2004)
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 35
Charm and low-x gluon dynamicspp
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 36
Charm and low-x gluon dynamicsGlobal fits of HERA data improved by adding nonlinear corrections to standard DGLAP partonic-evolution equationsArise 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
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].
QQyQQ es
Mx
2,1
For charm at pt 0:
x1~x2 ~ 10-4 at y = 0
min(x1,x2) ~ 10-5 at y = 3
pp
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 37
Gluons: DGLAP nonlinear
x = 10-3
Q = 2 mc = 2.4 GeV
222
2
2
2
),(log
),(
log
),(QxxgO
Q
Qxxg
Q
Qxxg
DGLAP
nonlinear (quadratic) correction has “–” sign Q2 evolution is slower
low Q2 and x has to be larger than in DGLAP
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 38
Charm: nonlinear/DGLAP vs pt
Strongly dependent on choice of mass and scale* (Q2)Varies from 1.5 to 5
“Enhancement” limited to pt < 2 GeV/c
ALICE could address it with D0 at low pt
Caveat: effect currently calculated only at LO
* R = F = Q
pp
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 39
How to detect the enhancement due to nonlinear effects?
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
A.D
. et a
l ., hep
-ph/0
40309
8.
Data:mc = 1.2 GeV, Q2 = 4mT
2
and enhancement
Data:mc = 1.3 GeV, Q2 = mT
2
and enhancement
pp
ALICE Physics Week - Erice, 06.12.2005 Andrea Dainese 40
TPC tracking parameterization: N.Carrer and A.Dainese, ALICE-INT-2003-011.
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 don’t affect detector performance (pt & d0 resol., efficiencies)