alice physics week - erice, 06.12.2005 andrea dainese 1 andrea dainese padova – università e infn...

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)


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 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


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


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


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


D0 K-+: Selection of D0 candidates

increase S/Bby factor ~103!


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


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


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


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


Statistical Errors

(R.Grosso)


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


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


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


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


20-60% 4x106 evts

0.0430x1210-14

0.0130x1205-14

0.0230x347-10

0.0130x745-7


20-60% 4x107 evts

see Francesco’s talk


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


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


EXTRA SLIDES


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


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


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


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


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


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


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


Results: resolutions

(x) = 55 m (y) = 55 m (z) = 90 m


Results: pulls

The covariance matrix of the vertex describes correctly the resolutions


Resolutions VS multiplicity


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)


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)


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


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


Study shadowing in p-Pb at 8.8 TeV?p-Pb

pp

R.Grosso, PhD thesis (2004)


Charm and low-x gluon dynamicspp


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


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


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


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


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)

alice physics week - erice, 06.12.2005 andrea dainese 1 andrea dainese padova – università e infn...

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