Detection of D 0 K - p + in pp collisions “Propedeutics for a feasibility study”

PPR meeting - November 8, 2002 Andrea Dainese 1

Detection of D0 K- + in pp collisions

“Propedeutics for a feasibility study”

Andrea DaineseUniversity of Padova


Table of contentsReconstruction of pp events

determination of z position of primary vertexparameterization of TPC tracking for ppresolution on track parameters (compared to Pb-Pb “6000”)

Status of the generation/reconstruction with AliEn of events for background studyProduction of charm particles in pp @ s = 14 TeV

cc cross section from NLO QCD calculationrates for D mesonsgeneration of charm signal (D0) with Pythia

Global properties of pp events with charm productionmultiplicitymean pT

event thrust and azimuthal correlations


Reminder: D0 detection strategy

Invariant mass analysis of fully-reconstructed topologies originated in secondary verticesSecondary vertex selection based on:

couple of tracks with large impact parametersgood pointing of reconstructed D0 momentum to the primary vertex

Resolution on track parameters and primary vertex is crucialThe feasibility study requires very large statistics (~106 pp events) for signal and background analysis at high pT

TPC tracking parameterization makes it possible


Primary vertexPb-Pb

bending plane: (x) = (y) 15 m given by transverse size of Pb beam

z direction: position measured using the correlation between clusters in the 2 pixel layers (Int. Notes by Ohio/LNL, Catania group)

(z) 1/dNch/dy (z) = 5.5 m @ 6000

ppbending plane: assume to have the same resolution as in Pb-Pb (for the moment)z direction:

1st step: use tracklets from pixels as in Pb-Pb, but … multiplicity lower by factor ~1000 (z) worse by factor ~ 30 !

2nd step: do tracking in TPC+ITS and use tracks to estimate zvtx

but not much expectation: tracks are more precise but fewer than tracklets


1st step: zvtx from trackletsSame method as in Pb-Pb (adapted to pp by M. Masera)

(z) = 155 m


2nd step: zvtx from ITS tracksTracking in TPC+ITSSelection of tracks from primary vertex (in bending plane):

|d0(r)| < 3 d0(r)(pT)

Estimate of zvtx from weighted average of z impact parameters d0(z) [weight is 1/d0(z)

2(pT) ]

Expected result:average # rec. tracks in pp = 7.5average pT of rec. tracks = 0.6 GeV/c

d0(z)(0.6 GeV/c) = 240 m

mmz zdvtx

905.7

240 tracksof#

)GeV/c 6.0()( )(0


Results (1)

(z) = 105 m

(z)1 track = 250 m


Results (2)

mdydN

zch

/2737)(


Comparison of the two methodsWith ITS tracks resolution improves by 50 m

Reconstruction chain:1. vertex from pixels2. tracking3. use tracks to improve vertex

position to be used for analysis (e.g. pointing angle)


Parameterization of TPC trackingParameterization of TPC Kalman filter response (efficiency, resolutions, covariance matrix, dE/dx) gives a large reduction in time/space for simulation (~factor 40)New in pp w.r.t. Pb-Pb:

use of AliTrackReferences at inner radius of TPC allows full coverage in all particles treated separately (, K, p, e, )

“Making of” presented at last Alice Week, here only some interesting “highlights”:

TPC tracking efficiency in pptests on tracking performance in ITS for “validation” of the tool


Tracking efficiency in the TPC (1)

Saturation at 0.93: 7 % of very high pT tracks that enter the TPC are lost no seed at end of TPC ?

Effect not seen at low pT


Tracking efficiency in the TPC (2)

<1/2 of the lost tracks don't have seed

others are lost when they change sector

very low pT: all have seed not lost when change sector

why do we loose tracks in pp ?

Muons, with seed,not reconstructed (pT = 0.7 GeV/c)they change sector!


Validation: fraction of prolonged tracks


Validation: resolutions after complete tracking


Resolutions: Pb-Pb VS pp

Momentum resolution ~ 25% better in ppImpact parameter resolution ~ same in Pb-Pb and in pp

primary tracks


Background for D0 with 5 million pp non-diffractive events generated in June:

“kinematic” window |y|<2, R < 85 cm (ITS only+AliTrackReferences at TPC)5000 pp events / file

(failures < 5 %)Reconstruction successfully executed in the last weeks:

TPC tracking parameterizedFast points in ITSVertex determination (smearing with resolution given by ITS tracks)tracking in ITS with standard Kalman V2 (6 points required)

Analysis and reconstruction of D0 candidates in progress:analysis macro + compiled library registered in cataloguecandidates built from ITS tracksonly small ROOT file with candidates being copied to my PC (~ 0.5 ‰ of total size)


Background for D0 with

Thanks to AliEn team (Pablo, Predrag)


Charm production in pp @ 14 TeVNLO QCD calculation (MNR):

Using : (115 in central Pb-Pb @ 5.5 TeV)

Yields for charm hadrons:

D0 K with K and in TPC acc.: 7.16 10-4 / event

mb 2.11cc

mb 70ppinel 0.16// pp

inelcccc eventN

Hadron(+ antiparticle) D0 D+ D+

S +C

dN/dy (|y|<1) 0.0196 0.0058 0.0038 0.0026


Generation of D0 signal Requirements for generation of D0 K signal in pp:

pT distribution as given by MNR calculationthe signal has to be generated together with “its own” pp event:

– want to study global properties of pp evts with charm– there can be “dangerous” backgrounds (one track from the D0

and one track from the underlying event)

Nicola has tuned Pythia to reproduce pT distribution of MNRtuning is meant to be used to generate charm stand-aloneif one keeps the full event, the total charged multiplicity in the barrel is lower than that given by pp min. bias

Recipe for generation:generate standard pp min. bias with Pythiakeep only those events that have a D0

weight D0s according to their pT in order to reproduce distribution given by NLO QCD


Global properties of charm eventsProduction of heavy quarks requires “hard” hard scatteringFragmentation of heavy quarks gives large multiplicity

Events with a cc pair should have larger multiplicity and larger mean pT w.r.t. to “ordinary” pp events

Maybe this cannot be used for detection of charm particles, but it’s worth having a lookGlobal event properties inspected:

multiplicitymean pT

thrust of the event (defined later)azimuthal correlations of D particle with all other tracks


Multiplicity and mean pT

Charged particles in ||<0.9 and pT > 200 MeV/c


… as a function of D0 pT

pT < 1 GeV/c 1 < pT < 2 GeV/c 2 < pT < 3 GeV/c

3 < pT < 5 GeV/c 5 < pT < 10 GeV/c pT > 10 GeV/c


Event ThrustDefinition:

thrust =

= is the thrust axis “axis which maximizes projected momenta”u

||max20

pupup


D0 w.r.t. thrust axis

pT < 1 GeV/c 1 < pT < 2 GeV/c 2 < pT < 3 GeV/c

3 < pT < 5 GeV/c 5 < pT < 10 GeV/c pT > 10 GeV/c


Azimuthal correlations of all particles (in acc.) w.r.t. to D0 flight-line (D0 decay products excluded)Particles weighted with their momentum

Looks like jets …Depends on (cc)…

could it give informations on this variable?


ConclusionsAll tools for reconstruction in pp readyBackground (~106 events) being analyzed using ALICE

distributed resources exclusivelySignal generation in progress

Survey of pp specific observables (multiplicity, thrust, etc…) events with charm have larger multiplicity clear jet topology for pT(D0) > few GeV/c

Possible applications:use thrust + invariant mass for D0 selection at high pT

– can be done on TPC tracks (increase statistics by ~50%)use azimuthal correlations for study of cc production kinematics

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Detection of D 0 K - p + in pp collisions “Propedeutics for a feasibility study”

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