V. Chiochia DIS 2003 Workshop Production of beauty quarks in deep inelastic scattering at HERA XI International Workshop on Deep Inelastic Scattering St

V. ChiochiaV. Chiochia DIS 2003 WorkshopDIS 2003 Workshop

Production of beauty quarks in Production of beauty quarks in deep inelastic scattering at HERAdeep inelastic scattering at HERA

XI International Workshop on Deep Inelastic Scattering St. Petersburg, 23-27 April 2003

Vincenzo ChiochiaDeutsches Elektronen SynchrotronDeutsches Elektronen Synchrotron

on behalf of the ZEUS Collaboration

Production of beauty quarks in deep inelastic scattering at HERAProduction of beauty quarks in deep inelastic scattering at HERA 2V. ChiochiaV. Chiochia DIS 2003 WorkshopDIS 2003 Workshop

OutlineOutline

• Event topology Event topology • Event selectionEvent selection• Signal extraction and cross section calculationSignal extraction and cross section calculation• QCD predictions and related uncertaintiesQCD predictions and related uncertainties• Results: total and differential cross sectionsResults: total and differential cross sections• Comparison with other measurementsComparison with other measurements• Summary and outlookSummary and outlook


Process topology in the Process topology in the ZEUS experimentZEUS experiment

laboratory frame

Inner muonchambers

Outer muonchambers

Barrel and rearcalorimeter

electronelectron protonproton

Central trackingdetector

(forward) (rear)


Summary of the event selectionSummary of the event selection

Data sample ZEUS 1999-2000 e+p (60 pb-1)

DIS cuts

Q2>2 GeV2

0.05<y<0.740<E-pz<65 GeV

box cut 12x24 cm

Muon cuts P(2)>1%p>2 GeV

30º<<160º

Jet cuts At least one jet in the Breit frame:At least one jet in the Breit frame:ET

Breit>6 GeV

-2<Lab<2.5

Jet associated to the muon:Jet associated to the muon:ET

Breit>4 GeV

QED radiation cutsQED radiation cuts

• Increase acceptance on signal• Increase background rejection

• Kinematic region• Background rejection andgood electron reconstruction

• Probability cut: remove fakemuons• Momentum cut: from inner andouter muon chamber requirement• Polar angle: good associated track


Extraction of the s.l. b-decays (1)Extraction of the s.l. b-decays (1)• After the selection cuts the data sample is a mixture of several processes:

– Fake muons: Punch-through, Sail-through, pions and kaons in-flight decays (cross checked with real data)

– Semi-leptonic decays of charmed hadrons– Semi-leptonic decays of beauty hadrons

• Introduce an observable to distinguish, on statistical basis, between the different processes: pT

rel

jet axisjet axisppTT

relrel

muonmuon

pe+

• Jet and and muon are associated using the jet algorithm


• 836 events after all cuts836 events after all cuts• Charm and light-quark ptrel

distributions are added together according to the respective cross sections

• The b-fraction is extracted with the Binned maximum likelihood method

ffbb = (25.3±4.8)% = (25.3±4.8)%• The algorithm takes in account The algorithm takes in account

the statistical errors of the the statistical errors of the data and MC samples in each data and MC samples in each binbin

Extraction of the s.l. b-decays (2)Extraction of the s.l. b-decays (2)

~210 beauty events


Comparison with the simulationComparison with the simulation

~25% of events~25% of eventsfrom b-decaysfrom b-decays

Good descriptionGood descriptionof the event kinematicsof the event kinematics

with the extracted with the extracted

fractionsfractions

muonmuonvariablesvariables

jetjetvariablesvariables


QCD predictions: NLO calculationsQCD predictions: NLO calculations

The calculation of the NLO QCD predictions proceeds in three steps:1) HVQDIS (B.Harris, J.Smith, hep-ph/9503484): *gbb, *gbbg, *qbbq, etc.2) Fragmentation of the b-quark into a B-meson3) Semileptonic decay of the B-meson

b quark

pQCDO(s)

corrections

Charmedhadron

Neutrino

Experiment

Lepton

Fragmentation

Peterson function with =0.002 Muon momentum spectrum

extracted from MC


Results and QCD predictionsResults and QCD predictions

ep e b b X e Jet X

Kinematic region:

QQ22> 2 GeV> 2 GeV22, 0.05<y<0.7, 0.05<y<0.7At least one jet in the Breit frame with:

EETTBreitBreit> 6 GeV, -2<> 6 GeV, -2<LabLab<2.5<2.5

At least one muon with:pp>2 GeV, 30<>2 GeV, 30<<160<160

Measured cross section:visvis = 38.7 = 38.7±7.7(stat.)+6.1(syst.)-5.0(syst.) pb±7.7(stat.)+6.1(syst.)-5.0(syst.) pb

NLO QCD (HVQDIS):NLONLO = 28 + 5.3 - 3.5 pb = 28 + 5.3 - 3.5 pb

R,F2 = Q2+4mb

2

mb=4.75 GeVPeterson frag. function

Proton parton distribution: CTEQ5F4

Model Program Cross Section (pb)

LO HVQDIS 14.4

ME+DGLAP RAPGAP 17.1

CDM RAPGAP 16.3

ME*+CCFM CASCADE 36.8

R,F2 = pT

2+4mb2

Data/NLO ~ 1.4 butData/NLO ~ 1.4 butin agreement within in agreement within exp. and theoreticalexp. and theoretical

uncertaintiesuncertainties


dd/dQ/dQ22

measured at HERA for the first time!measured at HERA for the first time!

NLONLO MCMC


dd/dlog(x/dlog(xbjbj))

NLONLO MCMC


NLO QCD uncertaintiesNLO QCD uncertainties

more sources:more sources:

Hadronization (jet): Hadronization (jet): ~ 10%~ 10%

Fragmentation (muon): Fragmentation (muon): 5-10%5-10%

Factorization and Factorization and renormalization scales renormalization scales b-quark mass: b-quark mass: 10-15%10-15%

Gluon uncertainty fromGluon uncertainty fromthe ZEUS NLO QCD fit:the ZEUS NLO QCD fit:~5%~5%

CTEQ5F4/ZEUS NLO FIT


A tricky business...A tricky business...

H1 DIS:H1 DIS:jets extrapolatedjets extrapolated

to the full phase spaceto the full phase spaceH1 Photoproduction:H1 Photoproduction:

jets extrapolated jets extrapolated to the full phase spaceto the full phase space

red: ICHEP02 results

ZEUS D*-muon:ZEUS D*-muon:extrapolated to the extrapolated to the

parton levelparton level

ZEUS PhotoproductionZEUS Photoproductionmuon extrapolated muon extrapolated

to the full phase spaceto the full phase spaceThe visible cross sectionThe visible cross section

agrees with the NLO!agrees with the NLO!

This analysisThis analysisvisible cross sectionvisible cross section

jets in the Breit framejets in the Breit frame


Summary and conclusionsSummary and conclusions

• The description of the b-quark production is still an open question. Cannot draw a unique conclusion from the existing results.

• We have measured a visible cross-section of b-quark production in DIS and single differential cross-sections (Q2, xbj) for the first time at HERA

• The cross section is measured with events containing the scattered positron, a hard jet in the Breit frame and a muon traversing the inner and outer muon chambers.

• Detailed analysis of the theoretical uncertaities has been performed

• The measured cross-section is a factor ~1.4 above the NLO QCD predictions but in agreement within the experimental and theoretical uncertainties

• Similar result in the ZEUS measurement in photoproduction


SPARE SLIDESSPARE SLIDES


Acceptance, efficiency, purityAcceptance, efficiency, purity

ME+PSCDM

ME+PS (e-p)ME*+CCFM


b-quark fragmentationb-quark fragmentation

[hep-ph/0204025]


Event simulation: Monte CarloEvent simulation: Monte Carlo

Parton shower with DGLAP Parton shower with DGLAP EvolutionEvolution

• Matrix Element with incoming on-shell gluon

• KT ordering in the gluon ladder

• RAPGAP:– Lund string model– Color dipole model and

ME+PS

Parton shower with Parton shower with

CCFM-like evolutionCCFM-like evolution

CASCADE• Matrix Element with incoming

off-shell gluon• Angular ordering in the gluon

ladder • Direct (pointlike) component

only• Gluon density unintegrated in

kT and extracted from a fit to the HERA structure function F2

• Lund string model

Incr

easi

ng v

irtu

alit

yIn

creasi

ng v

irtu

alit

y

Incr

easi

ng e

mis

sion a

ngle

Incr

easi

ng e

mis

sion a

ngle

Light quarks: ~120·106 events (200 pb-1)Charm quarks: ~7.5·106 events (94 pb-1)Beauty quarks: ~430·103 events (540 pb-1)

...more samples generated...more samples generatedto understand the modelto understand the model

dependancedependance


Visible vs extrapolated measurementsVisible vs extrapolated measurements

• Visible cross sectionVisible cross section: the kinematic region is close to the detector acceptance

• Example: ep bbx Jet Jet X• Model assumptions are not absorbed in the experimental result but in the

corresponding QCD predictions → published measurements are then safe

• Extrapolated cross sectionsExtrapolated cross sections:1)1) Extrapolation to an unmeasured kinematic regionExtrapolation to an unmeasured kinematic region:: the unmeasured phase

space is taken from a Monte Carlo simulation (for example: extrapolation to the jet or muon full phase space)

• It is nice to quote more general and inclusive measurements, but...• Is the and ET spectrum of the extrapolated region the same in the

NLO as in the Monte Carlo?• How large are the extrapolation factors? Are they reported in the

published results?• Is the extrapolation considered in the estimation of the systematic

uncertainties?2)2) Extrapolation from detector to parton/hadron levelExtrapolation from detector to parton/hadron level:: the corresponding

correction factors are extracted from a Monte Carlo simulation

Documents

V. Chiochia DIS 2003 Workshop Production of beauty quarks in deep inelastic scattering at HERA XI International Workshop on Deep Inelastic Scattering St