B-jet production cross section at CDF

1
B-jet production cross section at CDF

• Monica DOnofrio
• University of Geneva
• WineCheese Seminar, September 9th 2005

2
In the last 15 years

• Study of events with b-quarks has led to
  important Tevatron results
• Discovery and study of the top quark
• B physics in general (spectroscopy, lifetimes
  measurement, sin 2b etc..)
• Measurement of quarkonium states and appreciation
  of color-octet-mediated production mechanisms
• These results mostly obtained
• when a factor 3 discrepancy
• was reported between theory
• predictions and experimental
• data by both CDF and DØ
• in b-hadron cross sections

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•  To claim that we need to understand b
  production in order to make new discoveries is
  therefore a bit exaggerated
• .. Nevertheless, lack of confidence in the
  ability to describe properties of events
  containing b quarks, in addition to raising
  doubts over the general applicability of pQCD in
  hadronic collisions, does limit our potential for
  the discovery of possible subtle and unexpected
  new phenomena  (M.Mangano, HCP2004)

Therefore, the study of b production properties
should be one of the main priorities for
RunII  also considering high statistics..
Ecm1.96 TeV s(bb) 50 mb ? few kHz event
rate!!
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Outline

• Recent advances in the theory of b production
  cross sections in hadronic collisions and
  application to experimental results related to
  B-hadrons.
• Exploring b-jets at CDF
• Inclusive b-jet cross section
• Event selection, experimental tools
• Results and preliminary comparison with
  theoretical calculations at Next-to-Leading
  order (NLO)
• More exclusive b-jet cross sections
• bb-jets correlations to disentangle b production
  processes
• Zb-jet cross section to probe b content of the
  proton
• Conclusions

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The theory and recent developments
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B-quark production in hadron collisions
Leading Order
Next to Leading Order
Q
g
Q
g
g
g
Flavor excitation
other radiative corrections..
Gluon splitting
Flavor creation
Experimental inputs are B-Hadrons or b-jets
rather than b-quark
Observed
Proton structure
Fragmentation
NLO QCD
Factorization theorem factorize physical
observable into a calculable part and a
non-calculable but universal piece
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Proton structure PDF
Parton Distribution Functions (PDFs) are
universal global fits to data on proton structure
independent of the process ? Momentum
distributions of the partons inside proton
gluon
u
d
x
Generally PDF uncertainties are estimated at
?15 Dominant contribution due to high x gluon
distribution
Uncertainty on gluon PDF (from CTEQ6)
x
8
Fragmentation functions Db ?B
Perturbative part probability to find a hadron
with fraction x of original parton momentum
Hadronization non perturbative QCD, need models
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Recent theory advances

• pQCD calculations
• resummation of aslog(pT/m) terms
• ? Fixed Order Next Leading Log (FONLL)
• pT gtgt m ? need large corrections
• Moment analysis to treat Dmeas
• for fragmentation
• (new approach Cacciari et. Al. 2002)

• Cross section very dependent
• on PDF evolution

now
lt1994
sbNLO(ylt1) (mb)
Release date of PDF
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Testing FONLL B hadron production

• J/? decays of B-Hadrons used to
• measure the b production cross section
• Find J/y inclusive cross section
• Extract fraction of J/y from decay of
• long-lived b-hadrons
• Find b-hadrons cross section for
• pT(B) down to 0 considering y(J/y)lt0.6

? J/y from B ? J/y X will be displaced
B ? J/y X shape from MC templates
Maximum likelihood fit on flight path to extract
b fraction as function of pT(J/y)
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B hadron production cross section
Good agreement with theory prediction
Total inclusive single b-hadron (Hb) cross
section
considering Br(Hb?J/yX) 1.16?0.10 and
Br(J/y?mm) 5.88?0.10
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Summary on theory advances

• Reduction of discrepancy is due
• to four basic points
• FONLL calculation brings 20 increase
• in intermediate pT region
• fragmentation step from perturbative
• b quark to B hadron at small pT was
• too strong 20 increase in this pT
• region
• Peterson fragmentation function was
• too soft use new LEP data ?0.002
• additional 20 increase
• PDF evolution (gt 20 increase)

RunI
RunII
comparison with RunI data y(Hb) lt 1, s(RunII)
multiplied by B fragmentation0.4
(Ecm rescaled)
Data moved 20 down (still within errors) Many
little changes combined together
? big effect in Data/Theory comparison
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Why b-jets?

• b-jets include most of quark fragmentation
  remnants
• ? small dependence on
  fragmentation
• wide PT spectrum

In RunI studies performed to measure bottom and
charm fraction in inclusive jet samples
5 - 6
s(pTgtpTmin, ylt1)(nb)
sb 19 2(stat) (syst) nb at
PTgt 35 GeV/c
3 - 1
PT(B) GeV/c
In RunI differential b-jet cross section using
semi-leptonic decays of the b (muon tagger)
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Inclusive b-jet cross section at CDF
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The Tevatron in RunII

• Peak luminosity in 2005 above 1032 cm-2 s-1
• CDF collected 1 fb-1 on tape!!
• (but 1.2 fb-1 already delivered)
• Analysis shown here use 300 pb-1

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Collider Detector Fermilab
The CDF experiment

• Muon Chamber (collision hall)
• position and pT
• 4 systems of scintillators and proportional
  chambers
• min scattering resolution 12/p25/p cm/p

• TOF
• time
• Scintillators
• 100 ns resolution

Solenoid (1.4 T)

• CENTRAL and PLUG Calorimeter
• energy and direction
• 2 systems of passive layers-scintillators

• COT
• position
• drift chamber
• spatial resolution
• 100 ?m

• Silicon Detector
• With 750,000 channels, the largest Silicon
  detector in the world!
• position
• 3 systems of single or double sided detector
• down to 10 ?m spatial resolution (3D)

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B-jet cross section

• Nitagged is the number of tagged jets
• eib-tag is the b-tagging efficiency
• fib is the fraction of b-jets among tagged jets
• Ciunfold are correction factors from Monte
  Carlo for acceptance
• and smearing effects
• DY is the rapidity range
• DpiT is the size of bin in transverse momentum
• ?L is the integrated luminosity

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Jet reconstruction
Final state partons are revealed through
collimated flows of hadrons called jets
Jet
Beam remnants
Hard scattering

• Two main type of jet algorithms (in CDF)
• Cone Algorithm
• ? JETCLU and MIDPOINT
• - KT algorithm

Multiple partons interaction

• Seed towers
• Only iterate over towers above certain threshold
  (3 GeV at CDF)
• MidPoint adds extra seed in centre of each pair
  of seeds ? Infrared safe
• Ratcheting (JetClu only)
• All towers initially inside a cone must stay in a
  cone

• Merging/Splitting

fmerge0.75
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Event selection

• MIDPOINT jets, Rcone 0.7, Yjetlt0.7
• PT range 30-360 GeV/c
• ? 38-400 GeV/c for corrected PT jets
• use 5 samples with different ETjet threshold
• Total luminosity used 300 pb-1

Trigger efficiency

• Inclusive calorimetric triggers
• Level 1 selection based on ET of cal towers
  (EMHAD)
• Level 2 accept tower clusters with ET above a
  fixed threshold
• Level 3 jets reconstructed (JETCLU, Rcone0.7,
  Zv0)

• Z primary vertexlt50 cm
• to assure good energy measurement, vertexing
  capability
• Cut on missing ET Significance ( ET/v?ET)
• implemented to reject to cosmic rays

Event Selection
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Jet corrections detector effects
For each calorimeter jet in Ylt0.7 look for the
corresponding hadronic (particle) jet to remove
dependence from detector effects

• need inclusive correction that takes into
  account the
• bias due to the tagger ? correction on tagged
  jets

Time
20 10

• b-quark-originated jets different from ordinary
  jets
• account for smearing effects for detector
  resolution
• ? apply unfolding correction bin by bin for
  b-jet
• from Monte Carlo hadronic b-jet

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Jet Corrections Pile up
UEM (PT) 0.9320.002 GeV
Main idea measure PT in a random cone in
Minimum Bias sample (central region) as a
function of primary vertices to define effect
due to multiple interactions
Average number of primary vertices as a
function of instantaneous luminosity
Jet samples
6 different slices of Instantaneous luminosity
Small dependence on Lum.
Effects in jet PT about -1 GeV/c per each
additional primary vertex
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B tagging algorithm
In general b-tagging procedures take advantage
of the long life-time of B hadrons ? ct 450 mm

• Looks for tracks associated with a jet
• the track selection is based on on
• measurement of impact parameter (d0)
• with respect to primary vertex

• Need two displaced tracks to reconstruct
• a secondary vertex (made in 2 steps)
• After secondary vertex reconstruction
• require to be well separated from primary vertex
  
• in r-f space by looking at Lxy and its error
• Jets passing those selections tagged

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B tagging efficiency (1)

• Use Monte Carlo simulation to cover the wide PT
  spectrum 38-400 GeV/c
• Measure efficiency scale factor to take into
  account simulation imperfections
• (tracking efficiencyresolution, B hadron decay
  models)
• For this purpose, use independent dataset

• Sample enhanced in b-jet content
• dijet events ? one e/m jet away jet tagged
• look e.g. at eb-jet for the muon jet

• Measure b-tagging efficiency
• in Data and MC

b-jet content extract using PTrel muon-jet
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B tagging efficiency (2)
eData b-jet/eMC b-jet ? Scale Factor (SF)
SF 0.909?0.06(statsyst)

• Systematic error due to hadronic VS semileptonic
  b-decay below 3

• Geometrical acceptance and energy dependence of
  the tagger ? from simulation
• Tagging rates parametrized as function of
  relevant variables to define systematic error on
  PT dependence ? 5

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b-fraction tagged jets

• Extract fraction of b-tagged jets from data using
  shape of mass of secondary vertex as
  discriminating quantity
• bin-by-bin as a function of jet pT
• 2 component fitb and non-b templates
• (Monte Carlo PYTHIA)

82 lt pTjet lt 90 GeV/c
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Systematics errors
JES only ()
Total Jet Reconstr. ()
3 ES ? Resolution ? Smearing ? effect on cross
section 20-40

• Main sources
• jet reconstruction
• Secondary vertex mass templates

• Uncertainty
• MC generator, fitting procedure
• Heavy quark multiplicity in jets
• Fragmentation

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Systematics on Secondary vertex Mass

• To estimate systematics
• ? Test fit stability depending on templates shape
  and statistic
• - also PYTHIA/HERWIG comparison
• effects of fluctuation in relative composition
  of 2b/1b and 2c/1c
• estimate from
  NLO calculation

1b/2b
1c/2c
148 lt pT lt 202 GeV/c

• Check on templates variation due to
  fragmentation scheme
• PYTHIA Lund model, ?0.0025 (default) VS Peterson
  model, ?0.006

Total systematic from fraction from 10 to 30
(but for last bin 50)
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b-jet cross section results
Differential b-jet cross section at particle
level (range pT 38-400 GeV/c)
Total systematic error 25 ? 70 in the last
bin

Ratio Data/Pythia MC(CTEQ5L)
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Preliminary comparison with NLO for inclusive
b-jet cross section
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b-jets _at_ NLO

• - NLO calculation for b-jets ManganoFrixione
• 2? 3 process, so jets are very simple 1 or 2
  partons inside

gg
total
Shape very sensitive to bb content from gluon
splitting (more likely to have 2 b inside same
jet)
qg
qq
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Scale dependence of b-jets _at_ NLO
Rate bb-jets / All jets

• Fraction of double b-quark ending up in the
  same jet depend on gluon splitting, only
  appearing at LO ?

45
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Strong scale dependence
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Hadronization and Underlying events
Before comparison with theoretical expectations ?
correct NLO b-jets for hadronization and
underlying events
Corrections that need to be added to
theory from PYTHIA Monte Carlo
20 correction for lowest bin None for b-jets
above 130 GeV/c
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Preliminary Data VS NLO b-jets
NLO theoretical expectation for b-jet corrected
at particle level

• in analogy to inclusive jet cross
• section measurements
• mb4.75 GeV/c2
• PDF Uncertainty 7 ? 20
• Merging/splitting issue
• RtheoryRdataRsep, Rsep1.3
• ? 10 uncertainty
• Include scale uncertainty
• from 40 ? 20
• (PTbjetgt250 GeV/c)

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Data/NLO ratio

• Ratio up to 1.5 above 100 GeV/c jets
• Poor agreement but still within
• systematics without considering
• scale uncertainty

if considering scale uncertainty overlap region
increase
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Comparison with Run I D? results

• Use mm0 for central theory value
• data close to upper band of systematic (mm0/2 ?
  PDF uncertainty)
• direct comparison of data not possible
• (different center of mass energy ?s, different
  jet algorithm, different
• rapidity range )

For jets below 100 GeV/c, D0 data in RunI showed
a similar pattern
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Issues on high PT b-jets

• Unknown impact of higher order contributions
• reduced scale dependence
• event has more partons in the final state, thus
  closer
• to the real world
• better description of the transverse momentum of
  
• final state due to double radiation of
  initial states
• Logarithmic log(pT/m) enhancement of higher order
  contribution due to gluon splitting is not
  included in NLO calculations (neither in MC_at_NLO)
• ? at low PT effects are small (range of B-hadron
  cross section)
• ? at high PT are very important and need to be
  considered
• Experimentally
• study of bb-jets correlation
• ? to disentangle different production mechanisms
• Zb jets and gb jets
• ? could help to constrain the b density in the
  proton

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More exclusive bjet cross sections
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bb jets cross section

• Small data sample used
• still preliminary
• Analysis with larger
• sample in progress

• dijet events
• JETCLU, Rcone0.7 jets
• ET1gt30GeV, ET2gt20GeV
• h jets lt 1.2 ? central jets more likely to be
• sensitive to flavor creation

Flavor excitation
Q
g
Flavor creation ( radiative corrections) predom
inantly back-to-back
Q
g
g
g
Gluon splitting

• Jets corrected at particle level
• for b flavor jets
• Tag 2 jets with b-tagging algorithm
• earlier described

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bb jets cross section results
b fraction from Mass secvtx fit (global fit all
ET range) Fbb0.83?0.04
Integrated cross section
Data results

10.3 - 10.7
34.5 ? 1.8 nb
A trigger acceptance 1
(stat.) (syst)
Monte Carlo prediction

• Need to add more statistics
• on MC_at_NLO with final
• Underlying Event tuning

Pythia (CTEQ 5l) s 38.71 ? 0.62nb
MC_at_NLO s 28.49 ? 0.58nb
35.7 2.0 nb with preliminary UE tuning

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bb jets correlations
Differential cross section as function of Df jets

• Predominantly back to back
• Explains agreement in cross section with Pythia
• LO MC deviates away at low Df (where statistics
  are still low)

Linear scale
Log scale
Compared to MC_at_NLO
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Zb jet production
In QCD, Zb can help constrain b density in the
proton
Important background for new physics such as
higgs search

Probe the heavy flavor content of proton
With HERA Fbb2 data CTEQ below MRST by down to
?1/2 and below data ? Zb jets can help
understand this picture
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Analysis strategy

• Leptonic decays for Z reconstruction Z ? ee-,
  mm-
• Signal defined as Z0,g?ll- events with
  66ltMlllt116 GeV/c2

Muon channel equivalent

• ? Z associated with jets
• - Rcone jet 0.7, hlt1.5, ETgt20 GeV
• Backgrounds
• - fake electrons/same-sign muons (Data)
• - Real signatures ee-/mm- bjet (MC)

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Mass of secondary vertex Fit

• Look for tagged jets in Z events
• same b-tagging algorithm as in
• previous analyses
• extract fraction of b-tagged jets
• from secondary vertex Mass
• Use negative tagged jets to
• better constrain light and c
• quarks
• Make no assumption on the
• charm content
• ? extract ?b NbData/NbMC

Main source of systematic uncertainty from
dependence of mass templates on b/bb content in
the jet
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Zbjets results
Zb jet cross section corrected at particle
(hadron) level

?b NbData/NbMC in Z events SF scale factor
for b-tag NZbMCHad MC particle b-jets in Z
evts NZMCHad total MC Z evts
NZMC MC evts passing Z cuts NZData Data evts
passing Z cuts ?meas(Z) CDF Z cross section
mMZ Uncertainty 10
changing scale
Central value for Zb cross section also above
theoretical expectations
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Summary and Conclusion

• In the last year many advances in theory
  calculation for b production at intermediate and
  low PT
• Good agreement with B-Hadron cross section
• Big effort in study of b-jets production in CDF
  RunII
• Inclusive b-jet cross section measurement done
  within a wide range in transverse momentum using
  300 pb-1 data
• Similar behaviour to D? RunI cross section w.r.t.
  theoretical expectations for jet PT below 100
  GeV/c
• Agreement with theory within uncertainties NLO
  b-jet cross section calculation still shows big
  scale dependences
• Study of more exclusive b-jet cross section could
  help
• bb correlations to disentangle production
  processes
• Zb to understand b content in the initial
  radiation

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Back up
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Impact parameter resolution
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B-jet cross section Data/Herwig MC
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bb jets differential cross section
Comparison Data VS MC _at_NLO UE tuning
Differential cross section as function of dijet
mass
Comparison Data VS MC _at_NLO UE tuning
Differential cross section as function of
leading jet ET
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Z? mm- b
Z ? mm- channel same-sign muons events with a
reconstructed jet
Specific background in muon channel
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NLO uncertainty for Zb
Scale dependence is small (? ?10) Big
uncertainty on b density in the proton
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gb/ gc production
2934 GeV
2529 GeV

• g Et gt 25 GeV (hlt1.0) jet with secondary
  vertex
• Determine b, c, uds contributions (fit secondary
  vertex mass)
• Subtract bkg, find cross-section as fn. g Et

4260 GeV
3442 GeV
Sec. Vertex mass (GeV)
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gb/ gc production
s(gb)
s(gc)
Results consistent with LO