First measurement of forward backward Asymmetry in bb Production - - PowerPoint PPT Presentation

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First measurement of forward backward Asymmetry in bb Production - - PowerPoint PPT Presentation

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First measurement of forward backward Asymmetry in bb Production at CDF on behalf of CDF collaboration P. Barto Comenius University Motivation Noticeable A FB seen in tt production Deviation from the predictions is CDF, L+J

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First measurement of forward – backward Asymmetry in bb Production at CDF

  • P. Bartoš

Comenius University

  • n behalf of CDF collaboration
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05/02/12

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Motivation

CDF, L+J analysis, parton level

➢ Noticeable AFB seen in tt production ➢ Deviation from the predictions is

  • bserved:

→ does it appear only at large Mqq ?

➢ Measuring of bb asymmetry does

not probe the large Mqq → it probes almost the same SM theory computation needed for predicting AFB in tt

➢ new physics in leptophobic scenarios

could appear at relatively low mass,

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Source of the asymmetry

➢ SM: LO strong interaction processes

: no AFB

➢ sources of AFB:

  • Fig. 1
  • Fig. 2

q qb b and gg b b

 radiative corrections involving either

virtual or real gluon emission:

✔ Interference of (a)+(b), (c)+(d) in Fig.1 ✔ Interference of flavor excitation

in quark+gluon processes (Fig.2)

 contribution from EW production

processes:

➢ No asymmetry in gluon fusion

interactions

q q,Z b b

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Theoretical expectations

Rodrigo, Kűhn 1999 Phys. Rev. D59 054017 → black dash-dotted line is differential AFB for

(Θ * - b quark production angle in qq rest frame)



s=400GeV

➢ b quark production at Tevatron: main source gluon fusion (no AFB) ➢ The expected SM global AFB from qq and qg (qg) reactions is

small due to a big symmetric gluon contribution

➢ However for some selection criteria

(which increase qq→bb fraction) sizable asymmetry is predicted :

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µ− νµ

b

Events selection

→ dijet events with soft µ inside one of jets:

✔ pT (µ) > 10 GeV, |η(µ)| < 0.6

→ muon jet (MJ) : ET (MJ) > 20 GeV

∣pT AJ −pT MJ ∣ max{ pT AJ , pTMJ } 0.6

→ away jet (AJ):

✔ ET (AJ) > 15 GeV, |η(AJ)| < 1.0 ✔ AJ back-to-back to MJ: |∆Φ|>2.8 ✔

→ both jets b-tagged by Secondary Vertex tagging algorithm

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Secondary vertex b-tagging

This technique takes advantage of the long lifetime of a b-hadrons => identify jets from the bottom quark hadronization through the presence of a decay vertex displaced from the primary interaction. Secondary vertex: → vertex displaced from the primary vertex → contains at least 2 tracks which passed a quality criteria

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Methodology

A FB= N  yb0−N  yb0 N  yb0N  yb0  yb=Q⋅  y AJ− yMJ

In our case:

 yb= yb−y

b

✔ Integrated asymmetry is defined:

A FB= N  yb0−N  yb0 N  yb0N  yb0

✔ Assuming CP conservation ( ) we can express AFB

using Lorenz invariant :

✔ The study is performed as a function of Mbb and |∆yb|

N −yb=N  y

b

* Q(µ) – charge of the muon, AJ – away jet, MJ – muon jet (yb – rapidity of the b-quark in qq rest frame)

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AFB corrections for non bb events

A FB

b b= A FB reco

f b 

b

, if we assume that A FB

non-b b=0

A FB

reco= N1 reco−N 2 reco

N1

recoN 2 reco , where N1 reco=N  yb0

N2

reco=N  yb0

→ b-tagging procedure is not 100% pure → mistag (assign the non-b jet as b-jet) → rate depends on jet ET and η → correction for non bb events are needed → let's denote reconstructed asymmetry as:

To obtain fraction of bb – need to know b fraction of muon and away jets!

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b-fraction on away jet side (Mbb [35, 75] GeV)

→ We fit the data distribution of secondary vertex mass, Mvtx, by MC templates for b and non-b jets: Mvtx > 1 GeV is used to suppress mistags

b template filled part - used for fit non-b template fill part - used for fit

Result of the fit:

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b-fraction on away jet side (Mbb [75, 95] GeV)

→ We fit the data distribution of secondary vertex mass, Mvtx, by MC templates for b and non-b jets: Mvtx > 1 GeV is used to suppress mistags

b template filled part - used for fit non-b template fill part - used for fit

Result of the fit:

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b-fraction on away jet side (Mbb [95, 130] GeV)

→ We fit the data distribution of secondary vertex mass, Mvtx, by MC templates for b and non-b jets: Mvtx > 1 GeV is used to suppress mistags

b template filled part - used for fit non-b template fill part - used for fit

Result of the fit:

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b-fraction on away jet side (Mbb > 130 GeV)

→ We fit the data distribution of secondary vertex mass, Mvtx, by MC templates for b and non-b jets: Mvtx > 1 GeV is used to suppress mistags

b template filled part - used for fit non-b template fill part - used for fit

Result of the fit:

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b-fraction distributions for away jet side

Mbb dependence |∆yb| dependence

We see small decreasing tendency, what is expected due to the b-jet mis-tagging No dependence is observed (as is expected) => distribution is fitted by constant function

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b-fraction on muon jet side (Mbb [35, 75] GeV)

→ We fit the data distribution of muon pT,rel by MC templates for (b→µ) and (b→c→µ + c→µ):

Result of the fit:

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b-fraction on muon jet side (Mbb [75, 95] GeV)

→ We fit the data distribution of muon pT,rel by MC templates for (b→µ) and (b→c→µ + c→µ):

Result of the fit:

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b-fraction on muon jet side (Mbb [95, 130] GeV)

→ We fit the data distribution of muon pT,rel by MC templates for (b→µ) and (b→c→µ + c→µ):

Result of the fit:

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b-fraction on muon jet side (Mbb > 130 GeV)

→ We fit the data distribution of muon pT,rel by MC templates for (b→µ) and (b→c→µ + c→µ):

Result of the fit:

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Corrections of b-fraction on muon jet side

f b pT ,rel1= Af b Af bAf c Af b=f b⋅∫

1 ∞

btempldpT ,rel

∫

∞

btempldpT ,rel Af c=1−f b ⋅∫

1 ∞

ctempldpT ,rel

∫0

∞

ctempldpT ,rel f bMJ , pT ,rel1  f b

trueMJ= f bMJ , pT ,rel1

1−f seccondaries

pT,rel > 1 GeV is used to suppress mistags → need to obtain b-fraction for this cut: → we calculate absolute fractions of b and c for the cut: → to get the true fraction of b-jets on muon jet side we also need to include the cascade decays (b→c→µ + and b→c→µ –) → the fraction of cascade decays is obtained from MC and scaled to PDG2011

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b-fraction distributions for muon jet side

f bMJ , pT ,rel1

|∆yb| dependence Mbb dependence

We see decreasing tendency, what is expected due to the b-jet mis-tagging No dependence is observed (as is expected) => distribution is fitted by constant function

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bb fraction

→ we obtain as follows:

1) if all non-b on muon jet side are paired with non-b on away jet side → we can set upper limit = fb(AJ) 2) if all non-b on muon jet side are paired with b on away jet side → we can set lower limit = fb(AJ) – [ 1 - fb

true(MJ) ]

3) bb-fraction is obtained as the average of the above limits

* MC does not contain certain contributions existing in the data.

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Fraction of secondary decays and mixing

fmix – fraction of B0-B0 mixing events fsecOS – fraction of events with secondary decays ( b→c→µ + ) … obtained from no-mixing events

f secmix=f secOS.1−f mix1−f secOS.f mix

fractions are obtained from MC => scaled to PDG 2011 values

increasing tendency comes from increasing of fsecOS with Mbb, what is caused by muon pT cut No dependence is observed

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fsecmix corrections of AFB

N1

b b=N1 true.1−f secmixN 2 true.f secmix

N2

b b=N 2 true.1−f secmixN 1 true.f secmix

A FB

true=

AFB

b  b

1−2.fsecmix AFB

true=

AFB

reco

f b

b.1−2.fsecmix

→ cascade decays and B0-B0 mixing change sign of the µ → ∆yb has opposite sign => this dilute our calculations → another correction is needed → (For now) let's assume that we have only true bb events:

N1

b  b=N b  b yb0

N2

b  b=N b  b yb0

where Total correction for reconstructed AFB

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Expected sensitivity to bb asymmetry (I)

A FB

true=

1 f b 

b.1−2.fsecmix⋅A FB reco

Mbb dependence |∆yb| dependence

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35

  • 75

X.YZ ± 0.96 (stat) ± 0.05 75

  • 95

X.YZ ± 1.15 (stat) ± 0.11 95

  • 130

X.YZ ± 1.57 (stat) ± 0.1 > 130 X.YZ ± 2.56 (stat) ± 0.68 Mbb (GeV) AFB (in %) (syst) (syst) (syst) (syst) 0.00

  • 0.50 X.YZ

± 0.82 (stat) ± 0.07 0.50

  • 1.00 X.YZ

± 0.99 (stat) ± 0.14 > 1.00 X.YZ ± 1.48 (stat) ± 0.12 |∆yb| AFB (in %) (syst) (syst) (syst)

Mdijet dependence: ∆yb dependence:

Systematics which comes from the obtaining of bb-fraction:

A FB=X.YZ±0.62(stat)±0.10(syst)

Integrated AFB :

Expected sensitivity to bb asymmetry (II)

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Conclusions

  • First analysis of AFB in bb production using CDF data performed
  • Corrections connected with bb fractions, cascade decays, and

B0-B0 mixing are carried out

  • Potential of CDF experiment to see AFB in bb production analyzed

→ The integrated AFB at a level of 2% can be seen with 3σ (if only statistical error is taken into account)

  • Systematics error connected with the bb fraction is evaluated
  • It would be nice to compare our results with updated bb asymmetry

predictions

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Thanks you!