Radiative b-decays with ATLAS

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HAL Id: in2p3-00020369

http://hal.in2p3.fr/in2p3-00020369

Submitted on 29 Jan 2004

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Radiative b-decays with ATLAS

F. Ohlsson-Malek, S. Viret

To cite this version:

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1 Œ Motivation

• Generation, simulation, reconstruction

Ž Analysis, results, and comments

• Conclusion & perspective

F. Ohlsson-Malek, LPSC Grenoble

on behalf of

S. Viret, LPSC Grenoble

Athens Workshop - B-physics session

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Radiative b-decays in ATLAS Œ Motivation Physical interest 1. A laboratory for QCD b s,d W

-γ

u,c,t

•New constraints on CKM matrix parameters (V_ts, V_td) could be obtained: x R x Φ Br(B → K*_γ ₎ Br(B → ργ )

=

|Vtd| |V_ts| 2 2

Form factors ratio

Phase space factor

•CP asymmetries in B→ ργ decays, isospin violation in B→ K*_{γ & B}_{→ ργ decays.}

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3

Radiative

b-decays in ATLAS

Œ

Motivation

Physical interest 2. New Physics influences

b s,d

γ

?

Modifications largerly model dependent New diagrams appearing (charged Higgs, chargino, squark

contributions)

Physical observables (branching ratios, CP asymmetries) modified

Useful constraints on new physics parameter space

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4

Radiative

Œ

Motivation

How to study B _{→ Xγ with ATLAS?}

Œ Inclusive measurements:

→ Involve a very precise knowledge of the ‘non-B’ background (using, e.g., off-resonance data).

→ Cannot be achieved (using classic methods) at LHC.

• Semi-inclusive measurements:

→ An hadronic system with a single kaon and few pions is searched, then B meson is reconstructed. This method leads with a good accuracy to the photon spectrum.

→ Quite difficult without an efficient K/π identification.

Ž Exclusive measurements:

→ Direct reconstruction of principal decays (B→ K*_{γ , B}_{→ ρ γ, …} _).

→ Certainly the more accessible to ATLAS

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5

Radiative

•

Generation, simulation,...

Event sample production

•Generation:

(Athena+PythiaBmodule+Model (

for signal

))

•5.0.0 release, (Pythia 6.2) :

30k B_d → K*0 γ

4 events

50k bb → µ6 X events

•Simulation:

(Atlsim)

•6.0.2 release, initial layout :

16.5k B_d → K*0 γ

4 events

50k bb → µ6 X events

•Reconstruction:

(Athena)

•6.0.3 release, initial layout : 16.5k B_d → K*0 _γ

4 events

Background coming soon…

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6

Radiative

Ž

Analysis

Strategy of the analysis

•Level 1:

(Muon trigger)

µ₆ selection: a 85% efficiency is assumed (muon reconstruction coming soon…)

•Level 2:

γ selection in the calorimeter: γ/π0_{isolation, shower shape, p} t cut

•Offline cuts:

K*0 reconstruction in the ID:_{Combination of pairs of “K}+_π-_{” tracks (Vertex fit),} and K*0_{invariant mass reconstruction. Cuts on tracks p}

t, on impact parameter, Fit likelihood,...

B_d invariant mass reconstruction: Cut on p_t, and invariant mass.

•Refined cuts:

Cuts on angular distributions, γ/K*0_{center of mass momentum,...}

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7

Radiative

Ž

Analysis

γ selection

γ cluster candidate is selected if :

Cluster Et > 4 GeV

0.5 strips ≤ cluster width ≤ 3.5 strips

Energy leakage in φ direction ≤ 9 % Energy leakage in η direction ≤ 9 %

Energy proportion of strips 2nd_maximum_{≤ 6%}

2nd_{maximum physical meaning}_{≤ 1.5%}

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Athens Workshop - B-physics session Radiative b-decays in ATLAS Ž Analysis K*0_{reconstruction} • π- _{selection :} •Negative charge •|η|< 2.5 •P_t> 0.8 GeV Œ K+ _{selection :} •Positive charge •|η|< 2.5 •P_t> 1.3 GeV

Vertexing of the 2 tracks. K*0 _{selected if :}

χ2 < 6 & L

tr(K*0) > 250µm

K*0_peak

2_{σ mass cut (σ = 90 MeV)}

8

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Radiative

Ž

Analysis

•Reconstruction efficiency with

off-cuts (in 3_σ mass window) :

ε(True B_d) = 10%

•Signal purity (in 3_σ mass window): True B_d

Selected B_d = 60% True B_d

Selected B_d

•For each event, B_dcandidate with best mass is selected.

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10

Radiative

Ž

Analysis

Refined offline cuts

K

*0

_{candidate is selected if :}

0.2 GeV < P* K+< 0.35 GeV 0.2 GeV < P* π- < 0.35 GeV Impact(K+_{)x Impact(}_π-_{) > 0} Angle between K*0 _p

t & Bd transverse length < 40°

B

_d

candidate is selected if :

1.8 GeV < P*

γ < 3.5 GeV

1.8 GeV < P*

K*0< 3.5 GeV

Minimal distance bet. γ trajectory & B_d decay vertex < 9cm

P* = center of mass momentum

Œ

•

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Athens Workshop - B-physics session Radiative b-decays in ATLAS B_d final reconstruction Signal only Ž Analysis

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•Reconstruction efficiency with all

the cuts:

ε(True B_d) = 3.85%

•Signal purity (in 4_{σ mass window):}

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Radiative

•

Conclusion & perspective

Summary & conclusion

12 Offline

80 MeV

8.5%

*

143k

**

55x10

6

_***

60%

σ(B_d) ε_reco(B_d)* N(Signal)/y N(Back)/y*** Signal purity

Improved

60 MeV

3.3%

*

55.5k

**

Not enough stat.

93%

* The L1 µ6 efficiency is included.

** L_inst= 2x1033_cm-2_s-1

*** Old results with release 2.4.1 and weaker cuts.

PRELIMINARY

S.VIRET

S

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Radiative

•

Conclusion & perspective

Perspective

13 Just preliminary results

→ still lot of work to do:

Œ

Process the physics background with initial layout

•

Perform cuts on K*0_{angular distributions}

Ž

Vertexing routine (Athena patch for displaced vertex, real magnetic field,...)

•

Look at Bd → ργ decay (CKM matrix constraints)

•

Test the feasibility of a L1 calorimeter guided trigger

(

γ

?

)

‘

High-luminosity feasibility

’

...

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Radiative

Is the reconstructed B_d true or fake ?

Calculated p_t’s are compared to real p_t’s (Using KINE)

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Radiative

γ selection 1. Shower shape Œ Total cluster width (over 20 strips)

LVL2 cut: 0.5 strips ≤ width ≤ 3.5 strips

• Shower shape in φ & η direction (ECAL 2nd_sampling)

LVL2 cut: ≤ 9 %

Œ

•

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Radiative

Search for a 2nd_{maximum in the strips:}

• γ: no 2nd_max

• π0_{: 2}nd_{max close to the 1}st_one

Œ Transverse energy of the 2nd_{max on}

total transverse energy in the strips LVL2 cut: ≤ 6%

• Physical meaning of the 2nd_{max (real}

max or 1st_{max artefact). Look at the}

minimum energy between the two max and compare it to 2nd_{max energy}

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Athens Workshop - B-physics session Radiative b-decays in ATLAS Ž Analysis K*0_{reconstruction}

Mass window determination

True B_d Selected B_d

S.VIRET

K*0_{candidates giving a} selected B_d are displayed

A 2_σ mass window cut on

M_K*0 is sufficient for correct

signal reconstruction

We then re-reconstruct the B_d using this mass cut

A raw K*0 _{reconstruction is} performed (no cuts)

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Athens Workshop - B-physics session Radiative b-decays in ATLAS Ž Analysis Refined cuts K*0_{angular distributions} TRUTH

d

Γ(K

*0

_→K

+

_π

-

₎

dcos(

_{θ )}

~ sin²(

θ )

C.M. P*_K K*0_direction θ P*_π

Still not used

(Small asymetry should be understood (vertexing ?))

Flat distribution

for combinatorial Interesting cut

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Athens Workshop - B-physics session Radiative b-decays in ATLAS Refined cuts Mass cuts (1/2) Ž Analysis P_Bd P_γ P_K*0 Lab. C.M. P*_K*0 P*_γ

|P

* K*0

|=|P

*γ

|=

2.56 GeV

|P

* K+

|=|P

*π-

|=

0.29 GeV

•In C.M.S. we have :

Cuts

on p

*

Natural

mass cuts

Use C.M.S. cuts instead of mass

windows classic cuts

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Radiative

Refined cuts

Minimal distance between B_d vertex and γ trajectory

Ž

Analysis

•No information on γ in the ID

Vertexing routine couldn’t be used • K*0_{time of flight very short}

K*0_{decay vertex = B}

d decay vertex

• γ trajectory should contain this vertex • Minimal distance between γ trajectory & B-decay vertex is calculated (L_min). •B_dcandidate is selected if:

L

_min

< 9 cm

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Radiative

N

_s

=

2

x

σ

_prod(signal)

x

ε

_{reconstruction}(signal)

x Br

(

B_d→ K*0₍→_Kπ)γ

)

x L

inst

N

_b

=

σ

_prod(background)

x

ε

_{reconstruction}(background)

x L

_inst

Don’t forget B_d→ K*0 γ !

σ

prod(signal)

=

0.15 µ

barn

σ

_prod(background)

=

2.89 µ

barn

Br

(

B_d→ K*0₍→_K_π)γ )

≈

4.44 x 10

-5

L

_inst

=

2x10

33

_cm

-2

_s

-1 N_s = 0.053 ε_reco(signal) N_b = 5800 _ε_reco(background) (from Pythia) S.VIRET

Event rates calculation

σprod should be confirmed