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B->hh stripping selection update

Angelo Carbone

CP measurements WG: gamma with loops

19th March 2009

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Stripping: where we are A pre-selection algorithm has been tuned

based on the online selection Minimum bias rate 22±2 Hz after L0 trigger Minimum bias rate 5±1 Hz after L0 and HLT1

trigger Assumed 1MHz L0 outputpre-sel/

(Offline& L0) %

pre-sel/(Offline&L0&

HLT1) %

Bd+- 98±1 99±1

BdK+- 98±1 98±1

Bs+K- 98±2 98±2

BsK+K- 98±1 98±1

b->p- 97±2 97±3

b->pK- 97±2 98±2

pre-sel cut values

Mass wind. [5-5.8] GeV/c2

min PT() > 1 GeV/c

max PT() > 3 GeV/c

min IP() > 0.06 mm

max IP() > 0.20 mm

2 com vert.

< 8

DF(B) > 1.8 mm

IP(B) < 0.1 mm

HLT1 vs offline selection HLT1 rejects minimum bias events better

then offline stripping, why? HLT1 efficiencies as function of cuts have

been investigated Used Bd->signal events

HLT1 (all lines) selects 77% of offline signal selected events passed L0 trigger 85% of them are from DiHadron lines Let’s investigate this line

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HLT1 DiHadron algorithm From Jose’s private e-mail, DiHadron algorithm

selects: a track with IP>0.1 mm PT>2.5 GeV and associated

to a L0 hadron cluster with ET>3.5 GeV… … that forms a secondary vertex with a second track

if DOCA<0.2 mm the second track should have an IP>0.1 mm and

PT>1 GeV The vertex mast be separated from the PV in z, with Z(SV-PV)>1.5 mm A pointing cut

the transverse momentum of the B candidate in the plane perpendicular to the flight B direction given by the SV and the PV divided by the sum of the pt of the tracks < 0.4

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HLT1 investigation

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Invariant mass

HL

T1D

iHad

ron

eff

i.

mm

IP B

DOCA

mm

HLT1 efficiency = (offline sel. & L0 &&

HLT1DiHadronDecision)/ (offline sel. & L0)

Flat distributions: no differences between DiHadron

line and offline selection

GeV

HL

T1D

iHad

ron

eff

i.H

LT

1DiH

adro

n e

ffi.

Distance of flight

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• DiHadron The vertex separated from the PV in z, Z(SV-PV)>1.5 mm•Offline selected and L0 signal distribution start at 1mm

• Slightly inefficiency due to different cut value• The cut Z(SV-PV)>1.5 mm seems to be smeared by online

resolution

mmmm

Signal offline selected and L0

HL

T1D

iHad

ron

eff

i.

Co

un

ts

IP

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IP min

mm

IP max

mm

Flat efficiency

• HLT1 apply a cut IP>0.1mm to both tracks•Signal offline selected events start at 0.06 mm

mm

Signal offline selected and L0

HL

T1D

iHad

ron

eff

i.

HL

T1D

iHad

ron

eff

i.

Co

un

ts

IP min

Transverse momentum

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PT min

GeV

PT max

GeV

•Offline selection cut•pT min > 1 GeV•pT max > 3 GeV

•pT max lower efficiency between 3-5 GeV•This is due to HLT1 DiHadron L0 confirmation and a cut ET>3,5 GeV on the associated tracks to L0 hadron cluster. •pT min shows a slightly inefficiency around 2 GeV: to be investigated, any idea?

HL

T1D

iHad

ron

eff

i.

HL

T1D

iHad

ron

eff

i.

Update on stripping selection Towards a unified stripping selection for B-

>hh and Bs-> channels relax the pT cut use DOCA instead of Chi2 on secondary vertex

Reject ghosts using chi2 of tracks Move the lower limit of invariant mass to

4.8 GeV in order to better study B3 body bkg and include Bpp

Tight some cuts, i.e. IP of pions and B IP on B cut (IP<0.1) was inherited from online

selection, but it can be tighter thanks to better offline resolution

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Example of stripping cuts

IPB<0.06 and Chi2(track)<5 will ensure high signal efficiency

Chi2 of tracks IP B

chi2

Arb

itra

ry u

nit

s

mm

soft pre-selected mbias event

Offline sel. & L0 signal

ghosts

Stripping selection

pre-sel cut Values (1) Values (2)*changed from (1)

Mass wind. [5-5.8] GeV/c2 * [4.8-5.8] GeV/c2

min PT() > 1 GeV/c * 0.7 GeV/c

max PT() > 3 GeV/c * 2.4 GeV/c

min IP() > 0.06 mm * 0.08 mm

max IP() > 0.20 mm 0.20 mm

2 com vert.

/DOCA < 8 * 0.08

DF(B) > 1.8 mm 1.8 mm

IP(B) < 0.1 mm * 0.06 mm

2/DOF of track < - 5

Stripping selectionpre-sel cut Signal[%]

(1)Signal[%] (2) mbias [%]

(1)mbias [%]

(2)

min PT() 100 100 50.5 79.5

max PT() 100 100 42.2 66.8

min IP() 99.9 98.9 52.9 33.4

max IP() 99.2 99.2 48.7 56.3

2 com vert /DOCA.

100 99.7 67.4 42.5

DF(B) 98.6 98.9 63.8 58.8

IP(B) 99.9 99.7 20.9 8.3

Mass wind. 100 100 51 64.6

2/DOF + 100 99.1 100 79.7

2/DOF - 100 98.8 100 84.3

Tot. 98% 95% Rate: 22Hz Rate: 8HzEfficiency are relative to the previous step

Minimum bias starting sample is done with a soft pre-selection

Results

Ghost rate means at least one of the two tracks is a not associated using the Link associator in DaVinci

Signal eff: pre-selected events over offline selected and passed L0 (&HLT1)trigger

Mbias rate calculated assuming an ouput of 1MHz after L0

(1) (1)&HLT1 (2) (2)&HLT1

Sig. eff. [%] 98 76 95 74

M bias rate[Hz] 22 5 8 3

Ghost rate [%] 73 63 56 33

Conclusion (I) The HLT1 signal efficiency has been

investigated in order to understand the reason of higher power of HLT1 to reject mbias events with respect to pre-selection With respect to offline selection HLT1 Di Hadron

line has a high IP min cut on tracks a high pT max cut (i.e. ET>3.5 GeV) high distance of flight cut

The stripping selection algorithm has been update in order to match Bs analysis requirements and to reduce minimum bias rate Rate reduced from 22Hz to 8Hz with signal eff.

moving from 98% to 95% Ghost rate reduced from 73% to 56%

Conclusion (II) Applying the HLT1 there still a reduction of

minimum bias rate from 8Hz to 3Hz with a reduction from 56% to 33% of ghost rate Pre-selection selects 43 events and 24 are

ghosts 19 events are not ghosts applying HLT1 we got 15 events and 5 ghosts

10 events are not ghosts

There are still room for improvement Reducing and understand high ghost rate after

pre-selection suggestion?