# Presentation: 135705

## Update on BDK fits in control samples

- Hung-Chung Fang
- for the
- LBL *B* group
- 24 Jul 2006

## Where we left off

- Last time (June 26):
- Full likelihood fit with PHOTOS radiative tail and PID on *D**+**X** *sample
- Discussion of combinatorial background (plots from Amanda)
- Today:
*D**+**X*/*D*X** ***control sample*** ***fits** after requiring SVT (with large *d**0*) on track from *B*
- A little more on the combinatorial background (Johannes and Amanda have more detailed studies to come)

## Recap: the D*X sample and baseline cuts

- Cuts optimized for *D**0**X*, but with 0.142< *Δm* <0.147 GeV cut
*L**XY**(B)* signif. > 8
*L**XY**(B**D)* signif. > -2
*Iso**1.0**(B)* > 0.5
- | *d**0**(B)* signif. wrt PV | < 3
- | *z* IP signif. wrt PV |   < 3
- Compared to *D**+**X* sample, *D*X* has
- Lower statistics
- Far less physics background (no *Λ**c* / *B**s* reflections)
- Almost no combinatorial background
- Determine MC to data mass scale
- MC higher than data by factor of 1.00025 +/- 0.00011
- MC to data resolution smearing
- 		6.4 +/- 2.1 MeV in quadrature
- From comparing widths of D*π peak in MC and data

-  3D *B* vertex prob > 10-3
- 	(χ2 scaled by 1/1.7 to obtain flat probability)
* **p**T* > 2 for lone track from *B *(for dE/dx)
-  1.84 < m(*D**0*) < 1.89 GeV

## Recap: the D+X sample and baseline cuts

- Amanda working on *B* lifetimes in partially reco’d modes
- Need input from mass fit  I switched to her cuts last time
- Cuts originally based on Mixing Analysis cuts
- To increase S2/(S+B), some cuts new, some cuts tightened
*p**T* > 2 for lone track from *B *(for *dE/dx*)
*p**T**(B) > 5.5*
*L**XY**(B)* signif. > 8
*L**XY**(B**D)*  > 0
- | *d**0**(B)* | < 60 μm
- | *z* IP signif. wrt PV |   < 3
- Default combinatorial templates:
- mass shape from fit (expo+pol0) on WS
- dE/dx from WS
- MC versus data mass scale from *D*X *
- MC higher than data by factor of 1.00025  0.00011
- MC to data resolution smearing also from *D*X *
- 6.4  2.1 MeV in quadrature

- χ2(*B*)XY  < 10
- χ2(*D)*XY < 15
*Iso**1.0**(B)* > 0.5
*ΔR* < 1.5
- Veto *Λ**c*
- Veto *D** satellite mode

## The SVT requirement 

- Suggestion made by Sandro last time to require SVT match to track
- Easier to compare to Paola’s results
- dE/dx calibrated on an SVT sample
- Require lone track from *B* to have SVT match with large *d**0*
- SVT-offline matching χ2 < 25
- SVT fit χ2 < 15
- 0.012  *d**0*(SVT)  0.1 cm
*p**T*(SVT) > 2.0 GeV	             (*p**T*(offline) > 2.0 GeV already imposed)
- Efficiency of this cut is ~80 % in both *D*X* /*D**+**X,* data and MC

## Mass templates: before & after

- Template statistics after SVT cut shown scaled to that before
- No significant difference – change in shape slight

- before after

## New fits on D*X sample

- New fits on *D*X* sample

- 0h

- 0i

- Z projection for m[5.17,5.26]

- sum    π     K

- mass projection

- 0d

- 0h

- 0i

- 0d

- (summary

- (summary on following pages)

## New fits on D+X sample

- New fits on *D**+**X* sample

- 0h

- 0h

- 0i

- 0i

- Z projection for m[5.17,5.26]

- sum    π  comb  K

- mass projection

- (summary on following pages)

## Fit quality

## D*X fit results

- fit range

- fit range

## D+X fit results

## Summary of fit results

- Fit quality: reasonable χ2 values for
- All samples: 0h/0i *D**+**X** *and 0d/0h/0i *D*X*
- Both *Z* projections in *DK* region and mass projection
- Fits after SVT cut perhaps slightly better (especially in 0d *D*X*)
- In *D*X* fits (very low background):
***D*K***/*D*π** *rel. BR* *high by 0.5 – 2.5 σ  -1.0 – 0.5 σ
*D*ρ/D*π**  *rel. BR* *high by 1.2 – 2.0 σ    0.1 – 1.6 σ
- PHOTOS higher than EvtGen prediction by 40–75%  95–140%
- PHOTOS + *D*K* remains roughly constant at 14–16% *D*π** *
- In *D**+**X* fits (substantial background):
***D******+******K***/*D**+**π**  *rel. BR* *high by   2.0 – 2.6 σ    1.1 – 1.9 σ
*D**+**ρ/D**+**π**   *rel. BR* *high by –0.1 – 0.7 σ    0.0 – 0.7 σ
*D**+*π/D*+*π** *rel. BR* *high by   1.3 – 1.4 σ    1.2 – 1.3 σ
- PHOTOS higher than EvtGen prediction by -25–5%  5–70%
- PHOTOS + *D**+**K* remains roughly constant at 14–16% *D**+**π** *

## Next up: combinatorial background

- (Detailed studies from Johannes and Amanda to come. Please stay tuned.)
- From last time – at least two broad categories of combinatorial background:
**Fake *****D*** combinatorial background
- Still plot mass-constrained *B* mass
- But perform all cuts (incl.  χ2(B) cut) on non-*D*-mass-constrained *B* candidate to recover *D* sidebands
- Use ***D***** sidebands (RS)** to model **fake *****D*** combinatorial backgrounds
**Real *****D*** combinatorial background
- Use ***D***** sideband subtracted**** ****WS** **D****+****X****+**** events**
- 	to model **real *****D*** combinatorial backgrounds

## Fake D* background in D*X

- This is not so important for *D*X *
*Δm* sidebands scaled to background size under *D** signal

- But for the *D**+**X ... *

## Combinatorial background in D+X

- Perform all cuts (incl.χ2(*B*) cut) on non-*D*-mass-constrained *B* candidate
- But still require *m(D)* be in [1.85,1.89]
- Amanda has shown that this does not compromise *m(B)* resolution
- Still plot mass-constrained B mass
- Combinatorial background models
**Fake D:*** D* sideband normalized from *D *mass plot
**Real D:** WS needs sideband subtraction in *D* mass plot (not **yet** performed here!)
- Need combination of above two models!
- Looks like it will reduce *DK* and PR modes substantially

- (from Amanda)

## Conclusions

- Performed* D**+**X*/*D*X** *fits after requiring SVT (with large *d**0*) on track from *B*
- Clear tradeoff between PHOTOS and *DK* components in both modes
- Fit now prefers PHOTOS **1-2x **compared to EvtGen prediction
- SVT cut decreases *DK* component – dE/dx effect due to XFT hit requirements?
- In *D*X* fits (very low background):
***D*K***/*D*π** *rel. BR* *now **even more consistent** with PDG (-1.0 – 0.5 σ )
*D*ρ/D*π**  *rel. BR is now becoming consistent with PDG
- In *D**+**X* fits (substantial background):
***D******+******K***/*D**+**π**  *rel. BR* *now* *high only by 1.1 – 1.9 σ (1σ better than before)
*D**+**ρ/D**+**π**  & **D**+*π/D*+*π** *rel. BR* *largely unchanged
- Combinatorial background model requires
**	****	****Fake** *D*: *D* sidebands (RS)
- 		**Real** *D*:* D*-sideband-subtracted WS D+X+ events

## Backup slides

## Combinatorial background (I)

- What is in the combinatorial background?
- Is *D* meson real or fake?
- Does the track come from bbar or ccbar or something else?
- How we’ve tried to model the combinatorial:
- Extrapolate high sideband (in RS) down to our signal region ***OR***
- Use wrong sign *D*-track combination as proxy      baseline choice
- (Cut hard to get rid of as much as we can)
- How *reliable* are these methods?
- One of the most difficult questions to answer in these analyses
- Even if RS/WS agree in the high sideband, is this still true
- 	in our signal region?
- Lifetime analyses using partially-reco’d modes
- Also confronted with these issues (see previous talks of Jeff Miles and Amanda)

## Combinatorial background (II)

- Petar’s comments from last week
*D* Δm* S/B = 11:1
- 	 *D*X* combinatorial level low
- Does this mean that most combinatorial is fake *D*?
- Is WS a good model?  What about *D* sidebands?

- D*X

*Δm*

## Combinatorial background (III)

- Amanda’s lifetime fit with partially-reco’d modes:
- 	sensitive to both size (shape) and lifetime composition of combinatorial background
- Show some of her plots relevant to this analysis (see also her talk tomorrow)
- Low *D* signal in RS/WS high sideband
- Combinatorial dominated by fake *D !*

- (from Amanda)

*m(D)*

## Combinatorial background (IV)

- We mass constrain the *D* in the final *B* vertex fit
- χ2 cut on a *D*-mass-constrained *B* fit sculpts even raw D mass distribution
- Tantamount to a ‘fancy D mass cut’
- Remove *D* mass constraint (or alternatively, χ2 cut) to recover *D* sidebands
- Can still cut on *non-D-mass-constrained* χ2(B)  to guarantee good vertex

- (from Amanda)

## Combinatorial background (V)

- Perform all cuts (incl.  χ2(B) cut) on non-D-mass-constrained *B* candidate
- Still plot mass-constrained B mass
- Scale WS and D sideband to high sideband
- Looks like it will reduce *DK* and PR modes substantially
- But this is not the whole story ...

- (from Amanda)

## Combinatorial background (VI)

- At least two possible normalizations:
- 1.   norm. to _*B*__ upper sidebands_
- overshoots in high tail of
- 	Dπ peak
- 2.   norm. in _*D*__ mass plot_
- undershoots upper sideband by 40% !
- Probably need 2. **+**
- 	extra shape from track+real *D*
- 	to saturate upper sideband, i.e.,
- 	combination of fake and real *D*’s
- We’re still looking into this!
- In any case, new background shape will
- Cause high tail of Dπ to be better described
- Produce fit with lower DK and PR mode BRs

**normalization ****1.**

**normalization ****2.**