# Presentation: 124580

- Bill Ashmanskas, U. Chicago,
- Rob Harr, Wayne State
- D0 is an FCNC decay, GIM suppressed in the Standard Model
**B(D0********) ******** 3********10****-13**** in SM,**
**but can be as large as 3.5********10****-6**** in some RPV SUSY models (squarks behave as leptoquarks).**
**Best limit (BEATRICE) is 4.1********10****-6**** @ 90% CL.**
**We think we can meet or surpass that with data in hand.  (How well depends on BG level.)**
**See CDF note 6273**

**D****0**** ******** ******

## Analysis strategy

- Leverage huge SVT fully-reconstructed charm yields
**D0 -> K pi ~ 6 nb**
- with D* tag ~ 2nb
**D0 -> pi pi ~ 210 pb**
- with D* tag ~ 70 pb
- Use SVT-triggered D0 -> K pi, D0->pi pi samples to understand acceptance, backgrounds, signal normalization
**D0->mu mu signal looks like D0->pi pi, with two muon tags, and about a 10 MeV (~1sigma) mass shift**
**Ideally, BG should be dominated by D0->pi pi (BR 1.4E-3), where both pions fake muons**
**~1.4% pi->mu fake rate**
- BG should be equivalent to BR=3E-7
- may be better if we use CMP, or at least use CMP at high momentum (we don’t do that yet)
- BG estimate currently much higher: BR ~ 2E-6
**need to do much better before this could yield a credible discovery; but it should be possible**

## MC spectra (mumu mass)

## Mass offset, K pi tail

## Ingredients

- Number of reconstructed D0->pi pi with both tracks fiducial in CMU (for normalization)
**would improve x1.5 by using CMX**
- Muon ID efficiency
- Expected background
**doubly-mistagged D0->pi pi**
**combinatorial BG (a few real muons possible?)**
- Number of signal events, or an upper limit, based on observed number of events

## Event selection

- Start with hbot0h on CAF1
**40M events, runs 138425-156116, good runs ~63.5 pb****-1**
- defTracks (pT>0.3), numCTHitsAx 25, numCTHitsSt25
- COTXFTSVT match (>5sigma window)
- d*d<0, q*q<0
**No use of offline impact parameters, except in computation of track momenta at intersection!**
- Use CTVMFT to calculate intersection, momenta (no 2 cut)
- 1.5 < m(< 2.05 GeV (use  hypothesis)
-  selects 8.6% of hbot0h, write binary microDST
- Add bachelor pion (use CTVMFT to get momenta)
**no XFT/SVT match, ********m < 0.17 GeV**

## Event selection

- Tighten m cut: 0.144 < m < 0.147 GeV
- At this stage, D*-tagged D0->Kpi yield is
**140K candidates, before good run list**
- Require track pT>2, 120um<=dSVT<=1000um, at least 3 SVXII layers used on each D0 track, 2degrees<dphi<90degrees, Lxy>0
**137K candidates, before good run list**
- After good run list (63.5/pb)
**113K D0->Kpi candidates with D* tag**

## D*: 0.144<dm<0.147

## Good run list

- 138425<=runnumber<=156116
- “status” flags == 1:
**runcontrol, shiftcrew, clc, svx, svt, l1t, l2t, l3t, offline**
- “offline” flags == 1:
**cot, cmu**
- 440 runs, 63.55/pb
**we have at least one D*-tagged D0->Kpi in our ntuple for each of these runs**
- Check: relax svx, svt status; require runnumber<=152625 (typo in note)
**confirm CDF 6288: 458 runs, 39.096/pb**

## Check effective D* xsec vs run

## Normalization signal

- See 4345+-90 D*-tagged D0->pi pi
**1583+-60 after CMU fiducial cuts (x0.36)**
**M(pipi) peak at 1.861, sigma=11MeV**
**M(mumu) peak at 1.851, sigma=11MeV**

## Relative efficiency

- pipi is kinematically  identical to D0
**acceptance effects cancel**
**need relative ********/******** efficiency**
**we steal it from other authors for now**
- CDF 6029 quotes 971%  stub efficiency after isFiducial() requirement
**We can check later using ********+SVT sample**
**loophole: muons too close in CMU**
- avoid by requiring ~4.5 drift cell separation in CMU
- CDF 6114 finds chsqXPosition<9 (a.k.a. MOXFTM) to be >98% efficient over entire 2-10 GeV range relevant to us
- CDF 6018 finds a 4.50.2%  reco inefficiency due to decays-in-flight and hadronic interactions
- We inflate the errors and take (/ to be 1.0120.045

## K->mu, pi->mu BG

- Avg Pi->mu fake rate is ~1.4% (~2.4% for K->mu)
- Thus, expect (naively) 1583*0.84*0.014**2=0.3 BG events
**0.84 comes from 1 sigma shift of 2 sigma window**

## BG handles

- Resonant BG:
**look at double-tagged Kpi events**
**look at single-tagged pipi events**
- Combinatorial BG:
**look at high-mass pipi with 1 muon tag**
**look at high-mass pipi with 2 muon tags**
- Having two handles on each allows us to use one for tuning the cuts and another for estimating the remaining BG

## First realistic BG estimate

- Ignoring sideband subtraction ...
- 39791 Kpi events 1.840-1.885 GeV
**1494 have 1 muon tag (2p(1-p))**
**49 have 2 muon tags (p**2 ?)**
- Naively expect (0.024+0.014)*39791=1512 (OK)
- Expect 0.024*0.014*39791=13 (oops, see 49)
**factor of 3.8+-1.1**
- Now look at single-tagged pipi peak
**crude sideband subtraction-> 38+-10 / 1583 / 2**
- 1.2+-0.3%, consistent with ~1.4%
- High-mass pipi sideband (1.90-2.05 GeV)
**558 events before muon ID**
**47 have 1 tag (47/558/2 = 4.2%) (real muons?)**
**7 have 2 tags (0.042**2*558=1, oops)**
- 3.8*0.3 + 7x44/150 = 3.2 total BG (oops)

## What is to be done?

- Hand-scan of double-tagged Kpi events reveals many cases in which 2 D0 daughters extrapolate very close together in CMU
**we swim tracks (crudely) to CMU, check dphi**
**~half of 2-tag, only 10-15% of 0-tag, have dphi<100mrad at r=347cm**

## Beating down the BG, episode 2

- Combinatorial BG should be less likely to point back at beamline than (primary) D0.  Also, D0 from B decay may have real muons nearby.
- Maybe also more displaced (Lxy)?

## Other cuts?

- Maybe B decay and gluon splitting to ccbar can produce nearby leptons, and also other nearby tracks?
- Try pt(D0) / sumpt(cone of 0.25)
- This is Kpi signal vs pipi high-mass sideband
- Maybe require CMP stub if a track is CMP fiducial and of sufficient momentum to reach CMP?
- Maybe cut on CMU slope match (thanks TJL)?
- NB (Luciano): another source of “correlation” is variance in tag rate: <x**2> = <x>**2 + var(x)
**pT dependence causes largest variance?**

## Backgrounds & cut optimization

- Giovanni Punzi derives figure of merit S/(1.5+B)
**Makes sense: ~S for small B, ~S/********B for large B**
**Can use S ******** N******
**Need to estimate B = expected BG**
- for optimization, we use double-tagged Kpi for resonant BG and single-tagged high-mass pipi sideband for combinatorial BG
**Interesting: isolation cut reduces BG but does not improve FOM (before good run list, it marginally improved FOM)**
- Optimal cuts depend on exposure

## The BG strikes back?

- After tuning, we check double-tagged high-mass sideband to estimate combinatorial BG and single-tagged pipi to estimate resonant BG
- See 5 double-tagged events in 1.90-2.05 GeV window.  Scale down by 44MeV/150MeV
**1.5+-0.7 events (combinatorial)**
- See 1429+-56 untagged pipi events after all cuts, 22+-8 with a single muon tag
**divide by 2 (2p(1-p)), x0.014 fake rate, scale up x2 (observe 21 double-tagged Kpi, expect 12)**
**get 0.3+-0.1 expected resonant BG**
- So total estimated BG is 1.8+-0.7 events
**a bit disappointing, but still an improvement**
**combinatorial BG estimate is based on very low statistics, could be a fluctuation**
**should we look harder before we open the box?**

## Final sensitivity (closed box)

- Fit 1429+-56 events in normalization mode
- Scale by 0.95 for 2sigma mass window
- Scale by 1.012+-0.045 for mumu/pipi efficiency
**yields 1374+-82 events**
- So-called single-event sensitivity is
**1.4E-3 / 1374 = 1.0E-6**
- If no event is found, 90%CL limit will be (ignoring systematics)
**1.4E-3 * 2.3 / 1374 = 2.3E-6**
- But we expect 2 events, so we would get
**1.4E-3 * 5.3 / 1374 = 5.4E-6**
- Bummer?
**Before looking at the signal region, we should be confident in the cut optimization and the figure of merit.  Also should decide a priori how to update cuts for coming months’ data.**

## What’s next?

- More checks possible:
**Ran on hadronic sample with J/psi mass window; can check some muon ID stuff**
**Attempting to get CMU slope info, to see if a cut significantly reduces muon misID**
**Can use “official” CMU extrapolation, probably get a cleaner separation for dphi(CMU)**
- Next week:
**Want to bless some P.R. plots so that Ivan can show our search potential at LaThuile**
- Future directions:
**Let more data roll in, hopefully reach a few E-7**
**Rob wants to do**** ******
**Bill wants to do**** e********ee**