# Presentation: 567833

## Slide 1

## A Study of Two Electron Taggers

- Lashkar Kashif
- Harvard Univ HEPL
- Group talk, August 9, 2004

## Outline

- Two opposite-side electron taggers from the *B* semileptonic group:
- i.	likelihood-based (in great detail)
- cut-based (in brief)
- (cut-based muon tagger: one slide)
- For each, will discuss
- 	- electron ID (or cut) variables used
- 	- Methods of background estimation and maximization of sample purity
- 	- Tagger performance evaluation

- Lashkar Kashif, Group talk, 08/09/04

## The B semileptonic group’s likelihood-based electron tagger

- Tags the flavor (*b* or *b* content) of a *B* hadron at production
- Opposite side tagger: identifies electron from decay of the *B* hadron produced at collision together with the *B* hadron of interest
- Charge of electron correlated with *B* hadron flavor, eg,  *b **c l*****
- Signal sample: lepton+SVT
- Likelihood, pTrel and *d0sign* used to identify electrons from direct *b* decays and to minimize background

- Lashkar Kashif, Group talk, 08/09/04

## Backgrounds and ID variables

- Three major sources of background:
- 	- hadrons faking electrons
- 	- electrons from sequential *b* decays
- 	- conversion electrons
- Nine variables used for electron ID, investigated and parametrized using:
- 	- real conversion electrons: from 4 GeV and 8 GeV single electron triggers
- 	- pions from *K*s decays for fakes

- Lashkar Kashif, Group talk, 08/09/04

## Description of electron ID variables

- HadE**/**EmE: ratio of 2-tower HAD energy to 2-tower EM energy (fig. 1)
- EmE**/**p: Ratio of 2-tower EM energy to 3-momentum of track (fig. 2)
- CES x2 : Comparison of shower profile in CES wire view (*r-***) with profile from test-beam. Scaled by (p/10) so that comparison is valid at energies <10 GeV, where
- exp(-p/15.0) – p/1000
- 4.	CES z2 : Comparison of shower profile in CES strip (*z*) view with profile from test-beam. Scaled similarly as x2.

- Lashkar Kashif, Group talk, 08/09/04

## CES qx/x: CES x is the distance in transverse plane b/w measured cluster position and extrapolated track. It is signed with the charge of the track (fig. 4)

6.	CES z/z: Distance in the r-z plane b/w measured cluster position and extrapolated track. 

7.	ECES/p*: ECES is 7-wire cluster pulse height measured in the CES; p* = 10(p/10)

Corrected QCPR: Pulse height in CPR, corrected for sin dependence. (Tracks at small sin interact with more material and produce secondary particles that deposit additional charge.)

dE/dx: Energy loss measured in the COT.

- 6.	CES z/z: Distance in the *r-z* plane b/w measured cluster position and extrapolated track.
- 7.	ECES**/**p*: ECES is 7-wire cluster pulse height measured in the CES; p* = 10(p/10)
- Corrected QCPR: Pulse height in CPR, corrected for sin** dependence. (Tracks at small sin** interact with more material and produce secondary particles that deposit additional charge.)
- d*E***/**dx: Energy loss measured in the COT.

- Lashkar Kashif, Group talk, 08/09/04

## Fig. 1: HadE/EmE for electrons (black) and pions (red) for isolated (top) and non-isolated (bottom) candidates. Left: 2 < pT < 4 GeV, right: pT > 4 GeV.

## Fig. 2: EmE/p for electrons (black) and pions (red) for isolated (top) and non-isolated (bottom) candidates. Left: 2 < pT < 4 GeV, right: pT > 4 GeV.

## Fig. 3: CES x for electrons (black) and pions (red) for isolated (top) and non-isolated (bottom) candidates. Left: 2 < pT < 4 GeV, right: pT > 4 GeV.Note the second bump for pions in all plots. What is it?

## Fig. 4: CES qx/x for electrons (black) and pions (red) for isolated (top) and non-isolated (bottom) candidates. Left: 2 < pT < 4 GeV, right: pT > 4 GeV.

## Fig. 5: ECES/p* for electrons (black) and pions (red) for isolated (top) and non-isolated (bottom) candidates. Left: 2 < pT < 4 GeV, right: pT > 4 GeV.

## Fig. 6: Corrected QCPR for electrons (black) and pions (red) for isolated (top) and non-isolated (bottom) candidates. Left: 2 < pT < 4 GeV, right: pT > 4 GeV.

## Fig. 7: The mean (left) and RMS (right) of the dE/dx pull Z/ for e+ (black) and  (red).  is the width of the dE/dx distribution,  and Z is defined as Z = log [(dE/dx)meas/(dE/dx)hyp],where (dE/dx)hyp is the expected energy loss for the electron hypothesis.

## Definition of likelihood

- Distributions of ID variables parametrized for both real electrons and fakes, and the fitting functions treated as PDFs
- Probability *S* that an electron object is a real electron is the product of nine signal PDFs; similarly for probability *B* of its being a fake
- Define likelihood as *L = S/*(*S *+ *B*)
*L ~* 1 for real electrons, *L ~* 0 for fakes (fig. 8)
- Likelihood distinguishes real electrons from fakes; pTrel discriminates direct *b* decay electrons from sequential ones

- Lashkar Kashif, Group talk, 08/09/04

## Fig. 8: Likelihood distributions in muon+SVT (top) and electron+SVT (bottom) samples.

## Use of d0sign to identify conversions

- But there are still a lot of conversion electrons
- Define *d0sign* = corrected d0 * track charge
- Conversion electrons have positive *d0sign*, since the photon points back to the primary vertex (fig. 9, *left*)
- Electrons from heavy flavors are symmetric in *d0sign* (fig. 9, *right*)
- There is still a noticeable contamination from “unseen” conversions (fig. 10, *right*)
- Dilution and *D*2 binned in d0sign as well as likelihood and pTrel

- Lashkar Kashif, Group talk, 08/09/04

## Fig. 9: Electrons from conversions (left) and from B decays (right). Conversion electron have predominantly positive d0sign, while heavy flavor decay electrons are symmetric in this quantity.

## Fig. 10: Distribution of d0sign for (a) “seen” conversions and (b) opposite-side electron candidates with L > 0.85 in muon+SVT (red) and electron+SVT (blue).

## Tagger performance evaluation

- Evaluate tagger performance by counting how many *B* mesons are correctly flavor-tagged
- To do this, need high-purity trigger-side *b* sample
- But trigger sample has backgrounds from semileptonic *c* decays, hadrons faking trigger lepton, and so on
- Penn group’s background subtraction procedure allows estimation of tagger’s performance in a sample of pure *b* events
- Enter (SVT), the signed impact parameter of the SVT track (not related to *d0sign*)

- Lashkar Kashif, Group talk, 08/09/04

## , the signed impact parameter

- Definition (note 6480): The signed impact parameter of a track has the magnitude of the impact parameter, but is signed according to whether the track crosses a reference vector in front, or behind, the primary vertex (fig. 11):
- |*d0*|*sgn*(**x.v**)
- In our case, the reference vector **v** is the vector sum of the trigger lepton and SVT track momenta
**x** is the direction from IP to point of closest approach of SVT track
- Then, (SVT)|*d0*|*sgn*(***d0.p***lep+SVT)

- Lashkar Kashif, Group talk, 08/09/04

## Fig. 11: Event topology leading to positive signed impact parameter (left), and negative signed impact parameter (right).

## Using (SVT) for background subtraction

- For most background components, is symmetric about zero, since the reference vector and the track are not correlated
- For *b* decays, the lepton and SVT tracks come from a common parent   has a large positive asymmetry due to momentum conservation (fig. 12)
- For any distribution, subtracting the component with (SVT)from that with (SVT)gives a distribution representative of a pure *b* sample
- Use this neat background subtraction in estimating tagger efficiency and dilution

- Lashkar Kashif, Group talk, 08/09/04

## Fig. 12: Signed impact parameter of the SVT track in e+SVT sample.

## Efficiency and dilution

- Define efficiency and raw dilution as
-       {NRS(NRS({NWS(NWS(NTOT(NTOT(
-          {NRS(NRS({NWS(NWS(*D**raw*{NRS(NRS({NWS(NWS(
*D**raw** *must be corrected for trigger-side *B* mixing and sequential decays  done using MC
- Find *D**trig** *= 0.6412, and then *D* = *D**raw **/ D**trig*

## The tagging power

- Dilution has dependence on pTrel as well as likelihood and *d0sign*
*D, *and the tagging power *D*2, are binned in all three variables, with six bins in *L, *seven in pTrel and two in *d0sign*
- In an analysis, can predict dilution for each event
- *D*2 reported as:
- 	 0.3135% in the muon+SVT sample
- 	 0.4186% in the electron+SVT sample
- The tagger has been blessed

- Lashkar Kashif, Group talk, 08/09/04

## The B group’s cut-based electron tagger

- Same physics goals, background, and study samples as for the likelihood-based tagger
- Cut variables are *HadE/EmE*, *EmE/p*, *E*CES/*p**, *CES ****x, CES ****z, ****x**2**, ****z**2**, CPR ****x, CPR*(p), and CES**S **/CES**W*
*CPR*(p) = *(0.001*CPR)**/**(*p*/10.0)0.3
*CES**S**/ CES**W *: Ratio of the strip pulse height to the wire pulse height. The authors claim that for soft *e*-s brem losses lead to S/W > 1, while for pions the wire view gets a large PH due to overlap with other photons in the event such that S/W << 1 (fig. 13)

- Lashkar Kashif, Group talk, 08/09/04

## Fig. 13: CES S/W PH for electrons (yellow) and pions (blue). Fig. 14: Efficiency variation with CES S/W cut value for electrons (red) and pions (blue).

## Cut values and cut efficiencies

- Table 1: Cut values that are 90% efficient for electrons. Corresponding pion efficiencies and *e*/ ratios are also shown.
- cut# tracks passing cut **/** # tracks fiducial in CES+CPR

## Tag sample selection and tagging cuts

- Conversion removal cuts:
- 	|cot| < 0.05, |XY| < 0.1
- Require that invariant mass of  tag *e*- and *l+*SVT 4-momentum > 5 GeV to ensure that they don’t come from same *b* quark
- If more than one tag *e**-** *in event, take the one with maximum CPR pulse height.
-   Table 2: Tagging cut values used

## Tagger performance and tagging power

- Dilution, tagger efficiency (*not* cut) and *D*2evaluated using (SVT)-based background subtraction, and binned in six pTrel bins for three sub-samples:
- 	Sample 1: Tracks passing all cuts with 3 or more silicon *r-****  *hits.
- 	Sample 2: Same as sample 1, with non-isolated *e*- tracks only
- 	Sample 3: Isolated *e*- tracks passing all cuts, but not fiducial in SVX
- *D*2 = 0.366%
- Tagger not brought to blessing since Tania left CDF

## Comparison b/w L-based and cut-based e- taggers

| Likelihood-based tagger | Cut-based tagger |
| --- | --- |
| Uses dE/dx | Does not |
| Does not use CES S/W (but will in a future version, a la Manfred) | Uses CES S/W |
| Signs CES x with track charge (different distributions for positive and negative tracks?) | Does not  |
| Divides CES x and CES z with scaling functions x and z | Does not |
| Uses d0sign to identify conversions | Does not |

## The B group’s cut-based muon tagger

*Not* a mature tagger
- Cut variables not tuned owing to poor sample statistics
- At this point, variables used are:
*	**- *For CMU stubs, |z| < 70 cm
- 	- For BMU stubs, |z| < (5 + 30/pT) cm
- 	- Candidates that are fiducial in CMP, and have a CMU stub, and pT > 3 GeV, must have a CMP stub
- Muons passing these criteria are ‘high quality’, rest are ‘low quality’
- Performance evaluation as before, efficiency and dilution binned in six pTrel bins and for the different muon systems

## Conclusion

- Correlations between lepton ID/cut variables not studied, but claimed to be small, at least in the case of the electron ID variables
- For electron tagging, I prefer a likelihood-based tagger: each cut used by a cut-based tagger may be ~90% efficient, but there are ~10 of them
- The CMU group is going to ‘generalize’ its likelihood-based *e*- tagger such that it can be used, e.g., by the top group (according to* *Manfred). Time scale: by the end of August.

- Lashkar Kashif, Group talk, 08/09/04