# Presentation: 848510

## Charm Mixing at BaBarRay F. CowanM.I.T.for the BaBar Collaboration

- Charm mixing phenomenology
- Hadronic *D*0 ! *K* decay analysis
- Semileptonic *D*0 ! *K**(*****)**e*** decay analysis
- Lifetime ratio analysis of *D*0 ! *K*+*K*, +  decays
- Summary & outlook

## Charm Mixing Phenomenology

- The *D*0 can decay to a final state *f  *via direct decay or via mixing
- Time evolution is given by an effective Hamiltonian
- The mass eigenstates *D*1, *D*2 are
- 	where

## Charm Mixing Phenomenology

- Mixing can be characterized in terms of the masses m1, m2 and widths 1, 2 of the mass eigenstates
- Mixing occurs if x or y is non-zero
- New physics models and the S.M. predict |x|, |y| < 10-2
- Define the mixing rate RM  ́ (x2+y2)/2 < 10-4

- Survey of mixing predictions

- (From A. Petrov, hep-ph/0311371.
- Open triangles: |*x*|; open squares: |*y*| )

**Notes:**
Box diagram SM charm mixing rate naively expected to be very low (Rmix~10-10) (Datta & Kumbhakar)
 Z.Phys. C27, 515 (1985)
CKM suppression → |VubV*cb|2
GIM suppression → (m2s-m2d)/m2W
Recent SM predictions can accom-modate high mixing rate (Falk et al.)
x,y ≈ sin2 C x [SU(3) breaking]2 ~1%
y: Phys.Rev. D 65, 054034 (2002)
x: hep-ph/0402204 (2004)
Possible enhancements due to new particles and interactions in new physics models:
gluinos, squarks
fourth generation quarks
lepto-quarks, etc.
No CP-violating effects expected in SM – CP violation observation would be unambiguous signal of new physics

## CP Violation and D0 Mixing

*CP* violation can be characterized as one of three types* *
- In mixing:
- In decay:
- In interference between decays with and without mixing:
- Observation of any sizeable *CP *violation would indicate new physics beyond the Standard Model
- Dominant tree-level *D*0decays described by real 2£2 Cabibbo matrix
- Higher-order penguin, other processes suppressed by small *V**cb**V***ub*(no higher mass, down-type quark available)

## CP Violation and D0 Lifetimes

- Mixing affects *D*0 lifetimes for *CP*-even (odd) final states
- If *CP *is conserved, then for *CP*-even final states
- Define effective lifetimes

- If no mixing:

- If no CP violation:

## The BaBar Experiment

- PEP-II is a charm factory: charm cross-section 1⁄4 1.4 nb
- > 1M recon- structed D0! K decays
- Used here:
- 	60–90 fb-1

## K Hadronic Mixing Analysis: Decay Modes

- Uses                         hadronic decays, 57 fb-1
- “Right-sign” (RS)                           Cabibbo-favored (CF)
- “Wrong-sign” (WS)
- Doubly Cabibbo-suppressed (DCS)
- Mixing would also create WS decays
- Assuming *CP *conservation and that |*x|*, |*y|* ¿ 1, the WS decay rate is approximately
*R*D* *is the WS decay rate relative to the RS decays, and we have taken into account any CF to DCS relative strong phase K by defining

## K Hadronic Mixing Analysis: CP Violation

- Accommodate any *CP*-violating new physics by fitting                       the              events separately for *T*WS(*t*)
- Fit for                            where + (-) indicates
- Combine results to form *CP *violation parameters
*A*D (*A*M)  0 implies CP violation in DCS decay (mixing)

## Event Selection

- Select charged tracks of good quality
- Pair oppositely-charged tracks
- Form *D*0 candidates
- Determine production vertex
- Intersect *D*0 direction with beam spot
- Use *D*0 reconstructed from                        decays
- Require CMS *p*(*D**) > 2.6 Gev/*c *to reject *B* events
- Obtain *D*0 proper decay time t and error t from production, decay vertices (typical t 1⁄4 180 fsec)

## mK and m Distributions 

| category | m |  mK | time |
| --- | --- | --- | --- |
|  signal |  peaking |  peaking |  long-lived |
|  combinatoric |  non-peaking |  non-peaking |  prompt & long-lived |
|  wrong slow  |  non-peaking |  peaking |  long-lived |
|  broken orswapped D0 |  peaking |  non-peaking |  long-lived |

## Hadronic Mixing Analysis: Signals and Backgrounds

- Use *m**K***, *m* distributions to distinguish signal, backgrounds
- Characterize & fit for three types of backgrounds in WS events
- True *D*0 decays matched with unassociated pions
- Combinatorics
- Double-misid (*K *and  id swapped)
- Converts RS event $ WS event
- Level from fit consistent with PID performance

- Signal

- Sideband

- Wrong-sign events

|  | Signal |
| --- | --- |
|  | Fake slow s bkg |
|  | Double mis-id bkg |
|  | Combinatoric bkg |

## Hadronic Mixing Analysis Results

- Wrong-sign Events Decay Time Distributions

- Signal region

- Sideband region

- Points: data.
- Histogram: fit.

|  | Signal |
| --- | --- |
|  | Fake slow s bkg |
|  | Double mis-id bkg |
|  | Combinatoric bkg |

## Hadronic Mixing Analysis: Final Results

- Phys. Rev. Lett. 91, 171801 (2003)

| Mixing, with CP
Violation | Mixing, without CP Violation |
| --- | --- |
| No Mixing, with CP Violation | No Mixing, no CP Violation |

- Fits performed for four scenarios

## Semileptonic Mixing Analysis

- Use decay chain
- Uses 87 fb-1
- Only electronic mode (for highest purity sample)
- Pion, electron charges tag production, decay flavors
- Right-sign (unmixed) signal has same-sign *K,* *e *charges
- Wrong-sign (mixed) signal has opposite *K,* *e *charges
- No doubly Cabibbo-suppressed mode to distinguish
- Time dependence of mixing signal (in small *x*,* y* limit)

## Semileptonic Mixing Analysis: Right-sign Signal

- Rmix = NWS/NRS signal events
- Unbinned, maximum-likelihood fits to signal plus backgrounds

**zero life**

- RS
- Lifetime

- RS M

- Neural net for *D*0 reconstruction
- Improves M resolution
- ( » 2.2 MeV/c2)

## Semileptonic Mixing Analysis: Wrong-sign Signal

**random D0**

**random D+**

**zero life**

**D0 sgnl**

**peakng D+**

- Fit Result: 114§61 WS events
- Normalize to #RS events, obtain
*R*mix=0.0023§0.0012(stat.)§0.0004(syst.)
*R*mix<0.0042 (90% C.L.)

- _Full lifetime projection_

- _Lifetime projection_

## Lifetime Ratio Analysis

- Use 91 fb-1 dataset
- Look for specific final states:
- Require *D*0 to be *tagged *via its production mode
- Also use untagged *K*-*K*+ sample

## Lifetime Ratio Analysis: Decay Time Fit

- Perform fit to *D*0 decay time distributions for each sample
- Unbinned, maximum-likelihood
- Independent variables mK, t, t from each event
- Use reconstructed *D*0 mass value to assign probability event is signal (determined in the mass fit)
- Histogram: fit
- Points: data

- Decay Time Fits

**Signal**

**Background**

## Lifetime Ratio Analysis: Results

- Dominant systematics
- Background models, for both *Y* and *Y*
- For the three samples separately
- Combined results

- Phys. Rev. Lett. 91, 121801 (2003)

## Summary

- Three separate charm mixing analyses
- No mixing or *CP *violation observed so far
- But not systematics-limited yet
- May be able to see a signal with more data (500 fb-1 by 2006)
- Observation of *CP* violation would imply new physics
*K*** decay analysis (Phys. Rev. Lett. **91**, 171801 (2003))
- Semileptonic analysis: *R*mix<0.0042 (90% C.L.)
- 	(Phys. Rev. D**70**, 091102 (2004))
- Lifetime ratio analysis (Phys. Rev. Lett. **91**, 121801 (2003))

## Coming in 2006

- Several charm mixing analyses are underway in BaBar
- K updating to 250 fb-1
- K0: new analysis
- Knew analysis
- K(*)eupdating to 250 fb-1
- Results will become available during the next year

## Backup Slides

## K Hadronic Mixing Analysis: 95% C.L. Contours

- Fit allows        to take values < 0
- Strong correlation of
- Employ frequentist method (similar to Feldman-Cousins) to obtain 95% C.L. contours
- Two of four scenarios illustrated here

- Phys. Rev. Lett. **91**, 171801 (2003)