Charm Semileptonic Decays New Results from FOCUS Angel M. López

1. Charm Semileptonic Decays New Results from FOCUS Angel M. López University of Puerto Rico (Mayaguez) Representing FOCUS • Outline • Interference in D+ Kpmn • BR D+ K*mn/K2p • BR Ds+ fmn/fp • BEACH2002 • June 27, 2002 - Vancouver

2. New results on D+ Kpmn WS-subtracted Our Kp spectrum (like everyone else’s) looks like 100% K*(890). This has been “known” for about 20 years. Data Fit charm bkg RS-WS events Right Sign Wrong Sign 21,370events backgrounds are pretty small MKp (GeV/c2) But a funny thing happened when we tried to measure the form factor ratios by fitting the angular distributions ...

3. Five observables are studied A 4-body decay requires 5 kinematic variables: Three angles and two masses. MKp MW2 q2 t left-handed m+ right-handed m+ Two amplitude sums over W polarization (“mass terms”) Wigner D-matrices H0(q2), H+(q2), H-(q2) are helicity-basis form factors computable by LGT

4. An unexpected asymmetry in the K* decay Yield 31,254 dataMC We noticed a forward-backward asymmetry in cosqV below the K* pole, but almost none above the pole. HUGEasymmetry! Could it be QM interference?

5. Simplest approach — Try an interfering spin-0 amplitude (plus mass terms) A exp(id) will produce3 interference terms We simply add a new constant amplitude : A exp(id) in the place where the K* couples to a helicity=0 W+ with amplitude H0.

6. Since A << B, interference will dominate.. Ignoring the “mass terms”, If we average over acoplanarity we only get the first term • This is the term that created our forward-backward asymmetry! • If our model is right: • The asymmetry will have a particular mass dependence: • The asymmetry should be proportional to sin2ql • The asymmetry should have a q2 dependence given by q2 H02(q2)

7. Studies of the acoplanarity-averaged interference Extract this interference term by weighting data by cosqV. Since all other c-averaged terms in the decay intensity are constant or cos2qv , they do not contribute. We begin with the mass dependence: Efficiency correction is small Our weighted mass distribution.. ..looks just like the calculation.. A=0 A constant 450 phase works great... ...but a broad resonance is fine as well. 0.36 exp(ip/4)

8. Dependence of asymmetry on cosql • We plot the asymmetry versus cos ql and expect a parabola in cos2 ql since sin2 ql = (1 - cos2 ql) A=0 0.36 exp(ip/4) Looks  - (1 - cos2 ql). Some modulation due to efficiency and resolution

9. q2 dependence of asymmetry Below the pole A=0 0.36 exp(ip/4)

10. Acoplanarity dependent interference terms The interference adds two new terms to the acoplanarity dependence. Without s-wave interference, the acoplanarity terms are even in c:Only cos cand cos 2cdependencies are present The interference producessin cterms which breakc to -csymmetry

11. The loss of c to -c symmetry is seen in the data. Same sign (incorrect) convention used for D+ and D- Can be mistaken for CP violation! Opposite sign (correct) convention used for D+ versus D- When sign of c is handled properly, the D+ and D- acoplanarity distributions become consistent.

12. The correct acoplanarity convention counter clockwise about  The sine of the acoplanarity is a product of five vectors. Under CP, D+ => D-. We want c to have the same value for CP transformed states but the formula above changes sign when all 5 vectors reverse. Thus we must take the opposite sign for D+ versus D- decays.

13. The model is not a unique solution. A broad resonant amplitude also works well. We can mimic the cosV dependence for a constant amplitude using a BW put in with a relatively real phase.For example use a wide width (400 MeV) and center it above the K* pole (1.1 GeV). cos qV term

14. But surely an effect this large must have been observed before? Although the interference significantly distorts the decay intensity.... ...the interference is nearly invisible in the Kp mass plot.

15. New results on D+ K*ln/ K2p branching ratio FOCUS total signal — RS— WS With tighter cuts

16. This rate is very much an open experimental and theoretical question. The recent CLEO num-bers are higher than previous measurements. G(K*l n)/G(Kpp)

17. Fitted Mass Histograms for the FOCUS DataSamples with baseline + out-of-material + secondary vertex isolation cuts. K*mn after subtracting wrong-sign and charm background (from MC). Backgrounds are well understood! Forthe Kpp normalization channel it is sufficient to fit a Gaussian peak over a polynomial background.

18. TheFOCUS Result Weighted average We multiply muon results by 1.05 to compare to electron results. Our number is 1.59 standard deviations below CLEO and 2.1 standard deviationsabove E691. G(K*l n)/G(Kpp) muons electrons This result takes into consideration the interference phenomenon which affects the reconstruction efficiency.

19. New FOCUS Ds+ fmn/fp Branching Ratio — data— charm background MC Once we demand a decay out of the target segments, the backgrounds are matched by our Monte Carlo. This is a “c,cbar” MC exclud-ing events that contain a fmn decay.

20. Comparison of Results • FOCUS value is 0.54 0.033 0.048. • There is very good agreement among experiments.

21. Summary (1) S-wave interference inD+ Kpmnof form The new amplitude is small:7% of BW peak amplitude in the H0 part. 6% of all Kpmn over the full Kp range • (2) D+ K*mn/K2p Branching Ratio • New FOCUS value is 0.602  0.010  0.021 (1.57s lower than CLEO) • (3) Ds+fmn/fp Branching Ratio • New FOCUS value is 0.54 0.033 0.048

22. Many interesting FOCUS results are on the way (1) rv and r2form factor measurements for K*mn and fmn (2)f(q2) measurement forD0  Kmn (3) Cabibbo suppressed ratios: D+ rmn/K*mn & D0 pmn/Kmn There are probably still many surprises (such as the interference in Kpmn) to be discovered in charm and beauty physics even in places where we least expect it.