Advances in D* Particle Tagging at ZEUS: A New Method for Impact Parameter Calculation

1. Looking for D*’s at ZEUS First Year Interview Presentation Philip Allfrey

2. This talk describes work done on a new method for tagging charmed particles at ZEUS 1. Impact Parameter Calculation 2. Finding D*’s 3.Using Impact Parameter to improve D* signal

3. Primary vertex Secondary vertex The ZEUS detector upgrade should allow tagging of heavy particles from secondary vertices Silicon Micro Vertex Detector (MVD) installed in ZEUS over 2000-1 shutdown Improves tracking close to interaction point Should be able to resolve particles decaying close to primary vertex These particles will have non-zero impact parameter

4. A method for calculating the impact parameter has been developed Impact Parameter: Signed distance of closest approach of track to primary vertex Numerical method used Primary Vertex Calculate distance from primary vertex to several points on helix Impact Parameter Find point p which gives minimum p Calculate distance for points around p Find minimum and repeat till convergence Track

5. 6 4 2 0 2 4 6 0.6 0.4 0.2 0 0.2 0.4 0.6 Fitting tracks inside the MVD allows impact parameters to be found more precisely With MVD MVD PR No MVD Impact parameter (mm)

6. A sample of charmed mesons was found for testing impact parameter methods Long term: Tag charm production via impact parameter or secondary vertices First steps: Reconstruct charmed mesons from decay products Apply cuts on significance of tracks to see if improves signal Specifically: Choose decay mode D* → D0 ps→ (Kp) ps Use existing D* algorithm (MVD only for pattern recognition) Apply standard cuts on pT, h of tracks

7. 450 400 350 300 250 200 150 100 50 0 0.14 0.17 0.145 0.15 0.155 0.16 0.165 D*’s were reconstructed from the Kppdecay mode D* candidates M(Kpp) – M(Kp) (GeV)

8. 200 175 150 125 100 75 50 25 0.14 0 0.17 0.145 0.15 0.155 0.16 0.165 Background is reduced with a higher pT cut D* candidates M(Kpp) – M(Kp) (GeV)

9. Impact parameter Significance: Error 300 250 200 150 100 50 0 -4 -3 -2 -1 0 1 2 3 4 Significance cuts were applied to D* candidates Gives less weight to poorly determined tracks If secondary vertices, expect excess of +ve significance Try cut: Signif. of K, p > x, signif of ps < x Reduces total numbers Doesn’t improve signal Significance for K in D* peak

10. Multiple scattering may significantly affect impact parameter of ps p ps ps has a momentum cut pT > 0.12 GeV p, K have momentum cut pT > 0.4 GeV Impact parameter and error distributions significantly broader for ps 0 0.5 1.5 1.0 2.0 Impact Parameter Error (cm) May be due to multiple scattering(ps “too slow” ) p ps Cutting on imp. par. or significance of ps may not be a good idea -2 -1 0 1 2 Impact Parameter (cm)

11. All the steps need to be completed before drawing conclusions Using MVD tracking Using MVD pattern recognition only Write Code Impact parameter calculation Write Code + alignment corrections Apply impact parameter cuts Find D* sample Draw Conclusions

12. Summary Method for calculating impact parameter developed D* signal found First impact parameter cuts tried Future Plans Use MVD tracks for all steps Refine impact parameter cuts Tag charm production directly

13. Impact parameter cuts were applied to D* candidates ps comes from D* decay, i.e. (essentially) from primary vertex → zero impact parameter K, p come from D0 decay, i.e. secondary vertex → non-zero impact parameter Try cut: IP of ps less than IP’s of K and p Only reduces total numbers by 90%

14. r-z r-Φ [ Takanori Kohno ] Resolution Includes both errors from hit reconstruction and track parameters.