Looking for D*’s at ZEUS

First Year Interview Presentation

Philip Allfrey

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

The ZEUS detector upgrade should allow tagging of heavy particles from secondary vertices

Primary vertex

Secondary vertex

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

A method for calculating the impact parameter has been developed

Numerical method used

Impact Parameter:

Signed distance of closest approach of track to primary vertex

Calculate distance from primary vertex to several points on helix

Find point p which gives minimum

Calculate distance for points around p

Find minimum and repeat till convergence

Track

Primary Vertex

Impact Parameter

p

Fitting tracks inside the MVD allows impact parameters to be found more precisely

With MVD

No MVD

MVD PR

Impact parameter (mm)

6 4 2 0 2 4 60.6 0.4 0.2 0 0.2 0.4 0.6

Choose decay mode D* → D0 s → (K) s

Use existing D* algorithm (MVD only for pattern recognition)

Apply standard cuts on pT, of tracks

First steps:

Long term:

Reconstruct charmed mesons from decay products

Apply cuts on significance of tracks to see if improves signal

A sample of charmed mesons was found for testing impact parameter methods

Tag charm production via impact parameter or secondary vertices

Specifically:

0.14 0.145 0.15 0.155 0.16 0.165 0.17

450

400

350

300

250

200

150

100

50

0

D*’s were reconstructed from the K decay mode

M(K) – M(K) (GeV)

D* candidates

0.14 0.145 0.15 0.155 0.16 0.165 0.17

200

175

150

125

100

75

50

0

25

Background is reduced with a higher pT cut

M(K) – M(K) (GeV)

D* candidates

Significance cuts were applied to D* candidates

If secondary vertices, expect excess of +ve significance

Gives less weight to poorly determined tracks

Significance:Impact parameter

Error

Signif. of K, > x, signif of s < x

Try cut:

Reduces total numbers

Doesn’t improve signal -4 -3 -2 -1 0 1 2 3 4

300

250

200

150

100

50

0

Significance for K in D* peak

Multiple scattering may significantly affect impact parameter of s

s has a momentum cut pT > 0.12 GeV

, K have momentum cut pT > 0.4 GeV

Impact parameter and error distributions significantly broader for s

Cutting on imp. par. or significance of s may not be a good idea

s

0 0.5 1.0 1.5 2.0Impact Parameter Error (cm)

May be due to multiple scattering(s “too slow” )

s

-2 -1 0 1 2Impact Parameter (cm)

Write Code

Impact parameter calculation

Find D* sample

Write Code

+ alignment corrections

Using MVD tracking

Using MVD pattern recognition only

Apply impact parameter cuts

Draw Conclusions

All the steps need to be completed before drawing conclusions

Method for calculating impact parameter developed

D* signal found

First impact parameter cuts tried

Summary

Future Plans

Use MVD tracks for all steps

Tag charm production directly

Refine impact parameter cuts

Write Code

Impact parameter calculation

Find D* sample

Write Code

+ alignment corrections

Using MVD tracking

Apply impact parameter cuts

Draw Conclusions

Write Code

Impact parameter calculation

Find D* sample

Write Code

+ alignment corrections

Using MVD tracking

Apply impact parameter cuts

Draw Conclusions

Impact parameter cuts were applied to D* candidates

s comes from D* decay, i.e. (essentially) from primary vertex → zero impact parameter

K, come from D0 decay, i.e. secondary vertex → non-zero impact parameter

Try cut:

IP of s less than IP’s of K and

Only reduces total numbers by 90%

ResolutionResolution

47 μm

64 μmrz

r

r-z r-Φ

1.5 mm

325 μmrz

r

Includes both errors from hit reconstruction and track parameters.

[ Takanori Kohno ]