K0s Reconstruction at ZEUS

Before and after the upgrade

Mark Bell

Introduction

HERA was upgraded in 2001 to deliver greater luminosity. ZEUS used the opportunity to install a Micro Vertex Detector (MVD).

After the upgrade: How has the performance of the tracking and vertexing changed? What impact has the MVD had?

To study this, use K0s:

Produced copiously at HERA. Easy to detect through π+ π- decay,using Central Tracking Detector (CTD)and MVD.

K0s

π+

π-

Tracking packages

Before the upgrade, CTD was the only major tracking component. Now MVD used as well. There are different software track finding packages to use different detector components.

• CTD: uses CTD only to find tracks and vertices using standard ZEUS VCTRAK package.

• REG: uses MVD as well as CTD using VCTRAK.

• ZTT: uses MVD and CTD but uses Kalman filtering to find tracks. This claims to fit tracks more accurately and hence should find more K0

s.

Event selection

Sample of data taken from before the upgrade (22.8pb-1), and one from after (6.3pb-1). Old data normalized to that of new data for results.

Data taken through a third level trigger for DIS and also requiring |zvtx| < 60 cm.

K0s → π+ π- found by looking for secondary vertex with two oppositely charged

tracks.

Make kinematic cuts on pion tracks and other variables in the event:1) |zvtx| < 50 cm – rejects events not from interaction region.

2) cos(α) > 0.99 – ensures K0s comes from primary vertex.

3) Pion tracks: pT > 0.1 GeV and reach at least SL3 – gives well measured tracks.

4) M(e+e-) > 0.1 GeV – eliminates photon conversion electrons.5) M(pπ) < 1.125 GeV – eliminates Λ’s.6) s < 35cm – reduces background from interactions with CTD.

K0s mass

peak

Half as many K0s in new

data using CTD tracking.

Width of signal wider innew data.

Yield of K0s recovered

when using MVD in REGtracking.

ZTT tracking fails to findas many K0

s.

Invariant K0s mass after selection cuts

CTD(2000)

REG ZTT

CTD(2003/4)

MVD effective close to interaction point

MVD finds K0s secondary

vertices closer to the interaction point than the CTD alone.

All extra K0s in new data are

found closer to the interaction point.

Distance from primary to secondary vertex

ZTTfailure

ZTT tracking comparable to REG tracking when few tracks in an event.

Finds fewer K0s as N

increases.

Problem identified to be an error in vertexing routine.

Has now been put right.

Distance from primary to secondary vertexN = total number of tracks in event

Conclusions

K0s found in old and new data – tracking and vertexing compared.

• Multiple scattering in the extra material of the MVD reduces the yield of K0

s when only the CTD is used for tracking and vertexing.

• Using the MVD as well as the CTD recovers the same yield of K0s.

• Using Kalman filtered tracks fails to find as many K0s because of an

error in the vertex finding algorithm.

• The MVD is successful in finding secondary vertices closer to the interaction point.

MVD enhances overall tracking and vertexing performance of the detector.

Future work

• Reanalyse the new data with the fixed ZTT vertexing routine to see if it is as or more effective than REG tracking for finding K0

s.

• The tools for tracking, vertexing and particle identification can be used in many areas of data analysis, opening up different avenues for potential research.