Ricardo Goncalo ZEUS Week - London June 2002 1

Neutral Current 99/00 e+p

Analysis Status

Ken Long

Matthias Moritz

Henning Scnurbusch

Ricardo Gonçalo

Thanks to Matthias for most plots in the

talk

ZEUS Collaboration Meeting

London - June 26th 2002

Outline

Analysis overview

Sinistra/EM comparison

Comparison of analyses

Our own y bump

Conclusions and outlook

Ricardo Goncalo ZEUS Week - London June 2002 2

The 1999/00 y bump – Matthias, Ricardo

Yes, we have a y-bump … but it’s different from the y-bump in 1996/97:

1996/97: low y, medium Q2

1999/00: Q2>5000 GeV2, y>0.65

@ Q2>5000 GeV2, y>0.65: 206 events in data / 158.4 in MC

Ricardo Goncalo ZEUS Week - London June 2002 3

The 1999/00 y bump

First seen by Xiang (EXO) and confirmed by all NC analyses

Seen with both EM and Sinistra

Seen with DA, JB and electron methods

Seen with both Corandcut and cells

No time (run) or dependence

Events were scanned and look OK

No clear spike in a distribution that could explain the bump

Migration? Q2 distribution OK but x and y shifted. There is a deficit at lower y…

Q2>5000GeV2 x>0.65

Ricardo Goncalo ZEUS Week - London June 2002 4

The 1999/00 y bumpQ

2>

5000G

eV

2;

y>

0.6

5

In bump region positrons are detected both in the forward BCAL and in the FCAL

Many events in the bump have a well measured positron track

Q2>5000GeV2

Q2>5000GeV2

X>0.65

e+ track not in CTD / in CTD acceptance

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The 1999/00 y bump

Monte Carlo

Data

Ee/EDA shows a shift in the FCAL: this may just be caused by energy loss in dead material – not a problem with DA method

On the side of the hadronic final state: bump events seem to have H ~90deg kinematics or detector effect?

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The 1999/00 y bump1

2

3

G

eV

/ev

We tried looking at the hadronic energy flow in Q2>5000GeV2 events

Migration of H to the centre of BCAL (H ~90deg) ?

But PThad/PT

DA versus H looks OK and bump still there with electron method cannot be the only cause

Ricardo Goncalo ZEUS Week - London June 2002 7

The 1999/00 y bump

B-F supercrack

Tried looking at the fraction of MC positrons in accepted events as a function of Z at the exit radius of CTD and as a function of FCAL radius. No obvious dip in efficiency except for the supercrack MC looks OK

Ricardo Goncalo ZEUS Week - London June 2002 8

The 1999/00 y bump

In summary:

Before anything else, we must see if the bump is a detector effect

There must be more than one effect: both the scattered positron and the hadronic final state are affected

But it may also be something else…

Can this be caused by the PDFs in the Monte Carlo?

Must check HERACLES

Reweight to ZEUS-only PDFs?

Many possibilities have been tried, but the work is still in progress …

Ricardo Goncalo ZEUS Week - London June 2002 9

Analysis overview

Since last Collaboration Meeting:

Systematic uncertainties were fixed (in connection with 98/99 NC and CC papers):

Hadronic and electron energy scales

Hadronisation (using MEPS)

Photoproduction normalisation & systematic uncertainty

Systematic variation of selection cuts

New cuts were added (in connection with 98/99 NC paper):

MC validity: yJB.(1-xDA)2 < 0.004 - excludes low W resonance region (equivalent to W > 20GeV cut)

Projection of hadronic system on FCAL (calculated with H and Zvtx)

must be at a radius R > 20 cm – “ cut”– excludes low y region affected by cross-talk from FPC

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Positron identification:

Sinistra (option 5): P>0.9

EM: P>0.001 (P>0.01 outside fwd CTD acceptance)

Electron energy: Ee > 10GeV

Isolation: Econe < 5GeV

If positron is in CTD acceptance:

Matched positron track with Ptrk>5GeV

Track to CAL cluster DCA < 10cm

Outside of forward CTD acceptance:

Transverse momentum: Pt,e = Ee sine > 30GeV

Event selection and reconstruction

45cm

45cm

Vertex-dependent CTD acceptance

Ricardo Goncalo ZEUS Week - London June 2002 11

Background rejection:

38 (44 in fwd) GeV < E-PZ < 65 GeV

ye < 0.95

Balance of transverse momentum: PT/ET < 4 GeV1/2

Acceptance cuts:

Positron Z pos. in BCAL (super crack): -98.5 cm < Z < 164 cm

Electron in RCAL: R < 175 cm

Chimney in RCAL: | x | < 12cm, y > 90cm

FCAL projection of hadronic system (H,Zvtx): R > 20 cm (“gamma cut”)

Event selection and reconstruction

Other cuts:

Zvxt close to IP: -50 cm < Zvtx < 50 cm

MC validity: yJB.(1-xDA)2 < 0.004

Kinematic variables reconstruction:

Double Angle method

Hadronic angle reconstruction:

CorandcutNew cuts

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Analysis results

Rejected due to cut on hadronic system projection on FCAL (gamma cut)

d/dxdQ2

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Analysis results: d/dxdQ2

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Systematic errors for d/dQ2

2%

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Systematic errors for d/dxdQ2 - Henning

Not exactly our final systematic uncertainties yet: no gamma max or FCAL island splitting, different hadronic energy scale, no gamma cut

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Comparison: Matthias, Henning,Ricardo

d/dQ2•Much work has been done in comparing the analyses, and the agreement is already very reasonable

•Some differences are still there, but the work is advancing fast

• Cuts & corrections have evolved fast, recently.

Different HAC scale corrections for MM (A.Tapper), than for RG & HS (C.Cormack)

CTD acceptance cuts still different in points shown

• Latest results will be compared soon

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Sinistra/EM comparison - Matthias

We intend to use EM for final event selection

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Sinistra/EM comparisonPh

otop

rodu

ctio

n

EM better suited for high-Q2 / Sinistra is more efficient, but takes more PhP

For most events the same candidate is picked by EM and Sinistra

For some events the electron cell list or the candidate are different migrations

Ricardo Goncalo ZEUS Week - London June 2002 19

Sinistra/EM comparisonC

uts

faile

d b

y E

M

candid

ate

s

candidate failed only this cut

Several cuts rejected event

Cuts fa

iled b

y S

inistra

ca

ndid

ate

s

Ricardo Goncalo ZEUS Week - London June 2002 20

Conclusions and outlook

Much effort done in comparing the three analyses. They are in good agreement and still converging.

The EM / Sinistra comparison is in a very good shape.

A lot of work has been done in understanding systematics. This has been useful also for other analyses.

Y-bump should be our last major problem and must be understood. Much work has been going into this and we expect to solve this one soon.