Recent Physics Topics from ZEUS

45th PRC meeting ZEUS collaboration/Y. Yamazaki 1

Recent Physics Topics from ZEUS

45th PRC meeting8 July 1998

Yuji Yamazaki (KEK/IPNS-Tanashi)

On behalf of ZEUS collaboration

Outlook:

Low-Q2/Low-x phenomenology of F2

Charm production in DIS

Charged particle production Multijets in photoproduction

High-Q2 neutral current and limits on contact interactions Charged current

Not covering topics on diffraction due to the short time


Low-Q2/low-x F2 phenomenology

F2 : observation at HERA/ZEUS

• Q2 >> QCD2 (ZEUS nominal vertex)

Steep rise of F2

• 0.1 < Q2 < 0.7 GeV2 (ZEUS BPC) The rise becomes slower, closer to Regge model

• Intermediate region: 1 < Q2 < 4 GeV2 (ZEUS shifted vertex) The slope start to decrease towards low Q2

- F2 behavior and QCD description in transition region

Title:(f2svtx95_dl_zeusfit.eps)Creator:Adobe Illustrator(TM) 5.5Preview:

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08.02

08.0

)1(2

tot*

~

~

)(

xF

W

W

Wpp

e.g. Donnache- Landshoff


… NLO QCD parameterization can fit the behavior

Question:

• How is the parton densities (x) and g(x) derived from the QCD fits ?

• Can we believe the NLO at low Q2 down to ~ 1GeV2 ?

Systematic study of slope in F2 xeff

dlnF2 / dln(1/x)|Q2 = eff : rise of F2, for each Q2

reflecting sea quark xx

dlnF2 / dln(Q2)|x : scaling violation for each x

driven by splitting

~ ,

reflecting gluon density

)(xgPqg qqg

• Fit with ZEUS + E665 data for wider arms on x• compared with

– GRV 94 parameterization both q(x) and g(x) valence like at starting scale Q0


d lnF2/d ln(1/x) = eff : how steep is the rise in x

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Observation:• At Q2 ~ 0 : slow rise• higher eff as Q2 higher

Comparison with models in intermediate Q2 region:

• GRV : tend to have large eff — slower rise of F2 in data

• NLO can fit wide range in Q2


F2 vs. Q2 : scaling violationTitle:(f2vsq2.eps)Creator:Adobe Illustrator(TM) 5.5Preview:



• Normal behavior of scaling violation down to x ~ 10-4

– decrease at high-x and rise at low-x

• However, slopes less steeper again at very low-x

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• “Turnover” observed in scaling violation around x ~ 10-4

sea quark production rate smaller in small x and Q2)(xgPqg


Parton densities x(x) and xg(x) from NLO fit

• The observed behavior of F2:

– modest rise of F2 at low-Q2

– small scaling violation slope at low-x/low-Q2

can be explained by valence-like gluon density (assuming NLO fit to be valid at low-Q2)


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— Of course under debate (large s)

Example: NLO is flexible enough to explain very low Q2 regionbut with large uncertainties prediction power not strong at low Q2

Interesting indication from HERA data, but not conclusive

Need inputs from other process, F2cc and FL

Is NLO valid in low-Q2 (~ 1GeV) range ?


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Charm production in DIS Gluon may be valence like at low-Q2 Any other process for more information ?

(Almost) direct measurement: Charm production

Measuring charm cross section

(i.e. )pin down the gluon densityvia D* production

)(2 xF cc

Comparison with NLO using charm density from GRV

• Good agreement overall with NLO• Theory uncertainty mainly from mc (1.3-1.7 GeV)

• High-(D*) ( ~ low-x(D*) ) — small excess

e +e +

(q )

g

p

c

c


Title:/data/33c/alex/talks/figures/f2z9697h194emcgrv1.3-1Creator:HIGZ Version 1.23/09Preview:



… and the extracted )(2 xF cc

• Good agreement with NLO (GRV94)

• Steep rise to low-x down to Q2 ~ 2.5 GeV2

but rise slower at lowest Q2 bin – even lower Q2 behavior ?


Title:/automount/data/talisker/uk/jane/95/fort/ln1xpevo.cCreator:HIGZ Version 1.23/07Preview:



Particle production in DIS

• In the fragmentation universal ? compare with e+e using Breit frame

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• 1994-1997 data precise test on universality

• MLLA fit describes the data

• consistent with e+e

Multiplicity distribution and fragmentation function

Current Targetp T

p L

-Q/2

Q/2

(1- x )Q/2 x

*(-Q)e

Evolution on ln(1/xp)


Title:/export/area18/data/jane/data+old2.colour.epsCreator:HIGZ Version 1.23/07Preview:



Title:/tmp_mnt/area18/data/jane/data+ee2.colour.epsCreator:HIGZ Version 1.23/07Preview:



• High-x : negative scaling violation (cf. High-x F2)

reflecting parton branching like in parton density

• Shapes agree with e+e , but 30% lower

• Using the same parameterization as e+ein fragmentation + NLO (CYCLOPS)

Good agreement with data

Universality in fragmentation

Fragmentation functions and Q2 evolution


Multijet production in photoproduction

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Jets in photoproduction: hard parton-parton collision (like pp scattering)

33345

234512/2/1

,cos

)()()

M

dPSMxfxfdxdxjjjABd BBAABA

3jet mass Jet angles

3-Jet mass distribution M345 > 50 GeVTitle:/export/area23/data/strcklnd/mjanal/anal/combi/tplo (Portrait A 4)Creator:HIGZ Version 1.23/07Preview:



　　　　　 calculations(LO for this process)reproducing theshape and the amplitude of the cross section

)( 2sO


Angular distribution: proving matrix elementsTitle:3body.epsCreator:fig2dev Version 3.2 Patchlevel 0-beta3Preview:



Title:/export/area23/data/strcklnd/mjanal/anal/combi/tplo (Portrait A 4)Creator:HIGZ Version 1.23/07Preview:



• cos3 : spin of primary exchanging partons (q, g )

• 3 : ~ 0 or radiated jet is close to the plane of beam and the highest energy jet

Title:/export/area23/data/strcklnd/mjanal/anal/combi/tplo (Portrait A 4)Creator:HIGZ Version 1.23/07Preview:



Angular distributionsnot described by phase space

described by MC orcalculations

)2( sO

Need colour coherence inQCD for describing the observed distribution


High-Q2 events (NC and CC) 1994-97 results reported in DESY seminar

Events before/after LP97 conference

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Neutral current:

No high-Q2 events( > 20000 GeV2)after LP97

Charged current:Two high-Q2 events( > 20000 GeV2)after LP97

Q 2min Nobs Nexp

10000 6615000 2035000 2

1460 217

02.029.0

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Q 2min Nobs Nexp

10000 22

15000 8

30000 1

7.52.517

9.16.19.3

08.004.006.0


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Charged current cross section 2/ dQd CC

• Good agreement with standard model prediction up to Q2 ~ 10000 GeV2

• Cross section tend to be high in Q2 > 10000 GeV2

but large uncertainty is large in this region

• e+p CC constrains d-quark density


Title:/automount/oxdata9/uk/quadt/highq2/xsec97/mac/xsec_ (Portrait A 4)Creator:HIGZ Version 1.22/09Preview:



Neutral current cross section 2/ dQd NC

• Good agreement with standard model prediction up to Q2 ~ 10000 GeV2 except two very high-Q2 events above 35000 GeV2

• Precision sufficient to constrain PDFs, and …


Contact interaction analysis - using NC data

Contact interaction Lagrangean :

Enormous numbers of combinations, but …

• Vector term only

• Constrains from atomic parity violation

• SU(2) symmetry implies: u-quark only term also tested (sensitive at HERA)

24 combinations of (L,R)(u,d)(+,)

0 qRR

qRL

qLR

qLL

diL

uiL

Title:moriond.dviCreator:dvipsk 5.78 p1.4 Copyright 1996-98 ASCII Corp.([email protected])Preview:




Limits on contact interaction termsTitle:/afs/desy.de/user/d/davidw/slug3/arnulf/foureta.epsCreator:HIGZ Version 1.23/09Preview:



• Comparable or higher than LEP and Tevatron limits

Title:moriond.dviCreator:dvipsk 5.78 p1.4 Copyright 1996-98 ASCII Corp.([email protected])Preview:



• Four examples

on fits


Summary of high-Q2 DIS cross section

Title:xsecs.ps (Portrait A 4)Creator:HIGZ Version 1.23/07Preview:



• This year: ep operation

Comprehensive studies on EW, PDFs (u/d densities etc.)


Summary

• Filling the gap between soft and hard scale in Q2

– intermediate region: NLO fit suggesting gluon density valence like ? • Extending the kinematical region utilizing high statistics runs in 1997

– F2cc : high statistics D*, gluon density ?

– Fragmentation : extended to high-x, similar to e+e

– NC/CC cross section: start to constrain PDFs Sensitivity to contact interaction term Electroweak sector

• Extending phase space in final state

– Multijets in photoproduction: QCD matrix element test See also ICHEP contributions.

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