jets and the hadronic final state at zeus thomas schörner-sadenius hamburg university heidelberg 23...

JETS AND THEHADRONIC FINAL STATE AT ZEUS

Thomas Schörner-SadeniusHamburg University

Heidelberg23 January 2006

http://www.desy.de/html/home/index.html

Heidelberg, 23 January 2006 TSS: Jets at ZEUS 2

OVERVIEW

¶ INTRODUCTION

¶ BASIC JET PHYSICS AT ZEUS

¶ MORE DETAILED STUDIES

¶ CONCLUSIONS AND OUTLOOK

I will not cover specific resonances, strange particles, pentaquarks, baryon production properties etc.


OVERVIEW

¶ INTRODUCTION • DESY, HERA and the ZEUS Experiment • Current Data Taking • (Jet) Physics: Introduction and Motivation





DESY, HERA UND ZEUSThe only ep collider world-wide!


THE ZEUS EXPERIMENT … still going strong, even with new components!

But a lot of care-taking necessary to keep it going stably.

Microvertex tracker MVD installed 2001.

Starting to profit from it.

6m tagger starts tocontribute to:

- lumi measurement- FL determination

Freshly equipped FDET (STT):

Inclusive,heavy flavour, HFS measurements?


EVENTS IN ZEUS… our event display


URANIUM-SCINTILLATOR CALORIMETERCompensating, good hadronic energy resolution

An FCAL modulewith 23 towers with

12 PMTs each.

23 (32) modulesin F/RCAL (BCAL).

Signal generationvia shower develop-ment in scintillator

and uranium.

- ~12000 channels (H1: ~45000)

- About 300 dead/disabled (now 200).- E/E~0.18/E for electrons (H1: 0.12)

- E/E~0.35/E for hadrons (H1: 0.50)


HERA PERFORMANCEHERA II programme is a success!

- Roughly increase of 5 in integrated luminosity – upgrade aim reached.- now about 150pb-1 on tape from HERA II.

ZEUS overall efficiency is about 75%; ranging from 40-90%.


HERA PERFORMANCEApproaching 5*1031cm-2s-1!!


ZEUS SUFFERS FROM BACKGROUNDS Threat to central tracker, problem for data taking

Typical fill situationwith proton beam spikes (dark green) and central

chamber trips (red).

Number of CTD tripsversus day of running.

HERA not always successful in providing good and stable running conditions.


BASICS OF ep SCATTERING AT HERA … in lowest order

,Z,W

k Lepton (e±)

ProtonP

k’(e±,)

p=xP

Q2=-q2=-(k-k’)2

y=1-E’/E: Inelasticity

Q2: resolution ~1/Q

For a given s only two independent variables: Q2 = s·x·y

x=Q2/2Pq: protonmomentum fraction

Ee = 27.5 GeV

Ep = 920 GeV

s = 4EeEp ~ 318 GeV


JET PHYSICS: BASICS IA variety of interesting processes

Leading-order QCD, O(s).

NLO QCD, O(s2)

Suppressed with increasing Q2

s

s

QPM

QCDCBGF

Real/virtualcorrections

“Resolved”

`boring’

Direct access to coupling and gluon

Interesting!

Difficult!


JET PHYSICS: MOTIVATIONpQCD, PDF universality, measurements of PDFs, S

2

1 ,,

),,/(ˆ),()(m gqqa

frBjfarms xf

Jetcross-

sections

• Access to strong coupling.

• Access to PDFs and test of their universality.

• Tests of factorisation in soft and hard parts of the inter- action.

• Test of pQCD predictions.

• Is jet physics independent of type of exchanged boson?

Manyoptions

… in pQCD assuming collinear factorisation:


JET PHYSICS: BASICS IIAlgorithms, reference frames, theory, uncertainties, EFOs

Jetalgos /

correction

• Almost exclusively longitudinally invariant kT in the inclusive mode (in Breit frame for DIS).• Applied to energy flow objects or calorimeter objects. Current uncertainty on hadronic energy scale: 1-3%

• Correction for dead material etc. in individual analysis.

Theory• DGLAP-based NLO calculations for 1/2/3 jets in DIS (2 jets in photoproduction).• Data correction for detector/QED/hadronisation effects done with LO MC programs. Consistency?• Looking forward to NLO+PS programs (MC@NLO).

Uncertain-ties

• Mostly now dominated by theoretical uncertainties. - Only NLO theory estimate effect by varying renormalization scale. - PDF uncertainties not negligible. Uncertainty on coupling?

• jet energy scale 1-3% 10-15% in Xsection.• Statistics is not the critical item. Progress in systematics?


OVERVIEW

¶ INTRODUCTION

¶ BASIC JET PHYSICS AT ZEUS (HERA I) • Tests of perturbative QCD with jets • Determinations of the strong coupling with jets • Contributions to PDF determinations from jets




INCLUSIVE JET CROSS-SECTIONS (DIS)`Simple’ measurement – take PDFs/S as given

Tests – factorisation– pQCD, – PDF universality

Selection – ~80pb-1 from 98-00

– 125 < Q2 < 5000 GeV2

– ET > 8 GeV (Breit)


INCLUSIVE JET CROSS-SECTIONS (DIS)Also double-differentially, comparison to NLO


DI / TRIJETS IN DISTesting higher orders in pQCD

Selection – ~80pb-1 from 98-00– ET > 5 GeV (Breit)– 10 < Q2 < 5000 GeV2

– Mjj > 25 GeV

In ratio of dijets to trijets someuncertainties cancel out(lumi, partly energy scales, etc.)

SSS

SS

DD

TT

dijets

trijets

2

21

32

21

)(

)(

Nice possibility to extract S!


DIJETS IN PHOTOPRODUCTIONIn principle access to photon PDF.

Selection: – 39pb-1 from 1996/97.– ET > 14 (11) GeV – separation power between different photon PDFs!Highest sensitivity at low ET subtraction of underlying event – but great model dependence!

Compare to NLO calculationsand exploit sensitivity to structure via x (gluon!)

Aim:

Problem:

21

21

2,1,

2,1,

2

1

2

1

eEeEE

x

eEeEyE

x

TTp

p

TTe

We need a dedicated study of the underlying event at HERA!


EXTRACTION OF S

Typically done using interpolation

Dependence of cross-section on s(MZ) in each bin from NLO pQCD with different input s(MZ) values.for example with MRST or CTEQ4 (3/5 different s values, 0.110 to 0.122).

Use function to map measured cross-section to value of s(MZ).

Functional dependence on s(MZ) then approximated by

-- Ai, Bi determined in fit.

ZsiZsiZsi MBMAM 2

Input:

Parame-trisation:

Result:


S FROM INCLUSIVE, 2, 3 JETSTheory starting to dominate measurement

World average: s(MZ) = 0.11870.0020H1 inclusive jets: s(MZ) = 0.11970.0016(exp)0.0047(theo)

ZEUS inclusive jets: s(MZ) = 0.11960.0025(exp)0.0023(theo)ZEUS R3/2: s(MZ) = 0.11790.0013(stat)

0.0037(exp)0.0055(theo)


SUMMARY ON HERA S (C. Clasman)

HERA : S(MZ)=0.1186±0.0011(exp.)±0.0050(th.) (only NLO)Bethke: S(MZ)=0.1182±0.0027 (all NNLO)

Nice demon-strating of running

in HERA data.

Consistency ofall results is an

important check!


ACCESS TO PDFs WITH JETS Hope: Improve gx(x,Q2) at high values of x

s

s

Uncertainties are very large: – 15% at x=0.3, – 200% at x=0.5.

Problem Gluon density xg(x) at high xbasically unknown – constrainedonly by momentum sum rules and Tevatron jets with large ET.

Idea Jet data provide access to high x gluon via BGF process.

Problems … exploiting this feature: … Many …

QCDC BGF


ACCESS TO PDFs WITH JETS Reminder: Thesis M. Wobisch (2000), xg(x) from jets


– from integration of PDF and hard scattering matrix element– to multiplication of constant PDF and tabulated and summation over all bins of x and f. 0.01s for NLO !!!!!

• Divide phase-space in small x-f bins.• Remove `constant’ PDF bit from integration in each bin,• integrate the in each bin for once and for good and• store the integrated values in ASCII table.

METHOD How to get jets NLO in <1s?

Assumption • PDFs are approx. flat in small bins of x and f.

Problem • Evaluation of NLO jet cross-sections: 8 hours for 50M events.• PDF fit requires O(1000) evaluations PROBLEM!

dSimplification

2

1

5

5 ,,,

,

),,/(ˆ),()(m a

jfriBjijfiarms

jfi

xdf

2

1

5

5

),,/(ˆ),()(m a

frBjfarms xfd

Flat!

Table!


THE METHOD WORKSfor DIS, photoproduction, heavy flavours, Tevatron …

Comparison of normal NLO calculation with NLO cross-sections calculated with the formulae on the slide before.

ZEUS inclusive jets.

D0 inclusive jets.


INPUT DATAin addition to F2

Photo-production

dijets

High-Q2

inclusive jets


RESULTS Reduction in gluon uncertainty, effect on coupling

F2 + jets

F2 alone

Simultaneous fit toboth S and PDFs.

Fractional gluon uncertainty.


EFFECT ON STRONG COUPLING Very much constrained by jets dataStrong correlation between s and the gluon density for in-clusive F2 data

large increase in gluon uncertainty when s free!

Jet cross sections directly sensitive to s via *g qqbar (coupled to gluon) and via *q qg (NOT coupled to gluon)

s NOT as strongly correlated to gluon

F2 + jets

F2 + alone


OVERVIEW

¶ INTRODUCTION


¶ MORE DETAILED STUDIES • Parton evolution in the proton • Transition from photoproduction to DIS • More selected topics from jet physics • Event-shapes and charged multiplicities



PARTON EVOLUTION SCHEMES The `forward jet’ question

DGLAP approximation resumming terms lnQ2 for parton evolution works very well for most of HERA regime (F2!)

DGLAP: kT ordering!

BFKL: x ordering!

kT large

x large

forward region: > 2 (close to proton) jet ET

2 ~ Q2 (suppressed in DGLAP) large xjet=Ejet/Eproton (realized in BFKL)

Breakdown expected at very low x! Can we distinguish the onset of BFKL-like evolution in ln1/x? “forward jets”.

Startingpoint

Question

Forwardjets

Design phase-space to suppress DGLAP and enhance BFKL:

All results so far: Problems at low x! But not firm conclusions drawn: -- NNLO terms missing? -- Resolved contribution? -- BFKL evolution scheme?-- pure kinematics?


PARTON EVOLUTION SCHEMES The `forward jet’ question

- Tighter cuts than H1 same statistics from 72pb-1. R=Q.- 20 < Q2 < 100 GeV2.

only 1997 data, 13.7pb-1, 5 < Q2 < 85 GeV2, R=ET, ET > 5 GeV All models below the data at low x; CDM and MC `dir+res’ best non-kt-ordered contribution mimicking BFKL, or higher orders? First distinction between dir+res MC and CDM if require additional dijet system: only CDM works.

H1

New ZEUS

No new experimental conclusions.

Theory (Sassot et al.): Kinematics!


FROM PHOTOPRODUCTION TO DIS The transition region and its problems

direct

resolved

x

xR

obs

obs

75.0

75.0

x=1“direct”

x<1“resolved”

Resolved relevant even ifQ2>100 GeV2. Scales?

p NLO, MC+PS+res okay!

Quantify amount ofresolved as function of Q2 and ET with R=res/dir.

For ET2>Q2 parton resolves

structure of photonResolved

Aim

Results

No NLO theory for resolved in DIS!

JetViP (Pötter) has problems.


COLOR DYNAMICS IN JET EVENTS Testing the underlying gauge group (SU(3)C) like LEP

23: angle between two lowest-energy jets in 3-jet events!

Sensitivity to different contributions hope to test gauge structure!

Investigate color dynamics and underlying gauge group using QCD color factors CF, CA, and TF.

At LO 3jet Xsection sensitive to various color factor combinations:

CF2

CFCA

CFTF TFCA

DAFCFFBAFAFjetsep CTTCCCC 2

3

Aim


COLOR DYNAMICS IN JET EVENTS in DIS and photoproduction

SU(3) favoured, but U(1)3 not excluded!

DIS: 1015 eventsp: 2233 events

DIS: 82 pb-1, Q2 > 125 GeV2, ET > 8 / 5 GeV, 3 jets. Photoproduction: all HERA I (127pb-1), ET > 14 GeV, 3 jets.


SUBJET DISTRIBUTIONSin high Q2 DIS – study QCD radiation pattern / jet structure

- Study QCD radiation pattern using LL MC models and fixed order QCD calculations. - Use high-ET jets to minimise fragmentation etc. effetcs test structure of radiaton implemented in NLO pQCD.

Test variables ETsub/ET

jet, sub-jet,|sub-jet| and orientation of subjets in - space with respect to proton beam.

Increasing ycut

Aim

Define subjets by applying k algo to objects of one jet as function of distance measure dcut=ycut·ET

2. Chosen here: Jets with two subjets at ycut=0.05 (hadronisation)!

Method

Note for experts: Running NLO O(S

2) in lab frame jets with 3partons from fixed-order calculation!


SUBJET DISTRIBUTIONSin high Q2 DIS – study QCD radiation pattern / jet structure

All distributions nicely described by NLO QCD within 10%

radiation pattern understood / correctly implemented!

Selection – ~80pb-1 from 98-00 – Q2 > 125 GeV2

– ET > 14 GeV (Lab)

- ratio of subjet ET to jet ET.- 2 entries per jet.- trend towards similar energies


INTERJET ENERGY FLOWin photoproduction dijet events with large rapidity gaps

Normally lots of activity between two jets color connection.

Events with little activity between jets: study color singlet exchange in pQCD regime (high ET!) pomeron gap

Idea

Compare to MC models with(out) color singlet (CS) contribution: -- PYTHIA: high-t- exchange (MPI); -- Herwig: BFKL-Pomeron (JIMMY) added to dir+res.

gapTdE

d

Globally 3-4% CS exchange needed to account for small Et

gap cross-section. Amount depends on rapidity separation of jets (up to 50%).

Result


INTERJET ENERGY FLOWin photoproduction dijet events with large rapidity gaps

Fraction of events with rapidity gap energy below cut value as function of gap width.

For low cut values data and CS contribution level out at higher values of gap width!– non-CS contributions fall off exponentially


EVENT SHAPES: MEANSas a means to test power corrections (in high Q2>80 GeV2)

p

pSpowpowPT QQfVVVV

),,( 0

Idea Alternative way to describe non-pert. hadronisation corrections to event shapes. PC depend on one universal parameter 0.Example: Thrust (long. collimation)

PC approach works for means … … but fit to S and 0 difficult.

ii

ii

p

npT


EVENT SHAPES: DIFFERENTIALLYTheory including resummation (NLL)

NLO+PC+NLL

)()(pow

PT

VVdV

dV

dV

d

On differential distributions effect is only lateral shift:

Differentialdistributions

Extractionsof 0 and S.

New ZEUS analysis: No consistent extraction of the parameters from the different event shape variables, in contrast to H1 and the LEP experiments.

Do PC not work at all? Just underestimation/neglection of theoretical uncertainties like matching scheme, scale variations etc.?


CHARGED MULTIPLICITIESAt LEP, Tevatron and HERA

Expectation: – The current region in the Breit reference frame looks like a e+e- hemisphere.

– LEP and Tevatron find the same dependence of multiplicity on available energy. – What does HERA observe? Naively: Q~see.

Analysis: – 39pb-1 from 96/97.– Q2 > 25 GeV2,– tracks with pT > 150 MeV– use 2Ecurr instead of Q.– Multiply mean charged multiplicity by 2 for HERA comparison with LEP.

Result: – HERA exhibits same energy dependence as LEP and Tevatron.

Mean charged multiplicity.


CONCLUSION AND OUTLOOKHERA is a great machine for QCD!

HERAZEUS

– Aim of lumi upgrade reached: 5-times higher lumi than HERA I

– Hoping to get 700pb-1 on tape before shutdown in 2007.

– Experiments and machine getting a bit old … need lot of caretaking.

QCD@HERA

– HERA is an ideal laboratory which allows to adjust the parameters with which one wants to investigate QCD.

– HERA contributing significantly to - tests of pQCD (factorisation, PDF universality).- determinations of QCD parameters (PDFs, S).

- tests of theories / models (PC, BFKL, …)

Future– More statistics will allow for higher precision and thus better insight into QCD in few places. .

– Theoretical progress might improve our understanding of some problematic aspects of QCD (NLO@MC).

– Hopefully more insight into difficult subjects like parton evolution,

DIS-photoproduction transition, … . And input for LHC!


ADVERTISEMENT : NLOLIBA common framework work for fixed-order calculations

Purpose • Unify access to various fixed-order calculations for ep, pp, ee.• Thereby simplify data comparison to various predictions.

Authors • Formerly T. Hadig and K. Rabbertz (formerly H1).• Currently K. Rabbertz and TSS.• T. Toll starting to contribute with FMNR.

Programs • DISENT and JetViP for ep DIS jet production. Problems with DISASTER++ and MEPJET.

• RacoonWW for 4f processes in e+e-.• NLOJET for jet production in pp collisions almost there.

• FMNR for photoproduction heavy flavour jets in ep: Work has just begun.

Getting it: • www.desy.de/~nlolib or one of the authors. • HERA-LHC proceedings contribution.


NLOLIB – Results Implementation of DISENT and JetViP


BACKUP


URANIUM-SCINTILLATOR CALORIMETERprovides good control on beam, timing …

Momentumdistribution.

F/B/RCALtiming.

Imbalance betweenleft and right PMT foreach EMC FCAL cell.


ENERGY FLOW OBJECTS IN ZEUS Combining tracks and calorimeter information

Idea • Make best use of both detectors calorimeter at high momenta, tracker at low momenta.

Result • Resolution in pT,h and (E-pz)h. improves by 20%.

Method • Combine cells to ‘cone islands’.• Match tracks to islands.• Decide which information to use.


JET ENERGY CORRECTIONSto account for mis/non-measurements

Idea • Detector effects lead to mis- measurements of jet energies:

• Use MC simulation to estimate the effect and derive corrections.

jethadT

jetT EE ,det,

Method • Fit straight lines to detector-versus-hadron distributions (differentially in !, possibly several bins in ET).• derive slope b and offset m and correct ET: b

m

b

EE

uncorrTcorr

T det,det,

offset ~1,slope ~0.8.

offset ~0.1,slope ~0.98.


EVENT CORRECTIONSto account for detector/QED/Z0 effects

Datadetektor

akzeptance, efficiency

Comparison

NLO

QEDrunning, radiation

Z0 exchange

hadroni-sation


INCLUSIVE JET CROSS-SECTIONS (DIS)1998-2000 versus 1996/97

– As expected increase from 96/97 to 98-00 at high ET and Q2.

– Change described by NLO everything consistent.


TRIJETS IN DISTesting higher orders in pQCD

Selection – ~80pb-1 from 98-00 ET > 5 GeV (Breit)– 10 < Q2 < 5000 GeV2 invariant dijet mass > 25 GeV


TRI- AND DIJETS IN DIS in the same phase-space.

Idea: In ratio of dijets to trijets someuncertainties cancel out(lumi, partly energy scales, etc.)

Theory:S

SS

SS

DD

TT

dijets

trijets

2

21

32

21

)(

)(

Nice possibility to extract thestrong coupling with rathersmall uncertainties!


S FROM INCLUSIVE JETSTheory starting to dominate measurement

World average: s(MZ) = 0.11870.0020H1 inclusive jets: s(MZ) = 0.11970.0016(exp)0.0047(theo)

ZEUS inclusive jets: s(MZ) = 0.11960.0025(exp)0.0023(theo)


S FROM THE RATIO R3/2.Cancellation of some uncertainties

ZEUS R3/2: s(MZ) = 0.11790.0013(stat) 0.0037(exp)0.0055(theo)


SUMMARY ON S FROM ZEUS(C. Clasman)


RESULTS Massive reduction in gluon density uncertainty


EFFECT ON STRONG COUPLING Very much constrained by jets data


PHOTON PDF FROM DIJETS… just to point out the problems …

Low ET jets (6 GeV) are sensitive to the gluon content in the photon.

Large contributions from underlying

events subtraction procedure.

Size of correction depends on model! large uncertainties!

Gluon density in the photon as extracted from H1 dijet p data. Agreement with other determinations; very large errors.

jets and the hadronic final state at zeus thomas schörner-sadenius hamburg university heidelberg 23...

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