The Hadronic Final State at HERA

The Hadronic The Hadronic Final State at Final State at HERAHERA

Rainer MankelRainer MankelDESYDESY

for the ZEUS & H1 collaborationsfor the ZEUS & H1 collaborations

C2CR Conference PragueC2CR Conference Prague 9-Sep-20059-Sep-2005

(Some Highlights from)(Some Highlights from)

9-Sep-2005 R. Mankel: The Hadronic Final State at HERA 2

Typical Structure of Hadronic Final Typical Structure of Hadronic Final States at HERAStates at HERA

Current jetCurrent jet

Proton Proton remnant remnant

[gap[gap]]

Q2 > 1 GeV2 : deep-inelastic scattering (DIS)

Q2 < 1 GeV2 : photo-production (PHP)

Diffractive process

Diffractive Diffractive systemsystem


Comparison of Final State Comparison of Final State StructureStructure

contains main features of contains main features of energetic hadron interaction energetic hadron interaction ((proton remnantproton remnant))

less complexless complex than hadron- than hadron-hadron interactionhadron interaction

clean reconstruction of clean reconstruction of kinematic variableskinematic variables

ideal laboratory for studying ideal laboratory for studying QCDQCD

Quark jetQuark jet Anti-quark jetAnti-quark jet

ee++ee interaction interactionQuark jetQuark jet

Quark jetQuark jet

Proton Proton remnantremnant


hadron-hadron hadron-hadron interactioninteraction


Quark jetQuark jet

eep interactionp interaction


ee pp

Colliding mode detectors Colliding mode detectors can generally measure can generally measure current jet & scattered current jet & scattered electron very well electron very well (“central region”)(“central region”) in these areas, also in these areas, also

theoretical models are theoretical models are tested & tuned besttested & tuned best

The The proton remnantproton remnant emerges close to beam emerges close to beam pipe & is pipe & is less accessibleless accessible these areas also pose big these areas also pose big

challenges to theorychallenges to theory Cosmic ray experiments Cosmic ray experiments

cannot distinguish cannot distinguish between proton (nucleus) between proton (nucleus) remnants & jetsremnants & jets

Colliding Beam DetectorColliding Beam Detector

scattered electronscattered electron

current jetcurrent jet

protonproton

remnantremnant


Some Questions Related to Some Questions Related to Hadronic Final StateHadronic Final State

How well do we understand the How well do we understand the workings of QCD in the workings of QCD in the forward forward areaarea??

How strong is the How strong is the diffractive diffractive componentcomponent at high energies at high energies

At which accuracy can we describe At which accuracy can we describe production of production of heavy flavorsheavy flavors & & resulting resulting leptonsleptons


OutlineOutline

IntroductionIntroduction Leading baryon productionLeading baryon production Jets in the forward areaJets in the forward area Diffraction at high QDiffraction at high Q22

Heavy flavor production Heavy flavor production SummarySummary


Leading Baryon ProductionLeading Baryon Production Sizeable fraction of events with leading baryonsSizeable fraction of events with leading baryons Production mechanism not entirely understoodProduction mechanism not entirely understood At HERA, At HERA, special forward detectorsspecial forward detectors allow precision allow precision

measurements (p, n)measurements (p, n) FPS, FNC (H1)FPS, FNC (H1) LPS, FNC (ZEUS)LPS, FNC (ZEUS)

Example: Leading Proton Example: Leading Proton SpectrometerSpectrometer

6 stations of roman pots in 6 stations of roman pots in downstream curve of proton beamdownstream curve of proton beam

each station with 6 Si detector planes each station with 6 Si detector planes acceptance extends in range acceptance extends in range

0.4 < 0.4 < xxLL <1 (x <1 (xLL= E= ELPLP / E / Epp) ) p ptt

22 < 0.5 GeV < 0.5 GeV22


Typical Production Typical Production MechanismsMechanisms

Hadronisation of proton Hadronisation of proton remnantremnant

Herwig (cluster model)Herwig (cluster model) MEPS (parton MEPS (parton

shower,SCI)shower,SCI) Ariadne (CDM)Ariadne (CDM)

,IR,IPN,P

N,P p’p’

Exchange of virtual Exchange of virtual particlesparticles

leading protons: leading protons: 00, , Pomeron, ReggeonPomeron, Reggeon

leading neutrons: leading neutrons: ++, , ++, , ……


Leading Proton Spectrum Leading Proton Spectrum (DIS)(DIS)

Cross section vs. xCross section vs. xLL = E = ELPLP / E / Epp. Very . Very preciseprecise data. data. Standard fragmentation models Standard fragmentation models fail to describefail to describe flat part flat part

between 0.6-0.95between 0.6-0.95

HerwigHerwig

AriadnAriadnee

MEPSMEPS (1-xL)1.0

(1-xL)1.0

(1-xL)1.4

TheoryTheoryDiffractive peak

Flat spectrum for xL<0.95

DataData


Leading Proton pLeading Proton pTT Spectra Spectra

Fit transverse momentum spectrum with Fit transverse momentum spectrum with Slope b hardly dependent on xSlope b hardly dependent on xLL

Well-established models with standard Well-established models with standard hadronizationhadronization fail fail to describe leading baryon productionto describe leading baryon production


Leading Neutron Leading Neutron ProductionProduction

Leading neutrons show Leading neutrons show entirely different behaviorentirely different behavior

steep increase of psteep increase of pTT slope with slope with increasing xincreasing xLL

equally inexplicable with equally inexplicable with proton remnant fragmentationproton remnant fragmentation

In case of neutrons, one non-In case of neutrons, one non-fragmentation process (fragmentation process (++ exchange) is expected to exchange) is expected to dominatedominate ideal ideal

process to process to test validity test validity of of exchange exchange modelmodel

protons

neutrons


Leading Neutron Production Leading Neutron Production (cont’d)(cont’d)

Factorize cross section Factorize cross section into pion flux from proton into pion flux from proton and pion-photon cross and pion-photon cross sectionsection

Precise data allow to Precise data allow to compare compare various various parameterizationsparameterizations of pion of pion fluxflux constrains parameters constrains parameters

on some modelson some models excludes other models excludes other models

),)1((),(),,,( 22

*/

22

QWxtxfdtdx

txQWdLLp

L

LeXnep

same model with three same model with three different parameter setsdifferent parameter sets


Leading Neutrons in Di-Jet Leading Neutrons in Di-Jet EventsEvents

Comparison of Comparison of di-jet eventsdi-jet events with & without with & without leading neutrons allows further tests of leading neutrons allows further tests of modelsmodels Elaborates further differences in production Elaborates further differences in production

mechanisms. Pion exchange models able to mechanisms. Pion exchange models able to describe the datadescribe the data

Is the production of the leading Is the production of the leading neutron independent of the neutron independent of the photon virtuality (factorization)?photon virtuality (factorization)? Di-jet events in Di-jet events in photo-photo-

productionproduction have lower leading have lower leading neutron rates than those in neutron rates than those in DISDIS

factorization violationfactorization violation Difference is most pronounced Difference is most pronounced

at lower neutron energiesat lower neutron energies


Leading Neutron in Di-Jet Events Leading Neutron in Di-Jet Events (cont’d)(cont’d)

Smooth transition Smooth transition between photo-between photo-production & DIS regimeproduction & DIS regime

Depletion of neutrons at Depletion of neutrons at low Qlow Q22 may be indicative may be indicative of of absorption / absorption / rescatteringrescattering processes at processes at workworkHigh QHigh Q22 Low QLow Q22


Leading Baryons: Leading Baryons: ConclusionsConclusions

HERA experiments provide precise HERA experiments provide precise measurements of leading baryon measurements of leading baryon production using dedicated forward production using dedicated forward detectorsdetectors

General purpose models fail to describe General purpose models fail to describe leading baryon production via standard leading baryon production via standard fragmentation of proton remnantfragmentation of proton remnant

Virtual particle exchange processes Virtual particle exchange processes improve the picture. Powerful constraints improve the picture. Powerful constraints on model parameters from HERA data.on model parameters from HERA data.


Forward JetsForward Jets Forward area is Forward area is

particularly sensitive to particularly sensitive to details in evolution of details in evolution of parton cascadeparton cascade

At low x, we do not probe At low x, we do not probe the valence structure of the valence structure of the proton, but rather the proton, but rather see universal structure of see universal structure of QCD radiationQCD radiation at work at work signature: signature: forward jetforward jet

This allows us to This allows us to examine different examine different mechanisms of parton mechanisms of parton cascade evolutionscascade evolutions


Dynamics of Parton Dynamics of Parton EvolutionEvolution

DGLAPDGLAPDokshitzer-Gribov-Lipatov-Dokshitzer-Gribov-Lipatov-

Altarelli-ParisiAltarelli-Parisi

Evolution in powers of ln Evolution in powers of ln QQ22

Strongly orderered in kStrongly orderered in kTT

Well established at high x Well established at high x and Qand Q22, but expected to , but expected to break downbreak down at low x at low x

Evolution in powers of Evolution in powers of ln 1/xln 1/x

Strongly orderered in Strongly orderered in xx

May be May be applicableapplicable at at low xlow x

BFKLBFKLBalitsky-Fadin-Kuraev-LipatovBalitsky-Fadin-Kuraev-Lipatov

CCFMCCFMCiafaloni-Catani-Fiorani-Ciafaloni-Catani-Fiorani-

MarchesiniMarchesini

Evolution in both ln QEvolution in both ln Q2 2 and and ln 1/xln 1/x

Bridge between DGLAP Bridge between DGLAP and BFKLand BFKL

Angular orderingAngular ordering May be May be applicableapplicable at low x at low x


Forward Jet Measurements Forward Jet Measurements (DIS)(DIS)

DGLAPDGLAP leading order leading order

suppressed by suppressed by kinematicskinematics

even with NLO, factor even with NLO, factor 2 2 below databelow data at low x at low x

xxBjBj<0.004, 7<0.004, 7oo<<jetjet<20<20oo, x, xjetjet>0.035>0.035

CCFMCCFM distribution distribution too hardtoo hard comparatively poor comparatively poor

description of the description of the datadata

CDM (similar to BFKL)CDM (similar to BFKL) generally goodgenerally good

DGLAP with resolved virtual photonDGLAP with resolved virtual photon similar to CDM, but fails to similar to CDM, but fails to describe forward+dijet sampledescribe forward+dijet sample

Cuts Cuts designed to designed to

enhance enhance BFKL effectsBFKL effects


Forward Jets SummaryForward Jets Summary

Limitations of the pure DGLAP approach Limitations of the pure DGLAP approach clearly seenclearly seen in the forward area in the forward area higher order parton emissions break higher order parton emissions break

ordering schemeordering scheme Calculations which include such Calculations which include such

processes (CDM) provide better processes (CDM) provide better descriptiondescription


Diffractive Final States at Diffractive Final States at High QHigh Q22

Hard diffractive process Hard diffractive process is characterized by is characterized by rapidity gap near rapidity gap near outgoing protonoutgoing proton caused by colorless caused by colorless

exchangeexchange It is an interesting It is an interesting

question if & how far question if & how far diffraction extends to the diffraction extends to the large Qlarge Q22 region region clean final statesclean final states at LHC, at LHC,

e.g. for Higgs?e.g. for Higgs?

Large rapidity

gap

Look for rapidity gaps in Look for rapidity gaps in neutral current eventsneutral current events

Comparison of charged Comparison of charged current / neutral current current / neutral current events events universal behavior? universal behavior?


Rapidity Gaps in NC Rapidity Gaps in NC EventsEvents

Forward Plug Calorimeter

(FPC)

Rap

idit

y ga

p

““Normal” DIS MC (Ariadne) Normal” DIS MC (Ariadne) clearly clearly insufficientinsufficient at low at low maxmax

Need Ariadne+RAPGAP Need Ariadne+RAPGAP (diffractive MC) to describe the (diffractive MC) to describe the datadata

with FPC veto (at beam pipe)

55


Rapidity Gaps in NC: QRapidity Gaps in NC: Q22 DependenceDependence

SizableSizable diffractive contribution diffractive contribution to NC cross sectionto NC cross section drops with rising Qdrops with rising Q22

still 2% at Qstill 2% at Q22=1500 GeV=1500 GeV22

NC and CC NC and CC compatiblecompatible

For comparison:

Low-Q2 Data

High -Q2 NC

x P<0.05


Muons from Heavy Flavor Muons from Heavy Flavor DecaysDecays

Apart of weak decays of pions Apart of weak decays of pions and other light mesons, heavy and other light mesons, heavy flavor final states contribute in flavor final states contribute in particular to the particular to the muon ratesmuon rates at at high transverse momentumhigh transverse momentum

Main challenge Main challenge taggingtagging of quark of quark flavorsflavors

decay impact parametersdecay impact parameters ppT T relative to jetrelative to jet di-muon eventsdi-muon events ( ( correlation) correlation)

Study of di-muon event Study of di-muon event signatures allows to use low psignatures allows to use low ptt

μμ thresholds, thresholds, measure the total measure the total bb cross sectionbb cross section

BB

BB--

DD

DD--

Interaction Interaction vertexvertex

(D)(D)--

--


Di-Muons: Data vs MCDi-Muons: Data vs MC

Good overall description with MCGood overall description with MC bb contribution ~2000 events, purity ~43%bb contribution ~2000 events, purity ~43%

Same-charge Same-charge combinations combinations used to used to normalize light-normalize light-flavor flavor backgroundbackground

non-isolated, ET>8 GeV


bb Cross Section from Di-Muon Eventsbb Cross Section from Di-Muon Events

NLO QCD predictions:NLO QCD predictions:PHP: 5.8 nb PHP: 5.8 nb

(FMNR,CTEQ5M)(FMNR,CTEQ5M)DIS: 1.0 nb DIS: 1.0 nb

(HVQDIS,CTEQ5F4)(HVQDIS,CTEQ5F4)

6.8 nb6.8 nb

NLO prediction NLO prediction lower lower than the datathan the data, though , though not entirely not entirely incompatibleincompatible within within errorserrors

nbsyststatGeVsXbbeptot )(8.43.5)(8.11.16)318)((

+3.0+3.0

––1.71.7

Compare with recent H1 Compare with recent H1 measurement of measurement of visvis

bbbb in in PHP using PHP using D*D* correlations correlations

pT(D*)>1.5 GeV, |(D*)|<1.5. p()>2 GeV, |()|<1.7, 0.05<y<0.75, Q2<1 GeV2H1:

similar trendsimilar trend


SummarySummary Wealth of measurements from HERA on Wealth of measurements from HERA on

structure of hadronic final statestructure of hadronic final state only a small selection presentedonly a small selection presented

Leading baryons & forward jets Leading baryons & forward jets probe QCD probe QCD dynamicsdynamics in vicinity of proton remnant in vicinity of proton remnant allows allows accurate distinctionsaccurate distinctions between different models between different models

Hard Hard diffractiondiffraction reaches up to reaches up to high Qhigh Q22

Measurement of open Measurement of open beautybeauty cross section cross section leptonsleptons at high p at high pTT


The EndThe End


Backup SlidesBackup Slides


Direct Comparison of Global Direct Comparison of Global Phase Space and BFKL-Sensitive Phase Space and BFKL-Sensitive RegimeRegime

Global phase space:Global phase space: CDM (BFKL) works wellCDM (BFKL) works well MEPS (DGLAP) slightly worseMEPS (DGLAP) slightly worse fixed-order QCD fixed-order QCD

underestimates data at high underestimates data at high jetjet (missing higher orders) (missing higher orders)

BFKL-sensitive phase space:BFKL-sensitive phase space: Steep falloff with Steep falloff with jetjet ((hh cut) cut) MEPS (DGLAP) fails to describe data MEPS (DGLAP) fails to describe data CDM (BFKL) works wellCDM (BFKL) works well NLO QCD is better than LO (t-NLO QCD is better than LO (t-

channel gluon exchange)channel gluon exchange)

QQ22 > 25 GeV > 25 GeV22

y > 0.04y > 0.04

EEee’>10 GeV’>10 GeV

EETTjetjet>6 GeV>6 GeV

-1<-1<jetjet<3<3

in addition:in addition:

hadhad>90>90oo

0<0<jetjet<3<3

0.5<(E0.5<(ETTjetjet))22//

QQ22<2<2

Glo

bal Ph

ase

G

lob

al Ph

ase

S

pace

Sp

ace

BFK

L Ph

ase

Sp

ace

BFK

L Ph

ase

Sp

ace


More Pieces to Pentaquark More Pieces to Pentaquark PuzzlePuzzle(1520) : both in forward & backward hemisphere

+: only in forward hemisphere + signal signal mainly mainly

from forwardfrom forward pseudo rapidity pseudo rapidity regionregion unlike regular unlike regular

baryons baryons (1520) and c

predominantly at medium Q2

similar to c

no sign of decuplet partners seen in –– and –+ ( NA49)


Di-Muon Mass Spectra (Data vs Di-Muon Mass Spectra (Data vs MC)MC)

low mass

low mass high mass

high mass

same B

(charm cascade)

+ J/

different B’s (signal!)

+ cc + light flavor BG

mainly light flavor BG

different B’s

(charm cascade + BB mixing)

+ light flavor BG

Good description with MCGood description with MC bb contribution ~2000 eventsbb contribution ~2000 events


bb from Di-Muons: Normalization bb from Di-Muons: Normalization of BG-MCof BG-MC

cc: normalize to D*mu analysiscc: normalize to D*mu analysis Bethe-Heitler, elastic charmonium: Bethe-Heitler, elastic charmonium:

normalize to data under isolation normalize to data under isolation cutcut

Light flavor: use like sign spectrum Light flavor: use like sign spectrum (minus bb MC)(minus bb MC)


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The Hadronic Final State at HERA

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