Einige Aspekte der QCD BEI HERA und was man dabei lernen kann Thomas Schörner-Sadenius Universität Hamburg Bonn, 11. Mai 2006 I will focus strongly on jets A few words on me: Studies in HH and M, diploma at OPAL, PhD with H1, then ATLAS and OPAL, now ZEUS and a little CMS so I suffered quite a bit from jets at HERA. - problems in changing sides never really accepted ;-)( …

R. Klanner und TSS, Physik Journal, Mai 2006, 41 ÜBERSICHT ¶ HERA UND ZEUS Vorbereitet auf Unvorhersehbares? Datennahme, Detektor(weiter)entwickung, unerwartete Probleme, Trigger … ¶ QCD BEI HERA Konsequenzen für LHC? F2, Partonverteilungen, Jets, Kopplung S, R. Klanner und TSS, Physik Journal, Mai 2006, 41 ¶ “ES TUT SICH WAS!”, GROSSE FRAGEN Wo werden wir in 10 Jahren stehen? Was lernen wir für die Zukunft (LHC)? NLO+PS, “resolved”, Parton-Dynamik, … ¶ AUSBLICK Introduction: DESY, HERA, ZEUS, current data taking, basics of ep and jet physics at HERA Basic jet physics: cross-sections, alphas, PDFs More detailed studies: parton evolution, transition region, specialised stuff, hadronic final state (evshapes etc.) Also others not mentioned: Like forward pions, alphas from jet shapes etc. – Auch ein “altes” Experiment ist noch spannend – LHC kann von der HERA-Erfahrung profitieren Bonn, 11. Mai 2006 TSS: QCD bei HERA

Ideen/TODOS QCD dann: Wieviele CAL-Kanaele hat ATLAS? NLO+PS einbauen und Referenz Irgendwie zum Atlas-Trigger kommen und Referenz (am Anfang, wenn es um Flexivilitaet geht …) Doktorarbeitsplots Neues Paper-Plots NLOLIB-Plots Praezision vergleichen udn sagen, dass nicht so gut wie z.B. AFB ;-) … Brotund butter: F2 (nicht so wichitg, aber PDFs, NNLO) Jets und E-Skala  alphas, uncertainty, bumps, contact interactions … Jets und alphas (unification) Jet-Algos, … QCD dann: Frameworks wie NLOLIB Jets in den Fits NLO+PS etc. und hoehere ORdnungen (Resolved) Partondynamik und forward physics Wieviele CAL-Kanaele hat ATLAS? Bonn, 11. Mai 2006 TSS: QCD bei HERA

HERA UND ZEUS ep-Kollisionen bei s = 318 GeV Ep with 318 GeV cms energy Bonn, 11. Mai 2006 TSS: QCD bei HERA

ep-STREUUNG BEI HERA … in niedrigster Ordnung der QCD y=1-E’/E: Inelastizität Ee = 27.5 GeV Ep = 920 GeV s = 4EeEp ~ 318 GeV ,Z,W k Lepton (e±) Proton P k’(e±,) Q2=-q2=-(k-k’)2 Q2: Virtualität Auflösung ~1/Q p=xP x=Q2/2Pq: Protonimpulsanteil Bei gegebenem s sind nur zwei Variablen unabhängig: Q2 = s·x·y Bonn, 11. Mai 2006 TSS: QCD bei HERA

DAS ZEUS-EXPERIMENT … es läuft und läuft und … entwickelt sich weiter … Microvertex-Detektor Erste Erträge (schwere Quarks). 6m-Tagger fängt an beizutragen: - Luminositätsbestimmung - FL-Bestimmung Aufgerüsteter Vorwärts -Tracker STT “Arbeitspferd” Uran- Kalorimeter Basic features: MVD  mostly for b so far; impact parameters resolution etc. CTD with resolution sigmapt/pt for more than 150 MeV and all nine superlayers: 0.0065 Plus 0.0058pT Plus 0.0014/pT CAL later, probably known from calibration time … Solenoid 1.43 T BAC Muon Personal feeling: We start to notice that people are leaving for LHC, analysiswise … Das Experiment braucht mehr und mehr Pflege. Bonn, 11. Mai 2006 TSS: QCD bei HERA

Koordination 2004/2005. URAN-SZINTILLATOR-KALORIMETER kompensierend, gute hadronische Energieauflösung Signalerzeugung durch Schauer- entwicklung in Szintillator, Uran FCAL-Modul mit 23 Towern mit je 12 PMTs. Depleted Uranium 238, compensates by energy loss from fast neutrons released in fission process. Energy loss due to nuclear break-up. Equal response to hadrons and electrons within 1-2%. F/B/RCAL, with 23/32/23 moduls, and 4 (1/2) EMC/HAC sections. Typical size 20*5 cm 12000 PMT EMC: 1 hadronic interactionlength ~25 X0 HAC: 3-6 lambda. Use U noise to calibrate. 12000 Kanäle (H1: 45000, ATLAS: ) - Ca. 240 tot (vormals 360!) - E/E~0.18/E für Elektronen - E/E~0.35/E für Hadronen Aber alte Stecker/Kabel, lose Ver- bindungen, Detektor als “Mülleimer” 23 (32) Module in F/RCAL (BCAL). Bonn, 11. Mai 2006 TSS: QCD bei HERA

Ende 2005 72m Kupfer-Kühlschlangen eingebaut STT – DER STRAW-TUBE-TRACKER … liefert nach langer Auszeit (Kühlung!) Daten! Temperaturmodell des inneren Detektors ohne/mit zusätzlicher Kühlung  “ohne” wird Belastung der Spule zu gross (T=230, jetzt 60)! Ende 2005 72m Kupfer-Kühlschlangen eingebaut Temperaturverhaltem jetzt zeitlich stabil Depleted Uranium 238, compensates by energy loss from fast neutrons released in fission process. Energy loss due to nuclear break-up. Equal response to hadrons and electrons within 1-2%. F/B/RCAL, with 23/32/23 moduls, and 4 (1/2) EMC/HAC sections. Typical size 20*5 cm 12000 PMT EMC: 1 hadronic interactionlength ~25 X0 HAC: 3-6 lambda. Use U noise to calibrate. Bonn, 11. Mai 2006 TSS: QCD bei HERA

6m-TAGGER Nutzen für die longitudinale Strukturfunktion FL. Proposal to PRC April 2006 6m-TAGGER Nutzen für die longitudinale Strukturfunktion FL. Ziele: – Bestimmung der Akzeptanz des Lumi-Systems – Bestimmung des Photoproduktionsuntergrundes in tiefunelastischer Streuung  besondere Bedeutung für Messung der longitudinalen Strukturfunktion FL bei hohen Inelastizitäten y. – keine/kaum Rekonstruktions- und Simulationscode vorhanden. – keine Effizienz/Akzeptanzabschätzung – Akzeptanzbestimmung “manuell” mit Matrixformalismus – in etwa Reproduktion alter Simulationsergebnisse zur Akzeptanz Probleme: Ansatz: Depleted Uranium 238, compensates by energy loss from fast neutrons released in fission process. Energy loss due to nuclear break-up. Equal response to hadrons and electrons within 1-2%. F/B/RCAL, with 23/32/23 moduls, and 4 (1/2) EMC/HAC sections. Typical size 20*5 cm 12000 PMT EMC: 1 hadronic interactionlength ~25 X0 HAC: 3-6 lambda. Use U noise to calibrate. Bonn, 11. Mai 2006 TSS: QCD bei HERA

HERA-PERFORMANCE Das HERA-II-Programm ist ein Erfolg! Durchschnittliche ZEUS-Effizienz ca. 75% (40-90%). Lumi at HERAII increased by factor 5, currently something like 4*10^31 feasible, specific lumi something like 10^30. Typical currents 90 and 40mA. Grob 5-fache Luminosität (bezogen auf HERA-I) – Upgrade-Ziel erreicht. - integrierte Luminosität: Ca. 150 pb-1 HERA-II-Daten auf Band. - Derzeit bis zu 5 pb-1 pro Woche. Bonn, 11. Mai 2006 TSS: QCD bei HERA

HERA-PERFORMANCE Wir erreichen 5*1031cm-2s-1!! Many small problems inhibit data taking for few hours – and this sums up. Together with the background situation … Ein Wort ueber den Titel des Displays? Bonn, 11. Mai 2006 TSS: QCD bei HERA

Run Coordinator seit Anfang 2005 ZEUS LEIDET UNTER UNTERGRUND Bedrohung für zentralen Tracker, Problem für Datennahme Typische Füll-Situation mit Proton-Spikes (dunkelgrün) und Tracker-Trips (rot). Zahl der Tracker-Trips als Funktion des Tages. ZEUS never recabled CTD – no going to 95% level as H1 … On and stable or off and nothing. HERA kann nicht immer gute und stabile Strahlen bereitstellen. Bonn, 11. Mai 2006 TSS: QCD bei HERA

FLEXIBLER TRIGGER IST WICHTIG! Was kann man z.B. für ATLAS lernen? TSS, Mod.Phys.Lett. A18 (2003) 2149 FLEXIBLER TRIGGER IST WICHTIG! Was kann man z.B. für ATLAS lernen? Selection 2·1033 cm-2s-1 1034 cm-2s-1 MU6(20?) (20) 23 (3?) 4.0 2MU6 --- (1?) 1.0 EM25i (30) 11 22.0 2EM15i (20) 2 5.0 J200 (290) 0.2 3J90 (130) 4J65 (90) J60+xE60 (100) 0.4 0.5 TAU25+xE30 2.0 MU10+EM15i --- others total ~ 44 (25?) ~ 40 Prinzip größtmöglicher Inklusivität gefährlich, weil anfällig für Noise und Untergrund? RoI-(Region-of-Interest)-Konzept sinnvoll – wieviel topologische Information nutzbar im CAL auf Level1 mit neuesten FPGAs?) ZEUS never recabled CTD – no going to 95% level as H1 … On and stable or off and nothing. Bonn, 11. Mai 2006 TSS: QCD bei HERA

FEUER BEI ZEUS ! … und auch sonst ist es nicht langweilig Seit Antritt als einer von vier “Run Coordinatoren” Anfang 05:  ein Feuer nahe am Experiment  mehrere Evakuierungsalarme  Gas-Alarms, … Vorbereitet auf Unvorhersehbares? ZEUS never recabled CTD – no going to 95% level as H1 … On and stable or off and nothing. Bonn, 11. Mai 2006 TSS: QCD bei HERA

F2, Partonverteilungen, Jets, Kopplung S, ÜBERSICHT ¶ HERA UND ZEUS Vorbereitet auf Unvorhersehbares? Datennahme, Detektor(weiter)entwickung, unerwartete Probleme, Trigger … ¶ QCD BEI HERA Konsequenzen für LHC? F2, Partonverteilungen, Jets, Kopplung S, ¶ “ES TUT SICH WAS!”, GROSSE FRAGEN Wo werden wir in 10 Jahren stehen? Was lernen wir für die Zukunft (LHC)? NLO+PS, “resolved”, Parton-Dynamik, … ¶ AUSBLICK Hier erwaehnen warum QCD: - genauigkeit der Ergebnisse im Vergleich zu z.B. AFB etc. - Bedeutung fuer LHC - fundamentale Fragen .. – Auch ein “altes” Experiment ist noch spannend – LHC kann von der HERA-Erfahrung profitieren Bonn, 11. Mai 2006 TSS: QCD bei HERA

DIE STRUKTURFUNKTION F2 Präzisions-QCD bei HERA Inklusive Messung: Winkel, Energie Photonimpuls q (Q2 = -q2) q Impulsbruchteil x Proton (Partonverteilungen fi, Ladungen qi) Erwaehnen, dass bei hohen x (Gluon) am schlechtesten QCD-Kopplung Strukturfunktion Gluon-Dichte Gute Beschreibung der Daten großer Erfolg QCD bei HERA ist Präzisionsphysik! Bonn, 11. Mai 2006 TSS: QCD bei HERA

PARTONVERTEILUNGEN und ihre Unsicherheiten ¶ Die verschiedenen Bestimmungen der PDFs weichen signifikant voneinander ab. Unterschiede schon in den Daten – nicht im Fit. Q2 = 200 GeV2: Fehler 10%, Q2 = 5 GeV2: Fehler 100%, ¶ Viele wichtige Kanäle bei LHC betroffen (ggH) ¶ PDF-induzierte Unsicherheit des Higgs-Wirkungs- querschnitts derzeit je 10% durch Quarks und Gluonen. Quark-Sensitivitaet durch qqVektorboson->Higgsstrahlung, auch 10% Stirling: PDFs do not agree within respective errors! Unterschied vor allem im Slope der PDFs? Reicht das, um Essenz des Higgs-Mechanismus zu bestätigen (tH~mt)? Kenntnis der PDFs (HERA) wichtig für Entdeckungen + Interpretation bei LHC! Bonn, 11. Mai 2006 TSS: QCD bei HERA

JET-PHYSIK BEI HERA pQCD, Partonverteilungen, Kopplung S, … Wirkungs- querschnitte Perturbative QCD, kollineare Faktorisierung: Theorie “nur” in nächst- führender Ordnung (NLO, Parton-Niveau ohne Parton- schauer)  oft von theo. Unsicherheit dominiert! – Untersuchung der unterliegenden Eichgruppe – Starke Kopplung. – Partonverteilungen und ihre Universalität. – Faktorisierung in weiche/harte Anteile der Wechselwirkung – Vorhersagen der perturbativen QCD (– Unabhängigkeit vom Typ des ausgetauschten Bosons) (– Partondynamik im Proton (DGLAP versus BFKL)) Tests In out picture of collinear factorisation of hard and soft contributions to the Xsection we can divide the calculation in the PDFs (input from inclusive measurements) and the hard partonic matrix element which we then convolute for each order in alphas and for each quark flavour and the gluons seprately. Depending on how you look at the Xsection you can do all the things mentioned. Fehler oft von Theorie dominiert; hervorragendes Verständnis der Jet-Energieskala (1-3%)! Bonn, 11. Mai 2006 TSS: QCD bei HERA

JET-PHYSIK: QCD-TESTS I Spin der Teilchen, Farbfaktoren ¶ QCD: DIE effektive Theorie der starken Wechselwirkung. ¶ Aber sehen wir wirklich SU(3)C? – nicht-abelsch  3-Gluon vertex? – Spin-1/2 (1) Quarks (gluonen)? – Farbfaktoren? ¶ Einige Tests von Spin, Farbfaktoren in e+e– and ep: Gluon-Prop. In out picture of collinear factorisation of hard and soft contributions to the Xsection we can divide the calculation in the PDFs (input from inclusive measurements) and the hard partonic matrix element which we then convolute for each order in alphas and for each quark flavour and the gluons seprately. Depending on how you look at the Xsection you can do all the things mentioned. Quark-Propagator Scheinbar QCD! Starke WW von SU(3)C beschrieben! Bonn, 11. Mai 2006 TSS: QCD bei HERA

JET-PHYSIK: QCD-TESTS II Wirkungsquerschnitte - Beispiele ZEUS Collab., DIS06. H1 Collab., PLB XXXXX (2002) In out picture of collinear factorisation of hard and soft contributions to the Xsection we can divide the calculation in the PDFs (input from inclusive measurements) and the hard partonic matrix element which we then convolute for each order in alphas and for each quark flavour and the gluons seprately. Depending on how you look at the Xsection you can do all the things mentioned. Dijets bei hohen Q2-Werten, Q2 > 125 GeV2 Inklusive Jets bei niedrigen Q2-Werten, 5 < Q2 < 100 GeV2 Über weite Bereiche beschreibt QCD in NLO die Daten hervorragend! Faktorisierung, perturbative QCD, Universalität der PDFs – alles gut! Bonn, 11. Mai 2006 TSS: QCD bei HERA

BEDEUTUNG VON JETS für LHC Hier erwaehnen: QCD-Untergrund fuer alle Suchen Erinnerung an Aufregung bei High-ET-Bump vor einigen Jahre  neue Signaturen in Bumps in Jet-Spektren? irgendein Plot? Mini black holes Bild? Treffen sich nicht wirklich unbedingt in einem Punkt … CDF: PDF uncertainty as large as exp. Syst (about). LHC: PDF error probably 3%, Skalen 7%  theo. Error 10% realistic (effect, not variation!)! Stenzel! Bonn, 11. Mai 2006 TSS: QCD bei HERA

JET-PHYSIK: QCD-TESTS III Bestimmungen der starken Kopplung Weltmittel (NNLO) S(MZ)=0.1187(20) [PDG] S(MZ)=0.1182(27) [Bethke] HERA-Mittel (NLO) S(MZ)=0.1186± 0.0011(exp)±0.0050(th) In out picture of collinear factorisation of hard and soft contributions to the Xsection we can divide the calculation in the PDFs (input from inclusive measurements) and the hard partonic matrix element which we then convolute for each order in alphas and for each quark flavour and the gluons seprately. Depending on how you look at the Xsection you can do all the things mentioned. ¶ konsistente Einzelmessungen in vielen Prozessen. ¶ “Laufen” der Kopplung wird von QCD beschrieben. ¶ HERA-Messungen: vergleichbare Genauigkeit. Bonn, 11. Mai 2006 TSS: QCD bei HERA

BEDEUTUNG VON S z.B. für die Große Vereinheitlichung Nur unter der Annahme von Supersymmetrie (Erweiterung des Teilchenspektrums!) vereinigen sich die drei Kopplungen des Standardmodells bei ca. 1016 GeV! Standard-Modell SUSY 1 = (5/3)·/cos2W 2 = /sinW 2 = S Treffen sich nicht wirklich unbedingt in einem Punkt … CDF: PDF uncertainty as large as exp. Syst (about). LHC: PDF error probably 3%, Skalen 7%  theo. Error 10% realistic (effect, not variation!)! Stenzel! Verhalten bei hohen Skalen nur durch Renormierung, Teilchenspektrum und Startwerte bei heute zugänglichen Skalen bestimmt. Zur Überprüfung ist möglichst genaue Kenntnis von S nötig! Bonn, 11. Mai 2006 TSS: QCD bei HERA

F2, Partonverteilungen, Jets, Kopplung S, ÜBERSICHT ¶ HERA UND ZEUS Vorbereitet auf Unvorhersehbares? Datennahme, Detektor(weiter)entwickung, unerwartete Probleme, Trigger … ¶ QCD BEI HERA Konsequenzen für LHC? F2, Partonverteilungen, Jets, Kopplung S, ¶ “ES TUT SICH WAS!”, GROSSE FRAGEN Wo werden wir in 10 Jahren stehen? Was lernen wir für die Zukunft (LHC)? NLO+PS, “resolved”, Parton-Dynamik, … ¶ AUSBLICK Introduction: DESY, HERA, ZEUS, current data taking, basics of ep and jet physics at HERA Basic jet physics: cross-sections, alphas, PDFs More detailed studies: parton evolution, transition region, specialised stuff, hadronic final state (evshapes etc.) Also others not mentioned: Like forward pions, alphas from jet shapes etc. – Auch ein “altes” Experiment ist noch spannend – LHC kann von der HERA-Erfahrung profitieren Bonn, 11. Mai 2006 TSS: QCD bei HERA

EVENT SHAPES mit Resummation (NLL) und “Power corrections” Alternativer Beschreibung der nichtperturbativen Hadronisierung. Hängen nur von universellem Parameter 0 ab: Thrust- Verteilung Bestimmung von 0, S. Einheitliches Bild? Unklar! Funktionieren PCs generell? Wurden theoretische Fehler unterschätzt? Anwendbarkat auf Hadron-Hadron-Maschinen? Bonn, 11. Mai 2006 TSS: QCD bei HERA

I. Borjanovic et al., hep-ex/0403021 “UNDERLYING EVENTS” (UE) +mehrfache Wechselwirkungen (“Multiple Interactions”) UE ist die Summe von – Remnant-Remnant-WW – weicher Strahlung – mehrfachen Kollisionen Nicht perturbativ zugänglich! I. Borjanovic et al., hep-ex/0403021 RCluster UE mt [GeV] UE ist großer Effekt! Wichtig für: – mtop-Cross-Checks (Bild). – “Jede” Jet-Physik – als Effekt an sich. Tests / Anpassung der Parameter bei Tevatron und HERA. Nicht pile-up – das haent von Lumi ab … Abhängigkeit von Behandlung weicher Wechselwirkungen, PDFs, Faktorisierung etc. … Bonn, 11. Mai 2006 TSS: QCD bei HERA

MODELL-ANPASSUNG z.B. an pT-Summen in “transversaler Region” Tevatron – Summe der Impulse in transversaler Region – Modelle in etwa an Daten angepasst. – Extrapolation zum LHC scheitert deutlich – Tieferes Verständnis des Phänomens nötig. LHC Photoproduktion bei HERA ist effektiv Hadron-Hadron-Streuung: HERA p Oben: Transversale Ansicht eines Ereignisses Nicht pile-up – das haent von Lumi ab … Tieferes Verstaendnis fuehrt dann weiter, Richtung Diffraktion, MI etc. …! Hadronischer Charakter des Photons ist “einstellbar” (Q2)  HERA idealer Ort für systematisches Verständnis! Bonn, 11. Mai 2006 TSS: QCD bei HERA

ERFAHRUNGEN BEI HERA Parameter-Anpassung und neue Ideen(?) Schon ältere Analysen (4-jet-Ereignisse in Photoproduktion) zeigten/quantifizierten UE-Bedarf. Neue 3-Jet-Analyse in Photoproduktion untersucht relevante Parameter und initiiert Diskussion mit Theoretikern! Mit UE Ohne UE Nicht pile-up – das haent von Lumi ab Nur bei HERA kann man das Phaenomen sinnvolls studieren; auch im Zusammenhang Diffraktion, multiple Wechselwirkungen … Effekt bei HERA hier so gross, weil das 4jets sind … ZEUS Collab., DIS06 Bonn, 11. Mai 2006 TSS: QCD bei HERA

THEORIE-FRAMEWORKS Wichtig für effiziente Vergleiche Theorie-Daten • Vereinheitliche Zugang zu verschiedenen MC-Programmen oder NLO-Rechnungen  einfacherer Abgleich Daten-Theorie Idee Beispiele • HZTOOL (MC-Programme für Physik bei HERA) • JetWeb: Web-basierter Zugang zu verschiedenen MCs. • NLOLIB: NLO-Rechnungen für ee, ep, pp. NLOLIB K.Rabbertz und TSS, Proceedings des HERA-LHC-Workshops • DISENT, JetViP (Jets in DIS, bald DISASTER++, MEPJET). • RacoonWW (4f-Prozesse in ee) • NLOJET (Jet-Production in pp) • bald: FMNR (Photoproduktion schwerer Quarks in ep) Zugang zu verschiedenen Rechnungen zu einer Analyse drastisch vereinfacht! Bonn, 11. Mai 2006 TSS: QCD bei HERA

PARTON EVOLUTION SCHEMES The `forward jet’ question 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. New ZEUS - Tighter cuts than H1  same statistics from 72pb-1. R=Q. - 20 < Q2 < 100 GeV2. CDM and resolved can break up strong kt ordering (because resolved start from top), and resolver also mimicks partly higher orders. H1 also first to show difference between reoslved and CDM by requiring dijet system in addition. Resloved fails, CDM works more or less. Problem from 3g states – not implemented in standard NLO? Q2 scale iontroduces much larger scale uncertaintie in ZEUS than Et scale in H1. So without more ideas and more exclusive analysis no new insights. But we will have - to have more higher orders - some kt unordering … - no clear picture from CCFM – although it contains kt nonordering. It has often shape problems … No new experimental conclusions. Theory (Sassot et al.): Kinematics! Bonn, 11. Mai 2006 TSS: QCD bei HERA 17

Benutzung von Jet-Daten in ZEUS-Fits verbessert Gluondichte drastisch! JETS UND PARTONVERTEILUNGEN Zugang zur Gluondichte bei hohen x mit Jets ZEUS Collab., XXXX Die Gluondichte xg(x) ist bei hohen x im wesentlichen unbekannt (Summenregeln, Tevatron-Jets) Benutzung von Jet-Daten in ZEUS-Fits verbessert Gluondichte drastisch! x HEUTE Jet-Daten haben Zugang zur Gluon-dichte bei hohen x (Boson-Gluon-Fusion): And this with only the limited stats from 9697! Stat not the problem! Simult fit of alphas and PDFs with and wihtout jets. Ende 2007 s Bonn, 11. Mai 2006 TSS: QCD bei HERA

NEUES AUS DER THEORIE … hinkt bei HERA dem Experiment hinterher … Problem Viele QCD-Messungen sind theoretisch limitiert (NLO!) - Kopplung S, Jetwirkungsquerschnitte, … Fortschritt – Rechnungen mit höheren Ordnungen da oder greifbar nahe. - 2-Loop-Splitting-Funktionen (F2 in NNLO) existieren! - NNLO für Jets bald? (Problem: Matching der Beiträge) – Kombination von NLO-Rechnungen mit Partonschauer- Algorithmen ist gross in Mode: - korrekte Beschreibung harter und weicher Prozesse in EINER Rechnung! - Problem 1: Negativer Gewichte in Partonschauern? - Problem 2: Vermeidung von Doppel-Zählungen von Gewichten? – Resummationsrechnungen z.B. für “Event shapes” bei HERA B. Pötter und TSS PLB XXXXX And this with only the limited stats from 9697! Stat not the problem! Simult fit of alphas and PDFs with and wihtout jets. Leider Viele Fortschritte in e+e– (“leicht”) oder pp (“wichtig”). – Vieles gar nicht / verzögert für HERA gerechnet – Schade, weil daraus resultierender Gewinn auch wichtig z.B. für LHC (PDFs, Partondynamik, …) Bonn, 11. Mai 2006 TSS: QCD bei HERA

ZUSAMMENFASSUNG Fehlt noch … And this with only the limited stats from 9697! Stat not the problem! Simult fit of alphas and PDFs with and wihtout jets. Bonn, 11. Mai 2006 TSS: QCD bei HERA

BACKUP Bonn, 11. Mai 2006 TSS: QCD bei HERA

DIJETS IN PHOTOPRODUCTION In principle access to photon PDF. Aim: Compare to NLO calculations and exploit sensitivity to  structure via x (gluon!) Selection: – 39pb-1 from 1996/97. – ET > 14 (11) GeV – separation power between different photon PDFs! Also useable for proton PDFs, especially xg(x) via xp = 1/2Ep Summe Et exp(+eta) Xgamma = 1/(2yEe) Summe Et exp(-eta) W between 134 and 277 Jets above 14/11 NLO frixione ridolfi … In principle these data provide separation power between different photon PDFs, But … Problem: Highest sensitivity at low ET  subtraction of underlying event – but great model dependence! We need a dedicated study of the underlying event at HERA! Bonn, 11. Mai 2006 TSS: QCD bei HERA

JET PHYSICS: UNDERLYING EVENT? In out picture of collinear factorisation of hard and soft contributions to the Xsection we can divide the calculation in the PDFs (input from inclusive measurements) and the hard partonic matrix element which we then convolute for each order in alphas and for each quark flavour and the gluons seprately. Depending on how you look at the Xsection you can do all the things mentioned. Bonn, 11. Mai 2006 TSS: QCD bei HERA

JET PHYSICS: BASICS II Algorithms, reference frames, theory, uncertainties, EFOs Jet algos / 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). Also EFOs already corrected. Breit frame because there can ensure, via ET cut, that QCD process was happening (modulo fragmentation …) Fixed order NLO: correct wide angle / hard physics description, no detector, no hadronisation, correct normalisation LO MC: detector, hadronisation, soft phenomena, small angle (jet structure). NLO+PS can combine the best of both worlds … Tylical uncertainty on data is 10% 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? Bonn, 11. Mai 2006 TSS: QCD bei HERA

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 ¶ MORE DETAILED STUDIES ¶ CONCLUSIONS AND OUTLOOK Bonn, 11. Mai 2006 TSS: QCD bei HERA

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) Use to extract alphas and PDFs (in old data )  wait minute Checked thoroughly since it is THE analysis. Also compared to old data (37pb-1 from 9697). Bonn, 11. Mai 2006 TSS: QCD bei HERA

INCLUSIVE JET CROSS-SECTIONS (DIS) Also double-differentially, comparison to NLO Input to QCD fits. xg(x). Say a word about the size of the uncertainties … Bonn, 11. Mai 2006 TSS: QCD bei HERA

DI / TRIJETS IN DIS Testing 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 some uncertainties cancel out (lumi, partly energy scales, etc.) Subtilitaet: asymetric ET cuts to have NLO H1 use Q2 as scale – has smaller scale uncertainties in this phasespace. Nice possibility to extract S! Bonn, 11. Mai 2006 TSS: QCD bei HERA

EXTRACTION OF S Typically done using interpolation Input: 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). Parame- trisation: Functional dependence on s(MZ) then approximated by -- Ai, Bi determined in fit. Result: Use function to map measured cross-section to value of s(MZ). Finally combine single points or evolve to correct scale to show running of alphas. Plot is stolen from H1. Bonn, 11. Mai 2006 TSS: QCD bei HERA

S FROM INCLUSIVE, 2, 3 JETS Theory starting to dominate measurement ZEUS inclusive jets: s(MZ) = 0.11960.0025(exp)0.0023(theo) ZEUS R3/2: s(MZ) = 0.11790.0013(stat) 0.0037(exp)0.0055(theo) Title means that theory is larger than stat or syst, not than both combined. In boxes up and down errors averaged For H1 R3/2 stat error is 0.0017, syst is 0.0050  syst dominates! For R3/2: Averaged up ad down exp error and theo error. H1 : pm 0.0017(stat) pm 0.0050. World average: s(MZ) = 0.11870.0020 H1 inclusive jets: s(MZ) = 0.11970.0016(exp)0.0047(theo) Bonn, 11. Mai 2006 TSS: QCD bei HERA

SUMMARY ON HERA S (C. Clasman) Consistency of all results is an important check! Nice demon- strating of running in HERA data. Bethke with world average includes only NNLO results (tau decay etc.), HEA only NLO  l;arger theo error. One should mention that more and more combined effort between H1 and ZEUS (alphas, F2, fits). HERA : S(MZ)=0.1186±0.0011(exp.)±0.0050(th.) (only NLO) Bethke: S(MZ)=0.1182±0.0027 (all NNLO) Bonn, 11. Mai 2006 TSS: QCD bei HERA

ACCESS TO PDFs WITH JETS Hope: Improve gx(x,Q2) at high values of x Problem Gluon density xg(x) at high x basically unknown – constrained only by momentum sum rules and Tevatron jets with large ET. Uncertainties are very large: – 15% at x=0.3, – 200% at x=0.5. Idea Jet data provide access to high x gluon via BGF process. s Problems … exploiting this feature: … Many … CTEQ is only simple parametrisation, no real fit – but they make a secret from it. QCDC BGF Bonn, 11. Mai 2006 TSS: QCD bei HERA

ACCESS TO PDFs WITH JETS Reminder: Thesis M ACCESS TO PDFs WITH JETS Reminder: Thesis M. Wobisch (2000), xg(x) from jets Bonn, 11. Mai 2006 TSS: QCD bei HERA

METHOD How to get jets NLO in <1s? Problem • Evaluation of NLO jet cross-sections: 8 hours for 50M events. • PDF fit requires O(1000) evaluations  PROBLEM! Assumption • PDFs are approx. flat in small bins of x and f. Flat! • 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. Grids become potentially very large, especially when complicated interpolations are tried Which migt be needed at large values of x where steep PDF behaviour … Table! Simplification – 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 !!!!! Bonn, 11. Mai 2006 TSS: QCD bei HERA

THE METHOD WORKS for 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. Actually now done also by Carli for HERA-LHC – with similar results. Bonn, 11. Mai 2006 TSS: QCD bei HERA

INPUT DATA in addition to F2 Photo- production dijets And this with only the limited stats from 9697! But stat not entscheidend! High-Q2 inclusive jets Bonn, 11. Mai 2006 TSS: QCD bei HERA

EFFECT ON STRONG COUPLING Very much constrained by jets data Strong correlation between s and the gluon density for in- clusive F2 data  large increase in gluon uncertainty when s free! F2 + jets F2 + alone When including jets the gluon error does not blow up so much when fitting alphas and gluon together. 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 Bonn, 11. Mai 2006 TSS: QCD bei HERA

OVERVIEW ¶ INTRODUCTION ¶ BASIC JET PHYSICS AT ZEUS ¶ MORE DETAILED STUDIES • Parton evolution in the proton • Transition from photoproduction to DIS • More selected topics from jet physics • Event-shapes and charged multiplicities ¶ CONCLUSIONS AND OUTLOOK Bonn, 11. Mai 2006 TSS: QCD bei HERA

PARTON EVOLUTION SCHEMES The `forward jet’ question Starting point DGLAP approximation resumming terms lnQ2 for parton evolution works very well for most of HERA regime (F2!) Question Breakdown expected at very low x! Can we distinguish the onset of BFKL-like evolution in ln1/x?  “forward jets”. Forward jets Design phase-space to suppress DGLAP and enhance BFKL: DGLAP: kT ordering! BFKL: x ordering! kT large x large forward region:  > 2 (close to proton) jet ET2 ~ Q2 (suppressed in DGLAP) large xjet=Ejet/Eproton (realized in BFKL) All results so far: Problems at low x! But not firm conclusions drawn: -- NNLO terms missing? -- Resolved contribution? -- BFKL evolution scheme? -- pure kinematics? Forward or Mueller-Navelet jets. New H1 results point to simple kinematics / missing ggg terms in NLO / New paper shows that …effectively forward jets start only at alphas^2 so that this is EFFECTIVELY THE lo only so that the problems are no wonder. H1 ggg fiding plays also along this line. And probably also the forward plus dijets system thing … Bonn, 11. Mai 2006 TSS: QCD bei HERA

FROM PHOTOPRODUCTION TO DIS The transition region and its problems Resolved For ET2>Q2 parton resolves structure of photon x=1 “direct” x<1 “resolved” Aim Quantify amount of resolved as function of Q2 and ET with R=res/dir. R as function of Q2 – because resolved suppressed with Q2 or Et2/Q2. Things seemto depend a lot on mur. Of course result depends on where you look. Remember H1 triple-diff result. Used: dijets in gammap and DIS Q2 <= 2000, Et > 7.5, 5. 38inverse pb DISASTER++ direct Frixione ridolfi. Problem for NLO analysis in DIS? Results Resolved relevant even if Q2>100 GeV2. Scales? p NLO, MC+PS+res okay! No NLO theory for resolved in DIS! JetViP (Pötter) has problems. Bonn, 11. Mai 2006 TSS: QCD bei HERA

23: angle between two lowest- energy jets in 3-jet events! COLOR DYNAMICS IN JET EVENTS Testing the underlying gauge group (SU(3)C) like LEP Aim 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: 23: angle between two lowest- energy jets in 3-jet events! CF2 CFCA CFTF TFCA Is QCD really the right thing – it is defined by the color factors.Fits from LEP for alphas point towars SU(3)C. What does HERA say? Color factors define dynamics of the gauge group. Various contributions differ in shape – separate! DIS: 98-00, 82inverse pb, kt in Breit, Q2 > 125 GeV. Et > 8 und 5. Gammap: all HERA I, 127 inverse pb, threejets above 14 geV, MJJ etc. cuts. Sensitivity to different contributions  hope to test gauge structure! Bonn, 11. Mai 2006 TSS: QCD bei HERA

COLOR DYNAMICS IN JET EVENTS in DIS and photoproduction DIS: 82 pb-1, Q2 > 125 GeV2, ET > 8 / 5 GeV, 3 jets. Photoproduction: all HERA I (127pb-1), ET > 14 GeV, 3 jets. DIS: 1015 events p: 2233 events SU(3) favoured, but U(1)3 not excluded! Bonn, 11. Mai 2006 TSS: QCD bei HERA

SUBJET DISTRIBUTIONS in high Q2 DIS – study QCD radiation pattern / jet structure Aim - 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. Method Define subjets by applying k algo to objects of one jet as function of distance measure dcut=ycut·ET2. Chosen here: Jets with two subjets at ycut=0.05 (hadronisation)! Increasing ycut Note for experts: Running NLO O(S2) in lab frame  jets with  3partons from fixed-order calculation! Number of subjets = number of objects resolved at a given clustering value y. Trick in NLO here: Run DISENT NLO O(alphas2) in lab frame  can have up to three partons in one jet. Artificially enrich the parton level calculation.  get closer to real life. Ycut value chosen because hadronisation corrections smallest. Test variables ETsub/ETjet, sub-jet,|sub-jet| and orientation of subjets in - space with respect to proton beam. Bonn, 11. Mai 2006 TSS: QCD bei HERA

SUBJET DISTRIBUTIONS in high Q2 DIS – study QCD radiation pattern / jet structure 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 Also used to derive the gluon and the quark initiated part – data dominated here by q-initiated part Also LO MC with PS and CDM can describe the data – but they have a rich and tuned inner life. All distributions nicely described by NLO QCD within 10%  radiation pattern understood / correctly implemented! Bonn, 11. Mai 2006 TSS: QCD bei HERA

INTERJET ENERGY FLOW in photoproduction dijet events with large rapidity gaps Idea 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 Compare to MC models with(out) color singlet (CS) contribution: -- PYTHIA: high-t- exchange (MPI); -- Herwig: BFKL-Pomeron (JIMMY) added to dir+res. Dijets in photoproduction events from 96/97. What is this useful for? Might help for better understanding of diffractive and underlying event. Study as function of amount of energy between jets Etgap. Selection: Photoproduction dijets with 6/5 GeV, |eta|<2.4 and large rap separation of jets between 2.5 and 4 units in rapidity. 38pb-1 von 96/97. 0.2<y<0.85. Normal MC: Problems if very little or no energy Etgap!  Try other models with CS contribution: PYTHIA high-t-gamma exchange (fake), HERWIG with BFKL-Pomeron. Necessary CS contribution 3-4% total distributions. If we increase CS contribution by requiring Etgap < Etcut=1 GeV  20-50% CS needed – depending on separation of jets – the farther apart the higher the CS contribution necessary to describe the data. As function of rapidity separation normal predictions fall exponentially – as you would expect since it becomes more and more improbable to NOT radiate anything the farther the two jets are apart. But data and CS models level out! Fall=-off only seen in data and CS MC at low ETCUt. Result Globally 3-4% CS exchange needed to account for small Etgap cross- section. Amount depends on rapidity separation of jets (up to 50%). Bonn, 11. Mai 2006 TSS: QCD bei HERA

INTERJET ENERGY FLOW in 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 Bonn, 11. Mai 2006 TSS: QCD bei HERA

EVENT SHAPES: MEANS as a means to test power corrections (in high Q2>80 GeV2) Idea Alternative way to describe non- pert. hadronisation corrections to event shapes. PC depend on one universal parameter 0. Example: Thrust (long. collimation) Idea from webber et al. So no need to do LO mC hadronisation corrections. Why not for jets etc.? Not inclusive enough an object? P is …???? Sum over all particles in current of Breit (except for y2 and kout, there all breit). PC approach works for means … … but fit to S and 0 difficult. Bonn, 11. Mai 2006 TSS: QCD bei HERA

CHARGED MULTIPLICITIES At 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. LEP uses sqrt(s), Tevatron sqrt of sum of squared differences of fourvectors of incoming and through-beampipe-escaping particles. Ecurr is the energy of all current hemisphere particles (tis takes into account QCD radiation, hadronisation, soft processes etc.). Result: – HERA exhibits same energy dependence as LEP and Tevatron. Mean charged multiplicity. Bonn, 11. Mai 2006 TSS: QCD bei HERA

CONCLUSION AND OUTLOOK HERA is a great machine for QCD! HERA ZEUS – 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! Laboratory because you can adjust the experimental conditions rather well. Bonn, 11. Mai 2006 TSS: QCD bei HERA

EVENTS IN ZEUS … our event display Actually H1 display nicer … Bonn, 11. Mai 2006 TSS: QCD bei HERA

Momentum distribution. URANIUM-SCINTILLATOR CALORIMETER provides good control on beam, timing … Momentum distribution. F/B/RCAL timing. Depleted Uranium 238, compensates by energy loss from fast neutrons released in fission process. Energy loss due to nuclear break-up. Equal response to hadrons and electrons within 1-2%. F/B/RCAL, with 23/32/23 moduls, and 4 (1/2) EMC/HAC sections. Typical size 20*5 cm 12000 PMT EMC: 1 interactionlength ~25 X0 HAC: 3-6 lambda. Use U noise to calibrate. Imbalance between left and right PMT for each EMC FCAL cell. Bonn, 11. Mai 2006 TSS: QCD bei HERA

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. Method • Combine cells to ‘cone islands’. • Match tracks to islands. • Decide which information to use. I think in H1 this used to be called CLIM or FSCOMB objects – any changes since then? Right top plot: energy/momentum resolution as function of particle E/pT. Clearly visible: Point where change from tracker to CAL is sensible. Bottom plot: left: Resolution for calorimeter (hadronic) E-pz as function of yjb. Result: EFOS (solid line best). Top line is cells alone. Bottom right: relative Error on hadronic E-pz: 20% improvement on yjb at low values. Result • Resolution in pT,h and (E-pz)h. improves by 20%. Bonn, 11. Mai 2006 TSS: QCD bei HERA

JET ENERGY CORRECTIONS to 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. 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: offset ~0.1,slope ~0.98. - Find for each detector jet a close hadron jet (1 or 0.5 in eta-phi space). - plot correlation and fit. offset ~1,slope ~0.8. Bonn, 11. Mai 2006 TSS: QCD bei HERA

EVENT CORRECTIONS to account for detector/QED/Z0 effects Data detektor akzeptance, efficiency QED running, radiation Comparison Z0 exchange hadroni-sation NLO Bonn, 11. Mai 2006 TSS: QCD bei HERA

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. 96/97 was about 36 inverse pb. Interest to measure in new data although no big improvement in precision expected – we are not dominated by theory. Bonn, 11. Mai 2006 TSS: QCD bei HERA

TRIJETS IN DIS Testing higher orders in pQCD Selection – ~80pb-1 from 98-00 ET > 5 GeV (Breit) – 10 < Q2 < 5000 GeV2 invariant dijet mass > 25 GeV Subtilitaet: asymetric ET cuts to have NLO H1 use Q2 as scale – has smaller scale uncertainties in this phasespace. Bonn, 11. Mai 2006 TSS: QCD bei HERA

TRI- AND DIJETS IN DIS in the same phase-space. Idea: In ratio of dijets to trijets some uncertainties cancel out (lumi, partly energy scales, etc.) Theory: Nice possibility to extract the strong coupling with rather small uncertainties! Bonn, 11. Mai 2006 TSS: QCD bei HERA

S FROM INCLUSIVE JETS Theory starting to dominate measurement ZEUS inclusive jets: s(MZ) = 0.11960.0025(exp)0.0023(theo) Title means that theory is larger than stat or syst, not than both combined. In boxes up and down errors averaged For H1 R3/2 stat error is 0.0017, syst is 0.0050  syst dominates! World average: s(MZ) = 0.11870.0020 H1 inclusive jets: s(MZ) = 0.11970.0016(exp)0.0047(theo) Bonn, 11. Mai 2006 TSS: QCD bei HERA

S FROM THE RATIO R3/2. Cancellation of some uncertainties Averaged up ad down exp error and theo error. ZEUS R3/2: s(MZ) = 0.11790.0013(stat) 0.0037(exp)0.0055(theo) Bonn, 11. Mai 2006 TSS: QCD bei HERA

SUMMARY ON S FROM ZEUS (C. Clasman) Bonn, 11. Mai 2006 TSS: QCD bei HERA

RESULTS Massive reduction in gluon density uncertainty And this with only the limited stats from 9697! Bonn, 11. Mai 2006 TSS: QCD bei HERA

EFFECT ON STRONG COUPLING Very much constrained by jets data Massive improvement on value and error. Also correlation a bit decoupled because with QCDC no correlation  In common fit of alphas and gluon gluon error does not blow up so much if jets are included.  next slide Bonn, 11. Mai 2006 TSS: QCD bei HERA

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! Difference between PYTHIA and PHOJET from regularisation procedure for the behaviour of soft particles  different behaviour  large uncertainty for softest jets  large uncertainty for gamma PDF extraction. Quarks in gamma well restricted from LEP, gluon more interesting. The plots old stuff: 7.2 pb^-1 from 96. Jets abouve 4 and 6 GeV, Mjj > 12. Gluon density in the photon as extracted from H1 dijet p data. Agreement with other determinations; very large errors. Bonn, 11. Mai 2006 TSS: QCD bei HERA