Luca Stanco - PadovaLow-x at HERA, Small-x Low-x AND Low Q 2 Luca Stanco – INFN Padova Small-x and Diffraction 2007 Workshop FermiLab, March 28-30, 2007 on behalf of H1 and ZEUS collaborations at HERA with JETS

Luca Stanco - PadovaLow-x at HERA, Small-x Processes at HERA Electron beam Proton beam x Bj < 1 DIFFRACTION x  = 1 Q 2 > 0 Photon* DIS x  < 1 photon RESOLVED  -PRODUCTION x  = 1 Q 2 = 0 photon DIRECT  -PRODUCTION gluons… quarks PARTONS (defined by Initial States of HARD Scattering) x Bj << 1 low-x regime

Luca Stanco - PadovaLow-x at HERA, Small-x Tevatron LHC HERA HERA exploits many (correlated) processes over large kinematic regimes ! X Bj Q2Q2 Low-x “regime” perturbative Non-pert To extrapolate to other regions need to take care of low-x effects: saturation multiple interactions QCD DYNAMICS (DGLAP vs BFKL evolutions) study at HERA:

Luca Stanco - PadovaLow-x at HERA, Small-x if nobody else have already shown... Gluons and QCD dynamics

Luca Stanco - PadovaLow-x at HERA, Small-x Outline of the talk: a little bit of theory, modelling, Monte Carlo’s… ”Forward” Jets in DIS H1 (1997 data, final analysis) ZEUS (1998/00 data, preliminary analysis) Multi Jets in DIS H1 (1999/00 data, preliminary analysis) ZEUS (1998/00 data, preliminary analysis) Hard FORWARD Jets (near proton direction where effects of gluon radiation should be largest - A. Mueller 1990)

Luca Stanco - PadovaLow-x at HERA, Small-x before going into physics, let us look at the environment... HeraI: H1 and ZEUS took about 130 pb “luminosity upgrade” 2002/03 background problems 2004 positrons (50 pb -1 ) 2005: electrons 2006: positrons 2007: from March HERA for Low Energy Run (F L measure) END of HERA  next PETRAIII Results on JETS mainly from HERA-I (up till now) H1 and ZEUS will end up with ~ 0.5 fb -1, each Luminosity growth at HERA (I: 92-00, II: 03-07)

Luca Stanco - PadovaLow-x at HERA, Small-x QCD dynamics: Evolution & Resummation HERA: DIS at low-x Bj down to  large  *p center of mass energy (W ~ 1/x )

Luca Stanco - PadovaLow-x at HERA, Small-x saturation Inclusive F2 measurement not able to discriminate between different QCD dynamics  Study of Hadronic Final States (JETS)

Luca Stanco - PadovaLow-x at HERA, Small-x if nobody else is going to show... Definitively, physics has to be clarified. even if we consider Structure Functions results at low-x

Luca Stanco - PadovaLow-x at HERA, Small-x  FORWARD JETS (proton direction) possibility to study parton dynamics, by looking for effects beyond DGLAP HERA extends x Bj until (with jets until ) Where ? in the BREIT frame: where virtual boson is enterely spacelike at lowest order, quark is back scattered Caveat: different Monte Carlo models not fully clear where applicable

Luca Stanco - PadovaLow-x at HERA, Small-x JETS observables in the BREIT Frame … more technically

Luca Stanco - PadovaLow-x at HERA, Small-x NLO pQCD calculations and interfaced to various PDFs (like CTEQ, MRST) Parton Level cross sections – need to rely on LO MC for hadronisation corrections

Luca Stanco - PadovaLow-x at HERA, Small-x LO-DGLAP fails completely ! NLO-DGLAP well below data at low-x Monte Carlo models with extra parton radiation provide an improved description of the data inclusion of a resolved-photon component (RAPGAP: DI+RES) parton emissions not ordered in k T (CDM) H1 data in the kinematic region: FORWARD JETS L=13.7 pb <x< and 5<Q 2 <85 GeV < (p T,jet ) 2 /Q 2 < 2 to suppress DGLAP and enhance BFKL

Luca Stanco - PadovaLow-x at HERA, Small-x ZEUS data less striking… FORWARD JETS L=81.8 pb -1 E  jet >5 GeV, 2<  jet < <x< and 20<Q 2 <100 GeV < ( E  jet ) 2 /Q 2 < 2

Luca Stanco - PadovaLow-x at HERA, Small-x Is it a HERA business ? or HERA phase space too limited to distinguish between different evolution models ? ZEUS data less striking… (NLO(  S 2 )-DISENT MC below the data but within theoretical errors)  needs of next NLO ? ( but ZEUS uses  R 2 =Q 2, while H1 takes  R 2 = ) FORWARD JETS

Luca Stanco - PadovaLow-x at HERA, Small-x Very detailed investigation from H1 to enhance BFKL phase space: Compute differential cross-sections of  1 &  2 Select dijets plus forward-jet different  1 and  2 regions affect: Kt-ordering Gluon radiation Evolution chain

Luca Stanco - PadovaLow-x at HERA, Small-x Data towards NLO too little Phase Space for NLO FORWARD JETS

Luca Stanco - PadovaLow-x at HERA, Small-x Data towards CCFM Data towards CDM and  res  res not sufficient … CDM may mimic BFKL evolution … NO description ! FORWARD JETS

Luca Stanco - PadovaLow-x at HERA, Small-x MULTIJETS (dijets, trijets) ANALYSIS Kinematic selection to enhance (unordered) gluon radiation and to study parton dynamics (at low-x) H1:  Ldt = 44.2 pb <x Bj <10 -2, 5<Q 2 <80 GeV 2 ZEUS:  Ldt = 82.2 pb <x Bj <10 -2, 10<Q 2 <100 GeV 2 Hadronic Center-of-Mass (like Breit-frame boosted) Comparisons with NLO at order O(  S 2 ) and O(  S 3 ) NLOJET, DISENT Conparisons with Monte Carlo’s LO-PS with/out  res (CDM-DJANGOH, RAPGAP) H1 compares to parton level, ZEUS compares to hadron level

Luca Stanco - PadovaLow-x at HERA, Small-x All the sample: H1 Low-x region: more DATA than NLO predicted MULTIJETS LO discrepancy E  *> 4 GeV, E  1 *+E  2 *> 9 GeV Overall systematics error of 19% not included on DATA -1 <  jet < 2.5

Luca Stanco - PadovaLow-x at HERA, Small-x H1 Select TWO subsamples with Forward JETS (  jet 0.035) Two central jets plus one forward One central jets plus two forward NLO++ comparison NLO at O(  S 3 ) describes well the TWO central jets, expected to be produced from hard scattering MULTIJETS -1 <  jet < 1 FORWARD jets are expected to be produced from gluon radiation

Luca Stanco - PadovaLow-x at HERA, Small-x LO Monte Carlo’s comparison H1 Resolved contribution NOT working ! RAPGAP: direct+resolved contributions MULTIJETS

Luca Stanco - PadovaLow-x at HERA, Small-x ZEUS Reasonable description by NLO JET MULTIJETS Hadron Level Jet energy scale uncertainty E*  1 > 7 GeV, E*  2,3 > 5 GeV -1 <  jet < 2.5  R 2 =  F 2 = ( 2 +Q 2 )/4

Luca Stanco - PadovaLow-x at HERA, Small-x ZEUS MULTIJETS NLO JET comparison with Dijets DATA NLO consistent within errors GeV GeV 2 O(  S 2 ) does not describe DATA

Luca Stanco - PadovaLow-x at HERA, Small-x ZEUS NLO JET comparison with Trijets DATA … but theor. errors (  F,  R ) too large Need next order MULTIJETS

Luca Stanco - PadovaLow-x at HERA, Small-x Conclusions low-x Bj regime unsettled at HERA Multijets (Forward) production measured in x BJ >10 -4 and Q 2 <100 GeV 2 Statistical ~ Theoretical errors (NLO) need more data and theory computations unordered gluon emission significant at low-x ? A VERY EXCITING FIELD WHERE TO LOOK DEEPLY INSIDE QCD PARTON EVOLUTION DYNAMICS

Luca Stanco - PadovaLow-x at HERA, Small-x Backup Slides

Luca Stanco - PadovaLow-x at HERA, Small-x Kinematics of Neutral Current (NC) Deep Inelastic Scattering (DIS) W:  *P c.o.m. energy

Luca Stanco - PadovaLow-x at HERA, Small-x WHY studying pQCD ? at least 4 answers: 1.NOT TRUE it is so well understood.. 2.Strong coupling,  S, measurement (see next talk) 3.Parton Density Functions measurements 4.Parton evolutions From HERA-LHC workshops, examples of uncertainties,

Luca Stanco - PadovaLow-x at HERA, Small-x Z.Czyczula et al., p T of Higgs (120 GeV) from different Parton-Showers (PS) models and QCD scale choices Uncertainties: up to 170% due to PS (which evolution type ? which ordering ?) up to 100% due to the choice of QCD scale (Q 2, m 2, ŝ etc.)  low-x and forward jets…

Luca Stanco - PadovaLow-x at HERA, Small-x two different parton ladders….  Expect more energetic jets in forward region (LEPTO, DISENT) (~ColorDipM) (CASCADE)

Luca Stanco - PadovaLow-x at HERA, Small-x JETS in DIS The BREIT frame ! Frame where virtual boson is enterely spacelike At lowest order, quark is back scattered In pratice, high Et jet events must be at least of order  S

Luca Stanco - PadovaLow-x at HERA, Small-x Jets Production in NC Deep Inelastic Scattering

Luca Stanco - PadovaLow-x at HERA, Small-x jets event at low-x (H1)