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Low-x AND Low Q 2

Small-x and Diffraction 2007 Workshop FermiLab , March 28-30, 2007. Low-x AND Low Q 2. at HERA with JETS. Luca Stanco – INFN Padova. on behalf of H1 and ZEUS collaborations. HERA exploits many (correlated) processes over large kinematic regimes !. Q 2. X Bj. LHC. Tevatron.

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Low-x AND Low Q 2

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  1. Small-x and Diffraction 2007 Workshop FermiLab, March 28-30, 2007 Low-x ANDLow Q2 at HERA with JETS Luca Stanco – INFN Padova on behalf of H1 and ZEUS collaborations Low-x at HERA, Small-x 2007

  2. HERA exploits many (correlated) processes over large kinematic regimes ! Q2 XBj LHC Tevatron • To extrapolate to other • regions need to take care • of low-x effects: • saturation • multiple interactions perturbative HERA study at HERA: Non-pert QCD DYNAMICS (DGLAPvsBFKLevolutions ?) Low-x “regime” Low-x at HERA, Small-x 2007

  3. ...if nobody else have already shown ... Gluons and QCD dynamics Low-x at HERA, Small-x 2007

  4. 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) Low-x at HERA, Small-x 2007

  5. before going into physics, let us look at the environment... Luminosity growth at HERA (I: 92-00, II: 03-07) • HeraI: H1 and ZEUS took • about 130 pb-1 • 2001 “luminosity upgrade” • 2002/03 background problems • 2004 positrons (50 pb-1) • 2005: electrons • 2006: positrons • 2007: from March HERA for • Low Energy Run (FL measure) • END of HERA  next PETRAIII H1 and ZEUS will end up with ~ 0.5 fb-1, each Results on JETS mainly from HERA-I (up till now) Low-x at HERA, Small-x 2007

  6. QCD dynamics: Evolution & Resummation HERA: DIS at low-xBj down to 10-5  large g*p center of mass energy (W ~ 1/x) Low-x at HERA, Small-x 2007

  7. saturation Inclusive F2 measurement not able to discriminate between different QCD dynamics  Study of Hadronic Final States (JETS) Low-x at HERA, Small-x 2007

  8. HERA extends xBj until 10-5 (with jets until 10-4) • FORWARD JETS (proton direction) possibility to study parton dynamics, by looking for effects beyond DGLAP • 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 Low-x at HERA, Small-x 2007

  9. JETS observables in the BREIT Frame … more technically Low-x at HERA, Small-x 2007

  10. NLO pQCD calculations and • interfaced to various PDFs (like CTEQ, MRST) • Parton Level cross sections – need to rely on LO MC for hadronisation • corrections Low-x at HERA, Small-x 2007

  11. FORWARD JETS H1 data in the kinematic region: L=13.7 pb-1 10-4<x<4.10-3 and 5<Q2<85 GeV2 • 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 kT (CDM) 0.5 < (pT,jet)2/Q2 < 2 to suppress DGLAP and enhance BFKL Low-x at HERA, Small-x 2007

  12. FORWARD JETS ZEUS data less striking… L=81.8 pb-1 4.10-4<x<5.10-3 and 20<Q2<100 GeV2 Ejet>5 GeV, 2<hjet<3.5 0.5 < (Ejet)2/Q2 < 2 Low-x at HERA, Small-x 2007

  13. FORWARD JETS ZEUS data less striking… (NLO(aS2)-DISENT MC below the data but within theoretical errors)  needs of next NLO ? ( but ZEUS uses mR2=Q2, while H1 takes mR2=<pT2> ) Is it a HERA business ? or HERA phase space too limited to distinguish between different evolution models ? Low-x at HERA, Small-x 2007

  14. Very detailed investigation from H1 to enhance BFKL phase space: Select dijets plus forward-jet • differentDh1 and Dh2 • regions affect: • Kt-ordering • Gluon radiation • Evolution chain Compute differential cross-sections of Dh1 & Dh2 Low-x at HERA, Small-x 2007

  15. FORWARD JETS Data towards NLO too little Phase Space for NLO Low-x at HERA, Small-x 2007

  16. FORWARD JETS Data towards CCFM NO description ! Data towards CDM and gres gres not sufficient … CDM may mimic BFKL evolution … Low-x at HERA, Small-x 2007

  17. MULTIJETS (dijets, trijets) ANALYSIS Kinematic selection to enhance (unordered) gluon radiation and to study parton dynamics (at low-x) H1: Ldt = 44.2 pb-1 10-4<xBj<10-2, 5<Q2<80 GeV2 ZEUS: Ldt = 82.2 pb-1 10-4<xBj<10-2, 10<Q2<100 GeV2 Hadronic Center-of-Mass (like Breit-frame boosted) Comparisons with NLO at order O(aS2) and O(aS3) NLOJET, DISENT Comparisons with Monte Carlo’s LO-PS with/out gres (CDM-DJANGOH, RAPGAP) H1 compares to parton level, ZEUS compares to hadron level Low-x at HERA, Small-x 2007

  18. MULTIJETS H1 All the sample: LO discrepancy E*> 4 GeV, E1*+E2*> 9 GeV Low-x region: more DATA than NLO predicted -1 < hjet < 2.5 Overall systematics error of 19% not included on DATA Low-x at HERA, Small-x 2007

  19. MULTIJETS H1 Select TWO subsamples with Forward JETS NLO++ comparison (qjet<200, xjet=E*jet/Ep > 0.035) Two central jets plus one forward One central jets plus two forward -1 < hjet < 1 NLO at O(aS3) describes well the TWO central jets, expected to be produced from hard scattering FORWARD jets are expected to be produced from gluon radiation Low-x at HERA, Small-x 2007

  20. MULTIJETS H1 LO Monte Carlo’s comparison RAPGAP: direct+resolved contributions Resolved contribution NOT working ! Low-x at HERA, Small-x 2007

  21. MULTIJETS ZEUS Reasonable description by NLOJET Jet energy scale uncertainty E*1> 7 GeV, E*2,3> 5 GeV Hadron Level mR2=mF2 = (<ET>2+Q2)/4 -1 < hjet < 2.5 Low-x at HERA, Small-x 2007

  22. MULTIJETS ZEUS NLOJET comparison with Dijets DATA O(aS2) does not describe DATA 20 < Q2 > 30 GeV2 10 < Q2 > 15 GeV2 NLO consistent within errors Low-x at HERA, Small-x 2007

  23. MULTIJETS ZEUS NLOJET comparison with Trijets DATA … but theor. errors (mF, mR) too large Need next order Low-x at HERA, Small-x 2007

  24. Conclusions • low-xBj regime unsettled at HERA • Multijets (Forward) production measured in • xBJ>10-4and Q2<100 GeV2 • 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 Low-x at HERA, Small-x 2007

  25. Backup Slides Low-x at HERA, Small-x 2007

  26. 5 jets event at low-x (H1) Low-x at HERA, Small-x 2007

  27. ...if nobody else is going to show ... even if we consider Structure Functions results at low-x Definitively, physics has to be clarified. Low-x at HERA, Small-x 2007

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