# Presentation: 529547

## Low-x QCD dynamics probedby multijets at HERA 

**Outline:**
**Introduction**
**Multijet production at low-x in DIS**
**Angular correlation between jets**
**Forward jets**
**Three and four-jet final states in PHP**
**Summary and Outlook**

## e±p scattering at HERA

***HERA (1992-2007)***
***820-920 GeV p ************ 27 GeV e******±***
***2001 HERA II Lumi upgrade + polarised e******±***
***Machine operation ended in June 2007***
***About 0.5 fb******-1****** per experiment collected***
***Q******2****** (photon virtuality):***
***Q******2******~0 (<1 GeV******2******) ************ Photoproduction: photon-proton scattering ***
***Q******2******>1 GeV******2  ******       ************ Deep Inelastic Scattering (DIS), probing the proton structure***

***e******±******p***** scattering at HERA**

***√******s*****   =  300-318 GeV (Energy C.M.)**
***Q******2*****    ****=  Photon Virtuality**
*** ******x******Bj  *****=  energy carried by the**
**            ****struck parton in the proton**

**Leading Order**

## Parton Dynamics at low x in e±p collisions

**Parton Dynamics at low x in *****e******±******p***** collisions**

***Different approximations to the summation of    ***
***the perturbative expansion of parton evolution***
***DGLAP ******Σ******(******α******s******ln Q******2******)******n ***
***strong ordering in virtuality, i.e.                                               k******t,1******2****** << k******t,2******2 ******<<...<< Q******2****** ***
***weak ordering in x, i.e. x******1****** > x******2****** >...> x******Bj****** ***
***works very well at large Q******2***
***expected to fail at low Q******2****** and x***
***BFKL ******Σ******(******α******s******ln 1/x)******n****** ***
***random walk in k******t***
***strong ordering in x, i.e. x******1****** >> x******2****** >>...>> x******Bj****** ***
***expected to work well at low x***
***CCFM ******α******s******ln Q******2****** & ******α******s******ln 1/x ***
***angular ordering, i.e. ******θ******1****** << ******θ******2****** <<...<< ******θ******n****** ***
***expected to work at high Q******2****** and                                        low Q******2****** and x***
*********  ******DGLAP well established at HERA (PDFs QCD fits, F******2******). Look at measurements with better sensitivity to BFKL effects ...***

**Saturation?**

## NLO QCD calculations and Monte Carlo Models 

## Multijet at low x

**  ****NLOjet++ O(********S****3****) gives very good description of the   **
**    ****data (with possibly the exception at very low-x)**
**  ****CDM (DjangoH) gives also a good description**
**  ****Rapgap (Dir+Res) description not as good**

**Multijet at low x**

**H1-prelim-06-034**

**H1 99-00 data (44 pb****-1****):**
**DIS: ****4 **** < Q****2 ****< 80 GeV****2****;    **
**        ****10****-4 ****< x****Bj ****< 10****-2**
**   ****At least 3 jets with:**
**        ****E****T jet1****>4 GeV ;      **
**        ****E****T jet2****+E****T jet3****>9 GeV**
**        ****-1<********lab****<2.5;    **
**        ****one jet -1< ********lab****<1.3**

## ZEUS 98-00 data (82 pb-1):
DIS: 10 < Q2 < 100 GeV2 
        10-4 < xBj < 10-2 
2 or 3 jets with:
         Etjet1 > 7 GeV ;       
         Et jet2(3) > 5 GeV
         -1< lab < 2.5

**ZEUS 98-00 data (82 pb****-1****):**
**DIS: 10 < Q****2 ****< 100 GeV****2**** **
**        ****10****-4 ****< x****Bj ****< 10****-2**** **
**2 or 3 jets with:**
**         ****E****t****jet1 ****> 7 GeV ;       **
**         ****E****t ****jet2(3) ****> 5 GeV**
**         ****-1< ********lab ****< 2.5**

**Look in the Hadronic Center of Mass frame:**
**	****	****|********jet1,2****T,HCM****| = ****	****azimuthal separation between the ****	****	****	****	****	****two jets with the ****highest hadronic ****	****	****	****	****	****center of mass E****T**

**Multijet at low x**

**DESY-07-062**

## Multijet at low x

**Multijet at low x**

**DESY-07-062**

**Data overall well described by pQCD**

**Description much improved ****O(****α****s****2****)********O(****α****s****3****)**

## Azimuthal Correlations in Dijets

**Azimuthal Correlations in Dijets**

**H1-prelim-06-032**

**H1 99-00 data (64 pb****-1****):**
**DIS:**** 5 **** < Q****2**** < 100 GeV****2**
**2 Jets with:**
**	****−****1 < ****η****jet**** < 2.5 (LAB)**
**	****E****∗****Tj**** > 5GeV (HCM)**

- Phase space extended toward lowest  Q2 and xBj
- One-parton-radiation (NLO 2-jet) not enough to describe data
- Two-parton-radiation (NLO 3-jet) systematically low for Δφ∗ < 150◦

## Azimuthal Correlations in Dijets

**Azimuthal Correlations in Dijets**

**H1-prelim-06-032**

- Monte Carlo Models:
- Cascade (J2003) describes data reasonably well except    in lowest xBj bin
- Cascade (A0) fails in all         xBj bins. A0 has too hard         kT spectrum
- Data show sensitivity to the unintegrated gluon density

## Forward Jets

**Forward Jets**

**New measurement ZEUS 98-00 data: 2 < ****η****jet**** < ****4.3 ****(FwdPlugCal)**
**DIS: 20 < Q****2****< 100  GeV****2**** ;     10****-4**** < x****Bj**** < 5 x10****-3 **** ; **
**x****jet**** > 0.036   ******** Enhance BFKL expected behaviour**
**LO-DGLAP fails completely**
**NLO-DGLAP well below data at low *****x***** (as seen by H1)**

**ZEUS: DESY-07-102**

** ****Eur. Phys. J. C46 (2006) 27 **

**ZEUS**

## Forward jet & dijet

**Forward jet & dijet**

**Δη****1**** = η****2****– η****1 **
**Δη****2**** = η****fwd**** – η****2**

**DESY-07-102**

**η****1**

**η****2**

**Discrepancy at low Δη****1**** and Δη****2****, i.e. when all 3 jets tend to go forward**
**Need for additional higher orders      or BFKL resummation**

***e******±***

***p***

## Forward Jets & dijet: QCD Models

**Forward Jets & dijet: QCD Models**

**CDM (Ariadne tuned) describes data reasonably well**
**The breaking of k****T**** ordering is best modeled by CDM**

**ZEUS**

**DESY-07-102**

## Exclusive Trijets in DIS

**Exclusive Trijets in DIS**

**H1-prelim-06-034**

**1 fwd & 2 central jets**

** ****2 fwd & 1 central jets**

**2 fwd-jets → mainly due to gluons according to MC studies (CDM)**
**discrepancy at lowest x****Bj**** due to unordered gluon emissions? (NLO also below data for forward η)**

- 3 jets:
-   Forward
-    ηjet > 1
-   Central:
-    − 1 < ηjet < 1

## Multi-Parton Interactions (MPI)

Multijet photoproduction (resolved ) sensitive to MPIs
4-jet PHP: the highest order process ever measured at HERA
No NLO calculation available yet

**Multi-Parton Interactions (MPI)**
- Multijet photoproduction (**resolved ******) sensitive to MPIs
- 4-jet PHP: the highest order process ever measured at HERA
- No NLO calculation available yet

**Direct**
**Photon**
**(x****γ****=1)**

**Resolved **
**Photon**
**(x****γ****<1)**

**Multijets in Photoproduction **

## ZEUS HERA I data (121pb-1)
Event selection:
Photoproduction 
	Q2<1   GeV2       
3 or 4 jets with
	||<2.4; ET>6 GeV

Good statistics of                      3- and 4-jet events 

- ZEUS HERA I data (121pb-1)
- Event selection:
- Photoproduction
- 	Q2<1   GeV2
- 3 or 4 jets with
- 	||<2.4; ET>6 GeV
- Good statistics of                      3- and 4-jet events

**Multijets in Photoproduction**** **

**DESY-07-100**

## Multijets in Photoproduction 

-  Two mass regions:
- 25  Mnj  50 GeV
-                Mnj  50 GeV
-    Low Mass more sensitive to
-       MPIs (resolved photon)
-  General improvement
-      adding MPIs to LO MC

**Multijets in Photoproduction**** **

**DESY-07-100**

## Conclusions and Outlook

**Conclusions and Outlook**

**HERA provides a versatile testing ground for pQCD**
**Multi- and forward- jets production has been measured in the region x****BJ****~10****-4 ****and low Q****2**** where NLO DGLAP pQCD is expected to fail **
**Agreement between data and QCD calculations well improved when going from O(********S****2****) to O(********S****3****). Nevertheless**** ****BFKL enhanced forward jet data at low x****Bj**** are not described by NLO**
**CDM as implemented in Ariadne (tuned) provides a good description  of most datasets**
**Cascade (CCFM) with currently used sets of unintegrated gluon densities fails to describe shape of most distributions; these data could be used to determine the unintegraded gluon distributions.**
**Multijet photoproduction data are better described adding Multi-Particle-Interactions to LO Monte Carlos**
**HERA data will push further development of the Monte Carlo Models        and hopefully stimulate NNLO DGLAP and  NLO BFKL calculations**
**More precise results expected with the full statistics of HERA O(1 fb****-1****) **