# Presentation: 859905

## QCD results from colliders

**2007 Aspen Winter Conference**
**Wesley H. Smith**
***U. Wisconsin - Madison***
**January 11, 2007**
_**Outline:**_
**Parton Density Functions & Structure Functions**
**Jets & Measurements of ********S**
**Multijets, Inclusive Jets, W + Jets, Dijets**
**Prompt Photons**
**Diffraction**
**This talk is available on:**
**http://www.hep.wisc.edu/wsmith/files/exp_qcd_smith_aspen07.pdf**

## HERA Parton Densities

**HERA Parton Densities**

- QCD evolution

- Zeus and H1 DGLAP analyses of HERA-I (130 pb-1) and fixed target) data:
- 	 H1 2000 PDFs
-  	 ZEUS-S PDFs

- HERA PDFs essential for LHC

**Notes:**
ICHEP talk E. Tassi

## Jets at HERA constrain gluon distribution  

**Jets at HERA constrain gluon distribution  **

**First fit using HERA incl and jets data**
**HERA-I inclusive cross sects**
**Inclusive jet cross sects in NC DIS**
**Dijet cross sects in photo-production**
**Data from one Experiment only:**
**Syst unc. well understood**
**No fixed target unc.**
**Valence quarks from high-Q****2**** NC and CC cross sections**
**Jet data help to constrain the gluon and ********s****:**
**Reduction of gluon unc. by  ~ factor 2 for  0.01 < x < 0.4**
**Precise determination of ********s**

**Notes:**
ICHEP Talks: Tassi, Kluth, Terron

## Measurement of FL at HERA  

- Directly connected to the
- gluon distribution dominant at low-x
- Predictions are still very uncertain

- Martin,Stirling,Thorne

**Measurement of F****L**** at HERA  **

- Need to measure cross sections at same *x,Q**2* and different *y*.
- To change *y=Q**2**/xs*, HERA changes *s* by lowering Ep to 460 GeV
- The price in integrated luminosity is a factor of 410 pb-1 requires 3 months

## HERA Charged Current Scattering:sensitive to u, d valence at high Q2

- HERA II
- 04-05

- HERA I

**HERA Charged Current Scattering:****sensitive to u, d valence at high Q****2**

**more e****-  ****data:****16 pb****-1**** ******** **
**122 pb****-1**
**Central****values****unchanged**
**Uncertainty on u****V**** reduced**** **

- HERA-I: 130 pb-1, HERA-II: 350 pb-1 with e polarization so far -- stops June 30.

**Notes:**
ICHEP talk A. Tapper

## Neutral Currents vs.lepton charge: xF3

** ****Add to the knowledge of valence quarks at lower x **

**e****±**

******-Z interference flips sign when e****+ ******** e****-****  **

- ,

** ****H1 & ZEUS combined data **

** ****HERA I + II **** **

**Neutral Currents vs.****lepton charge: xF****3**

**e****±**

**Notes:**
H1 and ZEUS combined results have been also obtained now for the dependence of the neutral currents vs lepton charge
which is related to the structure function xF3
The g-z interference changes it sign when goes from positrons to electrons due to the electric charge (here at the vertex)
This can be used to obtain a better knowledge of the valence quarks in the proton 
So to do so you add up all e+p data and separately the e-p data, correct for residual polarisation effects and then you obtain the cross sections sigma+ and sigma - 
And then you cnostruct the gZ-interference structure funtionc xF3gz by subtracting the two cross sections and by dividing with these factors
This is approximately equal to the 2 times uv + dv times this factor
Again the results from H1 and ZEUS are shown first separately here, given for a Q2 value of 1500 GeV^2
And then you see here the results of the H1 and ZEUS combined data
And you see that they can really add to the knowledge of valence quarks at lower x.
For comparison also the predictions from h1 2000 PDF and ZEUS-JETS PDF are shown here.

## Electroweak tests:Charged current vs Pe

****** ****Demonstrates absence of RH charged currents **

****** ****M(W****R****) > ~180-208 GeV ****with current precision**

** **

**Electroweak tests:****Charged current vs P****e**

- Polarized electron-proton scattering at HERA

**Notes:**
We leave now the area of searches and enter the electroweak tests
The first result is the measurement of the charged current vs the longitudinal polarisation of the lepton beam 
The SM predicts a linear dependence of the W exchange cross sections as a function of the polarisation
For e+ data is expected to rise and for e- data to fall with increasing polarisation
And this is beautifully verifed with the data from the H1 and ZEUS experiment which are shown here
So this is a textbook plot which demonstrate the absence of righthanded W- (i.e. That the cross section --> 0 for Pe=+- 1)
These results can be translated into a lower mass limit for hypothetical Wr of 180-200 GeV with the current experimental precision.

## Electroweak tests:Neutral currents vs Pe 

** ****First observation of parity viol. in NC e****±****p data at R < 10****-18**** m**

** ****Use prelim. H1 and ZEUS e****±****p  data from 2003-2005   **

** **terms** **** **

** ****H1 & ZEUS combined data **

**Electroweak tests:****Neutral currents vs P****e**** **

**e****±**

**e****±**

**Notes:**
We continue the electroweak tests and study the neutral currents versus the lepton polarisation
As for the charged currents any dependence on the polarisation is a direct sign of parity violation 
this manifests itself here in the interference of gamma and Z exchange 
H1 and ZEUS have combined now their results in order to achieve a higher significance of this effect
Thanks to the ZEUS colleagues for the nice cooperation
So what is done is the following the preliminary H1 and ZEUS data from 2003-2005 are taken 
Then separately for the e+p and e-p data polarisation asymmetries are build 
So you take the cross sections with positive polarisation and subtract the ones measured with negative polarisation 
Then dividing by the sum and correcting for the average polarisation value
This is proportional  the axial coupling of the electron to the Z0 and the gamma Z interference structure function F2gZ divided by F2
This is proporitional to parity voilation terms ae vq where vq are the quark vector couplings to the Z0
So you see here first the results for H1 and ZEUS separately which already look nice
Then the H1 and ZEUS cross sections are combined before building the asymmetries
For this for each data point the weighted averaged cross section from H1 and ZEUS is taken
For each data point the full error is taken, bin to bin correlations are neglected- so this is somwhat simplified
The results are shown here and look really more impressive than  the separate results
If one calculates the probability that the results are ain agreement with no parity violation one finds if one uses only the data points above Q2>5000 a probability of less than a percent
In conclusion this demonstrates the first observation of parity violation in NN e+p data at spatial dimenstions smaller than 10^-18 m. (i.e. at high Q2)

## High pT jets at the Tevatron

- In 2005: CDF published both central cone and kT jets with 400pb-1
- Now: new preliminary results with full rapidity coverage for 1fb-1

- Midpoint Searchcone Algorithm

- kT Algorithm

## Tevatron Inclusive Jets: cone & kT 

- Data are well-described by NLO pQCD
- Experimental Uncertainties: Smaller than PDF Uncertainties!!
- (only shown for kT Algorithm)

- Midpoint Searchcone Algorithm

- kT Algorithm

## Inclusive Jet Cross Section

**Inclusive Jet Cross Section**

- DØ preliminary (800pb-1)

- Compare to NLO pQCD + 2-loop
- Threshold Corrections (Kidonakis, Owens)

-  Sensitive to PDFs!! -- esp. Gluon**!**

## New DIS Jet data: gluons & S  

- These measurements will provide additional constraints for the gluon distribution at middle *x*; and give a measurement of S

- Jet energy scale uncertainty of 1%

**New DIS Jet data: gluons & ********S****  **

## charm contribution to F2

**                         ****or**** **

****** ****obtain virtual charm content**
**     ****(charm PDF) of proton**

- (Q2 >> mc2)

- c massive

- c massless

**charm contribution to F****2**

**Notes:**
talk D. Pitzl

##   Beauty contribution to F2

- first NNLO calculation
- data in agreement with NLO
- and NNLO
- checks b PDF for LHC:

**  ****Beauty contribution to F****2**

**HERA**

**LHC, e.g.**

**H,Z**

- Q2 >> mb2

**Notes:**
talk D. Pitzl

## Probe b PDF with Z+b-jet at Tevatron

- CDF

- b

- b

- b

- b

- Z+b-jet/Z+jet :
- CDF:  2.4+-0.7(stat) +-0.5(sys) %
- D0:    2.1+-0.4(stat) +0.3-0.2(sys) %
- NLO:  1.8+-0.4 %   (CTEQ6)

- OK

**Notes:**
ICHEP talk K. Hatekayama

## can Tevatron results be used for LHC?

- Tevatron

**LHC**

- HERA

**can Tevatron results be used for LHC?**

**Are PDFs measured thus far applicable?**
**Does DGLAP QCD evolution work sufficiently to extrapolate?**
** ****Need to take care of low-x effects:**
**saturation**
**multiple interactions**
**BFKL vs. DGLAP**
****** ****study at HERA**

**Notes:**
talk M. Wielers talk C. Marquet talk J. Bartels talks T. Gosau, F. Eisele,  H Jung, R. Sassot

## Evolution & Resummation

- Expect more energetic jets in forward region

- (LEPTO, DISENT)

- (~ARIADNE)

- (CASCADE)

## ZEUS Forward Jets

## H1 forward jets

**LO is suppressed by kinematics, **
**NLO is a factor of 2 too low, Cascade somewhat better**

## Multijets in photoproduction:3-jet & 4-jet events from ZEUS   

**Multijets in photoproduction:****3-jet & 4-jet events from ZEUS****   **

**Test pQCD at higher order of ********S**
**NLO calculations for O(********S****2****): 3 jets**
**4 jets measure O(********S****3****)**
**Test of MC models (LO+PS) & Multiple Parton interactions**
**MPIs and Multi-jet HFS will be abundant at the LHC**
**Test of parton showers (LLA) used****to simulate multi-jet states in****(LO ME+PS) Monte Carlos. **
**What’s new:**
**7.5 ******** more lumi than existing 3-jet photoproduction results. **
**3-jets studied in more inclusive****phase-space region. **
**No published 4-jet photoproduction results by ZEUS or H1 yet. **

- Xobs < 1(resolved)

- Xobs = 1(direct)

## ZEUS 3-jet photoproduction

## ZEUS 4-jet photoproduction

## Study parton dynamics with DIS multijets at HERA

**three jet**
**events **

- “NLO”
- “LO”

**four jets:**

**or need unordered**
**gluon radiation? **

**O(********s****3****) (“NLO”)**** **
- much better than
-  **O(********s****2****) (“LO”)**
- but not perfect

-  **need**
**NNLO**
**   ****?**

** ****NLO: not available!**
** ****=> use LO+PS**
** ****Color Dipole Model OK**
** ****(DJANGOH: CDM: ****k****T**** ****unordered  gluons)**
** ****DGLAP parton shower fails**
**(Rapgap: ****k****T**** ordered parton showers & resolved photon component****)**

**Study parton dynamics with DIS multijets at HERA**

**Notes:**
ICHEP Talks talk F. Eisele, R. Sassot

## W+n Jets                  W+2 Jets

**W+n Jets                  W+2 Jets**

**Cross Section for Restricted W Phase Space**
****** ****Avoid Model-Dependent Acceptance Corr.**

- CDF preliminary (320pb-1)

- Shape-Comparison with LO Alpgen+PYTHIA:
- Reasonable Agreement!

## Z+n Jets

**Z+n Jets**

**Smaller ******** than (W+Jets) but cleaner exp. signature**

- Data are described by:
- MCFM: NLO for Z+jet, Z+2-jet
- Madgraph:  ME+PS (Z+ ≤ 3 parton) tree-level  + PYTHIA parton shower
- ALPGEN (LO ME) (pT Dependence)

- D0 preliminary (340pb-1)

- Ratios to total inclusive Zee

- (normalized at n=1)

- (normalized)

## Dijet Azimuthal Decorrelation

- Compare with theory:
- LO has Limitation >2/3
-        & Divergence towards 
- NLO is very good – down to /2
-        & better towards 
-        ... still: resummation needed

## Dijet Azimuthal Decorrelation

- Compare with theory:
- HERWIG is perfect “out-the-box”
- PYTHIA too low in tail but can tune

- SHERPA (ME+PS+CKKW match) good
- ALPGEN (LO ME) looks good – but low efficiency  large stat. flucts.

**Notes:**
Improve modeling of W+jets, Z+jets, WW+jets, ttbar+jets
  Test matching ME+PS in several ways with data
  Public StdHep files with MADGRAPH+PYTHIA (S. Mrenna)
  MLM matching scheme with CDF/D0 implementation
  SHERPA from Frank Krauss
  ALPGEN new version from Michelangelo Mangano

## DIS Event Shapes

**Use power corrections to correct for non-perturbative effects in infrared and collinear safe event shape variable, F:**

- Used to determine the hadronization corrections

- Valid for event shape means and differential distributions

**Power correction:**** **
**Independent of any fragmentation assumptions**
**            ****Universal “non-perturbative parameter”****       * – (Dokshitzer, Webber, phys. Lett. B 352(1995)451)**

-  - Collinear & infrared safe combination of sphericity eigenvalues

## S & 0 from DIS event shapes

- Extracted values of S are in good agreement with world average(shown as yellow band)

## DIS Jets, Event Shapes & S

- Precise measurement of S at HERA, error dominated by theory
- Running measured from jets and event shapes in a single experiment
- New world measurement incl. HERA Jets: S = 0.1189  0.0010 (hep-ex/0606035)

**DIS Jets, Event Shapes & ********S**

## Isolated Photon Cross Sect.

**Isolated Photon Cross Sect.**

- |y|<0.9

**p****T******** (GeV)**

- L = 326 pb-1

**p****T******** (GeV)**

-  PDF sensitivity requires:
-   Reduced exp. uncertainties – dominated by purity uncertainty
-   Improved theory (resummation / NNLO)

- data/theory (NLO: JETPHOX): good agreement over 23<pT<300GeV

- DØ  hep-ex/0511054, submitted to PRL

- Direct photons directly probe hard scattering dynamics:

## Di-Photon Cross Section

-  **DIPHOX**:
-   - NLO prompt di-photons
-   - NLO fragmentation (1 or 2 ****)
-   - NNLO gg**** diagram
-  **ResBos**:
-  - NLO prompt di-photons
-  - LO fragmentation contribution
-  - Resummed initial state gluon
-     radiation (important for qT)
-  **PYTHIA** (increased by factor 2)

- CDF Collab., Phys. Rev. Lett. 95, 022003, 2005. (207pb-1)

-  Pseudorapidity < 0.9
-  Photon pT> 13 & 14 GeV

**M******** (GeV/c****2****)**

## Di-Photon Cross Section

**Di-Photon Cross Section**

**NLO fragmentation contribution**
**    ****- only in DIPHOX **
**    ******** ****at high q****T****, low ****, low mass**
**Resummed initial-state gluon radiation – only in ResBos ******** at low q****T**

**************(rad)**

- _Important:_
- Combined Calculation with
- NLO Fragmentation
- & Initial State Resummation

**Statistical fluctuations: much larger data set soon - 1 fb****-1**

## Inclusive Prompt  at HERA:effective quark to photon fragmentation fn.

- Prediction for prompt in ep DIS: Gehrmann et al. (hep-ph/0601073,0604030)
- LO(3) calculation withlepton radiation (LL), quark radiation (QQ - incl. non-perturb. frag. functionbased on ALEPH data)
- & interference (QL):

- QQ

- LL

- LL dominates at small 
- QQ dominates at large 
- Reasonable description of data

- Q2 > 4 GeV2

- Q2 > 30 GeV2

- LL

- LL

- QQ

- QQ

- 

- 

- ET

## Photoproduced Prompt  + Jet

**Photoproduced Prompt ******** + Jet**

**Sensitive to k****t**** unintegrated parton densities in the proton, resolved ******

**E****T****Jet**

******Jet**

**E****T******

********

**Photon Kinematics**

**Jet Kinematics**

| KZ | 23.31 pb |
| --- | --- |
| FGH | 23.52 pb |
| LZ | 30.73 pb |

| PYTHIA | 19.98 pb |
| --- | --- |
| HERWIG | 13.54 pb |

**Underestimate data:**

**KZ****: ****GRV p & ******** SFs**
**FGH****:****MRST01 p & AFG02 ******** SFs**
**Both below data for Low E****t****jet ****& deviate from data at low E****t********LZ****: ****K****t****-factorization & unintegrated q/g densities**
**Better in forward jet region & for E****t******** &low **********

**also seen by H1:****hep-ex/0407018**

- 5.0 ≤ t ≤ 16.0 GeV-0.7 ≤  ≤ 1.1

- 0.2 ≤ YJB ≤ 0.8
- Q2 <1 GeV2

- 6.0 ≤ Etjet ≤ 17.0 GeV
- -1.6 ≤ jet ≤ 2.4

**Notes:**
K.Krawczyk & A.Zembrzuski (KZ): GRV parametrisation:  photon structure function proton structure function fragmentation function
Fontanaz, Guillet & Heinrich (FGH): MRST01 proton structure function AFG02 photon structure function
A.Lipatov & N.Zotov (LZ): Kt-factorization approach Unintegrated quark/gluon densities using Kimber-Martin-Ryskin prescription
Prompt photon PYTHIA 6.3 & HERWIG 6.5 Monte Carlo models;
Only prompt photon subprocesses. Generated with default parameters CTEQ5L proton structure function & SaS-2D parameterization for photon structure
Both direct and resolved events were generated: Full detector simulation Improved dead material map Also used for hadronisation correction of KZ, FGH and LZ. Fully inclusive photoproduction sample w/o Prompt photon subprocesses Used to model hadronic background i.e. Photons originating from the parton shower & resonance decays. Combine with prompt photon MC according to BPRE distribution MC samples were re-weighted in Et and h for hadronisation and acceptance corrections

## Color Non-Singlet and Singlet Exchange in Photoproduction

**Color Non-Singlet Exchange:**
**Final state partons color connected**
**Space between final state partons****filled with final state particles **
- No Gap between jets
**Color Singlet Exchange:**
**Final state partons not color connected**
**Space btw final state partons empty **
- Rapidity Gap between jets--measure ETGap

**Color Non-Singlet Exchange**

**Color Singlet Exchange**

**Jet**

**Jet**

**Jet**

**Jet**

- gap

- proton remnant

- jet 1

- jet 2

**E****T****Gap**

-   color-flow
- (gap from fluct.)

-  gap

- proton remnant

- W,Z,

- *

- *shorthand for gluon ladder

**Notes:**
IP here really means a gluon ladder

## Evidence for Color Singlet

**Inclusive dijet gap cross section vs. E****T****Gap**** for  Herwig (x 3.3) & Pythia (x 1.8) with & w/o color- singlet exchange. **
**MCs w/o color-singlet do not describe data in two lowest E****T****Gap ****bins.**
**Color-singlet needed for best fit to data is ~3-4% of total inclusive dijet gap cross section for E****T****Gap****  from 0 - 12 GeV**

**Indicates variation in gap probability (i.e. survival) from Tevatron-like low x******** to pointlike DIS at x******** = 1.**

**Fraction of events with less than 1 GeV in gap between jets vs. fraction of photon energy participating in hard scatter. **

## Diffractive DIS & Factorization

- xIP - momentum fraction of proton carried by color singlet exchange
- zIP - momentum fraction of color singlet carried by parton entering hard sub-process
-  - momentum fraction of color singlet carried by struck quark

- Assumption: proton vertex factorization: shape of diffractive
- PDFs independent of xIP and t
- (needed?)

**Notes:**
Durham group showed proton vertex factorization is not necessary nor appropriate -- see hep-ph/0609273

## DPDFs from Inclusive Diff DIS

- ~70% gluons
- Integrated over *z*

- Fit DPDFs (** & *Q**2* dependences) using NLO QCD.

## Inclusive DPDFs at high z

**Inclusive DPDFs at high z**

- Fit B: Cg = 0:
- Gluon constant
- at Q02

- gqq

- qqg

## Add Diff. Dijets to DPDF fit

** ****Obtain better constraint on****high z gluon **

**Jet**

**g**

**Combined fit of ****F****2****D  ****and dijet data  **

**Jet**

**Dijets **

**Add Diff. Dijets to DPDF fit**

-   **Both datasets well described by combined fit**

**LHC**

**Caveat:****Gap****survival**
**(reinter-****actions)**

## Test factorization:  diffr. charm  

**Test factorization:  diffr. charm****  **

****** ****DIS: D* &  H1 Lifetime method data consistent with NLO QCD**
****** ****Photoprod: D* data consistent with NLO QCD within scale error**

- DIS & Direct Photo.   Resolved Photo.

- H1: hep-ex/0610076

- H1: hep-ex/0610076

- x=1dominates

- 0 < x< 1smallfraction
- (~10%)

**Notes:**
GLP: same effect seen in diffractive dijets: data are a factor of ~ 2 above the predictions from the ZEUS Mx GLP fit - weaker Q2 dependence in ZEUS diffractive Mx data - smaller glupn

## Inclusive Diffraction at CDF

**No Q****2**** dependence **
**(100 < Q****2**** < 10000 GeV****2****)**
**Diffractive***** SF***** ~ proton SF**

**Slope of d**********/d*****t***** indep. of Q****2**** in SD dijets:**

**Use high***** p******T***** jets to measure ***Diffractive Structure Function,*** ***F**jj**D*

*** **= P**pomeron**/ P**proton*

*t = ( P**i** - P**f **)**2*

*** **= P**parton**/ P**Pomeron*

**Notes:**
IP is just short-hand for proton probed diffractively(either on the slide or simply when you give the talk). At HERA we have shown that there is no particle-like IP, we simply look at the PDFs of the proton through a lens that filters out all but the vacuum quantum numbers.

## QCD Mediated Dijet Production

*Strategy:***  **

- Similar for DPEMC

**select DPE dijets: **
**examine the *****dijet mass fraction R******jj***

- ExclusiveQCD:
- DPEMC
- ExHuME

- InclusiveQCD:
- POMWIG

- Similar to central exclusive Higgs production
- ExHuMEbased on
- Khoze, Martin, Ryskin calculation used for Higgs

- Study tail

**Notes:**
the mechanism leading to exclusive dijets is the same as that leading to exclusive Higgs production(the only difference being that in the latter case a top loop is produced as opposed to light quarks). Incidentally, Exhume is based on the Khoze,Martin, Ryskin calculation which is currently taken to be the best estimate of the Higgs cross section.

## Experimental QCD Summary

**Jet data constrains gluon & PDFs at high-x & ********S**
**Final HERA PDFs should improve by 50% from inclusive measurements, jets, cross-sections, NNLO fits, etc.**
**Improved measurements of heavy flavor PDFs**
**QCD evolution has been studied at lower x**
**Region over which DGLAP works is extended, but may need BFKL**
**Better understanding of extrapolation of PDFs to the LHC**
**Multijets measure higher order pQCD processes**
**Need theoretical calculations to higher order**
**Multiple Parton Interactions are important**
**NLO QCD describes new jet data & prompt ******** well.**
**Diffractive DIS factorizes into DPDF + hard scatter ******
**Consistent with NLO QCD**

## Acknowledgements

**Many thanks are due to the following for suggestions, comments & material used:**
**Michele Arneodo, Olaf Behnke, Gregorio Bernardi, Duncan Brown, Brendon Casey, Mary Convery, Massimo Corradi, Sridhara Dasu,**** Elisabetta Gallo, Max Klein,Regis Lefevre,****Vivian O’Dell,  Frank Petriello, Sascha Savin**