# Presentation: 656907

## The challenge to extract

***The challenge to extract***
*********G/G from HERMES data***

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## HERA @  DESY

***HERA @  DESY***

***HERMES @ HERA***

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## The HERMES Spectrometer

**Kinematic Range: ****0.02 < x < 0.8 at Q****2**** > 1 GeV****2**** and W > 2 GeV**

**Reconstruction:**** ********p/p < 2%, ******** < 1 mrad**

**Particle ID:**** ********TRD, Preshower, Calorimeter **
**                       ********1997: Cherenkov, 1998********: ****RICH ****+ ****Muon-ID**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Particle Identification

**hadron separation**
**Dual radiator RICH for** ******,** **K, p**

***Particle Identification***

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## The golden channels

**Idea: Direct measurement of ********G**

**Isolate the photon gluon fusion process (PGF)**

** **

**Open Charm production**
**          ****HERMES       = 7.2GeV           **
**                   ****too low count rates **

** **

**high p****t ****hadrons or pairs of hadrons**

**go to low Q****2            ****increase statistics**** **

**scale of hard sub-processes **

** **

**Data on tape:**
**polarized proton data 50pb****-1**
**polarized deuterium data 140pb****-1**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## What is needed to get G/G

** **

**Double spin asymmetries for high p****t  ****hadrons and pair of hadrons**

** **

**A theoretical model describing **
**          ****all underlying sub-processes**
**          ****fragmentation**

**two possibilities:**

**QCD calculation a la**
**B. Jaeger, M. Stratmann, W. Vogelsang hep-ph/0505157**
**Advantage:**** can be done to order ********s****2**
**Disadvantage: ****to include detector effects (acceptance,..)**
**                          ****difficult**

**Monte Carlo simulation (PYTHIA-6)**
**Advantage: ****detector effects can easily be included**
**Disadvantage:**** max. order ********s****1**** **

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## The Asymmetries

** **

**Inclusive hadron double spin asymmetries:**

** **

**2 kinematical regions:**
**          ****“****Photo-production”:**** anti-tagging of scattered lepton**
**          **

*** ******Event kinematics unknown***

*** ******p******t****** vs beam-axis***

**“****Transition region”: ****tagging of scattered lepton**

*** ******p******t******  defined relative to virtual photon direction***

*** ******Q******2****** > 0.1 GeV******2***

** **

**high p****t ****pairs of hadrons**

**“****Photo-production” + “Transition region”:**

*** ******p******t****** vs beam-axis***

*** ******Event kinematics unknown; only 10% events have e’***

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## The Asymmetries („Photo-production“)

**P-asym.’s > 0**
**D-asym.’s ~ 0 at small p****t**
**Positive h have positive asym.**
**negative h have 0 or negative asym. at large p****t**

-  **Blue: Proton data**
-  **Red: Deuteron data**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## The Asymmetries („Transition“)

-  **Blue: Proton data**
** ****Red: Deuteron data**

**At p****t****>1 GeV: Q****2 ******** 1.5 GeV****2****, x ******** 0.05**
**A******** relatively flat in p****t**
**Follows average behavior in SIDIS**
**Small difference of positive and negative h**
**Statistics at p****t****>1 GeV limited**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## The Asymmetries (pairs of hadrons)

**Proton**

**Deuteron**

**A******** relatively flat in ********p****t****2**
**Proton asym. > 0 for all ********p****t****2**** **
**Deuteron asym. < 0 for ********p****t****2**** > 2 GeV **
**Statistics at ********p****t****2**** > 2 GeV limited**

**Define:**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## The Model in PYTHIA-6

**„****Soft“ VMD:**
**Elastic, diffractive, non-diffractive („minimum bias“,**
**„****low-p****T****“****) **
**processes**
**Hard VMD:**** **
**QCD 2********2 processes**

**QCD-Compton**
**PGF**
**LO DIS (virtual photons****)**

**Splitting of **
**********qq**** **
**QCD 2********2 **
**processes**

**Model:**** Mix processes with different photon characteristics**
**Small Q****2****: VMD+anomalous (=„resolved“) photon dominate**
**Large Q****2****: LO DIS dominates**
**Choice of hard process according to hardest scale involved:**
**If all scales are too small: „soft“ VMD (diffractive or „low-pT“)**
**The „resolved“ part is modeled to match the world data on ********(********p) **

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## The Challenges

**Intrinsic k****T ******** 0.4 GeV**
**p****T**** from fragmentation ******** 0.35 GeV**
**Q****2**** small **
**    ******** ****expect contribution from **
**         ****VMD processes**
**Small scales (*****O*****(1 GeV****2****)) **
**    ******** ****Important:**
**    ****non-perturbative contributions, **
**    ****NLO corrections**
**Fraction of hard gluon initiated processes small **
**    ******** ****large background**
**All effects are folded with acceptance**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Adapting Pythia to Hermes Kinematics

***Adapting Pythia to Hermes Kinematics***

** **

**Check PYTHIA VMD model to exclusive ********0**** production**

**In “Transition” region as full kinematics known; Q****2**** > 0.1GeV****2**

** **

**Default PYTHIA: ********(PYTHIA) ~ ****6*************(HERMES)**

** **

**Changes:**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Adapting Pythia to Hermes Kinematics

** **

**The exclusive ********0**** cross section data-MC agrees now within ****10%**

*** ******Diffractive contribution ~10%***

** **

**The PYTHIA ********0**** cross section also matches the “photo-production”**
**data well**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Fragmentation

**DATA:**
**unpolarised p-target**
**Q****2 ****> 1GeV****2****, W****2****>10 GeV****2****, z>0.2, 2<p****h****<15GeV**

** **

**Tuning of JETSET FF parameters to multiplicities in acceptance**

**MC:**
**PYTHIA in combination with JETSET;  PDF:CTEQ-6**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Fragmentation-Purities

** **

**Proton target purities**

**K****-**

**K****+**

******+**

******-**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Contamination from excl. VM

***Contamination from excl. VM***

** **

**Exclusive VM production **
**different process than SIDIS**

** **

** **

**estimate the contribution from **
**Pythia-6**

******large contribution at high z**
**K:**** moderate contribution vs. z**

** **

******contribution growing with**
**          ****x****B**** decreasing**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Experimental Multiplicities

** **

** **

** **

**systematic uncertainty mainly from hadron PID correction**

**Q****2**** > 1GeV****2****, W****2 ****> 10GeV****2**

**EMC: Fragmentation Function       ****(Nucl. Phys. B321 (1989) 541)**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Cross Section vs Q2 and z

***Cross Section vs Q******2 ******and z***

** **

**Implement QED radiative corrections into PYTHIA using RADGEN**

** **

**Cross section for “Transition” region Q****2 ****> 0.1GeV****2**

**green curve relevant one**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Cross Section vs pt

** **

**HERMES standard values:**

**M. Anselmino et al. Phys. Rev. D71,074006**

** **

**Cross section for “Transition” region Q****2 ****> 0.1GeV****2**

**Has the cross section from**
** ****PYTHIA and Data to agree**
**at p****t ****> 1GeV ???**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Cross Section vs pt (II)

** **

**Process fractions for “Transition” region Q****2**** > 0.1GeV****2**** :**

**p****t **** > 1GeV:  (********s****1****) process start dominating **

**what about (********s****2****) corrections ??? **

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## NLO Corrections

**Polarized and unpolarized **
**cross sections and **
**k factors**

**Scale dependence of polarized cross section at LO and NLO**

** **

**Calculations from B. Jaeger et al. hep-ph/0505157 for HERMES:**

**“****Photo-production” kinematics**

**big NLO corrections**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## k-factors and PYTHIA

**Cross section vs pt in “Photo-production”:**

** **

**The cross section from**
** ****PYTHIA and Data have can not agree**
**at p****t ****> 1GeV !!!**
**PYTHIA includes only ********s****1**** processes**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## The Method

**Measured asymmetry is an incoherent superposition of different sub-process asymmetries:**
**Signal: Gluon of the nucleon in the initial state**
**Background: all other sub-processes**
**The gluon polarization is then:**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## PYTHIA itself gives only unpolarized xsec
Calculate asymmetry weight for each event

Takes kinematical distribution into account
Asymmetries of hard processes known
Correlation of pt and hard pt for pt> 1 GeV
Assume something about
Polarization of partons in  resolved photon
Asymmetry of „soft“ VMD processes 

**Calculate asymmetry weight for each event**
**Takes kinematical distribution into account**
**Asymmetries of hard processes known**
**Correlation of p****t ****and hard p****t ****for p****t****> 1 GeV**
**Assume something about**
**Polarization of partons in  resolved photon**
**Asymmetry of „soft“ VMD processes**

***Results – MC Asymmetries***

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Results – MC Asymmetries

** ****Large uncertainty in assigning asymmetry to low-p****t **
**    ****process**
** ****If it is VMD, it should not have an asymmetry**
** ****If it has an asymmetry, it should not be VMD**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Photon PDFs

**Anomalous and hard VMD processes enter in background**
**q(photon)+q(nucleon)********2 „jets“**
**and in signal**
**q(photon)+g(nucleon)********2 „jets“**
**g(photon) negligible**
**Only maximal and minimal scenarios for Δq/q (photon) available**
**using Δq/qmax=1 and **
**     ****Δq/qmin=-0.6**** **
**Large part of total uncertainty, scales with central value**

**M. Glück et al., Phys. Lett. B503**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Results G/G

**P**
**r**
**o**
**t**
**o**
**n**

**D**
**e**
**u**
**t**
**e**
**r**
**o**
**n**

**DIS suppressed at high ********p****t****2****, <10%**
**PGF+hard g ****~ 3-4*****QCDC**** (~10%)**
** ********p****t****2**** >2.5 GeV**
**„****soft“ VMD dominates at low ********p****t****2**** **
**Hard VMD processes ****~1****5%, ********p****t****2**** ****>2 GeV**

** “****Photo-production” high p****t**** hadron pairs**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Results – Sub-process Fractions

**DIS rises and dominates at high p****t**
**PGF+hard g**** **** ~ 2-3*****QCDC****, ~25%, **
**p****t**** >1** **GeV, ****Hard g**** ~ 40% of signal**
**„****soft“ VMD dominates at small p****T****, **
**~40%, p****T ********1 GeV**
**Hard VMD processes 10%, p****T ****>1 GeV**

**DIS suppressed at high p****t****, 20%**
**PGF+hard g ****~ 2-3*****QCDC****, ~10-25%**
** ****p****t**** >1 GeV**
**„****soft“ VMD ~15%, p****T ********1 GeV**
**Hard VMD processes ****~1****0%, p****T ****>1 GeV**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Results – ΔG/G

**„****Photo-production“ reg**.
**Scale Q****2********1.25 GeV****2****, x********0.15**
**At p****T****<1.5 GeV results for**
**Proton > Deuteron**
**„****Transition“ region**
**Scale Q****2********1.77 GeV****2****, x********0.16**
**2.8 ******** difference between**
**h****+**** and h****-**** Deuteron**

** ****4 independent data sets (p, d, h****±****)**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Results - ΔG/G

** “****Photo-production”**

** ****combined data sets**

** ****Combined data in both regions**
** “****Transition”**
**    ****only Deuteron data; p****t ****> 1 GeV**
** “****Photo-production”; p****t ****> 1.35 GeV**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Open Points

** ****Very wide correlation between p****t ****and x****g**

** ****Can the mean value theorem be used to calculate ********G/G**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer

## Conclusion

**Hermes can extract ΔG/G from tagged and anti-tagged high-p****t**** ****hadron asymmetries with good statistics and**
**Big model (MC) dependence**
**not so small estimated systematic uncertainties within one model**
**Not yet accounted for:**** **
**Large NLO corrections to pol. and unpol. cross sections**
**Statistical uncertainties of pol. and unpol. PDF used (including ****g(x)****)**
**Analysis of high-p****t**** hadron pairs consistent**
**Find solution to open points**
**For less model dependent measurements:**
**Higher energy (****collider****) – need high luminosity**
**For ep or ********p: Measure pol. photon PDFs**
**Clean process (****prompt photon at RHIC?****)**** and/or heavy quarks (****open charm?****)**

- Funding Agency Meeting-DESY 2004

- E.C. Aschenauer