# Presentation: 644181

## New Insights into Hermes Data

- From g1 to G
- Patricia Liebing
- RIKEN-BNL Research Center
-  for the                       collaboration

- Annual RHIC and AGS Users Meeting, BNL, June 05-09 2006

## Outline

- This is going to be a (very incomplete) compilation of our ever better understanding of our double spin asymmetries and unpolarized data
- Hermes and its evolution
- The basics: g1
- Semi-inclusive asymmetry analysis
- Multiplicities and Fragmentation
- s revisited
- Towards G

**Notes:**
Relying on naïve assumptions

## A Quick Reminder

- HERA ep collider at DESY (Hamburg)
- Polarized e with 27.5 GeV
- Hermes cms energy (fixed target) 7.2 GeV

## Hermes Evolution

- 1996-97

## Hermes Evolution

- 1998-2000

- Internal longitudinally polarized D target P0._x0010_85
- Flip every 90s

## Hermes Evolution

- 2001-2005

- Internal transversally polarized H target P0._x0010_75
- Flip every 90s

## Hermes Evolution

- 2006-2007

- Internal unpolarized target

**Recoil detector to measure**
**exclusivity in DVCS**

**Notes:**
Overview->Longitudinal Data

## Statistics for Longitudinal Running

- Accumulated Luminosity
- (in #DIS events Q2>1 GeV2)

- Best proton data: 1997
- Golden year: 2000 (deuteron)

- Beam: <PB>≈50%, Target <PT>≈85%,
- Scale uncertainty on double spin asym. <≈6%,

- Average Beam Polarization
- Per HERA Fill

## Statistics for Transverse Running

- Accumulated Luminosity
- (in #DIS events Q2>1 GeV2)

- Average Beam Polarization
- Per HERA Fill

- Golden year: 2005 (proton)

- Beam: <PB>≈30-40% 5%
- Target <PT>≈75%, 3%

**Notes:**
Overview->longitudinal pol. data

## g1 Results

- g1p,d corrected (unfolded)  for instrumental smearing and QED radiative effects
- Measured on neutron (3He, 1995), proton and deuteron targets
- Better precision for g1n from g1d-g1p

## QCD Analysis (g1 only)

- QCD Analysis (g1 only)

- QCD fits so far do not include Hermes 2D data
- Will increase precision
- Deuteron may be more sensitive to g (?)
- Inclusive data have no sensitivity to sea quarks
- Need assumptions on sea quark polarization

- NLO PDFs at Q2=4 GeV2

## Analysis of SIDIS Data

- Quark polarizations obtained from DIS+SIDIS (identified pion, kaon) asymmetries on proton and deuteron, using purity formalism:
- Purities determined by MC
- Caveat: LO analysis, dependent on MC fragmentation model

- A. Airapetian et al,
- Phys. Rev D 71
- (2005) 012003

## Measured Asymmetries

- All asymmetries unfolded
- to correct for smearing
- and QED radiative effects

**Notes:**
All asymmetries qualitatively the same, except k-

## NLO QCD Analysis of SIDIS Data 

- D. de Florian et al.
- Phys.Rev. D71 (2005) 094018

- NLO QCD Analysis of SIDIS Data

- Fit to inclusive and Hermes/SMC SIDIS asymmetries
- With error analysis
- Additional input: Fragmentation functions
- SIDIS data constrain sea quark densities better
- Uncertainties still very large

- Results very sensitive to choice of fragmentation function
- Need more precise FF from data

## Unpolarized Data

- Knowledge of unpolarized input to PDF fits very important
- SIDIS multiplicities of identified hadrons (together with e+e--data from BELLE+LEP) provide valuable information for new, more precise Fragmentation functions
- Understanding the unpolarized cross section (e.g., F2) helps evaluating the validity of pQCD assumptions

- Sorry, later ...

## Extraction of multiplicities at HERMES

- Correction for RICH efficiency
- and misidentification

- Unpolarized H and D data

- MC input

- Unfolding of smearing and
- QED radiative effects
- Acceptance correction

- Evolution to common Q2

- Multiplicities at Q2=2.5, 25 GeV2

## SIDIS Multiplicities (Pions)

- VMD correction large at z>0.7
- +/ - ratio 1.2

- Multiplicities on Proton

## SIDIS Multiplicities (Kaons) 

- VMD correction small
- k+/ k- ratio 2

- Watch the scale!

- Multiplicities on Proton

## SIDIS Multiplicities vs. Q2

- Pions

- Kaons

## Model Independent Extraction of s

- Model Independent Extraction of s

- The deuteron inclusive and charged kaon asymmetries are isoscalars:_x0013_

- }

- }

## Inclusive and Charged Kaon Asymmetries

- Asymmetries measured on deuteron,
- unfolded to correct for smearing, PID efficiency (RICH)
- and QED radiative effects

## Model Independent Extraction of s

- From A1incl and A1k we can extract Q and S assuming only
- Isospin invariance
- Charge conjugation symmetry of FF
- Use charged kaon multiplicities to get DQk and DSk
- Use CTEQ6LO for unpolarized PDFs

**Notes:**
Could use MC or FF to get Fragmentation function, but use our own data

## Charged Kaon Multiplicities

- Multiplicities measured on deuteron, unfolded to correct for smearing, PID efficiency (RICH) and QED radiative effects

- Very large difference between FF fit to data and Kretzer (PRD 62 054001 (2000)) or KKP (Nucl. Phys. B582, 514(2000)) predictions
- Have to regard light quark suppression in k fragmentation

## Results for Q(x) and S(x)

- Comparison with Previous
- Hermes Results

- Results on S(x) consistent with published 5 parameter fit
- Statistical and systematic uncertainties significantly reduced
- Polarized strange quark density consistent with 0
- No hint that it is negative

- Published isoscalar method:
- Assumed FF from BKK
- (Phys. Rev. D 52, 4947 (1995))

## And Now to G

- (Semi-) inclusive asymmetries vs. x are not very sensitive to G
- But: rather strong dependence of (semi-)inclusive hadron asymmetries vs. pT on G
- Can make use of totally inclusive (charged) hadrons (don’t bother about scattered e) large statistics

**Notes:**
Vs.pT because of the nature of NLO processes, that generate significant pT

## Extraction: Compare measured asymmetry             with MC calculated one:

Need to know individual process contribution and their asymmetries
Even worse:

    i.e., we need to know subprocess kinematics, too.
Understanding and trusting the MC is even more important

- Need to know individual process contribution and their asymmetries
- Even worse:
-     i.e., we need to know subprocess kinematics, too.
- Understanding and trusting the MC is even more important

- Towards G

**Notes:**
Fing that Deltag/g which makes MC asymmetry agree with data

## Modifying/Tuning PYTHIA

- Changes to the VMD model in PYTHIA
- The Q2 slope of the total
- (resolved) *p cross section:
- The parametrization of
- The –VM couplings
- The angular distribution of the
- 0 decay products
- And implementing QED radiative corrections ...

**Notes:**
Not only understand, but also modify Pythia
VMD important contribution to data

## PYTHIA Results for Hermes

- With modifications to VMD cross section
- Excl. 0 cross sections agree within 10%excellent tool to estimate VM fraction in SIDIS data
- Default PYTHIA: MC/DATA6
- With fragmentation tune to SIDIS multiplicities: semi-inclusive hadron cross sections agree within 10% for Q2>0.1 GeV2

## Cross Section vs. x, Q2 and z

- Cross Sections on Deuterium within Hermes Acceptance

## Cross Section vs. pT

- Inclusive charged hadrons with e-veto, i.e, enhance quasi-real photoproduction
- At pT>1 GeV MC underestimates data by a factor 4
- Turn up <kT> and/or <jT>? - would make agreement at low pT worse
- Consistent with <kT> 0.5 GeV from fit to p data (M. Anselmino et al., Phys.Rev.D71: 074006, 2005)
- NLO/LO ratio estimated in: B.Jager et al. (Eur.Phys.J. C44 (2005) 533-543)4!

- Cross Section on Deuterium
- within Hermes Acceptance

- Applying k-factor to LO MC cross section makes MC more consistent w/data

## What now?

- Large NLO corrections and scale dependence are of course problematic
- But: most of the cross section is between 0.1<x<0.3
- There’s our chance
-     to learn something
-     about G at large
-     x0.2
- Use these data in
-     Global fit

- X-distribution for gluon initiated
- processes from MC

## Summary

- Hermes has collected a wealth of unpolarized and longitudinally polarized data on H and 2D targets
- Analyses are being refined, including more and more understanding of the data (experimentally) and the physics (theoretically)
- E.g.,LUND parameter tuning and error scan
- “Overload” purity method for q, including 0, k0, asymmetries
- More multiplicities to come (maybe Fragmentation functions)
- Moving towards G

## BACK UP

## Study of Lund Fragmentation

- LUND string fragmentation parameters are tuned by minimizing the difference of MC and data (identified) hadron multiplicities
- Find agreement within ~10% (for pions)
- But: What are the errors?
- Estimate systematic uncertainties of values extracted using LUND MC (e.g. on q/q from purity method)
- Usual method: Compare an old “historical” fit with the current one normally gives large uncertainties
- Error scan using Hessian matrix (similar to CTEQ)

## LUND Error Scan

- 2-contours in correlated
- LUND parameter space

- 2-contours in uncorrelated
- Hessian parameter space

## Result of Error scan

- Black line = total systematic error estimate on q/q
- Grey line =  contribution from comparing old and new tunes
- Colored points = Sample q/q with parameters along 68% Hessian contour
- “True” error from LUND tune is much smaller than estimate
- But: The overall 2/ndf 20 for the best tune  MC model itself is not perfect
- Uncertainty due to “use of MC” unknown
- Studies are ongoing
- Try to avoid MC

## NLO SIDIS Fit Errors

## MC and data vs. pT Q2>0.1

- Cross section on deuteron,
- within Hermes acceptance
- Semi-inclusive hadrons, pT
- measured w.r.t.  direction