determination of the gluon polarisation at hermes n. bianchi on behalf of: the hermes collaboration...

Determination of thegluon polarisation at HERMES

N. Bianchion behalf of:

The HERMES Collaboration&

The main analyzers (P.Liebing, E.Aschenauer, R.Fabbri, V.Mexner, …)

N.Bianchi, Pacific SPIN07, Vancouver BC

How to measure G: indirect


])Q,(g)Qq(x,[e2

1)Qx,( g

2qq

22q

2LO1

2NLO1 CxCegg

For fixed target exp. small x-Q2 lever arm:

g (and q) very badly determined : G 0,5 1

How to measure G: direct (general)


Method: Photon-Gluon-Fusion

t h/2mq

Charm-production : PGF dominated and hard scale by the mass of c-Quarks

Open charm: clean process (no charm quarks in the nucleon wave function)

How to measure G: direct (high pt)


(Pairs of) hadrons with high transverse momenta (Hard scale: pt = 1 – few GeV range)

Open charm needs very high energy to access to charm production (CERN, RHIC experiments)

At HERMES hidden charm (J/) is produced and identified : •low statistics•less clean channel due to VMD contribution and FSI

Best direct way for HERMES to measure G:

First HERMES measurement


First longitudinal double spin asymmetries for 2 hadrons

Historical plot : first HERMES data and future projections

A. Airapetian et al, Phys. Rev. Lett. 84 (2000) 2584

Old HERMES data


Following SLAC pioneristic measurement on un-tagged single hadron asymmetry ..

HERMES preliminary 2001

…. the differences between the curves (BBS) are less than the differences between any of the curves and the data. This makes it impossible to draw any conclusions about G(x). ….The present data will provide valuable experimental constraints on such models, and perhaps lead to constraints on the gluon polarization in the nucleon in the future. (E155 -

Phys.Lett.B458 (1999) 536)

Proton Deuteron

New HERMES data


• improved statistics• both H and (high statistics) D longitudinally polarized target• new anti-tagged analysis • improved a lot the MC knowledge and tuning• systematic studies• different channels (anti-tagged, tagged, pairs)

Asymmetries (anti-tagged)


•Anti-tagged data:‣Scattered lepton notin acceptance‣pt measured withrespect to beam axis‣for pt>1.05 GeV : 1272k(419k) for deuteron (proton) sample

•Curves from MC+asymmetry model using:‣Δg/g(x)=0 : central‣Δg/g(x)=-1 : upper‣Δg/g(x)=+1 : lower

Δg/g(x)=0 asymmetry is due to quarks (DIS at large Q2 and x at large “fake” pT)Gluons become important for above pt ≈ 1 GeV

Asymmetries (tagged)


•Tagged data:‣Scattered lepton detectedin acceptance‣pt measured withrespect to virtual photon‣Q2>0.1 GeV2, W2>4 GeV2

‣for pt>1 GeV : 53k (19k) for deuteron (proton) sample

•Curves from MC+asymmetry model using:‣Δg/g(x)=0 : central‣Δg/g(x)=-1 : upper‣Δg/g(x)=+1 : lowerΔg/g(x)=0 : large and stable asymmetry is due to quarks in DIS

eventsaveraged in the HERMES acceptance

Asymmetries (hadron pairs)


•Anti-tagged data for pairs of charged-hadrons:‣No regards on scattered lepton (10% are detected)‣pt measured with respect to beam axis‣pt(h1,h2) > 0.5 GeV ‣for >2 GeV2 :60k (20k) for deuteron (proton) sample‣plotted vs. lower cut on:

•Curves from MC+asymmetry model using:‣Δg/g(x)=0 : central‣Δg/g(x)=-1 : upper‣Δg/g(x)=+1 : lower

Extraction(general)


•Measured asymmetry is an incoherent superposition of different hard and soft subprocess asymmetries:

Signal: Gluon of the nucleon in the initial state

Background: all other sub-processes ➟ MC

Lepto : LO and NLO DIS but no photoproductionPythia : DIS but also non perturbative model for photoproduction

MC Models


•MC model‣PYTHIA 6.2 ,tuned and adapted for HERMES datafragmentation process, intrisic kt, exclusive ρ0 cross section (VMD)

•Provides‣kinematics of the hard subprocess‣relative contributions fi of the background and signal subprocesses in the relevant pt range‣background asymmetries and the hard subprocess asymmetries-weight calculated for every MC event-PDFs (unpol/pol): Hard process CTEQ5L/GRSV2000 (nucleon)Hard resolved photon processes SaS2/GRS (photon)-Asymmetry assumptions for soft processes:

A=0 for exclusive/diffractive processesA~A1(low x) from world data for soft nondiffractive (“low-pT”)

•Vary PDFs/assumptions for syst. error

Subprocesses


Subprocesses


VMD (elast.+diffr., soft low-pT):decreasing with pT

DIS:DIS: increasing (dominating) with pincreasing (dominating) with pT T

QCDC/QCD2->2(q): QCD2->2(q): increasing with pincreasing with pTT

Signal processes are PGF and Signal processes are PGF and QCD2>2(g) (resolved photon)QCD2>2(g) (resolved photon)

Antitagged, Charge combined, Deuteron data

Asymmetries of Subprocesses


Antitagged, Charge combined, Deuteron data

DIS increasing with pDIS increasing with pTT (x):(x):

positivepositive

QCDC/QCD2->2QCD2->2,VMD: flat and small but important for

background asymmetry!

|PGF| increasing with p|PGF| increasing with pTT :negativenegative QCD2->2(g):QCD2->2(g):opposite to PGF, smallopposite to PGF, small

g/g extraction:methods I and II


• Method I:– Factorize

– Assumes • No sign change in â(x)• “flat” g/g(x)

– No information on <x> of measurement

– Gives average g/g over covered x range (0.07<x<0.7)

• Method II:– Fit: find a g/g(x) such

that

– Assumes functional form for g/g(x)

– Only small range in pT

– Gives g/g(x) and average x of measurement

G from method I


h+,h- antitagged: 4 points between1.05<pT<2.5 GeVh+,h- tagged: 1 point for pT>1 GeVPairs: 1 point for GeV2

Assuming g(x)/g(x) const over x :

Only statistical errors are shown

• Results for different data samples (diff. mixtures) agree within statistics• Consistency between the two hadron charges and the two targets• Dominating sample: Deuteron antitagged -> Used for Method II and syst. error analysis (charge combined)

G from method II


•Light shaded area: range of all data•Dark shaded area: fit center of gravity (span of the 4 pt bins)

• Several test functions• Final 2 functions used are

polynomials with 1(2) free parameters

• Fix:- g/gx for x0 - g/g1 for x1

• |g/g(x)|<1 for all x• Difference between

functions is a systematic uncertainty

(Anti-tagged only)

G from method II


2/ndf5 mainly due to highest pT point•Model systematic is not included in fit •1-2 parameter function is too smooth• function 1 used as default and function 2 for systematics

Model systematic


PYTHIA 6.2 has been tuned:• fair agreement in tagged region(see plot vs kinematic variables)• less agreement in anti-tagged region• some failures in pt dependence• checks with LO pQCD (collinear)

• Uncertainties from each group – PYTHIA params.– PDFs– low-pT asym.

summed linearly to “Models” uncertainty

• Experimental (stat.+syst.) added in quadrature– syst. uncertainty (beam&target)

from 4% scaling uncertainty to 14% on g/g

Results vsworld data


•Black and blue curves:pQCD fits to g1

•Black data points:CERN exp results

•Red data point:Prel. HERMES Method I

•Red curves Prel. HERMES Method II: fit Δg(x)/g(x) with 2 functions such that

Conclusions


g/g(x,2) = 0.071 ± 0.034(stat) ± 0.010 (sys-exp) (sys-model)

g/g(x,2) = 0.078 ± 0.034(stat) ± 0.011 (sys-exp) (sys-model)

g/g has been extracted by HERMES using two different methods

Method I

Method II

+0.125

-0.082

-0.105

-0.127

Syst. model uncertainties still dominating (PDFs, PYTHIA model)

• G/G is likely small • G/G is unlikely to solve the puzzle of the nucleon missing spin

Back up slides


determination of the gluon polarisation at hermes n. bianchi on behalf of: the hermes collaboration...

Documents

vancouver bc slide

pacific spin07

p t pgf

acceptance p t

p t qcd2

gev slide

direct high p t n

deuteron data slide