Title: Measurements of the Spin-dependent Structure Function g1(x,Q2) at COMPASS
1Measurements of the Spin-dependent Structure
Function g1(x,Q2) at COMPASS
d
Helena Santos LIP - Lisboa On Behalf of the
COMPASS Collaboration
- The nucleon spin
- The COMPASS experiment
- Inclusive asymmetries
- The g1 structure function
XXI Rencontres de Physique de La Vallée dAoste
2The Nucleon Spin
naïve parton model ?? ?u?d 1
Gluons, sea and c quarks are important
complete description orbital angular
momenta
EMC (1988)
?? 0.12 ? 0.09 ? 0.14 ?s?s 0.14 ? 0.03
SN ½ ½?? ?G Lq Lg (h1)
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
3Deep Inelastic Scattering
?(E,p)
?(E,p)
?
Q2 q2 (p p)2 ? E E x Q2/2M? y ?/E
X
N(M,P)
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
4Polarised Deep Inelastic Scattering
photon-nucleon asymmetry
?q(x) q(x) q(x) q(x) q(x) q(x)
quark ?? nucleon quark ?? nucleon
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
5The COMPASS Experiment at the CERN-SPS
COmmon Muon and Proton Apparatus for Structure
and Spectroscopy
LHC
SPS
M2
N
Luminosity 5 . 1032 cm-2 s-1 Beam intensity
2?108 µ/spill (4.8s/16.8s) Beam momentum 160
GeV/c
230 physicists from 12 countries
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
6The COMPASS Spectrometer
50 m
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
7Kinematic Domain
6075 of inclusive triggers over the full
COMPASS x range Semi-inclusive triggers dominant
at low x and low Q2 Calorimetric triggers
dominant for Q2 gt 30 GeV2
H. Santos (helena_at_lip.pt)
8The Longitudinal Photon-Deuteron Asymmetry
(d ? deuteron)
? is small in the COMPASS kinematic range, as
well as A2
N ? Number of detected events PB ? Beam
polarisation PT ? Target polarisation f ?
Dilution factor D ? Depolarisation factor
?? ? 1st cell (polarised anti-paralell to beam),
?? ? 2nd cell (polarised parallel to beam)
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
9Inclusive Asymmetry, Q2 lt 1 GeV2
- A1d asymmetry compatible with 0 at low x range
(0.0005ltxlt0.02) - At low x A1d has been measured only by COMPASS
and SMC - Systematic errors are mainly due to false
asymmetries
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
10The g1(x) Structure Function
F2(x)
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
11Inclusive DIS Asymmetry
CERNPHEP/200629
COMPASS 2002, 03 04 data 89 x 106 events
Q2 gt 1 (GeV/c)2
d
- A1 compatible with 0 for x lt 0.05
- Large asymmetry at large x
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
12Inclusive DIS Asymmetry
Q2 gt 1 (GeV/c)2
- Good agreement with previous experiments
- Improved significantly statistics at low x
- No tendency towards negative values at x lt 0.03
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
13The g1(x) Structure Function
N
Q2 gt 1 (GeV/c)2
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
14QCD Analyses
?? ?u ?d ?s, ?q3 ?u - ?d, ?q8
?u ?d - 2?s
DGLAP equations
Input parameterisations (xdependence at a fixed
Q02)
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
15QCD Fits
Data well described by two solutions ?G gt 0 and
?G lt 0
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
16Towards Structure Functions
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
17Towards Structure Functions
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
18Polarised Parton Distributions
- Very small sensitivity of x(?q?q) to x?G
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
19QCD Fits Results
(world data)
CERNPHEP/200629
Quark polarisation
?G gt 0 ?G lt 0
?? 0.27 0.01 0.32 0.01
(error ? factor 2 larger without COMPASS data)
Gluon polarisation (indirect determination via
DGLAP)
- Solutions with ?G gt 0
- Solutions with ?G lt 0
?G ? 0.2 - 0.3
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
20Gluon Polarisation ?G/G
- Direct measurement of gluon polarisation with NLO
QCD fits to g1 - Unpolarised G(x) from MRST
- Bands correspond to statistical errors of ?G
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
21First Moment of g1
(COMPASS data only)
CERNPHEP/200629
- in literature (S.A. Larin et al., PLB404 (1997)
153)
(from Y. Goto et al., PRD62 (2000) 034017 a8
0.585 0.025)
â0 is interpreted as the fraction of the nucleon
spin carried by
the quarks, ?? ?u ?d ?s
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)
22Conclusions Outlook
- Longitudinal A1 inclusive asymmetries measured
using 2002, 2003 2004 COMPASS data - From the first moment of g1d, we extract the
quark contribution to the nucleon spin (COMPASS
data only) -
- Semi-inclusive analysis towards flavour spin
distributions is in progress
- QCD fits to world data give for quark and gluon
polarisation
?G ? 0.2 0.3
XXI Rencontres de Physique de La Vallée
dAoste, March, 4th 10th 2007
H. Santos (helena_at_lip.pt)