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TRANSVERSE SPIN MEASUREMENTS IN

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Small Angle Spectrometer (SM2) tracking, calorimetry, PID. high energy beam ... splitting target cells and spectrometer, making bins in z and pt , changing F binning, ... – PowerPoint PPT presentation

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Title: TRANSVERSE SPIN MEASUREMENTS IN


1
TRANSVERSE SPIN MEASUREMENTS IN
Anna Martin University of Trieste and INFN
Trieste on behalf of the COMPASS Collaboration
Villa Mondragone, June 14, 2006
2
CONTENT
  • THE EXPERIMENT
  • PHYSICS RESULTS three different ways to access
    transversity

Collins and Sivers asymmetries positive and
negative hadrons p, K Two hadron
asymmetries all pairs leading hadrons ?
polarimetry
  • SUMMARY AND OUTLOOK

3
the COMPASS experiment
  • fixed target experiment at the CERN SPS
  • 240 physicists from 28 institutes, 11 Countries
  • approved in 1998 with a broad physics programme
  • hadron beams p,K,p (2004) 2007, …
  • muon beam 2002-2004, 2006, ….
  • transversity
  • gluon contribution to nucleon spin
  • g1, DS, Dq flavour decomposition
  • ….
  • r 0 production ? A. Sandacz

SPS
muon beam momentum 160 GeV/c longitudinal
polarisation -76 intensity 2108 µ/spill
(4.8s/16.2s)
4
the spectrometer
  • two stages spectrometer
  • Large Angle Spectrometer (SM1)
  • Small Angle Spectrometer (SM2)
  • tracking, calorimetry, PID

MuonWall
SciFi Silicon Micromegas GEMs
Straws SDC MWPC W45
SM2
E/HCAL
E/HCAL
SM1
MuonWall
Polarised Target
RICH
m beam
  • high energy beam
  • broad kinematical range
  • large angular acceptance

5
the target system
solid target operated in frozen spin mode
  • 6LiD
  • polarization PT 50
  • dilution factor f 0.38
  • relaxation time gt 2000 hrs
  • during data taking with
  • transverse polarization
  • dipole field always ?
  • polarization reversal in the 2 cells after 5
    days (systematics)

two 60 cm long cells with opposite polarization
(systematics)
dN/dz
4000
2000
0
zvtx (mm)
0
-1000
1000
6
the RICH detector
photon detectors
  • threshold momenta
  • pp 2 GeV/c
  • pK 9 GeV/c
  • pP 17 GeV/c

fully efficient for transverse data in 2003 and
2004
7
data taking 2002-2004
  • transversely polarised deuteron target
    20 of the running time

Collins and Sivers asymmetries published results
2002 prel results 2002-2004 with RICH
PID Two hadron asymmetries prel results
2002-2004 ? polarimetry prel results 2002-2003
8
PHYSICS RESULTS
Collins and Sivers asymmetries positive and
negative hadrons p, K Two hadron
asymmetries all pairs leading hadrons ?
polarimetry
9
single hadron asymmetries
unpol B and T
transvely polarised target
  • SIDIS cross-section

long pol B and/or T
  • ?h azimuthal angle of the hadron
  • ?s azimuthal angle of the transverse spin of the
    struck quark
  • ?s azimuthal angle of the transverse spin of the
    initial quark

?s p - ?s
  • ?C ?h - ?s
  • Collins angle
  • ?S ?h - ?s Sivers angle

10
single hadron asymmetries
unpol B and T
transvely polarised target
  • SIDIS cross-section

long pol B and/or T
  • ?h azimuthal angle of the hadron
  • ?s azimuthal angle of the transverse spin of the
    struck quark
  • ?s azimuthal angle of the transverse spin of the
    initial quark

?s p - ?s
at leading order
Collins
Sivers
beam polarization
independent angles ? independent extraction of
the asymmetries
11
Collins asymmetry
  • minimizes acceptance effects
  • spin independent terms cancel at 1st order

AColl evaluated as function of x, z and pt
for Leading Hadrons and All Hadrons
12
Sivers asymmetry
f target polarisation dilution factor
PT target polarisation
Sivers distribution function
ASivl evaluated as function of x, z and pt
for Leading Hadrons and All Hadrons
13
event selection
DIS events Q2 gt 1 GeV2 W2 gt 25 GeV2 0.1 lt y lt
0.9 hadrons pht gt 0.1 GeV/c z gt 0.2
(all h) z gt 0.25 (leading h)
?Q2? 2.4 (GeV/c)2
2002 data
?W? 9.4 GeV/c2
?xBj? ? 0.035
14
event selection cont
?pt??0.51 GeV/c
2002 data
?z? ? 0.45
15
Asymmetries 2003-2004 vs 2002
results from 2003-2004 data released in April
(DIS2006) comparison with the published 2002 data
PRL 94 (2005) 202002
  • increase in statistics by a factor of 7!

16
systematics
  • extensive studies, in view of the small
    statistical errors
  • false asymmetries
  • splitting target cells and scrambling the data
  • stability in time of acceptance / efficiency
  • R-test
  • stability of the measured asymmetries
  • dividing periods in time,
  • splitting target cells and spectrometer,
  • making bins in z and pt ,
  • changing F binning, …
  • use of different estimators
  • 2-dimensional fit
  • quality of the fit
  • ….

17
systematics (cont)
  • ….
  • consistency of the results from the different
    data taking periods

CONCLUSION all the effects are compatible with
the statistical fluctuations ? our systematic
errors are considerably smaller than the
statistical errors
expected normal standard distribution
18
Collins Asymmetries 2002-2004
  • small errors (1)
  • small asymmetries
  • cancellation between p and n
  • (lh and ah)

19
Sivers Asymmetries 2002-2004
  • small errors (1)
  • small asymmetries
  • cancellation between p and n
  • (lh and ah)

20
Collins - comparison with theory
some of the recent phenomenological studies
  • W. Vogelsang and F. Yuan
  • PRD72 (2005) 054028
  • DTq from saturation of Soffer inequality
  • 2 parameter fit to HERMES p data to extract the
    Collins FF
  • ? unfav FF - fav FF
  • (c2/n 0.8)
  • ? very small asymmetries for d target comparison
    with COMPASS 2002 data ok

COMPASS 2002 data
21
Collins - comparison with theory
  • A.V. Efremov, K. Goeke and P. Schweitzer, PRD73
    (2006) 094025
  • including transverse momenta
  • DTq from chiral quark soliton model
  • fit to HERMES data to get Collins FF
  • ? unfav FF - fav FF
  • agreement (absolute values) with analysis of
    BELLE
  • DTd unconstrained
  • ? comparison with COMPASS 2002 data ok

22
Sivers - comparison with theory
  • W. Vogelsang and F. Yuan
  • PRD72 (2005) 054028
  • 2 parameter fit to HERMES p data to extract
    Sivers function (u and d)
  • ? d - 2 u due to the large p-
    asymmetry
  • (c2/n 1.2)
  • ? comparison with COMPASS 2002 data ok

COMPASS 2002 data
23
Sivers - comparison with theory
  • M. Anselmino et al., PRD73 (2006) 094025
  • including transverse momenta
  • FF from literature
  • 13 parameter fit to HERMES and COMPASS 2002 data
    to extract Sivers functions ? (c2/n
    1.06)

24
PHYSICS RESULTS
Collins and Sivers asymmetries positive and
negative hadrons, p, K Two hadron
asymmetries all pairs leading hadrons ?
polarimetry
25
p, K Asymmetries 2003-2004
same DIS event selection and hadron definition as
before plus PID based on RICH response
final sample
additional systematic tests RICH stability ?
systematic errors smaller than statistical ones
26
p, K Sivers Asymmetries 2003-2004
GPD2006 last week
cancellation between p and n ?
27
p, K Collins Asymmetries 2003-2004
GPD2006 last week
again, difficult to see an effect …
just released …
28
comparison with HERMES
29
comparison with HERMES
30
PHYSICS RESULTS
Collins and Sivers asymmetries positive and
negative hadrons, p, K Two hadron
asymmetries all pairs leading hadrons ?
polarimetry
31
Two Hadron Asymmetries
looking at two hadron production, a different
asymmetry can be measured
FRS fR - fs
A. Bacchetta, M. Radici, hep-ph/0407345 X. Artru,
hep-ph/0207309
32
Two Hadron Asymmetries 1
  • selection of DIS events
  • Q2 gt1 (GeV/c)2
  • 0.9 gt y gt 0.1
  • W gt 5 GeV
  • and hadrons
  • z1, 2 gt 0.1 and xf1, 2 gt 0.1
  • z1 z2 lt 0.9
  • all combinations of ve (h1) and ve (h2)
    hadrons (1.3/ev)
  • presently no RICH PID

total statistics 2002-2004 6.1 M combinations
33
Two Hadron Asymmetries 1
K0
r0
final sample
34
Two Hadron Asymmetries 1
  • all combinations of ve (h1) and ve (h2) hadrons
  • small errors (order of )
  • small asymmetries! cancellation?

35
PHYSICS RESULTS
Collins and Sivers asymmetries positive and
negative hadrons, p, K Two hadron
asymmetries all pairs leading hadrons ?
polarimetry
36
Two Hadron Asymmetries 2
  • different hadron pairs selections have been tried
  • still based on the string fragmentation model
  • pT - leading hadron pairs h1 ve hadron with
    largest pT, h2 -ve hadron with second largest
    pT and h1 -ve hadron with largest
    pT, h1 ve hadron with second largest pT

1 entry/event
? SPIN 2005
37
Two Hadron Asymmetries / 2
  • z-leading hadron pairs

CIPANP06
as for p and K, just released …
38
PHYSICS RESULTS
Collins and Sivers asymmetries positive and
negative hadrons, p, K Two hadron
asymmetries all pairs leading hadrons ?
polarimetry
39
L polarimetry
event selection
40
L polarimetry
2002-2003 data
TRANSVERSITY 2005
41
L polarimetry
TRANSVERSITY 2005
2002-2003 data
2004 data analysis ongoing
42
  • SUMMARY AND OUTLOOK

43
summary
  • New precise DIS deuteron data from COMPASS are
    now available
  • In all the channels investigated in so far
  • the measured deuteron asymmetries are very small,
    compatible with zero
  • SURPRISING RESULTS FROM KAON ASYMMETRIES

Collins and Sivers asymmetries ve and -ve
hadrons, p, K Two hadron asymmetries all pairs,
leading hadrons ? polarimetry
Phenomenological studies can describe at the same
time the BELLE (FF), the HERMES (proton) and 2002
COMPASS (deuteron) Collins and Sivers asymmetries
Is the emerging picture satisfactory ?!!
44
outlook 1
  • in the near future COMPASS will produce some new
    results
  • from the present deuteron data
  • plus
  • transverse effects from longitudinal data
  • other two asymmetries from single hadron data
  • g2
  • Cahn effect
  • exclusive r production on transversely polarised
    target
  • ….

45
outlook 2
  • in 2006 COMPASS plans to take data with a
    transversely polarised proton target
    (NH3)
  • with 30 days running, precision deuteron
    in 2006, new PT magnet (larger acceptance
    at large x)
  • The COMPASS Collaboration
  • is considering a new proposal () based on
    spectroscopy,
  • GPDs, and TRANSVERSITY
  • SIDIS essential for flavour separation !

  • ? H.
    Fischer
  • () Input to CERN Council Strategy Group (Jan.
    15, 2006)
  • SPSC-EOI-005 (Jan. 18, 2005)
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