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COMPASS results: TRANSVERSITY. Paolo Pagano On behalf of the COMPASS collaboration. Talk outline. The physics case of transverse spin effects Probing transversity in SIDIS The COMPASS experiment The results on the deuteron for 1 hadron SSA and interpretation - PowerPoint PPT Presentation
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Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 1
COMPASS results: TRANSVERSITY
Paolo PaganoOn behalf of the COMPASS collaboration
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 2
Talk outline
1. The physics case of transverse spin effects2. Probing transversity in SIDIS 3. The COMPASS experiment4. The results on the deuteron
for 1 hadron SSA and interpretationfor hadron pairs SSA and interpretation
5. Conclusions and Outlook
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 3
Spin structure functions
3 distribution functions are necessary to describe the spin structure of the nucleon at LO:
All of equal importance !
TTq(x)q(x)
q(x)q(x)
q(xq(x))
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 4
Measuring ΔTq(x)
How to access it:SIDIS(e.g. COMPASS and HERMES )
Hard scattering (e.g. RHIC)–Drell-Yan–Single Spin Asym (e.g. p↑p→πX)
Hard scattering (e.g. GSI)–Drell-Yan
FFxqT )(
qq TT
NN NN qq TT
Chiral-odd: requires another chiral-odd partnerChiral-odd: requires another chiral-odd partner
Impossible in DIS Direct Measurement
ppl+l-X epe’hX
Convolution with fragmentation function
epe’X
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 5
Transversity
In the last ten years:• Great development in the theory
of transversity;• Remarkable role of ΔTq(x),
notably complementary to Δq(x).
In the last couple of years:• Role of the kT structure functions
clarified (Cahn and Sivers effects, …).
• Tensor charge (1991 – 1992):
in analogy with:
• Soffer inequality (1995):
• Leader sum rule (2004):
in analogy with:
)()(2
1)( xqxqxq TT
)()( xqxqdxg TTT
)()( xqxqdxgA
gqq
Tqq
T Lxqdx,,,
)(2
1
2
1
zz LGS 2
1
Key features of transversity:- Probes relativistic nature of quarks- No gluon analog for spin-1/2 nucleon- Different Q2 evolution and sum rule
than Δq(x)- Sensitive to valence quark polarization
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 6
•
)
Transverse Spin Physics at COMPASS
Possible quark polarimeters suggested using SIDIS:
• Azimuthal distribution of single (leading)
hadrons
• Azimuthal dependence of the plane containing
hadron pairs
• Measurement of transverse polarization of spin ½
baryons (e.g. hyperon)
Published RESULTS
2005
PreliminaryRESULTS
2005
Not covered in this talk
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 7
Azimuthal asym’s of single hadrons
• Collins effect predicts an azimuthal asymmetry in the quark fragmentation;
• Look at leading hadron (the most sensitive to the parent struck quark);• The larger z = Eh/, the stronger the signal;• An azimuthal asymmetry can also come from the un-polarised quarks; namely from
an azimuthal modulation of quark transverse momentum for a transversely polarized nucleon (Sivers effect)
Left Right
Non vanish Lq
The quark prefers to fragment in one direction
00 hqTT Dq
ChqTD sin ~FF 0
0
sin ~DF
0
0
hq
T
ST
Dq
q
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 8
Collins and Sivers asym’s
q
hqq
q
hq
Tq
Siv
q
hqq
q
hqTTq
Coll
Dqe
DqeA
Dqe
DqeA
S
C
2
02
sin
2
02
sin
Asym’s in C and S extracted independently;
Asym’s calculated as function of x, z and pt and for “Leading Hadrons” and for “All Hadrons”
s'hC
shS
Collins angle:
Sivers angle:
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 9
Azimuthal dependence of the plane containing hadron pairs • Another quark polarimeter comes from interference in fragmentation
• The struck quark fragments in two hadrons. The fragmentation function has a component dependent on the transverse spin momentum (as for the Collins function):
The hadron pair prefers to fragment in one direction
0 qT Hq ALeft Right
0sin :FF RSqH
A
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 10
02
h 2
sin
q
hqq
q qTq
UT Dqe
HqeA RS
Hadron pair azimuthal asymmetry
Calculated as function of x, z and Mh for all pairs of positive and negative hadrons.
s'RRS Azimuthal angle:
A
A. Bacchetta and M. Radici,Proceedings of DIS 2004,hep-ph/0407345
COMPASS Fixed target experiment at CERN SPS240 physicists from 12 countries28 institutes2000 technical run2001 commissioning run2002-04 physics runs (muon program)2005 SPS stop2006-… resume physics
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 12
With the muon beam
Gluon Polarization G/G
transverse spin distribution
functions Tq (x)
Flavor dependent polarized
quark helicity densities q(x)
physics
Diffractive VM-Production
Physics Goals of COMPASS
With hadron beams
Primakoff-Reactions- polarizability of and K
glueballs and hybrids
charmed mesons and baryons- semi-leptonic decays- double-charmed baryons
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 13
SM1 dipoleSM1 dipole
SM2 dipole
Polarised Target
HCAL1
Muon-filter1,MW1
Micromegas,DC,SciFi
Gems,SciFi,DCs,straws
MWPC Gems Scifi
trigger-hodoscopes
Silicon
Spectrometer 2002 2004
RICH_1 Gem_11
ECAL2,HCAL2
straws,MWPC,Gems,SciFi
straws Muon-
filter2,MW2
DW45
SciFi
Veto
Beam: 160 GeV µ+
2 . 108 µ/spill
(4.8s/16.2s)
P ~ 76%
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 14
The polarized 6LiD-Target
two 60 cm long Target-Containerswith opposite polarizationDuring data taking for transversitydipole field always
Relaxing time > 2000 hrs
superconductive
Solenoid (2.5 T)
3He – 4He Dilution refrigerator (T~50mK)
Dipole (0.5 T)
reversed once a week
1
2
For transversity:
Polarization: 50%
Dilution factor: 0.38
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 15
COMPASS data sample• In so far ( 3 years )
– only DIS off 6LiD– only runs with transverse polarized target being analyzed
runs good runsused events
in the analysis (1h)
2002 47545300 SPS
spills1.6 · 106
2003 47942900 SPS
spills~4•(2002)
2004 49694000 SPS
spills~2•(2003)
RICH
E-Calo
• Data sample increased in years 2003/4:– trigger system
upgraded;– DAQ upgraded;– 2004 longer run.
• 2003 and 2004 data production over(analysis in progress)
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 16
Kinematical distributions and general cuts
Q2=2.4 (GeV/c)2
Y=0.33
W=9.4
GeV/c2
xBj0.035
hadronshadrons
p
E
muomuonsns
Use of the calorimeters
To access DIS region and get rid of radiative corrections
2002 data
Q2 1 (GeV/c)2
0.1 Y 0.9
W 5
GeV/c2
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 17
Cuts for Collins and Sivers SSA
pT0.51
GeV/c z 0.45
2002 useful statistics Lead had’s
Positive hadrons 8.7 ·105
Negative hadrons 7.0 ·105
Total 1.6 ·106
2002 data
pT0.1 GeV/c z 0.25 (0.2 ah)
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 18
Extraction of Collins and Sivers asymmetries• Evaluating the
population as a function of Φ, independently in both target cells;
• Extracting 2 asymmetries and calculating the weighted mean.
)sin1()()( jjjjjjaFnN
j = C, S(Φj calculated always with spin = ↑)
F is the muon fluxn the number of target particlesσ the spin averaged cross-sectionaj the product of angular acceptance
and efficiency of the spectrometer;
jj
jjjj
jjjj
NrN
NrN
sin)()(
)()(
SivTS
CollNNTC
APf
ADPf
const)(
)(
jj
jj
a
a
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 19
Evaluation of systematics • Stability of the asymmetries:
i) in time;ii) in two halves of the target cells;iii) according to the hadron momentum.
• Evaluated 3 different estimators and compatibility in the results checked.
Look at the paper:
“First Measurement of the Transverse Spin Asymmetries ofthe Deuteron in Semi-Inclusive Deep Inelastic Scattering”(The COMPASS Collaboration)
Phys. Rev. Lett. 94, 202002 (2005)
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 20
Collins and Sivers effects (1)
Systematic errors are smaller than the quoted statistical errors.
• Asymmetries as a function of x, z, pt
• Only statistical errors shown
Phys. Rev. Lett. 94, 202002 (2005)
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 21
Collins and Sivers effects (2)
Systematic errors are smaller than the quoted statistical errors.
• Asymmetries as a function of x, z, pt
• Only statistical errors shown
Phys. Rev. Lett. 94, 202002 (2005)
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 22
Interpretation of Collins SSA
• Small asymmetries. Two explanations:– Cancellation between proton and neutron;– Too small Collins mechanism.
• If and large as from preliminary measurement by BELLE (see hep-ex/0507063), this is evidence for cancellation in iso-scalar target;
• Also a new phenomenological fit of the data by
Vogelsang and Yuan (see hep-ph/0507266).
00 hqTD
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 23
COMPASS (deut.) vs. HERMES (protons) Positive hadrons
• HERMES:– Negative Collins
asymmetries;– Positive Sivers
asymmetries.
HERMES data points for Collins effect flipped by a phase of π to match COMPASS notation !
HERMES data points from: A. Airapetian et al, Phys. Rev. Lett. 94 (2005) 012002[DC53] (hep-ex/0408013)
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 24
COMPASS (deut.) vs. HERMES (protons) Negative hadrons
• HERMES:– Large Positive
Collins asymmetries;
– No Sivers effect.
HERMES data points for Collins effect flipped by a phase of π to match COMPASS notation !
HERMES data points from:A. Airapetian et al, Phys. Rev. Lett. 94 (2005) 012002[DC53] (hep-ex/0408013)
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 25
Theoretical work on Sivers SSA
• “Extracting the Sivers function from polarized SIDIS data and making predictions”–Phenomelogical model whose parameters are constrained by HERMES proton measurements;– COMPASS 2002 preliminary results for Sivers effect are in agreement with the model.
M. Anselmino et al.
hep-ph/057181
(including HERMES
Prel. Results shown at DIS05)
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 26
Prediciton of Sivers asymmetries on a proton target
M. Anselmino et al.
hep-ph/057181
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 27
Expected accuracy for Collins asymmetries
AColl statistical errrors:Positive hadrons
(to the left):
2002 - Red;
2002 – 2004 Blue;
2006 Blue.
Negative hadrons
(to the right):
2002 - Red;
2002 – 2004 Blue;
2006 Blue.
• Accuracy from total statistics accumulated with LiD (deuteron) target;
• Projections from 30 days of data taking (included in COMPASS schedule for 2006) with NH3 (proton) target :
Taking into account:
– Variation of statistical errors:
– taking into account the variation of:
)(34.1)( 63 LiDNH AA
fPT
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 28
Kinematics from the analysis on hadron pairs
2002-2003:
2.8 106
combinations
Select all combinations of positive (h1) and negative (h2)
9.0
1.0,
1.0,
21
21
21
zzz
xx
zz
FF
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 29
Extraction of asymmetries for hadron pairs• Evaluating the
population as a function of ΦRS, independently in both target cells;
• Extracting 2 asymmetries and calculating the weighted mean.
)sin1()()( RSRSRS aFnN
ΦRS calculated always with spin = ↑; F is the muon fluxn the number of target particlesσ the spin averaged cross-sectiona the product of angular acceptance and efficiency of the spectrometer;
RS
RSRS
RSRS
NrN
NrN
sin)()(
)()(
RSUTNNT ADPf sin
const)(
)(
jj
jj
a
a
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 30
Effects on hadron pairs
• Asymmetries as a function of x, z, Mh
• Only statistical errors shown
Systematic errors are smaller than the quoted statistical errors. Presented for the first time @ DIS2005
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 31
Theoretical work on hadron pair (asym vs. invariant mass)
R. L. Jaffe, X. Jin and J. Tang, Phys. Rev. Lett. 80, 1166 (1998)
Radici, Jakob, Bianconi, PRD 65, 074031
H∢ (
z,M
2
+-)∼
sin 0
sin 1
sin
(0-
1)
AM
h
sin
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 32
Theoretical work on hadron pairs (asym vs. xbj and z)
Axsi
n
(
z)
∢
Radici, Jakob, Bianconi, PRD 65, 074031
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 33
Summary and Outlook
• Collins and Sivers SSA shown from 2002 data published by PRL:– The asymmetries are small and compatible with zero; – Making use of all the deuteron data, accuracy will
improve by a factor ~ 3. • SSA from hadron pairs calculated from 2002 + 2003 statistics:
– The asymmetries are small and compatible with zero;– Using 2004 data, accuracy improves by a factor 1.4.
• Stay tuned because:– New channels in hadron pair production are under study.– PID is being included in all the analyses;– New results on Lambda transverse polarization are coming.
• Data (of comparable statistics) will be collected on a transversely polarized proton target (NH3) in 2006.
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 34
Thank you!
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 35
Save slides
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 36
Estimators
Weaker assumptions on acceptance effects
less sensitive to distorsions set by Cahn effect
)sin41(cost)(
)(
)(
)( 2
,
,
,
,jj
jdj
jdj
juj
juj
N
N
N
N
jj
jjjjjjjj
jjjjjjjj
NNNN
NNNN
sin )()()()(
)()()()(3
Reconstruction time independent in the solid angle
Prague (CZ), August 1st Paolo Pagano (INFN - Trieste) 37
Why a proton run?
Flavour decomposition of Tq:
1. proton target, combining + and -
2. deuterium target:
dudu1 DDDDD dudu2 DDDDD
21
21TTTTpTp
p11 DD
ΔDΔD
du4d4u
dΔu4ΔdΔu4ΔDPfA
21
21vTvTpTp
p21 DD
ΔDΔD
du4d4u
dΔu4ΔDPfA
21
21TTTTdTd
d11 DD
ΔDΔD
dudu
dΔuΔdΔuΔDPfA
21
21vTvTdTd
d21 DD
ΔDΔD
d)ud5(u
)dΔu3(ΔDPfA
Flavour decomposition of Tq:
1. proton target, combining + and -
2. deuterium target:
dudu1 DDDDD dudu2 DDDDD
21
21TTTTpTp
p11 DD
ΔDΔD
du4d4u
dΔu4ΔdΔu4ΔDPfA
21
21vTvTpTp
p21 DD
ΔDΔD
du4d4u
dΔu4ΔDPfA
21
21TTTTdTd
d11 DD
ΔDΔD
dudu
dΔuΔdΔuΔDPfA
21
21vTvTdTd
d21 DD
ΔDΔD
d)ud5(u
)dΔu3(ΔDPfA
vTvT
T
T
du
dd
uu
,
)(
)(
To measure the shape of