flavor separated quark polarizations at hermes
Post on 23-Jan-2016
29 Views
Preview:
DESCRIPTION
TRANSCRIPT
1
Flavor Separated Quark Polarizationsat HERMES
W. Lorenzon (Michigan)NAPP 2003, Dubvrovnik, Croatia
Collaboration
28 May 2003
• Flavor structure of proton – some phenomenological models
• Flavor tagging and purity analysis
• Experimental results
• Summary and Outlook …
2
Flavor Structure of Proton
• Constituent Quark Model Pure valence description: proton = 2u + d
• Perturbative Sea sea quark pairs from g qq should be flavor symmetric:
u = d
E866: d > u
3
Flavor Structure of Proton - II
Non-perturbative models: alternate d.o.f.
Meson Cloud Models Chiral-Quark Soliton Model Statistical Model
Quark sea from cloud of 0 mesons:
d > u
• quark d.o.f. in a pion mean-field• nucleon = chiral soliton• one parameter: dynamically generated quark mass
• expand in 1/Nc:
d > u
d > u
• nucleon = gas of massless partons• few parameters: generate parton distribution functions• input: QCD: chiral structure DIS: u(x) and d(x)
important constraints on flavor asymmetry for polarization of light sea
4
Spin Structure of the Proton
½ = ½ + G + Lq + Lg“You think you understand something? Now add spin …” -R. Jaffe
• Constituent Quark Model pure valence description of constituent quarks:
u = + 4/3, d = - 1/3
• Relativistic Quark Model relativistic current quarks with light masses: orbital angular momentum is important, and accounts for the deficit of .
• From NLO-QCD analysis of inclusive DIS measurements… SMC, PRD 58 (1998) 112002
(in AB scheme) s = -0.02 to –0.15 (model dependent) slight neg. sea-quark polarization?
0.60 – 0.75
= 1
Lq=½(1-)
= 0.38
G= 1.0 +1.9 –0.6
5
Polarized Deep-Inelastic Scattering
Note: inclusive DIS is sensitive only to ef2
There are no neutrino DIS measurements on polarized targets (yet!)
inclusive DIS cannot distinguish quark from anti-quark q := (q + q)
21
21
and
1
2
1
2
: :
F e q
g e q
q q q q q q
1/ 2 q
3/ 2 q
One measures double spin asymmetries:
In QCD Parton Model:
1/ 2 3/ 21
1/ 2 3/ 2
A
22 220
2 11 2 2 2
1
( , )g ( , )( , )
( , ) ( , )
gqq
e q x Qx QA x Q
F x Q e q x Q
6
Comparison of Data for g1p(x)
Note: inclusive DIS is sensitive only to ef2
There are no neutrino DIS measurements on polarized targets (yet!)
inclusive DIS cannot distinguish quark from anti-quark q := (q + q)
21
21
and
1
2
1
2
: :
F e q
g e q
q q q q q q
1/ 2 q
3/ 2 q
One measures double spin asymmetries:
In QCD Parton Model:
1/ 2 3/ 21
1/ 2 3/ 2
A
22 220
2 11 2 2 2
1
( , )g ( , )( , )
( , ) ( , )
gqq
e q x Qx QA x Q
F x Q e q x Q
7
Anti-quark Spin in the Proton
Meson Cloud Models Chiral-Quark Soliton Model Statistical Model Li, Cheng, hep-ph/9709293 Goeke at al, NPA 680 (2000) 397 Bourelly et al, EPJ C23, (2002) 487
Light sea quarks polarized:
with … but …
qvalence > 0 qsea < 0, but … “higher-order” cloud ofvector mesons can gene- rate a small polarization.
q = 0
u – d >
0
d < 0, u > 0
u(x) – d(x) > d(x) –
u(x)
u(x) – d(x) ~ d(x) –
u(x)
8
Quark Polarization from Semi-Inclusive DIS
In semi-inclusive DIS a hadron h is detected in coincidence with the scattered lepton
Goal: Flavor Separation
of quark and anti-quark helicity distributions
Technique: Flavor Tagging z=Eh/
The flavor content of the final state hadrons is related to the struck quark through the agency of the fragmentation function Dqh(z,Q2). In LO
QCD: 2
, .
2
,..
2( ( ,, ) )h hq
q u u
hq
d dq x D zQe
dz dzQ
target fragmentationregion
current fragmentationregion
9
Is HERMES Fragmentation “Universal”?
Compare pion multiplicities: HERMES vs EMC (both at Q2=25 GeV2)
Assumptions for flavor tagging (in LO QCD):• factorization (DIS, fragmentation)• fragmentation functions exhibit scaling
Good agreement, despite order of magnitude difference in energy
10
Purity Analysis of Asymmetries
• Perform a simulateneous global analysis of all A1h(x,Q2)’s
• The photon-nucleon asymmetry is:
• The hadron quark purity Pqh(x,Q2) is the probability that a
quark q was struck in an event e+N e’+h+X
• Need at least six independent asymmetry sets A1h(x) to
determine six unknown helicity distributions• Purity equations are generally under determined add symmetry assumptions (HERMES 2002)
max
min
max
min
max
min
max
min
2 2 2
'
2 2
2 2
2
2
''
2 2
1
2
2
2
( , )
( ,
'( , ) ( , )
( , )
( , ))( , )
( ,
() ,
)( )
,
z h
q qq z
z h
q qq z
z h
q qq z
z hq q
h
q
z
e
e D
q x Q dzD z Q
q z Q dz
e D z Q dz
x Q
q x Q
e q x Q dzD
A x Q c
cxq
z qQQ
Δu(x), u(x), Δd(x), d(x), Δs(x), s(x)
Δs(x) s(x)
s(x) s(x)
FL contribution to unpolarizaed PDF’s: 2
2
1 ( , )
1
R x Qc
11
Generation of Purities
• Use Monte Carlo model of DIS process (LEPTO), fragmentation process (LUND model in JETSET) and detector• Systematic uncertainties from
– Variation of fragmentation parameters– Use of alternative PDF set GRV98LO vs. CTEQ5L
12
Purities
Systematic uncertainties from PDF sets (GRV98LO, CTEQ5L) and LUND parameters
13
Semi-Inclusive Asymmetries on the Deuteron
Systematic uncertainties from PDF sets (GRV98LO, CTEQ5L) and LUND parameters
• • is an all-sea object• Covered range: 0.023 x 0.6 0.2 z 0.8, xF 0.1
• Asymmetries are independent of z is this range
1 ( ) 0KA x
( )K us
14
Latest q Results from HERMES
First 5-flavor fit to q(x) ( assumed)
Results show:
• u-quark positively polarized
• d-quark negatively
polarized• light sea quark polarization small
• s-quark polarization somewhat positive (1)
( ) / ( ) ( ) / ( )s x s x s x s x
15
(A)symmetry of Polarized Light Quark Sea
• No significant SU(2)f breaking
• data disfavor QSM of Dressler et al.
• statistical model & Meson Cloud model consistent with BUT…
( ) ( )u x d x
0u d
16
Isoscalar Extraction of s
• A1K provide largest sensitivity to s: K+=(u,s), and K-=(u,s)
– but: large systematic uncertainties on PsK
• Alternative approach:
– use only total kaon flux K+ + K- on deuterium target (isoscalar)
– s’ = s + s is also isoscalar quantity
• Can make measurement of s using inclusive and semi-
inclusive
– use fragmentation functions from e+e- collider experiments,
directly compute purities PsK (no MC model needed)
• Provides independent check on strange polarization result
• Two component analysis also favors positive s
1, ( )dA x 1,K KdA
2( , )K Kq qD z Q
17
Remarks and Conclusions
• This is first “direct” 5 component decomposition of quark helicity
distributions using flavor tagging
• Helicity distributions confirm inclusive DIS results:
light sea in unpolarized:
• No indication for
• Can make measurement of using inclusive and semi-
inclusive
– use fragmentation functions from e+e- collider experiments
– provides independent check on strange polarization result
– two component analysis also favors positive
• The singlet strength, agrees to within 1 with the
Bjorken
sum rule
0, 0u d 0u d
( ) 0s s
s 1, ( )dA x1,K KdA
s
3 ,q u d
18
Quark Polarization: Status & Outlook
• quark polarization q(x) : -> first 5-flavor separation from HERMES -> q(x) consistent with zero, in contrast to QSM model predictions•
top related