Transcript Kein Folientitel

HERMES: status and selected recent results
Workshop on Hadron
Structure and Spectroscopy
Paris, 1-3 March 2004
K. Rith





The quark helicity distributions
Transversity
The Pentaquark +
DVCS
The tensor asymmetry AT and structure function b1d
The HERMES Spectrometer
The HERMES Spectrometer
The HERMES internal polarised atomic gas-target
The HERMES dual-radiator RICH
Silica aerogel: n = 1.03,
C4F10:
thresh = 4.2
n = 1,0005, thresh = 32
1
0.5
P
PK
Pp 
PK
PKK
Pp K
Pp
PKp
Pp p
0
0.8
0.5
0
0.8
0.5
0
5
10
15
5
10
15
5
10 15
P (GeV)
Quark helicity distributions, semi-inclusive asymmetries
 = E - E‘
z = Eh/
Leading hadron originates with large probability from struck quark
D(z):= Fragmentation function
Semi-inclusive asymmetries-1
In leading order:
A1h(x,z) =
=


2
q zq q(x)
q
Dq
zq2q(x) Dqh(z)
zq2q(x) Dqh(z)

q
q ' zq‘
P.L. B 464 (1999) 123
h(z)
2q‘(x)
q(x)
Dq‘h(z)
q(x)
Quark-‘Purity‘ Phq
Different targets and hadrons h:
Solve linear system for Q with

A = (A1,p, A1,d, A1,p  , A1,d
A=PQ
,

A1,p K )
Semi-inclusive asymmetries from the Deuteron
 ,K, p asymmetries identified with RICH
Pions
Kaons
P.R.L.92 (2004) 012005

Statistics sufficient for 5-parameter-fit
Q(x) = (u(x)/u(x), d(x)/d(x), u(x)/u(x), d(x)/d(x), s(x)/s(x) )
Purities
(Probability that observed hadron originates from quark of type f)
 Adequate degree of orthogonality:
 Shaded bands: systematic uncertainties
 Kaons have about 10% sensitivity
to
Extracted quark helicity distributions
P.R.L.92 (2004) 012005
u > d ?
The HERMES data are

s < 0 ?
x
consistent with flavour symmetry of
spin-dependent sea
d(x)  - 0.4 u(x) (!?)
What is the dynamics behind this??
 Data with much higher statistical
accuracy urgently needed
Transverse quark polarisation , ‘Transversity‘ h1
Complete description of nucleon in leading order QCD: 3 distribution functions
f1 =
g1 =
h1 =

Quark momenta,  q q



-

longitudinal quark spin, ,  q   5q HERMES 1995-2000
transverse quark spin, ,  q 5 q
HERMES 2002.....
h1 is chiral odd, can only be measured in conjunction with other chiral odd
distribution (pol. Drell-Yan) or fragmentation function (SIDIS)
See reviews by P. Mulders and P. Ratcliffe tomorrow
Single Spin Asymmetries (SSA)
ep(d) e‘hX
AUL( ) =
N+() - N-( )
N+() + N-( )
U: unpol. e-beam
L: long. pol. Target
Fit sin() to asymmetries
AULsin()
transverse spin component:
ST  sin  (15% - 20%)
SSA from longitudinally polarized target
Deuteron
Proton
P.L. B 562 (2003) 182
+
P.R. D 64 (2001) 097101.
o
o
+
-
-
0
2.4 M DIS

+
K+
-
0
8.9 M DIS

+
AULsin() from longitudinally polarized target
Deuteron
Proton
+
o
-
K+
AULsin() from longitudinally
polarized target
+
AULsin  SL(M/Q) za2 x [hLa(x) H1a(z) - x h1La(x) Ha(z)/z + ..]
- ST  za2 x h1a(x) H1a(z)
SL >> ST
Collins fragmentation function
o
Theory predictions seem to
explain the data well
A.V. Efremov et al., Eur. Phys. J. C 24 (2002) 47
B. Ma et al., P.R. D66 (2002) 094001
-
but contain a lot of assumptions:
magnitude of H1/D1 4 ...12%
only part of Twist-3 contribution
taken into account
no Sivers contribution
taken into account
K+
Azimuthal asymmetries: Collins vs Sivers effect
2 different possible sources for azimuthal asymmetry:
 product of chiral-odd transversity distribution h1(x) and chiral-odd
fragmentation function H1(z) (Collins)
 product of naive time-reversal odd distribution function f1T and
familiar unpolarised fragmentation function D1(z) (Sivers) (requires
orbital angular momentum of quarks)
Longitudinally polarised target:
Collins and Sivers effect indistinguishable
Transversely polarised target:
2 azimuthal angles,
Collins andSivers effect distinguishable
AUTsin( + s )
AUTsin( - s )
Data taking with transverse target polarisation
 Transverse target magnet installed end of 2000
 Since then: rather unsatisfactory performance of HERA
2002: 600 k DIS-events with polarised H-target (present analysis)
2003: 450 k Dis events
2004: > 300 k DIS events (until now)
(2000 > 9 M DIS events from pol. D-target)
 We still are hoping for
substantial improvements
 Possibly continuation until summer 2005
First measurement of transverse asymmetry - H target
„Collins“ moments
„Sivers“ moments
ph/M-weighted azimuthal asymmetries
„Collins“ moments
„Sivers“ moments
Interpretation of transverse asymmetries
 Sivers function nonzero  orbital angular momentum
Sivers flavor separation possible
 Collins asymmetries show an unexpected pattern
Expect A+  Ao  0 and A -  0 and smaller in magnitude
HERMES data for AULCollins show
A+  0 but Ao  A-  0 and larger in magnitude !
 Interpretation: forthcoming paper
 Extraction of transversity distributions underway
HERMES contribution to the Pentaquark story
u
d
s
u
d
Theoretical motivation and experimental status:
see reviews by M. Polyakov and M. Ostrick tomorrow
Exotic Hadrons
All until now observed Hadrons are (qq)- oder (qqq)-states
But
Model predictions do very often not agree with measured
masses
Many ‚missing‘ resonances
QCD allows additional „exotic“ hadrons:
Glueballs
(gg)
Hybrid states (qqg)
Multiquark mesons (qqqq)
Multiquark baryons, eg. Pentaquark (qqqqq)
Di-baryons [(qqq)(qqq)]
Again and again there were announcements of the discovery of such
states. None did survive so far
Theoretical prediction for 5-quark states [qqqqq + ‚sea‘]
Bag models [R. Jaffe (1977), J.J De Swart (1980) et al.]
Skyrme model [M. Praszalowicz (1987) et al.]
Prediction: Lightest 5-q state has M = 1530 MeV
Baryon-meson states [H.J. Lipkin (1987) et al.]
Chiral Soliton Model [D. Diakonov, V. Petrov, M. Polyakov(1997) et al.]
Excitement of chiral field in baryon: additional qq-pair
Reproduces mass splittings in baryon-octet and decuplet within <1%
Prediction: New anti-decuplet with + (uudds), M = 1530 MeV,
positive parity, width < 15 MeV
Diquark-pair model [R.L. Jaffe, F. Wilzcek (2003)]
Strongly correlated diquark-pairs plus antiquark: ([qiqj]2q)
5-Quark states in the SM
S
D. Diakonov, V. Petrov and M. Polyakov,
Z. Phys. A 359 (1997) 305
uud ds
+(1530) Prediction
1
sdu..
-1
duu(dd+ss)
sdd..
sdu..
*-
ssd du
1
*0
ssd (uu+dd)
I3
N(1710)
180 MeV
suu(dd+ss)
-1
(1890)
*0
ssu..
ssu ud
3/2(2070)
Experimental evidence for +(1530)
Last year: after 30 years of futile search,
sudden explosion of experimental evidence
Experiments
Resultats
Mass
(MeV)
LEPS
DIANA
CLAS
SAPHIR
ITEP (’s)
1540105
15392”few”
154225
154042
15335
World average
15392.5
Prediction
1530
Width
(Mev)
G 25
G 8
FWHM  21
G 25
G 29
Significance
()
4.61
4.4
5.30.5
4.8
6.7
G15 I=0 S=+1 JP=½+
Experimental Evidence from HERMES
Reaction:
e D -> p K0sX
-> p + - X
Ee = 27.6 GeV,
Target: pol/ unpol D
Momentum range:  (1-15 GeV), p (4-9 GeV)
Cuts: data quality, distance between + - -, Ks - p, + - beam
Ks: decay length > 7 cm, 485 MeV< M(Ks) < 509 MeV
L(1116) excluded: reject event if M(-p) within 1 of nominal L mass
KS -> + -
optimized yield of Ks peak in M(+-) while minimizing background
NO constraints optimized to increase significance of signal in final M(p+-)
Detector Calibration with KS , L, L, -, L*
Particle
observed mass
PDG mass
[MeV]
[MeV]
KS  +-
496.8  0.2
497.67
L  p -
1115.7  0.1
1115.68
-  p - +
1321.5  0.3
1321.31  0.3
L* p K-
1522.7  1.9
1519.5  1.0
Excellent Proton identification by RICH:
K+ and + contamination negligible for 4 GeV< P p< 9 GeV
L, L, -, L*... well identified
Monte Carlo Simulation of pKS  p +-
Input: Resonance at 1540 MeV with width  = 2 MeV, decay into pKs
Full detector simulation
Results: Mass M = 1540  0.3 MeV
Width  = 6.2  1 MeV, FWHM = 14.6  2.4 MeV
Masses are well reconstructed, apparative resolution determines
width of the signal
M(+-p) Spectrum - fit with polynomial background
hep-ex/0312044
Fit: 4th-order polynomial
Resonance is observed at
M(+-p) = 1528  2.6  2.1 MeV
Width: FWHM = 19.5  5  2 MeV
somewhat larger than exp. res.
Naive significance: 56/144  4.7 
True significance: 59/16  3.7 
Unbinned fit is used; result does
not depend on size of bin and
starting point
M(+-p) Spectrum - efforts to reproduce background
hep-ex/0312044
Mixed event background
PYTHIA6 simulation
(no resonances (+ or *+)
in mass range 1.4 – 1.7 GeV)
Remaining strength due to ‘known’
broad resonances ((1480), (1560),
(1580), (1620), (1660), (1670))
plus new structure
M(+-p) = 1527  2.3  2.1 MeV
Width: FWHM = 22  5  2 MeV
Naive significance: 74/145  6.1 
True significance: 78/18  4.3 
Significance
• Naïve estimator:
  Ns / Nb  72 / 164  5.6
– neglects uncertainty in background -> overestimates sign. of peak
– statistics books:   N s / Nb + var( Nb ) stress 2nd factor
• Second estimator:
  Ns / Nb + Ns  72 / 236  4.7
– gives somewhat lower value
– ??
• “Realistic” estimator:
  Ns /  Ns  72 /17.4  4.1
– Ns are of peak from fitting function, Ns its fully correlated uncertainty
– measures how far peak is away from zero in units of its own stand. dev.
– all correlated uncertainties, incl. of bkg parameters, are accounted for
Mass and width of the signal
+ mass
FWHM
[MeV]
[MeV]
a) 1527.0  2.3  2.1
22  5  2
74
145
6.1 
78  18
4.3 
a‘) 1527.0  2.5  2.1
24  5  2
79
158
6.3 
83  20
4.2 
b) 1528.0  2.6  2.1
19  5  2
56
144
4.7 
59  16
3.7 
b‘) 1527.8  3.0  2.1
20  5  2
52
155
4.2 
54  16
3.4 
a) Fit with 4th-order polynomial
Ns
Nb
naive
in  2 in  2 sign.
Total
signif,
Ns  Ns
b) PYTHIA6 + fit to resonances
`) with invariant mass of pKs-system, M(Ks) constrained to PDG-value
Experimental width larger than detector resolution of 14.6  2.4 MeV
Comparison with other Experiments
World average:
M(+ ) = 1536.2  2.6 MeV
(taken syst. uncertainty for
DIANA and ITEP: 3 MeV)
HERMES result for mass 2.1 below world average
Isosinglet vs isotensor
Clear L*(1520) signal in pK- mass spectrum
cross section estimate 62  11 (stat) nb
No peak structure in pK+ mass spectrum,
Gaussian + pol. fit give 0 counts with 91% CL
No indication of ++ , rule out isotensor,
observed state is very likely isosinglet
Pentaquark summary
A narrow exotic resonance has been observed by the HERMES
experiment in quasireal photoproduction via eD  KspX
Mass:
M = 1528  2.6  2.1 MeV,
this is by 2.1 below world average
Width: FWHM = 19  5  2 MeV,
this is somewhat larger than the experimental resolution of the
spectrometer
Preferably this is an isosinglet state as no peak structure is seen in
the pK+ mass spectrum
A production cross section of (100 - 220 nb)  25% is estimated
Orbital angular momentum contributions Lq,g to nucleon spin ?
½ = ½  + Lzq + G + Lzg
0,10
> 0,6
?
‘No one knows how to measure it‘ (R. Jaffe)
one hope: Exclusive processes,
Generalised parton distributions (GPDs)
X.Ji:
t  0
*
p

1
Jq = ½  + Lz = lim ½ dx x [H(x,,t) + E(x,,t)]
q
-1
*


DVCS
p
p
, K, 

,, 
p
DVCS
Example: DVCS (Interference of DVCS and Bethe-Heitler)
Azimuthal asymmetries:
LU beam polarisation,
C beam charge,
UL target polarisation
Beam Polar.
P.R.L. 87 (2001) 182001
Beam Charge
DVCS - deuteron target
Deuteron is Spin-1
9 GPDs
Beam Polar.
Target Polar.
DVCS - nuclear targets
Neon is Spin-0
1 GPD
DVCS
HERMES Recoil-Detector
Ready for installation this summer
2 years of data taking
Expected accuracies for
2 years of data taking
AT, b1 and b2 - deuteron
 Deuteron is spin-1 target
V = Pz = p+ - p- , Pz  1
T = Pzz = p+ + p- - 2p0 , -2  Pz z  +1
 More structure functions
Proton
Deuteron
F1
½ zq2 [q+ + q-]
1/3  zq2 [q+ + q- + q0]
F2
2xF1
2xF1
g1
½ zq2 [q+ - q-]
½ zq2 [q+ - q-]
b1
½ zq2 [2q0 - (q+ + q-)]
b2
2xb1
meas = u [1 + PbVA + ½ T AT ]
A  g1/F1 [ 1 + ½ T AT ]
AT  2/3 b1/F1
AT, b1 and b2 - deuteron
 First measurement, only possible with
atomic gas target
Model: K. Bora, R.L. Jaffe, PRD 57 (1998) 6906
AT, b1 and b2 - deuteron
 Deuteron is spin-1 target
 A  10
See e.g.:
b
- N.N. Nikolaec & W. Schäfer, P.L. B398 (97) 245
T
-2
little impact on det. of g1
1 is sizeable !
and interesting by itself
d
 related to
- nuclear binding
- D-state admixture
- diffractive nuclear shadowing
- nuclear excess pions in D
- VMD + double scattering
-
- P. Hoodboy et al., N.P. B312 (89) 571
- R.L. Jaffe & A. Manohar N.P. B321 (89) 343
- X. Artru & M. Mekhfi, Z. Phys. C45 (90) 669
- J. Edelmann et al., Z. Phys. A357 (97) 129,
P.R. C57 (98) 3392
- K. Bora & R.L. Jaffe, P.R. D57 (98) 6906
-
Further results - Outlook
Many more results:
 hadronisation in nuclei (P.L. B 577 (2003) 37- 46)
 DIS on nuclear targets (P.L. B567 (2003) 339-346)
 quark hadron duality in A1p (P.R.L. 90 (2003) 092002)
 Q2 dependence of GDH-integral (Eur. Phys. J. C26 (2003) 527-538)
 DSA for exclusive VM production (Eur. Phys. J. C29 (2003) 171-179)
 Nuclear attenuation of coherent and incoherent ‘s (coherence length,
colour transparency) (P.R.L. 90 (2003) 052501)

pion multiplicities and fragmentation functions
 longitudinal
and transverse L polarisation

vector meson production

hyperon production
   
Hadronisation in nuclei
 Hadron multiplicity ratios for different nuclei contain
information about the space-time development of the
hadronisation process
 hadron formation time q  h
 nuclear medium dependent fragmentation functions D(z,A)
 induced energy loss by multiple scattering and gluon radiation

Hadronisation in nuclei
Data for N and Kr from 12 GeV and 27.5 GeV

Strong reduction of hadron
multiplicities for low 
 Attenuation goes away with increasing 
(when hadrons are formed outside of the
nucleus)
 Attenuation stronger for h- than for h+
 Attenuation much stronger for Krypton
than for Nitrogen:
ratio  A2/3
See also: Eur. Phys. J. C 20 (2001) 479
Hadronisation in nuclei
P.L. B 577 (2003) 37
Hadronisation in nuclei
P.L. B 577 (2003) 37
Hadronisation in nuclei
P.L. B 577 (2003) 37
Cronin effect
Nuclear effects in structure functions
P.L. B567 (2003) 339
Nuclear effects in structure functions
P.L. B567 (2003) 339
Nuclear effects in structure functions
P.L. B567 (2003) 339
Nuclear effects in structure functions
P.L. B567 (2003) 339
Double spin asymmetry of exclusive  and 
Eur. Phys. J. C 29 (2003) 171
Double spin asymmetry of exclusive  and 
Eur. Phys. J. C 29 (2003) 171
Double spin asymmetry of exclusive  and 
Eur. Phys. J. C 29 (2003) 171
Q2 dependence of nuclear transparency in
14N
P.R.L.90 (2003) 05251
Q2 dependence of nuclear transparency in
14N
P.R.L.90 (2003) 05251
Uncertainty from VM decay products
Pion sample contains ,  decay
products
Dilutions not negligible