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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 A T and structure function b 1 d. - PowerPoint PPT Presentation
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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 b1
d
The HERMES Spectrometer
The HERMES Spectrometer
The HERMES internal polarised atomic gas-target
The HERMES dual-radiator RICH
Silica aerogel: n = 1.03, thresh = 4.2
C4F10: n = 1,0005, thresh = 32
P
PKK
Ppp
Pp
PK
PK
PpK
Pp PK
p
P (GeV)
5 10 15
5 10 15
5 10 15
1
0.5
0
0.8
0.5
0
0.8
0.5
0
Quark helicity distributions, semi-inclusive asymmetries
Leading hadron originates with large probability from struck quark
D(z):= Fragmentation function
= E - E‘
z = Eh/
Semi-inclusive asymmetries-1
zq2q(x) Dq
h(z)
A1h(x,z) =
zq2q(x) Dq
h(z)
q
q zq
2q(x) Dqh(z)
q(x)
=
zq‘2q‘(x) Dq‘
h(z) q(x)
q 'q
Quark-‘Purity‘ Phq
Different targets and hadrons h: Solve linear system for Q with
A = (A1,p, A1,d, A1,p , A1,d
, A1,p K )
A = P Q
P.L. B 464 (1999) 123
In leading order:
Semi-inclusive asymmetries from the Deuteron
,K, p asymmetries identified with RICH
Pions Kaons
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) )
P.R.L.92 (2004) 012005
Purities
Shaded bands: systematic uncertainties
Adequate degree of orthogonality: Kaons have about 10% sensitivity to
the strange sea
(Probability that observed hadron originates from quark of type f)
Extracted quark helicity distributions
u > d ?
s < 0 ?
The HERMES data are 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
P.R.L.92 (2004) 012005
x
g1 = - longitudinal quark spin, , q 5q HERMES 1995-2000
Transverse quark polarisation , ‘Transversity‘ h1
Complete description of nucleon in leading order QCD: 3 distribution functions
f1 = Quark momenta, q q
h1 = - 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‘hX N+() - N-()
AUL() =
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
P.R. D 64 (2001) 097101. +
-
K+
Proton
DeuteronP.L. B 562 (2003) 182
2.4 M DIS
o
o
+
-
- ++0
0-
8.9 M DIS
AULsin() from longitudinally polarized target
Proton
Deuteron+
o
-
K+
AULsin SL(M/Q) za
2 x [hLa(x) H1
a(z) - x h1La(x) Ha(z)/z + ..]
- ST za2 x h1
a(x) H1a(z)
SL >> ST Collins fragmentation function
AULsin() from longitudinally
polarized target+
o
-
K+
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
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 ) AUT
sin( - 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+ Ao 0 and A - 0 and smaller in magnitude
HERMES data for AULCollins show
A+ 0 but Ao A- 0 and larger in magnitude !
Interpretation: forthcoming paper
Extraction of transversity distributions underway
HERMES contribution to the Pentaquark story
u
d
d
us
Theoretical motivation and experimental status:
see reviews by M. Polyakov and M. Ostrick tomorrow
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
Exotic Hadrons
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)
Theoretical prediction for 5-quark states [qqqqq + ‚sea‘]
uud ds
sdd.. sdu.. suu(dd+ss)
*0
*- *0
S
I3
(1890)
3/2(2070)
+(1530)
5-Quark states in the SM
1-1
-1
N(1710)
ssu udssd du ssd (uu+dd) ssu..
duu(dd+ss)
sdu..
1
180 MeV
Prediction
D. Diakonov, V. Petrov and M. Polyakov,Z. Phys. A 359 (1997) 305
Experimental evidence for
15392.5World average
1530 15 I=0 S=+1 JP=½+Prediction
1540105 25 4.6115392”few” 8 4.4154225 FWHM 21 5.30.5 154042 25 4.815335 29 6.7
LEPSDIANACLASSAPHIRITEP (’s)
ResultatsMass Width Significance(MeV) (Mev) ()
Experiments
Last year: after 30 years of futile search, sudden explosion of experimental evidence
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
(1116) excluded: reject event if M(p) within 1 of nominal mass
e D -> p sX-> p +
- X
Reaction:
Ee = 27.6 GeV,
Target: pol/ unpol D
Experimental Evidence from HERMES
optimized yield of Ks peak in M() while minimizing background
NO constraints optimized to increase significance of signal in final
M(p)
KS -> + -
Detector Calibration with KS , , , -, *
Excellent Proton identification by RICH: K+ and contamination negligible for 4 GeV< P p< 9 GeV , , -, *... well identified
Particle observed mass PDG mass
[MeV] [MeV]
KS +- 496.8 0.2 497.67
p - 1115.7 0.1 1115.68
- p - + 1321.5 0.3 1321.31 0.3
* p K- 1522.7 1.9 1519.5 1.0
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
MeVsomewhat larger than exp. res.
Naive significance: 56/144
4.7
True significance: 59/16 3.7
Unbinned fit is used; result doesnot depend on size of bin and starting point
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
hep-ex/0312044
M(p) Spectrum - efforts to reproduce background
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:
– neglects uncertainty in background -> overestimates sign. of peak– statistics books: stress 2nd factor
• Second estimator: – gives somewhat lower value– ??
• “Realistic” estimator: – 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
/ var( )s b bN N N
mass FWHM Ns Nb naive Total signif,
[MeV] [MeV] in 2 in 2 sign. Ns Ns
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
Mass and width of the signal
a) Fit with 4th-order polynomial 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: ) = 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 *(1520) signal in pK- mass spectrumcross 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 + Lz
g
0,10 > 0,6
‘No one knows how to measure it‘ (R. Jaffe)
one hope: Exclusive processes, Generalised parton distributions (GPDs)
p pp p
?
* *
DVCS
, K, ,,
X.Ji: Jq = ½ + Lzq = lim ½ dx x [H(x,,t) +
E(x,,t)]t 0
1
1
Example: DVCS (Interference of DVCS and Bethe-Heitler)
Azimuthal asymmetries:
LU beam polarisation,
C beam charge,
UL target polarisation
DVCS
P.R.L. 87 (2001) 182001
Beam Polar. Beam Charge
DVCS - deuteron target
Deuteron is Spin-1 9 GPDs
Target Polar.
Beam Polar.
DVCS - nuclear targets
Neon is Spin-0 1 GPD
DVCS
HERMES Recoil-DetectorExpected accuracies for
2 years of data taking
Ready for installation this summer
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 zq
2 [q+ + q-
+ q0]
F2 2xF1 2xF1
g1 ½ zq2 [q+ - q-] ½ zq
2 [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
AT 10-2
little impact on det. of g1
b1d is sizeable !
and interesting by itself
related to - nuclear binding - D-state admixture - diffractive nuclear shadowing - nuclear excess pions in D - VMD + double scattering - - -
See e.g.:
- 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
- N.N. Nikolaec & W. Schäfer, P.L. B398 (97) 245
- 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 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
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
Data for N and Kr from 12 GeV and 27.5 GeV
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
Eur. Phys. J. C 29 (2003) 171
Double spin asymmetry of exclusive and
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