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