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Results from HERMES in view of future pp experiments

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Results from HERMESin view of futurepp experiments

Michael Düren

Universität Gießen

—pp-QCD Workshop, ECT* Trento, July 3-7, 2006—

HERMES physics: Modest aims and rich harvest

HERMES technology: polarization and novel techniques

Physics results:

Spin structure of the nucleon

Hard exclusive reactions

Quarks in nuclei

Future prospects for pp

Conclusions

Some transparencies are stolen from Aschenauer, Hasch, Nowak, Ji, …

M. Düren, Univ. Giessen

HERMES physics

In 1987 we discovered in the EMC the “spin puzzle”:

Violation of the Ellis Jaffe sum rule

A large negative strange sea polarization

Both results were based on inclusive DIS data and validity of SU(3)f

Original physics program of HERMES:

In 1989 we decided to solve the “spin puzzle” with a semi-inclusive DIS experiment doing a flavor decomposition of the quark spin

Re-measure inclusive polarized DIS on proton and neutron:

Ellis-Jaffe sum rule

Bjorken sum rule

g1, g2

Measure semi-inclusive polarized DIS on p and n:

u(x), d(x), s(x)

Results (in short):

Ellis-Jaffe sum rule: is really broken

Bjorken sum rule: is fine (fortunately)

g1, g2 are well known nowadays

u(x) is large and positive

d(x) is smaller and negative

s(x) is approx. zero i.e.

SU(3)f is broken and

only ~30% of the spin of the nucleon is due to the spin of the quarks

up

down

u

d

strange

M. Düren, Univ. Giessen

Latest results!

~30% up and down quark contributionin valence region

<3% strange quark

contribution

M. Düren, Univ. Giessen

Flavor decomposition of quark spin (done)

Gluon spin contribution (first result but large errors)

Transversity, Collins and Sivers functions (first non-zero results)

Orbital angular momentum of quarks (first results, but model dependent)

Generalized parton distributions (first results on BCA, BSA, TTSA,…)

Fragmentation process in vacuum and nuclear medium

Spin matrix elements of vector meson production

Positive Pentaquark signal

...

In many areas HERMES

contributed as a

pioneering experiment

M. Düren, Univ. Giessen

HERMES technology

HERMES requires:

Large polarization of beam and target; pure target

Relatively large luminosity (Much larger than EMC)

Relatively high beam energy (Q²>1 GeV²; larger than Jlab)

Relatively large acceptance (Much larger than SLAC)

Strangeness identification (kaons)

Recoil protons (for exclusive reactions)

Solution:

High HERA beam polarization (pushed by HERMES) and ABS

Storage cell technique (new at that time)

HERA fixed target

- Standard open spectrometer

- RICH upgrade in 2000 (well working RICH)
- Recoil upgrade in 2006 (for GPD program)

M. Düren, Univ. Giessen

M. Düren, Univ. Giessen

M. Düren, Univ. Giessen

M. Düren, Univ. Giessen

M. Düren, Univ. Giessen

- Novel technologies (at time of proposal)
- Unique facility (energy, luminosity, precision, …)
- Polarization (that is where one can falsify models)
- Flexibility for upgrades
- Electromagnetic processes that can be directly related to QCD
My message:

Panda at Fair will also measure completely different things compared to what is in the proposal today!

M. Düren, Univ. Giessen

HERMES physics: Modest aims and rich harvest

HERMES technology: polarization and novel techniques

Physics results:

Spin structure of the nucleon

Hard exclusive reactions

Quarks in nuclei

Future prospects for pp

Conclusions

M. Düren, Univ. Giessen

Quark density

Helicity distribution

Transversity

- The transverse polarization of quarks in a transversely polarized proton is not identical with the longitudinal polarization of quarks in a longitudinally polarized proton(Rotation and boost do not commute)
- There are three leading-twist structure functions:

M. Düren, Univ. Giessen

See talk by Anselmino

- The azimuthal angular distributions of hadrons from a transversely polarized target show two effects:
- Collins asymmetry in +s
- Sivers asymmetry in -s

Target spin

M. Düren, Univ. Giessen

- The azimuthal angular distributions of hadrons from a transversely polarized target show two effects:
- Collins asymmetry in +sProduct of the chiral-odd transversity distribution h1(x)and the chiral-odd fragmentation function H1(z) related to the transverse spin distribution of quarks
- Sivers asymmetry in -s

First results from Belle

Target spin

M. Düren, Univ. Giessen

- The azimuthal angular distributions of hadrons from a transversely polarized target show two effects:
- Collins asymmetry in +sProduct of the chiral-odd transversity distribution h1(x)and the chiral-odd fragmentation function H1(z) related to the transverse spin distribution of quarks
- Sivers-Asymmetrie in -s:Product of the T-odd distribution function f1T(x)and the ordinary fragmentation function D1(z) related to the orbital angular momentum of quarks

First results from Belle

Target spin

M. Düren, Univ. Giessen

See talk by Metz

- at HERMES(H) significantly positive/negative for +/-
- at COMPASS(D)compatible with zero

- at HERMES(H)positive/zero for +/-
- at COMPASS(D)compatible with zero

The results are unexpected

but not inconsistent

Collins and Sivers

Kaon asymmetries

- at HERMES (H)
and

- at COMPASS (D)
are similar

except for the

Sivers K+ asymmetry

M. Düren, Univ. Giessen

- Parton distributions accessible -similar (complementary) to DIS
- Asymmetries of polarized beam and/or target experiments give access to transversity
- if the energy is sufficient(energy scale ~√(x1x2s))
- if the beam polarizations are sufficient

- No knowledge of polarized fragmentation functions needed! (Contrary to DIS)

M. Düren, Univ. Giessen

See talk by Anselmino

Hard exclusive reactions

Quantum phase-space „tomography“ of the nucleon

- A classical particle is defined by its coordinate and momentum (x,p): phase-space
- A state of classical identical particle system can be described by a phase-space distribution f(x,p). The time evolution of f(x,p) obeys the Boltzmann equation.
- In quantum mechanics, because of the uncertainty principle, the phase-space distributions seem useless, but…
- Wigner introduced the first phase-space distribution in quantum mechanics (1932)
- Wigner function:

M. Düren, Univ. Giessen

- When integrated over x, one gets the momentum density.
- When integrated over p,one gets the probabilitydensity.
- Any dynamical variable can be calculated from it!

!

The Wigner function contains the

most complete (one-body) infoabout a quantum system.

- A Wigner operator can be defined that describes quarks in the nucleon
- The reduced Wigner distribution is related toGeneralized parton distributions (GPDs)

M. Düren, Univ. Giessen

- A proton matrix element which is a hybrid of elastic form factor and Feynman distribution
- Depends on
x: fraction of the longitudinal momentum carried by parton

t=q2: t-channel momentum transfer squared

ξ: skewness parameter

There are 4 important GPDs (among others):

Limiting cases:

- t0: Ignoring the impact parameters leads to ordinary parton distributions

- Integrating over x: Parton momentum information is lost, spatial distributions = form factors remain

z

by

bx

Fits to the known form factors and parton distributions with additional theoretical constraints (e.g. polynomiality) and model assumptions

M. Düren, Univ. Giessen

- Elastic form factors charge distribution (space coordinates)
- Parton distributions momentum distribution of quarks (momentum space)
- Generalized parton distributions (GPDs) are reduced Wigner functions correlation in phase-space e.g. the orbital momentum of quarks:
- Angular momentum of quarks can be extracted from GPDs: Ji sum rule:
- GPDs provide a unified theoretical framework for various experimental processes

M. Düren, Univ. Giessen

Hard exclusive reactions at

M. Düren, Univ. Giessen

- QCD handbag diagram

Deeply virtual Compton scattering (DVCS)

Hard exclusive meson production (HEMP)

M. Düren, Univ. Giessen

DVCS is the cleanest way to access GPDs: *N N

Factorization theorem is proven!

Handbag diagram separates

hard scattering process (QED & QCD) and

non-pertubative structure of the nucleon (GPDs)

x+x: longitudinal momentum fraction of the quark

-2x: exchanged longitudinal momentum fraction

t :squared momentum transfer

GPDs = probability amplitude for N to emit a parton (x+x) and for N’ to absorb it (x-x)

M. Düren, Univ. Giessen

FF

DVCS

Bethe-Heitler (BH)

Kinematics:

- x[-1,1] xB/(2- xB)
- t=(q-q‘)2 Q2=-q2

DVCS-BH interferenceIgives

non-zero azimuthal asymmetry

Use BH as a vehicle to study DVCS.

(Belitsky/Mueller)

M. Düren, Univ. Giessen

Fourier decomposition of interference term:

charge

spin

- Access to real and imaginary part of helicity conserving amplitude M1,1
- (GPDs enter in linear combinations in amplitudes)
- beam spin asymmetry (BSA)
- beam charge asymmetry (BCA)

HERMES is the only experiment which measures BCA

M. Düren, Univ. Giessen

BSA:

Significant sin() dependence

BCA:

Significant cos() dependence

BCA and BSA on proton

M. Düren, Univ. Giessen

first model dependent extraction of Ju possible

Code: VGG

Jdassumed to be zero

M. Düren, Univ. Giessen

same statistics with electron beam on tape

independent data set to constrain (Ju+Jd)

M. Düren, Univ. Giessen

- Beam spin asymmetries in nuclear DVCS (Neon)
- More to come: 2H,4He,14N,20Ne,82-86Kr,129-134Xe:

A-dependence of coherent DVCS processes to study quarks in nuclei

M. Düren, Univ. Giessen

Pion form factor

Dq

HEMP cross section ep e’n p+

GPD Model: Vanderhaeghen, Guichon, Guidal

-Clean signal without background subtraction;-DIS-background well described by MC;

HEMP target spin asymmetry ep e’p r0

M. Düren, Univ. Giessen

M. Düren, Univ. Giessen

Quarks in nuclei

understanding of the space-time

evolution of the hadron formation

process

K

p

M. Düren, Univ. Giessen

2-hadron production in nuclei

disentangle absorption and energy loss models

leading hadron: z1 >0.5

subleading hadron: z2< z1

M. Düren, Univ. Giessen

Hard exclusive reactions and GPDs in futurepp experiments

For exclusive processes, „crossing“ allows to measure the same matrix elements with completely different experiments and probes

Instead of having an initial electromagnetic process, a final state electromagnetic process is selected

Example: Compton scattering

Annihilation

M. Düren, Univ. Giessen

Experimental difficulty: the total cross section of a hadronic beam is typically much larger than the one of the exclusive electro-magnetic process

The experiment needs a huge background suppression factor as the majority of events have a hadronic final state

Example: Compton scattering

Annihilation

M. Düren, Univ. Giessen

What is the dominant mechanism?

Large pT in this exclusive reaction defines hard scale: parton picture

Emission of a single photon from one (quasi-free) parton is suppressed

Emission of two photons from the same parton is allowed

At intermediate energies(s~10 GeV) the hard interaction of one parton can be separated from the soft part which is parameterized by General Distribution Amplitudes:handbag diagram

Quasi-free partons cannot emit single photons (suppressed)

Handbag diagram: one quark emits both photons (allowed)

Soft part of annihilation

See talk by Kroll

M. Düren, Univ. Giessen

e-, m-

e+, m+

Q2 small: like Crossed-channel wide angle Compton scattering (large pT)

Q2 large: additional degrees of freedom

hard gluon

p, r, f, ...

Hard exclusive meson production (large pT)

M. Düren, Univ. Giessen

- No handbag diagram
- Here the photons and the pion are produced in forward direction!
- Measure „Transition distribution amplitudes“

explores the pion cloud

explores the cloud

explores the photon cloud

(Study next to lowest Fock state of the proton)

See talk by

B. Pire and L. Szymanowski

M. Düren, Univ. Giessen

Comparewith the time inverted process

Available at e+e- machines: quasi real photons

Results e.g. from Belle:

- Asymmetric e+e- collider
- High luminosity machine

M. Düren, Univ. Giessen

Energy dependence

Angular dependence

(GPD curve from Kroll/Schäfer)

M. Düren, Univ. Giessen

hard gluon

p, r, f, ...

- Much larger cross section (compared to ) makesit easier to access!
- 3- final state

Hard exclusive meson production (large pT)

M. Düren, Univ. Giessen

(curve from Kroll/Schäfer)

2

E760 results

- wide kinematic range accessible at PANDA (up to q²~20 GeV²)
- high statistical precision possible
- separate GM and GE

q2>0

q2<0

M. Düren, Univ. Giessen

- PAX wants to use a polarized H,D storage cell target (like HERMES)
- PANDA could (in principle) use a polarized 3He storage cell target (like HERMES in 1995) where the polarized 3He is injected from outside the solenoid into the cell
- A more attractive option is to run HESR in collider mode (to obtain high enough energy especially for Drell Yan) with both beams polarized
- The PANDA detector would be suitable for the asymmetric collider mode, so (part of) the PAX and ASSIA program could be done in an extended PANDA detector (“PANDAX”)

M. Düren, Univ. Giessen

The polarization of the final state can be determined in some special cases:

- Using parity violation of weak decays (e.g. of ´s) by observing angular distributions of decay products
- Angular distribution of decay products without parity violation (e.g. decay of vector mesons or of virtual photons)
- New idea: angular distribution of the nuclear interaction of proton in calorimeter can be used to determine the transverse proton spin (from the azimuthal distribution of the shower profile in the calorimeter)

M. Düren, Univ. Giessen

Some spin physics possible with out polarized target!

M. Düren, Univ. Giessen

- HERMES has done and still does unique and pioneering measurements in the fields of
- Spin structure of the nucleon
- Exclusive reactions and GPDs
- Hadronization studies in nuclei
- …

- Part of the program can be completed by the measurement of complementary reactions inpp annihilation experiments at PANDA, PAX and ASSIA
- An upgrade of HESR to a polarizedpp-collider seems ideal to follow up the HERMES program.

M. Düren, Univ. Giessen