Argonne National Laboratory is managed by The University of Chicago for the U.S. Department of Energy

Probing Hadronic Structure with Baryonic Probes – Drell-Yan Measurements

Donald Geesaman

Physics Division

Argonne National Laboratory

2

How to make progress on hadron structure?

Observables– <p|O|p>– Spectroscopy

Parton distributions fit into the first category as

Deep inelastic scattering

Flavor sensitivity from

neutrinos or semi-inclusive flavor tagging

p J J p| ) 0( ) ( |

)(22 xxqeW ii i

Graphic stolen from JLab

3

Recent Progress

Spin and orbital angular momentum– sea carries little spin (HERMES)– anticipating gluon measurements at

RHIC and Compass– Lots of interest on transverse spin

distributions

Strange quark content to Electric and Magnetic form factors

.02<xbj <1

4

Drell-Yan Experiments on the horizon

FNAL E906 – 120 GeV proton induced Drell-Yan– in the proton– in nuclei– parton energy loss

J-PARC – 50 GeV proton induced Drell-Yan– in the proton– parton energy loss

RHIC – polarized proton Drell-Yan – W production at higher energies (s1/2 ~ 500 GeV)

GSI – FAIR antiproton-induced Drell-Yan (with polarization?)– Transversity– Sivers Function

ud /u

ud /

5

Proton-induced Drell-Yan scattering (Fixed Target):A laboratory for studying sea quark distributions

Lead

ing

Ord

er

xtarget xbeam

proton

proton

}X

}X

-

+

Detector acceptance chooses range in xtarget and xbeam.

xF = xbeam – xtarget > 0 high-x Valence Beam quarks. Low/interm.-x sea Target quarks.

6

Method to study flavor dependence of anti-quark distributions

Assumptions

Use full parton distributions in analysis. The approximate sensitivity is

7

pQCD - Gluon splitting? Meson Cloud? Chiral Solitons?

Instantons? Models describe

well, but not —pQCD becoming dominant?

LA-LP-98-56

Soon lattice moment analysis

may also weigh in.

Structure of the nucleon: What produces the nucleon sea?

Peng et al.

8

The models all have close relations between antiquark flavor asymmetry and spin

• Statistical Parton Distributions )()()()( xuxdxdxu

9

Fermilab Accelerator Complex: Fixed Target Program

Fixed Target

Beam lines

Tevatron 800 GeV

Main Injector 120 GeV

E866 vs. E906: 800 vs. 120 GeV

Cross section scales as 1/s– 7 x that of 800

GeV beam Backgrounds (J/

decay) scale as s– 7 x Luminosity

for same detector rate as 800 GeV beam

5050 x x statistics!! statistics!!

13

Does deuterium structure affect the results at higher x

Important to also extend nuclear results to higher x

14

Projected results on ratio of d-bar/u-barParton Distributions

PDF fits are and uncertainties completely dominated by E866.

E906 will significantly extend these measurements and improve on uncertainty.

Absolute cross sections on deuterium give d-bar+u-bar

Impact Collider/LHC sensitivity for tests of the Standard Model—Background. Origins of the Proton Sea—Models explain d-bar > u-bar. No theory (model)

expects the results seen for x > 0.3.

15

Structure of nucleonic matter: How do sea quark distributions differ in a nucleus?

Comparison with Deep Inelastic Scattering (DIS)

Antishadowing not seen in Drell-Yan—Valence only effect?—better statistical precision needed—E906.

Intermediate-x seasea PDF’s set by -DIS on iron—unknown nuclear effects.

What can the sea parton distributions tell us about nuclear binding?

16

Structure of nucleonic matter: Where are the nuclear pions? Nucleon motion in the nucleus

tends to reduce parton distributions – f(y) peaked below y=1.

Rescaling effects also reduce parton distribution for x>0.15

Antiquark enhancement expected from Nuclear Pions.

18

Effects beyond leading order Interpretability of Drell-Yan based on factorization theorem

<pt2> ~ 0.4 GeV2 ~ 70 GeV on Pb, 23 GeV on Ca

How low in mass can we interpret continuum spectrum as Drell-Yan?

Parton Energy Loss– radiative or collisional

Decay Angular Distribution of Drell-Yan

pQCD = 1, , = 0 ... More generally Lam-Tung relation +2 = 1 not satisfied in pion-induced Drell-Yan at high x.Related to chiral odd quark transversity function h1

t(x,pt)

13/2

min / xApP

2cossin

2sincos1

31

431 222

dd

19

Parton Energy Loss

Colored parton moving in strongly interacting media.

Only initial state interactions are important—no final state strong interactions.

E866 data are consistent with no energy loss Treatment of parton propagation length and shadowing are critical

– Johnson et al. find 2.2 GeV/fm from the same data Energy loss 1/s—larger at 120 GeV Important to understand RHIC data.

20

Role of J-PARC (Letter of Intent L15)

Obviously can in principle reach higher x.

Energy may restrict nuclear parton dependence to light nuclei.

Other nuclear effects would be interesting – but would need higher energy data to separate effects

If polarization possible, whole new ball game

21

Energy Loss at 50 GeV

Even a suggestion that nuclear spin-orbit interaction

has significant effect

22

GSI – FAIR Drell-Yan with anti-protons

In anti-proton-proton collisions, the focus is on valence quark distributions

Initial plans are ~15 GeV antiprotons– This really restricts di-muon mass range– Are di-lepton masses below 4 GeV interpretable as Drell-Yan?– Also important question for RHIC

There are ideas for asymmetric collider to get some reach into “safe” Drell-Yan region--- s1/2= 5.5 GeV s1/2=14.7 GeV

POLARIZED ANTIPROTON-PROTON collisions– Novel polarization technique looks feasible– Emphasis on transverse distributions– double spin – Twist two transversity distribution– single spin – h1

t(x,pt)

23

GSI: GSI: Phase II (PAX@HESR)Phase II (PAX@HESR)

EXPERIMENT:1. Asymmetric collider:

polarized antiprotons in HESR (p=15 GeV/c)polarized protons in CSR (p=3.5 GeV/c)

2. Internal polarized target with 22 GeV/c polarized antiproton beam.

Physics: Transversity

Second IP with minor interference with PANDA

24

Drell-Yan kinematicsDrell-Yan kinematics

Statistics concentrates at low-M2 x~(PpPpbar)-1/2

x1x2s = M2 = Q2 x1-x2 = xF = 2 pL √s

25

PAX-Precision of hPAX-Precision of h11 measurement measurement1 year of data taking

15 + 3.5 GeV/c Collider

L = 2∙1030 cm-2s-1

hundreds of events/day

10 % precision on the h1u (x) in the valence region

26

Summary Fixed-Target Drell-Yan

is the ideal way to study the quark sea.

What is the structure of the nucleon?–d-bar/u-bar at intermediate-x

–Parton distributions as x1 What is the structure of

nucleonic matter?–Where are the nuclear pions?–Is antishadowing a valence effect?

Do partons lose energy?

Many of these problems have been with us for years, but they are still at the heart of hadron structure

PRELIMINARY