Probing Hadronic Structure with Baryonic Probes DrellYan Measurements

1
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
• ltpOpgt
• Spectroscopy
• Parton distributions fit into the first category
  as
• Deep inelastic scattering
• Flavor sensitivity from neutrinos or
  semi-inclusive flavor tagging

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

.02ltxbj lt1
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

5
Proton-induced Drell-Yan scattering (Fixed
Target)A laboratory for studying sea quark
distributions
Leading Order

• Detector acceptance chooses range in xtarget and
  xbeam.
• xF xbeam xtarget gt 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
Structure of the nucleon What produces the
nucleon sea?

• pQCD - Gluon splitting?
• Meson Cloud? Chiral Solitons? Instantons?
• Models describe well, but
  not pQCD becoming dominant?

Peng et al.
Soon lattice moment analysis may also weigh in.
8
The models all have close relations between
antiquark flavor asymmetry and spin

• Statistical Parton Distributions

9
Fermilab Accelerator Complex Fixed Target
Program

• 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
• 50 x statistics!!

Fixed Target Beam lines
10
FNAL E906 Collaboration
Abilene Christian University Donald Isenhower,
Mike Sadler, Rusty Towell Argonne National
Laboratory John Arrington, Don Geesaman, Kawtar
Hafidi, Roy Holt, Hal Jackson, Paul E. Reimer,
David Potterveld University of Colorado Ed
Kinney Fermi National Accelerator
Laboratory Chuck Brown University of
Illinois Jen-Chieh Peng Co-Spokespersons
Los Alamos National Laboratory Gerry Garvey,
Mike Leitch, Pat McGaughey, Joel Moss Rutgers
University Ron Gilman, Charles Glashausser,
Xiaodong Jaing, Ron Ransome Texas A M
University Carl Gagliardi, Bob Tribble, Maxim
Vasiliev Thomas Jefferson National Accelerator
Facility Dave Gaskell Valparaiso University Don
Koetke
11
Drell-Yan Acceptance

• Programmable trigger removes likely J/? events
• Transverse momentum acceptance to above 2 GeV
• Spectrometer could also be used for J/?, ?0
  studies

12
Projected errors on ratios of D to H
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-bar

• Parton 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-baru-bar

• Impact
• Collider/LHC sensitivity for tests of the
  Standard ModelBackground.
• Origins of the Proton SeaModels explain d-bar gt
  u-bar. No theory (model) expects the results
  seen for x gt 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-YanValence only
  effect?better statistical precision neededE906.
• Intermediate-x sea PDFs set by ?-DIS on
  ironunknown 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 y1.
• Rescaling effects also reduce parton distribution
  for xgt0.15
• Antiquark enhancement expected from Nuclear
  Pions.

17
Drell-Yan Absolute Cross Sections Proton
Structure as x? 1
MRST and CTEQ d/u ? 0 as x ? 1
PRELIMINARY

• Reach high-x through beam protonLarge xF)large
  xbeam.
• Proton-Protonno nuclear corrections4u(x) d(x)
• Proton-deuterium (cross check) agrees with
  proton-proton data.
• Parton distributions overestimate cross section.
• Working with CTEQ to incorporate data in global
  PDF fits.

18
Effects beyond leading order

• Interpretability of Drell-Yan based on
  factorization theorem
• ltpt2gt 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
  h1t(x,pt)

19
Parton Energy Loss

• Colored parton moving in strongly interacting
  media.
• Only initial state interactions are importantno
  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/slarger 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 ? s1/214.7 GeV
• POLARIZED ANTIPROTON-PROTON collisions
• Novel polarization technique looks feasible
• Emphasis on transverse distributions
• double spin Twist two transversity distribution
• single spin h1t(x,pt)

23
GSI Phase II (PAX_at_HESR)
Physics Transversity
EXPERIMENT 1. Asymmetric collider polarized
antiprotons in HESR (p15 GeV/c) polarized
protons in CSR (p3.5 GeV/c) 2. Internal
polarized target with 22 GeV/c polarized
antiproton beam.
Second IP with minor interference with PANDA
24
Drell-Yan kinematics
x1-x2 xF 2 pL vs
x1x2s M2 Q2
Statistics concentrates at low-M2
x(PpPpbar)-1/2
25
1 year of data taking 15 3.5 GeV/c Collider
L 21030 cm-2s-1 hundreds of events/day
PAX-Precision of h1 measurement
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

PRELIMINARY

• Parton distributions as x?1
• 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