Exploring Proton Structure with Drell-Yan Scattering Hadron Physics - PowerPoint PPT Presentation

Exploring Proton Structure with Drell-Yan Scattering Hadron Physics Seminar Darmstadt, December 13, 2017 M. Grosse Perdekamp, University of Illinois

Overview o Exploring Proton Structure Drell Yan vs Deep Inelastic Scattering o Quark and Gluon Structure of the Proton Momentum distributions Spin (helicity) distributions o Transverse momentum dependent proton structure A challenge to QCD? Drell-Yan measurements o Meson Structure from Drell-Yan 2 Exploring Proton Structure with Drell-Yan

Motivation: The Proton as QCD Laboratory The proton is the fundamental bound state of QCD; Quarks and gluons are the Constituents: Can we understand the wave function of the proton from first principles QCD ? Present (modest) status: Description of proton in hard scattering processes with parton distribution functions. 3 Exploring Proton Structure with Drell-Yan

Proton Structure: Momentum Distributions Constituents: quarks = u, d, s and gluons = ( ) quark momentum distributi on q x = q (x) anti - quark momentum distributi on = gluon momentum distributi on G(x) p x = quark small x ~ sea quarks, gluons p proton medium - high x valence quarks 4 Exploring Proton Structure with Drell-Yan

Probing the Quark Structure of Hadrons DIS space-like mapping of virtual valence quarks photon DF ⊗ FF electron- time-like electron virtual photon collisions FF ⊗ FF time-like virtual photon Drell-Yan mapping of sea quarks (DY) DF ⊗ DF time 5 Exploring Proton Structure with Drell-Yan

CTEQ Fits to DIS and Drell-Yan CTEQ Phys.Rev. D 93, 033006 (2016) Both DIS and Drell-Yan processes are tools for probing the quark and anti-quark structure of hadrons. The data stretch over a wide range in Q 2 and test evolution.

Quark and Gluon Momentum Distributions from CTEQ CTEQ Phys. Rev. D 93, 033006 (2016) up-quark uncertainties for example: d(x,Q 2 = 2 GeV 2 ) is the number density for down quarks Exploring Proton Structure with Drell-Yan 7

Quark and Gluon Momentum Distributions from CTEQ CTEQ Phys. Rev. D 93, 033006 (2016) anti-up uncertainties up-quark uncertainties for example: d(x,Q 2 = 2 GeV 2 ) is the number density for down quarks Exploring Proton Structure with Drell-Yan 8

Quark and Gluon Momentum Distributions from CTEQ CTEQ Phys. Rev. D 93, 033006 (2016) ATLAS Drell-Yan cross section vs invariant mass anti-up uncertainties up-quark uncertainties for example: d(x,Q 2 = 2 GeV 2 ) is the number density for down quarks Exploring Proton Structure with Drell-Yan 9

E866: Isospin Broken in the Anti-Quark Sea • Inclusion of E866 σ pd / σ pp into global Fermilab E866/NuSea 1998 fits: dramatic impact of sea-quark dis. from QCD analysis of hard scattering data! } parameterizations } excluding E866 data parameterization including E866 data ? 10 Exploring Proton Structure with Drell-Yan

Current Fermilab E906/SeaQuest extending sea-quark measurements to larger x by using 120 GeV protons from Fermilab Main Injector. 25% of total expected beam current from Paul Reimer’s ECT talk, 10-2017 11 Exploring Proton Structure with Drell-Yan

Proton Structure: Spin (Helicity) Distributions Constituents: quarks = u, d, s and gluons ⇒ Total Quark Spin : = ( ) quark momentum distributi on q x = 1 ↑↑ ↑↓ x ∆ = (x) - q (x) ∑ ∫ q(x) q ∆ ∑ = ∆ (x) q spin dependent quark distributi on , = q q 0 x ⇒ Total Gluon Spin : ↑↑ ↑↓ ∆ = (x) - G (x) G(x) G = 1 x ∫ ∆ = ∆ p (x) spin dependent gluon distributi on G G x = quark = 0 x p proton 12 Exploring Proton Structure with Drell-Yan

Proton Structure: Helicity Sumrule De-composition of the Proton Spin 1 1 = 2 ∆ ∑ + ∆ G + L z 2 Orbital Angular Quark Spin momentum p Gluon Spin x = quark p proton 13 Exploring Proton Structure with Drell-Yan

Quark and Gluon Helicity Distributions from NNPDF E. Nocera et. Al. Nucl.Phys. B887 (2014) 276-308 For example: Up and down quark helicity distributions are known. Still large uncertainties for gluon and anti-quarks. RHIC: evidence for non-zero gluon spin contribution! 14 Exploring Proton Structure with Drell-Yan

Transverse degrees of freedom: Transverse proton/quark spin Intrinsic transverse momentum of quarks k T Transverse momentum in hadron fragmentation p T

Quark Helicity Distributions from Deep Inelastic Lepton-Nucleon Scattering spin Spectator spin System electron or muon probe d proton target Magnetic Spectrometer eg. COMPASS to measure Momentum of final state Leptons and hadrons 16 Exploring Proton Structure with Drell-Yan

Quark Helicity Distributions from Deep Inelastic Lepton-Nucleon Scattering spin Spectator spin System electron or muon probe d proton target e - e - current factorize processes in the high quark jet energy lepton-quark scattering from target related processes spectator system proton 17 Exploring Proton Structure with Drell-Yan

How is Transverse Spin Different? spin Spectator System electron or muon probe d proton target Are the quark distributions changed by a spin rotation? At high probe energy: yes! boosts and rotations do not commute! 18 Exploring Proton Structure with Drell-Yan

Optical Theorem in Hard Scattering Forward Elastic Scattering Amplitude Cross Section Optical Theorem initial state final state e - photon, gluon pQCD, hard e - current quark scattering quark jet Factorization q(x,Q 2 ), G(x,Q 2 ) spectator system proton proton Process independent quark and gluon distri- butions 1 ∑ Operator product expansion µ ν σ 2 ... ~ ( ) | | C Q p O p = − in twist parameter t, t=d-n u..v − t d n / 2 t µ ν Q ... µ ν ... Wilson coefficients Operator matrix element 19 Exploring Proton Structure with Drell-Yan

Helicity Amplitudes in Hard Scattering H i h i H f h f Forward Scattering Amplitude initial state final state 1 1 1 1 Helicity is → hard probe: conserved 2 2 2 2 gluon, photon 1 1 1 1 → - - 2 2 2 2 Quark, h i Quark, h f ⇒ 2 2 ( , ) , q x Q F (x,Q ) helicity average 1 , 2 ∆ 2 2 ( , ), q x Q g (x,Q ) helicity difference 1 1 1 1 1 → - - helicity flip proton, H f proton, H i 2 2 2 2 ⇒ δ 2 ( , ) q x Q transverse spin distributions h: quark helicity In initial and for quarks: transversity H: proton helicity final state 20 Exploring Proton Structure with Drell-Yan

Decomposition of Helicity Flip Amplitudes at Leading Twist Transverse Momentum Dependent (TMD) TMD independent 21 Exploring Proton Structure with Drell-Yan

Proton Transverse Spin Structure: Transversity, Sivers and Boer-Mulders Transversity : correlation between transverse 2 ( , ) h T x k ⊥ 1 proton spin and quark spin δ ( x ) q or S p – S q – coupling ? Sivers : correlation between transverse proton ⊥ q 2 ( , ) f x k ⊥ 1 spin and quark transverse momentum T S p -- L q – coupling ? Boer/Mulders: correlation between transverse quark ⊥ 2 q ( , ) h x k 1 spin and quark transverse momentum ⊥ S q -- L q – coupling ? Insight in spin-orbit structure of quarks in the proton … 22 Exploring Proton Structure with Drell-Yan

First Experiment: Single Transverse Spin Asymmetries (SSA) in Hadron-Hadron Collisions 23

Single Transverse Spin Asymmetries (SSA) A N in Polarized Proton-Proton Scattering Example: Inclusive π production ↑ +  → π + one proton p p X is polarized! in polarized p-p π s p N R : pions to the right  s p Correlation proton spin p π ⊥ p p S p vs p π ⊥  p π transverse momentum π ⊥ π N L : pions to the left N L - N R Single transverse A N = = 0 ? spin asymmetries A N N L + N R 24 Exploring Proton Structure with Drell-Yan

For High Energy Reactions: A N  0 QCD Test ! (Kane, Pumplin, Repko, 1978) α m − ∝ = = ≈ s q 4 example, m 3 , 20 , 10 A MeV s GeV A q N N s 25 Exploring Proton Structure with Drell-Yan

Experiment: Large SSA Observed over Large Range of Scales ! Experiment: A N >> 10 -4 for 4 GeV < √s < 200 GeV for charged pions ! ZGS √s=4.7 GeV AGS √s=6.5 GeV FNAL √s = 20 GeV RHIC √s = 200 GeV π + π - Soft effects due to QCD dynamics in hadrons from Christine Aidala, Spin 2008 and remain relevant up to scales where pQCD can Don Crabb & Alan Krisch in then Spin be used to describe the scattering process! 2008 Summary, CERN Courier, 6-2009 26 Exploring Proton Structure with Drell-Yan

Origin of Large SSA  Inspect Factorized Components of Cross Section π + frag ragmen mentat tation δ ( 1 ) q i x  x 1 P proce ocess 1 s p 1 proton ton ij Can initial and/or final state a struct cture LL x 2 P effects generate large transverse 2 -1 ) spin asymmetries? (A N ~10 Jet hard s scatte tterin ring ( ) G x react ction ion 2 P 2 pQC pQCD ( ) ↑ σ → σ ↑ ↑ → π + 3 ˆ 3 ( ) d q q q q d pp X ↑ ∝ ⋅ × × i j k l ( , ) ( ) ( , ) q x k G x FF z p i 1 q , T 2 q h , T dx dx dz dx dx . k l 1 2 1 2 Init nitia ial s l state – Kane, Pumplin, Final state – hadron pr proto ton s stru tructu ture Repko  a LL ~10 -4 fragmentation 27 Exploring Proton Structure with Drell-Yan