Theory Introductionto

Semi-Inclusive Physics

Jianwei QiuBrookhaven National Laboratory

Jefferson Lab (Jlab) 2014 Joint Hall A/C Summer Meeting JLab, Newport News, VA, June 4-5, 2014

DIS vs. SIDIS Inclusive DIS – one scattering plane:

EE’

Localized probe:

Two independent variables:

Semi-inclusive DIS (SIDIS) – two scattering planes: Leptonic plane:

Hadronic plane:

Angle between two planes:

Path of the color flow

Semi-inclusive DIS Naturally, two scales:

high Q – localized probeTo “see” quarks and gluons

Low pT – sensitive to confining scaleTo “see” their confined motion

Confinedmotion

Theory – QCD TMD factorization

Spin-motion correlation:

4 spin combinations

4 spin combinations

Various TMDs: vector, axial vector, tensor

Needs

TMDs – role of spin and motion Rich quantum correlations:

8 leading power (twist) quark TMDs:

Similar for gluons

Quantum correlation between hadron and parton

Sivers effect – between hadron spin and parton motion:

Hadron spin influences parton’s transverse motion

Sivers functionoObservedparticle

Parton’s transverse spininfluence its hadronization

Collins functionTransversity

Collins effect – between parton spin and hadronization:

Observedparticle

JLab12, COMPASS, and low energy EIC for valence, EIC@US covers the sea and gluon!

SIDIS – the best for probing TMDs Naturally, two planes:

1( , )

sin( ) sin( )

sin(3 )

l lUT h S

h SSiverCollins

Pretzelosi

UT

tyU

sUT h S

h ST

N NAP N

A

ANA

1

1 1

1

1 1

sin( )

sin(3 )

sin( )Co

PretzelosityU

SiversUT

llins

T h S T

h S

UT

UT h S

TU

UT

TA

H

f

A

D

A h H

h

Collins frag. Func. from e+e- collisions

Separation of TMDs:

Very hard, if not impossible, to separate TMDs in hadronic collisions

Using a combination of different observables (not the same observable): jet, identified hadron, photon, …

QCD corrections Sources of parton kT at the hard collision:

Gluon shower

Confined motion

Emerge of a hadronhadronization

Parton kT generated by the shower caused by the collision:

Input distribution for the Q2 evolution - nonperturbative

“True” parton structure:

Has very little to do with the kT in hadron wave function – hadron structure At large Q2 and collision energy (large phase space), the shower generated kT could be perturbative Q2 – evolution of the cross section

Factorization for SIDIS Leading power contribution:

Low PhT – TMD factorization:

High PhT – Collinear factorization:

PhT Integrated - Collinear factorization:

TMD fragmentation

Soft factors

TMD parton distribution

Modified Universality of TMDs TMD distributions with non-local gauge links:

Parity + Time-reversal invariance:

The sign change is a critical test of TMD factorization approach

Evolution of TMDs Evolution in the b-space – Fourier transform of

kT:

RG equations:

Evolution equations for Sivers function:

CS:RGs:

Boer, 2001, 2009, Idilbi, et al, 2004Aybat, Rogers, 2010Kang, Xiao, Yuan, 2011Aybat, Collins, Qiu, Rogers, 2011Sun, Yuan, 2013…

Numerical “prediction” for evolution Aybat, Prokudin, Rogers, 2012:

Sun, Yuan, 2013:

Huge Q dependence

Smaller Q dependence

No disagreement on evolution equations!Issue: extrapolation to non-perturbative large b-region

choice of the Q-dependent “form factor” – more work needed!!!

World effort on TMDs

Partonic scattering amplitudeFragmentation amplitudeDistribution amplitude

proton

lepton lepton

pion

Drell-Yan

BNLJPARCFNAL

proton

proton lepton

antilepton

EIC

SIDIS

electron

positron

pion

pion

e–e+ to pions1 1(SIDIS) (DY)h h

BESIII

1 1(SIDIS) (DY)q qT Tf f

Test of the sign change!

Transition from low pT to high pT

TMD factorization to collinear factorization:

TMD Collinear Factorization

Two factorization areconsistent in the overlap

region where

Quantum interference – high pT region (integrate over all kT):

Non-probabilistic quark-gluon quantum correlation

Single quark state quark-gluon composite state

(Spin flip)interfere with

Kang, Yuan, Zhou, 2010

Twist-3 correlation functions Twist-2 parton distributions:

Unpolarized PDFs:

Polarized PDFs:

Two-sets Twist-3 correlation functions:

Kang, Qiu, PRD, 2009

Role of color magnetic force!

Evolution of twist-3 correlation functions

Closed set of evolution equations (spin-dependent):

Plus two more equations for:

and

Kang, Qiu, 2009

Sample scale dependence of twist-3 correlations

Follow DGLAP at large x Large deviation at low x (stronger correlation)

Kang, Xiao, Yuan, 2011Matching between low pT (resum) and high pT (fixed)

Kang, Qiu, 2009

Single-spin asymmetry in hadronic collisions Consistently observed for over 35 years!

ANL – 4.9 GeV BNL – 6.6 GeV FNAL – 20 GeV BNL – 62.4 GeV

BNL – 200 GeV Definition:

Do we understand it? Early attempt:

QCD factorization at twist-3:

2Kane, Pumplin, Repko, PRL, 1978

Cross section:

Asymmetry:

Too small to explain available data!Qiu and Sterman, NPB, 1991

Sivers - type

Collins - type

Kang, Qiu, Vogelsang, Yuan, 2011 Sivers function and twist-3 correlation:

+ UVCT

“direct” and “indirect” twist-3 correlation functions: Calculate Tq,F(x,x) by using the measured Sivers functions

direct

directindirect

indirect

A sign “mismatch” if keeps only Sivers-type

Important role of Collins’ effect to single pion production – twist-3 FFsMetz & Pitonyak, 2013

SIDIS – separate two effects by difference in angular distribution

Flavor structure of the proton sea The proton sea is not SU(3) symmetric!

Violation of Gottfried sum rule Confirmed by Drell-Yan exp’t

Why ? Why does change sign?

Future experiments:

Challenges for d(x) – u(x) All known models predict no sign change!

Meson cloud Chiral-quark soliton model Statistic model

Fermilab E906

What is the ratio asx increases?

Very important non-perturbative physics

Asymmetry between strange and up/down sea?

LO and NLO QCD global fitting to DIS data:for x > 0.1

New LHC data on W/Z data:with

HERMES data:

Does not follow the shape of u(x) + d(x)?Why strange sea behave so different?

PT-integrated SIDIS:Need FFs

Hadronization puzzle

How do hadrons emerge from a created quark or gluon?How is the color of quark or gluon neutralized?

Emergence of hadrons:

Need a femtometer detector or “scope”:Nucleus, a laboratory for QCDEvolution of partonic properties

Strong suppression of heavy flavors in AA collisions:

Nucleus as a “detector”

THE “VERTEX” DETECTORAT A FERMI SCALE

Need a good control of the kinematics of fragmenting parton

Almost impossible for a hadron machine

Nucleus in SIDIS is

an ideal “vertex” detector

Color neutralization – energy loss Unprecedented ν range at EIC:

semi-inclusiveDIS

Heavy quark energy loss:- Mass dependence of fragmentation

pion

D0 Need the collider energy of EICfor heavy flavors

Control of ν and medium length!

π

D0

PT broadening of leading hadron in SIDIS

Definition:

Color fluctuation – azimuthal asymmetry Preliminary low energy data: Hicks, KEK-JPAC2013

Contain terms in cos(φpq) and cos(2φpq) only statistical uncertainties shown Classical expectation:

Any distribution seen in Carbon should be washed out in heavier nuclei Surprise:

Azimuthal asymmetry in transverse momentum broadeningSpin-”orbital” correlation + soft multiple scattering Qiu & Pitonyak

In preparation

SIDIS’ role in probing the gluon saturation

Strong suppression of dihadron correlation in eA@EIC:

Never been measured! Directly probe Weizsacker-Williams (saturated) gluon distribution in a large nucleus A factor of 2 suppression of away-side hadron-correlation! No-sat: Pythia + nPDF (EPS09)

ϕ12

Theory Simulation

Summary

SIDIS in eP offers many more better controlled observables to probe QCD’s confining features and hadron’s partonic structure

A future EIC@US could help continue to keep the US’s leadership position in nuclear physics and …

SIDIS in eA collision is ideal for probing “hadronization”, “color neutralization”, QCD energy loss, …

Thanks!

JLab12 is excellent for the valence region, while a future EIC will cover the sea and gluon

From 3D confined motion to quantum interference of different parton statesBest channel for probing TMDs

Electron-Ion Collider (EIC) A giant “Microscope”

A sharpest “CT”

To “see” quarks and gluons

To “cat-scan” nucleons and nuclei