lepton hadron scattering, an overview abhay deshpande riken bnl research center snowmass 2001...

Lepton Hadron Scattering, An Overview

Abhay Deshpande

RIKEN BNL Research Center

RIKEN BNL Research Center

Snowmass 2001Working Group M5

July 3, 2001

Lepton Hadron Colliders, Snowmass01

July 3, 2001 2

Experiments and The Standard Model

Standard Model (SM) of High Energy Physics: 1. Electroweak Physics: Electro-Magnetic-Weak Interactoins mediated by gauge

bosons: W(+/-) and Z. Interactions between charged particles: quarks & leptons (3 families)

2. QCD Physics: Strong Interactions mediated by gauge boson: “g”, the gluon. Carry color charge! Distinct from EW carriers of forces in the fact that they can interact with other gluons “g-g” interaction

3. Gravitational Physics no direct connection found yetExperimental Tests of the SM 1. Lepton-Lepton Scattering 2. Lepton-Hadron Scattering: Probing the structure of matter and strong

interactions: Topic of this working group 3. Hadron-Hadron ScatteringAll three methods are complementary. Investigate the universality of physical laws

leading towards a better understanding of structure of matter and nature of above interactions.


July 3, 2001 3

Lepton Hadron Scattering Deep Inelastic Scattering

• A point like probe as in a lepton probes the inside of the hadron at high enough energies… exchange of or Z for neutral currents and W’s (not shown) for charged currents

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July 3, 2001 4

Lepton Hadron Scattering Deep Inelastic Scattering!• Fixed target experiments:

e-p, e-A scattering Unpolarized & Polarized DIS (SLAC/DESY)

-p, -A scattering Unpolarized and polarized DIS (CERN/FNAL)

-p, -A scattering Semi-un-polarized (FNAL)

• Collider Experiments

e-p scattering Semi-un-polarized ONLY (DESY)

Unpolarized Scattering Semi-un-polarized Scattering Polarized Scattering


July 3, 2001 5

HERA at DESY

Proton Electron Proton Electron


July 3, 2001 6

H1 Detector SchematicZEUS Detector Schematic

HERA Collider Detectors

Very close to 4acceptanceGood calorimetry, tracking & vertex resolution


July 3, 2001 7

HERA Kinematic Range with Collider Detectors

• Significant increase in kinematic range beyond the fixed target experimental era.

• Many new aspects of QCD were discovered and understood because of the extended range

• Unresolved problems…..


July 3, 2001 8

Experimental Excitement from HERA Future Experimental Measurements!Highlights from HERA I• Proton structure & QCD

Unexpected rise of F2, Role of gluon

Jet physics, Precise S determination

Electroweak physics

• Low x phyisics

High parton density matter,

Diffractive physics

• Photon structure

Jet physics, Prompt photons

• Physics beyond SM

Lepto-quarks, Contact interactions, excited leptons etc….

HERA II• Luminosity Upgrade

• Spin rotators for electron beams

for ZEUS and H1

Polarization of electron beams

• H1 and ZEUS Detector upgrades

GeV 5.27

GeV 920820

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p

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Apology: Will not cover all that HERA has done!


July 3, 2001 9

Proton Structure function F2(x,Q2)

• Scaling violation explicitly seen… Beyond the fixed target regime• H1 and ZEUS data in agreement. Further,

pQCD predictions at NLO describe data impressively over many decades in x and Q2.

• Studies have resulted in the determination of gluon distribution, precise determination of S

Rise in F2 at low x


July 3, 2001 10

Gluon Determination from F2 QCD Analysis

Perturbative QCD analysis ofF2 allows us to access the gluonDistribution in the proton.

Recent results from DIS2001, Bologna


July 3, 2001 11

FL Longitudinal Structure Function

• Presnt extractions of FL based on extrapolations of pQCD in to a region of high y

• Ideally, to get FL free of assumptions, one needs data at different CM energies

• It has been suggested to run HERA at lower proton beam energy… 400 GeV

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RUNNING ACCELERATORS AT LOWER ENERGYCONSEQUENCES TO THE LUMINOSITY?HOW CAN THE EFFETCS BE MINIMIZED?


July 3, 2001 12

Transition region: Low Q2 Photoproduction pQCD

U. Fricke, ZEUS, DIS2001


July 3, 2001 13

Observations….

• Is it really a hint of something new happening at low x?

• If so we need to go past the HERA kinematics in low x and at the same time keep a “respectable” Q2 so that some of these perturbative calculations/fitting procedures are still valid THERA/eVLHC

• Other ways: There are models that predict that at low x the partonic densities might be very large, and as such the DGLAP formalism breaks down…. Predominantly gluonic matter at very high density.

• Remember gluons are bosons, and at high enough densities they might form “Bose-Einstein Condensates” which form the high density matter.

Some have called this: Color Glass Condensate: A yet undiscovered state of matter…. When this (Glass) shatters…Quark Gluon Plasma is formed…

• Bottom line is: to create the high gluon density objects we might as well work with HEAVY IONS for DIS. This would be effectively going to the high parton densities I.e. LOW x! Electon Ion Collider (BNL)


July 3, 2001 14

Diffractive Events at HERA

Early observation at HERA

The diffractive cross sections is unexpectedly large at HERA:~typically 5-6% Explanations based on pomeron exchange exist.. But..


July 3, 2001 15

Diffraction at HERA…

• Larger than expected Gluonic in origin

• Consequences to interpretation at low x, low-intermediate Q2

• Saturation models (Color Glass Condensate) do indeed account for the higher than expected diffractive events

and predict that if e-A collisions are performed, due to even higher density of gluons in the nuclei, the expected diffractive cross sections might be as large as 40%

• A very clear and robust diffractive signal could be seen.

• Since these SAME high parton density models also address the low x structure function behavior in details… they provide a compelling alternative to the pomeron ideas…. How are they connected?

• THERA/eVLHC and e-A physics at high enough energies such as EIC would allow us to directly look at this.


July 3, 2001 16

SDetermination from jet measurements…

ZEUS H1


July 3, 2001 17

SDetermination from inclusive jet measurements…


July 3, 2001 18

Universality!• Determination of the

strong coupling constant from different methods.

• Lead to astonishingly consistent values

• We must be doing this right.. Beauty and complimentarity of measurement techniques!

S. Betke

The smallest and the third smallestUncertainty comes from DIS!


July 3, 2001 19

Electroweak Physics at HERA

•At large Q2 the electromagnetic and weak currents assume equal strength.•At high Q2 situation limited by statistics•Over 8 orders of magnitude these measurements have been compared with the predictions from SM•H1 and ZEUS data in excellent agreement•Further, data are in excellent agreement with the Standard Model.

These and such tests will improve further in HERA II. AND HERA will access Physics beyond the SM… if it exists…


July 3, 2001 20

xF3 Measurement at HERA• First measurement of xF3

at HERA• Agrees well with the

PDFs evolved from much lower Q2

• Statistics limited• HERA II will consolidate

this measurement

The polarized partner of xF3 isStructure function g5. Can be measuredIf we have polarized ep colliders…Capable of having e+/e- beams!

Can we assure that future e-p facilities wouldAlways have e+ and e- beams?


July 3, 2001 21

Rutherford would have been proud of this measurement…..

W. Marciano’s talk: Critical dimensions below which physics beyond the SM might appear are 10 e –17 cm


July 3, 2001 22

Excess of high pT lepton events vs. SM

SMSM

H1Detector


July 3, 2001 23

SM ProcessesConsidered asPossible sourcesOf events


July 3, 2001 24

High pT leptons


July 3, 2001 25

Other searches of Beyond SM

• Contact Interaction• Various SUSY searches• R-parity violating lepto-quarks• Compositeness• Searches of extra-dimensions• ….. All of these would be possible with the large

luminosity at HERA II and with future machines like THERA/eVLHC


July 3, 2001 26

• Sensitivity to s• Color Dynamics• Sensitivity to Photon Structure function• Sensitivity to Proton structure function

Why high ET jets?•Soft effects minimized•Fragmentation/Hadronization effects small•Hard Scale reliable NLO

A Great Test of QCD!

Photon Structure from High ET Photoprodction


July 3, 2001 27

Photon Structure… (Cont)

• NLO QCD predictions using available PDFs describe the measured cross sections.

• ZEUS sees deviations • Further interest and

studies needed and expected…. HERAII

• Ideal measurement for future colliders (polarized and un-polarized!)


July 3, 2001 28

Summary Slide… for future unpolarized DIS

• Great anticipation based on present data towards HERA II• This however will be only a luminosity upgrade• Interesting physics may lie beyond the kinematic reach of HERA• Higher energy machines need to be designed… Plans for THERA, DIS with VLHC?• High density partonic matter could be found in high energy e-A

scattering… Diffractive signals from eA physics at high energies might tell us something about the very low x region

Plan for EIC at RHIC? Discovery of a new state of matter… Color Glass Condensate…? Detailed studies of diffraction, FL, in general gluons in heavy ions

Upcoming talks will tell us what these futuremachines could do….


July 3, 2001 29

Polarized DIS

• Electron beams up to 50 GeV/c on fixed (solid and gaseous) targets were predominantly used at SLAC (EXXX serise) and DESY (HERMES)

• Muon beams 100-200 GeV/c on fixed (solid) targets were used at CERN(EMC,SMC)

Compared to un-polarized DIS, the kinematic rangeis extremely limited!


July 3, 2001 30

How far does polarized DIS have to go!


July 3, 2001 31

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Transversity Distributions:

Polarized and Unpolarized DIS


July 3, 2001 32

Physics Goals of Past Experiments


July 3, 2001 33

Polarized DIS … Towards a Collider.

• These goals needed only inclusive DIS measurements Have lead to results which can only be improved by making new

measurements at low x…. Higher sqrt(s) … higher energy beams and in collider mode:

Polarized Collider necessary• As experiments got more ambitious they started looking at the target

fragmentation… and tried to identify the final states using sophisticated detectors: tracking, particle ID, vertexing etc. on hadronic final states.

Lea to the results on semi-inclusive DIS mostly relevant in intermediate to high x

Next step could be to extend these to low x and a detailed study of target fragmentation region which would benefit a lot in Colllider mode DIS.


July 3, 2001 34

pQCD analysis results using world data set

QCD Analysis of available Data Sets: (SMC Phys, Rev. D58:112002, 1998)

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July 3, 2001 35

Polarized Parton Distributions

• Polarized Singlet quark distributions and the nonsinglet quark distributions determined rather accurately

• Polarized gluon distribution has largest uncertainty

• Principle uncertainties come from: a) assumed initial parton distribution functional form b) uncertainty in S c) factorization and renormalization scale etc…

• All related to “lack of data in the low x” region.


July 3, 2001 36

Lack of low x data… consequencesQ2 = 10 GeV2

5.00 as )0(1 xxg

Regge/QCD

SMC Results


July 3, 2001 37

Neutron structure function… E154/SLAC

• Consequence:• Unertainties in low x for

Bjorken sum rule…• Acceptable? No…

measure low x!


July 3, 2001 38

Ellis Jaffe Sum Rule Violated!


July 3, 2001 39

Ellis Jaffe Sum Rule….Violated!


July 3, 2001 40

Bjorken Sum Rule Confirmed!


July 3, 2001 41

How to access gluons…?

We know the quarks carry small amount of spin… and that gluons may have a significant role in this….

We saw what we need to determine gluon distributions… what other ways can we access gluon distributions? …..

Next Generation Experiments


July 3, 2001 42

G from Photon-Gluon-Fusion (PGF)

• Photon Gluon Fusion in DIS: Di-Jet events 2-High-pT hadron events HERMES Collaboration• At high Sqrt(s) the theoretical

interpretation is without ambiguities or uncertainties!

• At low Sqrt(s) however things may not be as simple…..


July 3, 2001 43

High pT Hadron:PGF HERMES Results

)???()(03.0)(18.041.0/ theorysyststatGG

No estimate of theoretical Uncertainty


July 3, 2001 44

G from high-pT hadron…. • PYTHIA used at very low

scales… caution!• VMD component’s

reliability questionable, and no reliable uncertainty estimate possible.

• High pT, possible in future lepton-hadron colliders will overcome this problems.

• H1 has already published such a result on G using high-pt hadrons.


July 3, 2001 45

Scale dependencies in this business…

• M. Stratmann & W. Vogelsang

Proceedings of SPIN2000, Osaka, Japan.

• At low energies (such as HERMES/COMPASS) result strongly dependent on the value of the scale

• At higher energies the scale dependence of the result significantly reduced… look at curves for eRHIC, HERA, RHIC, Tevatron…

Need a polarized high energy collider!


July 3, 2001 46

Semi-Inclusive Physics: HERMES/SMC

SMC: Tags only charge of hHERMES: Has particle ID

Flavor separated valence quark distributions in nucleon accessed


July 3, 2001 47

Semi-Inclusive & Inclusive Asymmetries

Newly installed RICH would do excellent particle ID expect to seeAsymmetries for K/instead of charged hadrons soon.


July 3, 2001 48

Transversity

• Probability of finding a quark oriented transverse to the proton momentum

• One of the last unmeasured spin structure function of the proton

• Unmeasured: because it’s a chiral odd structure function and does not couple directly to observables in experiments

• Suggested by Jaffe et al.: Why not measure a PRODUCT Transversity and something else the product of the two is a chiral even structure function Then if you can unfold that other function, to get at transversity!

• Approach will be tried by HERMES, COMPASS and RHIC Spin

• The “other function” which needs to be measured is the Collin’s function. Hermes collaboration recently measured this for the first time for pions.


July 3, 2001 49

Recent excitement in polarized DIS: Single spin azimuthal asymmetries…HERMES

• First attempt at measuring Collin’s function

• Measured for positive and negative pions

• Found that positive pion asymmetries are positive, while the negative pions are consistent with zero.

• This now in future can be used for getting to the transversity measurements.


July 3, 2001 50

Crossing the x-Q2 Barrier in DIS: Low-x surprises!

• Elastic e-p scattering (SLAC, 1950s)

Q2 ~ 1 GeV2 Finite Size of proton

• Inelastic e-p scattering (SLAC, 1960s)

Q2 > 1 GeV2 Parton structure of the proton

• Inelastic -p scattering off p/d at CERN (1980s)

Q2 > 1 GeV2 Unpolarized EMC effect

• Inelastic e-p scattering at HERA/DESY (1990s)

Q2 > 1 GeV2 Unexpected rise of F2, Study of pQCD and QCD through various physics processes, diffraction…

What NEXT? A higher energy collider? Or Denser Target “A”?


July 3, 2001 51

•Stern & Gehrlach (1921): Space quantization associated with direction•Goudschmidt & Ulhenbeck (1926): Atomic fine structure and electron spin magnetic moment•Stern (1933): Proton anomalous magnetic moment •Kusch (1947): Electron anomalous magnetic moment •Prescott & Yale-SLAC Collaboration (1978): EW interference in polarized e-d DIS, parity non-conservation•EMC (1989): Proton spin crisis/puzzle Low x!•HERMES (1999): ???? Transverse spin asymmetries ????

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Physics with “Spin” full of Surprises!

Future spin experiments may be best doneIn collider mode at high energies


July 3, 2001 52

How can we build upon all this?• The accelerator physics will pave the way… -- High energy, high luminosity lepton hadron colliders, high

polarizations of beam Access new kinematic regions for polarized and unpolarized DIS

-- Variable beam energies… without much effect on luminosity… -- Ability in Linac as well as rings to have high -polarized e+/e-

beams… -- Highly dense interaction regions• We should know this months from all the specialists who have

gathered here: -- How we can achieve this… and -- What journey we can embark upon within and outside of the

Standard Model with such exotic tools…

Challenges for Accelerator Physicists

lepton hadron scattering, an overview abhay deshpande riken bnl research center snowmass 2001...

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