Physics Case For Electron Ion Collider Abhay Deshpande Abhay Deshpande AA Physics Capabilities of an EIC Detector Mini-Workshop at BNL September 19, 2002

Physics Case For Physics Case For EElectron lectron IIon on CColliderollider

Abhay DeshpandeAbhay Deshpande

Abhay Deshpande

AA Physics Capabilities of an EIC Detector

Mini-Workshop at BNL

September 19, 2002

Riken BNL Research Center

http://www.bnl.gov/eic

A

BNL Sept. 02 Physics Case of EIC 2

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 m-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?

Low x


•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!•E704, AGS pp scattering, HERMES ep (1999): ???? Transverse spin asymmetries ????

79.2/ Np 000119.1 e

Physics with “Spin” full of Surprises!


Deep Inelastic Scattering Kinematics

• Observe scattered electrons and hadrons• Observe spectator or remnant of the interaction??


Advantages of a Collider Experiment

Historically polarized & un-polarized DIS fixed target experiment

However colliding beam configuration has advantages

• Better angular separation between scattered lepton & nucl. Fragments -- Better resolution for electro-magnetic probe -- Recognition of rapidity gap events (recent history of diffraction)• Better measurement of nuclear fragments• Higher Center of Mass (CM) energies reachable• Tricky integration of beam pipe and accelerator components


The EIC w.r.t. Other Experimental Facilities

• New kinematic region to be explored

• EIC = eRHIC + EPIC

• Kinematic Reach for DIS:

• High Luminosity!

GeVs

GeVEGeVE pe

10020

25030,103

42

422

101

7.010,1

Q

xGeVQ

233 cmsec//10*)0.13.0(~ L


The EIC w.r.t. Other Facilities

Large luminosity and high CM Energy makes EIC unique!

Variable CM energy, ion species

and polarizability of hadron beams enhances

its versatility!

TESLA-N


Scientific Frontiers Open to the EIC

• Nucleon structure: Spin structure: polarized quark and gluon

distributions the nucleon Unplarized quark and gluon distributions in the nucleon Correlations between partons• Role of quarks and gluons in the nuclei• Hadronization in nucleons and nuclei• Partonic matter under extreme conditions


The EIC Detector A “4” Detector

• Scattered electrons to measure kinematics of DIS

• Scattered electrons at small (~zero degree) angles to tag photo-production events

• Central hadronic final state for kinematics, jet measurements, quark flavor tagging, fragmentation studies….

• Central hard photon and particle/vector meson detection (DVCS)

• Zero angle photon measurement to control radiative corrections and in e-A physics to tag nuclear de-excitations

• Missing ET for neutrinos in final state (W physics)

• Tagging forward nuclear fragments

• Tagging forward particles for diffractive physics & target depedence

• Variable energies & species of ions• Polarized beam species: p, d, He• High luminosity

The EI Collider will provide:


Where do electrons and quarks go?

p q,e 10 GeV x 250 GeV1770 1600

scattered electron scattered quark

10 GeV

5 GeV900

5 GeV

100


Electron kinematics… some details…

scattered electron

10 GeV x 250 GeV

At HERA:Electron method: x/x ~E/(y.E) Limited by calorimeter resolutionHadron method: Limited by noise in calorimeter (E_noise/E_beam)

At EIC:Measure electron energy with tracker (< 20 GeV, large kin. region) p/p ~ 0.005-0.0001 (2-4T Magnet)Design low noise calorimeter Crystal or SPACAL


Electron, Quark Kinematics

scattered electron scattered quark

5 GeV x 50 GeVpq,e


Physics with Unpolarized e-p Collisions

Large kinematic region already covered by HERA but additional studies at the EIC are possible & desirable!

Uniqueness of EIC: High luminosity, variable Sqrt(s), deuterons, improved detector & IP

Will enable precision physics studies:• d beams neutron structure, d/u x0, dbar(x)-ubar(x)• precision S(Q2)• flavor separation (charm, strangeness)• slopes in dF2/dlnQ2

• precision gluon distribution x 0.001 0.5-1• exclusive reaction measurements• transition region physics Q2=0 few GeV• nuclear fragmentation region


Topics of Interest for polarized DIS

• Spin structure function g1p/g1n with high precision and at low x

Bjorken sum rule with high precision

G(x,Q2) from pQCD analysis, Di-Jet/high pT hadron events in PGF processes

• Precise determination of S from g1 scaling violations alone…

• Polarized structure of the photon from photo-production studies

• Electroweak structure functions g5(+/-) from W(+/-) production in polarized ep scattering

• Flavor separation of PDFs through semi-inclusive DIS

• Transversity • DVCS• Contribution to GDH sum rule

at very high υ• Tgt/Current fragmentation • Etc……. Many other measurements

A robust program which would require a challenging detector & interaction region design


Spin Structure Function g1 at low xA. Deshpande, V. W. Hughes

~5-7 days of data

No present/future approved experiment will do as well.

3 years of data

Studies included statistical errors & detector smearing


Low x Measurement of g1 of the NeutronA. Deshpande, V.W.Hughes

• With He+2 or Deuteron in hadron ring: g1n measurable

• ~2 weeks of EIC data (only the low x data shown)

• Shown is the present SMC data with various low x scenarios and uncertainties from possible HERA data in 3 years

• Combined with g1p this would enable a “hyperfine” test of Bjorken sum rule which is a fundamental result of QCD

(~0.5-1.0 % accuracy estimated)

EIC 1 inv.fb


Polarized Gluon Distribution

Polarized Gluon distribution is being pursued at various various experimental facilities as we speak…. Each has its weaknesses and strong points.

At the Electron Ion Collider this will be pursued with very different physics interactions: Different Systematics/Different Kinematics

Deep Inelastic Scattering Kinematics with EIC:1. Perturbative QCD analysis of the g1 spin structure of the data2. 2+1 Jet production in photon gluon fusion (PGF) process3. 2-high pT opposite charged hadron tracks (PGF) Photoproduction (real photon) Kinematics with EIC:1. Single jet production in PGF2. Di-Jet production in PGF3. Open charm production4. ….


First Moment of the G(x)A.Deshpande, V. W. Hughes & J. Lichtenstadt

• pQCD analysis of g1 structure function at NLO gives the first moment of the polarized gluon distribution. Present value and uncertainty is: (at Q2 = 1 GeV2)

1.0 (stat) (exp.sys.) (theory/low-x)

• Major source of uncertainty from low x unmeasured region: Theory completely unconstrained in this region.

• If EIC data (~1 week) is obtained and the analysis is repeated, the theoretical uncertainties are estimated to improve by factor of ~3-5; the statistical uncertainty improves by factor ~5. Study to be repeated for 1-3yr data.

+1.0 + 0.4 +1.4 - 0.4 - 0.2 - 0.5

Complimentary determination of G to that from RHIC Spin


Other “direct” methods to get GPhoton-Gluon-Fusion (PGF)

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

interpretation is without ambiguities or uncertainties!

• Method already tried at HERA -- NLO calculations exist -- G(x,Q2) extracted and

published (H1 & ZEUS) -- Consistent with G(x,Q2) from pQCD analysis of F2

Signal: PGF

BackgroundQCD Compton


Result of Di-Jet analysis at NLOG. Radel & A. De Roeck, A. Deshpande, V. W. Hughes, J. Lichtenstadt

• Easy to differentiate between different scenarios of G: Improves G by factor of ~2-3?• Combined analysis: Di-Jet + pQCD analysis of g1:G constrained by these two

together further improve the uncertainties by an additonal factor of ~3 Effectively a ~5% or better measurement of G might be expected

Statistical accuracy shown for EIC for 2 luminosities

Detector smearing effects studied

NLO analysis for Di-Jet considered in the past


G(x)/G(x) EIC vs. Rest of the World

EIC Di-Jet DATA 2fb-1

Good precisionClean measurementRange 0.01 <x< 0.3Constrains shape!!


Polarized Parton Distribution of the Photon

• Photoproduction studies with single and di-jet and one and 2 high pT opposite charged hadrons.

• At high enough energies the photon can resolve itself into its parton content• With polarized protons asymmetries related to the spin structure of the photon can be extracted! A UNIQUE measurement! • Asymmetries sensitive to the gluon structure as well!

Resolved PhotonDirect Photon


Spin structure of polarized photon!M. Stratmann & W. Vogelsang

• Statistical uncertainty with 1 inv.fb.

• ~2wks running for EIC

• Single and double jet asymmetries

• ZEUS Acceptance cuts

• Will resolve the photon pdfs easily!

Direct Photon Resolved Photon


Parity Violating Structure Functions g5

• Unique measurement with EIC polarized HERA • Experimental Signature:

missing (neutrino) momentum: huge asymmetry in detector

• Complementary measurement to RHIC SPIN

For EIC kinematics


Measurement Accuracy PV g5 with EICJ. Contreras & A. De Roeck

Assume:1) Input GS Polarized PDFs2) xF3 is measured well by that time3) 4fb-1 luminosity

If e+ and e- possible then one can have g5(+) as well.

Separate flavors Delta u, Delta d etc.


Drell-Hearn-Gerasimov Sum RuleS. Bass, A. De Roeck, A. Deshpande

• DGH sum rule:

• At EIC range: GeV-few TeV range

• Although contribution to integral is small: explore energy dependence of cross-section.

• Complementary to JLAB, MAMI Experimental effort

Electron Tagger:

GeVs

GeVQ

8525

1010 2262

Inclusive Photoproductionmeasurement


DVCS/Vector meson production

• Hard exclusive DIS process

(default) but also pions, vector mesons….

• Remove a parton & put another one back in Microsurgery of Baryons!

Claim: Possible access to skewed/off forward PDFsPolarized structure: Access to quark orbital angular momentum(??)

? H(x,,t), E(x,,t) Jq =q+Lq ?

• An ongoing debate….R. Jaffe/X.Ji ……• Will need a large kinematic coverage in the data EIC’s variable energy scheme should help

?? ??


Deeply Virtual Compton Scattering at EICD. Hasell, R. Milner et al.

• DVCS has been observed at HERA (ZEUS,H1), recently by HERMES and later by Jlab

• Potential for getting to Nucleon Angular Momentum???

• Reflection of processes in the nucleon

• Could be measured with EIC with considerable x,Q2 range.

Extrapolations and generalizations involved may become clearer by then?

EIC: 5 GeV e on 50 GeV proton: Much large range possible….


DVCS at EIC (preliminary)

10 x 250 GeV

Q2> 1 GeV2

20<W<95 GeV0.1<|t|<1.0 GeV2

Full curve: all eventsDashed curve: accepted events Q2>1 GeV2: 50K events/fb-1

A. Sandacz

Acceptance enhancedZEUS-like detector Add Roman pots a la PP2PP at RHIC


Target & Current Fragmentation RegionT. Londergan & P. Moulders

• Target and current fragments separated naturally in a collider mode compared to the fixed target experiment

• Exclusive/Semi-Inclusive measurements need high luminosity

• EIC will have both capabilities• Numerous measurements

leading to detailed study of different fragmentation functions using the different species of beams at EIC both in polarized and unpolarized DIS.


Polarized & Unpolarized Strange Quark Distribution Measurements

• Assuming that u and d quark distributions are measured by the time EIC comes along….• A detector with a good particle ID for pion/kaon… separation could access strange quark distribution• Upper left plot shows statistical accuracy for Asymmetries measured for events with Kaons with 1fb-1 luminosity at EIC• Lower left plot shows the accuracy on strange quark distribution possible

E. Kinney, U. Stoesslein


Collider helps: Target Fragmentation Studies

D. De Florian, G. M. Shore & G. Veneziano

Experimetal Measurement:Different targets,Forward detectors,Detect positive & negative hadrons,Provide good PID


Low x Physics with e-A Collider

• …and extend into a novel high parton density region:

Color Glass Condensate

• Study e-A in a collider mode for the first time!

• QCD in a different environment!

• Clarify & reinforce the physics studied so far from e-A/-A in fixed target experiments including target fragmentation region…..

• Physics to be explored can be broadly categorized into three x regions 1) High x

2) Intermediate x

3) Low x 01.0)2/(1 AN Rmx

1.0)2/(1)2/(1 NNAN RmxRm

1.0)2/(1 NN Rmx


Why e-A in Collider Mode?

• Highest energy fixed target experiment (NMC @ CERN, E665 @ FNAL) used secondary muon beams and achieved

• Beam intensities:

• Target thinkness: 600 gm/cm2

• Luminosity:

• Electron DIS experiments at SLAC & DESY: Large beam currents but limited beam energies:

GeVs 30~s/102 6

-1-1-232 Nscm106

• Thick targets do not allow a good measurement of the target fragments!

ONLY INCLUSIVE MEASUREMENTS

GeVs 8

An e-A Collider with appropriately chosen beam energies can overcome all these limitations!

Ee ~5-10 GeV, EA~100 GeV/nucleonLuminosity > per e-NThis corresponds to approx. 85pb-1 per day!

-1-233 scm10


Experimental Hints of the Unusual in Nuclei

M. L. Leicht et al. for FNAL E866

• 800 GeV Proton-fixed target A collisions: Comparison of Drell-Yan di-muon production vs production and then decay of J/Y and Upsilon

• Nuclear medium enhances high parton density effects!

Also more recent results from RHIC…


DIS on Nuclei is Different!E665, NMC & SLAC Collaborations

• Fermi motion

• EMC effect

• Enhancement

• Shadowing

• Saturation?

Region of Shadowing and saturation hardly have data with Q2> 1 GeV2

An e-A Collider will measure these regions!

F2D/F2A

Low Q2!


Statistical Precision possible with EICT. Sloan

Statistical Precision Possible with EIC:• NMC data F2(Ca/D)• EIC data with L=1 pb-1

•Recall expected rates at EIC are ~85 pb-1/day• Also extends the measurements in to the low x region keeping the Q2>1 GeV2! • Region of saturation?

EIC 1 inv. pb

T. Sloan


Why high Q2 important at low x?

• Before HERA: for Although low x, the coupling constant too large to make

predictions and extract information in this region from theory. Lack Lack of high Q2 a stumbling block for understanding QCD.of high Q2 a stumbling block for understanding QCD.

• HERA: Ep = 820-925 GeV, Ee=27.6 GeV, = 300 GeV For• At HERA in good portion of low x region of interest.

Coupling weak: computations in conventional pQCD possible & tested with data but interpretation is ambiguous.

• Hard (BFKL) Pomeron: In QCD it comes from gluon ladder diagrams 1) LO BFKL predicts cross sections rising faster than the HERA data 2) NLO corrections large and negative: Resummations necessary!

Confusion! Interest! Formulate QCD at small x: High Parton Density PhysicsFormulate QCD at small x: High Parton Density Physics

s224 10110 GeVQx

QCDQx 22410~

1)( 2 QS


Lessons from HERA: Hints of something new?• At high Q2 (>>10 GeV2), the rise of gluon

structure function at small x is well understood in pQCD framework.

• extracted reliably!• However this is not extremely low x! extremely low x! • In region Q2 (1-10) GeV2 issues less clear:

Although fits accommodate data well, the interpretation problematic!

Gluon too small, and sea quark distribution more than glue!

)/1ln())/ln(ln(exp~

),(ln/),(ln

22

2222

xQ

QxGQdQxFd

QCD

)( 22ZS MQ

410 fewx

Is the pQCD approach breaking down? If so Why?Is the pQCD approach breaking down? If so Why?


What could be wrong with the low Q2

evaluations of HERA pQCD fits?

Coupling strength is still weak in 1-10 GeV2 region!Screening effects due to large parton densities need

to be considered specially!

Phenomenological models that take these into accountExplain inclusive and diffractive data together!

Evidence of possible high parton density phenomena


In Summary….

• As parton densities become too high, standard pQCD breaks down.

• Even though the coupling is weak, the physics may be non-perturbative due to high field strengths generated by large number of partons

• A novel state of matter?

To experimentally explore this novel state of matter an e-A collider with LARGE luminosity and HIGH energy beams is essential!


High Parton Density Matter(HPDM)

• For a fixed external probe the number of partons per unit area grows rapidly with increasing energy (decreasing x)

• QCD field strengths grow as Small coupling implies large field strengths… non-linearities of theory are manifest and significantly change the properties of distributions in high energy collisions.

• Calculations indicate that the rapid rise of gluon distributions (as seen at HERA) will saturate.. i.e. grow slowly….. perhaps as slowly as ~ln(1/x).

)( 22QCDQ

SF /1~2

Will form a “Color Glass Condensate! ”


Color Glass Condensate….(CGC)L. McLerran, R. Venugopalan et al.

• Why Glass? At small x, partons are rapidly fluctuating gluons interacting weakly

with each other, but strongly coupled to the high x parton color charges which act as random-static-sources of color charge.

Analogous to a spin glass system of condensed matter: a disordered state of spins is coupled to random magnetic impurities.

• Why Condensate? Gluon occupation number very large. They form a condensate. Being

bosons large numbers can occupy the same state. A Bose-Einstein condensate leads to a huge overpopulation of the

ground states

A new state of matter at high energies would display dramatically different, yet simple, property of glassy

condensates.


Inclusive Signatures Of CGC/HDPM

The structure function F2(x,Q2), dF2/dlnQ2, dF2/dlnx. dF2/dlnQ2 at fixed x @ high Q2 is the Gluon Distribution. Predictions from saturation models VERY DIFFERENT from those in conventional pQCD EIC: Large luminosity and substantial x-Q2 coverage: Precise measurements possible!

Longitudinal structure function FL = F2 –2xF1

Needs variable beam energies Possible @ EIC/eRHIC – Provides independent measurement of the gluon distribution

Measurement of Nuclear Shadowing Quark Shadowing (F2A/A*F2N) in fixed tgt experiments: Observed Gluon Shadowing (GA/A*GN) indirect evidence through pQCD fits but are at low Q2! Gluon shadowing expected to be VERY LARGE at low x and perturbatively reliable Q2s. EIC with high energy can do precise measurements! In addition, direct measurements

possible using semi-inclusive channels (see next slide)

Relation between Shadowing and Diffraction Relation between diffraction off nucleons and shadowing in nuclei will be very

different in high parton density environment. EIC with different hadron beams will explore this.


Semi-Inclusive Signatures of CGC

• Hard Diffraction: Large rapidity gap between fragmentation region of electron and that of

the target. At HERA 7-10% of the cross section corresponds to hard diffraction!

Models with Saturation effects can explain this. These models predict that in eA scattering, the hard diffractive cross section could be huge! (~30-40%). EIC will clearly see this!

• Coherent & Inclusive vector meson production: For light vector mesons, the diffractive cross-section is ~1/2 inclusive

cross-section. For heavier vector mesons, this factor decreases (finally to 1/ln(Q2)). EIC will measure (for different A) Jcross sections

• Gluon distribution measurements using jets and other semi-inclusive probes

• Large multiplicity fluctuations on event-by-event basis

22 ;'*' QCDXMAXeAeAe


Implications of EIC for A-A and p-A Collisions

• Goal of RHIC is to discover and study properties of QGP Possible scenario: QGP formed when CGC “shatters”!• Bottom line: gluon distribution in nuclei important for

initial conditions for QGP formation in A-A collisions. • Physics of p-A complementary to that of e-A

Consequences of the absence of evidence of CGC Should we be probing even deeper?

We learnt something already! A) What was it that we saw hints of at HERA?

B) Is there a possibility that such a state of matter indeed does not exist?

What does this mean for QCD at high energies?


A Case for the EIC

• Explore a new regime of QCD: An e-A collider will provide a unique opportunity to explore fundamental and universal aspects of QCD.

• Measurements would be essential to fully understand the QGP already being pursued at RHIC now, and that would be studied at RHIC later…. And in future at LHC.

• An e-A collider will allow us to explore with great precision inclusive measurements that have not been pursued beyond the fixed target experimental era. It would also enable, for the first time, a wide range of semi-inclusive and exclusive measurements in the nuclear environment .


A Case for the EIC (Continued)…

• A polarized e-p scattering facility with variable sqrt(s) will allow

• a) at high sqrt(s) a new region in the x-Q2 to be explored for the pol. DIS. It will address issues that no other present or future approved facility will address

b) in the already measured x-Q2 range the high luminosity of EIC will allow us to settle lots of yet uncertain issues regarding the nucleon spin related to semi-inclusive and exclusive measurements…

http://www.bnl.gov/eic

DETECTOR DESIGN SHOULD BEGIN IN EARNEST


Time Line for EIC (past)

• September 2000: Electron Ion Collider grew out of a joining of forces from two communities:

1) Polarized eRHIC (ep and eA at RHIC) -- BNL, DESY, UCLA, Yale & CERN fixed target & HERA Collider Experiment Community 2) Electron Polarized Ion Collider (EPIC) (3-5 GeV e X 30-50 GeV polarized light ions.) -- Colorado, IUCF, MIT/Bates, IUCF & HERMES Community• February 2002: A white paper was submitted to NSAC Long Range Planning

Review Received enthusiastic support as a next R&D project beyond RHIC II at BNL

** Steering Committee 7 prominent scientists from DoE labs and DoE supported university groups & 1 contact person ** Physics Working Group, Accelerator Working Group (BNL-MIT-Budker Institute) Detector simulations about to begin with various detector concepts in mind


EIC Time Line… (future)….

“Predictions are very difficult to make, especially when they

are about the future…..” - Albert E.

• Expected presentation & approval in next LRP (2006) • R&D money for detector could start (2007)• Construction of IR and Detector components (2008)• 3 yrs for construction of IR & Detector (??) without interfering with the on going RHIC program• First collisions (2011) ??

If you know how to get it done earlier… -- I am listening….


EIC at BNL vs. What else is there?Machine Luminosity/year

(pb-1)

Sqrt(s)

(GeV)

HERA III 150 300-320

EIC 2000 30-100

THERA 40-250 1000-16000

• HERA III after 2006(?), new detector or/and H1 detector effort in progress No effort towards polarized protons yet, none on nuclei but seems possible Primary focus of new detector: low angle e-p scattering • EIC needs polarized electron beam facility & an experiment• THERA= TESLA * HERA Need TESLA, polarized protons/nuclei in HERA, new detectors and interaction point design > 2015-8

What is important? High CM or High Luminosity?Depends on what physic one wants to do….


)/1ln( x

Low x Resummation

• The g1p at low x dominated by double logarithmic terms like For Unpolarized case:

(J. Bartels et al. Z. Phy. C70 (1996) 236; C72 (1996) 627)

• Predictions for g1p using LO DGLAP equations + Resummed terms in splitting and coefficent function are now available. (J. Kwiecinski & B. Ziaja, HERA Workshop proceedings DESY-1999-02)

• Data from EIC would easily be able to distinguish between the LO, DGLAP or LO+Resummed term scenario.

)/1(ln2 x

)/1(ln2 x


Low x Resummation Scenario and EIC Data

A. Deshpande, A. De Roeck, J. Kweicinski & B. Ziaja

• Shown are: a) LO calculation for g1p b) LO DGLAP +

Resummed calculation c) HERA measurements

with 3yrs running d) EIC uncertainty ~2wks• Otherwise this physics

could be pursued robustly only at TELSA*HERA Collider at DESY

EIC

Documents

Physics Case For Electron Ion Collider Abhay Deshpande Abhay Deshpande AA Physics Capabilities of an EIC Detector Mini-Workshop at BNL September 19, 2002