# Presentation: 848506

## From HERA to eRHICA. Caldwell, Max-Planck-Institut f. Physik 

## eRHIC vs. Other DIS Facilities

**eRHIC**

- DIS

- eRHIC would cover a kinematic range which has already been measured ...

## Wide range of A
High luminosity
Polarized beams

Full acceptance det

- High luminosity
- Polarized beams
- Full acceptance det

- 3D structure of nuclear matter
- Spin structure
**QCD dynamics in much greater detail**

**Special eRHIC feature**

**Detector feature**

**Physics**

**The physics program is broad and extremely interesting**

## Optimized detector design will make a big difference in the physics which can be accessed.  Towards Bjorken’s FAD.

- See:
- I.~Abt, A.~Caldwell, X.~Liu and J.~Sutiak,
- arXiv:hep-ex/0407053

## Selected HERA results

- There are many physics topics addressed by HERA which I will not cover:
-  electroweak
-  searches for BSM physics
-  precision tests of QCD (S, heavy quark production, jet rates, ...)
-  all the results from HERMES and HERA-B
- Instead, I will focus on the unique aspects of HERA collider physics, which is the small-x physics, and also discuss the high-x end.  We will see what eRHIC could add

## Cross sections measured over wide kinematic range ! Precise determination of PDFs in context of NLO DGLAP.

- Inclusive cross sections

## Blue: total error < 4%

- Red: total error > 4%

- Dominated by statistical uncertainty

- Dominated by syst uncertainty

- ZEUS 1997 F2 measurement

- y=1

- y=0.01

## Structure function data used to parametrize parton densities in proton.  Impressive precision achieved.

## From Pumplin, DIS05

- There are signs that DGLAP (Q2 evolution) may be in trouble at small x (negative gluons, high 2 for fits).
- How well do we understand the small-x physics ?  So far, no theory which predicts the x-dependence of cross sections (PDFs).  Guided by data.

- But all is not well ...

## High y cross sections

- The data can be fit consistently with NLO DGLAP by H1 **assuming** no gluon saturation.  The turn-over is due the negative contribution from FL. **MRST, CTEQ have trouble fitting the H1 low Q****2**** data consistently at NLO DGLAP.**

- Note the turn-over of the cross section with decreasing x at small x in the H1 data.

## In QPM: hadron is made up of quarks with zero PT

- L=0

- QCD radiation introduces quark PT, L0

- LO pQCD

- Helicity conservation

- We will make an FL measurement at HERA in 2007 ...

## Measuring FL

- Small Q2, ignore F3

- For best sensitivity, maximize lever arm (y-range)

## Expected precision on FL HERA:

- Expected precision on FL HERA:

- eRHIC cannot push the small-x limit, but should provide much more accurate FL measurements.  Could be crucial in understanding the physics of gluons.

## FL: eRHIC vs. Other DIS Facilities

**eRHIC**

- DIS

- FL measurement from eRHIC+HERA

- FL measurement from eRHIC+fixed target

- eRHIC is in an optimal energy range to extract FL via cross section comparisons to previous experiments.

## x<0.01 measure universal structure of QCD radiation.  

-  x>0.1  measure hadronic structure.  Non-perturbative boundary conditions.  Eventually get these from the lattice ?

## The behavior of the rise with Q2

**The behavior of the rise with Q****2**

**Below Q****2**** ********0.5 GeV****2****, see same energy dependence as observed in hadron-hadron interactions.  Observe transition from partons to constituent quarks in data. Distance scale ******** 0.3 fm ??**

**Hadron-hadron scattering energy dependence (Donnachie-Landshoff)**

- eRHIC could probe this region with high precision (with the right detector)

## Probing the parton-hadron transition

-  eRHIC

- ALLM parametrization

- Extremely precise measurements possible in this interesting region

## Diffraction - the big surprise

- Large diffractive cross section came as a surprise:

- Still no understanding from a pQCD approach.

## Diffraction

**There is a large diffractive cross section, even in DIS (ca. 20 %)**
**The diffractive**** and**** total cross sections have similar energy dependences.  Data suggests simple physics – ****what is it**** ?**

- Key detector issues:
-  Need to guarantee proton intact.
-  Cover full W range
-  Good MX resolution
- Experience: measuring scattered proton gives cleanest measurements, but acceptance limited in PT,xL.

## Notes on diffraction

- Measurements have been performed with and without measuring the outgoing proton

- With proton
-  can measure t
-  clean elastic sample
-  small acceptance (%)

- Without proton
-  no t measurement
-  proton dissociation into low mass state difficult to estimate
-  large acceptance

- Ideal: measure the outgoing proton with large acceptance.  Will be easier at eRHIC because of the smaller EP, but needs detailed discussions with accelerator experts !

##  Exclusive Processes (VM and DVCS)

-  Exclusive Processes (VM and DVCS)

- 

- VM

**Clean process - has been measured for many different vector mesons differentially in many variables - wealth of information**

## BG=4 GeV-2 corresponds to an rms impact parameter of 0.56 fm.  smaller than the proton charge radius of 0.870 [PDG] ... 

-  eRHIC should aim to perform these measurements with the best possible precision (detector requirements)

- Curves from dipole model analysis of Kowalski, Motyka, Watt Hep-ph 0606272

## There is limited data on cross sections at high-x and high Q2

- There is limited data on cross sections at high-x and high Q2

- BCDMS has measured F2 up to x=0.75
- H1, ZEUS have measured F2 up to x=0.65

- Small-x is not the only frontier ...

**x**

**Q****2**

## The PDF’s are poorly determined at high-x.  Sizeable differences despite the fact that all fitters use the same parametrization            xq(1-x).  Is it possible to check this ?

## HERA high-x

-  At high Q2, scattered electron seen with 100% acceptance
-  For not too high x, measure x from jet:
-  For x>xEdge, measure

## HERA Kinematics

- No jet found

- Jet found

## Results

- 99-00 e+P

- Red line is expectation from CTEQ6D

## 99-00 e+P

- 99-00 e+P

- Good agreement with CTEQ6D in previously measured region. Data tend to lie above expectations at highest x.
- With the right detector, eRHIC could make precision measurements at high-x !

## Measurements close to x=1 possible with good precision !

- Limited by MC generator

**eRHIC with 100 pb****-1**** and FAD**

## HERA  eRHIC

- Precise scan of the transition region between partonic & hadronic behavior.  Something changes there - can we understand it ? Need acceptance in electron direction.
- Make precision measurements at high-x to understand the valence quarks. Need acceptance in proton direction.
- Make FL a highlight  of the program - much more direct access to gluon density than via F2 scaling violations.  Needs high precision measurements - good resolution, small systematics.  Note: FL can also be derived from comparison with HERA, fixed target.
- Focus on clean, high acceptance diffractive and elastic scattering measurements.  Needs high efficiency rejection of proton dissociation and high acceptance proton spectrometer.

## Physics Picture

-  b ~ 0.2 fm/sqrt(t) 		t=(p-p‘)2
- Exclusive processes yield matter profile of hadron.

- r ~ 0.2 fm/Q  (0.02 – 2 fm for 100>Q2>0.01 GeV2) transverse size of probe.  Scan across transition from partons to hadrons !

- ct ~ 0.2 fm (1/2MPx)  (<1 fm to 1000‘s fm).
- For x<0.01, ct>10 fm.  Study universal features of QCD radiation (short time scale fluctuations).
- For x>0.1, ct<1 fm.  Study proton structure (long times).

## Big picture

- eRHIC would allow a precise 3D mapping of nuclear structure at different distance scales, permitting the study of the transition from partonic constituents to hadrons.
- The short time-scale fluctuations of QCD which became visible at HERA could be studied in much greater detail, and with different targets.  More input needed for theoretical understanding.
- Both topics are fundamental, and need new data for a deeper understanding.