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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. 3D structure of nuclear matter
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From HERA to eRHICA. 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 3D structure of nuclear matter Spin structure QCD dynamics in much greater detail Special eRHIC feature Physics Detector feature 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
Inclusive cross sections Cross sections measured over wide kinematic range ! Precise determination of PDFs in context of NLO DGLAP.
ZEUS 1997 F2 measurement Dominated by statistical uncertainty y=1 y=0.01 Dominated by syst uncertainty Red: total error > 4% Blue: total error < 4%
Structure function data used to parametrize parton densities in proton. Impressive precision achieved.
But all is not well … 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.
High y cross sections Note the turn-over of the cross section with decreasing x at small x in the H1 data. 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 Q2 data consistently at NLO DGLAP.
Expected to dominate at small-x In QPM: hadron is made up of quarks with zero PT Helicity conservation L=0 QCD radiation introduces quark PT, L0 LO pQCD We will make an FL measurement at HERA in 2007 …
F2 r F2-FL 0 1 y2/Y+ Measuring FL Small Q2, ignore F3 For best sensitivity, maximize lever arm (y-range)
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 FL measurement from eRHIC+HERA eRHIC FL measurement from eRHIC+fixed target DIS 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 Below Q20.5 GeV2, see same energy dependence as observed in hadron-hadron interactions. Observe transition from partons to constituent quarks in data. Distance scale 0.3 fm ?? eRHIC could probe this region with high precision (with the right detector) Hadron-hadron scattering energy dependence (Donnachie-Landshoff)
Probing the parton-hadron transition eRHIC Extremely precise measurements possible in this interesting region ALLM parametrization
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 • Without proton • no t measurement • proton dissociation into low mass state difficult to estimate • large acceptance • With proton • can measure t • clean elastic sample • small 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) VM Clean process - has been measured for many different vector mesons differentially in many variables - wealth of information
Curves from dipole model analysis of Kowalski, Motyka, Watt Hep-ph 0606272 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)
Small-x is not the only frontier … 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 Q2 x
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 Jet found No jet found
Results 99-00 e+P Red line is expectation from CTEQ6D
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 !
eRHIC with 100 pb-1 and FAD Limited by MC generator Measurements close to x=1 possible with good precision !
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 ct r * b 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). b~ 0.2 fm/sqrt(t) t=(p-p‘)2 Exclusive processes yield matter profile of hadron.
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.
