From HERA to e RHIC A Caldwell MaxPlanckInstitut

From HERA to e. RHIC A. Caldwell, Max-Planck-Institut f. Physik

e. RHIC vs. Other DIS Facilities e. RHIC DIS e. RHIC would cover a kinematic range which has already been measured …

Special e. RHIC feature • Wide range of A • High luminosity • Polarized beams Detector feature • Full acceptance det Physics • 3 D structure of nuclear matter • Spin structure • QCD dynamics in much greater detail 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, ar. Xiv: 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 e. RHIC could add

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

ZEUS 1997 F 2 measurement Dominated by statistical uncertainty 1 = y y= 01. 0 Dominated by syst uncertainty Blue: total error < 4% Red: total error > 4%

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

But all is not well … There are signs that DGLAP (Q 2 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. From Pumplin, DIS 05

High y cross sections Note the turn-over of the cross section with decreasing x at small x in the H 1 data. The data can be fit consistently with NLO DGLAP by H 1 assuming no gluon saturation. The turn-over is due the negative contribution from FL. MRST, CTEQ have trouble fitting the H 1 low Q 2 data consistently at NLO DGLAP.

In QPM: hadron is made up of quarks with zero PT L=0 Helicity conservation QCD radiation introduces quark PT, L 0 LO p. QCD Expected to dominate at small-x We will make an FL measurement at HERA in 2007 …

Measuring FL Small Q 2, ignore F 3 F 2 r F 2 -FL 0 y 2/Y+ 1 For best sensitivity, maximize lever arm (y-range)

Expected precision on FL HERA: e. RHIC cannot push the small-x limit, but should provide much more accurate FL measurements. Could be crucial in understanding the physics of gluons.

FL: e. RHIC vs. Other DIS Facilities e. RHIC DIS FL measurement from e. RHIC+HERA FL measurement from e. RHIC+fixed target e. RHIC 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. Nonperturbative boundary conditions. Eventually get these from the lattice ?

The behavior of the rise with Q 2 Below Q 2 0. 5 Ge. V 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 ? ? e. RHIC could probe this region with high precision (with the right detector) Hadron-hadron scattering energy dependence (Donnachie-Landshoff)

Probing the parton-hadron transition e. RHIC 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 p. QCD approach.

Diffraction 1. 2. 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, x. L.

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 e. RHIC 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 Ge. V-2 corresponds to an rms impact parameter of 0. 56 fm. smaller than the proton charge radius of 0. 870 [PDG] … e. RHIC 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 Q 2 BCDMS has measured F 2 up to x=0. 75 Q 2 H 1, ZEUS have measured F 2 up to x=0. 65 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 Q 2, scattered electron seen with 100% acceptance • For not too high x, measure x from jet: • For x>x. Edge, measure

HERA Kinematics Jet found No jet found

99 -00 e +P Results Red line is expectation from CTEQ 6 D

99 -00 e+P Good agreement with CTEQ 6 D in previously measured region. Data tend to lie above expectations at highest x. With the right detector, e. RHIC could make precision measurements at high-x !

e. RHIC with 100 pb-1 and FAD Limited by MC generator Measurements close to x=1 possible with good precision !

HERA e. RHIC 1. Precise scan of the transition region between partonic & hadronic behavior. Something changes there - can we understand it ? Need acceptance in electron direction. 2. Make precision measurements at high-x to understand the valence quarks. Need acceptance in proton direction. 3. Make FL a highlight of the program - much more direct access to gluon density than via F 2 scaling violations. Needs high precision measurements - good resolution, small systematics. Note: FL can also be derived from comparison with HERA, fixed target. 4. 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>Q 2>0. 01 Ge. V 2) transverse size of probe. Scan across transition from partons to hadrons ! ct ~ 0. 2 fm (1/2 MPx) (<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 e. RHIC would allow a precise 3 D 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.