MEMO

Questions about parameters of detectors versus DIS eA observables

  by M. Strikman (Penn State)

Introduction

During the workshop we need to address the questions of matching between the observables which one needs to study in eA scattering and current/future capabilities of various experiments. Clearly this is not a trivial problem given symmetric kinematics for AA/pp collisions and highly asymmetric kinematics for collisions of 10 GeV electrons with 100 GeV per nucleon nuclei or 100-250 GeV protons.

The list below meant to be a first pass on these observables. Ideally it would be adjusted/expanded interactively with experimental groups coming back with their own suggestions of other possible observables which are feasible with their detectors. Clearly many of the questions cannot be answered in one month left before the workshop. However it is better to try to answer at least some of them.

List of quantities to measure

tex2html_wrap_inline62 Measurement of inclusive cross sections.

The main objective of measurements of tex2html_wrap_inline64 is to study these quantities at as small x as possible (keeping tex2html_wrap_inline66 ) to study nuclear response in the kinematics of maximal parton densities (up to a factor of two larger than in ep collisions at HERA) and look for manifestations of high parton densities in the tex2html_wrap_inline70 evolution of the nuclear structure functions.

tex2html_wrap_inline72 What is interval in tex2html_wrap_inline74 in which electrons can be detected? Can quasi-real kinematics tex2html_wrap_inline76 can be probed as well?

Here tex2html_wrap_inline78 where tex2html_wrap_inline80 is the angle between initial and final electron momenta. Denoting tex2html_wrap_inline82 the invariant energy tex2html_wrap_inline84 of the produced hadronic system we can write tex2html_wrap_inline86 . Introducing s as invariant energy squared of ep collision we can also write tex2html_wrap_inline92 .

Note that with a sufficiently hermetic detector it is also possible to measure x via detection of hadrons by determining the sum over tex2html_wrap_inline96 for the produced hadrons. Only hadrons produced in the electron direction contribute (this method is described in various HERA publications).

tex2html_wrap_inline98 Is it possible to detect photons emitted along the initial electron direction? This is necessary for an accurate measurement of the ratios of cross sections of scattering off light and heavy nuclei at smallest x since otherwise radiative corrections for heavy nuclei become too large and would not allow high precision measurements.

For comparisons of different detectors it would be convenient to have plots of tex2html_wrap_inline102 or tex2html_wrap_inline104 acceptance.

tex2html_wrap_inline62 Gluon small x dynamics

It is expected that nonlinear effects would be manifested at larger x for gluons. Hence study of the processes directly sensitive to gluons is very important. In addition to exclusive hard processes which are discussed below this includes production of two jets in tex2html_wrap_inline110 and production of D mesons.

tex2html_wrap_inline72 Is it feasible to detect two jets with tex2html_wrap_inline116 in the photon fragmentation region?

tex2html_wrap_inline98 Is it feasible to detect D-mesons?

tex2html_wrap_inline62 Study of the parton propagation through nuclei.

Currently the prime observable for study of parton propagation through nuclei at high energies is tex2html_wrap_inline124 broadening of the leading hadron spectrum.

tex2html_wrap_inline72 What is the tex2html_wrap_inline124 , z interval for typical tex2html_wrap_inline102 where measurements are possible? What is resolution in tex2html_wrap_inline124 ? ( tex2html_wrap_inline124 broadening effect are expected to be of the order 300-500 MeV/c for heavy nuclei). Here z is Feynman x for the hadron along the virtual photon momentum.

tex2html_wrap_inline98 For small x the changes of the z distribution are also expected. What is the z interval for the measurements.

Perhaps the best thing would be to plot acceptance in tex2html_wrap_inline152 plane for several intervals in tex2html_wrap_inline102

tex2html_wrap_inline62 Color transparency, color opacity phenomena.

A number of color transparency and color opacity phenomena for coherent scattering off nuclei namely the processes like tex2html_wrap_inline158 where X is a diffractive state for tex2html_wrap_inline162

The simplest final states X are tex2html_wrap_inline166 , tex2html_wrap_inline168 . Lack of the breakup of the nucleus for heavy nuclei would require use of detection of break up of the nucleus as anti coincidence. For gold use of zero degree neutrons would be an effective tool.

tex2html_wrap_inline72 What is tex2html_wrap_inline102 interval in which detection of tex2html_wrap_inline174 in this kinematics is feasible?

tex2html_wrap_inline98 Is it feasible to have anti coincidence with the nucleus break up.

tex2html_wrap_inline178 What is the acceptance for heavier (multihadron) diffractive states? (Typical tex2html_wrap_inline180 for RHIC)

tex2html_wrap_inline62 Hadron fluctuations at midrapidities

An important indicator of the dynamics is the A-dependence of the multiplicity of hadrons produced at the central (in tex2html_wrap_inline184 frame) rapidities. There exist predictions of multiplicity growing with A for small x where shadowing is observed as well as of increase of width of the distribution tex2html_wrap_inline186 - the multiplicity of hadrons produced in a narrow tex2html_wrap_inline188 interval of rapidity.

tex2html_wrap_inline72 For what tex2html_wrap_inline192 measurements of inclusive A dependence are feasible?

tex2html_wrap_inline98 Are measurements of tex2html_wrap_inline186 distribution feasible?

tex2html_wrap_inline62 Nucleus fragmentation region

Pushing observations of hadron production close enough to the nuclear rapidities would be critical for the addressing the question of hadron time formation. At tex2html_wrap_inline202 (where tex2html_wrap_inline204 is hadronic scale tex2html_wrap_inline206 ) hadrons are expected to be formed inside the nucleus, leading to a change of the pattern of hadron A-dependence.

tex2html_wrap_inline72 What is minimal tex2html_wrap_inline212 which could be measured?

tex2html_wrap_inline62 Inclusive tex2html_wrap_inline216 production

Inclusive tex2html_wrap_inline216 production is sensitive tool of the gluon dynamics which allows to probe both the final state interactions of tex2html_wrap_inline220 pairs and, for large tex2html_wrap_inline222 kinematics, the gluon densities in nuclei.

tex2html_wrap_inline72 What is the tex2html_wrap_inline152 range in which tex2html_wrap_inline216 can be measured?