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The space-time evolution of the hadronic matter produced in the central rapidity region in extreme relativistic nucleus-nucleus collisions is described. We find, in agreement with previous studies, that quark-gluon plasma is produced at a temperature &200 -300 MeV, and that it should survive over a time scale & 5 fm/c. Our description relies on the existence of a flat central plateau and on the applicability of hydrodynamics.

Fixed-target experiments are ideally suited for discovering new MeV-GeV mass U (1) gauge bosons through their kinetic mixing with the photon. In this paper, we identify the production and decay properties of new light gauge bosons that dictate fixed-target search strategies. We summarize existing limits and suggest five new experimental approaches that we anticipate can cover most of the natural parameter space, using currently operating GeV-energy beams and well-established detection methods. Such experiments are particularly timely in light of recent terrestrial and astrophysical anomalies (PAMELA, FERMI, DAMA/LIBRA, etc.) consistent with dark matter charged under a new gauge force. I. NEW GAUGE FORCESThe interactions of ordinary matter establish that three gauge forces survive to low energies. Two striking features of these forces -electroweak symmetrybreaking at a scale far below the Planck scale and apparent unification assuming low-energy supersymmetry -have driven model-building for a quarter-century. But the strong and electroweak forces need not be the only ones propagating at long distances. Additional forces, under which ordinary matter is neutral, would have gone largely unnoticed because gauge symmetry prohibits renormalizable interactions between Standard Model fermions and the other "dark" gauge bosons or matter charged under them.There is an important exception to the above claim: new "dark" Abelian forces can couple to Standard Model hypercharge through the kinetic mixing operator can be generated at any scale by loops of heavy fields charged under both U (1) and U (1) Y , and the A can acquire mass through a technicolor or Higgs mechanism. A mass scale near but beneath the weak scale is particularly well-motived -U (1) symmetry-breaking may be protected by the same physics that stabilizes the electroweak hierarchy [2]. Indeed, if the largest symmetry-breaking effects arise from weak-scale supersymmetry breaking, then the U (1) symmetry breaking scale is naturally suppressed by a loop factor or by √ , leading to MeV to GeV-scale A masses [2,3,4,5,6]. An A can be produced in collisions of charged particles with nuclei and can decay to electrons or muons. The production cross-section (σ A ) and decay length (γcτ ), σ A ∼ 100 pb /10 −4 2 (100 MeV/m A ) 2(1)γcτ ∼ 1 mm (γ/10) 10 −4 / 2 (100 MeV/m A ) (2) vary by ten orders of magnitude for the 's and masses m A we consider. This wide range calls for multiple experimental approaches, with different strategies for confronting backgrounds. Beam-dump searches from the 1980's exclude the low-mass and small-parameter range, and other data constrains large . In this paper we suggest five scenarios for fixed-target experiments sensitive to distinct but overlapping regions of parameter space (see Figure 1). Together they can probe six decades in A coupling and three decades in A mass with existing beam energies and intensities. Dark matter interpretations of recent astrophysical and terrestrial anomalies provide a further impetus to search for new U (1)'s. An...

By combining the qO-im method for asymptotic sum rules with the P -CQ method of Fubini and Furlan, we relate the structure functions W2 and WI in inelastic lepton-nucleon scattering to matrix elements of commutators of currents at almost-equal times at infinite momentum.We argue that the infinite momentum limit for these commutators does not diverge but may vanish. If the limit is nonvanishing we predict . . vW2(v,q2)-f2 v 0 2 as v andq2 tendto 00. From a similar analysis for neutrino processes, we conclude that at high energies the total neutrino-nucleon cross sections rise linearly with neutrino laboratory energy until nonlocality of the weak current-current coupling sets in. The sum of up and Yp cross sections is determined by the equal-time commutator of Cabibbo-current with its time derivative, taken between proton states at infinite momentum.