The differential cross section $dsigma/dq^2$ of diffractive electroproduction of heavy quarkonia on protons is a sensitive study tool for the interaction dynamics within the dipole representation. Knowledge of the transverse momentum transfer $vec q$
provides a unique opportunity to identify the reaction plane, due to a strong correlation between the directions of $vec q$ and impact parameter $vec b$. On top of that, the elastic dipole-proton amplitude is subject to a strong correlation between $vec b$ and dipole orientation $vec r$. Most of models for $b$-dependent dipole cross section either completely miss this information, or make unjustified assumptions. We perform calculations basing on a realistic model for $vec r$-$vec b$ correlation, which significantly affect the $q$-dependence of the cross section, in particular the ratio of $psi^{,prime}(2S)$ to $J/psi$ yields. We rely on realistic potential models for the heavy quarkonium wave function, and the Lorentz-boosted Schrodinger equation. Good agreement with data on $q$-dependent diffractive electroproduction of heavy quarkonia is achieved.
Polarized pp elastic scattering at small angles in the Coulomb-nuclear interference (CNI) region offers a unique opportunity to study the spin structure of the Pomeron. Electromagnetic effects in elastic amplitude can be equivalently treated either a
s Coulomb corrections to the hadronic amplitude (Coulomb phase), or as absorption corrections to the Coulomb scattering amplitude. We perform the first calculation of the Coulomb phase for the spin-flip amplitude and found it significantly exceeding the widely used non-flip Coulomb phase. The alternative description in terms of absorption corrections, though equivalent, turned out to be a more adequate approach for the Coulomb corrected spin-flip amplitude. Inspired by the recent high statistics measurements of single-spin asymmetry in the fixed-target HJET experiment at the BNL, we also performed a Regge analysis of data, aiming at disentangling the Pomeron contribution. However, in spite of an exceptional accuracy of the data, they do not allow to single out the Pomeron term, which strongly correlates with the major sub-leading Reggeons. A stable solution can be accessed only by making additional ad hoc assumptions, e.g. assuming the Pomeron to be a simple Regge pole, or fixing some unknown parameters. Otherwise, in addition to the STAR data at $sqrt{s}=$200 GeV new measurements, say at 100 GeV or 500 GeV, could become decisive.
We present a perturbative QCD based model for vacuum and in-medium hadronization. The effects of induced energy loss and nuclear absorption have been included. The main objective is the determination of the relative contribution of these mechanisms t
o the multiplicity ratio observable, measured in semi-inclusive deep-inelastic scattering off deuterium and nuclear targets. This is directly related to the determination of the production length, $Lp$, necessary for a quark to turn into a prehadron. We compare our results with HERMES data for multiplicity ratio and $p_t$-broadening, and show that the description of the whole data set, keeping the model parameters fixed, puts strong constrains on $Lp$. Contrary to induced-energy-loss based models, we find an important contribution from nuclear absorption at HERMES energies. Finally, we discuss some consequences of our study for the LHC physics, and we present the model predictions for the future EIC experiment.
High-multiplicity pp collisions exhibit features, traditionally associated with nuclear effects. Coherence motivates to treat high-multiplicity pp, pA and AA collisions on an equal footing. We rely on the phenomenological parametrization for mean mul
tiplicities of light hadrons and J/psi, assuming their linear dependence on N_{coll} in pA collisions. The results of this approach underestimate the recently measured production rate of J/psi at very high hadronic multiplicities. The linear dependence of J/psi multiplicity on N_{coll} is subject to predicted nonlinear corrections, related to mutual boosting of the saturation scales in colliding dense parton clouds. A parameter-free calculation of the non-linear corrections allows to explain data for pT-integrated yield of J/psi at high hadronic multiplicities. Calculations are in a good accord with data binned in several pT-intervals as well. As was predicted, Upsilon and J/psi are equally suppressed at forward rapidities in pA collisions. Consequently, their fractional multiplicities at forward rapidities in pp collisions are equal as well, and their magnitude agrees with data.
Brand-new high-precision data for single-spin asymmetry $A_N(t)$ in small angle elastic $pp$ scattering from the fixed target experiment HJET at BNL at $E_{lab}=100$ and $255 mbox{ GeV}$, as well as high energy STAR measurements at $sqrt{s}=200 mbox{
GeV}$, for the first time allowed to determine the spin-flip to non-flip ratio $r_5(t)$ in a wide energy range. We introduced an essential modification in the Coulomb-nuclear interference (CNI) mechanism, missed in previous analyses. It can be formulated either as a modification of the Coulomb phase, which is much larger for the spin-flip compared with non-flip amplitudes, or as absorptive corrections to the electromagnetic interaction of hadrons. The Regge analysis singles out the Pomeron contribution to the spin-flip amplitude, which steeply rises with energy. We found the spin-flip to non-flip ratio of the Pomeron amplitudes to be nearly $-10%$, steeply rising with energy in accordance with theoretical expectations.
The color field of a quark, stripped off in a hard reaction, is regenerated via gluon radiation. The space-time development of a jet is controlled by the coherence time of gluon radiation, which for heavy quarks is subject to the dead-cone effect, su
ppressing gluons with small transverse momenta. As a result, heavy quarks can radiate only a small fraction of the initial energy. This explains the peculiar shape of the measured heavy quark fragmentation function, which strongly peaks at large fractional momenta z. The fragmentation length distribution, related to the fragmentation function in a model independent way, turns out to be concentrated at distances much shorter than the confinement radius. This implies that the mechanisms of heavy quark fragmentation is pure perturbative.
We analyse the origin of dramatic breakdown of diffractive factorisation, observed in single-diffractive (SD) dijet production in hadronic collisions. One of the sources is the application of the results of measurements of the diagonal diffractive DI
S to the off-diagonal hadronic diffractive process. The suppression caused by a possibility of inelastic interaction with the spectator partons is calculated at the amplitude level, differently from the usual probabilistic description. It turns out, however, that interaction with the spectator partons not only suppresses the SD cross section, but also gives rise to the main mechanism of SD dijet production, which is another important source of factorization failure. Our parameter-free calculations of SD-to-inclusive cross section ratio, performed in the dipole representation, agrees with the corresponding CDF Tevatron (Run II) data at $sqrt{s}=1.96$ TeV in the relevant kinematic regions. The energy and hard scale dependences demonstrate a trend, opposite to the factorisation-based expectations, similarly to the effect observed earlier in diffractive Abelian radiation.
Absorptive corrections, known to suppress proton-neutron transitions with large fractional momentum $zto1$ in pp collisions, become dramatically strong on a nuclear target, and push the partial cross sections of leading neutron production to the very
periphery of the nucleus. The mechanism of $pi$-$a_1$ interference, which successfully explains the observed single-spin asymmetry in polarized $ppto nX$, is extended to collisions of polarized protons with nuclei. Corrected for nuclear effects, it explains the observed single-spin azimuthal asymmetry of neutrons, produced in inelastic events, where the nucleus violently breaks up. The single-spin asymmetry is found to be negative and nearly $A$-independent.
We describe production of heavy quarkonia in pA collisions within the dipole approach, assuming dominance of the perturbative color-singlet mechanism (CSM) in the $p_T$-integrated cross section. Although accounting for a nonzero heavy $Q$-$bar Q$ sep
aration is a higher twist correction, usually neglected, we found it to be the dominant source of nuclear effects, significantly exceeding the effects of leading twist gluon shadowing and energy loss. Moreover, this contribution turns out to be the most reliably predicted, relying on the precise measurements of the dipole cross section at HERA. The nuclear suppression of quarkonia has been anticipated to become stronger with energy, because the dipole cross section steeply rises. However, the measured nuclear effects remain essentially unchanged within the energy range from RHIC to the LHC. A novel production mechanism is proposed, which enhances the charmonium yield. Nuclear effects for the production of $J/psi$, $psi(2S)$, $Upsilon(1S)$ and $Upsilon(2S)$ are calculated, in agreement with data from RHIC and LHC. The dipole description offers a unique explanation for the observed significant nuclear suppression of $psi(2S)$ to $J/psi$ ratio, related to the nontrivial features of the $psi(2S)$ wave function.
Leading neutron production on protons is known to be subject to strong absorptive corrections, which have been under debate for a long time. On nuclear targets these corrections are significantly enhanced and push the partial cross sections of neutro
n production to the very periphery of the nucleus. As a result, the A-dependences of inclusive and diffractive neutron production turn out to be similar. The mechanism of pi-a_1 interference, which successfully explained the observed single-spin asymmetry of neutrons in polarized pp interactions, is extended here to polarized pA collisions. Corrected for nuclear effects it explains well the magnitude and sign of the asymmetry A_N observed in inelastic events, resulting in a violent break up of the nucleus. However the excessive magnitude of A_N observed in the diffractive sample, remains a challenge.
