arXiv subjects

Ivan Schmidt

Publications and source records attributed to Ivan Schmidt.

At least 181 records · Page 10Linked to original sources

arXiv2008

Quantum-mechanical description of in-medium fragmentation

We present a quantum-mechanical description of quark-hadron fragmentation in a nuclear environment. It employs the path-integral formulation of quantum mechanics, which takes care of all phases and interferences, and which contains all relevant time scales, like production, coherence, formation, etc. The cross section includes the probability of pre-hadron (colorless dipole) production both inside and outside the medium. Moreover, it also includes inside-outside production, which is a typical quantum-mechanical interference effect (like twin-slit electron propagation). We observe a substantial suppression caused by the medium, even if the pre-hadron is produced outside the medium and no energy loss is involved. This important source of suppression is missed in the usual energy-loss scenario interpreting the effect of jet quenching observed in heavy ion collisions. This may be one of the reasons of a too large gluon density, reported by such analyzes.

hep-ph↗

arXiv2008

Higgs Hadroproduction at Large Feynman x

We propose a novel mechanism for the production of the Higgs boson in inclusive hadronic collisions, which utilizes the presence of heavy quarks in the proton wave function. In these inclusive reactions the Higgs boson acquires the momenta of both the heavy quark and antiquark and thus carries 80% or more of the projectile's momentum. We predict that the cross section ${d σ/d x_F}(p \bar p \to H X)$ for the inclusive production of the Standard Model Higgs coming from intrinsic bottom Fock states is of order 150 fb at LHC energies, peaking in the region of $x_F \sim 0.9$. Our estimates indicate that the corresponding cross section coming from gluon-gluon fusion at $x_F = 0.9$ is relatively negligible and therefore the peak from intrinsic bottom should be clearly visible for experiments with forward detection capabilities. The predicted cross section for the production of the Standard Model Higgs coming from intrinsic heavy quark Fock states in the proton is sufficiently large that detection at the Tevatron and the LHC may be possible.

hep-ph↗

arXiv2008

Azimuthal angle dependence of di-jet production in unpolarized hadron scattering

We study the azimuthal angular dependence of back-to-back di-jet production in unpolarized hadron scattering $H_A+H_B \to J_1 + J_2 +X$, arising from the product of two Boer-Mulders functions, which describe the transverse spin distribution of quarks inside an unpolarized hadron. We find that when the di-jet is of two identical quarks ($J_q+J_q$) or a quark-antiquark pair ($J_q+J_{\bar{q}}$), there is a $\cos δϕ$ angular dependence of the di-jet, with $δϕ=ϕ_1-ϕ_2$, and $ϕ_1$ and $ϕ_2$ are the azimuthal angles of the two individual jets. In the case of $J_q+J_q$ production, we find that there is a color factor enhancement in the gluonic cross-section, compared with the result from the standard generalized parton model. We estimate the $\cos δϕ$ asymmetry of di-jet production at RHIC, showing that the color factor enhancement in the angular dependent of $J_q+J_q$ production will reverse the sign of the asymmetry.

hep-ph↗

arXiv2008

$\cos 2 ϕ$ asymmetries in unpolarized semi-inclusive DIS

We use the Boer-Mulders functions parameterized from unpolarized $p+D$ Drell-Yan data by the FNAL E866/NuSea Collaboration combined with recently extracted Collins functions to calculate the $\cos 2 ϕ$ asymmetries in unpolarized semi-inclusive deeply inelastic scattering (SIDIS) processes both for ZEUS at Hadron Electron Ring Accelerator (HERA) and Jefferson Lab experiments (JLab), and to compare our results with their data. We also give predictions for the $\cos 2 ϕ$ asymmetries of SIDIS in the kinematical regime of HERMES Collaboration, and the forthcoming JLab experiments. We predict that the $\cos 2 ϕ$ asymmetries of semi-inclusive $π^-$ production are somewhat larger than that of $π^+$ production. We suggest to measure these two processes separately, which will provide more detail information on the Boer-Mulders functions as well as on the Collins functions.

hep-ph↗

arXiv2008

Leading neutrons from polarized pp collisions

We calculate the cross section and single-spin azimuthal asymmetry, A_n(t) for inclusive neutron production in pp collisions at forward rapidities relative to the polarized proton. Absorptive corrections to the pion pole generate a relative phase between the spin-flip and non-flip amplitudes, which leads to an appreciable spin asymmetry. However, the asymmetry observed recently in the PHENIX experiment at RHIC at very small |t|~0.01GeV^2 cannot be explained by this mechanism.

hep-ph↗

arXiv2008

Damping of forward neutrons in pp collisions

We calculate absorptive corrections to single pion exchange in the production of leading neutrons in pp collisions. Contrary to the usual procedure of convolving the survival probability with the cross section, we apply corrections to the spin amplitudes. The non-flip amplitude turns out to be much more suppressed by absorption than the spin-flip one. We identify the projectile proton Fock state responsible for the absorptive corrections as a color octet-octet 5-quarks configuration. Calculations within two very different models, color-dipole light-cone description, and in hadronic representation, lead to rather similar absorptive corrections. We found a much stronger damping of leading neutrons than in some of previous estimates. Correspondingly, the cross section is considerably smaller than was measured at ISR. However, comparison with recent measurements by the ZEUS collaboration of neutron production in deep-inelastic scattering provides a strong motivation for challenging the normalization of the ISR data. This conjecture is also supported by preliminary data from the NA49 experiment for neutron production in pp collisions at SPS.

hep-ph↗

arXiv2008

Extracting Boer-Mulders functions from $p+D$ Drell-Yan processes

We extract the Boer-Mulders functions of valence and sea quarks in the proton from unpolarized $p+D$ Drell-Yan data measured by the FNAL E866 Collaboration. Using these Boer-Mulders functions, we calculate the $\cos 2 ϕ$ asymmetries in unpolarized $pp$ Drell-Yan processes, both for the FNAL E866/NuSea and the BNL Relativistic Heavy Ion Collider (RHIC) experiments. We also estimate the $\cos 2 ϕ$ asymmetries in the unpolarized $p\bar{p}$ Drell-Yan processes at GSI.

hep-ph↗

arXiv2008

Handedness of direct photons

The azimuthal asymmetry of direct photons originating from primary hard scatterings between partons is calculated. This can be accounted for by the inclusion of the color dipole orientation, which is sensitive to the rapid variation of the nuclear profile. To this end we introduce the dipole orientation within the saturation model of Golec-Biernat and Wüsthoff, while preserving all its features at the cross-section level. We show that the direct photon elliptic anisotropy v2 coming from this mechanism changes sign and becomes negative for peripheral collisions, albeit it is quite small for nuclear collisions at the RHIC energy.

hep-ph↗

arXiv2008

Azimuthal Asymmetry of Prompt Photons in Nuclear Collisions

The azimuthal elliptic asymmetry v2 observed in heavy ion collisions, is usually associated with properties of the medium created in the final state. We compute the azimuthal asymmetry which is due to multiple interactions of partons at the initial stage of nuclear collisions, and which is also present in $pA$ collisions. In our approach the main source of azimuthal asymmetry is the combination of parton multiple interactions with the steep variation of the nuclear density at the edge of nuclei. We apply the light-cone dipole formalism to compute the azimuthal asymmetry of prompt photons yield from parton-nucleus, proton-nucleus and nucleus-nucleus collisions at the RHIC energy.

hep-ph↗

arXiv2008

Azimuthal anisotropy of direct photons

The electromagnetic bremsstrahlung produced by a quark interacting with nucleons or nuclei is azimuthally asymmetric. In the light-cone dipole approach this effect is related to the orientation dependent dipole cross section. Such a radiation anisotropy is expected to contribute to the azimuthal asymmetry of direct photons in pA and AA collisions, as well as in DIS and in the production of dileptons.

hep-ph↗

arXiv2007

Perturbative fragmentation

The Berger model of perturbative fragmentation of quarks to pions is improved by providing an absolute normalization and keeping all terms in a (1-z) expansion, which makes the calculation valid at all values of fractional pion momentum z. We also replace the nonrelativistic wave function of a loosely bound pion by the more realistic procedure of projecting to the light-cone pion wave function, which in turn is taken from well known models. The full calculation does not confirm the (1-z)^2 behavior of the fragmentation function (FF) predicted in for $z>0.5$, and only works at very large z>0.95, where it is in reasonable agreement with phenomenological FFs. Otherwise, we observe quite a different z-dependence which grossly underestimates data at smaller z. The disagreement is reduced after the addition of pions from decays of light vector mesons, but still remains considerable. The process dependent higher twist terms are also calculated exactly and found to be important at large z and/or pT.

hep-ph↗

arXiv2007

Evidences for two scales in hadrons

Some unusual features observed in hadronic collisions at high energies can be understood assuming that gluons in hadrons are located within small spots occupying only about 10% of the hadron's area. Such a conjecture about the presence of two scales in hadrons helps to explain: why diffractive gluon radiation so much suppressed; why the triple-Pomeron coupling shows no t-dependence; why total hadronic cross sections rise with energy so slowly; why diffraction cone shrinks so slowly, and why $α^\prime_P\llα^\prime_R$; why the transition from hard to soft regimes in the structure functions occurs at rather large $Q^2$; why the observed Cronin effect at collider energies is so weak; why hard reactions sensitive to primordial parton motion (direct photon, Drell-Yan dileptons, heavy flavors, back-to-back di-hadrons, seagull effect, etc.) demand such a large transverse momenta of the projectile partons, which is not explained by NLO calculations; why the onset of nuclear shadowing for gluons is so much delayed compared to quarks, and why shadowing is so weak.

hep-ph↗

arXiv2007

Quenching of high-pT hadrons: Alternative scenario

A new scenario, alternative to energy loss, for the observed suppression of high-pT hadrons observed at RHIC is proposed. In the limit of a very dense medium crated in nuclear collisions the mean free-path of the produced (pre)hadron vanishes, and and the nuclear suppression, R_{AA} is completely controlled by the production length. The RHIC data are well explained in a parameter free way, and predictions for LHC are provided.

nucl-th↗

arXiv2007

Direct photons and dileptons via color dipoles

Drell-Yan dilepton pair production and inclusive direct photon production can be described within a unified framework in the color dipole approach. The inclusion of non-perturbative primordial transverse momenta and DGLAP evolution is studied. We successfully describe data for dilepton spectra from 800-GeV pp collisions, inclusive direct photon spectra for pp collisions at RHIC energies $\sqrt{s}=200$ GeV, and for $p\bar{p}$ collisions at Tevatron energies $\sqrt{s}=1.8$ TeV, in a formalism that is free from any extra parameters.

hep-ph↗

arXiv2007

Direct photons at LHC

The DGLAP improved color dipole approach provides a good description of data for inclusive direct photon spectra at the energies of RHIC and Tevatron. Within the same framework we predict the transverse momentum distribution of direct photons at the CERN LHC energies.

hep-ph↗

arXiv2007

Jet lag effect and leading hadron production

We propose a solution for the long standing puzzle of a too steeply falling fragmentation function for a quark fragmenting into a pion, calculated by Berger [1] in the Born approximation. Contrary to the simple anticipation that gluon resummation worsens the problem, we find good agreement with data. Higher quark Fock states slow down the quark, an effect which we call jet lag. It can be also expressed in terms of vacuum energy loss. As a result, the space-time development of the jet shrinks and the $z$-dependence becomes flatter than in the Born approximation. The space-time pattern is also of great importance for in-medium hadronization.

hep-ph↗

arXiv2007

Lepton Flavor Violating Photoleptonic Effect

We study lepton flavor violating analogs of the photoelectric effect, with a final $μ$ or $τ$ instead of an electron: $γe\to μ$ and $γe\to τ$. On the basis of the general parametrization of the matrix element of the electromagnetic current we estimate the upper limits for the cross sections and event rates of these processes, imposed by the current experimental bounds on $μ\to e γ$ and $τ\to e γ$ decays.

hep-ph↗

arXiv2007

Double transverse spin asymmetry in the $p^\uparrow\bar{p}^\uparrow$ Drell-Yan process from Sivers functions

We show that the transverse double spin asymmetry (DSA) in the Drell-Yan process contributed only from the Sivers functions can be picked out by the weighting function $\frac{Q_T}{M^2}(\cos(ϕ-ϕ_{S_1})\cos(ϕ-ϕ_{S_2})+3\sin(ϕ-ϕ_{S_1})\sin(ϕ-ϕ_{S_2}))$. The asymmetry is proportional to the product of two Sivers functions from each hadron $f_{1T}^{\perp(1)}\times f_{1T}^{\perp (1)}$. Using two sets of Sivers functions extracted from the semi-inclusive deeply elastic scattering data at HERMES, we estimate this asymmetry in the $p^\uparrow\bar{p}^\uparrow$ Drell-Yan process which is possible to be performed in HESR at GSI. The prediction of DSA in the Drell-Yan process contributed by the function $g_{1T}(x,\Vec k_T^2)$, which can be extracted by the weighting function $\frac{Q_T}{M^2}(3\cos(ϕ-ϕ_{S_1})\cos(ϕ-ϕ_{S_2})+\sin(ϕ-ϕ_{S_1})\sin(ϕ-ϕ_{S_2}))$, is also given at GSI.

hep-ph↗