62 research outputs found
Hard diffraction -- 20 years later
The idea of diffractive processes with a hard scale involved, to resolve the
underlying parton dynamics, was published 1985 and experimentally verified
1988. Today hard diffraction is an active research field with high-quality data
and new theoretical models. The trend from Regge-based pomeron models to
QCD-based parton level models has given insights on QCD dynamics involving
perturbative gluon exchange mechanisms, including the predicted BFKL-dynamics,
as well as novel ideas on non-perturbative colour fields and their
interactions. Extrapolations to the LHC include the interesting possibility of
diffractive Higgs production.Comment: 14 pages, 19 figures. Invited talk at XXII International Symposium on
Lepton-Photon Interactions at High Energy, Uppsala, Sweden, June 30-July 5,
200
QCD rescattering mechanism for diffractive deep inelastic scattering
We present a QCD-based model where rescattering between final state partons
in deep inelastic scattering leads to events with large rapidity gaps and a
leading proton. In the framework of this model the amplitude for multiple gluon
exchanges is calculated in the eikonal approximation to all orders in
perturbation theory. Both large and small invariant mass M_X limits are
considered. The model successfully describes the precise HERA data on the
diffractive deep inelastic cross section in the whole available kinematical
range and gives new insight into the density of gluons at very small momentum
fractions in the proton.Comment: 19 pages, 14 figures. Version to appear in PR
The 750 GeV excess from photon-photon and quark-quark processes
The observed excess in the diphoton mass spectrum around 750 GeV at the 13
TeV LHC possibly indicates the presence of a photonphilic resonance. We show
that the excess can be explained by a scalar of the type involved in
Bekenstein's framework for varying electromagnetic coupling theories. The
scalar, in our model, couples dominantly to photons and is mainly produced by
the quark-quark fusion at the LHC. In addition, it can also be produced in
photon-photon fusion. Our model has only two free parameters, the mass of the
scalar and the scale of the new physics, which are fixed by the LHC excess to
750 GeV and 1.5 - 2 TeV, respectively. The scalar has a large three-body decay
to a fermion pair and a photon, which provides an interesting search channel
with a dilepton-photon resonance around 750 GeV.Comment: 4 pages, 2 figures, 4 tables. Presented at the 51st Rencontres de
Moriond, QCD and High Energy Interactions, La Thuile, 19-26 March 201
Constraining minimal anomaly free extensions of the Standard Model
We consider a class of minimal anomaly free extensions of the
Standard Model with three generations of right-handed neutrinos and a complex
scalar. Using electroweak precision constraints, new 13 TeV LHC data, and
considering theoretical limitations such as perturbativity, we show that it is
possible to constrain a wide class of models. By classifying these models with
a single parameter, , we can put a model independent upper bound on the
new gauge coupling . We find that the new dilepton data
puts strong bounds on the parameters, especially in the mass region
.Comment: 23 pages, 4 figures. Modified model parametrization, exclusion limits
with new dat
Parton Energy Loss Without Transverse Momentum Broadening
The JEWEL 1.0 Monte Carlo simulates jet evolution in a medium with a
microscopic description of splitting and scattering processes. In the framework
of this model we investigate the transverse momentum broadening due to medium
effects in different scenarios. Depending on assumptions about hadronisation,
we observe either a small increase or even a slight decrease of the mean
transverse momentum, but no sizeable broadening. This appears to be a natural
consequence of a model formulation which conserves energy and momentum
microscopically at each splitting and at each scattering.Comment: 4 pages, 2 figures, Contribution to the 20th International Conference
on Ultra-Relativistic Nucleus-Nucleus Collisions: Quark Matter 2008 (QM2008),
Jaipur, India, 4-10 Feb 2008, to be published in the Indian Journal of
Physic
String model description of polarisation and angular distributions in pbar p -> Lambdabar Lambda at low energies
The observed polarisation of Lambda hyperons from the inclusive pA -> Lambda
X reaction at high energies has previously been well described within the Lund
string model through polarised ss quark pair production in the string breaking
hadronisation process. This model is here applied to the exclusive pbar p ->
Lambdabar Lambda reaction at low energies and compared to available data sets
down to an incident beam momentum of 1.835 GeV/c. This required an extension of
the diquark scattering model to involve three components: an isotropic part
relevant close to threshold, a spectator part and a forward scattering part as
in pA -> Lambda X at high energies. The observed angular distributions are then
reproduced and, for momentum transfers above |t'| = 0.2 GeV^2, agreement with
the measured polarisation is also obtained.Comment: 12 pages, 4 figures, submitted to Eur. Phys. J.
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