50 research outputs found
The heavy neutral Higgs signature in the gamma gamma to Z Z process
If the Standard Model (SM) Higgs particle is sufficiently heavy, then its
contribution to gamma gamma to Z Z should be largely imaginary, interfering
with the also predominantly imaginary SM "background" generated by the W-loop.
For standard model Higgs masses in the region 200 \lsim m_H \lsim 500 GeV, this
interference is found to be constructive and increasing the Higgs signal. In
the minimal SUSY case an interference effect should also appear for the
contribution of the heavier CP-even neutral Higgs boson H^0, provided it is
sufficiently heavy. The effect is somewhat reduced though, by the smallness of
the H^0 width and the gamma gamma and ZZ branching ratios. The interference is
again found to be constructive for part of the parameter space corresponding to
sfermion masses at the TeV scale and maximal stop mixing. For both the SM and
SUSY cases, regions of the parameter space exist though, where the interference
may be destructive. It is therefore essential to take these effects into
account when searching for possible scalar Higgs-like candidates. To this aim,
we present the complete analytic expressions for both resonance and background
amplitudes.Comment: version submitted to EPJC; 34 pages and 6 figures. e-mail:
[email protected]
The gamma gamma to Z Z process and the search for virtual SUSY effects at a gamma gamma Collider
We study the helicity amplitudes of the process gamma gamma to ZZ in the
Standard Model at high energy. These amplitudes receive contributions from the
W and charged quark and lepton loops, analogous to those encountered in the
gamma gamma to gamma gamma, gamma Z cases studied before. But gamma gamma to ZZ
also receives contributions from the Higgs s-channel poles involving the
effective Higgs-gamma gamma vertex. At energies of about 300 GeV, the
amplitudes in all three processes are mainly helicity-conserving and almost
purely imaginary; which renders them a very useful tool in searching for New
Physics. As an example, a SUSY case is studied, and the signatures due to the
virtual effects induced by a chargino-, charged slepton- or a lightest
stop-loop in gamma gamma to ZZ, are explored. These signatures, combined with
the analogous ones in gamma gamma to gamma gamma and gamma gamma to gamma Z,
should help identifying the nature of possible New Physics particles.Comment: 28 pages and 6 figures; version to appear in E.P.J.C e-mail:
[email protected]
The gamma gamma to gamma gamma process in the Standard and SUSY models at high energies
We study the helicity amplitudes of the process \gamma \gamma \to \gamma
\gamma at high energy, which in the standard and SUSY models first arise at the
one-loop order. In the standard model (SM), the diagrams involve W and charged
quark and lepton loops, while in SUSY we also have contributions from chargino,
charged sfermion and Higgs loop diagrams. The SUSY contributions are most
important in the region above the threshold for producing the supersymmetric
partners; since there, they interfere most effectively with the primarily
imaginary SM amplitudes. Simple expressions for the relevant 1-loop functions
are given, which provide a direct overview of the behaviour of the helicity
amplitudes in the whole parameter space at high energies. The various
characteristics of a large set of observables are studied in detail.Comment: 18 pages and 7 figure
The processes gamma gamma to Z H in SM and MSSM
The process gamma gamma to ZH first arises at the 1-loop level, and as such
it provides remarkable tests of the structure of the electroweak Higgs sector.
These tests are complementary to those in the gauge sector involving gamma
gamma to gamma gamma, gamma Z, ZZ. We show that in the Standard Model (SM)
where H=H_{SM}, as well as in the supersymmetric case where H=h^0, H^0 or A^0,
observables exist (like e.g. the energy dependence, angular distribution,
photon polarization dependence or final Z polarization) which present rather
spectacular properties. Such properties involve strong threshold effects with
steps, bumps or peaks, reflecting the type of Higgs and heavy quarks and
chargino masses and couplings predicted by the SM and supersymmetric models.Comment: 27 pages and 6 figures. e-mail: [email protected]
Neutralino-neutralino annihilation to gammaZ in MSSM
The 1-loop computation of the processes \tchi_i \tchi_j \to \gamma Z has
been performed at an arbitrary c.m. energy for any pair of MSSM neutralinos. As
an application suitable for Dark Matter (DM) searches, the
neutralino-neutralino annihilation is studied at the limiting case of vanishing
relative velocity, describing the present DM distribution in the galactic halo;
and at a relative velocity of about 0.5, determining the neutralino relic
density contributions. The most useful situation is obviously for , but
the case of non-identical neutralinos may also be useful in some corners of the
parameter space. Our results are contained in the FORTRAN code PLATONdmgZ,
applying to any set of real MSSM parameters. Numerical results are also
presented for a sample of 6 MSSM models, describing the various possible
neutralino properties. A comparison with other existing works is also made.Comment: 14 pages, 7 figures, Version to appear in Physical Review
Null Branes in Curved Backgrounds
We consider null bosonic p-branes in curved space-times. Some exact solutions
of the classical equations of motion and of the constraints for the null
membrane in general stationary, axially symmetrical, four dimensional, gravity
background are found.Comment: 19 pages, LaTeX, no figures. Extended version. To appear in Phys.
Rev.
Single Neutralino production at CERN LHC
The common belief that the lightest supersymmetric particle (LSP) might be a
neutralino, providing also the main Dark Matter (DM) component, calls for
maximal detail in the study of the neutralino properties. Motivated by this, we
consider the direct production of a single neutralino \tchi^0_i at a
high/energy hadron collider, focusing on the \tchi^0_1 and \tchi^0_2 cases.
At Born level, the relevant subprocesses are q\bar q\to \tchi^0_i \tilde g,
g q\to \tchi^0_i \tilde q_{L,R} and q\bar q'\to \tchi^0_i\tchi^\pm_j; while
at 1-loop, apart from radiative corrections to these processes, we consider
also gg\to \tchi^0_i\tilde{g}, for which a numerical code named PLATONgluino
is released. The relative importance of these channels turns out to be
extremely model dependent. Combining these results with an analogous study of
the direct \tchi^0_i\tchi^0_j pair production, should help in testing the
SUSY models and the Dark Matter assignment.Comment: 22 pages and 12 figures; version to appear in Phys.Rev.