S e a r c h e s
		Search for charginos mass degenerate with the LSP

The channel under investigation is :

• e⁺ e^- --> Chi⁺ Chi^-, with Chi^+- --> Chi⁰₁ + f-fbar

Event selection

• For extremely low Delta(M) (typically below 400-500 Mev/cē) the chargino gets a lifetime long enough to pass through the whole detector without decaying (and it can be detected by its anomalous dE/dx or by the RICH veto) or to decay inside the mean tracking devices (search for kinks).
• For 0.5<Delta(M)<3 Gev/cē we search for charginos produced after the emission of a photon from the initial state. Requiring the presence of this photon, although strongly reducing the efficiency obtainable for the signal, is necessary in order to kill the gamma-gamma background and also to raise the trigger efficiency, since the charged particles originated in the chargino decay carry a very low momentum at small Delta(M).

Production and decay of mass-degenerate charginos

• M2>>|mu|: the lightest chargino and neutralino are both higgsino. The relation between M1 and M2 can be whatever (in particular, also the GUT relation M1~1/2*M2).
• |mu|>>M2: the lightest chargino and neutralino are both gaugino. Mass degeneracy can be present only if M1>=M2. When the chargino is a gaugino, its cross-section and decay width (i.e. lifetime) depend on the mass of the sneutrino: in correspondence of sneutrino masses close to the chargino one the cross-section diminishes, and so do the lifetime (that is, also the probability to identify it as a quasi-stable particle diminishes).

Degenerate chargino cross section in the MSSM versus its mass :



In the figure is shown the expected cross-sections for e⁺ e^- --> Chi⁺ Chi^- at the four centre-of-mass energy used in the analysis and when the difference between the masses of the chargino and of the lightest neutralino is below 3 GeV/cē. The upper bands refer to the gaugino-like scenario; the lower bands to the higgsino-like scenario. The width of the two bands allows for a variation of the parameters within the intervals: 0.5<Delta(M)<3 GeV/cē; 1<tan(beta)<50; M(snu)>500 GeV/cē (gaugino) or M(snu)>M(chi) (higgsino).

Chargino branching ratios and lifetime versus Delta(M) :



Branching ratios (a) and lifetime (b) of a 50 GeV/cē chargino decaying into a neutralino plus standard particles, as a function of the mass difference between the chargino and the lightest neutralino (LSP). The sneutrino mass has been set to 1 TeV/cē, to avoid the contribution of the sneutrino in the decay. The branching ratios of SUSYGEN 2 (dashed line) are compared to those obtained with a dedicated analytical calculation (full line) and they differes only in a small intervall around the Delta(M) corresponding to the production of the hadronic resonances.

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Mass limits

Limits obtained with the lifetime methods : (preliminary)



Regions in the plane (M(chi) , Delta(M)) excluded at the 95% C.L. by the search for charginos with a visible decay length. (a) Higgsino-like scenario. (b)(c) Gaugino-like scenario, with the sneutrino mass set to 500 GeV/cē (b) or left free to vary above M(chi) + 2 GeV/cē and M(chi) + 1 GeV/cē (c).

Limits obtained with the ISR method : (preliminary)



Lower limit at the 95% CL of the difference between the number of chargino events expected and the ones allowed in the data (it depends on the candidates found and on the background computed), as a function of the chargino mass. Whenever this limit is above 0, the point can be excluded at 95% C.L.

Combined limits obtained by DELPHI in the search for charginos mass degenerate with the lightest neutralino : (preliminary)



The figure summarizes the previous two plots and compares these results with the ones obtained with the 'standard' search for charginos at larger Delta(M). The three plots represent:

• Higgsino. The low cross-section does not allow to use the ISR to improve the limits already set at LEP1. The narrow green band shows how it is possible to improve the limit of the 'traditional' search for charginos in correspondence of Delta(M)=3 GeV/cē, by combining it with the search which uses ISR.
• Gaugino + heavy sneutrino. It is the most favorable case. Here too, the narrow green band above the ISR region shows the combined limit at Delta(M)=3 GeV/cē.
• Gaugino + light sneutrino. The low cross-section does not allow to use the ISR to improve the limits already set at LEP1. In correspondence of any chargino mass, the mass of the sneutrino has been let vary, but only for values of the mass greather than (mass of the chargino + 2 GeV/cē): for lower sneutrino masses, the lifetime shorten quickly, thus removing any significance also to the limits obtained with the decay length methods.