Update of the search for charginos nearly mass-degenerate with the lightest neutralino

Abstract

The data collected by DELPHI in 1998 at the centre-of-mass energy of 189 GeV have been used to update the search for charginos nearly mass-degenerate with the lightest supersymmetric particle, which is assumed to be the lightest neutralino. Mass differences below ΔM=3 GeV/c² are considered. No excess of events with respect to the Standard Model expectation has been observed, and exclusions in the plane of ΔM versus chargino mass are given. The new ΔM independent lower limit on the mass of the chargino is 62.4 GeV/c² in the higgsino scenario (which includes the gaugino mass unification scenario), if all sfermions are heavier than the lightest chargino. In the approximation of large sfermion masses the limit is 59.8 GeV/c², independently of the field content.

Introduction

This paper updates the results of the search for charginos ($\text{χ}\text{̃}{}_1{}^{\mathrm{\pm }}$) nearly mass-degenerate with the lightest neutralino ($\text{χ}\text{̃}{}_1{}^0$) reported in Ref. [1], with the data collected by DELPHI in 1998 at the centre-of-mass energy of 189 GeV.

The experimental techniques used depend on the mass difference ΔM between the chargino and the lightest neutralino (assumed to be the Lightest Supersymmetric Particle, LSP), as described in Ref. [1]. When ΔM is below the mass of the pion, the chargino lifetime is typically long enough to let it pass through the entire detector before decaying. This range of ΔM can be covered by the search for long-lived heavy charged particles. For ΔM of few hundred MeV/c² the $\text{χ}\text{̃}{}_1{}^{\mathrm{\pm }}$ can decay inside the main tracking devices of DELPHI. Therefore, a search for secondary vertices or kinks can be used to explore this region. With increasing mass difference, the mean lifetime falls and it becomes difficult to distinguish the position of the $\text{χ}\text{̃}{}_1{}^{\mathrm{\pm }}$ decay vertex from the initial interaction point. In this case, the tagging of a high energy Initial State Radiation (ISR) photon can help in exploring the ΔM region between a few hundred MeV/c² and 3 GeV/c².

Compared to Ref. [1], the search using a tagged ISR photon has been improved with the use of an additional cut and a wider range of mass differences between the chargino and the lightest neutralino explored. Moreover, the selection cuts were optimized for each point in the plane , depending on the kinematics of the signal in that point. This significantly improved the sensitivity in a region of the space of the SUSY parameters that will probably never be covered by the searches at hadron machines [2].

All the new data have been combined with the samples already used in Ref. [1]. In the search which uses ISR, all the old data-sets have been re-analysed according to the new prescriptions.

Three SUSY scenarios were considered, depending on the values of the SU(2) gaugino mass M₂, the U(1) gaugino mass M₁, the Higgs mixing parameter μ, and the mass of the sneutrino $\text{M}{}_{\mathrm{<mtext>\nu </mtext><mtext>̃</mtext>}}$:

• M_1,2≫|μ| (higgsino-like);

• |μ|≫M₁≥M₂ and heavy sneutrino (gaugino-like);

• |μ|≫M₁≥M₂ and light sneutrino (gaugino-like).

Gauginos couple to $\text{ν}\text{̃}$, thus heavy and light sneutrinos define two phenomenologically different gaugino scenarios, with different cross-sections (because of the possible chargino production through $\text{ν}\text{̃}$ exchange in the t-channel), lifetimes and branching ratios. In the following, in the heavy sneutrino scenario $\text{M}{}_{\mathrm{<mtext>\nu </mtext><mtext>̃</mtext>}}\text{>500}$ GeV/c² is assumed. In all other cases the assumption is .

Also the charged sfermions couple to the gauginos, and if light they can modify the lifetimes and the branching ratios considered. In the following, the heavy charged sfermions approximation will be used for the two gaugino scenarios, while for the higgsino it is enough to consider for all sfermions.

A charged gaugino (cases 2 and 3) can get a mass GeV/c² only if the constraint of gaugino mass unification at the GUT scale, implying the electroweak scale relation $\text{M}{}_1\text{∼1/2}\text{M}{}_2$, is released. Several interesting scenarios without gaugino mass unification or with near mass-degeneracy between the lightest supersymmetric particles have been proposed [3], [4], [5], [6], [7], and all of them can be studied by using the techniques reported in the present paper.