Update of the search for charginos nearly mass-degenerate with the lightest neutralino
Introduction
This paper updates the results of the search for charginos () nearly mass-degenerate with the lightest neutralino () 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 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 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 :
- 1.
M1,2≫|μ| (higgsino-like);
- 2.
|μ|≫M1≥M2 and heavy sneutrino (gaugino-like);
- 3.
|μ|≫M1≥M2 and light sneutrino (gaugino-like).
Gauginos couple to , thus heavy and light sneutrinos define two phenomenologically different gaugino scenarios, with different cross-sections (because of the possible chargino production through exchange in the t-channel), lifetimes and branching ratios. In the following, in the heavy sneutrino scenario 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 , 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.
Section snippets
Data samples and event generators
The DELPHI detector is described in [8]. The integrated luminosity collected by DELPHI at 189 GeV was approximately 158 pb−1, out of which 155.3 pb−1 were used in the searches for long-lived particles and 152.9 pb−1 in the search for soft particles accompanied by an ISR photon.
SUSYGEN [9] was used to generate all signal samples and to calculate cross-sections. The decay modes of the chargino when ΔM<2 GeV/c2 were modelled using the computation of [3], while the widths given by SUSYGEN were used
Search for heavy stable charged particles
The results of the search for heavy stable charged particles at 189 GeV are described in Ref. [18], where all the details on the techniques used and on the efficiency can be found. The efficiencies for selecting heavy stable particles presented there were then convoluted with the expected distribution of the decay length of the chargino in a given scenario, in order to derive an event selection efficiency for long-lived charginos as a function of their mass and lifetime. One event was selected
Search for charginos with ISR photons
With respect to the analysis described in Ref. [1] a new variable was taken into account to better discriminate between nearly mass-degenerate charginos and the dominant two-photon background. This variable is the ratio between the missing transverse momentum (PTmiss) and the visible transverse energy (ETvis) in the event. Another improvement in the analysis at 189 GeV is that for ΔM<1 GeV/c2 the requirement of at least two charged tracks consistent with coming from a common primary vertex was
Limit on the mass of nearly degenerate charginos
The results of the searches for long-lived charginos and for soft particles accompanied by a high pT photon at 189 GeV have been combined with the results of the searches at lower energies to obtain the excluded regions in the plane (,ΔM) shown in Fig. 5. The same figure shows, for comparison, also the region excluded by the independent search in DELPHI for charginos with larger ΔM [20].
By simply superimposing the regions excluded with the search for long-lived charginos and the regions
Conclusions
Charginos nearly mass-degenerate with the lightest neutralino were searched for in DELPHI using the data collected at 189 GeV. Two different searches for long-lived charginos were complemented with a search for nearly mass-degenerate charginos that exploits the tag of an ISR photon. An improved selection was used for the search with the ISR photon tag, as compared with the previous analysis. No evidence of a signal was found. The results of the searches at 189 GeV were combined with those
Acknowledgements
We thank M. Drees for valuable comments and suggestions. We are also greatly indebted to our technical collaborators, to the members of the CERN-SL Division for the excellent performance of the LEP collider, and to the funding agencies for their support in building and operating the DELPHI detector. We acknowledge in particular the support of Austrian Federal Ministry of Science and Traffics, GZ 616.364/2-III/2a/98, FNRS–FWO, Belgium, FINEP, CNPq, CAPES, FUJB and FAPERJ, Brazil, Czech Ministry
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Now at DESY-Zeuthen, Platanenallee 6, D-15735 Zeuthen, Germany.