|
|
S e a r c h e s |
|
|
|
Chargino at 161+172 GeV |
The channel under investigation is :
- e+ e- --> Chi+ Chi-,
with Chi+- --> Chi01 + f-fbar
where the Neutralino is assumed to be stable (LSP)
The results for a Neutralino decaying into Gravitino + gamma are reported
in the page
Gravitino.
Chargino selection
Chargino detection efficiencies versus (Mcha-Mneu) : (preliminary)
|
| stable neutralino
|
The efficiencies estimated from Monte Carlo simulation at a center of
mass energy of 172 GeV.
The plots show respectively the efficiency for the JJl, the JJJJ, the ll
channels and the total. It includes also the detection efficiency in case of
the second to lightest chargino production and in case the decay to second to lightest
neutralino is open.
The total efficiency is computed by summing the three modes.
The figure is for a stable neutralino.
Selected events and background estimates : (preliminary)
Two different situations are considered :
- the non-degenerate case, where the mass difference between chargino
and neutralino is large, Mcha-Mneu >10 GeV or Mcha-Msneu >10 GeV if the sneutrino
is lighter than the chargino;
- the degenerate case, where 5 < Mcha-Mneu < 10 GeV.
|
| 161 GeV (preliminary)
| 172 GeV (preliminary)
|
|
| jjl
| jets
| ll
| jjl
| jets
| ll
|
| Non-degenerate selection
|
| Candidates
| 0
| 2
| 0
| 0
| 0
| 1
|
| Background
| 0.03+-0.01
| 1.80+-0.3
| 0.39+-0.06
| 0.20+-0.05
| 1.40+-0.2
| 0.80+-0.2
|
| Degenerate selection
|
| Candidates
| 0
| 0
| 0
| 0
| 1
| 0
|
| Candidates
| 0.47+-0.07
| 0.53+-0.08
| 0.51+-0.08
| 0.28+-0.04
| 0.27+-0.05
| 0.32+-0.07
|
Examples of selected chargino candidates :
- r068301e001680, unbalanced q qbar at 161 GeV
| 
| 
|
| with shading
| xy view
| zy view
|
Unbalanced q-qbar event which was selected as candidate for neutralino and
for chargino.
The two-jet invariant mass, corrected for particle losses,
is (72+-11) GeV/c**2 and the recoil mass (59+-26) GeV/c**2.
The most likely interpretation of this event is a t-channel (W e nu),
where the electron is lost, and of which 0.6 events are expected.
The event shows a clear 2-jet topology. This, in fact, excludes the
interpretation as chargino. It was kept as candidate because the explicit
4-jet requirement was not part of the selection criteria.
Cross-section and mass limits
Chargino cross section in the MSSM versus its mass : (preliminary)
|
The exclusion limits on the mass are derived from the 95% C.L. upper
limit on the cross section.
The cross section as function of the chargino mass has been computed by
varying uniformly the SUSY parameters M and Mu in the intervals
0 < M <800 GeV and -400 < Mu <400 GeV for values of tan(beta)
equal 1, 1.5 and 35.
In the upper figure, limits are given for the non-degenerate case
Mcha-Mneu >10 GeV or Mcha-Msneu >10 GeV if the sneutrino
is lighter than the chargino.
Two possibilities were assumed for the sneutrino mass, a light sneutrino
with 41 < m_snu < 100 GeV and a heavy sneutrino with
m_snu > 300 GeV.
The lower cross section in case of a light sneutrino is due to a
negative interference with the sneutrino exchange diagram.
The lower figure is for the degenerate case of Mcha-Mneu =5 GeV.
In this scenario the chargino is Higgsino-like, which leads to a strong
suppression of its production via sneutrino exchange.
The behaviour of the cross section is therefore the same as for a heavy
sneutrino.
Limits on Chargino mass and Cross-sections : (preliminary)
Lower limit on the mass and upper limit on the cross-section at the 95% C.L.
in regions of the chargino-neutralino mass difference, Dm,
and of the s-neutrino mass, Msnu
|
| Stable neutralino (preliminary)
|
|
| Mcha1 (GeV)
| X-sect (pb)
|
| Dm>10, Msnu>300
| 84.3
| 0.82
|
| Dm>10, 41<Msnu<300
| 67.6
| 0.65
|
| Dm=5, any Msnu
| 80.3
| 1.84
|
The results for (Dm>10, Msnu>300) are based solely on 172 GeV data;
the other results use all the LEP high energy data.
Region excluded in the (Mcha,Mneu) plane from Chargino search : (preliminary)
at the 95% C.L. for tan(beta) = 1,1.5,35.
The region in blue is excluded when it
is heavy, m_snu> 300 GeV.
The white region inside the triangle is kinematically allowed but cannot be
excluded at the
95% C.L. and the region outside is kinematically or theoretically (dashed)
not reachable .
MSSM exclusion regions
Regions excluded in the (Mu,M) plane from Chargino and Neutralino searches : (preliminary)
at the 95% C.L. for values of tan(beta) = 1.0, 1.5, 35.
A special effort was spent to exclude the region (Mu<0, M2=-Mu, Mu->0, small tan(beta)).
In that region the second to lightest chargino production and the decay to the second to lightest
neutralino are possible. The second to lightest neutralino disintegrates dominantly in one photon plus
the lightest neutralino. In addition we have that
MChi+- 1,2 = MW+-
and MChi01,2 = 0.
The final state is then indistinguishable from
W+W- production.
The analysis used to exclude those regions are the following:
- single photon production at LEP1;
- WW anomalous production;
- standard chargino and neutralino searches;
- radiative chargino search (look in the gravitino section).
Mass limit on the lightest Neutralino : (preliminary)
We can translate the excluded region (Mu,M,tan(beta)) space in a limit on the lightest neutralino.
The value we obtain in the case of heavy sfermions and for any value of tan(Beta) is the following:
REFERENCES :
The latest results on this analysis are :
Search for charginos, neutralinos and gravitinos at LEP
Paper submitted to the HEP'97 Conference, Jerusalem, August 19-26,
paper #348.
(PS)
Earlier results were presented by :
F.Richard,
DELPHI Results at 172 GeV,
Cern seminar, 25/02/97.
P.Rebecchi,
Sparticles Search at LEP2 in DELPHI,
Cern seminar, 09/12/96.
