Minutes of SUSY meeting Thursday 13 april
2000
More news on stau "excess", updates on new analysis, and organization of SC and
y2K online analysis.
More on stau's (F. Hoelldorfer):
Frank presented the checks he made on the stau "excess".
-The number of stau candidates from ECM of 192, 196, 200 and 202 GeV
is the following for the low DM selection, the high DM selection and the
logical .and. between the two regions:
low DM
.and. high DM
data
10
20
16
bg subs. (=WW bg)
8.6
11.
6.7
bg non subs.
1.7
2.9
3.1
-Frank showed the comparison between data and MC for:
1. the tau ID (at high and at low DM), using loose and tight cuts.
2. the tau mass (at high and at low DM).
3. the tau energy (at high and at low DM).
-> Conclusion: the excess does not show up in a particular bin/region.
-Using loose stau pre-cuts, a comparison between data and MC is presented.
It shows a bad agreement: the MC has too many bhabha events. However, it is not
too serious as the bhabha events are not the main background after all
selection cuts.
- Comparison with the chargino analysis:
the number of candidate for the channels e-e, mu-mu, e/mu-tau,
tau-tau and tau-tau (tight) is:
for ECM=196: 87 candidates, for 84.5 background events.
for ECM=200: 97 candidates, for 95.4 background events.
for ECM=202: 39 candidates, for 44.2 background events.
In the particular tau-tau channel, with purely hadronic tau (which contains
only few event) the description data/MC is fine at 196 and 202 GeV.
For ECM=200 GeV, a slight excess is observed, however the candidates lie in the
background region in the plane (p1, p2).
Status report from the LEPSUSYwg: "the stau excess" (F.Cerutti):
Fabio presented a status report from the LEPSUSYwg on the stau excess, and
the perspectives (possible strategies) for data taking of 2000.
- The excess:
For the combined 4 LEP experiments (ECM = 189 -> 202 GeV), the
number of candidates and background events, and the excess probability was
presented, in the M_chi, M_stau plane.
To quantify the excess, a Gedanken experiment was performed, using the
following estimator: E= fraction of point with CL (excess)<0.002.
The probability P = N(E>E_data)/N_tot = 0.001.
Using the Aleph data only, this probability is 0.07.
-the reference point in plane (M_stau, M_chi):
Using the stau (right) cross section, the best stau mass compatible with the
excess is M_tau(right) = (86.7 +- 1.4 ) GeV (for a chi mass in the range 15-35 GeV).
The point M_tau=85 GeV, M_chi=20 GeV is then used as the "reference" point.
For this point, the number of candidate is 226, the number of background events
is 179.5. The Probability (bg, N>Ncand)= 0.0005.
At the reference point, all four experiment observe an access of events.
Using the estimator defined above, for ALEPH data only, the compatibility of the
excess with the signal is estimated at the reference point:
Probability(bg+signal)=0.47 (rappel: Probability for the background only = 0.07).
-Study of the systematics:
The background is mostly the 4fermions processes (around 90%),
then the gamma-gamma-lept-lept and the tau-tau (gamma) processes.
The systematics on the background is around 2-4% (mainly MC statistics).
If 4% is added to the background at the reference point, the
probability (bg, N>Ncand)= 0.003
(and the Gedanken experiment for the full M_tau,M-chi plane,
with background+1 sigma systematics gives P = N(E>E_data)/N_tot = 0.012
- Comparison data/MC:
The 4-vector of the candidates of the
4 LEP experiments were exchanged in order to make further checks.
The distribution of E_max, E_min, Mass_tau_tau, Pt_miss, acollinearity,
E_max versus E_min, M_tau1 versus M_tau2, tau id1 versus tau id2 were presented.
The excess is more concentrated at low E2 and high (medium) E1
indicating two taus with asymmetric energies.
No particular behavior is found in the other distributions.
- Comparison with the charged Higgs searches:
The number of candidate is (for ECM from 192 to 202 GeV):
16 for H+ only, 10 for stau only, 20 for H+.and.stau.
-> the two analysis are not incompatible.
The background events have still to be checked.
There are 9 events selected by the stau analysis
and not by the H+ analysis, these event have a
not identified energetic lepton -> this need to be checked.
- Perspective for y2K run:
Using the estimation of the LEP performance by Patrick, 18
different configurations were presented: taking different tau masses,
with or without mini-ramps, with or without 2 weeks extension,
with or without 6 months extension.
In each case the signal/sqrt(bg) is given as a function of the
energy beam in 2000.
At the July LEPC, significance could increase from the actual 2.9 sigma
to 3.3-3.5 sigma (assuming a successful mini-ramp run).
A 4 to 5 sigma significance is reachable if the emphasis to the
luminosity (run at ~200 GeV) is given and if 6 months
extension is obtained.
Status of absolute selectron limit (M.Azzouz):
Mustapha presented a status report on his work on the absolute selectron limit.
In order to give an absolute limit on selectron mass, two pathological
regions were found, called problem I and problem II (see minutes of the
SUSY meeting Thursday 27th January).
Mustapha investigated the problem II, where M(selectron_right) > m (chi_2).
The neutralino multi-lepton analysis is used to close some
of the unexcluded regions.
A new set of cuts is defined in order to select topologies:
chi2 -> chi1 + l + l.
Using the preliminary results of this analysis, the corridor
between chargino 2body and 3body decay can be mostly excluded.
Further studies still need to be done: include all the energies
and applied to various tan(beta) and mu.
The problem I is investigated by Ulla.
A medium DM analysis for selectron (F.Hoelldorfer):
Frank presented an optimization of the mass depending cut using the
acollinearity, for the selectron searches.
In order to give a lower possible limit on the LSP, it is important
to have a very efficient search for slepton at medium DM. The cut for the
slepton searches were then optimized: the old mass dependent cut
( -1 < cos(alpha) < 1) is now optimized using the information of the acollinearity.
and the following cut: kin > -0.2, is added to the cut on alpha.
This value of the cut was obtained by doing a scan on the cos(alpha)
and the kin variables and minimizing N_95.
This new selection reject more background events with keeping all the
signal events.
The plan is to use this cut also for the muon searches.
Status of stau mixing analysis (J.Vollinga):
Jens presented a status report of his study of the impact of the stau mixing
on the chargino searches.
He presented a dedicated analysis optimized for:
chargino +- -> slepton neutrino, with
slepton -> lepton neutralino.
This canal is relevant in the low m_0 region or in the
regions with large mixing.
The basic ACOLZ cuts are used and the sliding-cuts are optimized
as a function of DM=(m_slepton-m_chi_0) and dm=(m_chi-m_chi_0).
The efficiency of the selection varies with DM, but not much with dm.
Using the results of this analysis the exclusion plot was presented
for 4 different mixing angles in the plane m_chi versus m_sneutrino,
the LSP limit being constrained for large mixing angle.
To do list: for the LSP limit a dedicated neutralino search is
probably needed, and the case of the mixing angle > 45 degrees
has to be investigated.
Status of the MSSM paper (G.Ganis):
Gerri presented the organization of the paper containing the
MSSM results.
The starting point of the paper is the Moriond conf. note.
The neutralino and the higgs analysis need to be included,
as well as an interpretation of the results in MSUGRA.
The mass exclusion plot was presented using the slepton, chargino,
higgs and Z width data, in the plane m_1/2 versus M_0.
The plan is to have a first draft by may 15th.
Sbottom at very low DM (G. Sguazzoni):
Giacomo presented the result of the following investigation:
can the stop at low DM analysis be used also for the sbottom ?
Giacomo remind us first the main results of the stop paper:
model independent limit: 59 GeV
MSSM limit: 63 GeV
The draft 1 is ready.
In order to investigate the low DM analysis for the sbottom,
3 region are defined:
1. DM > m_B
sbottom -> b chi
2. m_K < DM < M_B
sbottom -> s chi
3. DM < m_K
sbottom -> d chi
The cases 1 and 3 are covered by the no lifetime and the stable
sbottom analysis respectively. The channel sbottom -> s chi is investigated.
For the no lifetime case, there is no sensitivity down to DM=M_K,
leading to no model independent limit.
Using the MSSM relation, the width of the sbottom is compared to
the width of the stop (using approximate formulae).
The width Gamma = epsilon**2 times a phase space factor.
The ratio of epsilon_sbottom / epsilon_stop is presented
as a function of tan(beta): it is >> 1 at small tan(beta) and
of the order of 1 for Tan(beta)=35. The width of the sbottom is then
typically larger than the width of the stop.
So the life-time case for the channel: sbottom -> s chi is only for high
tan(beta).
The setting of the absolute limit is then not straightforward.
More work is needed and there is no time to include the sbottom
investigation in the stop at low DM paper.
SUSY results for the summer 2000 conferences (G. Taylor):
Gary recalled the already submitted results to winter 2000.
The analysis that will be send to the summer conferences were
listed:
MSSM: absolute slepton mass limit
MSSM: effects of stau mixing on chargino searches
GMSB: cascade decay
OTHER: chargino searches at very low mass difference.
OTHER: goldstino searches
OTHER: SUSY search at 20X GeV: a single paper with update of all
existing analysis with new data.
The organization of the run up to summer conference and
the responsible for each activities were finally presented.
Search for charginos nearly mass degenerate
with the lightest neutralino (N. De Filippis):
Nicola presented his results on the search for chargino at very low DM
(DM=M_chargino - M_neutralino), (typically DM < ~5 GeV).
The very small DM region is possible in:
- MSSS: if there is large
M_1,2 scenario (higgsino region), or a low M_2 scenario (gaugino region)
relaxing the relation between M_1 and M_2 due to gaugino mass unification
at GUT scale.
-in AMSB: the small DM is always possible due to the ratio M_1/M_2 >> 1.
The chargino production cross section was presented as a function of the
chargino mass for 3 values of m_0 and tan(beta)=1.41 and mu=-1 TeV.
The chargino decays in heavy neutralino (for DM < ~ 5 GeV):
chi+ -> W chi_0 and W-> f fbar.
The difficulty in studying such a topology is the low visible
energy in the final state.
The existing limit on the chargino mass is ~M_Z/2.
-Chargino decay at low DM:
The hadronic and the leptonic branching ratio of the chargino was presented
as a function of DM for M_chargino=92 GeV, mu=-900 GeV and tan(beta)=1.4.
For the hadronic decay, at DM>2 GeV, one can use perturbative QCD to describe
the hadronic final state, while for DM<2 GeV, exclusive hadronic final
states is used. In the last case, the parametrisation of Kuehn and
Santamaria is used to describe the pionic current (with the assumption
that the rho and the a_1 propagators dominate).
The Kuehn and Santamaria parametrisation was tested using the data for
tau decaying into neutrino and 2 pions and for tau decaying into neutrino and
3 pions. The distribution of the pion invariant mass was presented and
is well described by the parametrisation (with DM=m_tau).
-Topologies:
Depending on the mean decay length and on the available energy,
3 different topologies can be observed:
1. for DM < m_pion: long lifetime chi+_1, leading to strongly ionizing particles
deeply into the detector.
2. for m_pion < DM < 5 GeV: to ensure the trigger of the event, one need to ask for
a ISR signature.
3. for DM > 5 GeV: this region is covered by the chargino standard analysis.
-Comparison SUSYGEN and Kuehn-Santamaria parametrisation:
The MC events from SUSYGEN are compared to the prediction of the
Kuehn-Santamaria parametrisation, in the case of chi+_1 -> chi0_1 pion+ pion0,
and in the case chi+_1 -> chi0_1 + 3 pions.
The distribution of the pion invariant mass for the MC events was
presented together with the Kuehn-Santamaria parametrisation for DM=1.5 GeV,
mu=-900 GeV tan(beta)=1.4 and m_0=1 TeV.
A clear discrepancy is observed, showing difficulty in managing small invariant mass
system. This will be corrected.
- The ISR analysis:
This analysis is based on the single photon Gary's analysis.
A photon with E > 1 GeV is required with cos(theta) < 0.95,
and transverse energy > 0.034*sqrt(s).
From 2 to 10 charged tracks is required,
no |d_0| and no |z_0| condition is asked.
Cuts on visible energy, visible p_t, missing mass, and theta_isol
are applied.
After all the cuts, the MC backgrounds describe well the data.
The selection efficiency for ISR photon + visible energy, for
the chargino mass of 66 GeV was presented as a function of DM
(DM in the range 0.2-5 GeV). The efficiency varies between 0.005 and 0.015.
The SM background expectation is 1.3 events (for ECM from 189 to 202 GeV)
coming mainly from ee-> gamma nu nubar (0.77 event).
The observed number of event is 2 in at ECM = 189 GeV, and 1 at 196 GeV.
The excluded region up to ECM=202 GeV was shown in the gaugino and in
the higgsino region. The expected and observed limit is given in
the DM and chargino mass plane.
-plans:
Sliding cuts need to be implemented and the results will to be
combined with the standard chargino analysis .
Submitted by: Barbara Clerbaux