1. SUSY Searches with ZEUS
Alessandro Montanari (DESY) on the behalf of the ZEUS Collaboration
Overview
Phyisical motivations
Stop production (MSSM and mSUGRA)
Gaugino production (MSSM)
Gravitino production (GMSB)
Summary and conclusions
04/04/2019
A. Montanari DIS2006 Tsukuba

2. A. Montanari DIS2006 Tsukuba
SUSY at HERA
New discrete, multiplicative, quantum number: R-parity
R=(-1)3B+L+2S ( +1 SM particles, -1 sparticles)
If R-parity is conserved:
Sparticles produced only in pairs
Lightest Supersymmetric Particle (LSP) candidate as dark matter
The most general Lagrangian does not exclude R-parity violation
Possibility to produce single sparticles
The RPV superpotential:
Lepton-Hadron Colliders
At HERA
interactions mediated
by l´1jk coupling
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A. Montanari DIS2006 Tsukuba

3. A. Montanari DIS2006 Tsukuba
Stop Search
Stop produced as a resonance in the
s channel via the l´131 coupling
To simplify the decay phenomenology:
only l´131 different by zero
not considered decays with a top quark in the
final state
Three main decays cosidered:
Direct decay via l´131 : e+-Jet
Gauge decay: e+-Multiple-Jets, n-Multiple-Jets
Two SUSY models considered:
MSSM: GUT relation between M1, M2, M3 (100 GeV < M2 < 300 GeV),
-300 GeV < m < 300 GeV, tanb=6, mass of other sfermions ~ TeV, heavy gluino.
mSUGRA: only four parameters and one sign
m0, m1/2, A, tanb and m s
04/04/2019
A. Montanari DIS2006 Tsukuba

4. Stop search: Preselection for e+ final state
ZEUS: 65 pb-1 e+p
One high energy positron
qe>0.3: Pt > 8GeV
qe<0.3: Pt > 20 GeV
Q2DA > 1000 GeV2
0.2 < yDA < 0.98
45 GeV < E-Pz < 70 GeV
MeX > 100 GeV
Non ep events rejection
(cosmic, halo-m, beamgas)
Major source of background: NC DIS
Good agreement data and SM prediction
Red line: Mstop=220 GeV
m=1000 GeV, M2=200 GeV, l´=0.1
04/04/2019
A. Montanari DIS2006 Tsukuba

5. Stop search: Preselection for n final state
No positron
Pt > 20 GeV
0.2 < yJB < 0.95
MnX> 100 GeV
Non ep events rejection
(cosmic, halo-m, beamgas)
Major source of background: CC DIS
Good agreement data with SM prediction
Red line: Mstop=220 GeV
m=1000 GeV, M2=200 GeV, l´=0.1
04/04/2019
A. Montanari DIS2006 Tsukuba

6. Stop search: final selections
Invariant mass distributions
For each decay channel, final selection
to minimize the limits on the cross section
e+-Jet
Q2DA > 3000 GeV2
yDA > *MeX
Pt,had/Et,had > 0.8
Efficiency ~0.2 – 0.55
e+-Multiple-Jets
Pt,had/Et,had < 0.8
Efficiency ~0.25 – 0.5
n-Multiple-Jets
E-Pz > 25 GeV
Pt,had/Et,had < 0.4
Efficiency ~
No evidence of resonances
04/04/2019
A. Montanari DIS2006 Tsukuba

7. Stop search: limits on l´131 on MSSM
Limits calculated using a Bayesian approach
Limits on l´131 are evaluated as a function
of the stop mass and in a range of SUSY
parameters:
100 GeV < M2 < 300 GeV
-300 GeV < m < 300 GeV
tanb=6
Yellow: exclusion at 95% CL
Red: partially excluded by the SUSY
parameters
Limits are compared to APV results:
tighｔer limits for masses up to 250 GeV
For l’131 =0.3 scenarios with a stop of
mass 270 GeV are ruled out
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A. Montanari DIS2006 Tsukuba

8. Stop search: limits on mSUGRA model
Exclusion at 95% C.L. in (m0,m1/2):
A=0, tanb=6, m<0
l´=0.3
Red: excluded at 95% C.L.
Blue: theoretically not allowed (REWSB
have no solution)
Yellow: scenarios where neutralino (LSP)
has mass lower than 30 GeV
(already excluded by LEP)
Dotted lines: curves of constant stop
mass 250 GeV and 260 GeV
Excluded scenarios with stop with
mass less than 260 GeV
NEW PRELIMINARY RESULTS!
04/04/2019
A. Montanari DIS2006 Tsukuba

9. Gaugino and gravitino search
If Msquark >> Mslepton: gaugino production in t channel via l´ijk
Possible decay channels:
Main contribution from up quark: only l´111
NC-like channel
electron
multiple jets
CC-like channel
neutrino
Gravitino channel
isolated photon
missing transverse energy
MSSM, mSUGRA
Neutralino is LSP
GMSB
Gravitino is LSP
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A. Montanari DIS2006 Tsukuba

10. Gaugino production: limits
No deviations from SM are observed in high discriminant region
Limits in m-M2 plane (red zone) compared to LEP limits (green zone)
Parameter scan:
|m| < 500 GeV
M2 < 250 GeV
Mc± ≤103 GeV
Modified frequentist method (T. Junk, NIM A434, p.435)
Multi-variate discriminant method
ALEPH/DELPHI limits from
search of chargino pair
production:
(J.Abdallah et al. Eur. Phys. J.
C37 (2004) )
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A. Montanari DIS2006 Tsukuba

11. A. Montanari DIS2006 Tsukuba
Gravitino production
Production mechanism
c produced in t channel via eL exchange
c (NLSP) decays into G (LSP) + g
Event topology
missing Pt (undetected G)
isolated g
at least one jet
RPV coupling involved
e+p: l´1j1, ,j=1,2
e-p: l´11k, ,j=1,2,3
Main SM background
CC DIS events with g ~ ~ ~ ~ ~ ~ l’
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A. Montanari DIS2006 Tsukuba

12. Gravitino production: event selection
Selection cuts
Q2JB > 700 GeV2, yJB>0.1
E-Pz < 65 GeV
≥ 1 jets (Pt > 6 GeV,
-0.5 < hjet < 2.5)
Non ep events veto
Signal efficiency: 66-79%
Good agreement data and
SM expectation
ZEUS: 65 pb-1 e+p
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A. Montanari DIS2006 Tsukuba

13. Gravitino: final selection
≥ 1 isolated g
Eg > 4 GeV
-2.8 < hg < 2.8
Signal efficiency: 59-71%
Multi-variate discriminant method
Signal: m(c0)= GeV, Dm=40 GeV
l´=1.0
Cut: D>0.7
04/04/2019
A. Montanari DIS2006 Tsukuba

14. Gravitino production: limits
No deviation from the SM has been observed
Limits in m(co)-Dm plane、obtained　using a frequentist approach (T.Junk,　Nucl., Inst., Meth. A 434, 435 (1999) )
Comparison vith H1 limits (H1 Collab., Phys. Lett. B616 (2005) 31-42): compatible results
H1 results
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A. Montanari DIS2006 Tsukuba

15. Summary and conclusions
Searches for SUSY particles (stop, gaugino and gravitino) have been performed by ZEUS using the HERA I data set.
No deviations from the Standard Model have been found in the explored parameter spaces
Limits have been set for:
Stop search: l´ 131 as a function of stop mass (MSSM) and exclusion region in the plane (m0,m1/2) (mSUGRA)
Gaugino search: improved the LEP limits in the m-M2 plane
Gravitino search: exclusion limits for m(co)-Dm plane
Taking new data from HERA II (expected 500 pb-1 at the end of HERA operation) → much more statistic
HERA has still something to say before LHC
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A. Montanari DIS2006 Tsukuba

16. A. Montanari DIS2006 Tsukuba
Backup slides
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A. Montanari DIS2006 Tsukuba

17. A. Montanari DIS2006 Tsukuba
HERA and ZEUS
Protons at 820/920 GeV
e- / e+ at 27.5 GeV
√s=300/318 GeV
Equivalent to 50 TeV
on fix target
Luminosity:
1.8x1031cm-2s-1
H1(ep)
ZEUS
ZEUS:
HEP multi-purpose detector
tracking detectors
calorimiter uranium
muon chambers
collected L=121 pb-1 during HERA I
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A. Montanari DIS2006 Tsukuba

18. Gaugino production: event selection
ZEUS data 96-00:121 pb-1
Q2JB>100 GeV2, yJB > 0.4
45 GeV < E-Pz < 62 GeV
Et > 75 GeV
≥ 2 jets (Et,jet> 10 GeV, -0.5 < hjet < 2.7)
≥ 1 e± candidate (Pt > 6 GeV, qe<90º)
Signal efficiency x BR: ~30%
Multi-variate discriminant method
Cut D>0.7
Signal: Msquark=1 TeV, Msel=100 GeV
l´=1, tanb=30
04/04/2019
A. Montanari DIS2006 Tsukuba