Search for Gravitinos in R-Parity violating Supersymmetry at HERA

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Search for Gravitinos in R-Parity violating
Supersymmetry at HERA
SLAC experimental seminar Claus Horn (DESY /
Univ. Hamburg)
Introduction HERA ZEUS SUSY processes at
HERA Analysis Summary Outlook
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SUSY Motivation

• Coleman-Mandula theorem /
• Haag-Lopuszanski-Sohnius theorem
• Unification of the forces
• Solution of the hierarchy problem
• Candidates for dark matter
• Necessary for quantum-gravity

SUSY is our last chance to discover a
fundamental space-time symmetry!
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HERA accelerator
e p collider, in Hamburg Protons 920
GeV Leptons 27.5 GeV CMS-Energy 320 GeV Length
6.3km HERA I (1992-2000) L1.6 1031 cm-2s-1
HERA II (2002-2007) L7.0 1031 cm-2s-1
p 920 GeV
HERA II polarised lepton beam also at H1 ZEUS.
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ZEUS detector

• Calorimeter Uranium Scintillator
• 3 lt q lt 178
• EMC DE/E 18/?(E/GeV) ? 1
• HAC DE/E 35/?(E/GeV) ? 1

• Central Tracking DetectorDrift Chamber
• 15 lt q lt 164
• 1.4 T magnetic field

Weight 3500T, Size 12m 10m 19m
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Supersymmetry
Postulate superpartner for each SM particle
with same QNs but spin different by ½.
Qbosongt fermiongt Qfermiongt bosongt
H,Q0
No superpartners with same masses are observed.
SUSY is a broken symmetry.
MSSM Minimal number of sparticles and couplings.
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Gauge Mediated SUSY Breaking Model
Super trace theorem SUSY breaking not possible
in visible sector. -gt Hidden Sector
Models
Example for generation of sfermion masses
GMSB parameters sqrt(F), Mmess, N, L, tan(b),
sign(m)
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R-Parity
1 for SM particles -1 for sparticles
Multiplicative discrete symmetry RP(-1)3BL2S
RPC sparticles pair-produced, LSP stable
Most general Lagrangian contains additional
trilinear terms in superpotential which violate
RP
Unique initial state HERA ideal place to look
for l couplings.
Former analyses looked for resonant squark
production.
squarks are heavy.
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SUSY Classification Scheme
Motivation Check all possible SUSY channels at
HERA before start of LHC.
Systematic approach

• List all possible diagrams with potentially high
  cross section.
• Include also R-parity violating vertices.

Particles are produced on-shell (same for all
SUSY models). Decay depends on sparticle spectra
of SUSY model.
Sparticle creation at HERA

HERA topologies Abstract notation
SUSY-flow graphs Fundamental vertices
Abstract diagrams
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HERA Topologies

• All topologically distinct graphs
• with up to three outgoing (s)particle lines
• Initial state is fixed to electronquark
• (g and g from proton are only considered with 2
  outgoing lines)

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SUSY-flow Graphs
Choose RPV vertices Mark sparticle lines with a
. In the case of RPC C-like loops result.
Example F, RPC
Number of SUSY propagators Number of SUSY
particles
discarded
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Abstract Notation Fundamental Vertices
Physics description on an abstract level to
reduce complexity.
All vertices of the MSSM ! (neglecting pure
bosonic SM vertices and Higgs)
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Results
After all cuts 55 abstract diagrams of sparticle
production.
Additionally consider dominant sparticle decays
Complete list of SUSY signatures at HERA.
Characteristic signatures for different SUSY
models / scenarios.
Example of new found diagram
(now investigated by new PhD student)
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Analysis

• Signal processes Topologies
• Event selection
• Discriminant method
• GMSB phenomenology
• Limits

Investigated data set (1996-2005) e- p L155
pb-1 e p L145 pb-1 Total L300
pb-1 (HERA I and HERA II)
First ZEUS thesis with complete data set!
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Signal Processes
Gaugino production via slepton exchange
Gravitino channel
R-parity violating decay channels
Electron channel
e multiple jets
Neutrino channel
Signature
n multiple jets
jet g missing energy
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Signal Topologies
Simulated events (SUSYGENGeant detector
simulation)
RPV decay
GMSB decay
p

• 3 hard forward jets
• low pT electron or
• neutrino (missing PT)
• jets nearly isotropic in r-f plane

• 1 hard forward jet
• isolated, high pT photon
• missing energy

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Event Selection Gravitino Channel
Loos selection to maximize signal efficiency!

• n trigger selection
• Q²JB gt 700 GeV
• ? 1 jet with pTgt6 GeV
• and 1.5 lt h lt 2.5

• PT miss gt 22 GeV
• Df(jet,g) lt 3.0
• Background rejection

Data/MC 4751/4787 e?70-77 -gt good
agreement.
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Gravitino Channel Final Selection
Additional cuts

• photon candidate, with
• E gt 4 GeV, 2.8 lt h lt 2.8
• DCA gt 30 cm (track cut)

Data/MC 1254/1275 e?61-68 -gt
good agreement.
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Signal to Background Optimization
One dimensional cuts do not maximize S/B (for a
given signal efficiency) if correlations between
variables exist.
Discriminant
Only select events in signal dominated areas!
Disadvantage A lot of MC needed.
Advantages compared to
Likelihood ratios

• Take into account all correlations.

Neural Networks

• No training needed.
• No interpolation into empty phase space.

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Dynamic Discriminant Method
Box size needs to be fixed before counting
starts, however counting several too small boxes
is faster than counting one too big box.
1-dim factor for which N? Nmin.
events /box (box_size)dim
46?4000
Advantages of variable bin size method
Less parameters have to be set by hand. More
events get classified. Faster calculation. More
accurate results.
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Gravitino Channel Discriminant Vars
Selection of best set of discriminant variables

• Chose characteristic
• variables.
• Calculate discriminants for
• all possible combinations.

Purity and efficiency after different
discriminant cuts.
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Gravitino Channel Discriminant
ZEUS data 1996-2005
No excess observed in signal region!
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RPV Electron Channel

• etrigger selection
• ET gt 60 GeV
• ? 2 jet with 0.5 lt h lt 2.7
• pTgt25 GeV (first jet)
• pTgt12 GeV (second jet)
• electron candidate with
• Egt10 GeV,
• 1.2 lt h lt 2.8,
• pTgt15 GeV (3ltqlt17)
• pTgt6 GeV (17ltqlt115)

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RPV Neutrino Channel

• n trigger selection
• ET gt 50 GeV
• PTgt20 GeV
• ? 1 jet with 0.5 lt h lt 2.7
• pTgt10 GeV
• reject electron with
• pTgt6 GeV,
• q lt 180

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RPV Discriminants
Electron channel
Neutrino channel
No excess observed in signal region!
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Parameter Dependence
Problem factorizes
Effects of model parameters sometimes
interchangeable, or have only small effect.
Set limits on process parameters.
Slepton mass treated as free parameter.
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Results


Limit set in mass plane of process particles
m(e)-m(c).
For l1111 sparticle masses of up to m(e) lt 360
GeV and m(c) lt 190 GeV can be excluded at 95CL.


Best existing limits in RPV GMSB!
Limits calculated for different strengths of l
coupling.
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GMSB Phenomenology
Dominating decay channels




BR(c-gtgG)BR(c-gteqq)BR(c-gtnqq) ? 100.
RPV decays get important

• Toward high sqrt(F)
• for stronger RPV couplings.

Contribution from different gauginos
Low sqrt(F) Lightest neutralino dominates. High
sqrt(F) Partly contribution from lightest
chargino.
NLSP
Neutralino is NLSP for low N and high tanb.
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Gaugino Composition
Gauginos are superposition
High cross section requires 1. Small higgsino
component (for large eec coupling) 2. Large
photino component (for GMSB decay into photon)


MSSM
GMSB
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Limit Variations
Variation of M and sign(m)
Variation of N
Different RPV couplings
Dependence on sqrt(F)
mSUGRA-like scenario
Typical GMSB scenario
Similar limits are valid in large part of GMSB
parameter space!
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Outlook
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SUSY Discovery at LHC
SUSY gauge couplings are the same as in SM. Cross
sections only surpressed by mass terms. At high
energies SUSY production rates are similar to SM!
Measure SUSY spectrum

• Masses
• QNs
• Lifetimes
• Decay modes

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Summary

• SUSY is a promising candidate for physics BSM.
• New methods
• Classification scheme for SUSY processes
• There are still open SUSY discovery
  channels at HERA
• Dynamic discriminant method
• Best existing limits in RPV GMSB
• LHC will give the final answer
• Be prepared to discover a new world !

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Backup slides
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Solution of the Hierarchy Problem
Corrections to the Higgs mass
SM
Cancelation requires fine tuning to 17 orders of
magnitude!
MSSM
Contributions of SM particles and their
superpartners compensate each other.
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Unification of the Forces
Renormalisation Group Equations describe running
of the coupling constants due to screening /
antiscreening.
Example
Slope depends on number and masses of
particles in the model.
Miracle!
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Status of SUSY Searches
Examples of best current limits
Neutralinos/Charginos
LEP m(c0) gt 45 GeV (RPV)
m(c) gt 103 GeV
Sleptons
selectronR gt 100 GeV smuonR gt 95 GeV stauR gt 86
GeV
LEP
D0 sneutrinoR gt 460 GeV
(l1320.05 l3110.16)
Squarks
D0 squark gt 320 GeV

gluino gt 232 GeV

HERA squark gt 275 GeV (l1j10.3)
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MSSM Parameters

• mA pseudoscalar Higgs boson mass
• tan(b) ratio of VEV of two Higgs doublets
• m Higgsino mixing parameter
• M1, M2, M3 gaugino mass terms
• All sfermion masses
• Ai all mixing parameters of squark and slepton
  sector

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Broken Supersymmetry
Explain origin of SUSY breaking!
Spontaneous SUSY breaking in SM sector not
possible supertrace theorem -gt sum rules between
particle and sparticle masses, e.g.
excluded!
Hidden sector models
mSUGRA, GMSB AMSB, gMSB, ...

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Data Set
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Investigated Production Processes
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Radiative EW Symmetry Breaking
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Slepton mass splitting
where the al are positively correlated with tanb.
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Example Application to type C Diagrams
RPC
RPV
SUSY-flow graphs
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Possible abstract diagrams
C3 disfavoured due to high limits on squark
masses C7 - C6 lepto-quark search /
contact interaction C5 -gt gaugino production
analysis !
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Sparticle Decays
Neutralino
RPC MSSM RPV MSSM
GMSB
stable LSP
missing energy
Chargino
RPC RPV
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Sparticle Decays
Sleptons
RPC
RPV
RPC MSSM missing E, e / m / t RPV MSSM 2 jets /
2 l / 2jets2l GMSB l g G
Squarks decay in the same way.
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Results
55 abstract diagrams.
Diagrams with squarks are neglected.
Characteristic signatures for different models!
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Results
With two outgoing lines C5 With three outgoing
lines and one sparticle F4-2 With three outgoing
lines and two sparticles D1
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Restrictions

• diagrams with gt 3 on-shell produced (s)particles
  are neglected
• diagrams with outgoing g, g, Z0 are not
  discussed
• diagrams with initial g/g and 3 outgoing
  particles are discarded
• u-channel diagrams are not stated explicitly
• diagrams with gt 1 sparticle propagator are
  discarded
• interactions of Higgs bosons are not considered
• vertices with only SM bosons are neglected
• diagrams with three RPV vertices are discarded

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GMSB Parameter Space
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HERA Kinematics
ep collision
Mandelstam variables
Bjorken variables
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Gravitino Channel Kinematics
Gamma not too forward (small dependence on m(c)),
PT Jacobian peak m(c) smeared out by LT.
Gravitino reconstruction (E-pz)G (E-Pz)DET 55
GeV
E²pT²pz²
Neutralino mass m(c)² (pgpG)²
Selectron Qe² (pe-pc)²

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Signal Cross Sections BRs
For low sqrt(F)
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Number of Expected Events
Different RPV couplings pick different quarks
from p, dependent on e-/e.
Data 96-00 (HERA I)
Example for x-section ratios
Data 96-05 (HERA I HERA II)
Ordering depends on L(e-)/L(e).
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Electron Control Sample
ZEUS data 96-00

• NC trigger selection
• zvtx lt 40 cm
• 45 lt E-pz lt 62
• Q²DA gt 400 GeV
• ? 1 jet with pTgt6 GeV
• and 1.5 lt h lt 2.5
• electron candidate with
• pTgt15 GeV and
• 1.2 lt h lt 2.8

Electron selection works fine.
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Gravitino Control Plots - HERA II
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Contribution from different Gauginos
GMSB decays at low sqrt(F)
Contribution from lightest neutralino dominates.
RPV decays at high sqrt(F)
Partly contribution from lightest chargino.
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Different NLSPs in the GMSB Model
Gaugino masses
Scalar masses
For high values of N stau NLSP is favoured.
Stau is NLSP for small M and high tanb
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Contribution from different Decay Channels
In region where lightest neutralino is NLSP
BR(c-gtgG)BR(c-gteqq)BR(c-gtnqq) ? 100.
Importance of GMSB-decay/RPV-decay depend on
sqrt(F) and strength of l coupling.
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Circularity
Circularity?0
Circularity?1
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Monte Carlo Simulation
Signal MC

• SUSYGEN
• - sqrt(F)233, 3300
• - m(c)50 GeV .. 210 GeV
• - various m(e)
• CompHEP (for systematics)
• 3 25K events

Background MC
(At least twice the data lumi.)

• NC DIS Ariadne (MEPS)
• Q² gt 25 GeV, .. ,Q² gt 50K GeV
• CC DIS Ariadne (MEPS)
• Q² gt 10 GeV, .., Q² gt 20K GeV
• PhP Herwig (direct/resolved)

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Systematic Uncertainties
CC SUSY
Energy scale of CAL cells
2.5 0.6 (fhac bhac 2, rhac
3, emc 1.5)
Luminosity measurement
2.25 2.25
Ariadne gt MEPS
1.7 (less) --
PDF uncertainty
7 7
Scale uncertainty
-- 10
Signal efficiency
-- 2.5
zVtx lt 40 cm 10 cm
1.9 2
Q²jb gt 700 GeV² 100 GeV²
1.9 0.4
Pt gt 20 GeV 4 GeV
5.6 2.5
Yjb gt 0.1 0.02
2.3 0.1
Df(jet,g) gt 3.0 rad 0.033 rad (2)
0.5 1.4
10.4 13.2
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Confidence Limit Calculation
Modified frequentist method by Thomas
Junk. Likelihood Ratio for multi-channel
analysis.Estimator function needed CL 1
P(sb) / P(b) Sytematic Uncertainties
Average over systematic variations on s and b in
all channels, assuming Gaussian distribution
with lower cutoff at zero.
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Supermultiplets
Chiral supermultiplets (fermion,sfermion)
(spin ½, spin 0) Vectorial supermultiplet
(gauge boson, gauginos) (spin 1, spin ½)
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Limit Calculation
Use discriminant bins for S, B and data for limit
calculation.
Straightforward approch
Generate events for each parameter point.
Costly!
Better Seperate event generation and parameter
scan!
Use slim or Nlim as interface.
For m produced particles decaying into n
different decay channels.
Difficult to handle!
Solution

• Calculate discriminants (efficiency and shape!)
• for different masses and interpolate.
• Seperate discriminants for each channel.

Advantages

• Amount of events to be generated highly reduced.
• Limits can be produced for an arbitrary large
  parameter space.

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Limit Calculation
1. Straightforward method

• Disadvantage
• Different events have to be
• generated for each parameter point.

2. Seperate event generation and parameter scan.
If different prod. Particles and different decay
channels contribute
Difficult to handle!
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Limit Calculation

• Important 1. Discriminant height (eff.)
• 2. Discriminant shape.
• (depends on prod. particle
  and decay channel.)
• Seperate event generation and parameter scan.

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SUSY at LHC
LHC 5s discovery curves
But
Complicated decay channels g -gt qq -gt cqq -gt
llqq -gt cllqq
Problem is to seperate different SUSY channels.
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Future Prospects - ILC
Higher luminosity at similar energy
Precision measurements of SUSY parameters!
LHC
ILC
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Future
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Analysis Framework