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SUSY Searches at LEP Selected Topics. Outline. Introduction Standard SUSY and the LSP Gauge Mediated SUSY Breaking SUSY A taste of R-parity violating SUSY Conclusions. J.B. de Vivie, on behalf of the LEP collaborations. ICHEP’04, Beijing. Introduction. - PowerPoint PPT Presentation
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SUSY Searches at LEPSelected Topics
J.B. de Vivie, on behalf of the LEP collaborations ICHEP’04, Beijing
Introduction
Standard SUSY and the LSP
Gauge Mediated SUSY Breaking SUSY
A taste of R-parity violating SUSY
Conclusions
Outline
Introduction
The LEP2 data sample (/experiment) : L ~ 700 pb-1 at Ecm GeV
Detectors :
~140 pb-1 / exp at Ecm 206 GeV
• particle identification e±, ±b (especially upgraded vertex detectors at LEP2
Higgs boson searches)• good hermeticity Energy Flow
(e.g. ALEPH, (E) = 0.6E/GeV) + 0.6 GeV)• Trigger efficiency ~ 100 % for Evis > 5 GeV
The SM processes e+e- collider clean environment, s well known intensive study of two and four fermion final states
mW, W, (WW,ZZ,ff), single W, …-
Background well under control
good description by MC simulations
But we are looking for rare processes Tails !!
Why SUSY ?
o Theoretical motivations (e.g. stabilizes the hierarchy MPl/mEW scalars are natural, includes gravity in its local version, …)
o Good agreement with EW precision data and gauge coupling unification
o Some models provide a natural Cold Dark Matter candidate
…even in the simplest models
o unfortunately, already from LEP1 (i.e. Z (invisible) width) : m mZ/2~
(old top mass)
R-parity Rp = (-1)L+3B+2S conservation : SUSY particles pair-produced / Lightest SParticle (LSP) stable (CDM)
LSP = lightest neutralino (or sneutrino, but )
Typical search : NLSP LSP + (SM particles), LSP undetected : Sensitivity : mNLSP ~ s /2
Four main topologies covering most of the possible final states …
Standard SUSY and the LSP
… from slepton, squark, chargino and neutralino production
All topologies crucially depend on M = mNLSP - m Visible Energy
SM background : low M : process High M : 4 fermion processes with
The way to the mass limit for the lightest neutralino
The relevant parameters : LEP-MSSM
o at mGUT, * gaugino unified mass : m1/2 M1, M2 and M3 at mZ
* sfermion (not Higgs) unified mass : m0
o mA and free
o trilinear couplings At, A (Ab)
o tan
For the LSP : interplay of various searcheso From charginos to the LSP, in a large part of the parameter space
m ~ M1 M2/2 ~ m/2
o From sfermions to the LSP, Mi appear in their masses through the RGEs
o From Higgs bosons to the LSP, through stop masses in radiative corrections
Let’s go : the ingredients and the recipe The heavy sfermion case : high m0, only chargino and neutralino
Heavier neutralinos relevant at low tan and small||
Excluded domain in the (, M2) plane
Mass limit for the chargino : > ~ kinematic limit 103.5 GeV/c2
> even beyond with neutralinos
degradation at high M2 : M
efficiency
background (Similarly for neutralinos, m + mj almost at kinematic limit)
The very low M loophole (I) : chargino searches
At high M2, small ||, m ~ m : standard searches inefficient (or in non unified models, when |M2|<<|M1|, e.g. AMSB)
2 specific searches :
> M < 150 MeV/c2, long lived, highly ionizing particles
> 150 MeV/c2 < M < 3 GeV/c2,
ISR tagging analysis : require a high p photon to reduce events
M, m > 91.9 GeV/c2
easily translated into a limit on m
> 39 GeV/c2 @ tan = 1
ISR
long lived
Finally, and i searches,
The light sfermion case : small m0
light sneutrinos, selectrons (smuons and staus) chargino production cross section leptonic branching ratio ( WW background )
difficult region : sneutrino-corridor Soft track : trigger ?
background ?
invisible
use slepton searches and
At m ~ 40 GeV/c2
meR > 99.9 GeV/c2
(Another very low M loophole…)m > 39 GeV/c2 @ tan = 1 robust…
l
,
The (very low M) loophole (II) : slepton searches
very soft lepton from almost invisible final state
for selectron, the gap is closed by the single electron search
For smuons, back to the Z width
For staus, not even sufficient due to decoupling (stau-mixing) (DELPHI dedicated searches : m1 > 26.3 GeV/c2 any mixing, any M)
Loopholes when cascade decays : ,
Dedicated searches
Absolute eR mass limitmeR > 73 GeV/c2
~
(t-channel exchange)
Including the Higgs boson searches : low tan From charginos, neutralinos and sleptons, LSP limit set at tan = 1
The Higgs cover the low tan and protect against low m0 at intermediate tan
m > 47 GeV/c2 @ high tan, in the sneutrino-corridor
• Model dependence ?
Profit from the sound LEP environment to exclude pathological regions experimental h limit : OK except for very unnatural cases
mtop = 180 GeV/c2
From experiment to interpretation : the excluded tan range has a strong dependence on mtop and the mh computation
OK ?
Best reach at Tevatron but LEP can improve at low M
Large mtop mixing maybe large : t may be the lightest squark (also in mSUGRA-type models, stop soft masses generically smaller that other squark masses)
acoplanar jets from
~
The stop and the very low M loophole (III) :
Also 3 body decay at small msneutrino
4 body decay
for M ~ 40 GeV/c2
Mstop > 95 GeV/c2
Very low M (< 5 GeV/c2) long lived stop-hadrons decay inside the trackingDedicated generator for stop-hadron formation, interaction and decay
= 56o, tan = 1.5 = -100 GeV/c2
Mstop > 63 GeV/c2 M
~st
ab
le acop. jets
high impact parameter
LEP1
Higgs
charginos
selectronsand staus
theory
Sta
ble
sta
us
Model dependence ? The stau mixing Until now, no stau mixing A = tan. Does it matter ? YES !
Impact in mSUGRA where stau mixing is built in mA and no longer free, A0 fixes A tan
new corridor at large tan : the stau-corridor Mstau ~ m
o staus ~ invisible
o charginos ~ invisible o selectrons and smuons too heavy
Again, exploit the clean LEP events to searchfor difficult topologies :
recycle the ISR taggingsingle tau or asymmetric tausMulti taus
The stau-corridor is closed : no more holes in the (m0,m1/2) planes
theory
Higgs
Z width
chargino
stable slepton
< 0 > 0and the LSP mass limit in mSUGRA
m > 50 GeV/c2
(mtop = 175 GeV/c2, any A0)
Stau mixing is a delicate issue in a Very Constrained MSSM worse in the LEP-MSSM
m > 29.7 GeV/c2 … … for very unnatural A values (CCB ?)
For not too unnatural A (<20 TeV/c2) 39 GeV/c2 (no Higgs, no mixing)
36.6 GeV/c2 (no Higgs, mixing)
ALEPH only
With stau mixing, low tan delicate… in the most conservative case
* no Higgs* stau mixing (|A| < 20 TeV/c2)
Mass limit for the lightest neutralino : Summary
In mSUGRA, m > 50 GeV/c2, any A0 but strong dependence on mtop
In LEP-MSSM, mtop < 180 GeV/c2, no stau mixing
m > 36.6 GeV/c2
m > 47 GeV/c2
all this without any radiative corrections in the gaugino-higgsino sector ~ 1-2 GeV/c2 uncertainty
The most important hypothesis : Gaugino mass Unification
What if M1 and M2 are NOT unified at mGUT ?
If at mGUT, |M1/M2| < 1 chargino constraints less stringent… m > ? e.g. |M1/M2| = 1/3,
In the worst case, heavy sleptons,|M2|, || >> |M1|
no limit from LEP !
If at mGUT, |M1/M2| > 1, m > 45 GeV/c2 should hold (the ISR-tagging analysis is very relevant to go beyond)
Solve the FCNC problem of generic Gravity mediated models
The LSP is the Gravitino G
Experimental topologies depend on the nature of the NLSP and its lifetime, determined by the Gravitino mass :
In minimal models, the lightest neutralino or the sleptons are in general the only Sparticles relevant for LEP2 searches (+ )
~
Gauge Mediated SUSY breaking SUSY
G~
A curiosity : in non minimal models the gluino can be the NLSP or LSP Search for light stable gluino at LEP1 (DELPHI, ALEPH)
Z width : mgluino > 6.9 GeV/c2
Search for R-hadrons in
mgluino > 26.9 GeV/c2
( much weaker if open)Neutralino NLSP :
Short lifetime : acoplanar photons Intermediate lifetime : single photon with high impact parameter
(“non pointing photon”) Results for the acoplanar photons
GMSB interpretationof CDF ee event:
at last dead !
Long lifetime : indirect from charginos and sleptons
(OPAL increased the sensitivity at shortlifetime with sleptons and charginos, e.g.
)
Slepton NLSP : At small tan, all sleptons are mass-degenerate: co-NLSP At large tan, the stau is lighter due to large mixing ( m tan)
Short lifetime : MSSM slepton searches for very high M acoplanar leptons
Intermediate lifetime : sleptons decay inside the tracking volume (kinks) or give tracks with high impact parameters
Long lifetime : search for pair-produced heavy stable charged particles
combining the three searches, for a stau NLSP
Mstau > 86.9 GeV/c2
increase sensitivity by looking for
(High cross-section since eR light, 50% events with 2 high E Same Sign leptons)
~
Interpretation in minimal models : 5.5 parameters needed
• F, the SUSY breaking scale (lifetime), • tan, sign()• Soft masses determined from• , the universal mass scale of SUSY
particles• N, the effective number of messenger pairs• Mmess, the mean messenger mass Excluded domain
in the (m,m) plane… from which one can infer a lower limiton as a function of tan
slepton, 0
for slepton NLSP
slepton, ±
for 0 NLSP
In these models MNLSP > 54 GeV/c2
> 16 TeV/c2 (N5)
A taste of R-parity violating SUSY
The General MSSM allows lepton and baryon number violating couplings:
45 new couplings (some of them constrained by low E processes) LSP can be any Sparticle and is unstable Sparticles can be singly produced
Searches assuming a single coupling is dominant Lots of topologies covered:
from 2 leptons (slepton production)
to many jets, many leptons and missing energy
(up to 10 quarks from chargino production) Very good test of the standard model with a very broad range of final states studied !
An example: single sneutrino production with e.g. 122
improvement over low energy constraint up to
msneutrino = 189 GeV/c2
Example of 6 jet event in ALEPH
Conclusions
Lots of SUSY searches performed by the four LEP experimentso large class of models studied, many analyses dedicated to potential loopholes : limits are robust
o No signal from SUSY : sfermion, chargino masses > 100 GeV/c2
o In LEP-MSSM with reasonable assumptions, m > 47 GeV/c2
The LEP legacy : e.g. in mSUGRA
minimal unified models : hard for Tevatron (trilepton very relevant !)
but still room for discovery at CDF/D0 Standard unification relations may not hold, Higgs coverage dependence on mtop, A0, …
eagerly wait for more Tevatron results and LHC start !