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 !