susy searches at lep selected topics

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 !