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Construction. Supersymmetry at ATLAS. Dan Tovey University of Sheffield. Supersymmetry. Supersymmetry (SUSY) fundamental continuous symmetry connecting fermions and bosons Q a |F> = |B>, Q a |B> = |F> {Q a ,Q b }=-2 g m ab p m : generators obey anti-commutation relations with 4-mom

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Supersymmetry at atlas

Construction

Supersymmetryat ATLAS

Dan Tovey

University of Sheffield

1


Supersymmetry
Supersymmetry

  • Supersymmetry (SUSY) fundamental continuous symmetry connecting fermions and bosons

    Qa|F> = |B>, Qa|B> = |F>

  • {Qa,Qb}=-2gmabpm: generators obey anti-commutation relations with 4-mom

    • Connection to space-time symmetry

  • SUSY stabilises Higgs mass against loop corrections (gauge hierarchy/fine-tuning problem)

    • Leads to Higgs mass £ 135 GeV

    • Good agreement with LEP constraints from EW global fits

  • SUSY modifies running of SM gauge couplings ‘just enough’ to give Grand Unification at single scale.

LEPEWWG

Winter 2006

mH<207 GeV

(95%CL)

2


Susy spectrum
SUSY Spectrum

~

~

~

~

A

H0

H0u

H0d

H+u

H-d

~

~

~

~

u

d

e

ne

~

~

~

~

c

s

m

nm

~

~

~

~

t

b

t

nt

~

~

~

g

Z

g

Z

~

g

g

~

~

~

~

~

~

~

G

c02

c03

c ±1

c04

c01

c ±2

  • SUSY gives rise to partners of SM states with opposite spin-statistics but otherwise same Quantum Numbers.

h

  • Expect SUSY partners to have same masses as SM states

    • Not observed (despite best efforts!)

    • SUSY must be a broken symmetry at low energy

  • Higgs sector also expanded

ne

u

d

e

c

s

m

nm

t

b

t

nt

G

3


Susy dark matter
SUSY & Dark Matter

mSUGRA A0=0,

tan(b) = 10, m>0

Ellis et al. hep-ph/0303043

Disfavoured by BR (b  s) = (3.2  0.5)  10-4 (CLEO, BELLE)

Universe Over-Closed

~

c01

t

~

c01

l

~

t1

~

~

t1

g/Z/h

lR

~

c01

l

0.094    h2  0.129

(WMAP-1year)

  • R-Parity Rp = (-1)3B+2S+L

  • Conservation of Rp (motivated e.g. by string models) attractive

    • e.g. protects proton from rapid decay via SUSY states

  • Causes Lightest SUSY Particle (LSP) to be absolutely stable

  • LSP neutral/weakly interacting to escape astroparticle bounds on anomalous heavy elements.

  • Naturally provides solution to dark matter problem

  • R-Parity violating models still possible  not covered here.

4


Susy @ atlas
SUSY @ ATLAS

  • LHC will be a 14 TeV proton-proton collider located inside the LEP tunnel at CERN.

  • Luminosity goals:

    • 10 fb-1 / year (first ~3 years)

    • 100 fb-1/year (subsequently).

  • First data this year!

  • Higgs & SUSY main goals.

  • Much preparatory work carried out historically by ATLAS

    • Summarised in Detector and Physics Performance TDR (1998/9).

  • Work continuing to ensure ready to test new ideas with first data.

  • Concentrate here on more recent work.

5


Model framework
Model Framework

LHCC mSUGRA Points

3

1

2

5

4

  • Minimal Supersymmetric Extension of the Standard Model (MSSM) contains > 105 free parameters, NMSSM etc. has more g difficult to map complete parameter space!

  • Assume specific well-motivated model framework in which generic signatures can be studied.

  • Often assume SUSY broken by gravitational interactions g mSUGRA/CMSSM framework : unified masses and couplings at the GUT scale g 5 free parameters

  • (m0, m1/2, A0, tan(b), sgn(m)).

  • R-Parity assumed to be conserved.

  • Exclusive studies use benchmark points in mSUGRA parameter space:

    • LHCC Points 1-6;

    • Post-LEP benchmarks (Battaglia et al.);

    • Snowmass Points and Slopes (SPS);

    • etc…

  • 6


    Susy signatures
    SUSY Signatures

    p

    p

    ~

    c01

    ~

    ~

    ~

    q

    ~

    c02

    l

    g

    q

    q

    l

    l

    • Q: What do we expect SUSY events @ LHC to look like?

    • A: Look at typical decay chain:

    • Strongly interacting sparticles (squarks, gluinos) dominate production.

    • Heavier than sleptons, gauginos etc. g cascade decays to LSP.

    • Long decay chains and large mass differences between SUSY states

      • Many high pT objects observed (leptons, jets, b-jets).

    • If R-Parity conserved LSP (lightest neutralino in mSUGRA) stable and sparticles pair produced.

      • Large ETmiss signature (c.f. Wgln).

    • Closest equivalent SMsignature tgWb.

    7


    Inclusive Searches

    • Use 'golden' Jets + n leptons + ETmiss discovery channel.

    • Map statistical discovery reach in mSUGRA m0-m1/2 parameter space.

    • Sensitivity only weakly dependent on A0, tan(b) and sign(m).

    • Syst.+ stat. reach harder to assess: focus of current & future work.

    5s

    5s

    ATLAS

    ATLAS

    8


    Inclusive susy
    Inclusive SUSY

    Jets + ETmiss + 0 leptons

    ATLAS

    • First SUSY parameter to be measured may be mass scale:

      • Defined as weighted mean of masses of initial sparticles.

    • Calculate distribution of 'effective mass' variable defined as scalar sum of masses of all jets (or four hardest) and ETmiss:

      Meff=S|pTi| + ETmiss.

    • Distribution peaked at ~ twice SUSY mass scale for signal events.

    • Pseudo 'model-independent' measurement.

    • With PS typical measurement error (syst+stat) ~10% for mSUGRA models for 10 fb-1 , errors much greater with ME calculation  an important lesson …

    10 fb-1

    9


    Exclusive studies
    Exclusive Studies

    p

    p

    ~

    c01

    ~

    ~

    ~

    ~

    q

    c02

    g

    lR

    q

    q

    l

    l

    • With more data will attempt to measure weak scale SUSY parameters (masses etc.) using exclusive channels.

    • Different philosophy to TeV Run II (better S/B, longer decay chains) g aim to use model-independent measures.

    • Two neutral LSPs escape from each event

      • Impossible to measure mass of each sparticle using one channel alone

    • Use kinematic end-points to measure combinations of masses.

    • Old technique used many times before (n mass from b decay spectrum, W (transverse) mass in Wgln).

    • Difference here is we don't know mass of neutral final state particles.

    10


    Dilepton Edge Measurements

    ~

    ~

    c01

    c02

    ~

    l

    l

    l

    • When kinematically accessible c02 canundergo sequential two-body decay to c01 via a right-slepton (e.g. LHC Point 5).

    • Results in sharp OS SF dilepton invariant mass edge sensitive to combination of masses of sparticles.

    • Can perform SM & SUSY background subtraction using OF distribution

      e+e- + m+m- - e+m- - m+e-

    • Position of edge measured with precision ~ 0.5%

      (30 fb-1).

    ~

    ~

    • m0 = 100 GeV

    • m1/2 = 300 GeV

    • A0 = -300 GeV

    • tan(b) = 6

    • sgn(m) = +1

    e+e- + m+m-

    - e+m- - m+e-

    e+e- + m+m-

    Point 5

    ATLAS

    ATLAS

    30 fb-1

    atlfast

    5 fb-1

    SU3

    Physics

    TDR

    11


    Measurements With Squarks

    ~

    qL

    ~

    ~

    c02

    ~

    c01

    q

    l

    l

    l

    ~

    qL

    ~

    ~

    c02

    c01

    q

    h

    lq edge

    llq edge

    b

    b

    1% error

    (100 fb-1)

    1% error

    (100 fb-1)

    • Dilepton edge starting point for reconstruction of decay chain.

    • Make invariant mass combinations of leptons and jets.

    • Gives multiple constraints on combinations of four masses.

    • Sensitivity to individual sparticle masses.

    bbq edge

    llq threshold

    1% error

    (100 fb-1)

    2% error

    (100 fb-1)

    TDR,

    Point 5

    TDR,

    Point 5

    TDR,

    Point 5

    TDR,

    Point 5

    ATLAS

    ATLAS

    ATLAS

    ATLAS

    12


    Full simulation
    Full Simulation

    4.20 fb-1

    No cuts

    ll endpoint

    ATLAS

    • Full Geant4 simulation of signals + backgrounds now routine.

    • Invariant mass of dilepton+jet bounded by Mllqmin=271.8 GeV and Mllqmax =501 GeV.

    • Leptons combined with two jets with highest pT

    • Flavour subtraction & efficiency correction applied

    Preliminary

    SU3

    Dilepton Edge = (100.3  0.4) GeV

    Larger of M(llq)

    • m0 = 100 GeV

    • m1/2 = 300 GeV

    • A0 = -300 GeV

    • tan(b) = 6

    • sgn(m) = +1

    Smaller

    of M(llq)

    Smaller of M(llq)

    Events

    ATLAS

    4.20 fb-1

    2j100+2l10

    Preliminary

    10

    30

    4.20 fb-1

    2j100+2l10

    ATLAS

    0

    Preliminary

    0

    0

    200

    400

    600

    M(qql) (GeV)

    200

    400

    600

    13


    Model independent masses
    ‘Model-Independent’ Masses

    Sparticle Expected precision (100 fb-1)

    qL 3%

    02 6%

    lR 9%

    01 12%

    ~

    ~

    ~

    ~

    ~

    ~

    c01

    lR

    ATLAS

    ATLAS

    • Combine measurements from edges from different jet/lepton combinations to obtain ‘model-independent’ mass measurements.

    Mass (GeV)

    Mass (GeV)

    ~

    ~

    c02

    qL

    ATLAS

    ATLAS

    LHCC

    Point 5

    Mass (GeV)

    Mass (GeV)

    14


    Sbottom gluino mass
    Sbottom/Gluino Mass

    p

    p

    ~

    c01

    ~

    ~

    ~

    ~

    b

    c02

    g

    lR

    b

    b

    l

    l

    ~

    ~

    m(g)-0.99m(c01)

    = (500.0 ± 6.4) GeV

    300 fb-1

    ATLAS

    • Following measurement of squark, slepton and neutralino masses move up decay chain and study alternative chains.

    • One possibility: require b-tagged jet in addition to dileptons.

    • Give sensitivity to sbottom mass (actually two peaks) and gluino mass.

    • Problem with large error on input c01 mass remains g reconstruct difference of gluino and sbottom masses.

    • Allows separation of b1 and b2 with 300 fb-1.

    SPS1a

    ~

    ~

    ~

    m(g)-m(b1)

    = (103.3 ± 1.8) GeV

    ATLAS

    ~

    ~

    ~

    ~

    m(g)-m(b2)

    = (70.6 ± 2.6) GeV

    300 fb-1

    SPS1a

    15


    Stop mass
    Stop Mass

    mtbmax= (443.2 ± 7.4stat) GeV

    Expected = 459 GeV

    ~

    ~

    ~

    ~

    ~

    ~

    120 fb-1

    • Look at edge in tb mass distribution.

    • Contains contributions from

      • gtt1tbc+1

      • gbb1btc+1

      • SUSY backgrounds

    • Measures weighted mean of end-points

    • Require m(jj) ~ m(W), m(jjb) ~ m(t)

    ATLAS

    LHCC Pt 5 (tan(b)=10)

    mtbmax= (510.6 ± 5.4stat) GeV

    Expected = 543 GeV

    • Subtract sidebands from m(jj) distribution

    • Can use similar approach with gtt1ttc0i

      • Di-top selection with sideband subtraction

    • Also use ‘standard’ bbll analyses (previous slide)

    120 fb-1

    ~

    ~

    ~

    ATLAS

    LHCC Pt 5 (tan(b)=10)

    16


    Rh squark mass
    RH Squark Mass

    ~

    ~

    c01

    qR

    q

    ~

    ~

    ~

    • Right handed squarks difficult as rarely decay via ‘standard’ c02 chain

      • Typically BR (qRgc01q) > 99%.

    • Instead search for events with 2 hard jets and lots of ETmiss.

    • Reconstruct mass using ‘stransverse mass’ (Allanach et al.):

      mT22 = min [max{mT2(pTj(1),qTc(1);mc),mT2(pTj(2),qTc(2);mc)}]

    • Needs c01 mass measurement as input.

    • Also works for sleptons.

    ~

    qTc(1)+qTc(2)=ETmiss

    ATLAS

    ATLAS

    30 fb-1

    100 fb-1

    30 fb-1

    Right squark

    SPS1a

    ATLAS

    SPS1a

    SPS1a

    Right squark

    Left slepton

    Precision ~ 3%

    17


    Heavy gaugino measurements
    Heavy Gaugino Measurements

    ATLAS

    • Also possible to identify dilepton edges from decays of heavy gauginos.

    • Requires high stats.

    • Crucial input to reconstruction of MSSM neutralino mass matrix (independent of SUSY breaking scenario).

    SPS1a

    ATLAS

    ATLAS

    ATLAS

    100 fb-1

    100 fb-1

    100 fb-1

    SPS1a

    18


    Measuring model parameters
    Measuring Model Parameters

    Point m0 m1/2 A0 tan(b) sign(m)

    LHC Point 5 100 300 300 2 +1

    SPS1a 100 250 -100 10 +1

    Parameter Expected precision (300 fb-1)

    m0 2%

    m1/2 0.6%

    tan(b)  9%

    A0 16%

    • Alternative use for SUSY observables (invariant mass end-points, thresholds etc.).

    • Here assume mSUGRA/CMSSM model and perform global fit of model parameters to observables

      • So far mostly private codes but e.g. SFITTER, FITTINO now on the market;

      • c.f. global EW fits at LEP, ZFITTER, TOPAZ0 etc.

    19


    Susy dark matter1
    SUSY Dark Matter

    • Can use parameter measurements for many purposes, e.g. estimate LSP Dark Matter properties (e.g. for 300 fb-1, SPS1a)

      • Wch2 = 0.1921  0.0053

      • log10(scp/pb) = -8.17  0.04

    Baer et al. hep-ph/0305191

    LHC Point 5: >5s error (300 fb-1)

    SPS1a: >5s error (300 fb-1)

    scp=10-11 pb

    Micromegas 1.1

    (Belanger et al.)

    + ISASUGRA 7.69

    DarkSUSY 3.14.02

    (Gondolo et al.)

    + ISASUGRA 7.69

    scp=10-10 pb

    Wch2

    scp

    scp=10-9 pb

    300 fb-1

    300 fb-1

    No REWSB

    LEP 2

    ATLAS

    ATLAS

    20


    Complementarity

    D.R. Tovey et al., JHEP 0405 (2004) 071

    No. MC Experiments

    scpsi

    Wch2

    • Measurements at ATLAS complementary to direct and indirect astroparticle searches / measurements

      • Uncorrelated systematics

      • Measures different parameters

    EDELWEISS

    Wch2

    log10 (scpsi / 1pb)

    CDMS

    scpsi (cm2)

    DAMA

    • Aim to test compatibility of e.g. SUSY signal with DM hypothesis (data from astroparticle experiments)

      • Fit SUSY model parameters to ATLAS measurements

      • Use to calculate DM parameters Wch2, mc, scpsietc.

    CDMS-II

    ZEPLIN-I

    CRESST-II

    scpsi

    ZEPLIN-2

    ZEPLIN-4

    EDELWEISS 2

    GENIUS

    XENON

    ZEPLIN-MAX

    mc

    Provides strongest possible test of Dark Matter model

    DM particle mass mc (GeV)

    21


    Susy dark matter2
    SUSY Dark Matter

    'Focus point' region: significant h component to LSP enhances annihilation to gauge bosons

    ~

    ~

    c01

    t

    ~

    c01

    l

    ~

    t1

    ~

    ~

    t1

    g/Z/h

    lR

    ~

    c01

    l

    mSUGRA A0=0,

    tan(b) = 10, m>0

    Slepton Co-annihilation region: LSP ~ pure Bino. Small slepton-LSP mass difference makes measurements difficult.

    Ellis et al. hep-ph/0303043

    • SUSY (e.g. mSUGRA) parameter space strongly constrained by cosmology (e.g. WMAP satellite) data.

    Disfavoured by BR (b  s) = (3.2  0.5)  10-4 (CLEO, BELLE)

    'Bulk' region: t-channel slepton exchange - LSP mostly Bino. 'Bread and Butter' region for LHC Expts.

    Also 'rapid annihilation funnel' at Higgs pole at high tan(b), stop co-annihilation region at large A0

    0.094    h2  0.129

    (WMAP)

    22


    Coannihilation signatures
    Coannihilation Signatures

    • Small slepton-neutralino mass difference gives soft leptons

      • Low electron/muon/tau energy thresholds crucial.

    • Study point chosen within region:

      • m0=70 GeV; m1/2=350 GeV; A0=0; tanß=10 ; μ>0;

      • Same model used for DC2 study.

    • Decays of c02 to both lL and lR kinematically allowed.

      • Double dilepton invariant mass edge structure;

      • Edges expected at 57 / 101 GeV

    • Stau channels enhanced (tanb)

      • Soft tau signatures;

      • Edge expected at 79 GeV;

      • Less clear due to poor tau visible energy resolution.

    • ETmiss>300 GeV

    • 2 OSSF leptons PT>10 GeV

    • >1 jet with PT>150 GeV

    • OSSF-OSOF subtraction applied

    100 fb-1

    ATLAS

    Preliminary

    ~

    ~

    ~

    • ETmiss>300 GeV

    • 1 tau PT>40 GeV;1 tau PT<25 GeV

    • >1 jet with PT>100 GeV

    • SS tau subtraction

    100 fb-1

    ATLAS

    Preliminary

    23


    Full simulation1
    Full Simulation

    Two edges from:

    20.6 fb-1

    No cuts

    20.6 fb-1

    No cuts

    ATLAS

    Soft lepton

    Preliminary

    MC Truth, lL

    Hard lepton

    MC Truth, lR

    MC Data

    ATLAS

    Preliminary

    • m0 = 70 GeV

    • m1/2 = 350 GeV

    • A0 = 0

    • tan(b) = 10

    • sgn(m) = +1

    Each slepton is close in mass to one of the neutralinos – one of the leptons is soft

    24


    Focus point models
    Focus Point Models

    ~

    ~

    ~

    ~

    c03 → c01 ll

    c02 → c01 ll

    Z0→ ll

    ATLAS

    300 fb-1

    Preliminary

    • Large m0 sfermions are heavy

    • Most useful signatures from heavy neutralino decay

    • Study point chosen within focus point region :

      • m0=3550 GeV; m1/2=300 GeV; A0=0; tanß=10 ; μ>0

    • Direct three-body decaysc0n → c01 ll

    • Edges givem(c0n)-m(c01) : flavour subtraction applied

    ~

    ~

    ~

    ~

    M = mA+mB m = mA-mB

    25


    Susy spin measurement
    SUSY Spin Measurement

    Measure

    Angle

    Point 5

    Spin-0

    Spin-½

    Straightline distn

    Polarise

    (phase-space)

    Spin-½, mostly wino

    Spin-0

    Spin-½, mostly bino

    • Q: How do we know that a SUSY signal is really due to SUSY?

      • Other models (e.g. UED) can mimic SUSY mass spectrum

    • A: Measure spin of new particles.

    • One proposal – use ‘standard’ two-body slepton decay chain

      • charge asymmetry of lq pairs measures spin of c02

      • relies on valence quark contribution to pdf of proton (C asymmetry)

      • shape of dilepton invariant mass spectrum measures slepton spin

    ~

    Point 5

    ATLAS

    150 fb -1

    mlq

    spin-0=flat

    150 fb -1

    ATLAS

    26


    Preparations for 1 st physics
    Preparations for 1st Physics

    • Preparations needed to ensure efficient/reliable searches for/measurements of SUSY particles in timely manner:

      • Initial calibrations (energy scales, resolutions, efficiencies etc.);

      • Minimisation of poorly estimated SM backgrounds;

      • Estimation of remaining SM backgrounds;

      • Development of useful tools.

      • Definition of prescale (calibration) trigger strategy

    • Different situation to Run II (no previous s measurements at same Ös)

    • Will need convincing bckgrnd. estimate with little data as possible.

    • Background estimation techniques will change depending on integrated lumi.

    • Ditto optimum search channels & cuts.

    • Aim to use combination of

      • Fast-sim;

      • Full-sim;

      • Estimations from data.

    • Use comparison between different techniques to validate estimates and build confidence in (blind) analysis.

    27


    Background estimation
    Background Estimation

    Jets + ETmiss + 0 leptons

    ATLAS

    • Inclusive signature: jets + n leptons + ETmiss

    • Main backgrounds:

      • Z + n jets

      • W + n jets

      • QCD

      • ttbar

    • Greatest discrimination power from ETmiss (R-Parity conserving models)

    • Generic approach to background estimation:

      • Select low ETmiss background calibration samples;

      • Extrapolate into high ETmiss signal region.

    • Used by CDF / D0

    • Extrapolation is non-trivial.

      • Must find variables uncorrelated with ETmiss

    • Several approaches being developed.

    10 fb-1

    28


    W z jets background
    W/Z+Jets Background

    (Zll)

    ATLAS

    • Z gnn + n jets, W g ln + n jets, W gtn + (n-1) jets (t fakes jet)

    • Estimate from Z g l+l- + n jets (e or m)

    • Tag leptonic Z and use to validate MC / estimate ETmiss from pT(Z) & pT(l)

    • Alternatively tag W g ln + n jets and replace lepton with n (0l):

      • higher stats

      • biased by presence of SUSY

    Preliminary

    (Wln)

    ATLAS

    Preliminary

    29


    Qcd background
    QCD Background

    • Two main sources:

      • fake ETmiss (gaps in acceptance, dead/hot cells, non-gaussian tails etc.)

      • real ETmiss (neutrinos from b/c quark decays)

    • Much harder : simulations require detailed understanding of detector performance (not easy with little data).

    • Strategy:

      • Initially choose channels which minimise contribution until well understood (e.g. jets + ETmiss + 1l)

      • Reject events where fake ETmiss likely: beam-gas and machine background, bad primary vertex, hot cells, CR muons, ETmiss phi correlations (jet fluctuations), jets pointing at regions of poor response …

      • Cut hard to minimise contribution to background (at expense of stats).

      • Estimate background using data

    Pythia dijets

    SUSY SU3

    30


    Qcd background1
    QCD Background

    jets

    MET

    • Work in Progress: several ideas under study

    • Step 1: Measure jet smearing function from data

      • Select events: ETmiss > 100 GeV, Df(ETmiss, jet) < 0.1

      • Estimate pT of jet closest to EtMiss as

        pTtrue-est = pTjet + ETMiss

    • Step 2: Smear low ETmiss multijet events with measured smearing function

    fluctuating

    jet

    Njets >= 4,

    pT(j1,j2) >100GeV,

    pT(j3,j4) > 50GeV

    ATLAS

    ATLAS

    NB: error bars expected errors on background.

    Preliminary

    Preliminary

    ETmiss

    31


    Top background
    Top Background

    • Jets + ETmiss + 1 lepton: cut hard on mT(ETmiss, l)

    • Rejects bulk of ttbar (semi-leptonic decays), W g ln + n jets

    • Remaining background mostly fully leptonic top decays (ttbblnln, ttbbtnln, ttbbtntn) with one top missed (e.g. hadronic tau)

    ATLAS

    Preliminary

    Edge at W Mass

    32


    Decay resimulation
    Decay Resimulation

    • Work in progress: several approaches under study

    • Goal is to model complex background events using samples of tagged SM events.

    • Initially we will know:

      • a lot about decays of SM particles (e.g. W, top)

      • a reasonable amount about the (gaussian) performance of the detector.

      • rather little about PDFs, the hard process and UE.

    • Philosophy

      • Tag ‘seed’ events containing W/top

      • Reconstruct 4-momentum of W/top (x2 if e.g. ttbar)

      • Decay/hadronise with e.g. Pythia

      • Simulate decay products with atlfast(here) or fullsim

      • Remove original decay products from seed event

      • Merge new decay products with seed event (inc. ETmiss)

      • Perform standard SUSY analysis on merged event

    • Technique applicable to wide range of channels (not just SUSY)

    • Involves measurement of (top) pT: useful for exotic-top searches?

    33


    Decay resimulation1
    Decay Resimulation

    Estimate

    5201

    m(lb)

    Zmm

    • Select seed events:

      • 2 leptons with pT>25GeV

      • 2 jets with pT>50GeV

      • ETmiss < <pT(l1),pT(l2)> (SUSY)

      • Both m(lb) < 155GeV

    • Solve 6 constraints for p(n)

    • Resimulate + merge with seed

    • Apply 1l SUSY cuts (but Njet >=2)

    cut

    ATLAS

    Preliminary

    GeV

    ETmiss

    Sample 5201,

    pT(top) > 200 GeV

    reco 11.0.4205,

    450 pb-1

    ATLAS

    Pull

    (top pT)

    Preliminary

    Estimate requires >=2 jets,

    Normalisation under study

    ATLAS

    Preliminary

    34


    Supersummary
    Supersummary

    The LHC will be THE PLACE to search for, and hopefully study, SUSY from 2007 onwards at colliders (at least until ILC).

    Complementary to direct/indirect astroparticle dark matter searches

    ATLAS: evidence / limits on SUSY

    Astroparticle: evidence / limits on DM

    SUSY searches will commence on Day 1 of LHC operation.

    Many studies of exclusive channels already performed.

    Lots of input from both theorists (new ideas) and experimentalists (new techniques).

    Big challenge for discovery will be understanding systematics.

    Big effort ramping up now to understand how to exploit first data in timely fashion

    35




    Top background to susy
    Top Background to SUSY

    With btag

    ttbar

    ATLAS

    • Reconstruct top mass in semi-leptonic top events

    • Select events in peak and examine ETmiss distribution.

    • Subtract combinatorial background with appropriately weighted (from MC) top sideband subtraction.

    Preliminary

    SUSY

    • Good agreement with top background distribution in SUSY selection.

    • With this tuning does not select SUSY events (as required)

    ATLAS

    Preliminary

    Histogram – 1 lepton SUSY selection (no b-tag)

    Data points – background estimate

    With btag

    38


    Top background to susy1
    Top Background to SUSY

    Estimate

    SUSY selection (top)

    SUSY selection (total)

    Estimate

    SUSY selection

    Work in progress

    • Fully simulated data.

    • No btag used

    • Background estimates altered by increased sideband

    • Large excess at ETmiss > 500 GeV

    • With 10 fb-1 obtain following no. events passing ETmiss cut:

      • No SUSY: Observed: 50 ± 7 (stat), Estimate: 55 ± 17(stat)

      • With SUSY: Observed: 503 ± 22 (est. stat), Estimate: 7 ± 35 (est. stat)

    ATLAS

    Preliminary

    ATLAS

    Preliminary

    T2

    T2 + SU3

    39


    Full simulation2
    Full Simulation

    20.6 fb-1

    No cuts

    qll edge

    ql(min) edge

    • m0 = 70 GeV

    • m1/2 = 350 GeV

    • A0 = 0

    • tan(b) = 10

    • sgn(m) = +1

    ATLAS

    ATLAS

    Preliminary

    Preliminary

    ql(max) edge

    qll threshold

    ATLAS

    ATLAS

    Preliminary

    Preliminary

    40


    Full simulation mass fit
    Full Simulation: Mass Fit

    • Fit to all distributions. Expected shape, acceptance corrections, detector resolution to be accounted for.

    • Need to understand cuts to reject SM background

    • Assume 1% error on lepton+jets endpoints

    20.6 fb-1

    No cuts

    ATLAS

    ATLAS

    Preliminary

    M(02) (GeV)

    Preliminary

    M(01) (GeV)

    M(01) (GeV)

    41


    Llq edge
    llq Edge

    ~

    ~

    ~

    c02

    ~

    c01

    qL

    l

    l

    l

    q

    • Dilepton edges provide starting point for other measurements.

    • Use dilepton signature to tag presence of c02 in event, then work back up decay chain constructing invariant mass distributions of combinations of leptons and jets.

    ~

    • Hardest jets in each event produced by RH or LH squark decays.

    • Select smaller of two llq invariant masses from two hardest jets

      • Mass must be < edge position.

    • Edge sensitive to LH squark mass.

    e.g. LHC Point 5

    ATLAS

    1% error

    (100 fb-1)

    Physics

    TDR

    Point 5

    42


    Lq edge
    lq Edge

    ATLAS

    • Complex decay chain at LHC Point 5 gives additional constraints on masses.

    • Use lepton-jet combinations in addition to lepton-lepton combinations.

    • Select events with only one dilepton-jet pairing consistent with slepton hypothesis

      g Require one llq mass above edge and one below (reduces combinatorics).

    Point 5

    Physics

    TDR

    ATLAS

    • Construct distribution of invariant masses of 'slepton' jet with each lepton.

    • 'Right' edge sensitive to slepton, squark and c02 masses ('wrong' edge not visible).

    1% error

    (100 fb-1)

    Physics

    TDR

    ~

    Point 5

    43


    Hq edge
    hq edge

    ~

    qL

    ~

    ~

    c02

    c01

    q

    h

    b

    b

    ~

    ~

    • If tan(b) not too large can also observe two body decay of c02 to higgs and c01.

    • Reconstruct higgs mass (2 b-jets) and combine with hard jet.

    • Gives additional mass constraint.

    ATLAS

    Point 5

    1% error

    (100 fb-1)

    Physics

    TDR

    44


    Llq threshold
    llq Threshold

    ATLAS

    Physics

    TDR

    ~

    • Two body kinematics of slepton-mediated decay chain also provides still further information (Point 5).

    • Consider case where c01 produced near rest in c02 frame.

      • Dilepton mass near maximal.

      • p(ll) determined by p(c02).

    ~

    Point 5

    ~

    • Distribution of llq invariant masses distribution has maximum and minimum (when quark and dilepton parallel).

    • llq threshold important as contains new dependence on mass of lightest neutralino.

    Physics

    TDR

    ATLAS

    Point 5

    2% error

    (100 fb-1)

    45


    Mass reconstruction
    Mass Reconstruction

    • Combine measurements from edges from different jet/lepton combinations.

    • Gives sensitivity to masses (rather than combinations).

    46


    High mass msugra
    High Mass mSUGRA

    • ATLAS study of sensitivity to models with high mass scales

    • E.g. CLIC Point K  Potentially observable … but hard!

    ATLAS

    1000 fb -1

    • Characteristic double peak in signal Meff distribution (Point K).

    • Squark and gluino production cross-section reduced due to high mass.

    • Gaugino production significant

    47


    AMSB

    ~

    ~

    • Examined RPC model with

      tan(b) = 10, m3/2=36 TeV, m0=500 GeV, sign(m) = +1.

    • c+/-1 near degenerate with c01.

    • Search for c+/-1 gp+/-c01

      (Dm = 631 MeV g soft pions).

    ~

    ~

    • Also displaced vertex due to phase space (ct=360 microns).

    • Measure mass difference between chargino and neutralino using mT2 variable (from mSUGRA analysis).

    48


    GMSB

    ~

    c02

    ~

    ~

    ~

    l

    G

    l

    c01

    l

    g

    • Kinematic edges also useful for GMSB models when neutral LSP or very long-lived NLSP escapes detector.

    • Kinematic techniques using invariant masses of combinations of leptons, jets and photons similar.

    • Interpretation different though.

    • E.g. LHC Point G1a (neutralino NLSP with prompt decay to gravitino) with decay chain:

    49


    GMSB

    • Use dilepton edge as before (but different position in chain).

    • Use also lg, llg edges (c.f. lq and llq edges in mSUGRA).

    • Get two edges (bonus!) in lg as can now see edge from 'wrong' lepton (from c02 decay). Not possible at LHCC Pt5 due to masses.

    • Interpretation easier as can assume gravitino massless:

    50


    R parity violation
    R-Parity Violation

    • Missing ET for events at SUGRA point 5 with and without R-parity violation

    • RPV removes the classic SUSY missing ET signature

    • Use modified effective mass variable taking into account pT of leptons and jets in event

    51


    R parity violation1
    R-Parity Violation

    Phase space sample 8j +2l

    • Baryon-Parity violating case hardest to identify (no leptons).

      • Worst case: "212 - no heavy-quark jets

    • Test model studied with decay chain:

    • Lightest neutralino decays via BPV coupling:

    • Reconstruct neutralino mass from 3-jet combinations (but large combinatorics : require > 8 jets!)

    52


    R parity violation2
    R-Parity Violation

    Test point

    • Use extra information from leptons to decrease background.

    • Sequential decay of to through and producing Opposite Sign, Same Family (OSSF) leptons

    Decay via allowed where m( ) > m()

    53


    R parity violation3
    R-Parity Violation

    • Perform simultaneous (2D) fit to 3jet and 3jet + 2lepton combination (measures mass of c02).

    ~

    No peak in phase space sample

    Gaussian fit:= 118.9  3 GeV, (116.7 GeV) = 218.5  3 GeV (211.9 GeV)

    • Jet energy scale uncertainty  3% 3 GeV systematic

    • Can also measure squark and slepton masses.

    54


    R parity violation4
    R-Parity Violation

    • Different ”ijkRPV couplings cause LSP decays to different quarks:

    • Identifying the dominant ” gives insight into flavour structure of model.

    • Use vertexing and non-isolated muons to statistically separate c- and b- from light quark jets.

    • Remaining ambiguity from d s

    • Dominant coupling could be identified at > 3.5 

    55


    Mass relation method
    Mass Relation Method

    • New(ish) idea for reconstructing SUSY masses!

    • ‘Impossible to measure mass of each sparticle using one channel alone’ (Slide 10).

      • Should have added caveat: Only if done event-by-event!

    • Assume in each decay chain 5 inv. mass constraints for 6 unknowns (4 c01 momenta + gluino mass + sbottom mass).

    • Remove ambiguities by combining different events analytically g ‘mass relation method’ (Nojiri et al.).

    • Also allows all events to be used, not just those passing hard cuts (useful if background small, buts stats limited – e.g. high scale SUSY).

    ~

    Preliminary

    ATLAS

    ATLAS

    SPS1a

    56


    Dc1 susy challenge
    DC1 SUSY Challenge

    Modified LHCC Point 5: m0=100 GeV; m1/2=300 GeV; A0=-300 GeV; tanß=6 ; μ>0

    ATLAS

    Preliminary

    • First attempt at large-scale simulation of SUSY signals in ATLAS (100 000 events: ~5 fb-1) in early 2003.

    • Tested Geant3 simulation and ATHENA (C++) reconstruction software framework thoroughly.

    SUSY Mass Scale

    ATLAS

    ll endpoint

    No b-tag

    With b-tag

    Dijet mT2 distribution

    ATLAS

    Preliminary

    Preliminary

    llj endpoint

    ATLAS

    Preliminary

    57


    Dc2 susy challenge
    DC2 SUSY Challenge

    m+m- endpoint

    • DC2 tested new G4 simulation and reconstruction.

    • Points studied:

      • DC1 bulk region point (test G4)

      • Stau coannihilation point (rich in signatures - test reconstruction)

    • 400k events fully simulated

    ATLAS

    Preliminary

    DC1 Point

    DC1 Point

    ll endpoint

    m+m- endpoint

    ATLAS

    Preliminary

    ATLAS

    Coannihilation Point

    Coannihilation Point

    Preliminary

    ll endpoint

    58


    Rome susy studies
    Rome SUSY Studies

    • Concentrate on signatures from WMAP-compatible points accessible with 10 fb-1 for Rome Physics Workshop (last week).

    • Several points studied with full simulation (~100k events/point):

      • SU1: Stau Coannihilation Region (m0=70 GeV, m1/2 = 350 GeV, A0 = 0, tan(b) = 10, m>0)

      • SU2: Focus Point Region (m0=3550 GeV, m1/2 = 300 GeV, A0 = 0, tan(b) = 10, m>0)

      • SU3: Bulk Region (m0=100 GeV, m1/2 = 300 GeV, A0 = -300 GeV, tan(b) = 10, m>0)

      • SU4: Low mass region (m0=200 GeV, m1/2 = 160 GeV, A0 = -400 GeV, tan(b) = 10, m>0)

      • SU5.x: Scan (mostly m1/2 = 600 GeV): Inclusive studies

      • SU6: Funnel Region (m0=320 GeV, m1/2 = 375 GeV, A0 = 0, tan(b) = 50, m>0)

    • Results hot off the press (1 week old)

    • More to be shown by I Aracena at SUSY2005 next month!

    59


    Methodology
    Methodology

    • Possible approach:

      • Select semi-leptonic top candidates (standard cuts – what btag available?);

      • Fully reconstruct top and W momenta using ETmiss and W mass constraint  reject (SUSY) background via reconstructed m(top);

      • Estimate background at high ETmiss with events in top mass peak

      • Normalise to data at low ETmiss.

    • Study with Rome background (T1, T2, A7) and SUSY signal (SU3) samples.

    • Used SusyPlot and AODs throughout (negative weights included).

    • Simplified 1 lepton SUSY selection (softer cuts to accept events / no mT cut applied):

      • ETmiss > 20 GeV (we will extrapolate to large ETmiss later)

      • At least 4 jets with pT > 40 GeV

      • Exactly 1 lepton with pT > 20 GeV

    • Similar to top selection cuts but without b-tag requirement (assume not available initially).

    • Note: nothing has been tuned excessively (no time!)  almost ‘blind’.

    60


    Top mass reconstruction
    Top Mass Reconstruction

    • Cannot use hadronic top as W reconstruction efficiency poor at high pT(W) (jets become collinear)

    • Reconstruct semi-leptonic top mass from lepton + ETmiss and W mass constraint

      • quadratic ambiguity

      • select solution nearest to top mass (only place m(top) used)

    • Reconstruct top from combination of W with one hard jet

      • Reduce combinatorics by selecting highest pT top candidate.

    ATLAS

    Preliminary

    61


    E t miss estimation
    ETmiss Estimation

    ATLAS

    Preliminary

    ETmiss:

    100 GeV

    - 200 GeV

    T2

    • M(top) reasonably uncorrelated with ETmiss.

    • Select events at low ETmiss and examine top mass distribution

    • Define

      • signal band (140 – 200 GeV)

      • sideband (200-260 GeV)

    ETmiss:

    100 GeV

    - 200 GeV

    A7

    • Scale sideband by 1.57 (from W+jets MC distribution  combinatorics + use of m(top))

    • Assume this factor will be known with negligible stat. error from MC in practice (not the case here  16% err.)

    • Assumes same factor for ‘bad’ top / SUSY

    ATLAS

    Preliminary

    T1

    62


    Normalising the estimate
    Normalising the Estimate

    • Will use signal band – sideband (scaled) ETmiss distribution as background estimate.

    • Still need to normalise to SUSY selection (accounts for relative efficiency of top selection).

    • Normalise to low ETmiss region (100 GeV – 200 GeV) where SUSY signal expected to be small

      • Use T1 sample and assume low stats (0.5 fb-1)

    • Scaling factor 4.13 ± 0.27

    ATLAS

    Preliminary

    Estimate

    SUSY selection

    T1

    63


    Background estimates
    Background Estimates

    Estimate

    SUSY selection

    Estimate

    SUSY selection

    ATLAS

    Preliminary

    T2

    • Use T2 sample (pT>500 GeV) to estimate precision.

    • Count events with ETmiss > 500 GeV in SUSY selection and background estimate.

    • With 10 fb-1 obtain:

      • SUSY: 50 ± 7 (stat)

      • Estimate: 55 ± 17(stat)  30%

    • With 44 fb-1 (full T2 sample) obtain:

      • SUSY: 174 ± 13 (stat)

      • Estimate: 198 ± 38(stat)  20%

    • Errors in estimates mainly from sideband stats (poor top selection efficiency)

    • Contribution from statistical error in normalistion (even with 0.5 fb-1) negligible.

    ATLAS

    Preliminary

    T2

    64


    SUSY

    Estimate

    SUSY selection

    Estimate

    SUSY selection (top)

    SUSY selection (total)

    ATLAS

    Preliminary

    T1 + SU3

    • So what happens when a SUSY signal is present?

    • Study by adding in SUSY sample SU3 (appropriately normalised).

    • Repeat previous steps.

    • Normalisation procedure OK for SU3 and 100 – 200 GeV window

    • Normalisation factor 4.16 ± 0.27

    • Sideband subtraction appears to work

    ATLAS

    Preliminary

    SU3

    65


    Estimates with signal
    Estimates with Signal

    Estimate

    SUSY selection (top)

    SUSY selection (total)

    ATLAS

    Preliminary

    • Background estimates affected by increased sideband

    • Large excess at ETmiss > 500 GeV

    • With 10 fb-1 obtain:

      • SUSY: 503 ± 22 (stat - estimate)

      • Estimate: 7 ± 35 (stat - estimate)

    • Actual stat error on estimate: ± 49

    • Clear excess (~13 s)

    T1 + SU3

    • With 44 fb-1 (full T2 sample) obtain:

      • SUSY: 2156 ± 46 (stat - estimate)

      • Estimate: -15 ± 75(stat - estimate)

    • Actual staterror on estimate: ± 204

    • Expected errors on estimate increase due to dilution of top in signal band by SUSY (high ETmiss).

    ATLAS

    Preliminary

    T1 + SU3

    66


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