BSM m id-term report SUSY Projects - PowerPoint PPT Presentation

BSM m id-term report SUSY Projects. L ight stop model Choudury , Galant i, Godbole , Gucha it ,

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BSM mid-term report

SUSY Projects

  • Light stop model
  • Choudury, Galanti, Godbole, Guchait, Lari, Polesello, Schumacher, Zhukov
  • tau polarization
  • Choudury, Godbole, Guchait, Heldmann, Mangeol
  • Focus-point studies and model discrimination
  • Galanti, Lari, Zhukov
  • CPV Higgs
  • Choudury, Godbole, Schumacher

People not on the list (did I forgot someone?) are encorauged to join. This

also applies on people attending the second session and those not attending

the workshp. Contact T. Lari ([email protected]) and see the BSM web

page for details.

Light stop: parameter space scan
  • MSSM (non-mSUGRA) model, discussed in
  • C. Balazs, M. Carena and C.E.M. Wagner, Phys. Rev. D70 015007
  • Searched for a phenomenology
    • Motivated by baryogenesis
    • With relic density equal to Dark Matter abundance
    • Consistent with LEP and Tevatron limits
    • With heavy squark and sleptons, light stop, intermediate mass gluino
  • Parameter space scanned. A few days to get everything right.
    • Running stop mass scale in ISAJET is sqrt[m(tR)m(tL)]. Set to m(Z) to use
    • zero tR mass.
    • Different codes use different levels of precision (radiative corrections):
    • the same soft SUSY parameters give different results. Used LO whenever
    • possible, still few GeV differences between ISAJET (masses and decays,
    • interface to HERWIG), MICROMEGAS+ISAJET (relic density), Guchait
    • private code (4-body decay BRs).
Light stop: Selected Point


Decays in t t

Decays in c01c (30%) andc01bW*(70%)

  • Low Stop-LSP mass difference: difficult for Tevatron
  • Stop pair production O(100 pb):
  • t t→ cccc(impossible?) or t t→ bbW*W*cc(easy?)
  • Gluino pair production O(1 pb)
  • gg → t t tt → bbWWccccor bbbb WW W*W*cc or bbbcWWW*cc
  • Squark pair (rare): mostly gluino pair plus two jets
  • Charginos and neutralinos not in any decay chain – only direct production possible
  • (difficult).







Light stop: status and plans
  • Generated 5000 events with HERWIG and ATLAS fast simulation.

Observation of SUSY production and reconstruction of mass edges to be studied.

  • CMS fast simulation production to be started. A number of people from CMS interested to study this point.
  • Parameter scan: select other points? Dependence on parameters?
  • Observation of 4-body decay at Tevatron?
  • Volunteers willing to study the ATLAS or CMS ntuples?
tau polarization
  • The energy distribution of the p produced in the decays t → n p as well as those in t → r n, t → aln depends on the handedness of the t and can be used to determine t polarisation. General tool for physics at LHC.
  • SUSY application: net helicity of t produced in decay t → c0 t depends on mixing of tL and tR and on gaugino content of c0.
  • mSUGRA: c01 ~ B
    • Small tan b, cos qt small → Pt +1
    • Large tan b, cos qt large → but still Pt > 0.9.
  • AMSB: c01 ~ Wino → Pt ~ -1
  • GMSB:if t is NLSP, t → t G and Pt = sin2 qt - cos2qt
  • M. Guchait, D.P. Roy and R. Godbole, [arXiv:hep-ph/0411306].
  • M. Guchait and D. P. Roy, Phys. Lett. B535(2002)243; B541(2002)356.
  • S. Raychaudhuri and D. P. Roy, Phys. Rev. D52(1995)1556; D53(1996)4902;

D. P. Roy, Phys. Lett. B459(1999)607.

  • S. Kraml, T. Gadosijk, R.G., JHEP 0409, 051 (2004)






t polarization: how to measure

LHC CMS study 1:


Use R = pT(p)/pT(t-jet)

Green: polarization -1

Red: polarization +1

LHC CMS study 2:

see next slides

LHC ATLAS study has


Opposite sign tau's

Like sign tau's

t polarization at LHC


c02→ t t → c01 tt

  • The t from the two decays istribution can be discriminated with their transverse momentum if the two mass differences are very different.
  • OS-SS distribution can be

used to subtract background

(2-tau invariant mass).

Generator level


t polarization: status and plans
  • First studies on the measurement of the polarization of t with the CMS detector have started. First results promising, more work is needed to assess how well can we measure the polarization in SUSY events and constrain the underlying model.
  • This study has started also on the ATLAS side.
  • Other (SM and BSM) physics which can be studied with τ polarization?


Focus point

+intermidiate region [m0 > 1 TeV]




Excluded h<114, bsy

FP Studies: Motivation

-Relic density WMAP constraints in mSUGRA

-light neutralino are preferable for indirect and

large m0 by direct DM search (complimentarity)

h2 [0.094 -0.129]

EGRET preferable

FP studies: bulk vs focus-point

Focus point (gg production)

Focus point (cc production)

3-body decays

Bulk region (cc production)

Bulk region (qq or gg production)

2-body decays

FP studies: general idea

FP regions:

scalars are heavy

1±1 are light

only gluino and gaugino production)

3-body decays

Bulk and coannihilation regions

scalars are light (abundant squark production and sleptons in decays)

2-body decays


Identify focus/bulk regions by topology - without assumptions on the mass spectrum of a specific point)

FP studies: 2 and 3-body decays

Neutralino decays


Branching c2->2l+c1

Branching c2 ->l l->2l+c1

Parton level study of pure leptonic mode

Assymetry of the pt in the tagged dilepton pairs



2 body

3 body














m1/2 mo tanb

LM1 60 250 10

LM7 230 3000 10


L1 Trigger

3 isolated leptons with 2 OSSF

no tracks Pt < 1.5 GeV/c R<0.3

Ecal+Hcal < 5 GeV R<0.3

: Pt > 10 GeV/c 

e : Pt > 15 Gev/c 

No Jets ET>20GeV 

Focus Point: trileptons selections

m1/2 mo tanb totLO,pb1±->3l fb

LM9 175 1450 50 25 95

CMS Full simulation.

Direct production of gaugino pairs

observable if mass not too high

FP: status and plans

Large m0 region ( msugra mo>1000, m1/2<500) compatible with the WMAP relic density constraints will be accessible at LHC via neutralino and gluino production.

The con , cn have 3 body decays only in this region and can be selected by assymetry and MET (sumET) cuts.

Gluino also has 3 body decays only in this region.


Optimize model-independent topologycal selections to discriminate different regions of parameter space (and SUSY from SM).

Reconstruction of mass spectra in the FP .

Understand NLO SUSY cross sections.


CPX-Scenario: Overall Discovery Potential with 300 fb-1

ATLAS preliminary at SUSY04 hep-ph/0410112

small uncovered area

MH1: < 70 GeV

MH2: 105 to 120 GeV

MH3: 140 to 180 GeV


small masses below 70 GeV

not yet studied in ATLAS

channels at the border:

ttH2, H2  bb

VBF, H2  tt

ttWb H+-b, H+-tn

Maybe close hole via: ttbW bH+-, H+- H1W, H1bbFirst very preliminary look with ATLFAST

W decays considered: 1st Wqq + 2nd Wmn

Background considered: ttbb (generated with ACERMC)

s = 3.9 pb for QQCD=shat

(s = 8.1 pb for QQCD=(Mtop+60GeV))


Signal Xsec includes all branching ratios

Signal generated with PYTHIA

Primitive Selection
  • >= 1 muon with pt > 20 GeV
  • >= 4 b-tagged jets with pt>20 GeV (ideal b-tagging used)
  • >= 2 non-b-tagged jets with pt>20GeV
  • reconstruct neutrinos PZ from MW constraint (>0 solutions)
  • make list of light jet pairs with |MJJ-MW|<25 GeV
  • reconstruction of top quarks:

find combination for t1 = b ln + t2 = b bb qq

or t1 = b qq + t2 = b bb ln

which minimises D=(mt-mt1)2 + (mt-mt2)2

  • require: (mt-mt1) < 25 GeV and (mt-mt2) <25GeV
Reconstructed H1 Mass: 3 entries per evt.

before cut on mtop

MH1 = 50 GeV

MH+- = 140 GeV

L = 30 fb-1




after cut on mtop



Eff(signal) = 2.0%

Eff(ttbb BG) = 0.2%

Reconstructed H+- Mass: 3 entries per evt.

Before cut on mtop

MH1 = 50 GeV

MH+- = 140 GeV

L = 30 fb-1




after cut on mtop



CPX Higgs: status and plans
  • Analysis appears promising.
  • Realistic b-tagging, add electrons, optimize cuts, other backgrounds…
  • Can we cover the whole hole in discovery reach?

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