SUSY at the LHC

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SUSY at the LHC
Yeong Gyun Kim (Sejong U. KAIST)
? 2 ? ??-CMS ?? Workshop (Feb. 13-14, 2008)
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Contents

• Introduction
• Measurement of SUSY masses
• Gluino mT2 variable
• Spin effects in SUSY decay chain

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Introduction
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• General features for SUSY at the LHC

50 pb for m_gluino500 GeV 1 pb for
m_gluino1000 GeV
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• The gluinos and squarks cascade down,
• generally in several steps, to the final
  states including
• multi-jets (and/or leptons) and undetected two
  LSPs

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• Characteristic signals of SUSY with Rp

• Invisible LSPs
• ? Missing Transverse Energy
• Decays of squarks and gluinos
• ? Large multiplicity of hadronic jets
• and/or
• Decays of sleptons and gauginos
• ? Isolated leptons

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• Effective mass
• (Multi-jet plus missing ET signature is generic
  in most of R parity conserving models)

An excess of events with large Meff could be the
initial discovery of supersymmetry (Model with
new colored particles decaying into neutral
stable particle)
A mSUGRA point with m0100 GeV, m1/2300 GeV ,
A0300 GeV, tanb2.1 Signal (open circles) SM
background (histogram) With 10 fb-1
(Hinchliffe etal. 1997)
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• LHC (5-sigma) Reach for mSUGRA

The LHC reach in the jet plus MET channel
extend to squark and gluino masses larger than 2
TeV with 300 fb-1
1.5 TeV with 1 fb-1 (one month at low luminosity)
1 TeV with 100 pb-1 (a few days at low
luminosity)
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• SUSY Mass Scale

The peak of the Meff mass distribution provides a
good first estimate of the SUSY mass scale
mSUGRA models
Meff peak 2 MSUSY Typical measurement error
20 for mSUGRA model for 10 fb-1 The
spread might be larger for a more general models
(Hinchliffe etal. 1997)
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Measurement of SUSY masses (Exclusive
Studies)

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• Measurement of SUSY masses

• Precise measurement of SUSY particle masses
• ? Reconstruction of the SUSY theory
• (SUSY breaking mechanism)

• SUSY events always contain two invisible LSPs
• ? No masses can be reconstructed directly

• One promising approach
• ? Identify particular decay chain and measure
  
• kinematic endpoints using visible
  particles
• (functions of sparticle masses)

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When a long decay chain can be identified,
various combinations of masses can be determined
in a model independent way
Five endpoint measurements Four unknown masses
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• The SPS 1a benchmark scenario

A favourable scenario both for LHC and ILC
M_gluino 595 GeV M_qL 534 GeV, M_uR 522
GeV M_N2 177 GeV, M_N1 96 GeV M_eR 143
GeV, M_eL 202 GeV (M_eR lt M_N2)
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• The Cut used to isolate the decay chain

• SM background is suppressed requiring
• - two leptons and large MET (QCD processes)
• - high hadronic activity and MET (Z/Wjets,
  ZZ/ZW/WW)
• - subtraction of OSOF events (t tbar)

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• Dilepton invariant mass distribution after the
  cut

SUSY background mostly from uncorrelated chargino
decays Removed by Opposite-Sign Opposite Flavor
(OSOF) subtraction
77 GeV
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• Other invariant mass edges

In total, five endpoint measurements
Four invovled sparticle masses can be obtained
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• Gluino mass measurement

• Add a quark to an identified squark decay chain
• Consider b squarks to reduce combinatorial
  background
• (b-jet can be tagged)

• The momentum
• near dilepton mass endpoint
• can be approximated by

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Gluino Mass
assuming nominal values for neutralino masses
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Cambridge mT2 variable
(Stransverse Mass)
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• Cambridge mT2

(Lester and Summers, 1999)
Massive particles pair produced Each decays to
one visible and one invisible particle.
For example,
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• MT2 distribution for

LHC point 5, with 30 fb-1,
Endpoint measurement of mT2 distribution
determines the mother particle mass
(Lester and Summers, 1999)
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The LSP mass is needed as an input for mT2
calculation But it might not be known in
advance mT2 depends on a trial LSP mass
Maximum of mT2 as a function of the trial
LSP mass
Can the correlation be expressed by an analytic
formula in terms of true sparticle masses ?
Yes !
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• Right handed squark mass from the mT2

m_qR 520 GeV, mLSP 96 GeV
SPS1a point, with 30 fb-1
(LHC/ILC Study Group hep-ph/0410364)
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• Unconstrained minimum of mT

Trial LSP momentum
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• Solution of mT2 (the balanced solution)

(for no ISR)
with
Trial LSP momenta
mT2 the minimum of mT(1) subject to the two
constraints mT(1) mT(2) , and
pTX(1) pTX(2) pTmiss
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• In order to get the expression for mT2max ,
• We only have to consider the case where
• two mother particles are at rest and all decay
  products
• are on the transverse plane w.r.t proton beam
  direction,
• for no ISR

(Cho, Choi, Kim and Park, 2007)
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(Cho, Choi, Kim and Park, 0709.0288)
Well described by the above Analytic expression
with true Squark mass and true LSP mass
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Some remarks on the effect of squark boost
In general, squarks are produced with non-zero
pT The mT2 solution is invariant under
back-to-back transverse boost of mother
squarks (all visible momenta are on the
transverse plane)
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Cos(theta) distribution
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Gluino mT2 variable
In collaboration with
W.S.Cho, K.Choi, C.B.Park
Ref) arXiv0709.0288, arXiv0711.4526
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• Gluino mT2 (stransverse mass)

A new observable, which is an application of mT2
variable to the process
Gluinos are pair produced in proton-proton
collision Each gluino decays into two quarks and
one LSP


through three body decay (off-shell
squark) or
two body cascade decay (on-shell squark)
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• For each gluino decay,
• the following transverse mass can be
  constructed

• With two such gluino decays in each event,
• the gluino mT2 is defined as

(minimization over all possible trial LSP momenta)
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• From the definition of the gluino mT2

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Each mother particle produces one invisible
LSP and more than one visible particles
Possible mqq values for three body decays of the
gluino
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In the frame of gluinos at rest
Di-quark momenta
Gluino mT2
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• The gluino mT2 has a very interesting property

? mT2 m_gluino for all mqq
This result implies that
( This conclusion holds also for more general
cases where mqq1 is different from mqq2 )
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Unbalanced Solution of mT2 appears
In some momentum configuration , unconstrained
minimum of one mT(2) is larger than the
corresponding other mT(1) Then, mT2 is given by
the unconstrained minimum of mT(2)
mT2(max) mqq(max) mx
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Gluino mT2 distributions for AMSB bechmark point
True gluino mass 780 GeV, True LSP mass
98 GeV
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• If the function can be
  constructed from
• experimental data, which identify the crossing
  point,
• one will be able to determine the gluino mass
  and
• the LSP mass simultaneously.

• A numerical example

and a few TeV masses for sfermions
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• Experimental feasibility

An example (a point in mAMSB) with a few
TeV sfermion masses (gluino undergoes three body
decay) Wino LSP We have generated a MC
sample of SUSY events, which corresponds to 300
fb-1 by PYTHIA The generated events further
processed with PGS detector simulation, which
approximates an ATLAS or CMS-like detector
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• Experimental selection cuts

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• The four leading jets are divided into two
  groups of dijets by hemisphere analysis

Seeding The leading jet and the other jet
which has the largest
with respect to the leading jet
are chosen as two seed jets for the
division Association Each of the remaining
jets is associated to the seed
jet making a smaller opening angle
If this procedure fail to choose two groups of
jet pairs, We discarded the event
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The gluino mT2 distribution with the trial LSP
mass mx 90 GeV
Fitting with a linear function with a linear
background, We get the endpoints mT2 (max)
The blue histogram SM background
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• as a function of the trial LSP
  mass
• for the benchmark point

Fitting the data points with the above two
theoretical curves, we obtain
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• Possible improvements (?) of kink method

• Instead of jet-paring with hemishpere analysis,
• we may calculate mT2 for all possible
  divisions of
• a given event into two sets and then minimize
  mT2 (Mtgen)
• Barr, Gripaios and Lester (arXiv0711.4008
  hep-ph)
• A Variant of gluino mT2 with explicit
  constraint from
• the endpoint of diquark invariant mass (M2C)
• Ross and Serna (arXiv0712.0943 hep-ph)

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Spin effects in SUSY decay chain
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Ref. PLB 596 (2004) 205, (hep-ph/0405052)
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Decay chain under investigation
Spin correlations can play a significant role in
the kinematics of the emitted particles Consider
invariant mass of the quark (from the squark
decay) and lepton (from chi_20 decay)
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decay
It is assumed that neutralino is largely
Wino, so the branching ratios
are highly suppressed compared to the above
decays
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decay
Right-handed lepton goes the same direction to
the quark direction
quark
Right-handed anti-lepton goes the opposite to the
quark direction
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Near lepton quark invariant mass distribution
angle between quark and lepton in
neutralino rest frame
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• Invariant mass distribution of quark (near)
  lepton
• at the parton level for a test point

(mSUGRA point with m0100 GeV, m1/2300 GeV,
A0300 GeV)
shows nice charge asymmetry !
(caused by spin correlations carried by the spin
½ neutralino)
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• Experimental difficulties
• in making such a measurement

• In the decay of an anti-squark
• the asymmetry in the lepton charge
  distribution is
• in the opposite sense to that from squark
  decays
• If equal numbers of squarks and
  anti-squarks were produced,
• no spin information could be obtained
• It will not be possible to distinguish the
  near lepton
• from the far lepton on an event-by-event
  basis

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• Squark Antisquark Production asymmetry

• In a pp collider,
• will produce more squarks than anti-squarks.
• ( The quark PDF is larger than that of the
  anti-quark due to
• the presence of the valence quark )
• For the test point (m_squark, m_gluino 700
  GeV)
• the above production processes sample the PDF
  region
• where valence quarks are significant

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For squark production in the test model,
dominant contribution comes from x 0.1 Twice
as many squarks are produced as anti-squarks for
this point
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• Far lepton quark invariant mass distribution

Slepton has been produced in the neutralino N2
decay, And so has a boost relative to the quark
which depends on its charge
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• The l-q and lq distributions

from both near and far leptons, and from squark
and anti-squark
Charge asymmetry
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Including Detector Simulation and exp. cuts
The charge asymmetry survives, and favours a
spin-½
(black dots with spin correlations, green dots
switched off the spin correlations yellow
parton-level asymmetry 0.6)
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• SUSY vs. UED (Smillie, Webber 2005)

Different spin structures of two models For
hierarchical mass spectra (SPS1a) a good chance
of distinguishing the two models Dashed
SUSY Solid/red UED
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We are entering exciting period in particle
physics. The LHC is about to explore for the
first time the TeV energy scale. The origin of
EWSB ? The nature of dark matter ? Supersymmetry
? Extra dimensions ?
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Tau Polarization in SUSY Cascade decays
In collaboration
with S.Y.Choi, K.Hagiwara, K.Mawatari, P.M Zerwas
Ref) PLB 648207 (2007) hep-ph/0612237
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• Much attention has been paid in the recent past
  to
• the SPS1a cascade

• So far, cascades have primarily been studied
  involving
• first and second generation leptons/sleptons.

• Explore how the polarization of tau leptons
• can be exploited to study R / L chirality and
  mixing effects
• in stau and neutralino sector

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• Single pion decays of tau as polarization
  analyzer

At high energies, the fragmentation functions for
pions
( z energy fraction transferred from the
polarized tau to the pion. R / L
tau chirality )

• Pion from the right-handed polarized tau-
• is harder than the one from
• left-handed polarized tau-

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Neutralino decay
results in hard pions

• On the contrary,
• results in soft pions.

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• Invariant mass distribution of tau-tau and pi-pi
  

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• Dependence of on the stau mixing angle

With pion momentum cut Without pion momentum cut
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• m (pi-pi) distribution for SUSY (SPS1a) and UED

SPS1a LR type UED LL type
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• m (pi-pi) distribution for SUSY (SPS1a) and UED

SPS1a LR type UED LL type
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We are entering exciting period in particle
physics. The LHC is about to explore for the
first time the TeV energy scale. The origin of
EWSB ? The nature of dark matter ? Supersymmetry
? Extra dimensions ?
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For ttbar events