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SUSY at the LHC

Yeong Gyun Kim (Sejong U. KAIST)

? 2 ? ??-CMS ?? Workshop (Feb. 13-14, 2008)

Contents

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

Introduction

- General features for SUSY at the LHC

50 pb for m_gluino500 GeV 1 pb for

m_gluino1000 GeV

- The gluinos and squarks cascade down,
- generally in several steps, to the final

states including - multi-jets (and/or leptons) and undetected two

LSPs

- 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

- 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)

- 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)

- 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)

Measurement of SUSY masses (Exclusive

Studies)

- 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)

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

- 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)

- 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)

- Dilepton invariant mass distribution after the

cut

SUSY background mostly from uncorrelated chargino

decays Removed by Opposite-Sign Opposite Flavor

(OSOF) subtraction

77 GeV

- Other invariant mass edges

In total, five endpoint measurements

Four invovled sparticle masses can be obtained

- 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

Gluino Mass

assuming nominal values for neutralino masses

Cambridge mT2 variable

(Stransverse Mass)

- 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)

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 !

- 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)

- Unconstrained minimum of mT

Trial LSP momentum

- 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)

(Cho, Choi, Kim and Park, 0709.0288)

Well described by the above Analytic expression

with true Squark mass and true LSP mass

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)

Cos(theta) distribution

Gluino mT2 variable

In collaboration with

W.S.Cho, K.Choi, C.B.Park

Ref) arXiv0709.0288, arXiv0711.4526

- 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)

- 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)

- From the definition of the gluino mT2

Each mother particle produces one invisible

LSP and more than one visible particles

Possible mqq values for three body decays of the

gluino

In the frame of gluinos at rest

Di-quark momenta

Gluino mT2

- 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 )

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

Gluino mT2 distributions for AMSB bechmark point

True gluino mass 780 GeV, True LSP mass

98 GeV

- 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

- 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

- Experimental selection cuts

- 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

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

- as a function of the trial LSP

mass - for the benchmark point

Fitting the data points with the above two

theoretical curves, we obtain

- 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)

Spin effects in SUSY decay chain

Ref. PLB 596 (2004) 205, (hep-ph/0405052)

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)

decay

It is assumed that neutralino is largely

Wino, so the branching ratios

are highly suppressed compared to the above

decays

decay

Right-handed lepton goes the same direction to

the quark direction

quark

Right-handed anti-lepton goes the opposite to the

quark direction

Near lepton quark invariant mass distribution

angle between quark and lepton in

neutralino rest frame

- 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)

- 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

- 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

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

- 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

- The l-q and lq distributions

from both near and far leptons, and from squark

and anti-squark

Charge asymmetry

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)

- 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

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 ?

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

- 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

- 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-

Neutralino decay

results in hard pions

- On the contrary,
- results in soft pions.

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

- Dependence of on the stau mixing angle

With pion momentum cut Without pion momentum cut

- m (pi-pi) distribution for SUSY (SPS1a) and UED

SPS1a LR type UED LL type

- m (pi-pi) distribution for SUSY (SPS1a) and UED

SPS1a LR type UED LL type

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 ?

For ttbar events