SUSY using b- and - signatures at LHC

1

SUSY using b- and - signatures at LHC

Magali GruwéCERN – Atlas

Workshop on b/ physics at the LHC30 May – 1 June 2002

Workshop on b/ physics at the LHC – 30 May 2002Magali Gruwé CERN - Atlas

2

Introduction

Quite a few Susy final states with b’s and/or ’s Good b/ reconstruction allows

full or partial reconstruction of Susy events determination of some sparticle masses

Various cases (various Susy models): One case will be detailed, as an example:

mSugra models with large tg (both and b rec. needed) Some other cases will be briefly described:

need either or b reconstruction methods and ideas rather similar expected results outlined

Some cases not shown here... Some cases not even studied yet...

Everything presented here: based on Atlas simulations of course valid for CMS as well...


3

Layout

Reconstruction (outlined) Some words about reconstruction and b tagging

Using and b reconstruction: In mSugra models with large tg

Using reconstruction: Lepton flavour violation GMSB with 1 as the NLSP

Using b reconstruction In mSugra models

h b b in cascade decays g b… b ll…

Conclusion

Described in detail

Only briefly shown

~

~ ~


4

Reconstruction

finder:Concentrate on hadronic decays

leptonic decays not used because:– origin of lepton not clear– lepton in general carries only a small fraction of the

momentum

Performances (of finder used for Susy) Rejection against jets: factor of 15 Probability for a jet to be correctly tagged as : 66% Probability for its charge to be correctly assigned:

92%When :

invariant mass distribution– ~gaussian– peaks at M = M

/M =


5

Reconstruction

b-tagging:Efficiency ~ 60%

Lepton identification (e or ):Efficiency ~ 90%


6

Layout






Conclusion

Described in detail

Only briefly shown

~

~ ~


7

Susy at the LHC

Susy rates dominated by the production of gg gq qq

(depending on Susy model) Lightest Susy particle (

) is stable neutral escapes the detector gives missing energy

Classical signature for Susy production: Excess of final states

with missing energy ( E ) with 2 hard central jets arising from qq, qg,...

~~

~

~~~~

~~

~~


8

Sugra models

Reminder:Minimal Sugra has 4 parameters + a sign:

m : common scalar mass at GUT scale

m: common gaugino mass at GUT scale

A: common trilinear Higgs sfermion-sfermion coupling at GUT scale

tg : ratio of the Higgs vacuum expectation values

sgn(): being the SUSY conserving Higgsino mass


9

Sugra models with large tg

is the lightest l If m small enough:

h, Z not allowed

If m small enough:

lR l not allowed

The only allowed 2-body decay is:

decays are dominant:Final states involving ’s must be used!

Use benchmark point 6:

m = 200 GeVm = 200 GeVA = 0tg = 45sgn() = -1

mg = 540 GeVmb

= 391 GeVm = 152 GeVm = 81 GeV

~ ~

~ ~ ~

~ ~

~ ~~


10

Decay chains

g b b

b

g b b

qL q

q q

Significant production of

significant production of pairs from the decay of qL or g

Significant production of b’s (from g)

~ ~

~

~~

~ ~~

~

~

~ ~~

~ ~

~~


11

Analysis procedure

I. Select Susy eventsII. Select final states

infer information on m

III. Conjunction with one b-jet reconstruct b get mb

IV. Conjunction with two b-jets reconstruct g get mg

V. Extract other sparticle masses (q, )

VI. Constrain underlying model parameters

Described here

Not shown here

~

~

~

~

~~ ~


12

Selection of events

Select Susy events (reject SM background)Typical cuts:

ET GeV

4 jets with pT GeV, pT GeV

ST (transverse sphericity)

No or isolated e with pTGeV, || 2.5

ETMeff

with Meff = ET + pT (sum over 4 hardest jets)

Find final states In addition:

at least 2 jets identified as hadronic taus MeffGeV


13

Pair of ’s

Aim: reconstruct

Search for’s decaying hadronically:Reconstruct invariant mass

Combinations of onereal and one fake

Combinations oftwo real ‘s

In a real experiment: observation is the sum of the two histograms

invariant mass

Peak: infer position of the endpoint arising from

:

Mmax = fn (m, m, m)

(m)2 (m)2

Mmax =m 1 1 (m

)2 (m)2

= 59.6 GeV (expected)

~ ~ ~

~ ~ ~

~ ~ ~

~

~

~

~

~


14

Pair of ’s

Previous plot: all charge combinations

Now: subtracted distribution:

eliminates background (also from fake ’s because their charges are not correlated)

Peak, allowing to reconstruct Mmax = fn (m, m, m)

Real excess(due to

or decays)

~ ~ ~

~ ~


15

Pair of ’s + tagged b jets

Aim: reconstruct g bb bb

bb bb

Other g decays possible... But concentrate on final

states with b quarks as BR larger (55%) combinatorial background

lower

In addition to previous cuts: Require two tagged b-jets,

each with pT 25 GeV Select events with a M

within 10 GeV of the peak

~ ~ ~ ~ ~

~


16

Pair of ’s + one tagged b jet

First: reconstruct b b

b b

Form invariant mass of the pair and b-jet: Mb

Edge expected at

Mmax ~ mb1 – m

= 310 GeV

Edge not sharp because: 3 particles lost:

2 neutrinos 1

distribution contaminated by decays from

and

There should be an edge at ~ mb1

– m =

310 GeV

b invariant mass

~ ~ ~ ~

~ ~

~

~ ~

~ ~


17

Pair of ’s + 2 tagged b jets

Partial reconstruction technique: Infer momentum of

assuming nominal m

m

p = 1 p

m

Combine with momenta of 2 measured b jets

Get:

mb mbb

related to mb related to mg

as b b as g bb bb

~ 450 GeVexp. mg = 540 GeV

~ 120 GeVexp. mgmb = 150 GeV

~~

~

~

~

~ ~ ~

~ ~~~~~~

~


18

Layout






Conclusion

Described in detail

Only briefly shown

~

~ ~


19

Lepton Flavour Violation

Motivation: to oscillationsmixing of the two mass eigenstates requires LFV

Susy can accomodate LFVFor / mixing, significant mixing needed

in / sector Search for direct signals from LFV in Susy

sector

~ ~


20

Decays

Use: an mSugra point some modifications to get slepton mass

matrix (with mixing terms in the / sector)

Decays:

Z 7%

eR e 13%

R 13%

66%g q ... 100%qL

... 30% Most copious source of dileptons:

l l , with

coming from q decay

Use mSugra point:

m = 100 GeVm = 300 GeVA = 300 GeVtg = 10sgn() = +1

with:

ML = 236.0 GeVML = 235.2 GeVMR = 153.3 GeVMR = 150.7 GeV = 360.2 GeVA = 94.9 GeV

~ ~

~

~

~

~

~

~

~

~ ~ ~ ~

~ ~


21

Effects of Lepton Flavour Violation

Main effect of LFV: add a component of L in

decays

are allowed add and

signals

asymmetry between / and e/e final states

BR(

) through

BR(

) through

mixing parameter

~ ~~ ~ ~

~~ ~

~

~

~

~


22

Counting events

SM: l/l/l/l

l/l

l/l

/ LFV with BR = 10%

Again: cuts to remove SM bkgd selection of events with

a hadronic an isolated lepton

in final state reconstruct l invariant mass

Count events around expected signal (between 50 and 100 GeV): For 10 fb:

N(e/e) = 501N(/) = 977N(/) N(e/e)

= 476 ± 39 = 12.2% excess

Could get limit on BR(

)~ ~


23

Layout






Conclusion

Described in detail

Only briefly shown

~

~ ~


24

GMSB

Reminder:Parameters of the minimal GMSB models:

= Fm / Mm : effective Susy breaking scale (30 TeV)

Mm : messenger scale (250 TeV)

N5 : parametrization of the SU(5) messenger fields (3)

tg : ratio of the Higgs vacuum expectation values (12)

sgn() : being the Higgsino mass term (+) Cgrav : gravitino mass scaling factor (1)


25

GMSB with as the NLSP

For large tg becomes the NLSP (tg = 12, here)

Decay chain:q

q

l l l ()

G

Very clear same flavour opposite charge dilepton signal with a hard

Due to limited phase space in the decay: this is very soft and undetectable

Missing energy

HardSoft but detectable (~ 10 GeV)

Very hard

~ ~~

~~

~

~


26

Analysis procedure

Selection: Again cuts to remove SM background Require same flavour opposite charge dilepton One hard hadronic (pT 75 GeV)

Slepton reconstruction Select l combinations either with

soft lepton (pT 15 GeV): lepton from l decay

hard lepton (pT 15 GeV): lepton from decay

Get invariant mass of l system Neutralino reconstruction

Get invariant mass of ll system (one hard and one soft leptons)

Squark reconstruction Get invariant mass of llj system

~

~


27

l system

Invariant mass of l system: with soft lepton: with hard lepton:

Mmax = (mlR)2 – (m + m)2

= 29.0 GeV

Mmax = (m)2 –(mlR

)2

= 51.7 GeV

~ ~ ~ ~


28

ll system

Invariant mass of ll system:

(mlR)2 (mm)2

Mmax =m 1 1

(m)2 (mlR

)2


(mlR)2 (mm)2

Mmax =m 1 1

(m)2 (mlR

)2


~

~

~

~

~

~

~

~

~

~


29

llj system

Invariant mass of llj system: with Mll 15 GeV: with 15 GeV Mll 46

GeV: (enhances contribution from (enhances

contribution from qR

q ) qL q )

(m)2 (mm)2

Mmax =mqR 1 1

(mqR)2 (m

)2

(m)2 (mm)2

Mmax =mqL 1 1

(mqL)2 (m

)2

~

~

~ ~

~

~

~

~ ~

~

~~~~


30

Fit

Enough mass relations to solve for 6 unknown masses:

Mmax(l, l soft) = fn (mlR, m)

Mmax(l, l hard) = fn (m, mlR

)

Mmax(ll, 1st edge) = fn (m, mlR

, m)

Mmax(ll, 2nd edge) = fn (m, mlR

, m)

Mmax(llj, Mll 15 GeV) = fn (mqR, m

, m)

Mmax(llj, Mll [15,46] GeV) = fn (mqL, m

, m)

~

~

~

~

~

~

~~

~ ~

~

~

~

~~

~


31

Layout






Conclusion

Described in detail

Only briefly shown

~

~ ~


32

h bb in cascade decays

Decay chainqL

q h

tob b to

Analysis procedure:Get clean sample of h bb

Reconstruction of h bb decay Get mh

Partial reconstruction of qL q h

q Get invariant mass of jbb system

– sensitive to mqL

Get pT distribution of 2nd hardest jet– sensitive to mqL

or mqR

Use benchmark points 1, 2 and 5~ ~

~

~ ~ ~

~

~ ~


33

Reconstruction of h bb decays

Expected events: About 20% of Susy events contain h bb About 90% of these contain only 1 h boson, with pT 200

GeV Selection:

Remove SM background: ET 300 GeV

Require 2 tagged b-jets with pT 50 GeV

Enhance signal purity (for more detailed studies on the structure of the Susy signal

events)

No additional b-jet with pT 50 GeV

Rbb = ()2 + ()2 2 Lepton veto 2 additional jets with pT 100 GeV One of them at least within 2.0

Enough for clear h bbsignal over bkgd


34

Pair of b’s

Invariant mass of bb system:

Select events with Mbb within 25 GeV of peak: SM bkgd 10% of signal Susy bkgd 20% of signal

Fit of peak:mh ~ 1 GeV

SM bkgd

SM bkgd +Susy bkgd

SM bkgd +Susy bkgd +Signal


35

Rec. of qL q

h q b b q

Reconstruct jbb in final state:pT distribution of the bb pair: Mjjb invariant mass:

sensitive to qL and masses sensitive to qL,

and masses

m = 325 GeV

m = 425 GeV

Combination giving minimal Mjjb

Both combinations(2 entries per event)

~

~

~ ~ ~ ~ ~

~~~~


36

Observability of the h bb channel

Observability 5-contours of h bb from Susy cascade:

Areas for S/B 5For: tg = 2 sgn() = + A0 = 0

Contours of expected # of signal events

BR( h

) = 0.5(opening of the channel)

~ ~


37

Layout






Conclusion

Described in detail

Only briefly shown

~

~ ~


38

Multi-lepton / multi-b channels

mSugra models in which q heavier than g g b b predominantly

Topology:q g b

bL b

q

l

b

l l

Analysis: Selection, get invariant

masses, partial reconstruction, etc...Get masses of sparticles

mbb – mb

mb

mg – mb

mg

~ ~

~ ~

~

~

~~

~

~ ~~ ~

~ ~

~


39

Conclusions

Many Susy models predict topologies containing b’s and/or ’s in the final state

These are used in many ways: for selection of signal events as starting point of analysis chain in cascades to reconstruct sparticle masses to constrain Susy models

More possibilities than the ones presented here: for example:

g t t, b b t b… tau polarization studies

~ ~ ~

Documents

SUSY using b- and - signatures at LHC