Probing Supersymmetry with Photons - Tevatronqianj/fnalwc98.pdf · 2000-05-19 · Probing...

Probing Supersymmetry with Photons

Jianming QianUniversity of Michigan

for the DØ Collaboration

IntroductionSearch for γγE/ T events

Search for γE/ T+≥2-Jet eventsSummary

Wine & Cheese Seminar, Fermilab

June 12, 1998

Motivations for Supersymmetry

W&C 98

Why supersymmetry?Or any theory beyond the Standard Model?

There are, however, theoretical problems withthe Standard Model associated with the disparities

in the known mass scales in physics

The Higgs boson receives radiativecorrections which are quadratically divergent

H H

V

H H

H

H Hf

Since the fermion and boson loops have opposite signs,the leading quadratic divergences will cancel if

there are equal numbers of bosons and fermions withidentical couplings

H H

V~

H H

H~

H Hf~

δmH2≈-( )(Λ2+m2

F)+( )(Λ2+m2B)≈O( )|mB

2-mF2|

g2F g2

B α4π2 4π2 π

Motivations for Supersymmetry

W&C 98

Historically, introducing new particles served us well

In 1928, Dirac proposed that each particlehad to have a partner - antiparticle

Charm quark was postulated to solve theK0→µ+µ- problem (GIM mechanism)

and was discovered in 1974

W boson was introduced to make σ(νee→νee) finiteand was discovered in 1983

σ ⇒G2

F s

π

W

σ ⇒G2

F M2W

π

⇒

We need the Higgs boson to make σ(W+LW-

L→W+LW-

L) finitethough it remains to be discovered

+H

Supersymmetry Models

W&C 98

Supersymmetry predicts a supersymmetricpartner (sparticle) for every Standard Model particle

Weak-scale supersymmetry predictsthe radiative breaking of the electroweak symmetry

Minimal Supersymmetric Standard Model (MSSM)is the simplest supersymmetric model

(1) add an extra Higgs doublet of opposite hypercharge(2) supersymmetrization of the gauge theory

Standard Model ParticlesGauge/Higgs Bosons: γ, Z0, W±, h0, H0, A0, H±, gLeptons/Quarks: (ν,e)L, eR, (u,d)L, uR, dR, ...

Supersymmetric ParticlesGauginos/Higgsinos: χ

~

10, χ

~

20, χ

~

30, χ

~

40, χ

~

1±, χ

~

2±,

Sleptons/Squarks: (ν~,e~)L, e

~

R, (u~,d~)L, u

~

R, d~

R, ...g~

Lots of free parameters⇒ theorists´ dream, experimenters´ nightmare...

Double the number of particles ⇒half of the particles remain to be discovered...


W&C 98

Within the MSSM, the gaugino-higgsino sectoris described by only four parameters

M1 the U(1) gaugino mass parameterM2 the SU(2) gaugino mass parameterµ higgsino mass parametertanβ ratio of VEV of the higgs doublet

(Gaugino mass unification M1= M2tan2θW)53

Most supersymmetric models assume that R-parity(R=+1 for the SM particles and R=-1 for their partners)

is conserved(1) supersymmetric particles are pair produced(2) heavy sparticles decay to lighter sparticles(3) the LSP is stable (no available decay mode)

⇒ missing transverse energy (E/ T)

Supersymmetry cannot be an exact symmetryIt is assumed to be broken in a hidden sector

A messenger sector transmits the SUSY breakingto the visible sector (SM particles and their superpartners)

The messenger sector interactions are assumed to be eitherof gravitational strength (gravity inspired models)or SM gauge interactions (gauge mediated models)


W&C 98

In gravity inspired models,the supersymmetry breaking scale is generally of

ΛSUSY∼ 109 TeV

Resulting in a massive gravitino (G~)

⇒ no role in low energy phenomenology⇒ LSP=the lightest SM superpartner (often χ

~01)

Have been the focus of experimental searchesthe standard signatures are leptons, jets (w/o leptons) and E/ T

In gauge mediated models,the supersymmetry breaking scale can be as low as

ΛSUSY∼ 100 TeV

Resulting in an exceedingly light gravitino⇒ gravitino is naturally the LSP⇒ the lightest SM superpartner is the NLSP⇒ NLSP is unstable and decays to G

~

Phenomenology depends on NLSP andmost models assume NLSP=χ

~01 or τ

~

χ~0

1→γ G~, τ

~→τ G

~

Not well explored experimentally until recently

Experimental Status of Supersymmetry

W&C 98

There are no confirmed data that disagreewith the Standard Model predictions

Searches for supersymmetry have all been negative

However, the apparent unificationof the three gauge coupling constants is suggestive

It is unlikely thatwe can ever exclude supersymmetry...

Photon as a Probe for Supersymmetry

W&C 98

A CDF event has generated considerabletheoretical and experimental interests in using

photons as probe for supersymmetry

In Gauge Mediated Models with NLSP=χ~0

1

χ~0

1→γ G~

occurs with almost 100% branching ratio ifχ~0

1 has a non-zero photino component

Any supersymmetric particle will producea photon and a G

~ in its decay chain

However, the χ~0

1 decay width

Γ∝m-2(G~)

χ~0

1 can have sizable decay distance

Pair production of supersymmetric particleswill result in γγE/ T+X events

if both χ~0

1 decay inside the detector

pp_→χ

~+χ~-→W+W-χ

~01χ

~01

pp_→e

~e~→eeχ

~01χ

~01

were proposed as possible explanations of the eventEllis et al., PRB 394 (1997), Ambrosanio et al., PRD 54, 5395 (1996), ...

Photon as a Probe for Supersymmetry

W&C 98

Within the framework of MSSM with the LSP=χ~0

1,a class of models with dominant

e~→e+χ

~

20 and χ

~

20→χ

~

10+γ

decays was also proposed as a plausible explanation of the event

pp_→e

~e~→eeχ

~02χ

~02→eeγγχ

~01χ

~01

Kane et al., Phys. Rev. D55, 1372 (1997)

In these models, M1∼ M2, tanβ∼ 1 and µ<M2χ~0

1 is mostly higgsino and χ~0

2 is mostly gauginoNo gaugino mass unification

The event kinematics and rate suggest thatm(χ

~02)-m(χ

~01)>20 GeV/c2

Br(χ~0

2→χ~0

1+γ)≈100%

γE/ T+jets events are expected frompp

_→q

~/g~→χ

~02+X processes

γγE/ T events are expected frompp

_→e

~e~, ν

~ν~, χ

~

20χ

~

20+X processes

Photon Identification

W&C 98

Isolated photons are identified through a two-step process

1) identification of isolated EM clusters2) rejection of electrons

Identification of EM clusters

1) Electromagnetic energy fraction > 0.952) Shower profile consistent with a EM shower3) Isolation = (Econe(0.4)-Econe(0.2))/Eγ < 0.1

For photons with ET>20 GeV, ε∼ 90%

0

200

400

0.9 0.925 0.95 0.975 1

Bad e dominated

Good e dominated

EM Fraction

0

200

400

600

800

0 100 200 300χ2

χ2 <100

Bad e dominated

Good e dominated


W&C 98

Electron Rejection

Events with large E/ T are dominatedby W productions with W→eν

Electron is rejected by the presence ofa reconstructed track or a large number of hits

Still, there will be one electron misidentifiedas a photon for every 220 identified electrons

About 30% of photons is also lostdue to random overlaps

0

50

100

150

0 10 20 30 40 50 60 70 80 90 100E/ T (GeV)

Eve

nts

Loose W events

After track-veto


W&C 98

Conversions

Many photons are lost due toconversion in the materials upstream

The conversion probability is about10% in |η|<1.1 (CC region) and about 30% in 1.5<|η|<2.0 (EC region)

determined using single photon Monte Carlo

Most of photons from high pT processesare in the central region

0

0.1

0.2

0.3

0.4

0.5

0 5 10 15 20η×10

Con

vers

ion

Pro

b.

CC

EC

Trigger and Luminosity

W&C 98

Trigger

(1) One E.M. cluster with ET>15 GeV(2) A second object with ET>10 GeV(3) E/ T>14 (10) GeV

The trigger is >95% efficientfor events of interest in these analyses

Luminosity

The data used in this analysis were takenduring the 1992-1996 Tevatron Run

The integrated luminosity for this analysis is∼ 100 pb-1

Search for γγE/ T Events

W&C 98

Two high ET photonsLarge missing transverse energy

with/without leptons/jets

There is almost no Standard Modelbackground at parton-level

But there are important instrumental backgrounds

(1) multijet, direct photon events

(2) W+γ, Z→ττ→ee, tt_→ee+jets


W&C 98

Event Selection

(1) Eγ1T >20 GeV |η|<1.1 or 1.5<|η|<2.0

(2) Eγ2T >12 GeV |η|<1.1 or 1.5<|η|<2.0

(3) E/ T>25 GeV

No requirements on jets or other objects were made

Two events survivedfrom a data sample of Ldt = 106.5±5.6 pb-1∫

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100E/ T (GeV)

Eve

nts

E/T >25 G

eVAfter Track-Veto

After Hit-Veto


W&C 98

QCD Background

Multijet and direct photon events withmisidentified photons and/or mismeasured E/ T

will fake γγE/ T events

This background was estimatedusing events with two EM-like clusters

By normalizing the observed E/ T distributionsa background of 2.1±0.9 events was obtained

W-Like Background

Events with genuine E/ T such as those fromW+γ, Z→ττ→ee, tt

_→ee+jets would fake γγE/ T

events if the electrons were misidentified as photons

We estimate their contribution using a sampleof e+γ events passing the kinematic requirements

Applying the electron rejection factor from the photon IDa background of 0.2±0.1 events was obtained

Total number of background events 2.3±0.9


W&C 98

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100E/ T (GeV)

Eve

nts

γγ sampleTotal Background

After Track-Veto

2

4

6

8

10

12

14

0 50 100 150 200E/ T (GeV)

Eve

nts

γγ sampleBackground(µ,M2)=(-160,400)(×10)(µ,M2)=(600,180)(×10)

After Hit-Veto


W&C 98

χ~

iχ~

j Pair Production

We interpret our null resultsin terms of chargino and neutralino pair production

pp_→χ

~

iχ~

j→χ~0

1χ~0

1+X→γγG~G~+X

within the framework of MSSM with LSP=G~

We explore the (µ,M2) parameter space within the MSSMassuming gaugino mass unification at the GUT scale

M1= M2tan2θW53

and keeping tanβ fixed.

For the most part of the parameter spacethe pair production is dominated by

pp_→χ

~

1±χ

~

1±, χ

~

1±χ

~

20 + X

The chargino/neutralino production and decaysare modeled using Spythia Monte Carlo program

The efficiency for a typical point of interestin the parameter space is about 25%


W&C 98

0

50

100

150

200

250

300

350

400

-750 -500 -250 0 250 500 750µ (GeV)

M2

(GeV

)

tanβ=2

0.1 pb0.1 pb

1 pb1 pb

10 pb10 pb

0

50

100

150

200

250

300

350

400

-750 -500 -250 0 250 500 750µ (GeV)

M2

(GeV

)

tanβ=2

150150

100100

50504040

80 80

χ~

1±

χ~

10


W&C 98

10-2

10-1

1

10

10 2

0 50 100 150 200 250ET (GeV)

Eve

nts

γ1

γ2

(µ,M2)=(500,40)(×1.0E-3)

γ1

γ2

(µ,M2)=(-140,300)

0

0.02

0.04

0.06

-4 -3 -2 -1 0 1 2 3 4η

(µ,M2)=(500,40)(µ,M2)=(-140,300)


W&C 98

Signal Efficiencies

Pair production of charginos and neutralinosis modeled using Spythia Monte Carlo program

µ (GeV) M2 (GeV) m(χ~

10) (GeV/c2) m(χ

~

1±) (GeV/c2) εK(%) ε (%)

-160 500 156 167 66.0 33.4

-600 160 83 166 58.0 18.4

200 300 118 160 66.8 27.9

800 170 83 162 58.7 25.4

0

100

200

300

400

500

-1000 -500 0 500 1000µ (GeV)

M2

(GeV

)

tanβ=2

Sampled points


W&C 98

Bounds in (µ,M2) Plane

Based on 2 events observed and 2.3±0.9 events expected,we set 95% C.L. upper limit on the cross section

The limit is typically ∼ 200 fb for the region of interest

We also set 95% C.L. lower mass limits

m(χ~

1±)>150 GeV/c2

m(χ~

10)>77 GeV/c2

Suggested: χ~+χ

~-→eeγγ ννG~G~

0

50

100

150

200

250

300

350

400

-750 -500 -250 0 250 500 750µ (GeV)

M2

(GeV

)

tanβ=2

Excluded

DØ boundsLEP bounds

Ellis et al.PL B394, 354 (1997)

m(χ~±

1)=150 GeV/c2

m(χ~0

1)=77 GeV/c2


W&C 98

tanβ Dependence

The bounds depend on the value oftanβ slightly, due to the tanβ dependence

of the expected cross section

As tanβ is increased, the limits becomestronger in the µ<0 half-plane

and weaker in the other half-plane

NLSP will be τ~ in most models for large tanβ values

0

100

200

300

400

500

-1000 -500 0 500 1000µ (GeV)

M2

(GeV

)

Excluded

tanβ=1.05tanβ=2.0tanβ=100


W&C 98

Limits for χ~

1±χ

~

1±, χ

~

1±χ

~

20 Productions

pp_→χ

~

1±χ

~

1±, χ

~

1±χ

~

20 dominates

pair production of charginos and neutralinos

For a large part of the parameter space

m(χ~

1±)≈m(χ

~

20)≈2m(χ

~

10)

For heavy massesthe upper cross section limit is ∼ 200 fb

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

80 100 120 140 160 180 200 220m(χ

~±1) (GeV/c2)

Cro

ss S

ectio

n (p

b)

χ~

1±χ

~

1±

χ~

1±χ

~

20

Predicted

95% C.L. Limits


W&C 98

e~e~, ν

~ν~, χ

~02χ

~02 Production

In the models of Kane et al.,

the e~e~, ν

~ν~, χ

~02χ

~02 production can also result γγE/ T events

with e~→eχ

~02, ν

~→νχ

~02 and χ

~02→χ

~01+γ

The event topology is largely determined bythe mass difference between χ

~

20 and χ

~

10

For a given m(χ~0

2)-m(χ~0

1)the efficiency is almost independent of the processes

0

5

10

15

20

25

30

0 10 20 30 40 50 60m(χ

~02)-m(χ

~01) (GeV/c2)

Tot

al E

ffici

ency

e~e~

ν~ν~

χ~0

2χ~0

2


W&C 98

Limits on e~e~, ν

~ν~, χ

~02χ

~02 Production

With two observed γγE/ T events and2.3±0.9 events expected from backgrounds, we set 95% C.L.

upper cross section limits on e~e~, ν

~ν~, χ

~02χ

~02 production

For m(χ~0

2)-m(χ~0

1)> 30 GeV/c2,the 95% C.L. upper cross section limit is ∼ 400 fb

almost independent of the processes

10-1

1

10

10 20 30 40 50 60m(χ

~02)-m(χ

~01) (GeV/c2)

95%

C.L

. σ (

pb)

ν~ν~

e~e~

χ~0

2χ~0

2


W&C 98

Theoretical Cross Sections

However, the theoretical cross sections forpp

_→e

~e~, ν

~ν~, χ

~02χ

~02→γγE/ T+X

production are small even with the assumptionsBr(e

~,ν~→e,ν+χ

~02)=100% and Br(χ

~02→χ

~01+γ)=100%

The experimental upper cross section limits are abovethe theoretical cross sections for the mass region of interest.

10-1

1

10

10 2

100 125 150 175 200 225 250 275 300m(e

~) (GeV/c2)

Cro

ss S

ectio

n (f

b)

pp_→e

~

Re~

R+e~

Le~

L

Search for γE/ T+≥2-Jets Events

W&C 98

One high ET photon, two or more jetsLarge missing transverse energy

There is almost no Standard Modelbackgrounds at parton-level

But there are important instrumental backgrounds(1) multijet, direct photon events(2) e+jets (W+jets, tt

_,...) and ν+jets events

Events with less than two jets are not considereddue to the large backgrounds from QCD and W→eν events


W&C 98

Selection of Base Sample

(1) EγT>20 GeV, |η|<1.1 or 1.5<|η|<2.0

(2) Two or more jets with EjT>20 GeV, |η|<2.0

(3) E/ T>25 GeV

A total of 378 events are selected(74 events with ≥3-jets and 10 events with ≥4-jets)

from a data sample of Ldt = 99.4±5.4 pb-1∫

1

10

10 2

10 3

0 20 40 60 80 100 120 140

Background

γ+≥2-jets

Cut

E/ T (GeV)

Eve

nts


W&C 98

Multijet Backgrounds

Multijet (with misidentified photon)and direct photon events with mismeasured E/ T

will fake γE/ T+≥2-jets events

E/ T mismeasurement can be modeled usingmultijet events with photon-like clusters

The estimated multijet background is370±36 events

e/ν+jets Backgrounds

Events with genuine E/ T such as those fromW(→eν)+jets and Z(→νν)+jets would fake γE/ T+≥2-jets

events if the electrons or jets were misidentified as photons

We estimate their contributions using the fakeP(e→γ) and P(jet→γ) probabilities

The estimated e/ν+jets background is 6±1 events

Total background 376±36


W&C 98

0

20

40

60

80

100

20 40 60 80 100 120 140 160 180ET

γ (GeV)

Eve

nts

BackgroundγE/ T+≥ 2-jets

0

20

40

60

50 100 150 200 250 300 350HT (GeV)

Eve

nts

BackgroundγE/ T+≥2-jets

HT=ΣETj


W&C 98

m(q~

)=m

(g~)

0

50

100

150

200

250

300

2 3 4 5 6 7 8Jet Multiplicity

Eve

nts

γE/ T+≥2-jetsBackground

020

4060

80100

120140

160180

20 40 60 80 100 120 140 160 180(All) Jet ET (GeV)

Eve

nts

Background

γE/ T+≥2-jets


W&C 98

Squark/Gluino Production

We interpret our results in terms ofsquarks/gluinos production within the models of Kane et al.

The production of pp_→(q

~, g

~, χ

~02) →χ

~02+X

are modeled using Spythia program

Br(q~/g~→χ

~02+X) depends on

the MSSM parameters: M1, M2, µ, and tanβ and scalar masses

About 60% of the events containing χ~0

2

10-2

10-1

1

10

10 2

150 200 250 300 350 400Mass (GeV/c2)

Cro

ss S

ectio

n (p

b)

q~/g~

χ~0

2+X

m(q~)=m(g

~)

q~/g~

χ~0

2+X

m(q~)«m(g

~)


W&C 98

χ~0

2→χ~0

1+γ Decay

The χ~0

2 decay is governed bythe four MSSM parameters: M1, M2, µ, and tanβ

The mass difference between χ~0

2 and χ~0

1affects photon ET and E/ T

The branching ratio of χ~0

2→χ~0

1+γ decaydirectly affects the γE/ T+≥2-Jets event rate

M1=M2 (GeV)

µ=-40 GeV, tanβ=2.0

40 50 60 70 800

0.2

0.4

0.6

0.8

1

Br(

χ~0 2→

χ~0 1+

γ)

Br

0

10

20

30

40

50

60

m(χ~

0 2)-m

(χ~0 1)

(G

eV/c

2 )

δm


W&C 98

Signal Simulation

We simulate pp_ → q

~q~, q

~g~, g

~g~, q

~χ~, g

~χ~ production

using the Spythia program1) M1=M2=60 GeV, µ=-40 GeV, and tanβ=2.02) heavy scalar leptons3) no stop production

for three different squark/gluino mass scenarios1) m(q

~)=m(g

~)

2) m(q~)»m(g

~)

3) m(q~)«m(g

~)

For the case m(q~)=m(g

~), the expected numbers of events are

351 for m(q~)=200 GeV/c2 and 19 for m(q

~)=300 GeV/c2

in the base sample


W&C 98

m(q~

)=m

(g~) 1

10

10 2

10 3

0 20 40 60 80 100 120 140E/ T (GeV)

Eve

nts

Backgroundγ+≥2-jets

m(q~)=200 GeV/c2

m(q~)=300 GeV/c2 (×10)

0

20

40

60

50 100 150 200 250 300 350HT (GeV)

Eve

nts

BackgroundγE/ T+≥2-jets

m(q~)=200 GeV/c2

m(q~)=300 GeV/c2 (×10)

E/ T>25 GeV


W&C 98

Selection Optimization

The base sample is dominated by multijet backgroundsEvents from supersymmetry are expected tohave very different E/ T and HT distributions

To increase sensitivity to supersymmetry, we optimizethe event selection in E/ T-HT plane

E/ T and HT cuts are varied to maximize the ratioε/σb for m(q

~)=m(g

~)=300 GeV/c2

The optimized cuts areE/ T>45 GeV and HT>220 GeV

For the optimized cuts, we observe5 data events while 8±6 background events are expected

No excess of events


W&C 98

Selection Efficiency

The efficiencies change by about 4% by varyingthe MSSM parameters (M1, M2, µ and tanβ) with the constraints

m(χ~0

2)-m(χ~0

1)>20 GeV/c2

Br(χ~0

2→χ~0

1+γ)=100%

For m(q~)=m(g

~)=300 GeV/c2,

11.3 events are expected for the optimized cuts

0

10

20

30

40

150 200 250 300 350 400Mass (GeV/c2)

Tot

al e

ffici

ency

m(q~)=m(g

~)

Optimized

Basic Selection


W&C 98

Interpretations

Without excess of events, we set 95% C.L. lower mass limitm(q

~)>311 GeV/c2 for m(q

~)=m(g

~)

m(g~)>233 GeV/c2 for m(q

~)»m(g

~)

m(q~)>219 GeV/c2 for m(q

~)«m(g

~)

with the constraintsm(χ

~02)-m(χ

~01)>20 GeV/c2

Br(χ~0

2→χ~0

1+γ)=100%

1

10

10 2

150 200 250 300 350 400m(q

~) (GeV/c2)

σ×B

r (p

b)

m(q~)=m(g

~)

Theory

95% C.L. limit

m(χ~0

2)-m(χ~0

1)>20 GeV/c2

Br(χ~0

2→χ~0

1+γ)=100%

Kane et al., PRL 76, 3496 (1996)


W&C 98

Interpretations

The fraction of events containing χ~0

2 depends onslepton and stop masses

The mass limit changes by about 10 GeV/c2

if slepton and stop masses are lowered to 80 GeV/c2

These results constrain(but do not exclude) the models of Kane et al.

40

60

80

100 150 200 250 300 350 400Slepton/Stop Mass (GeV/c2)

χ~0 2

Fra

ctio

n (%

)

Stop

Slepton

Summary

W&C 98

We have searched for supersymmetryin γγE/ T and γE/ T+≥2-Jet final states

No excess of events was found

Within the MSSM with a light G~,

we set 95% C.L. lower mass limitsm(χ

~±1)>150 GeV/c2 and m(χ

~01)>77 GeV/c2

These limits exclude the region ofparameter space suggested

for the chargino interpretation of the CDF event

In the models of Kane et al.,we obtain a 95% C.L. lower mass limit of311 GeV/c2 for q

~/g~ assuming m(q

~)=m(g

~)

No sign of supersymmetryIf we cannot exclude it, can we discover it?

Date post:	28-Dec-2019
Category:	Documents
Upload:	others
View:	5 times
Download:	0 times

Probing Supersymmetry with Photons - Tevatronqianj/fnalwc98.pdf · 2000-05-19 · Probing...

Documents