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Search for New Physics in Jets plus Missing Transverse Energy Final States at CDF. Monica D’Onofrio IFAE-Barcelona On behalf of the CDF collaborations XXXIV International Meeting on Fundamental Physics IMFP06, Madrid 5 th April 2006.

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search for new physics in jets plus missing transverse energy final states at cdf

Search for New Physics in Jets plus Missing Transverse Energy Final States at CDF

Monica D’Onofrio


On behalf of the CDF collaborations

XXXIV International Meeting on Fundamental Physics

IMFP06, Madrid 5th April 2006

Expecting small excesses from SM many studies has to rely on simulation to test standard model prediction

One of our best handles: transverse energy imbalance (Missing ET, or MET)

MET associated with jets is a very good signature to search for New Physics beyond Standard Model


The Standard Model has been a great success but ...

hierarchy problem

how to explain dark matter, etc.

Tevatron is currently only place to search for new high pT physics!


Extra Dimensions



  • Monojet + MET configuration:
  •  Possible signature for Large Extra Dimensions (LED)
  • Multijets + MET configuration:
    •  Golden signature to search for squarks and gluinos in SUSY scenario

IMPF06, Madrid 5/4/2006

large extra dimensions add scenario
Proposed as a solution to the hierarchy problem

Extra dimensions are compactified or

inaccessible to some part of SM

Model of Arkani-Hamed, Dimopoulos, and Dvali (ADD)

n extra dimensions (≥ 2) compactified at radius R

SM constrained to a 4-d brane in higher dimensional

space  Gravity exists in (4+n)-d “bulk”

Effective Planck scale:

Large Extra Dimensions (ADD scenario)

M2Planck ~ Rn(MD)n+2 ,MD ~ 1 TeV

  • Gravitons appear to have mass, m2=m02+p2
  • Tower of KK modes with splittings m ~ 1/R
  • Each mode couples with strength, MPl-1, but there are many: cross section summed over all modes

IMPF06, Madrid 5/4/2006

hadron collider signatures of led

CDF event:

  • ET (jet) = 361 GeV
  • Missing ET = 350 GeV
Hadron Collider Signatures of LED

Graviton production mechanism

Real Graviton undetected lead to Missing Transverse energy + one recoiling jet

 Monojet

IMPF06, Madrid 5/4/2006

current results on monojet search at cdf
ET (leading jet) > 150 GeV

Missing ET > 120 GeV

Current results on monojet search at CDF

Current limit based on ~ 370 pb-1

  • Results:
  • 265 ±30 events predicted
  • 263 events observed

No excess observed

IMPF06, Madrid 5/4/2006

analysis strategy
Analysis strategy

Effect due to pT cut

in MC generation

Using the ~1fb-1 now available:

 LED would give an overall excess

above SM contributions

 explore lower jet/MET region

make data-driven estimates

of the major backgrounds.

  • SM Backgrounds that also produce Jet(s)+MET
    • Z + jets with Z (looks just like the signal)
    • W + jets with Wl and the lepton undetected
      • Wt is the most significant, followed by Wm
    • QCD dijets, where one jet is lost or mismeasured
    • Ztt
  • Z is irreducible:
    • Important to have good Monte Carlo simulation
    •  Use data to evaluate it, considering Z  e+e-

IMPF06, Madrid 5/4/2006

z 1 jets event topology
MC samples: Z( e+e-) + 1 parton


pThat > 5 GeV/c

Tune A, LQCD = 0.146



No Tune A

Z + ≥ 1 jets event topology

PT profile in f in

Z+≥1 jets events

  • Using tracks with:
  • pT > 0.5 GeV/c
  • nHits > 20
  • |z0| < 1.5 d0 < 2.0


transverse plane

  • Dominated by UE.
  • Independent of the pT of the jets.


Jet fragmentation

+ Underlying Event

Pythia is the simulation that better reproduces the jet fragmentation

IMPF06, Madrid 5/4/2006

jet shapes in z 1 jets events
Jet Shapes in Z+≥1 jets events

 Energy distribution within jets

Using the ET of the calorimeter towers

in jets with pT > 25 GeV/c and |Y|< 2.1

Integral shape

Differential shape in steps of DR = 0.1

Pythia  more accurate than Herwig in reproducing

jet fragmentation + effects due to UE

IMPF06, Madrid 5/4/2006

inclusive p jet t distributions
Inclusive pjetT distributions

pT of the jets for Z + ≥ n jets


Every MC distribution is normalized to the data  shape comparison only



Less jets at low pT due to lack of UE in Herwig MC

Good agreement

with Pythia MC

 Further investigation on going using Alpgen+Pythia MC

 First results expected for Summer 2006

IMPF06, Madrid 5/4/2006

squarks and gluinos

Missing ET


s (pb)

Multiple jets



Missing ET







Squarks and Gluinos
  • At the Tevatron: mainly Squarks and Gluinos pair production
  • Golden signature at LHC
  • Investigated decays: energetic jets + MET (from LSP)
  • Strong interaction  large production cross section
    • for M(g) ≈ 300 GeV/c2:
      • 1000 event produced
    • for M(g) ≈ 500 GeV/c2:
      • 1 event produced
  • Consider mSUGRA scenario:
    • A0 =0, m < 0, tan b = 3 or 5
    • 5 flavors degenerate



IMPF06, Madrid 5/4/2006

analysis strategy1




Analysis strategy
  • Chosen region not excluded by other experiments
  • Use Isajet but moving on for PYTHIA
  • third generation removed from 2  2 process
  • Chosen 3 bench-mark points to optimize analysis selection criteria
  • “Blind Analysis”
  • Define a signal region (Blind Box)
  • Make sure MC is in agreement with data outside this region (Control Region)
  • “Open” the Blind Box

Signal region (blind box)

determined by optimizing S/√B

IMPF06, Madrid 5/4/2006


Backgrounds dominate Need to be specifically rejected:


MET physics backgrounds

MET due to jet energy mismeasurements

  • W/Z+jets with Wl or Z, DiBoson:
  • reject isolated electrons and muons
  • Top:Signatures similar to W+jets (rejection more challenging)

QCD multijets:

reject jet close to MET direction

Z+jets with Z->e+e-(m+m-)

IMPF06, Madrid 5/4/2006

backgrounds 2
Backgrounds (2)


  • Generated with PYTHIA in different pT bins
  • Selected regiondominated by Jet events in the data with low MET
  • Compared distributions MC events to data to find NLO factor and obtained scale factor to the MC ~1.0

CDF Run II preliminary

Dedicated study to verify the LO to NLO scale factors for the normalisation of the samples

 “k-factors”


estimations from Monte Carlo

IMPF06, Madrid 5/4/2006

event selection
Event selection

Signal region

  • Selection:
    • 3 jets with ET>125 GeV, 75 GeV and 25 GeV
    • Missing ET>165 GeV
    • HT=∑ jet ET > 350 GeV
    • Missing ET not along a jet direction  Avoid jet mismeasurements

MET distribution after applying all the cuts except MET >165 GeV

HT distribution after applying all the cuts except HT >350 GeV

IMPF06, Madrid 5/4/2006


XY view of event with large MET

HT = ET(1st) + ET(2nd) + ET(3rd) = 404 GeV

ET(1st) = 172 GeV

ET(2nd) = 153 GeV

ET(4th) = 65 GeV

ET (3rd) = 80 GeV

Missing ET = 223 GeV

IMPF06, Madrid 5/4/2006



In L = 245 pb-1 of data  Background expectations inside the Blind Box:4.1  0.6  1.4 events.

Opening the blind box3 eventshave been found

  • No evidence for excess of events:
    • Exclude squarks and gluinos for

bench-mark masses values

    • D0 excluded gluinos up to 230 GeV

(VERY PRELIMINARY, no PDF unc. included)

    • CDF:
      • Limit expected in one week
      • Expected similar to D0

IMPF06, Madrid 5/4/2006

  • In 2005, Tevatron achieved the 1 fb-1 goal
  • Delivered total luminosity 1.6 fb-1
    • 1.2 fb-1 on tape ready for data analyses!
  • Very rich searches physics program ongoing at CDF
    • Tevatron is currently the only place to search for new physics
    • Missing transverse energy is one of our best handles: in association with jet(s) constitute i.e. good signature for
      • Large Extra Dimension (monojet)
      • Supersymmetry (multijets)
    • Final results foreseen for Summer Conferences
  • Very important for the LHC

IMPF06, Madrid 5/4/2006

the tevatron
The Tevatron

Highest-energy accelerator currently operational

Peak luminosity

1.8 *1032 cm-2 s-1

Integrated luminosity/week

 about 25 pb-1

CDF and D0:

~1.2 fb-1on tape

Analyses shown here use 0.3 – 1.0 fb -1


IMPF06, Madrid 5/4/2006

randall sundrum scenario
Only one extra dimensions limited by two 4-dimensional brains.

SM particles live in one of the brains.

Graviton can travel in all 5 dimensions.

mG = xn k exp(-πkrc)

The only scenario with extra dimensions implemented in Pythia.

Free parameters:

Graviton mass (mG)

Dimensionless coupling (k/MPl)

Randall-Sundrum Scenario

IMPF06, Madrid 5/4/2006