Search for neutral MSSM Higgs bosons decaying to pairs in pp collisions at 1.96 TeV (CDF)

Search for neutral MSSM Higgs bosons decaying to pairs in pp

collisions at 1.96 TeV (CDF)

Jessica LeonardOct. 23, 2006

Physics 835

10/23/06 Jessica Leonard 2

MSSM Higgs How do particles have mass?

SM Higgs mechanism Problems with SM Higgs may be solved in MSSM

Five Higgs in MSSM: 2 charged, 3 neutral This paper deals with neutrals ():

1 CP-odd: A 2 CP-even: h (lighter), H (heavier)

MSSM parameters: mA

tan = vu/vd, where vu, vd are vev’s of neutral Higgs that couple to up- and down-type fermions


MSSM Higgs For large tan b, one CP-even Higgs has

similar mass, couplings to A Dominant production mechanisms:

Gluon fusion bb fusion

Dominant decay modes for A, mass-degenerate CP-even state: -> bb (~90%) -> (~10%)


The search at CDF Tevatron: CM energy 1.96 TeV Data collected at CDF II between 2002-04 Integrated luminosity 310 pb-1

-> for 90 < mA < 250 GeV/c2

One -> lepton + neutrinos (e, ) Other -> hadrons + neutrino (had)


CDF detector

=-ln(tan(/2))


Event selection For the -> search, each event must contain a

lepton candidate (e or ) and hadrons consistent with a decay.

Pre-select with a “lepton plus track” condition Lepton and track must both point into central cal. Must have azimuthal separation of > 10o

e/ determination: Electrons only deposit energy in EM calorimeter Muons seen in muon chambers outside cal.

Momentum of neutrinos: missing transverse energy


Event selection Hadron reconstruction

Energy from decays are deposited in a small, contiguous region

Four-momentum of this jet determined from tracks of charged particles and energies of 0’s within “signal cone”

Particles in a ring outside the signal cone must have very low transverse energy (< 1 GeV total). This isolation cuts out quark and gluon jets.


Event selection Hadron reconstruction

The jets must have 1 or 3 tracks (charged particles), and sum of charges 1

Invariant mass must be < 1.8 GeV/c2 ( = m) Require (Ecl/Psig

trk)(0.95-f)>0.1 to reject electrons Ecl: value of energy deposit in the calorimeter f: ratio of electromagnetic to hadronic energy Psig

trk: scalar sum of track momenta in signal cone

Require ETcl > 15 GeV to reject muons


Event selection Hadron reconstruction efficiency:

38% at pThad = 15 GeV/c

46% at pThad > 25 GeV/c

Probability of misidentifying quark or gluon jet 1.5% at ET

jet = 20 GeV 0.1% at ET

jet = 100 GeV


Monte Carlo Simulation Event generation: PYTHIA, with CTEQ5L

PDFs Tau decays: TAUOLA package Detector response: GEANT-based model


Backgrounds Dominant background: Z/* -> Second-highest: “fake” ’s (misidentified

quark or gluon jets) from processes like di-jet, multi-jet, W+jets, +jets

Third: Z/* -> ll, WW, WZ, ZZ, tt Background contributions determined by

Monte Carlo


Eliminating backgrounds Cut out low-energy multi-jet backgrounds

Each e/ candidate must pass pT threshold of 10 GeV/c

Each had candidate must pass pT threshold of 15 GeV/c, must also have opposite charge

Require |pTl|+|pT

had|+|ETmiss| > 50 GeV


Eliminating backgrounds Cut out W+jets backgrounds

Define: p

vis = (pl + phad) p

ETmiss = ETmiss

Require pETmiss > 0.6p

vis - 10 GeV/c Removes ~85% W+jets events, keeps ~95% of signal

Cut out Z -> ll decays with misidentified lepton: reject events with lepton + 1-prong had invariant mass within 10 GeV/c2 of Z mass


Signal Acceptance For mA = 90 GeV/c2, combined acceptance

for ehad and had channels is 0.8% For mA = 250 GeV/c2, combined acceptance

is 2.0%


Uncertainties Systematic uncertainties for particle ID

efficiency: had 3.5%, e 1.3%, 4.6% Uncertainties in trigger efficiency: ehad 2.1%,

had 1.4% Jet -> misidentification 20% (3% bg. overall) Uncertainty in signal acceptance

Due to event-level cuts < 2% Due to PDFs: 5.7%


Expected events from SM Number of expected standard model events

passing all cuts: 4965(stat)28(sys)25(lumi)for a range of 438 to 554 events

Breakdown: 405 from Z/* -> 75 from jet -> misidentification 16 from all remaining background sources


Data 487 observed events Reconstructed the mass (mvis) for each event:

invariant mass of visible tau decay products and ET

miss

NOTE: This is incorrect! Binned the data to compare with background

and hypothetical Higgs data No signal evidence observed for mA = 90

through 250 GeV/c2


Example plot: mA = 140 GeV/c2


Exclusion limits


Excluded regions Plotted excluded regions in parameter space

for four situations mh

max vs. no-mixing > 0 and < 0


Excluded regions


Conclusions LEP excluded mA < 93 GeV/c2, higher mA for

small tan This search also excluded regions of higher

tan Results similar to those of D0

Documents

Search for neutral MSSM Higgs bosons decaying to pairs in pp collisions at 1.96 TeV (CDF)