Search for h -> with Muons at D Mark Owen Manchester HEP Group Meeting January 2006

Mark Owen Manchester Christmas Meeting Jan 2006 1

Search for h -> with Muons at D

Mark OwenManchester HEP Group Meeting

January 2006

• Outline:– Introduction & Motivation– Tau Identification at D– Backgrounds & Selection– Trigger Efficiencies– Conclusions & Plans


• Introduction to Higgs Searches

– The Standard Model predicts the existence of a neutral, spin-0 particle known as the Higgs boson.

– In the Standard Model the best channels for searching for the Higgs boson are the associated production channels at low mass, for example hW -> bb l.

– At higher Higgs masses, modes such as h -> WW become more important.

– To see the Standard Model Higgs Boson D0 & CDF need to accumulate more luminosity, currently we have ~1fb-1 on tape.


• Super Symmetric Higgs Bosons

– One common extension to the standard model is Super Symmetry, in particular the Minimal Super Symmetric Standard Model (MSSM).

– In MSSM, the production cross-section for gg -> H gets enhanced by a factor of tan2, where tan is one of the parameters of MSSM.

– This makes it possible to see Higgs production at the Tevatron with less luminosity than in the Standard Model.


• Decay Channels of Higgs within MSSM– The two main decay channels of the Higgs boson are bb (~90%) and

(~8-10%).– The large multi-jet QCD background makes the bb channel

problematic, unless you look at associated production.– This leaves the channel and since leptons decay almost

immediately in the detector, this has several sub-channels of its own:

Channel Br (%) Comment

+ / e + e 3 Very large background from Z/ -> / ee

+ hadronic 22 Promising channel

e + hadronic 22 Promising channel

+ e 6 Low branching ratio

Di-hadronic 44 Large multi-jet background & difficult to trigger

My channel

See Matthews talk


• Existing results for SUSY Higgs

– D has published results for SUSY higgs in the associated production channel bh / bbh -> bbb / bbbb with 260 pb-1.

– CDF has preliminary results for h -> with 310pb-1.

– The aim of my analysis is to conduct a search for h -> at D0 with the 1 fb-1 dataset.


• Hadronic Tau Identification

– Taus are first reconstructed as calorimeter clusters with a simple cone algorithm and matched with tracks in the central tracker.

– The tau candidates are divided into three types, by their detector signature.

Type 1:

Single track, no EM-sub clusters.

Mainly - -> -

Type 2:

Single track, EM-sub clusters.

Mainly - -> - -> -0

Type 3:

More than 1 track, can have EM-sub clusters

Mainly - -> --+


• Distinguishing Taus from QCD Background

– At this stage, tau leptons look very similar to narrow, low multiplicity jets and further identification is needed.

– At D0 a Neural Network for each tau type is used to differentiate real taus from fakes.

Profile = ETTower 1 + ET

Tower 2 / ET Iso = ETR < 0.5 - ET

R < 0.3 / ETR < 0.3

Combine Several Variables with Neural Network

Black: QCD

Red: Z -> MC


• Backgrounds to Signal process

– The backgrounds to the signal process can be split into three main types:

• Z -> background, this is very similar to the signal, the main distinguishing variable will be the mass.

• Backgrounds that can be simulated from MC where another particle fakes a tau, e.g. W+jets, Z -> , di-boson, top pair production.

• Background from all other multi-jet processes (‘QCD background’) that need to be estimated from data.

– Strategy is to first develop cuts against MC processes then look at the QCD background.


• Pre-selection

– Aim to select events containing an isolated muon & a tau candidate.• Require exactly 1 isolated muon, with a central track match, pT > 12 GeV.

– Sum of track pT in cone 0.5 around muon track < 2.5 GeV.– Sum of transverse energy in the calorimeter in annulus between the cones 0.1

& 0.5 < 2.5 GeV.

• Require tau candidate with loose pT cuts.– pT > 10 GeV for types 1 & 3, 5 GeV for type 2

• Require that the tau tracks do not match the muon track.

– Look at number of events by tau type for background processes.Key:

Z -> Z -> W -> W -> WW -> lltt -> inclusive


• Neural Network

– As expected before NN cut there is a very large background from W+jet events.

– Apply NN cut to remove jet fakes.– Cut at 0.9 for types 1 & 2, 0.95 for type 3.

Type 1 Type 2 Type 3


• Tau pT

– Expect the signal to be higher pT than the backgrounds.– Look at tau pT for the various signal processes after applying a cut

of NN > 0.9.– Cut at pT > 15 GeV for types 1 & 2, 20 GeV for type 3.– Also apply cuts on the sum of the track pT for types 1 & 2 (expect

track pT ≈ calorimeter pT). Cut at ∑pTtracks > 15 GeV.

Tau type 1 pT Tau type 2 pT Tau type 3 pT


• Muon Contamination

– Recall that some Z -> events peak close to 1 in the NN. While some of these may be Z + jets, it is likely the tau is faked by a muon, particularly for types 1 & 2.

– Look at distance, dR = √(∆2+∆2), between tau candidate and nearest loose muon (no pT or isolation requirement).

Type 1 Type 2 – Reject any tau candidates matched to a loose muon within dR < 0.4


• Rejecting W Events

– Since the kinematics of W events are significantly different from the signal it should be possible to reject them.

– Construct ‘Transverse Mass’, MT,• MT = √(2 ET

MET ET (1 - cosΩ))

• Where Ω = Angle between missing energy vector & muon.

Key:

Z -> Z -> W -> W -> WW -> ll tt -> inclusive

– Require MT < 20 GeV. – Expect to be able to cut tighter, but

need to check distribution for heavy Higgs.


• Status After All Cuts

– Apply all the cuts & see what remains. Plot background by tau type & plot the ‘visible mass’ = invariant mass of muon + tau + missing energy.

– Assume L= 500 pb-1 xBr = 10 pb-1 & MH = 120 GeV for signal (~ limit set by CDF).


• Summary for MC Backgrounds

– Results look reasonable, however still have a reasonable background from Z -> and W -> (remember these should be reducible).

– W background should be reducible by tightening transverse mass cut.

– May need a new cut for Z -> .– However, before doing any more optimization, need to deal

with QCD background.


• QCD Background

– Idea is to estimate QCD background from data.– Approximate the background in the signal region by looking

at taus recoiling from ‘semi-isolated’ muons.

– Normalize the events in the semi-isolated region by comparing number of events in signal region to prediction for the backgrounds from MC.

– Need to do this in a region where the signal is negligible (e.g. at preselection or with a cut, NN for instance, inverted.)

Calorimeter Isolation (GeV)

Track

Isolation

(GeV)

2.5 7

2.5

7Semi-isolated events

Signal region


• Trigger Studies

– In order to predict the number of events present in the signal region, need to apply a trigger efficiency to the MC events.

– Single muon triggers are the triggers that will be used for this analysis.

– D0 trigger system has 3 levels:

– Muon trigger efficiencies can be estimated by looking at a sample containing Z -> and using the ‘tag and probe’ method.

– Measure each term for each level separately & then combine to give real trigger efficiencies.

L1 L2 L3Collision rate 1.7MHz

1.9 kHz 900 Hz 50 Hz to tape


• Tag and Probe Method

– First select a sample of di-muon events from Z -> . Since there are two muons in the event, can use the pair to measure efficiencies.

– Plot the efficiencies with respect to eta, phi, pT, luminosity and time (run number) to investigate what dependences exist.

– This method can also be used to measure Muon ID efficiencies and examine differences between data & MC.

Control muon. Must be matched to Single Muon trigger

Test muon. Test if this muon has passed the muon trigger terms.


• Single Muon Triggers at D0– The single Muon Triggers have evolved through Run IIa as the

peak luminosity of the Tevatron has increased.– Three main divisions:– Triggerlists up to and including v12.xx

• MUW_W_L2M3_TRK10– L1: Tight Scintillator & loose wire condition for || < 1.5– L2: Medium muon, pT > 3 GeV– L3: Track, pT > 10 GeV

– Triggerlists v13.xx:• MUH1_LM15

– L1: Tight scintillator & L1 track, pT > 10 GeV for || < 1.6– L3: Loose Muon, pT > 15 GeV

• MUH1_TK12_TLM12– L1: Tight scintillator & L1 track, pT > 10 GeV for || < 1.6– L3: Loose Muon, pT > 12 GeV & Track pT > 12 GeV

– Triggerlists v14.xx (currently online), similar to v13 but also have Mu + Tau triggers.


• Example plots: v12 L1 Tight Scintillator

• Eta-Phi map needed.

• Efficiency looks flat wrt pT and luminosity

• Efficiency of runs 165-167k is ~10% lower than runs 167-170k

• Efficiency of runs 170-195k is flat.


• Example plots: v13 L3 Track, pT > 12 GeV

• Parameterize in terms of eta.

• See dependence on luminosity

• Efficiency drops as run number increases (probably due to luminosity dependence).


• v13 L3 Tracking dependence wrt Luminosity

– The luminosity dependence in the v13 data was puzzling - there was no dependence in v12.

– The dependence turned out to be due to a trigger that used a different tracking algorithm at level 3. This trigger term was included in the efficiency calculation and since it was pre-scaled at higher luminosities it resulted in a bias.

– After removing L3 tracks constructed with this algorithm the dependence was removed:


• Example Plots: v13 L3 Loose Muon, pT > 12 GeV

• Efficiency is flat wrt pT above ~20 GeV, below this have strong dependence.

• Efficiency looks flat wrt luminosity.

• Maybe slightly lower efficiency before run 195k.


• Example Plots: v13 L3 Loose Muon, pT > 15 GeV

• Efficiency is flat wrt pT above ~25 GeV, below this have very strong dependence (~34-68%)

• Efficiency looks flat wrt run number and luminosity.


• Trigger Studies - Summary

– All the trigger terms for v12 & v13 triggerlists look well behaved.

– The pT dependence seen for the L3 muon terms in v13 will have an effect on the signal.

– Still need to look at v14 data.– If we want to use Mu + Tau triggers from v14, need to

estimate the tau terms with a different technique.


• Conclusions & Plans

– Selection criteria for h -> had is progressing well.– Backgrounds from W & Z production have been suppressed.– Trigger efficiency studies are progressing well.– Need to now look at the data & estimate the QCD

background.– Once QCD background is understood can optimize the

selection.– Then look at the signal region & obtain a result!

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Search for h -> with Muons at D Mark Owen Manchester HEP Group Meeting January 2006