Ritva Kinnunen, Sami Lehti, Lauri Wendland: CMS physics simulation at HIP1 Physics Simulation Studies in HIP Outline: –Introduction –Pre-TDR physics studies

Ritva Kinnunen, Sami Lehti, Lauri Wendland: CMS physics simulation at HIP 1

Physics Simulation Studies in HIPPhysics Simulation Studies in HIP

• Outline:– Introduction– Pre-TDR physics studies – Full simulation studies for the CMS Physics TDR

• MSSM H± jet+X• MSSM H/A e+X• MSSM H/A jet+jet+X

– Educational activities– Publishing activities– Future plans


IntroductionIntroduction

• Main fields of interest: – Higgs boson production and the experimental methods related to

Higgs boson searches at the LHC

• Principal framework of physics studies studies: – Minimal Supersymmetric Standart Model (MSSM)

• The group has particular experties in the following experimental methods:

– identification– tagging with impact parameter and vertex measurement– b tagging


Pre-TDR physics studies duringPre-TDR physics studies during1992 - 20041992 - 2004

• These studies were based on particle level simulation, fast simulation and partially full simulation

• The work started 1992 with the first Higgs boson simulation work in CMS:

– R. Kinnunen, H. Plothow-Besh and J. Tuominiemi, ”Search for H ZZ* 4l± with the CMS detector at the LHC”, CMS TN-1992/008.

• Followed with studies on – the SM Higgs boson searches, H ZZ/ZZ*, H gg, H WW, H .. – the searches of the MSSM Higgs bosons with H and H± – experimental methods, b tagging, in-situ calibration, trigger studies

• The fast simulation era was finished with a summary work on the CMS studies on the Higgs boson searches under the responsability of HIP group,

– ”Summary of the CMS Discovery Potential for the Higgs Boson” , European Physical Journal C, Particles and Fields, vol. 39, (2005) 41-61

• Specialization to physics and on the H and H± discovery channels started early in HIP…


H/A H/A and H and H± ± channels channels

• Due to the tan enhancement of the Higgs couplings to down type fermions, these channels are the principal discovery channels for the heavy MSSM Higgs bosons at the LHC:

– H/A in the associated production with b quarks, gg bbH/A– H± in the associated production, gg tbH±

• In these searches:– the bbH process can be disentangled from the large Drell-Yan production of

pairs through tagging one b jet– the hadronic jet background can be suppressed with identification

• identification is based on isolation and pT cuts at the High Level Trigger and in offline analysis, and on vertex reconstruction and impact parameter measurement in the offline analysis

• identification and b tagging are the basic methods for the four discovery channels studied for the TDR by the HIP group with full simulation and reconstruction :

– fully hadronic H± ,– fully hadronic H/A 2 jets+X,– semileptonic H/A electron+jet+X and– fully leptonic H/A ll+X


Hadronic Hadronic trigger trigger

• HIP group has been involved in developing hadronic trigger algorithms

• Hadronic trigger needed for– fully hadronic H± ,

• single + missing ET trigger– fully hadronic H/A 2 jets+X and

• single + missing ET trigger, double + missing ET trigger– semileptonic H/A electron+jet+X

• e + trigger, single e trigger

• HLT trigger algorithm:– Start with L1 central calorimeter jets– Then either

• ECAL isolation + pixel track isolation or – Fast algorithm; gives good performance– Preferred approach in H/A 2 jets+X

• Tracker isolation (regional track reconstruction)– Slower algorithm, but gives a more accurate track pT estimation– Useful in channels like H±


Full simulation study on heavy charged MSSM Full simulation study on heavy charged MSSM Higgs bosons with the hadronic HHiggs bosons with the hadronic H±± decay decay

R. KinnunenR. Kinnunen

• There are few possiblities to discover the charged Higgs bosons at the LHC:

– the dominant H± -> tb decay channel difficult due to large background systematics

– H±-> decay promising,

• The H±-> decay channel can be used– for mH± < mtop in tt events through t->bH± with leptonic triggers with

a discovery up to almost the kinematical limit, mH± < mtop - mb

– for mH± > mtop , in fully hadronic final states from associated production with top

• Advantages of the fully hadronic channel:– possibility to exploit helicity correlations, – large missing transverse energy and– possibility to reconstruct a transverse Higgs boson mass

• Production of t + H± through gb -> tH± (LO) and gg(qq) -> tbH± (NLO) processes

• Merging the two processes is not possible in the full simulation:gg(qq) -> tbH± used


Backgrounds and triggerBackgrounds and trigger

• Signal events are characterized with: one energetic jet, large missing transverse energy, one b jet and 2 hadronic jets

• Main backgrounds are from genuine ’s in multi-jet events:– tt, t1 -> , t2->qqb

– Wt W1 -> , W2->qq

– W+3 jets, W->– and from fake ’s in QCD multi-jets

• The H±-> decay in fully hadronic final state can be triggered with:

– Single + missing transverse energy trigger

• CMS trigger for single : – Narrow calorimeter jet at Level-1 (ET > 93 GeV)

– Full regional track reconstruction, isolation and pT cut at the High Level Trigger

• Efficiencies for the signal 9 – 40%, with jet purity around 90%


Event selectionEvent selection

• A quasi two-body situation between the jet and the missing transverse energy, for a good transverse mass reconstruction, can be obtained only if

• ETmiss originates from H±->

• Other sources of missing transverse energy: W-> l and semileptonic b decays

• Start offline selection with a veto on isolated leptons, exploiting tracker isolation and electron identification in the calorimetry

• Results:– Measured ”BR(W->)” = 8.9%, purity 84% – Measured ”BR(W->e)” = 7.9%, purity 93%


Fraction of momentumcarried by the leading

Signal

tt background

Event selection 2Event selection 2

identification for H±:• Two scopes in this channel:

– suppress efficiently hadronic jets and– the genuine ’s from W ->

• Identification based on calorimeter jet,motivated with a large fraction of -> ± + n0 + decays in the signal (~ 52%)

• Reconstruction of the jet around theHLT direction in a cone of 0.4

• Select identification cuts to exploithelicity correlations in H± ->

• Decay angular distributions in the CMS frame:– H± ->, ->± +: N/cos ~(1+cos)– W± ->, ->± +: N/cos ~(1-cos)

• Harder charged pion from H± -> than from W->– For decays through vector mesons more– complicated but still harder pions from H±


Event selection 3Event selection 3

• identification cuts: – leading track within R < 0.1 the jet direction– small signal cone around the leading track, R = 0.04– isolation in the cone 0.04 < R < 0.4

– pleading track/ET jet > 0.8 to exploit the helicity correlations

Further event selections:

• Top mass reconstruction with minimization of 2 = ((mjj –mW)/W)2 + ((mjjj –mtop)/top)2 from all hadronic non- jets, ET > 20 GeV, with W = 10 GeV, top = 17 GeV

• B tagging with a probabilistic secondary vertex algorithm with discriminator cut


Transverse mass reconstructionmT = (2 ET

jet ETmiss (1 - jet, ET

miss))1/2 Discovery potential

Transverse mass and discovery Transverse mass and discovery reachreach

• An almost background-free signal can be reached in the signal area defined by ( jet, ET

miss) > 60o or by mT( jet, ETmiss) > 100 GeV

• Sources of systematic uncertainty in the background determination:ET

miss and jet scale, identification, b tagging, cross section uncertainties


Study of MSSM H/A Study of MSSM H/A jet+jet+X jet+jet+X

L. WendlandL. Wendland

• Channel studied in collaboration with S. Gennai and A. Nikitenko

• Final state search strategy:– two isolated jets,– one b jet,– veto for other jets and– missing energy

• Trigger with– single trigger (ET > 92 GeV)

– double trigger (ET > 76 GeV)

• Main backgrounds:– QCD di-jets, Z/*, tt

MC event visualization for bbH(500)->->2jet

jet 2

b jet 1

b jet 2

jet 1


identification in jet + jet final identification in jet + jet final statestate

• identification algorithm:– leading track within R < 0.1 the jet direction– small signal cone around the leading track, R = 0.07– isolation in the cone 0.07 < R < 0.4– number of reconstructed tracks: one or three

– pT cut for leading track, pT > 35 GeV

– quality cuts for the leading track• at least 8 hits, 2 at most 10

– upper limit on impact parameter

• IPT < 0.3 mm, IP < 1.0 mm


Impact parameter reconstructionImpact parameter reconstruction

• The principle:– Taus have IP > 0 because they fly

several mm before decaying; QCD jets come from interaction point

• New result:– A significant number of QCD jets

with only one reconstructed track have a large IP value; explanation: track contains hits from different tracks

– Effect is increased with jet ET and sensor displacement

• Significance of full IP is used

• Background rejection– One track final state: ~1.5-5.5 at

greater than 92 % signal level– Three track final state: ~2.2 at

greater than 85 % signal level

Transverse IP, mm Full (3D) IP significanceEfficiency curves based on

cut on the significance


Vertex reconstructionVertex reconstructionin 3-prong in 3-prong jets jets

• The principle:– Taus have an average flight path of

several mm before decay; QCD di-jets are produced at the interaction point

• Challenges:– kinematically very difficult final

state; the first few hits can be merged as one because of high track collimation

– background with long lived particles(B or D mesons) could be enhanced

• Significance of full flight path is used

• Sign of flight path is used

• Systematics from simulated sensor misalignment very small

• Background rejection ~4 at 80 % signal level

• Masters thesis in 2005

Signed full reco fl.path in mmSigned tr. reco fl.path in mmEfficiency curves based on

cut on the significance

Hit mergingon first

pixel layer from H(500)

QCD jet,ET 80-120

GeV

c jet,ET 80-120

GeV


Event selectionsEvent selections

SIGNAL mA = 200 GeV/c2

tan =20mA = 500 GeV/c2

tan =30mA = 800 GeV/c2

tan =40

× BR (fb) 3831 190 49.8

after selections (fb)

0.96 0.46 0.19

N ev. at 30 fb-1 28.8 13.8 5.7

BACKGROUND

QCD di-jet>170 GeV/c

QCD di-jet120-170 GeV/c



tt Z/*m 130-

300 GeV/c2

(fb) 1.33 × 108

5.03 × 108

2.94 × 109

2.08 ×1010

after selections (fb)

0.69 1.28 1.35 0.144

N ev. at 30 fb-1 20.7 38.4 40.5 4.3 0.3 1.9Most dangerousbackground


Higgs mass reconstructionHiggs mass reconstruction

Signal

Background

Signal + background


Discovery reachDiscovery reach


Full simulation study on Full simulation study on MSSM Higgs bosons with H/A MSSM Higgs bosons with H/A e + e + jet + X jet + X

R. Kinnunen and S. Lehti R. Kinnunen and S. Lehti

• Final state searched for: isolated electron, jet and a b jet

• These events are triggered with – a single e trigger (ET > 26 GeV) and with a combined electron +

trigger– Double and combined trigger in CMS: narrow jet at Level-1, regional

track reconstruction in the Pixel detector, isolation and soft pT cut for the leading track

– Efficiencies for the signal: 8 – 30% for mA = 130 – 500 GeV

• Background from– genuine ’s: Z,* , bbZ,* , tt, Wt– fake ’s: W+jet, tt, Wt – electrons: Z,* e+e-, bbZ,* e+e- – fake electrons and fake ’s: QCD multi-jet

• Special feature of this channel: good separation electron versus hadronic jet, versus hadronic jet and electron versus mandatory


ET of the most energetic HCAl cell in the jet

Variables for electron /jet separation

pion momentun over HCAL energy

identificationidentification

• To exploit the + n0 + decay modes (~52% of hadronic decays) jet reconstruction is used at the offline also

• identification algorithm:– leading track within R < 0.1 the jet direction– small signal cone around the leading track, R=0.04– isolation in the cone 0.04<R<0.4– pT cut for leading track, pT > 20 GeV – quality cuts for the leading track, transverse impact parameter < 0.3 mm, at

least 8 hits– veto on electrons: ET(max HCAL cell) > 2 GeV, 0.35 < p/EHCAL < 1.5


Several electron candidates from offline reconstruction: ~ 1.3 electron candidates/event

Identification starts with Tracker isolationno track with pT> 1 GeV within R < 0.4 around the electron candidate

Calorimeter identification needed in particular at low part of the pt spectrum and is done with - track-ECAL(super cluster) matching: |track-SC|, |track-SC|, ESC/ptrack, |1/ESC - 1/ ptrack|

- HCAL/ECAL energy ratio - ECAL profile cuts : E3x3/E5x5,

Cut optimization for good efficiency and maximal puritywith electrons from ->eagaist hadrons from ->hadrons+An identified electron (pT >20 GeV) found in 81.2% of signal events,purity for genuine electrons 97.5%

electron from electron from ->e->e

hadron from hadron from ->hadrons+->hadrons+

Electron identificationElectron identification

Example of electron identificaton variables:ECAL energy over track momentum


Further background suppression Further background suppression with with

• tagging one b jet– with secondary vertex method and discriminator cut

• veto on addional central jets, tt suppression ~ 10 • upper bound on the transverse mass from the electron and missing

transverse energy• further suppression of tt and W+jet backgrounds• Higgs boson mass reconstruction with collinear neutrino approximation:

– ’s emitted along the directions of electron and jet, excluding back-to-back configurations with (electron, jet) < 175o

• Efficiency for rejection of negative neutrino solutions: 60% for signal, ~ 40% for tt background, mass resolution ~ 22%


Signal and total backgroundfor mA = 200 GeV/c2, tan = 20

CMS discovery potential forH/A electron+jet+X

Higgs boson mass and discovery Higgs boson mass and discovery reachreach

• Background under signal mainly from Z,* -> ee and bbZ,* -> ee

• Estimate of the QCD multi-jet background: ~ 10% of the total background

• Sources of systematic uncertainties on background determination:– the jet scale (4%), ET

miss scale (10%), b tagging and mistagging (5%), cross section measurements


Study of MSSM H/A Study of MSSM H/A e e+X+XS.LehtiS.Lehti

• Final state search for:– isolated electron, isolated muon and a b jet

• Events are triggered in CMS with– a single electron trigger (Et > 26 GeV)– a single muon trigger (Et > 19 GeV)– Efficiency for signal 75-85% for mA = 140-250GeV

• Background from– Z/*, tt, tW, bb, WW/WZ/ZZ


Lepton identificationLepton identification

• Muon reconstruction and identification from muon chambers and tracker

• Electron reconstruction fromcalorimeters and tracker,identification is based on

– Track-ECAL matching– HCAL/ECAL energy ratio– ECAL profile cuts

• Identification optimized againstW+jet with jet faking an electronused

• Lepton isolation : no other trackspT>1GeV within R<0.4 around thelepton. Lepton isolation and pT cutsagainst backgrounds with soft leptons(bb,cc,...).


Tau impact parameterTau impact parameter

• Tau’s from Higgs travel couple of mm before they decay

• Impact parameter, the minimumdistance between the track trajectory and the primary interaction point

• Transverse ip used

• Significance of the two impact parameter combined into one variable

• The combined variable is found to suppress efficiently tt events with no genuine tau

• The fraction of tt events with two intermediate tau’s irreducible


B taggingB tagging

• Two associated b quarks in bbH

• Against Z,* for which the associated jets are mostly light quark and gluon jets

• Signal: soft jets. Tagging efficiency not very high

• Here a b tagging algorithm based on track ip and secondry vertices used.

• Jets in tt more energetic, more central, easier to reconstruct and b tag

• Only 1 jet b tagged, jet veto to suppress tt


Central jet vetoCentral jet veto

• tt events have more jet activity than bbH

• Veto on additional jets in the trackeracceptance region coming from theprimary vertex


Mass reconstructionMass reconstruction

• Mass reconstruction using collinear approximation: neutrinos assumed to be emitted along the leptons

• Events not back-to-back selected

• Events with neutrinos in opposite direction to leptons rejected: tt suppressed by a factor of 6, signal eff 40%

• Mass resolution can be improved by higher jet Et cut and stronger (e,) cut, but statistics is decreased significantly


Discovery reach including Discovery reach including systematicssystematics


bbZ as a benchmark for MSSM bbH searchbbZ as a benchmark for MSSM bbH searchS.LehtiS.Lehti

• Scope of this study– bbZ cross-section measurement– measurement of b jet and Z spectra– Z mass reconstruction with collinear approximation

• Z boson production in association with b quarks topologically similar to bbH

• If the theoretical predictions are verified for bbZ, the predictions for bbH should apply, too


Verification of Monte CarloVerification of Monte Carlo

• Verification of Z pT, associated b jet ET and distributions and cross section measurement


Mass reconstructionMass reconstruction

• Z boson mass known: collinear approximation tested with Z/* events

• Electron+muon final states chosen to select Z/*

• Purity is not important, Z /*+jet events accepted in addition to bbZ/* events

• Can be used to verify the detector calibration


Measurement of the H/AMeasurement of the H/A cross section and cross section and possible constrants on tanpossible constrants on tan

R.Kinnunen, S.Lehti, F.Moortgat, A.Nikitenko, M.SpiraR.Kinnunen, S.Lehti, F.Moortgat, A.Nikitenko, M.Spira

• One of the most important parameters to be determined in MSSM is tan

• The dominant part of the ggbbH,H proportional to tan2

• tanb measured from event rates

• Uncertainty of tan half of the uncertainty of the event rate

• Results from different H/A final sates combined


• Uncertainty of the cross section (times BR) measurement

=sqrt(Ns+Nb)/NsL/Lsel/selNbsyst/Ns

• Uncertainty of the tan measurement

tan/tan = ½ ½ theor/theor

Measurement uncertainty on tanMeasurement uncertainty on tan


Educational work of the HIP physics Educational work of the HIP physics groupgroup

• Academic degrees:– Sami Lehti, Prospects for the detection of neutral MSSM Higgs bosons

decaying into tau leptons in the CMS detector, PhD Thesis 2001– Lauri Wendland, Discovery potential for H,A → with 3-prong vertex

reconstruction in the CMS detector, Masters Thesis, 2005– Lauri Wendland, PhD Thesis under preparation (H,A → →2jet+X)

• Student program:– One summer student / year, working on subjects closely connected on

the group studies and with proper documentation at the end of the work (CMS note or internal CMS note)

• Other educational activities:– Help and guidance on groups of young physicists from other countries

(Turkey, India, Egypt, Pakistan, Estonia) according to the human resources available

– Introductory lectures at different high schools


Publications and conference talks Publications and conference talks during last 10 yearsduring last 10 years

• Publications in– International journals: 18– CMS public notes and preprint series: 53– Conference proceedings: 3– CMS internal notes: 5

• Plenary talks in international conferences: 5

• Invited talks in international conferences: 10

• Contributions to CMS TDRs

• Referee activities:– R. Kinnunen, PhD Thesis opponent, University of Stockholm, December,

2005– for articles in International journals: one article in EPJCdirect– for CMS public notes: 5


Future plansFuture plans

2006 Introduction and testing of the new CMS software CMSSWContribution to the ”1 fb Physics” simulationPreparation of calibration, alignment and physics studies with real

data

2007 Start up of measurements for calibration constants andbackground processes: W and Z production, tt, bb, hadronic jets

2008 Expect an integrated luminosity of few fb-1. Intensive measurement ofour background processes and search for the MSSM Higgs bosons in

the channels at large tan (~50) may start

2009-2010 Expect to reach 30 fb-1. The physics program presented here can befulfilled. Exploration of tan down to ~ 10 around mA = 200 GeV/c2

Measurement of tan with a precision of < 20%

The strong contribution of the HIP group to tau physics and Higgs boson is important to CMS and will continue

Documents

Ritva Kinnunen, Sami Lehti, Lauri Wendland: CMS physics simulation at HIP1 Physics Simulation Studies in HIP Outline: –Introduction –Pre-TDR physics studies