1. Physics Simulation Studies in HIP
Outline:
Introduction
Pre-TDR physics studies
Full simulation studies for the CMS Physics TDR
MSSM H±  tn  jet+X
MSSM H/A    e+X
MSSM H/A    jet+jet+X
Educational activities
Publishing activities
Future plans

2. Introduction 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:
t identification
t tagging with impact parameter and vertex measurement
b tagging

3. Pre-TDR physics studies during 1992 - 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  mm..
the searches of the MSSM Higgs bosons with H  tt and H±  tn
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 t physics and on the H  tt and H±  tn discovery channels started early in HIP…

4. H/A  tt and H±  tn channels
Due to the tanb 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  tt in the associated production with b quarks, gg  bbH/A
H±  tn in the associated production, gg  tbH±
In these searches:
the bbH process can be disentangled from the large Drell-Yan production of tt pairs through tagging one b jet
the hadronic jet background can be suppressed with t identification
t 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
t 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±  tn,
fully hadronic H/A  tt  2 jets+X,
semileptonic H/A  tt  electron+jet+X and
fully leptonic H/A  tt  ll+X

5. Hadronic t trigger
HIP group has been involved in developing hadronic t trigger algorithms
Hadronic t trigger needed for
fully hadronic H±  tn,
single t + missing ET trigger
fully hadronic H/A  tt  2 jets+X and
single t + missing ET trigger, double t + missing ET trigger
semileptonic H/A  tt  electron+jet+X
e + t trigger, single e trigger
HLT t 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  tt  2 jets+X
Tracker isolation (regional track reconstruction)
Slower algorithm, but gives a more accurate track pT estimation
Useful in channels like H±  tn

6. Full simulation study on heavy charged MSSM Higgs bosons with the hadronic H±  tn decay R. 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±->tn decay promising,
The H±->tn 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 t 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

7. Backgrounds and trigger
Signal events are characterized with: one energetic t jet, large missing transverse energy, one b jet and 2 hadronic jets
Main backgrounds are from genuine t’s in multi-jet events:
tt, t1 -> tn, t2->qqb
Wt W1 -> tn, W2->qq
W+3 jets, W->tn
and from fake t’s in QCD multi-jets
The H±->tn decay in fully hadronic final state can be triggered with:
Single t + missing transverse energy trigger
CMS trigger for single t:
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 t jet purity around 90%

8. Event selection
A quasi two-body situation between the t jet and the missing transverse energy, for a good transverse mass reconstruction, can be obtained only if
ETmiss originates from H±->tn
Other sources of missing transverse energy: W-> ln 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->mn)” = 8.9%, purity 84%
Measured ”BR(W->en)” = 7.9%, purity 93%

9. Event selection 2 t identification for H±: Two scopes in this channel:
suppress efficiently hadronic jets and
the genuine t’s from W -> tn
Identification based on calorimeter jet, motivated with a large fraction of t -> p± + np0 + n decays in the signal (~ 52%)
Reconstruction of the t jet around the HLT t direction in a cone of 0.4
Select t identification cuts to exploit helicity correlations in H± -> tn
Decay angular distributions in the CMS frame:
H± ->tn, t->p± +n: dN/dcosq ~(1+cosQ)
W± ->tn, t->p± +n: dN/dcosq ~(1-cosQ)
Harder charged pion from H± ->tn than from W->tn
For decays through vector mesons more
complicated but still harder pions from H±
Fraction of t momentum
carried by the leading p±
tt background
Signal

10. Event selection 3 Further event selections: t identification cuts:
leading track within DR < 0.1 the jet direction
small signal cone around the leading track, DR = 0.04
isolation in the cone 0.04 < DR < 0.4
pleading track/ET jet > 0.8 to exploit the helicity correlations
Further event selections:
Top mass reconstruction with minimization of c2 = ((mjj –mW)/sW)2 + ((mjjj –mtop)/stop)2 from all hadronic non-t jets, ET > 20 GeV, with sW = 10 GeV, stop = 17 GeV
B tagging with a probabilistic secondary vertex algorithm with discriminator cut

11. Transverse mass and discovery reach
Transverse mass reconstruction
Discovery potential
mT = (2 ETt jet ETmiss (1 - Df(t jet, ETmiss))1/2
An almost background-free signal can be reached in the signal area defined by Df(t jet, ETmiss) > 60o or by mT(t jet, ETmiss) > 100 GeV
Sources of systematic uncertainty in the background determination: ETmiss and jet scale, t identification, b tagging, cross section uncertainties

12. Study of MSSM H/A    jet+jet+X L. Wendland
Channel studied in collaboration with S. Gennai and A. Nikitenko
Final state search strategy:
two isolated t jets,
one b jet,
veto for other jets and
missing energy
Trigger with
single t trigger (ET > 92 GeV)
double t trigger (ET > 76 GeV)
Main backgrounds:
QCD di-jets, Z/*, tt
MC event visualization for bbH(500)->tt->2jet
t jet 2
b jet 1
b jet 2
t jet 1
Trigger rate 3.0 kHz, low luminosity (CMS-IN 2006/008)
The thresholds have to be this high to kill QCD background

13. t identification in jet + jet final state
t identification algorithm:
leading track within DR < 0.1 the jet direction
small signal cone around the leading track, DR = 0.07
isolation in the cone 0.07 < DR < 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, c2 at most 10
upper limit on impact parameter
IPT < 0.3 mm, IP < 1.0 mm
The signal cone size is slightly larger than in the e + jet final state
The pT cut for leading track is slightly larger than in the e + jet final state

14. Impact 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: ~ at greater than 92 % signal level
Three track final state: ~2.2 at greater than 85 % signal level
Efficiency curves based on cut on the significance
Transverse IP, mm
Full (3D) IP significance
N.B.: the tail in the IP occurs only in the one track final state!
The transverse and full IP significances are very similar
The tail is larger for the higher ET bins, which causes the efficiency curves to be separate (amount of tail up to 85 % for the highest ET bin)
Most of the rejection comes from the upper cut on the IP and IPT value; a small amount comes from a lower limit cut on the full IP significance (all of these are shown in the efficiency plot; the dots correspond to different significance cut values)
The sign is not used for the IP (the pt of the leading track is large  the leading track pt and jet direction are collinear  the jet direction and the IP vector are at a right angle and the sign becomes negative in almost half of the events both for signal and for background

15. Vertex reconstruction in 3-prong t 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
Efficiency curves based on cut on the significance
Signed full reco fl.path in mm
Signed tr. reco fl.path in mm
QCD jet, ET GeV
Because of the kinematical topology (i.e. highly collimated tracks), this final state is used as a benchmark for track and vertex fitting
The isolation requirement is killing away most of the QCD jets with B or D mesons  they are not a problem
The vertical axes on the left and middle plots are normalized to 100 (i.e. arbitrary units)
Sign assignment: if secondary vertex is reconstructed behind primary vertex (compared to jet direction), a negative sign is assigned
Hit merging on first pixel layer
t from H(500)
c jet, ET GeV

16. Event selections SIGNAL BACKGROUND Most dangerous background
mA = 200 GeV/c2 tan b =20
mA = 500 GeV/c2 tan b =30
mA = 800 GeV/c2 tan b =40
s × BR (fb)
3831
190
49.8
s 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-jet GeV/c
QCD di-jet GeV/c
QCD di-jet GeV/c tt
Z/g* mtt GeV/c2
s (fb)
1.33 × 108
5.03 × 108
2.94 × 109
2.08 ×1010
s 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
1.9
The 3 track final state is used at the moment only in the mA=200 GeV case; for heavier mA values the background is too large
Most dangerous
background

17. Higgs mass reconstruction
Signal + background
Background
Signal
3 track

18. Discovery reach

19. Full simulation study on MSSM Higgs bosons with H/A  tt  e + t jet + X R. Kinnunen and S. Lehti
Final state searched for: isolated electron, t jet and a b jet
These events are triggered with
a single e trigger (ET > 26 GeV) and with a combined electron + t trigger
Double and combined t 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 t’s: Z,g* , bbZ,g* , tt, Wt
fake t’s: W+jet, tt, Wt
electrons: Z,g*  e+e-, bbZ,g*  e+e-
fake electrons and fake t’s: QCD multi-jet
Special feature of this channel: good separation electron versus hadronic jet, t versus hadronic jet and electron versus t mandatory

20. t identification Variables for electron /t jet separation
To exploit the t  p + np0 + n decay modes (~52% of hadronic decays) jet reconstruction is used at the offline also
t identification algorithm:
leading track within DR < 0.1 the jet direction
small signal cone around the leading track, DR=0.04
isolation in the cone 0.04<DR<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, < pp/EHCAL < 1.5
Variables for electron /t jet separation
ET of the most energetic HCAl cell in the t jet
pion momentun over HCAL energy

21. Electron identification
Several electron candidates from offline reconstruction: ~ 1.3 electron candidates/event
Identification starts with Tracker isolation: no track with pT> 1 GeV within DR < 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: |ftrack-fSC|, |htrack-hSC|, ESC/ptrack, |1/ESC - 1/ ptrack|
- HCAL/ECAL energy ratio
- ECAL profile cuts : E3x3/E5x5, shh
Example of electron identificaton variables:
ECAL energy over track momentum
Cut optimization for
good efficiency and maximal purity
with electrons from t->enn
agaist hadrons from t->hadrons+n
hadron from t->hadrons+n
electron from t->enn
An identified electron (pT >20 GeV)
found in 81.2% of signal events,
purity for genuine electrons 97.5%

22. Further background suppression 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:
n’s emitted along the directions of electron and t jet, excluding back-to-back configurations with Df(electron, t jet) < 175o
Efficiency for rejection of negative neutrino solutions: 60% for signal, ~ 40% for tt background, mass resolution ~ 22%

23. Higgs boson mass and discovery reach
Signal and total background
for mA = 200 GeV/c2, tanb = 20
CMS discovery potential for
H/A  tt  electron+jet+X
Background under signal mainly from Z,g* -> ee and bbZ,g* -> ee
Estimate of the QCD multi-jet background: ~ 10% of the total background
Sources of systematic uncertainties on background determination:
the jet scale (4%), ETmiss scale (10%), b tagging and mistagging (5%), cross section measurements

24. Study of MSSM H/A    e+X S.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 = GeV
Background from
Z/*, tt, tW, bb, WW/WZ/ZZ

25. Lepton identification
Muon reconstruction and identification from muon chambers and tracker
Electron reconstruction from calorimeters and tracker, identification is based on
Track-ECAL matching
HCAL/ECAL energy ratio
ECAL profile cuts
Identification optimized against W+jet with jet faking an electron used
Lepton isolation : no other tracks pT>1GeV within R<0.4 around the lepton. Lepton isolation and pT cuts against backgrounds with soft leptons (bb,cc,...).

26. Tau impact parameter
Tau’s from Higgs travel couple of mm before they decay
Impact parameter, the minimum distance 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

27. B 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

28. Central jet veto tt events have more jet activity than bbH
Veto on additional jets in the tracker acceptance region coming from the primary vertex

29. Mass 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,m) cut, but statistics is decreased significantly

30. Discovery reach including systematics

31. bbZ as a benchmark for MSSM bbH search S.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

32. Verification of Monte Carlo
Verification of Z pT, associated b jet ET and h distributions and cross section measurement

33. Mass 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

34. Measurement of the H/A cross section and possible constrants on tan R.Kinnunen, S.Lehti, F.Moortgat, A.Nikitenko, M.Spira
One of the most important parameters to be determined in MSSM is tanb
The dominant part of the ggbbH,H proportional to tan2b
tanb measured from event rates
Uncertainty of tanb half of the uncertainty of the event rate
Results from different H/A final sates combined

35. Measurement uncertainty on tanb
Uncertainty of the cross section (times BR) measurement
=sqrt(Ns+Nb)/NsL/Lsel/selNbsyst/Ns
Uncertainty of the tanb measurement
tanb/tanb = ½   ½ stheor/stheor

36. Educational work of the HIP physics group
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 → tt with 3-prong t vertex reconstruction in the CMS detector, Masters Thesis, 2005
Lauri Wendland, PhD Thesis under preparation (H,A → tt →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

37. Publications and conference talks during 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

38. Future plans
Introduction and testing of the new CMS software CMSSW Contribution to the ”1 fb Physics” simulation Preparation of calibration, alignment and physics studies with real data
Start up of measurements for calibration constants and background processes: W and Z production, tt, bb, hadronic jets
Expect an integrated luminosity of few fb-1. Intensive measurement of our background processes and search for the MSSM Higgs bosons in the t channels at large tanb (~50) may start
Expect to reach 30 fb-1. The physics program presented here can be fulfilled. Exploration of tanb down to ~ 10 around mA = 200 GeV/c2 Measurement of tanb 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
Remember to emphasize, that the MC studies will be of importance even when analysing real data (to estimate the backgrounds, ...); the point is: we have gained already now a lot of experience that can be used when analyzing the real data