Measurement of production cross section
of Z boson with associated b-jets
and
Evaluation of b-jet energy corrections
using CMS detector at LHC
Aruna Kumar Nayak
Thesis Supervisor : Prof. Tariq Aziz
13th July 09
Synopsis Seminar
1
Overview
 Standard Model of Particle Physics
The reach of LEP and Tevatron
 The Large Hadron collider
CMS detector
Ability of CMS detector : physics objects reconstruction
 Cross section measurement of bbZ, Z → ll process
 Evaluation of b-jet energy corrections
 Study on cosmic muon charge ratio using CRAFT data
 Jet plus tracks algorithm : performance study using Test beam data
 Higgs boson search in CP violating MSSM like model
2
The Standard Model
SM Building blocks
The SM is based on
SU(3)C
X
SU(2)L X U(1)Y gauge symmetry
Strong : QCD Electroweak
Gluon
W ±, Z / 
SU(2)L X U(1)Y
U(1)Q
Electroweak Symmetry Breaking (Higgs mechanism),
Responsible for generating particle mass
3
The SM : Symmetry Breaking
The potential is of the form
The 2nd choice does the spontaneous
breaking of gauge symmetry
The strength of the interactions of the
particles with the Higgs field determines
the mass of the particles
e.g. in case of Z boson :
4
Success of Standard Model
Success :
LEP EWK page
W, Z were discovered at SPS, CERN in 1980s
Tevatron, Fermilab discovered Top quark
(the heaviest among all) in 1995
Most of the SM parameters, like masses and
gauge couplings have been measured very precisely
at LEP, CERN and matching well with
SM predictions
Yet Unknown :
The only parameter yet unknown in SM is the mass of
Higgs boson : the fundamental ingredient of the model
hep-ph/0503172
Limits to the Higgs boson mass :
From Experiments :
Indirect limit : LEP precision EWK measurements :
191 GeV upper limit (at 95% CL)
LEP-II direct limit : 114 GeV lower limit
From Theory : bound from Triviality and
Vacuum stability
5
Is There any New Physics?




THE SM has quite a few shortcomings, e.g. :
The SM is silent about the Gravitational force (the 4th fundamental force)
It does not explain the pattern of fermion masses
In SM, the higher order corrections to the Higgs boson mass diverges,
unless a fine adjustment of the parameters is performed.
 Possible candidates for New Physics :
Supersymmetry : predicts the existence of a super partner for each SM
particles (with spin difference ½) , Extra dimension etc…
 The LHC can explore all the possibilities upto TeV scale and can answer
some of the unknowns. Also precision EWK measurements, mtop etc.
One of the EWK measurements : cross section of Z + b-jets production
6
The large Hadron Collider
~27 KM ring, The LEP tunnel
proton-proton collision : 14 TeV CM energy
25 ns bunch crossing : 2808 bunches
with ~1011 protons in a bunch
Design luminosity : 1034 cm-2s-1 => 100 fb-1/year
7
The Compact Muon Solenoid Detector (CMS)
Design Objectives :
Example of few important Higgs discovery modes :
H → 
H → ZZ → 4 m
H → ZZ → 4 e
H → ZZ → 2e and 2m
1) Very good and redundant
Muon detection system
2) The best possible measurement of e/
3) Good resolution of hadronic jets and
missing transverse energy
4) High quality central tracking
Total weight : 14500 t
Diameter : 14.60 m
Length : 21.60 m
Magnetic Field : 4 Tesla
Size of 1 event : 1 MB
100 events / second
(stored in tape)
8
Detector Components (I)
Magnet :
The choice of magnetic field is key to the design of any
HEP detector in collider experiments.
CMS Magnet : Superconducting Solenoid
Field strength : 3.8 Tesla, Length : 13 m, Inner R = 2.95 m
operating current : 20 kA
Advantage : Compact and small detector
good resolution in inner tracking,
good muon momentum resolution
Tracker :
Geometry : r ~ 110 cm, L ~ 540 cm, || < 2.4
66 million pixels, 9.6 million silicon strips
Pixel : r ~ 10 cm, Particle flux ~ 107/s,
size of pixel : 100 mm X 150 mm
occupancy : 10-4 /pixel/bunch crossing
spatial resolution : ~10 mm in r- and ~20 mm in r-z
Strip : 20 < r < 55 cm, size : 10 cm X 80 mm
occupancy : 2-3% / bunch crossing
TIB resolution : 23-34 mm in r- and 230 mm in z
r > 55 cm, size : 25 cm X 180 mm
occupancy : 1% /bunch crossing
TOB resolution : 35-52 mm in r- and 530 mm in z
TID : 3 disks
TEC : 9 disks, 120 cm < z < 180 cm
9
Detector Components (II)
ECAL :
Compact, Hermatic, homogeneous,
61200 lead tungstate (PbWO4),
X0 = 0.89 cm, Rm = 2.2 cm
Fast : 80% light yield within 25 ns
Radiation hard : 10 Mrad
Barrel (EB) : Rin ~ 129 cm, 36 Super modules
0 < || < 1.479,
Each crystal 0.0174 X 0.0174 (, ),
front face ~ 22 X 22 mm2, L = 230 mm (~25.8 X0)
Endcap (EE) : Zin ~ 314 cm, 1.479 < || < 3.0 ,
crystal : 28.6 X 28.6 mm2, L = 220 mm ( 24.7 X0)
Preshower (ES) : 2 layers of Si strip (1.9 mm pitch),
behind lead (2X0 , 3X0)
The energy resolution is of the form
S : stochastic term, N : noise term, C : constant
10
Detector Components (III)
HCAL :
Layers of plastic scintillator tiles,
stacked within layers of absorbers (brass).
Light read out using WLS fibre.
WLS fibres are connected to clear fibres
outside the tiles.
Barrel (HB) : 32 towers, || < 1.4, 2304 towers in total
0.087 X 0.087 (, ) , 15 brass plates of 5cm,
2 steel external plates, front scint. plate 9 mm,
others 3.7 mm
Eencap (HE) : 14  towers, 1.3 < || < 3.0,
outer 5 towers :  ~ 0.087,  ~ 50 , Inner 8 towers :  ~ 0.09-0.35,  ~ 100
Forward (HF) : steel/quarz fibre calorimeter. 3.0 < || < 5.0, Zin ~ 11.2 m
13  towers ~ 0.175,  ~ 100
Outer (HO) :
Plastic scintillator, 10 mm, 2 layers in ring 0 separated by an iron absorber
of thickness 18 cm, 1 layer each in ring +/- 1,2. towers size same as HB.
|| < 1.26 . Increases the effective thickness of HCAL to 10 .
11
Detector Components (IV)
Muon :
Drift Tube (barrel), Cathode Strip Chambers (endcap),
Resistive Plate Chambers.
Barrel (MB) : || < 1.2, low radiation, low muon rate,
low residual magnetic field.
4 station : MB1-MB4, 12 sectors , single point resolution 200 mm
each station : 100 mm  precision (1 mrad in direction).
Endcap (ME) : || < 2.4 , high muon rate, higher magnetic field as well.
486 CSCs in 2 endcaps, trapezoidal shape, 6 gas gaps in each
chamber, strip resolution 200 mm,  resolution 10 mrad.
RPC provides fast response (few ns) and good time resolution.
But has coarser position resolution w.r.t DT and CSC.
Use to identify correct bunch crossing.
RPC and DT, CSC provide independent and complementary
information for L1 trigger.
12
Detector Components (V)
CMS Trigger :
Example of a Level-1 Jet Trigger
L1 Trigger : Electronics modules
e.g. Look up Tables
(RAM, ASIPs)
L1 Rate : ~25 kHz (at 2X1033 cm-2s-1)
L1 decision time < 1 ms
HLT : computer farm,
partial reconstruction of physics objects
HLT Rate : ~100 Hz
13
Physics Objects Reconstruction : Electrons
Reconstructed from the information of Tracker and ECAL
Electron Id : Robust (cut based) Electron Id (to discriminate against Jets)
H/E < 0.115(barrel), 0.150(endcap), s < 0.0140(barrel), 0.0275(endcap)
Din < 0.090(barrel), 0.092(endcap), Din < 0.0090(barrel), 0.0105(endcap)
H/E : Hadronic to electromagnetic energy deposit ratio.
Din :  difference between the electron supercluster and the electron track
at vertex
Din :  difference between the electron supercluster and the electron track
at vertex
14
Physics Objects Reconstruction : Electrons
Isolation : track isolation
S (pT(track)/pT(electron))2 < 0.02 (in cone 0.02-0.6, track pT > 1.5 GeV, )
(This isolation criteria is only for Z measurement study)
Efficiency calculated by matching MC electron to Reco electron in 0.1 cone
Electrons from Z decay
15
Physics Objects Reconstruction : Muons
Reconstructed from the information of Tracker and Muon Chamber
Isolation : S pT(tracks) (0.3 cone) < 3 GeV
Efficiency calculated by matching MC muon to Reco muon in 0.1 cone
Muons from Z decay
16
Physics Objects Reconstruction : Jets
Jets are reconstructed from calorimeter energy using
IterativeCone algorithm of cone size 0.5
 dependent & pT dependent corrections are used.
Reconstruction efficiency of jets Vs generated
Jet pT and  for Z + bb events.
17
Cross section Measurement of
pp → Z+bb, Z→ll process
CMS PAS EWK-08-001
CMS AN-2008/020
CMS CR-2008/105
(CMS approved result)
 Measurement of Zbb production is an important test of
QCD calculation
 Background to Higgs discovery channels at LHC, like SM
H → ZZ → 4l, SUSY bbF, F → tt (mm)
 bbZ measurement can help reduce the uncertainty in
SUSY bbH calculation
Dominant at LHC
 Z + 1 b-jet has been measured both at CDF & D0
 Z + 2-bjet may be observed for the 1st time
 The possibility of observing and measuring the
production of Z + 2 b-jet at LHC has been studied aiming
at early 100 pb-1 of CMS data at 14 TeV center of mass
energy.
~ 15% of bbZ total s
18
Cross section and Event generation
Signal llbb (Zbb) :
CompHEP events with pT(b) > 10 GeV, ||(b) < 10 , mll > 40 GeV, ||(l) < 2.5
were generated and fully simulated in CMS with 100 pb-1 calibration and mis-alignment
Cross section calculated using MCFM, NLO s (llbb) = 45.9 pb , l = e, m, t
PDF : CTEQ6M, scale mR = mF = MZ
LO cross section calculated using PDF : CTEQ6L1 and same values for scale K (NLO) = 1.51
19
Cross section and Event generation
Backgrounds
tt~ + n jets, n >= 0 :
Generated using ALPGEN
Cross section normalized to NLO inclusive tt~ cross section 840 pb
llcc + n Jets, n>= 0 (Zcc) :
Generated using ALPGEN
Normalized on NLO s (using MCFM) 13.29 pb, k factor = 1.46
pT(c) > 20 GeV, ||(c) < 5, mll > 40 GeV
with cuts :
ll + n Jets, n >= 2, (Zjj) :
Generated using ALPGEN
Normalized on NLO s (using MCFM) 714 pb , k factor = 1.02 with cuts :
pT(j) > 20 GeV, ||(j) < 5, mll > 40 GeV
All events are passed through full CMS detector simulation and reconstruction chain,
with appropriate alignment and calibration uncertainties corresponding to early 100 pb-1
of integrated luminosity.
20
Primary Event selections
Trigger selection : single isolated electron or
muon
Level-1 threshold 12 GeV, 7 GeV & High-Level threshold 15 GeV, 11 GeV
Corresponds to low luminosity period L = 1032cm-2s-1
Lepton Selection :
Two high pT, isolated,
opposite charged leptons
||(e) < 2.5, ||(m) < 2.0,
lepton pT > 20 GeV
Jet Selection :
Two or more jets with
corrected ET > 30 GeV ,
|| < 2.4
Jet corrected using Monte Carlo
jet energy correction
(as described earlier)
21
b-Jet Tagging
Lepton, jet selections + double b-tagging
with b-discriminator > 0.
b-discriminator of 2nd highest discriminator jet
Jets are tagged using “Track Counting b-tagging”
Which uses the 3-dimentional impact parameter
significance , of 3rd highest significance track,
as the b-tagging discriminator
i.e.
No. track (3D IP significance cut) >= 3
Effective to supress the Z+jets
backgrounds.
22
b-tag efficiency
b-tagging efficiency for b, c, light jets
after applying cut on b-discriminator > 2.5
*statistical error bars are not shown
23
ETmiss selection
Lepton, jet selections + double b-tagging with b-discriminator > 0
Missing ET reconstructed from calorimeter
and corrected for Jet Energy scale and
muons.
Type-1 ETx,ymiss = - (ETx,ycalo + Sjets(ETx,ycorr – ETx,yraw))
Muon corr. = - (Smuons (px,y – Ex,y (calo. deposit)))
Effective to supress the tt~+jets
backgrounds
Cut ETmiss < 50 GeV
24
Event Selection details
Two Leptons, pT > 20 GeV, ||(e) < 2.5 , ||(m) < 2.0
Two or more Jets , ET > 30 GeV , || < 2.4
Two b-tagged Jets
Missing ET < 50 GeV
Initial and final cross sections after all selections
Process Name
s NLO (pb)
Final s (fb)
Electron
Muon
Zbb
46
176 ± 3.3
212 ± 3.6
tt~ + jets
840
173 ± 9.0
178 ± 8.7
Z+jets
714
5.5 ± 2.8
5.5 ± 3.1
Zcc+jets
13.3
4.3 ± 1.63
5.1 ± 1.93
*More details for each selection cuts are in backup
25
Expected Measurement for 100 pb-1
events scaled to 100 pb-1
Purity of b-tagging in Zbb events
The points are the result of random selection of exactly 100 pb-1 of data
26
Expected Measurement for 100 pb-1
Z → ee final state
Z → mm final state
27
tt~ background Estimation
Dilepton mass region
Signal : 75-105 GeV (Z)
Side band : 0-75 GeV & 105 – above (no Z)
NZ(tt) = (eZ(tt)/enoZ(tt)) X NnoZ(tt)
DNZ(tt)/NZ(tt)= 1/√NnoZ(tt)
NZ(tt) = expected no. of tt~ events in signal region
NnoZ(tt) = measured no. of tt~ events out side
signal region
eZ(tt) = selection efficiency of tt~ in signal region
enoZ(tt) = selection efficiency of tt~ outside signal region
DNZ(tt) = uncertainty of the expected number of tt~ events in the signal region.
Uncertainty on eZ(tt)/enoZ(tt) is negligible compared to the statistical uncertainty on
NnoZ.
Assuming side band contains only tt~ background. Other possible backgrounds are negligible
28
Systematics
Uncertainty due to Background and double b-tagging.
NZbb and DNZbb are determined as follows.
NZbefore b-tag = NZjj + NZcc + NZbb
NZafter b-tag = el X NZjj + ec X NZcc + eb X NZbb
Where,
NZbefore b-tag = measured number of Z/* → ll
events after all selections except b-tagging under
Z mass peak (75-105 GeV). Contribution of tt~ is
negligible (~1%).
NZafter b-tag = measured number of Z/* → ll
events after all selections including b-tagging with tt~
subtracted
NZjj is unknown number of ll+jets (u, d, s, g) events before
double b-tagging.
NZcc is unknown number of Zcc events before double b-tagging.
NZbb is unknown number of Zbb events before double b-tagging.
(after all selections except b-tagging)
eb, ec, el are the efficiency of double b-tagging for Zbb, Zcc and Z+jets events
( Ratio of number of events before and after double b-tagging)
29
Systematics Contd ......
Reduce the no. of variables to two using the Ratio
where
is ratio of selection efficiencies
Solving the equations
The Uncertainties on NZbb is calculated from uncertainties of
NZafter b-tag (uncertainty due to tt~ subtraction),
R and uncertainties on eb, ec, el
* Calculation of systematics due to JES and MET scale
and others are in backup
30
Total Uncertainty on Measurement
Source of uncertainty
Value used (%)
(s(Zbb)) (%)
Jet energy scale (JES)
7
7.6
Type 1 missing ET scale
10 (unclustered ETmiss) + 7 (JES)
7.4
MC pTjet, jet dependence
-10, +0
-10, +0
b-tagging of b-jets (eb)
8
16
mistagging of c-jets (ec)
8
0.5
mistagging of light jets (el)
7.6
0.5
NZafter b-tag due to tt~ subtraction
4
4.6
R (Zcc / Zjj)
5
0.4
lepton selections
0.5
0.5
luminosity
10
10
Total cross section is expected to be measured in 100 pb-1 of data with uncertainty
s = +21%, - 25% (syst.) , +/- 15% (stat.)
31
Evaluation of the b-jet energy corrections from data
CMS Note-2007/014
using bbZ, Z->ll process
CMS AN-2006/106
(CMS approved result)
Why do we need It :
b-Jets in final state of many processes at LHC
b quark fragmentation function is different than
light quark and gluon
Production and decay of heavy hadrons in the b-jet
Part of the energy will be carried by neutrinos
in semi-leptonic decays.
c1
c1pxb1 + c2pxb2 = -pxZ
c1pyb1 + c2pyb2 = -pyZ
= (pyZpxb2-pxZpyb2) / (pxb1pyb2-pyb1pxb2)
c2 = (pyZpxb1-pxZpyb1) / (pxb2pyb1-pxb1pyb2)
c1 and c2 are mere scale factors
Assumption :
Exact pT balance in the event
(but there is effect of radiated jets)
Jets reproduce the parton direction :
Effect of detector, Algorithm
In Ideal case
It will be exactly 1
32
Applying to Generator level Jets
Ideal : ISR off in PYTHIA
ISR Effect
D = 
separation
between
Jet and quark
The error in direction
measurement of one jet
affects the other.
Ctrue = ET(jet)/ET(quark)
33
Very much similar
Selections,
Detector level Jets
10 fb-1 of data
(LO cross section used
for Zbb sample)
1000 total events
after selections
Jet veto improves
pT balance
75% signal and
25% background
(detail in backup)
Selected events with
DR > 1.2
ET and  of veto jets
34
Measured pT balance between di-b jets and di-leptons
The effect of background on pT balance is small ( < 1 %)
(if we fit around the peak)
4th Dec 06
Physics meeting, CMS Week
35
Extraction of energy corrections
Because of ISR,
Z boson and two
b quarks are not
perfectly balanced
in the transverse
plane. Jet veto does
not reduce completely
this effect.
When the jet
deviates from
the original b-quark
direction that error
propagates in the
pT balance equation
and gives a wrong
correction coefficient
36
Getting the functional form:
10 fb-1 “data”
10 fb-1 “data”
S and S+B points are within
~ 2 s stat errors
4th Dec 06
Physics meeting, CMS Week
37
How correction function works on bbZ events :
As a first test, the b-jets in
the same gg->bbZ process
has been corrected using this
correction function.
The plot shows pT ratio of
Z boson to that of combined
two b-jets, dashed plot is for
uncorrected jets and solid plot
is for corrected jets.
The correction restores the
pT balance and also makes
the distribution narrower
compared to uncorrected jets.
4th Dec 06
Physics meeting, CMS Week
38
How correction function works on h->bb in
tth, h->bb, W->ln events :
- restore Higgs boson mass to nominal value
- improve resolution by ~ 25 %
4th Dec 06
Physics meeting, CMS Week
39
b JES Uncertainty
Fit Uncertainty with
10 fb-1 of data
Uncertainty of Mbb
Mbb = 122.0 ± 8 (syst) GeV
Generated Mbb = 120 GeV
4th Dec 06
Physics meeting, CMS Week
40
Cosmic Muon Charge Ratio
(ongoing)
• Cosmic muon Charge ratio :
90% of proton in cosmic ray
Production of more p+ and K+ in
Shower than p- and K-.
• Data used : 300 M triggered
events taken last year in CMS :
100 M good events
(tracker used in the run)
• Studying cosmic physics is not
CMS aim : not designed for it.
• But it helps understanding the
detector by measuring this which
has been measured very precisely
in earlier dedicated experiments
and also confirms CMS capability.
41
Cosmic Muon Charge Ratio
(ongoing)
• Muon Selection for Charge Ratio
studies
•
•
•
•
•
•
•
•
Example of a cosmic muon
passing CMS detector
Global muon two Leg,  < 0
(downward)
pT (at PCA) > 10 GeV, pT = 1/C (curvature)
C = (1/2)(q1/pT1 + q2/pT2)
at Point of Closest Approch (PCA)
Does not share tracker track
No. CSC Hits, TEC Hits = 0
No. of DT Hits (per leg) >= 20
No. of TOB Hits (per Leg) >= 5
No. of DT SL2 (Z) Hits >= 3
Net q = Sign(q1/pT1 + q2/pT2)
42
Charge ratio Vs Zenith Angle
pT measured at PCA from the curvature of two Legs
a : Zenith angle measured at the entry point (CMS detector surface)
43
Cosmic Muon Angular Resolution
(ongoing)
• Muon selection :
•
Point of measurement
2 Leg Barrel muons
 (muon) < 0.,
same track charge for both leg ,
# of total track hits >= 25, 15
for upper and Lower legs.
Track propagation
• The Lower Leg track is propagated to
the closest approach to the 1st hit
point (inner most point as convention)
of the upper Leg track, using
SteppingHelixPropagator in opposite
to momentum.
Upper Leg
Lower Leg
• The difference of the measured angle
(, q, zenith angle) at the entry point
are studied.
44
 Resolution GLB muon
D =  (Extp Lower Leg) –  (Upper Leg)
Data
MC 10GeV
Fitted with double
gaussian function
May be due to difference in magnetic field map
45
a (Zenith angle) Resolution GLB muon
Da = a (Extp Lower Leg) – a (Upper Leg)
MC 10GeV
Data
46
Selection Efficiency from data Using Tag & Probe (ongoing)
Muon Selection
Tag Muon : Lower Leg
 < 0, pT (at PCA) >= 10, no. DT Hits >= 20,
no. of TOB Hits >= 5,
No. of CSC Hits = 0 , no. of TEC Hits >= 0
Compatible lower tracker track and
lower Stand alone muon track
Probe
Probe Muon : Upper Leg
 < 0, pT (at PCA) >= 10, no. DT Hits >= 20,
no. of TOB Hits >= 5,
No. of CSC Hits = 0 , no. of TEC Hits >= 0
Compatible upper tracker track and
upper Standalone muon track
Tag
Q(lower leg) * Q(upper leg) > 0.
47
Efficiency from Tag & Probe
Data
MC
< 2% difference in most of the bins.
48
Jet Plus Tracks performance study using Test Beam 2007 data
Main JPT steps:
subtract average response of
“in-calo-cone” tracks from calo
jet E and add track momentum.
- add momentum of “out-of-calocone” tracks (1,2,3 on figure) to
jet E
CMS AN-2008/111
Particle Energy Response :
ECAL (7 X 7 crystal )
HCAL (3 X 3 Tower)
Without Zero Suppression
ECAL Calibration using 100 GeV
electrons
HCAL Calibration using muon
and wire source
Jets are made from Charged pions
only, by randomly picking
6 pi of 5 GeV, 4 pi of 6 GeV
2 pi of 7 GeV, 1 pi of 8 GeV
True Jet Energy : 76 GeV
( pT = 28 GeV, eta = 1.653)
Track Correction : for each particle
subtract average (EE+HE) response
and add true energy.
Calo Correction : multiply each Jet
energy by True energy / Emeanraw
49
Higgs search at CMS in CPV MSSM Model
CMS AN-2008/025
arxiv:0803.1154 (hep/ph)
(part of 2007 Les houches study)
Because of the suppressed H1ZZ coupling, LEP
could not exclude the presence of
a light Higgs boson at low tanb (~ 3.5 to 10)
(LEP Higgs working group, hep-ex/0602042)
Because of the suppressed H1VV coupling one of the pseudoscalar Higgs state is very light
Since there is correlation between the mass of charged Higgs
and that of the pseudo-scalar Higgs state in MSSM, => a light
charged Higgs, with MH+< Mtop .
The traditional decay mode H+->tn is suppressed over an
order of magnitude.
M(H1) = 51 GeV, M(H+) = 133 GeV, M(top) = 175 GeV
F(CP) = 90o, tan(b) = 5
(133 GeV)
(51 GeV)
(Ghosh, Godbole, Roy hep-ph/0412193)
s* BR = 2 * 840 pb * 0.01 (BR(t->bH+)
* 0.567 (BR(H+->H1W) * 0.99 (BR(t->bW))
* 0.92 (BR(H1->bb) = 8.675 pb
Main backgrounds : tt + >= 2jets & ttbb+jets
50
pT distribution of b-quarks
b-quarks from H1 are very soft ,
36% events have both two b-quarks from H1 with pT above 20 GeV
51
Quarks distribution in (, ) space
DR between two closest quarks
The final state of the event
consists of 6 quarks and so
6 or more jets.
The two closest quarks in
the event fall very close to
each other and so it makes
difficult to reconstruct 6-jets
in the event.
0.5
52
Full Event Reconstruction
• Leptonic decayed W was reconstructed from lepton and missing ET. The
z-component of missing energy was calculated using W mass constraint.
This yields real solutions in 66% events. Events with imaginary solutions
are rejected. There are two possible candidates for each leptonic W.
• Hadronic decayed W was reconstructed from jets not tagged as b-jets.
All jet pairs with invariant mass within mw ± 20 GeV were considered as
possible candidates for W.
• Two Tops were reconstructed simultaneously from 4 b-Jets, two leptonic
W candidates and N (any number) possible hadronic W candidates. The
jets and W were assigned to Tops by minimizing
DM = sqrt( (mtop1 – mtop)2 + (mtop2 – mtop)2 + (mW (hadronic) – mW)2 )
where mtop1 = 1 b-Jet + 1 W
mtop2 = 3 b-Jets + 1 W ,
mtop = 175 GeV, mW = W boson mass.
events with mtop1 and mtop2 within mtop ± 30 GeV were selected.
53
Top Mass
>= 3 jets in top :
Wrong combinations
+
Uncert. In JES and JER
54
Results for 30 fb-1 of data
110 signal events and 203 ± 60 tt + Njets
events in 30 fb-1 data : 6.78 < S / B < 9.2
Systematic uncertainty on tt+jet background = 22.5%
Signal significance s = S/√(B+DB2) = 110 / √(263 + 592) = 1.8
Large syst. uncert. dut to
b-tagging, JES and
MET scale
The theoretical uncert.
on LO cross section
of tt+≥2jets is ≥50%
MH1 = 51 GeV
All 3 possible combination of
b-Jet pair out of 3 b-Jets from
2nd Top
The analysis was limited by the unavailability of
sufficient background events.
55
Summary
 The process, Zbb, Z -> ll has been studied aiming for the first LHC data.
The cross section of this process can be measured with first 100 pb-1 of
data within 30% uncertainty .
 Zbb, Z->ll process provides a data driven technique to evaluate dedicated
b-jet energy corrections with higher integrated luminosity.
 A study is being carried out for the measurement of Cosmic muon charge
ratio as function of zenith angle
 The performance of calorimeter response subtraction method for charge
paticles in Jet Plus tracks algorithm has been studied using the Test Beam
data (a data driven method which could be used to correct b-jets in Zbb).
 Studied the possibility of discovering a light Higgs in CPV MSSM model in
higher int. luminosity. The study is limited by the unavailability of large
background statistics (large stat. uncert.) and also large systematics. The
systematic uncertainty can be reduced with data driven background
measurement.
 A trigger study for H+ -> tn channel was performed with the updated MC
datasets for the Physics TDR.
56
Thank You
57
Trigger Selection for H+ -> tn, t hadronic decay
CMS IN-2006/008
Level-1 1Tau trigger : 1Tau > 93 GeV
HLT Tau trigger : HLT MET > 60 GeV
+ HLT Trk Tau ( pT = 25)
For Rate calculation : QCD 30-470 GeV
Data Challenge 04 samples
mH0 = mH+ = 200 GeV
Matching with DAQ TDR results
HLT rate : 0.7 Hz (1 Hz in DAQ TDR)
* Tables of efficiency and rates are in backup
New thresholds
to keep L1 rate
of 1T Or 2T 3 kHz
(with DAQ TDR cuts,
it was 3.6 kHz)
58