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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. 1. Overview. Standard Model of Particle Physics

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aruna kumar nayak thesis supervisor prof tariq aziz

Measurement of production cross section of Z boson with associated b-jetsandEvaluation of b-jet energy corrections using CMS detector at LHC

Aruna Kumar Nayak

Thesis Supervisor : Prof. Tariq Aziz

Synopsis Seminar


  • 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
the standard model
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 : QCDElectroweak

Gluon W±, Z / 

SU(2)L X U(1)Y U(1)Q

Electroweak Symmetry Breaking (Higgs mechanism),

Responsible for generating particle mass

the sm symmetry breaking
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 :

success of standard model
Success of Standard Model

Success :

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

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

LEP EWK page


is there any new physics
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

the large hadron collider
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

the compact muon solenoid detector cms
The Compact Muon Solenoid Detector (CMS)
  • Design Objectives :
  • Example of few important Higgs discovery modes :
  • H →gg
  • H → ZZ → 4 m
  • H → ZZ → 4 e
  • H → ZZ → 2e and 2m
  • Very good and redundant
  • Muon detection system
  • The best possible measurement of e/
  • Good resolution of hadronic jets and
  • missing transverse energy
  • 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)

detector components i
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 m X 150 m

occupancy : 10-4 /pixel/bunch crossing

spatial resolution : ~10 m in r- and ~20 m in r-z

Strip : 20 < r < 55 cm, size : 10 cm X 80 m

occupancy : 2-3% / bunch crossing

TIB resolution : 23-34 m in r- and 230 m in z

r > 55 cm, size : 25 cm X 180 m

occupancy : 1% /bunch crossing

TOB resolution : 35-52 m in r- and 530 m in z

TID : 3 disks

TEC : 9 disks, 120 cm < z < 180 cm


Detector Components (II)


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

detector components iii
Detector Components (III)


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 .


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 m

each station : 100 m  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 m,  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.


Detector Components (V)

Example of a Level-1 Jet Trigger

CMS Trigger :

L1 Trigger : Electronics modules

e.g. Look up Tables


L1 Rate : ~25 kHz (at 2X1033 cm-2s-1)

L1 decision time < 1 s

HLT :computer farm,

partial reconstruction of physics objects

HLT Rate : ~100 Hz

physics objects reconstruction electrons
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), shh < 0.0140(barrel), 0.0275(endcap)

Dfin < 0.090(barrel), 0.092(endcap), Dhin < 0.0090(barrel), 0.0105(endcap)

H/E : Hadronic to electromagnetic energy deposit ratio.

Dfin : f difference between the electron supercluster and the electron track

at vertex

Dhin : h difference between the electron supercluster and the electron track

at vertex


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

physics objects reconstruction muons
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

physics objects reconstruction jets
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 h for Z + bb events.

cross section measurement of pp z bb z l process
Cross section Measurement of pp → Z+bb, Z→l process

CMS PAS EWK-08-001

CMS AN-2008/020

CMS CR-2008/105

(CMS approved result)

Dominant at LHC

~ 15% of bbZ total s

  • 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
  • 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.


cross section and event generation
Cross section and Event generation

Signal llbb (Zbb) :

CompHEP events with pT(b) > 10 GeV, |h|(b) < 10 , mll > 40 GeV, |h|(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


cross section and event generation1
Cross section and Event generation


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 with cuts :

pT(c) > 20 GeV, |h|(c) < 5, mll > 40 GeV

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, |h|(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.


primary event selections
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

|h|(e) < 2.5, |h|(m) < 2.0,

lepton pT > 20 GeV

Jet Selection :

Two or more jets with

corrected ET > 30 GeV ,

|h| < 2.4

Jet corrected using Monte Carlo

jet energy correction

(as described earlier)


b jet tagging
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


No. track (3D IP significance cut) >= 3

Effective to supress the Z+jets



b tag efficiency
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

e t miss selection
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


event selection details
Event Selection details

Two Leptons, pT > 20 GeV, |h|(e) < 2.5 , |h|(m) < 2.0

Two or more Jets , ET > 30 GeV , |h| < 2.4

Two b-tagged Jets

Missing ET < 50 GeV

Initial and final cross sections after all selections

*More details for each selection cuts are in backup

expected measurement for 100 pb 1
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


expected measurement for 100 pb 11
Expected Measurement for 100 pb-1

Z →ee final state

Z →mm final state

tt background estimation
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



Uncertainty due to Background and double b-tagging.

NZbb and DNZbb are determined as follows.

NZbeforeb-tag = NZjj + NZcc + NZbb

NZafterb-tag = elX NZjj + ec X NZcc + eb X NZbb


NZbeforeb-tag = measured number of Z/g* → ll

events after all selections except b-tagging under

Z mass peak (75-105 GeV). Contribution of tt~ is

negligible (~1%).

NZafterb-tag = measured number of Z/g* → ll

events after all selections including b-tagging with tt~


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.

eb, ec, elare the efficiency of double b-tagging for Zbb, Zcc and Z+jets events

( Ratio of number of events before and after double b-tagging)

(after all selections except b-tagging)


systematics contd
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),

dR and uncertainties on eb, ec, el

*Calculation of systematics due to JES and MET scale and others are in backup


total uncertainty on measurement
Total Uncertainty on Measurement

Total cross section is expected to be measured in 100 pb-1 of data with uncertainty

ds = +21%, - 25% (syst.) , +/- 15% (stat.)


evaluation of the b jet energy corrections from data using bbz z ll process
Evaluation of the b-jet energy corrections from data using bbZ, Z->ll process

CMS Note-2007/014

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.

c1pxb1 + c2pxb2 = -pxZ

c1pyb1 + c2pyb2 = -pyZ

c1 = (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


applying to generator level jets
Applying to Generator level Jets

Ideal : ISR off in PYTHIA

ISR Effect

Df = f



Jet and quark

The error in direction

measurement of one jet

affects the other.

Ctrue = ET(jet)/ET(quark)

detector level jets
Detector level Jets

Very much similar


10 fb-1 of data

(LO cross section used

for Zbb sample)

1000 total events

after selections

75% signal and

25% background

(detail in backup)

Jet veto improves

pT balance

Selected events with

DR > 1.2

ET and h of veto jets


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)

Physics meeting, CMS Week


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


Getting the functional form:

10 fb-1 “data”

10 fb-1 “data”

S and S+B points are within

~ 2 s stat errors

Physics meeting, CMS Week


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.

Physics meeting, CMS Week


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 %

Physics meeting, CMS Week

b jes uncertainty
b JES Uncertainty

Fit Uncertainty with

10 fb-1 of data

Uncertainty of Mbb

Mbb = 122.0 ± 8 (syst) GeV

Generated Mbb = 120 GeV

Physics meeting, CMS Week

cosmic muon charge ratio ongoing
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.


cosmic muon charge ratio ongoing1
Cosmic Muon Charge Ratio(ongoing)

Example of a cosmic muon

passing CMS detector

  • Muon Selection for Charge Ratio studies
  • Global muon two Leg, f < 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)
charge ratio vs zenith angle
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)

cosmic muon angular resolution ongoing
Muon selection :

2 Leg Barrel muons

f (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.

The difference of the measured angle (f, q, zenith angle) at the entry point are studied.

Cosmic Muon Angular Resolution(ongoing)

Point of measurement

Upper Leg

Lower Leg

f resolution glb muon
f Resolution GLB muon

Df = f (Extp Lower Leg) – f (Upper Leg)


MC 10GeV

Fitted with double

gaussian function

May be due to difference in magnetic field map


a zenith angle resolution glb muon
a (Zenith angle) Resolution GLB muon

Da = a (Extp Lower Leg) – a (Upper Leg)


MC 10GeV


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 Muon : Upper Leg

f < 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

Q(lower leg) * Q(upper leg) > 0.



efficiency from tag probe
Efficiency from Tag & Probe



< 2% difference in most of the bins.

jet plus tracks performance study using test beam 2007 data
Jet Plus Tracks performance study using Test Beam 2007 data

CMS AN-2008/111

Main JPT steps:

subtract average response of

“in-calo-cone” tracks from calo

jet E and add track momentum.

- add momentum of “out-of-calo-cone” tracks (1,2,3 on figure) to jet E

Particle Energy Response :

ECAL (7 X 7 crystal )

HCAL (3 X 3 Tower)

Without Zero Suppression

ECAL Calibration using 100 GeV


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


higgs search at cms in cpv mssm model
Higgs search at CMS in CPV MSSM Model

(133 GeV)

(51 GeV)

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)

Because of the suppressed H1VV coupling one of the pseudo-scalar 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.

(LEP Higgs working group, hep-ex/0602042)

  • M(H1) = 51 GeV, M(H+) = 133 GeV, M(top) = 175 GeV
  • F(CP) = 90o, tan(b) = 5
  • * 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


(Ghosh, Godbole, Roy hep-ph/0412193)

p t distribution of b quarks
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

quarks distribution in h f space
Quarks distribution in (h, f) 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.


full event reconstruction
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.


top mass
Top Mass

>= 3 jets in top :

Wrong combinations


Uncert. In JES and JER

results for 30 fb 1 of data
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.



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.

trigger selection for h tn t hadronic decay
TriggerSelection 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)

New thresholds

to keep L1 rate

of 1T Or 2T 3 kHz

(with DAQ TDR cuts,

it was 3.6 kHz)

* Tables of efficiency and rates are in backup