# Presentation: 824242

## Introduction

**1e****-11**

- t

*W*

*b*

*b*

***W*****0**
- Longitudinal fraction
**F****0**** ~70%**

***W*****-**
- Left handed fraction
**F****-**** ~ 30%**

- Spin

- Momentum

*W*

- A precise measurement of the helicity of the W boson examines the nature of the decay vertex tbW* *and provides a stringent test of Standard Model. The standard top quark has a V-A charged-current weak interaction. To conserve angular momentum, the emitted b quark (essentially massless, with its helicity dominantly negative, i.e., spin pointing opposite to its line of flight in the rest frame of the top quark) can point either along the top spin, with the spin projection of the W vanishing (i.e., longitudinally polarized), or in the direction opposite to the top spin, in which case the W must be left-hand polarized (negative helicity). Hence, for massless b quarks, a top quark can decay to a left-handed W (negative helicity W_) or a longitudinal W (zero helicity W0). Therefore, in the Standard Model (using mb=0, Mtop=175 GeV/c2 and MW=80.4 GeV/c2), top quarks decay to longitudinal, left handed and right-handed W bosons with a branching ratio

-  It is defined only in terms of the main background (W+jets, 85%), PW+jets(x),  which proves to be an adequate representation for multijet background
-  The background probability for each event is calculated using VECBOS subroutines for W+jets [5]
-  It uses the same transfer functions for modeling the jet resolutions as used for signal events
-  All the permutations are considered, together with all possible values of the *z *component of the momentum of the neutrino
-  The integration is done over the jet energies

**Background Probability P****background****(x)**

**Introduction**

**Matrix Element Method**

***W(x,y)*** is the probability  of measuring** **a** set of variables *****x***** **when** **a** set of variables *****y ***was produced ** (*****x*** jet variables, ***y***** **parton variables). It is taken as a  function for quantities that correspond to well measured objects. Due to the excellent granularity of the D calorimeter, angles are considered well measured. Also, since energies of electrons are measured much better than for jets, the momenta of electrons will also be considered well measured. For W->, the muon momentum is often not well measured, so the muon momentum resolution has to be included. The effect is taken into account by integrating numerically over the resolution of the muons. For the e+jets final states, we write:
-  where ***p******y******e*** and ***p******x******e*** are the produced and measured electron momenta, ***E******y*** and ***E******x*** arethe parton and jet energies, and *********y*** and *********x******j*** are the quark and jet angles.

**Asymmetric**

- To calculate the signal probability of ttbar->lepton+jets, Pttbar(x;), an integration must be performed over 20 variables, corresponding to the vector momenta of the six final-state particles and the longitudinal momenta of the incident partons. Inside the integrals there are 15 -functions. Four for total energy and momentum conservation and eleven from the transfer functions (see **Transfer Functions**). The calculation of Pttbar(x;) therefore involves a five-dimensional integral.  The 2 and 3 parton invariant masses, and the absolute value of momentum of one of the quarks were chosen as the integration variables. The reason for this is that the value of the matrix element |M|2 is negligible, except near the four peaks of the Breit-Wigners corresponding to the two top and W decays.

- where ******1 ** is the momentum of one of the jets, **m****1**, **m****2**  are the top masses, and **M****1**, **M****2** are the W masses in the event, **f(q****1****),f(q****2****)** represent the parton distribution functions (CTEQ4) for incident partons, **q****1**,**q****2** are the initial parton momenta, ******6** is the six particle phase space, **W****jet****(x,y)** is the probability of measuring **x **when y was produced in the collision (see **Transfer Functions**), **|M|****2** is the ttbar->lepton+jets matrix element [4].

-  The probability of an event as a function of a parameter  that we would like to extract is proportional to the differential production cross section. When the reaction is initiated by partons inside a proton, and the resolution of the detector cannot be ignored, then the cross section has to be folded over the parton distribution functions and detector resolution, and integrated over all the possible parton variables** *****y*** leading to the observed set of variables* ****x ***(I.e. jets and leptons):
- where ***d******n******σ ***is the differential cross section, ***f(q)*** are the parton distribution functions, and ***W(y,x)*** is the probability that a parton level set of variables ***y*** will show up in the detector as the set of variables ***x***.
-  All the processes that can contribute to the observed final state must be included in the probability. Therefore, we include the background probability
- where i=1,2,...,K represent all possible contributions to the final state under study
-  The probability that an event is accepted depends on the characteristics of the event, and not on the process that produced it. When the effects of the detector acceptance are included, the “measured probability ***P(x;************)***” can be related to the “production probability ***P******0******(x;************)***”:
- where ***Acc(x)*** include all conditions for accepting or rejecting an event
-  We insert last equations into a likelihood function for ***N*** events, which compares our prediction with the data. The best estimate of a parameter , will therefore minimize this likelihood function

**Helicity of the W boson**

**Matrix Element Method**

**Approximations in the probabilities definitions (things to do better with more statistics)**
** ****Only qqbar production**: it does not include 10% of ttbar events that are produced by gluon fusion
** ****Only W+jets background**: that is ~85% only of the background
** ****Leading-Order ttbar matrix element**: no extra jets, constrains our sample to have only 4 jets
- After these approximations, the likelihood function is
- where ***c******1*** and ***c******2*** are minimized at each point of  .

**Results – M****top**** and F****0**

**We present a method developed at D******** for extracting information from data through a direct calculation of a probability for each event. This probability, which is a function of any parameter of interest, is calculated by convoluting the differential cross section with the resolution and acceptance of the detector.  The method is used to re-measure the mass of the top quark and to extract the fraction of longitudinal W bosons in the lepton + jets ttbar sample previously collected by the D******** experiment during Run I of the Fermilab Tevatron. The new method yields a top mass of M****top****=180.1 ******** 3.6 (stat) ******** 4.0 (syst) GeV/c****2****, which corresponds to a significant reduction in the uncertainty on M****top****. Assuming Standard Model coupling in the tbW vertex, we extract for the first time at D******** the longitudinal fraction F****0****=0.56 ******** 0.31 (stat+M****top****) ******** 0.04 (syst)****.**

**Top Production and Decay **
- The top quark is detected indirectly via its decay products. It decays via the weak interaction, and according to the Standard Model is almost always expected to decay to a b quark and a W boson. This is followed by the W decaying into two quarks or a lepton and a neutrino.
- The final state of the ttbar system has different topological classifications that depend on the decay of the W. This analysis uses the “lepton+jets” channel, and corresponds to one W decaying leptonically (into a e or a ), while the other W decays hadronically. This channel has a branching fraction of about 30%.

***W*****+  **
- Right handed fraction
**F****+**** ~ 0%**

- where = MW2/Mtop2

**Transfer Functions W(x,y)**** **

**Signal Probability P****signal****(x;********)**

- Using LO approximation and parameterized showering, we calculated the event probabilities, and measured:
**                    ****M****top ****(preliminary) =180.1 ******** 3.6 (stat) ******** 4.0 (syst) GeV/c****2****  **
- Significant improvement to our previous analysis, and is equivalent to having 2.4 times more data
**     ****F****0**** (preliminary) = 0.56 ******** 0.31(stat+M****top****) ******** 0.04 (syst)**
- => First F0 measurement done at **D******
- We have a method that allows us to optimize information to extract Mtop and F0. The statistical power comes from:
-  Correct permutation is always considered (along with the other eleven)
-  All features of individual events are included, thereby well measured events contribute more information than poorly measured events
-  The probability depends on _**all measured quantities**_ (except for unclustered energy)
-  This method offers the possibility of increasing the statistics for F  using  **both W decay branches**
-  For higher statistics, one clearly needs to improve the calculation of the probabilities, but this method is a **better way** to do the analysis
- To consider for the future:
-  A very general method (other top quark properties, Higgs searches, etc.)

**Conclusions**

**p**

**tbar**

**t**

**b**

**W****+**

**bbar**

**W****-**

**l**

****

**ubar,sbar**

**d,c**

**Top Mass**

**W helicity**

**pbar**

- W-

- t

- bbar

- l

- 

- 

*W rest frame*

- and the angular factor of the matrix element  for top decay is contained in

- and similar term for the top decaying to quarks.

- Since we cannot distinguish which jet is associated with which quark, all possible combinations that can lead to the observed final state in the detector are included in the calculation. In addition, there are more than one solution to the neutrino kinematics, and all solutions that are consistent with energy and momentum conservation are taken into account. This effectively increases the ttbar cross section by a factor of 12 and requires the additional factor of 12 in the denominator.

**Acceptance Integrals**

**ttbar@175GeV/c****2**

**W+jets**

- Every criterion used for the selection of the events introduces a bias as a function of the parameter we are measuring, . The acceptance integrals correct for this bias. They are performed via Monte Carlo method of integration.

**Selection Criteria**
- A set of selections was introduced to improve acceptance for lepton+jets from ttbar relative to background [1]. The standard requirements are:
-  **Lepton:** ET>20 GeV,|e|<2,||<1.7
-  **Jets:** 4, ET>15 GeV, ||<2
-  **Missing E****T** > 20 GeV
-  **“****E****T****W ****’’**> 60 GeV ; **|********W ****|****<**2
- We also applied specific selections for this analysis:
-  Since we use a leading-order matrix element in our calculation of the probabilities, we restrict our sample to **only 4 jets**
-  To increase the purity of signal, we apply a selection in the background probability,  **P****background****<1.10****-11**

**P****ttbar ****as a function of M****top****   **

**1e****-11**

-    This new technique improves the statistical error on Mtop from 5.6 GeV/c2 [PRD 58, 52001, (1998)] to 3.6 GeV/c2
-     Decrease in the statistical error is equivalent to a factor of  2.4 in the number of events
-    0.5 GeV/c2 shift has been applied, based on Monte Carlo studies

**M****top**** = 180.1 ******** 3.6****stat**** ******** 4.0****syst**** GeV/c****2**** **

**assuming  F****0****=0.70**

**F****0**** = 0.60 ******** 0.30****stat **

-    First F0 measurement done at **D******
** **  **CDF **measurement of F0 = 0.91  0.37 (stat)  0.13 (syst) (*PRL 84,216*) using 108 leptons (70% signal) [PRL 84,216, (2000)]
-   Corrected for response

**Top Quark Mass**

**Helicity of the W Boson**

| Signal model | 1.5 GeV/c2 |
| --- | --- |
| Background model | 1.0 GeV/c2 |
| Noise and multiple interactions PRD 58 52001, (1998) | 1.3 GeV/c2
 |

| Jet Energy Scale | 3.3 GeV/c2 |
| --- | --- |
| Parton Distribution Function | 0.2 GeV/c2 |
| Acceptance Correction | 0.5 GeV/c2 |

- Determined from MC studies with large  event samples:

- Determined from data:

_**Systematic Uncertainties for top quark mass**_

| Signal model | 0.020 |
| --- | --- |
| Background compositeness | 0.010 |
| Noise and multiple interactions | 0.006 |

| Jet Energy Scale | 0.014 |
| --- | --- |
| Top-antitop spin correlation | 0.008 |
| Parton Distribution Functions | 0.007 |
| Acceptance Correction | 0.021 |

- Determined from MC studies using Run I statistics:

- Determined from data:

_**Systematic Uncertainties W boson helicity**_

- Because the extracted F0 is required to lie inside the physical region, its total uncertainty cannot be greater than 0.34, that is the 68.27 interval for the region between 0 and 1. Adding all the separate components in quadrature, may in fact, overestimate the total uncertainty, and perhaps bring it above the limit of 0.34 for the case of no information whatsoever on F0. A fully Bayesian approach, integrating over all uninteresting parameters, would provide the best way of estimating the uncertainty, but this is not always possible. Therefore, if it is possible we should calculate the uncertainties “internally” so this limit could be attained.
- We calculate the the systematic error due to the uncertainty on the top mass, integrating over it (from 165 GeV/c2 to 190 GeV/c2) and using no prior knowledge of the top quark mass. Therefore, the statistical error in F0  and the systematic effect of the uncertainty in Mtop are estimated simultaneously by projecting this likelihood onto the F0  axis.

_**References**_
- [1] CDF Collaboration, F. Abe *et al*., Phys. Rev. Lett. {74}, 2626 (1995)
- [2] DØ Collaboration, S. Abachi *et al*., Phys. Rev. Lett. {74}, 2632 (1995).
- [3] DØ Collaboration, B. Abbott, *et al*., Phys. Rev. D {58}, 052001 (1998).
- [4] G. Mahlon, S. Parke, Phys. Lett. B 411, 133 (1997), G. Mahlon, S. Parke, Phys. Rev. D 53,4886 (1996).
- [5] F. A. Berends, H. Kuijf, B. Tausk and W. T. Giele, Nucl. Phys. {B357}, 32 (1991).
- [6] R. H. Dalitz and G. R. Goldstein, Proc. R. Soc. Lond. A {\bf 445}, 2803 (1999),
- and referenced therein; K. Kondo *et al*., J. Phys. Soc. Jap. {\bf 62}, 1177 (1993).
- DØ Collaboration, B. Abbot t, et al., Phys. Rev D {\bf 60}, 052001 (1999).
- [7] “Maximal use of Kinematic Information for the Extraction of the Mass of the Top Quark in single-lepton ttbar events at DØ”, J. Estrada, Ph.D. dissertation, University of Rochester, 2001.
- [8] “Helicity of the W in Single-lepton ttbar Events”, F. Canelli, , Ph.D. dissertation, University of Rochester, 2003.

***Background probability for the data sample (circles). Only those events on the left of the vertical line, are kept for further analysis. The black histogram corresponds to a Monte Carlo simulation with the measured fractions of signal (red) and background (blue) events.***

***Signal  probability as a function of M******top****** for data event with high values of background probability (background like events).***

***Signal  probability as a function of M******top****** for data event with low values of background probability (signal like events).***

***Final likelihood for the data sample as a function of M******top******. The right plot shows the Gaussian fit to the likelihood from which we extract the most probable mass and its statistical uncertainty.***

***Final likelihood for the data sample as a function of F******0******. ***

***Difference between the energy of the reconstructed jet and the associated parton in Monte Carlo simulated ttbar events.***

***Three jet invariant mass for the decay products of the top quark. The  fully simulated and reconstructed Monte Carlo HERWIG events (histogram) are compared with a prediction using parton model and transfer functions (solid line). The dashed line corresponds to a transfer function with a variant parameterization.  (a) Only the right combination among jets is considered. (b) Three invariant mass using all possible combination among jets and restricting the sample to only 4 jets without requiring jet-parton matching.***

***Two dimensional likelihood as a function of the mass of M******top  ******and F******0******. The estimation of the systematic effect in F******0****** due to the uncertainty in top mass is obtained by integrating over the top mass.***

_**Acknowledgements**_
- We thank T. Ferbel and G. Gutierrez for their contributions to this analysis.** **

**Top Quark Production**
- In proton antiproton collisions at Tevatron energies, top quarks are primarily produced in pairs, either via qqbar or gluon fusion. At the Tevatron, the main contribution to the ttbar yield during Run I was from qqbar annihilation. This is purely the result of the fact that the parton distribution functions (PDFs) favor this channel at s=1.8 TeV. In fact, about 90% of the top quarks are produced through the quark interaction.

**Top Quark Mass**

- Although its value is not predicted, Mtop is a fundamental parameter in the Standard Model. The best value of the top quark mass found from combining all channels at the Tevatron is,
- The systematic error is dominated by the uncertainty on the jet energy scale. The measurement of MW will improve significantly in the future, with an uncertainty of 27 GeV/c2 being a realistic goal for Run II of the Tevatron. To be able to make maximum use of this precision measurement to constrain the mass of the Higgs, we need to measure the top mass with an uncertainty of less than 3 GeV/c2. This will yield a prediction for the Higgs mass with an uncertainty of 40%.

**Mapping between jet and parton energies - W****jet****(Eparton,Ejet)**
- The mapping between parton and jet energies is determined by the transfer function Wjet(Eparton,Ejet). This function models the smearing in jet energies from effects of *radiation, hadronization, measurement resolution, and jet reconstruction algorithm,* taking into account the shape of the E =Eparton-Ejet to avoid underestimation the jet energy. The function is obtained from Monte Carlo event sample, and parameterized using 2 Gaussians, one to account for the peak and the other to fit the asymmetric tails,
- where

- We compare the three invariant masses calculated directly using Monte Carlo HERWIG jets after **full D****Ø**** reconstruction**, using the  standard criteria, with predictions based on the transfer functions applied to the parton level.

- (a)

- (b)

***Angular momentum conservation in the decay of the top quark. ***

- The top quark was first observed in Run I of the Fermilab TeVatron, in ~100 events/pb of integrated luminosity collected at the CDF and D experiments [1,2].  Yet, because of its relatively large mass, and only recent discovery, the properties of the top quark are not well known. Because the top quark is so much more massive than the other fermions, it has been speculated that it may play a unique role within the Standard Model. It is therefore very important to understand the properties of the top quark, their degree of consistency with the Standard Model, and to check whether or not the top quark is truly exceptional.

**Abstract**

**This method calculates the probability of each event of being signal as a function of the parameter we are measuring (F****0**** and M****top****) ****[6,7,8]. The probability for e     ach event being background is also calculated. The results are combined in one likelihood for the sample. These probabilities are functions of all momenta for the measured lepton and jets. In the previous analyses [3], the data was compared with two-dimensional templates and ****the features of individual events were averaged over the variables not considered in the template.**

**P****background****<1.10****-11**

***Selection applied in order to increase the purity of the signal. Only those events on the left of the dashed line, are kept for further analysis.***

***Acceptance curves for ttbar, W+jets and multijet events as a function of the top mass, M******top******, after ****P**background**<1.10**-11** was applied.*

***Acceptance curves for ttbar events as a function of F******0******, after all the selections****** were applied.***

- t

- t

*W*

*b*

**D******** ****Run I Data**
- DØ statistics from Run I corresponds to 125 pb-1. After applying the standard selection criteria, **91** ttbar candidates remained. Only **71** of these remaining events have 4 jets. And from this sample only **22** have a background probability larger than 1.10-11. We proceed to extract Mtop and F0 from these 22 events.
- The minimization of the –ln* L* as a function of only c1 and c2 results in
**n****ttbar****= 12 ******** 3**
- and a purity = nttbar/(nttbar+nW+jets)=0.54

**P****ttbar ****as a function of F****0**** and M****top**** ****   **

**assuming  M****top****=175 GeV/c****2**

***lbbar **is the angle between the lepton and the spin direction of the top quark in the rest frame of the W boson.**** ***

**F****0**** = 0.56 ******** 0.31****stat+Mtop**** ******** 0.04****syst**

**integrating over  M****top**

**Optimal Use of Information to Measure Top Properties    - ****F. Canelli, J. Estrada **