Search for Third Generation Vector Leptoquarks in Run II


	Search for Third Generation Vector Leptoquarks in Run II

We search for pair produced 3rd Generation Vector Leptoquarks (VLQ3), each decaying to b tau. The signature is a di-tau plus di-jet final state where one tau decays leptonicly and the other hadronicly. The model assumes that the branching ratio of VLQ3 to b tau is 100%. This analysis uses 322 pb^-1 of data collected by lepton+track triggers and requires the presence of at least one central electron candidate of E_T>10 GeV, or one central muon candidate of p_T>10 GeV/c, and at least one hadronic tau candidate of p_T>15 GeV/c. Acceptance, identification, and isolation requirements are imposed on the individual lepton and hadronic tau candidates.

At the event level, we require opposite sign lepton and tau candidates, and remove events consistent with photon-conversions, Drell-Yan (di-electron or di-muon), and cosmics. A minimum MET > 10 GeV is required. The scalar sum of the transverse momenta of the lepton, hadronic tau, 2 highest E_T jet candidates, and MET is formed, and a cut: H_T=p_T(l)+p_T(tau)+p_T(jets)+MET > 400 GeV/c is imposed. The backgrounds are estimated using a combination of data and Monte Carlo simulation, and control regions (defined below) are used to validate these estimates.

In the signal region we expect 0.25 ^+0.21_-0.06 (stat) +/- 0.05 (syst) events in the tau_electron tau_hadronic channel and 0.24 ^+0.22_-0.05 (stat) +/- 0.05 (syst) events in the tau_muon tau_hadronic channel. No events are observed in the signal region. With a 95% C.L. we set an upper limit on the VLQ3 pair production cross section of 344 fb, and set a lower limit on the VLQ3 mass of 317 GeV/c², assuming Yang-Mills couplings and Br(VLQ3 --> tau b) = 100%. If the uncertainties on the theoretical cross section are applied, the upper limit on the cross section is 354 fb and the lower limit on the mass is 303 GeV/c². For a theory with so-called Minimal couplings, the upper limit on the cross section is 493 fb and the lower limit on the mass is 251 GeV/c² for the nominal choice of parton distribution functions and Q² scale, while the corresponding limits are 555 fb and 235 GeV/c² if the theoretical uncertainties on the cross section are applied.

Contacts: Takashi Akimoto, Max Chertok, Stanley Forrester, Teruki Kamon, Yukihiro Kato, Dick Lander, Sung-Won Lee, Alexei Safonov, J.R. Smith, Aron Soha, and Soushi Tsuno
Data: Run II, 322 pb^-1
Documentation: Public note

Supporting Material:

Definitions of Signal and Control Regions
GRACE/GR@PPA VLQ3 Pair Production vs Mass
	For the first time, a matrix element generator (implemented in GRACE/GR@PPA) has been used to model the pair production of vector leptoquarks, incorporating the amplitude summation and propagating helicity information through to the resulting taus. This plot shows the total pair production cross section, for the case of Yang-Mills couplings, as a function of VLQ3 mass, and the individual contributions from quark/anti-quark annihilation (dominant) and gluon-gluon fusion. Plot is available in EPS or GIF format.
	This schematic shows the definitions of control and signal regions. Control region CR0J requires 0 jets and H_T > 80 GeV, control region CR1J requires 1 jet and H_T > 80 GeV, and control region CR2J requires 2 jets and 80 < H_T < 250 GeV. The signal region (SR) requires 2 jets and H_T > 400 GeV. The search has been optimized for this region. In addition, we define a safety (SAFE) region with 2 jets and 250 < H_T < 400 GeV. The safety region provides a buffer between CR2J and the signal region (a VLQ3 with a mass below the existing limit of 225 GeV/c² could not appear in CR2J). Both the signal region and safety region are used in the final fit. Schematic is available in EPS or JPG format.

Plots and Tables:

Total Efficiency vs Mass
	This plot shows the total efficiency (including acceptance, particle identification, particle isolation and separation, trigger, and event selection) as a function of VLQ3 mass for the electron (tau_e tau_h) channel and the muon (tau_mu tau_h) channel, for the two combinations of couplings called Yang-Mills and Minimal. Plot is available in EPS or GIF format.
Efficiency at VLQ3 Mass = 320 GeV/c²

The above table is for the case of Yang-Mills couplings. The table is also available in JPG format.
Ht for Signal and Backgrounds
	The left plot shows the signal and background distributions of H_T, where H_T = p_T(lepton)+p_T(tau_h)+p_T(jets)+MET, and illustrates the ability of H_T to discriminate between signal and the backgrounds. Plot is available in EPS or JPG format (for the EPS file, be sure that antialiasing is OFF in your viewer). The right plot shows the optimization ( S/Sqrt(S+B) ) that was used to determine the requirement of H_T>400 GeV for the signal region. Plot is available in EPS or JPG format.
Plots of Ht With Individual Background Contributions
	Ht plot spanning the 2-jet control region (CR2J), the safety region (SAFE), and the signal region (SR), for the tau_e tau_h channel. Plot is available in EPS or JPG format. Ht plot spanning the 2-jet control region (CR2J), the safety region (SAFE), and the signal region (SR), for the tau_mu tau_h channel. Plot is available in EPS or JPG format.

Control (and Signal) Region Plots of Jet Multiplicity
z	Plot of number of jets, including the CR0J, CR1J, and CR2J control regions, as well as the safety and signal regions, for the tau_e tau_h channel. The hatched area is the uncertainty on the summation of the backgrounds. Plot is available in EPS or JPG format (for the EPS file, be sure that antialiasing is OFF in your viewer). Plot of number of jets, including the CR0J, CR1J, and CR2J control regions, as well as the safety and signal regions, for the tau_mu tau_h channel. The hatched area is the uncertainty on the summation of the backgrounds. Plot is available in EPS or JPG format (for the EPS file, be sure that antialiasing is OFF in your viewer).

Control Region Number of Tau Prongs
	Plot of number of tracks for the hadronic tau in control region CR0J, for the tau_e tau_h channel. Plot is available in EPS or JPG format. Plot of number of tracks for the hadronic tau in control region CR0J, for the tau_mu tau_h channel. Plot is available in EPS or JPG format.

Systematic Uncertainties on Signal
	This table shows the systematic uncertainties that are applied to the signal acceptance, for three example VLQ3 masses, for the tau_e tau_h channel. Table is available in JPG format. This table shows the systematic uncertainties that are applied to the signal acceptance, for three example VLQ3 masses, for the tau_mu tau_h channel. Table is available in JPG format.

Event Yield Expectations
Expected number of background events, by category, in each region, CR0J, CR1J, CR2J, safety, and signal, as previously defined, for tau_e tau_h. Table is available in JPG format.
Expected number of background events, by category, in each region, CR0J, CR1J, CR2J, safety, and signal, as previously defined, for tau_mu tau_h. Table is available in JPG format.
Cross Section and Mass Limits
	This plot shows the pair production cross section as a function of VLQ3 mass, and includes the theory predictions, with bands for uncertainties due to the PDF and Q² choices, as well as the experimental limits based on 322 pb^-1, including both the tau_e tau_h and tau_mu tau_h channels. At 95% C.L., the cross section is less than 344 fb and the mass is greater than 317 GeV/c² for the case with Yang-Mills couplings. If the uncertainties on the theoretical cross section are applied, the upper limit on the cross section is 354 fb and the lower limit on the mass is 303 GeV/c². For a theory with Minimal couplings, the upper limit on the cross section is 493 fb and the lower limit on the mass is 251 GeV/c² for the nominal choice of parton distribution functions and Q² scale, while the corresponding limits are 555 fb and 235 GeV/c² if the theoretical uncertainties on the cross section are applied. The CDF Run I mass limits are also indicated on the plot. Plot is available in EPS or GIF format.

Page was last updated March 9th, 2007 by Aron Soha, Stan Forrester, and Takashi Akimoto.
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