Search for scalar leptoquarks and T-odd quarks in the acoplanar jet topology using 2.5 fb-1 of ppbar collision data at sqrt(s)=1.96 TeV

A search for new physics in the acoplanar jet topology has been performed in 2.5 fb-1 of data from ppbar collisions at sqrt(s)=1.96 TeV, recorded by the D0 detector at the Fermilab Tevatron Collider. The numbers of events with exactly two acoplanar jets and missing transverse energy are in good agreement with the standard model expectations. The result of this search has been used to set a lower mass limit of 205 GeV at the 95% C.L. on the mass of a scalar leptoquark when this particle decays exclusively into a quark and a neutrino. In the framework of the Little Higgs model with T-parity, limits have also been obtained on the T-odd quark mass as a function of the T-odd photon mass.

of events with exactly two acoplanar jets and missing transverse energy are in good agreement with the Standard Model expectations. The result of this search has been used to set a lower mass limit of 205 GeV at the 95% C.L. on the mass of a scalar leptoquark when this particle decays exclusively into a quark and a neutrino. In the framework of the Little Higgs model with T-parity, limits have also been obtained on the T-odd quark mass as a function of the T-odd photon mass.
At hadron colliders, new colored particles predicted by various extensions of the Standard Model (SM) would be abundantly produced if they are light enough. The final state with jets and missing transverse energy (/ E T ) resulting from the decay of those particles is a promising channel to discover physics beyond the SM.
In this Letter, a search for new particles in the topology consisting of exactly two jets and / E T is presented using 2.5 fb −1 of data collected at a center-of-mass energy of 1.96 TeV with the DØ detector during Run II of the Fermilab Tevatron pp Collider. The result of this search has been used to constrain two categories of models.
The first category corresponds to models predicting the existence of leptoquarks (LQ) [1]. Those are scalar or vector particles carrying both a lepton and a baryon quantum number. They are predicted by many extensions of the SM attempting to explain the apparent symmetry between quarks and leptons. To satisfy experimental constraints on flavor changing neutral current interactions, leptoquarks couple only within a single generation. Leptoquarks decay into a charged lepton and a quark with a branching ratio β, or into a neutrino and a quark with a branching ratio 1 − β. Pair production of leptoquarks assuming β = 0 therefore leads only to a final state consisting of two neutrinos and two quarks. The most stringent previous limit at 95% C.L. on the scalar leptoquark mass PLB:25267 Please cite this article in press as: DØ Collaboration, Search for scalar leptoquarks and T-odd quarks in the acoplanar jet topology using 2.5 fb −1 of pp collision data at Those limits, as well as the results presented in this Letter, apply for first-and second-generation scalar leptoquarks. For the thirdgeneration, tighter limits were obtained by increasing the signal sensitivity using heavy-flavor quark tagging [4]. The second category is the Little Higgs (LH) model [5], which provides an interesting scenario for physics at the TeV scale, predicting the existence of additional gauge bosons, fermions, and scalar particles with masses in the 100 GeV-5 TeV range. Electroweak precision constraints are satisfied by introducing a discrete symmetry called T-parity [6]. This symmetry is constructed such that all the SM states are even, while most new states of the LH model with T-parity (LHT) are odd. In the LHT model, six new Dirac T-odd quarks (T-quarks orQ ) are the partners of the left handed T-even quarks of the SM. In most of the parameter space, the lightest T-odd particle (LTP) is the so-called "heavy photon" (Ã H ) which is stable and weakly interacting. From SM precision measurements, it is possible to set a lower mass limit of ∼80 GeV on the mass ofÃ H [7]. The new particle spectrum of the LHT model has similar properties to spectra of supersymmetric models. The LTP, just as the Lightest Supersymmetric Particle in SUSY models with R-parity conservation, is a dark matter candidate which escapes undetected. There are, however, important differences: the new T-odd particles have the same spin as their SM partner; and in the LHT model, some SM states, for example right-handed SM fermions or gluons, have no partners. In the following, the mass of the T-quarks from the first two generations is assumed to be degenerate, and pair production of those four T-quarks is considered. As the T-odd gauge bosons other than theÃ H are relatively heavy, T-quarks decay into a quark andÃ H in most of the parameter space accessible at the Tevatron. It will be assumed in the following that this branching ratio is 100%. Pair production of Tquarks therefore leads to a final state with two quarks and two LTP, giving the missing transverse energy signature. The only direct constraint from collider data on the T-quark mass is the ∼100 GeV lower limit on the mass of the supersymmetric partner of the first two generations quarks from LEP [8] which can also be applied to T-quarks. Prospective studies [9] have shown that the Tevatron can be sensitive to T-quark masses up to ∼400 GeV. This sensitivity is severely reduced when the mass difference between the T-quarks and the LTP becomes small.
The DØ detector has been described in detail in Ref. [10]. Tracks are reconstructed in a silicon microstrip tracker and a central fiber tracker (CFT), both located within a 2 T superconducting solenoidal magnet. The liquid argon and uranium calorimeter consists of three cryostats. The central one covers pseudorapidities 8 |η| 1.1, and the two end sections extend the coverage up to |η| ≈ 4.2. The calorimeter is designed in projective towers of size 0.1 × 0.1 in the (η, φ) plane, where φ is the azimuthal angle in radians. The outer muon system, covering |η| < 2, consists of tracking detectors and scintillation trigger counters in front of 1.8 T iron toroids, followed by two similar layers after the toroids.
Jets were reconstructed with the iterative midpoint cone algorithm [11] with cone radius R = ( φ) 2 + ( y) 2 = 0.5 in azimuthal angle φ and rapidity y = 1 energy scale (JES) corrections were derived from the transverse momentum balance in photon-plus-jet events. The / E T was calculated from all calorimeter cells, and corrected for the jet energy scale and for the transverse momenta of reconstructed muons. 8 The pseudorapidity η is defined as − ln[tan(θ/2)], with θ being the polar angle with respect to the proton beam direction.
In events from SM processes, the presence of neutrinos from W or Z decay in the final state generates large / E T . The main irreducible SM background in this search for new particles is therefore the Z (→ νν) + jets process. The W (→ lν) + jets events also exhibit the / E T signature, but their contribution can be significantly reduced by rejecting events with an isolated electron or muon. However, the charged lepton can escape detection in uninstrumented regions of the detector, fail identification criteria, or be a tau lepton decaying hadronically. To further suppress that background, events containing an isolated high p T track are rejected. The other SM backgrounds for this search are the pair production of vector bosons (W W , W Z, Z Z ) and the production of top quarks, either in pairs (tt) or via the electroweak interaction. Finally, multijet production when one or more jets are mismeasured also leads to a final state with jets and / E T ("QCD background").
Events from SM processes and signal events were simulated using Monte Carlo (MC) generators and passed through a full geant3-based [12] simulation of the detector geometry and response. They were subsequently processed with the same reconstruction chain as the data. The parton distribution functions (PDFs) used in the MC generators are the CTEQ6L1 [13] PDFs. A data event from a randomly selected beam crossing was overlaid on each event to simulate the additional minimum bias interactions and detector noise. The alpgen generator [14] was used to simulate W /Z + jets and tt production. It was interfaced with pythia [15] for the simulation of initial and final state radiation (ISR/FSR) and of jet hadronization. Pairs of vector bosons and electroweak top quark production were simulated with pythia and comphep [16], respectively. The next-to-leading order (NLO) cross sections were computed with mcfm5.1 [17]. The QCD background was not simulated, since it can be conservatively neglected in the final stage of this analysis.
Leptoquark pair production and decays were simulated with pythia and the CTEQ6L1 PDFs. The LQ mass in the MC simulation ranged from 60 to 240 GeV. The NLO cross sections of this process were computed from a program based on [18] with a renormalization and factorization scale (μ r,f ) equal to the LQ mass, and using the CTEQ6.1M PDF sets.
For the LHT model, it has been shown in [9] that T-quark pair production and decay to qÃ H is very similar to squark pair production and decay to qχ 0 1 , whereχ 0 1 is the lightest neutralino. Signal efficiencies were therefore determined using MC events generated with pythia corresponding to the production and decay of these supersymmetric particles. It has been checked that the spin differences between the T-odd particles of the LHT model and the supersymmetric particles do not modify the signal efficiencies. Therefore, MC simulations of such events were performed to cover thẽ Q -Ã H mass plane accessible at the Tevatron. Concerning the signal normalization, the cross section of first and second generation T-quark pair production is equal to four times the cross section of heavy quark pair production, if no other new particles predicted by the LHT model are involved in the T-quark production. The NLO cross sections of this signal were therefore calculated using mcfm5.1, with μ r,f equal to the T-quark mass, and the CTEQ6.1M PDF sets.
The analysis strategy follows closely the "dijet" analysis from Ref. [19]. Events were recorded using triggers requiring two acoplanar jets and large / E T or / H T , where / H T is the vector sum of the jet transverse momenta (/ H T = | jets p T |). The trigger requirements evolved during the Run II data taking period in order to take into account the increasing peak instantaneous luminosity of the Tevatron. At the last stage of the trigger selection, the requirements were typically the following: (1) / E T or / H T greater than 30 GeV and their separation from all jets greater than 25 • ; (2) an azimuthal angle between the two highest p T jets less than 170 • . Offline, events where / E T was higher than 40 GeV were then selected.  The best primary vertex (PV0) was defined as the vertex with the smallest probability to be due to a minimum bias interaction [20]. The longitudinal position of PV0 was required to be less than 60 cm from the detector center to ensure efficient vertex reconstruction. Good jets were defined as jets with a fraction of energy in the electromagnetic layers of the calorimeter lower than 0.95. The acoplanarity, i.e. the azimuthal angle between the two leading jets, jet 1 and jet 2 , ordered by decreasing transverse momentum, was required to be less than 165 degrees. Then, the two leading jets were required to be in the central region of the detector, with |η det | < 0.8, where η det is the jet pseudorapidity calculated under the assumption that the jet originates from the detector center. After this preselection, the transverse momenta of the two leading jets had to be higher than 35 GeV. Finally, jets were required to originate from the best primary vertex, based on their associated tracks [19]. This was accomplished by requiring CPF0 > 0.75, where CPF0 is the fraction of track p T sum associated with the jet which comes from PV0, CPF0= p track At this stage, the QCD multijet background is still largely dominant. To further reject those events, the selection criteria on / E T was increased to 75 GeV. The requirement that the azimuthal angle between the / E T and the first jet, φ(/ E T , jet 1 ), exceeds 90 degrees, was used to remove events where a jet was mismeasured and generating / E T aligned to that jet. Also, the minimal azimuthal angle φ min (/ E T , any jet) and the maximal azimuthal angle φ max (/ E T , any jet) between jets and / E T directions had to be greater than 50 degrees and lower than 170 degrees, respectively.
To suppress W (→ lν) + jets events, a veto on events containing an isolated electron or muon with p T > 10 GeV was applied.
Events with an isolated track were then rejected to further reduce that background. Isolated tracks were required to have p T > 5 GeV, to originate from PV0 with DCA(z) < 5 cm and DCA(r) < 2 cm, where DCA(z) and DCA(r) are the positions of the projection of the distance of closest approach between the track and PV0 on the beam direction and in the plane transverse to the beamline, respectively. The number of hits in the CFT used to reconstruct the track was required to be at least 8. Finally, good quality tracks were selected by requiring the χ 2 /dof of the track-fit reconstruction to be lower than 4. A hollow cone with inner and outer radii of 0.06 and 0.5 was constructed around each track that passed those criteria. If no other track with p T > 0.5 GeV and the same quality criteria as above was found in this hollow cone, the track was considered isolated. The use of a hollow, rather than full cone also allowed rejection of tau leptons decaying into three charged particles.
Events with exactly two jets with p T > 15 GeV and |η det | < 2.5 in the final state were then selected. This criterion rejects a large fraction of the remaining tt events, and increases the signal sensitivity at large T-quark and leptoquark masses once large / E T and H T are required, with H T = jets p T , where the sum is also over all jets with p T > 15 GeV and |η det | < 2.5. Table 1 Table 2. In all cases, the contribution of the QCD multijet background was estimated to be small enough to be conservatively neglected. The number of events observed are in good agreement with the SM expectations. The uncertainty coming from the JES corrections on the SM backgrounds and signal predictions ranges from 5% for low H T and / E T cuts to 10% for high H T and / E T cuts. The uncertainties due to the jet energy resolution, to the jet track confirmation, and to jet reconstruction and identification efficiencies range between 2% and 4%. The systematic uncertainty due to the isolated track veto was measured to be 3%. All these uncertainties account for differences between data and MC simulation, both for signal efficiencies and background contributions. The trigger was found to be fully efficient for the event samples surviving all analysis requirements within an uncertainty of 2%. The uncertainty on the luminosity measurement is 6.1% [22]. All of these uncertainties are fully correlated between signal and SM backgrounds. A 15% systematic uncertainty was set on the W /Z + jets and tt NLO cross sections. The uncertainty on the signal acceptance due to the PDF choice was determined to be 6%, using the forty-eigenvector basis of the CTEQ6.1M PDF set [13]. Finally, the effects of ISR/FSR on the signal efficiencies were studied by varying the pythia parameters controlling the QCD scales and the maximal allowed virtualities used in the simulation of the space-like and time-like parton showers. The uncertainty on the signal efficiencies was determined to be 6%.
The nominal NLO signal cross sections, σ nom , were computed with the CTEQ6.1M PDF and for the renormalization and factorization scale μ r,f = Q , where Q was taken to be equal to the leptoquark or T-quark mass. The uncertainty due to the choice of PDF was determined using the full set of CTEQ6.1M eigenvectors, with the individual uncertainties added in quadrature. The effect of the renormalization and factorization scale was studied by calculating the signal cross sections for μ r,f = Q , μ r,f = Q /2 and μ r,f = 2 × Q . The PDF and μ r,f effects were added in quadrature to compute minimum, σ min , and maximum, σ max , signal cross sections.
For the leptoquark search, Fig. 2 shows the 95% C.L. observed and expected upper limits on scalar leptoquark production cross sections. The intersection with the minimal NLO cross section gives a lower mass limit of 205 GeV for β = 0. The corresponding ex-    Fig. 3 shows the 95% C.L. excluded regions inQ -Ã H mass plane assuming that the branching fraction of the decayQ → qÃ H is 100%. The largest excluded T-quarks mass, 404 GeV, is obtained for large mass difference between the T-quarks and the LTP.
In summary, a search for scalar leptoquarks and for T-quarks produced in pp collisions at √ s = 1.96 TeV has been performed with a 2.5 fb −1 data sample. This search was conducted in events containing exclusively two jets and large missing transverse energy. The results are in good agreement with the SM background expectations, and 95% C.L. limits have been set on the leptoquark and T-quark masses. For a single-generation scalar leptoquark, a lower mass limit of 205 GeV has been obtained for β = 0, improving the previous limit by 69 GeV. In the LHT model, limits on T-quark mass were obtained as a function of theÃ H mass assuming 100% branching ratio for the decayQ → qÃ H . T-quark masses up to 404 GeV are excluded when the mass difference between T-quarks and the LTP is large. Those are the most stringent direct limits to date on the T-quarks mass.