Collider Detector At Fermilab Research Articles

Global analysis of experimental constraints on a possible Higgs-like particle with mass ∼ 125 GeV

Abstract We perform a global analysis of the constraints on a possible Higgs-like particle with mass ∼ 125 GeV that are provided by the ATLAS, CDF, CMS and D0 experiments. We combine the available constraints on possible deviations from the Standard Model Higgs couplings to massive vector bosons and to fermions, considering also the possibilities of non-standard loop-induced couplings to photon and gluon pairs. We analyze the combined constraints on pseudo-dilaton scenarios and on some other scenarios in which the possible new particle is identified as a pseudo-Nambu-Goldstone boson in a composite electroweak symmetry-breaking sector.

Electroweak Beautygenesis: Fromb→sCPViolation to the Cosmic Baryon Asymmetry

We address the possibility that CP violation in B(s)-B(s) mixing may help explain the origin of the cosmic baryon asymmetry. We propose a new baryogenesis mechanism--"electroweak beautygenesis"--explicitly showing that these two CP-violating phenomena can be sourced by a common CP phase. As an illustration, we work in the two-Higgs-doublet model. Because the relevant CP phase is flavor off diagonal, this mechanism is less severely constrained by null results of electric dipole moment searches than other scenarios. We show how measurements of flavor observables by the D0, CDF, and LHCb collaborations test this scenario.

Implications of a largeBs→μ+μ−branching fraction for the minimal supersymmetric standard model

Recently, the CDF Collaboration reported the first nonzero measurement of the ${B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ branching fraction. The LHCb, CMS and ATLAS, collaborations have reported upper limits that are in tension with the CDF result. We consider the implications of these measurements for the specific case of the minimal supersymmetric standard model. We also discuss the implications of these measurements for neutralino dark matter and the supersymmetric contribution to the anomalous magnetic moment of the muon.

RevisitingBs→μ+μ−andB→K(*)μ+μ−decays in the MSSM with and withoutRparity

The rare decays ${B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ and $B\ensuremath{\rightarrow}{K}^{(*)}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ are sensitive to new particles and couplings via their interferences with the standard model contributions. Recently, the upper bound on $\mathcal{B}({B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})$ has been improved significantly by the CMS, LHCb, CDF, and D0 experiments. Combining with the measurements of $\mathcal{B}(B\ensuremath{\rightarrow}{K}^{(*)}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})$, we derive constraints on the relevant parameters of the minimal supersymmetic standard model with and without $R$ parity, and examine their contributions to the dimuon forward-backward asymmetry in $B\ensuremath{\rightarrow}{K}^{*}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ decay. We find that (i) the contribution of $R$-parity violating coupling products ${\ensuremath{\lambda}}_{2i2}^{\ensuremath{'}}{\ensuremath{\lambda}}_{2i3}^{\ensuremath{'}*}$ due to squark exchange is comparable with the theoretical uncertainties in $B\ensuremath{\rightarrow}K{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ decay, but still could be significant in $B\ensuremath{\rightarrow}{K}^{*}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ decay and could account for the forward-backward asymmetry in all dimuon invariant mass regions; (ii) the constrained mass insertion $({\ensuremath{\delta}}_{LL}^{u}{)}_{23}$ could have a significant contribution to $d{\mathcal{A}}_{\mathrm{FB}}(B\ensuremath{\rightarrow}{K}^{*}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})/ds$, and such effects are favored by the recent results of the Belle, CDF, and LHCb experiments.

Search for anomalous production of multiple leptons in association withWandZbosons at CDF

This paper presents a search for anomalous production of multiple low-energy leptons in association with a W or Z boson using events collected at the CDF experiment corresponding to 5.1 fb$^{-1}$ of integrated luminosity. This search is sensitive to a wide range of topologies with low-momentum leptons, including those with the leptons near one another. The observed rates of production of additional electrons and muons are compared with the standard model predictions. No indications of phenomena beyond the standard model are found. A 95% confidence level limit is presented on the production cross section for a benchmark model of supersymmetric hidden-valley Higgs production. Particle identification efficiencies are also provided to enable the calculation of limits on additional models.

Prompt photon and associated heavy quark production at hadron colliders with k T -factorization

In the framework of the kt-factorization approach, the production of prompt photons in association with a heavy (charm or beauty) quarks at high energies is studied. The consideration is based on the O(\alpha \alpha_s^2) off-shell amplitudes of gluon-gluon fusion and quark-(anti)quark interaction subprocesses. The unintegrated parton densities in a proton are determined using the Kimber-Martin-Ryskin prescription. The analysis covers the total and differential cross sections and extends to specific angular correlations between the produced prompt photons and muons originating from the semileptonic decays of associated heavy quarks. Theoretical uncertainties of our evaluations are studied and comparison with the results of standard NLO pQCD calculations is performed. Our numerical predictions are compared with the recent experimental data taken by the D0 and CDF collaborations at the Tevatron. Finally, we extend our results to LHC energies.

Constraint on compactification scale via recently observed baryonic Λ b → Λℓ + ℓ − channel and analysis of the Σ b → Σℓ + ℓ − transition in SM and UED scenario

We obtain a lower limit on the compactification scale of extra dimension via comparison of the branching ratio in the baryonic $\Lambda_b\rightarrow \Lambda \mu^+ \mu^-$ decay channel recently measured by CDF collaboration and our previous theoretical study. We also use the newly available form factors calculated via light cone QCD sum rules in full theory to analyze the flavour changing neutral current process of the $\Sigma_b \rightarrow \Sigma \ell^+ \ell^-$ in universal extra dimension scenario in the presence of a single extra compact dimension. We calculate various physical quantities like branching ratio, forward-backward asymmetry, baryon polarizations and double lepton polarization asymmetries defining the decay channel under consideration. We also compare the obtained predictions with those of the standard model.

Combination of CDF and D0 measurements of theWboson helicity in top quark decays

We report the combination of recent measurements of the helicity of the W boson from top quark decay by the CDF and D0 collaborations, based on data samples corresponding to integrated luminosities of 2.7 - 5.4 fb^-1 of ppbar collisions collected during Run II of the Fermilab Tevatron Collider. Combining measurements that simultaneously determine the fractions of W bosons with longitudinal (f0) and right-handed (f+) helicities, we find f0 = 0.722 \pm 0.081 [\pm 0.062 (stat.) \pm 0.052 (syst.)] and f+ = -0.033 \pm 0.046 [\pm 0.034 (stat.) \pm 0.031 (syst.)]. Combining measurements where one of the helicity fractions is fixed to the value expected in the standard model, we find f0 = 0.682 \pm 0.057 [\pm 0.035 (stat.) \pm 0.046 (syst.)] and f+ = -0.015\pm0.035 [\pm 0.018 (stat.) \pm 0.030 (syst.)]. The results are consistent with standard model expectations.

Weak-triplet, color-octet scalars, and the CDF dijet excess

We extend the standard model to include a weak-triplet and color-octet scalar. This `octo-triplet' field consists of three particles, two charged and one neutral, whose masses and renormalizable interactions depend only on two new parameters. The charged octo-triplet decay into a W boson and a gluon is suppressed by a loop factor and an accidental cancellation. Thus, the main decays of the charged octo-triplet may occur through higher-dimensional operators, mediated by a heavy vectorlike fermion, into quark pairs. For an octo-triplet mass below the t\bar{b} threshold, the decay into Wb\bar{b} or Wb\bar{s} through an off-shell top quark has a width comparable to that into c\bar{s} or c\bar{b}. Pair production with one octo-triplet decaying into two jets and the other decaying into a W and two soft b jets may explain the dijet-plus-W excess reported by the CDF Collaboration. Using a few kinematic distributions, we compare two mechanisms of octo-triplet pair production: through an s-channel coloron and through the coupling to gluons. The higher-dimensional operators that allow dijet decays also lead to CP violation in B_s - \bar B_s mixing.

CDF silicon tracking detectors, 1988–2011

On September 30, 2011, the Collider Detector at Fermilab (CDF) finished physics data-taking at the Tevatron proton–antiproton collider. The original CDF silicon tracking detector, proposed in 1981 (SVX) and later replaced and updated (SVX′), was again replaced for Run-2 in 2002–2011 (SVX-II, ISL, L00). These systems operated successfully for many years, performing essential roles in exploring physics at the energy frontier, most notably the discovery of the top quark.

Z′-gauge bosons as harbingers of low-mass strings

Massive Z'-gauge bosons act as excellent harbingers for string compactifications with a low string scale. In D-brane models they are associated to U(1) gauge symmetries that are either anomalous in four dimensions or exhibit a hidden higher dimensional anomaly. We discuss the possible signals of massive Z'-gauge bosons at hadron collider machines (Tevatron, LHC) in a minimal D-brane model consisting out of four stacks of D-branes. In this construction, there are two massive gauge bosons, which can be naturally associated with baryon number B and B-L (L being lepton number). Here baryon number is always anomalous in four dimensions, whereas the presence of a four-dimensional B-L anomaly depends on the U(1)-charges of the right handed neutrinos. In case B-L is anomaly free, a mass hierarchy between the two associated Z'-gauge bosons can be explained. In our phenomenological discussion about the possible discovery of massive Z'-gauge bosons, we take as a benchmark scenario the dijet plus W signal, recently observed by the CDF Collaboration at Tevatron. It reveals an excess in the dijet mass range 150 GeV/c^2, 4.1\sigma beyond SM expectations. We show that in the context of low-mass string theory this excess can be associated with the production and decay of a leptophobic Z', a singlet partner of SU(3) gluons coupled primarily to baryon number. Even if the CDF signal disappears, as indicated by the more recent D0 results, our analysis can still serve as the basis for future experimental search for massive Z'-gauge bosons in low string scale models. We provide the relevant cross sections for the production of Z'-gauge bosons in the TeV region, leading to predictions that are within reach of the present or the next LHC run.

Improved squark and gluino mass limits from searches for supersymmetry at hadron colliders

Squarks and gluinos have been searched for at hadron colliders in events with multiple jets and missing transverse energy. No excess has been observed to date, and from a comparison of experimental cross section limits and theoretical cross section predictions one can deduce lower bounds on the squark and gluino masses. We present an improved analysis of squark and gluino mass bounds which is based on state-of-the-art cross section calculations including the summation of large threshold corrections. For illustration, we consider experimental data obtained by the CDF collaboration at the Fermilab Tevatron and discuss the impact of the improved cross section predictions on the squark and gluino mass limits.

Formula omitted] production via photon fragmentation at hadron colliders

The transverse momentum (pt) distributions of production and polarization for J/ψ measured by CDF Collaboration are still challenging our understanding of the heavy quarkonium production mechanism even with recent significant theoretical progresses on the next-to-leading-order (NLO) QCD calculation. We investigate an interesting mechanism for J/ψ production at hadron collider, pp(p¯)→γ⁎(J/ψ)+X with J/ψ from a virtual photon γ⁎ fragmentation. Our calculations show that the J/ψpt distribution in the photon fragmentation process up to NLO in αs will be larger than that of the conventional J/ψ production from the color-singlet mechanism at NLO when pt>26 (35) GeV at the Tevatron (LHC) and reach about 6 (10) times of the conventional one when pt=50 (100) GeV at the Tevatron (LHC), in spite of a suppression factor (α/αs)2 that is associated with the QED and QCD coupling constants. In addition, it also has large impact on the pt distribution of J/ψ polarization in large pt region. Therefore, it is an important mechanism for J/ψ production at large pt region especially for the LHC.

Search for standard model Higgs boson production in association with aW boson using a matrix element technique at CDF inpp¯collisions ats=1.96 TeV

This paper presents a search for standard model Higgs boson production in association with a $W$ boson using events recorded by the CDF experiment in a dataset corresponding to an integrated luminosity of 5.6 fb-1. The search is performed using a matrix element technique in which the signal and background hypotheses are used to create a powerful discriminator. The discriminant output distributions for signal and background are fit to the observed events using a binned likelihood approach to search for the Higgs boson signal. We find no evidence for a Higgs boson, and 95% confidence level (C.L.) upper limits are set on the Higgs boson production rate. The observed limits range from 3.5 to 37.6 relative to the standard model expectation for Higgs boson masses between 100 and 150 GeV. The 95% C.L. expected limit is estimated from the median of an ensemble of simulated experiments and varies between 2.9 and 32.7 relative to the production rate predicted by the standard model over the Higgs boson mass range studied.

Top-pair forward-backward asymmetry from loops of new strongly coupled quarks

We examine loop-mediated effects of new heavy quarks $Q=({t}^{\ensuremath{'}},{b}^{\ensuremath{'}})$ on $t\overline{t}$ production at hadron colliders, using a phenomenological model with flavor off-diagonal couplings of charged and neutral scalars $\ensuremath{\phi}=({\ensuremath{\phi}}^{\ifmmode\pm\else\textpm\fi{}},{\ensuremath{\phi}}^{0})$ to $Q$. We show that an invariant-mass-dependent asymmetry, in the $t\overline{t}$ center of mass, consistent with those recently reported by the CDF collaboration, can be obtained for quark masses around 350--500 GeV, scalar masses of order 100--200 GeV, and modest-to-strong Yukawa couplings. The requisite strong interactions suggest a nonperturbative electroweak-symmetry-breaking mechanism and composite states at the weak scale. A typical prediction of this framework is that the new heavy quarks decay dominantly into $t\ensuremath{\phi}$ final states.

Exotic Charmonium and Bottomonium-like Resonances

Many new states in the charmonium and bottomonium mass region were recently discovered by the BaBar, Belle and CDF Collaborations. We use the QCD Sum Rule approach to study the possible structure of some of these states. In particular we identify the recently observed bottomonium-like resonance Z+b(10610) with the first excitation of the tetraquark Xb(1++), the analogue of the X(3872) state in the charm sector.

Inclusive jet production at the tevatron collider in the Regge limit of quantum chromodynamics

We consider the inclusive hadroproduction of jets, prompt photons, and b-quark jets in the quasimulti-Regge kinematics approach based on the hypothesis of gluon and quark reggeization in t-channel exchanges at high energies. The data taken by CDF and D0 collaborations at the Fermilab Tevatron collider are well described in the region of \(x_T = 2p_T /\sqrt s \lesssim 0.1\) without the introduction of any free parameters. In numeric calculations we use the Kimber-Martin-Ryskin prescription for unintegrated gluon and quark distribution functions with Martin-Roberts-Stirling-Thorne collinear parton distribution functions taken as input.

Effects of a Family Non-universal Z′ Boson in B̄d → μ+μ− and B− → π−μ+μ−, ρ−μ+μ− Decays

Motivated by the recently updated experimental measurement on B̄d → μ+μ− and B− → π−μ+μ− decays by CDF and Belle collaborations, we revisit these decays, as well as B− → ρ−μ+μ− decay, within the Standard Model, and evaluate the effects of a family non-universal Z′ boson. Under the constraint from ℬ(B− → π−μ+μ−), we find the ranges of the Z′ couplings SLRμμ < −5.2 × 10−2 or DLRμμ < −8.1 × 10−2 are excluded. Within the allowed Z′ parameters spaces, comparing with the SM predictions, we find that ℬ(B̄d → μ+μ−), ℬ(B− → π−μ+μ−), and ℬ(B− → ρ−μ+μ−) could be enhanced by a factor about 226%, 245%, and 254%, respectively, by Z′ contributions. However, they are hardly to be reduced. Furthermore, the zero crossing in the normalized forward-backward asymmetry spectrum of B− → ρ−μ+μ− decay at low dimuon mass always exists.

Search for the rare radiative decayW→πγinpp¯collisions ats=1.96 TeV

We present a search for the rare radiative decay \wpigamma\ using data corresponding to an integrated luminosity of 4.3 \invfb\ of proton-antiproton collisions at a center of mass energy of 1.96 TeV collected by the CDF experiment at Fermilab. As no statistically significant signal is observed, we set a 95% confidence level upper limit on the relative branching fraction \bratio\ at $6.4\times 10^{-5}$, a factor of 10 improvement over the previous limit.

AMC@NLO predictions for Wjj production at the Tevatron

AbstractWe use aMC@NLO to predict theℓν + 2-jet cross section at the NLO accuracy in QCD matched to parton shower simulations. We find that the perturbative expansion is well behaved for all the observables we study, and in particular for those relevant to the experimental analyses. We therefore conclude that NLO corrections to this process cannot be responsible for the excess of events in the dijet invariant mass observed by the CDF collaboration.