Search

We put constraints on unparticle physics, specifically on the scale $\Lambda_\U$ and the scale dimension $d_\U$ of unparticle operators, using (i) measurements of atomic parity violation as well as (ii) branching ratio and CP asymmetry measurements in some rare non-leptonic B decay channels., Comment: 11 pages, 3 figures; v2: minor typos settled, to appear in Phys Lett B; v3: minor changes in discussions following Eq.(8), matches PLB version

The existence of an exactly scale invariant sector possessing a non-trivial infrared fixed point at a higher energy scale and its possible communication with the Standard Model particles through a heavy messenger sector has been shown to lead to curious unparticle effects. We demonstrate that top physics at the Tevatron can already constrain such theories. We also consider possible improvements at the LHC and delineate some striking signatures., Comment: 5 pages

Recently Georgi has discussed the possible existence of `Unparticles' describable by operators having non-integral scaling dimensions. With the interaction of these with the Standard Model (SM) particles being constrained only by gauge and Lorentz symmetries, it affords a new source for lepton flavour violation. Current and future muon decay experiments are shown to be very sensitive to such scenarios., Comment: 13 pages, 6 figures

Recently proposed Little Higgs models present a viable solution to the naturalness problem of the Standard Model. An additional discrete symmetry, called T-parity, has been included in the simplest Little Higgs models to evade the constraints arising from electroweak precision data. The Littlest Higgs model with T-parity (LHT) not only predicts a set of new fermions in addition to the heavy gauge bosons of the original Little Higgs model, but also provides a new candidate for dark matter. In this paper, we study two particularly interesting signatures of T-odd fermion pair production at the LHC, namely, (a) jj + l^+ l^- + missing E_T and (b) jj + b \bar b + l^\pm + missing E_T. Using a parton level Monte Carlo event generator, we evaluate both the signal as well as the standard model background profile for a selected set of model parameters thereby developing a good discriminator. Finally, we scan the parameter space and delineate the possible discovery region in the same., Comment: 21 pages, 7 figures

In the CP-violating Minimal Supersymmetric Standard Model (MSSM), for certain values of the CP-violating phases associated to the universal trilinear couplings $(A_t, A_b)$ and the gluino mass $(M_{\tilde g})$, e.g., $\Phi_{\rm{CP}} = 60^\circ$ or $90^\circ$, for $M_{H^+} \lsim 140 $ GeV and $\tan \beta \sim 2-5 $, the lightest Higgs boson mass ($M_{H_1}$) is $\lsim 50 $ GeV. This mass interval is still allowed from standard LEP Higgs searches because of a strongly suppressed $H_1ZZ$ coupling. However, in the same region of parameter space in which these two conditions occur, the $H_1 H^\mp W^\pm $ coupling is enhanced because the two mentioned sets of couplings satisfy a sum rule. In this paper we probe such a light Higgs scenario at the Large Hadron Collider (LHC) by studying $H^\pm H_1$ associate production, leading to a $4b + \ell^\pm + \mET $ signal. We show that the latter is readily accessible at the CERN hadron collider, upon the application of suitable selection cuts against the Standard Model (SM) backgrounds. Our parton level Monte Carlo (MC) analysis yields $\sim 15-45 $ signal events, completely free of SM background, for ${\cal L} = 10-30 {\rm fb}^{-1}$ of accumulated luminosity, after taking into account the overall efficiency for tagging four $b$-jets., Comment: 14 pages, 5 postscript figures. One sentence added, typo corrected

The CP-violating MSSM allows existence of a light neutral Higgs boson ($M_{H_1} \lapp 50$ GeV) in the CPX scenario in the low $\tan \beta (\lapp 5)$ region, which could have escaped the LEP searches due to a strongly suppressed $H_1 Z Z$ coupling. This parameter space corresponds to a relatively light $H^\pm$ ($M_{H^\pm} < M_t$), which is predicted to decay dominantly into the $W H_1$ channel. Thus one expects to see a striking $t \bar t$ signal at the LHC, where one of the top quarks decays into the $bb \bar b W$ channel, via $t \to b H^\pm, H^\pm \to W H_1$ and $H_1 \to b \bar b$. The characteristic correlation between the $b \bar b$, $b \bar b W$ and $b b \bar b W$ invariant mass peaks is expected to make this signal practically free of the SM background. Our parton level Monte Carlo simulation yields upto 5000 events, for ${\cal L} = 30$ fb$^{-1}$, over the parameter space of interest, after taking into account the b-tagging efficiency for three or more b-tagged jets., Comment: 13 pages, 4 figures, LaTeX. Version to appear in Phys. Lett. B. Extended discussions of the QCD and combinatorial backgrounds

We study constraints on new physics from the recent measurement of Br ($K^+ \to \pi^+ \nu \bar \nu)$ by the E787 and E949 Collaborations. In our analysis we consider two models of new physics: $(a)$ extra down type singlet quark model (EDSQ) and $(b)$ R-parity violating Minimal Supersymmetric Standard Model(MSSM). We find that $K^+\to \pi^+ \nu\bar \nu$ along with other processes like $K_L \to \mu^+ \mu^-, \epsilon^\prime/\epsilon $ provide useful bounds on the parameter $U_{sd}$, characterizing the off-diagonal $Z-d-\bar s $ coupling of model $(a)$. The bounds on the ${\rm Re}(U_{sd})$ from $(K_L \to \mu^+\mu^-)_{\rm SD}$ and ${\rm Im}(U_{sd})$ from $\epsilon^\prime/\epsilon$ are so tight that the branching ratio of $K^+\to \pi^+ \nu \bar \nu $ can exceed the standard model value by at most a factor of two For model b), we also obtain stringent bounds on certain combinations of product of two $\lp_{ijk}$ couplings originating from $L$ number violating operator $ L_i Q_j D^c_k $ using $K^+ \to \pi^+ \nu \bar \nu $ and $K_L \to \mu^+ \mu^-$ processes. Even with the stringent constraints on $U_{sd}$ (in model $(a)$) and on products of two $\rp$couplings (model $(b)$), we find that the branching ratios for $K_L \to \pi^0 \nu \bar \nu$ and $K_L \to \pi^0 e^+ e^-$ can be substantially different in both the above models from those predicted in the standard model., Comment: K_L --> pi^0 e+ e- section has been altered, new references to this process are cited. One Figure has been removed and other Figure modified

The models of large extra compact dimensions, as suggested by Arkani-Hamed, Dimopoulos and Dvali, predict exciting phenomenological consequences with gravitational interactions becoming strong at the TeV scale. Such theories can be tested at the existing and future colliders. In this paper, we study the contribution of virtual Kaluza-Klein excitations (both spin-0 and 2) in the process $e^+e^- \to HHZ$ and $ e^+e^- \to HH\gamma $ at future linear collider (NLC). We find that the virtual exchange KK gravitons can modify the cross-section $\sigma(e^+e^- \to HHZ)$ significantly from its Standard Model value and will allow the effective string scale to be probed up to 6.6 TeV. The second process is absent at the tree level in the standard model, and can therefore be used to put limits on the effective string scale of 7.4 TeV., Comment: One new reference is added, some minor corrections, version to appear in Physical Review D

Recent measurement of time dependent asymmetries in $B \to \phi K$ are indicative of a new source of CP violation. We examine squark mixing in SUSY as this new source, and using QCD improved factorization method to describe $B \to \phi K $ decay, find the allowed range of parameter space for $\rho$ and $\psi $, the magnitude and phase of the down type LR(RL) squark mixing parameter $\delta^{bs}_{LR(RL)}$. We then study $B \to \phi K^{*}$ and calculate the expected CP asymmetries in the same range of parameter space. We find that this asymmetry is in the range 15% to $20% $ for acceptable value of $B \to phi K^{*}$ branching ratio. We also predict the helicity dependent CP asymmetries in the same parameter space., Comment: 21 pages, Latex, 2 postscript figures. Comments on B_s mixing is added. Typos are corrected in the text and figure label. Figure 1 is corrected. More references are added. Version to be published in Phys. Rev. D

In light of current atmospheric neutrino oscillation data, we revisit the invisible decay of the standard model Higgs boson and other pseudoscalar mesons which can be enhanced because of large number of KK modes in models with right handed singlet neutrinos in large extra dimensions. We find that the invisible decay rate of Higgs can be as large as $H\to b \bar b $ decay rate only for a very restricted region of parameter space. This parameter space is even further restricted if one demands that the dimensionless neutrino Yukawa coupling $\l$ is O(1). We have also studied the scenarios where singlet neutrino propagate in a sub-space, which lowers the string scale $M_{\ast}$ and keeps neutrino Yukawa coupling O(1). We have also considered decays of other spin-0 mesons to $\nu \bar\nu$ and found the rates to be too small for measurement., Comment: 14 pages, Latex, 4 postscript figures. Includes further discussions of invisible decay of Higgs and other mesons in the asymmetric scenario, where singlet neutrinos propagate in extra-dimensions with the largest size. Final conclusion remain unchanged. More references added, version to appear in PLB

We study the signatures of minimal anomaly mediated supersymmetry breaking in an e^- gamma collider. We demonstrate that the associated production of a sneutrino with the lightest chargino leads to a substantially large signal size. The background is negligibly small, though. Even more interestingly, a measurement of the fundamental supersymmetry breaking parameters could be possible., Comment: 25 pages, LaTex, 7 PS figures and 1 table (included)

The novel feature of a Higgs-triplet representation is a nonzero tree-level coupling of $H^+ W^- Z$, which is absent in all Higgs-doublet models. We study the associated production of a singly-charged Higgs boson of the Higgs-triplet representation with a $W$ or $Z$ boson at hadron colliders, followed by the $H^+ \to W^+ Z$ decay. We find that the $2\ell+4j$ final state gives an interesting level of signal with a negligible background, plus it allows a full mass reconstruction of the charged-Higgs boson. The cover range of the charged-Higgs mass is between 110 and 200 GeV., Comment: JHEP style, 15 pages, 4 figures, some typos are corrected

We study contributions of quark-squark-gluino interactions in Minimal Supersymmetric Standard Model (MSSM) to B->PP (PP =Kpi, pipi, KK) decays using QCD improved factorization method for the evaluation of the hadronic matrix elements and taking into account renormalization group running of the Wilson coefficients from SUSY scale (~ m_{\tilde q}) to the low energy scale (= m_b) applicable to B decays. Using the most recent experimental data we obtain constraints on flavor changing Supersymmetric (SUSY) parameters (\delta_{ij})_{LL, RR}. For \Delta S = -1 processes, b-> s gamma is usually considered to give the strongest limits. We, however, find that in some part of the parameter space B-> Kpi processes give stronger bounds. Implications for B^0_s -\bar B^0_s mixing is discussed. We also study cons traints obtained from \Delta S = 0 processes B-> pipi, KK, rho gamma and B_d -\bar B_d mixing. In this case, in a large part of the parameter space B_d - \bar B_d provides the best bound, but B-> K^- K^0, rho gamma can still give interesting constraints in a complementary region of the parameter space., Comment: 27 pages, 11 figures,new experimental data on B decays are used

We investigate single sneutrino production in the context of R-parity-violating Supersymmetry at future $\gamma\gamma$ linear colliders. The sneutrino is produced in association with fermion pairs and it is shown that its decays into two further fermions will lead to a clean signal. We also discuss possible backgrounds and the effects of beam polarisation., Comment: 31 pages, LaTeX, 10 postscript figures. Title has been modified. Two new figures and one appendix added. Detailed SM background estimations were made. A new reference added. Version to appear in PRD

We report on the possibility of observing Higgs sector CP violation of the minimal supersymmetric standard model at a hadron machine. The CP phase dependent cross-sections for the $VH_i$ associated production processes are given for the Fermilab Tevatron and CERN LHC. Substantial production cross-sections for channels of all three Higgs bosons simultaneously are shown to be possible, giving a drect indication of the CP violation. The observability of the Higgs signals are discussed., Comment: 16 pages, LaTeX, 4 postscript figures, some clarification, and misprints are corrected. To appear in Phys. Lett. B

We investigate the role of R-parity violating interaction in the non-leptonic decays of B mesons into two light mesons B --> PP. The decay amplitudes are calculated using the QCD improved factorization method. Using the combined data on B decays from BaBar, Belle and CLEO, we obtain strong constraints on the various products of R-parity violating couplings. Many of these new constraints are stronger than the existing bounds., Comment: 19 pages including two eps figures

Though the minimal model of anomaly mediated supersymmetry breaking has been significantly constrained by recent experimental and theoretical work, there are still allowed regions of the parameter space for moderate to large values of $\tan\beta$. We show that these regions will be comprehensively probed in a ${\sqrt s} =1$ TeV $e^+e^-$ linear collider. Diagnostic signals to this end are studied by zeroing in on a unique and distinct feature of a large class of models in this genre: a neutral winolike Lightest Supersymmetric Particle closely degenerate in mass with a winolike chargino. The pair production processes $e^+e^- \to {\tilde e}_L^\pm {\tilde e}_L^\mp$, ${\tilde e}_R^\pm {\tilde e}_R^\mp$, ${\tilde e}_L^\pm {\tilde e}_R^\mp$, ${\tilde \nu} {\bar {\tilde \nu}}$, $\widetilde \chi^0_1 \widetilde \chi^0_2$, $\widetilde \chi^0_2 \widetilde \chi^0_2$ are all considered at $\sqrt s = 1$ TeV corresponding to the proposed TESLA linear collider in two natural categories of mass ordering in the sparticle spectra. The signals analysed comprise multiple combinations of fast charged leptons (any of which can act as the trigger) plus displaced vertices $X_D$ (any of which can be identified by a heavy ionizing track terminating in the detector) and/or associated soft pions with characteristic momentum distributions., Comment: 25 pages, LaTeX, three PS figures and one EPS figure

In this paper we study non-commutative (NC) QED signatures at photon colliders through pair production of charged leptons $(\ell^+ \ell^-)$ and charged scalars $(H^+ H^-)$. The NC corrections for the fermion pair production can be easily obtained since NC QED with fermions has been extensively studied in the literature. NC QED with scalars is less studied. To obtain the cross section for $H^+H^-$ productions, we first investigate the structure of NC QED with scalars, and then study the corrections due to the NC geometry to the ordinary QED cross sections. Finally by folding in the photon spectra for a $\gamma \gamma$ collider with laser back-scattered photons from the $e^+ e^-$ machine, we obtain 95% CL lower bound on the NC scale using the above two processes. We find that, with $\sqrt{s} = 0.5, 1.0$, and $ 1.5$ TeV and integrated luminosity $L = 500(fb^{-1})$, the NC scale up to 0.7, 1.2, and 1.6 TeV can be probed, respectively, while, for monochromatic photon beams, these numbers become 1.1, 1.7, 2.6 TeV, respectively., Comment: 16 pages, 7 figures

We study the pair-production and decay of right handed selectrons within a Gauge Mediated Supersymmetry Breaking Model in polarised $e^-e^-$ interaction. Detailed analyses of the possible signals and backgrounds are performed for a few selected points in the parameter space. A judicious choice for polarisation of the initial electron beam helps eliminate almost the entire Standard Model background. We also show that phase space distributions can be used to distinguish such a supersummetry breaking scheme from the supergravity inspired models., Comment: 17 pages, LaTeX, 17 postscript figures

Selectron (smuon) pair-production in a next generation Linear Collider, yielding a fast electron (muon) trigger, a visible heavily ionizing track and/or a resolved soft pion impact parameter and overall $\mET$, is shown to provide a smoking gun signature for Anomaly Mediated Supersymmetry Breaking models with a neutral Wino as the Lightest Supersymmetric Particle, nearly mass-degenerate with the lighter chargino., Comment: 13 pages, LaTeX, 3 postscript figures. Though the results and conclusions remain unchanged, the text and the figures have been substantially modified. Some typos have been corrected and new references added. Discussion added of Standard Model backgrounds which turn out to be very small. The paper will be published shortly in JHEP