• Issue front cover thumbnail

      Volume 79, Issue 4

      October 2012,   pages  511-940

    • Preface

      Rohini Godbole Naba K Mondal

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    • Higgs physics: Theory

      Abdelhak Djouadi

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      The theoretical aspects of the physics of Higgs bosons are reviewed focussing on the elements that are relevant for the production and detection at present hadron colliders. After briefly summarizing the basics of electroweak symmetry breaking in the Standard Model, the Higgs production at the LHC and at the Tevatron is discussed, with some focus on the main production mechanism, the gluon–gluon fusion process, and the main Higgs decay modes and the experimental detection channels are discussed. Then the case of the minimal supersymmetric extension of the Standard Model is briefly surveyed. In the last section, the prospects for determining the fundamental properties of the Higgs particles are reviewed, once they have been experimentally observed.

    • Standard Model Higgs boson searches with the ATLAS detector at the Large Hadron Collider

      Aleandro Nisati on behalf of the ATLAS Collaboration

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      The investigation of the mechanism responsible for electroweak symmetry breaking is one of the most important tasks of the scientific program of the Large Hadron Collider. The experimental results on the search of the Standard Model Higgs boson with 1 to 2 fb-1 of proton–proton collision data at $\sqrt{s} = 7$ TeV recorded by the ATLAS detector are presented and discussed. No significant excess of events is found with respect to the expectations from Standard Model processes, and the production of a Higgs boson is excluded at 95% Confidence Level for the mass regions 144–232, 256–282 and 296–466 GeV.

    • Quantum chromodynamics effects in electroweak and Higgs physics

      Frank Petriello

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      Several examples of the often intricate effects of higher-order quantum chromodynamics (QCD) corrections on predictions for hadron-collider observables, are discussed, using the production of electroweak gauge boson and the Standard Model Higgs boson as examples. Particular attention is given to the interplay of QCD effects and experimental cuts, and to the use of scale variations as estimates of theoretical uncertainties.

    • Quantum chromodynamics results from HERA and JLAB

      Katja Krüger

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      Recent QCD results from electron–proton interactions at HERA and JLAB are presented. Inclusive cross-section measurements as well as studies of the hadronic final state like jet production or the production of heavy quarks are discussed. The results are compared with perturbative QCD predictions and their impact on the determination of the parton density functions of the proton as well as of the strong coupling α s is discussed.

    • Light hadron, charmonium(-like) and bottomonium(-like) states

      Hai-Bo Li

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      Hadron physics represents the study of strongly interacting matter in all its manifestations and understanding its properties and interactions. The interest in this field has been revitalized by the discovery of new light hadrons, charmonium- and bottomonium-like states. In this paper, the most recent experimental results from different experiments are reviewed.

    • Gauge boson production at colliders - Predictions for precision studies

      Giulia Zanderighi

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      The status of today’s theoretical description of Standard Model (SM) processes involving gauge bosons at the Tevatron and the LHC, and of the tools that are used in their phenomenological studies are reviewed. A few recent ideas to further improve on the way technically challenging calculations can be performed also are discussed.

    • Production of electroweak bosons at colliders

      Matthias U Mozer

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      The collider experiments at the Tevatron and LHC are accumulating samples of electroweak bosons of unprecedented size. These huge samples can be used to observe rare processes, such as diboson production which have the potential to show enhancements due to new physics. Alternatively, the great statistical power of the samples allows for detailed studies of electroweak production mechanisms and correspondingly QCD and the proton structure.

    • Top quark theory

      Eric Laenen

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      The theoretical aspects of a number of top quark properties such as its mass and its couplings are reviewed. Essential aspects in the theoretical description of top quark production, singly, in pairs and in association, as well as its decay related to spin and angular correlations are discussed.

    • Top production at hadron colliders

      Albert De Roeck

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      New results on top quark production are presented from four hadron collider experiments: CDF and D0 at the Tevatron, and ATLAS and CMS at the LHC. Cross-sections for single top and top pair production are discussed, as well as results on the top–antitop production asymmetry and searches for new physics including top quarks. The results are based on data samples of up to 5.4 fb-1 for the Tevatron experiments and 1.1 fb$−1$ for the LHC experiments.

    • Top quark properties

      Yuji Takeuchi

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      Since the top quark was discovered at Tevatron in 1995, many top quark properties have been measured. However, the top quark is still interesting due to unique features which originate from the extremely heavy mass, and providing various test grounds on the Standard Model as well as searches for a new physics. Though the measurements of the top quark had been performed only at Tevatron so far, LHC is now ready for measurements with more top quarks than Tevatron. In this article, recent measurements of top quark properties from Tevatron (CDF and DØ) as well as LHC (ATLAS and CMS) are presented.

    • Electroweak symmetry breaking beyond the Standard Model

      Gautam Bhattacharyya

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      In this paper, two key issues related to electroweak symmetry breaking are addressed. First, how fine-tuned different models are that trigger this phenomenon? Second, even if a light Higgs boson exists, does it have to be necessarily elementary? After a brief introduction, the fine-tuning aspects of the MSSM, NMSSM, generalized NMSSM and GMSB scenarios shall be reviewed, then the little Higgs, composite Higgs and the Higgsless models shall be compared. Finally, a broad overview will be given on where we stand at the end of 2011.

    • Searches for phenomena beyond the Standard Model at the Large Hadron Collider with the ATLAS and CMS detectors

      Henri Bachacou on behalf of the ATLAS and CMS Collaborations

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      The LHC has delivered several fb-1 of data in spring and summer 2011, opening new windows of opportunity for discovering phenomena beyond the Standard Model. A summary of the searches conducted by the ATLAS and CMS experiments based on about 1 fb-1 of data is presented.

    • Searches for physics beyond the Standard Model at the Tevatron

      Alberto Annovi for the CDF and D0 Collaborations

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      During the last 10 years, the Fermilab Tevatron has produced $p \bar{p}$ collision at a centre-of-mass energy of 1.96 TeV that the CDF and DO Collaborations have scrutinized looking for new physics in a wide range of final states. Here, recent updates of new physics searches are reported using a data sample of up to 9 fb-1

    • Heavy ions: Results from the Large Hadron Collider

      Tapan K Nayak

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      On November 8, 2010 the Large Hadron Collider (LHC) at CERN collided the first stable beams of heavy ions (Pb on Pb) at the centre-of-mass energy of 2.76 TeV/nucleon. The LHC worked exceedingly well during its one month of operation with heavy ions, delivering about 10 $\mu𝑏^{−1}$ of data, with peak luminosity reaching to $L_{O} = 2 × 10^{25}$ cm-2 s-1 towards the end of the run. Three experiments, ALICE, ATLAS and CMS, recorded their first heavy-ion data, which were analysed in a record time. The results of the multiplicity, flow, fluctuations and Bose–Einstein correlations indicate that the fireball formed in nuclear collisions at the LHC is hotter, lives longer, and expands to a larger size at freeze-out as compared to lower energies. We give an overview of these as well as new results on quarkonia and heavy flavour suppression, and jet energy loss.

    • Heavy ions: Report from Relativistic Heavy Ion Collider

      Sonia Kabana

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      We review selected highlights from the experiments at the Relativistic Heavy Ion Collider (RHIC) exploring the QCD phase diagram. A wealth of new results appeared recently from RHIC due to major recent upgrades, like for example the $\Upsilon$ suppression in central nucleus-nucleus collisions which has been discovered recently in both RHIC and LHC. Furthermore, we discuss RHIC results from the beam energy scan (BES) program aiming to search for a possible critical point and to map out the QCD phase diagram.

    • Sign-posting the phase diagram of quantum chromodynamics

      Sourendu Gupta

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      The good agreement between lattice predictions and data for the shape of the distribution of event-by-event fluctuations of the baryon number is discussed. Such comparisons can give fine probes of thermalization, and can be used to provide a direct determination of the cross-over temperature $T_{c}$ QCD. The logic of these comparisons and the systematics involved are discussed. The same methods can be used to further explore the phase diagram.

    • Low-energy neutrino measurements

      Davide D'angelo

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      Low-energy solar neutrino detection plays a fundamental role in understanding both solar astrophysics and particle physics. After introducing the open questions on both fields, we review here the major results of the last two years and expectations for the near future from Borexino, Super-Kamiokande, SNO and KamLAND experiments as well as from upcoming (SNO+) and planned (LENA) experiments. Scintillator neutrino detectors are also powerful antineutrino detectors which can detect neutrinos emitted by the Earth crust and mantle. First measurements of geoneutrinos have occurred which can bring fundamental contribution in understanding the geophysics of the planet.

    • Neutrinoless double beta decay

      Kai Zuber

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      The physics potential of neutrinoless double beta decay is discussed. Furthermore, experimental considerations as well as the current status of experiments are presented. Finally, an outlook towards the future, work on nuclear matrix elements and alternative processes is given.

    • Theoretical aspects of neutrino mass and lepton flavour violation

      Graham G Ross

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      We consider lepton flavour violation (LFV) in the charged lepton sector both from the bottom-up effective Lagrangian approach and from the top-down approach via various case studies that have been analysed. The implications for LFV studies at the LHC is briefly discussed. Finally the nature of LFV in the neutrino sector is considered, paying particular regard to the implications of the recent measurements of $\theta_{13}$.

    • A new observable to measure the top quark mass at hadron colliders

      Simone Alioli Juan Fuster Adrian irles Sven Moch Peter Uwer Marcel Vos

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      The $t\bar{t} + 1-{\text{jet}} + X$ differential cross-section in proton-proton collissions at 7 TeV centre of mass energy is investigated with respect to its sensitivity to the top quark mass. The analysis includes higher order QCD corrections at NLO. The impact of the renormalization scale $(\mu_{\text{R}})$, the factorization $(\mu_{\text{F}})$ scale and of the choice of different proton's PDF (parton distribution function) has been evaluated. In this study it is concluded that differential jet rates offer a promising option for alternative mass measurements of the top quark, with theoretical uncertainties below 1 GeV.

    • Measurement of the production cross-section of pairs of isolated photons in $pp$ collisions at $\sqrt{s} = 7$ TeV

      Sudha Ahuja

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      The integrated and differential cross-sections of the production of pairs of isolated photon were measured in proton-proton collisions at a centre-of-mass energy of 7 TeV using the CMS detector at the LHS. Data corresponding to an integrated luminosity of 36 pb-1 were analysed. A next-to-leading-order perturbative QCD calculation was compared to the measurements which showed an overall satisfactory agreement. A discrepancy was observed in the regions of phase space populated by photons radiated at relatively small angles.

    • Data-driven performance evaluation method for CMS RPC trigger system using 2011 data at LHC

      A Sharma S B Beri on behalf of the CMS Collaboration

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      The compact muon solenoid (CMS) is one of the four experiments which is getting and analysing the results of the collision of protons at LHC. The CMS trigger system is divided into two stages, the level-1 trigger and high-level triggers, to handle the large stream of data produced in collision. The information transmitted from the three muon subsystems (DT, CSC and RPC) are collected by the Global Muon Trigger (GMT) Board and merged. A method for evaluating the RPC system trigger efficiency with data from $pp$ collision was developed using the features of GMT. The results of the study with the real data of 2011 are shown and discussed here along with the comparison of Monte Carlo results

    • Search for Standard Model Higgs boson in the decay channel $H \rightarrow ZZ \rightarrow l^{+}l^{−} q\bar{q}$ at CMS

      Arun Kumar for the CMS Collaboration

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      A search for the Standard Model boson decaying to two 𝑍 bosons with a subsequent decay to a final state with two leptons and two quark jets, $H \rightarrow ZZ \rightarrow l^{+}l^{-} q\bar{q}$, is presented. Data corresponding to an ntegrated luminosity of 1.6 fb-1 of LHC proton-proton collisions at the centre-of-mass energy of 7 TeV were collected and analysed by the CMS experiment. The selection to discriminate between signal and background events is based on kinematic and topological quantities, which include the angular spin correlations of the decay products. The events are classified according to the probability of the jets to originate from quarks of light or heavy flavour or from gluons. No evidence for a Higgs boson is found and upper limits on the Higgs boson production cross-section are set in the range of masses between 226 and 600 GeV/c2.

    • Measurements of the electron and muon inclusive cross-sections in proton–proton collisions at $\sqrt{s} = 7$ TeV with the ATLAS detector

      Moritz Backes on behalf of the ATLAS Collaboration

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      We present the measurements of the differential cross-sections for inclusive electron and muon production in proton–proton collisions at a centre-of-mass energy of $\sqrt{s} = 7$ TeV, using ∼ 1.4 pb-1 of data collected by the ATLAS detector at the Large Hadron Collider. The muon cross-section is measured as a function of muon transverse momentum $p_{\text{T}}$ in the range $4 < p_{\text{T}} < 100$ GeV and within pseudorapidity $|\eta| < 2.5$. In addition, the electron and muon cross-sections are measured in the range $7 < p_{\text{T}} < 26$ GeV and within $|\eta| < 2.0$, excluding $1.37 < |\eta| < 1.52$. After subtraction of the$ W/Z/\gamma^{*}$ contribution, the differential cross-sections are found to be in good agreement with theoretical predictions for heavy-flavour production obtained from fixed order NLO calculations with NLL high-$p_{\text{T}}$ resummation, and to be sensitive to the effects of NLL resummation.

    • A search for charged massive long-lived particles at D0

      Sudeshna Banerjee for the D0 Collaboration

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      Results are presented on a search for charged massive long-lived particles (CMLLPs), based on 5.2 fb-1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron $p\bar{p}$ collider. This search selects events in which one or more particles are reconstructed as muons but their speed and ionization energy loss (d𝐸/d𝑥) are different from muons produced in beam–beam collisions. CMLLPs have been predicted by several theories of physics beyond the Standard Model. In this search we exclude pair-produced long-lived gaugino-like charginos with masses below 251 GeV and higgsino-like charginos with masses below 230 GeV at 95% CL, as well as long-lived stop quarks with masses below 265 GeV. We place cross-section limits on long-lived staus as 0.04 to 0.006 pb for stau masses in the range 100 to 300 GeV.

    • Non-singlet spin structure function $g^{NS}_{1} (x,t)$ in the DGLAP approach

      Neelakshi N K Borah D K Choudhury P K Sahariah

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      An analytical solution of the non-singlet polarized parton distribution $\Delta q^{NS} (x, Q^{2}) = (\Delta u(x, Q^{2}) − \Delta d(x, Q^{2}))$ is obtained by solving the DGLAP (Gribov and Lipatov, Sov. J. Nucl. Phys. 15, 438 (1972); Lipatov, ibid, 20, 94 (1975); Dokshitzer, Sov. Phys. JETP 46, 641 (1997); Altarelli and Parisi, Nucl. Phys. B126, 298 (1977)) equation by the method of characteristics. We then evaluate the non-singlet spin-dependent structure function $g^{NS}_{1}$ . The result is compared with the data from HERMES (Airapetian et al, Phys. Rev. D75, 012007 (2007)).

    • Dijet physics with CMS detector at the Large Hadron Collider

      Suvadeep Bose on behalf of the CMS Collaboration

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      The results from various dijet distributions in proton-proton collisions at a centre-of-mass energy of 7 TeV, with 2010 and 2011 data from the CMS experiment, are presented. The measurements of the dijet mass spectra, centrality ratio, azimuthal decorrelation and angular distribution are shown. Sensitivity of the phenomenological parameters used to model different event generators is also investigated. Prospects for observing evidence for new physics in these distributions are presented.

    • Search for the Standard Model Higgs boson produced in the decay channel $H \rightarrow ZZ \rightarrow 2l2\tau$ with CMS detector at $\sqrt{s} = 7$ TeV

      Chhibra Simranjit Singh on behalf of the CMS Collaboration

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      Search for the Standard Model Higgs boson in the decay mode $H \rightarrow ZZ \rightarrow 2l2\tau$, where $l = \mu, e$, is presented based on CMS data corresponding to an integrated luminosity of 1.1 fb-1 at $\sqrt{s} = 7$ TeV. No evidence is found for a significant deviation from Standard Model expectations anywhere in the $ZZ$ mass range considered in this analysis. An upper limit at 95% CL is placed on the product of the cross-section and decay branching ratio for the Higgs boson decaying with Standard Model-like couplings, which excludes cross-sections of about ten times the expected value for Higgs boson masses in the range 200 < $m_{H}$< 400 GeV/c2.

    • Flavoured co-annihilation

      D Choedhury R Garani S K Vempati

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      In minimal supergravity (mSUGRA) or CMSSM, one of the main co-annihilating partners of the neutralino is the lightest stau, $\tilde{\tau}_{1}$. In the presence of flavour violation in the right-handed sector, the co-annihilating partner would be a flavour mixed state. The flavour effect is two-fold: (a) It changes the mass of $\tlide{\tau}_{1}$, thus modifying the parameter space of the co-annihilation and (b) flavour violating scatterings could now contibute to the cross-sections in the early Universe. In fact, it is shown that for large enough $\delta \sim 0.2$, these processes would constitute the dominant channels in co-annihilation regions. The amount of flavour mixing permissible is constrained by flavour violating $\tau \rightarrow \mu$ or $\tau \rightarrow e$ processes. For $\Delta_{\text{RR}}$ mass insertions, the constraints from flavour violation are not strong enough in some regions of the parameter space dur to partial cancellations in the amplitudes. In mSUGRA, the regions with cancelations within LFV amplitudes do not overlap with the regions of co-annihilations. In non-universal HIggs model (NUHM), however, these regions do overlap leading to significant flavoured co-annihilations. At the LHC and other colliders, these regions can constitute for interesting signals.

    • Top physics with 0.70–1.08 fb-1 of $pp$ collisions with the ATLAS detector at the LHC

      Markus Cristinziani on behalf of the ATLAS Collaboration

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      With data collected during the first half of the 2011 $pp$ run of the Large Hadron Collider at $\sqrt{s} = 7$ TeV, a substantial data sample of high $p_{T}$ triggers, corresponding to an integrated luminosity of 1.08 fb-1, has been collected by the ATLAS detector. Measurements of the productions of top-quark pairs and single top quarks in different channels, the top-quark mass, the top-quark pair charge asymmetry and spin correlations, and the 𝑊 helicity fractions in top-quark decays are presented, as well as two searches for new physics ecffects involving top-quark pairs.

    • Pion showers in highly granular calorimeters

      Jaroslav Cvach on behalf of the CALICE Collaboration

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      New results on properties of hadron showers created by pion beam at 8–80 GeV in high granular electromagnetic and hadron calorimeters are presented. Data were used for the first time to investigate the separation of the neutral and charged hadron showers. The result is important to verify the prediction of the PFA algorithm based up to now on the simulated data only. Next, the properties of hadron showers were compared to different physics lists of GEANT4 version 9.3.

    • Quarkonia production at forward rapidity in Pb + Pb collisions at $\sqrt{^{s}NN} = 2.76$ TeV with the ALICE detector

      Debasish Das for the ALICE Collaboration

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      The study of formation of heavy quarkonia in relativistic heavy-ion collisions provides important insight into the properties of the produced high-density QCD medium. Lattice QCD studies show sequential suppression of quarkonia states with increasing temperature; which affirms that a full spectroscopy can provide us a thermometer for the matter produced under extreme conditions in relativistic heavy-ion collisions and one of the most direct probes of deconfinement. Muons from the decay of charmonium resonances are detected in ALICE experiment in $p + p$ and Pb + Pb collisions with a muon spectrometer, covering the forward rapidity region $(2.5 < y < 4)$. The analysis of the inclusive $J/\psi$ production in the first Pb + Pb data collected in the fall of 2010 at a centre of mass energy of $\sqrt{^{s}NN} = 2.76$ TeV is discussed. Preliminary results on the nuclear modification factor $(R_{AA})$ and the central to peripheral nuclear modification factor $(R_{CP})$ are presented.

    • A natural connection between neutrino mass generation and the lightness of a next-to-minimal supersymmetric Standard Model pseudoscalar

      Debottam Das Asmaa Abada Gautam Bhattacharyya Cédric Weiland

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      One of the attractive properties of the NMSSM is that it can accommodate a light pseudoscalar of order 10 GeV. However, such scenarios are constrained by several experimental results, especially those related to the fermionic decays of the pseudoscalar. In this work, extending the NMSSM field content by two gauge singlets, with lepton number +1 and $−1$, we generate neutrino masses via the inverse see-saw mechanism at one hand and on the other hand a very light pseudoscalar becomes experimentally viable by having dominant invisible decay channels which help it to evade the existing bounds.

    • Search for heavy resonances decaying to tau pairs with the CMS detector at the Large Hadron Collider

      N Dhingra on behalf of the CMS Collaboration

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      A search for heavy resonance production decaying to tau pair is performed in the proton–proton collisions at the center-of-mass energy $\sqrt{s} = 7$ TeV using 36 pb-1 of data collected with CMS detector at the LHC during the year 2010. The number of observed events are in good agreement with the predictions of Standard Model background processes. Therefore, an upper limit on the resonance cross-section times the branching ratio to tau pair is obtained as a function of the resonance mass. Ditau resonance Z' with Standard Model couplings having mass less than 468 GeV/c2 is excluded at 95% confidence level.

    • Search for new physics in dijet mass and angular distributions in $pp$ collisions at $\sqrt{s} = 7$ TeV measured with the ATLAS detector

      Thorsten Dietzsch on behalf of the ATLAS Collaboration

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      We present a search for physics beyond the Standard Model in proton–proton collisions at a centre-of-mass energy of $\sqrt{s} = 7$ TeV, performed with the ATLAS detector at the Large Hadron Collider (LHC). No evidence for new physics is found in dijet mass and angular distributions and stringent limits are set on a variety of models of new physics, including excited quarks, quark contact interactions, axigluons, and quantum black holes.

    • Ultraheavy Yukawa-bound states of fourth-generation at Large Hadron Collider

      Ts Enkhbat

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      A study of bound states of the fourth-generation quarks in the range of 500–700 GeV is presented, where the binding energies are expected to be mainly of Yukawa origin, with QCD subdominant. Near degeneracy of their masses exhibits a new `isospin'. The production of a colour- octet, isosinglet vector meson via $q\bar{q} \rightarrow \omega g$ is the most interesting. Its leading decay modes are $\pi_{8}^{\pm} W^{\mp}$, $\pi_{8}^{0} Z^{0}$, and constituent quark decay, with $q\bar{q}$ and $t\bar{t'}$ and $b\bar{b'}$ subdominant. The colour octet, isovector pseudoscalar $\pi_{8}$ meson decays via constituent quark decay, or to $W g$. This work calls for more detailed study of fourth-generation phenomena at LHC.

    • Unification of 3-3-1 models

      R Martinez F Ochoa

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      A 3-3-1 model is constructed for three families that can be embedded into a single $SU (8)$ unified model. Assuming appropriate branching rules and symmetry-breaking pattern, a complete fermion content is found within irreducible representations of $SU (8)$, where light Standard Model fermions, heavy 3-3-1 fermions and superheavy fermions may be distinguished.

    • SuSeFLAV: A program for calculating supersymmetric spectra and lepton flavour violation

      Debtosh Chowdhury Raghuveer Garani Sudhir K Vempti

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      The program $\mathnormal{SuSeFLAV}$ is introduced for computing supersymmetric mass spectra with flavour violation in various supersymmetric breaking scenarios with/without see-saw mechanism. A short user guide summarizing the compilation, executables and the input files is provided.

    • Constraints on the $K_{l_{3}}$ form factors from analyticity and unitarity

      Gauhar Abbas B Ananthanarayan Irinel Caprini I Sentitemsu Imsong

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      The $K \phi$ form factors are investigated at low energies by the method of unitarity bounds adapted so as to include information on the phase and modulus along the elastic region of the unitarity cut. Using as input the values of the form factors at $t = 0$, and at the Callan–Treiman point in the scalar case, stringent constraints are obtained on the slope and curvature parameters of the Taylor expansion at the origin.

    • Physics beyond the Standard Model through $b \rightarrow s\mu^{+} \mu^{-}$ transition

      Diptimoy Ghosh

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      A comprehensive study of the impact of new-physics operators with different Lorentz structures on decays involving the $b \rightarrow s\mu^{+} \mu^{-}$ transition is performed. The effects of new vector– axial vector (VA), scalar–pseudoscalar (SP) and tensor (T) interactions on the differential branching ratios, forward–backward asymmetries ($A_{\text{FB}}$ ’s), and direct CP asymmetries of $\bar{B}_{s}^{0} \rightarrow \mu^{+} \mu^{-}, \bar{B}_{d}^{0} \rightarrow X_{s} \mu^{+} \mu^{-}, \bar{B}_{s}^{0} \rightarrow \mu^{+} \mu^{-} \gamma, \bar{B}_{d}^{0} \rightarrow \bar{K} \mu^{+} \mu^{-}$, and $\bar{B}_{d}^{0} \rightarrow \bar{K}^{*} \mu^{+} \mu^{-}$ are examined. In $\bar{B}_{d}^{0} \rightarrow \bar{K}^{*} \mu^{+} \mu^{-}$, we also explore the longitudinal polarization fraction $f_{\text{L}}$ and the angular asymmetries $A_{\text{T}}^{2}$ and $A_{\text{LT}}$, the direct CP asymmetries in them, as well as the triple-product CP asymmetries $A_{\text{T}}^{\text{(im)}}$ and $A_{\text{{LT}}^{\text{(im)}}$. While the new VA operators can significantly enhance most of the observables beyond the Standard Model predictions, the SP and T operators can do this only for $A_{\text{FB}}$ in $\bar{B}_{d}^{0} \rightarrow \bar{K} \mu^{+} \mu^{-}$.

    • Limits on anomalous trilinear gauge couplings at the CMS with 7TeV Large Hadron Collider data

      Bhawana Gomber

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      Diboson production in proton–proton collisions presents an opportunity to study the self-interaction between gauge bosons via anomalous trilinear gauge couplings (aTGC). The values of these couplings are fully fixed in the SM by the gauge structure. Thus, any deviation of the observed strength of the TGC from the SM prediction would indicate new physics. This paper presents the limits on anomalous $WW\gamma$, $ZZ\gamma$ and $Z\gamma \gamma$ trilinear gauge couplings in proton–proton collisions at the centre of mass energy of 7 TeV with the CMS detector.

    • Dipole moments of the tau neutrino via the process $e^{+} e^{-} \rightarrow \nu \bar{\nu} \gamma$ in a 331 model

      A Gutiérrz-Rodríguez

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      The limits on the anomalous magnetic and electric dipole moments of the $\nu_{\tau}$ through the reaction $e^{+} e^{-} \rightarrow \nu \bar{\nu} \gamma$ at the $Z_{1}$-pole, and in the framework of a 331 model are obtained. The results are based on the data reported by the L3 Collaboration at LEP. We find that the bounds are almost independent of the mixing angle 𝜙 of the model in the allowed range for this parameter.

    • $W +$ jets in $pp$ collisions at 7 TeV with ATLAS

      Giovanni Zevi Della Porta on behalf of the ATLAS Collaboration

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      Measurements of differential cross-sections of 𝑊 boson production in association with jets, and jets containing 𝑏 hadrons are presented. The measurements are based on 35 pb-1 of $pp$ collisions at $\sqrt{s} = 7$ TeV collected with the ATLAS detector at the LHC, using the electron and muon decay channels of the 𝑊 boson. The data are found to be compatible with next-to-leading order predictions. In addition, a study of the invariant mass distribution of jet pairs produced in association with a 𝑊 boson is presented, based on an integrated luminosity of 1.02 fb-1. The measured dijet mass shows no significant excess over the Standard Model expectation.

    • Inclusive $b + Z, Z \rightarrow \mu^{+} \mu^{-}, e^{+} e^{-}$ production at CMS

      Natalie Heracleous for the CMS Collaboration

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      The production of 𝑍 bosons in association with at least one 𝑏-jet and 𝑍 decaying into muons or electrons is studied. This analysis is done by using proton–proton collisions delivered by the Large Hadron Collider (LHC) at a centre of mass energy of 7 TeV and with data recorded by the CMS detector in 2010, representing an integrated luminosity of 36 pb-1. A final state of two wellidentified muons or electrons and at least one 𝑏-tagged jet are required in order to perform this study. The event yields and shapes of kinematic variables are compared with Monte Carlo predictions from MADGRAPH. The ratio $\sigma (Z + b)/\sigma (Z + j)$ is found to be $0.054 \pm 0.016 (0.046 \pm 0.014)$ in the data for the $Z \rightarrow ee (Z \rightarrow \mu\mu)$ selection, compared to $0.043 \pm 0.005 (0.047 \pm 0.005)$ estimated from NLO theory predictions.

    • One-loop corrections to the baryon axial vector current

      M A Hernández-Ruíz

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      The symmetry breaking corrections to the pion–baryon couplings vanish to first order in $1/N_{c}$, where $N_{c}$ is the number of colours. Loop graphs with octet and decuplet intermediate states cancel to various orders in $N_{c}$ as a consequence of the large-$N_{c}$ spin-flavour symmetry of QCD baryons. The baryon axial vector current is computed at one-loop order in heavy baryon chiral perturbation theory in the large Nc limit. $1/N_{c}$ corrections in the case of $g_{A}$ in QCD are presented here.

    • Leptonic minimal flavour violation in warped extra dimensions

      Abhishek M Iyer Sudhir K Vempati

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      Lepton mass hierarchies and lepton flavour violation are revisited in the framework of Randall–Sundrum models. Models with Dirac-type as well as Majorana-type neutrinos are considered. The five-dimensional 𝑐-parameters are fit to the charged lepton and neutrino masses and mixings using $\chi^{2}$ minimization. Leptonic flavour violation is shown to be large in these cases. Schemes of minimal flavour violation are considered for the cases of an effective LLHH operator and Dirac neutrinos and are shown to significantly reduce the limits from lepton flavour violation.

    • Measurement of the cross-section of $Z\gamma$ and limits on ADD models at the CMS with 7 TeV Large Hadron Collider data

      Sandhya Jain

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      The measurement of the inclusive cross-section for $Z\gamma$ production at LHC with 7 TeV proton–proton collision is presented. The electron and muon decay modes are used to reconstruct the 𝑍 boson. The total cross-section is measured for photon transverse energy greater than 10 GeV and with photon and charged lepton separation in the pseudorapidity-azimuthal angle plane greater than 0.7. This study is extended by a measurement of $Z(\nu \bar{\nu})\gamma$ cross-section. A search is performed for extra dimensions in the Arkani-Hamed, Dimopoulos, Dvali framework using the final state of a graviton and photon. The limits are extended with MD $> 1.25–1.31$ TeV for $n = 2-6$. The measurement is found to be in agreement with the Standard Model prediction.

    • Search for excited leptons in pp collisions at $\sqrt{s} = 7$ TeV

      Shilpi Jain on behalf of the CMS Collaboration

      More Details Abstract Fulltext PDF

      A search for excited leptons, carried out with the CMS detector in $pp$ collision at the LHC with $\sqrt{s} = 7$ TeV, is presented. The search has been performed for an associated production of a lepton and an oppositely charged excited lepton $pp \rightarrow ll^{*}$, followed by the decay $l^{*} \rightarrow l\gamma$ , resulting in $ll\gamma$ final state, where $l = e,\mu$. No excess above the Standard Model expectation is observed in the data. Interpreting the findings in the context of $l^{∗}$ production through four-fermion contact interactions and $l^{∗}$ decay via electroweak processes, upper limits are reported for $l^{∗}$ production at this collision energy and the exclusion region in the $\Lambda - M(l^{*})$ parameter space is extended beyond the previously established limits.

    • Search for the Higgs boson in the $H \rightarrow WW \rightarrow \ell \nu jj$ decay channel in $pp$ collisions at $\sqrt{s} = 7$ TeV with the ATLAS detector

      Dilip Kumar Jana

      More Details Abstract Fulltext PDF

      A search for a Higgs boson has been performed in the $H \rightarrow WW \rightarrow \ell \nu jj$ channel using 1.04 fb-1 of $pp$ collision data at $\sqrt{s} = 7$ TeV recorded with the ATLAS detector at the Large Hadron Collider. No significant excess of events is observed over the expected Standard Model (SM) background and limits on the Higgs boson production cross-section are derived for a Higgs boson mass in the range 240 GeV $< m_{\text{H}} < 600$ GeV. The best sensitivity is obtained for $m_{\text{H}} = 400$ GeV, where the 95% confidence level upper bound on the cross-section for $H \rightarrow WW$ production is 3.1 pb, or 2.7 times the Standard Model prediction.

    • Measurement of the Drell–Yan differential cross-section $d\sigma/dM$ at $\sqrt{s} = 7$ TeV

      M Jindal on behalf of the CMS Collaboration

      More Details Abstract Fulltext PDF

      The Drell–Yan differential cross-section is measured in proton–proton collisions at $\sqrt{s} = 7$ TeV, from a data sample collected with the CMS detector at the LHC and corresponding to an integrated luminosity of $36 \pm 1.4$ pb-1. The measured cross-section is normalized to the cross-section of the 𝑍-peak region, for both dimuon and dielectron final state, in the dilepton invariant mass range of 15–600 GeV/c2. The normalized cross-section values are quoted in the full phase-space and within the detector acceptance. The effect of final-state radiation is also studied and the measurements are correted for this. The measurements are compared to the theoretical predictions and are found to be in good agreement.

  • Pramana – Journal of Physics | News

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      Posted on July 25, 2019

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