Integrated Luminosity Research Articles

Top channel for early supersymmetry discovery at the LHC

Arguably the best-motivated channel for early LHC discovery is events including a high multiplicity of third generation quarks, such as four top quarks. For example generic string theories compactified to four dimensions with stabilized moduli typically have light gluinos with large branching ratios to $t$- and $b$-quarks. We analyze signals and background at 7 TeV LHC energy for $1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity, suggesting a reach for gluinos of about 650 GeV. A non-standard model signal from counting $b$-jets and leptons is robust, and provides information on the gluino mass, cross section, and spin.

Leptoquarks decaying to a top quark and a charged lepton at hadron colliders

We study the sensitivity of the Tevatron and the 7 TeV LHC to a leptoquark $S$ coupling to a top quark and a charged lepton $L$ ($=e$, $\ensuremath{\mu}$, or $\ensuremath{\tau}$). For the Tevatron, we focus on the case ${m}_{S}<{m}_{t}$, where the leptoquark pair production cross section is large, and the decay is three-body: $S\ensuremath{\rightarrow}Wb{L}^{\ifmmode\pm\else\textpm\fi{}}$. We argue that existing Tevatron observations could exclude ${m}_{S}\ensuremath{\lesssim}160\text{ }\text{ }\mathrm{GeV}$. For ${m}_{S}>{m}_{t}$, we show that the LHC experiments with low integrated luminosity could be sensitive to such leptoquarks decaying to $t{l}^{\ifmmode\pm\else\textpm\fi{}}$ with $l=\ensuremath{\mu}$ or $\ensuremath{\tau}$.

Searches for thet′of a fourth family

We study the detection of the ${t}^{\ensuremath{'}}$ of a fourth family during the early running of LHC with 7 TeV collision energy and $1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity. By the use of a neural network we show that it is feasible to search for the ${t}^{\ensuremath{'}}$ even with a mass close to the unitarity upper bound, which is in the 500 to 600 GeV range. We also present results for the Tevatron with $10\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. In both cases the search for a fourth family quark doublet can be significantly enhanced if one incorporates the contribution that the ${b}^{\ensuremath{'}}$ can make to a ${t}^{\ensuremath{'}}$-like signal. Thus, the bound on the mass of a degenerate quark doublet should be stronger than the bounds obtained by treating ${t}^{\ensuremath{'}}$ and ${b}^{\ensuremath{'}}$ in isolation.

Determining the squark mass at the LHC

We propose a new way to determine the squark mass based on the shape of di-jet invariant mass distribution of supersymmetry (SUSY) di-jet events at the Large Hadron Collider (LHC). Our algorithm, which is based on event kinematics, requires that the branching ratio $B(\tilde{q} \rightarrow q \tilde{z}_1)$ is substantial for at least some types of squarks, and that $m_{\tilde{z}_1}^2/m_{\tilde{q}}^2 \ll 1$. We select di-jet events with no isolated leptons, and impose cuts on the total jet transverse energy, $E_T^{tot}=E_T(j_1)+E_T(j_2)$, on $\alpha = E_T(j_2)/m_{jj}$, and on the azimuthal angle between the two jets to reduce SM backgrounds. The shape of the resulting di-jet mass distribution depends sensitively on the squark mass, especially if the integrated luminosity is sufficient to allow a hard enough cut on $E_T^{tot}$ and yet leave a large enough signal to obtain the $m_{jj}$ distribution. We simulate the signal and Standard Model (SM) backgrounds for 100 fb$^{-1}$ integrated luminosity at 14 TeV requiring $E_T^{tot}> 700$ GeV. We show that it should be possible to extract $m_{\tilde{q}}$ to within about 3% at 95% CL --- similar to the precision obtained using $m_{T2}$ --- from the di-jet mass distribution if $m_{\tilde{q}} \sim 650$ GeV, or to within $\sim 5$% if $m_{\tilde{q}}\sim 1$ TeV.

LHC Higgs production and decay in theT′model

At $\sqrt{s} = 7$ TeV, the standard model needs at least $10\, (fb)^{-1}$ integrated luminosity at LHC to make a definitive discovery of the Higgs boson. Using binary tetrahedral ($T^{'}$) discrete flavor symmetry, we discuss how the decay of the lightest $T'$ Higgs into $\gamma\,\gamma$ can be effectively enhanced and dominate over its decay into $b \, \bar{b}$. Since the two-photon final state allows for a clean reconstruction, a decisive Higgs discovery may be possible at 7 TeV with the integrated luminosity only of $\sim 1\, (fb)^{-1}$.

Electron-to-tau lepton flavor violation at the Electron-Ion Collider

We analyze the potential sensitivity of a search for $e\rightarrow\tau$ conversion at a proposed electron-ion collider (EIC) facility. To that end, we calculate the cross sections for $e\rightarrow\tau$ events in a leptoquark framework assuming that the leptoquark masses are on the order of several hundred GeV or more. Given present limits on leptoquarks from direct searches at HERA and rare decay processes, an EIC sensitive to 0.1 fb $e\rightarrow\tau$ cross sections could probe previously unexplored regions of parameter space for these lepton flavor violating events (assuming 90 GeV center-of-mass energy and 10 fb$^{-1}$ integrated luminosity). Depending on the species of leptoquark and flavor structure of the couplings, an EIC search could surpass the HERA and rare process sensitivity to $e\rightarrow\tau$ conversion amplitudes by as much as an order of magnitude or more. We also derive updated limits on quark flavor-diagonal LFV leptoquark interactions using the most recent BaBar $\tau\rightarrow e\gamma$ search. We find that limits from an EIC $e\rightarrow\tau$ search could be competitive with the most recent $\tau\rightarrow e\gamma$ limit for a subset of the quark flavor-diagonal leptoquark couplings. Using an SU(5) GUT model in which leptoquark couplings are constrained by the neutrino masses and mixing, we illustrate how observable leptoquark-induced $e\rightarrow\tau$ conversion can be consistent with stringent LFV limits imposed by $\mu\rightarrow e\gamma$ and $\mu\rightarrow e$ conversion searches.

Dijet angular distributions at $\sqrt{s}=14$ TeV

We present a Monte Carlo study of dijet angular distributions at $\sqrt{s}=14$ TeV. First we perform a next-to-leading order QCD study; we calculate the distributions in four different bins of dijet invariant mass using different Monte Carlo programs and different jet algorithms, and we also investigate the systematic uncertainties coming from the choice of the parton distribution functions and the renormalization and factorization scales. In the second part of this paper, we present the effects on the distributions coming from a model including gravitational scattering and black hole formation in a world with large extra dimensions. Assuming a 25% systematic uncertainty, we report a discovery potential for the mass bin $1 < M_{jj} < 2$ TeV at 10 pb$^{-1}$ integrated luminosity.

Direct Photon-Hadron Correlations Measured with PHENIX

Direct photon-hadron correlations greatly improve our ability to perform jet tomography in heavy-ion collisions because the momentum of the direct photon can be used to constrain the initial momentum of the opposing jet. By comparing the spectrum of away-side hadrons observed in heavy ion collisions to the spectrum seen in nucleon collisions we can quantify the medium modification to the fragmentation function due to energy loss of the away-side parton. High p T direct photon-hadron correlations have been measured with the PHENIX detector using a statistical subtraction method to remove the photon contribution from meson decays. The increased integrated luminosity in the most recent Au+Au RHIC run at s N N = 200 GeV provides substantially improved statistical precision and enhances the kinematic reach. These measurements are compared to PHENIX p+p results and several theoretical models of energy loss. In addition, we compare direct photon-hadron and π 0 -hadron correlations.

Heavy quarks above the top at the Tevatron

Recent developments in models with warped extra dimensions have opened new possibilities for vectorlike quark studies at hadron colliders. These new vectorlike quarks can mix sizably with light standard model quarks without violating low energy constraints. We perform a model-independent analysis to determine the Tevatron reach in the search for new quarks. We find that the Tevatron has great potential to observe such quarks via their electroweak single production due to their mixing with valence quarks. With $4(8)\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity, one may reach a $5\ensuremath{\sigma}$ statistical significance for a heavy quark of mass $580(630)\text{ }\text{ }\mathrm{GeV}$ if the heavy quark-Standard Model quark mixing parameter is order one.

GeV Majorana neutrinos in top-quark decay at the LHC

We explore the $\ensuremath{\Delta}L=2$ same-sign dilepton signal from top-quark decay via a Majorana neutrino at the LHC in the top anti-top pair production samples. The signature is same-sign dilepton plus multijets with no significant missing energy. The most optimistic region lies where the Majorana neutrino mass is between $15--65\text{ }\text{ }\mathrm{GeV}$. For $300\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity, it is possible to probe ${S}_{ij}$, the effective mixing parameter, to $\mathcal{O}({10}^{\ensuremath{-}5})$.

Unravelling strings at the CERN LHC

We construct LHC signature footprints for four semirealistic string/M theory vacua with a minimal supersymmetric standard model visible sector. We find that they all give rise to limited regions in LHC signature space and are qualitatively different from each other for understandable reasons. We also propose a technique in which correlations of LHC signatures can be effectively used to distinguish among these string theory vacua. We expect the technique to be useful for more general string vacua. We argue that further systematic analysis with this approach will allow LHC data to disfavor or exclude major 'corners' of string/M theory and favor others. The technique can be used with limited integrated luminosity and improved.

Early Standard Model physics and early discovery strategy in CMS

Although hadron colliders like the LHC are constructed for exploring new physics at higher energies, datasets will be largely dominated by Standard Model processes. Especially in the early stage of running, it will be important to understand this overwhelming Standard Model continuum prior to any dedicated search for new physics. This paper covers CMS perspectives for early Standard Model physics and early discovery of new processes. It is shown how key Standard Model processes can be measured at low integrated luminosity. Plans for early discovery of physics beyond the Standard Model with 1 fb −1 or less are then discussed.

Search for excited quarks inqq¯→γγat the CERN LHC

If quarks are composite particles, then excited states are expected to play a r\^ole in the Large Hadron Collider phenomena. Concentrating on virtual effects, and using a large part of the compact muon solenoid detection criteria, we present here a realistic examination of their effect in diphoton production at the CERN LHC. For various luminosities, we present the 99% confidence limit (CL) achievable in $\ensuremath{\Lambda}\ensuremath{-}{M}_{q*}$ parameter space where $\ensuremath{\Lambda}$ is the compositeness scale and ${M}_{{q}^{*}}$ the mass of the state. For a ${q}^{*}$ of mass 0.5 TeV, $\ensuremath{\Lambda}\ensuremath{\le}1.55(2.95)$ can be excluded at 99% CL with 30 $(200)\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity.

DetectingH→hhin the mirror model at the CERN Large Hadron Collider

The Higgs sector may play an important role in detecting mirror particles, which can be the candidates of dark matter and appear as missing energy in the detectors at the LHC. In this paper we worked out the Higgs boson spectrum and the Higgs couplings for the symmetric vacuum, namely ${v}_{1}={v}_{2}=v$, in the mirror model, and investigated the constraints from electroweak precision observables. Our study showed that electroweak precision observables have already constrained the Higgs boson sector severely. We then explored the Higgs boson phenomenology, and focused on the scenario that the heavier Higgs boson $H$ can decay into a pair of lighter Higgs bosons $h$. We proposed to study the invisible decay of the Higgs boson via the pair production of them, in which one Higgs boson decays into bottom quarks and the other decays invisibly. Our detail simulation for signals and backgrounds showed that the observation of the signal can reach $5\ensuremath{\sigma}$ significance for ${m}_{H}=260\text{ }\text{ }\mathrm{GeV}$ and ${m}_{h}=115\text{ }\text{ }\mathrm{GeV}$ with $10\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity at the LHC. Moreover the possible method to further suppress dominant $Zb\overline{b}$ background was discussed. We also simulated the signals and backgrounds for $H\ensuremath{\rightarrow}hh\ensuremath{\rightarrow}4b$. Our results showed that it is very difficult to isolate the signals from huge QCD continuum backgrounds.

Signatures for Majorana Neutrinos at Hadron Colliders

The Majorana nature of neutrinos may only be experimentally verified via lepton-number violating processes involving charged leptons. We explore the Delta L = 2 like-sign dilepton production at hadron colliders to search for signals of Majorana neutrinos. We find significant sensitivity for resonant production of a Majorana neutrino in the mass range of 10-80 GeV at the current run of the Tevatron with 2 fb(-1) integrated luminosity and in the range of 10-400 GeV at the CERN LHC with 100 fb(-1).

Electroweak precision measurements and collider probes of the standard model with large extra dimensions

The elementary particles of the standard model may reside in more than $3+1$ dimensions. We study the consequences of large compactified dimensions on scattering and decay observables at high-energy colliders. Our analysis includes global fits to electroweak precision data, indirect tests at high-energy electron-positron colliders (CERN LEP2 and NLC), and direct probes of the Kaluza-Klein resonances at hadron colliders (Fermilab Tevatron and CERN LHC). The present limits depend sensitively on the Higgs sector, both the mass of the Higgs boson and how many dimensions it feels. If the Higgs boson is trapped on a $(3+1)$-dimensional wall with the fermions, large Higgs boson masses (up to 500 GeV) and relatively light Kaluza-Klein mass scales (less than 4 Tev) can provide a good fit to precision data. That is, a light Higgs boson is not necessary to fit the electroweak precision data, as it is in the standard model. If the Higgs boson propagates in higher dimensions, precision data prefer a light Higgs boson (less than 260 GeV) and a higher compactification scale (greater than 3.8 TeV). Future colliders can probe much larger scales. For example, a 1.5 TeV electron-positron linear collider can indirectly discover Kaluza-Klein excitations up to 31 TeV if $500 {\mathrm{fb}}^{\ensuremath{-}1}$ integrated luminosity is obtained.

The Large Magellanic Cloud Globular Cluster NGC 1866: New Data, New Models, New Analysis

We present a new deep (down to V ~ 24) photometry of a wide region (~6' × 6') around the Large Magellanic Cloud globular cluster NGC 1866. Our sample is much larger (by more than a factor of 3) than any previous photometry and with a main sequence (MS) that may be considered complete, down to at least 3 mag below the brightest MS star; such an occurrence allows a meaningful and robust comparison with various theoretical scenarios produced by means of models computed with the evolutionary code FRANEC. Both age and present-day mass function slope, α, are derived by a fit to the available MS and by the use of the parameter Δσ, which is simply the difference, in σ, between the observed and the predicted integrated MS luminosity functions. Our main conclusions are as follows: (1) The adoption of standard models (i.e., computed by adopting the Schwarzschild criterion to fix the border of the convective core) allows a fair fit to the MS for an age of the order of 100–140 Myr and a present-day mass function having a slope α between 2.3 and 1.9, the exact values depending on the adopted distance modulus. It is moreover possible to reproduce the average He clump luminosity while the total number of stars predicted in the He clump is twice the observed value; this means that we reobtain and confirm the first finding of Becker & Mathews, according to whom the simple adoption of a classical scenario leads to a neat discrepancy concerning the prediction of the number of stars in the He clump. (2) The adoption of models computed by increasing the size of the convective core by a certain amount—i.e., 0.25Hp—leads to a fair fit to the MS only for a visual distance modulus (m - M)V 18.6, an age t 200 Myr, and a mass function slope α 2.2. In this case, the total number of He clump stars is well reproduced, although the luminosity function of the He clump itself is predicted to be systematically less luminous than observed. The previous conclusions are based on the assumption that there is no appreciable population of binaries in NGC 1866. Though there are not yet sufficient data on the frequency of binary systems in these clusters, we analyze how the previous scenarios would change if a consistent (30%) population of binary systems were present in the cluster. This choice is based on the fact that a fraction of binaries of the order of 30% has already been found in NGC 1818, a cluster similar to NGC 1866. The inclusion of a 30% binary population leads to the following additional conclusions: (3) The adoption of the standard models now leads to a good fit to the entire luminosity function—i.e., MS, turnoff, and He clump stars—for a visual distance modulus (m - M)V = 18.8, an age t 100 Myr, and a mass function slope α 2.4, thus largely removing the classical discrepancy between observed and predicted number of stars in the He-burning clump. The quoted visual distance modulus constrains the unreddened distance modulus (m - M)0 within 18.50 and 18.62, depending on the reddening (whose most common values available in the literature range from 0.05 to 0.10). (4) At variance with the last point, the fit obtained by using models computed with an enlarged convective core gets worse when a binary component is taken into account. This is because of the fact that the presence of binary systems increases the existing discrepancy between the observed and predicted clump luminosity, since the He clump is predicted to be even less luminous than in absence of binaries. As a consequence of this analysis, we think that the next step toward a proper understanding of NGC 1866 and similar clusters, must include the accurate determination of the frequency of binary systems, which we hope will be performed with the incoming Cycle 8 Hubble Space Telescope observations of NGC 1866.

Search for Chargino-Neutralino Associated Production at the Fermilab Tevatron Collider

We have searched in $p \bar{p}$ collisions at $\sqrt{s}$ = 1.8 TeV for events with three charged leptons and missing transverse energy. In the Minimal Supersymmetric Standard Model, we expect trilepton events from chargino-neutralino ($\chione \chitwo$) pair production, with subsequent decay into leptons. We observe no candidate $e^+e^-e^\pm$, $e^+e^-\mu^\pm$, $e^\pm\mu^+\mu^-$ or $\mu^+\mu^-\mu^\pm$ events in 106 pb$^{-1}$ integrated luminosity. We present limits on the sum of the branching ratios times cross section for the four channels: $\sigma_{\chione\chitwo}\cdot $BR$(\chione\chitwo\to 3\ell+X) < 0.34$~pb, M$_\chione > $ 81.5 \mgev\sp and M$_\chitwo > $ 82.2 \mgev\sp for $\tan\beta=2$, $\mu =-600$~\mgev\sp and M$_\squark=$ M$_\gluino$.

SUPERSYMMETRY AT LINEAR COLLIDERS: THE IMPORTANCE OF BEING e-e-

Advantages of the e-e- option at linear colliders for the study of supersymmetry are highlighted. The fermion number violating process [Formula: see text] provides unique opportunities for studies of slepton masses and flavor mixings. In particular, slepton mass measurements at the 100 MeV level through threshold scans of scalar pair production may be possible. Such measurements are over an order of magnitude better than those possible in e+e- mode, require far less integrated luminosity, and may lead to precise, model-independent measurements of tan β. Implications for studying gauginos and the importance of accurate beam polarimetry are also discussed.

Unveiling palomar 2: The most obscure globular cluster in the outer halo.

view Abstract Citations (10) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Unveiling palomar 2: The most obscure globular cluster in the outer halo. Harris, William E. ; Durrell, Patrick R. ; Petitpas, Glen R. ; Webb, Tracy M. ; Woodworth, Sean C. Abstract We present the first color-magnitude study for Palomar 2, a distant and heavily obscured globular cluster near the Galactic anticenter. Our (V,V-I) color-magnitude diagram (CMD), obtained with the UH8K camera at the CFHT, reaches V(lim) =~ 24 and clearly shows the principal sequences of the cluster, though with substantial overall foreground absorption and differential reddening. The CMD morphology shows a well populated red horizontal branch with a sparser extension to the blue, similar to clusters such as NGC 1261, 1851, or 6229 with metallicities near [Fe/H] =~ -1.3. From an average of several indicators, we estimate the foreground reddening at E(B-V) = 1.24 +/- 0.07 and obtain a true distance modulus (m-M)_0 = 17.1 +/- 0.3, placing it =~ 34 kpc from the Galactic center. We use starcounts of the bright stars to measure the core radius, half-mass radius, and central concentration of the cluster. Its integrated luminosity is M_V(t) =~ -7.9, making it clearly brighter and more massive than most other clusters in the outer halo. Very rough arguments based on its half-mass radius and radial velocity suggest that Palomar 2 is now moving in toward perigalacticon on a highly elliptical orbit (e gtrsim 0.7). Publication: The Astronomical Journal Pub Date: September 1997 DOI: 10.1086/118534 arXiv: arXiv:astro-ph/9707049 Bibcode: 1997AJ....114.1043H Keywords: GALAXY: HALO; GLOBULAR CLUSTERS: INDIVIUAL: PALOMAR 2; Astrophysics E-Print: 25 pages, aastex, with 8 postscript figures full text sources arXiv | ADS | data products SIMBAD (13)