Early LHC Research Articles

Supermodels for early LHC

We investigate which new physics signatures could be discovered in the first year of the LHC, beyond the expected sensitivity of the Tevatron data by the end of 2010. We construct “supermodels”, for which the LHC sensitivity even with only 10 pb−1 useful luminosity is greater than that of the Tevatron with 10 fb−1. The simplest supermodels involve s-channel resonances in the quark–antiquark and especially in the quark–quark channels. We concentrate on easily visible final states with small standard model backgrounds, and find that there are simple searches, besides those for Z′ states, which could discover new physics in early LHC data. Many of these are well-suited to test searches for “more conventional” models, often discussed for multi-fb−1 data sets.

Dilepton and four-lepton signals at the LHC in the Littlest Higgs model with T-parity violation

In the presence of the T-parity violating Wess-Zumino-Witten (WZW) anomaly term, the otherwise stable heavy photon A_H in the Littlest Higgs model with T-parity (LHT) decays to either Standard Model (SM) gauge boson pairs, or to SM fermions via loop diagrams. We make a detailed study of the collider signatures where the A_H can be reconstructed from invariant mass peaks in the opposite sign same flavor dilepton or the four-lepton channels. This enables us to obtain information about the fundamental symmetry breaking scale f in the LHT and thereby the low-lying mass spectrum of the theory. In addition, indication of the presence of the WZW term gives us hints of the possible UV completion of the LHT via strong dynamics. The crucial observation is that the sum of all production processes of heavy T-odd quark pairs has a sizeable cross-section at the LHC and these T-odd particles eventually all cascade decay down to the heavy photon A_H. We show that for certain regions of the parameter space with either a small f of around 500 GeV or relatively light T-odd quarks with a mass of around 400 GeV, one can reconstruct the A_H even at the early LHC run with \sqrt{s}=10 TeV and a modest integrated luminosity of 200 pb^{-1}. At \sqrt{s} = 14 TeV and with an integrated luminosity of 30 fb^{-1}, it is possible to cover a large part of the typical parameter space of the LHT, with the scale f up to 1.5 TeV and with T-odd quark masses almost up to 1 TeV. In this region of the parameter space, the mass of the reconstructed A_H ranges from 66 GeV to 230 GeV.

QCD motivated approach to soft interactions at high energy: Inclusive production

We extend our two-component Pomeron, Gotsman-Levin-Maor-Miller (GLMM) model, for soft high-energy scattering to single inclusive cross sections. To this end we present a suitable formulation which also includes the semienhanced Pomeron-particle vertex corrections. The available data on single inclusive density $(1/{\ensuremath{\sigma}}_{\mathrm{in}})d\ensuremath{\sigma}/dy$ in the c.m. energy range of 200--1800 GeV are well reproduced by our model. The just-published ALICE Collaboration point at 900 GeV is in excellent agreement with the calculations of our model. We also present predictions covering the complete LHC energy range which can be readily tested in the early low-luminosity LHC runs. The results presented in this paper provide additional support to our Pomeron model approach.

Non-universal minimal Z′ models: present bounds and early LHC reach

We consider non-universal 'minimal' Z' models, whose additional U(1) charge is a non-anomalous linear combination of the weak hypercharge Y, the baryon number B and the partial lepton numbers (L_e, L_mu, L_tau), with no exotic fermions beyond three standard families with right-handed neutrinos. We show that the observed pattern of neutrino masses and mixing can be fully reproduced by a gauge-invariant renormalizable Lagrangian, and flavor-changing neutral currents in the charged lepton sector are suppressed by a GIM mechanism. We then discuss the phenomenology of some benchmark models. The electrophilic B-3L_e model is significantly constrained by electroweak precision tests, but still allows to fit the hint of an excess observed by CDF in dielectrons but not in dimuons. The muonphilic B-3L_mu model is very mildly constrained by electroweak precision tests, so that even the very early phase of the LHC can explore significant areas of parameter space. We also discuss the hadrophobic L_mu-L_tau model, which has recently attracted interest in connection with some puzzling features of cosmic ray spectra.

New Physics at ATLAS and CMS Experiments with the First Data

A new era in particle physics is about to begin. The long-awaited pp collisions at LHC will take place later this year. The two general purpose experiments, ATLAS and CMS, will collect first physics data in collisions of 5 TeV proton beams, by which ∼200 pb−1 total data is expected to be delivered by the end of the first 2009-2010 run. The ATLAS and CMS detectors are designed as discovery machines for the missing piece in Standard Model – the Higgs boson – and for physics beyond the Standard Model. Both experiments have conducted detailed analyses to show the discovery potential for new physics, including SUSY, extra dimensions, and new heavy resonances. In this note, after giving a brief overview of the status of the two experiments, I will present results from ATLAS and CMS studies for the expected new physics reach with early LHC data.

Systematic event generator tuning for the LHC

In this article we describe Professor, a new program for tuning model parameters of Monte Carlo event generators to experimental data by parameterising the per-bin generator response to parameter variations and numerically optimising the parameterised behaviour. Simulated experimental analysis data is obtained using the Rivet analysis toolkit. This paper presents the Professor procedure and implementation, illustrated with the application of the method to tunes of the Pythia 6 event generator to data from the LEP/SLD and Tevatron experiments. These tunes are substantial improvements on existing standard choices, and are recommended as base tunes for LHC experiments, to be themselves systematically improved upon when early LHC data is available.

MinimalZ′ models: present bounds and early LHC reach

We consider `minimal' Z' models, whose phenomenology is controlled by only three parameters beyond the Standard Model ones: the Z' mass and two effective coupling constants. They encompass many popular models motivated by grand unification, as well as many arising in other theoretical contexts. This parameterization takes also into account both mass and kinetic mixing effects, which we show to be sizable in some cases. After discussing the interplay between the bounds from electroweak precision tests and recent direct searches at the Tevatron, we extend our analysis to estimate the early LHC discovery potential. We consider a center-of-mass energy from 7 towards 10 TeV and an integrated luminosity from 50 to several hundred pb^-1, taking all existing bounds into account. We find that the LHC will start exploring virgin land in parameter space for M_Z' around 700 GeV, with lower masses still excluded by the Tevatron and higher masses still excluded by electroweak precision tests. Increasing the energy up to 10 TeV, the LHC will start probing a wider range of Z' masses and couplings, although several hundred pb^-1 will be needed to explore the regions of couplings favored by grand unification and to overcome the Tevatron bounds in the mass region around 250 GeV.

Likelihood functions for supersymmetric observables in frequentist analyses of the CMSSM and NUHM1

On the basis of frequentist analyses of experimental constraints from electroweak precision data, (g−2) μ , B-physics and cosmological data, we investigate the parameters of the constrained MSSM (CMSSM) with universal soft supersymmetry-breaking mass parameters, and a model with common non-universal Higgs masses (NUHM1). We present χ 2 likelihood functions for the masses of supersymmetric particles and Higgs bosons, as well as BR(b→s γ), BR(B s →μ + μ −) and the spin-independent dark-matter scattering cross section, σ SI . In the CMSSM we find preferences for sparticle masses that are relatively light. In the NUHM1 the best-fit values for many sparticle masses are even slightly smaller, but with greater uncertainties. The likelihood functions for most sparticle masses are cut off sharply at small masses, in particular by the LEP Higgs mass constraint. Both in the CMSSM and the NUHM1, the coannihilation region is favored over the focus-point region at about the 3-σ level, largely but not exclusively because of (g−2) μ . Many sparticle masses are highly correlated in both the CMSSM and NUHM1, and most of the regions preferred at the 95% C.L. are accessible to early LHC running, though high-luminosity running would be needed to cover the regions allowed at the 3-σ levels. Some slepton and chargino/neutralino masses should be in reach at the ILC. The masses of the heavier Higgs bosons should be accessible at the LHC and the ILC in portions of the preferred regions in the (M A ,tan β) plane. In the CMSSM, the likelihood function for BR(B s →μ + μ −) is peaked close to the Standard Model value, but much larger values are possible in the NUHM1. We find that values of σ SI >10−10 pb are preferred in both the CMSSM and the NUHM1. We study the effects of dropping the (g−2) μ , BR(b→s γ), Ω χ h 2 and M h constraints, demonstrating that they are not in tension with the other constraints.

Light octet scalars, a heavy Higgs and minimal flavour violation

It is widely believed that existing electroweak data requires a Standard Model Higgs to be light while electroweak and flavour physics constraints require other scalars charged under the Standard Model gauge couplings to be heavy. We analyze the robustness of these beliefs within a general scalar sector and find both to be incorrect, provided that the scalar sector approximately preserves custodial symmetry and minimal flavour violation (MFV). We demonstrate this by considering the phenomenology of the Standard Model supplemented by a scalar having SUc(3) ? SUL(2) ? UY(1) quantum numbers (8,2)1/2 ? which has been argued [13] to be the only kind of exotic scalar allowed by MFV that couples to quarks. We examine constraints coming from electroweak precision data, direct production from LEPII and the Tevatron, and from flavour physics, and find that the observations allow both the Standard Model Higgs and the new scalars to be simultaneously light ? with masses ~ 100?GeV, and in some cases lighter. The discovery of such light coloured scalars could be a compelling possibility for early LHC runs, due to their large production cross section, ? ~ 100?pb. But the observations equally allow all the scalars to be heavy (including the Higgs), with masses ~ 1?TeV, with the presence of the new scalars removing the light-Higgs preference that normally emerges from fits to the electroweak precision data.

Lepton flavors at the early CERN LHC experiments as the footprints of the dark matter producing mechanisms

The minimal supergravity (mSUGRA) parameter space corresponding to light sleptons well within the reach of LHC and relatively light squarks and gluinos (mass {<=}1 TeV) has three regions consistent with the Wilkinson Microwave Anisotropy Probe data on dark matter relic density and direct mass bounds from LEP 2. Each region can lead to distinct leptonic signatures from squark-gluino events during the early LHC experiments (integrated luminosity {approx}10 fb{sup -1} or even smaller) due to the distinct features of the 2-body leptonic decay modes of the electroweak gauginos, produced in squark-gluino decay cascades, in each region. In the much studied stau-LSP coannihilation region with a vanishing common trilinear coupling (A{sub 0}) at the grand unified theory scale, a large fraction of the final states contains electrons and/or muons and e-{mu}-{tau} universality holds to a good approximation. In the not so well-studied scenarios with nonvanishing A{sub 0} both lightest supersymmetric particle (LSP) pair annihilation and stau-LSP coannihilation could contribute significantly to the dark matter relic density for even smaller squark-gluino masses. Our simulations using Pythia indicate that the corresponding signatures are final states rich in {tau}-leptons while final states with electrons and muons are suppressed leading to a violation of lepton universality.more » These features may be observed to a lesser extent even in the modified parameter space (with nonzero A{sub 0}) where the coannihilation process dominates. We also show that the generic m-leptons+n-jets+Ee{sub T} signatures without flavor tagging can also discriminate among the three scenarios. However, the signals become more informative if the {tau} and b-jet tagging facilities at the LHC experiments are utilized.« less

CMS search plans and sensitivity to new physics with dijets

The compact muon solenoid (CMS) experiment will use dijets to search for physics beyond the standard model during early LHC running. The inclusive jet cross section as a function of jet transverse momentum, with 10 pb−1 of integrated luminosity, is sensitive to contact interactions beyond the reach of the Tevatron. The dijet mass distribution will be used to search for dijet resonances coming from new particles, for example an excited quark. Additional sensitivity to the existence of contact interactions or dijet resonances can be obtained by comparing dijet rates in two distinct pseudorapidity regions.

Search for TeV scale physics in heavy flavor decays

The subject of heavy flavor decays as probes for physics at and beyond the TeV scale is covered from the experimental perspective. Emphasis is placed on the more traditional beyond the standard model topics that have potential for impact in the early LHC era, and that are in anticipation of the B factory upgrade(s). The aim is to explain the physics, without getting too involved in the details, whether experimental or theoretical, to give the interested non-expert a perspective on the flavor/TeV link. We cover the forefront topics of CP violation in b→s transitions involving penguin and box diagrams, and probes of charged Higgs, right-handed and scalar interactions. We touch briefly on ϒ decay, D 0 mixing, rare K decays, and lepton flavor violating probes in τ decay. Our own phenomenological work is often used for illustration.

Proton dissociation into three jets

We explore the possibility to observe hard exclusive three-jet production in early LHC runs, corresponding to diffractive dissociation of the incident proton into three jets with large but compensating transverse momenta. This process is sensitive to the proton unintegrated gluon distribution at small x and to the distribution of the three valence quarks in the proton at small transverse distances. The corresponding cross section is calculated using an approach based on kt factorization. According to our estimates, observation of hard diffractive three-jet production at LHC is feasible for jet transverse momenta q⊥∼5 GeV.

Early LHC measurements to check predictions for central exclusive production

We show how the early data runs of the LHC can provide valuable checks of the different components of the formalism used to predict the cross sections of central exclusive processes. The ‘soft’ rapidity gap survival factor can be studied in electroweak processes, such as W+gaps events, for which the bare amplitude is well known. The generalised gluon distribution, in the appropriate kinematic region, can be probed by exclusive Υ production. The perturbative QCD effects, especially the Sudakov-like factor, can be probed by exclusive two- and three-jet production. We discuss the possible role of enhanced absorptive corrections that would violate the soft–hard factorisation implied in the usual formalism, and we suggest ways that the LHC may explore their presence.

LHC "Price Tag"

The dollar figures quoted in Alexander Hellemans' article “CERN sets sights on an early LHC [Large Hadron Collider]” (News & Comment, [3 Jan., p. 19][1]), could be misinterpreted in the absence of appropriate clarifications. In particular, the $2 billion quoted as LHC's “price tag” includes the accelerator, but not the detectors, and is based on accounting practice at CERN (the European particle physics center). On the other hand, the $530 million from the United States is to fund equipment and materials for both accelerator and detectors, and the amount of that equipment and materials is determined on the basis of U.S. Department of Energy (DOE) and National Science Foundation (NSF) accounting practices. Unlike the DOE-NSF costing procedure, CERN accounting excludes a number of elements such as research and development and a substantial fraction of labor costs, which CERN covers and accounts for separately. Thus, the figures of $2 billion and $530 million are not at all comparable. DOE-NSF accounting practices would place the cost for the LHC accelerator and detectors at about $6 billion, with about two-thirds for the accelerator and one-third for the detectors. It is against these figures that the proposed U.S. contribution should be compared, namely, $200 million by DOE for the LHC accelerator and $330 million (DOE and NSF) for the two detectors. All of the U.S. accelerator funding and the vast majority of the detector funding will be used to provide materials and equipment from U.S. industry, national laboratories, and universities. [1]: /lookup/doi/10.1126/science.275.5296.19