arXiv subjects

JoAnne L. Hewett

Publications and source records attributed to JoAnne L. Hewett.

33 records · Page 2Linked to original sources

arXiv2011

LHC Predictions from a Tevatron Anomaly in the Top Quark Forward-Backward Asymmetry

We examine the implications of the recent CDF measurement of the top-quark forward-backward asymmetry, focusing on a scenario with a new color octet vector boson at 1-3 TeV. We study several models, as well as a general effective field theory, and determine the parameter space which provides the best simultaneous fit to the CDF asymmetry, the Tevatron top pair production cross section, and the exclusion regions from LHC dijet resonance and contact interaction searches. Flavor constraints on these models are more subtle and less severe than the literature indicates. We find a large region of allowed parameter space at high axigluon mass and a smaller region at low mass; we match the latter to an SU(3)xSU(3)/SU(3) coset model with a heavy vector-like fermion. Our scenario produces discoverable effects at the LHC with only 1-2 inverse femtobarns of luminosity at 7-8 TeV. Lastly, we point out that a Tevatron measurement of the b-quark forward-backward asymmetry would be very helpful in characterizing the physics underlying the top-quark asymmetry.

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arXiv2008

Effects of the Noncommutative Standard Model on WW scattering

We examine W pair production in the Noncommutative Standard Model constructed with the Seiberg-Witten map. Consideration of partial wave unitarity in the reactions WW to WW and e+e- to WW shows that the latter process is more sensitive and that tree-level unitarity is violated when scattering energies are of order a TeV and the noncommutative scale is below about a TeV. We find that WW production at the LHC is not sensitive to scales above the unitarity bounds. WW production in e+e- annihilation, however, provides a good probe of such effects with noncommutative scales below 300-400 GeV being excluded at LEP-II, and the ILC being sensitive to scales up to 10-20 TeV. In addition, we find that the ability to measure the helicity states of the final state W bosons at the ILC provides a diagnostic tool to determine and disentangle the different possible noncommutative contributions.

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arXiv2008

General Features of Supersymmetric Signals at the ILC: Solving the LHC Inverse Problem

We present the first detailed, large-scale study of the Minimal Supersymmetric Standard Model (MSSM) at a $\sqrt s=500$ GeV International Linear Collider, including full Standard Model backgrounds and detector simulation. We investigate 242 points in the MSSM parameter space, which we term models, that have been shown by Arkani-Hamed et al to be difficult to study at the LHC. In fact, these points in MSSM parameter space correspond to 162 pairs of models which give indistinguishable signatures at the LHC, giving rise to the so-called LHC Inverse Problem. We first determine whether the production of the various SUSY particles is visible above the Standard Model background for each of these parameter space points, and then make a detailed comparison of their various signatures. Assuming an integrated luminosity of 500 fb$^{-1}$, we find that only 82 out of 242 models lead to visible signatures of some kind with a significance $\geq 5$ and that only 57(63) out of the 162 model pairs are distinguishable at $5(3)σ$. Our analysis includes PYTHIA and CompHEP SUSY signal generation, full matrix element SM backgrounds for all $2\to 2, 2\to 4$, and $2\to 6$ processes, ISR and beamstrahlung generated via WHIZARD/GuineaPig, and employs the fast SiD detector simulation org.lcsim.

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arXiv2006

Black holes in many dimensions at the LHC: testing critical string theory

We consider black hole production at the LHC in a generic scenario with many extra dimensions where the Standard Model fields are confined to a brane. With $\sim 20$ dimensions the hierarchy problem is shown to be naturally solved without the need for large compactification radii. We find that in such a scenario the properties of black holes can be used to determine the number of extra dimensions, $n$. In particular, we demonstrate that measurements of the decay distributions of such black holes at the LHC can determine if $n$ is significantly larger than 6 or 7 with high confidence, and thus can probe one of the critical properties of string theory compactifications.

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arXiv2004

Signatures of long-lived gluinos in split supersymmetry

We examine the experimental signatures for the production of gluinos at colliders and in cosmic rays within the split supersymmetry scenario. Unlike in the MSSM, the gluinos in this model are relatively long-lived due to the large value of the squark masses which mediate their decay. Searches at colliders are found to be sensitive to the nature of gluino fragmentation as well as the gluino-hadron interactions with nuclei and energy deposition as it traverses the detector. We find that the worst-case scenario, where a neutral gluino-hadron passes through the detector with little energy deposition, is well described by a monojet signature. For this case, using Run I data we obtain a bound of $m_{\tilde g} > 170$ GeV; this will increase to 210(1100) GeV at Run II(LHC) if no excess events are observed. In the opposite case, where a charged gluino-hadron travels through the detector, a significantly greater reach is obtained via stable charged particle search techniques. We also examine the production of gluino pairs in the atmosphere by cosmic rays and show they are potentially observable at IceCube; this would provide a cross-check for observations at hadron colliders.

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arXiv2004

Higher dimensional models of light Majorana neutrinos confronted by data

We discuss experimental and observational constraints on certain models of higher dimensional light Majorana neutrinos. Models with flavor blind brane-bulk couplings plus three or four flavor diagonal light Majorana neutrinos on the brane, with subsequent mixing induced solely by the Kaluza-Klein tower of states, are found to be excluded by data on the oscillations of solar, atmospheric and reactor neutrinos, taken together with the WMAP upper bound on the sum of neutrino masses. Extra dimensions, if relevant to neutrino mixing, need to discriminate between neutrino flavors.

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arXiv2002

Phenomenology of Supersymmetric Large Extra Dimensions

We study the phenomenology of a supersymmetric bulk in the scenario of large extra dimensions. The virtual exchange of gravitino KK states in selectron pair production in polarized \epem collisions is examined. The leading order operator for this exchange is dimension six, in contrast to that of graviton KK exchange which induces a dimension eight operator at lowest order. Some kinematic distributions for selectron production are presented. These processes yield an enormous sensitivity to the fundamental higher dimensional Planck scale.

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arXiv2002

Supersymmetric Extra Dimensions: Gravitino Effects in Selectron Pair Production

We examine the phenomenological consequences of a supersymmetric bulk in the scenario of large extra dimensions. We assume supersymmetry is realized in the bulk and study the interactions of the resulting bulk gravitino Kaluza-Klein (KK) tower of states, with supersymmetry breaking on the brane inducing a light mass for the zero-mode gravitino. We derive the 4-d effective theory, including the couplings of the bulk gravitino KK states to fermions and their scalar superpartners. The virtual exchange of the gravitino KK states in selectron pair production in polarized \epem collisions is then examined. We find that the leading order operator for this exchange is dimension six, in contrast to that of bulk graviton KK exchange which induces a dimension eight operator at lowest order. The resulting kinematic distributions for selectron production are dramatically altered from those in D=4 supersymmetric scenarios, and can lead to a enormous sensitivity to the fundamental higher dimensional Planck scale, of order $20-25\times \sqrt s$.

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arXiv2001

Indirect Signatures of CP Violation in the Processes $γγ\to γγ$, $γZ$, and $ZZ$

This paper studies the utility of the processes $γγ\to γγ$, $γZ$, and $ZZ$ in searching for sources of CP violation arising from energy scales beyond the production thresholds of planned future colliders. In the context of an effective Lagrangian approach we consider the most general set of CP odd SU(2) $\times$ U(1) operators that give rise to genuinely quartic gauge boson couplings which can be probed in $2 \to 2$ scattering processes at a $γγ$ collider. We study each process in detail, emphasizing the complementary information that is obtained by varying the initial beam polarizations. Finally, we compare our results to other constraints in the literature on CP odd gauge boson interactions and quartic gauge boson couplings; the search reaches obtained here are typically stronger and nicely complement previous studies which have focused primarily on $W$ boson, top quark, or Higgs production.

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arXiv1998

Top-Charm Associated Production in High Energy $e^+e^-$ Collisions

The possibility of exploring the flavor changing neutral current $tcZ/tcγ$ couplings in the production vertex for the reaction $\epem\to t\bar c + \bar tc$ is examined. Using a model independent parameterization for the effective Lagrangian to describe the most general three-point interactions, production cross sections are found to be relatively small at LEP II, but potentially sizeable at higher energy \epem colliders. The kinematic characteristics of the signal are studied and a set of cuts are devised for clean separation of the signal from background. The resulting sensitivity to anomalous flavor changing couplings at LEP II with an integrated luminosity of $4\times 500$ pb$^{-1}$ is found to be comparable to their present indirect constraints from loop processes, while at higher energy colliders with $0.5-1$ TeV center-of-mass energy and 50-200 fb$^{-1}$ luminosity, one expects to reach a sensitivity at or below the percentage level.

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arXiv1997

Don't Stop Thinking About Leptoquarks: Constructing New Models

We discuss the general framework for the construction of new models containing a single, fermion number zero scalar leptoquark of mass $\simeq 200-220$ GeV which can both satisfy the D0/CDF search constraints as well as low energy data, and can lead to both neutral and charged current-like final states at HERA. The class of models of this kind necessarily contain new vector-like fermions with masses at the TeV scale which mix with those of the Standard Model after symmetry breaking. In this paper we classify all models of this type and examine their phenomenological implications as well as their potential embedding into SUSY and non-SUSY GUT scenarios. The general coupling parameter space allowed by low energy as well as collider data for these models is described and requires no fine-tuning of the parameters.

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arXiv1997

Much Ado About Leptoquarks: A Comprehensive Analysis

We examine the phenomenological implications of a 200 GeV leptoquark in light of the recent excess of events at HERA. Given the relative predictions of events rates in e^+p versus e^-p, we demonstrate that classes of leptoquarks may be excluded, including those contained in E_6 GUT models. It is shown that future studies with polarized beams at HERA could reveal the chirality of the leptoquark fermionic coupling and that given sufficient luminosity in each e^\pm_{L,R} channel the leptoquark quantum numbers could be determined. The implications of 200-220 GeV leptoquarks at the Tevatron are examined. While present Tevatron data most likely excludes vector leptoquarks and leptogluons in this mass region, it does allow for scalar leptoquarks. We find that while leptoquarks have little influence on Drell-Yan production, further studies at the Main Injector are possible in the single production channel. We investigate precision electroweak measurements as well as the process e^+e^-\to q\bar q at LEP II and find they provide no further restrictions on these leptoquark models. We then ascertain that cross section and polarization asymmetry measurements at the NLC provide the only direct mechanism to determine the leptoquark's electroweak quantum numbers. The single production of leptoquarks in γe collisions by both the backscattered laser and Weisacker-Williams techniques at the NLC is also discussed. Finally, we demonstrate that we can obtain successful coupling constant unification in models with leptoquarks, both with or without supersymmetry. The supersymmetric case requires the GUT group to be larger than SU(5) such as flipped SU(5)\times U(1)_X.

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arXiv1997

Discovery Potential for New Phenomena

We examine the ability of future facilities to discover and interpret non-supersymmetric new phenomena. We first explore explicit manifestations of new physics, including extended gauge sectors, leptoquarks, exotic fermions, and technicolor models. We then take a more general approach where new physics only reveals itself through the existence of effective interactions at lower energy scales. [Summary Report of the New Phenomena Working Group. To appear in the Proceedings of the 1996 DPF/DPB Summer Study on New Directions for High Energy Physics - Snowmass96, Snowmass, CO, 25 June - 12 July 1996.]

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arXiv1996

Searching for supersymmetry in rare B decays

We quantify the ability of B-Factories to observe supersymmetric contributions to the rare decays B -> X_s γand B -> X_s l+ l-. A global fit to the Wilson coefficients which contribute to these decays is performed from Monte Carlo generated data on B(B -> X_s γ) and the kinematic distributions associated with the final state lepton pair in B -> X_s l+ l-. This fit is then compared to supersymmetric predictions. Evaluation of the Wilson coefficients is carried out with several different patterns of the superpartner spectrum. We find that the B-Factories will be able to probe regions of the SUSY parameter space not accessible to LEPII, the Tevatron, and perhaps the LHC. We also employ the recent NLO calculation of the matrix elements for B -> X_s γand find the bound mH+ > 300 GeV in two-Higgs-doublet models using present data.

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arXiv1995

Radiative Weak Decays of Charm Mesons

We address Standard Model predictions for flavor-changing radiative transitions of the pseudoscalar charm mesons. Short-distance contributions in $D$ radiative transitions are contrasted with those in B decays. A full analysis is presented of the $c\to u+γ$ electromagnetic penguin amplitude with QCD radiative corrections included. Given the importance of long-range effects for the charm sector, special attention is paid to such contributions as the vector dominance and pole amplitudes. A number of two-body final states in exclusive charm radiative decays is considered and the corresponding branching ratio predictions are given.

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