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Xerxes Tata

Publications and source records attributed to Xerxes Tata.

At least 109 records · Page 6Linked to original sources
arXiv1998

The Reach of CERN LEP2 and Fermilab Tevatron Upgrades for Higgs Bosons in Supersymmetric Models

Luminosity upgrades of the Fermilab Tevatron pbar-p collider have been shown to allow experimental detection of a Standard Model (SM) Higgs boson up to $m_{H_{SM}}\sim 120$ GeV via $WH_{SM} \to \ellνb\bar{b}$ events. This limit nearly saturates the parameter space for many models of weak scale supersymmetry (SUSY) with a minimal particle content. It is therefore interesting to examine the SUSY Higgs reach of future Tevatron experiments. Contours are presented of Higgs boson reach for CERN LEP2 and Tevatron luminosity upgrades for three models of weak scale SUSY: the Minimal Supersymmetric Standard Model (MSSM), the minimal Supergravity model (mSUGRA) and a simple Gauge Mediated SUSY Breaking Model (GMSB). In each case we find a substantial gain in reach at the Tevatron with integrated luminosity increasing from 10 fb^{-1} to 25-30 fb^{-1}. With the larger integrated luminosity, a Higgs search at the Tevatron should be able to probe essentially the entire parameter space of these models. While a discovery would be very exciting, a negative result would severely constrain our ideas about how weak scale supersymmetry is realized.

hep-ph↗
arXiv1998

Probing Minimal Supergravity at the CERN LHC for Large $\tanβ$

For large values of the minimal supergravity model parameter $\tanβ$, the tau lepton and the bottom quark Yukawa couplings become large, leading to reduced masses of $τ$-sleptons and $b$-squarks relative to their first and second generation counterparts, and to enhanced decays of charginos and neutralinos to $τ$-leptons and $b$-quarks. We evaluate the reach of the CERN LHC $pp$ collider for supersymmetry in the mSUGRA model parameter space. We find that values of $m_{\tg}\sim 1500-2000$ GeV can be probed with just 10 fb$^{-1}$ of integrated luminosity for $\tanβ$ values as high as 45, so that mSUGRA cannot escape the scrutiny of LHC experiments by virtue of having a large value of $\tanβ$. We also perform a case study of an mSUGRA model at $\tanβ=45$ where $\tz_2\to τ\ttau_1$ and $\tw_1\to \ttau_1ν_τ$ with $\sim 100%$ branching fraction. In this case, at least within our simplistic study, we show that a di-tau mass edge, which determines the value of $m_{\tz_2}-m_{\tz_1}$, can still be reconstructed. This information can be used as a starting point for reconstructing SUSY cascade decays on an event-by-event basis, and can provide a strong constraint in determining the underlying model parameters. Finally, we show that for large $\tanβ$ there can be an observable excess of $τ$ leptons, and argue that $τ$ signals might serve to provide new information about the underlying model framework.

hep-ph↗
arXiv1998

Searching for Bottom Squarks at Luminosity Upgrades of the Fermilab Tevatron

Because of their Yukawa interactions, third generation squarks may be substantially lighter than those of the first two generations. Assuming that $\tb_1 \to b\tz_1$ and $\tz_1$ escapes experimental detection, we show that experiments at the Main Injector upgrade (integrated luminosity of $2 fb^{-1}$) of the Tevatron should be sensitive to $b$-squark masses up to 210 GeV for $m_{\tz_1} \leq 120$ GeV. For integrated luminosities of $10 fb^{-1}$ (25 $fb^{-1}$) the sbottom mass reach increases by 20 GeV (35 GeV). If the channel $\tb_1 \to b\tz_2$ is also accessible, the reach becomes model-dependent and may be degraded relative to the case where only the decay to $\tz_1$ is allowed. In models with gaugino mass unification and $μ$ much larger than gaugino masses, we argue that this degradation is unlikely to be larger than 30-40 GeV.

hep-ph↗
arXiv1998

Developments in Supersymmetry Phenomenology

We survey strategies generally employed for SUSY discovery at colliders and then discuss how these may have to be altered for SUSY searches at the Tevatron if $\tanβ$ is large. We also discuss the reach of the Tevatron and the LHC in gauge-mediated SUSY breaking scenarios, assuming that the NLSP decays into photons. Finally, we briefly recapitulate measurements (which serve to guide us to the underlying theory) that might be possible at future colliders if supersymmetry is discovered.

hep-ph↗
arXiv1998

LHC Reach for Gauge Mediated Supersymmetry Breaking Models Via Prompt Photon Channels

We evaluate the supersymmetry reach of the Large Hadron Collider within the gauge-mediated low energy supersymmetry breaking framework, assuming that a neutralino is the second lightest sparticle and that it decays promptly into a gravitino which escapes detection. We find that the maximum reach is obtained via a search for inclusive $γγ+\eslt$ events coming dominantly from chargino and neutralino production. Assuming an integrated luminosity of 10~$fb^{-1}$, we find that LHC experiments will be able to probe values of the model parameter $Λ\alt 400$ TeV, corresponding to $m_{\tg} \leq 2.8$ TeV. A measure of the model parameter $Λ$ may be possible from the photon $p_T$ spectrum.

hep-ph↗
arXiv1998

Supersymmetry Reach of Tevatron Upgrades: The Large $\tanβ$ Case

The Yukawa couplings of the tau lepton and the bottom quark become comparable to, or even exceed, electroweak gauge couplings for large values of the SUSY parameter $\tanβ$. As a result, the lightest tau slepton $\ttau_1$ and bottom squark $\tb_1$ can be significantly lighter than corresponding sleptons and squarks of the first two generations. Gluino, chargino and neutralino decays to third generation particles are significantly enhanced when $\tanβ$ is large. This affects projections for collider experiment reach for supersymmetric particles. In this paper, we evaluate the reach of the Fermilab Tevatron $p\bar p$ collider for supersymmetric signals in the framework of the mSUGRA model. We find that the reach via signatures with multiple isolated leptons ($e$ and $μ$) is considerably reduced. For very large $\tanβ$, the greatest reach is attained in the multi-jet$+\eslt$ signature. Some significant extra regions may be probed by requiring the presence of an identified $b$-jet in jets$+\eslt$ events, or by requiring one of the identified leptons in clean trilepton events to actually be a hadronic 1 or 3 charged prong tau. In an appendix, we present formulae for chargino, neutralino and gluino three body decays which are valid at large $\tanβ$.

hep-ph↗
arXiv1997

$b\to sγ$ Constraints on the Minimal Supergravity Model with Large $\tanβ$

In the minimal supergravity model (mSUGRA), as the parameter $\tanβ$ increases, the charged Higgs boson and light bottom squark masses decrease, which can potentially increase contributions from $tH^\pm$, $\tg\tb_j$ and $\tz_i\tb_j$ loops in the decay $b\to sγ$. We update a previous QCD improved $b\to sγ$ decay calculation to include in addition the effects of gluino and neutralino loops. We find that in the mSUGRA model, loops involving charginos also increase, and dominate over $tW$, $tH^\pm$, $\tg\tq$ and $\tz_i\tq$ contributions for $\tanβ\agt 5-10$. We find for large values of $\tanβ\sim 35$ that most of the parameter space of the mSUGRA model for $μ<0$ is ruled out due to too large a value of branching ratio $B(b\to sγ)$. For $μ>0$ and large $\tanβ$, most of parameter space is allowed, although the regions with the least fine-tuning (low $m_0$ and $m_{1/2}$) are ruled out due to too low a value of $B(b\to sγ)$. We compare the constraints from $b\to sγ$ to constraints from the neutralino relic density, and to expectations for sparticle discovery at LEP2 and the Fermilab Tevatron $p\bar p$ colliders. Finally, we show that non-universal GUT scale soft breaking squark mass terms can enhance gluino loop contributions to $b\to sγ$ decay rate even if these are diagonal.

hep-ph↗
arXiv1997

What Is Supersymmetry And How Do We Find It?

In these Lectures, we present a pedagogical introduction to weak scale supersymmetry phenomenology. A basic understanding of the Standard Model and of the ideas behind Grand Unification, but no prior knowledge of supersymmetry, is assumed. Topics covered include: What is supersymmetry and why do we bother with it? Working with a supersymmetric theory: A toy example Construction of supersymmetric Lagrangians The Minimal Supersymmetric Model The mSUGRA Model: A paradigm for SUSY phenomenology Decays of supersymmetric particles Production of supersymmetric particles at colliders Observational constraints on supersymmetry Supersymmetry searches at future colliders Constraining supersymmetry models at future colliders R-parity violation Gauge-mediated supersymmetry breaking

hep-ph↗
arXiv1997

Collider Phenomenology for Supersymmetry at Large $tanβ$

If the parameter $tan β$ of the minimal supersymmetric model is large, then $b$ and $τ$ Yukawa interactions are important. These can significantly modify the masses and decays of sparticles. We describe new calculations which allow a reliable exploration of large $tan β$ values, and discuss implications for collider experiments. For large values of $tan β$, charginos and neutralinos may dominantly decay to $τ$-leptons or $b$-quarks. The usual cross sections for multilepton signatures may be greatly reduced, but SUSY may be detectable via new signals involving $τ$'s or $b$'s in the final state.

hep-ph↗
arXiv1996

Signals for the Minimal Gauge-mediated Supersymmetry Breaking Model at the Fermilab Tevatron Collider

We investigate the experimental implications of the minimal gauge-mediated low energy supersymmetry breaking (GMLESB) model for Fermilab Tevatron collider experiments. We map out the regions of parameter space of this model that have already been excluded by collider searches and by limits on $b\to sγ$. We use ISAJET to compute the cross sections for a variety of topological signatures which include photons in assocation with multiple leptons, jets and missing transverse energy. The reach in the parameter $Λ$, which fixes the scale of sparticle masses, is estimated to be $\sim 60$, 100 and 135 TeV for Tevatron integrated luminosities of 0.1, 2 and 25 fb$^{-1}$, respectively. The largest signals occur in photon(s) plus lepton(s) plus multi-jet channels; jet-free channels containing just photons plus leptons occur at much smaller rates, at least within this minimal framework.

hep-ph↗
arXiv1996

Impact of Hadronic Decays of the Lightest Neutralino on the Reach of the CERN LHC

If $R$-parity is not conserved, the lightest supersymmetric particle (LSP) could decay via lepton number violating or baryon number violating interactions. The latter case is particularly insidious since it leads to a reduction of the $\eslt$ as well as leptonic signals for supersymmetry. We evaluate cross sections for jets plus $\eslt$, 1$\ell$, $2\ell$ (same-sign and opposite sign) and $3\ell$ event topologies that result from the simultaneous production of all sparticles at the CERN Large Hadron Collider (LHC), assuming that the lightest supersymmetric particle $\tz_1$ decays hadronically inside the detector via $R$-parity violating interactions. We assume that these interactions do not affect the production rates or decays of other sparticles. We examine the SUSY reach of the LHC for this ``pessimistic'' scenario, and show that experiments at the LHC will still be able to search for gluinos and squarks as heavy as 1 TeV, given just 10 fb$^{-1}$ of integrated luminosity, even with cuts designed to explore the canonical SUSY framework with a {\it conserved} $R$-parity.

hep-ph↗
arXiv1996

Supersymmetry Studies at Future Linear $e^+e^-$ Colliders

We examine various aspects of supersymmetric particle production at linear $e^+e^-$ colliders operating at a centre of mass energy of $\sqrt{s}=500$ GeV, and integrated luminosity of $\int {\cal L}dt=20-50$ fb$^{-1}$. Working within the framework of the minimal supergravity model with gauge coupling unification and radiative electroweak symmetry breaking (SUGRA), we study various signatures for detection of sparticles, taking into account their cascade decays, and map out the regions of parameter space where these are observable. We also examine strategies to isolate different SUSY processes from another. In addition, we perform four detailed SUGRA case studies and examine the detectability of sparticles when several SUSY processes are simultaneously occuring. We show that precision mass measurements of neutralinos, sneutrinos and top-squarks are possible, in addition to previously studied precision mass measurements of sleptons and charginos.

hep-ph↗
arXiv1996

Simultaneous Search for Two Higgs Bosons of Minimal Supersymmetry at the LHC

The prospects of detecting the CP-odd Higgs pseudoscalar ($A$) in the minimal supersymmetric model via its decay into a $Z$ boson and the lighter CP-even Higgs scalar ($h$) at the CERN Large Hadron Collider are investigated. The final state of $Z \to l^+l^-$ and $h \to b\bar{b}$, may provide a promising way to simultaneously detect the $A$ and the $h$. The compact muon solenoid detector performance is adopted for a realistic study of observability. In this discovery channel, the masses of the $h$ and the $A$ can be reconstructed. The impact of supersymmetric decay modes is considered.

hep-ph↗
arXiv1996

Sonoluminescence in Neutron Stars

After a brief discussion of a possible relationship between the electroweak phase transition in highly compressed matter and gravitational collapse, we examine the speculative possibility that the electroweak phase transition might be contemporarily occurring in processes in neutron stars. We conjecture that adiabatic compression of neutron star matter due to focusing of the energy from a supernova bounce into a very small volume could result in extreme densities, and Fermi levels or temperature above $\cal{O}$ (100 GeV). We propose a qualitative scenario for sonoluminescence in neutron stars and discuss possible observable consequences.

astro-ph↗
arXiv1995

Signals for Minimal Supergravity at the CERN Large Hadron Collider II: Multilepton Channels

We use ISAJET to perform a detailed study of the multilepton signals expected from cascade decays of supersymmetric particle produced at the CERN LHC. Our analysis is performed within the framework of the minimal supergravity model with gauge coupling unification and radiative electroweak symmetry breaking. We delineate the regions of parameter space where jets plus missing energy plus 1, 2 (opposite sign and same-sign dileptons), and 3 isolated lepton events should be visible above standard model backgrounds. We find that if any $\eslt$ signal at the LHC is to be attributed to gluino and/or squark production, and if $m_{\tg} \alt 1$~TeV, then several of these signals must be simultaneously observable. Furthermore, assuming $10\,\fb^{-1}$ of integrated luminosity, we find that the reach for supersymmetry in the 1$\ell + \jets +\eslt$ channel extends to $m_{\tg}\sim 2300\ (1600)$ GeV for $m_{\tq}\sim m_{\tg}$ ($m_{\tq} \sim 1.5m_{\tg}$), and exceeds the corresponding reach in the $0\ell+\eslt$ channel. We show that measurements of the various topological cross sections, jet and $B$-hadron multiplicities in these events, together with the charge asymmetry for single lepton and same-sign dilepton events, and flavor asymmetry for opposite sign dilepton events, serve to narrow the allowed range of underlying SUGRA parameter values. We also delineate parameter regions where signals with clean isolated dilepton (from slepton production) and trilepton events (from chargino/neutralino production) are visible at the LHC, and examine the extent to which these signals can be separated from other SUSY sources.

hep-ph↗
arXiv1995

Supersymmetry Phenomenology: A Microreview

We briefly review the current status and future prospects for supersymmetry searches at colliders, and discuss strategies by which further information about sparticle properties may be obtained at the LHC.

hep-ph↗
arXiv1995

Supersymmetry: Where it is and how to find it

We present a pedagogical, but by no means complete, review of weak scale supersymmetry phenomenology. After a general introduction to the new particles that must be present in any supersymmetric framework, we describe how to write down their interactions with one another as well as with the particles of the Standard Model. We then elucidate the assumptions underlying the Minimal Supersymmetric Model as well as the more restrictive minimal supergravity GUT model with the radiative breaking of electroweak symmetry. These models serve to guide our thinking about the implications of supersymmetry for experiments. To facilitate our study of signatures of supersymmetric particles at high energy colliders, we describe the decay patterns of sparticles as well as their production mechanisms in $e^+e^-$ and hadron-hadron collisions. We then discuss how sparticles may be searched for in on-going experiments at the Tevatron and at LEP. We review phenomenological constraints on supersymmetric particle masses from non-observation of any signals in these experiments, and also briefly discuss constraints from low energy experiments and from cosmology. Next, we study new strategies by which supersymmetric particles may be searched for at supercolliders, and also what we can learn about their properties (masses, spins, couplings) in these experiments. A determination of sparticle properties, we will see, may provide us with clues about the nature of physics at the ultra-high scale. After a brief discussion of possible extensions of the minimal framework and the implications for phenomenology, we conclude with our outlook for the future.

hep-ph↗