arXiv subjects

Xerxes Tata

Publications and source records attributed to Xerxes Tata.

At least 37 records · Page 2Linked to original sources

arXiv2017

Natural SUSY with a bino- or wino-like LSP

In natural SUSY models higgsinos are always light because μ^2 cannot be much larger than M_Z^2, while squarks and gluinos may be very heavy. Unless gluinos are discovered at LHC13, the commonly assumed unification of gaugino mass parameters will imply correspondingly heavy winos and binos, resulting in a higgsino-like LSP and small inter-higgsino mass splittings. The small visible energy release in higgsino decays makes their pair production difficult to detect at the LHC. Relaxing gaugino mass universality allows for relatively light winos and binos without violating LHC gluino mass bounds and without affecting naturalness. In the case where the bino mass M_1<~ μ, then one obtains a mixed bino-higgsino LSP with instead sizable w_1-z_1 and z_2-z_1 mass gaps. The thermal neutralino abundance can match the measured dark matter density in contrast to models with a higgsino-like LSP where WIMPs (weakly interacting massive particles) are underproduced by factors of 10-15. If instead M_2<~ μ, then one obtains a mixed wino-higgsino LSP with large z_2-z_1 but small w_1-z_1 mass gaps with still an under-abundance of thermally-produced WIMPs. Portions of the light wino parameter space may already be excluded by the IceCube upper bound on the spin-dependent neutralino-nucleon cross section. We discuss dark matter detection in other direct and indirect detection experiments and caution that the bounds from these must be interpreted with care. Finally, we show that LHC13 experiments should be able to probe these non-universal mass scenarios via a variety of channels including multi-lepton + etmiss events, WZ+etmiss events, Wh+etmiss events and W^\pm W^\pm +etmiss events from electroweak chargino and neutralino production.

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arXiv2016

Indirect Searches of the Degenerate MSSM

A degenerate sfermionic particle spectrum can escape constraints from flavor physics, and at the same time evade the limits from the direct searches if the degeneracy extends to the gaugino-higgsino sector. Inspired by this, we consider a scenario where all the soft terms have an approximately common mass scale at $M_{\text{SUSY}}$, with splittings $\lesssim \mathcal{O}(10\%)$. As a result, the third generation sfermions have large to maximal (left-right) mixing, the same being the case with charginos and some sectors of the neutralino mass matrix. We study this scenario in the light of discovery of the Higgs boson with mass $\sim$ 125 GeV. We consider constraints from $B$-physics, the anomalous magnetic moment of the muon and the dark matter relic density. We find that a supersymmetric spectrum as light as 600 GeV could be consistent with all current data and also account for the observed anomalous magnetic moment of the muon within $2σ$. The neutralino relic density is generally too small to saturate the measured cold dark matter relic density. Direct detection limits from XENON100 and LUX put severe constraints on this scenario which will be conclusively probed by XENONnT experiment.

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arXiv2016

Gluino reach and mass extraction at the LHC in radiatively-driven natural SUSY

Radiatively-driven natural SUSY (RNS) models enjoy electroweak naturalness at the $10\%$ level while respecting LHC sparticle and Higgs mass constraints. Gluino and top squark masses can range up to several TeV (with other squarks even heavier) but a set of light Higgsinos are required with mass not too far above $m_h\sim 125$ GeV. Within the RNS framework, gluinos dominantly decay via ${\tilde g} \to t{\tilde t}_1^{*},\ \bar{t}{\tilde t}_1 \to t\bar{t}{\widetilde Z}_{1,2}$ or $t\bar{b}{\widetilde W}_1^-+c.c.$, where the decay products of the higgsino-like ${\widetilde W}_1$ and ${\widetilde Z}_2$ are very soft. Gluino pair production is, therefore, signalled by events with up to four hard $b$-jets and large ${\not\!\!{E_T}}$. We devise a set of cuts to isolate a relatively pure gluino sample at the (high luminosity) LHC and show that in the RNS model with very heavy squarks, the gluino signal will be accessible for $m_{\tilde g} < 2400 \ (2800)$ GeV for an integrated luminosity of 300 (3000) fb$^{-1}$. We also show that the measurement of the rate of gluino events in the clean sample mentioned above allows for a determination of $m_{\tilde g}$ with a statistical precision of $2.5-5\%$ (depending on the integrated luminosity and the gluino mass) over the range of gluino masses where a $5σ$ discovery is possible at the LHC.

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arXiv2016

Multi-channel assault on natural supersymmetry at the high luminosity LHC

Recent clarifications of naturalness in supersymmetry robustly require the presence of four light higgsinos with mass ~100-300 GeV while gluinos and (top)-squarks may lie in the multi-TeV range, possibly out of LHC reach. We project the high luminosity (300-3000 fb^{-1}) reach of LHC14 via gluino cascade decays and via same-sign diboson production. We compare these to the reach for neutralino pair production \tz_1\tz_2 followed by \tz_2\to\tz_1\ell^+\ell^- decay to soft dileptons which recoil against a hard jet. It appears that 3000 fb^{-1} is just about sufficient integrated luminosity to probe naturalness with up to 3\% fine-tuning at the 5-sigma level, thus either discovering natural supersymmetry or else ruling it out.

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arXiv2015

Same-sign Higgsino Production at the CERN LHC: How Not to Hunt for Natural Supersymmetry

We examine the prospects for detecting light charged higgsinos that are expected to be a necessary feature of natural SUSY models via $pp\to {\widetilde W}^\pm{\widetilde W}_1^\pm jj+X$ processes arising dominantly from $W^\pm W^\pm$ fusion at LHC13. The signal will be a pair of same-sign leptons ($e$ or $μ$) in events with two relatively forward, hemispherically-separated jets with a large rapidity gap. We find that even though the higgsinos have a full-strength $SU(2)$ gauge couplings to $W$-bosons, the LHC13 cross section for the production of same sign higgsino pairs is smaller than 0.02 fb over most of the interesting range of natural SUSY parameters, even before leptonic branching fractions of the chargino are included. This cross section is strongly suppressed because the two neutral Majorana higgsinos can be combined into a single Dirac neutralino if the bino and the winos are much heavier than the higgsinos, as is the case in natural SUSY models: in this limit, higgsino couplings to $W$-bosons exhibit an emergent (approximate) $U(1)_{\rm ino}$ global symmetry that suppresses same sign higgsino production by vector boson fusion. We conclude that this channel is not a viable way to search for natural SUSY even at the high luminosity upgrade of the Large Hadron Collider.

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arXiv2015

Supersymmetry: Aspirations and Prospects

The realization in the early 1980s that weak scale supersymmetry stabilizes the Higgs sector of the spectacularly successful Standard Model led several authors to explore whether low energy supersymmetry could play a role in particle physics. Among these were Richard Arnowitt, Ali Chamseddine and Pran Nath who constructed a viable {\em locally} supersymmetric Grand Unified Theory (GUT), laying down the foundation for supergravity GUT models of particle physics. Supergravity models continue to be explored as one of the most promising extensions of the Standard Model. After a quick overview of some of the issues and aspirations of early researchers working to bring supersymmetry into the mainstream of particle physics, we re-examine early arguments that seemed to imply that superpartners would be revealed in experiments at LEP2 or at the Tevatron. Our purpose is to assess whether the absence of any superpartners in searches at LHC8 presents a crisis for supersymmetry. Toward this end, we re-evaluate fine-tuning arguments that lead to upper bounds on (some) superpartner masses. We conclude that phenomenologically viable superpartner spectra that could arise within a high scale model tuned no worse than a few percent are perfectly possible. While no viable underlying model of particle physics that leads to such spectra has yet emerged, we show that the (supergravity-based) Radiatively-driven Natural Supersymmetry (RNS) framework serves as a surrogate for a phenomenological analysis of an underlying theory with modest fine-tuning. We outline the phenomenological implications of this framework, with emphasis on those LHC and electron-positron collider signatures that might point to the underlying natural origin of gauge and Higgs boson masses. We conclude that the supergravity GUT paradigm laid down in 1982 by Arnowitt, Chamseddine and Nath, and others, remains a vibrant possibility.

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arXiv2015

Supersymmetry, Naturalness, and Light Higgsinos

We compare and contrast three different sensitivity measures, $Δ_{EW}^{-1}$, $Δ_{HS}^{-1}$ and $Δ_{BG}^{-1}$ that have been used in discussions of fine-tuning. We argue that though not a fine-tuning measure, $Δ_{EW}$, which is essentially determined by the particle spectrum, is important because $Δ_{EW}^{-1}$ quantifies the minimum fine-tuning present in any theory with a specified spectrum. We emphasize the critical role of incorporating correlations between various model parameters in discussions of fine-tuning. We provide toy examples to show that if we can find high scale theories with specific correlations amongst parameters, the value of the traditional fine-tuning measure $Δ_{BG}^{-1}$ (which differs significantly from $Δ_{HS}^{-1}$ only when these correlations are important) would be close to $Δ_{EW}^{-1}$. We then set up the radiatively driven natural SUSY framework that we advocate for phenomenological analyses of natural models of supersymmetry, and review the implications of naturalness for LHC and ILC searches for SUSY as well as for searches for SUSY dark matter.

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arXiv2015

Same sign diboson signature from supersymmetry models with light higgsinos at the LHC

In supersymmetric models with light higgsinos (which are motivated by electroweak naturalness arguments), the direct production of higgsino pairs may be difficult to search for at LHC due to the low visible energy release from their decays. However, the wino pair production reaction \tw_2^\pm\tz_4\to (W^\pm\tz_{1,2})+(W^\pm\tw_1^\mp) also occurs at substantial rates and leads to final states including equally opposite-sign (OS) and same-sign (SS) diboson production. We propose a novel search channel for LHC14 based on the SS diboson plus missing E_T final state which contains only modest jet activity. Assuming gaugino mass unification, and an integrated luminosity > 100 fb^{-1}, this search channel provides a reach for SUSY well beyond that from usual gluino pair production.

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arXiv2015

Radiatively-driven natural supersymmetry at the LHC

Radiatively-driven natural supersymmetry (RNS) potentially reconciles the Z and Higgs boson masses close to 100 GeV with gluinos and squarks lying beyond the TeV scale. Requiring no large cancellations at the electroweak scale in constructing M_Z=91.2 GeV while maintaining a light Higgs scalar with m_h 125 GeV implies a sparticle mass spectrum including light higgsinos with mass 100-300 GeV, electroweak gauginos in the 300-1200 GeV range, gluinos at 1-4 TeV and top/bottom squarks in the 1-4 TeV range (probably beyond LHC reach), while first/second generation matter scalars can exist in the 5-30 TeV range (far beyond LHC reach). We investigate several characteristic signals for RNS at LHC14. Gluino pair production yields a reach up to m_{\tg} 1.7 TeV for 300 fb^{-1}. Wino pair production -- pp\to\tw_2\tz_4 and \tw_2\tw_2 -- leads to a unique same-sign diboson (SSdB) signature accompanied by modest jet activity from daughter higgsino decays; this signature provides the best reach up to m_{\tg} 2.1 TeV within this framework. Wino pair production also leads to final states with (WZ\to 3\ell)+\eslt as well as 4\ell+\eslt which give confirmatory signals up to m_{\tg} 1.4 TeV. Directly produced light higgsinos yield a clean, soft trilepton signature (due to very low visible energy release) which can be visible, but only for a not-too-small a \tz_2-\tz_1 mass gap. The clean SSdB signal -- as well as the distinctive mass shape of the dilepton mass distribution from \tz_{2,3}\to\tz_1\ell\ell decays if this is accessible -- will mark the presence of light higgsinos which are necessary for natural SUSY. While an e^+e^- collider operating with \sqrt{s} 600 GeV should unequivocally reveal the predicted light higgsinos, the RNS model with m_{1/2}> 1 TeV may elude all LHC14 search strategies even while maintaining a high degree of electroweak naturalness.

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arXiv2014

Monojet plus soft dilepton signal from light higgsino pair production at LHC14

Naturalness arguments imply the existence of higgsinos lighter than 200-300 GeV. However, because these higgsinos are nearly mass degenerate, they release very little visible energy in their decays, and (even putting aside triggering issues) signals from electroweak higgsino pair production typically remain buried under Standard Model backgrounds. Prospects for detecting higgsino pair production via events with monojets or mono-photons from initial state radiation are also bleak because of signal-to-background rates typically at the 1% level. Here, we consider the possibility of reducing backgrounds by requiring the presence of soft daughter leptons from higgsino decays in monojet events. We find that LHC14 experiments with an integrated luminosity of 300 fb^{-1} should be sensitive to light higgsinos at the 5σlevel for μ< 170 GeV with a S/B ~ 8.5%. For an integrated luminosity of 1000 fb^{-1} (which should be possible at a high luminosity LHC) the corresponding sensitivity to μextends to over 200 GeV though the systematic uncertainty would have to be controlled to considerably better than 5%. The corresponding reach from measurements of the rate asymmetry between monojet events with same flavour vs. opposite flavour dileptons is ~10-15 GeV smaller, but does not suffer the systematic uncertainty from the normalization of the background.

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arXiv2014

Physics at a Higgsino Factory

Naturalness arguments applied to supersymmetric theories imply a spectrum containing four light higgsinos \tz_{1,2} and \tw_1^+- with masses ~ 100-300 GeV (the closer to M_Z the more natural). The compressed mass spectrum and associated low energy release from \tw_1 and \tz_2 three-body decay makes higgsinos difficult to detect at LHC14, while the other sparticles might be heavy, and possibly even beyond LHC14 reach. In contrast, the International Linear e^+e^- Collider (ILC) with \sqrt{s}>2m(higgsino) would be a {\it higgsino factory} in addition to a Higgs boson factory and would serve as a discovery machine for natural SUSY! In this case, both chargino and neutralino production %which give rise to distinct event topologies, occur at comparable rates, and lead to observable signals above SM backgrounds. We examine two benchmark cases, one just beyond the LHC8 reach with \tw_1(\tz_2)-\tz_1 mass gap of 15 (21) GeV, and a second more difficult case beyond even the LHC14 reach, where the mass gap is just 10 GeV, close to its minimum in models with no worse than 3% fine-tuning. The signal is characterized by low visible energy events together with MET in the one or two jets +1\ell channel from chargino production, and in the opposite sign, same-flavour, acoplanar dilepton channel from \tz_1\tz_2 production. For both cases, we find that the signal is observable above backgrounds from the usual 2-> 2 SM events and from γγcollisions with just a few fb^{-1} of integrated luminosity. We also show that with an integrated luminosity of 100~fb^{-1}, it should be possible to extract \tw_1, \tz_2 and \tz_1 masses to percent level. These measurements would point to higgsinos as the origin of new physics and strongly suggest a link to a natural origin for W, Z and h masses.

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arXiv2014

Monojets and mono-photons from light higgsino pair production at LHC14

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arXiv2014

Naturalness implies intra-generational degeneracy for decoupled squarks and sleptons

The SUSY flavor, CP, gravitino and proton-decay problems are all solved to varying degrees by a decoupling solution wherein first/second generation matter scalars would exist in the multi-TeV regime. Recent models of natural SUSY presumably allow for a co-existence of naturalness with the decoupling solution. We show that: if sfermions are heavier than $\sim 10$ TeV, then a small first/second generation contribution to electroweak fine-tuning (EWFT) requires a rather high degree of intra-generational degeneracy of either 1. (separately) squarks and sleptons, 2. (separately) left- and right-type sfermions, 3. members of SU(5) multiplets, or 4. all members of a single generation as in SO(10). These (partial) degeneracy patterns required by naturalness hint at the necessity of an organizing principle, and highlight the limitations of models such as the pMSSM in the case of decoupled first/second generation scalars.

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arXiv2013

Dark Matter in the Coming Decade: Complementary Paths to Discovery and Beyond

In this Report we discuss the four complementary searches for the identity of dark matter: direct detection experiments that look for dark matter interacting in the lab, indirect detection experiments that connect lab signals to dark matter in our own and other galaxies, collider experiments that elucidate the particle properties of dark matter, and astrophysical probes sensitive to non-gravitational interactions of dark matter. The complementarity among the different dark matter searches is discussed qualitatively and illustrated quantitatively in several theoretical scenarios. Our primary conclusion is that the diversity of possible dark matter candidates requires a balanced program based on all four of those approaches.

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arXiv2013

Leaving no stone unturned in the hunt for SUSY naturalness: A Snowmass whitepaper

Imposing electroweak scale naturalness constraints (low Δ_{EW}) on SUSY models leads to mass spectra characterized by light higgsinos ~100-300 GeV, highly mixed top-squarks and gluinos at the 1-5 TeV scale and allows for m_h ~125 GeV. First and second generation squarks can easily live at the 5-20 TeV scale, thus providing at least a partial solution to the SUSY flavor/CP problems. For such models at the LHC, gluino pair production is followed by cascade decays to t- and b-quark rich final states along with multileptons. The reach of LHC14 with 300 fb^{-1} is computed to be around m_{\tg} 1.8 TeV. However, the small magnitude of the μ-parameter-- a necessary condition for naturalness-- leads to a unique hadronically quite same-sign diboson (W^\pm W^\pm) signature from wino pair production. In low Δ_{EW} models with unified gaugino masses, this signature yields a somewhat higher reach up to m_{\tg} 2.1 TeV. The smallness of |μ| implies that the ILC should be a higgsino factory in addition to a Higgs factory, and a complete search for SUSY naturalness seems possible for \sqrt{s} 600 GeV. Since a thermal under-abundance of higgsino-like WIMP dark matter (DM) is expected, there is ample room for an axion DM contribution. A thorough search for higgsino-like WIMPs can be made by next generation WIMP detectors, such as those with ton-scale noble liquid targets.

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arXiv2013

SUSY discovery potential of LHC14 with 0.3-3~ab^{-1} : A Snowmass whitepaper

We examine the discovery reach of LHC14 for supersymmetry for integrated luminosity ranging from 0.3 to 3 ab^{-1}. In models with gaugino mass unification and M_1,\ M_2<< |μ| (as for mSUGRA/CMSSM), we find a reach of LHC14 with 3 ab^{-1} for gluino pair production extends to m_{\tg} 2.3 TeV while the reach via \tw_1\tz_2 \to W h +ETmiss extends to m_{\tg}\sim 2.6 TeV.

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arXiv2013

Snowmass whitepaper: Exploring natural SUSY via direct and indirect detection of higgsino-like WIMPs

Supersymmetric models with low electroweak fine-tuning contain light higgsinos with mass not too far from m_h\simeq 125 GeV, while other sparticles can be much heavier. In the R-parity conserving MSSM, the lightest neutralino is then a higgsino-like WIMP (albeit with non-negligible gaugino components), with thermal relic density well below measured values. This leaves room for axions (or other, perhaps not as well motivated, stable particles) to function as co-dark matter particles. The local WIMP abundance is then expected to be below standard estimates, and direct and indirect detection rates must be accordingly rescaled. We calculate rescaled direct and indirect higgsino-like WIMP detection rates in SUSY models that fulfil the electroweak naturalness condition. In spite of the rescaling, we find that ton-scale noble liquid detectors can probe the entire higgsino-like WIMP parameter space, so that these experiments should either discover WIMPs or exclude the concept of electroweak naturalness in R-parity conserving natural SUSY models. Prospects for spin-dependent or indirect detection are more limited due in part to the rescaling effect.

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arXiv2013

Naturalness, Supersymmetry and Light Higgsinos: A Snowmass Whitepaper

We show that the electroweak fine-tuning parameter Δ_{\rm EW} derived from the well-known electroweak symmetry breaking condition written in terms of weak scale parameters leads to {\it a bound on fine-tuning in the MSSM} and explain its utility for phenomenological analyses. We argue that a small magnitude of the mu parameter, and the concomitant presence of light higgsinos, is the most basic consequence of naturalness in SUSY models, and list the resulting implications of this for experiments at the LHC and at future e^+e^- colliders.

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