Searcharxiv⌕ Search

arXiv subjects

Mikael Berggren

Publications and source records attributed to Mikael Berggren.

50 records · Page 3Linked to original sources
arXiv2017

The Potential of the ILC for Discovering New Particles

This paper addresses the question of whether the International Linear Collider has the capability of discovering new particles that have not already been discovered at the CERN Large Hadron Collider. We summarize the various paths to discovery offered by the ILC, and discuss them in the context of three different scenarios: 1. LHC does not discover any new particles, 2. LHC discovers some new low mass states and 3. LHC discovers new heavy particles. We will show that in each case, ILC plays a critical role in discovery of new phenomena and in pushing forward the frontiers of high-energy physics as well as our understanding of the universe in a manner which is highly complementary to that of LHC. For the busy reader, a two-page executive summary is provided at the beginning of the document.

hep-ph↗
arXiv2016

SUSY model and dark matter determination in the compressed-spectrum region at the ILC

It is an appealing possibility that the observed dark matter density in the universe can be fully explained by SUSY. The current experimental knowledge indicates that this possibility strongly favors a co-annihilation scenario. In such scenarios, the mass difference between the next-to-lightest SUSY particle (the NLSP) and the lightest one (the LSP) is quite small, which assures that the annihilation cross-section is sufficient not to predict a too large abundance of dark matter. However, the small mass difference also means that observing SUSY becomes hard at hadron colliders, where the observation hinges on the tell-tale signature of missing transverse energy: if the mass difference NLSP-to-LSP is small, only little energy is carried away by the invisible LSP. This is also true even if several other SUSY particles are within the kinematic reach, since these states would to a large extent decay via cascades ending with an NLSP to LSP decay. A lepton collider does not have this problem. The clean environment and known initial state at such machines assures that SUSY can be detected even if the mass difference is very small, provided the center-of-mass energy is sufficiently high. We present prospects for observation and precision characterization of SUSY with small mass differences at the ILC, based on detailed simulations of the ILD detector concept. The resulting possibility to predict the dark matter relic density is evaluated and compared to the precision obtained from the Planck mission. Taking a specific model as an example, we also discuss the synergies from combining ILC and HL-LHC results.

hep-ph↗
arXiv2016

Naturalness and light higgsinos: A powerful reason to build the ILC

A core prediction of natural Supersymmetry is the existence of four light higgsinos not too far above the mass of the $Z$ boson. The small mass splittings amongst the higgsinos -- typically 5-20\,GeV -- imply very little visible energy release from decays of heavier higgsinos. In particular, if other SUSY particles are quite heavy, as can be the case in SUSY with radiatively-driven naturalness, the higgsinos are extremely hard to detect at hadron colliders. The clean environment of electron-positron colliders with $\sqrt{s} > 2m_{\mathrm{higgsino}}$, however, would allow for a decisive search for the required light higgsinos. Thus, $e^+e^-$ colliders should either discover or exclude natural SUSY. We present a detailed study of higgsino pair production at the proposed International Linear $e^+e^-$ Collider which is under consideration for construction in Japan. A variety of precision measurements should allow for extraction of underlying parameters and provide a window onto physics at the grand unified scale.

hep-ph↗
arXiv2016

Dark matter relic density from observations of supersymmetry at the ILC

Supersymmetry can explain the observed dark matter relic density with a neutralino dark matter particle and a coannihilating, almost mass-degenerate sparticle. If this were the case in nature, a linear electron positron collider like the ILC could discover the two sparticles if their masses are in the kinematic reach of the collider. This contribution discusses which observations are necessary at the ILC for predicting the dark matter relic density correctly and for confirming that the observed lightest neutralino is the only kind of dark matter. We take the case of stau coannihilation as an example.

hep-ph↗
arXiv2013

Tackling light higgsinos at the ILC

In supersymmetric extensions of the Standard Model, higgsino-like charginos and neutralinos are preferred to have masses of the order of the electroweak scale by naturalness arguments. Such light $\widetildeχ^0_1$, $\widetildeχ^0_2$ and $\widetildeχ^{\pm}_1$ states can be almost mass degenerate, and their decays are then difficult to observe at colliders. In addition to the generic naturalness argument, light higgsinos are well motivated from a top-down perspective. For instance, they arise naturally in certain models of hybrid gauge-gravity mediation. In the present analysis, we study two benchmark points which have been derived in the framework of such a model, which exhibit mass differences of O(GeV) in the higgsino sector. For chargino pair and neutralino associated production with initial-state photon radiation, we simulate the detector response and determine how accurately the small mass differences, the absolute masses and the cross sections can be measured at the International Linear Collider. Assuming that 500/fb has been collected at each of two beam-polarisations $P(e^+,e^-)=(\pm 30\%,\mp 80\%)$, we find that the mass differences can be measured to 40-300 MeV, the cross sections to 2-5%, and the absolute masses to 1.5-3.3 GeV, where the range of values correspond to the different scenarios and channels. Based on these observables we perform a parameter fit in the MSSM, from which we infer that the higgsino mass parameter $μ$ can be measured to a precision of about $Δμ=$2--7 GeV. For the electroweak gaugino mass parameters $M_1$, $M_2$, which are chosen in the multi-TeV range, a narrow region is compatible with the measurements. For both parameters independently, we can determine a lower bound.

hep-ph↗
arXiv2013

Electroweakino Searches: A Comparative Study for LHC and ILC (A Snowmass White Paper)

We make a systematic and comparative study for the LHC and ILC for the electroweakino searches in the Minimal Supersymmetric Standard Model. We adopt a general bottom-up approach and scan over the parameter regions for all the three cases of the lightest supersymmetric particle being Bino-, Wino-, and Higgsino-like. The electroweakino signal from pair production and subsequent decay to Wh (h to b\bar b) final state may yield a sensitivity of 95% C.L. exclusion (5sigma discovery) to the mass scale M_2, mu ~ 250-400 GeV (200-250 GeV) at the 14 TeV LHC with an luminosity of 300 fb^{-1}. Combining with all the other decay channels, the 95% C.L. exclusion (5sigma discovery) may be extended to M_2, mu ~ 480-700 GeV (320-500 GeV). At the ILC, the electroweakinos could be readily discovered once the kinematical threshold is crossed, and their properties could be thoroughly studied.

hep-ph↗
arXiv2013

Simplified SUSY at the ILC

At the ILC, one has the possibility to search for SUSY in an model-independent way: The corner-stone of SUSY is that sparticles couple as particles. This is independent of the mechanism responsible for SUSY breaking. Any model will have one Lightest SUSY Particle (LSP), and one Next to Lightest SUSY Particle (NLSP). In models with conserved R-parity, the NLSP must decay solely to the LSP and the SM partner of the NLSP. Therefore, studying NLSP production and decay can be regarded as a "simplified model without simplification": Any SUSY model will have such a process. The NLSP could be any sparticle: a slepton, an electroweak-ino, or even a squark. However, since there are only a finite number of sparticles, one can systematically search for signals of all possible NLSP:s. This way, the entire space of models that have a kinematicall y reachable NLSP can be covered. For any NLSP, the "worst case" can be determined, since the SUSY principle allows to calculate the cross-section once the NLSP nature and mass are given. The region in the LSP-NLSP mass-plane where the "worst case" could be discovered or excluded experimentally can be found by estimating background and efficiency at each point in the plane. From experience at LEP, it is expected that the lower signal-to background ratio will indeed be found for models with conserved R-parity. In this document, we show that at the ILC, such a program is possible, as it was at LEP. No loop-holes are left, even for difficult or non-standard cases: whatever the NLSP is it will be detectable.

hep-ph↗
arXiv2013

Physics Case for the ILC Project: Perspective from Beyond the Standard Model

The International Linear Collider (ILC) has recently proven its technical maturity with the publication of a Technical Design Report, and there is a strong interest in Japan to host such a machine. We summarize key aspects of the Beyond the Standard Model physics case for the ILC in this contribution to the US High Energy Physics strategy process. On top of the strong guaranteed physics case in the detailed exploration of the recently discovered Higgs boson, the top quark and electroweak precision measurements, the ILC will offer unique opportunities which are complementary to the LHC program of the next decade. Many of these opportunities have connections to the Cosmic and Intensity Frontiers, which we comment on in detail. We illustrate the general picture with examples of how our world could turn out to be and what the ILC would contribute in these cases, with an emphasis on value-added beyond the LHC. These comprise examples from Supersymmetry including light Higgsinos, a comprehensive bottom-up coverage of NLSP-LSP combinations for slepton, squark, chargino and neutralino NLSP, a stau-coannihilation dark matter scenario and bilinear R-parity violation as explanation for neutrino masses and mixing, as well as generic WIMP searches and Little Higgs models as non-SUSY examples.

hep-ph↗
arXiv2012

Characterising WIMPs at a future $e^+e^-$ Linear Collider

We investigate the prospects for detecting and measuring the parameters of WIMP dark matter in a model independent way at the International Linear Collider. The signal under study is direct WIMP pair production with associated initial state radiation $e^+e^- \rightarrow χχγ$. The analysis accounts for the beam energy spectrum of the ILC and the dominant machine induced backgrounds. The influence of the detector parameters are incorporated by full simulation and event reconstruction within the framework of the ILD detector concept. We show that by using polarised beams, the detection potential is significantly increased by reduction of the dominant SM background of radiative neutrino production $e^+e^- \rightarrow ννγ$. The dominant sources of systematic uncertainty are the precision of the polarisation measurement and the shape of the beam energy spectrum. With an integrated luminosity of 500 fb the helicity structure of the interaction involved can be inferred, and the masses and cross-sections can be measured with a relative accuracy of the order of 1 %.

hep-ex↗
arXiv2012

SGV 3.0 - a fast detector simulation

The need for fast simulation programs is emphasised, both in terms of the need for "rapid response" to new results - in particular from the LHC - and new theoretical ideas, and in terms of how to cope with multi-billion simulated event samples. The latter would arise both from the need to be able to simulate significantly more events than expected in the real data, also for high cross-section processes, and the need to scan multi-parameter theories. The {\it Simulation à Grande Vitesse}, SGV, is presented, and is shown to be able to address these issues. The tracking performance of SGV is shown to reproduce very closely that of the full simulation and reconstruction of the ILD concept. Preliminary results on how to also closely emulate the calorimetric performance from full simulation is presented. The procedure is parametric, with no the need to simulate the detailed shower development, and promises to be many orders of magnitude faster than such approaches. Contrary to what is often the case with fast simulation programs, the procedure gives a somewhat {\it pessimistic} result, compared to the full simulation and reconstruction.

physics.ins-det↗
arXiv2010

Prospects for the study of the \stau-system in SPS1a' at the ILC

The prospects for the analysis of tau channels at the SUSY benchmark point SPS1a', especially from e+e- -> stau_1 stau_1 and e+e- -> stau_2 stau_2, have been studied in full simulation of the ILD detector concept forseen for the International Linear Collider. All accessible SUSY channels as well as all Standard Model backgrounds were generated at a centre-of-mass energy E_CMS=500 GeV, including the beam energy spectrum and beam backgrounds expected for nominal ILC beam parameters. With an integrated luminosity of 500 fb-1 delivered to the experiment, the masses of the stau_1 and stau_2 can be measured to 107.69^{+0.03}_{-0.06} GeV +- 1.1 δM_LSP and 183 ^{+11}_{-5} GeV +- 18 δM_LSP, respectively, while the true values in SPS1a' are 107.9 GeV and 194.9 GeV, respectively. The cross sections for stau_1 and stau_2 pair production could be obtained with a precision of 3.1% and 4.2%, respectively. Combining the mass and cross section measurement in the stau_2 channel allows to determine the LSP mass with a relative error of 1.7%, assuming a known mixing angle. In stau_1 -> tau LSP decays, the tau polarisation is measured to be 91 +- 9% and 87.0 +- 3.4% in the pi and rho decay channels of the tau, respectively. The true value in the simulated SPS1a' sample is 89.6%.

hep-ex↗
arXiv2010

ILC Beam-Parameters and New Physics

A brief overview of the linear collider design is given, with emphasis on the elements of particular importance for the performance. The modifications of the RDR design suggested in the SB2009 proposal are presented, once again with emphasis on those item that have most impact on the performance. In particular, the effects on New Physics channels are studied, by two examples: the analysis of the properties of $\stau$:s in the SUSY benchmark point SPS1a', and the model-independent Higgs recoil mass analysis. It is shown that for both these cases, the SB2009 design performs significantly worse than the RDR design: For the \stau ~analysis, the uncertainties on both the mass and cross-section determination increases by 20 \% (or 35 \% if the travelling focus concept is not deployed). For the Higgs analysis, the corresponding increase in uncertainty is found to be 70 \% both for cross-section and mass (or 100 \% without travelling focus). For both channels, the deterioration is to a large part due to the move of the positron source to the end of the linac.

hep-ex↗
arXiv2009

Summary of ILD performance at SPS1a'

The performance of the ILD detector at the ILC for the analysis of $μ$ and $τ$ channels at the SUSY benchmark-point SPS1a' has been studied with full detector simulation. It is concluded that if 500 fb$^{-1}$ is delivered to the experiment, $Δ(\MXN{1}) = 920 \MeVcc$, $Δ(\msmul) = 100 \MeVcc$, $Δ(\MXN{2}) = 1.38 ~\GeVcc$, and $Δ(σ(\eeto \smul \smul))$ = 1.35 fb can be achieved from the $μ$ channels alone. The preliminary results from the $\stau$ channels, indicates that $Δ(P_τ) = 13 $ % is also achievable.

hep-ex↗