Discovering bottom squark coannihilation at the ILC

Gordana Laštovička-Medin,

Montenegro University

TAM2010, Budva

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What do we study?

We study the potential of the international linear collider (ILC) at s=500GeV to probe new dark matter motivated scenario where the bottom squark (sbottom) is the next-to-lightest supersymmetric particle.

For this scenario, which is virtually impossible for the LHC to test, the ILC has a potential to cover a large fraction of the parameter space.

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Motivation

Study is motivated by recent measurements of the neutralino relic density suggest a small mass splitting between the neutralino and the next to lightest SUSY particle (NLSP) assuming that neutralino accounts for the measured dark matter content of the universe.

The challenge is due to a very low energy of jets, below 20-30 GeV, which pushes the jet clustering and flavor tagging algorithms to their limits.

The process of sbottom pair production was studied within the SiD detector concept. The study was done with the full SiD simulation and reconstruction chain including all standard model and beam backgrounds.

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What is the mechanism responsible for electroweak symmetry breaking and the generation of mass?

How do the forces unify?

Does the structure of space-time at small distances show evidence of extra dimensions?

What are the connections between the fundamental particles and forces and cosmology?

The Silicon Detector (SiD) has been designed to address questions of fundamental importance to progress in particle physics:

Why ILC/SiD ?

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ILC Physics Menu

These questions are addressed through precision measurements by SiD at the International Linear Collider (ILC) of the following:

Higgs boson properties;

Gauge boson scattering;

Efects resulting from the existence of extra dimensions;

Supersymmetric particles; and

Top quark properties.

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SiD Detector Concept

Three International Linear Collider (ILC) detector concepts were requested to submit their Letters of Intent by the end of March 2009.

SiD is a linear collider detector concept designed for precision measurements of a wide range of possible new phenomena at the ILC.

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SiD is based on a silicon pixel vertex detector, silicon tracking, silicon-tungsten electromagnetic calorimetry, and a highly segmented hadronic calorimetry.

EMC is made of plates of tungsten with 1 mm gaps for silicon detectors with pixels ~3.5 mm across. This arrangement largely preserves the Moliere radius (9 mm) of tungsten and permits separation of close energetic electromagnetic showers.

SiD also incorporates a high field solenoid, iron flux return, and a muon identification system.

Particle Flow Analysis (PFA) is an important consideration for the basic philosophy and layout of the detector.

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The Expected LHC-ILC Accuracy

Figure shows the expected accuracy of ILC and LHC for predicting the amount of dark matter in the universe by analysing the SUSY data observed at these colliders. The chosen point is relatively friendly for what concerns LHC. In comparison are shown the WMAP and the expected PLANCK SURVEYOR accuracies.

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Standard Cosmological Model

23%of universe composed of “Dark Matter”(DM)

Does not emit or reflect EM radiation.

Presence inferred by cosmic microwave background (CMB) (e.g. WMAP), gravitational effects (e.g. rotational curves), collision of galaxies, etc.

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• The existence of CDM is a crucial argument in favour of SUSY, and, at the same time, an important SUSY constraint.

Wilkinson Microwave Anisotropy Probe (WMAP) and galaxy survey data puts the most stringent constraint on the ratio of the dark matter density to the critical density

ΩCDMh2 = 0.111+0.011−0.015 (at 95%CL)

where h = 0.74±0.03 is the normalized Hubble constant

SUSY and Cosmology

In most of the parameter space of SUSY models, the value of ΩCDMh2

is well above the WMAP bound. In particular, in case of well explored minimal supergravity (mSUGRA) model which is defined by universal soft SUSY breaking scalar masses (m0), gaugino masses (m1/2) and A-terms (A0) at GUT scale, the CDM is the lightest neutralino.

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mSUGRA

Within the minimal supergravity model (mSUGRA) the parameter space can be strongly restricted requiring that the abundance of the lightest neutralino, which is stable in this model, is consistent with the dark matter density measured by WMAP.

In the so called “bulk region” all superpartners are light and many are visible at the LHC and the ILC.

In the “coannihilation region”the mass difference between the lightest neutralino, and the lighter stau, is very small so that the stau-decay particles that are visible by the detector have only a very small momentum.

In the “focus point region” the neutralino gets a significant Higgsino component enhancing its annihilation cross section. This leads to relatively heavy scalars, probably invisible at the ILC and the LHC.

Other regions, like the “rapid annihilation funnel” are characterised by special resonance conditions, increasing the annihilationrate. All these special regions tend to be challenging for both machines.

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SUSY and Cosmology

Neutralino is a very attractive CDM candidate.

During Universe expansion at some point the lightest SUSY particle (LSP) is out of thermal equilibrium and intensively annihilates till its freeze-out; the rate can be calculated (ISARED, micoMEGAs, ...)

The most of the SUSY parameter space is excluded – there are still too many neutrinos left providing too high relic density.

Cold Dark Matter favours some particular SUSY scenarios

one of them is co-annihilation scenario, when neutralino effectively co-annihilates with others quasi-degenerate SUSY particles into SM ones.

Neutralino-sbottom co-annihilation scenario has not been studies previously.

This scenario is virtually impossible for LHC while feasible but challenging at the ILC.

The small mass split between neutralino and sbottom leads to small energy release and softness of the visible particles.

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Sbottom and Neutralino

If sbottom (stop) and neutralino have a small mass split they can account for co-annihilation in early Universe through this type of diagrams:

Sbottom can be produced at ILC via s-channel photon or Z-boson exchange, then it decays to b and neutralino:

b~

0~b

,Z

b~

b~

0~t

W

t~

tb ~,~e

e

Z

tb ~,~

0~

bb~

If the mass split is low (as suggested) this would lead to very soft b-jets and missing pT.

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LEP and CDF/D0 Results

CDF/D0 – measurement at high masses but still relatively hard jets (due to triggers) which are not favored by the dark matter scenario.

LEP – able to measure in the region where the mass difference is only few GeV.

ILC should not be much worse but at higher masses.

Small (meaning tiny) mass splitting is not accessible at ILC.

ILC

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For more details see also the papers

C. Pallis, Nucl. Phys. B678, 398 (2004), hepph/0304047

S. Profumo, Phys. Rev. D68, 015006 (2003), hepph/0304071

It was concluded that in case of minimal sfermion non-universality (mSFNU) defined by (m10, m5, A0, m1/2, tan , sign(μ)) parameter space the SBC scenario could be realised, where the left-right squark and sfermion mass parameters at the GUT scale extend the universal m0 mSUGRA parameter

Sbottom Co-Annihilation scenario is done by Alexander Belyaev.

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Analysis, event selection

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Why are Soft b-jets Difficult to Analyse?

Tagging efficiency is dropping down quickly at low energies.

It is about 75% above 60 GeV, and is falling steeply for lower jet energies. The energy of b-quarks is determined by the mass difference between m(sbottom) and m(meutralino).

.

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Why are Soft b-jets Difficult to Analyse?

Jet finding algorithms begin to break.

Large gamma-gamma and gamma-e backgrounds.

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Why are Soft b-jets Difficult to Analyse?

The cross section of sbottom pair production depends strongly on how close the sbottom mass is to ILC kinematical limit, dropping quickly down for masses approaching 250 GeV.

The mass of the lightest bottom squark mass, and the mixing angle of the lightest and heaviest bottom squarks, cosΘb, completely determine the cross section of pair production of lightest bottom squarks.

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Data Samples

√s = 500 GeV; 1000 fb-1 luminosity; ~ 200k events /sample (CalcHEP)

Five points close to ILC limits

– (MNE1, Msbottom ) = (220,210) , (230,220) - mass difference 10 GeV

– (MNE1, Msbottom ) = (230,210) , (240,220) - mass difference 20 GeV

– (MNE1, Msbottom ) = (240,220) - mass difference 30 GeV

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Two-Photon and other SM background

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22bbbbbbR

is the difference in pseudorapidity of two leading jets is the diference in azimutal angles of leading jets

Powerful variable or not…….?

Ee->bb processes can”leak” into Rbb < region due to detector resolutionand event reconstruction effects.

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Analysis

Events are pre-selected using few basic quantities

– , , ,

– Veto on electrons or photons in forward detectors (>10mrad)

For the final selection Neural Net is trained with additional inputs.

Example plots for point (230,210) – signal (line) was multiplied by 105

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Additional kinematic variables used as neural net input variables

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RESULTS The measurement is interpreted in terms of signal significance calculated as S / √(S + B) and depending on a particular neural net output cut.

Points (230,210) and (220,210) both reach above 4σ level.

Other points are more difficult (low x-section, jet softness) but they all can be excluded @ 95% CL.

Summary

We study new cosmologically motivated sbottom co-annihilation scenario can be uniquely probed at the ILC.

.

Sbottom-quarks degenerate with LSP neutralinos can be identified with 95% CL even at very corners of the ILC kinematic plane.

Presence of very forward detectors is crucial for a decent measurement.

More details and references can be found in SiD LoI:http://silicondetector.org/display/SiD/LOI

Discovering bottom squark coannihilation at the ILCPhys. Rev. D 81, 035011 (2010)

Alexander Belyaev School of Physics & Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK and Particle Physics Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom

Tomáš Laštovička and Andrei Nomerotski University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, United Kingdom

Gordana Laštovička-Medin University of Montenegro, Cetinjska bb, 81 000 Podgorica, Montenegro