Measurement of the Superparticle Mass
Spectrum in the Long-Lived Stau Scenario atthe LHC
Takumi Ito
Tohoku University
In collaboration with R. Kitano and T. Moroi,
arXiv:0910.5853[hep-ph]

特定領域「フレーバー物理の新展開」研究会 20102010.2.23

Introduction
Supersymmetry : an important target of the LHC experiments.
Signature of SUSY at the LHC strongly depends on
what the LSP in the MSSM sector (= “MSSM-LSP” ) is!
Popular candidates of MSSM-LSP are:
* The lightest neutralino* The lightest stau
* ...

Large missing pT
It may have a long lifetime enoughto escape from the detector.
→ we can observe its track.
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Introduction
Long-lived stau scenario↔ Stau behaves as a “heavy muon” in high energy experiments.
* We can measure stau's charge, momentum, velocity and energy.

→ The stau mass can be measured. [Ambrosanio et al.(01),Ellis et al.(06,07)]→ All final state particles (= SM particles & stau) in the eventare visible. (Of course, except ν.)* A slow-moving charged track (= stau) informs us a production ofsuperparticles.

→ we can reduce SM backgrounds significantly.
→ We have a opportunity to probe the SUSY in detail!

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Introduction
Long-lived stau scenario* It is naturally realized if tanβ is large.
* Cosmological problems can be avoided if there isweekly interacting LSP (i.e. gravitino, axino, ...).
This scenario is not an unrealistic scenario.
→ We may observe stau's track at the LHC.
We discuss a determination of the superparticle masses
at the LHC experiments in the model with the long-livedstau.
4Assumptions and Outline
Assumptions* Stau is stable in the detector.

[Ambrosanio et.al.(01)]* Stau with v < 0.9c can be identified as “stau”, not muon.

* No SM backgrounds. ← Since there is at least one stau in
the SUSY event.
* The stau mass has been measured. (accuracy)[Ambrosanio et al.(01), Ellis et al.(06,07)]

Mass Measurements
1. Neutralino
2. Sleptons
3. Squarks

Charge information of
the stau is useful.
The knowlidge of the neutralinomass helps our reconstruction.

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Sample Model
GMSB [Dine,Nelson,Shirman(95)]Monte Carlo Analysis* Event Gen. by HERWIG6.510* Fast Detector Sim. by PGS4
* 67k events are generated.
(↔ 100 fb-1 , 14 TeV)

Reduced1 GeV
by hands

The decay
products canbe observed.

MSSM-LSPmasses are in GeV

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Neutralino Masses
The lightest neutralino (bino) decaybino → τ + stau
We consider hadronic decay mode of τ-lepton.
↔ Some part of the energy of τ is taken away by τ-neutrino.

Event Selection

Endpoint⇕
Bino mass(1) At least one stau with 0.4c < v < 0.9c
(2) At least one τ-tagged jet with pT > 15 GeV
7Neutralino Masses
To reduce BGs, we adopt a charge subtraction:
OS (τ stau) – SS (τ stau) = Signal= 241.1 ± 1.0 GeV(239.5 : underlying)
Uncertainties* Statistics± 1 GeV* From the error of stau mass
± 100 MeV

→ Bino mass measurement :8

Neutralino Masses
Also, wino mass can be measured by using charge subtraction.

Endpoint
Endpoint

This endpoint is not clear before we adopt charge subtraction.
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Neutralino Masses
Also, wino mass can be measured by using charge subtraction.

Endpoint= 430.0 ± 4.5 GeV(425.9 : underlying)
Uncertainties* Statistics± 5 GeV* From the error of stau mass
± 100 MeV

→ Wino mass measurement :10

Selectron & Smuon Masses
2-step SUSY decay chain:bino → ℓ + slepton ,slepton → ℓ + τ + stau ; followed by τ → τ-jet + ντCollinear approximation;
The value ofis determined by the condition:

Then,Event Selection(1)(2) At least one pair of stau and τ-tagged jet (OS)(3) At least one pair of leptons(SF,OS) with pT > 15 GeV
11Selectron & Smuon Masses= 194.5 ± 0.1 GeV(194.4 : underlying)= 193.4 ± 0.1 GeV(193.4 : underlying)
Uncertainties* Statistics± 100 MeV
* From the error of stau mass
± 100 MeV
* From the error of bino mass
± 1 GeV→ Slepton masses :12

Selectron & Smuon Masses
The mass difference of two sleptons,,

is not sensitive to the uncertainty of the bino mass.

→ Mass difference :Good accuracy!
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Selectron & Smuon Masses
The mass difference measurements with the good accuracy.

→ Implications for the SUSY breaking mechanism* Loop effects to the slepton masses (Yukawa interaction)→for the large tanβ* The size of SUGRA effects are estimated→ ifa few GeV,
→ These effects are detectable.
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Squark Masses
2-step SUSY decay chain:squark → q + bino,bino → τ + stau ; followed by τ → τ-jet + νττ's momentum is reconstructed byThen,Event Selection(1)(2) At least one pair of stau and τ-tagged jet(3) At least one jet with pT > 100 GeV(4) No isolated leptons with pT > 15 GeV

15Squark Masses
We use upto leading 4 high-pT jets, and perform charge subtraction.
= 1172 ± 1 GeV(1180 : Squark(R) )Uncertainties* Statistics + Systematics(± 1 GeV) + (- 8 GeV)
* From the error of stau mass
± 100 MeV
* From the error of bino mass
± 1 GeV→ Squark mass measurement :16

Conclusions
We have discussed mass measurements of superparticles
in the long-lived stau scenario at the LHC experiments.
* Neutralino Masses
by endpoint analysis :charge subtraction method is useful.
* Selectron & Smuon Masses()

by peak analysis:* mass difference :* Squark Masses

informations for the SUSY

by peak analysis:
17backup18

Another Example; Sweet Spot Model [Ibe,Kitano(07)]slepton masses

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Another Example; Sweet Spot Model [Ibe,Kitano(07)]bino mass
vsslepton mass

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Another Example; Sweet Spot Model [Ibe,Kitano(07)]The size ofthe mass difference(for the fixedN5 and Λ )21Introduction
Supersymmetry : A famous extension of the Standard Model
There are several reasons to consider SUSY seriously.
* Gauge coupling unification at the very high energy scale (GUT)* Candidates of Dark Matter of the Universe* Predicts a light higgs (prefered by EW precision measurements)* Naturally solves gauge hierarchy problem→ It will appear around TeV scale.
↔ in the energy range of the LHC experiments!
So, Question is
What is a signature of SUSY at the LHC?

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