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Shigeki Matsumoto (IPMU). Studying Very Light Gravitino at ILC. Collaborators: T . Moroi ( Tokyo ) [for basic idea: arXiv:1104.3624] & K. Fujii (KEK), T. Moroi (Tokyo), T. Suehara (ICEPP) [for realistic evaluations.].

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Studying very light gravitino at ilc

Shigeki Matsumoto (IPMU)

Studying Very Light Gravitino at ILC


T. Moroi (Tokyo) [for basic idea: arXiv:1104.3624]


K. Fujii (KEK), T. Moroi (Tokyo), T. Suehara (ICEPP)

[for realistic evaluations.]

Q. Is it possible to study the property of the very light gravitino (~ O(10) eV) at the ILC?

  • A. Yes it is by observing the distribution ofthe impact parameter, which is obtainedby the decay products of the stau NLSP!

The gravitino lsp


Gravitino = Super partner of Graviton! Its spin is 3/2!

Gravitino Mass = <F>/Mpl/31/2, & <F> = SUSY breaking scale!

Its interactions are suppressed by 1/<F> ~ 1/(Mpl m3/2)

The Gravitino LSP

Structure Formation of Universe

Vacuum Stability

LEP, SN cooling

10 –7

10 –13

10 –11

10 –9

10 –12

10 –10

10 –8

T. Moroi, hep-ph/9503210

Very Light Gravitino, whose mass is O(1)-O(10) eV, is quite attractive from the viewpoint of Cosmology!

What happens at the ilc


Next Lightest Supersymmetric Particle (NLSP) is living long!

How long?  t~ 100 x Mpl2 x m3/22/mNLSP5~ O(10–13 ) sec.  ct~ 100 mm !!! (Decay length is not long!)

What happens at the ILC?

We focus on the casethat the NLSP is Stau!



1st layer



e+ e– Stau pair

Impact parameter d


Hadronic decay

(p±, K±, etc.)

Impact parameter d is one of convenient quantities to measure the lifetime of the NLSP in such a circumstance.




Impact Parameter d



parameter d

Signal & Background processes



(+ ISR)

[Signal] e+e–t+t–

[gg-BG] e+e– e+ e–gg e+ e–t+t–

(+ ISR)

Kinematical cuts


100% efficiency for tagging and no contamination assumed.

(Evis = Total energy of charged particles.)

Kinematical Cuts

To reduce gg-BG. Tagging forward e± will also be useful.

(q = Scattering angle of the t-jet.)

t, W, and Z in BG are likely to be produced with high rapidity.

(f = Azimuthal angle of the t-jet.)

(L-lim.) Two ts in [tt-BG] are almost back-to-back even w/ ISR.

(U-lim.) To reduce t leptons from the Z boson decay.

( is the momentum of isolated g (> 30 GeV))

Two ts and isolated g in [tt-BG] should be on one plane.

Stau Mass = 120 GeV

Luminosity = 100 fb-1

CM Energy = 500 GeV



Stau Mass = 120 GeV

CME = 500 GeV


Stau Mass = 120 GeV

CME = 500 GeV

Lum = 100 fb-1


Bin size = 20 mm (0mm < d < 2mm)



Summary discussions


It is possible to study the lightgravitino

with the mass of O(10) eV at the ILC.

When the lifetime is longer than ~10-14

sec, the lifetime can measured accurately.

Information of the lifetime translated to

the scale of the SUSY breaking <F>.

CME = 500 GeV

Lum = 100 fb-1

Summary & Discussions

Stau Mass = 120 GeV

  • How can be large the efficiency of the t-tagging?

  • Polarization of the incident e± can reduce the BG?

  • Is there the effect of the magnetic field to detect the signal?

  • Changing the CME increases the significance to detect the signal?

  • 3-prong decay of t can be used to detect the decay point?

 Go to a full simulation with realistic detector setup.