Search for Stopped Gluinos

Split SUSY -> heavy scalars -> long-lived gluino

Lifetime <100s due to nucleosynthesis constraints, but >10ns if MSUSY~106 GeV

Gluinos hadronize into “R-hadrons”

Most are neutral, lose a small fraction of their momentum, and escape undetected

Not many reconstructed tracks -> bad for classic charged massive stable particle search

Gluinos can radiate gluons

This would create a monojet signature, but it would be hard to resolve the nature of the monojet signal from the monojets alone

R-hadrons can become charged when undergoing nuclear interactions, and then some may lose enough momentum to stop (most likely in the calorimeter), see hep-ph/0506242

These “stopped gluinos” would later decay to jets+MET, giving a very unique, one-sided, jet signal, uncorrelated with beam-interactions

If both gluinos eventually stop (~30% of the time) and are detected, the time-coincidence would be a way (the only way!) to measure the gluino lifetime, and thus MSUSY

Other new particles could also give rise to a stopped-gluino-like signal, like the stau NLSP, see hep-ph/0409278

Another simple motivation: this is an interesting “final-state” which has never been explored before (to my knowledge)

Production

Gluinos are “copiously” produced

Some ~10-20% or so would stop

30% of that time, both would stop

About 300 stopped gluinos in 2/fb for mg=300 GeV

The stopped gluinos are mostly central
(|eta|<1)

stop

LHC
100/fb

Tevatron

2/fb

Signal / Data

Large “jet” (E>100 GeV)

No tracks pointing to the jet

All energy on one side of the detector (even if the other gluino stops and decays, it's unlikely it would be during the same bunch-crossing!)

Unrelated to a p-pbar interaction

Require no min-bias interaction or underlying event to eliminate the beam-related background

Use the DIFF skim, selecting only triggers:
GapSN+CJT(2,3)+L3JET(15) – about 1Hz
GapSN+CJT(2,5)+L3JET(45) – about 0.1Hz

I've processed about 100/pb of PASS2 data: 3.6M events

Require MET>100 GeV

Most of the events are not gluinos. Backgrounds include:

Cosmic muons (about half of all events)

Beam muons (the other half of the events)

Detector effects (50% of events that have no muon segs)

Other?

Cosmic neutrons?

Beam and cosmic neutrinos (yes, I'm serious!)?

Double-diffractive interactions?

Cosmic Muons

Most of these are pretty clear...

Usually at least one muon segment (nseg=-3,-2,-1)
There are two chances to see the muon (on the way in, and going out)

Showers in the calorimeter can be quite large (>1 TeV !)
Hard Bremstralung photon
Narrower than “jets”:

More energy in single cells / towers

Smaller phi-width and eta-width

Usually an observable MIP trail

MIP

muon segment

narrow
shower

muon segment

narrow
shower

muon

Cosmic Muons

The observed rate is consistent with expectations

Spectrum at sea-level is known:
About 10-3/cm2/s above 100 GeV

Rate of hard Bremstralung is known:
About 0.1% will lose >10% of their energy

10m2 x 10-3/cm2/s x 0.1% = 0.1Hz

The muon and shower distributions are reasonable

Phi, Eta, and timing of muon scintillators
Muons are coming from the sky, and are being blocked by the earth!

top

bottom

Beam Muons

There are a large number of events (about as many as cosmic muons) which I believe are “beam muons”

Often a forward muon segment, or scintillator hits, or MDT hits

Scintillators are “in time” (|t|<10ns)

Protons (or anti-protons) hit gas in the beampipe or the beampipe itself to create pions, which decay to muons

Muons get bent by the dipoles, but have less momentum than the protons, so get bent to the “inside” of the ring -> phi=0
Another class of beam muons can escape the beampipe earlier on the beam curve and end up at the “outside” ->phi=pi
Some beam muons are at the other phi locations, rather evenly, but 99% are within dphi=.2 of phi=0 or pi

The MIP trail of the muon is usually visible

The shower is very narrow in phi
(phiwidth<0.05)

A typical jet has phiwidth>0.1

MIP

forward
muon segment

narrow
shower

“Detector Effects”

Detector problems tend to be in isolated runs, and manifest themselves in isolated regions of the detector

These are easily cleaned... even if it is “by eye”

Run 193803 has lots of events like this...

There are ~100 of these events with

a “jet” at eta=~.7 and phi=~1.35

Not in any isolated set of runs...

Background Estimation

Understanding the number of expected background events which would pass the muon, MIP, and jet shape cuts is crucial

We can make progress by assuming that the probability of passing each of these cuts is uncorrelated

This assumption can be tested, since we have 3 variables... signal free.
For jets failing shape cuts, is there a correlation between losing the muon segments and losing the MIP trail?

Look at muon events (with a reconstructed muon) and look at how often we miss the MIP trail and how often we fail the jet shower width cut

Look at very narrow jet showers with a MIP trail, and measure how often we miss the muon

The final probability to fail all 3 cuts is then just:
Pback = P(no muon) x P(no MIP) x P(good jet)

The final number of expected background events is then simply:
Nback = Nmuon /(1-Pback) x Pback

Other Backgrounds

Cosmic neutrons reach sea-level.
No muon segment, no MIP trail, wide shower!

The rate is 1/1000th of cosmic muons at the same energy... i.e. ~1/hour on detector

They would have to get through the iron toroids this seems difficult

Neutrons that did get through would deposit most of their energy in the Coarse Hadronic calorimeter (on the outside), and at the top of the calorimeter

This would be a good discriminant

Neutrinos (!) would be a small background:
Assuming a 1/1 ratio of muons/neutrinos from cosmic and beam sources
0.1 Hz / 0.1% Brem * 6e23 * 17g/cm3 / 238 g/mole * 400cm * 10e-38cm2/GeV * 500 GeV =
10e-8Hz = 0.1/year

Could double-diffractive events slip through, giving jets with no tracks? How would you get large MET? No sign of these leaking through, so far.

cosmic neutron flux at sea level

neutrino-nucleon cross-section

Signal Efficiency

The events have high energy deposits, trigger ~100%

Offline, conservatively:

90% have no MIP trail, 90% pass jet shower shape cuts, 90% have no muon segments.
Overall efficiency: ~70 +- 20 %.
More careful studies could be done...

However, the energy deposits are “out of time” !

We only sample the energy ONCE, when the shaped signal is expected to be at its maximum

If the energy is “early” then we have to worry about BLS removing part of the signal energy

What happens at L1??? I've got rough answers from Dean, but need to talk to Dan Edmunds.
The shaping time is much shorter (~120ns?).
But there's no BLS...
An estimate could be made using unbiased muon-triggered cosmic events.

No falloff observed in the number of muon-induced calorimeter showers, vs. muon time

But muon window only goes out to ~60ns, and I would expect the calorimeter response to start to fall off after ~140ns...

The number within the muon scintillator time window (10-50ns) in a run is 2983 and outside there are 17254. Assuming the full event time is 396ns gives an estimate of the falloff:
[(17254-2983)/2983] / [(396ns-40ns)/40ns] =~ 50%
Comparable to a 50% decrease in measured energy.

320ns

396ns

Current Results / Future Studies

I am left with ~5 events in the 100/pb with:

E>200GeV

No muon segments or muon hits which line up with the calorimeter cluster

Jet phi width >0.05

No observable MIP trail

Does not fall into a “detector effect” pattern

Assuming that each cut is 2% inefficient, the number of observed events is roughly explained by the number of background events we would expect to pass all the cuts

Future plans / work:

Optimized and automated muon-segment,
jet-shower, and MIP-trail cuts

A more precise estimate of the “muon backgrounds”

An estimate of the contributions from “other backgrounds”

A more precise estimate of the L1 trigger and offline inefficiency due to out-of-time energy

Look for time-coincidences in candidates (since 30% of the time, both gluinos stop, if one does)

Search for “di-jet” signature, since G->q+q+MET ~50% of the time

Claim discovery or set limits!

Andy Haas

2004-07-07T15:59:13