Search for Stopped Gluinos

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

Gluinos hadronize into “R-hadrons”

Charged fraction is a free parameter of the hadronization model, but likely is ~1/2

Most could be neutral, lose a small momentum fraction, and escape:
See hep-ph/9806361

R-hadrons can also become charged when undergoing nuclear interactions

Some charged R-hadrons lose enough momentum through ionization to stop in the calorimeters

They later decay into jets+MET (LSP)

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

See hep-ph/0506242 – Dimopoulos, et al.

An interesting “final-state” which has not been carefully explored before experimentally

30% of the time, if one gluino is stopped in the calorimeter, the other one (which was pair-produced) is also stopped (since their pT is correlated)

Using this time-coincidence would be the only way to measure the gluino lifetime, and thus MSUSY – if split SUSY is reality !!!

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

LHC
100/fb

Tevatron

2/fb

~500 stopped gluinos in 2/fb for mg=300 GeV

~5 stopped gluinos in 2/fb for mg=500 GeV

Simulated Stopped Gluinos

To optimize S/B of analysis cuts and determine signal efficiency, need a Monte-Carlo simulation. Problems:

No gluino decays in pythia or other available MC
Use Z(->nunubar)+jet events (MSEL=13, force decay to IDC=183,185,187)
Also tried StdHEP->Pythia events, setting quark/gluon four-vectors “by hand”
More work needed to make spectrum flat in theta, phi

“interaction point” is not at (0, 0, 0+-25) cm
but is at all points in the calorimeters
Use “copyRCP” command in mc_runjob to overwrite the standard d0_mcpp/rcp/mcvertex.rcp file
-> (0+-20, 120+-20, 0+-100) cm
More work needed to simulate a realistic stopped gluino distribution

Don't want the “beam” going through the calorimeter!
Hacked mcpp_gen/src/MCgetEvent.cpp and used “CustomExecutable” override in mc_runjob to filter out particles from the beam and underlying event
Is there an easier way to do this? Pass a single quark or gluon to Pythia for hadronization / fragmentation?

Used 250<pT<300 in Pythia to set energy range of jets

This roughly corresponds to a 500 GeV gluino (half the energy is lost to the LSP)

at standard
interaction
point

at interaction
point inside
calorimeters

A Closer Look at Stopped Gluino MC

Gluino jets look much like normal jets...

They are fairly wide (phiwidth,etawidth > 0.08)

Good discriminant against muon-induced showers (as we'll see later...)

Gluino jets often have significant leakage / punchthrough into the muon A-layers

They may be in the CH when they decay!

About 10% of the time, a particle (maybe a muon) gets into the BC-layers, forming a good muon segment

Sometimes the jets are not contained and shoot back through the tracking volume to the other side of the calorimeter

A few tracks are reconstructed (with large impact parameters)

Sometimes two jets are reconstructed, near each other in dR

A-layer punchthrough

gluino-jet
cluster

gluino-jet
cluster

Stopped Gluino MC Plots

I've only made 100 events so far

Most of these plots are after the final selection criteria

Total efficiency was 37% at this stage

Trigger / Data

We assume that the gluino decays long after the p-pbar interaction that created it
(lifetime > ~few microseconds)

Require no min-bias interaction 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

These are nearly always unprescaled

I've processed the full PASS2 p14 dataset:
~350/pb, 7.9M events

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 (about half of the events that have no muons reconstructed)

Cosmic neutrons

Cosmic neutrinos (yes, I'm serious!)

Double-diffractive interactions?

Initial cuts:

Exactly one JCCB jet

Jet energy > 90 GeV (uncorrected)

Jet energy < 900 GeV (detector problems)

Jet |eta| < 0.9 (other areas too noisy for now)

No reconstructed PV

(to remove diffractive events)

Cut out hot spots
(-.55<eta<-.75 && 1.3<phi<1.5)

Jet phiwidth and etawidth < 0.25
(to remove some major detector failures)

Additional selections are made later to separate gluinos from background...

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

The MIP trail of the muon is usually visible

The shower is very narrow in phi (<0.08)

But is usually relatively wide in eta (>0.05)

MIP

forward
muon segment

narrow, long
shower

beam muons

cosmic muons

Cosmic Muons

These, along with “beam-muons” are the major background, due to their high rate

Usually there's at least one muon segment
(nseg=-3,-2)

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 is the dominant process at high energy loss

Narrower showers than “jets”:

More energy in single cells
(n90 < 5)

Smaller phi- and eta-width
(<0.08)

A visible MIP trail about half the time

MIP

muon segment

narrow
shower

muon segment

narrow
shower

muon

Cosmic Calculations

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

Detector Effects

Detector problems tend to be in isolated eta-phi regions

These can be removed

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...

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 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? There's no sign of this background.

Require no tracks in event (no PV)

Require large MET (1 jet, E>90 GeV)

cosmic neutron flux at sea level

neutrino-nucleon cross-section

Background Estimation

A basic assumption is that the probability to not reconstruct a muon in the muon system is independent of how that muon showers in the calorimeter

measure P(no mu) in the narrow jet sample (etawidth and phiwidth < 0.08) - that we know are nearly all cosmics

apply P(no mu) to the wide-shower sample (etawidth and phiwidth >0.08) containing muons - that we know are also cosmics - to obtain an estimate of the background, wide jets with no muons

Some tricky details:

The narrow shower sample has some detector effects (hot spots) not present in the wide jet sample

they must be removed by not allowing contributions from regions with more than 50 counts in this plot...

The narrow jets contain a contribtuion from beam muons, which have a different spectrum and a different probability to not find a muon

they must be removed by requiring

short shower in eta

not regions around phi = 0 or pi

narrow shower
got muon

narrow shower
not got muon

wide shower
got muon

ESTIMATED

wide shower
not got muon

P

P

eta

phi

Results

Final selection criteria:

Jet etawidth and phiwidth > 0.08
(wide showers)

Jet n90 >= 10
(to prevent wide shower fakes from MIP trails)

No nseg = -2 or -3 muons

(these are mostly cosmics, although we do lose some signal efficiency here)

dphi between A-layer muons (nseg=-1) < 1.5

(gluino jets often leak into the A-layer a bit, but we want to veto on back-to-back A-layer muons)

dphi betwenn A-layer muons and the jet < 1.5

(we want to veto a case where the A-layer muon is on the other side of the detector from the jet)

Signal Efficiency in Data

The events have high energy deposits, trigger ~100% ?

But we also require a GAPSN... which is dead if there is a min-bias interaction.

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

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

Wish we had those ATLAS electronics! :)

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

The shaping time is much shorter (~120ns?) at L1.
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%

320ns

396ns

Current Results / Future Studies

Andy Haas

2004-07-07T15:59:13