The Search for
Stopped Gluinos

Andy Haas

Columbia University

All D0 Meeting

Feb. 2, 2007

Outline

What's a stopped gluino?

Simulating the stopped gluino signal

The data used

Background types

Event selection

Estimation of remaining background

Comparison of data to remaining background

Signal efficiency

Systematic uncertainties

Limits on stopped gluinos

SU/SY

Supersymmetry

gluons <-> gluinos

gluinos are neutral, colored, but spin ½

Split Supersymmetry

Normal SUSY has the mass of scalars near the EW scale

This prevents the Higgs mass from being unstable due to loop corrections

This was kind of the point of SUSY to begin with!

But what if the Higgs mass is stabilized by something else?

Chance! -> Anthropic principle.

Works for the cosmological constant, why not the Higgs mass?

Scalars should be very heavy -> near the GUT scale

Fermions are protected by chiral symmetries -> gluinos are still “light”

You still get all of the other nice things from SUSY

Unification of gauge forces (doesn't depend on scalar masses!)

Dark matter (there's still a “light” neutralino -> spin 1/2)

The right symmetries for string theory, at high energy

And even a bonus!

Natural suppression of all the normal SUSY stuff we don't see! :)

FCNC's, EDM's, stop quarks, tri-leptons, etc...

What's a Stopped Gluino?

ppbar -> gluino pair production

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

Gluinos hadronize into “R-hadrons”

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

They later decay into gluon+LSP (or maybe qqbar+LSP)
(BR to gluon+LSP assumed to be 100% for this analysis.)

Lifetime depends on MSUSY but is >10ns if MSUSY >106 GeV
(Lifetime assumed to be >10 us and <100 hours for this analysis.)

See hep-ph/0506242, J. Wacker et al.

Signature:

Large, isolated energy deposit in the calorimeter

rest of the “event” should be very empty

no tracks or primary vertex

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

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

production

decay (>10us later)

Signal Simulation

The stopped gluino decay produces a single high-energy gluon (balanced by missing energy)

Model with Pythia Z+g/q (MSEL=13) and
force Z->nunu

1000 events each, with M ~ 200,300,400,500

|eta|<0.1, but rotated jet to random 3D direction

Boost along Z removed

Switch off ISR and multiple parton interactions (no underlying event)

Remove remaining beam particles by removing in MCKineChunk if |pz/E|>.95

Decay location chosen to be Gaussian-distributed:
(x,y,z)=(0+-20, 120+-20, 0+-80) cm

In the middle of the calorimeter

Then, further weighting of events by decay location to match the distribution expected from stopping gluinos...

at standard
interaction
point

at interaction
point inside
calorimeters

Signal Simulation

The expected radial distribution in the calorimeter is not well known theoretically

Depends on the conversion rate from neutral->charged R-hadrons in matter

Events are weighted such that “r” is proportional to an average of the distributions expected for 0.3 -> 30mb

Nearly flat

Gluinos produced near threshold at Tevatron, and also those near rest are most likely to stop

Isotropically distributed, proportional to cos(theta) : tanh(eta)

Events are weighted such that z is proportional to tanh(eta)

Getting a Feel for Gluinos

Typical

Signal Distributions

Gluino jets look much like normal jets...

They are fairly wide
(phiwidth,etawidth > 0.08)

Good discriminant against muon-induced showers !

Gluino jets often (30% of the time) have significant leakage / punch-through into the inner muon layer (“A-layer splash”)

They may be near the outside of the calorimeter when they decay

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

Rarely (<5% of the time) the jets are not contained and shoot back through the tracking volume to the other side of the calorimeter

Sometimes two jets are reconstructed, near each other in dR (about 10-20% of the time)

Energy

Cluster “n90”

eta

phi

eta
width

phi
width

Plots from mG=400 GeV sample

E (jet) = (400^2-90^2)/(2*400) ~ 190 GeV

A-layer “splash”

Tracks

Data and Backgrounds

Data -> p14 PASS2:

Use “GAPSN” triggers – already in place for diffractive physics

Require no hits in either the North or South luminosity counters (scintillators)

Also require a ET > 45 GeV jet

7.9M events, ~0.5Hz (!)

Pre-selection cuts:

Good muon, calorimeter, and tracking runs

0.5 cone jets (JCCB)
(no selection criteria, no JES)

Require one jet with E>90 GeV

Veto on any other jets (above ET of 8 GeV)

Backgrounds remaining:

Cosmic muons

Beam muons (halo)

Diffractive events from beam

Detector problems

Cosmic muon.

Data!

Cosmic Muons

These are the major background

Usually there's at least one good (BC layer) muon

There's two chances to see the muon
(on the way in, and going out)

Showers in the calorimeter can be quite large (>1TeV!)

Hard Bremsstrahlung 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...

No algorithm available for identifying them (unless they are projective)

A difficult reconstruction problem!

Can be faked by gluino jets as well

Not used in the analysis

Cosmic muon

Beam Muons

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

Often a forward muon segment, or scintillator hits

in time with the beam! (|t|<10ns)

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

Why are they focused at integer values of phi/pi ?

Accelerator magnet configuration ?

Plane of the beam ?

Muon shielding at D0 and/or CDF ?

The shower is very narrow in phi (<0.08) but relatively wide in eta (>0.05)

beam muons

cosmic muons

phi

Diffractive Background

Could “double-diffractive” events slip through?

Require no Primary Vertex

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

Background is negligible after these cuts

diffractive

rest is cosmics

With vertex

Detector “Problems”

Detector problems tend to be in isolated eta-phi regions or isolated data-taking periods

These can be removed

They really don't look like jets at all

Background is negligible after these obvious problems have been removed

There are ~100 of these events with

a “jet” at eta=~.7 and phi=~1.35

Not in any isolated set of runs...

Event Selection

Jet |eta|<0.9 (gluinos stop centrally, and forward calorimeters had more DQ problems)

No Primary Vertex (remove diffractive events)

“Wide jet showers”:

d_eta > 0.08 and d_phi > 0.08

muon-showers tend to be narrow

n90 >= 10

muon showers tend to have most of their energy in a few cells

“No-muon showers”

No |NSEG|=2,3 muons

dphi between any A-layer muons < 1.5

Accept A-layer muon splash, but reject back-to-back A-layer muons

dphi (muon, jet) < 1.5

Accept A-layer splash, but only if it is geometrically consistent with the jet

A candidate signal event passes both the “wide-jet” and “no-muon” criteria

Signal Candidate

Background Estimation

Background is wide-showering, cosmic-muon type events for which we don't reconstruct the muon

Probability to not reconstruct a cosmic muon in the muon system, P(nomu), is independent of how that muon showers in the calorimeter

measure P(nomu) in the narrow jet sample
-- we know these are cosmics --

apply P(nomu) to the wide-shower sample containing reconstructed muons
-- we know these are also cosmics --

All other backgrounds have been argued to be negligible after proper cuts

This is conservative when setting limits...

narrow shower
got muon

narrow shower
not got muon

wide shower
got muon

ESTIMATED

wide shower
not got muon

P

P

Data vs. Background

Signal Candidate

Signal Efficiency

The events have high energy deposits

Jet trigger efficiency ~100%

But we also require a GAPSN... which is dead if

min-bias interaction

trigger gap

GAPSN efficiency measured w.r.t. a non-GAP high-ET jet trigger

Averaged over all inst. luminosities

Corrections made for non-linear effects due to inst. luminosity profile

Inefficiency due to bunch structure is simple:

22% due to “cosmic gaps” could be recovered

11% in “sync gap” is not recoverable

Tevatron
bunch

structure

Out-of-time Calorimeter Energy

The energy deposits are “out of time”!

Calorimeter electronics sample the shaped pulse only once per 396ns – at the assumed signal peak

Out of time energy will be under-estimated

Also need to consider baseline subtraction

L1 response important for deciding which crossing gets triggered

No falloff seen in plot of energy vs. shower time (as measured using muon scintillators)

Unfortunately, muon timing only good from -10 -> 50 ns... limited by electronics

396ns

Calorimeter
precision-readout
shaper response

Calorimeter L1
shaper response

Systematic Uncertainties

Systematics on signal efficiency come from various sources

Geometrical / kinematic (20%)

jet shape cuts

jet energy cut

2nd jet veto

eta cut

muon cuts

Calorimeter response

Data/MC (9%)

Out-of-time response (12%)

Trigger efficiency (5-15%, depends on gluino lifetime...)

The GAPSN requirement

Inefficiency due to min-bias overlap

Dependence on luminosity profile

Background normalization (10%)

Luminosity (6.5%) - still using old value!

Limits

4 jet energy ranges were chosen, corresponding to the energies of the jets in the various signal mass samples

M-RMS/2 < E < M+RMS*2

Asymmetric because of exponentially falling background and symmetric signal

~80% of signal events passing cuts are accepted in the energy range

Overall efficiency is around 10%

Count the number of signal, bknd., data events in each energy range

Set limits using Bayesian calculator

Gluino Limits

There is only one observable (E), and two model parameters (mG, mLSP)

Given the simulated values of E, translate to other mG, choosing 3 fixed values of mLSP = 50,90,200 GeV

Compare to theoretical prediction, which is a function of mG

theory

stopped gluino limits

(m_LSP = 50,90,200 GeV)

Limits for mLSP=90 GeV

Assumes a short gluino lifetime...

Gluino Lifetime

Preliminary results assumed the gluino lifetime was < ~ 3 hours

Longer than this, and luminosity differences between the creation time and decay time start to become significant!

Production rate is dependent on luminosity at production time

Trigger efficiency is dependent on current luminosity at decay time

These effects are now modeled, up to 100 hours

Method:

Choose production time randomly,
according to inst. lum. distribution

Choose decay time = production time
+ random contribution from half-life
distribution

Figure out if we're still taking data:
assume a 24 hour store
assume a 50% chance of another store
starting, at least 6 hours later

Calculate the GAPSN efficiency at the current decay time
- modeled by exp(-0.3*lum),
where lum is the relative inst. luminosity)

Translate to ROOT

Gluino Lifetime

Get an estimate of the trigger efficiency which depends on the gluino lifetime

The efficiency for small lifetime (70%) is consistent with the original estimate used for preliminary results (60+-15%)

Limits can be set on stopped gluino cross-section, as a function of gluino lifetime, for an assumption of the LSP mass (50 GeV – the best-case scenario):

Range estimated for
preliminary results (0.6+-0.15)
(lifetime<~3 hours)

Conclusions

First search for out-of-time jets appearing from within a calorimeter

A very challenging analysis...

Non-standard signal simulation

Many signal efficiencies and backgrounds were studied for the first time

Thanks to all who have helped me to understand the detector, the MC, and the physics!

Peter Tamburello, John Parsons, Dean Schamberger, Dan Edmunds, J.F. Grivaz, Arnd Meyer, Catherine Biscarat, Yuri Gershtein, Dave Cuts, and anyone I'm forgetting...

Theorists: Jay Wacker and Matt Strassler

Data agrees well with the cosmic muon background

Limits on the cross-section for stopping gluinos vs. mG

for various mLSP, and as a function of gluino lifetime (up to 100 hours)

Compare to the expected cross-section from theory

sensitive to mG ~ 300 GeV, but there is large theory uncertainty

Finalizing treatment of out-of-time calorimeter response

PRL is under EB review

Cosmic Checks

The observed rate is consistent with expectations

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

Rate of hard Bremsstrahlung 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

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)

This would be a good discriminant

We don't trigger on CH energy

No excess of wide no-muon showers on the outside of the calorimeter is seen

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

cosmic neutron flux at sea level

neutrino-nucleon cross-section

Narrow Cosmic Showers

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

Simply don't consider any hot spots when deriving P(nomu)

If a bin of the eta/phi histogram is > 50, ignore the region corresponding to that bin

The narrow jets contain a contribution from beam muons, which have a different energy spectrum, come into the detector at strange angles, and therefore have a different P(no mu)

Fortunately, we can study the properties of beam muons, and their difference from cosmics, by looking at “in time” vs. “out of time” muons!

Beam muons are removed by requiring:

not in regions around integer values of phi/pi

looking at high energy showers (beam muons' energy spectrum falls off faster than cosmics)

eta

phi

Fraction of “in-time” muons
vs. jet shower energy

Background Estimation / Beam-muons

beam muon
dominated

cosmic muon

dominated

Andy Haas

2004-07-07T15:59:13