Results of the
Search for Stopped Gluinos
using the DØ Detector

Andy Haas

Columbia U. / Nevis Labs

DØ / ATLAS

Columbia Seminar

Oct. 18, 2006

Outline

What's a stopped gluino?

The Tevatron and D0 detector

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

Future improvements

SU/SY

You've heard of Supersymmetry

gluons <-> gluinos

so gluinos are neutral, but colored, and spin ½

What's “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 stablized by something else?

Chance! -> Anthropic principle -> works for the cosmological constant, why can't it work for the Higgs mass???

Then the scalars could be very heavy -> near the GUT scale

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

You still get all of the other nice things from SUSY

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

Dark matter (there's still a 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 >10us and <~1hour 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)

The Tevatron at Fermilab

1 km radius p-pbar super-synchrotron

World's most powerful: 1.96 TeV

36 bunches of p/pbar, 396 ns crossing period

25 cm RMS interaction region

About one “store” per day

Inst. luminosity ~ 40e31/cm^2/s

About 20/pb per (good) week

>1/fb on tape so far

This analysis uses just the first 0.35/fb

DØ

A

B

C

Muon
layers

Luminosity
counters

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 vertex location to match the distribution expected from stopping gluinos

at standard
interaction
point

at interaction
point inside
calorimeters

Signal Simulation

Events are weighted such that “r” is proportional to the 3mb “Wacker distribution”

Other distributions (0.3 -> 30mb) are used for systematic uncertainties

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 : tanh(eta)

Simulation

Typical

Muon

Tracks

A-layer “splash”

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 (maybe a 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

A few tracks are reconstructed (with large impact parameters)

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

Unfortunately, all these odd signatures are also present in the background, at similar rates!

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

Data and Backgrounds

Data:

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 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, fake energy

Other : cosmic neutrons / neutrinos?

Cosmic muon.

Data!

Cosmic Muons

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

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

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 dedicated algorithm available for identifying them

A difficult reconstruction problem!

Can be faked by gluino jets as well

Not used in the analysis

Cosmic muon

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

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, or MDT hits...

scintillators are “in time” (|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)

No sign of this background after cuts

diffractive

rest is cosmics

PV eff. is high in diffractive events

(ineff. mostly due to cosmics)

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

No sign of remaining detector malfunctions after cuts

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)

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

Event Selection

Jet |eta|<0.9 (gluinos stop centrally, and forward calorimeter 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

We want to know the fraction of wide-showering, cosmic muon events for which we don't reconstruct the muon

This is our background!

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

If this is true, then we can:

measure P(nomu) in the narrow jet sample (etawidth and phiwidth < 0.08) - that we know are cosmics

apply P(nomu) to the wide-shower sample (etawidth and phiwidth >0.08) containing reconstructed muons - that we also know are cosmics - to obtain an estimate of the background

Sounds nice, but there's some tricky details...

narrow shower
got muon

narrow shower
not got muon

wide shower
got muon

ESTIMATED

wide shower
not got muon

P

P

These are all cosmic muon events...

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

Data vs. Background

beam muon
dominated

cosmic muon

dominated

Signal Candidate

Signal Efficiency

The events have high energy deposits

jet trigger eff. ~100%

But we also require a GAPSN... which is dead if there is a min-bias interaction, or we're in a 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:

Just count the bunch crossings we're live for

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 in each cell only once per 396ns – at the assumed signal peak

Out of time energy will be under-estimated

No falloff seen in plot of energy vs. shower time (as measured using muon scintillators – resolution ~5ns)

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

Look at cosmic muon events within the muon scintillator timing window, and compare to the number of cosmic muons observed outside the window

If the calorimeter energy is lower, we won't observe events as often when they are outside the window (either due to trigger inefficiency or energy falling below offline 90 GeV threshold)

Determine effect to be an average 15+-5% decrease in measured energy

396ns

Calorimeter shaper
timing

0

10

50

386

396ns

scint.

no scint. info

beam-muon

contaminated

Systematic Uncertainties

Systematics on signal efficiency come from various sources

Geometrical / kinematic

jet shape cuts

jet energy cut

2nd jet veto

eta cut

muon cuts

Data / MC calorimeter response

Simulation of the out-of-time response

Trigger efficiency

The GAPSN requirement – inefficiency due to min-bias overlap – dependent on luminosity profile

The background of wide-cosmic-showers has a normalization of +-10%, due to statistics of the narrow-cosmic-shower sample at high energy

There's an uncertainty on the total integrated luminosity of 6.5%

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 of the energy ranges

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

gluino theory

stopped gluino limits

(m_LSP = 50,90,200 GeV)

Limits for mLSP=90 GeV

Improvements

The following have been done, but are under review (will be in the publication)

Limits can be set as a function of gluino lifetime

A model of stores has been created, and limits have been set up to 100 hours

Care has been taken in modeling the out-of-time calorimeter energy response

Instead of just modeling the average loss of efficiency, the resolution effects are taken into account

Data/MC energy scale differences are taken into account

Somewhat important since we're using uncorrected jet energies

There's 4x more integrated luminosity on tape

And it's at higher instantaneous luminosity -> proportionally less cosmic background

But a new trigger is needed at high luminosity, since min-bias overlap is introducing significant inefficiency for the GAP trigger

Jet showers are sometimes wide in both phi and eta, but are narrow in some other linear combination of phi and eta... need to reconstruct the minimal shower width

This would require a dedicated study, but could reduce backgrounds by ~2x

Since ~30% of gluinos have a pair-produced partner which also stopped, the time-coincidence of candidates should be studied

Conclusions

First search (that I know of!) for out-of-time jets appearing from within a calorimeter

A very challenging analysis...

Non-standard signal simulation (“beam” inside calorimeter!)

Many signal efficiencies and backgrounds were studied for the first time

New method for estimating the remaining cosmic background

Limits were set in the stopped gluino model on mG vs. mLSP

Analysis is already sensitive to mG ~ 300 GeV

There are still several ways to improve the sensitivity of the analysis

Things only get easier at ATLAS ?

14 TeV

Underground cavern (1/10th the cosmic rate)

Higher bunch-crossing rate (25-75 vs. 396ns) and inst. luminosity (100x)

5 samples per crossing in LAr electronics -> ~1ns time resolution

But will pose new challenges...

Trigger?

More overlap with previous events

No Iron toroid magnet

Andy Haas

2004-07-07T15:59:13