Unusual Signatures at Hadron Colliders

Andy Haas

Columbia U. / Nevis Labs

DØ / ATLAS

Columbia “Particle Fest” '07

October 5, 2007

Introduction

Most known particles decay quickly

Exception, weak-decays:

b, tau ~ mm

s ~ cm's to km's

muon, neutron ~ even longer

New particles could be short
or long lived

Decay only possible through a heavy virtual intermediate?

beampipe

1cm

M

squark

gluon

gluino

M

d

u

e

nu

W

Introduction

New long-lived particles look unusual

Dedicated analysis, triggers, and (sometimes) detectors required!

Difficult, but interesting

Not my day job (I do Higgs)!

Two examples of long-lived particles in this talk:

Stopped gluino

Neutral scalar -> b+b

Many more possibilities:

Magnetic monopoles

Charged heavy long-lived particle

Long-lived particles decaying to photon, Z

...

1cm

Z'

tracker

SU/SY

Normal SUSY has the mass of scalars near the EW scale

Stabilize the Higgs mass

Higgs mass is stabilized by something else?

Anthropic principle?

Scalars not protected by symmetries

could be heavy (GUT-scale)

Fermions protected by chiral symmetries

gluinos, neutralinos still light (Weak-scale)

Still get all the other nice things from SUSY

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

Dark matter

The right symmetries for string theory, at high energy

And a bonus! Suppression of stuff we don't see!

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

Stopped Gluinos

Gluino is long-lived

-> hadronizes into “R-hadrons”

Some lose enough momentum through ionization

-> stop in the calorimeters

Decay later into gluon+LSP

Lifetime depends on MSUSY

See hep-ph/0506242, J. Wacker

Signature:

large, isolated energy deposit in the calorimeter

rest of the “event” very empty

First search where the decay is in a different bunch crossing than the production!

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 (>30us 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

About one “store” per day

~3/fb on tape so far

DØ

A

B

C

Muon
layers

Luminosity
counters

Stopped Gluino Simulation

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

Simulate using Pythia and our detector simulation

Decay location is in the middle of the calorimeter

Custom modifications needed

Further weighting of events by vertex location to match the distribution expected from stopping gluinos

ATLAS simulates the full production, stopping, and decay - fancy!

at standard
interaction
point

at interaction
point inside
calorimeters

Simulation

Typical

Muon

Tracks

A-layer “splash”

Signal Distributions

Gluino jets look like normal jets...

They are wide
(phiwidth,etawidth > 0.08)

Good discriminant against muon-induced showers !

Peak in the jet energy, related to gluino and neutralino masses:

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 Sample

Want an empty event with a large calorimeter shower

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

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

Require a ET > 45 GeV “jet”

Offline pre-selection cuts:

Require one jet with E>90 GeV

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

No vertex found using tracking detectors

~1 million events

What are all these?

Normal event

Selected data

Cosmic Muons

A major background, due to high rate

Usually at least one (BC layer) muon segment

Two chances to see the muon

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

Hard Bremsstrahlung photon is dominant

Narrower showers than “jets”:

More energy in single cells (n90 < 5)

Smaller phi- and eta-width (<0.08)

But hadronic showers are also possible

Visible MIP trail from muon about half the time

No dedicated algorithm available

A difficult reconstruction problem

Can be faked by gluino jets as well

Not used in the analysis

Cosmic muon

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

In the plane of the beam!

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

Often a forward muon segment, or scintillator hits... “in time” (|t|<10ns)

Phi

Beam Muons

Beam Muons

Negligible contribution to the wide-shower data

But narrow-shower data contain a contribution from beam muons

Study the properties of beam muons by looking at “in time” vs. “out of time” muons

Beam muons are removed from the narrow-shower data by requiring:

not in regions around integer values of phi/pi

looking at high energy showers

Diffractive Background

Double-diffractive events?

Require no Primary Vertex

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

Negligible background after cuts

diffractive

rest is cosmics

With vertex

Detector “Problems”

Detector problems tend to be

in isolated eta-phi regions

in isolated data-taking periods

They really don't look like jets at all

No sign of remaining detector issues after cuts

Cosmic Neutrons

Cosmic neutrons reach sea-level...

No muon segment, wide shower!

Rate is 1/1000th of cosmic muons at the same energy... ~1/hour on detector

Have to get through the iron toroids

Neutrons would deposit most of their energy in the Coarse Hadronic calorimeter (on the outside)

We don't trigger on CH energy

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

cosmic neutron flux at sea level

Cosmic Neutrinos

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

neutrino-nucleon cross-section

Candidate Selection

“Wide jet showers”:

d_eta > 0.08 and d_phi > 0.08

n90 >= 10

“No-muon showers”

No B,C -layer muons

dphi between A-layer muons <1.5

dphi (muon, jet) <1.5

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

Signal Candidate

Cosmic Background Estimation

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

Detector inefficiency / acceptance

Measure probability to lose muon in the narrow-showering, cosmic muon data sample

narrow shower
got muon

narrow shower
not got muon

wide shower
got muon

ESTIMATED

wide shower
not got muon

P

P

Data / Background Comparison

Remove beam-muon contribution to narrow-jet data

Left with a cosmic-muon narrow-shower data sample

Measure P_nomu = 0.11+-.01

Good data/background agreement in wide-shower energy spectrum

Signal Efficiency

Trigger “dead” if a ppbar collision takes place during the stopped gluino decay

Efficiency measured relative to a non-GAP high-ET jet trigger -> ~60%

Averaged over all luminosities

Corrections made for non-linear effects due to luminosity profile

+ normal event

gluino decay

Signal Efficiency

Trigger is “dead” in the “gaps”

Just count the bunch crossings:

22% due to “cosmic gaps”
-> could be recovered

11% in “sync gap”
-> not recoverable

Tevatron
bunch

structure

Out-of-time Calorimeter Energy

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

Out of time energy under-estimated!

Look at cosmic muon events within the muon scintillator timing window

Compare to the number of cosmic muons observed outside the window

Simulate using info from calibration pulses

396ns

Calorimeter shaper
timing

0

10

50

386

396ns

scint.

no scint. info

beam-muon

contaminated

Visible jet energy fraction

Stopped Gluino Limits

Four jet energy ranges were chosen

Count the number of signal, background, and data events in each energy range

Set limits using Bayesian calculator

Translate to limits on gluino mass, for given neutralino masses

Determine dependence on gluino lifetime

Compare to theory predictions

Phys. Rev. Lett. 99, 131801 (2007)

Future Stopped Gluino Searches

~10x more integrated luminosity on tape

Higher instantaneous luminosity

proportionally less cosmic background!

But new trigger is needed
-> few empty crossings!

Trigger during empty crossings
-> looking into this possibility at D0

~30% of gluinos have a pair-produced partner which also stopped

time-coincidence of candidates

Future Stopped Gluino Searches

Some things get easier at ATLAS

14 TeV -> larger signal cross-section

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

Underground cavern
(1/100th the cosmic rate)

But no Iron toroid magnet

5 samples per crossing in LAr electronics

But search faces new challenges

Must trigger during empty crossings!

How empty will empty crossings be?

Beam-muon background?

Cavern background?

...

Hidden Valley

Neutral, long-lived scalars

Prefer to decay to heavier things

Helicity suppressed

like pion -> mu

Study decay to bb

Strassler et al., hep-ph/0604261

Hidden Valley Signal

b-b vertex far from the beam

>4 cm in XY

Beyond distance from direct b production

Maybe another on the other side of the event

Depends on lifetime

Can trigger on muon from the decay of a b-quark

Vertices in Data

Many vertices at large radius!

Dominated by material interactions with the detector

For low number of tracks (<4)

Photon conversions

In-flight decays (of known particles)

For high number of tracks (>5)

Nuclear scattering...

cm

Background Simulation

GEANT does a good job of simulating these nuclear interactions!

Distributions of vertex mass, number of tracks, angles, etc. are in good agreement

Simulated

with

GEANT

Analysis Plans

Still finalizing the analysis selections

Will depend on particle mass and lifetime

Several handles to separate S / B

Track multiplicity, angles, mass...

Near detector material?

Muon matched to far vertex?

Could require a second far vertex

Far Vertex Collinearity

Angle between vertex momentum (sum of its tracks' momenta) and vertex direction

Good discrimination for slow-moving, low mass decays

primary

vertex

decay

vertex

Far Vertex Mass

Invariant mass (^2) of the tracks' 4-momenta (assuming they are pions)

Good discrimination, especially for high mass decays

Conclusions

Many models of new physics predict particles with unusual properties

Long lifetime is one example

Stopping gluinos

Long-lived scalars -> bb

Specialized analyses are required

Typically interesting and fun!

Shame to miss a signal that the detector is capable of seeing

Room for finding weird things at the Tevatron is narrowing

Studies are being extended to LHC

Large ATLAS hidden-valley group

Andy Haas

2004-07-07T15:59:13