supersymmetry measurements with atlas tommaso lari (cern/infn milano) on behalf of the atlas...

Supersymmetry measurements with

ATLAS

Tommaso Lari (CERN/INFN Milano)On behalf of the ATLAS Collaboration

After we have discovered New Physics, can we understandwhat it is?

Tommaso Lari 2January 5th-9th, 2009

Overview

Supersymmetry. What we might know from inclusive searches.

Measurements possible with very first data (~1 fb-1) Some of the possibilities with high luminosity Beyond masses: spin measurements Conclusions


Wanted (live or dead): SUSYAdd to each SM boson (fermion) a fermionic (bosonic) partner.

Partners should not be too heavy (< 1 TeV) to solve the hierarchy problem

MSSM: ~100 free parameters (all possible SUSYbreaking terms in the EW scale effective lagrangian) Constrained models have few parameters, with assumptions

mSUGRA parameters


Typical LHC scenario Abundant production of strongly interacting scalar quarks and gluinos They decay to some SU(2)xU(1) gaugino and jets Decay chain ends with stable, invisible LSP

Signatures: Missing energy+jets+somethingExamples of something: nothing, 1,2,3 leptons (e,m), , , Z, hCorresponding searches sensitive to a large number of SUSY

models/parameters, but also to other new physics with similar signatures


What might we know from inclusive analyses?

ATLAS

10 fb-1

ATLAS 10 fb-1

First step: establish excess over Standard Model expectations, make sure it is from new physics

The Atlas Collaboration, Observation of events with large transverse missing energy and high pT jets in pp collisions at s=1x TeV

Points to production of strongly interacting particles with undetectable particles in final state. It might be SUSY or something else.


Information to establish SUSY?

Each SM particle has a superpartnerTheir spin differ by ½ The couplings are the sameSUSY mass relation holds

Information desiredProduction cross sectionsMasses of new particlesAngular distribution of decaysBranching ratios

Observables

Inclusive observables are for example cross sections, rates of specific search channels, average pt of photons, etc. Exclusive analysis (this talk) isolate specific decay chains. Most ofthe work so far aims at measuring the masses of new particles. Spin measurements from angular distributions also possible in some cases.


Some comments on models SUSY models have typically long decay chains with several

particles in the final state The SUSY combinatorial background is usually much larger (and

less known!) than the Standard Model background For a realistic study of the feasiblity of a measurement technique,

simulation of the decay chain of interest is not enough. All the SUSY production cross section for a specific point in a model parameter space is needed

The results I show have been obtained with mSUGRA benchmarks The techniques should be applicable whenever the relevant decay

chain is open But the precision of the measurements IS model dependent


mSUGRA benchmarks Benchmarks have been chosen requiring that neutralino relic density

matches DM constraints SUn = mSUgra benchmark n (no reference to simmetry groups!)


Benchmarks details

For this talk I will show results forSU3: in bulk region, squark and gluino masses 600-700 GeVSU4: just beyond Tevatron limits, squark and gluino masses ~400 GeV


Some references Many results shown are from the recently published The ATLAS Collaboration, Measurement from Supersymmetric events, in

Expected Performance of the ATLAS experiment, CERN-OPEN-2008-020, pages1611-1636.

Summarizes three years of studies by the collaboration, focus is on initial data (~1 fb-1, moderately understood detector), all results are with full simulation

I will present also some earlier published work to show what else may be done with more (~300 fb-1) integrated luminosity

B.K. Gjelsten et al., A detailed analysis of the measurement of SUSY masses withthe ATLAS detector at LHC, ATL-PHYS-2004-007M. Biglietti et al., Study of the second Lightest neutralino spin measurement with The ATLAS detector at LHC, ATL-PHYS-PUB-2007-004G. Polesello and D.R.Tovey, JHEP 05 (2004) 071.U. De Sanctis et al., Eur. Phys. J. C52, 743.


The edge method With two undetected particles with unknown mass in the final state

it is not possible to reconstruct mass peaks The typical approach is to look for minima (thresholds) and

maxima (edges) of visible invariant mass products 2 two-body decays: the invariant mass of p,q (massless SM particles) has a maximum at and a triangular shape if the spin of particle b is zero.

3 successive two-body decays• Four invariant mass combinations of the three visible particles: (12), (13), (23), (123)• For the first three minimum is zero: only one constraint. The last has both non-trivial minimum and maximum: five constraints in total on four unknown masses.

If sufficiently long decay chains can be isolated and enough endpoints measured, then the masses of the individual particles can be obtained


The two-lepton edgeExperimentally very clean Lepton 4-momentum measured with good resolution and very small energy scale uncertainty

(ultimate ~0.1%) Lepton flavour unambiguos The combinatorial background cancels in the flavour subtracted distribution:

ATLASPhysics TDR

Mll (GeV)

The relevant decay chain is open in a large fraction of SUSY parameter space.


Dilepton edge

SU3 (bulk point), two body decaysFitting function: triangle smeared with a gaussian

SU4 (low-mass point near Tevatron limits), three body decay.Fitting function: theoretical three-body decay shape with gaussian smearing

In reality more luminosity is needed to discriminate two-body and three-body decays from the shape of the distribution. With 1 fb-1 both fitting functions give reasonable 2.


Lepton+jets combinations Lepton+jets combinations give

further mass relations The two jets with highest pT are

likely from squark decay – but which one belongs to the right decay chain?


Lepton+jets combinationsllq edge

llq threshold

lqmax edge

lqmin edge

For this particular benchmark (bulk point SU3) all constraints measurable with 1 fb-1 !


Mass and parameter fitsFrom these edges it is possible to derive the masses of particles in the decay and place limits on

parameters of constrained models. Large statistical errors with 1 fb -1. Mass differences better measured than absolute masses.

Sparticle Expected precision (100 fb-1) qL 3% 0

2 6% lR 9% 0

1 12%

~

~

~

~

ATLAS

SPS1a, fast simulation, 100 fb-1SU3, full simulation, 1 fb-1


Tau lepton edges Taus experimentally more difficult than electrons and muons

Can only identify hadronically decaying taus, with smaller efficiency and larger jet fake rate than for first two generations

Neutrino energy not measured – no sharp edge! However they carry unique information

Information on the mass of the scalar tau in the decay chain Tau BRs are enhanced over first two generations at large tan, and it may be

that 2 → is the only two-body decay open.

The polarization of taus also carries interesting information (different in various SUSY breaking models). Feasiblity of polarization measurements still under investigation.

~~


Measurement of edge The inflection point of the

invariant mass fit function is in a linear relation with the endpoint

Systematics from the (unknown) tau polarization

Measurement of both endpoint and polarization is under investigation

SU3, full sim., 1 fb-1


An hadronic-only signature If A is pair produced and A → B LSP, the endpoint of

is the mass of A (if true m(LSP) is used). Applicable to mSUGRA qR as BR(qR → q 0

1 ~ 1) Analysis requires two hard jets and large missing energySU3, full sim., 1 fb-1

Sharp endpoint is visibleA linear fit gives while true qR mass is 611 GeV

~ ~ ~

~


A 3rd generation example Using the low-mass SU4 point with large BRs in 3rd

generation squarks

Study decay chain Fully reconstruct hadronic top, and subtract jjb combinatorial

background with jet pairs in W sidebands SU4, full sim., 200 pb-1

• For this very low mass point, the tb edge is in principle visible with very low statistics• In practice, need good undertanding of detector (b-tagging, jet reconstruction) before attaching this channel


High luminosity possibilities With 1 fb-1, many measurements may already be possible for

favourable SUSY scenarios The high luminosity potential studied in the past in fast simulation,

for example for SPS1a point in B.K.Gjelsten et al., ATL-PHYS-2004-007 With 300 fb-1 many measurements are

limited by JES sistematics Scalar lepton, gluino, scalar bottom masses also measured

Parameter Expected precision (300 fb-1) m0 2% m1/2 0.6% tan() 9% A0 16%

Parameter constraints (assuming mSUGRA)


Dark Matter connection The unseen LSP particle is a natural DM candidate. Within a given model, we can determine the parameter space

compatible with measurements and compute the corresponding the relic density

Exercise done in JHEP 05 (2004) 071 using SPS1a 300 fb-1 simulated measurements, and within mSUGRA.

h2 = 0.1921 0.0053 log10(p/pb) = -8.170.04


Focus Point study Interesting information possible from few measurements In Focus Point region relic density ok because gaugino mass parameters (M1, M2, ) are of the same order giving a large Higgsino component to 0

1 For SU2 benchmark, two lepton edges observable. Using only this info, a fit of gaugino mass parameters, assuming unification relation M1 = 0.5 M2 (but not mSUGRA) tells that indeed ~ M1

ATLAS 300 fb-1

→

l+l-

→

l+l-

M1 (GeV) M1 (GeV)

(G

eV)

tan

Eur. Phys. J. C52, 743


Test of spin hypothesisImportant to measure spin of new particles; it’s a fundamental

check to ensure that what we have discovered is SUSY!

The charge asymmetry is diluted because:1. Usually not possible to discriminate neat and far leptons: we sum qlfar and qlnear distributions2. The charge coniugate decay gives the opposite asymmetry. Cancellation not exact at a pp collider however.


Spin measurement

Cuts on EtMiss and jet pt to reject SM 2 opposite sign electrons or muons; combinatorial background subtracted using For SU3 point, 10 fb-1 already enough to exclude charge symmetry

SU3 point: 19.3 pb x 3.8%Ratio squarks/antisquarks ~3

ATLAS-PHYS-PUB-2007-004

ATLASATLAS


Conclusions

If SUSY discovery, long path to understand the nature of the involved signal

In favourable scenarios (gluino or squark mass of the order of 600 GeV) ATLAS has the potential to isolate specific decay chains and measure several kinematic endpoints already with an integrated luminosity of the order of 1 fb-1 (assuming well understood detector).

The reconstruction of a (large part of) the SUSY mass spectrum and a clue on the underlying physics model (including whether it is really SUSY) will require exploiting the full high luminosity potential of the LHC


Backup slides


Gluino and sbottom mass peaksOnce the mass of

is known, it is possible to get the four-

momentum using p(= ( 1-m(

m(ll) ) pll

valid for lepton pairs with invariant mass close to the edge. The

can be combined with b jets to get the gluino and sbottom masses in the decay chain g → bb → bb

~ ~

~

~

ATLASSPS1a100 fb-1

SPS1a, fast sim., 300 fb-1 SPS1a, fast sim., 300 fb-1

supersymmetry measurements with atlas tommaso lari (cern/infn milano) on behalf of the atlas...

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