Measurement of the lsp mass
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Measurement of the LSP Mass. Dan Tovey University of Sheffield On Behalf of the ATLAS Collaboration. Contents. Motivation: Why measure the LSP mass? Will assume LSP ≡ lightest neutralino in this talk Natural in many SUSY models (constrained MSSM etc.)

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Measurement of the LSP Mass

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Measurement of the lsp mass

Measurement of the LSP Mass

Dan Tovey

University of Sheffield

On Behalf of the ATLAS Collaboration

1


Contents

Contents

  • Motivation: Why measure the LSP mass?

    • Will assume LSP ≡ lightest neutralino in this talk

    • Natural in many SUSY models (constrained MSSM etc.)

    • Will also assume R-Parity is conserved (RPV beyond scope of this talk)

  • SUSY particle mass measurements at the LHC

  • Measurement technique

  • Measurements using invariant mass 'edges'

  • Measurement combination: extracting particle masses

2


Why measure the lsp mass

Why Measure the LSP Mass?

10-3

10-4

10-5

10-6

DAMA

Allanach et al., 2001

  • Using mass of lightest neutralino and RH sleptons can discriminate between SUSY models differing only in slepton mass.

  • Use as starting point for measurement of other masses (gluino etc.)

  • SUSY Dark Matter

  • Lightest Neutralino LSP excellent Dark Matter candidate.

    • Test of compatibility between LHC observations and signal observed in Dark Matter experiments.

  • etc …

3


Neutralino mass measurement

Neutralino Mass Measurement

_

3H+g3He+ + e- + ne

  • Following any discovery of SUSY next task will be to measure parameters.

  • Will not know a priori SUSY model chosen by Nature g model-independent measurements crucial.

  • In R-Parity conserving models two neutral LSPs (often the lightest neutralino) / event

    • Impossible to measure mass of each sparticle using one channel alone

  • Instead use kinematic end-points to measure combinations of masses.

  • Old technique used many times before:

    • n mass from b decay end-point

    • W mass at RUN II using Transverse Mass.

  • Difference here is that we don't know mass of neutrals (c.f. n).

LHC mSUGRA Points

3

2

1

4

5

4


Dilepton edge

Dilepton Edge

~

~

c02

c01

l

l

Hinchliffe, Paige et al., 1998

~

  • Classic example (and easiest to perform): OS SF dilepton edges.

  • Important in regions of parameter space where three-body decays of c02 dominate (e.g. LHC Point 3).

  • Can perform SM background subtraction using OF distribution

    e+e- + m+m- - e+m- - m+e-

  • Position of edge measures m(c02) - m(c01) with precision ~ 0.1%.

Physics

TDR

ATLAS

Point 3

~

~

5


Dilepton edge1

Dilepton Edge

~

~

c02

~

c01

l

l

l

Polesello et al., 1997

~

~

  • When kinematically accessible c02 canundergo sequential two-body decay to c01 via a right-slepton.

  • Also results in sharp OS SF dilepton invariant mass edge sensitive to combination of masses of sparticles.

  • Can perform SM & SUSY background subtraction using OF distribution

    e+e- + m+m- - e+m- - m+e-

  • Position of edge (LHC Point 5) measured with precision ~ 0.5% (30 fb-1).

e+e- + m+m-

- e+m- - m+e-

e+e- + m+m-

5 fb-1

FULL SIM

Point 5

ATLAS

ATLAS

30 fb-1

atlfast

Modified Point 5 (tan(b) = 6)

Physics

TDR

6


Llq edge

llq Edge

~

~

~

c02

~

c01

qL

l

l

l

q

Bachacou et al., 1999

  • Dilepton edges provide starting point for other measurements.

  • Use dilepton signature to tag presence of c02 in event, then work back up decay chain constructing invariant mass distributions of combinations of leptons and jets.

~

  • Hardest jets in each event produced by RH or LH squark decays.

  • Select smaller of two llq invariant masses from two hardest jets

    • Mass must be ≤ edge position.

  • Edge sensitive to LH squark mass.

e.g. LHC Point 5

ATLAS

1% error

(100 fb-1)

Physics

TDR

Point 5

7


Lq edge

lq Edge

Bachacou et al., 1999

ATLAS

  • Complex decay chain at LHC Point 5 gives additional constraints on masses.

  • Use lepton-jet combinations in addition to lepton-lepton combinations.

  • Select events with only one dilepton-jet pairing consistent with slepton hypothesis

    g Require one llq mass above edge and one below (reduces combinatorics).

Point 5

Physics

TDR

ATLAS

  • Construct distribution of invariant masses of 'slepton' jet with each lepton.

  • 'Right' edge sensitive to slepton, squark and c02 masses ('wrong' edge not visible).

1% error

(100 fb-1)

Physics

TDR

~

Point 5

8


Hq edge

hq edge

~

qL

~

~

c02

c01

q

h

b

b

~

~

  • If tan(b) not too large can also observe two body decay of c02 to higgs and c01.

  • Reconstruct higgs mass (2 b-jets) and combine with hard jet.

  • Gives additional mass constraint.

ATLAS

Point 5

1% error

(100 fb-1)

Physics

TDR

9


Llq threshold

llq Threshold

Bachacou et al., 1999

ATLAS

Physics

TDR

~

  • Two body kinematics of slepton-mediated decay chain also provides still further information (Point 5).

  • Consider case where c01 produced near rest in c02 frame.

    • Dilepton mass near maximal.

    • p(ll) determined by p(c02).

~

Point 5

~

  • Distribution of llq invariant masses distribution has maximum and minimum (when quark and dilepton parallel).

  • llq threshold important as contains new dependence on mass of lightest neutralino.

Physics

TDR

ATLAS

Point 5

2% error

(100 fb-1)

10


Mass reconstruction

Mass Reconstruction

Allanach et al., 2001

  • Combine measurements from edges from different jet/lepton combinations.

  • Gives sensitivity to masses (rather than combinations).

11


Mass reconstruction1

Mass Reconstruction

Sparticle Expected precision (100 fb-1)

qL 3%

02 6%

lR 9%

01 12%

~

~

~

~

Allanach et al., 2001

  • Numerical solution of simultaneous edge position equations.

  • Gives pseudo model-independent measurements

  • Note interpretation of chain model-dependent.

  • Powerful technique applicable to wide variety of R-Parity conserving models.

~

~

c01

lR

Point 5

Point 5

ATLAS

ATLAS

Mass (GeV)

Mass (GeV)

~

~

c02

qL

Point 5

Point 5

ATLAS

ATLAS

Physics TDR

Point 5

Mass (GeV)

Mass (GeV)

12


Summary

Summary

  • Lightest Neutralino is the Lightest SUSY Particle in many models.

  • Measurement of SUSY particle masses in R-Parity conserving models complicated by presence of two LSPs in each event.

  • Use of kinematic edges and combinations of edges necessary to reconstruct individual masses.

  • Will allow test of SUSY model (CMSSM / mSUGRA, MSSM etc.).

  • Will also provide useful test of SUSY Dark Matter hypothesis.

13


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