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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

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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