hepph0405052 SUSY Spin Measurements with ATLAS Alan Barr

hep-ph/0405052 SUSY Spin Measurements with ATLAS Alan Barr So … is it really supersymmetry? “If it quacks like SUSY …” “What else could it possibly be? ” “Don’t be so sure … ” 12 May 2004 Alan Barr UK ATLAS Physics

(S)particle reminder SM Spin-1/2 Spin-1 Spin-0 SUSY quarks (L&R) leptons (L&R) neutrinos (L&? ) Z 0 W± gluon h 0 H 0 A 0 H± 12 May higgs 2004 sector Extended Alan Barr (2 doublets) B W 0 squarks (L&R) sleptons (L&R) sneutrinos (L&? ) Bino Wino 0 Wino± gluino Spin-0 “Left squark” = scalar SUSY partner of left quark After Mixing 4 x neutralino Spin-1/2 ~ H 0 ~ H± gluino 2 x chargino UK ATLAS Physics

[RPC SUSY] Gaugino mass term (Ge. V) 1 -year low-lumi m. SUGRA reach • Sparticle decays generate: – Missing ET (LSP) – Jets (squarks/gluino) – Leptons (sleptons) Plot by D. Tovey 12 May 2004 Alan Barr • “If we see these at LHC we have discovered SUSY” • Not necessarily… Scalar mass term (Ge. V) UK ATLAS Physics

Universal Extra Dimensions Cheng, Matchev hep-ph/0205314 “How to get fooled at the LHC” • Te. V-scale universal extra dimension model • Kaluza-Klein states of SM particles – same QN’s as SM Radius of extra dimension ~ Te. V-1 – mn 2 ≈ m 02 + n 2/R 2 [+ boundary terms] – same spin as SM KK tower of masses n=0, 1, … – KK parity • Lightest KK state stable, and weakly interacting • 1 st KK mode pair-produced • First KK level looks a lot like SUSY 12 May 2004 Alan Barr UK ATLAS Physics Dubbed “Bosonic Supersymmetry”

Cheng, Matchev UED KK spectrum hep-ph/0205314 1 st excited KK level (example masses) After adding boundary terms -> SUSY-like spectrum 12 May 2004 Alan Barr Example decay spectrum Stable lightest KK particle (weakly interacting) ATLAS Physics Need to UK measure spins!

SUSY decay chain • Use m. SUGRA LHC point 5 as example • Well studied in context of sparticle masses • Nice leptonic signal at LHC Spin-0 Spin-½, mostly wino Spin-0 Final state = jet + l+ + l- + ET ( + decay of other sparticle) 12 May 2004 Alan Barr UK ATLAS Physics Spin-½, mostly bino

Spin projection factors In rest frame of with +ve z-axis aligned with q. L 12 May 2004 SM particles as massless UK ATLAS Physics Approximate Alansince Barr m « p -> okay P S

Spin projection factors P In rest frame of with +ve z-axis aligned with q. L S Σ=0 S Spin-0 12 May 2004 SM particles as massless UK ATLAS Physics Approximate Alansince Barr m « p -> okay

Spin projection factors In rest frame of with +ve z-axis aligned with q. L θ* 12 May 2004 SM particles as massless UK ATLAS Physics Approximate Alansince Barr m « p -> okay p S

Spin projection factors P In rest frame of with +ve z-axis aligned with q. L S Σ=0 θ* S Spin-0 12 May 2004 SM particles as massless UK ATLAS Physics Approximate Alansince Barr m « p -> okay mql - measure p S Spin projection factor

invariant mass (1) near lepton quark θ* l+ Phase space χ 20 Phase space -> factor of sin ½θ* Spin projection factor in |M|2: l+q -> sin 2 ½θ* l-q -> cos 2 ½θ* 12 May 2004 Alan Barr Back to back in 20 frame Probability near l q UK ATLAS Physics l- m/mmax = sin ½θ*

lnearq invariant mass (2) (Lepton from 2 decay only – not directly measurable) 0 squark decay 0 1 l+ 1 anti-squark decay l+ l- Obvious asymmetry! Opposite asymmetry! -> if same # squarks as anti-squarks effect cancels 12 May 2004 Alan Barr l- UK ATLAS Physics

How to measure? • lnearq shows nice charge asymmetry – Excellent probe of spin of • Experimental problems in measurement: – Can’t tell near lepton from far lepton • Plot l+q and l–q instead -> contributions from both near and far – Can’t tell quark jet from anti-quark • Sum of q and q-bar is measurable • pp collider -> get squark asymmetry from PDFs 12 May 2004 Alan Barr UK ATLAS Physics

Production asymmetry qg -> squark + gluino x 1 and x 2 sampled by MC Valence quarks Peak near 0. 1, 0. 1 Parton distribution function has more q than q-bar for Bjorken x ≈ 0. 1 12 May 2004 Alan Barr -> pp collider produces more squarks than anti-squarks UK ATLAS Physics (Twice as much for our point)

Parton Level l- l+ spin-0=flat Charge asymmetry Experimentally measurable -> q and q-bar -> near and far leptons 12 May 2004 Alan Barr difference/sum Shape indicates that 0 UK ATLAS Physics 2 is spin-½

l- Change in shape due to chargeblind cuts l+ parton-level * 0. 6 Charge asymmetry, Events After detector simulation (ATLFAST) Invariant mass sp in ½ spin-0 detector-level -> Charge asymmetry survives detector sim -> Same shape as parton level (but with BG and smearing) 12 May 2004 Alan Barr UK ATLAS Physics For cuts see C. Lester thesis

Cross-check No asymmetry if spin correlations turned off • Monte Carlo is HERWIG (6. 5) • Can turn off spin correlations – distribution for “scalar 20” – consistent with flat – not consistent with spin-½ 20 of previous page (linearly increasing) spin-0=flat spin-½ -> spin-0 -> 12 May 2004 Alan Barr production asymmetry UK ATLAS Physics

Further evidence - slepton spin • Dilepton invariant mass. Straight line distn (phase-space) Back-to-back in slepton frame Shape after different-family subtraction 12 May 2004 Alan Barr – “Right-handed” slepton – l+ and l- are right handed – might expect pronounced spin effects (as for lq distributions) – none because slepton is scalar • Scalar particle carrying lepton number – sounds like a slepton to me! UK ATLAS Physics

Conclusions • Method for measuring spins of SUSY particles at the LHC • Measure l±q invariant mass distributions – equivalent to angular distributions in • Look for lepton charge asymmetry • Requires initial asymmetry in squark vs antisquark production – valence quarks in PDFs at x ~ 0. 1 • Measure spin-½ nature of 20 • Also spin-0 slepton from mll • Should be enough to kill “straw man” e. g. universal extra dimensions 12 May 2004 Alan Barr UK ATLAS Physics