Initial determination of the spins of the gluino and squarks at LHC Jing Shao University of Michigan Based on work with Gordon Kane, Alexey Petrov and Liantao Wang Madison, Apr. 28, 2008
Motivation • The upcoming LHC experiment new degree of freedom beyond SM SUSY? Extra dimension? …… ? • It is essential to confirm the spin of the new particle as predicated by these BSM physics. • Rigorous determination of the spins -- need a detailed analysis of the decay angular correlations -- difficult and complicated, particularly early with limited statistics A.J. Barr; J.M. Smillie & B.R. Webber; L.T. Wang & I. Yavin; P. Meade & M. Reece; C. Csaki, J. Heinonen & M. Perelstein; S.Thomas …… • Can we get any information about spin from early LHC data ?
Basic Idea • The information of the production cross section is extremely helpful in the initial determination of the spin. (though need to be confirmed later by the rigorous method) • The basic observation is that the cross sections of particles with different spin will differ significantly in many cases • If one production mechanism dominates, e.g. gluino at LHC, then measuring the cross section and mass of the new particle would immediately imply the spin.
Top at Tevatron • The production of top quark is dominated by s-channel q qbar g t tbar • For spin-0 or spin-1, there is a chirality suppression. But not spin-1/2. • Large difference in the cross sections spin of top was measured by σ + M μR= μF= s
Pair production of gluon partners • The production cross section of color octet is completely determined by the spin and its mass (assume color triplets are decoupled). • Very different for different spin -- g’: same-spin gluon partner (e.g.KK gluon) -- gs: spin-0 gluon partner -- scale: μR= μF= mass PDF: CTEQ6L
Pair production of quark partners • Cross section of squark pair production again depends on the spin and mass. (Assume color octets are decoupled) • Include first and second generation quark partners μR= μF= m q’: same-spin quark partner PDF: CTEQ6L
Caveats • The cross sections are calculated at tree-level. NLO calculation will enhance the cross section. However the K-factors for cases with different spin are not very different. – should not change the hierarchy of the cross sections • There could be uncertainty in measuring the production cross sections. • Measuring the mass of the new color particle is also not trivial – usually require high statistics (many talks in this section) • What should we do at early stage of LHC? -- first make hypothesis about the model and the spin -- then test it by fitting kinematical distributions as well as the cross section to the data. • No ambiguity about the spin if there is no degeneracy.
Degeneracy? • Typical transverse variables (Ht, Pt) are mainly sensitive to the mass differences between initial and subsequent particles in the decay chain mass measurement is hard (with low statistics) “degeneracies” (mass could be implied from other experiments, e.g. dark matter) • If one production channel is dominant, there may be “degeneracies” -- can be seen in the following few slides • However, generically there will be multiple production channels. -- probably no degeneracy, in the sense of distinguishing models with different spin assignment
Simulation • Same-spin scenario is implemented using MadGraph user model. -- spin-1 gluon partner g’ -- spin-1/2 quark partners q’ (u’,d’,c’,s’) • Parton-level events generated by MG/ME; use CTEQ 6L • Particles are decayed using BRIDGE and then showered and reconstructed using pythia + PGS4 • Use Level I trigger; mssing ET > 90GeV, Jet Pt > 50 GeV, η< 2.5
Pair production of quark partner • Squark mass = 550 GeV, same-spin partner q’ mass = 900 GeV cross sections are matched • Decay: q + Bino ; q’ q + B’ ΔM =M - MBino, B’ Same ΔM adjusted ΔM
Pair production of gluon partner • gluino mass = 800 GeV, same-spin partner g’ mass = 1060 GeV • Decay: q qbar + Bino ; g’ q qbar + B’ Same ΔM adjusted ΔM
More production channels • There are generically other production channels. For example in SUSY, pp / / are usually all important. • The relative difference in the cross sections of these channels are different for different spin scenarios – generically cannot be matched at the same time by adjusting the mass parameters. • This provides a way to break the degeneracy and determine the spin if the cross sections of each channels can be measured.
Cross sections of multiple channels • Compare SUSY with Same-spin scenario. • Match g’ pair production rate with that of gluino fix g’ mass
Cross sections of multiple channels • Compare SUSY and Same-spin scenario. • Match g’ pair production rate with that of gluino fix g’ mass • vary q’ mass to match gluino-squark production mq’=780 GeV • σ (squark)=8pb; σ(q’)=19pb -- cannot be matched! msquark = 550 GeV
More realistic study • In the real situation, it’s hard to measure cross section of each channel separately. • However, the differences could be seen in the jet multiplicity, if each channel leads to different number of jets (or other kinematical quantities). • This difference in jet multiplicity may be smeared by varying the masses if isolation cuts are imposed. However, even this happens, there could be other kinematical distributions which are different. • To show whether the degeneracy exist, we simulate a SUSY model and exam whether mass parameters in an alternative scenario can be tuned to fit to that data.
Example (Mgluino < Msquark ) • SUSY: gluino mass 630GeV, squark mass 800GeV. σ ~ 11 pb gluino q qbar + squark, squark gluino + Bino • match total cross section and jet-multiplicity mg’=950GeV, mq’=1120GeV,mB’=600GeV • HT distribution cannot be matched (notice the difference in width).
Conclusion • The cross section information can be used to determine the spin in the early stage. The result can be checked later by examining the spin correlation. • It works well in simple situations, but may need more work for complicated situations. • Degeneracies may occur and often can be dealt with by including several channels at once. • There could be no degeneracy if the mass of the lightest partner, e.g. LSP, is constrained by other experiments, e.g. dark matter relic density.
Example (Mgluino > Msquark ) • SUSY: gluino mass 608GeV, squark mass 550GeV. gluino q + squark, squark q + Bino • match total cross section and jet-multiplicity (not perfect) mg’=900GeV, mq’=800GeV,mB’=550GeV • Ht distributions are different (notice the difference in width).