determining spin in hadron colliders itay yavin in collaboration with lian-tao wang harvard...
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Determining Spin in Hadron Colliders
Itay Yavin
In collaboration with Lian-Tao Wang
Harvard University
The road ahead
• Introduction
• Simple decays, developing some tools.
• HERWIG – simulating spin correlations
• Where shall spin be found
• Conclusions
IntroductionThe LHC is about to go on-line very soon and discerning new physics is not going to be easy.
-Resonances
-End points
-Edges
-Lepton, jets
-what have you.
LNSM
for a recent accessment of the challanges in correctly interpreting the data, see:
N.Arkani-Hamed, G.L.Kane, J.Thaler and L.T.Wang, arXiv:hep-ph/0512190
We are also not the first to think about spin determination:A. Barr hep-ph/0405052 , hep-ph/05111115
P. Meade and M. Reece, hep-ph/0601124
Cascades
Observable
Unobservable
Observable
Observable
Unobservable
Observable
In this talk I will concentrate on measuring the spin through cascade decays.
Spin???
Rules for spin correlations
-1 -0.5 0.5 1
0.2
0.4
0.6
0.8
1
cos
Circular Longitudinal
cos-1 -0.5 0.5 1
1.2
1.4
1.6
1.8
2
Longitudinal Circular
LH, RH (RH) (LH)2
2
Em
-1 -0.5 0.5 1
0.5
1
1.5
2
cos121 P
cos121
When do particles get polarized?To understand how a particle might get polarized consider its rest frame
1) Production through Z0:Z0
ZaZa cc
RL
2) Decay through massive gauge-boson
Z’
RH
RH
long. dominate
RH LH
circ. dominate
3) Decay through a Majoranna fermion
RH
LH
LH
RH
LH
RH
LH
a)
b)
different
4) Decay through a Dirac fermion – must involve chiral interactions
Both must be chiral vertices
Approximations with MC simulators: (not with MadGraph and CompHEP)
The spin information in the propagator’s numerator is lost:
)(1 22
22222mp
mmmp
NWA
22 mp
mp
Narrow width approximation:
Monte-Carlo SimulatorsThere are many available: Pythia, HERWIG, ISAWIG, MadGraph, CompHEP
SPIN CORRELATIONS IN MONTE CARLO SIMULATIONS, Peter Richardson, JHEP 0111:029,2001 and references therein
Designed to simulate hadronic emissions with special emphasis on gluon interference.
HERWIG1
Utilizes a spin-correlations algorithms:
inoutinoutinpropout NWA
ininoutoutinpropout ''',
2
ininD '
',',
outoutD '',
[1] G. Marchesini, B.R. Webber, G. Abbiendi, I.G. Knowles, M. H. Seymour and L. Stanco, HERWIG: a Monte Carlo event generator for simulating hadron emission reactions with interfering gluons. Version 5.1 - april 1991, Comput. Phys. Commun. 67 (1992) 465.
Massive Gauge-BosonsIn order to implement spin-correlations for massive gauge-bosons in HERWIG, we need to write the polarization in terms of massive spinors:
)()()(2121 22
1),( pvpup
m
And massive spinors can be expressed in terms of mass-less spinors as usual and implemented into Herwig.
021
),(),(
L ),( R ),(
Where shall spin be found?We will assume for the moment that no leptonic partners are light enough to be produced.
Vs.
q~~kkW
2coslongP
012
2
2amama qwqw M
2)( Wqqw ppm
LaP cos121 Racos12
1
0222)( amaa qwRL MVs.
Comparison of theory with MC
2
22222~
max 4
),,()( 21
M
mmMMmm LSPWq
qw
Theory MC
1001 M
5002 M
80010tan
1000~ qm
)( 2P
2M
M1=180
160
120
100
We performed an initial scan of the parameters M1 and M2 keeping all the other parameters (mq, , tan) fixed
bins i
KKi
SSi nn
22
GeVHT 300
4.0R
Why is it a good channel?
1. Very sharp contrast between SUSY and KK signal.
2. Results in only few jets so combinatorics is not terrible.
3. 2 of the jets can (in principle) be set apart from the rest by reconstructing the W.
4. The signal is still strong even when averaged over Jnear and Jfar.
5. Assume no sleptons in the spectrum so fairly generic.
6. Standard model background?
Using SleptonsA. Barr originally considered,
q~ q~
q
2
l
l
LSP
q
2
l
l
LSP
Process 1 Process 2
Two problems:
1) You don’t know which lepton is which.
2) There is an equivalent chain starting with an anti-squark having all the arrows opposite which tends to wash out correlations
q~
q
l
LSP
q~
q
l
LSP
We can solve the second problem by considering instead:
Information about the charge is kept in the leptons.
1001 M
5002 M
800
10tan
1000~ qm
300~ l
m
KK
2lqm
SS
2lqm
Conclusions• The tools to investigate the possibility of measuring spin are available.• The channels we considered seem promising, but farther study (background, cuts,
smearing etc.) is warranted. • Spin determination through production channels (ala’ A. Barr) might prove to be a
cleaner signal.• Global studies combining different methods to cover the full parameter regions are
needed.• Gluon partner’s spin? Seems pretty hard!!!