1. Using Tau polarization for Charged Higgs boson & SUSY searches @ LHC
D. P. Roy
Homi Bhabha Centre for Science Education
Tata Institute of Fundamental Research
Mumbai, India

2. Outline τ Polarization effect SUSY and SUSY-Higgs searches at LHC
Charged Higgs boson signal in the τ channel
Using τ polarization for H± search
SUSY signal in the τ channel
Using τ polarization for SUSY search

3. τ-Polarization: V = ρ,a1 => 90% of 1-pr. hadronic decay
Best channel for τ id. is its 1-prong hadronic decay channel with BR ≈ 50%
=> 90% of 1-pr.
hadronic decay
V = ρ,a1
 τ (Pτ =+1) gives hard τ-jet from π,ρL, a1L
 τ (Pτ = -1) gives hard τ-jet from ρT , a1T
Can be distinguished
from X = pπ± / pτ-jet

4. Raychaudhuri & Roy PRD ‘96
ρT & a1T => even sharing of E/p among decay π’s.
ρL & a1L => uneven sharing, where the π± carries little or most of the E/p
Hardness of π± reqd for τ-id
=> X > 0.3
(low-X peaks of ρL , a1Linaccessible)
X > 0.8 cut will retain the high-X ρL & π conts ( Pτ = + Signal),
while effectively suppressing the ρT ,a1T conts ( Pτ = – bg ).
It will also suppress the fake τ bg from QCD jets effectively.

5. Natural Soln to the Hierarchy Problem of EWSB scale
SUSY and SUSY Higgs Boson LHC:
The Minimal Supersymmetric Standard Model (MSSM) is the most popular extension of the SM for 4 reasons:
Natural Soln to the Hierarchy Problem of EWSB scale
Natural (Radiative) Mechanism for EWSB
Natural Candidate for the cold DM (LSP)
Unification of Gauge GUT Scale
We hope to probe this model at LHC via search for
1) Supersymmetric (SUSY) particles, and
2) The extended Higgs boson sector of MSSM and in particular the H±.

6. 8 States - 3 Goldstone = 5 Physical St : h0, H0, A0& H±
H± LHC:
8 States - 3 Goldstone = 5 Physical St : h0, H0, A0& H±
H± carries unambiguous hallmark of MSSM Higgs sector
All the MSSM Higgs masses & couplings => MA & tan β

7. mt > MH± => large tbH+ BR at tan β  1 & tan β  mt/mb
LL QCD Corr => mq  mq(MH±) => mt =170, mb = 3, mc =1 GeV
Pert. Lim. of H±tb coupling upto GUT Sc. => 1 < tan β < mt /mb
MH= 140 GeV
H→τν dominate
at tanβ > 1
t→bH→bτν Sig
t→bW→bτν Bg

8. τ-Polarization: X > 0.8
Sig cross-sections satisfying Sig/Bg ratio > 1 shown for
MH = 80, 100, 120, 140, 150 & 160 GeV. (Raychaudhuri & Roy PRD ’96)

9. Assamagan, Coadou & Deandrea
mH > mt => gb→tH+
H+→ tb (leading): QCD Bg
H+→τν (BR~15%): Viable Sig
Viability of H±  tb channel is not supported by the full simulation study:Lowette, D’Hondt &Vanlaer

10. Heavy H± (MH>200GeV) Signal at LHC in hadronic τ decay channelb t g t
Sig has much harder pTτ-jet > 100 GeV (Enhanced by τ pol effect).
Azimuthal angle between τ-jet and missing-pT is peaked in backward
direction for signal ( forward direction for background).
Transverse mass of τ-jet and missing-pT —> MH for Sig (MW for Bg)

11. Guchait, Kinnunen & Roy
EPJ ‘07
PYTHIA+TAUOLA
+ CMSJET
Hardness of π± reqd for
τ-id => X > 0.3
X > 0.8 cut retains most of
the remaining Signal, while
effectively suppressing the Bg.

12. 3-prong τ-jet without π0
R3 > 0.8 or < 0.4 retains
most of the Signal, while
suppressing the Bg.

14. mT > 200 GeV cut
effectively suppresses
the Bg without
affecting the Signal.
Provides estimate of
H± mass.

15. H± discovery limit at LHC with luminosity of 30 fb -1 (solid)
and 100 fb -1 (dashed lines).

16. SUSY LHC:
mSUGRA: SUSY Br in HS communicated to the OS via grav. Int.
 m0 , m ½ , tanβ, A0 , sign (μ) at GUT scale ( A0 = 0 & +ve μ)
RGE ( Weak Sc masses)

17. Chattopadhyay et al
m0 ~ m1/2  TeV (Bino LSP)
mh > 115 GeV  m1/2 > 400 GeV (M1>2MZ)
 also large sfermion mass
Bino does not carry any gauge charge
 Pair annihilate via sfermion exch
Large sfermion mass  too large Ωh2
Except for the stau co-ann. region

18. τ is soft, but Pτ≈+1 Guchait & Roy Co-annihilation region μ >0
BR ≈ 1
BR = 1
τ is soft, but Pτ≈+1
μ<0
One can use Pτ to detect the
Soft τ coming from

19. = X
1-prong hadronic τ decay (BR≈0.5)
With pT > 20 GeV cut for the τ-jet the τ misid. Probability
from QCD jets goes down from 6% for R > 0.3 (pTπ±> 6 GeV)
to 0.25% for R > 0.8 (pTπ± > 16 GeV), while retaining most
Of the signal.

20. Soft τ signal from
decay in the co-annihilation region
Godbole, Guchait and Roy ‘08

21. N(15-25 GeV)/N(25-40 GeV) = 2.3 for Signal and 1.0 for BG.
So the steep rise of the Signal at the low PT end can be used to
distinguish it from the BG and also to measure the small mass
difference