Status of Linear Collider Status of Linear Collider Physics Studies in IndiaPhysics Studies in India

Sreerup Raychaudhuri

Indian Institute of Technology @ Kanpur

LC Physics Study MeetingKEK, March 6, 2005

The ILCWG

► Indian Linear Collider Working Group

• Formed in 2001

• Four meetings: 2001, 2002, 2003, 2004

• Hosted 6th ACFA Conference (2003)

► Contributions in three main areas

• Physics possibilities

• Machine design/development

• Data analysis (when there is data…)

Seven participating Institutions1. Tata Institute, Mumbai A. Gurtu

2. Indian Institute of Science, Bangalore R.M. Godbole

3. Physical Research Laboratory, Ahmedabad S.D.Rindani

4. Harish-Chandra Research Institute, Allahabad B. Mukhopdhyaya

5. Centre for Advanced Technology, Indore S. Krishna Rajagopal

6. University of Delhi D. Choudhuri

7. Indian Institute of Technology, Kanpur S. Raychaudhuri

+ Some other individuals in other institutions and some postdocs in foreign

countries

Modus Operandi is flexibleIndividual researchers are encouraged to work in LC physics and to collaborate, but no targets are set

► Experimental Research:

• Still to take off properly

• Some accelerator studies in CAT

• HEP experimentalists in India are heavily comitted to D0, CMS, ALICE, BELLE, INO

• Larger financial commitment required

► Good News (?)

• This has now come to the notice of policy-makers who control the funding…

Indian involvement in accelerator-based HEP experiments goes back very far…

1958 Cyclotron developed by M.N.Saha et al in Kolkata

1960s & 1970s

Attention shifted to cosmic ray studies, culminating in the Kolar Gold Field (KGF) experiment to detect proton decay

1980s Experimental participation in UA2 Collider Monte Carlo simulations pioneered by D.P.Roy, R.M.Godbole

1990s Participation in LEP-1 and LEP-2, D0, CMS…Growth of a school of collider phenomenologists

2000s Time to participate in a major way in the international efforts…

Why does the (Indian) HEP Why does the (Indian) HEP community find the idea of community find the idea of a high energy linear ea high energy linear e++ e e- -

collider socollider so exciting?exciting?

Physics advantages of a linear collider:

• e+e- offers a clean environment compared to a hadron collider

• Energy of each event is sharply defined• Laboratory frame is centre-of-mass frame• Possibility of beam polarization• Possibility of very high luminosities• Can be run in four different modes:

– e+e- usual mode– e-e- requires beam division– e requires laser back-scattering requires laser back-scattering

Major areas of research in electroweak physics:

• Standard Model: precision measurements• New gauge bosons:

– W , Z – Exotic charges, exotic couplings

• Non-standard Higgs bosons– MSSM & Two-Higgs doublets– Little Higgs models

• Exotic fermions– Exotic quantum numbers– Exotic couplings

• Supersymmetry– Breaking schemes: mSUGRA, GMSB, AMSB, gaugino-MSB– R-parity violation

• Extra Dimensions ● Split SUSY– Large extra dimensions (ADD) ● UED– Warped spacetime (Randall-Sundrum)

Standard Model at a linear collider:

• We expect a linear collider to have a very high luminosity, typically 10 - 1000 fb-1

• This should lead to copious production of W and Z bosons, through – e+e- W+W-– e+e- Z Z

• We can then study the decay products of W and Z, e.g. `gold-plated’ Z l+l- to determine the mass and width of the W and the Z

• A linear collider can also act as a top quark factory– e+e- t t high luminosity leads to copious production– Precision measurement of top quark mass and width possible

• Can also study the SM Higgs boson (discussed later)• No special Indian contributions to this area

• P.Mathews, V. Ravindran, K. Sridhar (hep-ph/0405292): NLO QCD corrections to two-jet production in ADD/RS gravity

New gauge bosons:

• Mixing of Z with exotic states is already strongly constrained by LEP-2; must consider new particle production and decays

• We can pair produce W/, Z/ just like WW or ZZ or γγ• We then typically focus on leptonic decays as trigger:

– W/ l+ ν and Z/ l+ l-

• Reconstruction of parent gauge bosons from final states is possible’– W/+ W/- l+ ν q q/ with quadratic ambiguity– Z/ Z/ l+ l- l+ l- with no ambiguity (`gold-plated’ signal)

• Presence of exotic states will show up in – ET distribution of leptons (both W/, Z/ )– Resonances in l+ l- invariant mass (Z/)

• In E6-type models, V and A couplings of Z/ are different from Z ; this will show up in AFB

• No special Indian contributions to this area

Gauge bosons with exotic charges and couplings:• W++ exists in some GUT models; usually heavy, but can be

made light using discrete symmetries– Decay chain W++ W+ l+ l+ l+ ν

• Possibility of exotic triple gauge vertices (TGV): assume SM is low-energy limit of some renormalizable theory: – WWγ, WWZ– Zγγ, ZZγ, ZZZ – 3γ is forbidden by C-invariance

• parametrize TGV in terms of form factors which are coefficients of dimension-5 and dimension-6 operators– 3 are CP-conserving, 2 are CP-violating

• Obtain bounds on these form factors by looking at– Total cross-sections (will pick up contributions from extra terms)– Kinematic distributions (will be different because of exotic operators)– CP-violating asymmetries

• Indian contribution: calculation of Zγγ, ZZγ, ZZZ vertices at one-loop in SM and MSSM– D.Choudhury, S.Dutta, S.D.Rindani (hep-ph/0001205 )

Higgs bosons I:

• Standard Model H0 is still to be discovered– Possible that it may be discovered at Tevatron Run-2 or at LHC

• Intermediate mass H0 (115 – 150 GeV) may still escape detection (according to famous CMS study)– This is the most interesting mass regime since MSSM (and minor

extensions, e.g. MSSM + Higgs singlet) predicts light Higgs in just

this region

• Linear collider at 500 GeV is ideal for this mass region– Higgs-strahlung process: e+ e- Z* Z H0 l+ l- b b

– Clean environment for detection of b-jets compared to hadron collider

• Lots of exotic Higgs models:– Two-Higgs doublet models, e.g. MSSM

– Singlet Higgses, coloured Higgses, composite Higgses

– Little Higgs models (latest fashion…)

Higgs bosons II:• Indian contribution: study of heavy Higgs bosons (MH > 2MZ )

- D.Choudhury, T.Tait, C.Wagner (hep-ph/0202162)-They consider

- e+e- Z* Z H0 (Higgs-strahlung)- e+e- ν ν H0 (W-boson fusion)- e+e- e+ e- H0 (Z-boson fusion)

- Higgs principal decay modes H0 WW, ZZ-Multi-lepton and multi-b-jet final states

-Involved combinatorics- They consider both SM and MSSM Higgs bosons- Conclude that with 1000 fb-1

• full parameter space of SM is observable (consistent with Higgs mass from precision electroweak measurements) • MSSM parameter space is observable up to kinematic limit

Higgs bosons III:

• Indian contribution: doubly-charged Higgses H++ at a γγ collider– S.Chakravarty, D.Choudhury, R.M.Godbole, B.Mukhopadhyaya

(hep-ph/9804297)– D.K.Ghosh, R.M.Godbole, B.Mukhopadhyaya (hep-ph/ 9605407)

• Higgs triplets have structure (H++, H+, H0 )• Exist in non-Standard models, e.g. L-R symmetric model• Decay mode: H++ H+ H+

– They consider γγ H++ H -- H+ H+ H - H -– Final states depend on the mass:

• If H+ is heavier than top quark, will decay into t b• If H+ is lighter than top quark, will decay into 2 jets (c s) or τ ντ

– All possible final states considered

– Conclude that large regions of parameter space are accessible• Accessibility increases with centre-of-mass energy• Accessibility increases with luminosity (no surprise!)

Higgs bosons IV:

• Indian contribution: use of tau polarisation in Higgs studies– Original idea due to Bullock, Martin and Hagiwara

• Consider one-prong decays of a tau lepton (mostly into a charged pion): accompanying neutral energy (mostly due to one or two neutral pions) will differ between L and R

• This can distinguish between tau’s arising through L-handed or R-handed couplings and hence can be used as a powerful discriminator between models

– CP properties: R.M.Godbole, R.K.Singh, S.Kraml (hep-ph/0501027)

– SUSY Higgs: R.M.Godbole, M.Guchait, D.P.Roy (hep-ph/0411306)

Indian contribution: Top Quark pairs and exotic couplings:

– S.D. Rindani & collaborators– (hep-ph/9809203, 0011321, 0211136, 0211134, 0204233, 0304046,

0309260, 0408083) – Several investigations of tt production in e+e- and – Mostly consider intermediate scalar or vector state

• Exotic t-t-scalar or t-t-vector couplings or leptoquarks• Form factors, both conserving CP and violating CP• Study lepton asymmetry in semi-leptonic top quark decays

– P.Poulose, S.D. Rindani, L.M.Sehgal (hep-ph/0111134)– They consider e+e- W+W- in BESS models

• Essentially parametrize Z-W-W coupling in strong limit in terms of form factors

• Measurement of lepton asymmetry will yield information on these form factors

– R.M. Godbole & collaborators– Several investigations of `total’ cross sections at e+e-

• Essentially add up beamstrahlung contributions• Use updated parametrizations for photon structure function

Supersymmetry:• SUSY has a very rich sparticle spectrum (awaiting discovery!)

– Gauginos • 2 charginos χ+, χ-• 4 neutralinos χ0

i (I = 1,4)• 8 gluinos ği ( i = 1,8 )

– Sleptons• Charged sleptons ěLi ěRi ( i = flavour index)• Sneutrinos (only left-chiral, one for each flavour)

– Squarks

• Squarks and gluinos are strongly interacting – QCD production at hadron colliders more promising

• Hence focus is on chargino, neutralino and slepton production at linear colliders

Signals for sparticles at a linear collider:• Some hundreds of production and decay channels

– Heavily dependent on mass spectrum of sparticles– Mostly encoded in Monte Carlo event generators:

• ISASUSY• SPYTHIA• HERWIG

– No longer worthwhile to simply calculate cross-sections!– More detailed study needed: detector simulations?

• Linear collider likely to become operational only after LHC has run for a few years– Early discoveries may be made at LHC

• Physics possibilities at a linear collider:– Try to pin-down parameters of the model in question using cross-sections etc.– Try to isolate exotic signals normally lost in background at a hadron collider– Particularly focus on regions of parameter space where hadron collider signals are not

clearly-defined

SUSY parameter space:

• MSSM has 124 unknown parameters– Hardly any predictive power; we require to reduce parameter space with

theoretical assumptions

• Most new parameters arise from SUSY-breaking scheme:– Gaugino masses M1, M2, M3

• (corresponding to U(1), SU(2), SU(3) sectors)– Squark and slepton mass parameters– Higgsino mixing parameter: – Tri-linear (scalar) couplings– Ratio of Higgs vev-s: tan

• Constraints on parameter space will come only if mechanism of SUSY-breaking (i.e. generation of these parameters) is considered– Usual idea: SUSY is broken spontaneously in a `hidden’ sector with very heavy

(unobservable) fields; communicated to `visible’ sector (observable fields) by some intermediate fields

Principal SUSY-breaking schemes I:

• mSUGRA: minimal supergravity– SUSY-breaking is carried from hidden sector to observable

sector by graviton-gravitino fields • We assume gauge unification at the GUT scale

• Electroweak symmetry is broken spontaneously by radiative corrections to Higgs sector, driving (one) Higgs mass parameter negative

– Model has 5 free parameters: • m0, m1/2, A, sign(), tan

– All sparticle masses are predicted in terms of these 5:• Usually the lightest neutralino χ0

1 is the LSP

(escapes detection leading to missing E and p)

• Usually we expect squarks and gluinos to be very heavy (~ 800 GeV)

• Usually we expect sleptons and snutrinos to be light (~100 GeV)

– LEP-2 constraints force us to take tan > 2 (approx)

– Tevatron constrains the m0 - m1/2 plane for different tan

Principal SUSY-breaking schemes II:

• GMSB: Gauge-mediated SUSY-breaking– SUSY-breaking is carried from hidden sector to observable sector by some heavy gauge fields– We assume gauge unification at the GUT scale– Electroweak symmetry may or may not be broken radiatively (both scenarios viable)

• Model has 5 free parameters– Mmess, Nmess, Λ, sign( ), tan –

mess stands for messenger gauge fields– Λ is the scale of SUSY-breaking

• Gravitino is the LSP and neutralino χ01 is the next-to-lightest sparticle

(NLSP)– Nearly massless gravitino escapes detection– Missing E and p signatures

– Typically χ01 γ Ğ

– Hard photons and missing energy are usually part of final state (useful to trigger on)

Principal SUSY-breaking schemes III:• AMSB: anomaly-mediated SUSY-breaking

– Inspired by brane-world scenarios – Invented to solve SUSY flavour problem– Place hidden and visible sector on different `branes’ I.e. four-

dimensional subspaces of higher dimensional space, separated by distance in extra dimensions

– SUSY-breaking is through conformal anomaly on hidden brane– Carried to visible brane across extra dimensions by graviton-

gravitino fields • Model has 4 free parameters

– M3/2, sign(), tan – Slepton mass-squared is negative sleptons become tachyonic!– Add on a slepton mass parameter m0 to make it positive

• Different spectrum from GMSB:– Neutralino LSP χ0

1 is nearly degenerate with chargino χ+1

– Chargino decay: χ+1 χ0

1 π+ (pion is very soft may not be detected)

– Chargino may just leave a track like a heavy lepton

Production processes at an e+e- collider:• Gauginos:

– Charginos: e+e- χ+i χ-

j- (i,j = 1,2)

• s-channel: photon, Z• t-channel: sneutrino

– Neutralinos: e+e- χ0i χ-

j0

• s-channel: Z• t-channel: selectron

• Sleptons:– Charged sleptons: e+e- ěLi ěLi , ěRi ěRi , ěLi ěRi

• s-channel: photon, Z• t-channel: neutralino

– Sneutrino pairs:• s-channel: Z• t-channel: chargino

Principal decay modes of sparticles:• All sparticles must ultimately decay to LSP, which escapes the detector.

– Origin of sparticle cascade decays– We detect the SM particles in the cascade

• Chargino Neutralino + W* Neutralino + f f/ – We detect the fermions and missing E, p– W can be reconstraucted only if it decays hadronically

• Neutralino (heavy) Neutralino (LSP) + Z* Neutralino + f f– We detect the fermions and missing E, p– Can reconstruct the Z from both leptonic and hadronic decays– If Z decays to neutrinos, heavy neutralino is also invisible

• Slepton Neutralino + Lepton of same charge, same flavour Chargino + Lepton of different charge, same flavour

– We detect the charged leptons and/or cascade decay products of chargino • This is a somewhat over-simplified picture: in practice we need to compute

branching ratios to all gauginos, all final state fermions– Signals can be quite messy

Indian contributions 1: parameter determination• Determination of parameters of chargino sector

– Y.Choi, M.Guchait, J.Kalinowski, P.MZerwas (hep-ph/0001175)

– Y.Choi, A.Djouadi, M.Guchait, J.Kalinowski, H.S.Song, P.M.Zerwas (hep-ph/0002033)

– They consider e+e- χ+i χ-

j- (i,j = 1,2)

– Measure 3 cross-sections– Measure several spin-correlations– Use these data to pin down parameters of chargino sector, viz.

• M1, M2, μ, tan – Discuss relative merits of different combinations of these– Consider radiatively-corrected cross-sections

• Determination of mixing angles of stau sector:– M.Guchait, J.Kalinowski, P.Roy (hep-ph/0103161)– SuperK results indicate mixing between muon neutrino & tau neutrino:

seems to indicate mixing between corresponding superfields, I.e. between muon and tau sneutrinos and smuon and stau through RG evolution

– Treat mixing angles as free parameters and measure from cross-sections for smuon pair production and stau pair production

Indian contributions 2: `invisible’ sparticles• Single photon signals for `virtual LSP’ scenarios

– A.Datta, A.K.Datta, SR (hep-ph/9605432)– Consider scenario when next-to-LSP is the sneutrino

– Decays to neutrino + neutralino (both invisible)– Consider radiative neutralino and sneutrino pair-production– Signal is photon + missing E, p– Should be observable over SM background ( νν with ISR γ) – Extend to scenario when next heavier sparticle is second neutralino

– Invisible decays to neutrino + sneutrino, LSP + Z* (neutrinos)

• Consider AMSB-inspired scenario when next-to-LSP is the chargino:– A.Datta, S.Maity (hep-ph/0104086)– Chargino decays to neutralino + soft pions (unobservable)– Consider radiative (ISR) chargino production– Again signal is photon + missing E, p

• Consider same AMSB scenario at a γγ collider– D.Choudhury, B.Mukhopadhyaya, S.Rakshit (hep-ph/0205103)– Final state is either photon + missing E, p or photon + π+π-

Indian contributions 3: GMSB signals• Single photon signals in e+e- collisions:

– A.Datta, A.K.Datta, A.Kundu, B.Mukhopadhyaya, S.Roy (hep-ph/9707239)

– They consider neutralino pair production e+e- χ01 χ-

10

– One neutralino decays χ01 γ Ğ

– Other neutralino decays χ01 Z Ğ ν ν Ğ

– Final state is photon + missing E, p– Use polarized beams– A.Ghoshal, A.Kundu, B.Mukhopadhyaya (hep-ph/9709431)– Consider left-right asymmetry in above signal– Use to probe photon-photino-gravitino & Z-Zino-gravitino couplings

• Tri-electron signals for GMSB at e colliders:– A.Ghoshal, A.Kundu, B.Mukhopadhyaya (hep-ph/9709431)– They consider e χ0

1 ĕR e ĕR ĕR

– Each selectron goes to electron + gravitino– Very small backgounds from SM and MSSM; – `smoking gun’ for GMSB

Indian contributions 4: AMSB signals• AMSB signals in selectron pair-production:

– D.K.Ghosh, P.Roy, S.Roy (hep-ph/0004127)– They consider e+e- ĕL ĕL

– One ĕL electron + neutralino (LSP), one neutrino + chargino– Chargino decays to pion + neutralino LSP– Two possibilities:

• Chargino leaves a heavy track in the detector• Chargino decays to pion with a displaced vertex

– Trigger on hard electron, missing E, p and look for heavy tracks or displaced vertices (claim `smoking gun’ for AMSB)

• AMSB signals in pair-production of sparticles:– D.K.Ghosh, A.Kundu, P.Roy, S.Roy (hep-ph/0104217)– More comprehensive study: They consider e+e- pairs of

Selectrons (L-L, L-R, R-R), Sneutrinos, Neutralinos (12, 22), Charginos– Final decay signal will still involve heavy tracks/displaced vertices

• AMSB signals in e colliders:– D.Choudhury, D.K.Ghosh, S.Roy (hep-ph/0208240)– They consider e sneutrino + chargino (rest is similar)

R-parity violation:• R-parity is a discrete quantum number

– R = (-1) L + 2S + 3B = +1 for SM particles = -1 for sparticles

– If R is not conserved, we will have rapid proton decay– R-parity conservation implies that sparticles always appear in pairs

at any vertex– R-parity conservation is responsible for stability of LSP– R-parity is assumed conserved in all of previous discussion

• Can R-parity be violated? Answer is YES – Ensure violation of either L or of B, but not both– Ensures proton stability is protected

• Three kinds of R-parity-violating operators:– λ LLĒ : 9 such λ’s– λ´ LQĎ : 27 such λ’s– λ ŪĎĎ : 9 such λ’s– Essentially 45 Yukawa-type couplings

• For phenomenological purposes we assume that only one coupling is dominant (different choices analyzed separately)

Indian contributions 5: R-parity violation

• Signals for LSP decay at e+e- collider– D.K.Ghosh, R.M.Godbole, SR (hep-ph/990233)– In R-violating scenario LSP neutralino decays

• LLE: dilepton + neutrino• LQD: dijet + neutrino, dijet + charged lepton• UDD: three jets

– Produce neutralino and chargino pairs: consider all possible cascade decays (84 channels !)

• LLE has very clear multi-lepton signals• LQD has interesting like-sign dilepton signals (Majorana LSP)• UDD must be detected in multijet scenarios

– Possibility of neutralino/chargino mass reconstruction• Signals for LSP decay at e collider:

– D.K.Ghosh, SR (hep-ph/ 9711473) Very similar study• Bilinear R-parity violation:

– B.Mukhopadhyaya, S.Roy (hep-ph/9612447)• Sneutrino resonances with associated photons:

– Choudhury, Rai, SR (2005)

Large Extra Dimensions:• Original idea due to Kaluza (1921), Klein (1926):

– unification of electromagnetism with gravity in 5 dim

• Embedded in superstring theory: – 10 dimensions, 6 compact

• Revived to solve hierarchy problem (1998):– N.Arkani-Hamed, S.Dimpoulos, G.Dvali

– There are d extra dimensions (d = 1- 6)

– Extra dim are all compact (radii upto 250 microns)

– Gravity free to propagate in 4+d dimensions

– Assume gravity is strong at TeV scale: cutoff for SM

– Newtonian gravity appears weak because graviton wave function spreads out into 4+d dimensions

• Observable consequences:– There exists a whole tower of closely-spaced massive graviton states

– Collective interaction of these builds up to near-electroweak strength

– Detection of gravitational effects at colliders is possible!

ADD phenomenology at linear colliders:• Each ADD graviton escapes detection

– Missing E, p signals

• Most important process for real gravitons is

– e+e- * G (Peskin et al) – Single-photon + missing energy signals

– Need to distinguish from all sorts of other new physics possibilities

• Indian contribution: – Part of ILCWG programme:

– confirmatory process: e+e- e+e- G– S.Dutta, P.Konar, B.Mukhopadhyaya, SR (2003)

– 2 3 process; 28 Feynman diagrams

– Calculation is long and messy Predict significant deviations from Standard Model

• Total cross-section

• Kinematic distributions

– Results of Peskin et al process and this one are correlated

Warped gravity models:

• L.Randall and R. Sundrum (1999)• Created to solve hierarchy problem without large dimensions

– Model has one extra dimension: orbifolded S1/Z2 (small)– There are two branes at each end: visible & invisible– Negative cosmological constant in bulk and visible brane– Fine-tuning of cosmological constants

• Leads to a unique `warped’ solution of Einstein equations• Gravity is strong (~ electroweak) on invisible brane, but graviton wave

function dies out exponentially (`warp factor) across bulk and reaches visible brane very weak - natural solution to hierarchy problem

• Observable consequences:– Again there is a tower of massive graviton states– Graviton masses are ~ electroweak scale– Each graviton couples with electroweak strength– Each graviton behaves like a WIMP

• Require a bulk scalar field to hold branes at correct distance apart– Leads to light scalar `radion’ field on visible brane– Higgs-like couplings with SM fields

RS graviton phenomenology at linear colliders:• RS gravitons must be heavier than 210 GeV (LEP-2)

• RS graviton width grows very rapidly with graviton mass– Only first three can form narrow resonances

– For large part of parameter space only first resonance is viable

• RS gravitons decay to all particle pairs– Maximum BR is to jets

– Width to WW and ZZ is also sizable

• Consider graviton resonances in SM processes, such as Bhabha scattering and e+e- +-

• Indian Contribution: RS graviton exchange in e-e colliders– D.K.Ghosh and SR (hep-ph/0007354)– Only t-channel graviton exchanges occur (no resonances)

– Deviations from SM in total cross-section and angular distributions

– Constraints on parameter space

• Indian Contribution: Single photon signals for RS gravitons– Part of ILCWG programme:

– S.K.Rai and SR (2003)– Process is e+e- : resonance is highlighted by spread in energy

Radion phenomenology at linear colliders:• Radion phenomenology is rather similar to Higgs

phenomenology for tree-level processes

• Indian contribution: • At one-loop, effect of kinetic terms in radion-fermion

couplings becomes important; can use to distinguish radions from Higgses (P.Das, S.K.Rai, SR: 2004)

• radion exchange effects in colliders – S.R.Choudhury, A. Cornell, G.C.Joshi– They mainly consider one-loop processes in SM (box diagrams)

which have tree-level contributions in RS model with graviton exchange

• (hep-ph/0007043) with radion exchange

• (hep-ph/0012043) ZZ with graviton & radion exchange

• (hep-ph/0202272)

Outlook

• Many exciting physics possibilities at a linear collider

• Major Indian participation: mostly beyond-SM physics

• Maximum contributions in SUSY and extra dimensions

• Encouraging: Lot of participation from younger people

• Crying need to make studies more realistic– Detector simulation, ISR, FSR, beamstrahlung, etc.

Lot of scope for collaboration between different countries, groups, institutions, individuals